Gasification Theory - Biomass Gasification, Pyrolysis and Torrefaction - page 199

Chapter 7
Gasification Theory
7.1 INTRODUCTION
The design and operation of a gasifier require an understanding of the gasifi-
cation process, its configuration, size, feedstock, and operating parameters
influence on the performance of the plant. A good comprehension of the
basic reactions is fundamental to the planning, design, operation, trouble-
shooting, and process improvement of a gasification plant, as is learning the
alphabet to read a topic. This chapter introduces the basics of the gasification
process through a discussion of the reactions involved and the kinetics of the
reactions with specific reference to biomass. It also explains how this knowl-
edge can be used to develop a mathematical model of the gasification
process.
7.2 GASIFICATION REACTIONS AND STEPS
Gasification is the conversion of solid or liquid feedstock into useful and
convenient gaseous fuel or chemical feedstock that can be burned to release
energy or used for production of value-added chemicals.
Gasification and combustion are two closely related thermochemical pro-
cesses, but there is an important difference between them. Gasification packs
energy into chemical bonds in the product gas; combustion breaks those
bonds to release the energy. The gasification process adds hydrogen to and
strips carbon away from the hydrocarbon feedstock to produce gases with a
higher hydrogen-to-carbon (H/C) ratio, while combustion oxidizes the hydro-
gen and carbon into water and carbon dioxide, respectively.
A typical biomass gasification process may include the following steps:
Drying
Thermal decomposition or pyrolysis
Partial combustion of some gases, vapors, and char
Gasification of decomposed products
Pyrolysis as explained in Chapter 5 is a thermal decomposition process
that occurs in absence of any medium. Gasification, on the other hand,
requires a gasifying medium like steam, air, or oxygen to rearrange the
molecular structure of the feedstock in order to convert the solid feedstock