DK201600437A1 - A process and a plant for the production of sulfuric acid from a coke oven gas desulfurisation product. - Google Patents

Abstract

A process for the production of sulfuric acid from a gas containing H2S and HCN is carried out in a process plant comprising an alkaline absorber, means for separation, an incinerator, a particulate removal device for separating solid particles from the process gas, and means for provision of concentrated sulfuric acid. The plant can further comprise means for obtaining a reduced NOx content and a sulfur dioxide converter comprising a material active in oxidation of SO2 to SO3 eventually being converted to sulfuric acid.

Description

Title: A process and a plant for the production of sulfuric acid from a coke oven gas desulfurisation product

The present invention relates to a process for removal of hydrogen sulfide and hydrogen cyanide from coke oven gas with associated production of concentrated sulfuric acid. More specifically, the invention relates to a process for the production of sulfuric acid from a gas comprising H2S and HCN.

When coke oven gas is produced from coal, significant amounts of sulfur and nitrogen are also released, mainly 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 vanadium, cobalt or iron to promote the catalytic 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 H2S04 and collected as concentrated sulfuric acid. Examples of such configurations may be found in the patents CN 101033060, CN 100497165 and CN 101092577 which involve dry filters and a wet sulfuric acid process, as well as CN 101734629 and CN 103072957 which involve a wet process gas cleaning unit and a dry gas sulfuric acid process.

In its broadest form, the present invention relates to a process for the production of sulfur trioxide from a gas comprising H2S and HCN, said process comprising the following steps : (a) directing said gas to an alkaline absorber containing a liguid ammonia absorbent solution and a dissolved material catalytically active in oxidation of H2S to elemental sulfur providing a liguid slurry comprising elemental sulfur and coke oven gas absorber liquid, (b) separating said elemental sulfur from coke oven gas absorber liquid by mechanical means of separation or by phase separation, (c) feeding said elemental sulfur stream and said coke oven gas absorber liquid in separate lines with a support fuel and a first oxidant gas in or upstream an incinerator, (d) incinerating said mixture, providing an incinerated gas comprising S02, (e) removing inorganic dust in a filter unit, and (f) contacting said process gas with a material catalytically active in oxidation of S02 to S03 providing an oxidized process gas.

The invention further relates to a process for the production of sulfuric acid comprising the above process steps (a)-(f) and a further step of: (g) withdrawing concentrated sulfuric acid by either cooling and condensation or by absorption of S03 in H2S04, providing oleum or concentrated sulfuric and a sulfur reduced process gas, and still further it relates to a process comprising the above process steps (a)-(f), in which the incinerated gas further comprises NOx, said process further comprising the steps of: (h) combining said incinerated gas with a stream comprising a ΝΟχ selective reductant taken from the group comprising ammonia, urea and precursors thereof, providing a process gas for selective reduction, and (i) reacting said process gas for selective reduction under selective N0X reduction conditions, providing a process gas having a reduced concentration of N0X, with the associated benefit of avoiding an emission of S02 and N0X and thus reducing the environmental impact of the process .

In a preferred embodiment, said material being catalytical-ly active in oxidation of H2S to elemental sulfur in step (a) is taken from the group comprising picric acid, thio-arsenate or quinones, such as a benzoquinone, naphthoquinone or anthraquinone, optionally together with one or more further constituents in ionic form, such as sodium, iron, vanadium or cobalt, with the associated benefit of catalysts having quinone structure or a similar structure 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 another embodiment, the process in step (a) further comprises the step of withdrawing a purge stream of said coke oven gas absorber liquid with the associated benefit of avoiding a buildup of sulfur containing ions such as S2032', SCN“ and SCq2- in the coke oven gas absorber 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 S03 with the associated benefit of the purge stream being converted to a commercial sulfuric acid product instead of having to send the purge stream to a waste water treatment plant.

In a still further embodiment, step (b) is carried out at a temperature below 150°C, with the associated benefit of a low heating requirement providing a very energy efficient process .

