Copyright is owned by the Author of the thesis. Permission is given for a copy to be downloaded by an individual for the purpose of research and private study only. The thesis may not be reproduced elsewhere without the permission of the Author. THE AIR GASIFICATION OF WOODY BIOMASS FROM SHORT ROTATION FORESTS Opportunities for small scale biomass-electricity systems A thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Agricultural Engineering at Massey University New Zealand KINGIRI A. SENELWA 1997 ABSTRACT Downdraft gasification of short rotation forestry (SRF) biomass was investigated to identify opportunities for small scale biomass-electricity systems. Case studies were conducted in Kenya to identify these opportunities by (i) defining the energy demand and supply structure to identify markets; (ii) evaluating the biomass resources available; and (iii) identifying the availability of other facilities required for such a system. At the same time, the yield potential of 1 2 SRF species, planted in Palmerston North, New Zealand in small plots at 3470 stems/ha a nd harvested after 2, 3, 4 and 5 years was evaluated. Samples were collected from each species to determine their energy properties. Data on tree growth, yield, and biomass properties were used to develop two multi-objective indices - the relative yield index (RYI) and the fuel wood value index (FVI) for evaluating SRF species. Biomass from the 4 year rotation harvest was used as feedstock to fuel a downdraft air gasifier rated 35 kW (electric). Feedstock gasification processes, gas quantity and quality were correlated with the biomass properties to define the characteristics of a good fuel wood species for gasification purposes. The Kenyan studies highlighted constraints in the energy sector and identified opportunities for new bioenergy technologies. Small scale biomass gasification systems showed potential but suitable sites were restricted to sawmills where processing residues could be used as gasifier feedstock. Field trials of SRF systems were recommended to evaluate tree species over different silvicultural treatments, and to intensify biomass production. A demonstration plant at one of the bigger sawmills was recommended to stimulate interest among investors. Species yields of the trial plantings in New Zealand in the 1 2 species assessed ranged from 6 ODtihaly for Alnus glutinosa to 73 ODtihaly for Eucalyptus globulus at 5 year rotations. A stocking density trial of E.saligna showed that 3 ,500 stems/ha managed on 4-5 year rotations provided the highest yields. Though these yields may not be achieved in field plantings or in Kenya, the study demonstrated the feasibility and methodology that could be applied. Like yield, the bioenergy properties varied between species. Higher heating values ranged from 19.6-20.5 MJ/kg for wood, 1 7 . 8-20.6 MJ/kg for bark, and 1 9 .5-24. 1 MJlkg for leaves. Gas yields varied between 1 . 88-2 .89 gIg dry wood due mainly to moisture content variations which also affected the composition of the gas. Gas heating values varied from 4.602 to 6. 1 12 MJlNm3, and were considered to be of sufficient quality to fuel internal combustion engines. Both RYI and FVI showed that yield factors outweighed bioenergy properties when identifying a good fuel wood species. The large differences in yields indicated the benefits that could be achieved by selecting appropriate species for a specific region. Although feedstock properties affected the gasification processes and products, their overall influence was not statistically significant. The inclusion of bark in the feedstock did not adversely affect the suitability of the feedstock. ii EXECUTIVE SUMMARY The energy supply and utilisation structure of many developing countries is constrained. Since the development of national economies is linked to the availability of appropriate energy resources, it is important that alternative energy supplies to fossil fuels that are environmentally friendly, economically sound, and those that will blend with the existing energy supply and utilisation technologies are identified. One such energy option is biomass when converted by thermal gasification for electricity generation. A successful application of this option requires (i) the identification of appropriate biomass energy applications and technologies; (ii) the identification of biomass resources to be supplied on a sustainable