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November, 1928

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Low temperature carbonisation is regarded by the colliery manager as one of those developments in coal utilisation which do not directly menace output. Many other processes that aim at more efficient utilisation of coal involve, by their very nature, a reduction in demand, at least initially and although in theory enhanced efficiency leads to lowered costs, expanded activity and eventually to increased demand, there is inevitably a period of restriction upon which colliery folk cannot be expected to look very favourably. Since low-temperature processes usually aim at the production of a substitute fuel, and incidentally extract by-products, there is no such qualifying sense of jealousy. In fact, as we have often pointed out, the industry is watching these developments closely and with great interest, in view of the added value which the extraction of by-products inevitably confers upon the raw product.

It has been known for some years that developments in connection with the pre-treatment of coal have been proceeding on the North-East coast, but a paper presented to the recent Fuel Conference, entitled "The Economic Utilisation of Fuel in the Production of Electricity," by Mr. R. P. Sloan, Chairman and Managing director of the Newcastle Electric Supply Company, contained the first particulars that have been released for publication. Subsequently Babcock & Wilcox Limited, the manufacturers of the equipment in question, have issued supplementary information. Great interest has been evinced in the enterprise and the following notes will enable our readers to appreciate its importance. Two points were in view in the design of the plant, viz., the desirability of establishing in this country a source of motor-spirit and similar derivatives, and the more economical generation of electricity. No attempt is being made to manufacture domestic smokeless fuel, the semi-coke produced being consumed beneath the power-station boilers; nor is any economy of coal, per se, an object, since the actual consumption of raw coal per kWh. must necessarily increase in proportion to the amount of by-products recovered. The new plant is solely intended to permit the manufacture of these products without any additional charge for fuel as regards the electricity generation, and indeed, it is claimed that some economy in this latter sphere is actually achieved.

At Dunston a self-contained boiler house has been erected to operate in conjunction with the coal-treatment scheme ; it houses four boilers, four retorts, and pulverising plant. Northumberland "rough slack" coal is used and is initially screened so as to remove all below ¼ in., this material being delivered to pulverising plant and utilised to fire one of the 30,000-lb. boilers along normal lines. By regulating the supply of fuel to this boiler, variations in steam demand can be met, thus permitting the other (coke-fired) boilers to operate under steady conditions in the actual conditions at Dunston, with the turbines electrically interconnected with a large network and several other plants, this feature is probably not of such great importance as it might be elsewhere, but it is undoubtedly advantageous. A second 30,000-lb. boiler is coke-fired with fuel derived from two retorts, each having a capacity of 30 tons per diem, while the other two boilers are operated as one unit with a capacity of 18,000 lb. per hour, using fuel from two 20-ton retorts. Thus, on full load the plant can furnish 78,000 lb. of steam per hour at 250-lb. pressure and 700° F., while the carbonising plant can handle approximately 100 tons of coal per diem. A separate building accommodates the gas-stripping and by-product plant, which is of normal design.

Two main classes of carbonising equipments can be distinguished, and the present design falls among those in which the heating is effected by the passage of hot gases through the body of the coal itself, as distinct from those in which the heat must penetrate walls of refractory material. Various expedients have also been adopted in other designs to accommodate or circumvent the effects of the swelling which occurs when most coals are heated ; collapsible plates, blending of the charge, and preheating have all their protagonists. In this case preheating is adopted, and it is effected very economically by the use of a portion of the chimney gases. Moreover, no new device has been introduced for the purpose ; the well proven chain grate has been utilised within a brickwork chamber. Conditions are adjusted to provide a temperature of about 120° C., and the coal takes about 15 minutes to traverse the chamber. Heating gases can be taken from either side of the boiler air-preheater and mixed to maintain the required temperature. Preheated fuel is then delivered to the retort by a screw conveyor, entering through a rotary sealing valve ; special gear within the retort levels the fuel as it arrives.

