Accepted Manuscript

Techno-economical study of biogas production improved by steam explosion

pretreatment

Marzieh Shafiei, Maryam Mohseni Kabir, Hamid Ziluei, Ilona Sárvári Horváth,

Keikhosro Karimi

PII: S0960-8524(13)01362-X

DOI: http://dx.doi.org/10.1016/j.biortech.2013.08.111

Reference: BITE 12310

To appear in: Bioresource Technology

Received Date: 20 June 2013

Revised Date: 16 August 2013

Accepted Date: 19 August 2013

Please cite this article as: Shafiei, M., Kabir, M.M., Ziluei, H., Horváth, I.S., Karimi, K., Techno-economical study

of biogas production improved by steam explosion pretreatment, Bioresource Technology (2013), doi: http://

dx.doi.org/10.1016/j.biortech.2013.08.111

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Techno-economical study of biogas production improved by

steam explosion pretreatment

Marzieh Shafiei1, Maryam Mohseni Kabir2, Hamid Ziluei1, Ilona Sárvári Horváth2,

Keikhosro Karimi1

1 Department of Chemical Engineering, Isfahan University of Technology,

Isfahan 84156-83111, Iran

2 School of Engineering, University of Borås, 50190 Borås, Sweden

Corresponding author:

Email: karimi@cc.iut.ac.ir

Tel: +983113915623

Fax: +983113912677

2 

Abstract

Economic feasibility of steam explosion pretreatment for improvement of biogas production

from wheat straw and paper tube residuals was investigated. The process was simulated by

Aspen plus®, and the economical feasibility of five different plant capacities was studied by

Aspen Process Economic Analyzer. Total project investment of a plant using paper tube

residuals or wheat straw was 63.9 or 61.8 million Euros, respectively. The manufacturing

cost of raw biogas for these two feedstocks was calculated to 0.36 or 0.48 € per m3 of

methane, respectively. Applying steam explosion pretreatment resulted in 13% higher total

capital investment while significantly improved the economy of the biogas plant and

decreased the manufacturing cost of methane by 36%. The sensitivity analysis showed that

5% improvement in the methane yield and 20% decrease in the raw material price resulted

in 5.5 and 8% decrease in the manufacturing cost of methane, respectively.

Keywords: Biogas; Pretreatment; Simulation; Steam explosion; Techno-economical

analysis.

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1 Introduction

Currently, one of the most important fundamentals for sustainable development is

production of fuels from biomass, as an alternative to fossil fuels. Bioethanol and biogas are

the two common biofuels (Börjesson and Mattiasson, 2008; Klass, 1998). While bioethanol

is currently produced from sugar or starch-containing materials, biogas is produced from

industrial or municipal wastes. The use of sugar or starch based materials for fuel

production has led to a “food vs. fuel” conflict. Therefore, several research and pilot scale

studies were performed using lignocellulosic biomass instead of food driven biofuels.

Lignocellulosic materials have the advantage of being available in very large quantities at

relatively low prices (Saddler, 1993; Taherzadeh and Karimi, 2008a).

The main drawback of using lignocellulosic materials is their rigid structure which is

resistant to enzymatic and microbial attacks. Therefore, a pretreatment step is required to

open up the structure and reduce the crystallinity of lignocelluloses (Taherzadeh and

Karimi, 2008a). Among the pretreatment methods, steam explosion, with or without

addition of a catalyst. This is one of the most thoroughly investigated methods, and it has

been reported to successfully increase the methane yield of different lignocellulosic

materials (Dereix, 2006; Dererie et al., 2011; Take et al., 2006). Steam explosion typically

operates within a temperature range of 160–260 °C for a few seconds to several minutes

(Taherzadeh and Karimi, 2008a; Vivekanand et al., 2013). This method produces low

amounts of waste and consumes moderate amounts of energy (Taherzadeh and Karimi,

2008a).

