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October, 1953

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The Whitehaven pits have always been troubled with a high rate of gas emission, and as early as 1733 Sir James Lowther presented a paper to the Royal Society describing the amount of firedamp encountered when sinking a pit in Cumberland. The first experiment at Whitehaven was most probably conducted in 1746 by an amateur scientist called Dr. Brownrigg who piped the gas from a neighbouring pit and used it to heat furnaces in his laboratory. Even in those days the town authorities were interested in using the gas for street lighting but this was not then possible. About a hundred years later Michael Faraday and Sir Charles Lyell proposed a scheme for extracting methane from the goaves following an explosion at Haswell colliery in Durham, but their ideas were rejected as being impracticable.

Since then there has been much comment on the great volume of gas discharged to waste from the mines, and a great deal of discussion* has taken place on how this methane could best be harnessed practically. It was not until after the Second World War however that the matter received serious consideration when the methane was extracted and utilised on the continent of Europe ; readers will recall a series of articles by L. T. Minchin, published in this journal for May, 1952, and June, 1952, and July, 1952, setting out what had been accomplished there. In Great Britain, Point of Ayr colliery in North Wales had the first installation to make use of methane for fuel purposes, and extended reference to this was made in an article by John Roberts, D.I.C., that appeared in our issue for November, 1951.

The decision to examine the possibility of a drainage scheme at Haig colliery, Whitehaven, was taken in April, 1948, and two years were spent in an extensive survey of the problem. To dilute the gas down to 1 per cent. in the main return it is normally necessary to circulate about ten tons of air for every ton of coal won, and it was obvious that the project could make an important contribution to safety by keeping as much as possible of the gas out of the airways. Among other things the investigation showed that the greatest emission at Haig is about 100 yards back from the face where a separation of the roof strata provides a ready outlet for the gas.

A pilot scheme was set up to test various techniques, and a few holes were bored in 1951 when the gas extracted was led out of the pit by the ventilation air. The pilot scheme gave valuable information and experience for developing the present programme which began in June, 1952. The first methane to be kept from the airways flowed by pipeline to the surface, the gas being moved by its own buoyancy. The extraction plant for drawing the gas came into operation on August 5, 1952, and has been working without interruption since September, 1952. About 500 cu. ft. of gas per minute of undiluted methane has been extracted in the past 11 months. During this time the life and yield of boreholes has varied considerably and one which was bored early in 1951 has yielded over 100 million cubic feet of gas — equivalent in heating value to over 4,000 tons of coal.

Over 76 boreholes have been completed and the drainage has been carried out from 65 holes in the Bannock Band seam and 11 from the Main Band seam. Of these 23 have been coupled to the surface, the remainder having ceased to be productive. The rate of flow to the surface is about 730 cu. ft. per min. at a purity of 74 per cent. CH4 which represents 540 cu. ft. per min. of methane.

The boreholes are usually drilled from the ribside gate of a longwall panel starting as close as possible to the face as indicated in the layout drawing Fig. 4. They are inclined over the waste, experience having shown that an angle of 60 deg. to the horizontal gives the best results, and the distance between holes is normally about 20-30 yards. It has been found that if they are less than 20 yards apart intercommunication between holes is established if the distance intervening exceeds 60 yards firedamp may reach the ventilating air between the two holes.

The boreholes are drilled for the first 30 ft. with a 115 mm. bit (4½ in.) to allow a standpipe of 3 in. diameter to be fitted. Subsequent boring is continued with a 65 mm. bit (2½ in.) and the effective diameter of the holes is therefore 2½ in. They are drilled in the Bannock Band seam to 190 and 200 ft. in length and the gas is usually obtained from between 120-130 ft. in a zone where the virgin seam is located. The bores in the Main Band have varied in length between 150 and 60 ft. mainly because of the convergence of the two seams in this area.

Fortschritt P IV/6 type machines have been used for drilling and these consist of a frame built up of steel sections along which slide a rotary drilling head and its compressed-air turbine motor and gearbox. There is a hand-operated device for holding the rods in position when one is being added or taken off. The drilling rods are in lengths of 5 ft. 6 in. with screwed couplings. Various types of drilling bits have been tried, all having tungsten-carbide tips. The type now in use is manufactured by Nusse and Grafer.

The first 30 ft. of the boreholes are drilled to accommodate 3 in. sealing pipe in lengths of 6 ft. When the sealing pipes have been inserted into the boreholes they are held in position by wooden wedges and sealed at the mouth of the borehole with clay. A second set of pipes of 2½ in. diameter is then inserted at the top of which is a wooden distributing cap of the same diameter as the inside pipe. Cement is blown up by compressed air and fills the annular space between the outside of the standpipe and the sides of the borehole. Two bags of cement are normally required for each hole. Each boring team consists of three men, and connecting the borehole to the gas line takes one shift. As Fig. 4 indicates, the boreholes conjoin into a 6-inch diameter pipe range which in turn is connected to a 9-inch diameter range in the district. For conveying the gas from the district to the upcast shaft the 14 in. diameter range shown in Fig. 1 is used and this has a length of 3¼ miles before reaching the shafts.

Surface equipment

On the surface, after entering the inlet calorimeter house the gas passes to the exhauster and outlet-calorimeter house shown in Fig. 2, which also shows the outlet chimney for venting the unused gas to atmosphere. Inside the gas-extraction house, a view of which is shown in Fig. 3, there are two gas extractors, each having a capacity of 450 cu. ft. per min. and driven by a 40-h.p. motor: provision has also been made for a further two units. On the outlet side of each extractor there may be noticed a cylindrical structure and this is a water separator to separate, from the discharged gas, water used for sealing the extractors.