According to another embodiment of the process, the elemental sulfur stream is a liquid, which is atomized into the incinerator by pneumatic (compressed air, steam), hydraulic or centrifugal means.

Other favourable embodiments of the process include the following: - The purged absorber liquid is concentrated by means of water evaporation before being introduced into the incinerator . - The purged absorber liquid is concentrated to a state, at which salts precipitate and can be withdrawn in their solid state . - The (concentrated) purged absorber liquid is atomized in the incinerator by pneumatic (compressed air, steam), hydraulic or centrifugal means. - One or both of said first and said second gas comprising oxygen is/are atmospheric air being preheated by heat exchange with a hot process stream such as the oxidized process gas. This has the associated benefit of contributing to an efficient thermal management of the process. - 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 that both the catalyst for selective NOx removal and the catalyst for S02 oxidation are highly active at this temperature. At the same time, any shifting of the equilibrium S02 + 0.5 O2 <-> SO3 towards S02 is avoided. - The process plant in step (f) further comprises a reactor downstream the addition of selective reductant, said reactor being configured for the process gas to contact a material, that is catalytically active in the selective reduction of NOx, with the associated benefit of the conversion of ΝΟχ to N2 being more efficient and less demanding in terms of process conditions. - The material being active in oxidation of S02 to SO3 in step (h) comprises vanadium with the associated benefit of being a highly effective and robust S02 oxidation catalyst. - The H20/S02 ratio in the oxidized process gas in step (i) is above 1.1, and the oxidized process gas is directed to a condenser for cooling and withdrawal of condensed concentrated sulfuric acid. Optionally the process also includes the step of addition of water in any position upstream said condenser, with the associated benefit of providing the required amount of water for hydratization of S03 to H2SC>4 and complete condensation of the sulfuric acid. - The process gas comprises less than 60 ppm water, and the oxidized process gas is directed to an absorber, wherein SO3 is absorbed in concentrated sulfuric acid and wherein concentrated sulfuric acid is withdrawn. The process also includes the step of removal of water in any position upstream the material, that is catalytically active in the oxidation of S02 to S03, with the associated benefit of providing the correct conditions for acid mist free SO3 absorption into concentrated sulfuric acid in the absorber and significantly reducing the sulfuric acid dew point temperature in the process gas. - The means for withdrawal for sulfuric acid is an absorber configured for receiving said oxidized process gas and a concentrated sulfuric acid and for absorbing SO3 in said sulfuric acid providing a concentrated sulfuric acid and a desulfurized process gas, with the associated benefit of such a process plant being efficient in providing a highly concentrated sulfuric acid.

The present invention also relates to a process plant comprising an alkaline absorber configured for contacting a gas comprising H2S with a liquid ammonia absorbent solution and a dissolved material catalytically active in oxidation of H2S to elemental sulfur providing a liquid slurry comprising elemental sulfur and coke oven gas absorber liquid, a means for separation configured for receiving an amount of said liquid slurry and configured for separating said elemental sulfur from coke oven gas absorber liquid by means of separation as an sulfur stream and a coke oven gas absorber liquid stream, an incinerator configured for receiving and incinerating said elemental sulfur and coke oven gas absorber liquid introduced via separate lines together with an support fuel and a first gas comprising oxygen, providing a process gas, a means of cooling such as a waste heat boiler for reducing the process gas temperature, a particulate removal device for receiving said process gas and separating solid particles from the process gas, and means for provision of concentrated sulfuric acid configured for receiving said oxidized process gas and for providing a concentrated sulfuric acid and a desulfurized process gas.

Optionally the process plant further comprises: means for addition of a selective N0X reductant into said process gas, providing a process gas having a reduced N0X content, and a sulfur dioxide converter configured for receiving said process gas having a reduced N0X content, and comprising a material active in oxidation of SO2 to S03 and configured for receiving said process gas and providing an oxidized process gas.

