Feedstocks, Processing and Safety

Theme Leader: Prof Jenny Jones

Feedstocks characterisation relates to biomass being derived from any dedicated energy crop, e.g. short rotation willow coppice, miscanthus, switchgrass etc., but also includes forestry and agricultural residues, e.g. straw, and conservation biomass (where biomass is harvested for conservation reasons e.g. maintenance of reedbed habitat for biodiversity). Research has also been undertaken for several decades into the potential for obtaining bioenergy from both microalgae and macroalgae (seaweeds). This includes the development of algal photo-bioreactors for renewable energy generation in the water industry. CO2 and nutrients are fed into the reactor in the presence of light, thus powering the growth of the algae. Various types of wastes can be used to generate energy, some of which have a biological origin. A large group at Leeds is involved in characterising these wastes and analysing their combustion properties (see Conversion).

Processing includes pre-treatment and densification technologies e.g. washing and torrefaction. The latter produces a substance more easily integrated into systems that were originally designed for coal fuel. Pelleting and briquetting similarly improve fuel quality. Clearly there is an energy input needed to for this, so Life Cycle Analysis (see Sustainability and Whole Systems) may be necessary to study the impact on efficiency of the overall process. Many low-cost residues and wastes have good greenhouse balances but cannot be used directly in conversion processes since their use results in many serious issues. For example, in combustion, fouling of boiler tubes or furnace walls is observed, as well as increased corrosion rates and potentially higher emission levels. Many companies who utilise solid biomass are now interested in how these very abundant but low quality resources can be upgraded into better fuels. Pre-treatment via washing is one such technology. Communition is the reduction in the average particle size of biomass through chipping, shredding, crushing and grinding in mills. Communition to small particle size is necessary prior to the production of pellets and briquettes and also in some industries, e.g. fast pyrolysis and large scale combustion. Processing also relates to nutrient recovery, soil quality, growth of crops and biochar.

Safety relates to self-heating and spontaneous combustion. Low temperature ignition and spontaneous combustion of biomass is a major safety concern in the utilisation of biomass. There have been several instances in the UK and elsewhere of explosions or fires during storage, milling or conveying; notable is the incident at Tilbury Power Station in February 2012. The dusts and gases associated with stored biomass can be a major safety hazard. A large well-established group at Leeds has a strong track record in research on fires and explosions which can be applied to this area. Stored biomass is also subject to microbial hazards; in addition the gases given off may contain pathogens. This area also looks at the emissions during cultivation, processing, transportation and storage.