EP0010792B1 - Process for gasifying particulate fuels - Google Patents

Description

The invention relates to a process for the gasification of solid, baking fuels under a pressure between 5 and 150 bar with free oxygen, water vapor and / or carbon dioxide-containing gasifying agents in a fixed bed, which slowly moves downwards, to which the gasifying agents are fed from below and under which non-combustible mineral components of the fuel are removed as solid ash or liquid slag, fine-grained, baking fuels with a liquid being formed into largely spherical pellets with a diameter of 5 to 50 mm and used as gasification fuels.

The gasification of granular coal is known and e.g. in Ullmann's encyclopedia of technical chemistry. 4th edition (1977) Volume 14, pages 383 to 386. Details of the gasification process with fixed ash can be found in US 3 540 867 and 3 854 895 and DE 22 01 278. The process variant with removal of liquid slag is explained in GB 1 507 905, 1 508 671 and 1 512 677. In these known processes, granular fuel with a grain size of approximately 3 to 60 mm is added to the gasification.

From US-A-3 692 505 and DE-A-25 40 165 one knows the gasification of pellets, which mainly contain fine coal. FR-A-1445980 describes the production of molded coke from coal pellets. To prevent caking, these pellets can be powdered with coke flour before coking.

The object of the invention is to design the pellets mentioned at the outset so that they have sufficient dimensional stability in the gasification, but without interfering with the attack of the gasification agents. According to the invention, this is achieved in that the pellets formed from fine-grained fuel with a grain size of less than 7 mm are gassed with a closed, 0.2 to 2 mm thick covering layer of fine-grained, non-baking material, namely cement. Foreign or own ash, dolimite, bentonite, lime, montmorillonite or zeolites can be provided. The casing materials are all mineral in nature. The fine-grained fuel can be produced when the fuel is extracted or can also be produced by comminution. A fine-grained fuel with a grain size of approximately 0.01 to 5 mm is preferably used for pelleting. Due to the pelleting, dust-like fuel can now also be used for the pressure gasification in the fixed bed, either alone or together with granular coal with a grain size above 3 mm to about 60 mm. The pellets preferably make up the majority of the fuel fed to the gasification.

If granular, baking coal is added to the fixed bed gasification, a slightly gas-permeable zone is formed in the upper part of the fixed bed when the softening temperature of the coal is reached, since the coal grains melt or at least become doughy. Up until now, stirring arms have been used to loosen up this layer of coal in the fixed bed gasification and thus to make it gas-permeable.

There has also been no lack of efforts to remove the baking properties of the coal, for example by pretreating it at temperatures of about 200 to 350 ° C. The aim was to use the entire grain of coal, among other things. also influenced by pore diffusion of oxygen in the desired manner. These efforts were somewhat successful, but treatment times were too long to oxidize the coal. A gasification plant with high throughput rates was therefore not economically feasible because too much effort was necessary.

Difficulties of this type are eliminated by the method according to the invention. The covering layer around the pellets now ensures that the pellets in the upper part of the fixed bed, where the temperatures are relatively low, retain a sufficiently stable shape. At the same time, the shell layer is gas-permeable and allows volatile constituents to escape from the interior of the pellet. If the temperatures continue to rise, the covering layer is largely preserved, and slight cracks are not disadvantageous if the fuel becomes plastic and only changes into a highly viscous mass. In the case of greater softening, the coating layer must be strengthened accordingly, or catalytically active substances which reduce the baking and expanding properties of the coal can be added to the coal before pelleting. After passing through the baking zone, the pellet casing tears open further due to the strong degassing of the coal substance, so that the coal coke which is now formed is exposed to the gasification reactions in its entire surface including the macro and microstructure. It is also an important result that the product gas from the gasification contains much less tarry constituents than was possible with the previously known gasification of baking coals under increased pressure.

The pelletes can be made in a known manner, e.g. in the inclined turntable or in the rotary tube. The pellets can also be provided with the desired coating layer using these devices. The pellets can have a diameter of 10 to 30 mm and preferably 8 to 15 mm.