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Biogas production from lignocelluloses has several advantages compared to bioethanol

production. Compared to ethanol, the overall energy efficiency is much higher in biogas

production (Murphy and Power, 2009). Furthermore, conversion of lignocellulosic pentoses

to ethanol is one of the obstacles in the process, since the ordinary industrial yeast species

are unable to assimilate pentoses (Karimi and Zamani, 2013). Therefore, one of the best

economical solutions is the assimilation of pentoses in a separate biogas production process

after bioethanol production (Shafiei et al. (2011). However, all suggested processes have

their own difficulties (Taherzadeh and Karimi, 2008b), hence the best solution could be to

produce biogas as the only product from lignocellulosic biomass. The advantage of this

single step process is the simultaneous hydrolysis and biogas production, which can be

applied principally for all types of fermentable materials. The produced biogas can be then

utilized in generation of power and heat, and it can also be upgraded to gaseous vehicle

fuels (Murphy and Power, 2009; Taherzadeh and Karimi, 2008b).

Techno-economical analysis is a useful tool to examine the profitability and performance of

the proposed process by combining process simulation with economic calculation. The

application of techno-economical analysis allows comparing alternative processes and

optimizing the process parameters such as capacity of the plant and composition of the raw

materials. Ekman et al. (2012) and Sassner et al. (2008) proved that steam treatment of

straw, spruce (a softwood), salix (a hardwood), and corn stover is economically feasible for

ethanol production. On the other hand, technical and economical analyses for biogas

production in the Netherland from different wastes showed that biogas production is not

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economically feasible unless subsidies are provided from the government (Gebrezgabher et

al., 2010). Nevertheless, to our knowledge, there is no reference detected in the literature on

techno-economical processes evaluating lignocellulosic biomass for biogas production.

The current study evaluates the economic feasibility of steam explosion pretreatment

applied for improvement of biogas production from wheat straw and paper tube residuals.

The process was simulated using Aspen Plus® simulation software (Aspen Tech, USA)

based on experimental data provided by previous studies (Bauer et al., 2009; Teghammar et

al., 2010). A process including a continuous pretreatment system combined with solid state

digestion was examined. Economical evaluations were conducted by Aspen PEA (Process

Economic Analyzer). The effects of plant capacity and price of raw materials and

byproducts on the economy of the process were also investigated.

2 Process development

Production of biogas from easily digestible materials is a well-developed technology, and

the solid state technology for utilizing the energy crops such as corn is already used in many

countries, e.g., Germany (Yliopisto, 2009). However, an efficient process for biogas

production from cellulose containing materials which are resistant to biodegradation is still

under development. An overview of a biogas production plant from paper tube residuals and

wheat straw using steam explosion pretreatment is presented in Fig. 1. The process includes

feedstock handling, steam explosion pretreatment, and anaerobic digestion. In order to

provide steam and electricity, it is assumed that the plant is located close to an existing

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power plant, and the byproducts, i.e., steam and solid residues as a fuel, considered as

valuable by-products in the main plant.

2.1 Plant size and feedstocks

Lignocellulosic wastes, e.g., straw, wood chips, paper residues, and grass as well as forest

and agricultural residues, can supply as the most abundant raw materials (Lin and Tanaka,

2006). The average of 5 year production of barely, oat and wheat in Sweden is 1.5, 0.7 and

2.3 million tons, respectively (FAO, 2013). The amount of produced straw is 1-1.2 fold of

the weight of these cereal grains. This amount is cultivated in the estimated area of 914,000

hectares.

Choosing the correct capacity for a plant depends on several parameters including

availability of raw materials, technical bottle necks, transportation costs for both raw

materials and products, as well as market demands for the products. In recent studies, the

capacity of plants for production of biogas (Murphy and Power, 2009) and bioethanol (Aden

and Foust, 2009; Aden et al., 2002; Ekman et al., 2012; Lohrasbi et al., 2010; Shafiei et al.,

2011) was selected to treat raw materials within the range of 100,000 to 600,000 tons

annually. An example of using lower capacity is found in a study evaluating the production

of biogas in the Netherlands using 70,000 ton/year of farm originated wastes. However, this

study showed that the process is not economically feasible unless subsidies are provided

from the government (Gebrezgabher et al., 2010). Therefore, a plant with a capacity of

200,000 ton/year of raw material (dry basis) was chosen as a base case for the present

investigations, which is similar to the one used in a previous study evaluating bioethanol

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production from wood in Sweden (Shafiei et al., 2011). Furthermore, different capacities

ranging from 50,000 to 600,000 tons/year were also evaluated. It was assumed that the plant

would be in full operation for 8,000 h/year (Shafiei et al., 2011).

