GB2031016A

GB2031016A - Processing a Crude Gas Produced by the Gasification of Solid Fuel

Info

Publication number: GB2031016A
Application number: GB7932017A
Authority: GB
Original assignee: Metallgesellschaft AG
Current assignee: GEA Group AG
Priority date: 1978-09-22
Filing date: 1979-09-14
Publication date: 1980-04-16
Also published as: DD150475A5; JPS5545784A; AU5099179A; DE2841182A1; ZA793774B; BR7905797A; CS211360B2; PL218373A2

Abstract

A crude gas, produced by the gasification of solid fuel under a pressure of 5 to 150 bars with gasifying agents comprising free oxygen and water vapour flowing in countercurrent to the fuel and having a temperature of 350 to 800 DEG C, is reacted under pressure in the presence of a hydrogenating catalyst. Various hydrogenating catalysts are described. In a preferred embodiment, substantially no oxygen is required to initiate cracking of the hydrocarbons in the crude gas, resulting in little loss of calorific value by partial oxidation of said gas. Also, most sulphur compounds are converted to hydrogen sulphide, rendering desulphurisation of the treated gas before methanation simple.

Description

SPECIFICATION Method of Processing a Crude Gas Produced by the Gasification of Solid Fuel This invention relates to a method of processing a crude gas produced by the gasification of solid fuel, particularly coal.

German Offenlegungsschriften Nos.

2532199 and 2532198 and the corresponding U.S. Patents Nos. 4056483 and 4082520 describe a process for treating hydrocarbons present in crude gas produced by the gasification of solid fuel under pressure with gasifying agents.

This treatment is effected with an addition of oxygen in a reactor which contains a fixed bed of inert solids or catalyst material. The added oxygen effects a partial oxidation of severai components of the crude gas so that the temperature rises and gasifying and cracking reactions are initiated. The treated gas is substantially free from tar, phenols, fatty acids and ammonia.

The pressure gasification of coal in a pressure reactor under a pressure in the range from 5 to 1 50 bars with formation of ash that remains solid is described in numerous publications, such as U.S. Patents 3540867 and 3854895 and German Offenlegungsschrift No. 2201278. In such processes, the coal is fed to the pressure reactor in granular form or as an agglomerate in the particle size of, e.g., 3 to 30 mm to form a bed and the coal travels gradually through a plurality of zones, which are not sharply defined and are at progressively increasing temperatures. Gasifying agents, which consist in most cases of oxygen and steam although carbon dioxide, e.g. may also be used, flow through the bed from bottom to top.

British Patent Specifications Nos. 1507905, 1 508671 and 1 512677 disclose for the pressure gasification of coal a modified process, which is carried out basically in the same manner but includes the withdrawal of ash in molten form.

The crude gas produced in this modified gasification process contains more carbon monoxide and less steam than the product of the gasification process in which the ash remains solid.

The crude gas produced by the pressure gasification of coal contains in addition to steam mainly hydrogen and carbon oxides as well as methane. Numerous additional substances are present in smaller quantities, such as condensible hydrocarbons, particularly aromatic compounds, paraffins and naphthenes having different boiling points and compositions.

It is an object of the invention to provide an economical and simple method by which the crude gas can be freed particularly from its contents of condensible hydrocarbons so that the purification of the gas and of aqueous condensate derived from the gas is greatly facilitated.

According to the present invention there is provided a method of treating a crude gas, said crude gas having been produced by the gasification of solid fuel, under a pressure of 5 to 1 50 bars with gasifying agents comprising free oxygen and water vapour flowing in countercurrent to the fuel, and having a temperature of 350 to 800QC, wherein said crude gas is reacted under pressure in the presence of a hydrogenating catalyst. With this method, the aqueous condensate which arises when the treated gas is cooled can be recycled to the pressure gasification process after a simple aftertreatment, e.g., by degassing or partial desalting. In order to control the salt content, part of the aqueous condensate may be supplied to a receiving stream without any purification.The arrangements for purifying the gas are also simpiified. In the presence method the treatment of the crude gas is preferably effected under pressure and temperature conditions which are approximately the same as those of the crude gas on exit from the gasifying step.

In the present method it is important that substantial quantities of gases which contain free oxygen and would result in a partial oxidation are not normally added in the treatment of the crude gas. Such partial oxidation caused by a supply of oxygen would undesirably convert a substantial part of the hydrocarbons, which have a high calorific value, to carbon dioxide and water so that the calorific value of the product gas would be decreased. For this reason, substantially no oxygen is added in the treatment of the crude gas in the present method. If the crude gas exits from the pressure gasifier at a very low temperature, a small addition of oxygen may be required to heat the crude gas to a temperature at which the catalyst is active.

