2010-05-28

Integration of first and second generation of ethanol production from

wheat

Projektering, KET050

Project mentor: Guido Zacchi, Ola Wallberg

Written by: Emelie Arvinius, Pia-Maria Bondesson, Ilia Komorin, Maria Navasa, Patrik Svensson

Abstract Nordisk Etanol & Biogas AB is planning to build a new ethanol plant that will produce 130 000 m3 of

ethanol per year from wheat. Ethanol is used more and more as fuel for cars. Most of the ethanol is

imported because only 200 000 m3 of ethanol fuel are produced in Sweden, mainly from wheat and

barley.

The goal is to study the integration of a straw based with an existing starch based ethanol plant, where

the stillage is used for biogas production. The lignin from the straw is combusted to obtain electricity

and heating for the process and the surplus electricity is sold. This was done by first designing an

ethanol plant based on the starch production of 130 000 m3 of ethanol/year and then designing a straw

based plant with a capacity of 200 000 tons of straw/year to produce ethanol and biogas. An economic

analysis was performed together with sensitivity analyses on the most uncertain values as cost of raw

material, selling price of products and the design of the simultaneous saccharification and fermentation

of the straw based ethanol process.

In the straw based process, there are many ways to pretreat the lignocellulosic material. It was thought

that steam explosion with dilute acetic acid was the most suitable setup. The acetic acid chosen is also

an intermediate in the biogas process so it will not harm the process downstream as other acids might

do and it is converted to biogas.

The integration of the processes can be done in two places of the processes, either in the fermentation

step or where the streams to the distillation are connected. In this study it was assumed that integration

in the distillation step was the best option.

When integrating the processes, the energy demand is higher than if considering having the two

processes separately. The stripper column is the element that requires most energy. However, the heat

integration study should be improved to lower the energy demand, especially in the distillation step.

The production of ethanol mainly comes from the first generation process while the biogas comes

mainly from the second generation process. When looking at the energy efficiencies, the best result was

obtained when integrating the two processes due to the larger amount of electricity produced.

From an economical point of view, building the first generation plant is a profitable investment due to

the low investment cost and the high output of ethanol. The income for the methane and the dried

destillers grain (DDG) is much lower than for the ethanol which makes the plant sensitive and

dependent on the ethanol selling price and the cost for wheat, which is the main cost in the process.

Building a second generation plant and integrating the process in the distillation step showed to be the

most profitable option. Even though the investment cost would be twice as high as for the first

generation plant, the production of electricity would become a major income (almost 4 times higher

than for DDG which is no longer produced). Methane income would also be more significant. This makes

the process less sensitive to market price fluctuations. This means that the integrated process is more

economically stable and more economically profitable, depending on the spread of costs and incomes.

Contents 1. Background ............................................................................................................................................... 1

1.1 The company: Nordisk Etanol & Biogas AB ......................................................................................... 1

1.2 The project task................................................................................................................................... 1

2. Ethanol/biogas production from wheat ................................................................................................... 2

2.1 Ethanol production with 1st generation, starch based production ..................................................... 2

2.1.1 Wheat ........................................................................................................................................... 2

2.1.2 Pretreatment of wheat ................................................................................................................ 3

2.1.3 Pre-Mixing step ............................................................................................................................ 6

2.1.4 Gelatinization ............................................................................................................................... 6

2.1.5 Liquefaction ................................................................................................................................. 7

2.1.6 Fermentation ............................................................................................................................... 8

2.1.7 Distillation .................................................................................................................................. 10

2.1.8 Alcohol dehydration technologies ............................................................................................. 11

2.2 Ethanol production with 2nd generation, straw based production ................................................... 13

2.2.1 Pretreatment methods .............................................................................................................. 14

2.2.2 Hydrolysis and fermentation ..................................................................................................... 17

2.2.3 Distillation .................................................................................................................................. 17

2.3 Biogas ................................................................................................................................................ 17

2.3.1 Biogas process [18] .................................................................................................................... 17

2.4 Possible integration steps ................................................................................................................. 21

3. Suggested process ................................................................................................................................... 22

3.1 Ethanol production with 1st generation, starch based production ................................................... 22

3.1.1 Pre-Mixing step .......................................................................................................................... 23

3.1.2 Gelatinization ............................................................................................................................. 23

3.1.3 Liquefaction ............................................................................................................................... 24

3.1.4 Fermentation ............................................................................................................................. 24

3.1.5 Distillation .................................................................................................................................. 24

3.2 Ethanol production with 2nd generation, straw based production ................................................... 25

3.2.1 Process overview ....................................................................................................................... 25

3.2.2 Pretreatment.............................................................................................................................. 26

3.2.3 SSF .............................................................................................................................................. 27

3.2.4 Distillation .................................................................................................................................. 28

3.2.5 Solid removal ............................................................................................................................. 28

3.2.6 Heat and electricity generation ................................................................................................. 28

3.3 Biogas ................................................................................................................................................ 29

3.4 Integration of 1st and 2nd generation ethanol production ................................................................ 30

4. Results and analysis ................................................................................................................................ 31

5. Economics and sensitivity analysis .......................................................................................................... 36

5.1 Economy ............................................................................................................................................ 36

Equipment cost ....................................................................................................................................... 36

Chemicals, raw material and products ................................................................................................... 39

Operational plant costs and investment analysis ................................................................................... 42

5.2 Sensitivity analysis ............................................................................................................................ 43

5.2.1 WIS concentration in the SSF ..................................................................................................... 43

5.2.2 Enzymatic yield .......................................................................................................................... 45

5.2.3 Product price .............................................................................................................................. 46

6. Discussion and conclusions ..................................................................................................................... 49

7. References .............................................................................................................................................. 51

Appendix ..................................................................................................................................................... 55

Appendix I ............................................................................................................................................... 55

Appendix II .............................................................................................................................................. 56

Appendix III ............................................................................................................................................. 57

Appendix IV ............................................................................................................................................. 61

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1. Background Ethanol is used more and more as fuel for cars, either as an additive or in FFV-cars (Flexible Fuel Vehicles)

which can run on regular unleaded gasoline or on a gasoline blended fuel like ethanol. Most of the

ethanol is imported because only 200 000 m3 of ethanol fuel are produced in Sweden, mainly from

wheat and barley.

Nordisk Etanol & Biogas AB is planning to build a new ethanol plant that will produce 130 000 m3 of

ethanol per year from wheat in the beginning. This plant will be located in Karlshamn, Blekinge. The

production will increase to reach 260 000 m3/year.

At the start, it is planned that the ethanol will be produced from starch (out of wheat) while biogas will

be produced from the cereal stillage (first generation plant). To get as high energy efficiency as possible,

it is planned later on to use straw for the production of ethanol and biogas (second generation plant).

From the sludge, biofuel and fertilizer products will be produced. The aim of this project is to look for

the best and most suitable method in which the straw based part can be integrated in a starch based

plant.

The interest for producing ethanol from straw is of big interest, because the raw material is available in

large quantities and it does not compete as a food source, like wheat.

The problem with using straw as a sugar source is that the structure is hard to brake in order to release

the cellulose and the hemicelluloses content. Therefore, the pretreatment step requires more effort.

1.1 The company: Nordisk Etanol & Biogas AB Nordisk Etanol & Biogas AB was founded in 2006. Their goal is to become Sweden’s biggest and most

effective production plant for bio based ethanol, with a clear focus on the environment, market

development and profitability. The company will contribute to a faster conversion towards renewable

fuel sources, both nationally and internationally. The company will also help Sweden drastically

decrease its need for importation of fossil fuels and the use of them [1].

The company will use the residue stillage which always occurs during ethanol production to produce

biogas which will be used as fuel for vehicles and it can also be used for producing green electricity [1].

1.2 The project task The goal is to study the integration of a straw based with an existing starch based ethanol plant, where

the stillage will be used for biogas production. The lignin from the straw will be combusted to obtain

electricity and heating for the process and surplus electricity will be sold.

The main task of his project is to design an ethanol plant based on the starch production of 130 000 m3

of ethanol/year. Moreover, different process options should be investigated in order to design a straw

based plant with a capacity of 200 000 tons of straw/year to produce ethanol and biogas. Operation and

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investments costs as well as project valuation and economic analysis should be performed. Finally,

sensitivity analysis of the most uncertain parameters should also be done.

2. Ethanol/biogas production from wheat

2.1 Ethanol production with 1st generation, starch based production A general flowsheet of a first generation ethanol plant is shown in Figure 1. Other starch based ethanol

production plants may differ a little but the basic components are the ones shown in the figure. The

production of ethanol consists of a grain pre-mixing step followed by a gelatinization and a liquefaction

step. Afterwards, the fermentation takes place where ethanol is produced. The ethanol is then purified

to obtain pure ethanol at 99.8%.

Figure 1 General picture of process for the first generation

2.1.1 Wheat

Wheat consists mostly of starch. Starch is composed of two polysaccharides, where one has a linear

chain structure of glucose units called amylose and the other has a branched structure called

amylopectin, see Figure 2. The ratio between these two polysaccharides gives starch its specific

properties.

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Figure 2 Structures of the starch components [2]

High Amylose content in the starch gives a low viscosity at a given temperature due to its unbranched

structure compared to the branched structure in amylopectin. Because of the higher temperature

required to make amylase gelatinize due to the linear structure, it is not economic defendable to use

this kind of starch because of the high energy requirement. On the contrary, starch with high amount of

amylopectin content swells more easily in solution and has a lower gelatinization temperature and is

more advantageous for ethanol production [2].

2.1.2 Pretreatment of wheat

When the wheat arrives to the factory it is inspected for mold infestation, moisture content, weight etc.

The wheat is then stored in silos. Afterwards, the wheat continues to the sieving step were it is cleaned

from stones and other undesirable substances. After the sieving, the wheat is grinded by hammer mills

[3].

The Dry Milling process

The purpose of the milling is to increase the surface area to make the slurry more accessible. By

breaking down the wheat to very fine particles the penetration of water increases in the cooking step [3],

as well as the different enzymes used will break down the starch more effectively in less time. This is

important for the economy of the process [4]. There is a lot of different milling equipment used to grind

cereals but the most popular for distilleries are the hammer mills [3], see Figure 3.

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Wheat is transferred from the silos to the hammer mill before the pre-mixing. The particle size

distribution is controlled by the size of the screen that covers the walls of the inside of the machine

where the hammers are rotating and preventing bigger particles to pass until they reach the proper size.

Figure 3 Hammer mill for milling grain [4]

It is important to check the screens regularly so the size distribution is correct. It can differ as much as 5-

10 % in ethanol yields if the screens are incorrect [3]. As the hammer mill is a crucial step, replacement

of mills or screens should be considered from time to time [3]. If the particles are too big, the enzymes

have difficulties to reach the starch and if the particles are too small, they can cause problems in the

stillage centrifuge [3] as well as they can cause coagulation in the slurry tank. As long as the particles are

in the right range, the yield will not be affected.

The temperature of the flour should not exceed certain temperatures (25-35 °C) because it can pre-

gelatinize which can result in starch that cannot be converted downstream.

The wet- milling process

Several steps are used in the wet milling process. The wet milling process is divided in two sections. The

first section is the millhouse which produces slurry consisting of starch and many by-products. The other

section is the “finishing” part that consists of different steps that process the starch slurry from the

millhouse. It begins with soaking where the water soluble compounds are removed. After this step, the

starch is impure and in the last step, it is washed with pure water countercurrent to produce a pure

product.

The steeping step (soaking)

The millhouse produces starch using a series of steps beginning with soaking. When the wheat is

cleaned, it is stored for 20-40 hours with acid containing about 1600 ppm SO2 at a temperature of 52 ⁰C.

The steeping process swells the kernel, softens the shell and the protein structure, which will make it

easier to separate the starch and the protein from the germ and the hull, see Figure 4.

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Figure 4 Nomenclature for the different parts of the wheat [3]

Germ system

The steeped corn passes through the mills to separate the germ from the rest of the kernel. The germ is

separated from the slurry in hydro cyclones. After two stages of milling and separation, the germ is

washed counter currently in three steps and then is dewatered and dried.

After these steps, the seed contains about 50 % of oil with fiber protein and starch. The whole germ is

processed to recover the oil. This can be done at the plant or the whole germ is sold.

Fiber system

After the removal of the germ, the remaining is slurry of starch, protein, fibers and particles of different

sizes. Screens are used to recover the fibers and the particles. The fine particles of starch and protein

are processed in the next step.

Gluten separation

The slurry which contains the starch and proteins is dewatered so that the viscosity gets higher and

centrifuged so there is a separation between the starch and the proteins. The stream of gluten is also

thickened, dewatered and dried. The product is gluten meal which is sold.

Starch washing

The starch that comes out from this step contains small amount of impurities which must be removed.

This is done by countercurrent washing with water and the use of hydro cyclones. The product is almost

pure starch.

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To make ethanol from starch, it must be broken down to fermentable sugars by the hydrolysis reaction

where water combines with starch.

When dry milled wheat is used, the whole wheat is fermented and there are a lot of other materials

within the wheat. When using the wet mill process, those materials are not present and pure starch is

hydrolysed and fermented. The yeast can be recovered and used many times instead of being wasted in

the stillage.

The advantages of the wet milling process are several: the fermentation starts immediately and fast

because of high yeast concentration. Due to the high concentration, other microorganisms are displaced

so they cannot infect the process that easy. Because of the yeast recirculation, there is not a significant

growth of new yeast cells, which means there is a saving of 4 % of sugars that can be converted to

alcohol instead of yeast growth.

Nowadays, almost all the wet milling plants use maize as the feed but wheat can be used as well.

2.1.3 Pre-Mixing step

In this step, flour is mixed with water and backset from the distillation towers. The thin stillage (backset)

is rich in nutrients from the fermentation and has a low pH which is optimal for the enzymes [5]. The

slurry from the distillation stripper is pumped to a decanter, the water from the decanter is called thin

stillage and is rich in nutrients, and the centrifuged biomass is used for biogas production [5]. This step is

critical for the process. The stillage also reduces the water consumption and lowers the pH to about

between 4.5- 5.5 which is the optimal pH for the fermentation step.

If the dry matter content is too low, it will result in poor economy of the process. On the other hand, if

the dry matter content is too high, the viscosity will be too high and that will cause problems with heat

exchangers, enzyme kinetics and fermentation. Reduced viscosity will reduce the energy needed for all

the process steps. That is something gained with lower viscosity, but the dry substance should be as high

as possible.

In this step, a small quantity of α-amylase is added (0.02 % v/v) to facilitate agitation because of the high

viscosity of the slurry.

