Method for removal of harmful sulphurous compounds from gas mixtures

Abstract

Waste gas mixtures produced and used in industry may contain harmful sulphurous compounds. The present disclosure provides a method for treatment of gas mixtures contaminated with harmful sulphurous compounds by using microorganisms capable of degrading said harmful sulphurous compounds which involves controlling nitrate levels in the medium in which microbiological conversion of harmful sulphurous compounds takes place at high levels.

Claims

1. A method for treating a raw gas mixture which contains harmful sulphurous compounds, comprising i) introducing into a first unit: a raw gas mixture containing harmful sulphurous compounds, and an acidic aqueous medium containing nitrate; ii) contacting in said first unit said raw gas mixture with micro-organisms capable of degrading said harmful sulphurous compounds in said aqueous medium; iii) passing said aqueous medium with said nitrate and said raw gas mixture through a synthetic structure comprising a biofilm of said micro-organisms for facilitating diffusion between said medium and said raw gas mixture, and allowing, under anaerobic conditions, said micro-organisms to metabolize in said aqueous medium at least part of said harmful sulphurous compounds to sulphuric acid (H.sub.2SO.sub.4) and to metabolize at least part of said nitrate into nitrite such that a gas product comprising a lower content of harmful sulphurous compounds compared to the raw gas mixture and an aqueous medium containing H.sub.2SO.sub.4 are obtained; iv) discharging aqueous medium containing H.sub.2SO.sub.4, said nitrite and said gas product from said first unit; wherein, in the acidic aqueous medium introduced into said first unit, the nitrate level is controlled at a level which is sufficient to allow conversion of at least part of said harmful sulphurous compounds to H.sub.2SO.sub.4 under said anaerobic conditions; v) passing said aqueous medium containing H.sub.2SO.sub.4 and nitrite from said first unit to a second unit in which, under aerobic conditions, nitrate is regenerated by an oxidation reaction of nitrite to nitrate to obtain an aqueous medium containing H.sub.2SO.sub.4 and nitrate; and vi) recycling at least part of said aqueous medium containing H.sub.2SO.sub.4 and nitrate from said second unit back to said first unit to function as said acidic aqueous medium containing nitrate of step i); wherein in said aqueous medium throughout the method the pH is maintained below 2.

2. The method according to claim 1, wherein, in the acidic aqueous medium introduced into said first unit, the nitrate level is controlled at a level which is sufficient to allow conversion of essentially all harmful sulphurous compounds to H.sub.2SO.sub.4 under said anaerobic conditions.

3. The method according to claim 1, wherein said harmful sulphurous compounds are H.sub.2S and/or CS.sub.2, and wherein, in the acidic aqueous medium introduced into said first unit, the nitrate level introduced into said first unit is controlled to a molar ratio with respect to the portion atomic sulphur in said H.sub.2S and/or CS.sub.2 introduced into said first unit of at least 5:1.

4. The method according to claim 1, wherein said raw gas mixture contains H.sub.2S, and wherein in step iii) said micro-organisms metabolize in said acidic aqueous medium under anaerobic conditions H.sub.2S to H.sub.2SO.sub.4, and wherein, in the acidic aqueous medium introduced into said first unit, the nitrate level is controlled to a molar ratio with respect to the portion atomic sulphur in said H.sub.2S introduced into said first unit of at least 5:1.

5. The method according to claim 1, wherein said raw gas mixture contains CS.sub.2, and wherein in step iii) said micro-organisms metabolize in said acidic aqueous medium under anaerobic conditions CS.sub.2 to H.sub.2SO.sub.4, and wherein, in the acidic aqueous medium introduced into said first unit, the nitrate level is controlled to a molar ratio with respect to the portion atomic sulphur in said CS.sub.2 introduced into said first unit of at least 5:1.

6. The method according to claim 1, wherein said raw gas mixture introduced into said first unit contains both H.sub.2S and CS.sub.2, and wherein in step iii) said micro-organisms metabolize in said acidic aqueous medium under anaerobic conditions H.sub.2S and CS.sub.2 to H.sub.2SO.sub.4, and wherein, in the acidic aqueous medium introduced into said first unit, the nitrate level is controlled to a molar ratio with respect to the portion atomic sulphur in said H.sub.2S and CS.sub.2 introduced into said first unit of at least 5:1.

7. The method according to claim 3, wherein, in the acidic aqueous medium introduced into said first unit, said molar ratio is at least 6.5:1.