It has been found that hydrocracking catalysts used in petroleum-refining technolqgy are capable of hydrogenating or cracking the tars contained in the crude gas. Suitable catalysts consist of one or more active components preferably present on a porous support. Such active components may be selected from cobalt, alone or in combination with molybdenum, nickel, montmorillonite, which has been acidified with dilute acid, and oxides of silicon, magnesium, calcium, iron, sodium, potassium and tungsten. The porous support may be alumina, a zeolite or coke. Alternatively, coke may be used as the hydrogenating catalyst.

A first suitable catalyst comprises 3 to 10% by weight of cobalt, alone or in combination with 5 to 15% by weight of molybdenum, on a porous support such as alumina, although a zeolite is also suitable. Known forms of such catalysts are the so-called Comox catalysts having surface areas in excess of 80 m2/g.

A second suitable catalyst comprises 5 to 40% by weight of nickel on a support, preferably with an addition of one or more oxides of silicon, magnesium, calcium, iron, sodium, potassium and tungsten. Known forms of such catalysts are the nickel catalysts used, e.g., to produce rich gas from evaporated liquid hydrocarbons or for methanation. The reaction can also be promoted by montmorrillonite which has been treated with dilute acid.

The treatment of the crude gas may be carried out in one or more reators in which the hydrogenating catalyst or catalysts are contained in a fixed bed or fluidized bed. A pressure of 5 to 1 50 bars is maintained in the treatment zone; and it is desirable to feed the crude gas from the pressure gasifying stage to the treatment zone substantially without a pressure relief.

Means for collecting coarse or fine dust from the crude gas are preferably arranged between the reactors for the pressure gasification of the fuel, particularly coal, and the treatment stage.

Such collection of coarse or fine dust may be effected, e.g., by means of cyclones or bag filters.

Liquid hydrocarbons, which are preferably formed into fine jets, and/or fuels in the form of dust, such as pulverized coal having a particle size below 2 mm, may be added to the crude gas fed to the treatment stage.

The treated gas contains virtually no hydrocarbons which condense at ambient temperature but may possibly contain traces of hydrocarbons boiling in the gasoline (or petrol) range. The treated gas contains more methane than the crude gas. The increase of the methane content depends mainly on the tar content of the gasified coal and on any vaporizable hydrocarbons which have been added to the crude gas. The increase of the methane content can be varied within wide limits.

On the hydrogenating catalyst used for treatment, the sulphur compounds in the crude gas are virtually completely converted to hydrogen sulphide. For numerous reasons it is essential in most cases to remove all or most of the hydrogen sulphide from the treated gas before the latter is used as a fuel gas, reducing gas or synthesis gas. The hydrogen sulphide may be removed by known chemical or physical scrubbing processes or by adsorption, e.g., on activated carbon or alumina. Hydrogen sulphide must also be removed if the treated gas is to be catalytically methanated in order to increase its calorific value so that it can be substituted for natural gas.The invention will now be illustrated by the following Examples: Example 1 1 5 Metric tons of coal per hour having a net calorific value of 21 275 kJ/kg and the following elemental analysis in % by weight: Carbon 57.6 Hydrogen 4.0 Oxygen 5.9 Nitrogen 1.1 Sulphur 2.1 Ash 29.3 were gasified in a pressure gasifier for coal, of known type ('Lurgi Gas Producer'), having a mean diameter of 2.6 metres. The gas producer was operated under a pressure of 20 bars and was fed with 270 standard m3 oxygen per metric ton of coal and with 4.9 kg water vapour per standard m3 oxygen as gasifying agents.

The crude gas produced at a rate of 1420 standard m3 per metric ton of coal had the following analysis in % by volume on a dry basis: CO2 27.5 CO 23.1 H2 37.5 CH4 9.9 CnHm 0.6 N2 0.6 H2S 0.8 The gas contained also 0.64 standard m3 water vapour per standard m3 of dry gas and was at a temperature of 650 C as it left the gas producer.

(If this crude gas were cooled to 250C, the condensate would contain, in addition to water, the following substances per 1 000 standard m3 of dry crude gas: Tar 34.3 kg Oil 15.5 kg Gasoline (petrol) 6.4 kg Phenols 4.8 kg Fatty Acids 0.7 kg Ammonia 4.9 kg) The gas was fed to a treatment reactor without being cooled and virtually without a pressure loss.

