Anaerobic digestion and bio-genic waste
Swan Centre research activity focuses on:
- uses for cellulose product from MSW steam autoclaving, including conversion to bio-gas;
- novel anaerobic digestion;
- difficult wastes; pre-digestion and post-processing;
- seaweed anaerobic digestion
- membrane bioreactor system and multi-stage processing for optimising bio-gas from AD;
- advising companies on how to improve their AD process;
- CO2 capture from flue gas using micro-algae;
- gasification of sewage sludge and expected emissions;
CREEL Project: Anaerobic Digester Unit at Cockle Park
Work is underway to establish an Anaerobic Digester (AD) unit at the University’s Cockle Park site as part of the Centre for Renewable Energy from Land (CREEL) project. This would be a commercial scale AD unit which would operate as a demonstration and research facility, along with associated experimental scale equipment required for research.
The UK has substantial biomass resources, which could be processed via anaerobic digestion in terms of; about 150 million wet tonnes of livestock slurry (pig and cattle), 3.4 million wet tonnes of used poultry litter and excreta, together with in excess of one million tonnes of food production residues (vegetable and dairy processing residues). In addition to which there is about 90Mt of waste produced in the UK each year with 62% of this waste stream said to be biodegradable (DETR, Limiting Landfill, 1999). This biodegradable waste will produce about 150m3/tonne of biogas at 60% methane concentration (Warmer Information Sheet, 1998). Using a process efficiency of 70%, a 70% load factor, and the known 37GJ/tonne energy content for methane, after accounting for the 20-40% of energy needed to maintain the temperature of the processor, anaerobic digestion could provide the UK with about 1.4GW of electricity representing about 1.9% of the UK's installed capacity.
UK agriculture contributes 7% of all UK greenhouse gas emissions, including 67% of nitrous oxide and 37% of methane. Main sources of GHG emissions are from animals (32%), manure management (20%) and soil breakdown (48%). There has been an overall decline of 14% in GHG emissions from agriculture since 1990 (due to reduced livestock numbers and reduced fertiliser use). During storage of animal manure significant GHG emissions occur particularly of N2O and CH4 as a result of uncontrolled anaerobic digestion processes. Anaerobic digestion exploits this process so that methane can be used as a fuel resulting in reduced emissions of approximately 90%. A well managed AD scheme aims to maximise methane generation, but not to release any gas to the atmosphere, thereby reducing overall emissions. In addition it provides an energy source with no net release of carbon. Anaerobic Digestion has considerable potential to contribute to the production of renewable energy on farms in addition to reducing the overall contribution of agriculture to global warming.
Anaerobic digestion offers considerable potential for farms and rural businesses in the production of energy and the management of animal manures and other waste streams with a high moisture content. However the number of facilities on farms in the UK is limited as the technology has proved somewhat unreliable and uneconomic. On-farm AD facilities have in the past predominantly been used for the management of animal manures rather than energy production. The potential for on-farm energy generation at regional and national levels is obviously huge bearing in mind that the UK has some 150,000 full-time and 200,000 part-time farms. The up-take of such technologies by a relatively small proportion of these businesses would lead to a significant contribution of renewable energy to the UK market.
Both capital and operating costs are high for AD and when just used for the purpose of manure management the technology has not proved to be economically viable. An AD project is more likely to be financially viable when treated as part of an integrated sustainable system aimed at maximising the energy production potential as is proposed here. Animal manure generally has a low biogas yield so the aim of the Cockle Park facility will be to maximise the biogas production from the facility via variation in feedstocks and in the operating conditions. Small scale laboratory digesters (3litres) will be utilised to maximise the efficiency of the anaerobic digestion process with relation to the substrates used, retention time for different substrates and the use of glycerol to increase the efficiency of the digestion process.
A full range of research projects will be based around the facility through a multidisciplinary programme exploring all dimensions of AD lifecycle including: feedstock production, processing & management, biogas production, output energy usage and application, and digestate usage and application.