Coke oven gas (COG) from the gasification of coal goes through several cleaning steps before it can finally 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 a fraction of 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 will 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 water soluble salts, including (NH4)2S203, (NH4)2S04 and NH4SCN, may be formed in the liquid phase accounting for up to half of the 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, Takahax, Thylox, Fumaks-Rhodacs and Stretford. The differences are primarily within the catalyst system and the 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 or thioarsenate. Typically these quinone type catalysts are supplied to the process in the form of sodium salts or together with ions of metals such as iron, vanadium or cobalt.

The present disclosure involves a method to convert the poor quality sulfur and purged coke oven gas absorber 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 can be converted to sulfuric acid, not only the elemental sulfur formed in the coke oven gas desulfurization plant.

In the coke oven gas desulfurization plant, some of the captured HCN and H2S will be converted into water soluble salts, of which S2032“, SCN“ and S042~ are the most abundant, but other sulfur containing ions will also be present in low concentrations. The formed elemental sulfur is easily separated from the aqueous solution, whereas the concentration of salts will increase over time if a purge stream is not withdrawn from the desulfurization plant. A too high salt concentration will result in decreased desulfurization efficiency and risk of salt precipitation in the plant, with the associated risk of plugging pipes and equipment.

If the purge stream is too large, there is an increase in loss of catalyst and consumption of fresh water. The optimal salt concentration in the absorber liquid is around 300 g salts/liter.

By mechanical means, such as settling, centrifugation or filtration, the elemental sulfur can be separated from the liquid phase. This separation is not very efficient, and some liquid will be present in the sulfur phase. This phase can be further separated in a so-called autoclave unit, in which the elemental sulfur is melted and the two liquid phases separate better and can be withdrawn as two relatively pure phases.

The elemental sulfur phase can be cooled to solidify and then stored and/or sold as a solid. The quality of the sulfur is not high, as impurities from the absorber liquid are still present.

The purged absorber liquid can be concentrated, such that salts precipitate and can be sold as solid NH4SCN, (NH4)2S04 and (NH4) 2S2O3 * Often the demand and price for these salts are too low for the process to be economical, and therefore this purge stream could be problematic to handle. Conver sion into sulfuric acid will however make this liquid a saleable product.

As in the prior art, described in CN 101092577, the mixture of sulfur and absorber liquid are fed to an incinerator via a single feed line, and the so-called slurry is atomized into the incinerator by means of compressed air. A more flexible solution is presented here: The two streams, separated in the so-called autoclave unit, are kept separated until they are fed into the incinerator, which will allow optimal preparation and injection of the two feed streams before being incinerated.

The sulfur phase from the autoclave is kept in the liquid phase, or melted in case an efficient mechanical separation device is used, and fed into the incinerator as a liquid. Liquid sulfur atomization is well known in the industry and can be done in several ways - either with pneumatic nozzles using steam or compressed air to split up the liquid sulfur phase, with hydraulic nozzles, where the liquid pressure will split up the liquid phase, or with a rotating cup/disc in which centrifugal forces will split up the liquid phase. All three atomization devices are used, and the choice depends on several factors, such as nozzle capacity, turndown requirements and incineration chamber design.

If desired, a storage tank for the melted sulfur can be added, from which sulfur can be withdrawn and fed to the incinerator at a constant rate. This is desirable if the coke oven gas production leads to variable amounts of coke oven gas with varying amounts of HCN and H2S in the gas. In this way, the operation of the sulfuric acid plant can be stabilized, ensuring optimal performance of the plant.

The purged absorber liguid contains a lot of water, which ends up in the process gas leaving the incinerator, increasing the process gas flow but also the support fuel consumption, as the heat of vaporization of water is substantial and the reguired energy must be supplied via the support fuel. Furthermore the concentration of the sulfuric acid produced in the sulfuric acid plant will increase when the water concentration in the process gas decreases.

