Another advantage of clean, beneficiated coal is that it gives a higher fuel value to the mixture and, hence, allows greater oil replacement (COM) and/or causes a smaller reduction in boiler capacity, i.e. The ash and sulfur content of the slurry fuel must be low enough to avoid particulate build-up inside the furnace, excessive erosion of boiler tubes or serious emissions in the stack gases. Higher concentrations also reduce settling. This will entail using additives to increase slurry stability or to grind the coal finer so that its settling rate is reduced. Settling of the slurries should be avoided, particularly if storage tanks and long transfer lines are used. Good combustion characteristics, flame stability and sufficiently rapid burning to give complete combustion during the short residence time, typical of boilers designed originally for burning oil, are also important and require the fuel droplet and coal particle sizes to be relatively small. Shear thinning behaviour (Bingham plastic, pseudoplastic) is desired, so as not to have rapid settling, yet low viscosity for good atomization under high shear rates. This essentially determines slurry rheology. The most important requirements of coal-slurry fuels are that they be pumpable and readily atomized in the furnace. Trass, in Advances in Clean Hydrocarbon Fuel Processing, 2011 Slurry properties Characterization and preparation of biomass, oil shale and coal-based feedstocks
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