Electric Heat Versus Heat From Fuel

Posted By Richard Jefferson on 14 November 2014

Posted in The Vintage Machinery Almanac

This article was originally published in Electrochemical and Metallurgical Industry Publication of August 1907. Information within this article is therefore correct as of 1907. The publication of this material aims to provide historical insight on the subject and its place in industry.

In the Synopsis of Current Literature in our present issue we notice the last reports of our San Francisco contemporary, The Mining and Scientific Press, on the progress of the Noble Electric Steel Co.'s work at Heroult-on-the-Pitt in California. As will be remembered from our former notices this undertaking of Mr. H. H. Noble is noteworthy in two respects from an industrial point of view. If successful it will mark the beginning of commercial manufacture of pig iron at the Pacific Coast, and at the same time it is the first commercial plant in the world in which the electric furnace enters into competition with the blast furnace. A concise analysis of the possibilities of electric heat versus heat from fuel seems timely, and it seems proper to first view the problem from the narrow point of view which is most unfavorable to the electric furnace and which simply inquires as to those relative costs of fuel and electrical energy for which the cost of the blast-furnace and of the electric-furnace treatments becomes the same. The immense importance of the element carbon in metallurgy is due to its double application as a strong reducing agent and as a fuel to produce, by its oxidation, the heat needed for the chemical reaction. In the case of reduction of iron from the ore, the blast furnace and the electric furnace require the same quantity of carbon to act as reducing agent and to combine with the oxygen in the iron oxide. But the blast furnace also requires carbon for producing the necessary high temperature, and it is only this last portion of carbon, the calorific value of which is replaced by electrical energy in the electric furnace. We will, therefore, first compare the cost of fuel and electrical energy to produce the same number of heat units.

One kilowatt-hour is equivalent to 860 kilogram-calories, which is approximately the full heat obtained from 100 grams of good coal when completely burnt to carbon dioxide. If the cost of a ton of coal (2,000 pounds or 907 kilograms) is as dollars, and if the efficiency of heat production by burning coal is x per cent, then the coal required for producing 860 kilogram-calories costs a -f- 90.7 x dollars. On the other hand, if the cost of 1 kilowatt-hour is b cents and if the efficiency of producing heat from electrical energy is y per cent, then the kilowatt-hours, required for producing 860 kilogram-calories, cost b -r- y dollars. Hence electrical heat will be cheaper than heat produced by combustion of fuel if a y is greater than 90.7 b x.

To go further, we need the efficiency figures x and y. By assuming definite figures we introduce, of course, an uncertainty into our comparison, but it will probably be considered fair if for a first approximation we assume a 25 per cent efficiency for fuel heating and a 75 per cent efficiency for the electric furnace. Then we conclude that electrical heat will be cheaper than heat produced from fuel if a is greater than 30.2 b, or in words, if the cost of a ton of coal in dollars is more than 30 times the cost of a kilowatt-hour in cents. For instance, to compete with coal at $6.00 per ton, the electric kilowatt-hour would have to cost less than 0.2 cent. This is clear evidence that if the electric furnace did not have other important features it could not compete with fuel heat under ordinary conditions.

We have purposely made this comparison on the basis of the cost of the ton of coal and of the kilowatt-hour, although it has become customary to use the electric horsepower-year as the unit of electrical energy in such estimates. Then the above condition can be stated in this way: electric heat will be cheaper than fuel heat if the cost of 1 ton of coal is more than one-half the cost of the electric horsepower-year. Stated in this form, the comparison looks more favorable for the electric furnace than stated, as in the preceding paragraph, But in reality it is not proper to make the comparison on the basis of the horsepower-year, since we thereby implicitly assume that the electric furnace is working continuously every hour all year around, which is in general a decidedly improper assumption.