GB2151253A - Methane from coal - Google Patents

Abstract

Methane-rich gases 8, suitable for use as SNG are produced by the hydrogenation of coal 2. By-product char 12 together with additional coal 11 are fed to a fixed bed ash slagging gasification stage 50 eg. a British Gas/Lurgi Slagging Gasifier to produce a CO/H2 mixtures which is subjected to CO-shift 60 to produce a hydrogenating gas 18 for the hydrogasifier. <IMAGE>

Description

SPECIFICATION Combined coal hydrogenation and slagging gasification processes This invention relates to the gasification of carbonaceous materials. More particularly the invention relates to the production of methane containing gases by the gasification of coal.

Processes are being developed for the production of methane by direct hydrogenation of coal. These hydrogenation processes generally suffer from a number of disadvantages - for example an inability to handle coal containing a large proportion of fine particles, sensitivity to coal quality, and the production of environmentally unacceptable by-products.

Efficient and complete utilisation of by-product char is also a major problem generally associated with coal hydrogenation schemes.

Fixed bed slagging gasifiers offer the potential advantages of high thermal efficiency and complete carbon conversion. They do however, generally require a lump fuel feed and so in the context of a coal hydrogenation facility by-product char would have to be briquetted (or pelletised) to allow it to be fed to the reactor. An example of such a technique is disclosed in UK Patent Specification 2044288A.

The present invention proposes an alternative to this general approach by recognising the possibility and advantages of feeding fine char particles directly to the ash slagging zone of a fixed bed slagging gasifier. To achieve this the char 5 is injected into the gasifier e.g. through the nozzles (tuyeres) thus eliminating the need for costly briquetting equipment, binder materials etc.

In accordance with the present invention there is provided a process for the production of methanecontaining gases which process includes the steps of subjecting a first portion of a coal feed comprising at least 50% of the total coal gasified, to reaction with a hydrogen-containing gas to produce a methane-containing product gas and char, wherein at least a portion of said hydrogen-containing gas is obtained by subjecting a feedstock comprising the remaining portion of said coal gasified and at least a portion of char to reaction with steam and an oxygen-containing gas in a fixed bed slagging gasifier in which the char introduced into the slagging zone by injection, and CO-shifting the resulting product gas.

The process of the present invention can be applied to coal containing a large proportion of fine particles wherein the first portion will comprise substantially the smaller particles of coal fines. Additional larger lump coal required for the first portion will be ground or pulverised to a size small enough for feeding to the hydrogenator. Typically, the particle size of the fines for the hydrogenator will not exceed 6mm.

The operating temperature of the hydrogenation stage may range from 700-1 100"C and the reaction pressure is preferably from 2-10 MPa. The reactants for the hydrogenation reaction are coal and a hydrogen-containing gas and the preferred coal to hydrogen (in the hydrogenating gas) ratio is from 5 - 1.7 kg daf per kg of hydrogen. (As used herein the abreviation "daf" means "dry and ashfree").

The second or hydrogen producing reactor is one conventionally employed for the gasification of coal under ash-slagging conditions wherein the gasifying agents are fed to the fixed bed reactor via tuyeres located towards the bottom of the reactor. The gasification agents include steam and an oxygen-containing gas such as air or oxygen itself.

The char, which contains mainly ungasified carbon and ash, obtained from the hydrogenator, is fed as fines into the ash slagging zone via the tuyeres.

The fixed bed slagging gasifier is operated in known manner, for example, as described in US Patent Specification 3390971, except that the char derived from the is hydrogenation stage is fed to the ash slagging zone via tuyeres along with other gasification agents. The operation is preferably carried out at a pressure ranging from 0.1 - 10 MPa. The preferred reactant parameters for gasification stage are given in the following table.

Char fines : Coal Ratio Upto 0.7 kg daf:1 kg daf Steam : Coal Ratio 0.3-0.7 keg: 1 kg daf Oxygen : Coal Ratio not less than 0.5 kg 1 kg daf Steam : Oxygen Ratio 0.9-2.00 mol 1 mol Tar & Liquor: Coal Ratio 0.05-0.4kg 1 kg daf* * Tars and liquors are obtained during gas clean-up from both the hydrogenation and fixed bed slagging gasification stages The present invention will be described in greater detail with reference to the accompanying drawings in which: Figure lisa schematic flow diagram illustrating the main features of the invention and, Figure 2 is a schematic flow diagram of a more detailed embodiment of the invention. Like numbers are used to indicate like parts.

Referring to Figure 1 a first portion coal feed eg. comprising 50 to 60% of the total reactant coal feed is pulverised and fed to a hydrogenator 22 via line 2. Hydrogen for the hydrogenator is supplied via line 3. The product of the hydrogenator comprises a solid char which is removed via line 4 and a gaseous component which is then subjected to gas purification and separation 30 to produce a hydrogen-rich stream 7 and product gas 8.

A second portion of the coal feed eg. the remaining 50 to 40% of the total and preferably comprising the larger lumps is fed via line 11 to slagging gasification section 50, eg. a British Gas/Lurgi Slagging Gasifier.

