DK201670723A1 - Production of sulfuric acid from coke oven gas desulfurization product - Google Patents

Abstract

The present invention relates to a process and a process plant for the production of sulfuric acid from a gas comprising H2S characterized by the following steps; directing said gas to an alkaline absorber containing a liquid ammonia absorbent solution and a combustible material catalytically active in oxidation of H2S to elemental sulfur providing a liquid slurry comprising sulfur and waste liquid, separating at least a portion of said elemental sulfur from waste liquid by mechanical means of separation as a sulfur sludge, combining said sulfur sludge with an ash-free support fuel and a first gas comprising oxy gen in or upstream an incinerator, incinerating said mixture, providing an incinerated gas comprising SO2, optionally adding a second gas comprising oxygen to said incinerated gas, contacting said process gas with a material active in oxidation of SO2 to SO3 providing an oxidized process gas, optionally adding water to said oxidized process gas and condensing concentrated sulfuric acid formed in said oxidized process gas with the associated benefit of the incinerated gas being substantially ash free, such that the process downstream the incinerator may be conducted with little or no filtration.

Description

Title: Production of sulfuric acid from coke oven gas desulfurisation product

The invention relates to a process for removal of hydrogen sulfide from coke oven gas, with associated production of concentrated sulfuric acid.

When coke oven gas is produced from coal significant amounts of sulfur and nitrogen are also released, typically in the form of H2S and HCN. These constituents must be removed from the coke oven gas before the coke oven gas is used as city gas or process gas. One process for cleaning the coke oven gas involves absorbing H2S and HCN in an alkaline scrubber, and oxidizing H2S to elemental sulfur by means of a liquid catalyst, which typically is a quinone type compound in combination with a metal such as cobalt or iron to promote the catalytical activity. The elemental sulfur is then separated from the slurry, and may either be sold as low quality sulfur, or may be transferred to a sulfuric acid production plant, where it is incinerated by the use of a support fuel, forming S02, filtered in a high temperature filter and transferred to a catalytic S02 to SO3 conversion unit, and subsequently hydrated to H2SO4 and collected as concentrated sulfuric acid. Examples of such configurations may be found in the patents CN101033060 and CN101092577 which involve dry filters and a wet sulfuric acid process, as well as CN101734629 which involve a wet process gas cleaning unit and a dry gas sulfuric acid process .

It has now been realized that a significant cost in this configuration is the removal of ash, e.g. by filtration or a wet quench process, and that a selection of conditions for providing ash free incineration products may reduce the overall cost.

By analysis of the process the key sources of particles have been identified as the alkaline solution and a dissolved metallic catalyst, and accordingly the present invention relates to a process in which the alkaline solution is ammonia base and the dissolved catalyst is provided in a metal free form.

For the present application the term catalyst shall be understood as any compound participating reversibly in a reaction. This shall be construed as including a dissolved compound which is reacting in one process position and being regenerated in a different process position.

For the present application the term combustible shall be understood as substantially ash-free, i.e. that particles are not formed during combustion.

In a broad form the present invention relates to a process for the production of sulfuric acid from a gas comprising H2S characterized by the following steps; directing said gas to an alkaline absorber containing a liquid ammonia absorbent solution and a combustible material catalytically active in oxidation of H2S to elemental sulfur providing a liquid slurry comprising elemental sulfur and waste liquid, separating at least a portion of said elemental sulfur from waste liquid by mechanical means of separation as a sulfur sludge, combining said sulfur sludge with an ash-free support fuel and a first gas comprising oxygen in or upstream an incinerator, incinerating said mixture, providing an incinerated gas comprising SO2, optionally adding a second gas comprising oxygen to said incinerated gas, contacting said process gas with a material active in oxidation of SO2 to SO3 providing an oxidized process gas, optionally adding water to said oxidized process gas and condensing concentrated sulfuric acid formed in said oxidized process gas by heat exchange with a cooling medium such as atmospheric air, providing a desulphurized process gas and a heated cooling medium with the associated benefit of the incinerated gas being substantially ash free, such that the process downstream the incinerator may be conducted with little or no filtration.