All the electrical apparatus in the outlet calorimeter house is either flameproof or intrinsically safe, and the plant has been designed for continuous running without anyone in constant attendance. Adequate precautions have been taken to guard against dangerous occurrences, and some of the instruments in use are shown in the accompanying illustrations. Fig. 6, for instance, shows the pressure-relief valve which allows any surplus gas to be vented up the 40 ft. chimney shown in Fig. 2. Also shown in Fig. 6 is an instrument panel on which are mounted pressure and temperature recorders, and the thermometers to indicate the temperatures of the gas and the sealing water. Between the two recorders is an indicator board with 12 droppers, and should any abnormal condition occur in the plant (e.g., a rise in the outlet-gas temperature) alarm bells ring and the appropriate dropper falls. One of these droppers is connected to the water-flow indicator shown in Fig. 7, and this is set to ring an alarm and operate the dropper should the water flow be reduced below a safe rate. To indicate the gas flow through the extractor there is the Kent meter shown in Fig. 9. A record of the rate of flow is made on the strip chart at the bottom of the meter, and the cumulative quantity of gas which has passed is shown on the cyclometer type recorder at the top of the instrument. For recording the methane content of the drained gas a Sigma recording calorimeter (shown in Fig. 10) is provided. The gas is burnt at a constant rate in the tube on the left-hand side of the calorimeter, thus providing a flame to heat a metal rod the expansion of which is proportional to the methane content of the gas; the movement of the rod is transmitted by a system of levers to the pen on the chart.

For safety, the main safeguard against an ignition of gas within the drainage system is a device which automatically shuts down the extraction pumps and isolates the underground and shaft gas mains should the methane content of the gas fall below a predetermined figure (normally 40 per cent.). To ensure a rapid sampling, gas from the top of the shaft main is circulated through a pipe ring-main by means of the diaphragm gas pump shown in Fig. 5. This ring main is fitted with electromagnetic isolating valves, flame arrestors, and a gas-pressure governing valve. From the ring main, gas is fed to the M.S.A.-C.R.E. methanometer shown in Fig. 8 and to an oxygen-detecting burner and an infra-red methanometer, these instruments being fitted with electric contacts arranged to shut down the plant if the drained gas contains too much air.

As will be apparent from Fig. 4, some of the gas is also used at the colliery boilers, and Fig. 11 shows one of the five Lancashire boilers which have been equipped with gas burners; two large water-tube boilers have also been fitted. Leaving the extraction pumps the drained gas has a gauge pressure of 3-5 lb. per sq. in. It then passes through one of the flame arrestors visible in the top right-hand corner of Fig. 12 to a gas-governing valve which reduces the pressure to about 5 in. w.g. From the reducing valve the gas passes to a small receiver and thence to the boiler house gas header seen in the top left-hand corner of Fig. 12. Each boiler is equipped with valves which automatically shut off the supply of gas to the burner if no flame is present at the burners. The shut-off valves also operate in the event of failure of the boiler forced or induced-draught fans, or if the gas-supply pressure is too high or too low. The gas burners seen in Fig. 11 have now been replaced by forced-draught burners which experience has shown to be essential for safe and efficient operation.

In the early stages, attention at Haig was concentrated on getting the gas out of the pit and at a recent press conference, held to mark the successful progress made in extracting methane and passing it to the gas consumers, great emphasis was placed on the fact that safety in the pits was the primary interest of the mining engineer. The presence of methane has in the past compelled collieries in this area to reduce their potential output of coal in order to be able to concentrate on the improvement of their ventilation systems, and Haig colliery which formerly produced 12,000 tons of coal a week has been limited to 7,500 tons weekly.

Both Mr. James Bowman, Chairman of the Northern (N & C) Divisional Coal Board and Mr. Crowther of the Gas Board affirmed that they would suborn their requirements to what could be extracted in conditions of complete safety. Sir Charles Ellis, who told the conference that 23 pits throughout the country were or would be engaged on similar experiments, thought that the National Coal Board's plans would not lead to a wholesale contribution to fuel economy but they would make pits more safe and make a noticeable difference to it by providing a useful fuel. Since the autumn of 1952 increasing quantities of gas have been used under the colliery boilers and about 7,500 tons of coal has been saved in this way. It is considered that the boilers can function well enough on low-grade solid fuels and more gas can then be passed to the Gas Board.

For home consumption it is necessary to modify the gas by catalytic reforming. A part of the gas is mixed with steam and passed over a nickel catalyst maintained at red heat. This converts the methane to a mixture of hydrogen and carbon monoxide — both normal constituents of town gas, and which when blended with pure methane, produces a final gas identical in heating value to the usual supply. The flame characteristic of methane is a lightness which sometimes allows it to be blown away from the burner. Methane has a low flame speed and the flame tends to float. Hydrogen has a high flame speed and a short flame.

Of the 730 cu. ft. per min. available about 700 cu. ft. per. min. is being burned at the colliery boilers and an average of 30 cu. ft. per min. is being passed to the Whitehaven Gasworks.

* See COLLIERY ENGINEERING, p 461, November, 1927.

Drawings and Photographs accompanying the article