NL8302281A - PROCESS FOR PREPARING A CARBON MONOXIDE AND HYDROGEN-CONTAINING GAS. - Google Patents

Description

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K 5680 NET

WEEKLY FOR PREPARING A CARBON MONOXIDE AND HYDROGEN-CONTAINING GAS

The invention relates to a process for preparing a carbon monoxide and hydrogen-containing gas from a solid carbonaceous fuel in which the fuel is gasified to form a first gas containing carbon monoxide, hydrogen, steam and organic impurities, and wherein at least a portion of the first gas is mixed with a stream of a second carbon monoxide and hydrogen-containing gas that has a higher temperature than said first gas.

The gasification of a solid carbonaceous fuel, such as lignite, bituminous coal, coke, etc., with an oxygen-containing gas produces synthesis gas, i.e. a gas mixture consisting mainly of carbon monoxide and hydrogen. Depending on the starting fuel and on the manner in which the gasification is carried out, the synthesis gas 15 may also contain other substances such as H 2 O, C 2, N 2 and COS. Nitrogen is mainly present when the gasification is carried out by partial combustion with air. The presence of nitrogen can be reduced by performing partial combustion using oxygen-enriched air or pure oxygen. Steam and carbon dioxide are by-products obtained as a result of the complete combustion of part of the fuel. In many partial combustion processes, steam is fed to the combustion reactor where it acts as a temperature moderator and delivers hydrogen through an endothermic reaction with the fuel. Depending on the type of process, up to 9 kg of steam per kg of oxygen can be added to the reaction mixture. Therefore, steam can be found in relatively large amounts in synthesis gas.

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In addition to the mentioned gases, synthesis gas can also contain organic impurities, such as methane. In some partial burnings, heavier organic impurities, such as phenol, oil and tar, are found in synthesis gas.

In conventional methods, the synthesis gas is cooled and purified from the impurities. Therefore, relatively expensive operations are required to remove the relatively useless organic impurities from the synthesis gas. In the method according to the invention this is prevented (at least in part) by mixing the synthesis gas with a stream of a hotter second gas resulting in the organic impurities and the steam in the synthesis gas (hereinafter referred to as 'the first gas') are heated to such a temperature that they react to carbon monoxide and hydrogen. In this way, the total yield of carbon monoxide and hydrogen is increased and the content of organic impurities is reduced.

To obtain the first gas, use is preferably made of the gasification of the fuel in a moving bed which moves in the opposite direction to the gas flow. The so-called Lurgi process is very suitable. This process is described in "Oil and Gas Journal", August 26, 1974, page 80. A variant of this process is the British Gas Co./Lurgi process in which liquid slag is formed.

In countercurrent gasification in a moving bed, the fuel is gasified with preferably oxygen and with a considerable amount of steam at a relatively low temperature, which is mainly between 650 and 900 ° C, and at a pressure of 1 to 100 bar. In the British Gas Co./Lurgi process, the temperature at the bottom of the combustion reactor is so high that the ash melts 30 (1300-1500 ° C), but in this case too, the temperature is in most of the moving bed about 650-900 ° C. At these temperatures, methane and other organic compounds such as phenols, oil and tar are obtained. These impurities are entrained with the carbon monoxide and hydrogen formed. In addition, the gas formed may contain carbon and ash dust.

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The method according to the invention is especially applicable if the obtained first gas contains tar as an organic impurity, since tar can easily be converted with steam into carbon monoxide and hydrogen. The first gas obtained suitably has a temperature of 250 to 700 ° C and a pressure of 1 to 100 bar.

In general, the amount of organic impurities in the first gas is 50 to 300 g per Nm (CO + H 2).

Preferably, this gas is mixed with the stream of the hotter gas, so that as many impurities as possible are converted.