Fig. 1 gives, in the general arrangement of the plant, a diagrammatic sectional elevation through one of the retorts, from which it will be seen that the fuel bed is relatively shallow and is supported upon a series of toothed rollers. The latter are hydraulically operated and extract the coke evenly over the bed, before the retort begins to contract towards the bottom. Thence the coke passes through further grinders and a sealing valve before entering a screw conveyor which takes it to the boiler hoppers, distributing it there with the usual oscillating movement. About three hours are occupied by the coal in passing through the retort.

The heating medium is a mixture of hot gases and low-pressure steam, the latter being supplied so far as possible from the quenching of the hot coke on its way to the stoker hoppers, supplemented to some extent by steam bled from the main turbines. The hot gases are provided by the combustion of fuel in a fire-brick chamber below each retort. At Dunston coke-oven gas is normally utilised for this purpose, though arrangements are made for the alternative use of either producer gas or pulverised coal. From this combustion chamber the gases pass through a brick-lined flue to an annular distributing duct around the retort, the steam being introduced into the flue and the temperature of the ensuing mixture being adjusted thereby to give between 650 and 700° C. Rich gas is withdrawn from the top of the retort by ordinary gas exhausters and thence passes to filters, condensers, tar extractors, &c. Light oils are absorbed from the gas by wash oil, which is a product of the distillation process, and this is subsequently distilled to separate various fractions ; the chemical recovery plant is on normal lines and furnishes by-products according to the chart given in Fig. 8. Surplus stripped gas is eventually burned at the back of the boilers, as indicated in Fig. 1 ; it has a calorific value of approximately 80 B.Th.U. per cub. ft. The gross yield of gas from the retorts, i.e., the rich gas derived from the coal plus the non-condensible heating gases is about 35,000-37,000 cub. ft. per ton of coal treated.

Since the semi-coke produced at the relatively low temperature employed is free-burning although containing comparatively low volatiles, its combustion in the boilers does not present much difficulty, particularly in view of the presence of the gas burner at the back of the combustion chamber. In this respect it is a very different proposition from the use of high-temperature gas or coke-oven coke, although even these fuels can, of course, be handled effectively with special furnace settings. The Dunston boilers are normal Babcock & Wilcox C.T.M. type with air preheaters and forced and induced draught ; the superheaters are interposed between the front generating tubes and the main bank. Compartment chain-grate stokers are used, with a suction ash system of standard type. Two 35-ton overhead bunkers are provided to handle the screened coal, feeding direct to the preheater hoppers, there being also chutes available to supply the main stoker hoppers and the gas producer.

As regards personnel there are in each of the three shifts at Dunston two boiler firemen, two retort attendants, one distillation plant attendant and one ash man, or probably three men additional to these required for a normal boiler house of similar capacity. Further, two coal men and one pitch man are required on one shift per diem only. Mr. Sloan states that a plant four times the capacity of the existing Dunston installation could be operated with but approximately 40 per cent, increase in labour cost and without any appreciable increase in the cost of supervision, thereby materially reducing the operation cost per ton of coal distilled. The power taken by all the auxiliaries used in the pre-treatment and distillation plant amounts to 0.88 B.H.P. per ton of coal carbonised per day and this also would be reduced in a plant operating on a larger scale.

Obviously the economic results of this equipment will receive close scrutiny. Mr. Sloan, in his paper mentioned previously, appears to be satisfied with the results, particularly from the national standpoint, and Babcock & Wilcox Ltd. have published the balance-sheet reproduced, which they point out is based, not on estimates, but on the actual results obtained at Dunston from the carbonisation of 27,300 tons of coal and on prices ruling in January last.

These figures show a profit on the process of nearly 3s. per ton of coal treated and tend to be conservative in regard to the prices of by-products as well as the value placed upon the coke and gas. Further, it has been calculated that with a plant of four boilers, each of 60,000 lb. evaporative capacity, equipped in this way, the operating and maintenance charges per ton would be reduced to 3s. 4d. per ton, thus increasing the credit balance to 4s. 3½d. per ton. On the other hand, it will be observed that the statement does not include any capital charges.

From the technical point of view the equipment is operating, we understand, with much satisfaction. No disguise is made of the fact that many and great difficulties were encountered in the early stages of its design, but these have been overcome, and the gear has operated for long periods during the past three years without a hitch.

Drawings and Photographs accompanying the article