Leeds Thesis Template

On the combustion of solid biomass fuels for large scale power generation Investigations on the combustion behaviour of single particles of pulverised biomass fuel Patrick Edward Mason Submitted in accordance with the requirements for the degree of Doctor of Philosophy The University of Leeds School of Chemical and Process Engineering JULY 2016 i The candidate confirms that the work submitted is his own and that appropriate credit has been given where reference has been made to the work of others. This copy has been supplied on the understanding that it is copyright material and that no quotation from the thesis may be published without proper acknowledgement. The right of Patrick Edward Mason to be identified as Author of this work has been asserted by him in accordance with the Copyright, Designs and Patents Act 1988. © 2016 The University of Leeds and Patrick Edward Mason ii Acknowledgements In undertaking the research presented in this report, the author gratefully acknowledges the following assistance: Guidance and support from my supervisory team: Professor Jenny Jones; Professor Alan Williams; Dr Leilani Darvell at The University of Leeds and Professor Mohamed Pourkashanian at The University of Sheffield; Training provided for the use of analytical equipment and laboratory hardware: Simon Lloyd; Dr Leilani Darvell; Dr Adrian Cunliffe; Dr Bijal Gudka; Stuart Micklethwaite; Sara Dona (The University of Leeds);Dr Phil Davies (TA Instruments); Matthew Clavey (Thermal Vision Research); Preparation of ash samples for XRF analysis: Peinong Xing. Performing flame oxygen concentration measurements: Eirini Karagianni. Machining, fitting and other laboratory technical support: Ed Woodhouse, Chris Day, Gurdev Boghal. Design and build of photometer amplifier circuit: Dr Iain Mosely, Converter Technology Ltd, Reading. Provision of various samples of fuels: Paul Straker and James Ashman (Drax Power); Susan Weatherstone (Eon); Ricky Chaggar (SSE); Carly Whittaker (Rothamstead Research). Cellulose fibre and lignin analyses – IBERS Analytical Laboratory, Aberystwyth University. Elemental analysis of coal samples - Environmental Scientifics Group Ltd, Bretby. General support, assistance and comradery as fellow PhD students in the field of biomass combustion: Dr Paula McNamee; Ben Dooley; Eddy Mitchell; Dr Farooq Atiku; Yee Sing Chin, Dougie Phillips and Aidan Smith. Colleagues in the Future Conventional Power Research Consortium: Dr Juan Riaza, Dr Hannah Chalmers, Dr David Waldron and Professor Simon Hogg iii Funding for research activities was provided through Research Councils UK Energy Programme (EPSRC) grant reference EP/K02115X/1. This was part of the Future Conventional Power Research Consortium, a multi-disciplinary and cross-institutional research project focussing on the use of conventional thermal power stations in the context of a low-carbon UK energy economy. For financial support, the author is very grateful the University of Leeds for the award of a Postgraduate Student Scholarship. Special thanks to my wife, Irene for inspiring me to study for a PhD, supporting my decision to do so full-time, and providing the essential encouragement needed during the process to convince me that I could achieve it. iv Abstract 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 v 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 vi 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. vii Table of Contents Acknowledgements ..................................................................................... ii Abstract ....................................................................................................... iv Table of Contents ....................................................................................... vii List of Tables .............................................................................................. xii List of Figures ........................................................................................... xiii List of Symbols and Abbreviations ....................................................... xviii Chapter 1 Use of solid biomass fuels for large scale power generation .......... 1 1.1 Introduction .................................................................................... 1 1.2 Renewable Energy......................................................................... 1 1.2.1 The demand for renewable energy ....................................... 1 1.2.2 UK renewable energy targets ..............................................

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