The treatment reactor was filled to approximately one-half of its volume with 6x6 mm tablets of a commercially available Comox catalyst comprising 6% by weight of cobalt and 10% by weight of molybdenum on an alumina support.

The treatment reactor had a shaftlike chamber which was 2 metres in diameter. The pile of bunk catalyst had a height of 4 metres.

The gas produced by the reaction left the treatment reactor at a temperature of 6700C and had the following composition in % by volume on a dry basis: CO2 45.8 CO 10.4 H2 28.3 CH4 14.4 H2S 0.1 N2+Ar 1.0 It was free from higher hydrocarbons and contained 0.7 standard m3 water vapour per standard m3 of dry gas.When the carbon dioxide and water vapour had been removed, the resulting fuel gas had a net calorific value of 1 5 540 kJ/standard m3 and the following analysis in % by volume: CO 19.2 H2 52.2 CH4 26.6 H2S 0.2 N2+Ar 1.8 Example 2 A coal having a net calorific value of 25 120 kJ/kg and the following elemental analysis in % by weight: Carbon 63.9 Hydrogen 4.1 Oxygen 7.3 Nitrogen 1.0 Sulphur 1.4 Ash 22.3 was fed to an arrangement as described in Example 1 and, under the same conditions in other respects, was gasified at 20 bars by a treatment with 1 500 standard m3 air per metric ton of coal and with 0.42 kg water vapour per standard m3 of air.

At a rate of 2 700 standard m3 per metric ton of coal, a crude gas was thus produced which was at 5850C and had the following composition in % by volume on a dry basis: CO2 10.6 CO 21.3 H2 19.8 CH4 4.2 CnHm 0.3 N2 43.5 H2S 0.3 The crude gas contained also 0.11 standard m3 water vapour per standard m3 of dry crude gas. (If this gas were cooled to 25"C, the following byproducts would become available per metric ton of coal: Tar, oil and light liquid hydrocarbons 37.1 kg Phenols 2.6 kg Fatty Acids 0.6 kg Ammonia 3.3 kg) Just as in Example 1, this gas was fed directly, without being cooled, to the treatment reactor and was treated therein. The catalyst consisted of 6x6 mm tablets of a commercially available nickel catalyst comprising 25% by weight of nickel on a silica support.The resulting gas was free from higher hydrocarbons and had the following composition in % by volume: CO2 13.8 CO 17.9 H2 16.0 CH4 9.8 N2 42.2 H2S 0.3 The gas contained also 0.1 standard mJ water vapour per standard m3 of dry gas. When the carbon dioxide and water vapour had been removed, the resulting fuel gas had a net calorific value of 680 kJ/standard m3 and the following composition in % by volume: CO 20.8 H2 18.6 CH4 11.4 N2 48.9 H2S 0.3 Example 3 1 7 Metric tons of coal were fed per hour to a pressure gasifier which had a mean diameter of 1.8 m and was operated under a pressure of 25 bars and from which ash was withdrawn in a molten form.The coal had the following elemental analysis in % of weight of water- and ash-free matter: Carbon 78.99 Hydrogen 5.25 Oxygen 8.75 Nitrogen 1.37 Sulphur 0.43 Chlorine 0.38 It had a net calorific value of 29 490 kJ/kg and contained 4.83% by weight of ash.

The gasifying agents fed to the gas producer consisted of 350 standard m3 of oxygen per metric ton of coal and of 1.1 kg water vapour per standard m3 of oxygen. The resulting crude gas produced at a rate of 1 890 standard m3 of dry gas per hour had the following composition in % by volume on a dry basis: CO2 2.3 CO 58.2 H2 29.3 CH4 6.8 QHm 0.6 H2S 0.1 N2+Ar 2.7 It contained also 0.12 standard m3 of water vapour per standard m3 of dry gas. The exit temperature of the crude gas was 6500C.

(If the crude gas were cooled to 250C, the following by-products would become available per 1 000 standard m3 of crude gas: Tar 39.0 kg Oil 8.3 kg Gasoline (petrol) 9.8 kg Ammonia 2.7 kg Phenols 2.3 kg Fatty acids 3.4 kg) The same treatment reactor as in Example 1 was used to treat the crude gas, without prior cooling, with 0.76 kg water vapour per standard m3 of crude gas.

The reactor was filled with about one-half of its volume with activated coke having a mean particle diameter of 30 mm. The reaction chamber was 2 metres in diameter and the pile of bulk coke had a height of 4 metres.

