GB2088896A

GB2088896A - Process for the Exploitation of Coal by Underground Gasification with a View to the Production of a Gas with High Hydrogen Content

Info

Publication number: GB2088896A
Application number: GB8135880A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1980-12-01
Filing date: 1981-11-27
Publication date: 1982-06-16
Also published as: DE3146681A1; FR2495179B1; BE886433A; FR2495179A1; GB2088896B; NL8105066A; DE3146681C2

Abstract

Process for the exploitation of coal by underground gasification, characterised in that it consists of simultaneously using a gaseous gasifying agent containing oxygen injected at the ambient temperature behind the gasification front (e.g. at 1) and a liquid gasifying agent containing water injected at high pressure in front of the gasification front (e.g. at 4). <IMAGE>

Description

SPECIFICATION Process for the Exploitation of Coal by Underground Gasification with a View to the Production of a Gas with High Hydrogen Content In modern gas generators, the production of a gas with high hydrogen content is generally achieved by using a gasifying agent consisting of a mixture of oxygen and steam.

In the exploitation by underground gasification at great depth, the use of such a mixture has various disadvantages which are enumerated in Belgian Patent Application No. 6/47289 and may be summarized as follows: The need to inject this gaseous mixture at a relatively high temperature (of the order of 2500C), which increases the cost of the injection bores and renders virtually impossible the use of any method of gasification in which the gasifying agent is conducted along galleries carved out by conventional mining techniques; overall reduction in energy yield due to the fact that the gasifying agent cannot be preheated in contact with the rocks surrounding the gas generator but may, on the contrary, give up part of its heat to them.

Another difficulty encountered in the development of all the underground gasification processes results from the fact that the coal subjected to the gasification reactions is in the form of a solid seam so that the surfaces of contact between the gas and solid are always very small compared with the surfaces of contact obtained in surface gas generators which operate on coal pieces of a limited size.

It is an object of the process according to the invention to overcome these various disadvantages.

For this purpose, it uses two types of gasifying agents simultaneously: A gaseous gasifying agent containing oxygen injected at the ambient temperature behind the gasification front and a liquid gasifying agent containing water, optionally mixed with soluble or liquid additives, injected at high pressure in front of the gasification front and reaching the reaction zone by infiltrating the coal.

This liquid may be injected at the ambient temperature or at an elevated temperature below the temperature of evaporation of water at the injection pressure used.

The process according to the invention is described hereinafter with reference to the attached figure which shows a section made at the surface of contact between the coal seam and the rocks covering it.

The bore for injecting the gaseous gasifying agent intersects the seam at point 1 situated behind the gasification front which extends between the points 2 and 3. The bore for injecting the liquid gasifying agent intersects the seam at point 4 situated in front of the gasification front.

This bore may subsequently be used for injecting the gaseous gasifying agent when the gasification front has advanced sufficiently.

The gaseous gasifying agent and the gases produced by the gasification reactions flow between the mass of coal 5 and the zone of debris 6 which results from subsidence of the subjacent rocks.

The liquid gasification agent injected at point 4 flows through the coal in front of the gasification front 2-3 where it emerges by seeping through the planes of stratification and the preexisting microfissures in the coal.

The rate at which the liquid is injected is adjusted to the desired value by acting on the injection pressure, which will always be more than 60% of the lithostatic pressure corresponding to the depth of the seam and it may without disadvantage reach 100% to 120% of this lithostatic pressure. For a seam situated at a depth of 1000 m, this corresponds to injection pressures in the region of 1 50 bar to 300 bar.

The liquid may be injected at ambient temperature but may equally well be preheated to a temperature of the order of 3000C to 3500C, which ensures that the coal mass will be preheated to a certain extent and facilitates the evaporation of water when the latter reaches the gasification front.

The gases resulting from the gasification reactions are evacuated by a known technique through a channel 7 which is formed in the thickness of the seam and connects the end of the gasification front to the bore 8 along which it is discharged to the surface.

The gaseous gasification agent injected at point 1 should contain a certain quantity of oxygen, which is necessary to maintain the gasification front at a sufficiently high temperature by virtue of the exothermic nature of the following reactions: O2+C=CO2 and 02+2C=2CO.

In practice, the gaseous gasification agent may be air, air enriched with oxygen, oxygen alone or oxygen mixed with CO2 in order to benefit from the advantages described in Belgian Patent Application No. 6/47289.

The liquid gasification agent may be water, water containing products such as ammonia which assist in the breakdown of coal into smaller pieces, or water containing residues which one wishes to get rid of, such as, for example, water containing phenol or tarry products which were used for washing the gas produced.

The advantages of the process according to the invention are various: It enables the diameter of the bores for injecting the gaseous gasifying agent to be substantially reduced. For example, for a given quantity of gasified coal, the volume injected may be reduced in the ratio of 6 to 1 if oxygen is injected at the ambient temperature instead of a mixture of steam and oxygen in the molar ratio of 2 to 1 being injected at 3000C.

An additional reduction in the cost of the injection bores is obtained by the fact that injection of a cold gas obviates the need to provide an internal heat insulating tube for reducing the heat losses and protecting the cement work of the casing tubes.

It obviates the need for boilers and their fuel supply and hence results in considerable economies in the investment costs and running costs of the installations on the surface used for the preparation of the gasifying agents.

An additional economy is possible by the elimination of installations for treating the waters used if these are recycled to the underground gas generator.

If the pressure of the emerging gas is maintained at a sufficiently high level, the process according to the invention enables the underground gas generator to function as a high performance heat engine. As a first approximation, it may be assumed that the reactions between the fixed coal and the gasifying agents take place in accordance with the following reaction scheme: Q+2H2011q.+2C=C02+H20vp.+C0+H2 This means that for one mol of oxygen, which must be compressed at the inlet to the gas generator, 4 mols of gas or steam under high pressure are obtained at the outlet.

The process also enables the efficiency of the gasification operations to be increased by increasing the surface areas of contact between gas and solid. It is well known that application of the techniques of infusion of water into a seam promotes breakdown of the coal which reduces its capacity to resist the pressures of the ground and assists crumbling and crushing of the seam.

This effect may be enhanced if the water injected contains certain chemical additives such as ammonia which assist in breaking down the coal.

Since the process according to the invention avoids the injection of any gasifying agent at a high temperature, it is generally applicable to the injection of gasifying agent from the surface and injection from below upwards from underground galleries hollowed out by conventional mining techniques in accordance with the methods which form the object of Belgian Patent Application No. 6/47288.

Claims

1. Process for the exploitation of coal by underground gasification, characterised in that it consists of simultaneously using a gaseous gasifying agent containing oxygen injected at the ambient temperature behind the gasification front and a liquid gasifying agent containing water injected at high pressure in front of the gasification front.

2. Process according to claim 1, characterised in that the gaseous gasifying agent is air, air enriched with oxygen, oxygen or oxygen mixed with carbon dioxide.

3. Process according to claim 1, characterised in that the liquid gasifying agent is water, water with the addition of ammonia or water obtained as residue from the installations for washing the gas produced.

4. Process according to claim 1, characterised in that the liquid gasifying agent is injected at a pressure of from 60% to 120% of the lithostatic pressure which corresponds to the depth of the seam.

5. Process according to claim 1, characterised in that the liquid gasifying agent is preheated before its injection to a temperature below the temperature of evaporation of water corresponding to the injection pressure employed.

6. Process for the exploitation of coal by underground gasification as described above and represented in the annexed drawing.