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144
Evaluation of Opportunities for Converting Indigenous UK Wastes to Wastes and Energy
AEA/ED45551/Issue 1
10 Thermochemical processes for generating
energy
Thermochemical processes for the generation of energy are taken as gasification and pyrolysis in the
Chapter. Combustion, gasification and pyrolysis all use heat to break down the structure of the feedstock
so that the chemical energy can be released either as heat or a fuel product. The conditions in the
reactor determine the products.
When combustible waste enters a high temperature environment it will first dry and then decompose into
volatile gas and char components.
A combustion process is always supplied with an excess of air so the char and volatile gas burn
completely. The energy in the waste is released within the combustion equipment and the flue gas
from it. It is recovered as hot water or steam.
Gasification processes use a limited supply of oxidant; usually air, to maintain both combustion and
reducing reactions in the same reactor. These reactions produce carbon monoxide (CO), hydrogen
(H2) and hydrocarbons as well as combustion products. Most of the energy in the fuel is transferred
into the calorific value (CV) of the gas leaving the reactor. This gas can be burned separately in a
boiler, engine or gas turbine. Alternatively it can be converted by a synthesis reaction to methane for
injection to the Natural Gas Grid or liquid fuels for transport.
In a pyrolysis process there is no oxygen and the char and volatile gas remain largely unchanged.
The energy in the waste is retained in the CV of the gas and char removed from the reactor. These
can then be burned separately in a boiler, engine or gas turbine. Some pyrolysis processes produce
gases that can be condensed into a liquid fuel. Pyrolysis products can also be further upgraded to
transport fuels although the technologies required are not yet commercially available.
There are a number of advantages of converting solid waste into a gas or liquid fuel.
• Cleaning a small fuel gas stream before combustion rather than a large flue gas flow as in an
incinerator reduces the size of the pollution control equipment.
• The controlled combustion in a gas flame, as opposed to a grate, may also reduce the extent and
complexity of final gas cleaning.
• The two points above make installations economically possible that are smaller than current
incinerators. These should be more acceptable for permitting and more compatible with the
throughputs of the newer mechanical separation and other recycling and recovery processes.
• Higher electrical output per tonne is possible if high-pressure boilers, gas turbines or engines are
used.
• Alternative energy products such as methane and liquid fuels can be manufactured by chemical
synthesis using the products of gasification or pyrolysis as a base.
In the sections below we discuss how thermal technologies are used to generate energy from waste,
what demands they make of the feedstock, their commercial status and the risks of deployment.
These advantages are often quoted by gasification and pyrolysis suppliers as solutions for perceived
weaknesses in the current market leader – combustion with energy recovery. These claims are shown in
Table 74 with our comments.