THE MANUFACTURE OF GAS

Otago Witness, Issue 4048, 13 October 1931, Page 65

THE MANUFACTURE OF GAS

DESCRIPTION OF THE PROCESS. How many people when they turn on their gas stoves in the morning give a thought to the production of gas, which so greatly facilitates the preparation of their breakfast? Indeed, one of the remarkable things about civilised man of to-day is his unquestioning acceptance of most of the things which go towards increasing his efficiency or easing his life. Who, for instance, of the thousands of users of gas heaters and gas cookers in Dunedin knows anything of what goes on at the Gasworks? —knows, for example,

that gas is washed and cleaned before it enters the holders and passes out through the mains to be distributed over the city? Behind the steady flame which burns so vigorously beneath the family porridge lies a history of production the romance and interest of which remain practically unknown. Our representative recently inspected the processes of the manufacture of gas from the time the coal is received at the works until the pure gas is pumped into the containers. He saw powerful grips seize a truck of coal bodily and slowly but irresistibly turn it over on its side until the contents were discharged into the bins, whence endless chains of buckets scooped up the coal and carried it away into the great stores where hundreds of tons lay heaped. He peered into glowing furnaces within whose fiery depths the temperature reached the seemingly incredible heat of 1300 degrees Centigrade, followed the great mains along which the gas was drawn, saw it bubbling through great vats of ammonia and then of water, and over beds of iron oxide, until finally it passed out into the mains for distribution throughout the city. Perhaps the strongest impression which remains after an inspection has been made of the Gasworks is the way in which everything possible has been done to guard against any interruption in the supply ot gas through a breakdown in any part ot the plant. Almost every part has been duplicated. In the building in which the coal is stored there is a sufficient supply to last for about four weeks without any addition being made to the stock. In case any breakdown should occur in the elevators conveying the coal and coke to the top of the retorts, a two days' supply is kept in the bins above. Although most of the power for the driving of the machinery in connection with the works is derived from Waipori, an auxiliary steam plant is kept in readiness to take

ip the running should any failure occur n that source of energy. In short, nohing but a succession of accidents could flirow the City Gasworks out of com-

mission. The average person has a vague idea that gas is produced by the beating of coal, thus driving off the gas which is simply collected in the containers. Of the various processes which it goes through, and of the great forethought and care necessary, the man in the street is almost completely ignorant. In the veryfirst stages—the choice of a type of coal from which to manufacture the gas—it is necessary to exercise great care. An important part of the revenue of the Gas Works is derived from the sale of coke, of which about eight hundredweight is sold to outside consumers for every ton of coal passed through the retorts. As various coals differ in the amount and the quality of the coke which is produced from them, preference naturally is given -—other factors being equal—-to those coals which will produce the greatest quantity’ of high grade coke. Undoubtedly the most interesting part of the works is that in which are situated the great vertical retorts, where under the fierce heat of the coke furnaces, the coal is heated to such a degree that the gases contained in it are driven off. Here amid the clanging of elevators and the rattle of coal and coke being discharged into the bins, grimy stokers armed with long iron bars prod the encrusting coal in the retorts, where the terrific temperature has passed far beyond the 1000 degrees mark. A plug is pulled from the side of a combustion chamber, and the flames belch out in a long arm of fire, indicating. the huge amount of inflammable gas within. Water gas is introduced into the retorts, and the combined mixture in the form of heavy brown fumes passes out along the mains. Here, with a falling temperature, the tar and ammonia compounds in the gas liquefy, and are collected in wells to be treated later. The gas is drawn along the mains, and is passed through large tanks, first of ammonia solution and then of water. By this means it is cleansed of the last traces of tar and ammonia which have been retained despite the gradual lowering of the temperature. There still remains, however, a considerable amount of sulphur in the gas, and in order to remove this it is drawn over large beds of iron oxide by which the sulphur is gradually absorbed, and finally the gas in a commercially pure form passes away to the great containers. Since from a ton of coal-about 16 gallons of crude tar are obtained during the processes of extracting and purifying the gas, the production of tar in a commercial form constitutes no small part of the activities which go on in the gasworks. By an ingenious method the tar is freed from the impurities which it contains sufficiently to bring it up to the standard required. One of the by-products from this process is a tar oil which is used in the specially-designed furnaces of the auxiliary’ steam plant, which, in addition to being a stand-by in case of the failure of power from Waipori. is used for the purpose of pumping the gas into the gasometers. By thus using what would otherwise be a practically’ useless product a considerable saving is effected in the cost of fuel. The choosing of the type of coal to be used at the works is by no means a haphazard affair. In the first place laboratory tests are carried out, with the object of discovering what amount of gas will be obtained per ton, what amount of byproducts, especially of coke, and what the quality of the coke will be. In order to ensure that the results obtained by the preliminary tests will correspond with the results under actual working conditions the laboratory at the works is fitted up with a miniature gas retort in which the gas is driven off and collected in practically the same manner as in the works themselves. After a coal has passed the laboratory tests, it is tried in this retort, and the amount of gas and by-products is noted. The various coals thus receive a thorough testing, and that wiving the best all-round results is selected. With the exception of the year 1928, the consumption of gas in Dunedin has shown a steady increase, and the amount used during last year was to a not inconsiderable extent the largest yet produced. The fact that there was no increase during the year 1928 was, it has been explained, in keeping with a general world-wide similar tendency about this period. About 1926 the automatically’ coni rolled gas stove came into common use in England and elsewhere, and by 1928 the

effects of this invention, which brought about a wonderfully increased efficiency and economy in the use of gas as a means of cooking, was felt to its full extent in Dunedin. The resultant lowering in the consumption of gas was, however, only’ temporary, as the new stove quickly found favour with scores of people who had previously used coal or other stoves in their homes, and since then so rapid has been the increase in demand that the departnient'has been forced to undertake the building of another gasometer to meet the demand.