The amount of the second gas relative to the first gas and the temperature of the second gas can vary within wide limits. After all, if the temperature of the second gas is very high, less gas will suffice to obtain a sufficiently high temperature of the mixture than if the temperature of the second gas is only slightly higher than the desired temperature of the mixture of first and second gas. Generally, the flow of the hotter second carbon monoxide and hydrogen containing gas has a temperature of 1100 to 1800 ° C at a pressure of 1 to 100 bar. Under these conditions, the molar ratio between the amounts of the hotter second gas with which the first gas is mixed, and the first gas, is preferably 0.2 to 10. The temperature of the resulting gas mixture immediately after mixing is selected such that the organic pollutants present can react with steam. This requires a higher temperature for methane 25 than for larger molecules such as, for example, tar. Due to the endothermic reactions of the impurities with steam, the gas cools down to a temperature at which the reactions no longer proceed. Preferably, the mixture obtained after mixing the first gas with the flow of the hotter second gas and after reacting the impurities with the steam from the first gas has a temperature in the range of 800 to 1300 ° C.

The mixture then still has a temperature that is high enough to generate a lot of high-quality steam. This is a significant advantage over conventional moving bed partial gasification processes. The temperature of the synthesis gas obtained there 8302281-4 is considerably lower. In addition, it must be washed quickly to remove the impurities, causing heat losses. As a result, only low-grade steam can be generated. High-quality steam is desirable, however, since a considerable amount of steam is required in the countercurrent gasification in a moving bed, as well as for the energy required for any oxygen preparation from air. In a conventional process, high-quality steam is usually generated in a separate installation. In the method according to the invention, the mixture obtained after mixing the first gas with a stream of the hotter second gas is preferably cooled in an indirect heat exchanger, generating steam. This steam is then advantageously used at least in part in the gasification in the formation of the first gas.

When the impurity content in the first gas is high and the steam content is low, it is advantageous to add steam to the first gas for mixing with the stream of the hotter second gas. So much steam is added that the conversion of the organic impurities to carbon monoxide and hydrogen can proceed almost completely. In order to prevent that the supplied steam does not or does not reduce the temperature of the first gas too much, the added steam preferably has a temperature of 250 to 700 ° C. The weight ratio between the amounts of steam and organic impurities is preferably 2 to 20.

The second hotter gas can include any carbon monoxide and hydrogen containing gas. Preferably, however, the flow of the hotter second carbon monoxide and hydrogen-containing gas is from a separate gasification process. This gasification process is preferably carried out at high temperatures. It is possible to use an oil gasification process. Suitable is, for example, the "Shell Gasification Process". This procedure is described in "Chemical Economy and Engineering Review", volume 5, December 1973, no. 12, pp. 22-28. Preferably, however, the gasification process comprises the co-flow gasification of a solid fuel. Very suitable is the coal gasification process described in "Chemical Engineering Progress", March 1980, pages 65-72.

The co-current gasification is suitably carried out in a reactor in which the fuel, oxygen and an amount of steam are supplied. The temperature is usually 1200-1900 eC and the pressure 1 to 100 bar. The product gas leaving the reactor has a high temperature (1100-1800 ° C) and contains few other gaseous components besides carbon monoxide and hydrogen. In many cases, it carries droplets of molten snail. When the gas is cooled, the droplets solidify. However, in the melting range, which can be hundreds of degrees Celsius, the slag is tacky.

Due to their tackiness, particles can cause problems in transit through pipes and / or heat exchangers. To avoid these potential problems, a cold gas is usually injected into the hot product gas, causing the droplets to cool rapidly and solidify completely. In the method according to the invention, a temperature drop relative to the second gas already occurs by mixing the hotter second gas with the cooler first gas. In addition, after mixing, endothermic reactions occur between the organic pollutants and steam present, thereby further cooling the resulting gas. It deserves the preference. to mix before injecting with a cold gas.

The temperature of the second gas, and therefore also of the mixture of first and second gas, is then as high as possible, so that the endothermic reactions proceed easily. In some cases, the gas mixture cools down so much that the slag droplets present fully solidify into non-sticky particles. An injection of cold gas is then no longer necessary.

The molar ratio of hydrogen to carbon monoxide in the gas produced in the co-current gasification is relatively low and is usually between 0.45 and 0.55. In the process of the invention, the ratio of hydrogen to carbon monoxide increases slightly as organic impurities are gasified with steam to form more hydrogen than carbon monoxide. If the first gas has a higher hydrogen / carbon monoxide ratio than the second, hydrogen enrichment also occurs through the mixing itself. A hydrogen-rich gas is desirable in the synthesis of ammonia, methanol and in a Fischer-Tropsch synthesis. .

