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June, 1955

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Natural gas, of which the chief constituent is methane, occurs in the earth's crust in rocks of widely differing age and nature. It occurs in recently formed marshes and hogs, where it is appropriately known as marsh gas and it is also found in ironstone mines1, gypsum mines and gold mines. Boreholes 200 to 300 ft. deep in the Wealden Beds at Heathfield in Sussex have yielded natural gas for over 50 years.2 Large accumulations of natural gas occur in many parts of the world, often, though not always, associated with deposits of oil, In the U.S.A., Italy, Canada, France and Germany, natural gas is tapped by means of deep boreholes and is transmitted by pipelines over long distances for use as a fuel for industrial and domestic purposes. In some countries, particularly the U.S.A. and Italy, it forms a significant proportion of the total consumption of fuels for heat and power.

In Great Britain the Gas Council is now drilling for natural gas; a trace has been reported from a borehole at Crowborough in Sussex, but the largest known source in this country is the coal mines. Natural gas in a coal mine is known as "firedamp."

Firedamp

When excavations are made in coal-bearing strata, firedamp is emitted into the workings from the seams or adjacent strata. The rate of discharge varies greatly from mine to mine; at some it is so small that naked lights can be safely used underground, but at others it may amount to 4,000 cu. ft. for each ton of coal worked. A large gassy mine may produce 3,000 cu. ft. or more of methane per minute.

The total quantity of methane discharged from all the coal mines in Great Britain is not known accurately, but it is probably of the order of 300 — 400 million cu. ft. per day. The heat in this methane amounts to about 1,000 million therms per annum, which may be compared with 2,339 million therms from all the gas works in Great Britain in 1953.

Historical. — A fairly complete account of the history of firedamp drainage in the North of England has been compiled by one of the authors and will shortly be published as a separate paper. The subject is of interest but not of great importance, the main difference between the modern work and that of the earlier investigators being that the latter confined their efforts to the removal of firedamp after it had accumulated in roadways, cavities and wastes, whereas the modern technique is directed to the collection of the gas before it has entered the working places of a mine.

In this report special mention is made of the development of firedamp drainage at the Haig Pit, Whitehaven, and it is of interest to record that probably the earliest attempts at work of this nature were made at Whitehaven approximately 200 years ago. In 1733, Sir James Lowther read a paper to the Royal Society on the gas encountered during the sinking of a shaft, demonstrated the properties of the gas and made an attempt to explain its occurrence. Sometime later gas from the pit was piped to the surface and used in a small laboratory.

In the early nineteenth century considerable gas troubles were encountered in many of the Tyneside collieries. At Wallsend, the firedamp obtained from a district of 5 acres in the Bensham seam was taken to the surface by a 4 in. pipe, the rate of flow being estimated as 120 cu. ft. per min. An attempt at utilisation was made and in 1840 a company was organised to supply Newcastle and several neighbouring townships with the gas. Pipes were laid to Carville station (Wallsend), but the low illuminating power of the methane caused the venture to fail.

After a disastrous explosion at Haswell Colliery, Co. Durham in 1844, Michael Faraday, together with Sir Charles Lyell, a geologist, was asked to report on the cause and to suggest methods for preventing such accidents. He concluded that the explosion was due to firedamp which had accumulated in the goaves and proposed that such accumulations should be drained by means of pipes connected with the upcast shaft. It was thought that this scheme would operate because of the buoyancy of the firedamp compared with air, but the use of mechanical suction devices was also envisaged. The scheme was studied by a committee of mining engineers and was dismissed as being impracticable.

From time to time the subject has been revived. At Point of Ayr Colliery in N. Wales, it has been the practice for more than 60 years to drain firedamp from unworked seams by means of cross-measure boreholes, but it was not until 1943, at Mansfeld Colliery in the Ruhr, that the present method of gas drainage was first applied on a large scale with success. Since that date progress has been rapid and firedamp drainage projects are now in operation at many collieries in the Ruhr, the Saar, Belgium, France and Great Britain.