In a further embodiment the content of metal in said liquid ammonia absorbent solution is less than 50 ppm weight/weight, preferably 10 ppm weight/weight and most preferably 3 ppm weight/weight, with the associated benefit of very low formation of ash particles during incineration, and thus no need for filtration of the incinerated gas.

In a further embodiment the content of particles in said incinerated gas is less than 20 mg/Nm3, preferably 10 mg/Nm3 and most preferably 2 mg/Nm3, with the associated benefit of such a low content of particles being non-problematic in the operation of the sulfur dioxide converter.

In a further embodiment said combustible material catalyti-cally active in oxidation of H2S to elemental sulfur is a quinone type, such as a benzoquinone, naphthoquinone or an-thraquinone compound optionally with one or more non-metal-lic substituents with the associated benefit of quinone catalysts being well known, tested, active, low cost, and known to generate only gaseous combustion products, such as C02, H20 and possibly N2, N0X, S02 if substituted with sulfur or nitrogen groups.

In a further embodiment said combustible material catalyti-cally active in oxidation of H2S to elemental sulfur is picric acid with the associated benefit of all said examples of catalysts being active, low cost, and known to generate only gaseous combustion products, such as N0X, C02, and H20.

In a further embodiment the process further comprises the step of withdrawing a purge stream of said waste liquid with the associated benefit of avoiding a buildup of sulfur in the waste liquid.

In a further embodiment at least a part of said purge stream is directed to the incinerator for conversion of sulfur containing anions to S02 and SO3 with the associated benefit of the purge stream being converted to commercial sulfuric acid instead of waste liquid.

In a further embodiment said material active in oxidation of S02 to SO3 comprises vanadium with the associated benefit of being a highly effective and robust S02 oxidation catalyst.

In a further embodiment one or both of said first and said second gas comprising oxygen is atmospheric air being preheated by heat exchange with a hot process stream such as the oxidized process gas with the associated benefit of contributing to an efficient thermal management of the process .

In a further embodiment the process further comprises the step of cooling said incinerated gas to a temperature in the range of 300 to 450°C, preferably 380 to 420°C with the associated benefit of the reaction being highly active at this temperature, while avoiding shifting the equilibrium towards SO2.

In a further embodiment in which NOx is present in said incinerated gas, the process further comprises the process steps of combining said incinerated gas with a gas comprising NH3 providing an SCR feed gas and contacting said SCR feed gas with a material catalytically active in selective reduction of NOx with the associated benefit of avoiding an emission of NOx and thus reducing the environmental impact of the process even further.

In a further embodiment the material catalytically active in selective reduction of NOx comprises a carrier, such as titanium oxide, and active catalytic components taken from the group consisting of oxides of base metals such as vanadium, molybdenum and tungsten, zeolites, and precious metals . A further aspect of the present invention relates to a process plant comprising an alkaline absorber configured for contacting a gas comprising H2S with a liquid ammonia absorbent solution and a combustible material catalytically active in oxidation of H2S to elemental sulfur providing a liquid slurry comprising elemental sulfur and waste liquid, a means for mechanical separation configured for separating at least a portion of said elemental sulfur from waste liquid by mechanical means of separation as a sulfur sludge, an incinerator configured for incinerating said sulfur sludge together with an ash-free support fuel and a first gas comprising oxygen, providing a process gas, a sulfur dioxide converter comprising a material active in oxidation of SO2 to SO3 and configured for receiving said process gas and providing an oxidized process gas a condenser, configured for receiving said oxidized process gas and a cooling medium and providing a concentrated sulfuric acid, a desulfurized process gas and a heated cooling medium, with the associated benefit of said process plant being simple and inexpensive.

In a further embodiment said does not include a gas filter operating at a temperature above 400°C, with the associated benefit of being a simple and inexpensive process plant.

Coke oven gas (COG) from the gasification of coal goes through several cleaning steps before it can leave the gasification plant and be used for e.g. chemical synthesis, combustion and heating purposes.

