Biomass Pyrolysis and Gasifier Designs
The biomass gasification process is an alternative approach for producing ethanol from lignocellulosic biomass. This method involves controlled burning of biomass to produce synthesis gas, or syngas, and then conversion of syngas to ethanol. Part 3 (Chapters 11-13) of this book is devoted to this approach. This chapter is an introduction to this process, where chemistry of syngas formation and gasifier designs that are used to make syngas from biomass is discussed. Syngas produced from biomass can be converted to ethanol by two alternative methods, as shown in Figure 11.1. The first method is to use microorganisms as biocatalysts to convert syngas to ethanol, and this approach is presented in Chapter 12. The second method is to use metal-based chemical catalysts to produce ethanol from syngas. This approach is presented in Chapter 13 of this book. One of the principal differences in this gasification approach and the aqueous-phase biomass hydrolysis process is that in theory all the carbon in cellulose, hemicellulose, and lignin can be transformed to carbon in ethanol in the gasification method, whereas in
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Figure 11.1 Biomass gasification, then syngas to ethanol conversion process. |
the aqueous phase route only cellulose and hemicellulose are used in making ethanol; lignin is separated as a byproduct.
Any form of biomass rich in carbon-like agricultural waste, forest residues, municipal wastes dedicated energy crops, and grasses can be converted to syngas by controlled burning in a gasifier. In fact, syngas technologies were originally developed as far back as the early 1800s for conversion of coal to syngas, and syngas can be used directly as a fuel in internal combustion engines and gas turbines as well [1].
Direct gasification of biomass with air produces a syngas of heating value in the range of 4-12 MJ/m3 [2,3]. Low energy density and difficulties in storage are the major drawbacks in using syngas directly as a fuel, however, the conversion of syngas to liquid fuels with high energy density and easy storage is an attractive proposition. There are a number of approaches for conversion of syngas mixture to liquid fuels like the Fisher-Tropsch method for conversion to gasoline, diesel-like hydrocarbon fuels as well as conversion to alcohol and ether-type oxygenated fuels. This chapter will focus on the production and properties of biomass-derived synthesis gas.