EP0560039B1 - Process for purifying gas obtained by gasification of carbonaceous material - Google Patents

Description

The invention relates to a method for cleaning a gas obtained by gasification of carbon-containing material, in particular fine-grained to dusty coal, which is to be burned in the combustion chamber of a gas turbine of a power plant.

The use of fuel gases obtained by gasification of carbonaceous material, in particular coal, to generate electricity will become more important in the future, since more than 90% of the energy stored in the fuel used can be used to generate electricity, provided that the gasification combined with a gas and Steam turbine power plant is operated. Here, the gas generated during gasification is burned in the combustion chamber of the gas turbine, while the process heat generated can be used to generate steam for the steam turbine. This process therefore achieves the highest efficiency compared to all other electricity generation processes.

However, the gas generated during the gasification of carbonaceous material contains nitrogen and sulfur compounds, such as ammonia (NH₃), hydrogen cyanide (HCN), hydrogen sulfide (H₂S) and carbon oxysulfide (COS), which in the gas turbine produce nitrogen oxides (NO _x ) and sulfur dioxide ( SO₂) are implemented. In the interest of protecting the environment as much as possible, it is therefore necessary to remove the above-mentioned compounds as much as possible from the gas before the same in the Combustion chamber of the gas turbine is burned. Another reason for an extensive removal of hydrogen sulfide from the fuel gas for the gas turbine can be seen in the fact that it sets a relatively low turbine exhaust gas temperature and thus the sensible heat of the turbine exhaust gas can be better used without reaching the SO₂ / SO₃ dew point.

The invention is therefore based on the object of providing a method for purifying a gas obtained by gasifying carbon-containing material, in which the nitrogen and sulfur compounds mentioned above are removed from the gas to such an extent that the purified gas can be removed without Damage to the environment can be used as fuel gas for the gas turbine of a power plant. The resulting waste products should not pose a burden on the environment and should at the same time be used as widely as possible.

The method of the type mentioned at the outset, which is used to solve this problem, is characterized by method steps a) to e) of the main claim.

In the procedure described in the main claim, only elementary nitrogen and sulfur are obtained as waste products, the sulfur being released in liquid form and being able to be further processed. The nitrogen leaves the plant essentially together with the cleaned gas, which thereby leans in terms of its calorific value. In the use of the cleaned gas as the fuel gas for the gas turbine this does not represent a disadvantage. In order to suppress the thermal NO _x formation in the gas turbine, the gas must be leaned anyway by admixing nitrogen.

If the production of elemental sulfur is not desired, the process can also be modified in accordance with the main claim in such a way that instead of process steps d) and e) the hydrogen sulfide driven off from the washing solution is processed together with the exhaust gas from the two-stage stripping to sulfuric acid, the Exhaust gas from the sulfuric acid plant is released into the atmosphere.

Further details of the method according to the invention will be explained below using the process flow diagram shown in the figure. Here, the flow diagram shows only the process steps that are absolutely necessary for the process explanation, while details of the upstream gasification plant and the downstream power plant plant with the gas turbine are not shown. Likewise, all secondary facilities, such as Additional heat exchangers, pumps, valves etc. and the material flows that are not important here are not listed in the flow diagram.

The raw gas coming from the gasification system is introduced via line 1 into the circuit water scrubber 2 used for dedusting the gas. Here, at a pressure between 15 and 25 bar and a temperature between 110 and 150 ° C, in addition to the dust, the ammonia and hydrogen cyanide present in the gas are almost completely washed out as well as the hydrogen sulfide partially, so that the gas after the cycle water wash still has the following residual levels: NH₃ 0.02 vol% CN 50 ppmV

The pressure required to carry out the cycle water scrubbing 2 is normally already predetermined, since the upstream gasification is carried out under pressure. The application of pressure enables the apparatus required for gas treatment to be kept correspondingly small. The required temperature can normally be set during the cooling of the raw gas following the gasification. Gas washers of conventional design, which can be provided with internals, can be used to carry out the cycle water scrubbing 2. In order to free the washing water from the entrained dust, which is removed from the gas in the circuit water scrubbing 2, the washing water is circulated over the solids separator 4, which is indicated in the flow diagram by the double arrow 3. Here, the entrained dust is separated in the solids separation 4 in a manner known per se, e.g. by filtration and / or sedimentation, separated from the wash water. The wash water then returns to the cycle water wash 2.