The absorber liquid can undergo several process steps before being introduced into the incinerator via a separate line with an atomization system designed for the liquid properties .

If low grade heat is available, the absorber liquid can be concentrated by means of water evaporation, either at atmospheric pressure or at a reduced pressure. A concentrated absorber liquid will reduce support fuel consumption and process gas flow in the incinerator.

Further evaporating of water from the absorber liquid will eventually result in precipitation of salts, of which NH4SCN, (NH4)2S04 and (NH4)2S203 will be the major salts.

They can be purified and sold if there is a demand for these salts. The precipitation can be partly, i.e. only precipitating the salts of interest. Then the concentrated liquid can be fed into the incinerator, further reducing support fuel consumption, process gas flow but also sulfuric acid production.

The absorber liquid can be fed into the incinerator with atomizing nozzles as described for the liquid sulfur, i.e. by pneumatic, hydraulic or centrifugal principles.

If large amounts of low grade heat are available, the absorber liquid can be completely dried and fed into the incinerator as a powder. This application requires special nozzles for introduction of the powder.

Keeping separate feed lines of sulfur and (concentrated) absorber liquid has the following advantages: 1. a more stable operation of the sulfuric acid plant as the sulfur flow can be adjusted as desired, provided that a sulfur tank is provided 2. option to precipitate valuable salts from the absorber liquid by means of evaporation of water from the absorber liquid 3. option to reduce support fuel consumption, process gas flow and thus sulfuric acid plant size by evaporating water from the absorber liquid, and 4. use of nozzles that are optimal for the atomization of the two stream, which have different flow properties .

According to the present invention, the sulfur and purged (and optionally concentrated) coke oven gas absorber liquid is fed to an incinerator via separate feed lines. Support fuel, which may be coke oven gas and oxygen, is fed to an incinerator, whereby the elemental sulfur is oxidized to SO2 and the sulfur-containing NH4+ salts are decomposed into S02, C02, N2 and N0X. The oxygen required for the combustion may be in the form of atmospheric air, pure oxygen or any other gas rich in oxygen. This combustion air may beneficially be pre-heated, e.g. by using heated cooling air from the sulfuric acid condenser and optionally further preheated air in a designated combustion air heat exchanger, as this will reduce the amount of support fuel required.

After the incineration step, the process gas may be cooled to 380-420°C in a waste heat boiler, producing saturated steam. Alternatively the process cooling can be carried out in a combination of a waste heat boiler, which produces saturated steam, with a combustion air heater, pre-heating combustion air to the incinerator.

The process gas then passes through a filtration device, which removes practically all dust from the process gas. If not removed, the dust will eventually plug the catalyst beds in the downstream S02 reactor. The filtration device could e.g. be an electrostatic precipitator or candle filters .

The ΝΟχ formed by incineration is reduced in a process for selective reduction of N0X by NH3, by 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 (selective catalytic reduction (SCR)). The NH3 needed for the reaction may be obtained from a washing process step, where NH3 is removed from the coke oven gas. Anhydrous ammonia, ammonia water (NH4OH) and urea ((NH2)2C0) are also frequently used as the source of NH3 for the SCR reaction.

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

The S02 oxidation may take place in two or three catalytic beds with inter-bed cooling; the exact layout depends on the S02 concentration in the process gas and the required S02 conversion. After the last bed of S02 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 S02 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 inter-bed coolers superheat the steam produced in the waste heat boiler and the process gas cooler, and the valuable superheated 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 then separated from the cleaned process gas. The hot cooling air from the sulfuric acid condenser may be used as an oxygen source in the incinerator, either as it is or further pre-heated in a combustion air heat exchanger, thus increasing the heat recovery and minimizing support fuel consumption.

The invention is further explained with reference to the Figures, where

Fig. 1 shows a flow sheet of a process plant according to the prior art,

Fig. 2 shows a flow sheet of a process plant according to a preferred embodiment of the present invention, and

Fig. 3 shows the details of the introduction of elemental sulfur and absorber liquid to the incinerator.

