Bio-CO2: Power Generation and Heat Recovery from Biomass with Advanced CO2 Thermodynamic Power Cycles and Novel Heat Exchanger Designs

In the UK, power generation is achieved mostly through the combustion of fossil fuels from remote power stations at a low-efficiency rate of 40%. This can lead to a large depletion of energy resources and pollution to environment. In reality, after taking into consideration long-distance power transmission and distribution losses, the generation efficiency tends to be further reduced to around 32% at the power supply end. To combat this problem, a local and decentralised combined heat and power (CHP) system may be used to attain not only 30% electrical efficiency but also over 50% heating efficiency, which would significantly improve the energy utilisation rate. In areas with simultaneous heating and electricity demand including supermarket and district heating, such systems would be a viable economic option. However, currently most CHP systems still require fossil fuel energy resources, which diminish both their energy-saving merit and potential CO2 emission reductions. Therefore, it would be highly desirable to promote the use of localised renewable resources, such as biomass fuels, with optimised CHP system engineering designs.

Currently, there are two main biomass CHP systems: biomass gasification with gas/steam turbines and biomass combustion with Organic Rankin Cycles (ORC). However, these biomass CHP systems cannot be further developed or extensively applied before the resolution of certain critical issues. These include achieving an acceptable thermal efficiency, compact system size, environmentally-friendly working fluid, advanced thermodynamic power cycles, optimal system design and control, and flexible operation etc. On the other hand, for power generation with medium to high temperature heat sources, CO2 supercritical Brayton cycles (S-CO2) can predominate over conventional ORCs in terms of thermal efficiency, environmental impact and system compactness. The S-CO2 systems have been applied in large-scale waste heat recovery of nuclear power plants but have not yet been utilised in biomass power generations due to various unsettled challenges. In this proposed project, a small-scale biomass power generation system with advanced CO2 supercritical Brayton cycles and novel heat exchanger designs will be investigated experimentally and theoretically. The investigation will address the challenges involved in the proposed system including innovative designs of thermal drive CO2 supercritical compressors, precise CO2 parameter controls at the S-CO2 compressor inlet, novel designs of supercritical CO2 heat exchangers and comprehensive understanding of the complex heat transfer and hydraulic processes involved. In addition, a detailed transient model of the biomass S-CO2 power generation system will be developed which will enable the system to be further optimised and scaled up for actual design and operation.

The impact of the research will be widespread and varied. It is highly relevant to many sectors including: (i) Industry - The success of this project will directly contribute the performance evaluations and improvements of developing technologies such as thermal driven CO2 supercritical compressors, biomass CO2 supercritical heaters, CO2 supercritical recuperators, as well as the application and control of CO2 supercritical Brayton cycles (S-CO2). It will have long-term influence on the biomass industry by enabling high efficiency power generation and more flexible operations. ii) End-users- UK domestic houses particularly in rural areas with limited power supply have rich energy resources of waste wood/pellet which can potentially be used by the proposed biomass-fired S-CO2 system as fuel for generating electricity and producing heat. This proposal will also benefit other end-user types such as schools, out-of-town retail, leisure developments ,district heating networks and other commercial buildings where biomass resources are available and both electricity and heating/cooling are simultaneously demanded. (iii) Academics - This project will contribute to the investigation and development of a new biomass-fired power generation systems with advanced CO2 supercritical Brayton cycles (S-CO2), its design and selection of main components, and social impacts and business infrastructure. (iv) Government - This project is a prompt response to the government's recent targets for CO2 emission reduction and enhanced utilisation of renewable energy. The proposed programme will support and enhance the achievements of these targets in order to have a significant impact on the policy implementation of biomass power generation and demand side management.

The industrial, end users' and government impact will be achieved through direct industrial interaction and knowledge transfer. Ashwell Biomass Ltd, Kelvion Searle , Entropea Labs Ltd , and SWEP International Limited which are the potential manufacturers of the proposed technology, are directly involved in the project. Residential houses in rural areas and district heating networks in urban areas are our potential users of the proposed product and technology. Our outcomes will also be disseminated through various knowledge transfer networks as well as application-oriented magazines to maximize impact. The academic impact will be achieved via article publications and conferences/seminars events. We will keep publishing our findings in a timely manner in top peer-reviewed journals and conferences. We will also report our outcomes to other relevant programmes such as Sustainable Energy Use in Food Chains' (CSEF) programmes to attract attention in this new area, with the ultimate goal of establishing a society for biomass-fuelled CO2 power generation research. In addition we will be involved in two new developed master programs in Renewable Energy Engineering at Brunel University.