basis; (iii) the definition of biomass feedstock quality requirements for the selected technology; and (iv) the determination of the optimum operating conditions for high quality products. These were all covered in this research programme. A case study was conducted in Kenya to identify the opportunities for modern biomass energy technologies like gasification for small scale biomass-electricity systems. At the same time, field trials of short rotation forestry (SRF) at Massey University, New Zealand were conducted to evaluate a selected range of tree species and silvicultural treatments (based on growth and yield). Samples were collected from each of the species grown under a range of silvicultural treatments and tested in the laboratory to characterise their energy properties. The data on growth, yield, and energy properties obtained were used to develop two multi­ objective indices used to evaluate SRF species - the relative yield index (RYI) and the fuelwood value index (FVI). Samples of biomass harvested at the age of four years from nine of the twelve SRF species were processed and used as feedstock in a 35 kW (electric) downdraft gasifier. The gas produced under a controlled set of conditions was analysed and correlated with the laboratory feedstock characteristics to determine any differences in the fuelwood species. In Kenya, fuelwood supplies more than 70% of the total national energy requirements. Oil provides 22-26%; coal 1 %; and electricity (hydro, thermal, geothermal and imports) 2-4%, being 82 1 and 672 MW (installed and effective capacity, respectively). Recommendations for the development and use of more renewable energy sources were made based on economi�, environmental and feasibility considerations, and favoured sustainable utilisation of biomass iii in modern conversion technologies like gasification for electricity production. However, the standing tree stock as assessed on private farms in the densely populated rural areas (0.7-4.6 m3/household) would be insufficient to supply such technologies for rural areas. The location of such plants is therefore limited to sawmills processing more than 720 tly of round wood and generating more than 1 t/day of solid residues (including slabs and offcuts but excluding sawdust). This quantity of residues could be supplied to a gasifier to generate on an annual basis more than 0.3 GWh (electric) and almost 0.6 GWh of heat in a co-generation plant. The 73 medium to large scale sawmills in Kenya have the potential to generate more than 100,000 t of solid residues per year with a producer gas potential of 221 x 1 06 normal cubic metres per year (Nm3/y), equivalent to 24,000 t of oil. This gas could be used to generate up to 76 GWh (electric) plus another 14 1 GWh of heat per year. A demonstration plant at any of the 73 sawmills was recommended to illustrate the potential of the technology. Also, field trials were recommended to (i) evaluate a range of high yielding SRF species suited to specific regions; (ii) evaluate a range of silvicultural treatments suited to the production of large quantities of high quality woody biomass; and (iii) intensify woody biomass production to supply gasifier installations in rural communities. Of the 1 2 SRF species evaluated in New Zealand, being planted at 3470 stems/ha in small plots and harvested at ;3, 4 and 5 year rotations, Eucalyptus globulus had the highest yields of 73 oven dry tonnes per hectare per year (ODt/haly), while E.nitens and Acacia dealbata had 59 and 49 ODt/haly respectively in the 5 year rotations. The lowest yielding were Eucalyptus sa ligna « 1 2 ODt/haly); Salix matsudana x alba ( 1 002), « 1 0 ODtlhaly); Alnus glutinosa, « 6 ODt/haly); and Paulownia tomentosa, « 7 ODtihaly) in all three rotations. Although the longer rotations had higher yields, the current annual increment (CAl) for most species was decreasing and approaching the mean annual increment (MAl) for the 5 year rotation. This indicated that the plots were nearing the optimum rotation length. In a "Neider" radial design trial of E.saligna, the 4-5 year rotations at a stocking density of about 3500 sternlha provided the optimum growing conditions for SRF systems in the �anawatu region of New Zealand. Results of the coppicing trial were not conclusive. A series of biomass properties were recorded - the harvest index, proportion of bark on the stem, basic density, fixed carbon content, extractives content and heating values. These iv varied significantly between species, and among the tree components tested (wood, bark and leaves) . Most properties did not vary significantly with different silvicultural treatments except for the proportion of bark which declined with cutting age, and the wood basic density which increased. B iomass properties did not vary significantly with sampling height up the stern. Higher heating values (HHV) ranged from 1 9.6-20.5 MJlkg for wood, 1 7. 