activity 3 final report

Activity 3:

Activity leader:

Instituto de Recursos Naturales. Universidad de León

Collaboration:

Agencia Energética Municipal de Valladolid

FINAL REPORT

Best Available Techniques

EBIMUN Evaluation of biomass resources for municipalities

Instituto de Medio Ambiente

y Recursos Naturales

Instituto de Medio Ambiente

y Recursos Naturales

Best Available Techniques in Pyrolysis 1

The document has two parts:

Part I

Best Available Techniques in Pyrolysis Instituto de Recursos Naturales. Universidad de León

Part II

Best Available Techniques in Combustion and

Gasification Agencia Energética Municipal de Valladolid


Part I

Techniques.

RTP Technology.

Dynamotive’s Fast Pyrolysis

Pyrolysis Reactor for bio-char production from pruning and agro-

residues. ULE

Pyrolysis with microwaves.

The Carbo-V® process


1. RTP Technology.

Description

RTP stands for Rapid Thermal Process, representing the technology and equipment used to convert non-

food-based feedstock into pyrolysis oil. It is a fast thermal process whereby biomass is heated rapidly by

contact with hot sand; the biomass is first vaporized and then rapidly cooled. The process, see Fig 1, occurs

in less than two seconds, typically yielding 65wt% to 75wt% pyrolysis oil.

This oil can then be used in the generation of electricity and for the production of process heat. Development

is underway to upgrade pyrolysis oil into green gasoline, green diesel and green jet fuel.

Fig 1. RTP process diagram

Achieved environmental benefits

When pyrolysis oil replaces fossil fuels to produce heat or steam, life cycle greenhouse gas emissions are

dramatically reduced by 70-88% depending on the biomass transport distance. The SOx emissions from

burning pyrolysis oil are similar to natural gas.

Applicability

Industries looking to reduce their carbon footprint, replacing fossil fuels, with greener fuels.


Economics

By way of example (producing electricity), a 30 MWe RTP/Dual Engine power plant would consume 400

dry tonnes per day (tpd) of biomass and would cost about $99 million (i.e., 30,000 kWe x $3,300 per kWe).

On the other hand, a 30MWe direct combustion/steam plant would cost over $110 million (i.e., 30,000 kWe

x $3,700 per kWe) and would consume about 690 dry tonnes of biomass per day.

Example plant

Renfrew RTP – 1 (Owned and operated by Ensyn), Renfrew, Ontario, Canada

Crystal glass factory RCR in Località Catarelli, Italy.

Reference literature

http://www.envergenttech.com/rtp.php

http://www.collenergia.com/eng/progetto.shtml

The white paper on power generation. http://www.ensyn.com/technology/biomass-energy-pathways/


2. Dynamotive’s Fast Pyrolysis

Description

The process starts when the feedstock is fed into the bubbling fluid-bed reactor, which is heated to 450–500

°C in the absence of oxygen. This is lower than conventional pyrolysis systems and, therefore, has the

benefit of higher overall energy conversion efficiency. The feedstock flashes and vaporizes like throwing

droplets of water onto a hot frying pan. The resulting gases pass into a cyclone where solid particles, char,

are extracted. The gases enter a quench tower where they are quickly cooled using pyrolysis oil (Bio Oil)

already made in the process. See Fig 2.

Fig 2. Dynamotive’s Fast Pyrolysis diagram.


The reduction of GHGs emissions co-firing pyrolysis oil instead of 100% fossil fuels.

No waste is produced in the process.

Operational data

As the non-condensable gases are used as energy to run the process, nothing is wasted and no waste is

produced. The uncondensed, flammable gases are re-circulated to fuel approximately 75% of the energy

needed by the pyrolysis process. Three products are produced: Pyrolysis oil (60-75% by weight), char (15-

20wt%) and non-condensable gases (10-20wt%). Yields vary depending on the feedstock composition.

Pyrolysis oil and char are commercial products and non-condensable gases are recycled and supply a major

part of the energy required by the process.


Applicability

This technology is easily applicable to process or industries where there is biomass availability, and want to

make a better use of the resource than direct combustion. And for the ones which are trying to reduce

dependency on fossil fuels.

Driving force for implementation

Tax and Investment Incentives

Numerous tax penalties for fossil fuel use, some dramatic tax incentives and direct subsidies for renewable

energy in many industrialized countries are also driving demand.

Energy Costs

The volatility and upward tendency of energy prices over recent years has forced both industrial and

individual consumers of energy to evaluate alternative sources of renewable energy.

Example plants

Lumber Kiln, Canfor Lumber Mill, Prince George, BC, Canada


Dynamotive Energy Systems. http://www.dynamotive.com/industrialfuels/biooil/


3. Pyrolysis Reactor for bio-char production from pruning and agro-

residues.ULE

Description

This reactor is designed to treat woody residues from vines pruning and some other suitable materials. Bio-

char is the principal desired product, in order to use it as soil amendment.

The reactor consists in an easy to transport unit. It reaches the working temperature by burning auxiliary fuel,

and then it runs on gases from the ongoing pyrolysis. A basic diagram is shown in Fig 3

Fig 3. Pyrolysis Reactor for bio-char production from pruning and agro-residues diagram

Environmental benefits

Transporting the reactor where the residues are, better than carrying low density biomass, reduces the GHGs

emissions.

Bio-char on soil is a carbon capture technique, due to its stability.

Applicability

The technology is suitable to operate almost anywhere, with biomass availability, particulars or associations

of producers. The purpose is to avoid the transportation of bulky biomass for long distances. Treat the

pruning residues near where they are and the application of the Bio-char to soil.



The improvement in yields of crops due to the application of Bio-char to the ground.

To position the final products ahead the competitors.


For further information, Institute of Natural Resources (IRENA) University of Leon, Spain.


4. Pyrolysis with microwaves.

Description

Microwave technology for heating has been shown to be more energy efficient than conventional methods in

many applications. Microwave irradiation is rapid and volumetric with the whole material heated

simultaneously. In contrast, conventional heating is slow and the heat is introduced into the sample from the

surface. This feature of microwaves is very important for processing poor thermal conducting materials such

as wood. Fig 4 shows the heating patterns.

Microwave heating can be controlled instantly and the power applied can be accurately regulated. This

allows safe and precise control, even when applying very rapid heating rates.

Microwaves also promote novel reaction pathways and can greatly accelerate reaction rates as a result of

specific interactions.

Fig 4. The micro waves heat the material up from the core to the outer.

Achieved environmental effects

Reduced CO2 burden and low product carbon footprints.

Production of alternative fuels to petrol with very low GHGs emissions.


Using microwaves in pyrolysis brings some benefits like:

- the use of mibile processor near to large biomass concentrations,

- flexibility in operation to favour the production of gas, liquids or solids,

- rapid, continuos processing and high efficient use of energy.


Example plants

Department of Chemistry, Green Chemistry Centre of Excelence, The University of York, U.K.


Microwave brochure, http://www.york.ac.uk/res/gcg/research/areas/microwave.html

http://www.york.ac.uk/res/gcg/research/areas/microwave.html


5. The Carbo-V® process

Description

It is a three phase process, a low temperature pyrolysis with a gasification agent, high temperature

gasification and endothermic entrained flow gasification. See Fig 5.

-First phase. Low temperature gasification (LTG)

The biomass is dried, to 15-20% moisture content, using process energy (waste heat). Then, it is carbonized

in the LTG through partial oxidation (low temperature pyrolysis) with air or oxygen as gasification agent, at

temperatures between 4000C and 500

0C. The carbonization gas is produced.

-Second phase. High temperature gasification (HTG)

The carbonization gas is post-oxidized in the high-temperature gasifier’s combustion chamber using air

and/or oxygen.

The heat released by oxidation heats the carbonization gas to temperatures higher than the fusion temperature

of the input fuels’ ash. In these conditions, the aromatics, tars and oxo compounds contained in the

carbonization gas are fully broken down. The main constituents of the gas are CO, H2, CO2 and H2O and, in

the case of gasification in air, N2.

-Third phase

The char is blown into the hot combustion gases in the lower section of the endothermic entrained flow

gasifier. The carbon reacts with the carbon dioxide and steam to form CO and H2. As a result of this

endothermic reaction, the gas temperature is instantly reduced to approx. 900 °C. This “chemical quenching”

allows highly efficient production of a tar-free raw gas.

Fig 5. Representation of the Carbo-V process.


Achieved environmental effects

The beta plant working with the Carbon-V process saves 40,000 tonnes of CO2 per year.[6]

Operational data

Thermal capacity: 160 MW, 45 MW

Operating pressure: 5 bar

Gasification agent: air or oxygen

Fuel throughput: 35 t/h, 9 t/h

Applicability

This process can be applied to produce chemical raw materials. It makes ecological sense to prioritize use of

biomass and residues for the production of chemical materials, and thus significantly reduce CO2 emissions.

Example plant

The CHORE´S Beta Plant. Freiberg, Saxony Germany.


http://www.choren.com/ http://www.choren.com/en/carbo-v/references/beta-plant/ (Jun 2011)

http://www.choren.com/

http://www.choren.com/en/carbo-v/references/beta-plant/

Best Available Techniques in Anaerobic Digestion 13

Techniques:

Batch-dry anaerobic digestion

Plug-flow digester

Continuously Stirred Tank (CSTR)

Upflow Anaerobic Sludge Blanket (UASB)


1. Batch-dry anaerobic digestion

Description

The batch-dry process involves a three-step treatment with aerobic-anaerobic-aerobic phases for the

production of biogas and quality compost, treating either farm or municipal wastes. The substrate is a solid-

like material which remains inside the reactor during the process without being stirred or mixed. Thus, the

anaerobic phase can be described as a batch-static-dry anaerobic digestion, while the aerobic phases are in-

vessel composting with forced aeration. The aim of the aerobic phases is the stabilization and sanitation of

the substrate in order to obtain a product valuable as compost (Blanco et al., 2007).