Origin and composition of firedamp

Most of the firedamp emitted in the workings of a coal mine comes from the seams of coal and carbonaceous rocks. Carboniferous shales, clays and sandstones usually have little free pore space: they are incapable of absorbing methane and consequently contain little methane per unit volume. Sometimes, however, sandstones occur which are either porous or contain fissures and open joints. Such strata may form gas reservoirs and may contain paths by which the gas can flow into the mine roadways, so producing blowers. It is sometimes possible by means of boreholes to drain large volumes of firedamp from porous or fissured rocks lying adjacent to seams of coal.3

Firedamp is one of the products of decomposition in the coalification of vegetable matter, a process by which an accumulation of vegetation is converted successively into peat, lignite, bituminous coal and anthracite, though the chemical reactions of the coalification process are still obscure, In the early stages it is probable that the cellulose was decomposed with the evolution of carbon dioxide, methane, hydrogen and water, most of which escaped into the atmosphere. In the later stages the seam of vegetation became covered with thick layers of silts and other deposits which formed a more or less impermeable seal to the gases formed in the coalification process. Pressure and heat also were important factors influencing the course of the chemical reactions. It is thought that the gases evolved in the later stages were mainly methane and carbon dioxide but it is difficult to explain the reason for the very low carbon dioxide content of present day firedamp. Outbursts of carbon dioxide in coal mines do occur, however, especially in France.

It is unlikely that the rocks overlying seams of coal can be completely impermeable to the flow of methane through them. Even if their permeability is very low any free gas in the enclosed coal would have escaped many ages ago, approximately 250 million years having elapsed since the Carboniferous era. It is probable, therefore, that the very large quantities of firedamp which may be found in seams of coal underground, sometimes amounting, at atmospheric pressure and temperature, to up to 40 times the volume of the coal, are retained in the coal substance as a result of the strata pressure on the seams.

The manner in which the methane is retained in the coal is not fully understood, but it is generally accepted that the gas exists partly in an adsorbed state and partly as free gas. The coal substance consists of colloidal particles with spaces between them forming submicroscopic pores. There is therefore a very large specific surface on which adsorption can take place. In the pore spaces it is thought that the gas phases are in equilibrium.

The main constituent of firedamp is methane, but other hydrocarbons are also present in varying amounts.

Analyses have been made by mass spectrometer of a number of samples of firedamp obtained from British coal mines. The hydrocarbons present were mainly paraffins and occurred in the proportions shown in Table I.

Firedamp normally consists of hydrocarbon gases (90 — 98 per cent.), carbon dioxide (0.2 — 6.0 per cent.) and nitrogen (trace to 6 per cent.), although recent experiments at one mine in County Durham have yielded a gas containing 20 per cent. of nitrogen.

Other gases which have been detected in small amounts include oxygen, hydrogen, argon and helium, so that the complete range of analytical figures may ultimately prove to be much wider than was at first thought. Firedamp is probably always saturated with water vapour.

Methods of draining firedamp

A block of coal lying in virgin strata is subject to a vertical stress roughly corresponding to the weight of the column of rocks between it and the surface, although this pressure may often be modified by tectonic stresses in the earth's crust. If the pressure on the coal is reduced by extracting minerals from above or below it, the volume of the submicroscopic pore space in the block of coal increases. The pressure of the free gas in the pores and the quantity of gas adsorbed will therefore also decrease. At the same time the coal is fractured and fissured, and the free gas in the pore spaces adjacent to the fissures escapes, so decreasing the free gas pressure and reducing the quantity of gas in the adsorbed state on the micelle surfaces. The coal therefore emits firedamp.