2.1.4 Gelatinization

After the pre-mixing step, the slurry is pumped to the gelatinization tank where the structure of the

starch is broken down. Starch gelatinization is a process that breaks down the bonds of the starch

molecule. This allows the hydrogen bonding sites to absorb water and swell. As the water enters the

starch, it decreases the number of crystalline regions and the heat added makes chains separate into an

amorphous form. When increasing the temperature over a specific value, the starch granules start to

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absorb water, swell and burst, releasing the starch into solution [3]. This process is irreversible and is

called gelatinization. The gelatinization temperature for wheat is between 58-65 ° C. At this

temperature, the starch grain swells and opens up so the enzymes in this step, the α-amylase, can

hydrolyze the long chains into shorter sugars. When the sugars are hydrolyzed to shorter chains, the

viscosity of the mash will decrease rapidly and will not increase again in the process, see Figure 5.

Figure 5 Gelatinization cycle with a decrease in viscosity [5]

If the temperature drops when the starch is gelatinized, it will re-crystallize and it cannot then be

degraded by α-amylase and thus, the starch will go through the whole process undegraded.

2.1.5 Liquefaction

As yeast cannot use the starch present in the process for fermenting, the starch needs to be hydrolyzed

first. During liquefaction, starch is converted to shorter carbohydrates by the presence of the enzyme α-

amylase, added to the slurry after the pH is adjusted [6] as explained before. The enzymes used in the

production of ethanol from wheat are the α-amylase and glucoamylase. α-amylase can be used to

hydrolyze the starch randomly to dextrin. The glucoamylase converts dextrin to glucose units by

hydrolyzing the α-1, 4 linkages by the non reducing end of the dextrins [6].

Enzymes are biological catalysts which can improve the fermentation by breaking down proteins into

amino acids and thereby, increase the yield. Moreover, they also reduce fouling downstream caused by

the proteins, which is very good in means that the system does not need to be cleaned that often.

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Liquefaction and gelatinization are generally carried out simultaneously. The viscosity is also reduced

because of the degradation to shorter carbohydrates (dextrins) [7]. The liquefaction is carried out at

about 88-93 °C and at a pH about 6-6.5. This is the optimum temperature for the enzymatic breakdown

of the starch to dextrins (carbohydrates). If the temperature is lowered, the energy demand is

decreased and pressure vessels for high temperature are not necessary. The enzymes catalyze the

hydrolysis reaction at this temperature for about 2 hours till the proper dextrose equivalent is obtained

[8].

Proteases are a group of enzymes that break down proteins. They are very important because they

break down the peptide bonds in proteins to liberate amino acids. They are able to break down almost

any kind of proteins [9]. The proteases can be used for increasing the capacity of the plant. They reduce

the viscosity so more wheat feed can be handled and energy consumption can be decreased in the plant.

The reaction time is proportional to the enzyme concentration. It is an important design factor if high or

low concentrations of enzymes are to be used and thus, the reactors must be designed for the correct

residence time [3].

Rhizozyme which is also an enzyme can be used instead of glucoamylase (0.5 kg/ton starch) or as a

supplement (0.1 kg/ton starch). With this, higher yields can be achieved. The optimal temperature for

Rhizozyme is 30-35°C and a pH around 3.5-4.5.

2.1.6 Fermentation

The most common modes of fermentation are batch fermentation and continuous fermentation. Most

of the processes use batch fermentation where the sugars can be converted to 75-95 % of the

theoretical and the final ethanol concentration going into the distillation tower is 10-16 % (v/v).

The continuous process does not have the drawbacks that are present in batch fermentation. It can be

run for longer periods without the need for shutting them down and for this reason the reactors can be

smaller in size. They can also be fully automated. To increase the productivity even further, two stirred

tank reactors in series can be used to avoid inhibition by ethanol. As the conversion of sugar will not

reach the maximum in the first reactor, it is pumped to the second.

To increase the productivity, the yeast cells are recycled. The recycle of the yeast cells creates a high

biomass concentration which reduces the time for the conversion of sugar to ethanol. This in turn

increases productivity because of the rapid fermentation. This process also has drawbacks because high

ethanol concentrations increase the death rate of the cells, has downtimes and is hard to automate.

With the continuous process with cell recycle, higher productivity is obtained because the high yeast

concentration can be maintained and the sugars can be fermented faster than if no cell recycle was

performed. The biomass leaving the reactor is separated by a centrifuge and pumped back to the

reactor. The cost of this system is higher than without recycling because of the cell separation.

The best yeast at the moment suited for the fermentation step is normal baker’s yeast, Saccharomyces

cerevisiae, under anaerobic conditions. The yeast Saccharomyces cerevisiae is classified as facultative

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anaerobic, which means that it can grow with or without the presence of oxygen. When the oxygen level

is sufficient and the sugar concentration is kept low, almost no ethanol is obtained. The sugars are

instead used for producing new cells. When the oxygen level is reduced or the sugar concentration is

over a certain value, ethanol is produced.

In a factory for producing ethanol, the substrate is fermented in an anaerobic way even if the

environment contains a small amount of oxygen. If oxygen is not supplied, the yeast will not produce

ethanol from sugars. The yeast will stop to grow and will not complete the fermentation. The yeast

requires this small amount of oxygen to synthesize vital molecules for the cell [3]. In order for the yeast

to grow in an optimal way, they need to have nutrients. If the right nutrients are not supplied, the yeast

will not grow fast enough and ferment well so the yield3will decrease.

Bacterial growth is also inhibited due to the lack of substrate. If the bacteria, that produce lactic acid,

are present in the system, they will start to consume the sugars and a decrease in the ethanol yield will

be observed.

In the separate hydrolysis and enzymatic fermentation (SHF) process, the enzymes catalyze the

breakdown of cellulose by the hydrolysis reaction to form glucose in one reactor. The product stream

then goes into a fermentor where the glucose is converted to ethanol due to the action of yeast.

The sequence steps for the simultaneous saccharification and fermentation process are the same as in

SHF except for that the hydrolysis and fermentation are combined in the same vessel.

Simultaneous Saccharification and Fermentation

Simultaneous Saccharification and Fermentation (SSF) has become a popular process on new plants and

is one process option for producing ethanol from starch. Many distillers changed from having a separate

saccharification reactor to adding the saccharification enzyme directly into the fermentor.

SSF has become the most low-cost process from which a high ethanol yield is obtained. The benefits

gained by performing the enzymatic hydrolysis together with the fermentation, instead of having a

separate step after the hydrolysis, is that the sugar concentration in this process will never reach any

high level that inhibits the enzymes used. As soon as the glucose molecules are formed, they are

consumed by the yeast and converted to ethanol.

The drawback of SSF is to find optimal conditions (pH and temperature) for both the yeast and the

enzyme. There are also difficulties to recycle the yeast and the enzymes. The temperature is usually kept

below 37 °C to maintain the yeast in a good growing phase [10]. The heat in the liquefaction slurry that

has to be cooled down to about 37 °C is reused by heat exchange in the process. The ethanol

concentration leaving the fermentor is about 10 % (w/w).

Both methods have their advantages and disadvantages but SSF was chosen as it is the method which

presents a higher ethanol yield and that is what was wanted to achieve [11].

For example, SHF has the possibility to operate the process at optimal temperatures as the two

processes are separated. These temperatures are around 30ºC for the yeast and 50-60 ºC for the

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enzymes. On the contrary, the temperature used in SSF is a compromise as low temperatures are

preferred by the yeast so no damage occurs but the enzymatic hydrolysis rate is slowed down.

Moreover, it is known that end-product inhibition occurs during the enzymatic hydrolysis step when the

concentration of glucose is high. In SHF, in order to avoid end-product inhibition, the solution must be

diluted or the enzymatic hydrolysis must take place longer time. Furthermore, in SHF the yeast cells can

be recycled as the liquid hydrolysate is separated from the solid residue before fermentation so, the

glucose consumption for the cell growth is kept low but the separation of the solid material requires an

extra washing step to avoid sugar losses. This means that the operating volume stream is higher and

thus, the sugar concentration is lower [11].

2.1.7 Distillation

Distillation is required to concentrate the low content of alcohol solution. By heating the liquid, ethanol

can be separated from water and other impurities in order to leave the distillation tower at 93 wt/wt%

of ethanol. There are different types of distillation systems; multi-stage, countercurrent, continuous.

The section above the feed entry is defined as the rectifying section and the part below the feed is called

the stripping section, see Figure 6 [3].

The good things about using distillation towers are a high alcohol recovery and that columns are easy to

scale. But it even has a downside. For example, if there is an azeotrope, the dryness specification cannot

be reached. The energy demand is higher for low feed concentrations of alcohol compared to high feed

concentrations [3].

Nowadays, a modern facility for producing ethanol from wheat can use as many as three separation

columns (stripper, rectifier and an additional rectifier). This system is used to improve the heat

integration. The heat from the first column is condensed and connected to the reboiler that drives the

second distillation tower and the same connection is made on the third column. If there is low grade

steam, it can be used and recovered from other parts of the facility, instead of producing expensive

steam [12].

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Figure 6 A typical distillation system [13]

In the stripper, the liquid going in is separated from all the ash, organic acids and other low boiling

products. The end product from the stripper is a mixture of ethanol and water with volatile compounds.

After the stripping column, the mixture comes into the rectifier column that removes most of the water.

[12].

The stillage from the stripper is centrifuged and the water that comes out is called thin stillage. Some of

the thin stillage can be recycled, where the recycled stream is called backset. The amount of thin stillage

recycled varies and is usually around 10-15 % by volume in the pre-mix step in order not to get

accumulation of substances in the process. The rest of the thin stillage can be used in the biogas

production. The wet cake can be used for biogas or for cattle feed.

2.1.8 Alcohol dehydration technologies

To use ethanol as transportation fuel, the ethanol content must be increased even further. A mixture of

ethanol-water forms an azeotrope at 95.6 wt % ethanol. To get beyond this point, a third component is

needed, the entrainer. Some techniques are explained through the text.

Nowadays, the most common method is to use molecular sieves, an adsorption method, see Figure 7.

They can selectively remove the water from the mixture and produce a very dry product. Molecular

sieves are granular and hard substances which can be spherical or cylindrical. The pores in the sieves are

typically 3 Angstroms (3*10-8 centimeters). The water molecules have a diameter of less than 3

Angstroms and the ethanol molecules have a diameter greater than 3 Angstroms. This means that the

water molecules can be adsorbed in the internal structure of the sieve and dehydration is achieved [13].

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By this technique, you can produce anhydrous ethanol of 99.9 % (wt) [13]. Usually a cyclic process is

used for loading and unloading the adsorbent, (water). To unload the water; vacuum or high

temperature can be used to desorb the water.

Figure 7 The adsorption process for both the loading and unloading cycle [13]

Another method to dehydrate the ethanol is to add some compounds to avoid azeotrope formation.

This can be done by the addition of solvents or salts. Examples of entrainers are benzene and ethylene-

alcohol.

Gas stripping is common in various chemical process industries in liquids containing volatiles substances.

Gas strippers achieve high recovery of ethanol. In gas stripping the gas is an inert gas that will not be

condensed in the condenser.

Another method is liquid-liquid extraction where the broth from the fermentation is placed in a mass

transfer contact with an organic liquid that can extract the ethanol from water. Because the extractant

has to be reused, the extracted ethanol must be removed in the regeneration unit, see Figure 8. This can

be done if a difference in volatility between the two liquids exists.

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Figure 8 Liquid-liquid extraction process [13]

Pervaporation is a membrane based process in which the fermentation broth is brought into contact

with a porous membrane, see Figure 9. A vacuum is applied on the other side. The use of pervaporation

was a very strong competitor to molecular sieve adsorption but today this system is rarely used. For

ethanol dehydration, a hydrophilic membrane is used which selectively transports water. The good thing

about these membranes is that they can remove water from alcohol even when the VLE (Vapor liquid

equilibrium) is unfavorable [13]. The components of the feed diffuse through the membrane and

evaporate into another vapor phase. Because different compounds have different diffusion

characteristics, some compounds will be enriched in the permeate.

Figure 9 The pervaporation process [13]

2.2 Ethanol production with 2nd generation, straw based production Instead of using starch for ethanol production, lignocellulosic material can be used. This can be wood,

waste products or crop residues. Lignocellulosic material contains mainly three different constituents;

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cellulose, hemicellulose and lignin. The production of ethanol from lignocellulosic material consists of

five steps; pretreatment, hydrolysis of cellulose, fermentation of hexoses, separation and effluent

treatment, see Figure 10.

Figure 10 General flowsheet for second generation ethanol production

2.2.1 Pretreatment methods

There are several different technologies available for ethanol production from lignocellulosic materials.

For the moment, the conversion processes, based on the use of cellulosic enzymes seem to be the most

promising ones for large scale application. The efficiency of this technology is limited due to the

chemical structure of lignocellulose biomass which inhibits the enzymatic breakdown of cellulose and

the solubilization of hemicellulose-derived sugars [14]. Cellulose strains are bundled together and tightly

packed in such a way that neither water nor enzyme can penetrate through the structure [15]. For this

reason, the pretreatment is an essential step for ethanol production in purpose of deconstructing the

structure to obtain an adequate yield of released fermentable sugars in the hydrolysis step [16]. In the

pretreatment, the solubilization of lignocelluloses components depends on temperature, pH and

moisture content18. In addition, this is considered to be one of the most expensive processing steps,

estimated to account for up to 33% of the total cost by the National Renewable Energy Laboratory [14].

Further on the goal is to achieve high sugar release pretreatment efficiencies; low production of toxic

compounds and low energy consumption as possible [15].There are four groups of processes that can be

used for pretreatment: physical, physico-chemical, chemical and biological.

Physical pretreatments

The first step in pretreatment of the wheat straw for ethanol production is size reduction through

milling, grinding or chipping. This should improve the efficiency of downstream processing. Both particle

size and moisture content are variables that influence the energy consumption as well as the

effectiveness of the following step in the process. Smaller particle size requires higher energy for

production while higher moisture content of straw will lead to higher specific energy consumption in

downstream steps [15].

Stillage

Distillation

Ethanol

Solid

Liquid

Fermentation Separation

Biogas

Straw Pretreatment

Anaerobic

digestion

15

Physico-chemical

In lignocellulosic materials, such as wheat straw, hemicellulose is the most thermal-chemically sensitive

fraction. Hemicellulose compounds start to solubilize into the water at temperatures higher than 150C,

where xylan is the most easily extracted.