8 -20.6 MJlkg for bark, and 1 9 .5-24 . 1 MJ/kg for leaves. The highest HHV (20.5 MJ/kg for wood and 2 0.6 MJ/kg for bark) was obtained from Pinus radiata, the only softwood tested. The different properties were correlated mdicating that the quality of biomass used as feedstock was defined by most of the properties . Therefore, each property of the potential feedstock must be considered when formulating guidelines for the design of biomass energy conversion equipment. Multi-objective techniques incorporating measured yields and energy properties for the range of species showed that yield factors outweighed the energy characteristics when identifying a good fuelwood species. The large differences in species yield indicated the large gains in terms of GJlhaly to be made: by selecting the most appropriate species for a specific region. Assuming that the average yields (27-39 and 4 1 -55 ODt/haly of stemwood and total biomass respectively) from the 4 best species over the three rotations would be achievable under Kenyan growing conditions, it was estimated that a gasifier suitable to produce electricity and hot water for a village community would require a minimum of 1 0 hectares of SRF harvested (2.5 halyear), to produce about 0. 5 t of dry feedstock per day. The processes and products of downdraft air gasification were influenced by (i) gasification temperature (ii) the equivalence ratio (ER) defined by the quantity of air used; and (iii) the feedstock moisture content. High temperatures (above l000°C) produced larger quantities of gas with high heating values. The ER values of 0. 1 95 -0.328 (with an average of 0.250) were less than the optimum value of 0. 275 and indicated that air tlow was not optimum for most runs, resulting in insufficient gasification reactions (tending towards pyrolysis), and lower gas yields. High feedstock moisture contents reduced the reaction temperatures; altered the optimum temperature profiles; reduced the available feedstock substrate through hydrolysis, thereby reducing the quantity of gas achievable; and produced a gas with high moisture content, high CO2 and N2 v content, but reduced CO and ClL content with consequently lower heating value. Although feedstock properties affected the gasification processes and products, their overall influence was not statistically significant. Similarly, the inclusion of bark in the feedstock did not adversely affect the suitability of the feedstock when compared to samples without bark. Downdraft alf gasification of biomass produced 1.88-2.89 grams of gas per gram of feedstock (equivalent to 2. 1-3 . 0 m3lkg of dry feedstock) . The gas composition varied but typically contained 51-62% nitrogen; ] 9-26% carbon monoxide; 8-13% hydrogen; 7.5 -10.6% carbon dioxide; and 1.8 -2.5% methane. The gas heating value varied from 4.602 to 6.112 MJINm3, and the heating value of the stoichiometric gas-air mixture ranged from 2.241 to 2.524 MJINm3 for air dry and oven dry feedstocks. The gas composition and heating values were considered to be of sufficient qUality to fuel internal combustion engines. The quantity of solid residues (and ash particulates) produced was not sensitive to the species or feedstock properties, reaction temperature, equivalence ratio (air supply) or the feedstock moisture content. The quantity of liquid residues (condensate) generated being 20-80 glkg of dry gas from air dry feedstock and oven dry feedstock decreased with increasing gasification temperatures and increased with increasing feedstock moisture contents. The quantity of condensate was however not sensitive to the species or to the equivalence ratio. The pH, COD, electrical conductivity and turbidity of the condensate showed that it would be unsafe to dispose of the residue into water-ways or onto land without pre-disposal treatment. An initial step would be to frlter the residue through the solid residues collected from both the ash port