The distance from any fissure from which firedamp drains into the fissure is probably very small. If a lump of coal is placed in vacuo it will emit firedamp at a decreasing rate until, after some weeks, the emission ceases. If the surface of the lump is now scraped it will give off more gas.4 A lump of coal which has been stored for many years will emit methane if it is fractured. The coal substance itself must therefore be impervious to the flow of methane.

It follows that a coal seam will not yield much firedamp unless it is fissured and unless the strata pressure on it is reduced. This is confirmed by experience, for boreholes drilled into virgin seams will yield little gas even though the coal may emit large quantities when it is worked. Similarly, boreholes drilled into the coal ahead of a working face in a gassy seam have been shown to yield little gas. The only known exception to this rule is Point of Ayr Colliery, North Wales, where boreholes drilled down into an unworked seam at any point yield large flows of gas at pressures up to 300 lb. per sq. in. The unusual conditions at this colliery are probably due to the seams being of exceptionally high natural permeability.

In order, therefore, to drain large quantities of firedamp from the strata in a coal mine the boreholes must be drilled into a zone in which the strata pressure in gas-bearing coal seams is relaxed. This fact was not appreciated until about 10 years ago, which explains why firedamp drainage from coal mine strata has been applied on a large scale only during the last 5 to 10 years.

The early instances of firedamp drainage mentioned in the historical section were all cases where the firedamp entered the working places of a mine before it was collected and piped away. Modern firedamp-drainage techniques aim at tapping the gas before it enters the ventilating air of a mine, the primary and most important purpose of drainage being to reduce the quantity of gas entering the ventilating air and so increase safety. The production of a valuable gas is a secondary purpose only.

The zone of relaxed strata pressure necessary for effective firedamp drainage is found in the roof, and to a lesser extent in the floor beds, behind a longwall face. Fig. 1 is a vertical cross-section through the strata at right angles to a longwall coal face and shows the approximate distribution of pressure on the roof beds. It will be seen that the most productive borehole is shown in a region in which the pressure on the strata is less than the pressure which existed before mining commenced. It is usually found that boreholes yield gas at the greatest rate when they are about 50 yards behind the face.

The removal of the coal seam by mining causes the formation of pressure arches in the roof and floor beds. The front abutment of these arches is situated 10 to 20 yards ahead of the coal face, whilst the back abutment is usually 100 to 150 yards behind the face where the stowed waste cavities have become consolidated. The front and back abutments of the pressure arches are the regions of high pressure shown in Fig. 1. The cores inside the two pressure arches are the zones of relaxed pressure from which firedamp is drained.

Two main techniques are used, the "superjacent heading" and the "cross-measure borehole" methods. In the superjacent heading method, a heading or roadway is driven in a seam lying above the coal which is to be worked, the heading being located directly over the area to be worked. The heading is then sealed at its open end. When the lower seam is extracted, the heading acts as a collecting duct for the firedamp emitted from the upper strata and the gas is drawn off from the heading by means of a pipe built through the seal.

In the cross-measure borehole method, the only one so far used in Great Britain, the firedamp in the strata above or below a seam being worked is drained by means of boreholes drilled into the relaxed cores of the pressure arches.

Detailed descriptions of the above methods have been published.5

Drainage boreholes. — The drainage boreholes are drilled as close to the face as possible and are usually inclined over the worked-out area at an angle of about 50 deg. to the strata bedding planes. They are usually 100 to 250 ft. long and 2½ in. diameter. The outer end of each hole is drilled to 4½ in. diameter for a length of 20 to 30 ft., and a steel tube of 3 in. bore is grouted with cement into the enlarged portion of the hole to enable the emission of gas from the hole to be controlled and also to minimise the leakage of air through fissures into the borehole.

The location of typical drainage boreholes with respect to the working face is shown diagrammatically in Fig. 2, which illustrates the complete project, both drainage and utilisation, at Haig Pit, Whitehaven.

