GB2112016A - Producing coal-containing shaped bodies for gasification - Google Patents

Abstract

Coal-containing shaped bodies for gasification are produced from a water-containing mixture of fine- grained coal having a particle size below 5 mm. and metal sulphate. The sulphate content of the mixture, calculated as SO4<-->ions, may be 0.1 to 8% by weight, preferably 0.2 to 3% by weight. In one embodiment, the coal and a metal compound are mixed in a dry state and moistened with sulphuric acid. Alternatively, the coal and metal sulphate may be mixed in a dry state and then be moistened with water. The total water content of the mixture is preferably in the range of about 5 to 30% by weight. The shaped bodies are dried before they are gasified.

Description

SPECIFICATION Process for producing coal-containing shaped bodies for gasification and gasification process This invention relates to a process for producing coal-containing shaped bodies for gasification and to a process for the gasification of such bodies.

The shaped bodies of this invention are particularly suited for gasification under a pressure of 5 to 1 50 bars with gasifying agents comprising free oxygen, stem and/or carbon dioxide in a slowly-descending fixed bed, to which the gasifying agents are fed from below, incombustible mineral constituents being withdrawn as solid ash or liquid slag from the lower end of the bed.

The gasification of granular coal has been described, e.g. in Ullmanns Enzyklopädie der technischen Chemi, 4th edition (1977), volume 14, on pages 383 to 386. Details of a gasifying process in which the ash remains solid are disclosed in U.S. Patents Nos. 3,540,867 and 3,854,895 and German Offenlegungsschrift No.

2,201,278. A modification of the process in which a liquid slag is withdrawn has been described in British Patent Specifications Nos. 1,507,905; 1,508,671; and 1,512,677. In this process the gasifying reactor is preferably supplied with granular fuel having a particle size of about 3 to 60 mm.

Coal in the form of powder and dust is not well suited for gasification in a fixed bed. It is desirable to separate a fine-grained coal fraction from the coal to be gasified and make shaped bodies from it. Processes for producing such shaped bodies have been described in German Offenlegungsschriften Nos. 28,51,370 and 28,53,389. In connection with these processes it is desirable that the proportion of caking coal in the shaped body should be as high as possible and that the shaped body should not be deformed in the upper portion of the fixed bed in which gasification occurs, where only volatilization takes piace at moderate temperatures. Shaped bodies which become too soft or melt in the upper portion of the fixed bed will obstruct or prevent the flow of gas through the fixed bed so that the gasifying operation may be suppressed.

It is an object of the invention to provide a simple and inexpensive process for producing shaped bodies which are to be gasified and have a sufficiently high dimensional stability to prevent or reduce deformation in the upper portion of the fixed bed.

According to one aspect of the present invention there is provided a process for producing coal-containing shaped bodies for gasification wherein a water-containing mixture is produced from fine-grained coal having a particle size below 5 millimetres and metal sulphate, and wherein shaped bodies are formed from said mixture and are subsequently dried.

Another aspect of the present invention resides in the coal-containing shaped bodies produced by the process just indicated.

According to a further aspect of the present invention there is provided a gasification process, wherein the coal-containing shaped bodies just indicated are subjected to gasification at a pressure of 5 to 1 50 bars in a slowly-descending fixed bed to which gasifying agents comprising free oxygen, steam and/or carbon dioxide are fed from below, incombustible mineral constituents being withdrawn as solid ash or liquid slag from the lower end of the bed.

The coal component used in the production of the present shaped bodies may consist of the coals which are usually gasified and particularly also of caking coal as well as brown coal. The mixture is preferably made to have a sulphate content (calculated as S04- - ions) of 0.1 to 8% by weight, preferably 0.2 to 3% by weight. The particle size of the fine-grained coal used to prepare the mixture is preferably blow 3 millimetres.

The sulphate content of the mixture may be provided by the addition of sulphuric acid and by the metal compounds in the ash content of the coal or by the separate addition of metal compounds. Alternatively, the mixture may be formed from a dry metal sulphate with an addition of water or with an aqueous solution of metal sulphate.

The total water content of the mixture is preferably 5 to 30% by weight but in any case is sufficient to allow the mixture to be shaped in a moist state. Shaping is preferably effected by briquetting, pelletizing or extruding.

Shaped bodies having a strength above the average and a desirable behaviour during gasification will obtained if the coal or the mixture of several coals has a caking capacity corresponding to a free swelling index of at least 1. If the mixture has an inadequate caking capacity, caking promoters, such as pitch, tar, caking coal and/or bitumen, may be added to the mixture. The caking promoter will increase the resistance of the shaped body to pyrolysis because the caking promoter contributes by sintering, melting and resolidifying process to the formation of a solid coke structure in the heated shaped body.