Various types of liquids can be used to form the pellets. Suitable are, for example, water, sulfite waste liquor, water glass, molasses, starch solutions, gas water from the gasification or coking of solid fuels, or lime milk. The pelleting liquid or the fine-grained one to be pelleted Solid fuel can be added to catalytically active substances to increase the reactivity. Such substances are in particular NaCl, KCI, NaC0 ₃ , K _Z C0 ₃ , oxides or sulfides of molybdenum, tungsten, tin, chromium, nickel, cobalt or iron or finely ground thin-layer zeolites. These catalytically active substances are able to effect desired conversions in contact with gases and vapors from the pellets, for example to make hydrocarbons with higher boiling and lower boiling points.

In order to reduce the baking and blowing properties of the solid fuel to be gasified, suitable substances can be added to the pelleting liquid or the fuel to be pelletized. Such substances are, for example, Na ₂ B ₄ O ₇ , NaN0 ₂ , K ₂ Cr0 ₄ , K ₂ C0 ₃ or KN0 ₃ .

Non-baking coal, ash, e.g. from gasification or from external production, dolomite, bentonite, lime, montmorillonite. Cement or other inorganic substances can be used. Mixtures of finely ground zeolites are also particularly suitable. It is best to add these substances in fine or dusty form before the end of pelleting.

The covering layer of the pellets should have a thickness of approximately 0.1 to 2 mm, preferably 0.2 to 0.5 mm. For economic reasons, efforts are made to keep the coating layer as thin as possible. However, it must be sufficiently strong and thick, especially in the case of heavily baking coals, in order to prevent flowable coal from breaking through.

If necessary, the coated pellets can be dried before being introduced into the gasification. This is best done at temperatures below the softening point of the coal, preferably between 40 and 150 ° C. The pelletized fuel generally has an incombustible mineral component content of 5 to 40% by weight and preferably 10 to 30% by weight.

The gasification product gas can be used in a variety of ways. After cleaning and conditioning, the gas can e.g. can be used for syntheses or as a hydrogenation gas. Another possibility is to use the gas as fuel gas, if necessary after removing disruptive solid or gaseous constituents, and to use it in a combined steam-gas turbine power plant.

Examples

A moderately baking and expanding gas flame coal with 20% by weight ash and 8% by weight moisture was used for experiments. The baking number according to Roga (ISO 335-1974) was 25, the blowing number according to DIN 51741 was 2.5. The coal had a grain size between 0 and 1 mm and had an average grain diameter of 0.291 mm. A pelletizing plate of 1 m in diameter with an inclination of approximately 50 ° and a speed of 20 revolutions per minute was used to form the pellets. In each case, pellets with a coating layer were produced, the diameter of which was in the range from 10 to 20 mm. The pellets were dried at 110 ° C. and the compressive strength was then measured. To simulate the conditions in the fixed bed gasification, coking was carried out with shock-like heating to 800 ° C. under a nitrogen pressure of 20 bar.

example 1

Water was used as the pelleting liquid, so that the water content of the finished pellets was 15% by weight. The shell layer had a thickness of 0.3 mm and consisted of finely ground own ash of the coal. The point pressure resistance of the dried pellets was approx. 70 N. After the pressure coking, the pellets were not baked together, the casing had burst, the pellet core was structural. The pellets were extremely suitable for pressure gasification.

Example 2

A 5% calcium sulfite waste liquor was used as the pelleting liquid, finely ground dolomite was used for the coating layer, the coating layer had a thickness of about 0.3 mm. The point compressive strength of the dried pellets reached about 80 N, the test results after the pressure coking matched those of Example 1. In addition, about 50% of the sulfur contained in the coal was bound by the dolomite layer.

Example 3

A 20% water glass solution was used as the pelleting liquid, cement was used for the covering layer and a thickness of about 0.4 mm was achieved. The dried pellets had a point compressive strength of about 60 N. After the pressure coking, the pellets did not adhere to one another and were structurally stable. The shell layer was cracked and partially chipped. The pellets of Examples 2 and 2 were suitable for gasification in a fixed bed without a special loosening device having to be provided in the upper part of the gas generator.

