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Bristol had a traditional waste
incinerator at the Avonmouth Industial Estate which closed down in 1993 because
the emissions exceeded the modern air quality standards. It was uneconomic to
upgrade the power station to meet these standards. Since then, most of the
domestic waste of Bristol has been transported by train to
a land fill site on the other side of the UK.
Several alternatives to waste disposal have been
investigated. One company called Compact Power has built a demonstration waste
disposal plant next to the site of the old incinerator which uses the new
technology of pyrolysis.
Concept
The plant is small and flexible enough to deal with waste
from a variety of sources and so can be economic. Large-scale incinerators,
which need 200,000 to 500,000 tonnes of waste per year, do not encourage the
policy of reusing or recycling as much of the waste as possible. The small
pyrolysis plants are cleaner and designed to deal with local waste in the
region of 32,000 to 64,000 tonnes per year.
The plant at Avonmouth is designed to deal with 8,000 tonnes per year of
difficult wastes provided by the City Council for demonstration purposes. The
waste can be standard municipal waste or priority waste such as scrap tyres and
clinical waste. The aim is to use waste which is left after all possible
recycling and reusing is complete.
Design
The plant is a modular design which used pyrolysis,
gasification and high temperature oxidation using carbon from a variety of
sources.
The process begins with a hopper and feed system to take
the waste into the pyrolysis chamber. The waste is heated to 800oC
with no oxygen present. Hydrocarbons are converted to simple gases leaving
residues of carbon solids, inert grit and heavy metals. The residues are
reacted inside a superheated steam box to produce hydrogen and carbon monoxide.
Gases from the pyrolysis and gasification processes are
reacted with air at a temperature greater than 1250oC for more than
two seconds to destroy any remaining pollutants and particulates.
Exhaust gases are passed though a steam boiler, which
recovers up to 80% of available energy, and power is generated by a steam
turbine or a steam reciprocating engine.
The Avonmouth facility consists of a combined pyrolysis
and gasification unit with two pyrolysis tubes (MT2). Each tube is capable of
processing 500 kg/hr of waste with an average energy conversion of 12 MJ/kg.
The size of the building is 40m x 40m with a roof height of 10m.
The aim of the project is to demonstrate the commercial
viability of the technology and show the public and environmental organisations
that the technology has a good environmental performance.
As the design of the plant is modular it can be built to
different scales. The table below gives an indication of the range of
parameters. The actual output figure will depend on the type of waste used.
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Model
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MT2
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MT8
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2xMT8
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Waste capacity
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8.000 tpa
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32,000 tpa
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64,000 tpa
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Tube details
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2 x 0.5m x 3.5m
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8 x 0.5m x 3.5m
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8 x 0.5m x 3.5m
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Gross thermal output
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1.8 MW
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12 MW
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30 MW
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Electrical output
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0.33 MWe
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2.7 MWe
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5.6 MWe
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Further information can be found at www.compactpower.co.uk
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