1023 November 15, 1918. THE COLLIERY GUARDIAN. per ton of dry coal—is available as surplus energy, and to overlook the fact that the internal require- ments of the plant must first be satisfied. In order to ascertain these, the following data may be taken for a battery coking 350 tons of washed slack per day —say, 310 tons in the dry state: — B.Th.Il- Surplus gas at 450 B.Th.U. per cubic foot, 2,475,000 B.Th.U. per ton of dry coal x 13 tons per hour 32,175,000 Deduct (a) Power required to run washer y, crushers, water and other pumps, exhauster, charging and discharging machinery, etc. Mean of day and night load, 400 brake-horse power, say 300 kw. at switchboard. 300 kw. x 15,075 B.Th.U. per kw.- hour ............ ......................... 4,522,500 (bj Steam for ammonia and benzol stills, boiler-feed pump, steaming out and miscellaneous purposes, including 10 per cent, steam distribution losses (3851b. per ton of coal coked), 5,000 lb. per hour x 3'5* cu. ft. of gas per lb. of steam x 450 B.Th.U. per cu. ft.... 7,875,000 ---------- 12,397,500 19,777,tOO * This is reduce 1 to 2’6 cu. ft. in Bonecourt boilers. (1) Regenerative Ovens and Gas Engines. The surplus energy available is therefore 19,777,500 B.Th.U. , i -i xx TT 1 •>-------xx — L^ll kilowatts. 15,075 B.Th.U. per kilowatt (2) Waste Heat Ovens and Boilers. A similar battery of waste heat ovens (the waste heat and surplus gas combined) will raise 2,000 lb. of steam per ton of coal. Lb. per hour. Total steam, 13 tons coal x 2,000 lb............ 26,000 Deduct (a) 300 kw. for internal require- ments as before, at 251b. of steam per kilowatt hour ...................... 7,500 (b) Steam for stills, etc., as above ... 5,000 ---- 12,500 13,500 The surplus energy in this case is accordingly 13,500 lb. - 25 lb. = 540 kw.* (3) Waste Heat Ovens and Gas Engines and Boilers Together. If the surplus gas is used in gas engines instead of under boilers, the waste heat ovens will yield a larger amount of power than 540 kw. The waste heat will raise 1,400 lb. of steam per ton of coal, and the surplus gas will amount to, say 2,200 cu. ft. per ton of coal. Lb. Total steam, 13 tons of coal x 1,400 lb. ... 18,200 Deduct internal requirements as before 12,500 Total ... ... 5,700 Kw. Power from steam 5,700 — 25 cu ft. ... 228 Power from gas -’.200 eu. ft. x 450 B.Th.U. 657 8 15,075 B.Th.U. Total power ... ... 885 If, then, the gas is burnt in gas engines, more power is derivable from the regenerative than from the waste heat oven, but the regenerative oven, which costs more, is not per se more economical. There is no economy in using surplus gas for raising steam, but rather the reverse. It should not be used for that purpose if coke breeze is available. The combination of regenerative ovens and boilers is therefore de- fensible only in the event of the boilers being remote from the battery, or if the full power is wanted during the day only, in which case the overnight make of gas may be stored. If the maximum kilowatt production from a given amount of energy were the only consideration, the advantage is overwhelmingly on the side of the gas engine. The greater cost of the gas plant, its higher operating cost and its low efficiency at low loads are of no consequence beside the outstanding fact that, as compared with the boiler and turbo-generator, the some amount of energy gives some 2^ times more power. But whilst the maximum of power may repre- sent the ideal point of view, in most instances there are other consideration to be weighed. For example, when the maximum power consumption is once estab- lished, the coke ovens must be made to live up to it, and there is no room for changes in the fuel market, strikes, irregularities of supply, or variations of any kind in the output of potential thermal units. The plant then becomes a gas producer plant. The gas engine has not kept -pace with the steam turbine in its developement, either in size of units or in sim- plicity. It is still subject to improvements or to inven- tions of even a revolutionary character. With coke*- oven gas as fuel, the reliability of the gas engine is more or less in doubt; thus, in an iron and steel works the undiluted gas from the coke ovens would not be preferred to blast-furnace gas for power purposes, but would be sent to the steel furnaces. Finally, as some steam is required at every coke oven plant, the size of the plant and proportion of steam necessary must be taken into consideration. For these and other reasons peculiar to each plant no general rule can be laid down. Producer Gas. The oven-gas may be released for other purposes by heating the ovens with producer-gas. In that event the steam required for the ammonia and benzol stills will be raised by direct firing with coal, along with the steam required for the producers, because direct firing is essentially more economical than gasifying for boilers. Owing to the large capital expenditure * These figures do not take into consideration the maxi- mum dry load, and cannot therefore be taken as a basis for the size of the generating sets. From that point of view the internal .consumption and surplus combined would indicate for the day load two gas engine sets of 800 kw. each for (1) the regenerative ovens, and two steam turbine sets of 500 kw. each for (2) the waste heat ovens, exclusive of stand-bys in each instance. involved, a producer installation with ammonia re- covery is not applicable to small coke oven plants. Assuming, therefore, a battery coking 620 tons per day, or 226,300 tons per annum, the producer fuel required will be 36,500 tons per annum, and the steam as follows : — Tons. For producers ... ... ... ... 63,875 ,, stills ... ... ... ... 