An important aspect in the choice for the capacity of a plant is the availability and

transportation cost of raw materials. The required amount of straw for the base case is only

4% of the total amount of produced straw in Sweden. In the case of paper tube residuals, the

required amount for the biogas plant is less than half of the total amount of collected paper

from households in Sweden, which is 383,000 tons (Weine, 2012). However, in Sweden,

besides recycled paper from households, recycling companies collect paper with different

qualities from industries too (STENA, 2013). This recycled paper can also serve as another

source for further biogas production.

In this study, the raw materials were considered for a plant with a single feedstock, and the

manufacturing cost was evaluated; however, running a plant with only one type of raw

material is practically questionable. Therefore, the design of the plant should be flexible

enough, so that primarily a raw material with higher profitability is used and the more

expensive raw material will only be used when the other one is not sufficient.

The composition of raw materials was defined based on previous experimental data (Bauer

et al., 2009; Teghammar et al., 2010). Moisture content of paper tube residuals and wheat

straw was 5% and 15%, respectively. Major components of paper tube residuals were 23%

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lignin, 60% cellulose, and 17% hemicellulose. Wheat straw contained 50% cellulose, 27%

hemicellulose, and 23% lignin.

2.2 Feedstock handling and pretreatment unit

After transportation of the raw materials to the plant, they are weighed and unloaded from

truck trailers. The received materials are reduced in size as a preliminary treatment, and then

dumped into storage piles. Conveyor belts are used for transporting the materials to a

continuous pretreatment system. The pretreatment process modifies the structure of the

cellulose, lignin, and hemicellulose. Thus, the carbohydrate bonds can be easily exposed by

the hydrolytic bacteria during the following anaerobic digestion process (Taherzadeh and

Karimi, 2008a). The pretreatment method is based on a steam explosion process developed

by Aden et al. (2002). This process was used for pretreatment of corn stover for

improvement of ethanol production. In this pretreatment unit, the feedstock is conveyed to a

pre-steamer. The pre-steamer is a horizontal vessel which is fed with variable frequency

drive set of screw conveyors. Low pressure steam is injected to the pre-steamer which

increases the temperature to 100°C and removes the non-condensable gases. Screw

conveyors are used for discharging the preheated materials to a flash vessel as well as for

discharging the material from the flash vessel to a continuous pretreatment reactor. This

reactor is a high pressure horizontal extruder using steam as a motive force. High pressure

steam is used to increase the pressure (Aden et al., 2002). Since continuous process is used

for the pretreatment, the retention time for heating-up to the desired temperature would be

reduced, two minutes residence time was therefore assumed for the treatment.

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2.3 Biogas production

The pretreated materials are subjected to the biogas production unit where solid state

digestion is used for biogas production based on the technology presented by DRANCO

(Germany). This technology is a well-established and reliable technology used for treatment

of solid organic wastes. Each digester has 3150 m3 volume, and the retention time of the

materials in the reactor is 2 to 4 days. The overall residence time before the materials are

fully digested is about 20 days (Baere, 2010). The digesters are continuous vertical plug

flow reactors with cone bottom shape. The semi-digested materials are extracted from the

middle of the digester by screw pumps, and mixed with the pretreated materials at ratio of

about 5:1 and then returned to the top of the digester. Gravity is the driving force for

movement of the solids toward the bottom of the digester. The fully digested materials are

extracted from the bottom of the digester and sent to the dewatering unit (Baere, 2010).

2.4 Process simulation and economic calculations

Mass and energy balances were calculated for the process (Fig. 1 and 2) using the Aspen

Plus® program. This software was chosen because of its high ability to handle solid state

materials. This feature together with its broad property data base makes this software the

best tool for simulation, design, and optimization of the processes (Aspen-Tech, 2013). The

results of rigorous evaluations, using detailed equipment models, are presented in Table 1

and 2. A data bank prepared by NREL (National Renewable Energy Laboratory, USA)

(Wooley and Putsche, 1996) for the calculation of physical and thermodynamic properties

of biomass components was introduced to Aspen Plus®.

