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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.
Evaluation of Opportunities for Converting Indigenous UK Wastes to Wastes and Energy AEA/ED45551/Issue 1 Table 74 Claims for gasification/pyrolysis with comments Incineration Gasification/pyrolysis Comment Large (Regional) size and fuel demand suppresses recycling. Small volume of Hazardous and toxic fly ash. Bulky residual ash can be used as a aggregate. Poor electrical efficiency and lack of suitable heat loads gives poor resource use efficiency. Can be built economically at the smaller sizes that can be integrated into local schemes. This remains to be demonstrated, but is probably true. No dioxin found in fly ash. This seems to be valid. Some options give low volume compact ash. Some options can give very high electrical efficiencies. Smaller sizes can match heat loads better. Particularly as these are often independent of the power supply. Electricity and or heat only Some options can be used to generate gas or transport fuels 10.1 Gasification RDF Waste wood Clean wood Ag residues Gasification Syn gas Valid. Some incinerators can give low volume slag like ash but at a very high energy cost for ash melting. Very few demonstrations of high electrical efficiency. Lack of enthusiasm for CHP. Installations in wood waste sector now making progress. True at very large scale. Process Description In simple terms gasification is the conversion of a solid fuel to a gas that has sufficient energy value to be useful as a fuel or feedstock in its own right. This concept is refined in the definition for the Renewables Obligation to the substoichiometric oxidation or steam reformation of a substance to produce a gaseous mixture containing two or all of the following: oxides of carbon, methane and hydrogen and which has a gross calorific value of at least 4.0 MJ/m 3 when sampled at the inlet to the generator and measured at 0.1 MPa and 25°C. 277 277 Renewables Obligation: Defining the different types of electricity generation using biomass and waste www.berr.gov.uk Methanation Grid Injected SNG Fischer Tropsch CHP 2 nd gen biofuels Heat, Power 145
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Figure 27 Orchid Environmental proc
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