In a process according to the prior art, as illustrated in Fig. 1, a coke oven gas 1 is directed to an alkaline absorber 2 fed with an alkaline solution with dissolved catalyst 4. On the gas side, a desulfurized coke oven gas 3 is released from the absorber, and on the liquid side a coke oven gas absorber liquid 5, comprising elemental sulfur and dissolved ions comprising sulfur, is withdrawn. In a reactor 7, the coke oven gas absorber liquid is continuing the sulfur forming reaction, and the catalyst is re-oxidized by means of air addition 14. The formed slurry is skimmed from the coke oven gas absorber liquid in a mechanical separator 11 into a sulfur sludge 15 and a coke oven gas absorber liquid 12. The sulfur sludge, including the elemental sulfur and part of the coke oven gas absorber liquid 15, is directed to an incinerator 30 with a support fuel 31, such as coke oven gas, and (hot) combustion air 61. The coke ov- en gas absorber liquid 12 is directed back to the alkaline absorber, possibly after being replenished with catalyst and alkaline solution 13. If the implicit purge of coke oven gas absorber liquid through line 15 is insufficient, an extra purge line 29 is required to maintain an acceptable low concentration of dissolved sulfur-containing ions (e.g. S2032~, S042-, SCN“) in the coke oven gas absorber liquid.

The incinerated sulfur sludge and the coke oven gas absorber liquid form a process gas 31, which is cooled in a waste heat boiler 32, and directed to a hot filtration device 36, that efficiently removes the dust formed from the inorganic compounds present in the feed to the incinerator, e.g. inorganic catalyst compounds, dissolved ions from the process water, corrosion products and compounds absorbed from the coke oven gas .

The dust free process gas 38 enters the S02 oxidation reactor 44, in this case comprising three catalytic beds (45a, 45b and 45c), two inter-bed coolers (46a and 46b) and an outlet heat exchanger 47. The S03 in the oxidized process gas 50 reacts immediately with water present in the oxidized process gas (or optionally added to the oxidized process gas) to form H2S04 which is condensed as concentrated sulfuric acid 52 in a condenser 51, cooled with air 58. A fraction of the heated air 59 from the condenser 51 may be used as combustion air 61 in incinerator 30. The remaining hot air can be combined with the clean gas 53 released from the condenser, which will have a very low sulfur concentration, and may be released to the atmosphere via a stack 56.

Fig. 2 shows a flow sheet of a process plant according to a preferred embodiment of the present invention: A coke oven gas 1 is directed to an alkaline absorber 2 fed with an alkaline solution comprising a catalyst 4. On the gas side, a desulfurized coke oven gas 3 is released from the absorber, and on the liguid side, coke oven gas absorber liguid 5, containing elemental sulfur and dissolved ions comprising sulfur, is withdrawn. In a reactor 7, the coke oven gas absorber liguid reacts to form elemental sulfur and sulfur containing ions in form of a slurry, which is skimmed from the coke oven gas absorber liguid in a mechanical separator 11.

The slurry 15 may be separated further (in a separator 16) into a concentrated sulfur stream 25 and a coke oven gas absorber liquid 17. If the purge stream of coke oven gas absorber liquid is not sufficient to keep the salt concentration at a stable value, a fraction of the stream 12 can be combined with stream 17.

The purged coke oven gas absorber liquid 17 then passes through an optional treatment plant 18, in which water is withdrawn (19) from the liquid, thus increasing the concentration of salts in the liquid. The simplest way of withdrawing water from the liquid is by evaporation, where heat must be supplied, preferably by means of low grade heat such as hot water, hot air and/or low pressure steam. If the liquid is further concentrated, salts will precipitate, and they can be withdrawn (20) and sold.

The (concentrated) absorber liquid is then introduced into the incinerator via line 21 and atomized, either by means of compressed air or steam via line 28, or by hydraulic or centrifugal principles.

