GB2167431A

GB2167431A - Coal gasification process

Info

Publication number: GB2167431A
Application number: GB08429513A
Authority: GB
Inventors: Keith Raymond Tart; Stephen John Ling
Original assignee: British Gas Corp
Current assignee: British Gas Corp
Priority date: 1984-11-22
Filing date: 1984-11-22
Publication date: 1986-05-29
Also published as: GB8429513D0

Abstract

Organic matter and water condensed from the product gas of the ash-slagging gasification of coal, are used to slurry coal fines for use in an entrained flow gasifier. High pressure steam is obtained from the entrained flow gasifier process for use as process steam in the ash-slagging gasification process and in upgrading the synthesis gas.

Description

SPECIFICATION Coal gasification process This invention relates to the production of methane-containing gases and, in particular to the production of methane-containing gases from carbonaceous, eg. coal feedstocks.

A number of processes have been demonstrated for the gasification of coal for the production of synthesis gases which may be used, for example, as fuel gases. Such synthesis gases may be upgraded to meet fully the specifications for substitute natural gas (SNG).

Fixed bed gasification processes are able to gasify coal at very high efficiency owing to the good heat recovery between feeds and products resulting from the counter current flow regime. The earliest work on fixed-bed coal gasification resulted in processes whereby lump coal of a size greater than about 6mm could be gasified but not coal particles of a size less than about 6mm.

Modern mining techniques produce large quantities of these smaller coal particles or fines and as a consequence research is being concentrated on the efficient gasification of fines as well as the larger lumps so that the whole run-of-the-mine (ROM) can be gasified.

One such proposal is disclosed in US Patent Specification No.4211540 wherein a coal gasification process scheme employs a combination of a moving bed gasifier, for example the British Gas Corporation-Lurgi Ash Slagging Gasifier and an entrained bed gasifier, for example the Texaco coal partial oxidation reactor. The principal advantages claimed for this scheme derive from three complimenary features of the gasifier types, viz: (i) The Ash Slagging Gasifier produces an aqueous effluent which could cause disposal problems whereas the Texaco gasifier requires an aqueous feed.

(ii) The preferred feed for the Ash Slagging Gasifier is lump coal whereas the Texaco Gasifier requires a fines feed.

(iii) The Ash Slagging Gasifier used to make SNG requires additional steam, e.g. from a fuel-fired boiler, whereas the Texaco Gasifier generates a surplus of steam. If the energy associated with the surplus steam is not utilised the gasification efficiency of the Texaco process alone becomes very low.

In general terms the process of USP 4211540 (assigned to the Fluor Corporation) seeks to provide for the gasification of ROM coal feedstocks utilising water management systems between the gasifier stages.

The effluent from the Ash Slagging Gasifier is obtained in the first place as a condensate from the reaction product gas. The condensate contains tars, oils and an aqueous liquor which comprises phenols, other organic polar species and inorganic salts either dissolved or solubilised to a greater or lesser extent in water.

In the Fluor process the condensate has to be first treated to separate the aqueous and non-aqueous fractions and it is only the aqueous fraction which is utilised to slurry the coal fines to form the feedstock for the Texaco Process. We have now found that all the benefits of the Fluor process as defined in USP 4211540 can be realised without incurring the disadvantages of separating the aqueous component from the condensate derived from the Ash Slagging Gasifier.

Thus, in a process for the gasification of coal to produce a methane-containing gas wherein a first portion comprising larger size coal particles is gasified in an ash-slagging gasifier and the product gas is cooled to produce a condensate having an aqueous component which is used to slurry a second portion of smaller size coal particles and the slurried coal is gasified in an entrained flow gasifier, the present invention provides an improvement wherein the slurrying of said second portion of smaller size coal particles is effected with a liquid vehicle which also comprises non-aqueous components derived from said condensate.

The present invention will be further described and illustrated by reference to the accompanying drawing which is a schematic block diagram of the process of the invention.