Liquid Hot Water Pretreatment

In the Liquid Hot Water (LHW) pretreatment, the process is kept under pressure to maintain the hot

water in a liquid state. Commonly used are temperatures in the range of 170-230C and pressures over

5 MPa [15]. To improve the efficiency, it is possible to homogenize the straw by mechanical

pretreatment (for example milling) and the slurry of water and straw is agitated. The pressure applied,

the ratio solid/liquid, the processing time as well as the temperature are parameters that have been

tested in different ranges but the conclusion is that a two-step LHW pretreatment gives the maximum

recovery of fermentable sugars. However, economic and energetic aspects have to be considered in

proportion to the gained ethanol. As no acids are used, the materials costs are remarkably reduced

when hydrothermal pretreatment that does not employ any catalysts, as LHW, is used. Two fractions

are obtained after filtration of the pretreated slurry, one cellulose-enriched (water-insoluble solids

fraction) and one rich in hemicellulose-derived oligomeric sugars (liquid fraction or prehydrolyzate) [14].

Steam Explosion

Steam explosion is one of the most cost-effective and widely used pretreatment methods for wheat

straw. For this method, size-reduced (by physical pretreatment) wheat straw is rapidly heated by high

pressure steam for a period of time. Then, the pressure is suddenly reduced which causes an explosive

decompression of the material. Most commonly temperatures in the range of 160-230C are used and

the time period varies from several seconds to few minutes. Addition of chemicals such as H2SO4 or SO2

can improve the rate and extent of hemicelluloses removal and lead to enhanced yield of sugar in

enzymatic hydrolysis at lower temperatures [15]. Here, the economical aspect in more expensive

materials in comparison to the higher ethanol yields should be taken into consideration.

Ammonia Fiber Explosion (AFEX)

In this pretreatment, the wheat straw is exposed to liquid ammonia at high temperature and pressure

for a period of time, followed by a sudden pressure release causing an “explosion” of the structure. For

AFEX, there is no need for size reduction and it does not produce any inhibitors that may affect later in

the process. A comparison between steam and ammonia explosion has been made and the results

showed that the enzymatic hydrolysis was more or less the same with both pretreatments for wheat

straw. The main drawback with pretreatment method is handling large amounts of ammonia under

pressure, which is a high risk. However AFEX pretreatment of wheat straw is rarely reported and more

experimental work is needed [16].

Chemical pretreatments

For the chemical pretreatment of wheat straw, different chemicals such as acids and oxidizing agents

are used. Most commonly used is H2SO4, but depending on which one is used, the effect on the

lignocellulosic material is different. Alkaline, ozonolysis, peroxide and wet oxidation pretreatments are

16

more effective in lignin removal whereas dilute acid pretreatment is more efficient in hemicellulose

solubilization [15].

Acid hydrolysis

When inorganic acids, often H2SO4 are used for pretreatment, the downstream enzymatic hydrolysis

improves. Depending on the dose of acid the method is categorized as dilute- and/or concentrated-acid

hydrolysis. When the latter method is used, the wheat is treated with high concentration of acids at

ambient temperatures, resulting in high yields if sugars (30C, 30 min). The advantage with this method,

except the high yields, is that no enzymes for saccharification are needed. On the other hand,

drawbacks such as high energy consumption, equipment corrosion, obligation of acid recovery and

longer reaction times should be taken into consideration.

In the dilute-acid method, low concentration acids (0,5-1%) and high temperatures (>180C) are used.

The high temperatures make it possible to attain acceptable rates of hemicellulose conversion into sugar

oligomers and monomers. Even though low concentrations are used, the high temperatures increase the

equipment corrosion but the main drawbacks of this method are the formations of many inhibiting

byproducts and the required pH-neutralization for downstream processes [16].

Dilute organic acids, often fumaric or maleic acid, have been investigated in comparison to sulfuric acid

using temperature ranges 150-170C for 30 minutes. The author’s conclusion was that maleic acid

pretreatment of wheat straw was almost as effective as sulfuric acid in respect to enzymatic digestibility

where more xylose and less furfural (a byproduct that inhibits the hydrolysis) is produced. Another

positive aspect is the fact that pH-adjustment is not needed to achieve a pH of 4-5 in the SSF as well as

the use of organic acids might improve the biogas production.

Alkaline

In the alkaline pretreatment dilute bases such as sodium, potassium, calcium or ammonium hydroxides

are used at lower temperatures (50-65C) and longer periods (up to 24h) to achieve an effective method

with high sugar yields. Sodium hydroxide is the most studied alkaline method, but due to the high cost

of alkali utilization, the cheaper option calcium hydroxide (lime) seems to be the most promising one

[15]. The down side with lime is its ability to precipitate and clog up the instrumentation. Another option

is the use of ammonia due to the positive effect it might have later in the biogas production process.

See the biogas section for detailed information.

Biological

In this pretreatment method, microorganisms such as brown, white or soft-rot fungus do a selective

degradation of both lignin and hemicellulose. The suitable fungi for biological pretreatment should have

higher affinity for lignin and degrade it faster than carbohydrate components. For now, the most

effective one is the white-rot fungi but the rate of hydrolysis reaction is very low (~5 weeks) and needs a

great improvement to be applied to a bigger scale. There is interest in further studies as the biological

pretreatment is safe, environmentally friendly and has rather low energy consumption in comparison to

other pretreatment methods [15].

17

2.2.2 Hydrolysis and fermentation

Once the pretreatment of the wheat straw has been performed, the enzymatic hydrolysis and the

fermentation steps take place. The hydrolysis (converts the complex polysaccharides to sugar

monomers) uses the same principles as for the first generation ethanol production, see Section 2.1.6.

The enzymes for the second generation ethanol production are not the same as those used in the first

generation. To hydrolyze cellulose, enzymes as cellulases are used to depolymerize cellulose to first

cellobiose, two glucose units linked together, and then to glucose. The fermentation also uses the same

principles as for the first generation production, see Section 2.1.6.

In the present case, SSF has been chosen to be used as method to perform the calculations. If recycle of

yeast was to be considered, separation of yeast from the lignin would be required as the lignin would

otherwise accumulate. So, new yeast has to be produced each time and this means that the glucose loss

due to yeast cell production is higher in SSF than in SHF [17].

2.2.3 Distillation

The product stream from the SSF contains ethanol, non-converted glucose, pentoses, water and several

other products. Distillation is the method of choice for the recovery of ethanol from the fermentation

broth in second generation ethanol production, see Section 2.1.7 for more information about ethanol

recovery techniques.

2.3 Biogas

2.3.1 Biogas process [18]

After the distillation, the stillage from the first and second generation process using SSF will go to the

biogas production site. Organic matter will be broken down under anaerobic conditions in the presence

of different microorganisms.

The process can be summarized into four stages, see Figure 11:

1. Hydrolysis

2. Acidogenesis

3. Acetogenesis

4. Methanogenesis

Figure 11 The different stages in the biogas production process [19]

18

All these stages may take place in the same reactor. The first step in the process is the hydrolysis, where

the complex organic (cellulose, starch, protein and fat) matter is broken down to simple sugars, amino

acids and fatty acids.

During the acidogenesis step, the products from the hyrdolysis step are broken down even further. The

products from this step are volatile fatty acids (VFA), ammonia, carbon dioxide, hydrogen sulfide and

some other. These products are further broken down in the acetogenesis step to mainly acetic acid,

carbon dioxide and hydrogen. The production of hydrogen can only be maintained if the concentration

remains at a low level. If it is not maintained at a low level, the anaerobic oxidation will stop and so will

the whole process.

The last stage in the biogas production is the methanogenesis step. The bacteria in this step use the

intermediate products from all previous stages to convert them to methane, carbon dioxide and water31.

The Acetotrophic methanogens are the dominating methane producing organisms, which use acetate as

a substrate. In their metabolism, the acetate is converted into two parts, where one carbon is used for

the production of methane and the other for carbon dioxide. As these organisms grow very slowly, this

is often the rate determining step in the biogas process. If the residence time is too short, less than 12

days, the organisms can be washed out because of too short time to reproduce. The residence time

depends on the reactor, if the methanogens are immobilized on a support material shorter residence

times can be used.

Process optimization

To get a good conversion, stirring is needed in order to obtain a good contact between the substrate

and the organisms and get an even temperature distribution in the tank.

For the biogas site to work well there is a need for right different environmental conditions for good

growth of the organisms. Examples are oxygen content and salt concentration. Every microorganism has

its own environmental conditions that need to be satisfied but they can adapt to the system

environment to some extent.

The bioprocess is best when the pH is neutral or between 7.0 and 8.5. To maintain this pH, a good buffer

(alkalinity) is needed and if high protein content is present in the substrate, with much nitrogen, high

alkalinity can be achieved due to the formation of ammonium bicarbonate.

When the microorganisms digest the substrate, the C/N (carbon/nitrogen) ratio has the highest

influence for the biogas production. If this ratio is too low, the process will stop due to inhibition by

ammonia. If the ratio is too high, the organisms’ nitrogen content will be too low. A good C/N ratio is

between 10-30, with an optimum between 15-25.

The temperature is a very important factor. The biogas process contains many kinds of microorganisms

that differ in the temperature range for optimal growth. Many microorganisms follow the growing

curves, see Figure 12. The biogas process is set after these temperatures, approximately around 35-40

ºC and 50-60ºC. At temperatures above 60 ºC, the activity of the methane producers is reduced and

accumulations of fatty acids in the process are obtained.

19

Figure 12 Growing phases for different organisms

When a microorganism uses a substrate, new cells are formed but also different toxic byproducts from

the cells are produced. Those byproducts cannot usually be used by that cell but can be a substrate for

another type of cells. That is what happens in the biogas process where different kinds of

microorganisms use each other’s byproducts.

In the absence of oxygen, fermentation or anaerobic respiration usually occurs. The end products from

this process are mainly acids, alcohols, hydrogen and carbon dioxide. If many electron acceptors are

available, the organisms that use the most energizing component will dominate. This is what happens in

the biogas process. In this process, the microorganisms that produce methane dominate. They use the

carbon dioxide as the final electron acceptor. The biogas process is very sensitive to oxygen. The

methanogenes are very sensitive and do not survive if they come into contact with oxygen and other

bacteria in the process.

During the anaerobic digestion, there is very little heat release from the organisms so the heat must be

supplied from another source because most of the energy released is obtained in the end product.

There are two temperatures used for the biogas digestion, which is around 37 ºC (mesophilic) and

around 55 ºC (thermophilic). Once the temperature is set, it should not vary more than +/- 0.5 ºC to get

the best results. If the temperature is lowered under the optimal temperature, the fermenting organism

will continue to produce acids and alcohols but the methane producing organisms will stop producing

and cannot take care of the fermentation products. The fermentation products will start to accumulate,

the pH will decrease and the process will stop.

The choice of temperature varies. At a higher temperature, the digestion is faster but it gets more

unstable because the optimal temperature for the organism is closer to the maximum temperature the

organism can tolerate. More heat is also needed to be supplied. Also ammonia is released at a higher

temperature that will slow down the organisms.

The process can be performed in different ways depending on which substrate is to be digested. During

continuous digestion, material is pumped in as slurry continuously, see Figure 13. It gives an even

20

amount of biogas production during the whole time. For this to work the slurry should not have a dry

substance of more than 15 % dry solids, otherwise the slurry cannot be pumped. The slurry is usually

pumped between 1-8 sets/day to the biogas process; this is called fed-batch production. The opposite is

batch production, where all the material is digested at the same time as it is maintained in the same

reactor during the whole process, see Figure 13. Afterwards, the material is taken out and new material

can be put in to be digested. In a batch production, when the material is degraded and new material is

put inside a certain amount of the digested material which contains sludge is used as inoculate. The

amount of inoculate varies and depends on the substrate, but it is between 20 and 70 % [20].

The decomposition rate is fast at the beginning of the process and decreases over time. It can be hard to

maintain an even digestion rate in this system if the dry matter content is high. Also the methane

production is greatest at the beginning of the production and declines over time.

Figure 13 Picture of a batch and continuous digestion

Either one or two digester chambers can be used for the different steps. The simplest case is when the

whole process occurs in one chamber. When two digester chambers are used, the process is divided into

two steps. In the first step only hydrolysis and fermentation take place and most of the acid production

is done. The second step is customized for methane production. This process is well suited when easily

digested products as monomer sugars, fatty acids and amino acids are present.

The temperature has a big influence on the residence time. In the mesophilic range, the residence time

is about 15 days or longer but in the thermophilic range it can be around 10 days. The methane

producing bacteria undergo mitosis during a time of 12 days, this means that the residence time of the

substrate cannot be lower than that value or otherwise, the bacteria will be washed out. If the methane

producing bacteria are immobilized on a support material, the residence time can be shorted down to

two days to get an economy in the system if the feed into the process is high. To use a system which has

a residence time for two days, the bottom products from the distillation are separated in a way that only

the liquid fraction will go for biogas production and the solids will be taken away and be sold as animal

feed if having a first generation ethanol production plant. If having a second generation ethanol

production plant, the solids will instead be used for production of heat and electricity.

21

Fertilizer

The degradation of organic material in the bioprocess gives biogas and if the remaining sludge has good

quality, it can be used as fertilizer. The different minerals that are available are concentrated in the end

product. Many factors contribute to the quality of the fertilizer like the type of substrate, pretreatment

method and process conditions.

The liquid fertilizer has a dry substance of 2-7 %. In some biogas plants the fertilizer is divided into a

solid and a liquid part. When the fertilizer is divided into two parts, the liquid part will contain more

nutrients and the solid part will contain more humus formation materials. Under the microbial

breakdown in a biogas process many different minerals are released. Bio fertilizer therefore contains

phosphor, potassium and manganese for the plant to take up. Bio fertilizer that will be used must

contain at least 2 kg/tone of ammonia and 3-4 kg/tone of total nitrogen content. The content of

nitrogen in the digestate can be increased if more proteins are added in the substrate. However, too

much protein is not good either because it will produce too much ammonia that will inhibit the methane

producers in the biogas process.

2.4 Possible integration steps There are different steps where it is possible to integrate the first and second generation ethanol

production, like the fermentation or the distillation step.

A good way to integrate the first and second generation plants is in the fermentation step. This can be

done because the same yeast is used and it operates at the same pH. The integration results in a higher

ethanol concentration from the fermentation and is better than running the both processes separately.

The advantage for getting higher ethanol concentration is that the energy consumption for the

distillation is reduced dramatically because less water needs to be removed. During the pretreatment,

several sugar degradation products that come from hexoses, pentoses and weak organic acids have

been shown to inhibit yeast and enzymes. Small concentrations of acids have a stimulating effect for

producing more ethanol. When mixing the pretreated slurry from the 1st generation plant with the one

from the 2nd generation plant, the inhibitor concentration will be diluted and the fermentation of

glucose from starch and cellulose will be improved [21].

Another process integration alternative that also works is to integrate the first and second generation

processes in the distillation step. The processes are run separately and combined in the distillation step

as the incoming feed to this step has the same physical properties. The first stripper acts to remove

unwanted products with low volatility, as ethanol and water mixture to concentrate the ethanol. In the

second distillation tower were only ethanol and water remains, the ethanol reaches the azeotropic point

which is the same for all integration possibilities.

