Biogas production

The microbial processes which naturally take place during metabolizing of manure components can be used in a controlled conversion in a technical process to produce biogas. A combination of positive effects results: Climate related emissions of CH4 are partly or fully prevented. The quality of the manure and its applicability as a fertilizer are improved. Easily degradable odor producing substances and aggressive organics are metabolized and therefore no longer emitted in large amounts. Thus air pollution is reduced and the fertilizing effect is improved.

It is an even more important benefit that biogas is a renewable energy source which can replace fossil fuels for the generation of electrical power and heat since it causes extra climate benefits by avoidance of fossil fuel burning GHG emissions. Effects are summarized and quantified in table 11.4.

Table 11.4 Climate related effects of biogas production (FVB, 2006)

Consequences of biogas production in agriculture

Quantitative or qualitative effects

De-centralized renewable energy supply

Calorific value (biogas with 60 percent methane): 6 kWh/m3

Power potential: 1.8-2 kWhel/m3 biogas

Mineral oil equivalent: 0.62 l/m3 biogas

Performance: 1-1.5 m3/animal and day

Biogas from manure of four cattle units equals to the energy need of one household

Reduced climate burdens by climate-neutral energy supply

Installed capacity of 1 kWel avoids 7,000 kg CO2-eq./a

Methane emissions by 1,500 kg CO2-eq. per cattle unit and year are prevented

Use of treated manure as fertilizer

Amount of nitrogen is equivalent to 20 kg N per cattle unit per year

Odor emissions during application of fertilizer are reduced

Improved sanitary and environmental situation at higher supply security

Regional economy improved by use of internal resources, empowerment of less developed regions

The principle technology of biogas production is already in widespread use in developed and developing regions of the world. In rural regions biogas is a traditional de-central energy source for cooking or heating which is often applied in families or households only on the basis of the manure of a low number of cattle applying a relative simple technology. In opposition, the modern biogas strategy aims at producing electricity on the scale of several hundreds kW up to the MW capacity applying a combined heat and power process. A diversity of substrates is used, including manure, organic waste of agricultural or industrial origin, and renewable crops. This type has been established in recent years, e.g. in Germany, where more than 3,700 biogas plants are in operation. The installed electrical capacity is 1,300 MW.

The efficiency of an agricultural biogas process depends on several factors amongst which the type of the raw material and the technological layout are most important. With reference to the raw material the organic content per mass unit is decisive for the biogas yield. Of manure, organic waste and renewable energy crops examples are given in figure 11.2.

Fat seperating residues Waste edible oil Grass Pig manure Chicken faeces Vegetable waste Cattle manure

Figure 11.2 Biogas yield of manure and renewable crops

Obviously best results will be achieved if a high proportion of fat residues is fermented. Only cattle manure results in a poor gas yield which is due to the utilization of energy rich substances during rumen digestion already.

A variety of technologies is applied dependent of the type of the substrate and the end usages of generated biogas and processed residues as well as cost and maintenance factors. It necessarily includes equipment for the preparation of the substrate, the conversion in a closed reactor under a mostly oxygen-free atmosphere, as well as the collection and cleaning of the biogas and the conditioning of the residues. The resulting biogas has to be transformed into power in an electrical generator. Only the electricity yield is 30 percent. The larger part will be lost as heat if not a combined heat and power (CHP) process takes place which is typical for modern sustainable solutions.

An example of a layout of such a biogas plant is displayed in figure 11.3. The plant is supplied by manure from 1,500 dairy cattles as well as an annual amount of 1,000 t of renewable crops such as maize. The electrical efficiency is 250 kW in a combined heat and power process. Heat energy is partly utilized for drying processes in a nearby factory. A special reactor design type is used in this facility where the gas is collected and stored under a plastic cover which is blown up by the gas pressure and afterwards transported to the power generator.

Figure 11.3 Biogas reactor with plastic cover for gas storage (Markert, 2008)
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