At some collieries substantial yields of gas are obtained from holes drilled into the strata below the seam being worked. The pressures of the gas in downward boreholes, measured with the borehole valve closed, are usually much higher than pressures in upward holes, doubtless due to the fact that the strata above a worked-out area are subjected to much greater movement and to a greater reduction of stress than the underlying strata.

Each borehole is fitted with a valve, an orifice for measurement of flow, and tappings for measurement of pressure and for taking samples.

Integrating gas-flow meters are occasionally fitted at the underground boreholes, as illustrated in Fig. 3, which shows a downward borehole equipped with two flow meters, thermometer, pressure gauge, water receiver and drain trap.

The machine normally used for drilling the boreholes is the Fortschritt P IV/6, Fig. 4, made in the Ruhr by Nusse and Grafer. It is powered with a 6 h.p. compressed-air motor .for rotating the drill-rods, and a 2 h.p. compressed-air motor for traversing the rods. It is capable of drilling a hole 150 ft. long and 2½ in. diameter in normal strata in one shift.

Underground Gas Ranges. — The main ranges underground usually consist of 8 in. bore pipes with Carlton-type joints, the subsidiary ranges being 6 in. bore. The shaft pipes are normally 10 in. bore with flanged joints. For the larger schemes, pipes of greater diameter may be necessary. For example, Fig. 5 shows a 14 in. bore gas main in an underground airway at Haig Colliery.

Provision must be made in the pipe ranges for gas sampling, for the drainage of condensed water and for the isolating of individual sections by means of valves.

Firedamp drainage at Haig Pit, Whitehaven

General. — The following account of the work carried out at Haig Colliery is given as typical of the development of firedamp-drainage schemes in Britain.

Haig is the largest of the Cumberland pits and is situated at the coast on the edge of the cliffs to the immediate south of Whitehaven. The colliery workings are all undersea and the main districts are now about three miles from the coast, so that ventilation is a problem of some magnitude.

In December, 1947, the average quantity of air circulated through the workings was 230,000 cu. ft. per min., and the concentration of methane in the main return at the upcast shaft was 1.25 per cent. This position was unsatisfactory and the working of coal was therefore restricted, with the result that by March, 1949, with the same volume of ventilating air, the concentration of methane had decreased to 1.08 per cent.

This concentration is much higher than that in most shafts in Great Britain and in order that the scale of gas emission may be fully appreciated the following facts are presented: —

(a) The reduced output of coal at Haig in 1948, was about 1,100 tons per day, but the daily weight of air circulated through the workings was 11,000 tons, i.e., 10 tons of air were circulated for each ton of coal produced.

(b) The total volume of methane emitted was about 4 million cu. ft. per day the weight of which is approximately 78 tons, i.e., the methane output was 7 per cent, by weight of the coal output of the pit.

It is clear, therefore, that the gas at the Haig Pit not only formed a constant source of danger and necessitated a restriction in coal output, but it also represented a large amount of energy which was being lost.

In 1949, it was decided to study the possibilities of firedamp drainage and as the first move a survey was made of the amounts of methane obtained from different sources in the pit. It was found that about 40 per cent. of the total gas is obtained from working sections and that, of this 40 per cent., between one-third and one-sixth is released on the face. It follows, therefore, that of the Haig daily make of 4 million cu. ft., less than 10 per cent, is removed at the working faces by the ventilation air.

Data obtained in the course of the ventilation survey indicated that during the normal course of coal winning an appreciable amount of gas drainage takes place between adjacent seams. Thus, the gas emitted by the Bannock Band seam is greater when the workings are above virgin Main Band seam than when the Main Band has previously been wrought.

The position in 1948, is shown in Table II.

Experimental work started in June, 1950, and for a considerable period the rate and quality of gas emission from each borehole was determined every two hours. The first boreholes had small positive pressures (about 3 in. w.g.) and yielded an average natural flow of 30 to 50 cu. ft. per min. of gas containing 85 to 95 per cent. CH₄.