22

3. Suggested process

3.1 Ethanol production with 1st generation, starch based production

Wheat grain composition used in the process is shown in Table 1 and is based on approximate figures and reactions from literature, see Table A1 Appendix I, it is also assumed that 3% of the fibers are soluble and labeled as fermentable sugars which react as if they were starch. The water content is assumed to be 15%.

Table 1 Dry solid composition of wheat grain composition

DS%

Starch 69

Fermentable sugars 3

Protein 13

Fiber 10

Fat 3

Ash 2

Glucose is not produced until the SSF step. The steps before SSF (see Figure 14) soften and break down the starch/amylopectin to shorter chains to lower the viscosity and so that glucoamylase can penetrate the structure more easily. The operation time for the plant is 8000 hours a year.

The pH in the first generation ethanol process is not taken into account in the calculations and is not adjusted during the process.

The overall reaction for the process is the following, for 72 DS% starch (69 DS% starch + 3 DS%

fermentable sugars). The theoretical yields for the reaction are shown in Table 2.

Table 2 Theoretical yields for the reaction from starch to ethanol. The yields are given in kg/1000 kg wet starch.

Yield kg/1000 kg

wet starch

Glucose 800

Ethanol 409

Carbon dioxide 391

The suggested process for the first generation ethanol production process is shown in Figure 14.

23

E-1

Silo

Wheat transportation

P-2

E-2

Hammer mill

Pre mixing tank Gelatinization/Liquifaction

35 % solids

vvxSSF

CO2 out

Warming stream to the pre-mix tank

Cold water

vvxPump

Stripper Rectifier

E-9

Condensor

Fuel ethanol

Bioreactor Bioreactor

Methane/CO2

Methane

Waste products

Glucoamlylose

Yeast

Water

Alfa-amylose

Centrifuge

H2O out

Upgrading plant

Solids for animal feed

Backset (10 %)

Condenser

Mixer

Water slurry

Slurry for biogas

Figure 14 Flowsheet for suggested 1st

generation ethanol plant

The wheat is kept in silos with an average temperature of 10°C. Before pre-mixing, the wheat grain is

milled in a hammer mill. This is done to gain more surface area for the enzymes to work. The milling is

important for the ethanol yield. [22]

3.1.1 Pre-Mixing step

Milled wheat flour, α-amylase, fresh water and backset water are mixed all together in a vessel and heated up to 55°C under stirring for 60 min, see Table A3 Appendix I. The slurry in the pre-mix vessel is mixed until a 35 wt% of solids is reached, based on the report from Novozyme. The α-amylase in the premix is purchased from Novozyme, the trade name is Termamyl SC/Liquozyme SC, the choice was based on information gathered and based on the specifications for the process. That is the temperature interval and the pH.

0.2 kg enzyme/ton starch is added in the pre mixing to lower the viscosity and make mixing with the enzymes better, even though the main work by the enzyme is done in the liquefaction step where the rest of the enzymes are added.

3.1.2 Gelatinization

After the premixing, the slurry is fed into the gelatinization vessel and heated up to 60°C. And then,

stirring takes place during 30 minutes for further swelling of the starch granules. The slurry contains 35%

of solids. This is an optimal composition that is used in first generation production in the industry and

recommended in many articles, see Table A4 Appendix I. It keeps the viscosity of the slurry acceptable

and result in high ethanol yields.

Keeping the temperature low keeps the starch granules from building coatings so that enzymes would

not be able to penetrate the structure and which leads to lower yields [22].

Heating is done by direct steam or heat exchanging with a hot process stream.

24

3.1.3 Liquefaction

In the liquefaction, the slurry is heated up to 90°C and stirred for 120 minutes. Heating is done by direct

steam or heat exchange. An additional 0.2kg α-amylase/ton starch is added. Retention times and

temperatures do not vary that much in the literature, see Table A5 Appendix I. The choice was based on

literature but also on the assumption that no other pretreatments were taken into account and the

shorter times in premixing and gelatinization.

3.1.4 Fermentation

Before the fermentation, the slurry needs to be cooled down. This is done rapidly to avoid

retrogradation. To save energy, the heated stream is heat exchanged with the cold water feed entering

the pre-mixer warming it up to 55°C.

The incoming stream to the fermentation enters one of the six fermentation vessels, where the slurry

then ferments for 50h. Each fermentor is working batch wise. The feed in and the feed out are adjusted

so the rest of process can run continuously. The fermentation process is chosen to be SSF as it results in

a higher yield, shorter reaction time and less process steps, according to literature, see Table A6

Appendix I.

The temperature is set to 31°C, as 30-33°C is the optimum. A temperature rise of 6-8°C is caused by the

reaction so 31°C is chosen to keep the temperature inside the yeast optimal temperature interval. For

cooling the mash, an external cooling jacket is used. The fermentation works best under yeast growth

which has an optimum at 28°C, so 31°C in the suggested fermentation and an adequate cooling will keep

the fermentation under good conditions.

The total yield for the suggested SSF process is set to be 84.3% of theoretical for starch to ethanol. For

calculations, the yields for starch to glucose and glucose to ethanol were divided into 98% and 86% from

given information and guides from Professor Guido Zacchi and Doctor Ola Wallberg from the

department of Chemical Engineering, Lund University. There have been documented higher total yields

but 84.3% was the best figure that could be found from full scale industrial ethanol plant with SSF, see

Table A2 Appendix I.

In SSF, both the enzyme for breaking up the oligosaccharides into glucose and the yeast to ferment

glucose into ethanol and CO2 are used simultaneously without inhibition of each other. The enzyme

chosen is Novozymes Spirizyme Plus® which is a glucoamylase with a dosage of 0.8 kg/ton starch and the

yeast used is Saccharomyces cerevisiae. The dosage is chosen to be 1.5 kg yeast /ton mash.

Except for the main reaction in the SSF, there are other byproducts formed where glycerol is the most

common and is taken into account. 4% of the glucose is assumed to be converted into glycerol [5]. The

byproduct does not affect any reaction or contributes to contamination and is later on used in the

biogas production. CO2 is of course a major product from the fermentation and is almost 1:1 with the

ethanol. The gas is purged out from the SSF.

3.1.5 Distillation

The slurry from the SSF is fed to the distillation unit, which was set to produce an ethanol stream where

the outgoing concentration was 93 w/w%. The distillation unit consists of two different columns. The

25

first unit is a stripper column consisting of 30 trays. The energy needed in the reboiler in the stripper is

generated in a heat and power plant, where bought pellets are burnt. To diminish the heat needed in

the reboiler in the stripper, the stillage from the stripper is used for heat exchange with the feed going

into the stripper.

The vapor stream from the stripper is condensed in the reboiler for the rectifier and then fed to the

rectifier column. The heat provided is used in the reboiler. The bottom product from the stripper is

centrifuged and some of the liquid is recirculated as backset to the pre-mixing step and the rest is taken

to the biogas plant. The solids with a dry matter content of 50% are used for cattle feed. These solids,

which are the remaining fermentation residues, are also called DDG (distilled dried grains). The DDG can

be used as animal feed after it has been dried. It is an excellent protein and energy source for cattle. It is

rich in residual yeast protein, minerals, vitamins, fibers and fat. It can also be used as fuel [23].

The product from the stripper, assumed only to contain water and ethanol, enters the rectifier in the

45th tray of a total of 50 trays. The distillate stream, having a concentration of 93 w/w%, is taken to the

dehydration step to remove the rest of the water. The dehydration step consists of molecular sieves.

This results in a 99.8 w/w% ethanol stream. 20 % of the ingoing ethanol was assumed to be used for

regeneration in the dehydration step. The bottom stream from the rectifier is taken into water

treatment since it is assumed to not contain more than water.

The calculations were performed with the flow sheeting program Aspen Plus®.

3.2 Ethanol production with 2nd generation, straw based production

3.2.1 Process overview

The straw based ethanol production process has a capacity of 200 000 tons straw/year, as defined by

NEAB. The working hours/year is assumed to be 8 000.

The wheat straw being used in this project was assumed to have a 90% dry matter content. The

composition of the straw is assumed to be as given in Table 3. This assumption is made from different

articles, where the composition of straw, has been analyzed. For cellulose, hemicelluloses, lignin and ash,

an average have been used. The leftover has been called residuals. The different compositions are listed

in Appendix II.

Since the majority of the hemicellulose is xylan and only a small part is arabinan and other sugars, the

hemicellulose is said to only contain xylan. Lignin consists of both acid soluble and acid insoluble lignin,

where the soluble part is small compared to the insoluble part. Therefore, lignin is assumed to contain

only acid insoluble lignin. The residuals are hard to determine. Proteins and fats are a part of the

residuals but also other components can be a part of it. In the project, assumptions are made that 50%

of the residuals are fat and 50% are proteins.

26

Table 3 Composition of wheat straw

Weight (%)

Cellulose 35

Hemicellulose 23

Lignin 23

Ash 5

Residuals 14

The process designed for straw based production is shown schematically in Figure 15. The straw is being

pretreated in a first step and then separated with a filter press where half of the liquid goes directly to

the biogas production and the rest goes with the solid part to the SSF. After the SSF, the products go to

the distillation step, where 93wt% ethanol is obtained in the distillate and the rest product, the stillage,

is fed to the biogas plant. Before the stillage enters the biogas plant, the solids are filtered and taken to

a heat and power plant for heat and electricity generation.

Straw transport

Straw in

HAC-Spray

vvxPretreatment

Steam in (20 bar)

Flash-vesselCondesor

Filter

SSF

CO2 out

Nutrient

Ammonia

Water

Enzyme

Yeast

vvx Pump

Stripper Rectifier

Condensor

P-15

H2O out

Dehydration

Fuel Ethanol

Bioreactor Bioreactor

Methane/CO2

Methane

Waste products

Filter Unit for solid

separation

Lignin for burning

Mixing tank

Upgrading plant

Kondensor

Heat for SSF

Heat

Water +volatiles

Filter unit

Figure 15 Schematic over 2nd

generation process

3.2.2 Pretreatment

As pretreatment method, steam pretreatment, of diluted acetic acid impregnated straw, at 20bar was

chosen. Using sulphuric acid, which is the most commonly used acid for this pretreatment method was

not an option since the sulphur affects the downstream process negatively. The sulphur inhibits the

27

methane production in the biogas plant [18]. Organic acids have the potential to achieve high yields as

using sulphuric acid. Organic acids also have the positive effect that smaller amounts of byproducts in

the pretreatment step are produced since it is not promoted by the organic acids, [24]. Since it is an

organic acid, it can be used in the downstream process as a substrate generating methane in the biogas

plant. Acetic acid is chosen because it is one of the last intermediates during the anaerobic digestion

where organic material is converted into biogas. The acetic acid being used is diluted to 1 weight-%

assuming that is gives the best yields in the pretreatment.

Since no data was found for pretreatment of wheat straw with this method, the parameters and yields

were taken from Linde where the pretreatment was made with sulphuric acid [25]. In Linde’s

experiments the highest yields for both cellulose and hemicelllulose after the pretreatment were

achieved when the material was heated up with steam to 190°C during 10 minutes. The yields of sugar

recovery from the pretreatment are almost the same but since Linde got more than 100 % yield, the

yields being used have been scaled down. The glucose recovery was scaled down and it was assumed

that no HMF was produced. This assumption was based on that smaller amounts of byproducts are

generated when using organic acids. Also less furfural is assumed to be generated compared to Linde’s

results. The assumed yields are listed in Table 4, where the pretreated wheat straw is divided into the

liquid part (hydrolysate) and the water insoluble solid part (WIS).

Table 4 Yields of sugar recovery in pretreatment where a stands for furfural

Yield (% of theoretical)

WIS Hydrolysate Byproduct

Glucose 96.0 4.0

Xylose 23.9 75.9 0.2a

Before feeding the straw into the pretreatment reactor, the straw is treated with the diluted acetic acid.

This can be done by soaking, where the material is impregnated in a batch and then pressed, or with

spraying, where the material gets sprayed with the acid. The soaking leads to an evenly impregnated

material. The spraying method involves lower amounts of chemicals and no pressing step [11]. This

makes spraying a more feasible alternative in big scale and was also chosen in the process. The spraying

is assumed to change the dry content of straw to 50%.

The steam used in the pretreatment plant was set to 20 bars (212°C). The energy losses in the

pretreatment reactor is said to be 10% of the steam required for heating up the material to 190°C.The

pressure is then lowered by a flash to 1 bar and the material is then cooled down to 35°C. The flashed

steam is condensed and taken to the biogas plant. The heat generated is used in SSF to heat up to and

maintain the needed temperature.

3.2.3 SSF

Before the slurry from the pretreatment is fed to the SSF, 50% of the liquid is taken away with a filter

press. The liquid is later mixed with the stillage stream from the stripper and taken to the biogas plant.

28

SSF has been chosen to be used as method to produce ethanol. Process data are based on Linde’s

results [25]. The temperature and pH was set to 35°C and pH 5 just as Linde. The residence time was set

to 55 hours since in Linde’s results the concentration of ethanol is almost as high as possible after that

time. Running for longer time is assumed to result in higher costs for the reactors than the extra amount

of ethanol gained.

In Linde’s experiments, the enzyme dosage was varied. Enzyme dosage of 12 FPU/g WIS was chosen

instead of the highest dosage of 14 FPU/g WIS since the difference between the results was not that big

and the enzyme costs get lower. Instead of Celluclast and Novozyme that Linde used, an enzyme

mixture, Cellic Ctech, which combines both enzymes that depolymerize the cellulose into cellobiose and

converts the cellobiose into glucose units, is used. This has an activity of 96 FPU/ml enzyme mixture.

The nutrients and the amounts of these are the same as in Linde’s study, 0.5 g/l (NH4)2HPO4 and 0.025

g/l MgSO4·7H2O. The yeast being used is normal baker’s yeast, which is cheapest. In this process there is

no yeast cultivation, all yeast that is being used is bought. The amount being added is decided to be 3 g/l

based on Linde’s experiment.

The WIS going into the reactor is set to 15% by diluting it with water, which is higher than the 5 % Linde

uses. Since the WIS is higher in this process, the yields are set lower, based on Linde’s results for various

WIS concentration in the SSF. [26]

The yield for the enzymatic hydrolysis was set to 60% of theoretical after the pretreatment and for the

fermentation 90% of the theoretical, which results in an overall ethanol yield in the SSF to 54%. Linde

achieved an overall yield of 62% after 55 hours.

Ammonia is used to set the pH to 5. In Linde’s experiments, sodium hydroxide was used but the nitrogen

in the ammonia is better for the downstream process. The nitrogen prevents the chance of too high

acidity in the biogas plant as an effect of the fermentation of the sugars in the anaerobic digestion [18].