and from the cyclone. The study demonstrated methodologies for ( i) identifying and evaluating opportunities for biomass to electricity systems in developing countries, and in isolated remote regions of developed countries; and ( ii) evaluating SRF species for fuelwood requirements using multi­ objective indices - the relative yield index (RYI) and the fuelwood value index (FVI). The methodologies may be applied to other regions of the world. In particular, the indices could be used to evaluate SRF species grown under Kenyan conditions. The species which performed well under the Manawatu conditions, (E.glabulus, E.nitens, A.dealbata and E.avata), and others which are known to grow well under tropical conditions should be evaluated alongside high yielding local species. vi Commercial gasifiers need to be compared and one selected for a demonstration to stimulate the concept in Kenya. A financial and economic evaluation of the demonstration plant would not necessarily indicate the true economic and environmental values of the venture as a learning process will be involved. However, given that the project wou ld be in the experimental / demonstration stages, it would enable a full cost - benefit analysis to be undertaken to provide useful data which would be accurate enough to be used when comparing electricity generat ion from SRF crops and sawmill residues with other electricity generating technologies such as traditional fossil fuel fired power plants, or even photo voltaic, solar, wind, or diesel generator sets. DEDICATION This Thesis is dedicated to my mother, posthumously, The Late t MAMA PAULINE MUHONJA KINGIRI t You got so close, yet (i) a few weeks shy of realising your own work ( ii) many years shy of reaping the full benefits of your long, hard and painful struggle ( iii) capable of much more, had you had the opportunities you gave us vii " . . . . . . the long hours of sleep after finishing school will never have the meaning you meant". ��� �49ddi Ht«I(9«fu t tY!JA!J the Almi9ht!J qoll rest her soul in eternAl peAee t viii ACKNOWLEDGEMENTS I would like to express my sincere gratitude to my Chief Supervisor, Associate Professor Ralph E.H. S ims, for his inspiration, guidance, thoughtful advice, kind assistance and support at every stage of the study. The friendly "atmosphere" during our discussions was a great asset. I could not have had better, and for all this, I am particularly grateful. My sincere thanks and appreciation are also due to Professor Gavin L. Wall , my Co-Supervisor for his inspiration and invaluable advice during the course of the study. I am also grateful to the entire staff of the Department of Agricultural Engineering. In particular, I want to thank Leo Bolter, Gerard Harrigan, Dexter McGhie, Russel Watson and Ian Painter. I doubt they have had a more nagging student before. Mrs. Helen Harker, Irene, and all the others were wonderful. The assistance by Tavale Maiava and Andrew is noted. Thanks are also due to the following who contributed immensely to the success of this study: Mr. Doug Williams, Director, Fluidyne Gasification, Auckland, New Zealand was only a phone call away for the much appreciated technical advice on the operation of the gasifier. The frequent advice, based on his long standing expertise and experience in the manufacture and operation of gasifiers was an asset. The advice with the statistical analysis from Jenny Edwards (Computing S ervices) and Dr. S iva Ganesh (Department of S tatistics) was appreciated. Ndugu Ndibalema Alphonce read through the draft script. The comments were invaluable. The M inistry of External Relations and Trade, Government of New Zealand awarded me the fees scholarShip which enabled me to come to New Zealand; Massey University awarded the Doctoral Scholarship; Massey University Graduate Research Fund provided funding for some of the experiments; and AGMARDT funded the purchase of the gasifier. I was also fortunate to receive a Helen E Akers postgraduate scholarship when all seemed to be "falling apart". Without these financial backing, I would never have accomplished the work. Thank you all . ix Moi University, Eldoret, Kenya is acknowledged for the grant of study leave to undertake this study; while the Moi University Deans Committee is acknowledged for partly funding the field work in Kenya. I am also grateful to the Department of Wood Science and Technology, Moi University, and all