PULVERISED COAL.

Press, Volume LV, Issue 16417, 10 January 1919, Page 8

PULVERISED COAL.

AN ECONOMY SYSTEM. The recont coal shortages in many ■countries have accentuated the need for burning fuel to tho greatest advantage and tho use of coal ground into lino powdet, in spocial furnaces, is being more actively dovelopod. In tho firing of furnaces with coal, whether for heating or industrial purposes, fuel is lost in two ways. Finely divided carbon, or soot, and unburn ed combustible gasos are lost up the stack, and carbon, or coke, is retained in the ash. The former can be reduced to an almost negligiblo quantity by intelligent firing in proporly constructed grates, but not so the latter. Even when the ashes are sifted, carefully picked over, and the carbonaceous part reburnou, there is a considerable loss in fine particles of combustible carbon that cannot be recovered economically. Engineers estimate this loss at rarely less than o per cent, of tho fuel used while where proper care is not exercised it will rise to 15 or 20 per cent. The loss vanes with tho quantity and nature of,the ash in • the coal, 'generally increasing directlv with the proportion of ash an<J with the readiness with which it fuses. "When a coal.has an excessive amount of ash the latter, as the coal burns, smothers particles of carbon, preventing access of atmospheric oxygon aud combustion. A readily fusible ash when it melts tends to surround particles or carbon and prevents their combustion. To remedy these very serious defects in the economic combustion of coal, engineers have been experfmenting with the firing of coal in a vory finely-pul-vorised state, and have been meeting with no small measure of success. Or course, firing with coal dust, is not new. It- has been used for many years with tho utmost snccess in large metallurgical furnaces where completo control or temperature and other furnace conditions are essential, but for boilers for domestic heating and for power purposes it is a comparatively new form of firing. The experiments to date show that°it will be as successful in the new use as it lias been in the old one, and already, some largo industrial companies" have adopted'it, and have found it to be entirely satisfactory. In firing with coal in an exceedingly finely-divided state, instead of in lump form, the conditions of its combustion are completely changed. It more.nearly approaches a gaseous fuel. In fact, it lias long been known that many of the explosions that have taken place in coal mines were the result of rapid combustion of coal dust, and not, as wa3 previously supposed, the comoustion of gases draining from freshly-ex-posed surfaces of coal. ... e Wo must consider the combustion or coal as a reaction between tho solid fuel and atmospheric oxygen. The speed of combustion 'will depend mainly on the amount of surface offered to tho oxidising influence. If, then, the coal is in an exceedingly finely-dmded form—for instance, so fine that the wholo of it will pass through-a scroen having 200 meshes to the linear men, which is tho size usually employed in coal-dust firing—the -area of surface offered to the atmosphere is increased many hundreds or thousands of times, and' the velocity of combustion. is correspondingly increased. Further, it is possible to so mix the finely-divided coal and air. that each minute particle of coal is surrounded with the proper proportion of air to ensure its complete combustion, and, in this way. the i maximum, of efficiency is obtained from the fuol. . it has been found, too, that be3ides , obtaining* the highest efficiency possible , from high-grade fuels, it is possible to use in the finely-divided state grades of , coal so low that previously they had been considered to be worthless. Screenings, refuse from* coal-washing plantsi dumpings from hand-picking belts, and , fuels that contained 30, 40, and 50 per cent J of inert ash, recovered from lako ' bottoms and other - unlikely places, are ■ ' being pulverised and used" in' this new I way. _ . . ' ' The main essential to success is that the ooal dust and air shall be properly [ proportioned and thoroughly mixed before entering the grate. The usual 1 form of burner consists of'a screw-con-voyor, which carries the dust from the rtceptacle in which it is stored .to a ' pipe, where it meets and becomes thor--1 oughly incorporated with a, current of ! air from a fan that carries it to the fur- » nace. As coal-dust firing bfecOmos more ' general it may be expected that con- ■ siderabie improvement will be made in " the burner. Accurate and intimate mixing of tho proper proportions of coal dust and air 1 are essential to success. A useless excess of air unnecessarily lowers the tem- * perature of the flame and thus wastes ■ heat. On the other hand, an excess i of coal dust results in incomplete com- ) bustion and consequent loss of. fuel. It will bo seen, then, that thp burner ia as i vital to the coal dust-burning furnace t as the carburetter is to the internal . combustion engine. In fact, the funcr tions of the two are identical. r Up to tho present, coal dust firing 3 has not been employed for private domestic heating. As the' fuel becomes more common, it is probablo that an > apparatus'will be designed for use in, at any rate, the large privato house. It \ has the disadvantage, however, of being \ an exceedingly dirty fuel to handle, and as it forms a highly explosive mixture | with air, if handled carelessly it may bo . vory dangerous.