FIELD

The application relates to a method defined in claim 1 and a process arrangement defined in claim 11 for removal of sulfur from gas, such as dry bed re moval of sulfur from gas. Further, the application re lates to a use of the method defined in claim 15.

BACKGROUND

Known from the prior art is to produce syn thesis gas, such as syngas, by means of gasification from biomass or coal wherein the synthesis gas typi cally also contains 5 to 5000 ppmv hydrogen sulfide, organic sulfur and 5 to 5000 ppmv ammonia. An acid gas comprising hydrogen sulfide is a catalyst poison and a corroding agent, and therefore the hydrogen sulfide needs to be removed from the syngas before various synthesis processes. Further, it is known from the prior art that acid gases can be removed by wet scrub bing. However, wet scrubbing of acid gases is often a capital and energy intensive process step, especially at smaller or medium scale. The hydrogen sulfide can selectively be removed with chemical or physical ad sorption, e.g. by means of metal oxide adsorbents, such as ZnO adsorbents, or modified activated carbons. The physical adsorption is believed to be the dominant mechanism for hydrogen sulfide uptake by the adsorbent materials in ideal conditions with vacuum, dry and in presence of no other gases. The modified activated carbons are processed high surface area carbons which are used, for example, in air, water and other gas pu rification applications. The activated carbon surface chemistry modification is an effective but expensive way to improve a removal capacity of non-regenerable adsorbents. The carbon surfaces can be modified with addition of impregnates in the manufacturing process. High capacity adsorbents, e.g. caustic impregnated carbons, are often too expensive for bulk hydrogen sulfide removal.

OBJECTIVE

The objective is to disclose a new type meth od and process arrangement for dry bed removal of sul fur from gas, e.g. from process gas with low to moder ate impurity levels, syngas or the like. Further, the objective is to disclose an effective method and pro cess arrangement for purifying the gas. Further, the objective is to remove sulfur by means of simple tech nology at ambient conditions. Further, the objective is to improve cost-effectiveness of the gas cleaning.

SUMMARY

The method and process arrangement and use are characterized by what are presented in the claims.

In the method and process arrangement, sulfur is removed from gas in a desulfurization device in which the removal of the sulfur is catalyzed by means of an ammonia of the gas and a standard activated car bon adsorbent.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a further understanding of the invention and constitutes a part of this specification, illus trate some embodiments of the invention and together with the description help to explain the principles of the invention. In the drawings:

Fig. 1 is a flow chart illustration of a pro cess according to one embodiment, and Fig. 2 shows results from labotarory-scale tests .

DETAILED DESCRIPTION

In the method for removing sulfur, e.g. hy drogen sulfide (H ₂ S) , from gas, the gas (1) which com prises ammonia (N¾) and sulfur, such as inorganic or organic sulfur, as impurities, and in one embodiment steam, is supplied to a desulfurization device (3) which comprises at least one bed comprising an acti vated carbon adsorbent, and the gas (1) is treated in the presence of oxygen in the desulfurization device (3) such that the sulfur of the gas reacts on the sur face of the activated carbon adsorbent, in more fa vourable reaction pathway and at improved reaction rate, with the oxygen in the presence of the catalyz ing ammonia of the gas, e.g. the catalyzing gaseous ammonia .

In this context, the sulfur means any sulfur or sulfur compound, e.g. inorganic or organic sulfur. In an embodiment the sulfur means sulfur compounds which can be oxidized to elemental sulfur. In one em bodiment, the sulfur is hydrogen sulfide, and then the hydrogen sulfide is removed from the gas. Alternative ly, the sulfur can be any other sulfur compound or sulfur-based compound, e.g. non-oxidized sulfur com pound .

In the method, the sulfur of the gas reacts, and most likely chemically reacts, with the oxygen by means of an irreversible oxidation reaction. The hy drogen sulfide of the gas (1) reacts with the oxygen by means of an irreversible oxidation reaction to ele mental sulfur according to the total reaction:

H ₂ S + 0.5 0 ₂ -> S + H ₂ 0

This reaction is catalyzed by the ammonia present in gas on the surface of the activated carbon. The oxida- tion of the hydrogen sulfide to elemental sulfur or any other sulfur compound significantly improves hy drogen sulfide uptake compared to physical adsorption. Other acidic sulfur gases, e.g. thiols, may oxidise in a similar path and thus experience similar improved removal rates. In the method, the impurity of the gas, i.e. ammonia, can be utilized to increase the rate of the adsorption and/or subsequent reaction kinetics of the hydrogen sulfide on the activated carbon surface. The ammonia may facilitate the dissociation of the hy drogen sulfide on the activated carbon surface. Then the improved adsorption capacity can be achieved. Fur ther, water, e.g. steam, adsorbed to the activated carbon surface enhances the hydrogen sulfide dissocia tion significantly and thus the removal of hydrogen sulfide from the gas. Further, a separate ammonia in jection is not required in the process.

