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EfW - Process: Pyrolysis Gasification (Gas Conversion)

The Converter consists of two chambers, an inner pyrolysis chamber (PC) and an outer heating chamber. The outer heating chamber totally encloses the inner PC and acts as a heating jacket.

The outer heating chamber is designed to increase the temperature of the inner PC to 850°C, it is heated initially by burning propane gas in the secondary converter (SC) . When the PC reaches temperature, and pyrolysis begins, some of the generated gas is then diverted to the SC and the propane is shut off.

The pyrolysis process requires an oxygen free atmosphere, hence an inert gas is introduced into the PC to replace the atmospheric gas, and nitrogen is used for this purpose. The nitrogen gas is fed into the unit by a network of pipes from an external storage vessel; the nitrogen is turned off once an oxygen free atmosphere is achieved.

The dry feed is then transferred, by mobile plant fitted with a front scoop, on to a conveyor, which transports the feed to a metering bin that feeds into a hydraulic compaction system that compacts to a solid state used to stop any atmospheric air entering the PC by squeezing it out of the waste.

A ram feeder, at one end of the PC, governs the rate of introduction of the dry feed into the PC from the compaction system. The feed is rapidly dissipated in the PC.

Due to the high temperature and the lack of oxygen the organic compounds are broken into short chain length organic compounds, which would be gaseous at room temperature, hence up to 99% by weight of the feed will vaporise to a gas. This process is known as thermal cracking.

This gas generated will then be passed through a filter to remove any remaining particulate material.

All non-converted elements (char) will be collected as carbon ash at the far end of the PC. There are rotary valves, which will allow the removal of the char without losing the inert atmosphere. This char (assumed to be rich in carbon) is quenched before being transferred to the SC stage where any remaining energy is utilised, leaving a vitrified slag to be used as building material.

Fuel inlet end
Gas side elevation
Top view
Gas side

The retention time of the gas varies as some feed will be vaporised near the top of the vessel and other will vaporise near the bottom of the vessel. However, the average retention time of the gas between the PC and the cooling equipment is approximately 8 seconds.

The gases releasing from the outer heating chamber of the PC (exhaust) will be carried to the dryer air-mixing box

An oxygen Analyser is installed in-stream of the gas leaving the PC as a safety device, it is to detect any leak from a break in the walls of the chamber or anywhere else in the pipeline. The oxygen analyser ensures that the oxygen level stays within the safe working parameters of the process.

All PCs have a water trap pressure relief valve, to avoid the build up of pressure in the vessel. The PCs will also be well insulated to avoid any heat losses.

The Converters are designed in a manner that in the event of a power failure or other problems, the Converter defaults off, i.e. the ram feeder will stop feeding the PC, a thermocouple which controls the temperature of the Converter will stop the combustion process in the heating chamber. Without this source of heat the strongly endothermic pyrolysis process will rapidly cool down and stop.

© Alternative Use Group plc, 2013