The plant has typically a reactor of 20 m3 and an adjacent control unit. The design allows the addition of one

or more reactors (modular system) associated to the same control unit. The process control is based on the

management of the liquid and gas phases instead of stirring or mixing. In the aerobic phases, a forced stream

of atmospheric air is channeled through the substrate (positive aeration) and exhausted air is evacuated to the

atmosphere through a biofilter. In the anaerobic phases, the gas in the reactor is recirculated and heated so

that mesophilic conditions (35ºC) can be kept (see the diagram of the gas/air circuit in Fig 7). Anaerobic

phases are started-up by spraying a liquid percolate into the top of the digester, and then allowing gravity

draining through the digesting material (Spencer, 2010). Over the digestion time, the percolate is repeatedly

drained and sprayed back into the digester. The diagram of the percolate circuit is shown in Fig 7 (Blanco et

al., 2007). Multiple cells can simultaneously operate in a plant in order to reduce the impact of reducing

methane production when a digester is refilled. Furthermore, the process can be controlled by exchanging

percolates from “fresh” and “mature” cells (Spencer, 2010).

Fig 6. Plant's overview


Fig 7. Diagrams of percolate and gas/air circuit in the prototype of batch dry aerobic-anaerobic treatment.


Batch-dry AD has several environmental advantages. Neither additional water nor wastewater takes part in

the process and, moreover, the digestate is a solid product more easily handled for fertilization and

reclamation purposes than liquid digestates. The digested material thus obtained can be composted in a

posterior stage without the need of removing any liquid from it. (Blanco 2009).

The 2nd

phase (anaerobic) – 3rd

phase (anaerobic) sequence fulfills the aim of the valorization and at the same

time doesn’t produce negative effects on the anaerobic conditions of the 3rd

phase (Blanco, 2009)..

An additional advantage is that dry systems have proven to be less sensitive to inhibition than wet ones. The

robustness of these systems is not completely explained, but it is speculated that the microorganisms in the

dry fermentation medium are protected against transitory high concentrations of inhibitors (Mata-Álvarez,

2002).

The above described characteristics of the process permit small size reactors, due to the high concentration of

biodegradable matter regarding the conventional wet anaerobic treatments. The process design also avoids

the wastewater production: the water contained in the raw material remains in the system (in a percolation

tank) and incorporates to the following treatment cycles, so the main water output takes the form of

condensations and is also expelled through the exhausted gasses stream.

Cross-media effects

The 1st phase takes an important role when saving energy to achieve the mesophilic conditions needed for the

following phase, even though it is not determined yet if, as a pretreatment, either improves or reduces the

subsequent biogas yield. Thus, the first step should last the minimal time to allow the treatment to achieve

the mesophilic conditions for the second step (Blanco, 2009).

Pilot plant trials have shown some lacks that should be equally amended aiming to a further commercial

development of this system. The main improvements should overcome the hermetic conditions particularly

considering the door locking as well as the control system in terms of alarms and some control circuits

(Blanco, 2009).

Substrate

Leachate

tank

Substrate

Heat exchanger

Biogas

extraction

Air inlet


Operational data

Anaerobic and aerobic phases duration

It has been established, with the help of the parametric fit of CH4 yield curve, an optimum anaerobic

digestion time of cattle manure in terms of reactor productivity of 37,5 days The mixed process, which has

to share its time between the anaerobic and the aerobic phases, is more effective in organic matter

degradation in the aerobic phases. In this regard, the optimization of the aerobic phase in terms of CH4 yield

is also justified for it allows to spend more time in composting, optimizing at the same time the organic

matter destruction. Oxygen concentration in the gas phase of the reactor should be kept above the values 6-

8% in order to avoid partial anaerobic conditions in some areas and diffusion limitations (Blanco, 2009).

Leachate recirculation regime

The study of leachate regime variation during the digestion shows that this regime has a small influence on

the process, so it is better to work rather with low recirculation frequencies (of 3 days) for operative matters

(Blanco, 2009).

Addition of co-substrates

The addition of structural ligneous materials to the substrates has a positive effect in the own substrate CH4

yield, but it is detrimental –above a certain level– to the use of the reactor space. Thus, a rate of structural

materials around 20-25% is recommended to be kept. Blending the manure with energy crop wastes such as

rapeseed or sunflower (rather than wood chips) is preferable since, apart from playing the role of structural

materials, they contribute to the CH4 production and to the improvement of the blend’s chemical properties

(Blanco, 2009)

Biological performance

Table 1shows the parameters of performance of batch-dry AD tests at semipilot scale (Blanco et al., 2011).

Table 1. Performance of batch-dry AD tests at semipilot scale

Substrates

Retention

time / HRT

(d)

OLR

(kgVS·m-3

·d-

1)

Specific CH4

yield

(l·kgVS-1

)

Volumetric

CH4 yield (l·

m-3

·d-1

)

CH4 content

in biogas

Sheep manure 94 0.81 184 150 55%

Sheep manure + potato

waste (3:1 VS) 94 0.93 323 297 58%


The competitiveness of the batch-dry processes, in terms of the use of substrates, has been demonstrated,

although the optimum use of the reactor space is still a challenge. The low OLRs and the long lag phases are

the main obstacles. Batch-dry AD kinetics may be negatively affected by NH4+-N and VFAs inhibition

which, in combination with high pH levels, may lead to an inhibited steady state (Blanco et al., 2011).

Applicability

Plant is available for farms whit more than 107 livestock units or 228 livestock units if compost can`t be

sold. For each reactor, it is necessary a concrete of 150m2 in the plant site for unloading and downloading

operations. It is necessary a water and electricity intake.

Economics

The economic assessment of a commercial plant based on the mix system shows the economic feasibility of

a relatively medium or short size livestock farm unit (up to 107 Livestock Units). Nonetheless, 250 LU is the

livestock farm size over which feasibility doesn’t depend on compost trade incoming. The optimal time for

the anaerobic conditions in economic terms (44-49 days) is longer than that one for the best reactor

performance (37.5 days) in the semi-pilot scale tests (Blanco, 2009).

Percolate, with lower solid content than heavy slurries from continuous dry and wet AD processes, is more

easily pumped and less power demanding than in wet systems (Spencer, 2010).


Batch-dry anaerobic systems can be implemented in small and medium farms or places where the

management of biowaste is expensive, like isolated or poorly communicated places. These systems are also a

source of renewable energy where there is lack of supply. Feasibility of this systems will be clearer where

compost trade and demand is well developed. It is not required any other special local condition.

Example plants

Two treatment plants have been installed, one in the University of León (Spain) and the other one in Weibern

(Austria) as result of the European 3A-biogas® project with the participation of German, Austrian and

Spanish partners (www.3a-biogas.com).



www.3a-biogas.com

Blanco, D. (2009). Farm waste aerobic-anaerobic-aerobic biological treatment for obtaining biogas and

compost. PhD thesis, Institute of Natural Resources, University of León, León, Spain.

Blanco, D., Bergmair, J., Gil, M.V., Calvo, L.F., Morán, A. Batch dry anaerobic digestion of biowaste:

optimization of the process. Oral communication. Workshop on anaerobic digestion in mountain area

and isolated rural zones. Chambery, June 2007.

Blanco, D., Gómez, X., Fernández, C., Martínez, E.J. and Morán, A. Anaerobic co-digestion of potato waste

and sheep manure: semi-continuous-wet vs. batch-dry processes. Oral communication.. IWA Water &

Industry 2011. Valladolid, 2-4 May 2011.

Carballo, T., Gil, M.V., Blanco, D., Morán, A. (2006). Metodología APPCC (Análisis de Peligros y Puntos

de Control Críticos) aplicada a la producción de compost en el proceso 3A-Biogás. RESIDUOS, nº93,

págs. 38-45.

Gil M.V., Blanco, D., Calvo, L.F., Aller, A. and Morán, A. 3A-biogas: Three step fermentation of solid state

biowaste for biogas production and sanitation. Poster: International European Compost Network

(ECN) Workshop: “Effective Compost Marketing and Compost Application in Practice”.

Aschaffenburg, November 2004

Mata-Álvarez, J., editor (2002). Biomethanization of the Organic Fraction of Municipal Solid Wastes. IWA

Publishing.

Spencer, R. (2010). High solids anaerobic digestion of source separated organics. BioCycle, 51(8), 46-50.

http://www.3a-biogas.com/


2. Plug Flow digester

Description

The basic design of a plug flow digester is a long narrow tank (typically 5:1 ratio; 5 times as long as the

width) made of reinforced concrete, steel or fiberglass, a gas tight cover to capture the biogas, and a heating

system. It is usually built below ground level.

This type of digesters are unmixed systems that works on a semi-continuous mode by regularly receiving

untreated wastes in one side of the reactor, and ejecting digested waste out at the end of the digester.

Fig 8. Plug flow digester scheme (AGStar)

Plug flow reactors are used in the treatment of wastes with high solid content 10-12% TS. (De Mes et al.