In order to avoid an undesirable accumulation of the dissolved pollutants in the wash water, a small partial flow of the same is drawn off after the solid separation 4 and fed to the two-stage stripping 6 via line 5. The amount of the partial stream withdrawn via line 5 is dependent on the pollutant content, in particular on the chloride content, of the fuel used for the gasification. To the extent necessary, the amount of wash water withdrawn from the circuit replaced by fresh water. In the two-stage stripping 6, a series connection of a stripper stage 7 operating in an acidic environment and a stripper stage 8 operating in a basic environment is provided. First, in the stripping stage 7 working in the acidic environment, the acidic components are driven off from the wash water. By adding acid, these components are made according to the reaction equations



CN⁻ + H⁺ → HCN




HS⁻ + H⁺ → H₂S



converted into the molecular form and then driven off by increasing the temperature. The acid required for this, such as hydrochloric acid, is metered in via line 9 to the partial stream in line 5 at the stripper inlet.

Then the washing water running out of the stripper stage 7 is transferred to the stripper stage 8. Here the ammonium ions contained in the wash water are added according to the reaction equation by adding lye



NH₄⁺ + OH⁻ → NH₃ + H₂O



converted into molecular ammonia, which is also driven out of the wash water by increasing the temperature. The lye required for this, such as sodium hydroxide solution, is metered in via line 10 to stripper stage 8.

Following the stripper stage 8, the correspondingly treated wash water can either be returned to the cycle water wash 2, or it is discharged from the process and to a wastewater treatment facility fed. Both options are not shown in the flow diagram. The two stripper stages 7 and 8 can either be carried out in separate columns connected in series, or both stages are combined to form a structural unit, the wash water running out of the stripper stage 7 being metered in with the alkali required for the stripper stage 8. The two-stage stripping 6 is carried out in stripping columns of conventional design, in which the temperature increase required for the removal of the pollutants from the wash water is brought about either by a bottom circulation boiler. The pollutants driven off from the wash water, ie HCN, H₂S and NH₃, are fed via lines 11 and 12 to the Claus system 13. In deviation from the separate feed provided in the flow diagram, the pollutant streams emerging from the two stripper stages 7 and 8 can also be combined and led to the Claus system 13 via a common line.

The dedusted gas emerging from the cycle water scrubber 2 is meanwhile fed to the carbon oxysulfide removal 15 via the line 14. Here the carbon oxysulfide present in the gas, which is difficult to wash out with water or other common solvents, is converted into hydrogen sulfide by catalytic hydrolysis. The carbon oxysulfide reacts in the gas phase with water vapor according to the following reaction equation:



COS + H₂O → H₂S + CO ₂



The catalyst used for this reaction contains aluminum oxide as an active component. The gas then passes via line 16 to the hydrogen sulfide scrubbing 17, in which the hydrogen sulfide present in the gas is washed out absorptively with a selectively acting washing solution. The hydrogen sulfide present in the gas can be removed to a residual content of approx. 7 ppm, while the co-absorption of the other gas components is only slight. An amine solution such as, for example, an aqueous methyldiethanolamine solution can be used as a particularly suitable washing solution. Following the hydrogen sulfide scrubbing 17, the treated gas is of sufficient purity and can therefore be fed via line 18 to the combustion chamber of the gas turbine.

