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April, 1931

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We have frequently stressed the necessity for the pre-treatment of raw coal before it is offered for domestic consumption. The gas industry already practises carbonisation on a large scale, conserving the valuable chemical derivatives and providing domestic fuels in the shape of gas and coke. Despite improved methods of using gas coke, there have been many attempts to provide a more easily ignitable solid fuel by means of low-temperature carbonisation. Most of these attempts prove difficult to adapt to large-scale operation, because they necessitate the treatment of the coal in a narrow retort or thin stream, which in turn involves the multiplication of small units for any considerable throughput. The electrical generating industry has hitherto taken little part in such developments, and has accordingly been reproached for the waste of by-products incurred by its straight use of raw coal. Conditions during the war brought special attention to this point. Messrs. Merz & McLellan, with the co-operation of the Newcastle-upon-Tyne Electric Supply Company, initiated experimental work on this problem as long ago as 1913, directed towards the combination of low-temperature carbonisation with ordinary power house practice. As a result an equipment has now been developed which not only serves this purpose but also supplies in some degree the demand for a good domestic fuel. It has proved capable of large-scale operation, and although it incorporates numerous ingenious points, it is straightforward and eminently practical in design. Such a plant will enable coal producers to provide domestic fuel that will not contribute to the smoke nuisance and at the same time secure a larger share in the oil fuel and chemical by-product markets.

In November, 1928, we described the Dunston low-temperature plant as it then was. The two 30-ton-day retorts are still in operation and have enabled many problems to be solved relating to the behaviour of different classes of coal and the handling of their derivatives. The Newcastle-upon-Tyne Electric Supply Company, and Babcock & Wilcox Ltd., who built these retorts, have now, however, carried the matter a step further by the erection of a single 60-ton per day retort on improved lines, and on the basis of the experience gained, Messrs. Babcock & Wilcox are now prepared to quote for plants capable of handling 300 tons of coal per 24 hours in one unit. The 30-ton retorts produced no domestic fuel, but from the more recent design some 40 per cent, of the 45 tons of semi-coke produced per 24 hours is available for this purpose. Since the remaining 60 cent, is suitable for boiler firing, either on stokers or in pulverised form, it is evident that the carbonising equipment is still eminently suitable for operation in conjunction with public or private electrical generating plant. It is not now, however, tied up to its boilers, as was the case with the 30-ton retorts which delivered all their coke direct to stoker hoppers with a view to retaining its sensible heat. The new equipment produces quenched and screened coke and is designed to run at full output for long periods without being affected by the normal variations of load on the boilers with which it operates, as the semi-coke can be stored in bunkers so that the peak loads may be carried therefrom.

In order to reduce capital and operating costs and to avoid a multiplicity of units it was necessary to increase the individual capacity of the carbonising retorts. To do this with the circular type used in the 30-ton equipments involved undesirably long shafts for the coke extractor rollers and a more complicated form of coal spreading gear. A rectangular construction has therefore been adopted, maintaining a similar depth of coal bed but allowing the use of repetition cast-iron sections for the sides instead of the specially shaped wrought plates used in the older designs. The brick lining is also much simplified. Each extractor roller in the new retort is of the same length, while the levelling device is simplified into a kind of drag-link conveyor. There are 6 extractor rollers in the 60-ton retort at Dunston, and if required the capacity of the retort can be increased to 75 tons per 24 hours by making it 28 in. wider. With the latter dimension four such retorts, erected without party walls except for the central support of the rollers, provide a unit for treating 300 tons per 24 hours without introducing any unknown factors.

Carbonisation tests have been carried out with many kinds of raw coal, but naturally the greatest experience has been with the production of local pits. The coal may be small, but should be screened to remove the dust run-of-mine coal and pieces up to 3 in. cube can be utilised if specified. Normally, of course, coking coal is not used in power stations, but is in demand for coke-oven and gasworks processes, and it is a special feature of this process that it employs non-coking coals. For the production of high-grade domestic semi-coke it is obviously desirable to use a clean coal with a low ash content. Investigations into the treatment of lignite in these retorts are now proceeding.

At Dunston the new retort has been erected in an existing building, necessitating placing the preheater to one side of the retort to enable coal to be fed from an existing bunker. The normal arrangement provides for the preheater being placed immediately above the retort as shown in Fig. 5, and this makes the whole equipment very compact. This arrangement also eliminates the screw conveyor now necessary to convey any riddlings from the preheater to the retort.

Preheating is an essential preliminary, as the drying and heating of the coal prevents the condensation of the oil vapours in the retort, which might otherwise impede the flow of the distilling gases. It also effects an economy in the heat required to distil the coal, as the preheating is carried out by heat in waste gases derived from the boiler plant. The preheater employed is a standard Babcock & Wilcox chain-grate stoker, preceded by a control gate and Lea coal meter. Heating is by means of flue gases at 200° C. drawn from the adjacent boiler plant by means of a fan.

Preheated coal, after being delivered into the retort proper through a rotary sealing valve, is levelled across the retort by a drag-link chain. An ingenious device ensures that the retort is kept fully charged. At the end of the retort remote from the coal feed there is a coal level indicator, which is raised as each link of the levelling conveyor passes. The position to which this indicator drops when it is released on to the top of the coal bed determines, by means of a simple mechanical device, the position of a rheostat, and this in turn controls the speed of the motor driving the valves of the hydraulic gear which operates the extractor rollers. Thus if the fuel bed tends to become lower, the action of the extractor rollers is slowed up, and vice versa. This device ensures that the level of the fuel in the retort is maintained automatically and relieves the operators from any necessity to make continuous small adjustments ; in fact the action of the control is analogous to that of the feedwater regulators frequently used on boilers.