It was therefore decided to investigate the effect of suction on gas yield. In view of the history of the pit every possible precaution was taken and the exhauster used for trials was designed for safety rather than for efficiency. It consisted of a 30 h.p. compressed-air motor driving a two-stage pump which delivered water (250 g.p.m. at 230 ft. head) from a tank into four ejectors arranged in parallel and fitted into the top of the tank. The ejectors withdrew gas from a 6 in. pipeline from the boreholes and delivered it, together with the water stream, into the water tank and thence to a 6 in. outlet pipe.

The apparatus worked satisfactorily and enabled sufficient data to be collected to plan the full scale project.

At the end of the first year (June, 1951), 14 holes had been bored and had given a peak gas yield of 465 cu. ft. per min. which fell fairly rapidly to 286 cu. ft. per min.

After 18 months (February, 1952), 32 boreholes had been completed and the position was as shown in Table III.

Main Project: Initial Stages. — Plans were made for a total methane recovery of 1,000 cu. ft. per min. with exhausters at the surface and a 14 in. pipeline from the shaft to the working districts.

On June 18, 1952, the 14 in. underground gas main was completed and firedamp flowed to the surface. On August 5, the first trials were carried out with the surface exhauster; the plant was operated intermittently until August 22, and then continuously from August 25 to August 29. After minor modifications had been made, continuous extraction of firedamp commenced on September 8, and has since been maintained without interruption. Intermittent experiments on boiler firing were also started on September 8; continuous firing started a week later and is still in progress.

Buoyancy drainage. — Prior to June 18, six boreholes on the west side of the Main West Bannock District had been connected to the 9 in. collecting range in the district and preparations had been made to couple the range to the 14 in. main to the surface. When this was done gas did not flow out, but, instead, air flowed in the reverse direction, i.e., down the pipeline and into the strata via the boreholes. This was rectified by priming the pipeline, using the suction of the upcast shaft. When the underground line and vertical shaft pipe were full of firedamp the buoyancy of the gas was sufficient to maintain flow to the surface. The first sample had a methane content of 95 per cent. and a flow (calculated to 100 per cent. CH₄) of 205 cu. ft. per min. which stabilised at 175 cu. ft. per min. within three hours. The systematic recording of gas quality and rate of flow then commenced and has since continued without interruption.

Induced drainage. — The first experiment with induced drainage was carried out on August 5, and intermittent runs up to 8 hours in duration were made until August 22. After the necessary information had been obtained relating to fluctuations in gas quality, effect of variation of suction on quantity and quality, etc., continuous extraction started on August 25, 1952, and since that date has proceeded without interruption.

Effect on Ventilation. — As soon as the firedamp drainage scheme came into operation it brought about a significant reduction in the methane content of the faces, tailgates and district returns. The figures obtained are summarised in Tables IV, V and VI.

Results obtained after development. — After this promising start the scope of the work was gradually increased by adding new boreholes and extending operations to other districts of the Bannock Band seam. The chief advance was made in February, 1953, when the system was extended to include the Main Band seam and thus to cover all the principal working districts of the mine.

Table VII summarises the results obtained since January, 1953, and illustrates the development of a satisfactory routine. It will be noted that as the project has developed and new boreholes have come into operation at a greater distance from the shaft, it has been necessary to increase the suction applied at the surface. In the early stages one exhauster was adequate, but three exhausters are now in use and it is intended in due course to increase the capacity of this section of the plant. As more gas has become available the quantity burned under the boilers has gradually increased, with a consequent saving of saleable coal; in addition, a supply has been provided for the Gas Board who are now operating an experimental catalytic reforming plant. In all, very little methane has been wasted. The present output is now over 1,100 cu. ft. per min. of gas or nearly 800 cu. ft. per min. of pure methane. During 1953 the rate of gas extraction increased and the quality of the gas improved. Progress in 1954 has also been satisfactory and the rate of extraction has increased by over 20 per cent.