8. The method according to claim 1, wherein the pH of the acidic aqueous medium is less than 1.

9. The method according to claim 1, wherein the raw gas mixture is selected from the group comprising boiler gas, landfill gas, flare gas and biogas.

10. The method according to claim 1, wherein the raw gas mixture contains inflammable material.

11. The method according to claim 10, wherein the inflammable material comprises CH.sub.4.

12. The method according to claim 9, wherein the raw gas mixture is biogas and wherein said first unit is provided downstream of a digester that produces biogas from organic matter.

13. The method according to claim 1, wherein said first unit is a biotrickling filter.

14. The method according to claim 2, wherein the raw gas mixture contains inflammable material.

15. The method according to claim 14, wherein the raw gas mixture contains CH.sub.4.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Many aspects of the invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present disclosure. Moreover, in the drawings, like reference numbers designate corresponding parts throughout the several views.

(2) FIG. 1 is a schematic representation of the method of the present invention; and

(3) FIG. 2 depicts an apparatus for practicing the method of the present invention.

DETAILED DESCRIPTION

(4) The method of the invention can be carried out by means of an apparatus as shewn in FIG. 1. FIG. 1 shows an schematic representation of an apparatus for treating a raw gas mixture which contains harmful sulphurous compounds, comprising a first unit (1), which when in operation is an anaerobic unit. This first unit comprises an inlet (2) for the acidic aqueous medium, an inlet (3) for receiving said raw gas mixture (which in a particular preferred embodiment contains methane and H.sub.2S); a first chamber (4) in connection with said inlet (2) for receiving said aqueous medium and said inlet (3) for receiving said raw gas mixture, said first chamber comprising a structure (5) for facilitating diffusion between said medium and said raw gas mixture, so as to enable at least partial dissolving harmful sulphurous compounds and at least partial converting harmful sulphurous compounds to sulfuric acid in the medium such that a gas product comprising lower content of harmful sulfuric compounds compared to the raw gas mixture (in a particular preferred embodiment a product stream comprising methane and lower H.sub.2S content compared to the raw gas mixture) is obtained; an outlet (6) for passing out said gas product, and an outlet (7) for said medium via which the medium can be passed to a second unit which is an aerobic unit under operational conditions. Microorganisms may be comprised in the acid medium introduced via inlet (2) and/or be present as biofilms on said structure (5).

(5) The apparatus of the invention further comprises a second unit (8) comprising an inlet (9) for an oxygen containing gas (such as air) in order to allow aerobic conditions in the second unit; an inlet (10) in connection with said outlet (7) for said medium of the first unit (1) for receiving slurry from said first unit; a structure (11) for facilitating diffusion of said slurry with the oxygen containing gas so as to allow aerobic conversion of any remaining nitrite dissolved in the slurry back to nitrate; an outlet (12) for said medium; and a gas outlet (13) in connection with the surroundings to allow release of gas. The units, outlets and inlets and other elements of the apparatus are interconnected or connected with the surroundings by connection means (26, 27, 23, 29, 30, 31)

(6) In the apparatus said cutlet (12) for said medium of the second unit is in connection with the inlet (2) for medium of the first unit so as to enable recycling of at least a part of said medium from the second unit to the first unit in order to provide nitrate to the first unit (1). The recycling of medium from the second unit to the first unit provides an anaerobic and acid environment which increases dissolving and conversion of harmful sulphurous compounds, such as H.sub.2S, in the slurry. In particular when the gas mixture contains methane and H.sub.2S the recycling of the major part of the slurry from the second unit to the first unit provides an anaerobic and acid environment which reduces biological loss of CH.sub.4 to a minimum, while increasing H.sub.2S dissolving and conversion in the slurry.

(7) The skilled person will understand that connections between the units, inlets, outlets can be provided by any connections means that enable a stream of gases, fluids or liquid such as tubings or pipes. Likewise the inlets and outlets of the apparatus can be provided with any suitable tubing or pipe that enables entry or exit of gases, fluids or liquids from chambers and/or units.

(8) The apparatus preferably further comprises in the first unit a second chamber which is in connection with the first chamber, comprising a structure for facilitating diffusion of water and said product stream and a means for providing an intermittent flow of water into the second chamber so as to enable removing any residual harmful sulphurous compounds from the product stream from said first chamber. The incorporation of this element provides the opportunity of applying a step to “polish” the product stream, which in a preferred embodiment contains methane, from the first chamber to obtain a further cleaned up product stream.

