GB2093476A

GB2093476A - Production of a Calorific Gas Mixture

Info

Publication number: GB2093476A
Application number: GB8105262A
Authority: GB
Original assignee: BOC Ltd
Current assignee: BOC Ltd
Priority date: 1981-02-19
Filing date: 1981-02-19
Publication date: 1982-09-02

Abstract

A gas producer has a shaft 104 with a grate 118 on which coal or other solid carboniferous fuel is burned. An ash zone 122, combustion zone 124 and reduction zone 126 are established in operation of the producer. In the combustion zone 124 the predominant chemical reaction is the oxidation of carbon to carbon dioxide, whereas in the reduction zone the predominant chemical reactions are the reduction of carbon dioxide and steam by the coal. The necessary air and steam are supplied from below the grate 118. Oxygen or oxygen- enriched air is introduced into tie combustion zone 124 so as to form an endothermic or reduction region bounded by the combustion zone 124 or so as to lower the boundary between the combustion zone 124 and the reduction zone 126 whereby the calorific value of the resulting fuel gas passing upwardly out of the reduction zone 126 is able to be increased. <IMAGE>

Description

SPECIFICATION Production of a Calorific Gas Mixture This invention relates to the production of a combustible gas mixture. In particular, it relates to a method of producing such a gas mixture by the partial or total oxidation of a solid carboniferous fuel in a gas producer (or gasifier). The invention also relates to a gas producer adapted to perform such a method.

A gas mixture having a sufficiently high calorific value for it to be used as a fuel gas in industry can, it is well known, be produced by the reactions of steam, air and a solid carboniferous fuel at temperatures typically up to 1 2000C. The carbon in the fuel reacts with steam to form carbon monoxide and hydrogen, and carbon dioxide formed by complete oxidation of the carbon is reduced again by reaction with the carbon to carbon monoxide. Thus, a gas mixture whose main constituents are carbon monoxide, hydrogen and nitrogen (from the air) is formed.

This mixture is sometimes known as 'producer gas'.

The gas mixture is typically formed in a gas producer which typically has a vertical shaft to whose upper end the carboniferous fuel is fed, and at whose lower end is provided a grate, on which a bed of the carboniferous fuel (for example coal or coke) is supported (above a layer of ash).

In operation, a combustion zone (or region) is established below a reduction zone (or region). In the combustion zone carbon (in the fuel) is oxidised to carbon dioxide by a blast of air, and ash is consequently formed. The heat generated in the combustion zone maintains the reduction zone at a suitable temperature. In order to prevent the ash or solid fuel itself from fusing or agglomerating it is necessary to control the temperature within the combustion zone. This is conventionally done by introducing steam through the grate with the air blast. The steam acts as a coolant in the combustion zone almost entirely by virtue of its relatively low temperature and not by virtue of endothermic chemical reaction with the fuel.

In the reduction zone the carbon dioxide, formed in the combustion zone, reacts with the fuel to produce carbon monoxide, and some of the steam also passing from the combustion zone reacts with the fuel to produce carbon monoxide and hydrogen.

There is no discrete boundary between the combustion zone and the reduction zone. On the contrary, in a vertically upward direction from the bottom of the combustion zone the reactions between carbon and steam and between carbon and carbon dioxide become gradually prevalent until they predominate over the combustion reaction (i.e. oxidation of carbon to carbon dioxide). In this specification the boundary between the combustion zone and the reduction zone is held to be that level in the shaft of the gas producer where the rate of formation of carbon dioxide by oxidation of carbon equals its rate or reduction.Regions where the rate of reduction of carbon dioxide exceeds its rate of formation are considered to be in the reduction zone, and regions where the rate of oxidation of carbon to carbon dioxide exceeds the rate of reduction of carbon dioxide are considered to be in the combustion zone.

If desired, a two stage gas producer, having a distillation zone above the reduction zone, may be employed. A proportion of the gas produced in the reduction zone is passed upwards through an additional retort to preheat, dry and distil off volatile organic materials from the coal. The gas (containing the volatile matter) is then recombined directly or after cleaning with the main gas stream withdrawn from above the reduction zone. Such an arrangement avoids volatile substances being contacted with gas at high temperature which can lead to formation of tar deposits and like substances inside the gas producer or the main pipeline for the producer zone.