The project has halved the concentration of methane at the faces and in the returns from the working places. It is difficult to obtain reliable figures relating to the improvement at the faces as predrainage data are available for only three of the faces which were worked during 1953, In the return airways, where the boreholes are situated, the efficiency of extraction, assessed by the volume of methane drained as a fraction of the total methane in the pipeline and airways, is 52 per cent. for 1953 and 68 per cent. for June, 1954, the highest figure yet achieved being 88 per cent. for No. 2 South, Bannock Band seam.

The above figures relate only to the working sections. Since firedamp drainage started 12 faces have stopped, but some of the boreholes in these districts continue to yield gas and have therefore not been disconnected from the drainage system. The overall drainage efficiency, including abandoned faces, is estimated as 38 per cent. for 1953 and 52 per cent. for June, 1954.

For the complete mine, as so much of the total gas is obtained from abandoned districts which are not affected by the drainage scheme, the overall efficiency is lower, the 1953 figure being 23 per cent. compared with 33 per cent. for June, 1954. It is hoped that as the gas emission from very old workings gradually decreases and the drainage of more recent workings continues it will be possible to increase the overall efficiency of firedamp drainage until it approaches the standard already obtained in the working districts.

Summary. — When investigational work started in 1948 the colliery output was 1,100 tons per day, with a total ventilation current of 230,000 cu. ft. per min. and a methane content of 1.25 per cent, at the upcast shaft. After two years of firedamp drainage it has been possible to increase the coal output by 55 per cent. (to 1,700 tons per day). The volume of ventilation air circulating has been increased by 20 per cent. and the methane content in the return air at the upcast shaft has decreased by over 50 per cent. (to 0.60 per cent.).

The utilisation of the gas has saved 300 tons of washed coal a week, in addition to which a small quantity has been sold to the Northern Gas Board for experimental work. Thus the scheme has considerably improved the conditions at the mine and at the same time has yielded a satisfactory profit.

Operational and plant control at Haig

Underground. — During the past year the firedamp drainage scheme has operated using 30 to 35 boreholes. The total number of holes completed is now more than 100, approximately three new boreholes coming into production each month. This is roughly sufficient to keep pace with the rate of advance of the longwall faces.

The life of the boreholes is variable and extends from completely unproductive holes which are never used to the other extreme, as instanced by one hole which was bored in February, 1951, and is still in use.

It is not possible to be precise, but the average life of all the Haig boreholes is about nine months, this figure including unproductive holes which were not connected to the drainage system. The average life of those holes which have now ceased to yield gas is eleven months, but this figure is not completely representative for some holes become inaccessible after a face is discontinued and the effective life of a borehole is often determined by the ability to maintain access to it after coal working has ceased.

Surface. — The surface equipment for the control of the firedamp drainage installation consists of an "inlet" calorimeter house adjacent to the fan house at the top of the upcast shaft and an exhauster house about 30 yards from the shaft with an "outlet" calorimeter house in the same building. Interior and exterior views of this are shown in Figs. 7 and 8 respectively. The general arrangement is shown diagrammatically in Fig. 2.

The gas passes from the shaft past the inlet calorimeter house where it is continuously sampled and assessed, through to the exhauster house and thence either to the colliery boilers or to the gas works, any small excess being discharged to atmosphere.

The exhausters are of the Nash Hytor type in which the rotor revolves without metallic contact in an elliptical casing containing water. The gas is drawn at a suction of up to 60 in. w.g. and fed to the boilers at a pressure of up to 5 lb. per sq. in., a relief valve passing gas to atmosphere should the pressure rise above this figure. The flow is measured on the suction side by a Kent flowmeter.

The safety of the plant is guarded by a system of interlocked electrical relays which is brought into operation if the gas quality falls below 50 per cent. CH₄ or the exhauster speed falls below 75 per cent. of the normal figure. A second circuit operates an alarm bell in conjunction with a "flag drop" indicator panel and warns the operator when items such as water flow to exhausters, gas pressure in outlet main, etc., are outside the predetermined ranges.