2003), like manure and household solid wastes. Theoretically, manure in a plug-flow digester does not mix

longitudinally on its way through the digester. It advances towards the outlet as a plug whenever new manure

is added. When the waste reaches the outlet, it discharges over an outlet weir arranged to maintain a gas tight

atmosphere, but still allow the effluent to flow out. Actually the waste does not remain as a plug, and part of

the manure flows through the digester faster than other that stays longer.

Biogas produced is used to heat the digester and the excess biogas can used to run an engine generator.

Fig 9. Plug flow reactor for manure treatment. (AGStar)


The plug flow reactor has the lowest operation and maintenance costs due to its simplicity. As the basic

design of this type of digester does not have any stirring system, the only mechanical components of the

digester are the heating system.


Due to its simplicity, plug flow reactors are wide used in developing countries, particularly India, Nepal,

China and Vietnam, providing a cheap source of fuel, and reducing diseases caused by the use of untreated

manure as fertilizer.(AIDG 2011)

It is suitable for difficult feedstock that can’t be treated in any other digester such CSTR, UASB, anaerobic

filter, etc.

Secure sanitation. It is difficult that shortcuts between inflow and outflow appear. The risk of discharge un-

decomposed substrate is lower than in other digesters. (Beddoes et al 2007)

Cross-media effects

Performance of the digester is lower than in other low rate digester like CSTR. The plug flow digester is a

growth based system, as bacteria are washed out with the effluent; a portion of the waste must be converted

to new bacteria, decreasing the biogas production and the performance of the reactor. (Denis & Burke 2001).

Other disadvantage of this reactor is the lack of homogeneity on the transversal section of the digester,

especially when it is a horizontal plug flow. It can be solved installing a stirring system, usually recirculating

biogas. (Salgado 2008).

As the retention time of the substrate in the reactors are quite high, thus bigger reactors are needed

comparing with other digester technologies.

Incompatible with manure containing sand and other wood-based bedding materials (Beddoes et al 2007)

Solids must be removed from the plug flow reactor. Since there is no easy way of removing the solids, the

reactor must be shut down during the cleaning period. The cost of cleaning can be considerable.

Operational data

Many different operational and performance data have been published. Performance of the digester will

depend of the operating conditions and the characteristics of the substrate. The operational data described

here are the most common parameters.

The solid content of the feedstock digested on plug flow reactors have a solid content from 11-14 % TS and

a OLR 1 - 6 kg COD m-3

d-1

according to Beddoes et al 2007.

Plug-flow digesters on dairy operations with HRT-value ranging from 21 to 40 days removed a quarter of the

total solids and a third of the influent volatile solids (Wright et al. 2004). These authors report that daily,

farm-scale biogas production ranged between 0.367 to 0.786 m3 m

−3 d

−1. This biogas had an average CH4

content of 64%.

Martin et al 2003, reported performance of a full-scale plug flow digester on a 550 dairy cattle farm. Under

steady-state conditions, the average reductions in total solids, total volatile solids, and chemical oxygen

demand were 25.1, 29.7, and 41.9 percent, respectively, with no loss of nitrogen or phosphorus. Reductions

in fecal coliform and M. avium paratuberculosis densities were approximately 99.9 and 99 percent

respectively. Biogas composition averaged 59.1 percent methane at an average production rate of 1,214 m3

per day. This translates to 0.34 m3 per kg of chemical oxygen demand destroyed. The annual income derived

from the use of the biogas produced to generate electricity is estimated to be $39,474 per year. Based on this

income estimate, the simple payback period for the capital invested is approximately 5.6 years


Applicability

Plug flow reactors are used on the digestion of organic wastes with high solids content. It is wide used on the

treatment of livestock manure, mainly cow manure collected by scraping, and also it is suitable for the

organic fraction of municipal solid wastes.


The basic design of the plug flow reactor has been modified and implemented to increase the performance of

the reactor. These are some of the most used technologies based on the plug flow reactor.

Kompogas process

Belonging to a Swiss company, the patented Kompogas process is based on a plug flow reactor, with a

pretreatment of the waste before the digestion stage and a recirculation flow (Fig 10).

The pretreatment consists on a size reduction stage, and the removal of the materials that are not suitable to

produce energy, like ferrous materials, plastics, etc.

The organic waste is mixed with water and the digested liquid phase, to reach a feed with 25-30% of solid

content, which is pumped to the plug flow digester. It is a thermophilic process (55-60ºC) and the retention

time is usually 14-20 days. The reactor has axial mixing.

After the digestion stage, the digested waste is dewatered with centrifuges, and a fraction of the liquid phase

is used to inoculate the fresh substrate.

Fig 10. Scheme of a Kompogas process (Hahn & Hoffstede 2010)

This dry fermentation system is wide used on the treatment of the organic fraction of municipal solid waste.

According to the manufacturer, the biogas production of the Kompogas system is 600-1000 kWh per ton of

waste.

The largest plant using Kompogas technology in Europe is established in Montpellier, and went into

operation in 2008 with a capacity of 100.000 t/y of the organic fraction of municipal solid wastes

(Kompogas).


Laran plug flow reactor

Formerly known as LINDE process, it is a single stage dry fermentation process of the Austrian construction

company Strabag. Its design is based on a plug flow type reactor horizontally arranged in which solid

substrates (15 to 45 % of solids content) are digested.

The reactor has an intermediate storage tank to provide a continuous feeding. This tank is aerated and is used

for an anaerobe hydrolysis of the material that leads to a self-heating of the waste, so that a further heating

prior to charge the material in the reactor is not necessary.

Feed is mixed with water to adjust the solid content to about 30%. The digester has a mixing system with

several agitators of transverse in-line arrangement (Fig 11), which prevent the formation of floating scum

and settlement of material. At the bottom of the reactor is a conveyer fixed which transfer the sediments to

the digester discharge.

The digester sludge leaves the reactor through several suction tubes with vacuum. The tubes at the bottom of

the reactor are used to remove sediments which are transferred by the conveyor. The average HRT of the

substrate is around 20 days (STRABAG)

Fig 11. LARAN plug flow reactor scheme

Dranco Process (Dry anaerobic composting)

Developed in Belgium, this process takes place in a vertical plug flow reactor without mechanical mixing.

Feed is introduced on the top of the reactor and digested wasted is removed on the bottom of the reactor. It

can handle high concentrations of pollutants and non-degradable in the substrate.

Fresh substrate is added at the top of the digester, and it needs 2-4 days to reach the bottom of the reactor. To

stabilize the process 70% of the digestate is recycled with the new feed. Can operate with solid concentration

in the digester up to 50%. (HAHN & HOFFSTEDE 2010)

Digester loading: 10 to 20 kg COD/m³ reactor per day

Temperature range: thermophilic: 48 to 57°C (or mesophilic: 35 to 40°C)

Retention time in the digester: 15 to 30 days

Biogas production: 100 to 200 Nm³ of biogas per ton of waste

Electricity production: 220 to 440 kWh per ton of waste


Fig 12. Basic Dranco process scheme

Example Plants

Lille Waste Treatment Plant. (Lille, France)

The Organic Recovery Centre (ORC) of Lille is designed to treat 108.000 tons of organic waste per year.

The feedstock consists of organic fraction of household waste, green waste and municipal organic wastes.

The anaerobic digester is a LARAN plug flow type that works at 57ºC and with a retention time of 21 days,

and the estimated production is 4,5m m3 per year..

About 7% of the biogas production is used to for heating the digester. After digestion, the digester residue is

mixed with wood chips from green waste and sent for post composting.

Vitoria Waste Treatment Plant (Vitoria, Spain)

The waste treatment plant of Vitoria treats 120.000 tons of

municipal solid wastes per year. After several pretreatments, the

organic fraction is digested on a biogas unit, and the residue of the

digestion is composted afterwards.

The digester is a Dranco plug flow reactor with a capacity of 1.170

m³ and was started up on 2006.

Fig 13. Dranco plant in Vitoria (Spain).

(www.ows.be)

References

AGStar Program Web Site. Environmental Protection Agency

http://www.epa.gov/agstar/anaerobic/ad101/anaerobic-digesters.html, accessed June 2011

AIDG , Appropriate Infrastructure Development Group (AIDG) - Biodigesters. Available:

http://www.aidg.org/biodigesters.htm [2011, 7/21/2011].

http://www.aidg.org/biodigesters.htm


Beddoes, J.C., Bracmort, K.S., Burn, R.B., Lazarus, W.F., 2007. An analysis of energy production costs

from anaerobic digestion systems on US livestock production facilities. Technical Note No. 1. USDA,

Natural Resources Conservation Service.

Dennis, A. Burke, P.E; (2001): Dairy Waste Anaerobic Digestion Handbook. Options for Recovering

Beneficial Products from Dairy Manure. Olympia: Environmental Energy Company.

www.mrec.org/pubs/Dairy%20Waste%20Handbook.pdf Accessed June 2011

De Mes, T.Z.D., Stams, A.J.M., Reith, J.H. & Zeeman, G. 2003, "Methane production by anaerobic

digestion of wastewater and solid wastes" in Bio-methane & Bio-hydrogen. Status and perspectives of

biological methane and hydrogen production, eds. J.H. Reith, R.H. Wijffels & H. Barten, Dutch

Biological Hydrogen Foundation, , pp. 58.

