Conversion

Theme Leader: Prof Paul Williams

Combustion is the first area within conversion, initially looking at combustion fundamentals. We are concerned with understanding, at a fundamental level, the thermal conversion behaviour (pyrolysis, gasification and combustion) of biomass and identifying and understanding problems different biomass might present. Aviation is a rapidly expanding sector worldwide, with a consequent growth in the use of petroleum derived fossil fuels (kerosene). From the viewpoint of CO2 emissions associated with climate change, and security of supply given the large concentration of petroleum reserves in potentially politically volatile locations, there is a great deal of interest in the use of alternative fuels in aviation, and the use of such fuels requires the fundamental combustion properties be understood. Fouling in boilers is also explored. There are significant challenges to implementing biomass combustion in boilers for the production of heat and power. Biomass contains significant amounts of alkali metals, which modify ash melting, slagging, fouling and deposition behaviour within power plant equipment. In addition to the problems with fouling, corrosion issues are a significant factor in biomass systems. Modelling through computational fluid dynamics also underpins most areas of study within these themes.

Gasification is also explored, primarily looking at gasification fundamentals. This relates to the use of high temperatures and a controlled environment to convert biomass into gas. This area focuses on gasification of solid biomass and gasification from liquids and volatiles. The transport sector faces increasing challenges to reduce its impact on both climate change and air quality. Strategies are likely to include the use of a wide range of fuels from renewable sources, at the same time as improving engine efficiency through new technologies e.g. turbo charging, homogeneous compression charge ignition and gasoline compression ignition. Legislation on vehicle emissions will necessitate reductions in both CO2 and a range of health related air pollutants, whilst maintaining vehicle reliability, efficiency and safety. Biofuels offer promising options since they can be derived from sustainable sources and can reduce emissions such as particulates when compared to conventional fossil fuels like diesel. However, differences in their properties compared to conventional fuels pose challenges for new engine technologies and it is essential that their combustion properties are well understood in order to inform practical engine design. Anaerobic digestion also comes under this area as it uses naturally occurring microorganisms to break down organic materials and produce biogas, a mixture of methane and carbon dioxide. The biogas can be combusted to produce renewable electricity, cleaned to pipeline natural gas standards, or further processed into compressed natural gas fuel. Through anaerobic digestion, many goals can be accomplished: Divert organic materials from landfills and incinerators; Generate clean, distributed, renewable energy; Restore and maintain healthy soils using compost products; Displace chemical fertilizers; Create green jobs. Advanced anaerobic co-digestion of algal biomass is explored under this area.

Liquefaction also comes under conversion. Catalytic pyrolysis is explored, which is the thermochemical decomposition of organic material at elevated temperatures in the absence of oxygen. Pyrolysis of organic substances produces gas and liquid products and leaves a solid residue richer in carbon content, char. Pyrolysis oils from biomass are high in oxygen, acidic and corrosive and are difficult to use directly in fuel systems. The production of biofuels from microalgae by hydrothermal liquefaction involves the processing of microalgae in hot compressed water and effectively accelerates nature to produce a bio-petroleum. This route has many advantages over competing technologies as it is capable of processing wet biomass, it produces a high energy density bio-petroleum and allows valuable nutrients to be recycled. There are still however challenges in order to improve the economic viability of this technology. Finally, biological conversion comes under this area. Potatoes can be used as a source of bioenergy (bio-ethanol), although controversial as they are a food crop. The main limiting factor in bio-ethanol production is the low sugar content in the fermentation broth, leading to low percentage alcohol mixtures after fermentation and high energy costs for distillation. Starch crops overcome this problem because starch granules can be recovered by wet-milling with water and differential sedimentation to separate cell wall pulp and protein juice. All fractions can lead to valuable products and high percentage starch suspensions can be obtained without the need for evaporating water, one of the highest energy requiring steps in bioenergy processing.