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In the next step, Aspen PEA was used for the economical evaluations. Key features in

Aspen PEA include interactive equipment sizing to determine operating costs and

investment analysis as well as automatic generation of block and process flow diagrams. It

also has interfaces to all major process simulation tools, including Aspen Plus (Aspen-Tech,

2013). Based on the results from simulation, the sizes of equipments were calculated using

Aspen Plus® or Aspen PEA, and then the costs of all major equipments were estimated

using Aspen PEA. The main equipments were crusher, flash vessels, conveyor belts, biogas

digester, belt press, pumps, vessels, and heat exchangers as well as auxiliary equipments

including piping, insulation, electrical equipment, and instrumentation. The costs for

pretreatment vessels were calculated according to an NREL report (Aden et al., 2002). The

main assumptions for all process scenarios are as follows:

The plant capacity is 200,000 tons/year of raw materials (based on dry weight).

The plant is located in Sweden.

Cost indexes at 2013 are used for equipments, chemicals, and labor costs estimations.

Straight line depreciation for 7 years with zero salvage value was used for calculation

of the plant depreciation.

The project is one hundred percent equity financed.

In the project investment, 15% contingency was considered.

The construction period is 64 weeks followed by 40 weeks start-up period.

Working capital is based on 5% of the total investment cost.

The desired internal rate of return in the process is 10%.

Project life is 15 years.

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Operating supplies and laboratory charges are respectively 18% and 15% of the

operating labor cost.

Taxation rate value is 30%.

These values were chosen based on the data available in the literature (Peters and

Timmerhaus, 1991; Turton et al., 2009). The prices of the raw materials and products are

presented in Table 3. The cost for production of methane was calculated based on the total

capital investment of the project and the annual operating costs. A discounted cash flow

analysis was used to evaluate the payback period of the plant.

2.5 Sensitivity analysis

The plant capacity affects the manufacturing cost as well as the total capital investment.

Therefore, plant capacities ranging from 50,000 to 600,000 tons/year of dry raw material

were evaluated using Aspen PEA program and compared with the base case (200,000

tons/year). In order to investigate the effect of pretreatment on the economy of the process,

simulation and economic evaluation for a plant without a pretreatment unit were also

conducted. Quality and type of feedstock as well as transportation costs of feedstock may

affect the manufacturing cost of the product. Thus, a sensitivity analysis was performed to

investigate the effect of feedstock price on the manufacturing cost. Changes in the

manufacturing cost with respect to other required materials and to the byproduct’s prices

were also evaluated.

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3 Results and Discussion

The economic feasibility of biogas production from wheat straw and paper tube residuals

improved by steam explosion pretreatment was studied. For this purpose, the process was

simulated using Aspen Plus® and the economical evaluation was performed by Aspen PEA.

3.1 Pretreatment process

The block flow diagram and process flow diagram of the steam explosion process for

production of biogas are presented in Figs 1 and 2, respectively. Based on the simulation

results (Fig. 2) and capacities suggested by Aden et al (2002), two identical units for

unwrapping and preparation of the raw materials are required. The amounts of material to

feed each unit were calculated to 26.3 and 29.4 ton/h for paper tube residuals and wheat

straw, respectively. Each of these units has bale unwrap and shredder feed conveyor, truck

scale, and truck unloading forklift, bale moving forklift, shredder, and magnetic separator.

Pretreatment temperatures of 180 and 190 °C are considered for pretreatment of wheat straw

and paper tube residuals , respectively (Bauer et al., 2009; Teghammar et al., 2010). After

pretreatment, 7-8% of the hexosan and 50% of the pentosan are hydrolyzed to their

corresponding monomers (Tables 1 and 2). The heat loss from the reactor was not accounted

in the energy balance calculations (Aden et al., 2002). Based on the simulation results, three

identical units for pretreatment consisting of pre-hydrolysis equipments, screw reactor

cyclone, conveyor belts and expansion tanks are required. Each of these units has the

capacity of 37.3 ton/h.

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After completion of the pretreatment, the material is conveyed to an expansion tank. The

tank is a cyclone with atmospheric pressure in which over 90% of the steam used for heating

is recovered as a vapor. Beside major changes in the structure of cellulose, a portion of

holocellulose is converted to glucose and xylose monomers and then degrades to

hydroxymethylfurfural (HMF) and furfural. About 70% of the furans are separated from the

pretreated solid and moved to the vapor phase (Aden et al., 2002). This steam is a low

pressure steam contains over 98% water; however, presence of furans decreases its value

compared to pure low pressure steam. The pretreated materials contain less than 0.05% of

furans.

