Pollution under control at Manchester
HD reports on a project to ‘clean’ gas used for waste incineration.
cwa Air received a 240,000 order to design, install and commission gas cleaning plant for the clinical waste incinerator at South Manchester’s Wythenshawe Hospital, to replace plant that had become increasingly expensive to operate.
In addition, the existing plant was unable to satisfy ever igh ten ng emission requirements. A more efficient and cost-effective replacement had to be found, and the project was put out to competitive tender. Acwa Air’s initial design proposal for a dry scrubbing plant, with integrated heat recovery, was considered to be the most appropriate and cost-effective submission, and with regard to present and future environmental legislation. Following more detailed investigations, these proposals were fully accepted. Acwa has arranged for’ the project to be directly financed by Asset Investment Capital, in a facility which will allow the South Manchester University Hospital Trust to operate the plant on leasehold.
In addition to cleaning the incinerator flue gases, the dry scrubbing system will also provide a degree of heat recovery, capable of preheating feed water for the hospital’s steam boilers. This is to be achieved by a heat exchange system in which hot flue gases from the incinerator are ducted through an economiser. As the flue gases are cooled from 450 deg C to 180 deg C – by pressurised water passing through horizontal tubes – the temperature of the water increases. This closed circuit pressurised water system is used to heat the boiler feed water in the hotwells.
Cooled gases discharged from the economiser are then injected with sodium bicarbonate, which reacts with the acid gases and powdered activated carbon (PAC) to adsorb contaminants such as dioxins, furans and heavy metals. In the next stage of the process, flue gases enter a gas filter installed with ceramic elements which filter out particulates and used reactants. Capable of withstanding high temperatures – in excess of 400 deg C claims Acwa – the filter elements hang vertically within a strong rigid housing, supported at both ends. A baffle distribution plate is installed at the filter inlet to achieve an even distribution of gas throughout the filter housing and protect the elements against large foreign objects in the gas stream.
A uniform distribution of a sorbent coating on all collection surfaces, enables high removal efficiencies, says the manufacturer. The ceramic elements will be cleaned periodically by a reverse pulse of compressed air to remove dust.
Copyright Wilmington Publishing Ltd. Oct 2001
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