GB2125429A

GB2125429A - Tar removal process

Info

Publication number: GB2125429A
Application number: GB8320940A
Authority: GB
Inventors: Frank Clifford Brown; Anthony Horton; Howard Geoffrey Hargreaves
Original assignee: Humphreys and Glasgow Ltd
Current assignee: Humphreys and Glasgow Ltd
Priority date: 1982-08-04
Filing date: 1983-08-03
Publication date: 1984-03-07
Also published as: GB2125429B; GB8320940D0

Abstract

A process for removing tar from the gaseous products of the thermal treatment of a carbonaceous material, which process comprises cooling the products to a temperature sufficiently high to keep essentially all of the water vapour present therein in the vapour phase, but sufficiently low to condense out at least a major portion of the tar therein, and then separating the condensed tar from the gaseous products without essentially any condensation of water. Preferably the condensed tar is separated by scrubbing the cooled products with a tar-containing liquid stream, which stream is maintained during scrubbing at a temperature sufficiently high to prevent the condensation of water. The process is particularly applicable for the gasification of coal.

Description

SPECIFICATION Tar removal process The present invention relates to a process for removing tar from the gaseous products of the thermal treatment of carbonaceous materials.

More particularly, it relates to the removal of tars from the gaseous products of the retorting of shales and, especially, of the gasification of coal.

The thermal treatment of carbonaceous materials often produces gaseous products which require cooling and very frequently cleaning, before they can be utilised. One major cause of the need for cleaning is the production during the thermal treatment of so-called "tars". The word "tar" has no well-defined meaning, but is recognised by those skilled in the art as referring to a relatively high boiling point mixture of high molecular weight hydrocarbons including aromatics of various structures, which mixture often includes inorganic components such as ammonia compounds and compounds of carbon.

The nature of any particular tar will depend upon the carbonaceous material from which it came and the thermal history of that material.

Very often water is produced concurrently with the tar or is added in the form of a water quench, and one of the first requirements in the processing of the gaseous products after cooling is frequently the separation of the tar/water mixture thereby formed.

In, for example, the pressurised gasification of coal using fixed beds, the gaseous product generally contains H2,CO,CO2,CH4,N2, water vapour, light hydrocarbon oils, tar, and entrained dirt.

Depending upon the detailed design of the gasifier the product can exit either at quite low temperatures, say 2000 C, or in some gasifiers even up to 8000C. Conventional practice is to separate the tar, dirt and hydrocarbon oils from the system concurrently with cooling the product by passing the raw dirty product through a circulating water quench system. The product is cooled and exits from the scrubber with the cooling liquor which now contains the major portion of the tar condensed in the water. This water/tar liquor is cooled by conventional heat exchange means and re-circulated to the scrubber system. A bleed of water/tar is taken from this circulating system and passed to a separator vessel wherein the heavy tar is allowed to settle and separate from the lighter water, these being discharged as separate liquid phases.

The main problem associated with this known type of system is the well-recognised difficulty of achieving a separation by gravity of tar and water.

If too much mechanical energy is imposed on the system either from the turbulence from within pumps or by pressure reducing devices, then there is a significant tendency for an emulsion to be formed which increases the difficulty in separating the two components.

The present invention seeks to provide a process for removing tar whereby the cooling of such gaseous products and the removal of tar therefrom are carried out in such a way as to avoid the necessity for a tar/water separation step.

In accordance with the present invention there is provided a process for removing tar from the gaseous products of the thermal treatment of a carbonaceous material, which process comprises cooling the products to a temperature sufficiently high to keep essentially all of the water vapour present therein in the vapour phase, but sufficiently low to condense out at least a major portion of the tar therein, and then separating the condensed tar from the gaseous products without essentially any condensation of water.

In its preferred embodiment the present invention provides a process of cooling and detarring the hot exit gas from a pressurised coal gasifier wherein the exit gas, after quenching by evaporating a stream of liquid water such that the said exit gas is maintained after quench at a temperature above its water dew point but sufficiently low to ensure that substantially the major part of the tar it contains is in liquid form, is passed into a scrubbing loop wherein the said gas is contacted in a scrubber with a circulating stream of tar so that the gas leaving the scrubber is substantially free of tar but containing in the vapour state substantially all the water used for the initial quench, and wherein some of the tar leaving the scrubber is re-circulated through the said scrubbing loop which operates substantially adiabatically, for further use as the scrubbing medium.