3.2.4 Distillation

For information about the distillation and the following dehydration system see Section 3.1.5

Distillation. The difference between the first and second generation ethanol plant is that the bottom

stream from the stripper is filtered and the solids are taken to the heat and power plant and no liquid is

used as backset. All the liquid is instead used for biogas.

3.2.5 Solid removal

The solids are removed before the biogas plant because some of them are not digestible. The complex

structure of lignin makes it for example impossible to digest. Also the time to digest the stillage is

shortened when removing the solids. The solids can be used for heat and electricity generation. It is

assumed that the solids are fed to the heat and power plant with a dry content of 50%.

3.2.6 Heat and electricity generation

The lignin and other solids, which are removed before the biogas step, are used for generating heat and

electricity, see Figure 16. The process is based on Sassner’s model [27]. Like in this model, the substrate

is burnt and the flue gases are used to produce 91 bar steam at 470°C. Into the burner, air is provided.

29

The flow rate of air is set to be 25% higher than that required for burning. The steam is expanded in the

turbines from which 20 and 4 bar steam is taken out to cover the energy demand in the process. The 20

bar steam is used in the pretreatment reactor. The 4 bar steam is divided in different streams, some are

used to preheat the sprayed straw to 95°C before it enters the pretreatment reactor and some are used

to produce that amount of heat required in the reboiler in the stripper. The rest of the 4 bar steam is

expanded in a condensate turbine at 0.1 bar to generate electricity. The condensates from the

preheating of the feed and the heating of the reboiler are taken back to the system and are being mixed

with the stream from the last turbine and then reused together with fresh water to produce new 91 bar

steam. The isentropic efficiencies are 85% for the different turbines and the mechanical efficiencies are

97%.

Combustion

Lignin-ash

Boiler

Flue gases

vvx

Warm air

Flue gases out, 150 degrees C

Turbine 1 Turbine 2 Turbine 3

Steam 91 bar

E-7

E-8 Steam 20 bar

E-9

Electricity

vvx

Steam 4 bar

E-11

Water, 10 degrees C

E-12Return condensate, 4 bar E-13

Steam 20 bar

Air in, 15 degrees C

Figure 16 Heat and power plant

In this model, more electricity is produced than being used in the process and is assumed to be sold as a

co-product. An alternative would be not burning more solids than are needed to supply the process

energy demand. Then the rest can be sold as pellets.

3.3 Biogas The biogas process is divided in two phases. In the first part all the steps, except for the methane

producing step, are active. In the second phase the methane is produced. The ingoing material contains

a high amount of easily digestible substrates like sugar monomers, for which the two phase process is

recommended [18]. The process is continuous where the methane is taken out and the rest products

from the anaerobic digestion are also taken out. The residence time is assumed to be 48 hours because

it is assumed that the most of the material has been digested at that time.

30

The sludge being used is assumed to be 10% of the reactor volume. The organisms are reproduced

during the anaerobic digestion so a part of them are taken out with the rest products.

The temperature is set to 37°C, a mesophile process, and the pH to 7. There are wider varieties of

organisms in mesophile sludge than in thermophile sludge and with more kinds of organisms the

process is more stable to changes [18]. Ammonia is used to set the pH to 7.

Since it is living organisms that digest the material, some of the material is used as substrate for the

organism growth. The maximum yield for the organism for each reaction in the biogas process is

assumed to be the same as for yeast in fermentation of glucose to ethanol as no data has been found.

The yield for fat is assumed to be much lower (30%) than the rest substrates. This is assumed because

the fat would need longer time than 48 hours to digest maximal. The reactions that are assumed to

occur in the process are defined as below with the assumed yield in % of theoretical in parenthesis. HAc

stands for acetic acid.

Glucose → 2Ethanol + 2CO2 (95%)

3Xylose → 5Ethanol + 5CO2 (85.5%) [28]

2Ethanol + CO2 → 2HAc + CH4 (95%)

2Glycerol → 2HAc + CO2 + CH4 + 2H2 (95%)

Furfural + 4H2O → 2HAc + CO2 + 2H2 (84%) [28]

Fat + 4CO2 + 2H2 + 8H2O → 9HAc + 4CH4 (30%)

HAc → CH4 + CO2 (95%)

The biogas is then upgraded in a biogas plant, which is not designed here, while the liquid part from the

rest products is taken to water treatment. The solid parts can be used as fertilizers. The raw biogas

obtained in the digestion is not high quality enough if it is planned to sell this gas or use it as a fuel. Thus,

by the use of a biogas upgrading or purification process, contaminants in the raw biogas are adsorbed or

scrubbed increasing the methane concentration.

3.4 Integration of 1st and 2nd generation ethanol production The integration was done in the distillation step. This was chosen since it would be easier to combine

the two processes in this step since the first generation ethanol plant already exists when the second

generation plant is planned to be built. Then it is only the distillation unit and the downstream process

with the biogas plant and the heat and power unit that has to be rebuilt.

Another option is to integrate the two processes in the SSF but then the SSF also has to be redesigned.

The best advantage with integrating in that step is the decreased inhibitor concentration. But since the

inhibitors are already diluted in the process, where half of the liquid is removed before the straw based

slurry is fed to the SSF with fresh water, integration in SSF is not necessary from that point of view.

31

The process flowsheet for the integrated process is shown in Figure 17.

Straw transport

Straw in

HAC-Spray

vvxPretreatment

Steam in (20 bar)

Flash-vesselCondesor

Filter

SSF

CO2 out

Nutrient

Ammonia

Water

Enzyme

Yeast

Pump

Stripper Rectifier

Condensor

H2O out

Dehydration

Fuel Ethanol

Bioreactor Bioreactor

Methane/CO2

Methane

Filter Unit for solid

separation

Lignin for burning

Mixing tank

Upgrading plant

Kondensor

Heat for SSF

Heat

Water +volatiles

Filter unit

E-28

Silo

Wheat transport

Hammer millPremixing/gelatinization Liquefaction

SSF

CO2 out

Cold water

Yeast

Glucoamylose

Amylase

Cooler

Integration vvx

vvx

Methane

Figure 17 Flowsheet for the integrated ethanol and biogas process

In the first generation ethanol plant, heat and electricity is generated by burning pellets, straw or what

else being the best alternative to produce that amount of electricity and heat being needed. In both the

second generation ethanol plant and in the integrated plant, heat and electricity is generated from rest

products from the ethanol plant. If not integrating the two plants, the electricity and heat demand in

both first and second generation can be satisfied by the production of heat and electricity from the heat

and power plant in second generation ethanol production plant.

4. Results and analysis In this project, a simulation has been made for a first generation plant, for a second generation plant

and for a plant working with both processes in order to see if there are advantages when integrating the

first generation with the second generation in the distillation step.

32

The energy demands have been calculated by Aspen in the most cases, except for the pretreatment

reactor in the 2nd generation ethanol process. This has been calculated with the flow of the 20 bar steam

(8300 kg/h) and the enthalpy of vaporization (1890 kJ/kg). According to the calculations performed, it

can be seen in Table 5 that the total primary energy demand in the integrated case is higher compared

to the sum of energy demand when considering two individual plants. However, this difference is not

very big and as it also can be seen, the system which demands more energy is the second generation.

This is due to the fact that high amount of water is needed in second generation ethanol process and

thus, higher flows must be taken into account which leads to high energy consumption when heating is

needed as in the stripper. From the results obtained in Table 5, one would say that there is no point in

having an integrated plant but an economic evaluation is needed before any conclusions can be

withdrawn.

Table 5 Total primary energy demand comparison

Energy demand integrated 29.7 MW

Energy demand 1st 13.7 MW

Energy demand 2nd 15.2 MW

Energy demand 1+2 sep 28.9 MW

Moreover, the step which is most heat demanding is the stripper step. The pretreatment in the second

generation plant is also very energy demanding because it uses vapor at 20 bar, see Table 6.

Table 6 Primary energy demand comparison by process steps

Primary energy demand (MW)

1st generation 2nd generation integrated

Heater in liquefaction step 1.5 1.7

Preheating of sprayed wheat straw 2.8 2.8

Pretreatment 2nd 4.4 4.4

Stripper 12.2 8.0 20.8

Total (MW) 13.7 15.2 29.7

Heat integration has been performed in the whole integrated plant. Stillage preheats the feed into the

stripper and the fresh water needed in the pretreatment of the first generation plant is heat exchanged

with the slurry after the liquefaction step. Also the heat needed to heat up the SSF in the second

33

generation plant is obtained by heat integration, where it is heated up by condensate the flashed steam

from the pretreatment step in the second generation plant. But probably more effective heat

integrations could be done. Lots of streams need to be cooled down with water and the heat generated

by condensate the flashed steam is more than is needed in the SSF. For energy demand and energy heat

release, see Table 7.

Table 7 Energy demand and release in the different process steps. The underlined values are those steps where the energy demand is satisfied with 4 and 20 bar steam. The other values are secondary energy used in the process. All the process steps marked with 1

st is meaning that it is a part of the first generation ethanol process, while 2

nd means that it is a part of the 2

nd

generation ethanol process.

Energy demand and release (MW)

1st generation 2nd generation integrated

Heater in liquefaction step, 1st (74 -

> 90°C)

1.5 1.7

Cooler before the SSF, 1st (40 ->

31°C)

- 0.8 - 0.8

Preheating of sprayed wheat straw,

2nd (10 -> 95°C)

2.8 2.8

Pretreatment, 2nd 4.4 4.4

Condenser, flashed steam 2nd - 5.0 - 5.0

SSF, 2nd 1.5 1.5

Cooler before filter the pretreated

slurry, 2nd (100 -> 35°C)

- 2.2 - 2.2

Stripper 12.2 8.0 20.8

Condenser after stripper - 11.5 - 3.2 - 11.1

Reboiler, rectifier column 9.2 1.7 10.7

Condenser, rectifier column - 10.0 - 1.9 - 11.8

Cooler before biogas plant 0.0 - 3.0 - 3.3

As it can be seen, there is a lot of energy that could be used from the condensed flash steam as well as

the pretreated slurry could be used to preheat some other stream diminishing the amount of cold water

and primary energy (steam) needed. If an efficient plant is desired, the distillation step should be

34

treated separately, looking more into the details in this step in order to optimize the energy demand.

The high energy consumption in this step is due, among other factors, that a lot of heat is required in

the stripper column because the feed does not enter the tower at its boiling point. Also the number of

trays and other process design parameters should be investigated into more detail to optimize the

energy demand in the towers.

When comparing the different steps between the integrated plant and the non-integrated, see

Appendix III for results, the results obtained were those expected. The data obtained from the

pretreatment and SSF in both first and second generation were the same as those obtained from the

integrated plant. This is obvious as it is the same process. Nevertheless, when the integration is done,

that is in the distillation step, values differ quite a lot. Of course the flow downstream increases but it is

mainly because of the high amount of water required which, as said before, turns into higher energy

consumption.

The same amount of ethanol is produced, 15.5 ton/h, when integrating the two different plants

compared to the case where plants are not integrated. The ethanol mainly comes from the first

generation plant, 13 ton/h, compared to the 2.5 ton/h from the second generation plant. These values

are the ethanol flows after dehydration. On the other hand, the biogas production is a bit higher when

integrating than if the two plants are considered. 8.1 ton/h is the biogas flow when integrated, 2.8 ton/h

from first generation and 5.0 ton/h from second. All biogas has 26% in weight in methane and 74% in

carbon dioxide, see Table 8. The higher amount of biogas produced when the two ethanol plants are

integrated is due to a design specification in the simulated process. It should be the same value as

considering the two processes working separately.

Table 8 Ethanol and Biogas production comparison

Ethanol-biogas production

1st generation 2nd generation integrated

Ethanol produced (ton/h) 13.0 2.5 15.5

Biogas produced (ton/h) 2.8 5.0 8.1

Mass fraction of methane 0.26 0.26 0.26

Mass fraction of carbon dioxide 0.74 0.74 0.74

From a mass and energy balances point of view, seems not to be any reason to integrate the first

generation and second ethanol production as the results obtained from the integrated plant, are almost

the same as considering the two plants working separately. However, the economical study will

determine if it is reasonable or not to integrate the two plants once it is shown that it is possible to

perform the integration. Before taking into consideration if the integrated plant should be built, other

35

studies should be performed, specially heat integration improvement and maybe integration in other

steps.

The heat and electricity produced in the process by burning the solids are given in Table 9. When the

two processes are integrated, both solids from the 1st and the 2nd generation ethanol process are used.

When not integrating, only the solids from the 2nd generation ethanol process are used. The column 2nd

+ 1st shows the products when the plant in the second generation is used for heat and electricity

generation for both the first and the second generation ethanol plant.

Table 9 Production of heat and electricity from burning solid residues. The electricity in brackets is the net electricity that can be sold. The first column is generation of heat and electricity from 2

nd generation plant to 2

nd generation. The second column

is generation of heat and electricity from 2nd

generation plant to 2nd

and 1st

generation. And the last column is the generation of heat and electricity from integrated plant.

Total energy production (MW)

2nd generation 2nd + 1st Integrated

20 bar steam 4.4 4.4 4.4

4 bar steam 10.8 24.6 25.2

Electricity 15.4 (10.6) 11.2 (0.4) 26.8 (17.0)

TOTAL (MW) 30.6 40.2 56.4

As can be seen, less electricity is generated when the heat demand for the stripper is supplied in both

the first and the second generation ethanol plant. Moreover, when not integrating the processes, the

solids from the 1st generation ethanol plant are sold as pellets which are not taken into account when

comparing the energy production and the energy efficiency. The pellets are calculated within the

economy analysis.

The energy efficiency for the plant is calculated as the ratio between the heat of combustion (lower

heating value) in the products divided by the heat of combustion in the raw material, see Appendix IV.

The electricity is used as it is and has not been corrected with any factor to have a comparable value

versus the heating value. The efficiencies for the different cases are shown in Figure 18.

36

Figure 18 Energy efficiency for the different cases based on lower heating value. The electricity is not recalculated to heat required for products. Calculated from the Appendix IV.

In the two cases, integrated process and separate plants; the integrated plant presents higher energy efficiency, 54.0% compared to 48.2%. The energy efficiency in the 1st generation plant is high (52.2%) due to the fact that the supplied heat and electricity are taken into account but not as the lower heating values for the pellets but as the energy needed in electricity and heat (6 MW + 13.7 MW) in the calculations. It should in reality be smaller than the case with separated plants.

5. Economics and sensitivity analysis

5.1 Economy The economical analysis was mainly done to investigate if an integration of first and second generation

ethanol production is profitable. Parameters that are taken into consideration are the amount of

ethanol, biogas and electricity that can be produced by building the second generation process. Three

scenarios were considered:

1. First generation ethanol and biogas production alone.

2. Second generation and first generation not integrated but with joint heat and biogas

production.