In order to reduce the proportion of nitrogen in the producer gas, and hence increase its calorific value, it has been proposed to introduce commercially pure oxygen into the air blast by mixing such oxygen with the air blast upstream of the grate. The enrichment of the air blast in oxygen upstream of the grate creates a need for a proportionally greater amount of steam to be introduced into the combustion zone so as to keep the temperature of this zone below that at which the fuel or its ash would agglomerate or fuse. (Typically, the temperature is maintained in the range 900 to 1 2000C preferably 1000 to 1 000C). Relatively large quantities of steam are required since the additional heat of the said enrichment of the air blast is removed by the extra steam almost entirely by sensible heating of the steam.A significant proportion of the steam thus passes through the higher zones of the gas producer and leaves it in the product gas. This, we believe, has the following drawbacks.

(a) a reduction in temperature in the reduction zone caused by undecomposed steam removing sensible heat as it leaves the zone; and thus a lower equilibrium concentration of carbon monoxide and hydrogen in the product gas; (b) a reduction in the proportion of additional exothermic heat that would otherwise be available to support the endothermic reactions producing carbon monoxide and hydrogen; (c) an increased carbon dioxide content in the producer gas; (d) a loss of process efficiency in view of the increased requirement for steam to be removed from the product gas; (e) a reduction in the residence time of the reactants in the reduction zone; (f) an increased ratio of hydrogen to carbon monoxide in the product gas, thereby reducing the net calorific value of the product gas (owing to the loss of the latent heat of water formed by combustion of hydrogen).

Notwithstanding such drawbacks, we believe that overall the enrichment of the air blast in oxygen upstream of the grate is advantageous giving a significant improvement over operation without such oxygen-enrichment.

The present invention aims at providing a method of producing a combustible gas mixture in a gas producer which makes it possible to enhance the calorific value of the gas mixture by reducing its nitrogen content while ameliorating or reducing the drawbacks listed above.

According to the present invention there is provided a method of producing a combustible gas mixture in a gas producer having a vertical shaft with a grate (or the like) at or near the bottom thereof, comprising the steps of: (a) establishing in the producer: (i) immediately above the grate, an ash zone in which ash produced by combustion of a solid carboniferous fuel collects, and from which ash is withdrawn; (ii) above the ash zone, a combustion zone in which solid carboniferous fuel is oxidised predominantly to carbon dioxide, and (iii) above the combustion zone, a reduction zone in which solid carboniferous fuel predominantly reduced carbon dioxide to carbon monoxide and any steam present to hydrogen; (b) feeding solid carboniferous fuel into the producer from the top of the shaft;; (c) causing a gas stream to flow vertically upwards from below the grate through the ash and combustion zones into the reduction zone, said gas stream comprising one or more of air, oxygen, steam and carbon dioxide; (d) introducing oxygen and one or both of steam and carbon dioxide from outside the shaft directly into the gas producer to form in the combustion zone a region in which endothermic reaction(s) predominate, or to extend downwards the boundary between the reduction zone and the combustion zone (in comparison to its location if there is no such direct introduction of oxygen and one or both of steam and carbon dioxide; (e) withdrawing a combustible gas mixture including carbon monoxide from the reduction zone.

The invention also provides a gas producer for performing such method, including a vertical shaft having a grate at or near its bottom; means for feeding solid carboniferous fuel into the shaft from thereabove; means for passing a gas stream vertically upwards through the shaft from below the grate; means for introducing oxygen and one or both of steam and carbon dioxide from outside the shaft directly into the gas producer to form in operation of the producer, in the combustion zone a region in which endothermic reactions predominate, or to extend downwards the boundary between the reduction zone and the combustion zone (in comparison to its location if there is no direct introduction of oxygen and one or both of steam and carbon dioxide); and means for withdrawing a combustible gas mixture including carbon monoxide from the reduction zone established in operation of the gas producer.

Preferably, the oxygen introduced directly into the gas producer (i.e. without passing through the grate or ash zone) is taken from a source of commerically pure oxygen or oxygen-enriched air.

It is then possible with appropriate selection of the composition of the gases introduced directly and/or indirectly into the combustion zone to produce a combustible gas mixture in such a way that the aim of the invention is met. We believe the creation of an endothermic region within the combustion zone or the extension downwards of the boundary between the combustion zone and the zone helps improve the cooling of the combustion zone and enables there to be a reduced requirement for the introduction of steam (and/or carbon dioxide) into such zone that would arise were all the oxygen to be introduced from beneath the grate. It is, however, necessary for some of the steam (and/or carbon dioxide) required to cool the combustion zone to be introduced directly into the combustion zone.

