GAS FOR INDUSTRIAL USE The gas manufactured for public supply finds extensive and increasing applications for industrial heating. Being thoroughly cooled and cleansed it can be controlled with nicety through taps and valves, which makes for convenience, cleanliness and efficiency in use. It is a smokeless fuel, and by its use ground space and expenditure on the cost of gas-making apparatus is saved. Since, too, such gas is of high calorific value and does not carry into its flame any large proportion of nitrogen or other inert constituents, it can be used for high temperature processes in simple apparatus without the necessity of providing for that pre-heating of the air or gas or both which is a necessity for such high temperature work when the leaner producer gas is employed. The simplest process of gas making is that used in making producer gas, and the great bulk of gasification effected for such purposes as the heating of steel-melting and other large industrial furnaces is conducted on this plan.
When air is passed through a deep bed of carbon maintained at a high temperature, above 1,000° C, such that complete contact with the carbon is ensured and equilibrium obtained, practically the whole of the carbon is obtained as car bon monoxide, according to the equation If the temperature is lower, even although the contact is complete and equilibrium is still attained, some carbon will be burned to
according to the equation If, however, the high temperature has been maintained and the carbon entirely converted to CO, it is plain that the gas will con sist of one-third carbon monoxide and two-thirds nitrogen, and the equation representing its formation may be called the ideal producer gas equation. If this producer gas is collected and burned with air, it will generate heat according to the equation It will be seen that even if the whole of the heat generated in making the producer gas by converting the carbon to CO were lost, sixty-eight ninety-sevenths of the total heat of combustion of carbon to
would still remain available for use by its com bustion of the gas. This large proportion of heat available for the second stage of the combustion of the carbon in burning carbon monoxide to carbon dioxide is the basis of producer gas practice. There are various factors which cause divergence in the compo sition of producer gas from that of the ideal producer gas equa tion. In the first place, when coal is used as a fuel and is red down on the fuel bed, it is at once subjected to a process of dis tillation or carbonization in the current of producer gas ascending from below, made by the action of the blast upon the carbonized fuel. Producer gas is so enriched to some extent with hydrogen and hydrocarbons, particularly methane, and the percentage of nitrogen correspondingly diminished. More important, however, is the modification in composition brought about by the steam consequent upon a lowering of the temperature of the fuel bed and the formation of water-gas by interaction with carbon. The more steam is used, the lower the temperature and the more car bon dioxide and hydrogen at the expense of carbon monoxide is formed. The percentage of nitrogen is further lowered by the ad mixture with water-gas. Moreover, as the quantity of steam is in creased and the temperature decreases, the rate of steam decom position by the carbon lessens and steam passes through the fuel bed undecomposed. The quantity of steam supplied is best con trolled by the temperature of the mixed blast at a point well be yond the introduction of the steam, so as to allow a thorough mix ing. The temperature of the blast rises with the proportion of steam. It will be understood that undecomposed steam, which be gins to occur in quantity as soon as the saturation temperature of 6o° has been exceeded, is an objectionable constituent in the pro ducer gas, since it is thermally useless and would tend to prevent the attainment of high temperatures on combustion on account of its high specific heat. Bone and Wheeler followed changes brought about in the composition and yield of producer gas, by The two columns "Weight of steam undecomposed per lb. of coal" and "Therms in gas per ton of coal" have been added by the writer.
As the saturation temperature was raised by more steam, the gas composition shows a rise in carbon dioxide from 5 to 13% and a change over from a carbon monoxide producer gas, in which that constituent is dominant to a hydrogen producer gas is explained. The nitrogen has fallen and the percentage of total combustibles has also fallen on account of the increase of carbon dioxide, resulting from the lower temperature of the fuel bed. The calorific value of the gas has slightly diminished, but the volumetric yield increased, so that the yield in therms contained in the gas per ton of coal gasified shows little change. The weight of steam undecomposed per lb. of coal has run up from 0.05 to 0.9 lb. per ton of coal.
