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Show 2 1.0 Introduction Coal-Vater Slurry (CVS) applications in steam raising plant represent a relatively new and developing technology. Vhile there have been some unpublished experiments carried out at the International Flame Research Foundation (IFRF) in Holland (1962) and at the VGB in Germany (1963), it was only recently that this new fuel attracted more widespread interest, primarily because of its potential as a replacement fuel in oil-designed utility and industrial boilers. Coal beneficiation to the level required by retrofit of oil-designed boilers requires fine grinding of the coal, and the mixing of the pulverized coal with water provides an attractive means for transporting, storing, and feeding the fuel (in the form of an atomized spray) into the combustion chamber. Recent studies have indicated that it is feasible to produce CVS with about 30/6 water content for boiler applications. The coal is generally ground to the usual specifications for pulverized coal, i.e., 80/6 < 74 Mm, although efforts are currently being made to reduce the coal particle size even further ("99/6 < 44 MHI). In addition, "micronized" coal (99/6 < 20 /urn), is under consideration for possible application in gas turbines. About 1/6 by weight of the mixture consists of chemical additives which act as dispersants, make the coal hydrophilic, reduce foaming tendencies, control the viscosity of the mixture, and serve as an algicide to permit long term storage without deterioration. EPRI projections of the production rate and cost of CVS are shown in Figure 1. The calculations are generally based on coal cost of $1.60 per MMBtu and it is thought that a margin of about $1.50 between the costs of oil and CVS must be maintained for the latter to be of sufficient attraction in boiler retrofit applications. Figure 2 presents a summary of characteristics of oil-designed boilers in the U.S., i.e., the potential market for retrofit with CVS. The units included in this survey represent 81/6 of the U.S. boilers that were designed for oil and which are greater than 50 MV in capacity. The total capacity is about 100,000 MV; it is thought that only a fraction of these units, perhaps 25/6, can be considered to be candidates for retrofit with CVS. Vhile issues connected with the preparation of CVS are strongly interwoven with those of the combustion technology, they fall outside the scope of this report and readers are referred to a paper by Ehrlich, Drenker and Manfred (1980) for further details. Combustion Engineering Problems and Potential Means for Their Solution Although amounting to a small fraction of the total heat input (~3%) f the water contained in coal-water slurries engenders practical difficulties in achieving ignition and flame stability that set this fuel apart from most others. Before combustion can be initiated, the water must be vaporized and the coal heated to ignition temperature. In order to achieve ignition and good flame stability a high degree of energy feedback from the flame and surroundings to the incoming fuel is required from radiative and convective heat transfer mechanisms. |
