Underground gasification of coal

Abstract

 A method of underground gasification of coal involves drilling at least two boreholes (8, 9) along the length of the seam, one borehole (8) serving to feed oxidant air or oxygen to the combustion zone (6), the other borehole (9) being to recover combustible gas from the combustion zone. The problem of back-burning of coal along the oxidant borehole is tackled by continuously or intermittently injecting water along the borehole.

Description

[0001] This invention relates to the underground gasification of coal and particularly to a method of gasifying coal which improves the efficiency and yield of the gasifying operation.

[0002] Gasification is a method of partially burning coal underground under conditions such as to generate combustible gas (usually a mixture of CO, H₂, CH₄ CO₂ and N₂).

[0003] Proposals for the underground gasification of coal seams which; for one reason or another, are unable to be mined economically by conventional methods, are not new, and much work has been carried out on the idea particularly in the U.S.S.R., but also in other countries.

[0004] In general the problems encountered hitherto, coupled with the ready availability and low cost of petroleum products until recently, have hindered the development of fully commercial operations for underground gasification. With the rapid escalation in the cost of petroleum, interest has been restimulated in the exploitation of coal seams by gasification.

[0005] Conventional methods of gasifying coal seams have involved the drilling on pairs of boreholes, either vertically or at a steep angle into the seam, linking there holes by a channel formed in the seams and passing a blast of air or oxygen/steam mixtures along one borehole and recovering combustible gas from the other borehole. The operation is illustrated schematically in Figure 1 and the scenario comprises essentially burning face or combustion zone, and a gasification or reduction zone. In the combustion zone, carbon and some of the tar and volatile hydrocarbons in the coal are burned to produce a high temperature gas stream containing carbon dioxide and water vapour. These react in the gasification zone with hot coke, both as broken material which has fallen from the coal face and the walls of the gas exit channel; and the surfaces of the exit channel. The raw coal behind the burning face is heated by conduction from the face; water vapour, tars and rich gas are discharged through the hot surfaces by which they may be partly cracked to hydrogen and carbon monoxide before mixing with the reacting gas stream. The combustion and gasification zones move forward into the virgin coal, "mining" the coal.

[0006] It has been found necessary to drill the boreholes at quite closely spaced intervals, which is costly. Moreover, such drilling arrangements can only be made on relatively flat terrain.

[0007] As a means of mitigating the above problems, boreholes have been drilled along the lower part of a seam, linked by various means at one end where air is introduced; gas is recovered from vertical boreholes intersecting the "in seam" holes.

[0008] A real problem with this method is that eventually the unsupported roof of the void thus created, will cave in, blocking the passage of air to the burning face. At depths greater than about 600 metres subsidence of the ground above the cavity may completely seal the cavity.

[0009] Further problems arise in thin seams, particularly of bituminous coal, namely in that it is difficult to obtain a wide burnout zone (permitting wide lateral spacing) and also as gasification continues, the gas quality falls as a result of air by-passing the combustion zone and of completion of carbonization.

[0010] As a means of avoiding the above problems it is proposed that using systems of in seam boreholes, air or other oxidant should be introduced along some of the boreholes and gas recovered from the remainder - in contrast to current practice where all the air is introduced at the end remote from the gas offtakes. For many conditions, particularly with high rank coals, it is advisable alternately to reverse the flow of gas and air in order to provide even burnout and to maintain a high temperature in the combustion zone.

[0011] Unfortunately, this method is subject to a problem commonly referred to as "burn-back". Thus the hot air passing along the inlet channel causes combustion at the face at the far end of the channel, but instead of burning out a void in the seam which progressively moves back along the seam, the coal tends to "burn-back" along the narrow inlet channel. This is of course highly inefficient as only a fraction of the available coal is burned.

[0012] According to the present invention a method for the underground gasification of coal seams which comprises drilling two boreholes into and along the seam and linking the boreholes at one end, passing an oxidant blast of air or oxygen along one of the boreholes from the unlinked end and recovering gas from the other borehole at its unlinked end, and injecting water either continuously· or intermittently into said one borehole so as to minimise the combustion of coal within the borehole and to control the position of the combustion zone.

[0013] It will be appreciated that although a minimum of two boreholes are required, any number greater than that may be employed. Thus boreholes may be arranged in pairs, respectively carrying oxidant blast and produced gas; alternatively boreholes may be provided in trios, oxidant being passed along one and gas recovered from the remaining two boreholes. Other arrangements may also be envisaged according to the size and character of the coal seam. Usually the ratio of oxidant to gas boreholes is from 1:1 to 1:5. Moreover, the role of the various boreholes may be reversed periodically, the oxidant blast channels being switched to conduct the gas produced and oxidant being forced down the previous gas-conducting channel.

[0014] The boreholes may be drilled from an underground heading or by deviation drilling frcm the surface. In particular, for a steeply dipping seam the boreholes may be drilled from the surface along the base of the seam.

[0015] Although the control water injection may be introduced continuously, preferably some is injected as an intermittent "slug" of water in an amount computer from the extent of combustion which has taken place in the system, the blast rate and the temperature and analysis of the gas produced, in order to reduce the "burning back" effect.

[0016] Waste water from the gas cleaning system used to clean the gas produced may be used as the control water. The control water may contain surplus tar byproduct, if desired. Thus dirty water may be used since the control water passes to the combustion zone.

[0017] This can be particularly advantageous where water is in short supply, or of bad quality, or where for other reasons a complete waste water purification installation is difficult or expensive. The water injection system eliminates the need for a large steam generator, which is very costly for high pressure operation. The system also permits re-injection to the combustion zone of particulate material recovered from the product gas.

