Microwave processing of lignocellulosic biomass for production of fuels Brigid Anne Lanigan Submitted for the degree of Doctor of Philosophy University of York Department of Chemistry 2010 Abstract Abstract Current environmental issues and resource demands are driving the global development of renewable energy. The work described in this thesis applies green and energy efficient microwave technology to transform lignocellulosic biomass into solid and liquid fuels suitable for application in coal burning power plants or upgrading into transportation fuels. Current thermochemical biofuel production (e.g. pyrolysis and gasification) suffer many drawbacks such as high energy consumption and poor flexibility. Herein, it is shown that by applying novel low temperature microwave processing, fuels can be produced at temperatures up to 190 oC lower than required in equivalent conventional thermal treatments. Studies on the microwave activation of the major components of biomass give insight into the mode of action. 180 oC was identified as the key temperature in the degradation of cellulose. Softening of the amorphous region of cellulose at this temperature enables microwave induced rearrangement increasing the efficiency of microwave interaction resulting in acid catalysed decomposition. It was shown possible to produce high calorific value chars at 150 oC lower than previously expected. A reduction of 100 oC was observed in the degradation temperature of hemicellulose. The technology is versatile, effective on a variety of biomass species, and has a favourable energy balance. In studies on whole biomass, the processing conditions and energy usage were found to be favourable when compared with conventional methods. Chars were produced at low temperatures with increased calorific values and material properties in parallel with high quality bio-oils. Pilot scale trials were also carried out proving the technology to be scalable and open to industrial application. This thesis shows for the first time the possibility to produce biofuels via microwave processing, while operating at temperatures below 300 oC. The impact of these findings is being further investigated at the dedicated microwave facility at the University of York. Contents Table of Contents Chapter 1: Introduction 1 1.1 Project Background .................................................................................. 2 1.2 Project Aims ............................................................................................. 3 1.3 Climate Change ........................................................................................ 4 1.4 Sustainable Development and Green Chemistry ..................................... 5 1.5 Biorefinery Concept ................................................................................. 7 1.6 Biomass composition ............................................................................... 8 1.7 Biomass for Energy Production ............................................................. 11 1.7.1 Biomass for liquid fuel ................................................................... 13 1.7.2 Biomass for solid fuel ..................................................................... 14 1.8 Biomass upgrading for energy production ............................................. 15 1.8.1 Thermochemical upgrading of biomass for energy production ..... 17 1.8.2 Gasification of biomass .................................................................. 18 1.8.3 Pyrolysis of biomass for liquid biofuel production ........................ 19 1.8.4 Torrefaction of biomass ................................................................. 21 1.9 Microwave Chemistry ............................................................................ 23 1.9.1 Microwave treatment of biomass ................................................... 25 1.9.2 Industrial application of microwave processes .............................. 28 1.10 Introduction to work in this thesis ....................................................... 28 1.11 References ............................................................................................ 30 Chapter 2: Microwave treatment of biomass components 33 2.1 Introduction ............................................................................................ 34 2.2 Composition of biomass ........................................................................ 34 2.2.1 Cellulose ......................................................................................... 35 2.2.2 Hemicellulose ................................................................................. 36 2.2.3 Lignin ............................................................................................. 37 2.3 Microwave treatment of biomass ........................................................... 38 2.4 Microwave treatment of cellulose .......................................................... 40 2.4.1 The “microwave-effect” ................................................................. 40 Contents 2.4.2 Effect of power and temperature on char formation ...................... 41 2.4.3 Cellulose char characterisation: Spectroscopic analysis .............. 44 2.4.4 Cellulose char characterisation: Elemental analysis .................... 49 2.4.5 Cellulose char characterisation: Thermal analysis ....................... 51 2.4.6 Cellulose char characterisation: Crystallinity ............................... 53 2.4.7 Isotope exchange studies ................................................................ 55 2.4.8 Discussion ...................................................................................... 56 2.5 Microwave treatment of hemicellulose .................................................. 59 2.5.1 Char formation ............................................................................... 59 2.5.2 Hemicellulose char characterisation: Spectroscopic analysis ...... 60 2.5.3 Structural analysis of hemicellulose: Elemental analysis .............. 62 2.5.4 Hemicellulose char characterisation: Thermal analysis ............... 62 2.5.5 Discussion ...................................................................................... 63 2.6 Microwave treatment of lignin .............................................................. 64 2.7 Conclusion ............................................................................................. 65 2.8 References .............................................................................................. 66 Chapter 3: Production of char through microwave treatment of biomass 69 3.1 Introduction ............................................................................................ 70 3.2 Process development .............................................................................. 70 3.3 Biomass for energy production .............................................................. 71 3.3.1 Dedicated energy crop: Miscanthus x giganteus ........................... 72 3.3.2 Agricultural residue: Wheat straw ................................................. 73 3.3.3 Industrial food waste: Cocoa husk ................................................. 74 3.4 Bio-char formation ................................................................................. 75 3.4.1 Properties of unprocessed biomass ................................................ 75 3.4.2 Bio-char from Miscanthus.............................................................. 76 3.4.3 Bio-char from wheat straw............................................................. 77 3.4.4 Bio-char from cocoa husk .............................................................. 80 3.5 Characterisation of bio-char ................................................................... 80 3.5.1 Miscanthus char ............................................................................. 81 3.5.2 Wheat straw char ........................................................................... 83 Contents 3.5.3 Cocoa husk char ............................................................................. 84 3.6 Fuel Properties ....................................................................................... 86 3.7 Conclusions & Future Work .................................................................. 88 3.8 References .............................................................................................. 90 Chapter 4: Production of bio-oil through microwave treatment of biomass 93 4.1 Introduction ............................................................................................ 94 4.2 Bio-oil from the microwave treatment of biomass ................................ 95 4.2.1 Experimental set-up ........................................................................ 96 4.2.2 Microwave treatment of wheat straw for production of bio-oil ....97 4.2.3 Properties of wheat straw bio-oil ................................................... 98 4.2.4 Composition of wheat straw microwave bio-oil ........................... 100 4.2.5 Influence of additives on microwave bio-oil ................................ 102 4.2.6 Influence of additives on bio-oil composition .............................. 104 4.3 Conclusions & Further work ................................................................ 109 4.5 References ............................................................................................ 111 Chapter 5: Short term studies 113 5.1 Introduction .........................................................................................