Maximising the potential of anaerobic digestion

Thesis

Solving climate change needs new renewable technologies that can be deployed at scale economically and sustainably. These technologies must also help stabilise the grid and follow demand rather than simply generate when nature provides.

Anaerobic digestion could be part of the solution. It produces gas that can be stored and fed to engines as needed to generate electricity. However currently it is too expensive, the feedstock resource is limited, and much of the feedstock comes from energy crops that displace food. This is clearly not sustainable long term. To fulfil its potential, anaerobic digestion needs a new resource base to add to the present ones, and it needs to be much lower cost.

Part 1 of this thesis examines the costs and resource base. It concludes that the resource base could be very substantially increased by growing hyper-water-efficient plants that use the crassulacean acid metabolism on degraded and semi-arid land globally. Between 5% and 15% of this land could provide as much electrical power as natural gas. The resource could be further increased by hybridisation with solar PV. Food production need not reduce, and may even increase. To become economic, however, needs a major reduction in capital costs. This can be achieved by increasing fourfold the volumetric power density of an anaerobic digester, principally by speeding up the rate of reaction.

Part 2 of the thesis compares and contrasts digestion in ruminants with conventional anaerobic digestion technology. It concludes that ruminants perform the rate limiting step 20-30 times faster than a commercial anaerobic digester, partly as a result of adopting different mechanical and chemical strategies. The thesis identifies and explores these differences, and proposes future work to understand and quantify them, to facilitate development of a new generation of low cost Advanced Anaerobic Digestion that meets the challenging cost reduction targets set in Part 1.

Authors: Mason, P

• Alternative title: Moving the bio-energy debate from 'fuel or food' to 'fuel and more food' in a way that is economic, large scale, and sustainable.
• DOI: 10.5287/ora-rbevwknxk
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Ruminants; Crassulacean Acid Metabolism; Semi Arid Agriculture; Anaerobic Digestion
• Subjects: Microbiology; Bioenergy; Renewable energy sources