Based on the simulation results, the moisture content of the raw material as well as the

amount of water added to the feedstock prior to the pretreatment are key factors that affect

the amount of steam required for the pretreatment. The amount of water and steam used in

the pretreatment process affects the size of the pretreatment equipment as well as the size of

the expansion tank. Therefore, in the simulation, the amounts of water and low pressure

steam are calculated in such a way that the preheated materials would contain 30% total

solids in all cases. For paper tube residuals with 5% moisture content, 108 and 59 kton/year

water and low pressure steam were required, whereas these amounts for wheat straw

(containing15% moisture) were 81.7 and 61.4 kton/year, respectively (Tables 1 and 2). The

pretreated material contained 70% moisture when they were fed to the digesters.

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3.2 Biogas production

The biogas digester was also simulated by Aspen plus® and the amount of produced

methane was calculated by the program. Totally, fifteen digesters with 3150 m3 capacity

containing all auxiliary equipments as well as feed pumps and screw pumps are required.

The input flow rate to each digester is 6.1 ton/h. A package for supplementation of nutrients

and control of pH was considered in the biogas production unit. The solid residue from the

digesters is dewatered and contains 30% dry matter. This residue mainly consists of lignin

(75% of dry weight) which remains intact during the digestion process and is considered as

a valuable byproduct for production of heat.

The most important advantage of solid state biogas production is less digester volume

required. Another advantage of this technology is the production of no or limited amount of

excess wastewater (Baere, 2010). The results from simulation have shown that the process

can run with 100% water recycling. The water inside the solid residue (70%) contains

excess soluble materials which will be purged from the process. Furthermore, the furans are

flashed out from the process in the pretreatment unit.

3.3 Total project investment and manufacturing costs

Five different plant sizes were analyzed for the total project investment as well as for the

manufacturing cost of the methane from the pretreated materials. Total project investment

for a biogas production plant with 200,000 tons/year of dry paper tube residuals and wheat

straw as feedstocks was calculated to be 63.9 and 61.8 million Euros, respectively, and the

corresponding manufacturing cost of methane was 0.36 and 0.48 €/m3 of methane (or 0.18

15

and 0.23 €/m3 of biogas). Investments for feedstock handling, pretreatment, and biogas

production units were contributed to 5, 13, and 82% of the total project investment,

respectively. Additionally, 0.10-0.21 €/m3 and 0.10 €/m3 are suggested to be able to cover

the costs for upgrading and pressurizing the raw biogas, respectively (Yliopisto, 2009).

Therefore, the manufacturing cost of pressurized methane from paper tube residuals and

wheat straw would be 0.56-0.67 €/m3 and 0.68-0.79 €/m3, respectively. These results are

comparable with the results of other studies producing biogas from easily degradable raw

materials. The price for upgraded and pressurized biogas from organic waste was reported to

be 0.55- 0.65 €/m3, while the price for biogas production using energy crops was 0.53-0.74

€/m3 (Mårtensson, 2007). The cost of biogas from energy crops were estimated to be 0.68-

0.85 €/m3 including addition of 0.16-0.27 €/m3 for the gas distribution.

In Table 4, comparison of some biogas production processes and their yearly profits are

presented. Yearly profit was defined according to the following equation:

(1)

The yearly profit were 0.27 and 0.14 (€/€.year) for paper tube residuals and wheat straw in

the current work (Table 4). In another study, the price for the production of biomethane

utilizing wheat grains and other crops in Ireland were estimated to be 0.83-0.90 €/m3;

however, this manufacturing price was not low enough to be economically feasible (Murphy

and Power, 2009). The yearly profit for the process presented by Tricase and Lombardi

(2009) was 0.13( €/€.year), while this value was 0.2 ( €/€.year) only if governmental support

is available for the process presented by Gebrezgabher et al (2010).

16 

The results of changes in manufacturing and total project investment’s cost for different

biogas plant sizes using paper tube residuals or wheat straw as feedstock are presented in

Fig. 3. Although the total project investment for both of these substrates were in the same

order, the manufacturing costs of methane from wheat straw was higher than that from

paper tube residuals, due to the higher price of the raw material in the case of wheat straw.