The elemental sulfur stream 25, preferably in the liquid phase, is directed to an optional and storage tank 26, from which the sulfur can be withdrawn and introduced into the incinerator 30 via line 27. The atomization of the sulfur can be carried out by means of compressed air or steam via line 28 or by hydraulic or centrifugal principles.

In the incinerator 30, the sulfur, absorber liquid, air and support fuel are combined, and the combination reacts to from a process gas 31, which is cooled in a waste heat boiler 32 and (optionally) further cooled in a combustion air heat exchanger 34 . Thereafter, the process gas 35 passes through a filtration device 36, in which the inorganic and possibly organic dust, formed in the incineration process, is separated from the process gas and withdrawn.

The cleaned process gas 38 is mixed with a stream containing ammonia 39 and directed to a selective catalytic reduction reactor 42, in which NOx formed during incineration is selectively reduced to N2 in the presence of a material, that is catalytically active in selective reduction of NOx, providing a process gas having a reduced concentration of NOx. Downstream the selective catalytic reactor the process gas 43 having a reduced concentration of NOx is directed to a S02 oxidation reactor 44, in this case comprising three catalytic beds (45a, 45b and 45c) and two inter-bed heat exchangers (46a and 46b) as well as an outlet heat exchanger 47. The SO3 in the oxidized gas 50 reacts immediately with water to form H2S04 which is condensed as concentrated sulfuric acid 52 in a condenser 51, cooled with air 58. A fraction of the heated air 59 from the condenser 51 is directed to the combustion air heat exchanger 34, and the hot air 61 is then directed to the incinerator 30. The clean gas 53 released from the condenser will have a very low sulfur dioxide concentration, and so it may be released to the atmosphere via a stack 56, possibly after dilution with heated air 62, not required in the incinerator.

In one embodiment of the present invention, the process does not employ cooling and condensation, but instead uses an absorber, in which S03 is absorbed in sulfuric acid. Typically this embodiment is related to configurations in which the process gas is dried, often downstream the SCR reactor, since water is produced during incineration and selective reduction of NOx.

The invention is illustrated further in the examples which follow.

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 desulfurization plant

Example 1

In an example according to the present invention, a process as shown in Fig. 2, involving an aqueous ammonia absorbent solution and a quinone type H2S oxidation catalyst, is used. The key parameters for treatment of a typical coke oven gas (given in Table 1) in a wet sulfuric acid process are shown in table 2. The overall process includes desulfurization of coke oven gas, pre-treatment of effluent from the desulfurization plant, incineration of elemental sulfur sludge and coke oven gas absorber liquid and sulfuric acid production in a wet type sulfuric acid plant. A coke oven gas is cooled to approximately 30°C and contacted with a liquid ammonia absorbent solution, which also contains a dissolved quinone-based catalyst. The alkaline absorber according to this 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 further oxidized to a large number of sulfur and oxygen containing ions, for simplicity considered as being NH4SCN and (NH4)2S2C>3. Deviations in the actual composition of NH4 salts will only slightly influence the composition of the process gas after incineration. The liquid slurry is sepa rated into a sulfur sludge and the coke oven gas absorber liquid. The sulfur is kept in a melted stage and introduced to the incinerator through a separate feed line, the atomization system being optimized for the specific properties of the liquid sulfur.

The purge of coke oven gas absorber liquid is fed to the incinerator through a separate feed line, the atomization system being optimized for the specific properties of the liquid feed. The purge stream can also be pre-treated before being fed to the incinerator, and examples of water evaporation and NH4SCN withdrawal are shown in Table 2.

In the incinerator, all S atoms are oxidized to SO2. The NH4+ ion is primarily decomposed to N2 and H20, but a small fraction will be oxidized to NO.

To maintain a combustion temperature around 1000°C in the incinerator, 0-0.3 % 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 a combination of the waste heat boiler, producing saturated high pressure steam, with a combustion air heater, pre-heating the cooling air form the sulfuric acid condenser. Excluded from that is the example with a very dry salt feed, where the heat content of the feed is sufficiently high to sustain the 1000°C incinerator temperature with 50°C combustion air.