the staff for the support. In particu lar, Professor W.N. Ringo, and Mr. R.N. Mugweru were immensely helpful . Mr J. Musisi (formally Head, MU Library Services), Professor B .M . Khaemba (MU), and Prof. Maleche (formerly Deputy Vice Chancellor, MU) for the initial inspiration. Professor David O. Hall (Kings' College, University of London) provided the foundation for the work on biomass energy. S .G. Abbas (Guzni) bhai, and bhabee Naheed, have been more than a big brother. Guo, Hamish and al l the others in the Department for everything. Others who have been invaluab le include Mr & Mrs F . & B . M'pelasoka and family, Mr. & Mrs. Alphonce & C.M. Ndibalema and family. Dr. & Mrs . Edward & Judy Wasige, Bryan and Claudia have been of immense support even though they were far away in Kassel , Germany. To the many other friends, I am grateful for the moral and or material support. I ran the risk of forgetting that I was 20,000 km away from home - thanks to Cathy S ims. There is a special place for the family, wide for an African, and expansive for a Luhya. I am indebted to my late mother, Mrs. Pauline Muhonja, for the constant prayers, inspiration, and all the opportunities she availed to me. I am also indebted to my father, Mr. Jonathan Kingiri Luvusi Bayi for being there. My brothers Luvusi H.K. , Ijusa E.K. , Luvusi P.K. , and Muganagani M.K; and my sisters Vuhugwa M'mbone B . , Kang'ahi Andisi J. and (the Late Kageha Rose); and all their families are acknowledged for the support, encouragement, and for taking care of home while I have been away. Ijusa gave up much more for the sake of his younger brothers and sisters. I trust all could have done better. The challenge to achieve greater is left to "Visukulu va Baulina na Bayi". Finally, I want to specifically thank Pamela M. Ingutia for being that patient under the "strange" work routines I adopted. Thanks too for the encouragement, understanding, and everything else. Last, but by no means least, Denue Kingiri (Mtoto) provided the much needed "distraction", and added the all important fresh"spice" into everything in the family. ITEM Abstract Executive summary Dedication Acknowledgements Table of contents List o f tables List o f figures List of appendices TABLE OF CONTENTS CHAPTER ONE: GENERAL INTRODUCTION 1 . 1 S tatement of the problem I .2 Objectives 1 .3 Justification 1 .4 Preview of chapters CHAPTER TWO: SMALL-SCALE BIOMASS-ELECTRICITY SYSTEMS IN KENYA 2. 1 Introduction 2. 1 . 1 The energy sector 2. 1 .2 Electricity potential, generation and use in Kenya 2. 1 .3 The rural electrification programme in Kenya 2 . 1 .4 Biomass energy options for Kenya 2. 1 .5 Objectives 2.2 Methodology 2.2 . 1 The forestry resource 2 .2.2 The household survey 2.2 .3 Wood processing residues 2.3 Results and discussion x PAGE II VB Vlll X X V III XX XXll 3 4 4 5 5 5 7 1 0 I 1 1 3 14 1 4 1 4 1 5 1 6 2 .3. 1 Woody biomass resources Inventory of the forest resource Procurement and utilisation of biomass in rural areas Socio-cultural factors in tree growing and utilisation activities Farm and short rotation forestry configurations Wood processing residues 2 .3 .2 Producer gas potential Potential from gazetted forests and from farm forestry activities Potential from wood processing residues 2 .3 . 3 Small-scale biomass electricity systems Demonstration unit requirements 2.4. Conclusions CHAPTER THREE: SHORT ROTATION FORESTRY SPECIES EVALUATION Preamble 3 . 1 Introduction 3 .1. 1 Objectives 3.2 Biomass yields and assessments 3 .2 . 1 Yield in short rotation forestry (SRF) 3.2.2 Influence of si lvicultural treatments on biomass production Stocking density Rotation length and periodic growth pattern in trees Coppicing 3.2 .3 SRF species evaluation - a multi-objective approach 3 .2.4 Tree weight equations in short rotation forestry 3 . 3 Materials and methods 3 .3 .1 Evaluation of SRF species for b iomass production 3 .3 .2 Growth patterns in juveni le trees 3 .3 .3 Silvicultural treatments - "NeIder" (radial) trial xi 16 16 16 19 2 0 21 21 22 22 23 25 28 30 3 0 3 0 3 1 32 32 34 34 37 39 40 41 42 42 44 44 xii 3. 3.4 Sampling and tree measurements 46 3. 3.5 AnaJysis of results 47 3.3.6 Multi-objective evaluation of SRF species for biomass production 48 3.4 Results 52 3.4 . 1 Short rotation forestry species growth and yield 52 Species evaluation 52 Growth patterns in juvenile trees 55 3.4 .2 Effect of s i lv icultural treatments 5 7 Coppice vs. establishment crop yields 59 Stocking density, rotation length, coppicing, and yield relationships 59 Yield optimisation 6 1 3.4 . 