OCR Text |
Show By reducing the fly ash particle size to well below 10 um, the collection efficiency of all heat-transfer surface subjected to inertial impact should be reduced to zero. Under these circumstances, the total ash collected in the furnace should be minimized, and very little ash should be collected in the furnace bottom. A reduction in coal size to 3 to 4 um should also enhance the oxidation rate of pyrites, which to a certain extent reduces the slagging potential of the iron. This could be offset by high flame temperatures exceeding the melting temperatures of species which otherwise might be quenched to a safe temperature. The increase in absorption rate reduces the thickness to which a deposit must grow before a given surface temperature is achieved. Thus although the rate of deposition may be reduced, the deposited ash may slag prematurely, offsetting the advantages achieved by size reduction-at least within the flame basket zone, where absorption rates are high. Running slag in a furnace designed for dry ash can result in slagging problems not directly associated with the transfer of heat. Some field trials have been performed with micronized coal, and the results reported are very encouraging regarding furnace deposits. However, the fuel types and their mineral content are not well documented, and the amount of iron that can be tolerated in the ash before the effects of micronizing are offset is uncertain. Claims have been made for an improvement in fouling as a result of fly ash size reduction, postulated on the basis of reduction in collection efficiency by inertial impact approaching zero. Fouling of convection surface for both coal and oil begins as the result of condensation of volatilized mineral matter, namely the alkalis and vanadium. Further growth proceeds with the deposition of 1-50 |