The fine-grained coal desirably has an ash content of at least 10 to about 40% by weight, and contains metal compounds which can form metal sulphates with sulphuric acid. If the content of metal compounds or metal components capable of forming sulphates is insufficient for the required purpose, the desired metal sulphate content of the mixture can be provided by adding material which contain, for example, magnesium sulphate, aluminium sulphate and/or iron sulphate.

Alternatively, at least one oxide, hydroxide or carbonate of aluminium, iron or an alkaline earth metal, particularly calcium, may be used to prepare the mixture for reaction with the added sulphuric acid.

The sulphuric acid which is used need not comply with special purity requirements and waste sulphuric acid can readily be used.

Owing to the use of sulphuric acid as a binder component, the resulting shaped bodies have a very low pH value. This may result in more aggressive corrosion of the machines and other equipment which are used in preparing the mixture and in shaping and for processing the shaped bodies. In order to increase the pH value close to a neutral value it may be desirable to add not only sulphuric acid but also the salt of a weak acid, such as calcium carbonate, to the coal mixture. This will result in the formation of gypsum, which constitutes in the shaped body an additional salt having a bonding activity.

The drying of the shaped bodies before their gasification may be effected at a temperature of about 1 1 OOC. In order to test the dried shaped bodies for their suitability for gasification in a fixed bed, they may be subjected in a laboratory apparatus to coking under pressure and at high heating-up rates of about 30 to 800C/min. up to about 600 to 8000C, so that the shaped bodies pass through the critical conditions to which the shaped body is subjected as it is charged into the fixed bed for gasification and subsequently descends into the devolatilization zone of the gasifier. An adequate dimensional stability of the shaped bodies is particularly important in the devolatilizing zone so that the gases can flow easily therethrough from bottom to top.

The invention will now be illustrated by the following non-limiting Examples. In these Examples, the water-containing mixture was compacted in the laboratory by means of a hydraulically operated ram press under an applied pressure of 500 bars and at room temperature to form cylindrical briquettes having a length of 25 mm. and a diameter of 50 mm. The resulting briquettes were dried in an air-circulating drying cabinet at 1 1 OOC for 4 hours. When the briquettes had cooled their compressive strength was measured between two planoparallel plates. A coking test was subsequently carried out under a pressure of 20 bars and at heating-up rates of about 500cumin and end temperatures of about 700--8000C.

Example 1 A slightly caking German long-flaming gas coal having a high ash content of 33% by weight, a moisture content of 2.2% by weight, a free swelling index of 1 T and a particle size below 3 mm. was mixed with 5% by weight of a sulphuric acid having a concentration of 20%. The dried briquettes had a compressive strength of 14.8 N./mm.2.

Example 2 The coal used in Example 1 was mixed with 5% by weight of a waste sulphuric acid, which contained about 20% by weight of free sulphuric acid and 10% by weight metal sulphate. The dried briquettes had a compressive strength of 14.7 N/mm.2.

Example 3 For comparison, the coal of Example 1 was mixed with 5% by weight of water. In this case the dried briquettes had a compressive strength of about 7.7 N./mm.2.

Example 4 A mixture of coal dust and filter sludge (mixing ratio 8 kg 1 kg) was used, which mixture had become available as the result of cleaning a longflaming gas coal containing 33% ash and 2.1% moisture and having a free swelling index of 1 , the long-flaming gas coal being different from that used in Examples 1 to 3.

The mixture was mixed in Case A with sulphuric acid as in Example 1, in Case B with waste sulphuric acid as in Example 2 and in Case C with water as in Example 3. The compressive strengths of the dried briquettes were measured with the fnllowing results: Case A: Compressive strength 9.0 N./mm.2.

Case B: Compressive strength 11.4 N./mm.2.

Case C: Compressive strength 5.3 N./mm.2.

The sulphuric acid-containing briquettes of Examples 1,2 and 4, Cases A and B, exhibited a very high dimensional stability in the coking test so that they were eminently suitable for gasification in a fixed bed. The briquettes of Examples 3 and 4, Case C, which were made without sulphuric acid, did not have sufficient dimensional stability to enable them to be gasified in a fixed bed.

Example 5 A German long-flaming gas coal having an ash content of 22% by weight, a moisture content of 3.7% by weight, a free swelling index of 1 and a particle size below 3 mm., was mixed in Case A with 10% by weight of water and in Case B with 2.5% by weight of calcium carbonate and 10% by weight of waste sulphuric acid (H2SO4 content 21.3%) and the resulting mixtures were briquetted. In Case B, the moist mixture had a pH value of about 4. The dried briquettes of Case A had a compressive strength of less than 3 N./mm.2. and an inadequate strength in transit.

The briquettes of Case B had a compressive strength of 8 to 9 N./mm.2; they had an adequate strength in transit and proved in the coking test that they were well suited to gasification in a fixed bed.

Example 6 A non-caking lean coal which has an ash content of 27% by weight and a moisture content of 0.6% by weight was mixed in a weight ratio of 3:1 with moderately caking long-flaming gas coal.

