DE3621041A1 - METHOD FOR THE GASIFICATION OF CARBONATED FUEL AND REMOVAL OF HCN, NH (ARROW DOWN) 3 (ARROW DOWN) AND COS FROM THE SYNTHESIC GAS formed thereby - Google Patents

Description

The present invention relates to a method for gasification of carbonaceous fuel.

In the partial combustion or gasification of a carbonaceous Material such as coal, this material is used at elevated temperatures implemented with a limited volume of oxygen, where this reaction advantageously in the presence of additives, such as water vapor, carbon dioxide or various other materials is carried out. This gasification reaction produces a gas which is also known as syngas and that mainly contains carbon monoxide and hydrogen. At the same time alternating quantities of other gases are also generated, such as Carbon dioxide and methane and also a wide variety of liquid and solid materials, such as small particles of ash. In general solid materials entrained in the gas become common  from the raw synthesis gas using a combination of Cyclones or separators or a water washing system which Washer cooler or Venturi scrubber or by means of filters or electrostatic precipitators or a combination of such systems.

The raw synthesis gas drawn off from the gasifier or the gasification zone contains, in addition to the materials mentioned above, also sulfur-containing gases, such as hydrogen sulfide and carbonyl sulfide, and small amounts of ammonia and hydrogen cyanide. The presence of HCN, NH ₃ and COS (carbonyl sulfide) in the synthesis gas formed when gasifying a carbonaceous fuel such as coal makes it difficult to remove additional impurities such as H ₂ S and / or CO ₂ , and leads to problems with product quality and what Regarding control requirements related to air pollution. Although, as already indicated, HCN, HN ₃ and COS are only present in small amounts, for example in amounts totaling less than 1 percent by volume, based on the total raw synthesis gas, these small amounts must nevertheless be eliminated before the synthesis gas is used in practice can be.

It is therefore an effective procedural measure that can be carried out in practice to remove these contaminants from large economic importance. It is therefore an object of the invention a process for the gasification of a carbonaceous material to provide at which these contaminants removed in a practicable and effective manner will.

The inventive method for the gasification of carbonaceous Fuel is characterized by the following process steps.

• a) Partial combustion of carbonaceous fuel in particulate form in a gasification zone, which comprises at least one gasification reactor, to produce a gas stream which contains synthesis gas, particulate material and small amounts of HCN, NH ₃ and COS;
• b) cooling the gas stream and separating at least the majority of the particulate material therefrom and washing the gas stream in a scrubbing zone with an effective amount of an aqueous scrubbing solution to remove HCN, NH ₃ , COS and residual amounts of the particulate material from the synthesis gas, resulting in a purified Synthesis gas and a loaded washing solution are obtained;
• c) stripping off the main amount of HCN, NH ₃ and COS from at least part of the loaded washing solution in a stripping zone to produce a gas mixture containing HCN, NH ₃ and COS and a stripped liquid mixture, and separating the gas mixture from the stripped liquid mixture and withdrawing the Gas mixture from the stripping zone;
• d) feeding in at least a portion of those formed in step (c) and withdrawn gas mixture in at least one gasification reactor the gasification zone.

By feeding the gas mixture into the gasification zone the above-mentioned impurities are almost completely in easily removable and / or reusable gaseous material converted. According to an advantageous embodiment of the Invention becomes at least the bulk of the stripped liquid Mixture returned to the wash zone to be therein as an aqueous Wash mix to be reused. It can also be particulate Material from the aqueous mixture before it is fed be separated into the stripping zone or such particulate Material can be made from the stripped liquid mixture be separated off before being fed back into the washing zone.

The particle combustion of carbonaceous materials such as Coal for the production of synthesis gas, which in consists essentially of carbon monoxide and hydrogen in itself well known and an overview of such known methods  is in "Ullmann's Encyclopedia of Technical Chemistry", Volume 10 (1958), pages 360 to 458. Several such procedures for the production of hydrogen and carbon monoxide containing Gases are also currently under development. Accordingly details of the gasification process are only so far indicated as being for understanding the present invention is required.