In an alternative embodiment the condensed tar can be re-cycled as the quench in place of water, and the separation step can be carried out using a cyclonic separator. Heat from the condensed tar can be extracted by heat exchange to raise steam, whilst leaving the tar fluid enough to be circulated.

Direct quench is preferred as the method of cooling, but it will be recognized that other forms of heat removal, e.g. countercurrent exchange, can be used.

Direct quench is particularly useful where the gaseous product is at a relatively high temperature, e.g. 8000 C. In such cases, an initial quench, e.g. with water, can be used before the main product cooling step in order to bring the product temperature down to suit pipe metallurgy in the main treatment equipment, e.g. to 5000C.

This water can then be recovered by condensation down-stream and recycled.

One embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawing, which illustrates a flow diagram for the removal of tar from the gaseous products of the gasification of coal. Coal is gasified in gasifier (1) and the gaseous products leave via its exit gas main (2).

Liquid water is sprayed (3) into the main to quench the product to the desired temperature, after which the cooled product is scrubbed with tar only in a conventional contact device, e.g. by a scrubber (4). The tar is circulated at a high rate through the scrubber and a holding tank (5) whilst the circuit is bled to maintain the correct tar inventory. No cooling device is necessary on this tar circulation. The bled tar then goes forward for further processing, e.g. cooling and depresurisation.

The gas (6) leaving the scrubber is sensibly free of tar but contains in the vapour state all the water that was used for the quench. This gas stream is passed to a conventional partial condenser (7), to recover that water which is then re-circulated via a holding tank (8), to the quench spray (3).

With correct design of the system it is possible to ensure that this water circuit requires a make up of water to maintain its mass balance. By this means any tar entrained from the tar scrubber into the water circuit is passed back via the water sprays and will eventually end up in the tar circuit, thus eliminating any tar/water separation step.

The heat from the partial condenser (7) is used to raise steam utilising steam drum (9), whilst the cleaned and cooled gasous product (10) goes forward for further processing, e.g. further cooling, hydrocarbon oil/water separation and depressu risation.

Claims

1. A process for removing tar from the gaseous products of the thermal treatment of a carbonaceous material, which process comprises cooling the products to a temperature sufficiently high to keep essentially all of the water vapour present therein in the vapour phase, but sufficiently low to condense out at least a major portion of the tar therein, and then separating the condensed tar from the gaseous products without essentially any condensation of water.

2. A process as claimed in Claim 1 wherein the condensed tar is separated by scrubbing the cooled products with a tar-containing liquid stream, which stream is maintained during scrubbing at a temperature sufficiently high to prevent the condensation of water.

3. A process as claimed in Claim 2 wherein the liquid stream is re-circulated in a scrubbing loop, and wherein the temperature reduction achieved in the cooling step is such that the scrubbing loop operates substantially adiabatically.

4. A process as claimed in Claim 1 wherein the separation step is performed by using a cyclonic separator.

5. A process as claimed in any one of the preceding claims wherein the cooling step is effected by quenching.

6. A process as claimed in Claim 5 wherein the quenching is performed using re-cycled condensed tar.

7. A process as claimed in Claim 5 wherein the quenching is performed using water.

8. A process as claimed in Claim 7 including the step of further cooling of the gaseous products after the tar separating step to condense out at least a major amount of the water vapour present therein, wherein this condensed water is used as the quench.

9. A process as claimed in Claim 8 wherein substantially all of the quench water is condensed water.

10. A process as claimed in any one of the preceding claims wherein shale is retorted.

11. A process as claimed in any one of Claims 1 to 9 wherein coal is gasified.

12. A process as claimed in Claim 11 wherein a relatively high pressure and a temperature in the range of 200 C to 8000C is used in the gasification step.

13. A process as claimed in Claim 12 wherein between the gasification step and the products cooling step, the gaseous products are subjected to an initial quench with water.

14. A process as claimed in Claim 1 substantially as hereinbefore described with reference to the accompanying drawing.