The process arrangement for removing sulfur, e.g. hydrogen sulfide (¾S) , from gas (1) comprises at least one desulfurization device (3) which comprises at least one bed comprising an activated carbon adsor bent and to which the gas (1) which comprises ammonia and sulfur as impurities, and possibly also steam, is supplied, at least one oxygen or air feed device for adding oxygen (2) to the gas, and the gas (1) is ar ranged to flow through the bed in the desulfurization device (3) such that the sulfur of the gas reacts with oxygen on the surface of the activated carbon adsor bent in the presence of the catalyzing ammonia of the gas .

One embodiment of the method and the process arrangement is shown in Fig 1.

In this context, the gas (1) means any gas which comprises ammonia and sulfur as impurities. The gas is a gas from a process, boiler or industrial treatment. The gas mainly consists of other components, and further comprises ammonia and sulfur as impurities. In one embodiment, the gas comprises sulfur, such as hydrogen sulfide, as low to moderate impurity level (ppm level) . Further, the gas may comprise steam. For example, syngas may comprise hydrogen and carbon monox ide, and further comprise carbon dioxide, methane and steam. Further, the gas may contain also other spe cies. In one embodiment, the gas is a gas from a gasi fication. In one embodiment, the gas is selected from the group consisting of a gasification gas, synthesis gas, syngas, process gas, biogas, biomethane, flue gas, coke oven gas, gas from metal industry or their combinations. In one embodiment, the gas is process gas .

In this context, the gasification in a gasi fier means any gasification process. The gasification is a process that converts starting material into gas eous products. This is achieved by reacting the start ing material at suitable temperatures with incomplete combustion, with a controlled amount of steam and/or additional oxygen.

In this context, the activated carbon adsor bent means any non-impregnated virgin activated carbon adsorbent. The activated carbon manufacturing process, surface area or pore size can vary in the activated carbon which is used in this process. The activated carbon acts as the medium for the reactions between sulfur and oxygen.

In one embodiment, oxygen (2) is added to the gas. In one embodiment, the oxygen (2) is added to the gas (1) before the desulfurization device (3) or in the desulfurization device (3) . In one embodiment, ox ygen is added to the desulfurization device. In one embodiment, oxygen is added to a feed of the desulfu rization device. In one embodiment, the oxygen is pre sent in the gas. In one embodiment, the oxygen can be added in the form of oxygen or air. The process ar rangement comprises one or more oxygen or air feed de vice, e.g. an injection device or oxygen feed device, for adding oxygen to the gas. In one embodiment, the oxygen is added such that the amount of oxygen is min imum 4 - 6 times the amount of sulfur, such as hydro gen sulfide, in percent by volume.

In one embodiment, the desulfurization device (3) is a reactor or tank which comprises at least one bed and in which at least sulfur can be adsorbed and removed from the gas (1) . In one embodiment, the reac tor or tank is selected from a tubular flow reactor, horizontal tubular flow reactor or tank, vertical tub ular flow reactor or tank, fluidized bed reactor or tank or their combinations. In one embodiment, the bed is a packed bed, basket bed or their combination. The bed is formed from the activated carbon adsorbent. In one embodiment, the activated carbon adsorbent is in the form of particles, pellets, extrudates or granu lates in the bed. Alternatively, the activated carbon adsorbent can be in any other form.

In the desulfurization device (3) , the gas (1) reacts with the oxygen, the irreversible oxidation reaction is carried out on the adsorbent to provide elemental sulfur, and said reaction is catalyzed by the ammonia of the gas on the activated carbon sur face. Then sulfur can be removed with a higher rate from the gas allowing for a higher bed hourly space velocity, GHSV.

In one embodiment, the gas (1) is supplied through one or more beds in which the sulfur is re moved. In one embodiment, the gas (1) is supplied through two beds wherein the sulfur is removed in the first bed and the ammonia of the gas is removed in the second bed by appropriate or suitable gas removal ad sorbents. In one embodiment, the gas is supplied through an additional bed for desulfurization after the first bed as a polishing step. In one embodiment, the desulfurization device (3) comprises at least one additional bed for improving the removal of the sul- fur .