A coal feed is first screened in unit 1 to separate it into particles or fines of less than about 6mm size, typically from 4-8 mm and those of greater size. If desired unit 1 could also comprise a crushing plant in order to provide greater amounts of fines.

The larger coal particles or lumps and possibly a modest amount of fines eg. 15% are fed to a conventional British Gas/Lurgi Ash Slagging Gasifier 2, which would typically operate at a steam to oxygen ratio in a rang: of about 0.9 to 1.3:1(molar). The ratio of lump to fines would depend upon the composition of the run-of-mine feed. However, it is preferred that the major proportion of the total coal feed be sent to After the primary gasification stages both product gases usually require purification and possibly upgrading for use as a fuel or synthesis gas. Whether the gas streams are combined prior to post-treatment or treated separately depends to some extent upon application of the gas product.For example, for SNG applications, a transmission pressure of about 68 bar maybe required and therefore, ideally both gasifiers should operate at pressures high enough to avoid product gas compression. However, practical constraints will usually result in a mis-match e.g. the slagging gasifier might, for example, be operated at about 32 bar, whereas operation of the entrained bed gasifier would be at about 82 bar. In US4211540, it is suggested that the entrained gasifier product be expanded through a power generating turbine prior to mixing with the slagging gasifier product. The resultant mixture was then post treated and had to be compressed before it could be transmitted. As an alternative we have found that advantages accrue if the product of the slagging gasifier is compressed prior to mixing since operation of down-stream plant becomes more economical.This is particularly advantageous if the upgrading process is carried out according to the teachings of our UK Patent Specification No. 1544245 since the duties of the recycle compressor in that process can be significantly reduced. Similarly, the use of high operating pressures reduces the utility demands on acid gas clean up processes such as Selexol and Rectisol.

The choice of the ratio of feedstock fed to the ash-slagging and entrained flow gasifiers is dependent upon: (i) the required specification for the product gas (ii) the balance of the heat requirements If it is desired to increase the output of the ash-slagging gasifier relative to the entrainedflow gasifier, adjustments may be made by altering the feed ratio but also by altering the amount of condensate liquor fed to the entrained bed gasifier. Thus it may be preferred not to utilise all the liquor in the entrained flow gasifier and a portion may be recycled to the ash-slagging gasifier. This recycled portion may be added either into the bed at the tuyere level, or, in the case of tar, may be added at the top of the bed where it will suppress dust carry-over.

This novel and inventive combination of gasification techniques has the following advantages: (i) elimination of the requirement for tar/aqueous liquor and oil/aqueous liquor separators, (ii) avoidance of cooling and reheating condensate from the ash-slagging gasifier (iii) more efficient tar, oil and fines gasification since waste heat is recovered from the entrained bed gasifier as high pressure steam and can therefore be used as process steam.

In another embodiment it may be preferred to feed all the fines to the entrained flow gasifier but to gasify a proportion in the ashslagging gasifier. In this case, the fines may be added directly to the top of the fixed bed or into the raceway region of the ash slagging gasifier. These fines may be added dry or in the form of a slurry; liquid condensate being a suitable vehicle for the slurry.

Claims

1. A process for producing methane-containing gases wherein a first portion comprising larger sized coal particles are gasified in an ash-slagging gasifier and wherein liquid condensate obtained from the product of such ash slagging gasification is used to slurry a second portion of smaller sized coal particles which are gasified in an entrained flow gasifier, characterised in that the slurrying of said second portion is effected with a liquid vehicle which also comprises non-aqueous components derived from said condensate.

2. A process as claimed in claim 1 in which a majority of the coal feed is gasified in the ash-slagging gasifier.

3. A process as claimed in claim 1 on claim 2 wherein a proportion of said liquid vehicle is recycled to said ash-slagging gasifier.

4. A process as claimed in any one of claims 1 to 3 wherein said first portion also comprises coal fines.

5. A process as claimed in claim 4 wherein said fines are added as a slurry.

6. A process as claimed in claim 5 wherein the liquid vehicle for said slurry is liquid condensate.

7. A process for the production of methanecontaining gases according to claim 1 and substantially as herein before described.