The solid slag particles are separated from the gas mixture after mixing, for example with one or more cyclones.

The carbon black optionally formed in the endothermic reactions and the ash and dust particles possibly entrained by the first gas are then partly removed from the gas mixture.

The fuel for the co-current gasification is usually finely divided; the average particle diameter is generally less than 0.1 mm. For example, the fuel is easy to carry with a carrier gas or transport fluid, and is reactive enough for rapid gasification. For gasification in a moving bed, on the other hand, the fuel is coarser; the particles 15 usually have a diameter of 5 to 50 mm. This prevents fuel from being carried along with gases that flow through the moving bed in counter-current. Particles with a diameter smaller than 5 mm cannot be processed in a moving bed. Therefore, in conventional moving bed processes, the particles of such small diameter are burned separately, usually for the generation of steam required in the moving bed gasification. In the method according to the invention, preferably the same type of fuel is used for the co-current gasification and for the counter-current gasification in a moving bed, the coarser pieces being used in the moving bed and the fine particles being added, if necessary after further milling, in the co-current gasification. used.

The relative quantities of fuel sent to the co-flow gasification, respectively. fed to the countercurrent gasification in the moving bed may vary within wide limits. The nature of the fuel is important for this. The amount of impurities obtained in the first gas depends, among other things, on the nature of the fuel. It is also important which part of the hotter gas produced in the co-flow gasification is mixed with the first gas. When all the 8302281 * <* - 7 - gas produced in the co-flow gasification is mixed with all the gas produced in the partial combustion in a moving bed, 0.5 to 10 times as much fuel is advantageously supplied to the co-flow gasification per unit time as to the 5 gasification in the moving bed.

The invention is further elucidated by means of the following example.

EXAMPLE

XO coal of the following composition was used as starting material in the following experiments: C 78.1 wt.% On dry and ash-free basis H 5.5 "0 10.9" N 1.2 "15 S 4.3" ash 12 .0 "water 2.0"

Experiment 1

In this experiment, the countercurrent coal was gasified in a moving bed. The particle size of the coal was 5 to 20 mm. 0.50 kg of oxygen and 2.94 kg of steam were added per kg of ash and anhydrous carbon. The gasification was carried out at temperatures between 570 and 1000 ° C and a pressure of 25 bar. The gas formed had a temperature of 577 ° C. Per kg ash and water-free coal, the gas contained 2.0 kg steam and 0.21 kg organic impurities, of which 0.03 kg tar.

Experiment 2

Coal of the same composition as that used in Experiment 1 was gasified in a co-current gasification process at temperatures between 1600 and 2000 ° C and a pressure of 20 bar. The particle size was 0.01 to 0.10 mm. 0.90 kg of oxygen and 0.08 kg of steam were added per kg of ash and anhydrous carbon.

The gas produced had a temperature of 1602 ° C. The gas, which contained no organic impurities, was cooled and 35 slag particles entrained with it were separated. After drying, the gas had the composition as stated in the Table.

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Experiment 3

In this experiment, one part by weight of coal was gasified in a moving bed under the same conditions as in experiment 1. Seven parts by weight of coal were gasified in a co-current gasification under conditions as stated in experiment 2. The gases produced were mixed. A reaction occurred between the steam present and the organic impurities. The temperature dropped to 1227 ° C. After further cooling, removal of slag, soot and ash particles, and drying, a clean hydrogen and carbon monoxide-containing gas was obtained.

Experiment 4

One part by weight of the coal was gassed in a moving bed under the same conditions as in Experiment 1, and two parts by weight were gassed in a co-current gasification under the same conditions as in Experiment 2. The gases produced were mixed. After mixing and the endothermic reactions between the steam and organic impurities, the gas mixture had a temperature of 927 ° C. The gas mixture was further cooled, stripped of solid particles and dried.

20 Experiment 5

As in experiment 3, in this experiment too, seven parts by weight of coal were gasified in a co-current gasification and one part by weight in a moving bed. The produced gases were separately cooled and dried, after which they were combined.

Experiment 6

As in experiment 4, in this experiment two parts by weight of coal were gasified in a co-current gasification and one part by weight in a moving bed. The two gases produced were cooled and dried separately, after which they were combined.

Results of the experiments are shown in the table.