Der anfallende Elementarschwefel wird hierbei über die Leitung 20 in flüssiger Form aus der Claus-Anlage 13 abgezogen und kann seiner weiteren Verwendung zugeführt werden. Bei der Claus-Anlage 13 handelt es sich um eine an sich bekannte Anlage, die aus einem Verbrennungsofen zur Durchführung der Reaktionen 1., 2. und 3. sowie einem ein- oder mehrstufigen Claus-Reaktor zur Durchführung der Reaktion 4. besteht. Im Verbrennungsofen ist dabei eine Katalysatorschicht zur Zersetzung der Stickstoffverbindungen gemäß den Reaktionen 1. und 2. vorgesehen. Da die H₂S-Konzentration des über die Leitung 19 zugeführten Gasstromes verhältnismäßig hoch ist, können die Anlagenvolumina der Claus-Anlage 13 entsprechend klein gehalten werden.The hydrogen sulfide wash 17 usually consists of an absorption and a desorption column. In the latter, the loaded washing solution is regenerated by stripping off the hydrogen sulfide taken up. This produces a gas stream with a high H₂S concentration, which reaches the Claus system 13 via line 19. Here, the gas flows from lines 11, 12 and 19 are further treated according to a modified Claus process, which also allows the catalytic decomposition of the nitrogen compounds (NH 3 and HCN) supplied via line 11 and 12. The following reactions take place:



1. 2 NH₃ → N₂ + 3 H₂





2. 2 HCN + H₂O → N₂ + CO + 2 H₂

The resulting elemental sulfur is withdrawn in liquid form from the Claus plant 13 via the line 20 and can be used for further use. The Claus plant 13 is a known plant which consists of an incinerator for carrying out reactions 1, 2 and 3 and a one- or multi-stage Claus reactor for carrying out reaction 4. A catalyst layer for decomposing the nitrogen compounds according to reactions 1 and 2 is provided in the incinerator. Since the H₂S concentration of the gas stream supplied via line 19 is relatively high, the system volumes of the Claus system 13 can be kept correspondingly small.

Since the reactions are equilibrium reactions according to equations 3 and 4, they do not proceed completely from left to right. Therefore, a so-called Claus residual gas is always produced in the Claus plant 13, which in addition to uncondensed elemental sulfur contains unreacted sulfur dioxide. Since this Claus residual gas cannot be released into the atmosphere easily because of its pollutant content, it must be subjected to an aftertreatment. The Claus residual gas emerging from the Claus system 13 is therefore fed to the aftertreatment 22 via the line 21. The one shown in the flow diagram The embodiment of the process is followed by post-treatment by catalytic hydrogenation in accordance with the reaction equations



SO₂ + 3 H₂ → H₂S + 2 H₂O




S _x + xH₂ → xH₂S



The resulting H₂S-containing gas is returned via line 23 and, after appropriate compression, is mixed with the gas stream in line 14 before it enters the carbon oxysulfide removal 15.

As an alternative to the method of operation described above, the residual Claus gas can also be fed to an afterburning system, the waste gas resulting from the afterburning being blown off into the atmosphere.

As has already been mentioned above, a further variant of the method according to the invention is that the gas streams from the lines 11, 12 and 19 are not worked up further in the Claus plant 13 but in a sulfuric acid plant with the production of sulfuric acid. The exhaust gas from the sulfuric acid plant is released into the atmosphere.

The effectiveness of the method according to the invention is demonstrated by the following exemplary embodiment.

A raw gas from a coal pressure gasification was treated, which in about 180,000 m³ / h 0.300 vol .-% H₂S 0.030 vol .-% COS 0.020 vol% NH₃ 0.005 vol .-% HCN
contains and is dedusted under a pressure of 25 bar.

The circuit water scrubber 2 used for dedusting is charged with 20,000 kg / h of circuit water. In addition to the solids contained in the gas, ammonia and hydrogen cyanide are almost completely washed out as well as hydrogen sulfide and carbon dioxide from the gas.