However, there might be limitations in availability of paper tube residuals; thus,

construction of a flexible plant with ability to use several types of raw materials is

inevitable. On the other hand, the price of the feedstock for plants with 2 and 3 folds

capacities compared with the base case was increased by 10 and 20%, respectively, because

of the increase in the transportation cost. As a result of this increase, the manufacturing cost

of methane for plants with 3 fold capacity using wheat straw as a raw material showed a

slight increase. The total project investment required for a plant does not increase linearly

with the plant size, but is believed to increase according to the “six-tenth” rule (Turton et al.,

2009). Total project investments for plants with different capacities were calculated by

Aspen PEA (Figure 3). Generally, a plant with larger size requires less investment per each

unit of product. Therefore, larger plants are more economically feasible unless technical or

logistical problems occur. However, there are some limitations that should be considered

here, e.g., technical bottle necks, availability of raw materials, transportation costs for both

raw materials and products, and market demands for the products.

In the construction of the plant, 42% of the fixed capital investment (FCI) is attributed to the

purchased equipment cost. In biogas production plants, large size for digesters are required

17 

in order to provide sufficient retention time for the materials, and beside the costs for the

equipments, the civil costs of the digesters is also high (10% of FCI). Moreover, since a

biogas production plant using lignocellulosic materials with the pretreatment is not a proven

technology yet, 15% of FCI was chosen for the contingencies and unpredicted charges. The

rest of the FCI is attributed for piping, instrumentation, electrical works, painting, contract

fee, and general and administrator over heads.

The total project investment for a plant without a pretreatment unit was 56.4 million Euros.

The average annual production of methane from these feedstocks and in the base case (i.e.,

200,000 tons/year capacity) was 70.5 million Nm3; however, if no pretreatment is used, this

amount reduces to 36.8 million Nm3/year. Although the total operating costs for

pretreatment and steam requirement are reduced from the methane manufacturing cost, it

will still increase to 0.56 €/m3 when pretreatment is not introduced. Thus, the results showed

that while the application of a pretreatment unit requires 13% increase in the total capital

investment, it reduces the manufacturing cost of the produced methane by 35.7%.

3.3.1 Cost distribution

Total manufacturing cost from paper tube residuals and wheat straw was 24.1 and 32.4

million €, respectively. These numbers were calculated by considering the credit of low

pressure steam and solid residue as byproducts of the pretreatment. The respective values

for the credit of by products were -1.20 and -1.54 million €. Figure 4 shows the breakdown

of the methane manufacturing cost. The most significant fraction of this cost is attributed to

the raw materials, mainly to the cost of lignocellulosic substrate. Utility cost, mainly due to

18 

the consumption of high pressure steam, is the second highest cost within the manufacturing

cost for the process.

3.3.2 Sensitivity analysis

Effect of feedstock price on the manufacturing cost of methane was evaluated and is

presented in Fig. 5. Furthermore, effect of prices for fresh water and high pressure steam

(HPS) as well as low pressure steam (LPS) and solid residue as products was also

investigated (Fig. 6). Moreover, effect of methane production yield on the manufacturing

cost of methane is presented in Fig. 6. The results of sensitivity analysis showed that

manufacturing cost was highly sensitive to the price of the cellulosic raw material. For

instance, 20% decrease in the price of these raw materials resulted in 7-8% reduction in the

manufacturing cost of methane, whereas the price of other raw materials used in the process

does not highly affect the manufacturing cost of the produced methane. This is in line of

previous studies (Murphy and Power, 2009). Furthermore, the yield of methane production

is a critical parameter affecting the economy of the process, whereas the value of the

additional byproducts has a minor effect on the profitability. When the yield improved from

90% to 95%, the manufacturing cost increased by 5.5%. Yield of methane production is

directly related to the amount of cellulose and hemicellulose in the raw materials as well as

the process operation conditions. Further development for purification and utilization of

furfural and HMF as valuable byproducts of the process can improve the profitability, and

as a result providing a clean steam free from the chemical contaminations, which can be

reused in the process.

19 

By selling the raw methane for 0.42 and 0.53 €/m3 from paper tube residuals and wheat

straw, respectively, the payback period of the plant would be 5 years. Accordingly, when the

selling price increases to 0.45 and 0.57 €/m3, the payback period would be 3 years.

