Characteristisation of particulates produced from chain grate and pulverised coal combustion processes
Date
2012-02-14
Authors
Molapo, Tsebo
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Abstract
Most of the electrical power produced in South Africa arises from the combustion of coal. The combustion of coal produces by-products in the form of fine particulates that are released to the atmosphere and these may cause respiratory infections in humans. The current literature studies have divided such particulates into three modes based upon particle size. For example, pyrolysis, i.e. that part of combustion of coal that leads to the emissions of gaseous and heavy hydrocarbon products, characteristically produces ultrafine fly ash particles smaller than 1 micron. The burning of porous char particles or particles that fragment lead to particle size ranges mainly between 1 and 3 microns; while dense chars and mineral forms that melt and coalesce with each other produce larger fly ash particles generally larger than 3 microns.
The original aim of this study was to characterise the three modes of coal fly ash formed by South African coals when combusted in a pulverised fuel (Pf) boiler and a chain grate stoker boiler. This, however, was not possible due to the lack of appropriate equipment for the capture of fly ash material in the three different sizes. For this reason, an alternate approach was taken. Namely, bulk samples of fly ash were taken from an electrostatic precipitator (ESP) of Pf boilers, chain grate boiler, from the out-going stack of a Pf boiler, and from the exit stack of a chain grate stoker. Particle size distribution analyses were undertaken on these samples and the <10 micron material was send out for further analyses using XRD, SEM EDS and XRF. The samples taken from the stack of the Pf boiler were iso-kinetically sampled and the fly ash was captured in the filter fibres of thimbles. Due to the small quantity of sample and the fact that the fly ash was effectively trapped within the fibers, no particle sizing analysis could be undertaken on these samples, only SEM EDS was used to analyse the fly ash particles trapped within the fibers. The feed coals were analysed using similar techniques to those used on fly ash particles.
The purpose of the amended investigation was to compare; (1) the size distributions and qualities of the <10 micron fly ashes from a Pf boiler ESP and the chain grate stoker relative to one another and to their feed coals, and (2) the Pf boiler ESP fly ash sample and the out-going stack sample from the same boiler.
The research has shown that the fly ash compositions reflect the feed coal mineral content, and that the ESP at the back end of the Pf boiler has efficiently captured most of 5-10 micron particles and less than 5 micron material through, as seen in the stack samples. The chain grate
stoker fly ash samples have not passed through an efficient pre-screening process, thereby allowing both ultra fine <5 micron and 5 to10 micron material into the atmosphere.