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Can micro & community scale anaerobic digestion ever be economically viable?
Can micro & community scale anaerobic digestion ever be economically viable?
Posted by
Richard Porter
at
January 15. 2010
Anaerobic Digestion (AD) is a hot topic in the UK at the moment, driven by the rising cost landfill & energy, but also by increasingly tight environmental legislation. Interest is particularly active in micro & community scale AD as farms seek to cut costs and increase income, but also for rural communities as they move towards self-sufficiency.
The Cornwall Agrifood Council recently commissioned research into suitable technology for AD on farms. The initial brief included CHP (combined heat & power), a cost of less than £3,000 per kW of capacity & target generation capacity of 50kW. Consultation with farmers added further design criteria, such as operation at ambient temperature (10-15 deg. C), simplicity of operation, low time input and no change to farm routines.
The project looked at 3 designs:
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Covered slurry lagoon. Capacity 1000 m³ and depth of 3m. Cover made of neoprene. Gas was drawn off and compressed before running through a CHP plant. Calculated additional benefits for farms in NVZs as capture & diversion of clean rainwater reduced the need for storage capacity. This was given as £19/m³ of capacity reduction.
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Flexible reaction tanks. These were made of 50mm neoprene, designed to hold the slurry. Unproven technology at the time. The gas was used as above. The system was more expensive to install, but more easy to manage as the temperature and nutrient levels could be easily adjusted.
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Fast liquid digestor. Proven technology at the time. Solids separated from the liquid before digestion. Faster than 1 & 2 because the bacteria doesn't need to break the solids down, so has a cycle time of 36 – 48 hours. Needed constant monitoring as the pH tended to move quickly and kill the bacteria.
Plants that run solely on cattle slurry don't return investment well because of relatively low COD (chemical oxygen demand) levels. For comparison, slurry gives 25 m³ of gas per wet tonne, animal fats 250 m³ & grain 700 m³. Farmers needed to add silage to increase gas production and only then achieved a 4 year pay-back on investment.
Installed cost came in over budget.
UK legislation allows farmers to vent methane from slurry pits direct to atmosphere, with no controls. However it's an irony that an environmental permit is needed to capture the methane and use it to generate power, thereby reducing the potential for global warming. The cost and complexity of permitting in the UK is likely to make most farm-scale plants unattractive.
Planning permission for permanently sited plant isn't needed if farmers are using their own wastes, as there is no “change of use” of the land. Planning permission is needed if commercial wastes are brought in and co-digested with the slurry, to generate income and to make the plant more productive.
There are no UK health & safety regulations that specifically cover the generation and use of bio-gas, but farmers would be wise to comply with the Dangerous Substances & Explosive Atmospheres Regs (DSEAR) 2002.
More on this research can be found at the link below: -
http://www.cornwallac.org/news/index.php#NewstudyintoLowCost,%27lowtech%27AnaerobicDigestion
There are several commercial designs available for packaged micro & community scale AD and I'd value the Group's experience of the pros & cons of the different processes & on if & how projects can be made to be economically viable.