Dust (in low concentration) is removed in a filtration device operating at 380-420°C, such as an electrostatic precipitator or candle filters.

The NO formed by NH4+ decomposition in the incinerator is removed by reaction with NH3 over an SCR catalyst before the process gas enters the wet type sulfuric acid plant.

In the wet type sulfuric acid plant, S02 is oxidized in three catalyst beds, separated with inter-bed coolers to provide an optimum between S02 oxidation reaction rates and S02 to S03 equilibrium. The temperature at the inlet of all of the three beds is approximately 400°C. The overall S02 conversion efficiency is usually between 99.0 and 99.9 %.

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; the concentration of the acid depends on the H20/SC>3 ratio in the process gas entering the condenser. A fraction of the hot air (200-240°C) from the sulfuric acid condenser is used as combustion air, either by feeding it directly to the incinerator or further pre-heating the air to 350-450°C in a designated combustion air pre-heater. Increasing the combustion air temperature increases the heat efficiency of the process, thus saving support fuel.

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

In Table 2 below, four cases are calculated and the cases are characterized by: • Base case: all elemental sulfur and the purge of absorber liquid are fed into the incinerator as received from the coke oven gas desulfurization plant. • Case 1: the purge absorber liquid is sent to an evaporation plant, in which 30% of the water is evaporated. Low grade heat is used for the evaporation . • Case 2: the purge absorber liquid is further refined such that 50% of the valuable NH4SCN salt is precipitated. The rest of the concentrated liquid is fed to the incinerator. • Case 3: the purge absorber liquid is almost fully dried and sent to the incinerator as a salt paste or wet powder.

Table 2

Key process parameters for a 100000 Nm3/h coke oven gas desulfurization plant and sulfuric acid production from the effluent of the desulfurisation plant

From the results in table 2 it is seen that a lot of water is being introduced to the incinerator, requiring significant heat input to evaporate the water and also increasing the process gas flow. Most of this energy is wasted as the water primarily exits the sulfuric acid plant as vapor. Thus, reducing the water injection rate will increase the heat efficiency of the plant, as less support fuel is required, but also reduce the size of the sulfuric acid plant, as the process gas flow is decreased.

It is assumed that there is low grade heat (e.g. hot air, warm water, low pressure steam) available at low cost compared to the cost of the coke oven gas.

In case 1, the support fuel consumption is reduced by 78% by evaporation of 30% of the water in the absorber liquid. The process gas flow is decreased by 17% while maintaining the full sulfuric acid production.

In case 2, the withdrawal of 50% of the NH4SCN both reduces the water intake and the NH4SCN intake, providing the lowest process gas flow of the four cases. Also the sulfuric acid production is decreased by 21%.

In case 3 where almost all water from the absorber liquid is evaporated from the purge absorber liquid, the heating value of the feed is sufficiently high to sustain 1000°C incinerator temperature without need for support fuel.

Claims (17)

