GB2061998A - Quenching tar vapours - Google Patents

Abstract

The tar vapours from carbonising coal, preferably in a fluidised bed, are treated by quenching with a hydrogen-donor solvent. Tar molecules prone to polymerisation upon condensation and upon distallation are partially hydrogenated; the resulting tar solution is of lower viscosity and less likely to coke or form pitch.

Description

SPECIFICATION Quenching process This invention concerns the treatment of gases or vapours resulting from the carbonisation of coal.

It is well known that the carbonisation (also known as pyrolysis) of coal evolves gases or vapour rich in tar. The tar contains a large variety of substances, and includes oil components. The oil components are known as solvents for coal in the liquefaction of coal and it has been suggested that liquid fuels and/or chemical feedstocks can be produced by upgrading the oils by hydrotreatment.

Our studies of coal tars have met with the problem of high molecular weight material in the tar; much of this material is formed by polymerisation of simpler molecules, which polymerisation can occur in the vapour phase and during and after condensation. The polymers tend to coke when tar is distilled or hydrogenated.

Hydropyrolysis, the carbonisation of coal under high pressure hydrogen, as well as upgrading the tar oils also reduces considerably the polymerisation reaction. However, hydropyrolysis has the severe disadvantages of the problems associated with feeding solids into and out of high pressure systems, and of large hydrogen consumption because of the formation of byproduct methane.

The present invention provides a method of treatment of tar vapours from the carbonisation of coal comprising quenching the vapours using a hydrogen donor.

The invention also provides a method of carbonising coal in which the tar vapours evolved are quenched using a hydrogen donor.

Preferably, the quenching cools the tar vapours to a temperature of below 4000 C, more preferably to below 2000 C.

The coal to be carbonised may be a bituminous or lower rank coal such as brown coal, but is preferably a high or medium-volatile bituminous coal.

The carbonisation may be carried out on a static charge of coal, or in a disperse phase, but is preferably carried out as a fluidised bed carbonisation. Temperature for carbonisation may be in the range 500 to 6500 C, and carbonisation is suitably carried out at or very close to atmospheric pressure. Carbonisation may be carried out under a reactive gas such as steam, carbon dioxide or hydrogen. Although these conditions are thought to be the most advantageous, it is believed that the present invention offers improvements in other processes of carbonisation such as flash pyrolysis.

Hydrogen donors are known as solvents for the degradation and liquefaction of coal. In the liquefaction of coal it is thought that available hydrogen from the solvent is donated to the degrading coal structure, thereby stabilising the soluble molecular fragments as they are formed.

The hydrogen donor in turn reverts to a stable unsaturated form which may subsequently or concurrently be regenerated by conventional hydrogenation techniques. Typical hydrogen donors are hydroaromatics, and solvents preferred for use in the present invention are hydrogenated tar oils and tetralin, although tetrahydroquinoline and o-cyclohexylphenol are also suitable.

Hydrogen donors may also be used which arise directly or by hydrotreatment of products arising from a process using the invention.

The quenching may be carried out in accordance with established chemical engineering principles, for example by spraying with the hydrogen donor. Conveniently, the invention is used as part of a process in which the 'used' donor in dehyrogenated form is regenerated by hydrogenation for re-use, and the regenerated donor is cooled before the quenching step.

Preferably, the quenching is carried out as soon as practicable after the formation of the vapours, in order to minimise the opportunity for polymerisation of the coke-forming precursors.

The invention provides as a product a stabilised tar solution comprising carbonisation tar and hydrogen donor in which the quantity by weight of tar to hydrogen donor is from 1 :0.5 to 1 :10, preferably 1:1 to 1:5. The presence of the hydrogen donor reduces the viscosity of the tar thus permitting easy further processing. In addition, the lower viscosity is of considerable assistance when the tar is from a fluidised carboniser in that the fine char contaminant can be more easily removed, for example by filtering or settling. It will be appreciated that the hydrogen donor donates hydrogen to the tar components having most need of it in the quenching step and therefore becomes itself dehydrogenated. The product is therefore different from that which would result from dissolving condensed tar (which already contains polymerised components) in a hydrogen donor.

The tar solution may be fractionated, for example under reduced pressure to remove the dehydrogenated donor. The donor and the recovered tar may be then each separately catalytically hydrogenated in known manner, the donor being recycled to the quenching step and the hydrogenated tar being processed to chemical feedstock and fuel. Alternatively, the tar solution may be catalytically hydrogenated and then fractionated to yield the regenerated donor for recycle and to yield hydrogenated tar oil. It is believed that this latter process would be especially advantageous because of the hydrogen transfer characteristics of the donor.

The invention will now be described by way of example only.

EXAMPLE A high volatile bituminous coal (from Linby colliery, England) was fed into a bed of fluidised sand heated to 6000C by external heaters, at a rate of 1 Kg/hr. The bed was fluidised with nitrogen. Immediately that the vapours evolved from the decomposition of the coal were carried, by the flow of nitrogen, out of the vessel containing the fluidised bed, they were quenched by a spray of tetralin from a spray head mounted in a bend in the tube carrying the vapours and arranged so that the spray completely filled the tube. The temperature of the vapours immediately before the quench spray was approximately 6000C and this was reduced to approximately 1 000C thereafter.

The quenched vapours were taken into a receiving pot cooled by a coil carrying cold water, from which a high quality tar was recovered and taken for analysis. Tetralin was recycled to the spray although it was found that the nitrogen gas still carried an amount of tetralin thus necessitating further cooling of the gas to avoid loss.

Claims (10)

1. A method of treatment of tar vapours from the carbonisation of coal, comprising quenching the vapours using a hydrogen donor.

2. A method according to claim 1, wherein the quenching cools the tar vapours to a temperature of below 4000 C.

3. A method according to claim 2, wherein the quenching cools the tar vapours to a temperature of below 2000C.

4. A method according to claim 1, 2 or 3, wherein the hydrogen donor is a hydrogenated tar oil or tetralin.

5. A method according to any one of the preceding claims, in which the hydrogen donor is used in an amount of weight of 10:1 to 0.5 :1 of the tar vapours.

6. A method according to claim 1, substantially as hereinbefore described.

7. A method of carbonising coal, in which the tar vapours evolved are treated using a method according to any one of the preceding claims.

8. A method according to claim 7, wherein the carbonisation is a fluidised bed carbonisation carried out at a temperature in the range 500 to 6500C.

9. A method according to claim 7 or 8, wherein the carbonisation is carried out in the presence of a reactive gas.

10. A stabilised tar solution comprising carbonisation tar quenched with hydrogen donor, in which the quantity by weight of tar to hydrogen donor is from 1 :0.5 to 1 :10.

1 A solution according to claim 10, in which the quantity by weight of tar to hydrogen donor is from 1:1 to 1:5.