The mixture was ground to a particle size below 3 mm. The long-flaming gas coal has a low ash content of 4% by weight. Then 2.5% by weight of limestone and 10% by weight of the waste sulphuric acid which was used in Case B of Example 5 were added to the mixture. The dried briquettes had an average compressive strength of 1 2.2 N./mm.2. The coking test showed that they are well suited to gasification in a fixed bed.

Example 7 The non-caking lean coal used in Example 6 was mixed with a strongly caking coal of the Leopold Mine (Ruhr area, Germany), 109/0 by weight of the waste sulphuric acid used in Example 5 and 3% by weight of calcium carbonate. The proportions by weight of the two coals were 1:1 in the first case and 3:1 in the second case. The dried briquettes had a compressive strength of 9.3 N./mm.2. in the first case and of 12.2 N./mm.2. in the second case. The briquettes retained their shape in the coking test.

The structure of the briquettes was slightly sandy but was generally strong so that the briquettes were suitable for gasification in a fixed bed.

The additional use of 3% coal pitch as a binder in the mixture to be briquetted produced virtually the same results regarding compressive strength and behaviour in the coking test.

Example 8 The coal of Example 5 was mixed with 12% by weight of an aluminium sulphate solution of 30% concentration and was then briquetted. The dried briquettes had compressive strengths of 8 to 9 N./mm.2. so that they had an adequate strength in transit and were well suited to gasification in a fixed bed.

Example 9 A German long-flaming gas coal which has an ash content of 4.5% by weight, a moisture content of 1.8% by weight, a free swelling index of 4 to 12 and a particle size below 3 mm. was mixed with 10% by weight of a waster sulphuric acid (20% free H2SO4) and 2.5% Fe203. The dried briquettes had compressive strengths of 7.5 N./mm.2. These briquettes were also suitable for gasification in a fixed bed.

When the coal to be briquetted was only moistened with water, the dried briquettes had a compressive strength of only 1.7 N./mm.2, which is entirely inadequate for gasification in a fixed bed.

Claims (14)

1. A process for producing coal-containing shaped bodies for gasification, wherein a watercontaining mixture is produced from fine-grained coal having a particle size below 5 millimetres and metal sulphate, and wherein shaped bodies are formed from said mixture and are subsequently dried.

2. A process as claimed in Claim 1, wherein the mixture has a sulphate content (calculated as S04- - ions) of 0.1 to 8% by weight.

3. A process as claimed in Claim 1 or 2, wherein the mixture has a sulphate content (calculated as 804- - ions) of 0.2 to 3% by weight.

4. A process as claimed in any one of Claims 1 to 3, wherein the mixture is prepared by mixing the coal and metal compound in a dry state and subsequently moistening the mixture with sulphuric acid.

5. A process as claimed in any one of Claims 1 to 3, wherein the mixture is prepared by mixing the coal and metal sulphate in a dry state and subsequently moistening the mixture with water.

6. A process as claimed in any one of Claims 1 to 5, wherein the mixture has a total water content of 5 to 30% by weight.

7. A process as claimed in any of Claims 1 to 6, wherein the coal has a caking capacity corresponding to a free swelling index of at least 1.

8. A process as claimed in any one of Claims 1 to 7, wherein the fine-grained coal has an ash content of at least 4 to 40% by weight.

9. A process as claimed in Claim 5 or any one of Claims 6 to 8 when appended to Claim 5, wherein said metal sulphate is megnesium sulphate, aluminium sulphate and/or iron sulphate.

10. A process as claimed in Claim 4 or any one of Claims 6 to 8 when appended to Claim 4, wherein said metal compound is at least one oxide, hydroxide or carbonate of aluminium, iron or an alkaline earth metal.

11. A process as claimed in any one of Claims 1 to 10, wherein waste sulphuric acid is used to prepare the mixture.

12. A process as claimed in any one of Claims 1 to 11, wherein the shaped bodies are made from the mixture by briquetting, pelletizing or extruding.

13. A process as claimed in any one of Claims 1 to 12, wherein a caking promoter is added to the mixture.

14. A process as claimed in Claim 13, wherein the caking promoter is pitch, tar, caking coal and/or bitumen.

1 5. A process in accordance with Claim 1 for producing coal-containing shaped bodies for gasification substantially as hereinbefore described in any one of Examples 1,2 and 4 to 9 of the foregoing Examples.

1 6. Coal-containing shaped bodies produced by the process claimed in any preceding claim.

1 7. A gasification process, wherein the coalcontaining shaped bodies claimed in Claim 1 6 are subjected to gasification at a pressure of 5 to 1 50 bars in a slowly-descending fixed bed to which gasifying agents comprising free oxygen, steam and/or carbon dioxide are fed from below, incombustible mineral constituents being withdrawn as solid ash or liquid slag from the lower end of the bed.