Increasing energy yield from the integration of anaerobic digestion and pyrolysis

Biomass-based energy processes are frequently cited as a great opportunity but also a risk because of their competition for productive arable land used for food production. Virgin crop biomass is much easier to use for energy production and for this reason is the basis of first generation biofuel production. Waste and mixed biomass sources are more difficult to process into usable fuel products, and the key research challenges therefore lie in developing technologies to provide second generation biofuels from these waste sources. The vision for the proposed research is to develop a synergy between biological (AD) and thermal (intermediate and fast pyrolysis) conversion processes in which the overall net energy yield from waste biomass can be improved, producing energy carriers that are both storable and transportable. These gains can be achieved as the thermal process allows access to lignin-bound components within the biomass which are otherwise unavailable for anaerobic biological conversion. In return, biological system can process the lower carbon chain molecules in pyrolysis oils and in the aqueous fraction to produce a fuel gas, thus improving the value of the liquid fuel fraction. The solid char also has potential for use as an energy carrier as well as other value-added uses.

The vision includes proving that this hybrid approach can provide a sustainable and societally acceptable means of recovering value from the non-source segregated organic fraction of municipal solid waste (MSW). This waste fraction currently poses the greatest challenge to the UK Government in terms of the requirement for diversion of organic materials generated by society, industry and commerce from landfill: whilst it also offers the potential to contribute significantly to renewable energy targets and to the offset of carbon emissions. This is proposal is therefore closely aligned to the aims of the SUPERGEN call 'Challenges in Bioenergy Technologies' and also to several areas within the RCUK research portfolio on energy, bioenergy and living with environmental change.

At the heart of the project is a new thermal processing technology, intermediate pyrolysis, which can use feedstocks with higher water content than fast pyrolysis thus, increasing the potential for direct process integration with commercial AD plants. This technology is currently available for use in the project at a large pilot scale (up to 100 kg hour-1), ensuring that adequate quantities of material will be available for in-house use in AD trials at a scale allowing realistic estimation of energy balances. Plans are already in place for up-scaling of the pyrolysis side of the process through the construction of five demonstration plants around Europe within the next three years.

Increasing the energy yield obtained from the initial feed stock (MSW) is vital, as future energy generation technologies will need to become more efficient compared to current technologies. Having localised energy generation systems (for example combined pyrolysis and AD) will reduce the transmission distance for electricity which will in turn reduce energy losses and reduce the UK reliance on foreign fuel supply. Focusing this project upon the utilisation of MSW will provide a possible use for the organic fraction of MSW and increase the UK's generation of sustainable energy. The proposed research is therefore ideally timed to develop bespoke AD solutions for the digestion of the different fractions produced from pyrolysis (oil phase, water phase, vapours and gaseous fractions), in order to optimise the energy yield obtained from a critically important waste feedstock. In addition the project will also address the range of social and regulatory factors that need to be better understood if this vision is to gain acceptance and deliver its perceived benefits.

The target of this project is to improve the overall net energy yield obtained from residual municipal solid waste, through a combination of thermal pyrolysis and anaerobic digestion. This will be undertaken in the context of producing a hybrid process that is technically efficient, economic, compliant to regulations and acceptable to society. Discussions have already taken place with specialist AD companies (for example Marches Biogas and ADBA), pyrolysis companies (Pyrotop and WSE) and with energy producers (EoN), providing demonstrable evidence of commercial interest for this synergy to be developed (please see statements of support). We also expect public-sector interest, for example from local authorities.

The project has also been designed to take into consideration the views of regulatory and market concerns/priorities, engaging regulators (DEFRA, Environment Agency), industry (ADBA, specific AD companies, fuel suppliers, engineers), financiers and local authorities (especially LGA and NGOs). Providing a new synergy approach for the disposal of MSW; will reduce the requirement for landfill, reduce GHG emissions from this waste and increase energy production without conflicting with food waste or the use of agricultural land.

The foreseen benefits to the AD industry include the production of new substrates from pyrolysis which are suitable for AD and a thorough understanding of the digestion conditions required to form protocols which maximise the energy yield obtained from these substrates. Similarly, this research would benefit the pyrolysis companies, providing additional commercial usages for pyrolysis products and an increased understanding of how different processing conditions affect the process and the composition of the products. With regard to energy companies and the local authorities, this research will provide companies/local authorities with alternative treatment solutions for MSW, increasing the energy yield obtained. If 50% of the collected MSW that would have been sent to landfill was used in an integrate pyrolysis/AD system, it would save the government an estimated £4.8 billion per year on landfill costs and an estimated 17 million MTCE per year. This is in addition to the energy produced from the process and production of digestate and char to be used on the land. Reduing landfill and GHG emissions would also benefit companies' compliance with producing greener energy, aid with meeting UK targets set under the Climate Change Act, waste management and carbon management.

The benefits to society will include the development of an alternative waste management process which will be more environmentally benign than current options. Commercial companies are currently interested in the development of this synergy approach and realise the benefits this technology could bring. Moreover, the proposal also has the potential to contribute to producing decentralised heat and power through energy-generation units that use waste materials in a more efficient way. This will reduce the UK's reliance on importing energy and minimise the fluctuations in energy prices which are associated with this. As the research project intends to increase the energy security of the localities whilst serving environmental aims, the proposed combination of technologies has international applicability in urban and rural regions. Indeed, with the potential to use animal, catering and municipal mixed waste, the latter posing a major, long-standing cost and environmental concern, this technology has global applications at various levels.