to read the full report - Ecolateral by Peter Jones

146
Evaluation of Opportunities for Converting Indigenous UK Wastes to Wastes and Energy
AEA/ED45551/Issue 1
Gasification has been successfully used with coal, in the American Great Plains project to generate
synthetic natural gas and in South Africa to generate transport fuels. There are also instances
internationally where gasification has been used with SRF waste as a feedstock.
A gasification for energy process has four steps, the gasifier itself that produces the gas, feedstock
preparation to get the fuel in the right form for the gasifier, the clean-up of the gas, and the utilisation of
the clean gas for energy. This is shown Figure 19.
Step
Material
state
Typical
equipment
and
process
Step 1
Pretreatment
Step 2
Thermal
reactor
Step 3
Gas
conditioning
Solid Waste Raw gas
Clean Gas
Separation
of glass and
metals.
Screening
Shredding
Pelleting or
briquetting
Fluidised
bed reactors
Externally
heated
reactors
Fixed bed
reactors
Entrained
flow
reactors
Gas
coolers
Filters
Liquid
scrubbers
Figure 19 Steps in a gasification process
Step 4
Conversion
to energy
Energy
products
Steam
boilers
Engines
Gas
turbines
Chemical
synthesis
The gasification process
The gasification reactions that produce the fuel gas components are endothermic and need a constant
supply of heat to sustain production. This heat is supplied by the combustion of part of the fuel. The
challenge is for the equipment design to transfer heat from exothermic combustion to endothermic
gasification. There are two main types although there are many variants in each category.
Direct – This process occurs in a single reactor where oxygen or air and usually superheated steam is
supplied to the same vessel so that some combustion occurs in situ providing the required heat to drive
the gasification reactions. The direct use of air dilutes the product gas and results in a low calorific value
fuel commonly referred to as producer gas.
Indirect – Here gasification and combustion process occur in separate fluid bed reactors. Heat is usually
transferred by circulating the fluidised bed material between the two beds. This concept is more complex
than the direct process but produces a gas with a higher calorific value.