GB2086930A

GB2086930A - Process for the Gasification of a Deposit of Coal or Lignite

Info

Publication number: GB2086930A
Application number: GB8117097A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1980-10-13
Filing date: 1981-06-04
Publication date: 1982-05-19
Also published as: DE3140027C2; GB2086930B; DE3140027A1; FR2491993B1; NL8104625A; BE885681A; FR2491993A1

Abstract

The process is of the type in which the preparation of the sites is accomplished from a network of galleries (5) in the virgin sites, formed by conventional mining techniques below the level of the strata (1,2) to be exploited. The protective roof layer, maintained between the galleries in the virgin sites and the layers to be exploited, has a minimum thickness of 40m to 50m. During the whole period of gasification, the sole connection between these galleries and the strata being exploited consists of a series of small diameter borings (11, 12, 13), drilled before firing of the site, and covered by tubes sealed to the rock by cementation. <IMAGE>

Description

SPECIFICATION Process for the Exploitation by Sub-Terranean Gasification of a Deposit of Coal or Lignite Various methods have been proposed with a view to carrying out the exploitation by subterranean gasification of a deposit of coal or lignite. These methods can be classified in three main categories; Mining methods in which the preparation of the sub-terranean gas-producer is achieved by means of wells and hollow galleries by conventional mining techniques; there may be mentioned, by way of example, the experiment carried out in Morocco, at the Djerada site.

The methods by boring from the surface which have been developed in the U.S.S.R., at the Angren and Yuzhno-Abinsk sites.

Mixed methods, which include preparatory work carried out by mining techniques and boring from the surface; there may be mentioned by way of example, the experiment carried out in Great Britain, at the Newman Spinney site.

None of the methods which have been developed up to the present is suitable for the exploitation of deep deposits.

The methods by boring carried out from the surface are handicapped by the high costs of very deep borings and their profitability can only be assured by an exploited area which extends for a long way from the boring itself.

Mining methods and mixed methods pose the problem of the safety of the personnel who are occupied with the preparation of the working sites. Indeed, the passage to great depths permits a large increase in the gasification pressure and the use of a high gasification pressure is of great value when the yield of the gasification process is considered, in the power developed and the cost price of the gas-produced; but it is necessary to ensure that this gas under pressure, which contains a substantial quantity of carbon monoxide, cannot seep into galleries which are accessible to personnel.

An object of the process according to the invention is to remedy these difficulties and permit safe and economic exploitation of deep deposits, by a mixed method in which a small number of borings from the surface serve as outlets for gas produced in a small number of sites of great extent.

The preparation of these sites is achieved from a network of galleries in a virgin site, formed by conventional mining techniques, below the level of the strata to be exploited and separated from them by a protective roof layer having a minimum thickness of 40m to 50m, so that these galleries remain outside the zone of influence of the subterranean gas-producer.

During the whole period of gasification the only connections between these galleries and the strata which are being exploited consist of a series of small-diameters bore-holes drilled before the firing of the site and covered by tubes sealed to the rock by cementation.

The process according to the invention is suitable for development in two variants: In a first variant, the development of the gasification sites is carried out in the form of galleries or borings in the vein, formed by conventional mining techniques, from an internal well bored across the protective roof layer which separates the vein, galleries in the virgin site being formed below the strata to be exploited.

Before firing of the site, this interior well is sealed by a concrete plug of great thickness, which acts as a dam and above which there is injected a volume of water several metres in height, to act as a hydraulic shield between the gas-producer and the galleries in the virgin site.

In a second variant, the development of the gasification sites is constituted by a series of channels in the vein, formed by the technique of retro-combustion, the comburent being injected by borings of small diameter, bored through the protective roof layer which separates the vein from the galleries at the excavated virgin site below the strata to be exploited.

The process according to the invention is illustrated, by way of example only, by the attached schematic drawings, in which: Figure 1 represents a vertical section through a coal deposit; Figure 2 represents a section of a gasification site through the plane X-Y; Figures 3 and 4 represents two variants on Figure 2.