Example 4

For this experiment, a moderately baking and expanding gas flame coal with 22 wt.% Ash and 8 wt.% Moisture was used. The baking number according to Roga (ISO 335-1974) was 25, the blowing number according to DIN 51 741 was 3. The coal had a grain size between 0-1 mm, the mean grain diameter being 0.280 mm.

The pellets produced on the pelletizing plate of the previous examples had a diameter of 10 to 20 mm. As a covering layer Dolomite was applied with a layer thickness of 0.3 mm. A 10% potassium carbonate solution was used as the pelleting liquid. Before the coating was applied, the pellets had a moisture content of approximately 15% by weight. Before being used in coking and gasification, the pellets had a potassium carbonate content of less than 0.6% by weight.

When examining the product gas, it was found that of the tar contained in the coal according to ISO 647 - 1974, only about 50% of the liquid hydrocarbons had reached the gas space after exiting the pellets and that the boiling end of these liquid hydrocarbons, which normally is above 450 ° C, only contained hydrocarbons that boiled below 240 ° C.

Furthermore, in a reactivity test under 20 bar and a constant temperature of 800 ° C., it was found that at a mass flow of 20 I _N / h 3.8 vol.% CO _{2 were} decomposed, while under otherwise identical conditions, but without using potassium carbonate, only values of 2.3 vol.% were reached.

Example 5

In a further experiment, a coal was used whose baking number according to Roga (ISO 335-1974) was 52, while the blowing number according to DIN 51 741 was 7. The coal had a grain size between 0-1 mm with an average grain diameter of 0.264 mm.

After pelleting, it was provided with fine-grained ash in a layer thickness of 0.25 mm. A 5% sodium tetraborate solution was used as the pelleting liquid. As a result of this measure, the baking and expanding capacity of the coal was reduced to such an extent that the pellets did not cake together during pressure coking. The reactivity of the pellets produced in this way was favorably influenced by this measure.

Claims (5)

1. A process of gasifying caking solid fuels under a pressure between 5 and 150 bars with gasifying agents which contain oxygen, water vapor and/or carbon dioxide, wherein the fuel formes a fixed bed which slowly moves downwardly, the gasifying agents are introduced into the fixed bed from below, and the incombustible mineral constituents of the fuel are withdrawn under the fixed bed as solid ash or liquid slag, fine-grained caking fuels are formed with the aid of a liquid into substantially spherical pellets which are 5 to 50 mm in diameter, characterized in that the pellets which are formed from fine-grained fuel with a grain size below 7 mm. before gasification are provided with a substantially closed covering layer of fine-grained non- caking material selected from cement, ash obtained from the same or a different fuel, dolomite, bentonite, lime, montmorillonite or zeolites, said covering layer has a thickness of 0.2 to 2 mm.

2. A process according to claim 1, characterized in that water, spent sulfite liquor, waterglass, molasses, starch solutions, aqueous condensate obtained by gasifying or coking solid fuels, or milk of lime is used in forming the pellets.

3. A process according to claim 1 or 2, characterized in that catalytically active substances serving to increase the reactivity and consisting particularly of NaCl, KCI, NaC0₃, K₂CO₃, or oxides of molybdenum, tungsten, tin, chromium, nickel, cobalt or iron are added to the pelletizing liquid or to the fine-grained solid fuels to be pelletized.

4. A process according to claim 1 or any of the following claims, characterized in that substances which decrease the caking and swelling properties of the solid fuel and consist particularly of Na₂B₄O₇, NaN0₂, K₂CrO₄, K₂C0₃ or KN0₃ are added to the pelletizing liquid or the fine-grained solid fuels to be pelletized.

5. A process according to claim 1 or any of the following claims, characterized in that the pellets provided with the covering layer are dried, particularly at temperatures below the softening point of the coal and preferably between 40 and 150 °C, before they are supplied to the gasifier.