The system for quality control of the gas is a little complicated, but may be summarised as follows

A small pump takes gas from the 14-in, main, circulates it through a ½-in, sampling line in the inlet calorimeter house and returns it to the main. On this sampling line are arranged a Sigma recording calorimeter, two North Thames gas burners with electronic amplifiers and relays, and an infra-red gas analyser.

The calorimeter is adjusted so that if the quality of the gas falls below 50 per cent. CH₄ an alarm bell rings in the colliery fan house and in the exhauster house. The time lag is about two minutes and the instrument therefore serves primarily to record gas quality. The main safety circuit is controlled by the North Thames burners, which are adjusted so that if the methane content of the gas falls to 50 per cent. the flame back-fires to the bottom of the burner tube where it fuses a wire. A further reduction of the methane content causes the flame to be extinguished. The absence or displacement of flame is detected by an electrode fixed above the burner and connected to electronic amplifiers and relays. If these devices operate in one burner an alarm is given, but if both flames fail or both fuses blow, or one flame in one burner and one fuse in the other burner are affected, then the alarm sounds and the plant automatically shuts down. This operation takes approximately second.

This arrangement may sound complicated, but in practice it has operated well: the use of two burners is recommended as it obviates unnecessary shutdowns due to dust in a gas jet, fracture of a fuse wire, etc. It also avoids the failure of the alarm system due to the breakdown of an electronic component.

The infra-red, apparatus, Fig. 6, controls an independent safety circuit and operates a system of green, amber and red lights arranged in the manner used for the control of street traffic. These signals can be adjusted to any predetermined levels of methane content; they are installed in the fan house and in the outlet calorimeter house in front of a partition window so that they are visible from the exhauster house. In the event of an alarm they show the operator whether the trouble is due to a deterioration in gas quality and enable him immediately to take the steps necessary to rectify matters, different procedures being adopted for different causes of stoppage.

The outlet calorimeter house contains a recorder for the infra-red apparatus, a second Sigma calorimeter and a recording methanometer.

The Sigma calorimeter Fig. 9 guards against in-leakage of air in the exhauster system. It is set so that an alarm is given and the plant closes down if the gas quality falls below 50 per cent. CH₄.

Results at Haig

The plant has now been in operation for two years. There have been a number of alarms and occasional shutdowns, but these have all been minor incidents, and during the past year the plant has been out of operation for only 7½ hours. On no occasion has a shutdown been caused by unforeseen variation in gas quality. Indeed, the quality and quantity of the gas have remained surprisingly constant as exemplified by the stoppage of the pit for a fortnight's holiday in August, 1953, when only a small diminution in the monthly figures was caused, as recorded in Table VII. This is a fortunate circumstance which cannot be accepted as a generalisation for all collieries, the constancy of gas made at Haig being associated, of course, with the fact that sections of the pit continue to emit gas for a considerable period after coal working has ceased.

Fig. 7 shows two of the gas extractors with their water separators, switchgear and instruments in the exhauster house at Haig Colliery. The outlet calorimeter room can be seen through the glass window at the far end of the room.

(To be continued)

REFERENCES

1 "Inflammable Gas in the Stratified Ironstone Mines of the Cleveland District of Yorkshire," H. J. Perrins. T.I.M.E. Vol. 113 p. 763, June 1954.
2 "The Recovery of Natural Gas in East Sussex." Q.J.G.S., 1898, 54, 564.
3 "Drainage of Gas by Surface Boreholes." Anon. Coal Age, May 1943.
4 Private Communication. R. L. Cawley.
5 "Drainage of Firedamp from Mines," Report of Joint Working Party of Ministry of Fuel & Power and National Coal Board, 1951.

Drawings and Photographs accompanying the article