Steam 13 and oxygen 14 are fed in together with a portion 12 of the char 4 removed from hydrogenator 22, through tuyeres (not shown). The gaseous product is treated in downstream section 60 where the gas is CO-shifted and separated to produce a hydrogen-containing stream 18 and a side-stream of carbon dioxide.

Streams 18 and 7 are combined to form the hydrogen feed stream 3.

The invention will be further illustrated by the following example and with reference to Figure 2 of the drawings.

A coal feed comprising 58% of the total was fed via line 1 to coal drying and pulverisation plant 21 and thence to a hydrogasifier 22 via line 2 at a rate of 103 tonnes hr1 and reacted with a hydrogenating gas fed at a rate of 21 tonne mol hrl into the reactor via line 3. The composition of the hydrogenating gas was: CH4 3.7% vol CO 1.6 H2 93.7 Higher Hydrocarbons and Inerts 1.0 The reaction product was then subjected to gas cooling and char separation in unit 31, to acid gas removal in unit 32 from line 5, and thence via line 6 to a cryogenic separation plant 33. In plant 33 a hydrogen-rich stream 7 is separated from the methane-containing stream 8. Stream 8 is then subjected to methanation in unit 40 to produce a final gas 10 of substitute natural gas specification.The gas compositions of streams 6, 8 and 10 are given in the following Table.

TABLE 1 Stream No. 6 8 10 Component Methane 26.76 75.38 96.49 Carbon Monoxide 2.93 5.58 Hydrogen 69.09 17.26 1.23 Inerts 1.22 1.78 2.28 The remaining 42% of the coal feed is fed through line 11 to the lock hopper of a British GaslLurgi ash-slagging gasifier 50 together with flux from storage 51. A portion of the char from separator 31 is also fed to the gasifier through line 12, together with steam and oxygen through lines 53 and 54. The char, steam and oxygen are fed into the ash slagging zone through tuyeres (not shown). Lock hopper make-up gas is provided from unit 52 through line 9. The reactant feed rates for gasifier 50 are shown in Table 2.

TABLE 2 Coal 79.2 tonne hf' Char 14.8 tonne hr1 Steam 2.3 tonne mol hil Oxygen 1.8 tonne mol h71 The product gas leaving the slagging gasifier is quenched and cooled to ambient temperature in a series of heat exchangers 61. Within the coolers the product separates into a liquid condensate phase comprising tars and aqueous liquors containing fatty acids, phenols etc. and a gas phase 15. The gas phase 15 is then subjected to purification in unit 62 wherein sulphur and the medium hydrocarbons eg. naphtha, are separated. The hydrocarbon condensate phases separated from units 61 and 62 can be recycled for consumption in the gasifier.

Gas stream 16 exiting unit 62 is then mixed with steam via line 23 and subjected to a CO-shift reaction to produce hydrogen rich stream 17 which, after acid gas (e.g. CO2) removal in unit 64, is sent via line 18 to admix with stream 7 to form hydrogenation gas stream 3.

Table 3 gives the gas compositions of the hydrogenation gas during the synthesis stages.

TABLE 3 Stream No. 16 17 18 7 3 Gas Composition % CH4 5.60 3.56 5.71 2.26 3.64 CO 59.10 1.06 1.66 1.59 1.62 CO2 4.50 39.38 - - H2 29.56 55.22 91.37 95.21 93.67 C2H5 0.41 0.34 0.52 - 0.21 C2H4 0.13 - - - - Inerts 0.70 0.44 0.74 0.94 0.86 Char which not employed in the slagging gasifier is sent via line 19 to be mixed with additional coal feed from storage 71 via line 20 to fuel power and steam generation.

The process of the present invention offers considerable technical advantages including: (i) The ability to accept raw coal, i.e. coal with a high fines (particles less than 6mm in size) content.

(ii) High thermal efficiency.

(iii) The ability to gasify all carbonaceous by-products of the process, eg. tar, oil, phenol, char, in the slagging gasifier section.

Claims (8)

1. A process for the production of methane-containing gases including the steps of subjecting a first portion of a coal feed comprising at least 50% of the total coal gasified, to reaction with a hydrogencontaining gas to produce a methane-containing product gas and a char, wherein at least a portion of said hydrogen-containing gas is obtained by subjecting a feedstock comprising the remaining portion of said coal feed and at least part of said char to reaction with steam and an oxygen-containing gas in a fixed bed slagging gasifier in which the char is introduced into the ash slagging zone by injection, CO-shifting the resultant product gas.

2. A process as claimed in Claim 1 wherein said first portion comprises up to 60% of the total coal feed.

3. A process as claimed in claim 1 or claim 2 in which the char is injected either dry or in admixture with aqueous or organic liquids.

4. A process as claimed in any of the preceding claims wherein the coal feed is raw coal and the coal particles of not greater than 6mm size comprise said first portion of coal feed.

5. A process as claimed in any one of the preceding claims wherein said resultant product hydrogen containing gas is purified by the removal of acid gases prior to reaction with said first portion of coal feed.

6. A process as claimed in any one of the preceding claims wherein said oxygen-containing gas is air.

7. A process as claimed in any one of the preceding claims, wherein a hydrogen-containing gas is recovered from said methane-containing gas.

8. A process for the production of methane containing gases according to Claim 1 and substantially as hereinbefore described.