Generally the gasification of carbonaceous material by means of partial combustion of such material with a limited Volume of oxygen at a temperature of usually between 800 and 2000 ° C and advantageous at a temperature between 1050 and 2000 ° C carried out. If a temperature in the range of 1050 to 2000 ° C is used, the product gas can be very small Amounts of gaseous by-products such as tars, phenols and condensables Contain hydrocarbons. To the carbonaceous Starting materials include lignite, bituminous coal, sub-bituminous Coal, anthracite, coke, wood and heavy petroleum residues. Lignites and coal types are particularly advantageous carbonaceous ones Raw materials. To be faster and more complete To achieve gasification, it is advantageous to use the solid carbonaceous To pulverize the raw material beforehand. The particle size can be chosen such that 70% of the fixed feed pass through a 200 mesh sieve. The gasification is advantageously carried out in the presence of oxygen and water vapor, the purity of the oxygen expediently at least 90 percent by volume, while impurities in the form of Nitrogen, carbon dioxide and argon are permitted. Although in general a ratio of oxygen to water vapor can be chosen can that 5 to 150 parts by volume of water vapor per volume of oxygen are present, the method according to the invention can also be used, if there are completely different proportions of oxygen be applied to water vapor. The oxygen used can be heated before contacting the carbonaceous material be, for example to a temperature in the range of about 200 to 500 ° C.

The details of the gasification reactor system are not the subject  of the present invention and suitable reactors are disclosed in British Patent No. 15 01 284 and in the US patent No. 40 22 591. The high temperature at which the gasification takes place can be adjusted by the carbon-containing Material with oxygen and water vapor in a reactor implement high flow rates. An advantageous linear Speed is in the range of 10 to 100 m per second, though higher or lower speeds can also be used. The pressure at which the gasification is carried out can vary within wide ranges, for example in the range from 1 to 200 bar. There is also a residence time a wide range of variations. Usual residence times are 0.2 to 20 seconds. With dwell times in the range of 0.5 to 15 seconds advantageous results are achieved.

After the starting materials have been reacted, the reaction product containing hydrogen, carbon monoxide, carbon dioxide and water and the above-mentioned impurities is withdrawn from the reactor. This gas, which usually has a temperature between 1050 and 1800 ° C., can also contain impurities such as ash and solid carbon-containing particles. To enable the impurities to be removed from the gas, the reaction product stream should first be cooled. A number of sophisticated techniques have been developed to cool the gas stream, the techniques in question generally being characterized by the use of a boiler in which water vapor is generated by the waste heat. Cyclones or other suitable separation techniques can be used to remove particulate matter from the gas stream. Although such measures can be carried out, a further reduction in the solids content is desirable in many cases. For this purpose, the gas stream can be sent through a washing zone in which it is washed out with an aqueous washing liquid. This washing zone can have one or more washing sections or washing towers. The term "aqueous washing solution" used here includes water, a wide variety of process streams and the solution from which HCN, NH ₃ and COS have been stripped, ie the circulating solution. The aqueous wash solution may also contain other materials, such as selectively selected amines, to facilitate removal of the contaminants. In addition, alkaline substances can be added in order to adjust the pH value and to enable optimal separation of the impurities. Water can be used to wash out HCN and NH ₃ and an amine solution can be used, if desired in a separate step, to remove COS. If more than one wash-out stage is used, the solutions in question can be combined in the stripping zone before being fed into them. The person skilled in the art is given the ability to adjust the pH, the composition and the volume of the aqueous wash solution to one another in such a way that practically the total amount of HCN, NH ₃ and COS are separated from the synthesis gas stream. A suitable washing device is described in British Patent No. 8 26 209. Such a washing treatment gives a gas which contains practically no solids and which has a temperature between 20 and 40 ° C.

It was pointed out above that the treatment with the aqueous washing solution not only removes the residual proportion of solid particles, but also substantial amounts of HCN, NH ₃ and COS from the gas stream. The present invention aims at the treatment of the contaminated or "loaded" washing solutions and solves the problem of substantially reducing the emission of HCN, NH ₃ and COS that has to be treated or removed. The term "load" as used herein is only intended to express that the wash solution (s) contain (contain) finite amounts of one or more of the gaseous contaminants mentioned when used to wash out the gas stream.