38,925 Total ______________ 102,800 The equivalent of this in boiler fuel will be 102,800 4- 6 = 17,133 tons, and the cost of gas will be as follows : — £ Producer fuel, 36,500 tons at 10s. per ton 18,250 Boiler fuel, 17,133 tons at 12s. per ton ... 10,280 Producer wages, 36,500 tons at Is. 6d. per ton_____________________ ________ 2,737 Boiler wages, 17,133 tons at 9d. per ton ... 642 Repairs, stores, and maintenance, 36,500 tons at 9d. per ton ______________ 1,368 Interest and depreciation, 10 per cent, on cost of plant, say, £120,000 ________ 12,000 Sulphuric acid, 1,140 tons at £2 per ton ... 2,280 Bags and packing sulphate, 1,140 tons at 7s. per ton___________________________ 399 £47,956 Less sulphate (70 lb. per ton), 1,140 tons at £13 net at works ______________ 14,820 Net cost of gas per annum_____________£33,136 At these (pre-war) prices the cost of producer gas is £33,136 4- 226,300 tons, equal to, say, 3s. per ton of coal carbonised. This represents the cost of releas- ing a further 5,000 cu. ft. of gas, or 2| million British thermal units per ton of coal carbonised, to add to the surplus 5,500 cu. ft. already obtained as a by- product. Low Temperature Carbonisation. Indefinite and incomplete carbonisation of coals of good coking quality is economically unsound, and (until after the demand for metallurgical coke is satis- fied) it is not to the best interests of the country, whether the coke is destined for domestic use or for gasifying in producers. The most liberal concessions as to the yields obtained of fuel oils and other pro- ducts do not justify it. The treatment, therefore, is most suitable for coals inconvertible into metallurgical coke, and has no place in this discussion. Seif-Sealing Doors. When a charging larry is part of the equipment, the winches necessary for oven doors of the lifting type require more room on top of the ovens than can be spared. The substitution of a door removable out- wards and sideways by the ramming machine, or by a travelling door crane, is a natural development, and presents no mechanical difficulties; but the “ self- sealing ” feature of the doors is not yet satisfactorily solved; and their maintenance* is a formidable matter. Coke Quenching and Handling. The inclined coke hearth finds most favour in this country. Although costly in the first instance, it is a ’ permanency and ideal for quenching, and a stock may be held on the bench to tide over interruptions in railway traffic. The coke is removable from the foot of the slope either by conveyor, the upkeep and drive* of which costs nearly 2d. per ton of coke, or by hand grips directly into trucks; but if there is sufficient natural fall in the ground upon which the battery is built, the coke might slide* off an inclined hearth on to a coke car, which would make a very perfect arrangement. The car is an alternative to the inclined hearth, and the lost stocking-room is in a measure compensated by sliding the coke* off the car on to a bench or screen. The relatively light weight of a car admits its eleva- tion by incline or hoist to the higher level required for screens. Drenched with quenching water and steam, it has only a short life, but it is cheap to re- place. It is preferably built in two parts, mounted on four standard railway axles and wheels, with a large floor area—not less than 50 by 13 ft. for an 8 ft. oven, and 60 by 14 ft. for a 10 ft. oven. The coke is more readily quenched, and steams off drier, if spread thinly on a large floor. Apparatus carrying motors and gearing, weighing ten times more than the coke it carries, and subject to the action of quenching water, breeze, and steam, is to be avoided unless the site is of such a nature as to make it obligatory. In that case the machine should be duplicated so as to avoid a complete inter- ruption when there is a breakdown. American practice differs from British, just as the conditions differ. A batch of coke weighing from 10 to 12 tons cannot be spread thinly without the dimen- sions of the car exceeding practical limits, and the car is therefore loaded deeper, with the inclined floor measuring only about 40 by 10 ft. It is mounted on double bogie wheels at each end, and is adapted to run on standard tracks with other traffic. Again, the batteries are often so large that only a few minutes elapse between the discharges, and there is no time to quench and steam off in front of the oven. The quenching is therefore done at a station at the end of the battery, with the advantage that there is no cloud of steam to obstruct the workmen and attack the oven ironwork. The coke is quenched in the car at the station in some instances; but in order to release the car for further service and preserve* its life, the present practice tends towards a partial quenching in the car, completed on a sloping bench or screen. If we con- template similar practice, the* arrangements must necessarily conform to those in America; the surface* of coke exposed to the air must be reduced, and the ramming out of the coke* and moving of the car must be more smartly done. Ammonia. Although up to 50 per cent, of the nitrogen may be liberated in coking, only from 13 to 16 per cent, are recovered as ammonia. There* is no control over the ___________________________ proportion recovered, and one has to be satisfied with what is yielded from any coal by the conditions of operation. All that is known is that the temperature at which it is formed differs with different coals. For- tunately, despite these limitations, it is recovered at a lower figure than the cost of fixing nitrogen in any other chemical combination. The thermal decomposition of the gases that are evolved is inappreciable