In one embodiment, the gas (1) is treated at a room temperature in the desulfurization device (3) . In one embodiment, the gas is treated at an around am bient temperature in the desulfurization device. In one embodiment, the gas is treated at a temperature of 10 - 70 °C. In one embodiment, the gas is treated at a temperature of 10 - 70 °C and at atmospheric pressure or higher pressure in the desulfurization device. In one embodiment, the gas is treated without an external heating in the desulfurization device.

In one embodiment, the relative humidity is 50 - 80 % during the treatment in the desulfurization device (3) for optimal removal efficiency. The mois ture or steam of the gas improves the adsorption in the desulfurization device.

In one embodiment, the gas hourly space ve locity, GHSV, is 3000 - 6000 1/h in the desulfuriza tion device (3) .

In one embodiment, the ammonia, N¾, is re- moved after the removal of the sulfur. In one embodi ment, the desulfurization device (3) comprises a sec ond bed comprising a suitable gas adsorbent or cata lyst, e.g. special activated carbon, for removing the ammonia from the gas after the bed or beds for removal of the sulfur. In one embodiment, the process arrange ment comprises a separate device for removing the am monia from the gas after the desulfurization device (3) .

In one embodiment, the gas (1) can be treated before the removal of the sulfur, e.g. before the desulfurization device (3) . In one embodiment, the am- monia can be partly removed before the removal of the sulfur to improve stoichiometrical relation to the sulfur concentration.

In one embodiment, an excess oxygen, such as an added oxygen, can be removed after the removal of the sulfur. In one embodiment, the process arrangement comprises an oxygen removal device (4) after the desul furization device (3) . In one embodiment, the oxygen is removed by using a suitable catalyst.

In one embodiment, a purified gas (5) can be supplied to a desired process, e.g. to a synthesis process, after the desulfurization.

In one embodiment, the process arrangement comprises at least one first feed inlet for supplying the gas (1) into the desulfurization device (3) . In one embodiment, the process arrangement comprises at least one first outlet for discharging the gas out from the desulfurization device (3) . The feed inlet may be any suitable inlet known per se, e.g. pipe, port or the like. The outlet may be any suitable out let known per se, e.g. pipe, outlet port or the like.

The method and process arrangement can be op erated as a continuous process.

In one embodiment, the method and process ar rangement are used and utilized in a gas purification process, a production of syngas, a purification of syngas, at treatment of carbon dioxide, biogas pro cess, biomethane process, production of biogas, iron industry, or their combinations.

Thanks to the invention, the gas cleaning process can be improved. Higher desulfurization capac ity at higher GHSV can be achieved. The improved ad sorption capacity at higher GHSV leads to smaller ad sorbent beds and lower process costs. In the present invention, cheap and standard virgin active carbons can be used to remove sulfur and to purify the gases. The method and process arrangement offer a possibility to purify the gases easily and cost-effectively.

The present invention provides an industrial ly applicable, simple and affordable way to remove sulfur from the gases. In the method of the invention, cheap and readily available standard activated carbon can be used as the effective ¾S adsorbent. Then spe cial activated carbons, such as microporous high sur face-area activated carbon or caustic impregnated or doped activated carbon, are not needed to remove sul fur. Further, the method and process arrangement are easy and simple to realize in connection with produc tion processes, and they allow improved process safety due to the lower temperature compared to, for example, caustic impregnated carbons.

EXAMPLES

Example 1

Fig. 1 presents the method and also process arrangement for removing hydrogen sulfide from the gas .

The process of Fig. 1 comprises a desulfuri zation device (3) which comprises at least one bed comprising an activated carbon adsorbent and to which the gas (1) which comprises ammonia and hydrogen sul fide as impurities, and also steam, is supplied. Fur ther, the process comprises an oxygen for adding oxy gen (2) to the gas (1) . The gas is arranged to flow vertically from top to bottom through the bed in the desulfurization device (3) such that the hydrogen sul fide of the gas reacts with oxygen by means of an oxi dation reaction, which is catalyzed with the ammonia of the gas on the activated carbon adsorbent. The oxygen (2) can be added to the gas (1) before the desulfurization device (3) or in the desul furization device (3) .

The desulfurization device (3) is a reactor or tank which is selected from a tubular flow reactor or tank, horizontal tubular flow reactor or tank, ver tical tubular flow reactor or tank, fluidized bed re actor or tank or their combinations. The bed is formed from particles, pellets, extrudates or granulates of activated carbon. The desulfurization device (3) can comprise at least one additional bed for improving the removal of the hydrogen sulfide.

The gas (1) is treated at an around ambient temperature and at atmospheric pressure in the desul furization device (3) . The relative humidity is 50 - 80 % during the treatment in the desulfurization de vice. GHSV is 3500 - 4500 1/h in the tubular desulfu rization device.