Preferably, at least some of such steam (and/or carbon dioxide) is premixed with the oxygen.

One or more tuyeres extending through the wall of the shaft are preferably used to introduce the oxygen directly into the gas producer. If two or more such tuyeres are used they are preferably spaced equally apart. If steam and/or carbon dioxide is to be introduced directly into the combustion zone separately from the oxygen, this may be accomplished by means of tuyeres similar to those used to introduce oxygen and preferably each oxygen tuyere has an associated steam and/or carbon dioxide tuyere disposed such that its longitudinal axis intersects that of the oxygen tuyere inside the combustion zone.

The or each oxygen tuyere is preferably disposed with its longitudinal axis perpendicular to the axis of the shaft or up to 200 with said perpendicular.

We believe that direct introduction of steam (or carbon dioxide) with oxygen into the combustion zone from tuyeres facilitates its endothermic reaction with carbon in or near the combustion zone and thus makes it possible for the steam to cool the combustion zone in a more effective manner than if it were introduced into the combustion zone from below the grate. Thus, for a given throughput rate of solid carboniferous fuel considerably less steam is required for the purposes of cooling the combustion zone than would be the case were all the oxygen for the combustion zone to be introduced from below the grate.

Preferably, the gas mixture introduced into the combustion zone from below the grate comprises a mixture of air and steam. If desired, the air can be enriched in oxygen, for example, by premixing it with commercially pure oxygen. The total amount of oxygen entering the combustion zone per unit time may be arranged to be substantially the same as it would be were all the oxygen to be introduced from below the grate.By using commercially pure oxygen as a source of the oxygen injected directly into the combustion zone it is thus we believe possible for a given rate of consumption of solid carboniferous fuel to reduce the amount of air passed into the combustion zone from below the grate (in comparison with what would be required if there were no direct injection of oxygen and steam into the combustion zone), thus reducing the amount of nitrogen in the combustible gas mixture produced by the method.

By injecting oxygen directly into the combustion zone, it is, we believe, also possible to increase the rate of consumption of the carboniferous solid and/or the calorific value of the combustible gas mixture.

If desired, the oxygen and/or steam (or carbon dioxide) injected directly into the gas producer may be pre-heated, for example, by heat exchange with a gas stream produced by the process.

If desired, oxygen can be introduced directly into the reduction zone (and above the region impinged upon by the oxygen directly introduced to lower the boundary between the combustion zone and the reduction zone or to create an endothermic zone in the combustion zone) from outside the shaft, that is, along a path in which none of the other said zones lies intermediate the exterior of the shaft and the reduction zone. The oxygen may be taken from a source of commercially pure oxygen or oxygen-enriched air.

It may be introduced into the reduction zone through one or more tuyeres which are preferably disposed perpendicularly to the axis of the shaft or at an angle in the range 0 to 200 with such perpendicular. Preferably, if such oxygen is introduced directly into the reduction zone, steam or carbon dioxide is similarly introduced and is, if desired, premixed with the oxygen. We believe that one advantage of adding oxygen in such a manner to the reduction zone is that the temperature of the reduction zone is raised. As a result, the rate of the reduction reactions in the reduction zone tends to be increased thus enabling the rate of production of the combustible gas to be increased.Even if there is no such injection of oxygen into the reduction zone there may be an advantageous raising of the average temperature in such zone in consequence of the direct oxygen introduction into the combustion zone. Moreover, for a given rate of consumption of solid carboniferous fuel, by reducing the rate of flow of air through the grate (in comparison to what would be needed if no oxygen were introduced directly into the combustion zone) the overall flow rate of gases into the shaft may be reduced with a consequential increase in the residence time of the gases in the reduction zone.

We believe that such an increase in residence time also favours an increased rate of production of calorific gas mixture and/or an increase in the rate of consumption of solid carboniferous fuel.