In the desulfurization unit H2S and HCN are efficiently absorbed into an alkaline absorbent liquid in which H2S is oxidized to elemental sulfur by means of a liquid catalyst. The catalyst is regenerated by air addition which optionally may be carried out in a separate process step. As side reactions some of the H2S may be further oxidized to a wide range of sulfur-oxides, including S2032“ and S042“ and HCN is converted into SCN-. With NH3 as the alkaline component, a wide range of salts, including (NH4)2S203, (NH4)2S04 and NH4SCN may be formed in the liquid phase accounting for up to half of sulfur in the H2S removed. The sulfur and dissolved NH4+ salts are fed to a unit where the sulfur is separated from the NH4+ salts. Most of the salt solution is returned to the desulfurization unit, but a purge stream is withdrawn to control the concentration of dissolved salts.

This COG desulfurization process is cheap and effective but the products are of poor quality and could be expensive to get rid of in an environmentally benign way. The process variants are numerous and known by trade names such as HPF, PDS, Perox, LoCat and Stretford. The differences are primarily within the catalyst system and alkaline compound.

In the prior art a wide range of organic catalysts for the oxidation of H2S to elemental sulfur are known, typically having a quinone structure such as a benzoquinone, naphthoquinone or anthraquinone compound optionally with one or more non-metallic substituents, or the structurally similar picric acid. Typically these quionone type catalysts are supplied to the process in the form of sodium salts or together with ions of metals such as iron or cobalt.

The present disclosure involves a method to convert the poor quality sulfur and waste liquid into a commercial quality concentrated sulfuric acid by incineration followed by treatment in a sulfuric acid plant. All of the H2S captured in the desulfurization unit is converted to sulfuric acid, not only the elemental sulfur formed.

According to present disclosure the sulfur and waste liquid is fed to an incinerator together with a support fuel which may be coke oven gas, whereby the elemental sulfur is oxidized to SO2 and the sulfur containing NH4+ salts are decomposed into S02, CO2, N2 and possibly some N0X. The oxygen required for the combustion may be in the form atmospheric air, pure oxygen or any other gas rich in oxygen. This combustion air may beneficially be preheated, e.g. by using cooling air of the condenser, since this may reduce the amount of support fuel required.

To avoid ash formation in the incinerator it is important that the coke oven gas desulfurization process is NH3 based and that the liquid catalyst is of organic origin without presence of metals, such that substantially no inorganic dust is present in the gas after incineration. This will simplify the process layout and reduce cost of the sulfuric acid plant.

After the incineration step the process gas may be cooled to 380 - 420 °C in a fire tube waste heat boiler, producing saturated steam.

The ΝΟχ formed by incineration may be catalytically reduced in a process for selective catalytic reduction (SCR) of NOx by NH3, which contacting the process gas with a catalyst comprising a carrier, such as titanium oxide, and active catalytic components which usually are either oxides of base metals (such as vanadium, molybdenum and tungsten), zeolites, or various precious metals. The NH3 needed for the reaction may often be obtained from a waster washing process step where NH3 is removed from the coke oven gas.

Downstream the SCR process the process gas enters the SO2 reactor where SO2 is oxidized by an appropriate catalyst, such as a vanadium-based S02 conversion catalyst supported on silica.

The SO2 oxidation may take place in two or three catalytic beds with inter bed cooling; the exact layout depends on the SO2 concentration in the process gas and the required SO2 conversion. After the last bed of SO2 conversion catalyst the gas is cooled in the process gas cooler, producing saturated steam.

One or more dust guard catalyst layers can be installed above the first catalyst layer in the SO2 reactor, as it is described in EP 1 114 669. Such guard layers will prolong the up-time of the sulfuric acid plant in the case that small amounts of inorganic ash are present in the process gas .

The interbed coolers superheat the steam produced in the waste heat boiler and process gas cooler and the valuable steam can be exported to other processes.

The process gas finally enters a sulfuric acid condenser where the sulfuric acid is condensed by cooling with air and separated from the cleaned process gas. The hot cooling air from the sulfuric acid condenser may be used as an oxygen source in the combustor, thus increasing the heat recovery and minimizing support fuel consumption.

Figures

Figure 1 shows a flow sheet of a process plant according to the present disclosure, and

Figure 2 shows a flow sheet of a process plant according to a preferred embodiment of the present disclosure.