Hahn, H., Hoffstede, U. Assessment report on operational experience. BIOGASMAX - Integrated Project No

019795. Fraunhofer IWES. http://www.biogasmax.eu, Accessed July 2011

Martin, J.H., Wright, P.E. Inglis, S.F., Roos, F.K. Evaluation of the performance of a 550 cow plug-flow

anaerobic digester under steady-state conditions, Animal, Agricultural and Food Processing Wastes IX:

Proceedings of the Ninth International Symposium, American Society of Agricultural Engineers, St

Joseph, MI (2003), pp. 350–359.

Salgado, J.M.F. 2008, Tecnología de las energías renovables, AMV. Madrid

STRABAG Environmental Technology- Technologies . Available: http://www.strabagumwelttechnik.com,

Accessed July 2011

Kompogas, Axpo Kompogas. Available: http://www.axpo-kompogas.ch, Accessed July 2011

Organic Waste Systems (OWS) . Available www.ows.be, Accessed July 2011

Penn State College of Agricultural Sciences http://www.biogas.psu.edu/. Accessed July 2011

Wright, P., Inglis, S., Ma, J., Gooch, C., Aldrich, B., Meister, A., Scott, N., 2004. Comparison of five

anaerobic digestion systems on dairy farms ASAE Annual International Meeting 2004, pp. 4693–4709.

http://www.sswm.info/sites/default/files/reference_attachments/BURKE%202001%20Dairy%20Waste%20Anaerobic.pdf

http://www.sswm.info/sites/default/files/reference_attachments/BURKE%202001%20Dairy%20Waste%20Anaerobic.pdf

http://www.mrec.org/pubs/Dairy%20Waste%20Handbook.pdf

http://www.biogasmax.eu/

http://www.strabagumwelttechnik.com/

http://www.axpo-kompogas.ch/

http://www.biogas.psu.edu/


3. Continuously Stirred Tank Reactor (CSTR)

Description

The CSTR reactors are the most common low rate digesters for large scale application. They are been wide

used on the digestion of wastes with a low solid content (between 5-10%), on what it’s known as “wet

digestion” (Hahn & Hoffstede 2010).

On this type of reactor, feed is introduced to a constantly stirred tank to ensure complete mixing of the

reactor content. At the same time an effluent flow is removed from the reactor. Retention time within the

reactor can be varied according to the nature of the feedstock and process temperature applied, which is

typically in the range of 15 - 25 days (Hahn & Hoffstede 2010, Cuesta et al 2009).

CSTR reactor is continuously stirred so there aren’t any concentration or temperature gradient. Stirring also

improves contact between anaerobic microorganisms and feedstock. There are basically two types of stirring

systems, through propellers, and recirculating the biogas generated on the reactor (Fig 14). Mixing creates a

homogeneous substrate, preventing stratification and formation of a surface crust, and it ensures that solids

remain in suspension.

Fig 14. Schematic CSTR reactor with two different stirring systems (De Mes et al. 2003)

The main characteristic of the CSTR reactor is that solids and liquid retention times on the reactor are equal.

When the load is increased, retention time decrease, and untreated solids and microorganism are washed out.

Reduction on initial hydrolysis reactions and washout of slow-growing bacteria, lead to an imbalance in

bacterial population that may cause the accumulation of VFA, and the failure of the system. (De Mes et al.

2003)

Digester volume ranges from around 100 m3 to several thousand cubic meters, often with retention times of

10-20 days, resulting in daily capacities of 6 m3 to 400 m

3 (De Mes et al. 2003). Usually, large reactors are

built in reinforced concrete that becomes gas tight due to the water saturation. They are also partially buried

to reduce the pressure of the digester content.


Better performance than other “low rate” digesters such plug flow reactors, and covered lagoons. Due to the

mixing system, a CSTR reactor can decrease the hydraulic retention time of the waste from months to

between 10 and 20 days, increasing the biogas production over 10-fold. (Lusk 1998).


Fig 15. Biogas performance from four different AD technologies. Error bars

shows the range of reported values. (Cantrell et al. 2008).

Due to its design, it is possible to treat a wide type of different wastes like slurries, swine, etc, making

possible to mix different wastes to improve the biogas production (co-digestion).

CSTR digesters can be installed on series, creating a multi-stage process where the two main stages of the

anaerobic digestion (hydrolysis and methanogenesis), take place in different reactors, allowing to keep the

best conditions to each type of microorganisms. Single stage process are more common than multi-stage due

to the lower costs (Hahn & Hoffstede 2010).

Cross-media effects

One of the drawbacks of this type of reactors is that longer retention times are required in

comparison with other anaerobic reactors (UASB, Anaerobic Filter, etc.). Thus bigger reactors are

needed for the same organic load.

Short-circuiting. Fractions of undigested feedstock can reach the outflow

With heterogeneous wastes, despite of the mixing system, different layers may appear on the reactor.

Heavies accumulate at the bottom of the reactor, and may damage the propeller or the pumps if they

are not removed before enter on the reactor. (Mata-Alvarez 2002)

Particularly sensitive to shook loads, as inhibitors spreads immediately in reactor

High consumption of water and energy for heating large reactors.

Complete mixing is difficult to attempt on large bioreactors. A floating layer can appear on the top

of the reactor, and heavier particles can accumulate on the bottom of the reactor, causing the failure

of pumps or the stirring system.

Operational data

Due to the versatility of this type of reactor, the operational parameters of the reactor can be very different

depends on the waste that is used.

Usually CSTR reactors treats manure with a 3% to 10% TS concentration (Beddoes et al, 2007), and the

biogas production is improved by the mixing system to between 1-1,45 m3m

-3d

-1 (Cantrell et at. 2008).


Co-digestion with lignocellulosic crops and food waste can increase the production even more. Lindorfer et

al.(2007) reported a biogas production of 2.91 m3m

-3d

-1 on a full scale CSTR reactor treating swine manure

with maize, rye, and wheat crop components. Another co-digestion example is a 675 cow farm were manure

and food wastes are digested together, reporting a biogas production of 3.25 m3m

-3d

-1 with 70% of methane

(Wright et al. 2004).

Applicability

Is the most used reactor in the treatment of the sludge generated on the wastewater treatment process. Also is

widely used in treatment of cattle slurry and organic fraction of municipal solid wastes.

CSTR systems are applied in practice for treating animal manure, sewage sludge, household waste,

agricultural wastes, faeces, urine and kitchen waste or mixtures of these substrates with a TS percentage of

approximately 2-15% (Cuesta et al. 2009). On the treatment of wastewater with a low solid content, there are

other technologies that allow working with higher loads and thus smaller reactor sizes.

Economics

An anaerobic digestion plant has high investment cost that have to be spread over several years. The size of

the plant has to be adapted to each situation and the input material must be produced nearby the facility. The

typically agricultural biogas plants sizes from 100 to 500 kWel. Larger plants are economic if the input

material is close to the plant, and size plays an important role for upgrading the gas to fit the natural gas

standards.(AEBIOM 2009)

The following table shows the main economic data of two small farm-scale digestion plants:

Table 2. Main data of two typical farm-scale co-digestion plant (Al Seadi 2004).

Year of construction 2000 2004

Digester volume (m3) 700 1.650

Digester system 2-step continuous stirred tank

reactor

Continuous Stirred tank

reactor

Mean Residence Time (d) 100 60 – 70

Digestion temperature 37-38 ºC 40º C

Main substrate

Corn silage, cow manure, fat

scraper contents and sun flower,

grass

Corn silage, manure, and

grain

Biogas use Electricity generation, 2 CHP,

150 kWel.

Electricity generation, 1

CHP, 500 kWel

(Double Fuel Injection

unit).

Use of digestate Fertilizer, own farm land Fertilizer, farm land

Investment costs (€) 250,000 280.000

Subsidies (€) 50,000 ----

Biogas Production (m3/yr.) 490,000

Energy prod. Electricity 757,000 kWh/yr. 840.000 kWh/yr.


Energy production heat 1,300,000 kWh/yr. ----

Electricity rates (€ cents / kWh) 10.23 11.50

Total Running Cost (€ / yr.) 58,200 -------

Labor 9,000 -------

Maintenance 750 -------

Various 48,450 -------

Income (Energy, € / yr.) 78,225 -------

Income (Co -substrates) 0 -------

Total Income (€ / yr.) 78,225 96.000

Net Margin (€ / yr.) 20,025 -------


There are many digestion processes that have been implemented from the CSTR technology, focused on

increase the performance of the digester and solve some of the drawbacks of this type of reactors. The

Anaerobic Contact reactor (AC) is one of these implemented techniques.

AC reactor design is based on a CSTR reactor with a clarifier tank where the suspended flocculent biomass

is settled and then recycled to the reactor (Fig 16). This sludge recycling increase the biomass population on

the reactor, and allow it to work with lower retention times than o a conventional CSTR system solving the

biomass wash-out problem.

Fig 16. Anaerobic contact reactor scheme (Veolia-Water)


Example Plants

Co-Digestion Plant in Osa de la Vega (Cuenca, Spain)

The plant, built in 2010by UTS Biogastechnik GmbH, belongs to Exporinsa, one of the biggest swine farms

in Spain. The aim of the plant is to manage the wastes generated at the farm and reduce its energy

consumption through the use of the biogas.

The plant has a combined heat and power unit (CHP) of 500 kWe. The heat is used to heat the farm and the

electricity is sold to the grid.

The plant has one reception tank (853 m3)

, one fermenter (2513

m³) and a secondary digester (4250 m³).

