Modern Gasworks Practice

THE HORIZONTAL RETORT BENCH 63 From the above it will be seen that the greater efficiency of the gaseous principle is not due to the faet that an inereased number of heat units are expelled, the reason is that the heat is obtained in that portion of the setting where it is most useful, and the bulk of gases resulting from total combustion is reduced to the smallest practicable limits. It is not proposed to deal here with. the calculation of combustion temperatures, as the procedure is explained in most textbooks on physics, and is of little practical use to the gas engineer. The ultimate limiting temperature is influenced— (a) By the calorific power of the fuel. (&) By the total weight of the products of combustion and their specific heat. (c) By the temperature of the mixture before combustion. Of these, the first is under control to only a limited extent, as the fuel used coke—has to be taken as it comes, the calorific power depending upon the type of coal carbonized at the time. It is largely by working on conditions (&) and (c) that the inereased firing efficiency of the present day has been obtained. AV ith. regard to the total weight of the products, it will be seen that the greater the quantity of air admitted to the furnace for the combustion of a definite amount of fuel the greater will be the weight of the waste products over which a fixed number of heat units will be spread. Hence, the ideal condition is that of admitting just sufficient air for combustion, and reducing the weights of the waste products to a minimum— in which case the heat units are concentrated in the smallest possible volume and a maximum temperature is obtained. . It will be as well to explain here the meaning of the term “ Calorific Intensity,” which must on no account be confused with calorific power, from which. it differs. Calorific intensity is measured in degrees Fahrenheit or Centigrade, and is the maxi-mum temperature theoretically attainable under the conditions of combustion prevailing ; hence it depends very largely upon the way in which the fuel is employed. Producer gas resulting from the combustion of coke should theoretically consist of 34-8 per cent, of carbon monoxide and 65-2 per cent, of nitrogen, by volume. In practice, however, ideal conditions are not to be looked for, and the ordinary individual retort-bench producer is considered to be giving satisfactory results when the carbon monoxide content of the gas averages 25per cent., the carbon dioxide amounting to about 5 per cent., although higher proportions of CO are occasionally found. The modern combustion chamber temperature rarely, if ever, exceeds 1,535° 0.(2,800° Fahr.), and under normal working conditions is usually found in the n’eighbourh’ood of 1,370° C., with an actual retort temperature of l,030°-l,100° C. Several causes account for the reduced intensity, among them being the variations in the depth of fuel-bed, the effeet of clinker on the velocity of travel of the gases, and the various sources of thermal losses. The actual temperature of the fire is of some importance ; and whilst the recog-nized gasworks rule is that of keeping the fuel-bed cool, this axiom can be carried too far, for the maximum proportion of carbon monoxide is reached at a fuel-bed temperature of about 75O°-8OO° C. The prevailing temperature is from 950°-l,000° C. Producer gas from retort furnaces seldom consists of merely CO, CO2) and nitro-