According to a simple embodiment of the present disclosure as illustrated in Figure 1, a coke oven gas 1 is directed to an alkaline absorber 2 fed with an alkaline solution of catalyst 4. On the gas side a desulfurized coke oven gas 3 is released from the absorber, and on the liquid side waste liquid 5 comprising sulfur. In a reactor 6 the waste liquid reacts to form elemental sulfur and sulfur oxygen compounds in form of a slurry which is skimmed from the waste liquid in a mechanical separator 8 into a sulfur sludge 12 and waste liquid 9. The waste liquid is directed to an incinerator with the sulfur sludge 12, a support fuel 17, such as coke oven gas, and hot combustion air 34. The waste liquid 13 may be replenished with catalyst solution 16 and directed to the alkaline absorber.

The incinerated sulfur sludge and waste liquid forms a process gas 19, which is cooled in a waste heat boiler 20, and directed to a SO2 oxidation reactor in this case comprising a single catalytic bed (25) and an outlet heat exchanger 27. The SO3 in the oxidized gas 28 reacts immediately with water to form H2S04 which is condensed as concentrated sulfuric acid 30 in a condenser 29, cooled with air 32. The heated air 33 from the condenser 29 may be combined with the process air 31 released from the condenser which will have a very low sulfur concentration, and may be released to the atmosphere in a stack 36.

Figure 2 shows a flow sheet of a process plant according to a preferred embodiment of the present disclosure. A coke oven gas 1 is directed to an alkaline absorber 2 fed with an alkaline solution of catalyst 4. On the gas side a desulfurized coke oven gas 3 is released from the absorber, and on the liquid side waste liquid 5 comprising sulfur. In a reactor 6 the waste liquid reacts to form elemental sulfur and sulfur oxygen compounds in form of a slurry which is skimmed from the waste liquid in a mechanical separator 8. The slurry may be separated further in 11 into a concentrated sulfur sludge 12 and waste liquid 13. A purge stream 14 may be separated from the waste liquid and directed to an incinerator with the sulfur sludge 12, support fuel 17, such as coke oven gas, and hot combustion air 34. The remaining waste liquid 15 may be replenished with catalyst solution 16 and directed to the alkaline absorber.

The incinerated sulfur sludge and waste liquid forms a process gas 19, which is cooled in a waste heat boiler 20, and is combined with ammonia 21 and directed to a selective catalytic reduction reactor 22, in which NOx formed during incineration is selectively reduced to N2. Downstream the selective catalytical reactor the gas is directed to a SO2 oxidation reactor in this case comprising three catalytic beds (25a, 25b, 25c) and two inter-bed heat exchangers (26a and 26b) and an outlet heat exchanger 27. The SO3 in the oxidized gas 28 reacts immediately with water to form H2SO4 which is condensed as concentrated sulfuric acid 30 in a condenser 29, cooled with air 32. The heated air 33 from the condenser 29 may be directed as combustion air 34 for the incinerator 18. The process air 31 released from the condenser will have a very low sulfur concentration, and may be released to the atmosphere in a stack 36.

Examples

An example of a coke oven gas composition entering the desulfurization plant and the removal efficiencies of the desulfurization plant is shown in Table 1.

Table 1: Typical coke oven gas composition and removal efficiencies in the coke oven gas desulfurisation plant.

Example 1

In an example according to the present disclosure a process involving a liquid ammonia absorbent solution and quinone type H2S oxidation catalyst is used. According to this example no ash is formed in the incineration step and no filter is required for ash removal.

The key parameters for treatment of a typical coke oven gas (given in Table 1) in a wet sulfuric acid process have been calculated and shown in table 2. The overall process includes coke oven gas desulfurization, incineration of sulfur sludge and waste liquid and sulfuric acid production in a wet type sulfuric acid plant. A coke oven gas is cooled to approximately 30°C and contacts a liquid ammonia absorbent solution, which also contains a dissolved non-metallic quinone based catalyst. The alkaline absorber according to the example operates at approximately atmospheric pressure. In the reactor operating at about 30°C approximately 50% of the H2S is converted into elemental sulfur forming a liquid slurry. The remaining H2S is for simplicity considered as being NH4SCN and (NH4)2S203, the actual composition of NH4-salts will only slightly change the process gas after incineration. The liquid slurry is separated into a sulfur sludge and waste liquid. A bleed of the waste liquid is combined with the sulfur sludge, and directed to the incinerator, while the major part of the waste liquid is recycled to the absorber after being replenished with catalyst solution.

