Farm Digesters And The Important Role of Biogas Reduction Of Greenhouse Gas Emissions
Agriculture is the largest source of methane emissions in Britain, adding the equivalent of over 17m tonnes of carbon dioxide to the atmosphere every year.
A very large proportion of that methane comes from livestock, especially cattle.
Anaerobic Digesters cannot prevent all those methane emissions. Some of the methane comes directly from the digestive systems of the animals themselves, especially ruminants such as cattle, by enteric fermentation.
But, AD Plants can reduce those emissions in a number of different ways.
- They trap and use the large amounts of methane which would otherwise be produced naturally when raw slurry or farmyard manure is stored, or spread on the land. This prevents it from escaping into the atmosphere.
- They allow the nutrients in livestock and other organic wastes to be more effectively used and recycled. This reduces the amount of fossil energy that is consumed, and therefore the greenhouse gases are given off, in the manufacture and transport of artificial fertilizer. (These emissions are in addition to the figure given above, of emissions relating directly to agriculture.) Recycling nutrients also helps, to some extent, to reduce emissions of nitrous oxide.
- They produce a compost fibre which can help to reduce the use of peat in horticulture. This saves on the carbon dioxide emissions given off through peat extraction and use.
The difference between these overall savings in emissions, and those saved by generating electricity from gas produced by energy crops, instead of fossil fuels, is very significant.
This is a quotation from the book "Farm Digesters", by Jonathan Letcher.
Citations in support of these statements - again from the above book:
This is clearly illustrated by a bar chart produced by the German Energy Department.
The bar chartcompares the net emissions produced by generating lkWh of electricity using three different sources of energy. These are 1) an average of the fossil fuel types usually used for power generation in Germany (coal, natural gas, etc) 2) a digester running purely on specially grown energy crops, such as maize silage and 3) a digester running on a mixture of 40 per cent energy crops and 60 per cent livestock slurry.
The bar chart offsets actual emissions against reductions, if any, created by the process in question. A credit is given in the chart for reductions in the use of artificial fertilizer, for instance, or of methane and other emissions which would have been given off by slurry if it had not been processed in a digester. The results are very revealing.
In generating kWh, the averaged fossil fuels give off emissions equivalent to 0.7kg of carbon dioxide, while the energy crops produce net emissions of 0.45kg. The net emissions from the mixture of slurry and energy crops, by comparison, are less than 0.1kg. Unfortunately, the chart does not include a digester running on livestock waste alone.
But a new study by Bangor University6 estimates that generating 1 kWh of electricity from a digester processing cattle slurry alone reduces overall greenhouse gas emissions by the equivalent of 3.27 kg of Carbon Dioxide.
In other words, for each lkWh of output, the digester running purely on energy crops adds 0.45kg of carbon dioxide to the atmosphere, whereas a digester processing livestock waste alone reduces net greenhouse gas emissions by 3.27 kg C02e.
These figures show that processing livestock waste in a digester is a more effective way to of reducing greenhouse gas emissions than using energy crops alone.
That is the outcome even though energy crops produce more energy from a given size of digester.