The extractor rollers are built up of cast-iron nine-toothed star wheels, threaded on to a square shaft. Their minimum clearance with the edges of the ports in the retort bottom is one inch. As the star wheels are nine-toothed it is possible to thread them on to the square shaft so as to produce a stepped helix, and the coke is thus continuously delivered instead of being dropped in batches ; Fig. 3 shows an assembled roller and one of the star wheels. The use of the same pattern for all the star wheels is of considerable interest, since it allows repetition castings to be employed and cheapens the construction accordingly. Opposed hydraulic rams, packed with stationary S.E.A. rings, are employed to actuate the extractor rollers, through ratchets of a very robust type, and all the drives are on one side of the retort, thus facilitating the placing of two retorts side by side without difficulty.

From the retort the semi-coke passes through sealing valves to an inclined quenching trough containing a drag-link conveyor ; the latter, after passing the coke under water sprays, conveys it through a water bath in which the quenching is completed and delivers it into the boot of an elevator for distribution to bunkers or railway wagons. The quenching trough is seen in Fig. 7. Screening of the larger size, which forms an excellent domestic fuel, is at present done on a stationary inclined screen located beneath the coke delivery chute. The large coke is bagged by hand for sale, while the small is used in the boilers. A special grading and quenching equipment has been designed for future plants, whereby the semi-coke will be automatically graded, quenched and delivered to the storage hoppers or railway wagons. During this process about half the low-pressure steam required for the distilling medium is produced from the sensible heat of the coke and any combustible gas given off by the hot coke is utilised for the production of the inert distilling gases.

As we pointed out in our earlier article the Dunston retorts are internally heated, various fuels being practicable for this purpose. The use of superheated steam as a distilling medium was made the subject of extensive trials, but was abandoned in favour of the present method owing to the lower operating and maintenance charges thereby obtained. The use of hot inert gas necessarily depreciates the quality of the gas derived from the retort, but, on the other hand, the stripped gases can be used to meet the heat requirements of the plant, thus making it self-contained. At Dunston the calorific value of the retort gas is of the order of 80-100 B.Th.U. per cu. ft. and is burnt under a boiler, while coke-oven gas, which is very cheap locally, is employed for heating the retort. The temperature resulting, in the special combustion chamber, is of the order of 1,100° C., depending upon the proportion of excess air admitted, which is controlled to suit the nature of the coal in the retort. This temperature is, as already mentioned, reduced by the admission of low-pressure steam recovered from the quenching equipment, and it is finally controlled, during its passage to the retort, by the use of a live (or exhaust) steam jet, the gases actually entering the retort, through the openings under the extractor rollers, at 650-680° C. The flow of gases through the retort is normally controlled by reference to the temperature of the gases leaving the retort, and adjustments are made to maintain the temperature between 300 and 350° C.

Careful studies have been made of the operating costs of the process, and the curves of Fig. 11 are based on the experience gained at Dunston. Three schemes are indicated, showing the costs (a) with the retort only, for the production of smokeless fuel and crude oil, (b) with the retort and tar plant, producing smokeless fuel, creosote, crude tar spirit, and pitch, and (c) with the retort, tar plant and refining equipment, giving as products smokeless fuel, light and heavy creosote, motor spirit and white spirit, tar acids, and pitch. The curves show the great advantage of operating on a large scale, but obviously flatten considerably after the 300-ton per 24 hours region they refer only to labour and supervision charges. Figs. 13 and 14 show how plants of 60 and 300 tons per day respectively, operating independently, would be normally laid out.

Gas and tar treatment at Dunston remains practically as it was for the 30-ton retorts, except that additional stills for refining the creosote and tar acids have been installed ; these are, however, of purely chemical interest. The products obtainable from a typical Northumberland coal are set out in Fig. 12, the three schemes indicated corresponding to those outlined in connection with Fig. 11.

The world market for by-products from both high and low-temperature carbonising plants has become very depressed, and we are of the opinion that the financial return from low-temperature carbonisation in conjunction with power plants will largely depend on the scale of a portion of coke in the domestic sphere. As manufactured at Dunston this coke is very suitable for ordinary grates ; it ignites readily and makes a definite contribution to the abolition of the smoke problem. Spirit produced from the plant is utilised by the fleet of motor vehicles belonging to the Newcastle Electric Supply Company and markets for other products have been found. That portion of the coke which is too small for domestic use is being used for steam-raising on chain-grate stokers, and provision is also being made for its utilisation in pulverised form. A 30,000-lb. Babcock & Wilcox boiler which is installed in a house adjacent to the retort has been fitted with a Fuller-Bonnot mill and Lodi burner, and has already operated very satisfactorily on this fuel. On the occasion of our visit to the plant this unit was out of commission for cleaning and the fitting of experimental apparatus in the chimney for arresting flue dust, but further experience is to be gained in due course.

There are many circumstances abroad where this combination of low-temperature carbonisation with a boiler plant would have undoubted financial advantages. It has been developed to a point in which the mechanical difficulties of large-scale operation can be said to have been overcome, the present 60-ton retort having been on full throughput for some seven months. It is compact and practical, simple and efficient, and employs no components that are not thoroughly understood. In this country we suggest that those municipalities which control both electrical and gas undertakings should investigate the possibilities of the plant ; they already have the organisation to market the domestic fuel and the chemical staff to supervise the by-product equipment. The electrical department would have no additional equipment to operate other than plant very similar to that with which they are already perfectly familiar.

From a national point of view municipalities would be doing good work by encouraging co-operation between their departments in this way, and if the work was undertaken on sufficiently large scale, profits should be realised. In any case the Dunston plant merits close attention if only because large-scale low-temperature carbonisation plant with low operating costs has now definitely become practicable.

Drawings and Photographs accompanying the article