CHAPTER XXV CHANGING FROM COAL TO OIL When it is contemplated to change from coal firing to oil firing, the first thing to be considered is the relative cost of the two fuels. This does not mean merely the cost of a ton of coal compared to the cost of a ton of oil, because oil has a far greater heating value than an equal weight of coal. Again, while oil has a fairly uniform heating value, there are great differences in the heating values of different kinds of coal. Consequently in making the comparison it is necessary to know the kind of coal under consideration, and its heating value per pound. Even then we have not gone quite far enough, for the boiler efficiency is not the same for all grades of coal, and is higher for oil than for coal. Thus, with a good grade of semi-bituminous coal an efficiency of 75 per cent. is readily obtainable, whereas with a low grade bituminous coal or lignite it is difficult to obtain more than 60 per cent. efficiency under ordinary methods of firing. With oil on the other hand, tests have shown net efficiencies of over 80 per cent. and with careful operation it is readily possible to maintain 78 per cent. efficiency in regular plant operation. Knowing the relative prices and heating values, and the probable boiler efficiency, it is a simple matter to calculate the saving that may be effected by changing from coal to oil. Suppose, for example, that the owner of a plant is purchasing coal at $6.00 per ton of 2000 lb. and that this coal contains 6 per cent. moisture and has a heating value of 13,000 B.t.u. per pound dry. He is considering changing over to oil which he can purchase for $1.50 per bbl. of 42 gal. The oil has a gravity of 16°Bé. and therefore weighs 336 lb. per bbl.; it contains 1 per cent. water and its heating value when free from water is 18,500 B.t.u. per pound. Since the coal contains 6 per cent. moisture it is 94 per cent. dry, and 1 ton of coal contains If both fuels could be burned with the same efficiency, then by dividing 24,440,000 by 6,153,840 we would find that one ton of coal is equivalent to almost 4 bbl. of oil. However, if the oil can be burned with an efficiency of 78 per cent. and the coal with an efficiency of only 69 per cent., we find that the useful heat in one ton of coal is and the useful heat in 1 bbl. of oil is 0.78 X 6,153,840 = 4,800,000 B.t.u. Consequently one ton of coal is equivalent for steaming purposes Price of Coal- Dollars per Ton of 2000 Lbs. FIG. 145.-Comparison of fuel oil with coal of various heating values. The cost of 3.5 bbl. of oil at $1.50 per barrel is $5.25, and since the coal costs $6.00 per ton, the saving would be $0.75 for each ton of coal. If the plant in question is a 100 h.p. plant, burning, say, 10,000 tons of coal per year, the saving would amount to $7500 per year for the particular conditions assumed. In the set of curves shown in Fig. 145 a comparison is given of fuel oil with coal of heating values varying from 10,000 to 15,000 B.t.u. per lb. The heating value of the oil is taken as 18,500 B.t.u. per lb. which is a fair average value for California oil, the variation from this value being small. The efficiency of 78 per cent. for oil firing assumed in these calculations, can readily be maintained in normal service provided proper attention is paid to the furnace design and the regulation of the fires. In order to make this comparison fairly correct for the different grades of coal, an efficiency of 60 per cent. has been assumed for coal having 10,000 B.t.u. and 75 per cent. for coal having 15,000 B.t.u. per lb. with intermediate values for coals that lie between these extremes. As the heating value of coal is usually given on the basis of dry coal, and as coal when purchased invariably contains a considerable proportion of moisture, it has been assumed that the coals considered in this comparison contain 6 per cent. moisture. In the case of fuel oil the water content does not usually exceed 1 per cent., and this value has been assumed in the comparison. It will be observed from the diagram that oil at $1.50 per bbl. is equivalent in price to 14,000 B.t.u. coal at $6.00 per short ton. Oil at $1.50 per bbl. is also equivalent to 12,000 B.t.u. coal at $4.60 per short ton. In addition to the saving in cost of fuel there will always be a saving in labor on changing from coal to oil, as the operation of firing is much simpler, there are no expensive coal elevators and conveyors to be kept up and there are no ashes to handle. On the other hand, the interest and other fixed charges on the investment required to change over, reduce the saving to some extent. Both of these items, however, are small compared to the cost of fuel, and as they tend to neutralize each other they may be safely neglected except in special cases. The apparatus required to change a coal burning plant into an oil burning plant consists of the oil storage tank, oil pumps, oil heater, oil burners with the necessary interconnecting piping, strainers, regulating valves, etc., all of which have been described in Chapter XXIV. In addition an automatic oil firing system may be installed if desired. The furnaces under the boilers must be altered to suit the new fuel. If the boilers are hand fired this is a simple matter, for all that is necessary is to cover the grates with firebrick, leaving suitable openings for the admission of air, and install the burners properly housed and protected from the heat. A furnace similar to that illustrated on page 158 may then be used, the grates acting as supports for the checkerwork in the furnace floor. Boilers larger than 300 h.p. should have a furnace length not less than 10 ft., so in many cases where the grates are shorter than this it will be necessary to extend them. For the additional length necessary pieces of pipe or I-beams may be used to support the furnace floor, instead of grates. FIG. 146.-Typical boiler room for easy changeover from oil to coal. Many power plant operators prefer to so design the fuel oil operation that quick change over to coal operation may be accomplished should oil later involve less economic operation than coal. The view shown is that of the boiler room of the steam electric plant for the lighting department of the city of Seattle-East Lake Avenue and Highland Place. |