| Summary: | This dissertation considers the behaviour of particulate fly ash produced during cocombustion
experiments of biomass materials with pulverized coal in a 1 MWth pilotscale
combustion test facility (PSCTF). Particular attention is generally given to fly
ash particles of diameters less than 10 and 2.5 microns (namely PM10 and PM2.5).
These small particles have the potential for affecting human health and forcing
climate change because of their ability to scatter and absorb light and also to act as
cloud condensation nuclei. South African coal has high ash content that consequently
affects the ash burden and the efficiency of ash removal system. Previous research
work reports increase of the concentration of fine particles during the co-firing of
biomass with coal, thus limiting the amount of biomass co-fired.
Coal and two types of biomass, grass and sawdust, were used in this study. The coal
chosen was representative on the basis of the annual average calorific value of coals
burned at ESKOM’S coal-fired power stations. For each biomass, the ratios of
biomass to coal used on an energy basis were 10%:90%, 15%:85% and 20%:80%,
resulting in a total of seven different tests including coal alone. Seven tests with
similar fuels were also carried out using a drop tube furnace (DTF) to determine their
reaction kinetics for the combustion simulation.
The experimental results revealed that the grass and sawdust blends showed decreases
of PM10, and PM2.5 particles percentages compared to the coal baseline. The grass
because of its high content of alumina-silicate showed considerable agglomeration
whereas sawdust blends gave minor increase of PM10 under high pressure condition
inside the combustion chamber. The pulverised-coal fineness, flue gas temperature
and excess air were found to affect the particulate matter behaviour. The fly ash
samples collected were also analysed by scanning electron microscope and
spectrometry; alkali metals released were observed to react with the alumino-silicate
phase. The fine sulphate enriched particles formation during combustion process was
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modelled based on the Glarborg-Marshall mechanism using CFD tool. The simulation
results were validated by the experimental data from the pilot-scale combustion test
facility.
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