The development of the apparatus in which the manufacture of producer gas is carried out can now be traced. It would appear that the earliest gas producers were deep shafts of brick-work, but the name most closely identified with the successful establishment of the gas producer is that of Siemens, and a diagram of his Producer (i 86i) is given in fig. 7. It illustrates how the coal falls from the hopper and lies in the producer above the step grate. The producer was connected to a furnace, and the air for gasification was drawn through the fuel bed by natural chimney draft, operative on the furnace, supplemented at times by a syphon effect, induced by the dis position of the main between producer and furnace.
modern conditions have, how ever, demanded an increased output per unit of space and grate area, and this has been met by putting the producer under positive blast. Such a high rate of working involves a greater tendency to clinker formation, which has been counteracted by the use of steam. Fig. 8 is an example of one of the many gas producers devised to work under these conditions. It is shown as a steel cylindrical shell, firebrick lined, and dipping into a water lute, which enables a pressure of blast to be maintained without escape, and ashes to be withdrawn through the water as required. The grate bars are shown arranged in a truncated cone, and the air-steam blast is admitted to the space between them and the casing. An alternative to this "side-blowing" is to deliver the blast up the centre line of the producer from the bottom, the blast escaping from under a mushroom head. This arrangement is shown in fig. io. The mushroom head in such a "centre-blown" producer must be kept in ashes below the hot gasifying coke to prevent its destruction. The side blown and centre blown arrange ments are occasionally combined for very wide producers, in order to give a penetration of the blast to all parts of the wide bed, but such combination is not at all common and is somewhat difficult to control. The simplest way of blowing a pressure pro ducer is to use an injector supplied with steam under pressure, which can be easily arranged so as to be adequate in amount for injecting the air and at the same time saturating the blast (fig. Io).
It will be readily understood that one of the principal objects to be attained in the design of a producer is that there shall be no accretion of semi-fused ash into large pieces of clinker, and maximum facility for removing the ash necessarily left when the gasification is complete, so keeping the producer in good working order and minimizing labour. Fig. 9 illustrates one mode of attaining these ends adopted in the Kerperly producer. The central grate used for admission of the blast revolves mechani cally in the ash bed, affording little hold for any pieces of clinker or bringing a shearing stress to bear upon them with a dis integrating effect. The outside shell of the producer is lined with fire-brick in the ordinary way in its upper half, but in the lower half is constituted by what is in effect an annular boiler, with no brickwork lining. Clinker cannot form in the same way on the comparatively cold metal surface of such a water jacket as it can on a hot surface of fire-brick, which is itself practically as hot as the coke and ash in contact with it. The steam raised by the annular boiler can be used for the blast. (This combination of mechanical grate and water-cooled sides is also coming into use for water-gas generators.) The ash in the arrangement as shown in fig. i I is delivered into a water trough below, which provides a seal and a convenient channel from which the ash is automatically scooped. Details differ and a dry revolving base without water seal is sometimes employed. The well-being of a producer depends upon keeping the distribution of the blast and the ascending gas current as uniform as possible across the section of the producer, so as to give satisfactory contact in all parts with the descending fuel. Imperfections in this respect, such as the existence of channels up the bed, have their effect in a deterioration in the quality of the producer gas made, indicated by a rise in tempera ture of the gas leaving the producer. For this reason, the top of the producer is usually provided with a number of poke holes, through which pokers are periodically inserted by the producer man, for the purpose of keeping the fuel bed level, filling up hollows or channels, and breaking up incipient clinkers. In some of the more modern designs this work has been minimized by the introduction of a mechanical revolving poker. In the form intro duced by Talbot, the poker was a central vertical shaft running the whole depth of the producer, with two arms, one revolving near the top of the fuel bed and the other just above the grate in the clinker-forming region. This form of poker was difficult to maintain in action over long periods, and it has been simplified by Talbot and others, as indicated in the diagram of the Chapman mechanical producer in fig. 9. There the horizontal arm of the poker is shown revolving near the top of the fuel bed. It is not intended to prevent clinkering of ash, but to keep the top of the fuel bed in good working order.
