OCR Text |
Show INTRODUCTION National energy goals stress the increased use of coal in preference to natural gas and oil. This is intended to minimize U. S. dependence on foreign oil and to reserve supplies of natural gas for residential use. Conversion of gas- and oil-fired units to coal has not been economically practical due to the need to significantly derate, modify and/or add equipment. Effected equipment includes the boiler, burner, sootblowers, and fuel supply and particulate collection systems. Micronized, or ultrafine, ground coal is being evaluated as a substitute fuel for oil- and gas-fired utility and industrial boilers, t-* because the finer coal may reduce the level of derating and/or equipment modifications. The driving force behind this investigation is based on the hypotheses that ultrafine grinding: Oj N! S 1) Increases the burning rate of coal to permit completion of combustion in the relatively smaller furnaces of oil- and gas-fired units. A \} 2) Reduces the mean particle size of the minerals in the coal, so that finer ash particles may be produced during combustion [1, 2] and follow flow streams around the tightly spaced tubes of gas- and oil-fired boilers. Deposits interfering with heat transfer, pluggage of the gas passage, and erosion of the tubes is reduced. If these hypotheses are valid, then burning yery fine (micronized) coal in units designed for oil and gas would have great potential in industrial and utility boilers. With the use of micronized coal, existing oil- and gas-fired boilers could be converted to fire coal with a minimum of derating and equipment modifications. An oil- or gas-fired boiler is smaller than a coal-fired boiler having the same heat input for two main reasons. First, the furnace volume in a coal-fired boiler must be greater because of the increased residence time required for complete coal combustion. Also, additional volume is required due to the larger heat transfer surface required in this region of the boiler to accommodate increased thermal resistance 9-2 |