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Anaerobic Digestion of farm and food processing residues GoodPractice Guidelines Page23 Alternatively, zoning can be used to exclude people from areas which may have health and safety risks. In general, it would be unwise to burn ‘as produced’ gas anywhere other than in an engine to produce electricity or in a boiler to produce heat. There are many different ways of cleaning the gas. The best advice is to choose the appropriate equipment, consult the manufacturer as to the best methods for specific plant requirements, and ensure that anyone using it is fully trained to operate it efficiently. Using the biogas Up to one third of the biogas energy will be needed to sustain the heat in the digester; the remaining two- thirds will be surplus and can be put to various uses: • The gas can be burned to generate heat, either on site or piped elsewhere. Small AD units are likely to produce heat only. The gas can be used for cooking (using safe equipment), to heat the farmhouse, dairy or other nearby buildings, without any need to convert it to electricity. However, contaminants in the gas may restrict its use for any purpose other than electricity generation unless it is cleaned. • The gas can be used as fuel for an engine connected to a generator to produce electricity which can then be used on site, marketed to the grid and to other networks. Details on the planning permissions needed, and on electricity connection, are given in the Project Development section of these guidelines. • If a combined heat and power (CHP) plant is used, all gas will be consumed to provide both electricity and heat. This is a more efficient use of the available energy; the whole digester heat requirement is normally provided from the heat recovered from the engine and exhaust gas. • The biogas is of a lower calorific value than natural gas and it would generally not be cost effective to store large volumes on site. However, the value of electricity produced may be enhanced if small volumes of gas are stored overnight (4 - 8 hours) and used for generation at peak demand times during the day. • It is also possible to use the gas (if carbon dioxide is removed through the use of water towers, and the gas is compressed), to run motor vehicle engines, which would need to be converted to use the gas. The processes to clean and compress the gas for this purpose, and the conversion of the vehicles, are expensive, but may become more viable in future. Electricity from renewable resources will continue to be a growth market. Currently, the Non-Fossil Fuel Obligation (NFFO) requires regional electricity companies to purchase a certain amount of energy from renewable sources, and NFFO contracts of up to 15 years, offering premium rates, may be awarded to suppliers. There are currently (1997) seven NFFO contracts for energy from AD which, if all are built, will provide a total of 7MW capacity. There are also increasing opportunities outside NFFO to sell energy from renewable resources to a growing niche market as energy markets are liberalised. Digestate The digestate which remains after the gas has been removed is pumped to a storage tank, after which it can be spread directly on to the land. However, this digestate can only be considered partially treated at this stage, and should be stored ready for applying to the farm land at the appropriate rate (see MAFF Code of Good Agricultural Practice for the Protection of Water, as well as specific guidelines in Nitrate Sensitive Areas and Nitrate Vulnerable Zones). It may be necessary to consult the Environment Agency. Alternatively, the digestate can be separated into fibre and liquor. Fibre The fibre produced from AD is bulky and contains a low level of plant nutrients. It can be used as a soil conditioner, and the most efficient use for the fibre is to return it directly to the land locally. This is much easier to do after processing than conventional land-spreading as spreading the fibre requires much less power and can be done using a small tractor rather than specialised equipment. It is also much less offensive to neighbours, having no odour after digestion. AD fibre can be used to condition the soil, and as a low grade fertiliser. In some cases the fibre from AD can be used as an alternative to peat, although it does not have exactly the same purpose as peat (which is nutrient-free). A possible strategy for increasing the value of the fibre is to further compost it aerobically after processing to produce a potting compost/growing medium. This process can be speeded up through careful management and control such as adding heat or insulating the composting bins. However, market development is required to make the commercial sale of fibre viable on any large scale. For any sale of fibre, access to local markets is crucial as it would not be economically viable or environmentally sound to transport it long distances.PDF Image | Anaerobic Digestion Of farm and food Processing residues The development of a sustainable industry
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