Preferably, the direct introduction of oxygen into the combustion zone is controlled by means of one or more temperature sensors located in that zone. For example, the supply of oxygen to the tuyeres or other means for introducing oxygen directly into the combustion zone may be controlled by one or more automatically operated valves operatively associated with the or each temperature sensor such that oxygen is introduced directly into the combustion zone at a rate such that the sensed temperature is maintained within a chosen range. Alternatively, the supply of steam (or carbon dioxide) directly to the combustion zone may be similarly used to control temperature. An optical pyrometer may alternatively be used to monitor the temperature.

Analogously, it is possible to use temperature sensors to control any direct introduction of oxygen or steam into the reduction zone.

If desired, a carbon dioxide containing gas stream produced by the process may, typically after further treatment to remove tar and pitch therefrom, be reintroduced into the combustion zone or the reduction zone. It is also possible to recycle heavy oils collected or condensed from the calorific gas mixture and introduced them into the combustion zone.

The method according to the present invention may be operated on a single-stage fixed bed gas producer operating at or slightly above atmospheric pressure or on a two-stage fixed bed gas producer operating at or slightly above atmospheric pressure.

The solid carboniferous fuel may typically be any kind of coal or coke. In addition, it is also possible to use lower grade solid carboniferous fuels. The solid carboniferous fuel may be of any convenient size. However, it is desirable not to use too small a size otherwise excessive quantities of dust may be produced.

The method and apparatus according to the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic sectional view of a single-stage gas producer, and Figure 2 is a schematic sectional view of a two-stage gas producer.

Referring to Figure 1 of the drawings, a gas producer has a cylindrical shell 102 defining a vertical shaft 104. The shell is surrounded by a water jacket 106 at the top of which there is a dome 108 for collecting steam. A pipe 110 leads from the dome 108.

At the top of the shaft 104 is a vertical feed channel 112 in which a rotary feeder 114 is located. The top of the channel 112 is connected to the base of a hopper 1 16. In operation of the gas producer lumps of coal or other solid carboniferous fuel may be fed from the hopper 1 6 into the shaft 104 by means of the rotary feeder 114.

In a lower region of the shaft 104 is a grate 1 18. The grate may be of any pipe employed in gas producers. For example, it may be of the continuously revolving eccentric kind. In order to start up the gas producer a bed of coal is established on the grate. It is then fired and the following zones are established in vertically ascending order. First, there is an ash zone in which ash from the combustion of the coal collects. Above the ash zone is a combustion zone 124. Above the combustion zone 124 is a reduction zone 126. In the combustion zone the predominant chemical reaction is between carbon and oxygen to form carbon dioxide. In the reduction zone 126 the predominant chemical reactions are the reduction of carbon dioxide and steam by the coal.Some combustion will inevitably take place in the reduction zone 126 and likewise some reduction will inevitably take place in the combustion zone 124. The boundary between these two zones is for the purpose of this specification is taken to be as follows. The combustion zone is taken to end and the reduction zone start at a level where the rate of oxidation of carbon to carbon dioxide is equal to the rate at which carbon reduces steam and carbon dioxide to hydrogen and carbon monoxide respectively.

At the top of the reduction zone is a gas offtake pipe 120.

The gas producer has an air main 128 for supplying a flow of air through the shaft in an upward direction from below the grate 118. The steam pipe 110 terminates in the air main 128 to enable the steam to be premixed with the air.

Extending through the water jacket 106 and the shell 102 of the gas producer are two opposed and horizontally-disposed tuyeres 130 which terminate in the combustion zone 124. The tuyeres are connected to an oxygen main 132 in which an automatic valve 136 is located. A steam pipe 134 extends from the pipe 110 and terminates in the oxygen main 132 downstream of the valve 136. Operatively associated with the valve 136 is a temperature sensor 138 located in the combustion zone in the vicinity of where the maximum temperature is likely to be attained. The valve 136 is programmed so that the rate of introduction of oxygen through the tuyeres 130 into the combustion zone 124 is varied to keep the temperature sensed by the sensor 138 between chosen values. (Alternatively, the temperature may be controlled by keeping the oxygen flow rate constant and varying the steam flow rate).

