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(11) | EP 0 000 442 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Process and apparatus for the gasification of coal |
| (57) A coal gasification process and apparatus therefor in which means are provided for
supplying fuel to and maintaining a fixed fuel bed in the upper region of a reactor
vessel, supplying additional fuel together with a gasifying agent to initiate and
maintain gasification in an entrained stage located in the lower region of the vessel,
the gasification products from the entrained stage passing up through and gasifying
fuel in the fixed bed, the slag from the fixed bed passing downwards to mix with slag
from the entrained stage and exits via a slag tap to a slag quench system located
below the reactor vessel. The fixed bed may be maintained in the upper region of the vessel by a grate which extends across the vessel. The grate may be cooled by the circulation of an appropriate coolant through the elements forming the grate. |
[0001] This invention relates to coal gasification plant, and more particularly to an improved and modified form of slagging gasifier said slagging gasifier being plants of the kind (hereinafter referred to as the kind specified) in which coal, or other carbonaceous fuel, is introduced into the top of a column-like gasifying vessel and is gasified under high pressure and temperature by means of oxygen and steam introduced near the fuel bed through tuyeres. The residual ash collects as a molten slag and iron in the hearth of the gasifier vessel from which it is discharged (commonly known as slag-tapping) downwardly through a slag tap outlet or orifice in the hearth into water contained in a quenching chamber. Usually, a pool of molten slag and iron is maintained in the hearth by directing hot combustion products from a burner located beneath the slag tap orifice up the tap orifice to retain the pool of slag and iron in the hearth, the tapping of the molten slag and iron being initiated and controlled by stopping or reducing the burner output and reducing the pressure in the quenching chamber by controlled venting to atmosphere through its venting system so as to produce a differential pressure between the quenching chamber and the gasifier vessel.
[0002] Examples of such slagging gasifier plant are those disclosed in United Kingdom Patent Specification No. 977,122 and the Gas Council Research Communications No's GC 50 and GC 112.
[0003] A slagging gasifier plant of the kind specified represents slagging gasification in its simplest form and is a form most appropriate to the fixed bed gasification of "lump" coal (nominally 2" -
) at high pressure in steam and oxygen. It is particularly suited for the production of substitute natural gas (SNG) and synthesis gas. The gasifier has an output two or three times greater than the well known and conventional grate-type gasifier; has a lower steam consumption; produces less aqueous liquor; has a greater thermal efficiency and, on the less reactive low fusion point ash coals, has a lower oxygen consumption. Neither the slagging or the grate-type gasifiers can operate with a coal feed which contains a high proportion of coal fines although the slagging gasifier will accommodate the injection of fine coal or char at the tuyeres up to about 10% of the total fuel supply. Future coal supplies may consist of even more fines than those of today and the ability to consume a total mine output will be an important requirement for conservation of resources.
[0004] SNG plants in the USA and those that might be built later in Europe are expected to have capacities of 250 MMSCFD and will require about 5) million tons of coal per year which will come from one or more mines dedicated to the plant. Strip mining produces a minimum of 20% material below 1" in size, mechanised deep coal mining produces as much as 40% while figures as high as 60% have been quoted for strip mining of brown coal in Germany. The ability to gasify fine coal therefore could become a major if not an essential requirement of future gasification systems. Powdered coal processes of the fully entrained type and of the fluidised bed type now under development can absorb total mine output but at the expense of grinding or pulverising all of the coal, of sacrifices in thermal efficiency in the former case and, in the latter case, of complications in reactor and process engineering, etc.
[0005] According to one aspect of the present invention a coal gasification process and apparatus therefor comprises means for supplying fuel to and separately maintaining a fixed bed fuel gasification zone in the upper region of a reactor vessel, supplying additional fuel together with a gasifying agent to initiate and maintain reaction in an entrained fuel gasification zone located in the lower region of the vessel, the gasicication products from the entrained fuel gasification zone passing up through and gasifying fuel in the fixed bed gasification zone, slag from the upper gasification zone passing downwards to mix with slag from the entrained stage and exits via a slag tap outlet to a slag quench system located below the reactor vessel.
