Energy recovery from spent mushroom compost and coal tailings.

Spent mushroom compost (SMC) is an agricultural waste disposed of in an unsustainable and environmentally degrading manner - mainly in landfills. For every 1 kg of mushrooms grown, approximately 5 kg of SMC is produced, where current generation in the UK is 200,000 tJa. Coal tailings, an industrial by-product from coal mining, are also discarded untenably in lagoons; removing these deposits will eliminate the associated environmental hazards. This project aimed to combine these waste materials into a suitable 'green' fuel for industry through thermal treatment, to produce energy from a sustainable source. Not only will this alleviate the issues regarding existing waste management strategies, but will also attempt to mitigate the environmental impacts of energy generation from nonrenewable sources, such as anthropogenic climate change, through the generation of renewable energy. This PhD research has shown that both materials had high moisture contents, which negatively impacted the calorific value (CV). Drying, though expensive, would thus be required prior to pelletisation and thermal treatment. Key pelletisation parameters were identified and manipulated to control product quality. Optimal values were experimentally-determined for pellet composition (50:50 wt% SMC:coal tailing ratio), moisture (10.5 %) and pressure (6000 psi/41 MPa); such pellets had a NCV of 16.11 MJ/kg. As these pellets were still friable, additional studies were carried out to further improve pellet quality, in tenns of density, tensile strength and durability. Elevated temperatures and steam were considered, in addition to the use of starch and caustic soda binders, which were all successful to varying degrees. Combustion, gasification and pyrolysis tests compared the raw SMC to SMC-coal tailing pellets, where pellet combustion performed better than the SMC alone, and fluidised-bed combustion was more efficient than the packed-bed. Although pyrolysis worked well, the CV of the fuel products were low, whereas gasification was unsuccessful. Consequently, in-depth studies into pellet combustion in a laboratory scale fluidised-bed were perfonned, examining: (i) combustionl fluidisation air flowrates (4.9-10.7 kg/hr); (ii) fuel pellet feedrates (2.02-4.58 kg/hr); and (iii) sand bed depths (0.22-0.30 m). The impacts on temperatures, combustion efficiency and gas concentrations, including acid gas species were analysed. The most favourable operating conditions resulted in high temperatures for efficient energy recovery, with minimal pollutants, although the addition of secondary air jets could further improve the already high combustion efficiencies. While gaseous pollutants are unlikely to be an issue, as the emissions produced generally conformed to the Waste Incineration Directive, efficient particulate collection will be required to remove flyash from the gas stream prior to release to the atmosphere. Industrial implications were explored for heat and power generation, where mass and energy balances for a theoretical furnace, boiler and turbine set-up were completed for various fuel throughputs. Excess heat from the process could be utilized to dry the initial materials, but the economic analysis showed this would be costly - totalling 7) of overall pelletisation expenses. Assuming an overall process efficiency of 18.6 %, a steam turbine could generate over 10 MWe, based on an SMC-coal tailing pellet feedrate of 400,000 tJa - to simulate a large, centralised energy-from-waste facility. FLUENT, a mathematical model, was able to effectively replicate the results of the experimentation and was then used to model particle elutriation and entrainment to assess the suitability of the transport disengagement height provided.