Also extending through the water jacket 106 and the shell 102 of the gas producer are two tuyeres 140 which terminate in the reduction zone 126. The tuyeres 140 are horizontally disposed and are connected to an oxygen main 142. An automatic valve 146 is located in the main 142. A steam pipe 144 branches from the pipe 110 and terminates in the main 142 downstream of the valve 146. A temperature sensor 148 is located in the reduction zone at a chosen region (for example located along the path followed by oxygen introduced from the tuyeres 140). The sensor 148 is operatively associated with the valve 146. The valve 146 is programmed so that the rate of introduction and the supply of oxygen through the tuyeres 140 is varied to keep the sensed temperature between chosen values.

(Alternatively, the temperature may be controlled by keeping the oxygen flow rate constant and varying the steam flow rate.) The grate 118 of the gas producer shown in Figure 1 is adapted to withdraw ash continuously from the ash zone 122 or has means associated with it for this purpose. The gas producer has a funnel shaped bottom section 1 50 into which the ash is discharged. At the bottom of the section 1 50 is an ash discharge valve 1 52 to enable the ash to be withdrawn from the gas producer as a whole.

In operation of the gas producer shown in Figure 1, lumps of coal are continuously fed by the rotary feeder 114 from the hopper 11 6 into the shaft 1 04. Ash is continuously withdrawn from the ash zone 122. Thus lumps of coal gradually move through the shaft 104 in a downward direction. An upward fluid flow through the grate 11 8 is created by passing air through the main 128 and mixing steam from the pipe 110 with it. Such steam is produced by water supplied to the water jacket 106 being boiled by the heat generated within the shell 1 02.

As the mixture of air and steam passes through the ash zone so its temperature is raised. When it encounters the combustion zone 124 the oxygen in the air reacts with the carbon to form carbon dioxide. Towards the top of the combustion zone 124 there will also be some reaction between steam and carbon to form carbon monoxide and hydrogen but this does not predominate over the combustion reaction. In the reduction zone 126 the predominant reactions are between the descending coal and carbon dioxide to form carbon monoxide and between the descending coal and steam to form carbon monoxide and hydrogen. A gas mixture comprising carbon monoxide, hydrogen and nitrogen is thus formed and this gas mixture leaves the gas producer through the pipe 120.

A mixture of commercially pure oxygen and steam is injected directly into the combustion zone through the tuyeres 1 30 so as to create a zone (within the combusion zone) in which endothermic reactions predominate. This injection makes it possible to reduce the flow of air into the gas producer from what would be regained if there is no direct injection of oxygen without adversely affecting the rate of production of the calorific gas mixture.

Since the reduction reactions are favoured by a relativeiy high temperature, it is desirable for the temperature in the reduction zone to exceed 10000C by a great a margin as possible. In both the combustion zone and reduction zone it is important to prevent so much heat being generated that the coal or ash is caused to fuse or agglomerate. In practice, therefore, it will generally not be possible to exceed a temperature of, say, 11 000C in the reduction zone. The introduction of oxygen and steam through the tuyeres is varied so as to achieve these objectives.

The combustible gas mixture comprising carbon monoxide, nitrogen and hydrogen leaving the gas producer through the pipe 120 may be subjected to further treatment in order to purify it.

The gas producer shown in Figure 2 differs from that shown in Figure 1 in that it is provided with an annular distillation zone above the reduction zone whereby volatile substances such as tars can be distilled off from the coal before it enters the reduction zone.

Referring to Figure 2, a two stage gas producer illustrated therein includes a shell 202 which defines the vertical shaft 204. The shell has at its bottom a grate 21 8. A portion of the shell 202 above the grate 218 is surrounded by a water jacket 206 having a steam dome 208 from which a pipe 210 leads. At the top of the shell 202 is a vertical feed channel 212 having a rotary feeder 214 in it. The feed channel 212 communicates at its upper end with a hopper 21 6. In operation, coal is fed from the hopper 21 6 into the top of the shaft 204. The coal then passes gradually down the shaft 204 passing successively through an annular distillation zone 260, a reduction zone 226 into a combustion zone 224.Ash from the combustion of the coal in the zone 224 collects in an ash zone 222 from which it is withdrawn and passes through the bottom section 250 of the gas producer under a water seal 266 and is then discharged.

An air main 228 is situated beneath the grate 218. The steam pipe 210 terminates in the air main 228. Thus, a mixture of air and steam may be passed upwardly through the shaft 204.

