On the combustion of solid biomass fuels for large scale power generation: Investigations on the combustion behaviour of single particles of pulverised biomass fuel

Biomass is classed as a renewable resource. Depending on the means of production, it can be sustainable and can provide net benefits regarding CO2 emissions by displacing fossil fuels as an energy source. A significant biomass energy conversion technology is combustion in conventional thermal power stations. This can be implemented in large scale plants such as those which dominated electricity generation throughout the 20th century. While these power stations were generally fuelled by the erstwhile ‘King Coal’, the technology is not exclusive to it. Coal consumption can be displaced in these types of plants by either co-firing biomass with coal or full conversion to biomass. Currently, in the UK, the vast majority of the biomass fuel consumed for power generation is imported pelletized forestry wood. However, sustainability and domestic energy security concerns have created interest in using other resources including energy crops such as short rotation coppice willow and miscanthus, agricultural by-products such as wheat straw and olive residue. The variation in the properties of these fuels presents a number of technical challenges which conventional power plant must overcome to achieve ‘fuel flexibility’. Along with other technical challenges regarding the operation of conventional thermal power plant, these formed the basis of the Research Councils UK funded consortium grant (EPSRC, 2012) entitled Future Conventional Power. As a consortium partner in this project, the University of Leeds led research tasks associated with fuel flexibility. Much of the research presented in this thesis was based on the objectives set out in the Future Conventional Power project and was financially supported though this grant. Two particular challenges provide the incentive for the investigations presented in this thesis and can be summarised as: • assessing the variability in fuel combustion behaviour and control of burn-out efficiency for different fuels • understanding the behaviour of potassium during the combustion of biomass fuels to aid in the prediction of ash behaviour, emissions and associated operational problems Both these points were addressed with a series of experimental studies. In addition, a model of the combustion of single particles was developed for validating and interpreting the results. A range of fourteen solid biomass fuels, typical of those likely to be used in large scale power plant, were selected for the experimental studies. The composition and fundamental characteristics of these fuels, obtained by standard analytical techniques, are presented. In the first experimental study, single particles were exposed to a methane flame, simulating biomass combustion in a furnace. Measurements of ignition delay, volatile burning time and char burn-out time were undertaken using high speed image capture. Particle surface temperatures were measured by infra-red thermal imaging. Analysis of the data identified correlations between the biomass fundamental characteristics, particle size, and the observed combustion profiles. Empirical expressions for the duration of each combustion stage are obtained from the data. From these, a “burn-out” index is derived which provides a useful indication of the relative milling requirements of different fuels for achieving effective burn-out efficiency. A similar experimental method was used in the second study in which the gas-phase potassium release patterns from single particles of various biomass fuels were measured by use of flame emission spectroscopy. The observed potassium release patterns for the various fuel samples are presented. The release patterns revealed qualitative differences between different fuel types. Relationships between the initial potassium content, peak rate of release and the fractions of potassium released at each stage of combustion were identified. These were subsequently used for comparing with results of modelled potassium release. A third experimental study investigated the variation in thermal conductivity between different types of solid biomass using a technique and apparatus developed specifically for the study. The results showed variation of thermal conductivity between different types of biomass which had been similarly homogenised and densified. The thermal conductivity of small particles of each fuel was derived. The resulting data provides useful values for thermal modelling of biomass particles and is used subsequently in a combustion model. Elements of each of the experimental studies were used in a detailed model of single particle combustion. In this, the particle was modelled as a series of concentric spherical layers which enabled calculation of internal mass diffusion and heat transfer. Devolatilisation and char oxidation were approximated with single step reaction kinetics. A volatilisation and diffusion mechanism was adopted to simulate the release of gas-phase potassium from the particle. The output from the model was compared and validated using data from the experimental studies. The modelling produced confirming evidence that the assumed mechanisms for gas-phase potassium release were valid and provided a tool for future investigation of the subject