3.1 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 is one of several reaction steps or zones observed in a gasifier. During pyrolysis, large complex hydrocarbon molecules of biomass break down into relatively smaller and simpler molecules of gas, liquid, and char (Figure 3.1).

Pyrolysis has similarity to and some overlap with processes like cracking, devolatilization, carbonization, 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 relatively low temperature range of 300 to 650 °C compared to 800 to 1000 °C for gasification.

Torrefaction is a relatively new process that heats the biomass in the absence of air to improve its usefulness as a fuel. Interest in torrefaction is rising on account of its several advantages.

This chapter explains the basics of pyrolysis and torrefaction. A brief dis­cussion of the design implications of the two is also presented.