The electricity production is about 4.300 MWe·h/yr., and

approximately 9% is used for self-consumption. The heat

generation is 2.500 MWth/yr., and it is used to heat the farm

and the digesters.

The digesters work with a HRT of 45 days and the digestate

production is around 51.700 m3/yr. with 4-6% TS.

Biogas co-digestion plant in (Schweinfurt, Germany)

Built by the German engineering company Krieg & Fischer Ingenieure GmbH, this co-digestion plant was

built in a large farm to digest the manure generated by their animals, mixed with other organic residues

produced nearby (fat and distillery wastes).

The plan has a CSTR digester of 800 m3 and a heating unit that keeps the temperature at 35ºC. Around the

digester there is a concrete ring tank of 1000 m3 where the digested waste is stored. At the same time this

tank insulates the digester, decreasing the surface that must be insulated on the construction stage. Under the

roof there is a gas storage membrane.

The biogas is burned in two CHP units of 90 kW and 200 kW. The generated electricity is sold to the grid,

and the heat is used at the distillery and to heat the digesters.

References

AEBIOM 2009, "A biogas roadmap for Europe", [Online], Available from: http://www.aebiom.org/ .

Al Seadi, T. (Ed.), 2004. Biogas from AD: BIOEXCELL Training Manual. Project Deliverable of the

BIOEXELL Project. Biogas Centre of Excellence.

Beddoes, J.C., Bracmort, K.S., Burn, R.B., Lazarus, W.F., 2007. An analysis of energy production costs

from anaerobic digestion systems on US livestock production facilities. Technical Note No. 1. USDA,

Natural Resources Conservation Service.

http://maps.google.es/maps?source=s_q&hl=es&geocode=&q=Kraft+co-digestion+plant&aq=&vps=1&jsv=363c&sll=40.396764,-3.713379&sspn=8.91552,21.643066&vpsrc=0&ie=UTF8&hq=Kraft+co-digestion+plant+loc:&radius=15000.000000&split=1&oi=geosplit&sa=X&ei=8HBfTqSxMcrRswa_zI3ABg

http://www.aebiom.org/wp/wp-content/uploads/file/Publications/Brochure_BiogasRoadmap_WEB.pdf


Cantrell, K.B., Ducey, T., Ro, K.S. & Hunt, P.G. 2008, "Livestock waste-to-bioenergy generation

opportunities", Bioresource technology, vol. 99, no. 17, pp. 7941-7953.

Cuesta Santianes, M.J., Martín Sánchez, F., Vicente Crespo, G. & Villar Fernández, S. 2009, "Informe de

Vigilancia Tecnológica madri+d: “Situación actual de la producción de biogás y de su

aprovechamiento”, [Online], Available from: http://www.madrimasd.org/citme/Informes/default.aspx .

Accessed June 2010

De Mes, T.Z.D., Stams, A.J.M., Reith, J.H. & Zeeman, G. 2003, "Methane production by anaerobic

digestion of wastewater and solid wastes" in Bio-methane & Bio-hydrogen. Status and perspectives of

biological methane and hydrogen production, eds. J.H. Reith, R.H. Wijffels & H. Barten, Dutch

Biological Hydrogen Foundation, , pp. 58.

Hahn, H., Hoffstede, U. Assessment report on operational experience. BIOGASMAX - Integrated Project No

019795. Fraunhofer IWES. http://www.biogasmax.eu, Accessed June 2011

Krieg & Fischer Ingenieure GmbH. Available: http://www.kriegfischer.de/ [Accesed June 2011].

Lindorfer, H., Corcoba, A., Vasilieva, V., Braun, R. and Kirchmayr, R., Doubling the organic loading rate in

the co-digestion of energy crops and manure- A full case study, Bioresource Technol. 99 (2007), pp.

1148–1156.

Lusk, P., 1998. Methane recovery from animal manure: a current opportunities casebook, third ed.

NREL/SR-25145. National Renewable Energy Laboratory Golden, CO.

UTS Biogastechnik. Available: www.uts-biogas.com [Accesed June 2011].

Veolia Water Solutions & Technology. Available: http://www.veoliawaterst.com/biobulk/ [Accesed June

2011].

Wright, P., Inglis, S., Ma, J., Gooch, C., Aldrich, B., Meister, A., Scott, N., 2004. Comparison of five

anaerobic digestion systems on dairy farms ASAE Annual International Meeting 2004, pp. 4693–4709.

http://www.biogasmax.eu/media/d2_11_biogasmax_iwes_vfinal_nov2010__095398400_1109_10022011.pdf

http://www.kriegfischer.de/

http://www.uts-biogas.com/

http://www.veoliawaterst.com/biobulk/


4. Upflow Anaerobic Sludge Blanket (UASB)

Description

The interest on anaerobic systems as the main biological step (secondary treatment) in wastewater treatment

was scarce until the development of the upflow anaerobic sludge blanket (UASB) reactor in the early 70s

(Lettinga et al., 1980; Lettinga & Vinken, 1980). This technology refers to a special kind of reactor concept

for the "high rate" anaerobic treatment of wastewater. A scheme of a UASB is shown in Fig 17 below.

The UASB reactor has four major components: 1) sludge bed, 2) sludge blanket, 3) gas–solids separator

(GSS) and 4) settlement compartment (Metcalf and Eddy, 2003;).

The sludge bed is a layer of biomass settled at the bottom of the reactor. The sludge blanket is a suspension

of sludge particles mixed with gases produced in the process. Wastewater is distributed into the tank at

appropriately spaced inlets. The wastewater passes upwards through an anaerobic sludge bed where the

microorganisms in the sludge come into contact with wastewater-substrates. The sludge bed is composed of

microorganisms that naturally form granules (pellets) of 0.5 to 2 mm diameter that have a high sedimentation

velocity and thus resist wash-out from the system even at high hydraulic loads (Fig 17b).

Fig 17. UASB reactor and sludge granule. (Source: www.uasb.org

The resulting anaerobic degradation process typically is responsible for the production of gas (e.g. biogas

containing CH4 and CO2). The upward motion of released gas bubbles causes hydraulic turbulence that

provides reactor mixing without any mechanical parts. At the top of the reactor, the water phase is separated

from sludge solids and gas in a three-phase separator (also known the gas-liquid-solids separator). The three-

phase-separator is commonly a gas cap with a settler situated above it. Below the opening of the gas cap,

baffles are used to deflect gas to the gas-cap opening.


o UASB treatment process requires no external input of energy. Even the required mixing is achieved

by up flowing waste water and rising gas bubbles.

o Nutrient requirement is less than (about half that needed) for aerobic treatment.


o Residuals generated by UASB treatment are much less in amount and well digested, requiring

reduced sludge handling and causing much less odors problems.

o Biogas, rich in methane, is generated as a valuable by product. Methane produced is about 0.15 to

0.35 Nm3/kg COD destroyed.

o Once a UASB reactor has been put in operation, acclimatised bacteria can survive without food in

the reactor for long durations. This enables easy start-up of the UASBR after prolonged periods of

being out of operation.

o UASB reactors are a noiseless, closed and covered unit.

Cross-media effects

Some of disadvantages due to implementation of this technique can be:

o Requires skilled staff for construction, operation and maintenance (control of feeding pump and

influent organic load).

o Only suitable for treatment of wastewaters with low solid content.

o Long start-up phase

o Not resistant to shock loading. Treatment may be unstable with variable hydraulic and organic loads

o Insufficient pathogen removal without appropriate post-treatment

o Constant source of electricity and water flow is required

o Not adapted for cold regions

The findings in the work of Vinod et al., 2005 based on effluent data of eight UASB and a long term

monitoring of the performance at Kanpur (India), present some disadvantages of the UASB reactors.

o The effluent from UASB is highly anoxic and it exerts a high immediate oxygen demand (IOD) on

the receiving water body or land. If discharged in to a water body, it immediately sucks up the

dissolved oxygen and undermines survival of aquatic life.

o If the raw sewage carries sulphates, it gets reduced to sulphides in the UASB reactor and upon

release into an aquatic body it contributes in exerting immediate oxygen demand due to its

conversion back to sulphate.

o The quantity of biogas produced in a small to medium sized UASB is not adequate enough to

guarantee favorable economics of bio-energy generation.

Operational data

The construction, the start-up phase as well as the maintenance of UASB requires skilled staff. As domestic

or municipal wastewater already contains the composition of nutrients and micronutrients required for

bacterial activity and growth, they are generally less problematic than industrial wastewaters.

To keep the blanket in proper position, the hydraulic load must correspond to the upstream velocity and must

correspond to the organic load. The latter is responsible for development of new sludge, this means that the

flow rate must be controlled and properly geared in accordance with fluctuation of the organic load. A

permanent operator is also required to control, monitor and repair the reactor and the dosing pump.

Desludging is infrequent and excess sludge needs to be removed only every few years (2 to 3 years) (Tilley

et al. 2008).


UASB reactors require several months to start up. Granular sludge forms chains and coagulate into flocs.

Many different factors have been found to affect this process and the overall efficiency of UASB reactors

such as: temperature, wastewater composition, mixing, pH, organic loading rate and toxicity (Lettinga and

Hulshoff,1997).

