Impacts of Fuel Inventory on Low Temperature Ignition Risk during Handling and Storage of Biomass

As modernisation takes place, fossil fuel burning is one of the quickest ways to meet the ever rising energy demand. The increasing emissions of greenhouse gases, particularly carbon dioxide, as a result of excessive fossil fuel burning had been blamed for global climate change. Vegetation-based biomass is a form of bioenergy and a recognised solid renewable fuel with potential to replace coal in combating anthropogenic climate change in the power generation sector. Nevertheless, it is not a straight forward case for biomass to replace coal since biomass is an extremely reactive fuel prone to self-heating leading to self-ignition. Spontaneous biomass ignition leading to disastrous fires during biomass handling and storage could be avoided if the causes of biomass low temperature ignition are well understood. Detailed studies on woody and herbaceous biomass fuels commonly used in UK power stations were examined according to several British Standards. On top of characterising all the biomass samples, BS EN 50281-2-1 and BS EN 15188 were adhered to specifically in investigating low temperature ignition during biomass handling and biomass storage respectively. Many power stations use a mix of different biomass in their fuel inventories which can lead to dusts of biomass mixtures. Thus the low temperature ignition characteristics of biomass blends have been studied. Other factors that may impact on ignition risks are binders (added to give strength to briquettes or pellets) and pretreatments (washing and torrefaction). Washing aims to improve ash properties towards the end of combustion process while torrefaction is used to increase the calorific value of biomass that is naturally lower than fossil fuels. The reaction kinetics of some biomass dust layers deposited on a constant temperature hot surface and corresponding ignition delay time were estimated mathematically. Results from minimum dust layer ignition temperature determination showed that all biomass, regardless of woody or herbaceous, with or without binder, before or after pre-treatments, had critically ignited within a very small temperature range. This was consistent with the results of self-ignition propensity risk ranking that concluded that biomass possess medium-high risk of self-igniting. An exception to this is torrefied biomass which had not sustained a much higher temperature before it critically ignited as compared with the untreated counterpart; unlike many anticipations and therefore, the low temperature ignition characteristics were discussed from many other aspects, mainly on the reduced particle size or dust layer density. For biomass storage, scaling up method and Frank-Kamenetskii method derived from Thermal Explosion Theory had been applied to forecast the critical ignition temperature and ignition delay time for large-scale industrial storage from smaller laboratory scale experiments. Non-negligible error was detected when extrapolating to industrial volume especially for the ignition delay time and appropriate recommendation was made as a possible remedy. Emissions when biomass smouldered and critically ignited that happened at 10˚C apart were examined with a three-stage emission sampling and compared, with the aims of obtaining a suitable biomass self-ignition indicator. Detailed studies were required since only one organic compound was detected to be consistently different between smouldering and critically-igniting biomass dust. Within this small temperature difference, different volatile species with respective intensities had been modelled with FG-BioMass software. Towards the end of this work, conclusions were drawn for each section and suggestion of combining both pre-treatments with binder addition were recommended for further studies. The work in the thesis provides a large data-set which will help inform power plant operators in their dust management risks. The laboratory-scale experiments give a useful risk-ranking for dust layer ignition, but uncertainties in ignition-delay times, especially for large biomass quantities, indicate that improvements are required to BS EN 15188 (biomass storage test) to enable scaling-up with more certainty.