The gas leaving the treatment reactor had a temperature of 670 C and a net calorific value of 12 730 kJ per standard m3 and had the following composition in % by volume on a dry basis: O2 33.6 CO 10.8 H2 32.0 CH4 21.0 H2S 0.1 H2+Ar 2.5 This treated gas was free from condensible hydrocarbons. It was cooled down to 300C in waste heat boilers and cooiers and then scrubbed with liquid methanol at about -250C for desulphurisation and removal of carbon dioxide.

After this treatment the gas had a net calorific value of 19 160 kJ per standard m3 and the following composition in % by volume: CO 16.3 H2 48.2 CH4 31.7 N2+Ar 3.8 The resulting gas was supplied to a plant for methanol synthesis.

Example 4 Coal was gasified under pressure as described in Exampe 3, and 0.28 standard m3 of oxygen, 0.8 kg water vapour were added to the resulting crude gas per standard m3 thereof as well as pulverized coal at a rate of 300 kg per metric ton of lump fuel and in a particle size of 0.03 to 0.3 mm. The pulverized coal had the same analysis as the coal in Example 3.

The crude gas, oxygen, water vapour and pulverized coal were reacted in the treatment reactor used in Example 3, but the hydrogenating catalyst was in the form of balls about 5 mm in diameter, and comprised 3.5% by weight of cobalt and 10% by weight of molybdenum on an alumina support.

The gas leaving the treatment reactor at a temperature of 9500C had the following composition in % by volume on a dry basis: CO2 20.7 CO 32.2 H2 45.0 CH4 0.4 H2S 0.1 N2+Ar 1.6 By the treatment described in Example 3, this intermediate gas was transformed into a synthesis gas.

Claims

1. A method of treating a crude gas, said crude gas having been produced by the gasification of solid fuel, under a pressure of 5 to 1 50 bars with gasifying agents comprising free oxygen and water vapour flowing in countercurrent to the fuel, and having a temperature of 350 to 8000C wherein said crude gas is reacted under pressure in the presence of a hydrogenating catalyst.

2. A method as claimed in claim 1, wherein no gases which contain free oxygen are added to the crude gas to be treated.

3. A method as claimed in claim 1 or 2, wherein the hydrogenating catalyst is contained in a fixed bed.

4. A method as claimed in claim 1 or 2, wherein the hydrogenating catalyst is contained in a fluidized bed.

5. A method as claimed in any of claims 1 to 4, wherein the hydrogenating catalyst comprises one or more active components selected from cobalt, alone or in combination with molybdenum, nickel, montmorillonite which has been acidified with dilute acid, and oxides of silicon, magnesium, calcium, iron, sodium, potassium and tungsten.

6. A method as claimed in claim 5, wherein the active component(s) is (are) present on a porous support.

7. A method as claimed in claim 6 wherein the porous support is alumina, a zeolite, or coke.

8. A method as claimed in claims 5 to 7, wherein the hydrogenating catalyst comprises 3 to 10% by-weight of cobalt.

9. A method as claimed in claim 8, wherein the hydrogenating catalyst comprises 5 to 1 5% by weight of molybdenum.

10. A method as claimed in claims 5 to 7, wherein the hydrogenating catalyst comprises 5 to 40% by weight of nickel.

11. A method as claimed in claim 10, wherein the hydrogenating catalyst comprises one or more oxides of silicon, magnesium, calcium, iron, sodium, potassium and tungsten.

12. A method as claimed in any of claims 8 to 11, wherein the catalyst comprises acidified montmorillonite.

13. A method as claimed in any of claims 1 to 4, wherein the hydrogenating catalyst consists of coke.

14. A method as claimed in any one of claims 1 to 13, wherein sulphur compounds are removed from the treated gas.

1 5. A method according to claim 14, wherein the treated gas from which the sulphur compounds have been removed is methanated.

16. A method as claimed in any one of claims 1 to 1 5, wherein hydrocarbons and/or dustlike fuels having a particle size below 2 mm are added to the crude gas to be treated.

1 7. A method as claimed in claim 14, wherein the treated gas from which the sulphur compounds have been removed and from which dust has been collected is fed as a fuel gas to a combined steam-gas turbine power plant process, or is used as a reducing gas for the reduction of ores or as a synthesis gas.

18. A method of processing a crude gas produced by the gasification of solid fuel substantially as hereinbefore described in any one of the foregoing Examples.