Two horizontally opposed tuyeres 230 extend through the water jacket 206 and the shell 202 into the combustion zone 224. The tuyeres 230 communicate with an oxygen main 232 into which a steam pipe extends from the pipe 210 downstream of automatic valve 236. The valve 236 is operatively associated with a temperature sensor 238. The temperature sensed by the temperature sensor 238 is kept within chosen limits by operating the valve 236 to vary the flow rate of oxygen.

There is an analogous arrangement for introducing a mixture of oxygen and steam directly into the reduction zone 226. Tuyeres 240 extend through the shell 202 into the reduction zone 256. There are two tuyeres opposed to one another and horizontally disposed. The tuyeres 240 are connected to oxygen main 242 which receives the outlet of a steam pipe 244 extending from the pipe 210. The outlet of the pipe 244 is situated downstream of an automatic valve 246 which is operatively associated with a temperature sensor 248 located in the reduction zone 226. The temperature is kept within chosen limits by operating the valve 246 to vary the flow rate of oxygen. (The higher the oxygen flow rate relative to that of steam the higher the resultant temperature, and the lower the oxygen flow rate relative to that of steam the lower temperature).

A combustible gas mixture comprising carbon monoxide, hydrogen and nitrogen is withdrawn from the reduction zone 226 by means of a cyclone 270 through a main gas off-tape pipe 220. In the cyclone dust is separated from the main product gas which is passed to a product gas pipeline 274. Some of the gas produced in the production zone 224 will pass into the distillation zone 260. This zone is defined by the walls of the shaft 204 and a central cruciform insert 262. In this zone volatile substances such as tar are distilled off from the coal and become entrained with the gas passing upwardly through the zone and are withdrawn from the gas producer through a pipe 264 by means of a cyclone 272 in which the tar is separated from the gas. The gas is then passed into the product gas outlet pipeline 274.

The operation of the gas producer shown in Figure 2 is substantially the same as that shown in Figure 2 with the exception that volatile organic substances forming part of the coal are distilled off from the coal in the distillation zone 260 before the coal enters the reduction zone 226.

In an example of a conventional gas producer the following results have been achieved.

Carboniferous fuel feed 1 500 Ib/hr Air rate 73,000 SCFH (standard cubic feet per hour) Steam rate 900 lb/hr Product gas rate 113,000 SCFH Gas composition (% v/v) CO2 5.9 CO 28.8 H2 14.2 N2 50.3 heating value 133 BTU/ft3 The abovementioned steam rate corresponds to a total usage of 18.5 Ibs of steam per thousand standard cubic feet of carbon monoxide and hydrogen produced.Approximately 75% of the steam is decomposed, and the remaining 25% passes through the gas producer unreacted; If the air is enriched in oxygen below the grate (i.e. conventional oxygen enrichment) by addition of pure oxygen to the air blast it is possible to achieve the following results:- Carboniferous fuel rate 1 900 lb/hr Enriched air rate 34,000 SCFH Oxygen concentration 50% v/v Steam rate 2100 lb/hr Product gas rate 97,000 SCFH Gas composition (% v/v) CO2 12.2 CO 40.3 H2 17.0 Heating value 214 BTU/ft3 In this case the steam rate corresponds to a total usage of 30 Ibs of steam per thousand standard cubic feet of carbon monoxide and hydrogen produced. Only 60% of the steam is decomposed. The remaining 40% passes through the gas producer unreacted.

If the concentration of oxygen is increased the quantity of steam required per thousand standard cubic feet of carbon monoxide and hydrogen produced increases steadily to a value of approximately 35 Ibs per thousand SCF (at 100% oxygen) while the quantity actually decomposed remains approximately constant at 20 Ibs per thousand standard cubic feet of carbon monoxide and hydrogen.

If the gas producer is adapted to perform the method according to the invention, we believe that an increase in the heating value of the gas produced may be achieved without a requirement for the large excess of steam used in the conventional oxygen enrichment process. It is envisaged that the total flow of air and oxygen into the producer operated in accordance with the invention need not be substantially in excess of the figure given above for the enriched air rate in a conventional enrichment process. The air supplied through the grate need not be enriched in oxygen. The rates at which oxygen and steam are introduced directly into the combustion zone (and if desired directly into the reduction zone) may be determined by simple experiment.

Similarly, the rates at which steam and air are passed through the grate may also be determined by simple experiment.