3.3.3 Energy balance

Energy efficiency was calculated by dividing energy outputs with the energy inputs of the

process (Sassner et al., 2008) (Table 5). The energy is produced in forms of methane, solid

residues, and low pressure steam. The energy input includes steam and electricity

requirements of the process as well as the energy available in the raw materials. The energy

balance of the plant is presented in Table 5. Total energy input of the base case is 153.9±0.4

MW. Total energy output is 123.1±0.5 MW in the form of lignin, low pressure steam, and

methane (Table 5). Therefore, total energy efficiency of the process is 80%. In another

study, it was shown that the energy efficiency of a biorefinery for production of bioethanol

and biogas from spruce wood was 79% (Shafiei et al., 2011). Furthermore, it was shown

that the overall energy efficiency of biogas production is much higher than that of ethanol

production (Murphy and Power, 2009). The total required energy for the processes utilizing

paper tube residues and wheat straw were 18.2 and 16.9 MW, respectively, which were 25%

and 23% of the fuel energy of the produced methane (72.6 and 72.5 MW) (Table 5).

Therefore, it can be concluded that methane production by the process evaluated in this

study can be a promising and economically feasible alternative for renewable energy

production.

20 

4 Conclusions

This study shows that steam explosion pretreatment of wheat straw and paper tube residuals

can significantly improve the economy of biogas production plants. The sensitivity analysis

shows that the methane yield and the raw material price were the most important parameters

affecting the manufacturing cost.

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25 

Figure Captions

Fig. 1. Block flow diagram of the process for biogas production from lignocellulosic raw materials using

steam explosion pretreatment

Fig. 2. Simplified process flow diagram for biogas production from lignocellulosic raw materials

improved by a steam explosion pretreatment step.

Fig. 3. Variation of the total project investment as well as manufacturing cost of methane with the

project capacity when paper tube residuals ( ) and wheat straw (▲) are pretreated by steam explosion

prior to anaerobic digestion.

Fig. 4. Breakdown of manufacturing cost of methane from paper tube residuals (a) and wheat straw (b).

Fig. 5. Effect of raw material price on manufacturing cost of methane from paper tube residuals

(triangles) and wheat straw (squares). For each raw material the base case is presented by empty points.

Fig. 6. Effect of prices for fresh water, high pressure steam (HPS), low pressure steam (LPS), and solid

residue price as well as the methane production yield on the manufacturing cost of methane from

pretreated paper tube residuals (colored bars), wheat straw (empty bars). Methane price for base case is

0.36 €/Nm3 of methane for paper tube residuals and 0.48 €/Nm3 of methane for wheat straw.

Table Captions

Table 1. Stream components in the process presented on Fig. 1. using paper tube residuals as feedstock.

Table 2. Stream components in the process presented on Fig. 1. using wheat straw as feedstock.

Table 3. Prices used in the economical evaluation for the raw materials and for the products.

Table 4. Comparison of results obtained during the current work results with results from similar

studies.

Table 5. Energy requirements for the process utilizing 200,000 tons/year raw material.

HPSteam

Pretreated MaterialsRaw Materials

Anaerobic DigestionFeedstock Handling

2

LP Steam 1

Water

8Pretreatment1

37

Biogas

Solid Residue

Water Recirculation

4

LP Steam 2

5

Nutrients

6

Fig. 1.

Figure 1

28 

Fig. 2.

29 

Fig. 3.

30 

Fig. 4

31 

Fig. 5.

32 

Fig. 6.

33 

Table 1.

Component Mass fraction 1 2 3 4 5 6 7 8

Water 0.050 1.000 1.000 1.000 0.996 0.713 0.095 0.701 Hexoses - - - - - 0.013 - - Pentoses - - - - - 0.024 - - Hexosan 0.570 - - - - 0.151 - 0.028 Pentosan 0.162 - - - - 0.024 - 0.004 Lignin 0.218 - - - - 0.061 - 0.224

Methane - - - - - - 0.242 - CO2 - - - - - - 0.663 -

Biomass - - - - - 0.011 - 0.041 Othera - - - - 0.004 0.003 - 0.002 Total

(Kton/year) 210.5 108.0 59.0 145.0 149.5 737.7 172.7 200.3 aOther components are mainly furfural, hydroxymethyl furfural and solubilized lignin

34 

Table 2.