The desulfurization device (3) can comprise a second bed comprising suitable adsorbent or catalyst for removing the ammonia from the gas after the bed or beds for the removal of the hydrogen sulfide. Further, the injected oxygen can be removed after the removal of the hydrogen sulfide in an oxygen removal device (4) using, for example, copper- or platinum-based de oxygenation catalysts.

A purified gas (5) can be supplied to a de sired process, e.g. to a synthesis process, after the desulfurization .

Example 2

In this example, the syngas desulfurization process was studied in a laboratory-scale vertical tubular reactor comprising an activated carbon packed bed. The gas flow direction is from top to the bottom. An ammonia catalyzed adsorption of hydrogen sulfide was studied, and the results were compared with the comparative results from a non-catalyzed ad sorption. In the ammonia catalyzed adsorption, the syngas comprised the ammonia and hydrogen sulfide as impurities and steam. In the non-catalyzed adsorption, the syngas did not comprise ammonia.

The syngas and oxygen were supplied to the reactor which comprises the activated carbon adsorbent pellets to form the bed. The particle size of the pel lets was 0.5 - 0.85 mm. In the reactor, the hydrogen sulfide was removed from the syngas by means of the oxidation and adsorption. The temperature was 50 °C and relative humidity 50 %. The concentration of ¾S was 140 ppm and the concentration of N¾ was 37 ppm in the inlet syngas. Additional process conditions and the results are shown in Table 1 and in Fig. 2.

Table 1

in which AC is activated carbon

The results show that the capacity of the ¾S recovery was 0.29 g¾S/gAC, when the syngas comprised the ammonia, and the capacity of the ¾S recovery was only 0.035 g¾S/gAC, when the syngas did not comprise the ammonia. Thus, the capacity improvement, compris ing eight-fold improvement, was realized. It was ob served from the tests that the removal of the hydrogen sulfide can be improved significantly from the syngas, when the syngas comprised the ammonia as impurity and the ammonia was not removed from the syngas before the desulfurization. FTIR gas analysis results after the bed showed almost immediate breakthrough of ammonia. Thus, ammonia was not observed to be consumed in the process. Further, it was observed that the ammonia which was as the impurity in the syngas improved reac tion kinetics and reaction speed of the hydrogen sul fide adsorption.

Further, it was observed that standard, non- impregnated, activated carbon has poor properties for adsorbing hydrogen sulfide. Physical adsorption on ac tivated carbon is based on van der Waals forces and is weak for low-molecular weight compounds, such as hy drogen sulfide. Further, non-regenerable impregnated and doped activated carbons are enpensive for bulk ¾S removal .

This invention allows for the use of lower grade activated carbons by utilizing a catalyst, i.e. ammonia, that is already present in the gas, for im proved sulfur adsorption.

Scanning electron microscope (SEM) images for the spent ammonia catalyzed adsorbent showed that the surface microstructure had not changed significantly from the fresh sample. Energy-dispersive X-ray spec troscopy (EDS) analysis was performed for the spent adsorbent at 7 different analysis points on the carbon surface for particles located at random bed heights. The analysis results are presented in Table 2 (Fresh and spent activated carbon average EDS results) . Table 2

According to the EDS analysis results, on av erage, the sulfur amount in the carbon matrix in creased 5-fold. The spent bed mass was 24 % higher compared to the fresh bed, therefore the analysis re sults agree well with the increase in bed mass during the run .

Example 3

In this example, a full-scale process is pre sented for the syngas desulfurization. The full-scale comprises a 150 MW biomass gasification process with a 55 ppm dry gas hydrogen sulfide concentration. The process arrangement comprises a desulfurization device with a packed bed for hydrogen sulfide removal. A standard activated carbon of the bed is exchanged about once per year during the plant's scheduled down time. The volume of the packed bed is about 250 m ³ as suming similar hydrogen sulfide removal capacity as in the lab-scale results of Example 2. The desulfuriza tion device can comprise several smaller beds contain ing particles, pellets or granulates of the standard activated carbon. The air consumption is 1 Nm ³ /h in the desulfurization device. If hydrogen sulfide con centration in the gasification syngas is higher, the process configuration could be changed to include two removal units, such as the desulfurization devices, in parallel for continuous operation.

The devices and equipments of the process used in these examples are known per se in the art, and therefore they are not described in any more de tail in this context.

The method and process arrangement are suita ble in different embodiments for removing sulfur and for cleaning gases.

The invention is not limited merely to the examples referred to above; instead many variations are possible within the scope of the inventive idea defined by the claims.