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128 Evaluation of Opportunities for Converting Indigenous UK Wastes to Wastes and Energy AEA/ED45551/Issue 1 Approximately a third of the heat production will be used to heat the digester vessel and maintain the bacterial cultures at their optimum temperature. Further heat may be necessary to pasteurise the solid residue before it can be sold as fertiliser or soil conditioner. Recently the National Grid Company has announced plans to encourage and develop projects that can inject natural gas quality biogas into the National Gas Grid. This will involve the removal of carbon dioxide and any acid gas components by a combination of liquid scrubbing and adsorption technologies. The resulting cleaned gas will then need to be compressed to either transmission pressure of approximately 30bar or for smaller installations the low pressure distribution network at 3.0 and 0.5 bar. 229 This concept has advantages for the operator of an AD plant if the incentives are sufficient, in that he will be able to sell the biogas into the grid all year round. Heat sales from CHP are always problematic with space heat demand varying throughout the year. Understanding the potential for AD requires study on a spatial basis The technology relies on the collection, transport and disposal of large volumes of materials and at each stage of the process there are constraints related to the location; • Wastes, crops and manure must be collected from a wide variety of sources and suppliers. • All feedstocks are high water content, low energy density, and expensive to transport. • The installation must be sited where traffic density and odour will not be a problem and where there is an electricity network connection and a heat customer. • The unit produces almost as much material as digestate for transport and disposal as it receives as feedstock. • The business will also need access to suitable agricultural land within a reasonable transport radius to dispose of the digestate. An accurate estimate of potential would need to be carried out by mapping waste arisings, energy demand, gas grid and the digestate holding capacity of the land as a GIS study. ADAS have carried out work on the holding capacity but this has not as yet been extended to digestate. Feed stocks suitable for anaerobic digestion Only high moisture content organic feedstock containing proteins, fats and carbohydrates can be processed successfully by anaerobic digestion. Cellulose and lignin digest far too slowly so it is not suitable for paper and wood. This does mean that a large variety of wastes ranging from farm manures and bio-solids to catering wastes, food wastes, meat, and bone meal can be treated, however the processing time (retention time) varies widely with the feedstock composition. The content of nitrogen compounds in the feedstock needs to be managed as they are converted to ammonia, which is toxic to anaerobic bacteria. The C:N ratio is also important as optimal growth of the bacteria is achieved when this is 20-30:1. Food wastes and manures contain high levels of pathogens such as Salmonella and E.coli which must be destroyed during the digestion or during the pasteurisation process once digestion is complete. Scale of operation Typically digesters based in rural locations and using manures, food waste and energy crops will have an output of 300kWe, which represents approximately 40k tonnes per annum of wet feedstock, such as the BioCycle Plant in South Shropshire and the Western Isles Integrated Waste Management Facility. Larger installations in semi-rural and urban locations that so far have focussed on supermarket and food processing waste may have an output up to 5MWe, for example the Wanlip Composting and AD Plant in Leicester. Sainsbury and PDM indicate that larger installations are unlikely. 230 229 http://www.nationalgrid.com/corporate/Our+Responsibility/News/newsbiogas.htm 230 Announcement by Sainsburys and PDM cooperation
Evaluation of Opportunities for Converting Indigenous UK Wastes to Wastes and Energy AEA/ED45551/Issue 1 By-products The digestate can be used as a fertiliser and soil conditioner. Recent practice has been to strike an arrangement between the plant operator and local farms to allow the spreading of digestate to their land in return for treatment of manures. More recently the establishment of the Quality Protocol for the digestate where it is to be spread on agricultural or forestry land, is intended to facilitate the use of the digestate, although care will still need to be taken in Nitrate Vulnerable Zones (NVZ), as discussed in the Protocol, as 68% of arable land in England and 4% in Wales are designated to be within NVZs and there are restrictions on how much nitrogen farmers can apply yearly. 231 A recent example of the challenges increased volumes of digestate will create is given by Summerleaze’s Holsworthy site in Devon, which may only spread the digestate on land between February and September during the grass growing season, as the surrounding farms are livestock ones. Therefore to operate at maximum capacity it must be able to store six months worth of digestate, totalling 42,000m 3 . Its current storage capacity is 15,000m 3 . 232 Technology status Anaerobic digesters are very well established in Germany and other EU Member states, which have or have had favourable policy instruments. The table below provides the number of AD plants in EU countries and their electricity generation capacities in 2005. It is clear from the table that there are a number of European countries in which the uptake of AD, either on-farm or centralised (CAD), by the agricultural industry has been much greater than in the UK, although it is also clear that Germany is quite exceptional. Table 68: Numbers of biogas plants in EU-countries producing electricity 233 Country Agricultural AD plants Installed capacity MWe Austria 159 29 +150 to end 2007 + 40 to end 2007 Belgium 6 12.3 Denmark 58 on-farm 20 CAD 40 France 3 n/a Germany > 3000 550 Great Britain
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South East 327 - - 55% of MSW by 20
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Appendix 3 MBT Methods Specific in
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Figure 27 Orchid Environmental proc
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