many purposes, producer gas can be used hot from the producer without further cleaning. Indeed, that is so with the great majority of uses to which pro ducer gas is put. If however the gas is generated for use in the gas engine, which is thermally much more efficient than the steam engine, it must be cleaned and cooled. The most com plete system for providing washed clean producer gas, and at the same time recovering as ammonia the nitrogen in the coal gasified (to the extent of some 90 lb. of sulphate of ammonia per ton) is due to the late Dr. Ludwig Mond (1889). The plant he elabo rated has been simplified by Lymn and Rambush and is shown in the simplified form in fig. r r. The producer carries a very much deeper fuel bed (say i o ft. against 5 ft.) than is ordinarily employed, and more steam is employed in the blast, r lb. per ton of fuel gasified. By this means, the temperature of the fuel bed is kept low, and the coal is given a long time of exposure to the ascending gas current, as it gradually descends towards the grate. These conditions favour the production and preservation of ammonia and of low temperature tars. The gas is washed, freed from ammonia, and cooled, by passing in turn through the three Lymn static washers, the ammonia being absorbed by a solution of ammonium sulphate, maintained slightly acid, and the after-cooling being effected by water. The Lymn washer, which replaced the towers of the original Mond plant, consists of a number of cones fixed to a central shaft, alternating with strips fixed to the outer case of the washer, so that the descending water or liquor is repeatedly forming a falling sheet or shower, through which the ascending gas has to pass. The air blast going to the producer ascends a washer "F," superposed on "C" and is there warmed and moistened by hot water which has been pumped from "C" of ter abstracting heat from the gas. Gas leaving such a plant is ready for use in furnaces, but centrifugal tar extractors and saw-dust scrubbers are employed in cleaning the gas more thor oughly for use in engines.
The following results have been reported by Lymn (1924) as long-period averages using a Durham coal containing 8.o% of ash : Carbon dioxide . . . . . .
Carbon monoxide . . . . . . 2 r •o%Gas analysis Hydrogen . . . . .
Methane . . . . . . . • 4'9%Nitrogen . . . . . . . • 45'3% B.T.Us. net to cu.ft. . . . . . . 178
Gas efficiency I Cal. val. of gas (netto) 1 8o% I Cal. val. of coal (netto) I Average gas yield (cu.ft.) per ton of dry fuel gasified 122,000 Average ammonium sulphate yield per ton of dry fuel gasified . go lb.
Average yield of dry tar per ton of dry fuel gasified . 2 r gal.
Steam into producer per lb. of dry fuel . . . 1 lb.
When ammonia recovery is not attempted, the process of wash ing and cooling is simplified. A washed producer gas has advan tages for many uses in that its control can be made so much more precise and its subdivision for heating purposes so much more readily effected than with gas straight from the producer. Distribution, moreover, is much simplified, so that for example in south Staffordshire a scheme is in operation for distributing pro ducer gas in pipes over a considerable area. Even in a works it is an advantage to be able to convey the gas in steel or cast-iron mains, which drain themselves of all impurities, rather than fire brick lined conduits, in which tarry and dusty deposits accumu late, necessitating periodical clearances, by burning out, or other methods.
Many producer-gas plants are made for power purposes (Dowson, National) and washed producer-gas finds one of its most characteristic applications in the provision of gaseous fuel for isolated gas engine units. In the smallest plants for this purpose, the so-called "suction gas plants," it is the suction stroke of the gas engine which is relied upon to draw air through the fuel-bed of the producer.
The gas on its way to the scrubber passes through a vertical water vaporizer, which provides all the steam required. Water is supplied to the scrubber and the gas passing up it is both cleaned and scrubbed.
The blast furnaces used for the pro duction of pig-iron may be regarded as very deep air-blown gas producers, from which the quality of the gas is lowered to some 90 or i oo B.T.U. per cu.ft., by the oxidation which much of the carbon monoxide undergoes in reducing oxide of iron to the metallic state.
There are some special types of gas producer which have not been described. In one, invented by Dowson, air is drawn in both at the top and bottom of the fuel bed and the gas is collected in nostrils, which open into the producer half way down, where the fuel is red hot. By this means, the amount of tar in the gas is lessened, but the gas of necessity leaves the producer very hot and there are difficulties in keeping the gas exits open for the free passage of gas. In another special type of producer, the ash is run off as a liquid slag. No steam is employed in that case, since a very high temperature is a working necessity, and it may even be necessary to add some flux along with the coal.
BIBLIOGRAPHY; Among books dealing with gas manufacture are: Bibliography; Among books dealing with gas manufacture are: Alwyne Meade, Modern Gasworks Practice; and W. B. Davidson, Gas Manufacture. Producer gas is treated of by N. E. Rambush, Modern Gas Producers; and Dowson and Larter, Producer Gas. More general are Bone, Scientific Uses of Coal; and Haslam and Russell, Fuels and their Combustion. The Transactions of the Institution of Gas En gineers and Proceedings of the American Gas Association, the Gas Journal and Gas World, are periodicals dealing specially with gas manufacture. (J. W. C.)