After the solids contained in the circulating water have been separated off by filtration, which works at 1.5 bar, the filtrate is largely returned to the circulating water wash. Only 6,000 kg / h of filtrate are fed via line 5 to the two-stage stripping 6. After adding 400 kg / h of 35% hydrochloric acid via line 9, the acidic gas components HCN and H₂ and CO₂ are released in stripper stage 7 and drawn off via line 11.

350 kg / h of 15% sodium hydroxide solution are added to the outlet from stripper stage 7 via line 10, whereupon the mixture in stripper stage 8 is stripped in a basic medium. The released ammonia leaves the stripper stage 8 via line 12. Both lines 11 and 12 are at 80 ° C under a pressure of 1.5 bar. The dedusted raw gas in line 14 is meanwhile combined with the gas stream from line 23 and fed to the COS removal 15, in which the COS present in the raw gas is converted to H₂S by catalytic hydrolysis to a residual content of about 5 ppm.

The gas, which has been cooled down to 40 ° C., is then treated in the hydrogen sulfide wash 17 with a circulating methyldiethanolamine washing solution, the residual H₂S content in the desulfurized gas being reduced to approximately 7 ppm H₂S. The desulfurized gas can then be warmed up in the heat exchange with the gas stream in line 16 and leaves the system via line 18 at approximately 130 ° C. and a pressure of 22.5 bar.

The hydrogen sulfide released during the regeneration of the washing solution is conducted at 40 ° C. and 1.5 bar via line 19 to the Claus system 13. Here 830 kg / h of liquid sulfur are generated from the gas streams entering via the lines 11, 12 and 19 and are discharged via line 20 in the liquid state. At the same time, the nitrogen compounds present in the gas are catalytically decomposed in the Claus plant 13. The residual Claus gas is withdrawn via line 21 and, after catalytic hydrogenation via line 23, is mixed with the gas stream in line 14.

Claims (3)

1. Process for purifying a gas produced by gasification of carbon-containing material, in particular fine-grained to dusty coal, which gas is to be burnt in the combustion chamber of a gas turbine of a power station, characterized by the following process steps:

   a) the gas coming from the gasification plant is subjected, at a pressure between 15 and 25 bar and a temperature between 110 and 150°C, to a dedusting by a circulated water scrubber, in which, at the same time, the ammonia and the hydrogen cyanide contained in the gas are virtually completely, and the hydrogen sulphide in part, scrubbed out, a respective part-stream of the scrubbing water being taken off from the circulation and, after separating off the entrained solids, being subjected to a two-stage stripping in an acidic and basic environment;

   b) the carbonyl sulphide contained in the gas is converted by catalytic hydrolysis into hydrogen sulphide;

   c) the hydrogen sulphide still present in the gas is removed from the gas by a selective scrubbing, whereupon the purified gas is delivered to the gas turbine, while the hydrogen sulphide is expelled from the loaded scrubbing solution;

   d) the hydrogen sulphide expelled from the loaded scrubbing solution, together with the exhaust gas arising in stage a) from the two-stage stripping is converted to elemental sulphur in a Claus plant, at the same time the nitrogen compounds present in the exhaust gas being catalytically decomposed; and

   e) the Claus residue gas arising in stage d) is subjected to an aftertreatment, the residue gas being either catalytically hydrogenated and then added back to the gas stream upstream of the entry into stage b) (COS hydrogenation), or the residue gas is fed to an afterburning unit, the exhaust gas arising there being discharged into the atmosphere.
2. Process according to Claim 1, characterized in that, instead of the use of the stages d) and e), the hydrogen sulphide expelled from the loaded scrubbing solution, together with the exhaust gas arising in stage a) from the two-stage stripping, is converted to sulphuric acid, the exhaust gas from the sulphuric acid plant being discharged into the atmosphere.
3. Process according to Claims 1 and 2, characterized in that a selectively acting amine solution, such as e.g. an aqueous methyldiethanolamine solution, is used for the hydrogen sulphide scrubbing in stage c).

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