Operational conditions of UASB reactors will be very different as there are many factors affecting the

process. As an overall value, a typical UASB reactor can remove between 60-80% of de COD and TSS of

the wastewater. The HRT of the reactor usually are between 4 to 20 hours and the biogas yield fluctuate on

0.02-0.3 Nm3/m

3reactor·day. (Naturgerechte 2001)

Applicability

The main application of UASB reactors are the treatment of industrial wastewater brewery, distillery, food

processing and pulp and paper waste and blackwater, even though its application to domestic sewage is still

relatively new and they are not resistant to shock loading and are not adapted for low strength influent.

Fig 18. Types of AD systems used for industrial wastewater

pre-treatment plants. (De Mes et al 2003)

Economics

The significantly lower level of technology required by the UASB process in comparison with conventional

advanced aerobic processes means that they are also cheaper in construction and maintenance than other

technologies. Capital costs for construction can be estimated as low to medium (and comparable to baffled

reactors (Sanimas 2005).

Construction costs for a municipal wastewater treatment plant based on UASB reactors can vary

significantly depending on the overall plant concept. Calculated on the basis of 50.000 p.e. investment costs

for UASB-treatment without gas use are about 20 US$/inhabitant (Naturgerechte 2001)

Example plants

Even though the number of UASB reactors treating domestic wastewater is increasing, most of the plants are

treating wastewater from food industry, breweries, etc. Two plants are described next


Example 1: UASB reactor in a potato processing plant

An American snack food producer with production facilities all over the world opened a potato

processing site in Kashira (Russia) in 2002. The treatment plant, consisting of three stages:

pretreatment, anaerobic treatment and aerobic post treatment.

Pretreatment realizes efficient suspended solids and FOG (fat, oil & grease) removal.

After pretreatment the wastewater is collected in a buffer tank. In the next step, the conditioning

element of the treatment, the wastewater is made ready for the anaerobic process by nutrient

addition and pH and temperature regulation. The anaerobic process takes place in three Biothane®

UASB reactors, with a design COD removal efficiency of 80%. A package scrubber removes H2S

from the produced biogas. The biogas is thus re-used in a boiler or burned in a flare.

In the aerobic post treatment step consisting of a final clarifier and a DAF (Dissolved

Air Flotation) unit, effluent polishing and N/P removal takes place.

Main characteristics of the wastewater:

Flow: 1200 m3/d (peaks of 100 m3/h)

COD load: 19.000 kg/d

TSS: 17.000 kg/d

TKN: 250 ppm

FOG: <100 ppm

Plant Characteristics:

2 Buffer tanks of 130 m3 and 190 m

3

3 Biothane® UASB of 270 m

3 (two of them), and 440 m

3

Biogas scrubber and an aerobic post-treatment (clarifier and DAF unit)

Example 2: EGSB reactor in a Brewery

The biggest beer producer in Portugal located close to Oporto, operates a beer and softdrinks production

facility. Due to the limited space available, they decided to install a EGSB reactor.

An EGSB reactor (Expanded Granular Sludge Bed) is a modified UASB reactor. The distinguishing feature

is that a faster rate of upward-flow velocity is designed for the wastewater passing through the sludge bed

The EGSB design is appropriate for low strength soluble wastewaters (less than 1 to 2 g soluble COD/l) or

for wastewaters that contain inert or poorly biodegradable suspended particles which should not be allowed

to accumulate in the sludge bed.

The present anaerobic Biobed® EGSB reactor can treat up to 19.800 kg of COD and 12.600 kg of BOD per

day, in a reactor with a surface area of only 113m3.


Main characteristics of the wastewater:

Flow: 6.000 m3/d

COD load: 19.800 kg/d

TSS: 1.000 mg/l

TKN: 80 mg/l

FOG: <100 mg/l

Plant Characteristics:

2 Buffer tanks of 1230 m3

1 Biobed® EGSB of 1.320 m

3

Aerobic post-treatment

After a pretreatment, where grains and other solid particles are removed, wastewater is collected a

conditioning tank that fed the anaerobic EGSB reactor. The anaerobic process removes a large part of the

COD wich is converted to biogas. The effluent from the EGSB is treated on a aerobic sequencing batsc

reactor to remove the residual COD and nutrients. After the treatment process an average of 80% of the COD

is removed (COD < 150 ppm) and the wastewater is discharged to process Leça river.

Reference

Anaerobic Granular Sludge Bed Technology Pages. http://www.uasb.org/discover/discovery.htm [Accessed:

July 2011]

Franklin, R. J. 2001. Full scale experience with anaerobic treatment of industrial wastewater. Wat. Sci.

Technol. 44(8):1-6.

Naturgerechte Technologien, 2001. Anaerobic methods of municipal wastewater treatment. Bau- und

Wirtschaftsberatung (TBW) GmbH. GTZ-GATE. Frankfurt. http://www.gate-

international.org/documents/techbriefs/webdocs/pdfs/w6e_2001.pdf Accessed July 2011

Lettinga, G., A. F. M. van Velsen, S. W. Hobma, W. De Zeeuw, A. Klapwijk 1980. Use of upflow sludge

blanket reactor concept for biological waste water treatment, especially for anaerobic treatment.

Biotechnol. Bioengineer. 22: 699-734.

Lettinga G. and Vinken J. N. (1980) Feasibility of the upflow anaerobic sludge blanket (UASB) process for

the treatment of low-strength wastes. Proc. 35th Annual Ind.Waste Conf. Purdue Univ. 625-634

Metcalf and Eddy, Wastewater Engineering: Treatment, Disposal, Reuse (fourth ed.), Metcalf & Eddy, Inc.,

McGraw-Hill, New York (2003).

Seghezzo, L., Zeeman, G., van Lier, J.B., Hamelers, H.V.M., and Lettinga, L. (1998), A review: The

anaerobic treatment of sewage in UASB and EGSB reactors, Bioresource Technology 65, 190-215.

Tilley, E.; Luethi, C.; Morel, A.; Zurbruegg, C.; Schertenleib, R. (2008): Compendium of Sanitation Systems

and Technologies. Duebendorf and Geneva: Swiss Federal Institute of Aquatic Science and Technology

(EAWAG). URL [Accessed: 15.02.2010]

http://www.gate-international.org/documents/techbriefs/webdocs/pdfs/w6e_2001.pdf

http://www.gate-international.org/documents/techbriefs/webdocs/pdfs/w6e_2001.pdf


Veolia Water Solutions & Technology. Available: http://www.veoliawaterst.com/biothane-uasb [Accesed

June 2011].

Vinod, T., Nema, A., UASB Technology – Expectations and reality. Available online:

http://unapcaem.org/Activities%20Files/A01/UASB%20Technology%20%E2%80%93%20Expectation

s%20and%20Reality.pdf, [Accessed July 2011]

http://unapcaem.org/Activities%20Files/A01/UASB%20Technology%20%E2%80%93%20Expectations%20and%20Reality.pdf

http://unapcaem.org/Activities%20Files/A01/UASB%20Technology%20%E2%80%93%20Expectations%20and%20Reality.pdf

Best Available Techniques in Combustion and Gasification 1

Best Available Techniques in Combustion and

Gasification

iq

Texto escrito a máquina

iq


Part II

iq



1. THERMAL BIOMASS

GENERAL OVERVIEW OF THE ENERGY RESOURCE

The use of biomass for thermal energy use is more widespread. The solid fuel direct combustion is the

process of thermal generation from biomass more commonly, other methods of using existing minority

consisting of liquid and gaseous biofuels by Thermochemical processes, biological or chemical, which was

later recovered thermal energy through combustion.

Thermal applications can take place in the industrial sector to generate process heat as steam, thermal oil, hot

water, use dryers or ovens, etc.., or buildings to service hot water, heating or cooling.

To provide services to buildings have been developing two systems: While the installation of thermal power

generation and supply neighbourhoods or districts through networks to users, without limitation extension

techniques (also known as District Heating), or solutions by Boiler of each building or adjacent users.

In the residential sector this can be accomplished by thermal production:

• Stoves, pellet or wood usually that heat a single room and generally acting as both decorative elements.

• Low power boilers for private homes or small buildings.

• Boilers designed for a block or apartment building, which act as central heating.

• Power plants that heat multiple buildings or facilities (district heating) or group housing.

For the use of biomass as solid fuel for thermal generation is carried out a series of pre-treatment steps,

which may not be necessary depending on the scope of action:

� Drying, the moisture removal of biomass increases the energy density of the biomass.

� Adequacy granulometric particle size reduction of biomass also improves the energy density and easy to transport, as is the case of chipping.

� Densification, in some cases interested in making a densification of the biomass to increase its energy density, lower transport costs and facilitate dosing in thermal generation systems.

� Storage and preservation. Finally in case of large concentrations of biomass, could be interesting to perform thermal processes such as roasting, to keep the properties of the biomass at time.

Commercial biomass types commonly used for heating are:

• Pellets and briquettes industrially produced.

• Chips, from the industries of the primary and secondary processing of timber or forest silvicultural treatments (pruning, thinning, woody energy crops, etc.).

• Agro-industrial wastes such as olive pits, nut shells, almond, pineapple, etc.

• Wood, which can produce the user or available in the market.

Below is a table summarizing the advantages and disadvantages of this biomass:

Wood Pellets


Advantages:

- High calorific value.

- Very low ash content, reducing

operation and maintenance

needs.

- Pellet boilers are high

efficiency condensing boilers are

even pellet.

- are traded internationally, with

a constant composition.

- Standard designs are used in

Europe.

Disadvantages:

- High price compared to other

biomass.

Considerations:

- Requires a secluded storage

and dry.

- No need any drying or

treatment once produced.

