Page 109 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

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Chapter 4

Torrefaction

4.1 INTRODUCTION

Biomass can provide a full range of convenient feedstock for energy, metal-
lurgical and chemical industries. This feedstock can be in the form of solid,
liquid, or gases. Production of solid fuels from biomass using carbonization
has been practiced for many thousands of years. It provided early people
with charcoal, the first convenient solid fuel as well as a feedstock for iron
extraction at a later date. The art of torrefaction (French word for “roasting”)
has been used in a host of industries for tea and coffee making, but only in
recent time, it has caught the attention of power industries for the production
of a coal substitute from biomass. Torrefaction is often called a pretreatment
process as it prepares biomass for further use instead of direct use in its raw
form. Torrefied biomass finds use in fields such as:
Cofiring biomass with coal in large coal-fired power plant boilers
Use as fuel in decentralized or residential heating system
Use as a convenient fuel for gasification
Potential feedstock for chemical industries
Substitute for coke in blast furnace for reduction in carbon foot print.
This chapter discusses the production of solid fuels from biomass, its
principle, technologies available, and design considerations.

4.2 WHAT IS TORREFACTION?
Though no generally accepted definition of torrefaction is available at the
moment, by examining various features of the process and attributes of the
product, one may describe torrefaction as:

a thermochemical process in an inert or limited oxygen environment where biomass
is slowly heated to within a specified temperature range and retained there for a
stipulated time such that it results in near complete degradation of its hemicellulose
content while maximizing mass and energy yield of solid product.
Typical temperature range for this process is between 200 C and 300 C

(Bergman et al., 2005). Though other ranges (Table 4.1)have been suggested,

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