1. Claims :

   1. A process for the production of sulfur trioxide from a gas comprising H2S, said process comprising the following steps : (a) directing said gas to an alkaline absorber containing a liquid ammonia absorbent solution and a dissolved material catalytically active in oxidation of H2S to elemental sulfur providing a liquid slurry comprising elemental sulfur and coke oven gas absorber liquid, (b) separating said elemental sulfur from coke oven gas absorber liquid by mechanical means of separation or by phase separation, (c) feeding said elemental sulfur stream and said coke oven gas absorber liquid in separate lines with a support fuel and a first oxidant gas in or upstream an incinerator, (d) incinerating said mixture, providing an incinerated gas comprising S02, (e) removing inorganic dust in a filter unit, and (f) contacting said process gas with a material catalytically active in oxidation of S02 to SO3 providing an oxidized process gas.
2. 2. A process for the production of sulfuric acid comprising the process steps of claim 1 and a further step of (g) withdrawing concentrated sulfuric acid by either cooling and condensation or by absorption of SO3 in H2S04, providing oleum or concentrated sulfuric and a sulfur reduced process gas.
3. 3. Process according to claim 1, in which the incinerated gas further comprises NOx, said process further comprising the steps of: (h) combining said incinerated gas with a stream comprising a N0X selective reductant taken from the group comprising ammonia, urea and precursors thereof, providing a process gas for selective reduction, and (i) reacting said process gas for selective reduction under selective N0X reduction conditions, providing a process gas having a reduced concentration of N0X.
4. 4. Process according to claim 1, in which step (b) is carried out at a temperature below 150°C.
5. 5. Process according to claim 1, wherein the stream of elemental sulfur is in the liquid state and in the temperature range of 120-150°C.
6. 6. Process according to claim 1, wherein the elemental sulfur is introduced into the incinerator and atomized by pneumatic, hydraulic or centrifugal principles.
7. 7. Process according to claim 1, wherein the coke oven gas absorber liquid is introduced into the incinerator through a separate line and atomized by pneumatic, hydraulic or centrifugal principles.
8. 8. Process according to claim 1, wherein the coke oven gas absorber liquid is concentrated by removing water from the liquid prior to being introduced into the incinerator.
9. 9. Process according to claim 1, wherein salts are precipitated from the coke oven gas absorber liquid by removing water from the liquid prior to being introduced into the incinerator.
10. 10. Process according to claim 9, wherein the salts are NH4SCN, (NH4)2S203 or (NH4)2S04.
11. 11. A process plant comprising an alkaline absorber configured for contacting a gas comprising H2S with a liquid ammonia absorbent solution and a dissolved material catalytically active in oxidation of H2S to elemental sulfur providing a liquid slurry comprising elemental sulfur and coke oven gas absorber liquid, a means for separation configured for receiving an amount of said liquid slurry and configured for separating said elemental sulfur from coke oven gas absorber liquid by means of separation as an sulfur stream and a coke oven gas absorber liquid stream, an incinerator configured for receiving and incinerating said elemental sulfur and coke oven gas absorber liquid introduced via separate lines together with an support fuel and a first gas comprising oxygen, providing a process gas, a means of cooling such as a waste heat boiler for reducing the process gas temperature, a particulate removal device for receiving said process gas and separating solid particles from the process gas, and means for provision of concentrated sulfuric acid configured for receiving said oxidized process gas and for providing a concentrated sulfuric acid and a desulfurized process gas.
12. 12. Process plant according to claim 11, said plant further comprising: means for addition of a selective N0X reductant into said process gas, providing a process gas having a reduced N0X content, and a sulfur dioxide converter configured for receiving said process gas having a reduced N0X content, and comprising a material active in oxidation of S02 to S03 and configured for receiving said process gas and providing an oxidized process gas.
13. 13. Process plant according to claim 11, in which said means for withdrawal for sulfuric acid is a condenser configured for receiving said oxidized process gas and a cooling medium and configured for cooling and condensing a concentrated sulfuric acid, a desulfurized process gas and a heated cooling medium, and which process plant optionally comprises a means for addition of water upstream said condenser .
14. 14. Process plant according to claim 11, in which said means for withdrawal for sulfuric acid is an absorber configured for receiving said oxidized process gas and a concentrated sulfuric acid and for absorbing S03 in said sulfuric acid providing a concentrated sulfuric acid and a desulfurized process gas.
15. 15. Process plant according to claim 11, in which water is removed from said coke oven gas absorber liquid before introduced to the incinerator.
16. 16. Process plant according to claim 11, in which at least a portion of salt is precipitated and removed from said coke oven gas absorber liquid before being introduced into the incinerator.
17. 17. Process plant according to claim 11, in which the elemental sulfur is introduced into the incinerator in melted form.