below 870-900 degs. Cent., and this fixes the temperature of operation at the top of the oven chamber. When the coal is wet, as compared with dry, the state of dilution of the gas with water vapour protects the constituents of the gas from dissociation into its elements at that tem- perature. The importance of equality of pressures in the oven is also known; if it were practicable to have more than one ascension pipe, and an even pressure throughout the length of the collecting main, the pressure in individual ovens might be more nearly uniform than is possible in our present practice, and this would result in larger yields of ammonia. A contributory improvement is furnished by avoid- ing high mains and long ascension pipes, which are liable to become coated internally with carbon, by charging so as to fill to the crown of the oven and minimise the hot surface of brickwork exposed, and by placing the ascension pipe in the middle of the oven, if possible, to equalise the travel of the gases. The catalytic effect of iron and its oxides at the temperature at which ammonia is dissociated is most unfavourable to the yield. In one case it has been observed that an increase of iron in the ash contents of a coal over a term of years had a corresponding adverse effect upon the yield of ammonia. Calcium compounds have a catalytic effect of a con- trary character, a fact that has proved attractive to investigators for many years. Campbell found that the addition of 2 per cent, of lime increased the yield of sulphate by 10 to 15 per cent.; Knublauch added 2J per cent, for an increase of 30 per cent; Schrieber added 3J per cent, for an increase of 20 per cent.; whilst Patterson obtained an increase of 13 per cent. But apart from the fact that part of the sulphur is fixed which would otherwise be volatilised, no process is admissible which adds to the ash contents of metal- lurgical coke. Dr. Diehl proposed the use of lime- stone instead of lime, on the supposition that the volatile sulphur is driven off before decomposition of the limestone takes place, and although this found support in the experiments of Stevenson, it is directly controverted by the researches of Campbell. Tar. As is the case with ammonia, the yield is favoured by a low temperature at the top of the oven and quick removal of the gas from the chamber. Coke oven tars vary widely in specific gravity and compo- sition, but there is no British publication which corre- lates their composition with the district in which they are produced, nor with the method and tempera- ture of recovery, such as is found in the series of analyses made by the United States Government (Department of Agriculture, Circular No. 97). It is to be supposed that in this country any differences are frequently attributed to the coal, which an impar- tial examination would probably more correctly trace to the treatment or process. But it should be empha- sised that the tar distiller finds his operations easier, shorter, and more profitable the lower the percentage of carbon, and there is no doubt that in buying he differentiates one tar from another. The maximum yield and (from the distiller’s point of view) the best composition are not obtainable from ovens equipped with dry mains operating at high temperatures. Hot Direct Process. The recovery of ammonia by the hot direct process made a very strong appeal when first introduced into this country, since it required no cooling water or condensers, and produced no effluent. At that time benzol was not recovered as a matter of course, as it is to-day, and the cooling of the gases was not necessary, The advantage over other processes is now limited to this—that somewhat less steam is required, and a smaller quantity of less noxious effluent water is pro- duced, since it contains no lime. The dry main and hot circulation essential to the process yield a tar low in naphthalene, light oils and creosote oils, and having 70 per cent, and upwards of pitch. The naphthalene remaining in the gas is dissolved (in the absence of tarry constituents) in anthracene oil. The chloride of ammonia is dissolved out of the tar by condensing for that purpose a small proportion of the liquor, and may be disposed of in one of two ways: it may be concentrated as a separate product, or (provided the formation of hydro- chloric acid is negligible) it may be run into the saturator. Semi-Direct Process. In the several variations of this process the ammonia of the condensed liquors is led indirectly through the still into the saturator, and only the ammonia remaining in the gas goes “ direct.” The pre-heat of the gas which the Koppers process demands was originally obtained by interchange with the hot crude gas, until the inventor began to use steam pre-heaters about 1911. Originally, also, > the still gases were delivered into the crude gas before the coolers, but this procedure was very properly given up; it can be shown to add to the work of the con- densers as well as to increase the weight of liquor to be distilled. As now practised by Koppers, the gas is re-heated after being cooled and passed through tar-extractors, and the still gases added to the stream entering the saturator at the following typical temperatures: Entering superheater, 27 degs. Cent.; leaving, 85 degs. Cent.; after addition of still vapours, 81 degs. Cent. ; outlet of saturator, 45 degs. Cent. One early application of the process at Mont Cenis Colliery, Herne, Westphalia, was not successful, and led to a modification which goes by the name of that colliery. In this the still gases are partially cooled (the condensed liquor being returned to the still) before being added to the oven gases. Average tempera-