[0018] In low-rank coal seams, however, it may be necessary to introduce the control water continuously in order to prevent the coal surface in the borehole from drying out.

[0019] Carbon dioxide, water and/or additional oxidant may be introduced through vertical or other boreholes in the burned out zone in order to recover heat from the previously burned-out region and to purge that zone of combustible gases.

[0020] It will be appreciated that the number, size and location of the boreholes is a matter which is determined by the size and character of the coal seam being worked, and by the operating pressure. Similarly the blast rate will depend upon the coal and seam properties and the pressure, but these are matters which can be readily and routinely optimised for any given coal seam by experiment.

[0021] The linking of the boreholes at the intended initial combustion zone can be achieved in a number of ways, the preferred method again depending to some extent upon the character of the coal. For example, linkage between boreholes may be achieved by air or oxygen injection, water injection, electrically or by drilling.

[0022] Embodiments of the invention will now be described with reference to the drawings wherein:

Figure 1 is a vertical cross section through a coal seam showing a method of gasification according to the prior art,

Figure 2 is a horizontal cross-sectional view of the prior art arrangement shown in Figure 1.,

Figure 3 shows a first embodiment of the present invention employing a continuous parallel blast arrangement,

Figure 4 shows a second embodiment of the present invention similar to the first embodiment, except that it is conducted cyclically, and

Figure 5 is a horizontal cross sectional view of a third embodiment employing intersecting boreholes.

[0023] A prior art underground gasification method is illustrated in Figures 1 and 2. Figure 1 shows air A being passed from the ground surface down through an oxidant borehole 1 to a coal seam 5 having a combustion zone 6. Combustible gas produced by partial combustion of coal passes along gasification bore 7 and is withdrawn along gas borehole 2. Reference numeral 3 indicates an initial oxidant blast borehole which has now been superseded as the combustion zone moves forward, and a further blast borehole 4 which will come into use when the combustion zone reaches it. In this way, the coal seam is progressively burnt out, usually at the rate of a few meters a day.

[0024] The gasification bores 7 have been drilled horizontally from a heading 14 shown in dashed lines. The heading formed the initial ignition and blast zone.

[0025] Figure 3 shows a first embodiment of the invention. A series of parallel boreholes 8, 9, 10 are drilled in the base of the coal seam 5 by drilling from a heading as before, cr by deviation drilling from the ground surface. The borehole 8 transmits blast air to the combustion zone 6 and boreholes 9 and 10 recover combustible gas from the combustion zone. Auxiliary gas, if required, is introduced via a vertical bore hole 12 into the burnt out region 13 behind the combustion zone.

[0026] There is a tendency for coal to burn backwards along the air blast borehole 8, rather than burning between the borehole 8 and the adjacent gas boreholes 9 and 10. This tendency is reduced by injecting water and/or steam into the borehole 8 along with air blast. The water may be injected continuously or intermittently. When oxygen is employed instead of air, water may be injected continuously with intermittent additions of slugs of water. This embodiment is particularly suitable for coals which shrink on drying (lower-rank coals).

[0027] Figure 4 shows a similar arrangement to Figure 3 wherein the role of the parallel boreholes 8, 9 and 10 is cyclically varied according to arrangements 1, 2 and 3. Each borehole is fitted at the surface with a valve (not shown) to permit the connection of the borehole to either air, product gas, control water or to a purge mains for purging.

[0028] The control water system may be combined with a conventional water quenching or washing system used to treat the gas exiting from the gas borehole. In this system each borehole is used alternatively for air blast and for gas production.

[0029] This embodiment allows any burn back which may occur on the air blast borehole to be equalised by switching the air blast between the various boreholes.

[0030] Figure 5 shows a third embodiment wherein the linking of adjacent boreholes is achieved by drilling along the seam angled boreholes 11 which intersect. in the desired combustion zone 6. As before, air A is introduced along one borehole and gas G is recovered along the other borehole. This technique removes the need to create linking channels between the various bores before combustion is initiated, and may be particularly advantageous for deep seam operation.

Claims

1. A method for the underground gasification of coal seams which comprises

drilling two boreholes (8,9) into and along the seam and linking the boreholes at one end, conibusting coal in a combustion zone (6) at the linked ends, passing an oxidant blast of air or oxygen along one of the boreholes (8) to the combustion zone, recovering combustible gas from the combustion zone along the other borehole (9), characterised in the step of injecting water either continuously or intermittently into said one borehole (8) so as to minimise combustion of coal within the borehole and to control the position of the combustion zone.

2 . A method according to claim 1 wherein the oxidant blast contains steam.

3. A method according to either preceding claim wherein for a steeply dipping seam the boreholes are drilled from the surface along the base of the seam.

4. A method according to any preceding claim wherein the ratio of the number of oxidant blast boreholes (s) to the number of produced gas borehole(s) is from 1:1 to 1:5.

5. A method according to any preceding claim wherein the role of the boreholes is periodically varied during operation such that some or all of the boreholes are used for both oxidant blast, and at a different time, for produced gas.

6. A method according to any preceding claim wherein carbon dioxide, water or additional oxidant are introduced into a burned out region behind the combustion zone via a furhher substantially vertical borehole (12) so as to recover heat and purge the region of combustible gas.

7. A method according to any preceding claim wherein the boreholes are either parallel to each other, or are arranged at an angle to intersect and thereby link.

8. A method according to any preceding claim wherein the oxidant is oxygen and water is injected continuously with intermittent injection cf slugs of water.

9. A method according to any preceding claim wherein. the water is waste water from a produced gas cleaning system.

Drawing