The coal deposit shown in Figure 1, comprises two strata 1 and 2, which it is proposed to exploit successively in descending order.

The deposit is intersected by twin wells 3 and 4 used respectively as inlet and outler wells for air.

A transverse gallery 5 is formed below the level of the strata to be exploited and a boring 6 is bored from the surface to the end of this gallery to provide it with an air return independent of the main wells of the mine.

In a first variant, the preparation of the gasification site situated in the vein 1 includes the formation of an internal well 7 and the formation of a passage or boring in the vein 8, which can extend for a length of several hundreds of metres, in a direction perpendicular to the plane of the section.

This communication in the vein is to serve as an evacuation route for gas between the gasification front and the foot of the boring drilled from the surface to act as the outlet for the gas during the whole duration of exploitation of the site. The compressed air and the gasifying agents to supply the underground gas producer are conveyed by a network of conduits, formed along the wells 3 and 4 and along the transverse gallery 5 as far as a passage 10 formed perpendicular to the plane of the section.

The gasifying agents are injected into the vein 1 by a series of vertical borings such as 11, drilled upwards from the passage 10 and covered by metal tubes sealed to the rock by cementation.

The preparation of the site can, possibly, be completed by the drilling of several oblique borings, such as 12 and 13, which are used to form the first connections in the vein, by the technique of retro-combustion.

Before firing the site, a very thick concrete dam 14, is formed in the internal well 7, so that the gasification site is completely isolated from the gallery 5. In order to avoid any risk of infiltration of gas, a volume of water several metres high is injected above this dam to serve as a hydraulic shield between the gas-producer and the access gallery.

In Figures 2 and 4, which represents a section of the gasification site, the plane X-X is situated at the roof of the stratum 1.

Any arrangement shown in Figure 2, the gasification site is bounded, at its lower part, by a passage in the vein 8, formed by the conventional mining technique, and the initial position of the gasification front is determined by the connection 1 5 which can be formed as a boring in the vein or as a riser formed from the base passage 8.

In the arrangement illustrated in Figure 3, the gasification site is bounded, at its lower part, by a boring of great length, drilled in the thickness of the vein, and the initial position of the gasification front is determined by the formation of retrocombustion channels between the end of this boring, where a firing operation takes place, and the end of the oblique borings 13, 12 and 11 a, through which the compressed air is injected under high pressure.

In a second variant illustrated in Figure 4, a preparation of the site is accomplished without resorting to the formation of any internal wells such as 7. The gasification site is bounded, in its lower part, by a series of channels in the vein 8 intended to serve as an evacuation route for the gas between the gasification front and the foot of the boring 9 drilled from the surface. These channels are formed by the technique of retrocombustion by carrying out a firing at the foot of the boring 9 and injecting air under high pressure successively through the borings 1 7a, 1 7b, 1 7c and 1 7d, driiled from bottom to top from the gallery 10 or from another similar gallery at the level of the transverse gallery 5.

The same retro-combustion technique is used between the borings 1 7d, 13 and 12 and 11 a, to form the channels which will constitute the starting line of the gasification front.

In these various alternatives a site is bounded at its upper part, by a line of borings such as 11 a, 1 1 b, 1 1 c, 1 1 d and 1 1 e, drilled from the base passage 10.

In a first exploitation phase gasifying agents are injected through the boring 11 a; firing is carried out at the end of this boring and the gasification gases which are formed along the front defined by the connection 1 5 or by the retrocombustion channels formed between the end of the borings 1 a, 12 and 13 and the end of the base boring 8 are evacuated via this boring 8 as far as the foot of the boring 9, which acts as the outlet for the site. The progression of the fire results in a progressive displacement of the gasification front and this displacement can be controlled, to a certain extent, by progressively displacing the injection of the gasifying agents from the boring 1 a towards the boring 11 b.

The exploitation of the panel proceeds according to the return method, by using successively the borings, 1 b, 1 c, 1 d and 1 e as injection borings for the gasifying agents.