3 The Relative yield index and multi-objective evaluation of SRF species 6 1 3.4.4 Yield estimation in short rotation forestry 64 3.5 Discussion 65 3.5 . 1 Growth and Yield in SRF species 66 Species periodic growth and the influence of s tocking density 70 3.5 .2 The influence of si lvicultural treatments 72 Stocking density 72 Rotation length 73 Coppicing 74 3.5 . 3 Yield optimisation 75 3.5.4 SRF species selection and The Relative Yield Index CRYI) 76 3.5 .5 SRF systems for a Village based Gasifier plant 77 3.5 .6 Yield equations in SRF systems 78 3.6 Conclusi ons and recommendations 80 CHAPTER FOUR: ENERGY CHARACTERISTICS OF WOODY BIOMASS 82 4 . 1 Introduction 82 4. 1 . ] Objectives 83 4 . 2 Fuelwood characteristics 4. 2. 1 Energy properties of biomass materials The proportion of bark on the stem (bark : wood ratio) Basic density Moisture in biomass materials Ash Volatile matter Fixed carbon Extractives Heating value 4. 2. 2 Influence of silvicultural treatments on biomass energy properties 4. 2. 3 Interaction of properties and fuelwood quality 4. 2.4 The fuelwood value index and species selection 4. 2. 5 Summary 4 .3 Materials and methods 4.3 . 1 Experimental variables 4.3 . 2 Sampling 4.3 .3 Laboratory procedures 4.3 .4 Analysis of results 4.3 . 5 Fuelwood value index and species selection 4.4 Results 4.4 . 1 Experiment A: Species variability 4.4. 2 Experiment B: The influence of silvicultural treatments "Neider" radial trial stock 4.4.3 Experiment C: Tree parts and components 4.4.4 Experiment D: Interaction of properties and fuelwood quality Statistical correlation analyses Influence of extractives - laboratory analysis Influence of moisture content - laboratory analysis Influence of particle size - laboratory analysis xiii 84 84 84 8 5 85 86 8 7 88 89 90 92 94 95 96 97 97 99 99 101 101 103 103 1 04 105 1 08 109 109 1 10 1 1 1 III 4.4.5 Regression analysis between properties Heating values 4.4.6 The fuel wood value index (FYI) Assumptions 4.5 Discussion 4.5 . 1 Energy properties of biomass materials Proportion of bark on the stem (Bark: wood ratio) Basic density Moisture in biomass materials Ash Volatile matter and fixed carbon contents Extractives Heating value 4.5 .2 Influence of silvicultural treatments on properties 4.5 .3 Interaction of properties and fuelwood quality Influence of moisture content on heating values Influence of extractives content on ash content, HHV and VM Effect of particle size on b iomass proximate analysis Regression models 4.5.4 The fuelwood value index and species selection 4.6 Conclusion CHAPTER FIVE: THE AIR GASIFICATION OF SRF WOODY BIOMASS 5 . 1 Introduction 5. 1 . 1 Historical overview 5.1 .2 Gasification processes 5. 1 .3 Thermal gasification of woody biomass 5. 1 .4 Objectives 5.2 Woody biomass gasification xiv 1 1 2 1 1 3 1 1 4 1 1 5 1 1 7 1 1 7 1 1 8 1 1 9 1 2 1 1 2 1 1 22 1 23 1 24 1 26 1 26 1 27 1 27 1 29 1 30 1 30 1 33 1 35 1 35 1 35 1 36 1 37 ] 39 1 40 5.2.1 Process ofthennal gasification of biomass - a theoretical background Heating and Drying Primary pyrolysis Secondary gasification reactions 5.2.2 Products of biomass gasification Producer gas Liquid residues 5 .2.3 Mass and energy balances 5.2.4 Feedstock for thermal gasification 5.2.5 Effect of feedstock properties Basic density Particle size distribution and bulk density Ash content Volatile matter content (VM) Feedstock charring and reactivity properties Feedstock moisture content 5.2.6 The role of air 5.2.7 The role of high temperature Temperature profiles 5.2.8 Sample (mass) loading 5.2.9 Gas residence time 5 . 3 Materials and methods 5 .3.1 Instrumentation and set-up Data- logging system 5.3 .2 Test materials 5.3.3 Experimental variables and set-up of the tests 5.3.4 Operation of the gasifier and general measurements 5 .3.5 Product gas sampling and analysis 5 .3.6 Liquid residues sampling and analysis 5 .3.7 Solid residues sampling 5 .3.8 Theory, definitions and derivations xv 140 141 142 144 145 145 146 147 148 149 150 150 1 51 lSI 152 152 153 154 155 156 156 157 157 16 1 16 1 162 163 164 164 165 165 5 .3 .9 Analysis of results 5 .4 Results 5 .4. 