Claims

1. A method of producing a gas mixture in a gas producer having a vertical shaft with a grate or like) at or near the bottom thereof, comprising the steps of: (a) establishing in the producer: (i) immediately above the grate, an ash zone in which ash produced by combustion of a solid carboniferous fuel collects, and from which ash is withdrawn; (ii) above the ash zone, a combustion zone in which solid carboniferous fuel is oxidised predominantly to carbon dioxide, and (iii) above the combustion zone, a reduction zone in which solid carboniferous fuel predominantly reduces carbon dioxide to carbon monoxide and any steam present to hydrogen; (b) feeding solid carboniferous fuel into the producer from the top of the shaft;; (c) causing a gas stream to flow vertically upwards from below the grate through the ash and combustion zones into the reduction zone, said gas stream comprising one or more of air, oxygen, steam and carbon dioxide; (d) introducing oxygen and one or both of steam and carbon dioxide from outside the shaft directly into the gas producer to form in the combustion zone a region in which endothermic reaction(s) predominate, or to extend downwards the boundary between the reduction zone and the combustion zone (in comparison to its location if there is no such direct introduction of oxygen and one or both of steam and carbon dioxide; (e) withdrawing a combustible gas mixture including carbon monoxide from the reduction zone.

2. A method as claimed in claim 1, in which one or more tuyeres extending through the wall of the shaft are used to introduce the oxygen directly into the gas producer.

3. A method as claimed in claim 2, in which the or each oxygen tuyere is disposed with its longitudinal axis perpendicular to the axis of the shaft or making an angle of up to 200 with the perpendicular.

4. A method as claimed in claim 2 or claim 3, in which the directly introduced steam and/or carbon dioxide is premixed with the oxygen upstream of the tuyeres.

5. A method as claimed in any one of claims 1 to 3, in which the directly introduced steam and/or carbon dioxide is introduced separately from the directly introduced.

6. A method as claimed in any one of the preceding claims, in which the oxygen and/or one or both of the steam and carbon dioxide introduced directly into the gas producer is preheated.

7. A method as claimed in any one of the preceding claims, in which the temperature of the combustion zone is monitored, and the rate of direct introduction of oxygen into the gas producer is varied so as to keep the monitored temperature between chosen values.

8. A method as claimed in any one of claims 1 to 6, in which the temperature of the combustion zone is monitored, and the rate of direct introduction of steam or carbon dioxide into the gas producer in varied so as to keep the monitored temperature between chosen values.

9. A method as claimed in any one of the preceding claims, in which further oxygen is introduced directly into the reduction zone above the region impinged upon by said oxygen directly introduced into the gas producer.

10. A method as claimed in claim 9, in which further steam and/or carbon dioxide is introduced into the reduction zone with or separately from said further oxygen.

11. A method as claimed in claim 9 or claim 10, in which a temperature in the reduction zone is monitored, and the rate of introduction of said further oxygen varied so as to keep the temperature monitored in the reduction zone between chosen values.

1 2. A method as claimed in claim 10, in which a temperature in the reduction zone is monitored, and the rate of introduction of said further steam and/or carbon dioxide is varied so as to keep the temperature monitored in the reduction zone between chosen values.

13. A method as claimed in any one of the preceding claims, in which the gas producer additionally includes a distillation zone.

14. A method producing a combustible gas mixture substantially as herein described with reference to Figure 1 or Figure 2 of the accompanying drawings.

1 5. A gas producer for performing the method claimed in any one of the preceding claims, including a vertical shaft having a grate at or near its bottom; means for feeding solid carboniferous fuel into the shaft from thereabove; means for passing a gas stream vertically upwards through the shaft from below the grate; means for introducing oxygen and one or both of steam and carbon dioxide from outside the shaft directly into the gas producer to form in operation of the producer, in the combustion zone a region in which endothermic reactions predominate, or to extend downwards the boundary between the reduction zone and the combustion zone (in comparison to its location if there is no direct introduction of oxygen and one or both of steam and carbon dioxide); and means for withdrawing a combustible gas mixture including carbon monoxide from the reduction zone established in operation of the gas producer.

16. A gas producer as claimed in claim 15, substantially as herein described with reference to, and as shown in, Figure 1 or Figure 2 of the accompanying drawings.

1 7. A method of producing a combustible gas including any one or any combination of the novel features described herein.

1 8. Apparatus for producing a combustible gas mixture including any one or any combination of the novel features described herein.