Anaerobic digestion from waste

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Anaerobic digestion (AD) is becoming increasingly important as a means of disposing of waste. Germany, with 1500 small farm-based digestion plants, and Denmark, with its centralized plants producing combined heat and power, are among the leaders.

In this process, wet waste comprising dung or sewage is transformed into a slurry with about 95% water content. This mixture is fed to a sealed digester where the temperature can be controlled. Digesters range in size from a domestic scale holding ~200 gallons to up to 1000 m3 for industrial-size installations. The digestion process involves the breaking down by bacteria of organic material into sugars and then into various acids. These

Figure 8.2 Diagram of a farm scale anaerobic digestion plant (courtesy of Renewable Energy World March/April 2004)

decompose to produce the final gas. The action of the bacteria generates heat and, in temperate climes, this usually has to be augmented to maintain a temperature of between 35 and 60°C. The heat can be provided by utilizing some of the biogas produced by the process.

The biogas consists of about 65% methane (CH4) and 35% CO2. Biogas can be used to provide heat and electricity. It can power conventional combustion engines to provide electricity and contribute to the heat required for digestion. If it is treated to remove hydrogen sulphide and CO2 it is virtually the same as natural gas and can be used to power vehicles. Most spark ignition engines can be modified to run on this fuel.

In total the energy content of waste from farm animals in the UK is around 100 PJ. However, the accessible energy is in the region of 10 PJ which relates to an installed capacity of -100MW (see Fig. 8.2).

Biogas can be processed to serve the piped network. To use natural gas pipelines it has to be processed as follows:

• removal of fine particles and other trace components

• desulphurization

The processing of municipal sewage and farm waste offers the greatest long-term potential for biogas. However, the main deterrent to widespread use is cost. In the current costings the environmental gains are not factored in. 'With legislation in place to protect the environment, biogas plants would be far more economic government support mechanisms are currently required to make biogas plants commercially attractive.'3

The technology has been proven for a considerable time but it suffers from the cost-effectiveness anomaly yardstick set by subsidised co-generation gas-fired plants.

Landfill methane

The focus in the UK for anaerobic digestion has been on landfill methane, which can be extracted at much less cost since the digestion process takes place naturally underground. However, on the European scale, this source is expected to diminish rapidly after 2006.4 The UK should not be much different.

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Figure 8.3 Biogas for Germany (courtesy REW)

Examples

At Laholm, Sweden, a community biogas plant was installed in 1992 to process 25,000 tonnes of liquid manure per year and 10,000 tonnes of other waste. In 2001 a processing plant was set up to raise biogas to natural gas quality. In 2003 the plant provided 30% of the gas distributed in Laholm. At Linkoping the municipal bus fleet of 63 vehicles and 132 other vehicles is powered by biogas. The biogas production plant operates simultaneously as a waste treatment facility producing vehicle fuel and a manufacturer of fertilizers.

At the present time Germany and Austria offer the best investment opportunities for anaerobic digestion, the reason being the relatively high feed-in tariffs: 15 Eurocents/ kWh guaranteed for 15 years for Germany and 14.5 Eurocents for 13 years for Austria. The effect can be judged by the development of the technology in Germany since 1999 (see Fig. 8.3).

The UK by comparison produces almost as much manure as Germany but only a small fraction is converted to energy. It is still a long way from realizing its potential which, according to the Royal Commission on Environmental Pollution, is 16 GW of energy5 (generating capacity is measured in watts).

This is particularly the case in terms of anaerobic digestion energy (Fig. 8.4).

Output over a given period or delivered energy is measured in joules (see p. 15).

Output is also measured in millions of tonnes of oil equivalent (mtoe) and kilowatt hours (kWh).

In the UK about 70% of all sewage is now treated by means of anaerobic digestion. By 1994 33 MW of electricity was produced under the Non-Fossil Fuel Obligation (NFFO). Following the abolition of the NFFO there have been very few new projects. However, many plants use the biogas to provide heat and electricity for site use.

One of the key problems for the UK is that there is no government biofuel strategy. 'There is considerable policy inertia because existing UK policy governs such large amounts of primary energy, some 225mtoe [2002], of which roughly 90% is fossil fuel based. Such a lop-sided fuel mix makes it difficult for new primary fuels to make inroads.'6

Note: in 2005 fossil fuels accounted for 72% of the energy mix.

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Market penetration (%) Q Manure production (million tonnes/year)

Figure 8.4 Market penetration of anaerobic digestion compared with manure production in the reviewed European countries (courtesy of REW)

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