Emissions from Residential Solid Fuel Combustion and Implications for Air Quality and Climate Change
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Emissions from Residential Solid Fuel Combustion and Implications for Air Quality and Climate Change Edward John Sproston Mitchell Submitted in accordance with the requirements for the degree of Doctor of Philosophy The University of Leeds Doctoral Training Centre in Low Carbon Technologies School of Chemical and Process Engineering May 2017 Declaration The candidate confirms that the work submitted is his own, except where work which has formed part of jointly-authored publications has been included. The contribution of the candidate and the other authors to this work has been explicitly indicated in section 1.3. The candidate confirms that appropriate credit has been given within the thesis where reference has been made to 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 Edward J. S. Mitchell to be identified as Author of this work has been asserted by him in accordance with the Copyright, Designs and Patents Act 1988. © 2017 The University of Leeds and Edward J. S. Mitchell i Acknowledgements First and foremost I would like to thank my supervisors Professor Jenny Jones, Professor Alan Williams, Dr Amanda Lea-Langton and Professor Piers Forster for their support throughout my studies. Huge thanks to all of my co-authors and everyone in our research group, most notably Farooq Atiku, Dougie Philips, Ben Dooley, Patrick Mason, Yee-Sing Chin and Paula McNamee. Special thanks also go to Dr Rob Johnson and Arigna Fuels who kick-started the project and kept me on my toes in the first few months. Many thanks also to Dr Bijal Gudka and Dr Leilani Darvell for guidance and advice. I am very grateful to University of Leeds technical and laboratory staff too for their support and patience. They are Ed Woodhouse, Dr Simon Lloyd, Dr Adrian Cunliffe, Sara Dona, Karine Alves Thorne, Dave Instrell, Bob Harris and Sam Burdon. Not to forget LEMAS who were very helpful and supportive with my electron microscopy escapades. Many thanks also to the EPSRC Doctoral Training Centre in Low Carbon Technologies (EP/G036608), as well as the DTC centre managers, James McKay, Emily Bryan-Kinns, Rachel Brown and David Haynes for everything they’ve done for me over the past four years. Many thanks also go to colleagues in other departments, most notably Professor Dominic Spracklen and Mr Edward Butt in Earth & Environment. Thanks also to Jes Sig Andersen and Dr Guy Coulson for hosting me in Denmark and New Zealand where I learned an enormous amount about the topic. The Supergen Bioenergy Hub and SHARE network deserves a special mention due to their kind support, guidance and financial contributions (EP/J017302). Finally I would like to thank my family and friends, who have supported me every step of the way throughout my combined nine years at University, and who can finally stop asking ‘haven’t you finished yet?’ ii Abstract Small scale biomass burning stoves and boilers are growing in popularity in the UK and abroad, owing in part to renewable heat incentives and policies. However, combustion in domestic scale appliances is often inefficient and uncontrolled in comparison to larger systems, leading to high emissions factors of gaseous pollutants such as CO, NOx and PAH, as well as fine particulate matter (PM). Evidence is presented from 105 source apportionment studies from 31 developed countries showing that the impact of residential solid fuel (RSF) combustion on air quality is more wide spread that previously thought. Wood burning contributes to up to 95% of wintertime ambient PM in some rural communities in New Zealand, which is used as a case study throughout this work. Modelling work has shown that emissions from heating stoves may be underestimated in global climate models (GCMs) and UK residential wood consumption is forecast to increase by a factor of 14 between 1990 and 2030. By 2030, annual emissions of black carbon (BC) from UK wood stoves and fireplaces are predicted to exceed 3000 tonnes which is higher than the traffic sector. BC is the most important component of RSF radiative forcing, accounting for over 77% total warming effect. Model inventory and literature emission factors (EFs) for over 150 pollutants have been compared and contrasted, and compiled into a new RSF emissions inventory. Results are presented from experimental work on the emissions testing of a 6 kWth multi-fuel stove and three high quality cook stoves, burning a range of over 25 conventional and novel fuels including wood, coal, agricultural residues and torrefied wood briquettes. Despite a large resource of agricultural residues being available with lower EFs than open burning, their suitability as a residential solid fuel is uncertain. For example, straw briquettes had a measured density less than half that of wood logs and reed briquettes showed evidence of ash melting in the stove bed due to a high sodium, silica and chlorine content. It was found that PM and CO emissions were correlated to the content and composition of volatile matter within the fuel and NOx is linearly dependent on fuel nitrogen content. Mean whole cycle PM EFs ranged from 2.1 g kg-1 for wood logs to 4.2 g kg-1 for coal and 0.5 g k-1 for smokeless fuel. Torrefaction of wood has the potential to significantly reduce emissions, with PM EFs 49% lower than wood logs. However, emissions from all fuels were highly dependent on the duration of the flaming phase of combustion, during which EFs may be a factor of 5-9 higher than the smouldering phase. Heat treatment such as torrefaction removes 10-15% of volatiles, shortening the flaming phase and removing key species involved in the chemical soot formation pathways, which are discussed in detail. iii The physical and optical properties of collected particulate samples collected were also examined using electron microscopy and spectroscopy, which are useful for GCMs. Flaming phase particles had a high average EC/TC ratio (>0.9), a high carbon:oxygen ratio (93:5.4) and an Ångström Absorption Exponent (AAE) near 1 (0.9-1.2). After emission, it was found that particles undergo a significant increase in branching and oxygenation (C:O 88:10). The morphology of particles was also found to change following the injection of plasma into the flue, which is evaluated as a promising retrofit abatement technology. Graphical abstract Cross section of a wood log before being split and bagged. ‘To poke a wood fire is more solid enjoyment than almost anything else in the world’ Charles Dudley Warner iv Contents 1 Declaration .................................................................................................................................. i Acknowledgements .................................................................................................................... ii Abstract ..................................................................................................................................... iii Contents ..................................................................................................................................... v List of Figures ......................................................................................................................... viii List of Tables ............................................................................................................................ xi List of Abbreviations .............................................................................................................. xiii 1 Motivation and Background............................................................................................. 15 1.1 Context ..................................................................................................................... 16 1.2 Aims and Objectives ................................................................................................ 18 1.3 Thesis structure ........................................................................................................ 19 1.4 List of Publications .................................................................................................. 21 1.5 References ................................................................................................................ 22 2 Critical Literature Review ................................................................................................ 23 2.1 RSF Combustion: History, Policy and Practice ....................................................... 24 2.2 Pollutant Emissions and Formation ......................................................................... 27 2.2.1 Particulate matter ............................................................................................. 27 2.2.2 CO, NOx and SOx ............................................................................................. 35 2.2.3 Organic gaseous emissions .............................................................................. 36 2.2.4 Chlorinated emissions ...................................................................................... 39 2.3 Variation in Emission factors ................................................................................... 40 2.3.1 Impact of fuel properties on emissions ............................................................ 40 2.3.2 Impact of combustion conditions on emissions ............................................... 46 2.3.3 Emissions inventories .....................................................................................