3. The integrated process.

The economical analysis is based on an estimation of the cost of the equipment and operating the plant

as well as an investment analysis. Calculations of equipment size, energy and raw material usage as well

as amount of products produced were based on figures simulated in Aspen Plus.

Equipment cost The estimation on major equipment in the process was based on calculations using the Ulrich method.

Some parts of the plant are not possible to estimate with Ulrich, that is why price proposals from similar

equipment are taken into account. The Ulrich method is based on 1982 figures, for actual prices the

0%

10%

20%

30%

40%

50%

60%

1stgeneration

2ndgeneration

1st + 2nd Integrated

Ene

rgy

eff

icie

ncy

Electricity

Methane

Ethanol

37

Marshall and Swift Equipment Cost Index is used where I1982=746, I2009,2ndQ=1462.9 [29] and the exchange

rate is 7 SEK/USD (2009 2nd Q) [30].

(

)

∑

The prices are then adjusted by multiplying with factors for contracting and unforeseen cost

(fcont/unfor=1,25), for extra sizing for equipment (fsize=1,15) as well as for help equipment, plant buildings,

surrounding etc. (frest=2,85). CBM is the estimated price for the equipment and is specifically calculated

from Ulrich method. For equipment based on price proposals the prices are only multiplied with the

factors.

By this method the following costs in Table 10 were calculated for building the plant(s) and the amount

of equipment needed (n):

38

Table 10 Equipment cost and amount for 1st, 2nd and integrated plant

Equipment n Price 1st generation

(MSEK)

n Price 1st+2nd generation

(MSEK)

n Price Integrated process

(MSEK)

Silos 3 4.6 5 7.3 5 7.3

Pre mixer 1 10.5 1 10.5 1 10.5

Gelatinization 1 7.1 1 7.1 1 7.1

Liquefaction 2 18.6 1 18.6 1 18.6

SSF (1st) 6 24.7 6 24.7 6 24.7

Acetic Acid pretreatmenta

- - 1

86 1

86

Filter - - 1 5 1 5

SSF (2nd) - - 7 31.4 7 31.4

Stripper 1 12 2 22.1 1 15.8

Distillation tower 1 17 2 32.1 1 19.5

Ethanol purifierb 1 20 2 40 1 20

Lignin separatorc - - 1 49 1 49

Bioreactors 1 3.1 1 9.3 1 9.3

Clarifier 1 5.5 1 5.5 1 5.5

Methane purifierd 1 20 1 20 1 20

Minor (HE, condenser, mills)

- 4.3 - 4.8 1 4.3

Total equipment cost

343.1 373,4 334

Total plant cost, (after multiplying with the factors, except heat and power plant)

604 1536 1368

Heated and power plante

1 200 1 250 1 400

Total investment: 804 1786 1768

a Price for the equipment for pretreatment of wheat straw is based on Andritz [31] price proposal where

impregnation with acetic acid, heat exchanging and flashing is included, which is 85 MSEK.

b Equipment cost for ethanol purification is estimated to 20 MSEK, based on information from Robert,

Jianlong Chemical.

c Price for the lignin separation equipment is from a Master Thesis from LTH found and recalculated by

Professor Guido Zacchi, which is 45 MSEK.

39

d The methane purifier equipment cost estimated to 20 MSEK, based on consultation with Doctor Tobias

Persson (Process manager for Biogas at Malmberg Water AB, Åhus).

e Heat and power plant is an estimation based on (referens per sassner page 69). The boiler used in the

Sassners process had an effect of 67 MW and cost roughly 300 MSEK. The first generation has a boiler

size of 25 MW (200 MSEK), second generation has 56 MW (250 MSEK) and the integrated has 96 MW

(400 MSEK).

Chemicals, raw material and products Prices for chemicals and raw materials can be seen in Table 11 and were taken from different sources

though it was not possible to find all prices summarized in one source. Therefore, prices may fluctuate

but for more accuracy, prices are no older than three years. Annual cost and income could be calculated

for each plant(s) based on flows (kg/h) from Aspen Plus and is seen in Table 12.

Table 11 Prices for chemicals and raw materials

Substance SEK/kg

Acetic acid [32] 5.61

Ammonia [32] 12.52

Amylas LIQUOZYME SC DS [33] 37.60

ENZYME 30.00

Ethanol [34] 6.36

Glucoamylas, SPIRIZYME PLUS [33] 29.88

Methane 13.44

Nutrient [27] 31.00

Wheat [35] 1.00

Wheat Straw [36] 0.69

Yeast [37] 24.00

Pellets from solids [38] 0.8

40

Table 12 Annual cost/income for consumed chemicals or raw materials and the products.

1st

generation 1st

+2nd

generation Integrated

ton/year MSEK/year ton/year MSEK/year ton/year MSEK/year

CONSUMED

Ammonia 910 11.4 1 817 22.8 1 817 22.8

Amylas, LIQUOZYME SC DS

89 0.6 88 0.6 88 0.6

ENZYME - - 18 0.5 18 0.5

Acetic Acid - - 3 200 17.9 3 200 17.9

Glucoamylas, SPIRIZYME PLUS

179 5.4 179 5.4 179 5.4

Nutrient - - 461 14.3 461 14.3

Wheat 354 481 354.5 354 481 354.5 354 481 354.5

Wheat Straw - - 200 000 138,0 200 000 138.0

Yeast 1 327 31.9 3 962 95.1 3 962 95.1

Total Cost - 403.8 - 649.1 - 649.1

PRODUCTS

Ethanol 103 295 657.2 122 872 781.8 121 813 775.0

Methane 5 866 78.9 16 514 222,0 17 074 229.6

DDG 45875 36.7 45875 36,7

NET INCOME 369.0 391.4 355.5

Other costs estimations were done for electricity and water consumption for the processes. The annual

water consumption can be calculated from the Aspen simulation. On the other hand, it is not possible to

calculate the electricity consumption (kWh/year) for the 1st generation plant, while 1st+2nd generation

and the integrated plants are self sufficient with both heat and electricity. An approximation is made

based on Murphys & Powers [39]here the size of the “Murphy” plant is similar to the one investigated

for first generation in Table 13:

41

Table 13 Comparison with investigated plant and Murphys & Power

Murphys & Power Investigated 1st generation Fraction (Invested/Murphys)

Wheat, (ton/year) 403 200 354 500 0.88

Water, (l/year) 603 722 300 460 800 000 0.76

Produced ethanol, (l/year) 150 930 600 103 300 000 0.68

Electricity requirement, (MWh/year)

48 300 n.a

Thermal requirement, (MWh/year)

362 200 109 600 0.5

The requirements and ethanol production for the reference plant are higher at the same time as the

thermal requirement is much lower for the investigated plant. So the electricity consumption for the

investigated plant is set to be 48 000 MWh/year for 1st generation plant. The need of thermal energy for

the 1st generation plant is 109 600 MWh/year, which are supplied by the heat production plant by

burning wood pellets or oil. While 1st+2nd generation and the integrated plant are producing an

electricity surplus that is later on sold and can be seen in Table 14.

Price for pellets is set to 550 SEK/MWh [40], which is the price for producing thermal energy and

electricity in the heat and power plant in first generation plant. The electricity surplus is assumed to be

sold for 1000 SEK/MWh.

Table 14 Income and cost for electricity

Electricity consumption (MWh/year)

Electricity surplus

(MWh/year)

Cost for electricity production

( MSEK)

Income for electricity

surplus (MSEK)

1st generation 48 000 - 157.6 -

1st+2nd generation

89 600 3 200 - 3.2

Integrated 78 400 136 000 - 136.0

The price for water is set to 20.8 SEK/m3[41], which gives the following cost in Table 15.

Table 15 Water cost

m3/year Cost (MSEK/year) Fixed price (MSEK/year)

1st generation 460 800 9.58 0,15

1st+2nd generation 1 364 640 28.38 0,15

Integrated 1 373 500 28.57 0,15

42

Operational plant costs and investment analysis A method based on Table 16 was used for the calculations of operational costs for the plant(s) [42]. The

annual cost is later used for the investment analysis. In all three scenarios the factors are the same.

Table 16 Costs for operational plant cost and costs estimations

Description of cost Factors

Frozen capital

Storage of raw material + chemicals

Delivery once a week

Storage of products Delivery twice a week

Direct variable costs

Chemicals + Raw material Cost of consumed chemicals and raw materials

Energy and water Cost of electricity, thermal energy and water consumption

Operators (Staff) 8000 h/year; Salary 110 SEK/h + 50% in workers taxes; 10 operators

Operator managment 15% of operator costs

Laboratory staff 15% of operator costs

Indirect variable costs

Overhead for personal 70% of operator costs for shift personal

50% of operator costs for day personal

Administration 25% of overhead for personal

Annual plant costs

3% of annual plant costs for license

5% of annual plant costs for selling & distribution

2% of annual plant costs for research and development

5% of annual plant costs for maintains and reparations

15% of maintains and reparations for spare parts

Total annual plant operation cost

All three options have an annual surplus. By doing an investment analysis it is possible to make a

conclusion of which option is most profitable. The annuity method is used to see the real annual profit

by taking the investment cost and interest into account. The economical life time (calculated time to pay

off the investment) is 15 years (N) with an interest of 8% (X). There is no rest value of the investment

after 15 years and the net payment each year is constant. The annuity factor is multiplied with the

investment cost to get the annual capital costs which are taken off from the annual net income, this

gives the real annual profit as in Table 17.

43

Table 17 Annual income, plant cost and profit

Ethanol income (MSEK)

Biogas income (MSEK)

Pellets from solids

income (MSEK)

Electricity income (MSEK)

Operational plant cost

(MSEK)

Investment cost

(MSEK)

Real annual profit

(MSEK)

1st generation

661.4 78.2 36.7 - 632.1 804.2 50.3

1st+2nd generation

788.6 218.1 36.7 3.2 845.4 1785.9 -7.0

Integrated 788.6 226.4 - 136.0 845.7 1768.1 99.9

The result is that building only the 1st generation plant makes the investment profitable.

Depending on the low investment cost and the high ethanol output.

Building the 1st and 2nd generation plants parallel is unprofitable depending on very high

investment cost and no extra gain from the integrated process.

Building an integrated process gives a profitable investment, depending on the high output of

products compared to a relative low investment cost.

So an extra investment cost to build the 2nd generation ethanol production is recommended considering

economical profit. While the 1st generation process will produce satisfying amount of ethanol, the 2nd

generation process will double the biogas income and replace the high energy consumption cost when

1st generation is operated alone, even making a high profit (10% of total income). Compared to the pay-

off time (Investment/net annual income), the 1st generation plant would have 5.1 years and the

integrated plant would have 6.4 years. From an economical point of view the pay-back times are

basically the same. The interesting though is the real annual profit which is twice higher for the

integrated plant, even though the investment is twice as high then for the 1st generation plant it is still

more profitable to integrate. Operational plant costs would of course be higher for the integrated

process, but the higher net income makes worth the extra cost.

The separated 1st and 2nd generation is not a profitable option. Depending on the extra investment made

in building double distillation and ethanol purifiers. While producing and extra income out of pellets

from solids and the same amount of ethanol and methane as in the integrated plant, the need for

energy in the separated process is just too high to make a profit. The pay-of time for the separated plant

is much higher than for the two others, which is 10.2 years.

5.2 Sensitivity analysis Sensitivity analyses have been performed for the WIS concentration and enzymatic yields in the SSF of

the straw based process in the integrated plant. Analyses have also been performed for the price

changes of straw, methane, ethanol, electricity, DDG and yeast.

5.2.1 WIS concentration in the SSF

How much the feed, going into the SSF in straw based process, has to be diluted is unsure because there

are not any available data for full scale production. Increasing the WIS can damage the equipment since

44

there are more solids going into the SSF. Because of less dilution of the feed, the risk of inhibition of the

reactions will increase. Furthermore, the stirring is affected since more power is needed when there are

more solids. When lowering the WIS, not so much energy for stirring is needed but much more water

has to be added which has then to be removed in downstream processing. In our sensitivity analysis, we

varied the WIS content from 5% to 20% and investigated how this affected the energy demand in the

SSF, the distillation unit, the amount of ethanol and biogas obtained and the electricity generation for

the integrated process. The results are shown in Table 18.

Table 18 Results for various WIS concentrations in the SSF in the integrated process. The line marked in grey shaded is the base case

WIS (%)

Energy demand

in SSF (MW)

Energy demand in stripper (MW)

Ethanol 99.8% (ton/h)

Biogas (ton/h)

Electricity to sell (MW)

(Total amount

produced electricity

(MW))

5 6.7 34.7 15.2 8.7 15.7 (25.5)

10 3.3 25.8 15.3 8.4 16.4 (26.2)

15 1.5 20.8 15.3 8.1 17.0 (26.8)

20 0.9 19.1 15.3 7.8 17.3 (27.1)

The same amount of ethanol is assumed to be produced. However the volume of the SSF is changed

with the WIS in the way that lower WIS concentration means bigger volume of the reactors. The biogas

product is larger with a lower WIS. This depends on the fact that the solids leaving the filter before the

biogas plant have a dry content of 50% and with more water in the SSF, less of the components in the

liquid goes with the solids to the burning plant and can instead be used in the biogas production.

However, less electricity is produced.

The energy demand in the SSF increases with decreasing WIS concentration like in the distillation unit,

mostly the stripper. The increase depends on the increase of water. The higher energy demand in the

SSF is supplied by the heat generated by condensing the flashed steam after the pretreatment unit like

before. The stripper is supplied with heat from the 4 bar steam produced from the heat and power plant.

Increasing the energy demand means less produced electricity that can be sold.

As seen in Table 19, an increase in WIS content is profitable due to the lower investment and water cost.

The change in the income is not affected by the lower methane production since more electricity is

produced then. A higher WIS content seems to be recommendable from this analysis but the risks with

higher solid contents in the SSF need to be investigated in lab scale.

45

Table 19 Economical analysis depending on WIS content

WIS Income products (MSEK/year)

Water cost (MSEK/year) Investment cost (MSEK/year)

Real annual profit (MSEK/year)

5 1 158 59 411 34

10 1155 39 360 78

15 1152 28 334 99

20 1145 24 324 103

5.2.2 Enzymatic yield

Like in the former sensitivity analysis, the enzymatic yield is unsure in the SSF in the straw based process

because of non existing data for full scale production. Also data of how the yield is affected with a WIS

of 15% is missing since most lab scale processes have a WIS of less than 10%. In our sensitivity we varied

the enzymatic yield from 50% to 80%. In this sensitivity analysis, the same parameters as in the

sensitivity analysis of WIS content have been checked. The results are shown in Table 20.