Component Mass fraction 1 2 3 4 5 6 7 8

Water 0.150 1.000 1.000 1.000 0.993 0.715 0.095 0.702 Hexoses - - - - - 0.010 - - Pentoses - - - - - 0.037 - - Hexosan 0.423 - - - - 0.125 - 0.023 Pentosan 0.229 - - - - 0.038 - 0.007 Lignin 0.198 - - - - 0.061 - 0.225

Methane - - - - - - 0.242 - CO2 - - - - - - 0.663 -

Biomass - - - - - 0.011 - 0.041 Othera - - - - 0.007 0.003 - 0.002 Total

(Kton/year) 235.3 81.7 61.4 125.0 128.8 743.5 172.5 202.1 a Other components are mainly furfural, hydroxymethyl furfural and solubilized lignin

35 

Table 3.

Materials Price (€/kg)

Raw materials

Fresh water 0.0001a

Wheat straw 0.08b Paper tube 0.05b High pressure steam 0.008c Low pressure steam 0.004c Nutrients/ pH control materials 0.6d

Products Steam 0.003e Solid fuel 0.02f

a Data from Turton et al (2009) corrected by cost index of 2013. b Data from Odhner et al (2012). c Data from Peters and Timmerhaus (1991) corrected by cost index of 2013. d Data from Lohrasbi et al (2010). e Estimated based on the data for low and high pressure steam. f Estimated based on the data from Shafiei et al (2011).

36 

Table 4.

Raw materials Pretreatment Biogas

production

Total Capital

Investment

(M€)

Capacity

of raw

material

Production cost Yearly profit

(€/€.year)

Paper tube 63.9 0.56-0.67 b,c 0.27 e Base case

Wheat straw

Steam

explosion

Solid state

fermentation 61.8 200 ktpy a

0.68-0.79 c,d 0.14 e

Wheat, sugar beet yes 205.6 988 ktpy a 0.9 c Murphy and

Power (2009) Wheat yes

Solid state

fermentation 220.3 714 ktpy a 0.84 c

Not

profitable f

Tricase and

Lombardi

(2009)

Animal sewage +

co-substrates no Conventional 0.237

5000 m3

1000 m3

43 kW electricity +

1295 GJ/year heat

38,868 €/year Total

cost

0.13

Gebrezgabher

et al. (2010)

Animal manure +

energy maize +

other

no Conventional

6.7 (for all

additional

units to an

existing plant)

70 ktpy a Electricity sales:

900,000 €/year.

Not

profitable

(0.2 if

subsidies are

provided) a ktpy: kilo ton per year. b The production cost of raw biogas is 0.18 €/m3 of biogas (or 0.36 €/m3 of methane). c Upgraded and pressurized methane. d The production cost of raw biogas is 0.23 €/m3 of biogas (or 0.48 €/m3 of methane). e The calculation are based on selling raw biogas at the price of 0.57 €/m3 of methane. f The project was not profitable with selling the pressurized methane at price of 1€/m3.

37 

Table 5.

kg/s MW

Paper

tube Wheat straw

Combustion Heat(J/kmol)

Mol. Weight MJ/Kg

Paper tube

Wheat straw

Input materials and energy for the process Cellulose 4.17 3.46 2.81E+09 162 17.3 72.3 60.0 Xylan 1.18 1.87 2.35E+09 132 17.8 21.0 33.3 Lignin 1.60 1.62 3.27E+09 122 26.7 42.8 43.4 HP Steam 5.03 4.34 11.0 9.5 LP Steam 1 2.05 2.13 4.2 4.4 Electricity a 3.0 3.0 Total 154.3 153.5

Output materials and energy for the process Lignin 1.57 1.59 3.27E+09 123 26.7 41.6 42.1 LP Steam 2 4.53 3.91 9.4 8.1 Methane 1.45 1.45 8.00E+08 16 50.1 72.6 72.5 Total 123.6 122.7 a Efficiency of electricity production was assumed to be 33%.

 

38 

 

39 

Highlights

The economy of steam explosion pretreatment for biogas production was studied.

Aspen plus and Aspen Process Economic Analyzer was used for the evaluation.

Total energy efficiency of the process is calculated to 80%.

Production cost of methane from wheat straw and paper tube was 0.36-0.48 €/m3.

The production cost of methane decreased by 36% using the pretreatment.