[0006] The fixed bed gasification zone may be maintained in the upper region of the vessel by a throat formed within the vessel to restrict the downward flow of fuel and support the fuel bed.
[0007] In an alternative arrangement the fuel bed may be supported on a grate which extends across the vessel. Said grate may be cooled by the circulation of an appropriate coolant through the elements forming the grate.
[0008] In a preferred form of the invention additional gasifying agents are introduced to the fixed bed gasification zone. Said gasifying agents can be introduced above or below the throat or grate.
[0009] Methods of carrying out the invention are described in detail below with reference to drawings which illustrate only two specific embodiments, in which
Figure 1 is a general cross-sectional view of a coal gasification reactor vessel showing the use of a restricted throat in the vessel,
Figure 2 is an enlarged view of part of a similar vessel showing the use of a grate within the vessel,
Figure 5 is a block flow diagram of a composite gasifier.
[0010] The coal gasifier as shown in Figure 1 comprises a reactor vessel 1 arranged to contain within the upper region of the vessel a fixed bed gasification zone 2 and within the lower portion of the vessel a fully entrained fuel gasification zone 3.
[0011] The gasifier is provided with coal supply chutes 25 which feed lump coal via coal lock hoppers 12 and 13 and coal chutes 14 into the upper region of the reactor vessel. The interior of the vessel 1 is lined with a refractory material 8 and the lower end of the vessel is provided with a slag tap outlet 4, through which liquid slag may be discharged into a water filled slag quench chamber 10. After cooling the granular like slag can be removed via a slag lock hopper 11.
[0012] A gas burner 5 located below the slag tap 4 provides hot combustion products which when directed up the slag tap orifice retains the molten iron and slag in the hearth area. One form of slag tap and burner is described more fully in British Patent Specification No. 1,512,677.
[0013] The refractory lining which extends into the hearth zone is provided with cooling coils 9 such that a suitable coolant can be passed through the refractory.
[0014] A gas off-take duct 15 is located at the top of the vessel. The refractory lining of the off-take and of the vessel is provided with cooling coils 16.
[0015] A restricted throat 18 may be formed of a cast refractory material and provided with coolant passageways 17. Other materials may be used. Tuyeres 6 are arranged to inject pulverised fuel and gasifying agents, eg steam, oxygen or oxygen enriched air, into fully entrained gasification zone 3 and tuyeres 7 located above the throat 18 may inject additional gasifying agents into the fixed bed gasification zone 2. Additional tuyeres (not show) or tuyres 7 may optionally be
located below the throat 18 to supply the additional gasifying agents.
[0016] The gasifier when operating comprises a fixed bed gasification zone 3 in the upper regions of the reactor vessel and a fully entrained gasification zone in the lower portion of the vessel.
[0017] The fixed fuel bed is supported on the restricted throat 18 below which is the combustion chamber wherein a fully entrained gasification zone is initiated and maintained by the injection of pulverised fuel and gasifying agents through the tuyeres 6. The products of gasification from the entrained zone consisting essentially of steam, carbon monoxide and hydrogen pass up through the throat to gasify the fuel in the fixed bed gasification zone above. The slag produced in the fixed bed gasification zone runs down through the throat and the slag produced from the pulverised fuel in the entrained zone below collects together and runs or is drained off through the slag tap into the water quench system. The combustion reactions in the fully entrained zone can take place in a vortex, the slag being thrown out on to the combustion chamber walls, or in the turbulence of opposing jets. Since the walls and floor of this chamber are lined with water-cooled coils with the water cooled throat forming the roof, all of the wall may be coated in frozen slag to some equilibrium thickness. The layer of frozen slag provides protection for the vessel internals and may facilitate the flow down the walls of slag from the upper gasification zone.