Table 20 Results for various enzymatic yields in the SSF in the integrated process. The line marked in grey shaded is the base case

Enzymatic yield (%)

Energy demand in SSF (MW)

Energy demand in stripper (MW)

(Energy demand in distillation unit

(MW))

Ethanol 99.8%

(ton/h)

Biogas (ton/h) Electricity to sell

(MW) (Total

amount produced

electricity (MW))

50 1.6 20.6 15.1 7.8 18.3 (28.1)

60 1.5 20.8 15.5 8.1 17.0 (26.8)

70 1.4 21.0 15.9 8.2 15.7 (25.5)

80 1.3 21.2 16.3 8.5 13.3 (23.1)

The results in Table 20 show that the energy demand does not vary so much with varying the enzymatic

yield. The amounts of ethanol and biogas produced increase with increasing yield while the electricity

produced decreases.

As seen in Table 21 an increase in enzymatic yield is profitable, which was expected after the discussion

above, due to the increased income from the products. This means that the biogas and ethanol pays off

better than electricity. A higher yield means more enzymes are needed. The higher income shows that it

is possible to have more enzymes in the process to gain higher amounts of ethanol and biogas.

46

Table 21 Economic analysis depending on enzymatic yield

Enzymatic yield (%)

INCOME products (MSEK/year)

50 1133

60 1152

70 1164

80 1183

5.2.3 Product price

Both the first generation process and the integrated process are economically profitable. Analyses have

been made below on different price changes for the sources of income and different cost of

expenditures to compare and see when the first and integrated process will be more profitable than the

other.

The major incomes for the processes are summarized as:

o Ethanol (largest income in both processes)

o Methane

o DDG (only first generation process)

o Electricity (only integrated process)

The major costs for the processes are summarized as:

o Wheat (highest cost in both processes)

o Straw (only in the integrated process)

o Yeast

o Electricity demand (only first generation plant)

Methane is an interesting product to examine, it showed that the integrated process is much more

sensitive to methane price fluctuations and the investment is as profitable as the first generation

process when the methane price is between 10-11 SEK/kg (if the methane price is fixed in the first

generation process). A break even is when the price on methane is 9 SEK/kg, see Figure 19 and then

both processes are as profitable. Such price decrease is relatively high (35% decrease) but is possible.

47

Figure 19 Real annual profit compared to price on methane

Comparing the fluctuation on ethanol price in the two processes is not interesting though ethanol is the

highest income in both process and would vary in the same way. More interesting was to examine at

what price the ethanol from the first generation and the integrated process become unprofitable and at

what price the integrated process becomes as profitable as in the first generation. The result can be

seen in Figure 20. To make the integrated process as profitable as in the first generation, the ethanol

price should decrease from 6.36 SEK/L to 5.95-6 SEK/L, which is a very possible scenario that should be

considered. The price of ethanol has to decrease to 5.55 SEK/L and to 5.87 SEK/L to make the integrated

process respectively the first generation process unprofitable. An expected decrease considering the

economical situation in the world, but it also shows that the integrated process is less sensitive to

ethanol price.

Figure 20 Real annual profit compared to price on ethanol

Wheat straw represents one of the major costs in the integrated process, the original price for wheat

straw is 0.69 SEK/kg. If the price of wheat straw would increase a break even between first generation

and the integrated process will occur at the price of 0.9 SEK/kg. The integrated process would be

unprofitable if the price would increase to 1.12 SEK/kg, see Figure 21. Such increase is unlikely at the

time considering that there is a lot of wasted wheat straw, but price can increase if ethanol from wheat

straw production will increase and price conflict will become an issue.

0

20

40

60

80

100

8 9 10 11 12 13 14

Re

al a

nn

ual

pro

fit

(MSE

K)

Methane price (SEK/kg)

1st

Int

1st (fix)

-40

10

60

110

160

5 5,5 6 6,5 7 7,5Re

al a

nn

ual

pro

fit

(MSE

K)

Ethanol price (SEK/L)

1st

Int

Int (fix)

48

Figure 21 Real annual profit compared to price on wheat straw

When using pellets as a fuel in the first generation and when electricity is one of the major costs, the

process gets sensitive to price change. Pellet price (electricity price) has to increase to 0.83 SEK/kWh to

make the first generation unprofitable and decrease to 0.3 SEK/kWh to be as profitable as the

integrated process, see Figure 22. While in the integrated process the electricity is one of the major

incomes. A change in the selling price from 1 SEK/kWh to 0.27 SEK/kWh would make the investment

unprofitable but is very unlikely to happen, while a decrease to 0.67 SEK/kWh will make the investment

as profitable as first generation process, see Figure 23.

Figure 22 Real annual profit compared to electricity selling price

-5

15

35

55

75

95

0,65 0,75 0,85 0,95 1,05 1,15

Re

al a

nn

ual

pro

fit

(MSE

K)

Wheat straw price (SEK/kg)

1st (fix)

Int

0

20

40

60

80

100

0,25 0,45 0,65 0,85 1,05

Re

al a

nn

ual

pri

ce (

MSE

K)

Electricity selling price (SEK/kWh)

Int

1st (fix)

49

Figure 23 Real annual profit compared to electricity price if buying pellets as fuel

To make the process unprofitable, the yeast price must be doubled in the integrated process. However,

the first generation can withstand an increase in the yeast price but not to a large extent. Further

examination is not made though such price change is mostly unreal.

The income from DDG in the first generation is one of the major incomes. After some examination, it is

concluded that the selling price for the DDG, which is 800 SEK/ton, has to increase to almost 2000

SEK/ton to be as profitable as in the integrated process, which is very unlikely. Even if no DDG is sold,

the first generation would still be profitable.

6. Discussion and conclusions To integrate the first generation and second generation ethanol production processes in the best and

economically feasible way is not an easy part because it has never been done before. The second

generation process exists only in the pilot scale stage and has not yet been commercialized.

In the second generation process there are many ways to pretreat the lignocellulosic material. It is

important to choose the right pretreatment method as it is the most costly part of the process. It was

thought that steam explosion with dilute acetic acid was the most suitable setup. The acetic acid chosen

is also an intermediate in the biogas process so it will not harm the process downstream as other acids

might do and it is converted to biogas.

The integration of the processes can be done in two places of the processes, either in the fermentation

step or where the streams to the distillation are connected. To integrate in the SSF might be a good

choice since there have been reports that say it would facilitate the ethanol production due to the yeast

cells at the right inhibitor concentration will increase the ethanol concentration. In our study we

assumed that the inhibitors are already diluted before the SSF in the straw based process so it was not

necessary in this case. Due to these conclusions the integration is done in the distillation step.

When integrating the processes, the energy demand is higher than if considering having the two

processes separately. The stripper column is the element that requires most energy. However, the heat

-5

15

35

55

75

95

115

0,15 0,35 0,55 0,75

Re

al a

nn

ual

pro

fit

(MSE

K)

Electricity buying price (SEK/kWh)

1st

Int (fix)

50

integration study should be improved to lower the energy demand, especially in the distillation step.

The production of ethanol mainly comes from the first generation process while the biogas comes

mainly from the second generation process. When looking at the energy efficiencies, the best result was

obtained when integrating the two processes due to the larger amount of electricity produced.

From an economical point of view, building the first generation plant is a profitable investment due to

the low investment cost and the high output of ethanol. The income for the methane and the dried

destillers grain (DDG) is much lower than for the ethanol which makes the plant sensitive and

dependent on the ethanol selling price and the cost for wheat, which is the main cost in the process.

This results in an economically profitable plant but unstable due to market price fluctuations on ethanol

and wheat.

Building a second generation plant and integrating the process in the distillation step showed to be the

most profitable option. Even though the investment cost would be twice as high as for the first

generation plant, the production of electricity would become a major income (almost 4 times higher

than for DDG which is no longer produced). Methane income would also be more significant. This makes

the process less sensitive to market price fluctuations. This means that the integrated process is more

economically stable and more economically profitable, depending on the spread of costs and incomes.

Even if the factors in the operational plant costs (Table 16) would be higher, depending on more

operators and more costs for maintenance because of new technology, the investment would be as

profitable as the first generation plant.

Building a second generation plant parallel to the first generation plant and integrating in the biogas

showed to be an unprofitable investment. Even if the investment cost is almost the same as for the

integrated process and incomes for methane and ethanol and the operational plant costs are also

almost the same. The main reason that the investment is not profitable is the surplus electricity which is

practically zero. If more electricity could be produced by perhaps more heat integration, the parallel

plant would also be a profitable option.

51

7. References

[1] Nordisk Etanol & Biogas AB. [on line].<http://www.nordisketanol.se/index.html> [29/03/2010]

[2] Wheat as a feedstock for alcohol production By T C Smith1, D R Kindred1, J M Brosnan2, R M

Weightman1, M Shepherd3, & R Sylvester-Bradley1. Research Review No. 61 by HGCA

[3] Energy analysis of ethanol production; A case study of Lantmännen Agroetanol’s production system in Norrköping, by Per Paulsson , Department of Biometry and Engineering, Examensarbete 2007:03, Uppsala 2007, ISSN 1652-3245 [4] Wikipedia, Hammermühle [on line]. <http://en.wikipedia.org/wiki/File:Hammerm%C3%BChle.jpg>

[30/03/2010]

[5] Sejr Olsen, Hans. “Using enzymes in ethanol production”, Novozymes A/S ©, 2004-13388-02. P-17

[6] Novozymes [on line].

<http://www.novozymes.com/en/MainStructure/ProductsAndSolutions/Starch/Liquefaction/Liquefactio

n> [12/04/2010]

[7] Swapmil Bhargava, Patentdocs. Liquefaction processes [on line].

<http://www.faqs.org/patents/app/20080254518> [12/04/2010]

[8] Maps. Enzymes for starch [on line]. <http://www.mapsenzymes.com/Enzymes_Starch.asp>

[12/04/2010]

[9] Enzyme Essentials. Protease [on line]. <http://www.enzymeessentials.com/HTML/protease.html>

[12/04/2010]

[10] Olofsson K, Bertilsson M, Lidén G. A short review on SSF - an interesting process option for ethanol

production from lignocellulosic feedstocks. NCBI, May 2008.

<http://www.ncbi.nlm.nih.gov/pubmed/18471273?log$=activity>[13/04/2010]

[11] Linde, Marie. Process Development of bioethanol Production from Wheat and Barley Residues,

Steam Pretreatment and SSF. Department of Chemical Engineering, Lund University (Sweden),

November 2007.

[12] Leland M.Vane PhD. “Separation technologies for the recovery and dehydration of alcohols from

fermentation broths, US Environmental Protection Agency”. Interscience wiley, Cincinnati, Ohio, October

21, 2008 P-553–588, 563-569

[13] P.W. Madson; D.A. Monceaux Katzen. Fuel ethanol production [on line].

<http://www.bioethanol.ru/images/bioethanol/Fuel%20ethanol%20production%20-%20Katzen.pdf>

[13/04/2010]

52

[14] J.A. Pérez, I. Ballesteros, F. Sáez, M.J. Negro, P. Manzanares. ”Optimizing Liquid Hot Water

pretreatment conditions to enhace sugar recovery from wheat straw for fuel-ethanol production”

[15] Farid Talebnia, Dimitar Karakshev, Irini Angelidaki. ”Production of bioethanol from wheat straw : An

overview on pretreatment, hydrolysis and fermentation”

[16] P. Alvira, E. Tomás-Pejó, M- Ballesteros, M.J. Negro. ”Pretreatment technologies for an efficient

bioethanol production process based on enzymatic hydrolysis: A review”

[17] Integrated Publishing. Separate Hydrolysis and Fermentation, Simultaneous Saccharification and

Fermentation, Direct Microbial Conversion. Alternative Fuels CD04 [on line]

<http://www.tpub.com/content/altfuels04/3479/34790010.htm > [29/03/2010]

[18] Mikrobiologisk handbok för biogasanlåggningar, Rapport U2009:03.Reviderad oktober 2009. ISSN

1103-4092

[19] Wikipedia. Picture, stages of anaerobic digestion [on line]. <http://en.wikipedia.org/wiki/Anaerobic_digestion> [12/04/2010] [20] Nordberg, Ulf; Nordberg, Åke. ”Torrötning-kunskapssammanställning och bedömning av utvecklingsbehov”. JTI-rapport, Lantbruk & Industry, 357, 2007, P-36-38. [21] Erdei Borbála et al “Ethanol production from mixtures of wheat straw and wheat meal” Lund

University, Department of Chemical Engineering. Budapest University of Technology and Economics,

Department of Applied Biotechnology and Food Science.

[22] D.R. Kelsall; T.P. Lyons. “Grain dry milling and cooking for alcohol production: designing for 23% ethanol and maximum yield”. In: Alcohol book. 1999 [23] KSGrains. Distillers grains can increase your profits [on line]. <http://www.ksgrains.com/ethanol/DDGS%20Flyer.pdf> [28/05/2010] [24] Kootstra et al, Comparision of dilute mineral and organic acid pretreatment for enzymatic hydrolysis of wheat straw. Biochemical Engineering Journal, 46(2009), p.126-131 [25] Linde, Marie. Steam Pretreatment of Dilute H2SO4-Impregnated Wheat Straw and SSF with Low Yeast and Enzyme Loadings for Bioethanol Production, Biomass and Bioenergy, 32(2008) p.326-332 [26] Linde, Marie. Simultaneous sacchaarification and fermentation of steam-pretreated barley straw at low enzyme loadings and low yeast concentration, Enzyme and Microbial Technology 40(2007) p. 1100-1107 [27] Sassner, Per. Lignocellulosic Ethanol Production Based on Steam Pretreatmen an SSF. Process Development through experiments and simulations. Department of Chemical Engineering, Lund University (Sweden), December 2007

53

[28] Rivard, Christopher J; Grohmann, Karel. Degradation of furfural (2-furaldehyde) to methane and carbon dioxide by an anaerobic consortium. Applied Biochemistry 1(1999) p.285-295 [29] Chemical Engineering. Economic Indicator [on line]. September 2009. <http://tekim.undip.ac.id/download/pci_20012008.pdf> [01/05/2010] [30] Avanza Bank. Index USD/SEK [on line]. <https://www.avanza.se/aza/aktieroptioner/indikatorer/indikator.jsp?orderbookId=19000&timeframe=year> [01/05/2010] [31] ADRITZ Pulp & Paper, “Wheat Straw Pre-Treatment System”, March 15 2010

[32] ICIS.com. Indicative chemical prices A-Z 2010 [on line]. <www.icis.com/StaticPages/a-e.htm>

[28/05/2010]

[33] Novozyme. Email: from Hans Erik Gording with price information. [08/04/2010]

[34] Matthew Bristow. Ethanol Fuel [on line]. Europe: 4th November 29.