1 Processes and products of gasification Material flows and products of downdraft air gasification Product gas Temperature Mass loading and rate of gasification Interaction of the process 5.4 .2 The influence of feedstock type - tree species Material flows and products distribution Gas composition and quality Process efficiency Interaction in gasification parameters 5.4.3 Influence of feedstock properties 5 .4.4 Influence of feedstock moisture content Gas composition and quality 5.4.5 Particle size distribution and bulk density 5 .4.6 Air flows and the equivalence ratio (ER) 5.4.7 Temperature Peak temperature Temperature fields 5 .4.8 Solid residues 5.4.9 Liquid residues (condensate) 5 .5 Discussion 5.5 . 1 Downdraft air gasification of SRF woody biomass Ga.<;ifier operating conditions Sample (mass) loading 5.5 .2 Products of biomass gasification Product gas Solid residues Liquid residues xvi 1 68 1 69 1 70 1 7 1 1 72 1 74 1 77 1 77 1 78 1 80 1 8 1 1 83 1 83 1 84 1 86 1 92 1 94 1 94 1 96 1 96 1 98 202 202 206 206 207 2 1 0 2 1 0 2 1 0 2 1 2 2 1 2 5.5.3 Mass and energy balances Energy balances 5.5.4 Energy conversion efficiencies 5.5.5 Feedstock characteristics Basic density Ash Volatile matter content and fixed carbon content Extractives Feedstock heating value Interactive effects 5.5.6 Tree species 5.5.7 Influence of feed particle size distribution and bulk density 5.5.8 The role of high temperature Variations in temperature 5.5.9 Air flow and equivalence ratios 5.5.10 Influence of feedstock moisture content 5.5.11 SRF species for downdraft gasification 5.6 Conclusions CHAPTER SIX: OVERVIEW DISCUSSION AND CONCLUSIONS 6.1 Identification of bioenergy application options 6.2 Feedstock supply options - short rotation forestry species evaluation 6.3 The gasification of SRF biomass 6.4 Short rotation forestry, gasification, and biomass-electricity systems LIST OF REFERENCES '- .. APPENDICES xvii 214 215 215 216 216 216 217 218 218 219 219 221 223 225 226 228 230 231 233 234 235 236 237 239 266 Table 2.1 2.2 2.3 3.1 3.2 3.3 3.4 3.5 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 5.1 5.2 5.3 5.4 5.5 5.6 5.7 LIST OF TABLES Energy end-use by sector and source for selected years ('000 toe) Generation and utilisation of electricity for selected periods The rural household economy and energy use in Vihiga/Kakamega region Mean plot and tree characteristics at 3, 4 and 5 years old Short rotation forestry species biomass production in age series p lots Influence of stocking density on early tree growth and yield in 4 species to 2 years SRF species and rotation lengths relative yield indices Relative y ield indices of the establishment harvest of E.saligna planted in a "Neider" radial trial Fuelwood properties of 3 year o ld SRF tree species The influence of rotation length on selected physical properties of SRF species Variability within tree parts and components in 5 SRF species Correlation analysis between properties The influence of extractives on ash, volatile matter and heat values on selected samples The influence of particle size on the proximate analysis of wood from 3 SRF species MUltiple regression models relating bioenergy properties Regression models relating HHV and other biomass properties Species fuelwood value index (FYI) and ranking The main reactions in biomass gasification Product gas from the air gasification of different materials Gasifier feedstock and wood chips properties Summary of the process and products of downdraft air gasification (An example of a sample run using oven dried P.tomentosa feedstock) Process variables in the gasification of SRF species (air dry or oven dry, and with or without bark) Gas analysis and process efficiencies from the gasification of SRF species (air dry or oven dry, and with or without bark) Correlation between feedstock properties and the processes and products of gasification xviii Page 7 8 17 53 54 56 62 63 103 104 108 109 110 112 113 114 116 144 146 169 176 172 182 184 xix 5.8 The effect of feedstock moisture content on downdraft gasification processes 190 5.9 The effect of feedstock moisture content on product gas composition, quality and gasification efficiencies 190 5.10 Maximum recorded temperatures from each run 197 5.1 1 Analysis of collected liquid residues (condensate) 205 5. 