In the process according to the invention, the aqueous washing solution in which HCN, NH ₃ and COS are dissolved is fed to a stripping zone or a stripper, in which case the gaseous impurities are stripped from the solution. This measure expediently takes place in that the solutions obtained from the washing zone are drawn off in the form of a secondary stream and fed into the stripping zone, in which suitable measures are taken to strip the gaseous impurities from the solution.

In the stripper, the wash solution can be stripped by heating, contacting with a non-reacting gas, or a combination of these two measures. The stripping forms a gas stream which contains HCN, NH ₃ and COS. The stripped solution may, as has already been pointed out, still contain solids, and these fines, which can be present in barely perceptible amounts up to a concentration of about 2 to 5 percent by weight, determine the further type of final treatment or removal of the solution. The stripped solution can be returned to the wash zone for reuse.

As indicated above, the wash solution can be either Heat or let flow through with a non-reactive Gas or stripped by a combination of both measures will. If you only use thermal energy for this purpose, then the amount of heat must be sufficient to dissolve the Release gases. It is not necessary for these to be stripped Gases are heated before being fed to the gasification reactor. Suitable devices for this purpose include, for example one filled with packing or one with replacement trays Column with a reboiler. Generally are Temperatures in the range of about 100 to 300 ° C and advantageous in the range of about 200 to 300 ° C suitable to the dissolved gases release.

If a non-reactive stripping gas is used, then this is supplied under a suitable pressure of, for example, 3 to 5 at, to remove the dissolved gases from the wash solution. It can use any suitable scraper for this purpose are used, for example a packed with packing Column or a column provided with exchange trays. In the Cases where the clogging is due to solid matter still present Other devices could be used as a problem regardless of whether stripping by heat, by gas flow or a combination of these measures.  

In any case, however, a suitable non-reactive gas must be used be used. The term "non-responsive" used here means that the gas in question is not to a significant extent reacts with the washing solution. Suitable gases, which under the conditions in the stripping zone can be used for example air, water vapor, carbon dioxide, oxygen, Nitrogen and other inert gases. However, water vapor is preferred because it also uses heat energy for stripping Provides and is easily condensable, making it a relative concentrated current is formed. The expert is given the volume and speed of the stripping gas settle in an appropriate manner. It has already been pointed out that additionally when using a stripping gas Thermal energy can be supplied to the stripping process facilitate.

The stripped gaseous contaminants are separated and removed from the wash solution and fed to a gasification reactor. If there are a plurality of gasification reactors in the gasification zone are present, so the gaseous impurities from the stripping zone into any or all of these Reactors can be fed in as required or is appropriate. If the gasification reactor is operated under high pressure as is usually the case, the pressure must the flow of gaseous contaminants before the gases enter be raised in the reactor. Suitable devices for this Purpose are well known to those skilled in the art and are such as such not a feature of the invention.

The invention will now be described in detail, for example of the accompanying drawing, in which the method is explained is shown schematically in the form of a flow chart, wherein Auxiliary devices, valves, pumps, etc. not shown are. All numerical values given are only for example or are calculated.