In the incinerator all S-atoms are oxidized to S02. The NH4+-ion is primarily decomposed to N2 and H20, but a small fraction is oxidized to NO.

To maintain a combustion temperature around 1100°C in the incinerator, around 0.5 % of the coke oven gas is used for support fuel - the coke oven gas can be withdrawn upstream the desulfurization plant.

The process gas is cooled to about 400°C in an ordinary waste heat boiler, producing saturated high pressure steam.

The NO formed by NH4+ decomposition in the incinerator is removed by reaction with NH3 over a SCR catalyst before the process gas enters the wet type sulfuric acid plant. A wide range of SCR catalysts are known to the person skilled in the art typically comprising a carrier, such as titanium oxide, and active catalytic components taken from the group consisting of oxides of base metals such as vanadium, molybdenum and tungsten, zeolites, and precious metals. This removal of NO may be omitted, with no effect on sulfur removal, but naturally this would result in an undesired emission of N0X.

In the wet type sulfuric acid plant, SO2 is oxidized in three catalyst beds, separated with inter bed coolers to provide an optimum between SO2 oxidation reaction rates and S02 to SO3 equilibrium. The temperature at the inlet of all of the three beds is approximately 400°C. An overall SO2 conversion of 99.67% is achieved, resulting in a SO2 concentration of 400 mg/Nm3 in the cleaned process gas. If one or two beds were used, the overall conversion would be slightly lower.

In the sulfuric acid condenser, the process gas is cooled in glass tubes by heat exchange with air flowing on the shell side. Concentrated sulfuric acid is withdrawn from the bottom of the condenser. A fraction of the hot air (240°C) from the sulfuric acid condenser is used as combustion air, thus increasing the heat efficiency of the process and saving support fuel. By using regular atmospheric air, the required amount of support fuel would be increased.

The heat withdrawn by cooling the process gas in waste heat boiler, inter bed coolers and process gas cooler is converted into valuable high pressure superheated steam.

Table 2: Key process parameters for coke oven gas desulfu-risation and sulfuric acid production from the effluent of the desulfurisation plant.

Example 2

In a comparative example, according to the prior art, a commonly used coke oven gas desulfurization process layout involves using NaOH to control the pH in the absorber solution. The salts formed by side reactions are primarily NaSCN, Na2S2C>3 and Na2SC>4. In the incinerator these salts are decomposed and form NaHSCg, Na2SC>4 and/or Na2S03, depending on the process gas conditions. These compounds have high fouling potential in the downstream waste heat boiler and the boiler must be designed to handle this, either by installing cleaning devices (such as soot blowers as described in CN101033060) or by using larger and less effective tubes.

According to Example 2, the process operates under conditions and in a manner similar to Example 1. Approximately 50% of the H2S is converted into elemental sulfur in the desulfurization plant.

Using the coke oven gas characteristics from Table 1 and assuming that the Na-salts are converted to Na2SC>4 in the incinerator, the process characteristics of the sulfuric acid plant treating the sulfur sludge and waste liquid are given in Table 3. In the incinerator, the O2 source is hot air from the wet sulfuric acid plant and coke oven gas is used as a support fuel to reach 1100 °C in the incinerator; the use of hot air saves coke oven gas consumption. A total of 825 kg/h Na2S04 salt is withdrawn in the hot filtration unit; the sulfur bound in the salt decreases the possible sulfuric acid production by 25%. To avoid or minimize the plugging of the catalyst in the wet sulfuric acid plant, the dust emissions from the filter must be 2 mg/Nm3 or less, requiring very high removal efficiency. SO2 is oxidized in three catalytic layers, each separated with heat exchangers to control the process gas temperature for optimal conversion of the SO2 in the catalysts. The heat exchangers either produce saturated high pressure steam or superheat the high pressure steam.

The 5500 kg/h high pressure steam export assumes that the process gas from the incinerator can be cooled to 400 °C without causing excessive fouling in the waste heat boiler.

