Renewable energy sources are playing an increasingly important role in world energy supply, which is facing an unprecedented dilemma in meeting the growing energy demand with depleting natural resources. Reducing the greenhouse gas emissions is also extremely important to energy utilisation based on combustion. In moving towards renewable and cleaner energy technologies, biomass is undoubtedly attractive because it is environmentally friendly and can be used as an alternative feedstock for energy resources in the future. The three key technologies for thermo-chemically converting biomass into heat, power and fuels are direct combustion, gasification and pyrolysis. Until now large biomass units (>300MWe) are economically unpractical due to its lower heating values and low bulk densities.
There are two types of gaseous renewable fuels related to biomass that are of particular importance: biogas and bio-syngas. Biogas is normally produced by the anaerobic digestion or fermentation of biodegradable materials such as biomass, manure, sewage, municipal waste, green waste, plant material, and agriculture crops. Meanwhile, synthesis gas from thermal gasification of biomass, or bio-syngas, could be potentially a cleaner fuel than biogas because of its high hydrogen content.
Despite being renewable fuels for combustion, biogas and bio-syngas have not been broadly used in power plants such as gas turbine combustors. The main reason is that the complex fuel composition leads to unpredictable combustion performance. Hazardous combustion phenomena such as combustion instability can happen. Different fuels may lead to very different flame distributions in a combustor. The wrong distribution may generate flashback or hot spots which might cause expensive damage to the combustor hardware. For gaseous biofuels, typical biogas comprises primarily methane (CH4) and carbon dioxide (CO2) and may have small amounts of nitrogen (N2), hydrogen sulphide (H2S), hydrogen (H2), moisture and siloxanes etc. Bio-syngas thermo-chemically converted from biomass gasification normally contains numerous gas components, including mainly carbon monoxide (CO), H2, CO2, and CH4 etc. with varying component percentages. The fundamental issue with biogas and bio-syngas combustion is associated with the significant variation in the fuel composition that changes the flame speed, heat release rate, local fuel consumption rate, pollutant formations, and more importantly flame stability mechanisms such as local extinction and re-ignition.
Recently, researchers working on the HPC4E project have presented a paper “Effects of fuel composition on biogas combustion in premixed laminar flames” (by Angelo Greco, Daniel Mira & Xi Jiang) at the 8th International Conference on Applied Energy (ICAE2016), Beijing, China, 8-11 October 2016. The paper has also been recommended for the further consideration in the special issue of the journal Applied Energy. The study can be further extended for numerical simulations under more realistic and practical conditions in order to develop guidelines for industrial combustors.
Prof Xi Jiang
Engineering Department, Lancaster University
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