12 Theoretical products distribution in thermochemical processing of b iomass 209 Figure 2.1 2.2 2.3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 LIST OF FIGURES Trends in total energy supply in Kenya (Mtoe) The Kenyan grid network (and the rural household survey region) B iomass-electricity systems demonstration unit requirements Reported maximum tree species yield data from different regions of the world I l lustration of the influence of stocking density on tree size and yield in SRF Determination of optimum rotation age (MAl/CAl curves) Species trials plots before the 3 year rotation sampling The "NeIder" (radial) design trial showing sub-plots of 2-5 year rotations The "NeIder" radial trial after the first harvest Sampling of fresh weight in the field Comparison of yields (MAl and CAl) in 12 SRF species over the 3 rotations SRF species height and diameter growths at 6,940 and 16,260 stems/ha 3.1 0 Tree characteristics in the establ ishment rotation of "NeIder" trial of Eucalyptus sa ligna xx Page 6 9 26 33 36 38 43 45 45 47 54 56 57 3.1 1 Effect of stocking density and cutting age on yield (MAl and CAl) 58 3.1 2 Establishment and coppice yields (MAl) harvested at 2, 3, 4 and 5 year rotations in a "NeIder" design trial of E.saligna 60 3.1 3 Ophelimus eucalypti ( leaf gal l wasp) infestation on E. saligna 68 3.14 Caterpi l lars of the emperor gum moths on Eucalyptus species 69 4. I The influence of rotation length, stocking density and coppicing on wood properties in a "NeIder" radial coppicing trial of E.saligna 107 4.2 Relationships between heat value and moisture content 111 5.1 Major types of Gasifier reactors 137 5.2 Representation of the biomass gasification mechanisms 140 5.3 Mechanisms of biomass pyrolysis 143 5.4 Vertical cross section the Fluidyne downdraft gasifier 157 5.5 Position of the high temperature probe in the gasifier 158 5.6 Gasifier and product gas clean-up and sampling set-up 159 5.7 The Fluidyne Gas-engine generator rig 160 xxi 5.8 Arrangement of the sieve trays 162 5.9 Example of material movements in a down draft gasifier 171 5.10 Example of variation in gas composition and heating value during a run of one hopper batch of feedstock 173 5.11 Example of a profile of temperature in a down draft gasifier 175 5. 12 Materials and products distribution in oven dry and air dry feedstocks 180 5.13 The effect of feed moisture content on gasification of S.kinuyanagi with bark 187 5. 14 The effect of feed moisture content on gasification temperature of S.kinuyanagi 188 5. 15 The influence of feedstock moisture content on gas heat values 192 5. 16 The influence of feedstock moisture content on operating conditions 193 5.1 7 The effect of feedstock moisture content on product gas quality 193 5.18 The effect of air flows on gas yield and heat values 195 5. 19 The effect of equivalence ratio (ER) on gas quality 195 5.20 The influence of temperature on input and output flows 198 5.21 Temperature fields in the Fluidyne downdraft gasifier 199 5.22 Top view of an un-disturbed oxidation/combustion zone within the gasifier 200 5.23 The influence of maximum gasifier temperature on gas yield and quality 201 6. 1 Summary of the main phases necessary for implementation of the b iomass gasification option for electricity generat ion 233 LIST OF APPENDICES Appendix 2. 1 2.2 3.1 3.2 4. 1 5 . 1 5.2 5.3 SA 5.5 R. l Rural households survey questionnaire Medium-large scale sawmills annual residues, gas and electricity production potential The influence of stocking density (stems/ha) and rotation length ( years) on tree size and yield in a "NeIder" radial trial of E.saligna Comparison of fIrst rotation and coppice yields (ODt/haly) and the influence of stocking density (stems/ha) and rotation length (years) in a "NeIder" radial trial of E.saligna Effect of stocking density, cutting age and coppicing on wood properties of 6 year o ld rootstock trees of E.saligna planted in a "NeIder" radial t rial Correlations between measured gasifIcation process variables over the time of a batch fuel wood conversion process (OD P. tomentosa) Correlations and interaction between processes in downdraft gasifIcation of SRF species woody biomass (All tests) The influence of particle size distribution and bulk density on down draft gasification processes and products The influence of feedstock particle size distribution on gas composition and gasification ratios Correlation between properties of liquid condensates and other gasification parameters Proof reading copy of paper to the Journal Biomass and Bioenergy: Senelwa K. and R.E.H. Sims ( 1997 a) Tree biomass equations for short rotation eucalypts grown in New Zealand. xxii Page 267 269 270 271 272 273 274 275 275 276 277