According to the diagram shown, powdered coal is fed via a line ( 1 ) to a coal dryer ( 2 ), in which the coal is dried, suitably at a temperature of about 220 ° C. This dry coal is then drawn off via line ( 3 ) and then passes into a gasification reactor ( 4 ), in which it is gasified at a temperature of about 1500 to 2000 ° C, a pressure of about 35 at abs. is maintained and the required oxygen is fed via a line ( 5 ). The gasification reaction produces a product gas which is withdrawn from the upper part ( 6 ) of the reactor while slag is removed from the lower part of the reactor via line ( 7 ). The gasification product is then passed via line ( 8 ) through a boiler or heat exchanger ( 9 ), in which it is cooled to a temperature of about 200 ° C. In this boiler ( 9 ), water supplied via line ( 10 ) is converted into high-pressure steam by indirect heat exchange, and this steam is drawn off via line ( 11 ). The cooled gasification product then passes via a line ( 12 ) into a device for the first removal of the solids, for example a series of cyclones ( 13 ), in which the majority of the solid particles (fly ash) are separated off. It then passes into a washing device ( 14 ) in which it is contacted with an aqueous washing solution. This washing device ( 14 ) is supplied with water via a line ( 15 a ) and / or recirculated aqueous washing solution via a line ( 15 ). In the washing device ( 14 ), the washing solution absorbs HCN, NH ₃ and COS from the gas stream and also removes remaining amounts of soot and ash particles in this gas stream. Purified synthesis gas is withdrawn from the washing device via line ( 16 ) and discharged from there for further treatment and / or recovery. The washing water containing the dissolved gases, ashes and soot is drawn off from the lower part of the washing device ( 14 ) and fed via a line ( 17 ) to an optional filtration zone ( 18 ) in which soot and ash particles can be removed. The washing solution drawn off from the filtration zone ( 18 ) is then fed via line ( 19 ) to a stripping zone ( 20 ). This stripping zone ( 20 ) comprises a stripper with contact plates, the liquid being fed in at the top of the column and a non-reacting gas being fed in via a line ( 21 ) at the bottom of the column. Water vapor with a suitable temperature of, for example, 150 ° C. can be used for stripping off the gases, so that the gases which flow out of the column ( 20 ) via a line ( 22 ) and are freed of impurities together with the water vapor in line ( 22 ) can be easily compressed by means of a compressor ( 23 ) and then fed directly via a line ( 24 ) into the gasification reactor ( 4 ). The inlet openings in the gasification reactor can be those openings which already exist and are used for feeding other reactants, or they can be provided specifically for feeding in the gas containing HCN, NH ₃ and COS. The inlet openings can be arranged so that an optimal conversion of the impurities into easily removable substances takes place. The stripped washing solution is fed back into the washing device ( 14 ) via line ( 15 ) or it can be removed as waste water from the circuit via line ( 15 a ).

Although the method according to the invention with reference to a specific Flow diagram and special devices have been explained for the person skilled in the art, it is readily apparent that equivalents or analog units or means can be used. The The term "zone" is used in the description and claims, to also consist of devices consisting of several parts or To designate plants that are operated in series, or below is the division of a single unit into several units understood to increase efficiency or restrictions regarding the size of a facility. For example, one number of washers arranged in series, where different aqueous solutions are used and at least the majority of the loaded solutions to one or more scrapers are fed. Also a parallel operation of device units naturally falls under the inventive concept.

Claims (4)

1. Process for the gasification of carbon-containing fuel, characterized by the following process steps:

• a) Partial combustion of carbonaceous fuel in particulate form in a gasification zone, which comprises at least one gasification reactor, to produce a gas stream which contains synthesis gas, particulate material and small amounts of HCN, NH ₃ and COS;
• b) cooling the gas stream and separating at least the majority of the particulate material therefrom, and washing the gas stream in a wash zone with an effective amount of aqueous wash solution to remove HCN, NH ₃ , COS and residual amounts of particulate material from the synthesis gas, thereby creating a purified synthesis gas and a loaded wash solution are obtained;
• c) stripping the bulk of HCN, HN ₃ and COS from at least a portion of the loaded wash solution in a stripping zone to produce a gas mixture containing HCN, NH ₃ and COS and a stripped liquid mixture and withdrawing the gas mixture from the stripping zone;
• d) feeding at least part of the gas mixture formed and withdrawn in step c) into at least one gasification reactor of the gasification zone.

2. The method according to claim 1, characterized in that at least the bulk of the stripped liquid mixture for reuse as an aqueous washing solution in the washing zone is returned. 3. The method according to claim 1 or 2, characterized in that particulate matter from the loaded wash solution is separated before it is fed into the stripping zone becomes. 4. The method according to claim 1 to 3, characterized in that particulate material from the stripped liquid Mixture is separated before this again in the washing zone is fed.