Page 170 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter 5

Pyrolysis

5.1 INTRODUCTION

Pyrolysis is a thermochemical decomposition of biomass into a range of
useful products, either in the total absence of oxidizing agents or with a limited
supply that does not permit gasification to an appreciable extent. It also forms
several initial reaction steps of gasification. During pyrolysis, large complex
hydrocarbon molecules of biomass break down into relatively smaller and
simpler molecules of gas, liquid, and char (Figure 5.1).
Pyrolysis has similarity to or overlaps with processes like cracking, devola-
tilization, carbonization, torrefaction, dry distillation, destructive distillation,
and thermolysis, but it has no similarity with the gasification process, which
involves chemical reactions with an external agent known as gasification
medium. Pyrolysis of biomass is typically carried out in a temperature range

of 300 650 Ccompared to 800 1000 C for gasification and 200 300 Cfor
torrefaction. A discussion on the difference between these processes is given
in Section 4.2.1 and Table 5.2.
Biochar is the solid product of biomass pyrolysis. This provides an alterna-
tive to complete burning of agricultural product or forest residues releasing the
carbon to the atmosphere. Biochar could retain a part of that carbon in
stable solid form in soil for hundreds of years. Owing to its growing impor-
tance for carbon sequestration, many institutions are taking a closer look at
this. A discussion on this is presented in Section 5.8.
This chapter explains the basics of pyrolysis. A brief discussion of the
design implications of the two is also presented.

5.1.1 Historical Background

Charcoal from wood via pyrolysis was essential for extraction of iron from
iron ore in the preindustrial era. Figure 5.2 shows a typical beehive oven
used in early times to produce charcoal from biomass using a slow pyrolysis
process. This practice continued until wood supplies nearly ran out and
coal, produced inexpensively from underground mines, replaced charcoal for
iron production.
The modern petrochemical industry owes a great deal to the invention
of a process for production of kerosene using pyrolysis. In the mid-1840s,

Biomass Gasification, Pyrolysis and Torrefaction.
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