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Description of the biogas production in an agricultural biogas plant

BIOS BIOENERGIESYSTEME GmbH, Graz

Biogas is a renewable resource consisting mainly of methane and carbon dioxide. It is produced during anaerobic micro bacterial degradation processes of organic material (e.g. manure from livestock or poultry farming, crop components or residues and waste material). For the production of biogas all microbiological degradable substrates can be used. Beside the agricultural sector the biogas production has been established for the stabilisation of sludge from waste water cleaning and for the preparation of high charged waste water from the food industry. Agricultural biogas plants mainly use manure for the biogas production, because it is more economic to mix this substrate with organic residues from agriculture and food industry or energy crops, because these cosubstrates offer a much higher biogas yield than agricultural fertilizer like manure.

Working principle of biogas plants

The multi-stage process of producing biogas needs a large amount of micro organisms, which are able to use the stored energy in carbon hydrates, fats and proteins under anaerobic conditions for their metabolism. Nearly 70% of the contained methane bacteria use acetic acid for their metabolism, while 30% of the known species use hydrogen and carbon dioxide. For an optimised setting a temperature of 30-40°C for mesophilic bacteria and 55 to 60°C for thermophilic bacteria should be adjusted. The pH-value should be neutral up to low alkaline.

The bacteria of every degradation stage are addicted to each other, because the metabolism products of one group form the agar for the next group. It is also possible that different groups can inhibit other groups of bacteria, depending on the amount of contained bacteria at a time. Due to the occurring order of the processes the production of biogas starts after 4 to 6 weeks after starting the fermentation process.

According to the requirements of the different substrates for the production of biogas different technologies are used. These technologies can be classified by the type of reactor into wet fermentation reactor (up to ~15% dry matter content) and dry fermentation reactor (~25-50% dry matter content), by the type of feeding (continuous or batch reactor), by the process temperature (psychrophilic operation up to 20°C; mesophilic operation 30-40°C; thermophilic operation 55-60°C) and by the number of stages (single-stage, two-stage, multi-stage operation).

In agriculture the wet fermentation is used generally, where over the intervening years several plant types have been developed. The wet fermentation technology became accepted due to the possibility of using liquid substrates like manure, which is available in most of the farms, which is ideal for fermentation. Also the technologies for spreading the liquid fertilizer (digestate) on the fields already exist. For the production of biogas from stackable biomass the dry fermentation was developed. This technology can be found less often than the wet fermentation.

Figure 1 shows the scheme of an agricultural biogas plant.

Fig. 1

The produced biogas is a mixture of about 50-70% methane and 30-40% carbon dioxide. It also contains several trace gases like hydrogen sulphide (H2S), nitrogen (N2), hydrogen (H2), ammonia (NH3) and carbon monoxide. This gas can be used in many ways (combustion for the production of electricity and heat, feed in into a gas grid, in fuel cells or even as fuel). The most common alternative is the utilisation in a gas engine for the production of electricity and heat. The produced electricity can be fed in to the public electricity grid, receiving funded tariffs for it. To optimize the utilisation of the produced biogas the utilisation of the excess heat from the combustion of the gas should be considered. This heat can be used as process heat for the fermentation process or for heating parts of the plant or adjacent agricultural buildings (e.g. stables). The best option for using the excess heat is to sell it to an external heat consumer.

In addition to biogas a fermentation product is formed, which can be used as a liquid fertilizer in agriculture or as compost after dehydration. A further advantage of this process is the decomposition of biological instable compounds up to 30 - 40 percent, whereby the annoying odour of the sludge can be decreased to a very low level. Therefore the unpleasant odour from fermentation compared to composting can be minimized, because the fermentation is operated in closed tanks. The produced biogas can be used as a renewable, flexible und comfortable energy resource for the decentralized power supply.

For the utilisation of the biogas for instance in a gas engine, the impurities contained have to be removed. The main problem is formed by the hydrogen sulphide contained in the biogas, which can be found in gas from agricultural plants using manure and agricultural residues in concentrations between 200 and 5,000 ppm. This compound has a high toxicity and causes corrosion when it reacts with steel. Due to this reasons an effective desulphurisation is very important, because even low excessive H2S amounts in the gas can cause damages at the succeeding conversion technology. To prevent damages at the combined heat and power plant and other equipment (heat exchanger, catalyst, gas engine, fuel cell), the hydrogen sulphide has to be removed from the gas, to reach the limits given by the manufacturer. The limits for H2S for biogas used in gas engines lie between 100 and 500 ppm, depending on the manufacturer. The lower the amount of H2S, the higher is the lifespan of the gas engine. Figure 2 shows a gas engine CHP plant with a nominal electric capacity of 500 kW.

Fig. 2

The technologies for cleaning the biogas can be classified into technologies using iron oxides (filters containing iron oxides or iron hydroxide, technologies working with feeding of iron sludge or iron salts into the substrates, high pressure water scrubbing technologies (absorption), pressure swing adsorption (adsorption) or biological desulphurisation units converting H2S to elemental sulphur using oxygen. Also molecular sieves are used for gas cleaning and to enrich the methane content in the gas.

In addition to the desulphurisation a drying of the biogas is necessary. This can be done by several ways. Often the most economic way, a long tube in the soil, is realised, where the gas is cooled down and the contained water condenses. If very low water contents are necessary a water separator in the gasholder or a condensation drying unit is used.

Relevant technical data and efficiencies of agricultural biogas plants

  • Biogas composition
    CH4: 50-70%
    CO2: 30-40%
    H2O: 2-7%
    N2: < 2%
    H2: < 1%
    H2S: 200-5,000 ppm
    NH3: < 500 ppm
  • Digester operating temperature and pH-value:
    mesophilic operation: 30-40°C
    thermophilic operation: 55-60°C
    pH-value: neutral to weak alkaline
  • Electric capacity: 100-1,500 kWel
  • Electric annual use efficiency: 20-30 %

Realised projects and proposals under design of biogas plants and biogas monitoring projects

  • Biogas CHP plant based on agricultural waste with gas engine / Zwettl (Lower Austria, Austria)
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  • Biogas CHP plant based on agricultural waste with fuel cell and integrated gas treatment
  • Biogas plant with integrated gas treatment for biogas injection into an existing natural gas grid
  • Agricultural biogas CHP plant based on a gas engine / Saaz (Styria, Austria)
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  • Biogas CHP plant based on agricultural waste with integrated gas treatment for biogas injection into an existing natural gas grid and utilisation in gas engines at the customer sites/ Bad Tatzmannsdorf (Burgenland, Austria)
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  • Combination of an anaerobic waste water treatment plant and a biogas CHP plant for the energetic utilisation of organic residues with biogas utilisation in a gas engine and feed-in of biogas into the company-internal natural gas grid / Hermann Pfanner Getränke Ges.m.b.H., Enns (Upper Austria, Austria)
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