- They are standardized, so they

have high operational reliability

and less stress for the operation

and maintenance of the boiler.

Wood Chips Advantages:

- The cost of production is lower

than the pellet due to lower

process required.

- clean of bark chips and dry

(Class 1) are usually of high

quality.

- has an average degree of

standardization at European

level.

Disadvantages

- They are less dense than the

pellets and olive pits, and so

require a larger space for

storage.

- Being less dense, transport is

justified only to a short distance

(<50 km).

Considerations:

- Its composition is variable.

- It is necessary to dry the raw

material of natural or artificial to

humidity below 45% or less than

30% in the case of the best Class

1 chips.

- They have an ash content less

than 1% (class 1) or 5% (class

2).

Agro-Industrial residues Advantages:

- Availability and types

(abundance of products and

quantities).

- Major productions in Spain.

- The cost of production is lower

due to the by-products of a

process.

- Usually have a high calorific

value, but caution should be

exercised with the quality of

biomass to be purchased,

avoiding unwanted waste

biomass with.

Disadvantages:

- The ash content, although

acceptable, is higher than the

pellet, so that maintenance work

will tend to be higher.

Considerations:

- biomass can be seasonal, so

that their supply, if the producer

directory, you should remember

during the season.

- Composition variable.

Wood and briquettes Its use is rare and almost exclusively for small boilers and a degree of automation means, there are wood

or briquettes to enter several times a day (the days of higher consumption). The cost of production of

briquettes is much higher than wood, although the calorific value of the first is clearly on top.

Furthermore, the briquettes produce less ash, making cleaning and maintenance of the boiler.

There are several reasons that justify the use of modern systems of heating and hot water from biomass.

Among these can include:

1. The facilities supplied with biomass in its various forms (pellets, chips, crushed olive stones, etc.)

are friendly to the environment by presenting a reduced emission of pollutants into the atmosphere

and contribute to the greenhouse effect by having a neutral balance CO2. The latter feature helps to

meet the climate change agreements.


2. At present, another reason is the lower price compared to other fuels and greater stability does not

depend on external fluctuations, although the initial investment cost of equipment is usually higher

than the computers that use conventional fuels.

3. The operation and maintenance of these systems is easy to be automatic with the incorporation of

electronic control. As an example may be noted that some boilers incorporate even on remotely via a

mobile phone message.

4. The cleaning of the equipment in the boilers with advanced technologies is fully automatic and ash

removal task rare.

5. The biomass boilers have a high wear resistance, long life and, most importantly, have energy

efficient, exceeding values between 75 and 90% efficiency according to the team.

6. From a policy perspective, biofuels have cited solid in the RITE own treatment, which significantly

promote the growth of the market for biomass facilities.

Technologies Classification

Processing technologies of biomass into heat energy can be grouped into two broad groups: direct

combustion in the form of solid fuel or by combustion of liquid or gaseous fuels obtained by other processes

from biomass.

Direct combustion of a solid fuel.

This heat generation from biomass stoves can be performed, where the heat transfer fluid is air or boilers

where water or steam used as heat transfer fluid.

The biomass boilers can be classified according to type of fuel in three types:

1. Specific Pellet Boilers

Usually small (up to 40 kW) and highly efficient. Stresses its compactness due to the stability of the fuel

supplied. The reason for these boilers makes sense because of its low cost, small size and high efficiency. In

some cases other biofuels can be used with similar characteristics, provided that the manufacturer guarantees

it.

2. Biomass Boilers

Its output ranges from 25 kW to hundreds of kW. Do not support multiple fuels simultaneously, but you can

change the fuel if scheduled well in advance to empty the bin, the new refill and reprogramming of the

boiler. Require modifications to the feed screw and grill.

3. Boilers Mixed or multi

Support several different types of fuel, changing from one to another quickly and efficiently, such as pellets

and chips. Are usually made for moderate power (about 200 kW) or larger.

Regardless of its power, RITE explicitly excludes the need to spread the power for biomass boilers.

Another classification of boilers is based on its technology, in which case they are divided into four groups:

4. Conventional boilers suitable for biomass

They are usually old coal boilers can be adapted for use with biomass or oil boilers with a biomass burner.

Although they are cheaper, their efficiency is low, at around 75-85%. They are usually semi-automatic

because, not being designed specifically for biomass systems do not have specific maintenance and cleaning.

In Spain there are several manufacturers for this type of boilers.

5. Standard Biomass Boilers

Designed specifically for a particular biofuels (pellets, chips, firewood,...), achieve yields of up to 92%,

although it is usually possible to use an alternative fuel at the expense of lower efficiency. This is usually


automatic boilers as they have automatic fuel feeding, cleaning the heat exchanger and extraction from the

ashes.

6. Boilers mixed

Firing units allow the alternative use of two fuels, making it possible to change from one to another if

economic conditions or the provision of a fuel so indicate. They need storage and supply system for each fuel

boiler, so the investment cost is higher than for other technologies. The yield is high, close to 92%, and is

fully automatic boilers.

7. Pellets to condensing boilers

Small, automatic and for the exclusive use of pellets, these boilers recover the latent heat of condensation

contained in the fuel gradually lowering the temperature of the gas until it condenses the water vapour in the

heat exchanger. With this technology, saving pellets is 15% compared to a standard combustion, thereby

achieving greater efficiencies in the market, with a yield of up to 103% compared to the calorific value

(NCV).

ADAPTATION OF TECHNOLOGIES TO THE NEEDS OF USE

The technology of direct combustion boiler is most suitable for the thermal use of biomass.

Thermal Biomass for domestic use

Use of biomass boilers in residential use, producing hot water, heating or even cooling.

A biomass system consists of the following teams, which meets the standards established by the Regulation

of Thermal Installations in Buildings (RITE), IDEA Technical Guide for thermal biomass systems, as well as

any other national, regional or local application is:

1. Fuel storage: it can be done using containers, textiles flexible silos, underground tanks, silos of work,

etc.

2. Power System through endless screw, pneumatic or gravity.

3. Boiler, which consists of combustion chamber, swap, ash and flue gas

4. Fireplace, drive system and distribution, regulation and control and other equipment similar or

identical to those used in existing facilities or for other fuels.

5. Absorption machine, for applying cooling biomass.

Depending on the type of application and use of biomass available is recommended to use a type of boiler or

other.

Thermal biomass for industrial purpose

In industrial boilers used high-power chips.

Technology features


Characteristics related to benefits and performance.

RITE established that the efficiency of the biomass boiler should exceed 75%, so it can only be installed in

buildings boilers class 3 (according to UNE-EN designation-303-5). The following table shows the generic

performance data for different types of boilers:

The average efficiency biomass boilers in Austria (where 13% of energy demand is covered by this source)

increased from 50% in 1980 to 93% in 2004. Their yields are getting higher.

As an example of performance data for pellet boilers nominal and partial load, the following table presents

the data from KWB boilers.

As emission limits to be met by biomass boilers in terms of its power, the UNE-EN-303-5 provides the

following:


The power range of the boiler to be used depends on the application, in the case of residential use in the

following table shows an estimate of thermal power necessary for typical buildings about medium-high heat

insulation.

The biomass boilers are usually pellets for power below 40 kW, boilers using wood chips or mixed

(multifuel) to higher powers.

Characteristics related to the investment and operating economy

Overall thermal generation using biomass boiler requires a greater initial investment, higher cost of

equipment and fuel storage requirements, compared to oil boilers, but the low cost of fuel is in the medium

term investment pays off. Here are some approximate values of the cost of thermal kWh for different fuels:

Diesel Propane Natural Gas Wood Pellets

0,068 €/kWh 0,056 €/kWh 0,040 €/kWh 0,036 €/kWh

Benefits of using this technology

The use of biomass for heat generation in the home presents a range of interests including:

1. Development of local biomass resources, such as lignocellulosic biomass.

2. Replacement of fossil fuel use of biomass heating, thereby reducing CO2.

3. Biomass boiler technology matures technologically possible to include cooling by absorption

machines.

4. Hybridization biomass boiler systems use solar thermal and / or geothermal.


2. Technologies to produce electrical and thermal energy (cogeneration).

General overview of the energy resources

The growing demand for energy in today's society, the high dependence on fossil fuels and the growing

awareness of the limitation of CO2 emissions, has sparked interest in the development of new systems that

allow the use of energy sources indigenous and renewable sources like biomass.

The main sources of biomass available for energy use are: agricultural waste, forestry, livestock, industrial

and urban. Another source of biomass for energy use are energy crops, which should be highlighted to be

produced specifically for this particular purpose. The interest of using biomass created when it comes to

products that can be considered as waste by not being used for other purposes.

In the case of residual biomass from agro-resources, their use has an added value, since it promotes forest

clearing with a consequent reduction in the risk of fire, the landscape, reduces pollution and contributes to

job creation.

Classification of technologies

Gasification

In this section, the study focuses on the generation of electricity and thermal cogeneration. Considering the

solid biomass from agro-forestry resources in Castile and Leon as the source of biomass for the production of

both electrical and thermal energy, which it intends to make gasification processing technology more

appropriate in this case, which happens to be justified below.

The gasification process was developed following four threads thermochemical biomass which moves from

its solid to a gas state. These processes are:

• Drying: The process in which biomass releases most of the moisture content starting at temperatures

slightly below 100 º C. This process is endothermic and to achieve acceptable yields in the overall process,

the moisture of biomass use is generally limited to 20-30% wet basis.