(9) The apparatus may suitably be provided with a blower to bring the right amount of ambient air into the second unit to allow for the full oxidation process of the anaerobic slurry from the first unit. Vents may also be added in the apparatus where gases need to be ventilated out of the apparatus.

(10) The apparatus may be suitably provided with pumps to pump the gases, fluids or liquids to and from the respective compartments.

Exemplary Embodiment

(11) In the following paragraph an exemplary embodiment of an apparatus which can be used for performing the method of the invention will be explained with reference to FIG. 2 in reference to an process in which a raw gas mixture comprising methane and H.sub.2S is cleaned.

(12) FIG. 2 shows an exemplary embodiment of the apparatus of the invention in which a raw gas mixture comprising methane and H.sub.2S is passed via piping (26) and inlet (3) into the first chamber (4) of a first unit (1). The first unit (3) also comprises an inlet (2) for receiving via piping (31, 31′) an aqueous slurry comprising anaerobic and aerobic micro-organisms. This slurry is introduced into the first chamber (4) in a strong and continuous flow. In the first chamber (4) synthetic and structured media (5) are installed which optimize diffusion between the raw gas mixture and the aqueous slurry and which allow for optimal exchange of components. In this treatment, at least part, but preferably most or even more preferably substantially ail of the H.sub.2S is absorbed or dissolved into the slurry and at least partially converted to other sulphurous compounds in the slurry. After this treatment the cleaned up gas is passed to a second chamber (14) which is situated above the first chamber. The slurry is passed out of the first chamber (4) via outlet (7) and piping (28). In the second chamber (14) the gas will be “polished” to take out remaining H.sub.2S. The second chamber (14) also comprises a structure (15) for facilitating diffusion of the water and gas. In the second chamber any residual H.sub.2S is removed from product stream by intermittently spraying the product stream with water by means (16) via piping (42) for providing an intermittent flow of water. The purified methane gas is released from the first unit via outlet (6) and piping (27). The first and the second chamber (4, 14) are separated by a water collector (17) which allows washing said structure for facilitating diffusion, for instance by rinsing the structure with a caustic mixture. The water collector is configured such that flow of the methane containing gaseous product stream from the first to the second chamber is possible, for instance in the form of drain gutters. The first chamber (4) is connected via outlet (7) and piping 28 with inlet (36) of buffer tank (24). The second chamber (14) is connected (optionally via water collector (17)) via outlet (7′) and piping (28′) with inlet (36) of the buffer tank (24). In the buffer tank the anaerobic wash slurry which contains the converted products of the biological conversion from the second chambers, i.e. nitrite and sulfuric acid, is collected. The buffer tank can be connected via outlet (37) and piping (33) to a drain sump to allow draining of excess slurry. From the buffer tank (24) the slurry is passed via outlet (39) and piping (28″) to a second unit (8). This unit is provided with an inlet (9) for supplying an oxygen containing gas via piping (29); an inlet (10) in connection via piping (28, 28′, 28′″) with said outlet of the first unit for receiving slurry from said first unit; a structure (11), preferably a synthetic structure, for facilitating diffusion of said slurry so as to allow aerobic conversion of nitrite to nitrate; an outlet (12) for said slurry; and a gas outlet (13) in connection with the surroundings via a pipe (30). In this pipe (30) a vent (25) may be connected to allow ventilation of gases. The outlet (14) for said slurry of the second unit is in connection with the inlet (2) for slurry of the first unit via piping (31, 31′) so as to enable recycling of said slurry from the second unit (8) to the first unit (1). The apparatus of this embodiment further comprises a means (18) for replenishing the slurry with fresh water and nitrate in the slurry positioned via inlet (40) and outlet (41) in piping (31, 31′) between the outlet (12) for said slurry of the second unit (8) and the inlet (2) for said slurry of the first unit (1). Means (18) may be connected to a water supply, wherein the water with a desired level of nitrate is present, via inlet (19) and piping (34) and a drain sump via outlet (20) and piping (35). Means (18) functions to enable control of the sulphuric acid and nitrate content in the slurry before it is recycled back the first unit (1). The apparatus is provided with pumps (22) and (23) to pump the gases, fluids or liquids to and from the respective compartments. The apparatus may also be provided with a blower to bring the right amount of oxygen containing gas (preferably ambient air) into the second unit (8) to allow for the full oxidation process of the anaerobic slurry from the first unit (1). It will be obvious that many changes can be made to the exemplary embodiment according to the invention in the paragraph above without being beyond the inventive idea as it is defined in the claims.