OCR Text |
Show Feedstock costs all but preclude the use of coal-oil mixtures since, at the upper physical limit (about 50 percent) of coal concentration, the cost of coal, oil and preparation exceed the price of competing fuels. There may be an exception to this generalization in the case of proprietary fuels containing about 25 percent oil, 15 percent water and 60 percent coal, particularly for modest scale heating plant and marine use, but overall the Canadian coal-liquid-mixture program has veered away from its early interest in coal-oil mixtures on economic grounds. Canada has the objective of decreasing its atmospheric emissions of sulphur oxides by 50 percent between 1980 and 1990 and the substitution of low sulphur coal for residual oil can materially assist in reaching this objective. The multistage cleaning process associated with coal-liquid mixtures can reduce medium-sulphur coal to this desirable state. However, even where such mixtures might be chosen as an oil substitute preferable to coal on environmental grounds alone, such as in a furnace originally designed to burn coal but later switched to oil (to minimize particulate as well as sulphur emissions), the competitiveness of clean-burning natural gas sets an upper limit to the price. In utilizing coal-oil mixtures in utility boilers, particularly those designed for oil-firing, many problems arise which must be overcome. Usually an oil-designed boiler is smaller, the steam-raising tube banks are configured differently and the gas velocities in the banks much higher than for an equivalent capacity coal-fired unit. In addition to this, coal slurry fuels contain ash which poses problems of tube erosion and slagging and/or fouling. The ignition, flame and heat transfer characteristics of coal-oil mixtures may be quite different from those of heavy fuel oil and therefore the heat release pattern from the flame may not be suitable for the oil-designed unit. The combination of all these factors usually means that the oil-designed unit will be derated; that is it will not be able to 12-9 |