[0018] Additional steam and oxygen (or oxygen enriched air) can be introduced through tuyeres 7 into the fixed bed gasification zone. Tars and oils can also be introduced through these tuyeres 7 or through tuyeres 6.
[0019] It is the ability to distribute the gasifying agent across the full cross-sectional area and to use large coal particularly in the upper bed which enable, with the less strongly caking coals, outputs to be achieved well in excess of the known slagging gasifier.
[0020] In the alternative embodiment shown in Figure 2 the fuel bed in the fixed bed gasification zone is supported on a grate 20 which extends across the reactor vessel. The vessel may also be provided with a restricted throat 21 of refractory material and the elements forming the grate and the throat are provided with internal coolant passageways 22 and 23.
[0021] A gasifier, constructed in accordance with the invention, has some of the advantages of both fully entrained and fixed bed gasifiers. It accommodates the total output of a mine with the minimum of preparation. The fixed bed gives the system "carbon capacity", by which we mean a large quantity of carbon present within the reactor which reduces if not eliminates the sensitivity which a fully entrained system has to a loss of fuel feed. It also adds counter-current flow of fuel to product gas and so provides some heat recovery absent in a fully entrained system. It also enhances the production of methane in passing the product through the bed.
[0022] A further potential advantage of the composite gasifier is that it may be arranged for operation on air or oxygen enriched air for the production of low Btu( 160 Btu/SCF) as a particulate and sulphur-free reducing of synthesis gas, and as a fuel gas for combined cycle power production or direct boiler firing.
[0023] Figure 3 is a block flow diagram of the process. Table 1 and Table 2 may be used in conjunction with Figure 3 to show the material balances estimates for cases in which there is high heat loss from the process and alternatively a low heat loss.
[0024] Tar removal from the product gas may be used to lay dust on top of the coal bed but used this way the tar will again be recovered in the tar recovery system. Tar may be gasified by injecting it preferably through the tuyeres 7 into the fixed bed gasification zone.
[0025] Conventional air-blown fixed bed gasifiers are handicapped by the low temperature or operation imposed by the need to avoid clinker formation a problem which can be eliminated if slagging operation can be employed.
[0026] The slagging gasifier is not particularly suited to operation on air, the nitrogen in the air tripling the volume of gases entering the combustion zone and tending to lead to instability and poor slagging conditions. The addition of fines below the fixed bed of fuel in the gasifier, constructed in accordance with the invention, will facilitate the use of air or oxygen enriched air for gasification under slagging conditions.
1. A coal gasification process and apparatus therefor comprising means for supplying
fuel to and separately maintaining a fixed bed fuel gasification zone (2) in the upper
region of a reactor vessel (1), supplying additional fuel together with a gasiyfing
agent to initiate and maintain reaction in an entrained fuel gasification zone (3)
located in the lower region of the vessel (1), the gasification products from the
entrained fuel gasification zone passing up through and gasifying fuel in the fixed
bed gasification zone, slag from the upper gasification zone passing downwards to
mix with slag from the entrained stage and exits via a slag tap outlet (4) to a slag
quench system (10) located below the reactor vessel (1).
2. A coal gasification process according to Claim 1, wherein additional gasifying
agents are supplied to the fixed bed gasification zone.
3. A coal gasification process and apparatus therefor according to Claim 1, wherein
the fixed bed gasification zone is maintained in the upper region of the vessel by
a throat (18) formed within the vessel to restrict the downward flow of fuel and support
the fuel bed.
4. A coal gasification process and apparatus therefor according to Claim 1, wherein
the fixed bed gasification zone is maintained in the upper region of the vessel by
a grate (20) extending across the vessel to restrict the downward flow of fuel and
support the fuel bed.
5. A coal gasification process and apparatus therefor according to Claim 4, wherein
said grate (2 ) is cooled by the circulation of an appropriate coolant through the
elements forming the grate.