<http://www.icispricing.com/il_shared/Samples/SubPage10100100.asp> [28/05/2010]

[35] FAOSTAT.com, Sweden-wheat-Producer price (USD/tone)-2007 [on line]. FAO16th

September 2009

<http://faostat.fao.org/site/570/DesktopDefault.aspx?PageID=570#ancor> [28/05/2010]

[36] Hay & Forage Grower. Prices Rise As Mercury Drops [on line]. February 1, 2002.

<http://translate.google.se/translate?hl=sv&sl=en&tl=sv&u=http%3A%2F%2Fhayandforage.com%2Fma

g%2Ffarming_prices_rise_mercury%2F&anno=2> [28/05/2010]

[37] Kronjäst. Jästbolaget. Email: from Lotten Nilsson with price information. [29/04/2010]

[38] Nilsson, Daniel; Bernsson, Sven. Pelletering och brikettering av jordbruksråvaror-en systemstudie.

Instutitionen för energi och teknik, Uppsala 2008

[39] Jerry D., Niamh M. Power. “How can we improve the energy balance of ethanol production from wheat?”. Fuel ,87 (2008) P-1799-1806 [40] Pelletsindustrins Riksförbund, Energy prices - pellets, oil and electricity [on line]. <http://www.pelletsindustrin.org/web/Pelletsprisindex.aspx> [15/05/2010] [41] Svensson, Christina. Karlshamns Kommun, VA-avgifter [on lne]. Karlshamn: 05/03/2009. <http://www.karlshamn.se/Kommunen/Samhallsbyggnad/Teknik-_och_fritidsavdelningen/Vatten_och_avlopp/VA-avgifter/>[15(05/2010] [42] Hans T. Karlsson, ”ProjekteringsHandboken 2007”, Instutionen för Kemiteknik [43] N. Arifeen et al. ”Development of novel wheat biorefining: Effect of gluten extraction from wheat

on bioethanol production”. Biochemical Engineering Journal, 43 (2009)

[44] Xiaosu et al. “Recycle Bioreactor for Biochemical Production from Wheat Starch I. Cold Enzyme

hydrolysis”. Energy sources,23 (2001), p. 417-425

54

[45] Montesinos, Thierry et al. “ Production of alcohol from raw wheat flour by Amyloglucosidase and

Sacchaomyces Cervisiae”. Enxymes and Microbial Technology,27 (2000)

[46] Arifen, Najmul et al. “Process Design and Optimization of Novel Wheat-based Continuous

Bioethanol Production System”. Biotechnol, 23(2007)

[47] Aldén, Anna. “Optimization of the Liquefaction Process in Bioethanol Production and Development

of Method for Quantification of Nonsolubilized Starch in Mash”. Master Thesis, University of Linköping,

2008

[48] Biatas, Wojciech et al. “Fuel ethanol production from granular corn starch using Saccharomyces

cervisiae in a long term repeated SSF process with full stillage recycling”. Bioresource Technology, 101

(2010)

[49] Murphy, Jerry D et al. “How can we improve the energy balance of ethanol production from

wheat?”. Fuel 87(2008) p1799-1806

55

Appendix

Appendix I Table A1 Content of wheat from literature

Novozyme [5] HGCA [2] Article 1 [43] Article 2 [44] Suggested

content

Water % 15 15 12.4 14 15

Starch DS% 67-70 69 68.5 75.6 69

Fermentable sugars DS% 1-2 3 - - 3

Biomass DS%

Protein 12-14 11.5 11 15 13

Fiber 10-13 11 - 7 10

Fat 2-3 2.5 - 3

Ash 2 2 - 2.4 2

Lignin - 1 - - -

Table A2 Yields in 1st

generation SSF from literature

Article 5 [48] HGCA [2] Article 3 [45] Article 4 [46] Suggested values

SSF yield [%] 83.38 84 87-89 82 84.3

Starchglucose - - - - 98

Glucoseethanol - - 90 - 86

Table A3 Literature values for the pre-mixer

Agro [47] Alcohol book [22] Novozyme [5]

Temperature (°C) 58 60 <60

Time (min) ~60 - 0-30

Enzyme (kg/ton

starch)

0.4-0.6 Amylase 0.2 Termamyl 120L Can be added

Table A4 Literature values for gelatinization

Agro [47] Alcohol book [22] Novozyme [5]

Temperature (°C) 73 58-64 58-65

Time (min) 120 6-120 -

56

Table A5 Literature values for liquefaction

Article 3 [45] Agro [47] Alcohol book [22] Novozyme [5]

Temperature 95 89 85 85-95

Time (min) 120 120 60-120 30-120

Enzymes (kg/ton

starch)

0.2 (Termamyl 120L) - 0.4-0.6 (Termamyl

120L)

0.2-0.4 (α-amylase)

Table A6 Literature values for 1st

generation SSF

Article 3 [45] Agro [47] Article 5 [48] Alcohol book [22] Novozyme [5]

Temperature (°C) 35 31.5 - 32 30-32

Time (h) 30 (with di-

ammonium

phosphate)

55 72 10-48 45-55

Yeast (kg/starch) 1.5 kg/ ton

(batch)

- - 0.1 kg/ton (batch) -

Living cells

(million/mL)

- - Start: 20 - Start: 10-15

- - - Top: 150 Top: 200

Enzyme (kg/ton

starch)

0.9 - - - 0.5-0.9

Appendix II Table A7 Composition of wheat straw from different articles

Composition of wheat straw (%w/w)

Article 1[43] Article 2 [44] Article 3 [45] Article 4 [46] Mean value used

Cellulose 36.3 35.9 32.6 35 35

Hemicelluloses 23 23.9 24.2 22.3 23

Lignin 25.5 19.3 26.5 15.6 23

Ash 6.7 4.1 4.6 6.5 5

Residuals 8.5 16.8 12.1 20.6 14

57

Appendix III Table A8 Calculated heat duties for the 1

st generation pretreatment

Pretreatment 1st generation

1st generation 2nd generation integrated

heater (MW) 1.6

1.7

cooler (MW) 0.8

0.8

Table A9 Calculated values for the 1st

generation SSF

SSF 1st generation

1st generation 2nd generation integrated

Heat duty (MW) -3.1

-3.1

Product (ton/h) 109.2

111.5

Mass fraction of EtOH 0.14

0.13

T(ºC) product 31

31

Pressure (bar) 1

1

The product stream consists of water, ethanol, glucose, xylose, furfural, acetic acid, carbon dioxide,

oxygen, glycerol, ammonia and nutrients. The mass fraction given in the table is the mass fraction of

ethanol in the product stream. The mass fractions obtained are too high to be considered. A common

value should be around 7-8% in weight. However, the results obtained will not affect the distillation

results very much.

Table A10 Calculated values for the 1st generation gas separator

Gas separator 1st generation

1st generation 2nd generation integrated

Heat duty(MW)

-0.2

Gas stream (ton/h) 13.0

12.9

Product (ton/h) 96.2

98.6

The main task of this separator is to separate the carbon dioxide from the stream outgoing the SSF. So,

the gas stream consists of 99.8% in weight of carbon dioxide and 0.2% of oxygen. The product stream is

formed by water, ethanol, glucose, xylose, furfural, acetic acid, glycerol, ammonia and nutrients.

58

Table A11 Calculated values for the 2nd

generation pretreatment

Pretreatment 2nd generation

1st generation 2nd generation integrated

Energy demand (MW)+10%

4.4 4.4

Energy released--> 10%

0.4 0.4

steam 20bar

8.3 8.3

Product

53.3 53.3

T(ºC) product

190.0 190.0

The energy demand is the heat duty required for the pretreatment. However, a 10% more of the

required heat duty was taken into account to guarantee that the temperature in the reactor reaches

190ºC. Assuming no heat losses and that the steam provided to the reactor covers the energy demand,

the 10% extra of the energy demand will be in excess, this value is referred to in the table as energy

released.

The product consists of water, glucose, xylose, acetic acid and furfural. The main component is water

with a mass fraction of 0.842, 0.136 of xylose, 0.011 of acetic acid and the same mass fraction for

glucose. The mass fraction for furfural is very small, 229ppm.

Table A12 Calculated values for the 2nd

generation cooling before prefilter

Cooling before prefilter

1st generation 2nd generation integrated

Heat duty (MW)

-2.2 -2.2

T(ºC) in

100 100

T(ºC) out

35 35

Pressure (bar)

1 1

After the pretreatment and before filtering the stream to remove 50% of the liquid, mainly water and

the stream needs to be cooled down with a heat exchanger.

Table A13 Calculated values for the 2nd

generation SSF

SSF 2nd generation

1st generation 2nd generation integrated

Heat duty (MW)

1.5 1.5

Supplied by condensed vapor 100ºC,1 bar(MW)

5.0 5.0

Net heat duty (MW)

-3.4 -3.4

Product (ton/h)

113.1 113.1

Mass fraction of EtOH

0.03 0.03

T(ºC) product

35 35

Pressure (bar)

1 1

59

The heat duty for the SSF is the energy required to preheat the feed stream in the reactor. This energy is

supplied by condensed vapor at 1 bar. As the energy supplied by the vapor is higher than the energy

required to preheat the ingoing feed, heat will be released from the SSF as shown in the table above.

The product stream of the second generation SSF contains water, ethanol, glucose, xylose, furfural,

acetic acid, carbon dioxide, ammonia and nutrients. The main component is water in 90.8%w/w. The

mass fraction of ethanol is the mass fraction in the product stream.

Table A14 Calculated values for the 2nd generation gas separator

Gas separator 2nd Generation

1st generation 2nd generation integrated

Heat duty (MW)

-0.003 -0.003

Gas stream (ton/h)

2.5 2.5

Product (ton/h)

110.6 110.6

The gas separator separates the carbon dioxide from the outgoing stream of the SSF. The gas stream is

the carbon dioxide while the product stream contains water, ethanol, glucose, xylose, furfural, acetic

acid, ammonia and nutrients.

Table A15 Calculated values for the stripper column

Stripper column

1st generation 2nd generation integrated

Number of stages 30.0 30.0 30.0

Pressure (bar) 3.0 3.0 3.0

Pressure drop (bar) 0.01 0.01 0.01

Q demand reboiler (MW) 11.9 8.0 20.8

T(ºC) reboiler 137.0 136.9 136.9

Feed (ton/h) 96.2 110.6 208.5

T(ºC) Feed 88.0 96.6 91.8

Vapor (ton/h) 20.8 6.9 28.5

T(ºC) Vapor 117.8 126.9 121.4

Waste-liquid (ton/h) 75.4 103.8 180.1

T(ºC) Waste-liquid 134.3 135.2 135.0

Vapor condenser (MW) 11.5 3.2 11.1

T(ºC) condenser in 118.0 126.9 121.4

T(ºC) condenser out 110.7 113.8 111.6

P (bar) condenser 3.0 3.0 3.0

Mass fraction of EtOH vapor 0.65 0.38 0.56

60

Table A16 Calculated values for the furfural separator

Furfural separator

1st generation 2nd generation integrated

Heat duty (MW) 0 0.004 0.13

EtOH-H20 (ton/h) 20.8 6.8 28.4

waste-furfural (ton/h) 0 0.004 0.01

The furfural separator is located after the stripper column. It takes the distillate from the stripper and

removes the furfural contained in that stream as it is an undesired byproduct.

Table A17 Calculated values for the rectifier column

Rectifier column

1st generation 2nd generation integrated

Number of stages 50 50 50

Pressure (bar) 0.5 0.5 0.5

Pressure drop (bar) 0.01 0.01 0.01

Reflux ratio 1.53 1.5 1.5

Feed stage 45 45 45

Feed (ton/h) 20.8 6.8 28.4

T(ºC) Feed 110.7 113.8 111.6

Ethanol (ton/h) 14.6 2.8 17.2

T(ºC) Ethanol 61.37 61.4 61.37

Water (ton/h) 6.2 4.1 11.1

T(ºC) Water 99.1 99.3 99.2

Mass fraction of EtOH vapor 0.93 0.93 0.93

Q condenser (MW) 10 1.9 11.8

T(ºC) condenser 61.4 61.4 61.4

Q reboiler (MW) 9.2 1.7 10.7

T (ºC) reboiler 99.1 99.3 99.2

Q left after using vapor condenser (MW) 1.8 1.5 0.4

Table A18 Calculated values for the biogas cooler

Biogas cooler

1st generation 2nd generation integrated

Heat duty (MW) 0.0 -3.0 3.3

Pressure (bar) 3 1 1

61

Table A19 Calculated values for the anaerobic digester 1

Anaerobic digester 1

1st generation 2nd generation integrated

Heat duty (MW) -0.5 -1.0 -1.5

Ammonia (ton/h) 0.5 0.5 0.5

Product (ton/h) 48 100.8 148.8

T(ºC) 37 37 37

Pressure (bar) 1 1 1

Table A20 Calculated values for the anaerobic digester 2

Anaerobic digester 2

1st generation 2nd generation integrated

Heat duty (MW) -0.05 -0.1 -0.1

Biogas (ton/h) 48 100.8 148.8

T(ºC) 37 37 37

Pressure (bar) 1 1 1

Mass fraction of CH4 0.02 0.01 0.01

Table A21 Calculated values for the gas separator biogas

Gas separator biogas

1st generation 2nd generation integrated

Heat duty (MW) -0.05 -0.08 -0.1

CH4-CO2 (ton/h) 2.8 5.0 8.1

non-CH4 (ton/h) 45.1 95.8 140.7

Mass fraction of CH4 0.26 0.26 0.26

Mass fraction of CO2 0.74 0.74 0.74

Appendix IV Table A22 Mass flow for raw material and products

Mass flow (ton/h)

IN 1st generation 2nd generation integrated

Wheat 44.3 44.3

Straw 25.0 25.0

OUT

Ethanol 12.9 2.4 15.2

Methane 0.7 1.3 2.1

62

Table A23 Lower heat of combustion for raw material and products

Stream Heat of combustion (MJ/kg)

Ethanol 27.1

Methane 50

Wheat (8.95 % moisture) 15.2

Wheat straw 16.5

Table A24 Energy content in the raw material and the products based on the lower heat of combustion

Energy content (MW)

IN 1st generation 2nd generation integrated

Wheat 187.1 187.1

Straw 114.6 114.6

OUT

Ethanol 97.2 18.4 114.6

Methane 10.2 18.5 29.6

Table A25 Electricity needed in the ethanol plant

Electricity needed (MW)

1st generation 2nd generation integrated

Electricity 6 4.8 9.8

The electricity value for the first generation is taken from an article [49] without considering the biogas

part, the value from second generation is an average from Sassners’ data [27] where the biogas part is

also not included and the integrated is the sum of the first and second generation values assuming that

1 MW is not needed when integrating.