• Pyrolysis: is an endothermic process, whereby at temperatures above 200 º C, the biomass starts to pyrolyze

produce charcoal, condensable gases and condensable gases, whose relative proportion depends mainly on

the heating rate, temperature and particle size biomass.

• Oxidation is an exothermic process that occurs whenever the type of gasifier is autothermal, between

pyrolysis products and / or solid biomass and oxygen introduced with the leavening agent in the gasifier.

This thread releases the energy required for reactions taking place drying, pyrolysis and reduction. The

amount of oxygen introduced into the type autothermal gasifiers is typically between 20 and 40% of

stoichiometric air required for complete combustion of biomass.

• Reduction: This thread covers the reactions of fuel gas formation itself, and developed globally

endothermic temperatures above 600 º C, taking the energy of the sensible heat of gas and charcoal

produced.

As a result of the overall process yields a fuel gas whose composition can be very different, depending on the

available biomass, the gasification agent, the type of gasifier used and operating conditions used in the same,

but a typical composition may be the Next: Nitrogen (50-54%), carbon monoxide (17-22%), carbon dioxide

(9-15%), Hydrogen (12-20%), methane (2-3%), with a calorific (NCV) between 4150 and 6000 kJ/Nm3.

Existing types of gasifiers can be classified by several parameters as follows in the following table:


Parameters Options Solid flow rate Fixed bed/Fluidised Bed

Leavening agent Air/Oxygen/vapour/Hydrogen

Operating pressure Atmospheric/high pressure

Heat supply Internal/External heating

Type of raw material

or feedstock Biomass/coal

Table. Parameters and gasification technologies.

Solid flow rate Leavening agent Operating pressure Heat supply Type of raw material

Setting a criterion for classifying the type of gasifying agent flow and the solid gasified inside the reactor,

gasifiers can be classified into fixed bed reactors and fluidized bed.

• Fixed bed: countercurrent (updraft) or uniflow (downdraft)

• Fluidized bed: either current or bubbly flow, which can be pressurized or not.

There are a number of fixed bed gasifiers uniflow in operation, with most of them simultaneous generation

of heat and electricity. Only India and China there are hundreds of gasifiers of this type on farms and small

industries for energy production locally. The following diagram shows the main systems necessary for the

energy use of such technologies.

Figure. General scheme gasification

ENERGIA ELECTRICA

Grupo Motogenerador

AIRE

ENERGÍA TÉRMICA

GAS ESCAPE

ALQUITRÁN PARTÍCULAS

Acondicionador Gas Pobre

CALOR GP limpio y

seco

AGUA

GP

BIOMASA LIGNOCELULÓSICA

Reactor Gasificación

CENIZAS

AIRE

In Europe, in countries like Germany, Finland and Spain, are known the existence of different operating

plants based on this technology, which range from electrical power up to 1MW 20kWe, while thermal power

ranging from 60kWt to 1.5MW, focusing on powers relating to the scope of the project.

In Castilla y León, CIDAUT FOUNDATION, develops gasification plants uniflow fixed bed for electric

power generation (100kW) and thermal (200kW) from lignocellulosic biomass from agricultural and forestry

residues. It should be noted this type of technology it is oriented to distribute and sustainable energy use

different types of agroforestry biomass.

The development of each part that integrates the whole has not only been carried out to ensure proper


functioning, but also the manufacturing costs have been considered the costs associated with operation and

maintenance times during continuous operation.

The plant consists of a number of subsystems: gasification system, gas conditioning and engine-generator,

which operates as a whole is governed by an automatic control system that allows continuous operation

reducing the presence of the operator at the time of operation, which directly affects the cost of the plant.

This type of plants allowed to be located next to the owner of the waste, making the link between energy

supplier and generator provide a business that is not subject to variations in the price of raw materials or

transportation costs.

This type of technology requires low investment costs and low operating costs, so are presented as the most

appropriate technology for exploiting the resources of the rural environment.

The plant installed in the town of Wet, as well as serving for the development and commissioning of various

systems of the plant, has allowed to evaluate the performance of the system with different types of biomass

such as poplar, oak chip, splinter pine, pine crushed pineapple hearts, pine, pine bark from sawmill residue,

etc., which can make the adjustments necessary technology associated with a particular biomass. Based on

these data, we can present the biomass gasification plant and CIDAUT foundation as an alternative for use in

the project area by the proximity of the technology, experience in the energy use of waste from a rural

environment and capacity of the plant to be placed with low investment and operating cost.

The fixed-bed technology can deliver upstream power levels much higher than the current parallel and no

contraindications for its application for the production of thermal energy in electric power boilers and

external combustion engines and steam turbines. The disadvantages of fixed bed technology in

countercurrent with respect to the parallel streams include:

1. Very important generation of tar which some authors may reach 100 g/Nm3. In the event that the gas

wants to be used in internal combustion engines are required tar removal systems for high

performance.

2. For certain biomass gas, methyl chloride can present to the output very significantly complicates the

use of the internal combustion engine.

Given the amount of biomass needed for a specific power to defining a location for a type of installation of

these features should be evaluated both the availability of biomass in concrete surroundings as transportation

costs for sourcing facilities that include these technologies, which will determine its use in a particular rural.

Technology of fixed bed gasification uniflow also called "downdraft”

Systems fixed bed gasification are those in which the particle bed biomass gasifier down slowly in the effect

of gravity, taking place on four threads in different physical zones of the gasifier. This type of gasification

has the advantage of not requiring a strict grain size reduction of biomass use, being valid for a wide range of

sizes of solids, such as shells, chips, etc, unlike the specific requirements of the bed gasification fluid, which

are developed thermochemical phenomena within a volume alkali at high temperature.

The uniflow or downdraft gasification, in which the gasification agent moves in the same way that advances

the thermochemical conversion process, is especially useful when searching for low power applications and

power generation application by use of internal combustion engines.

The technology for fixed bed gasification is valid for a wide range of sizes of solids, is modular and

relatively small construction periods, allowing its implementation in multiple locations and in different sizes

appropriate to a model of energy supply based on demand. Its simple construction and operation that will


result in a lower initial investment, both manpower for its operation, its high percentage of conversion to gas

and especially the low presence of tar in the gas within the product will stand out as optimal for applications

in internal combustion engine

Combustible

bruto

Zona de

secado

Zona de

pirólisis

Zona de

reducción

Zona de

oxidación

Zona de

cenizas

COMBUSTIBLE

AIRE

GAS

AIRE

Figure: Operating diagram uniflow gasification technology

Given its main features, this technology is mostly applied for the production of electrical energy by internal

combustion engines, and given its limited scale, is used in small-scale units for biomass processing

capabilities between 3 and 1,000 kg / h or (assuming a calorific value of biomass of 14 MJ / kg, with

approximately 22% moisture), and thermal power (referring to the incoming biomass gasifier) between 11.5

and 3,900 kWt.

Advantages

• Its main advantage is the low tar content of product gas, which makes it suitable for applications in electric

power generation using internal combustion engines without the need for expensive cleaning systems.

• Technology adaptable to a wide range of sizes of solids (skins, chips, etc..)

• Allows implementation in different sizes appropriate to a model of energy supply based on demand.

• Low investment and operation cost.

Disadvantages

• Biomass to use needs to meet very stringent specifications in terms of uniformity of size, low fines content,

low ash, high specific gravity and low moisture content, mainly.

• For a given design does not accept too many variations in the characteristics of biomass.

Internal Combustion engines

In the case of the use of reciprocating internal combustion engines, engine mechanical power is harnessed to

generate electricity through a generator. Simultaneously and through a series of heat exchangers, it is

possible to recover much of the residual heat contained in the water cooling engine shirts (50% of the total


and thermal level of 80-90 º C) in engine oil (10% of the total and thermal level of 90-120 º C) and exhaust

(40% of the total and thermal level of 400-600 º C).

The type of engine can be spark ignition (Otto cycle or MEP with compression ratios up to 12:1) and

compression ignition (MEC with compression ratios up to 22:1 or higher). Within these, there are the so-

called pilot injection where it is possible to use high octane gas fuels, although it is necessary to inject up to

10% diesel to cause the ignition of the mixture.

The main advantages of this technology are:

• Wide range of powers, including 5KWe (micro) to 5-10MW

• Heat Engines with higher conversion efficiency of thermal energy into mechanical

• Reliable technology, competitive and a very mature market

• Operating flexibility, allowing modular power almost instantaneously with acceptable variations in the

specific consumption

• modular and easily expandable Equipment

Among the major problems are:

• Lifetime limited to about 45000-60000 hours

• Heat recovery on three levels and low temperature

• Significant maintenance needs for the high wear of moving parts

Adaptation of technologies to user needs

For choosing the most appropriate technology takes into account three main aspects as:

• The type and amount of biomass available.

• The final application is intended, ie what is intended to meet energy needs.

• Other factors such as environmental and economic.

Given the central aim of the project is harnessing the resources of the rural environment, as well as its

advantages, will be selected for use technologies that meet the low power:

• Greater simplicity of logistics supply of the plant. This means that projects can be promoted by the owners

of biomass, ensuring the supply, is also interesting to promote these projects through the forest or

agricultural cooperatives.

• Greater use of thermal energy, as it is more feasible to have thermal demands near the plant the lower the

power output.

• Easier on the location of points of connection to the grid, and may even be able to dump BT.

• Less visual and environmental impact.

• Distributed generation: its low power and its location near the intake point.

activity 3 final report

Documents