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TABLE

Experiment 1 2 3 4 5 6

Gas composition after drying (vol.X): E ^ 38.6 32.1 35.6 37.7 33.0 34.4 CO 19.8 65.0 58.2 48.9 59.0 49. 2 CH4 11.0 0.0 0.0 1.8 1.5 3.8 CO2-ffl2S 30.2 2.2 5.6 11.1 5.9 12.0 N2 + Ar 0.4 0.7 0.6 0.6 0.6 0.6

Nm3 dry gas per kg ash and anhydrous coal 2.35 2.19 2.33 2.38 2.21 2.24

Nm3 H2 + CO per kg ash and anhydrous carbon 1.37 2.13 2.19 2.06 2.03 1.87 E / CO ratio 1.94 0.49 0.61 0.77 0.56 0 , 70

Organic contaminants other than methane per kg ash and anhydrous coal (kg) 0.03 0.000 0.000 0.000 0.004 0.01

Consumption per Nm3 H2 + C0 of ash and anhydrous coal (kg) 0.730 0.471 0.457 0.485 0.493 0.535 oxygen (kg) 0.37 0.42 0.38 0.37 0.42 0.41 steam (kg) 2.16 0.04 0.14 0.36 0.15 0.40 8302 281 - 10 -

f I

Of experiments 1-6, only experiments 3 and 4 have been carried out according to the invention. The others are included for comparison.

When comparing experiment 3 with experiment 5 and experiment 4 with experiment 6, it appears that per kg of fuel more gas is produced in the experiments according to the invention. The yield of CO and CO is higher, and the content of C and other organic impurities is lower. In addition, the H ^ / CO ratio is higher.

3 10 Finally, it also appears that per Nm of carbon monoxide and hydrogen less oxygen and less steam is required.

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Claims (17)

A method for preparing a carbon monoxide and hydrogen-containing gas from a solid carbonaceous fuel wherein the fuel is gasified to form a first gas containing carbon monoxide, hydrogen, steam and organic impurities and at least part of the first gas is mixed with a stream of a second carbon monoxide and hydrogen-containing gas having a higher temperature than said first gas. The method of claim 1, wherein the first gas is obtained from the gasification of the fuel in a moving bed which moves in opposite directions to the gas flow. The method of claim 1 or 2, wherein the organic contaminants comprise tar. Method according to one or more of claims 1-3, wherein the first gas has a temperature of 250 to 700 ° C and a pressure of 1 to 100 baf. A method according to any one or more of claims 1-4, wherein the amount of organic impurities in the first gas is 50 to 300. per Km (CO + H ^). The process according to any of claims 1 to 5, wherein the stream of the hotter second carbon monoxide and hydrogen containing gas has a temperature of 1100 to 1800 ° C and a pressure of 1 to 100 bar. The method of any one of claims 1 to 6, wherein the molar ratio between the amounts of the hotter second gas with which the first gas is mixed and the first gas is 0.2 to 10. The method of any one of claims 1 to 7, wherein the mixture obtained after mixing the first gas with the hotter second gas stream has a temperature in the range of 800 to 1300 ° C. 8302281 - 12 - 1 * \% * The method of claim 8, wherein the mixture is cooled in an indirect heat exchanger while generating steam. 10. A method according to any one of claims 1 to 9, wherein steam is added to the first gas before mixing with the stream of the hotter second gas. The method of claim 10, wherein the added steam has a temperature of from 250 to 700 ° C. 12. A method according to any one of claims 1-11, wherein the weight ratio between the amounts of steam and organic impurities is 2 to 20. The method of any one of claims 1-12, wherein the stream of the hotter second carbon monoxide and hydrogen-containing gas is from a separate gasification process. A method according to claim 13, wherein the gasification process comprises the co-flow gasification of a solid fuel. The method according to one or more of claims 2-14, wherein the same type of fuel is used for the co-flow gasification and for the countercurrent gasification in a moving bed. A method according to claim 15, wherein 0.5 to 10 times as much fuel is supplied to the co-flow gasification per unit of time as to the partial combustion in a moving bed. The method of claim 1, as described above under special reference to the Example. 18. Carbon monoxide and hydrogen-containing gas, as far as obtained according to the method according to one or more of claims 1-17. BJRH04 / EO / NL 8302281