After achieving the exploitation of the stratum, 1, the same means will be adopted for the exploitation of the stratum 2. The borings 11 a, 1 Ib, 1 Ic, 1 Id and 1 le can be reutilised for the ; exploitation of this second stratum, with the injection of a plug of cement into the part of these borings situated between the two layers and with the cutting off of the covering at the plate where the borings intersect the layer 2.

It may be emphasized that, in the process according to the invention, the extent of the deposit exploited from the single boring, accomplished from the surface, can reach 5 to 10 hectares. It results from this that the effect of the cost of these borings on the cost price of the gas produced is practically negligible.

This limitation on the number of borings has also the effect of reducing to the minimum the effect of the exploitation at ground level and this advantage can be particularly important in regions having a high density of population, as well as in the case of coal deposits which extend under the sea.

The economic advantages which can be gained from using the process according to the invention, can be rapidly assessed.

On the basis of the facts available from the literature, it can be estimated that, in a conventional mining exploitation, up to one third of the cost price of coal is made by the cost of the preparatory work, and two thirds by the cost of the exploitation sites, underground transport, and extraction.

In the conditions of exploitation in Western Europe, the costs involved in the conversion of coal to synthetic gas represent an additional expense of the order of 50% of the coal extracts; the energy yield of the conversion being of the order of two thirds, the cost price per energy unit .

produced in the form of gas can be expressed as-a function of the cost price per energy unit in the form of coal by the relationship:- c

In the case of a sub-terranean gasification process according to the invention, the cost of the mining works can be reduced to that of the only preparatory works and the cost of the surface treatments can be reduced by half owing to the suppression of the gas producers and the sorters washers. If the energy yield of the gasification operation is designated byp it follows:-

if the energy yield of the sub-terranean gasproducer is of the order to 50%, the price of synthetic gas can be half as dear as the price obtained by the conventional techniques of exploitation by mining followed by gasification of the extracted coal.

If the energy yield reaches 60%, the price per unit of energy of synthetic gas can be less than the price per unit energy of the coal extracted by conventional methods.

It should also be noted that the method according to the invention involves only a very slight risks for the personnel occupied in the subterranean works.

The work involved in preparing the sites, carried out before any sub-terranean firing of the gas producer, does not differ at all from conventional preparatory operations.

By situating the access routes at a depth sufficiently below the strata to be exploited, they cannot be effected by relaxation movements of the rocks resulting from the progress of the exploitation and, by equipping the passages with diagonal ventilation, with return of the air direct to the surface, in the case of accident any link of toxic gas to the access wells of the personnel is avoided.

Claims

1. Process of exploitation by sub-terranean gasification of a deposit of coal or lignite, in which the preparation of the sites is accomplished from a network of galleries in a virgin site formed by conventional mining techniques and below the level of the strata to be exploited, characterised in that a protective roof layer, maintained between the galleries in the virgin sites and the strata to be exploited, has a minimum thickness of 40m to 50m and in that, during the whole period of gasification, the only connections between these galleries and the strata being exploited consists of a series of small diameter borings drilled before the firing of the site and covered by tubes sealed to the rock by cementation.

2. Process of exploitation according to claim 1, in which the working at the sites is in the form of galleries or borings in the vein, formed by conventional mining techniques, from an interior well drilled through the protective roof layer which separates the vein from the galleries in the virgin sites formed below the strata to be exploited, characterised in that, before firing of the site, this internal well is sealed by a thick concrete plug which acts as a dam and above which there is injected a volume of water several metres in height.

3. Process of exploitation according to claim 1, characterised in that the working of the gasification sites comprises a series of channels in the vein, formed by the retro-combustion technique, the comburent being injected by the borings drilled through the protective roof layer which separates the vein from the galleries in the virgin site formed below the strata to be exploited.

4. Process of exploitation according to claim 1, characterised in that the network of galleries in the virgin site formed below the layers to be exploited is ventilated by a current of fresh air from air inlet wells of the mine, and in that the evacuation of this air, in the zone close to the gasification sites, is ensured by one or more borings directly connecting the network of galleries to the surface.

5. Process of exploitation by sub-terranean gasification of a deposit of coal or lignite, as herein above described and shown in the attached drawings.