Table 3: Key process parameters for coke oven gas desulfu-risation and sulfuric acid production from the effluent of the desulfurisation plant, using NaOH for pH control.

Claims (14)

1. A process for the production of sulfuric acid from a gas comprising H2S characterized by the following steps : a. directing said gas to an alkaline absorber containing a liquid ammonia absorbent solution and a combustible material catalytically active in oxidation of H2S to elemental sulfur providing a liquid slurry comprising elemental sulfur and waste liquid, b. separating at least a portion of said elemental sulfur from waste liquid by mechanical means of separation as a sulfur sludge, c. combining said sulfur sludge with an ash-free support fuel and a first gas comprising oxygen in or upstream an incinerator, d. incinerating said mixture, providing an incinerated gas comprising S02, e. optionally adding a second gas comprising oxygen to said incinerated gas, f. contacting said process gas with a material active in oxidation of S02 to SO3 providing an oxidized process gas, g. optionally adding water to said oxidized process gas, and h. condensing concentrated sulfuric acid formed in said oxidized process gas, by heat exchange with a cooling medium such as atmospheric air, providing a desulphurized process gas and a heated cooling medium.

2. A process according to claim 1 in which the content of metal in said liquid ammonia absorbent solution is less than 50 ppm weight/weight, preferably 10 ppm weight/weight and most preferably 3 ppm weight/weight.

3. A process according to claim 1 or 2 in which the content of particles in said incinerated gas is less than 20 mg/Nm3, preferably 10 mg/Nm3 and most preferably 2 mg/Nm3.

4. A process according to claim 1, 2 or 3 in which said combustible material catalytically active in oxidation of H2S to elemental sulfur is a quinone type compound, such as a benzoquinone, naphthoquinone or anthraquinone compound optionally with one or more non-metallic substituents.

5. A process according to claim 1, 2 or 3 in which said combustible material catalytically active in oxidation of H2S to elemental sulfur is picric acid.

6. A process according to claim 1, 2, 3, 4 or 5 further comprising the step of withdrawing a purge stream of said waste liquid.

7. A process according to claim 1, 2, 3, 4, 5 or 6 in which at least a part of said purge stream is directed to said incinerator for conversion of sulfur containing anions to S02 and S03.

8. A process according to claim 1, 2, 3, 4, 5, 6 or 7 in which said material active in oxidation of S02 to SO3 comprises vanadium.

9. A process according to claim 1, 2, 3, 4, 5, 6, 7 or 8 in which one or both of said first and second gas comprising oxygen is atmospheric air being preheated by heat exchange with a hot process stream such as the oxidized process gas.

10. A process according to claim 1, 2, 3, 4, 5, 6, 7, 8 or 9 further comprising the step of cooling said incinerated gas to a temperature in the range of 300 to 450°C, preferably 380 to 420°C.

11. A process according to claim 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10, in which N0X is present in said incinerated gas, further comprising the process steps: i. combining said incinerated gas with a gas comprising NH3 forming an SCR feed gas, and j . contacting said SCR feed gas with a material catalyt-ically active in selective reduction of NOx.

12. A process according to claim 11 wherein the material catalytically active in selective reduction of NOx comprises a carrier, such as titanium oxide, and active catalytic components taken from the group consisting of oxides of base metals such as vanadium, molybdenum and tungsten, zeolites, and precious metals .

13. A process plant comprising an alkaline absorber configured for contacting a gas comprising H2S with a liquid ammonia absorbent solution and a combustible material catalytically active in oxidation of H2S to elemental sulfur providing a liquid slurry comprising elemental sulfur and waste liquid, a means for mechanical separation configured for separating at least a portion of said elemental sulfur from waste liquid by mechanical means of separation as a sulfur sludge, an incinerator configured for incinerating said sulfur sludge together with an ash-free support fuel and a first gas comprising oxygen, providing a process gas, a sulfur dioxide converter comprising a material active in oxidation of SO2 to SO3 and configured for receiving said process gas and providing an oxidized process gas, a condenser, configured for receiving said oxidized process gas and a cooling medium and providing a concentrated sulfuric acid, a desulfurized process gas and a heated cooling medium.

14. A process plant according to claim 13 which does not include a gas filter operating at a temperature above 400 °C.