WO2019091521A1 - Process for purification of biogas without removing co2 or ch4 from the gas - Google Patents

Process for purification of biogas without removing co2 or ch4 from the gas Download PDF

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WO2019091521A1
WO2019091521A1 PCT/DK2018/000158 DK2018000158W WO2019091521A1 WO 2019091521 A1 WO2019091521 A1 WO 2019091521A1 DK 2018000158 W DK2018000158 W DK 2018000158W WO 2019091521 A1 WO2019091521 A1 WO 2019091521A1
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gas
sulfuric acid
acid
biogas
concentrated sulfuric
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French (fr)
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Peter Carl Sehestedt Schoubye
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Peter Carl Sehestedt Schoubye
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/96—Regeneration, reactivation or recycling of reactants
    • B01D53/965—Regeneration, reactivation or recycling of reactants including an electrochemical process step
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/46—Removing components of defined structure
    • B01D53/48—Sulfur compounds
    • B01D53/50—Sulfur oxides
    • B01D53/507—Sulfur oxides by treating the gases with other liquids
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/74—General processes for purification of waste gases; Apparatus or devices specially adapted therefor
    • B01D53/86—Catalytic processes
    • B01D53/8603—Removing sulfur compounds
    • B01D53/8612—Hydrogen sulfide
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2251/00—Reactants
    • B01D2251/10—Oxidants
    • B01D2251/102—Oxygen
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2251/00—Reactants
    • B01D2251/10—Oxidants
    • B01D2251/106—Peroxides
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2251/00—Reactants
    • B01D2251/50—Inorganic acids
    • B01D2251/506—Sulfuric acid
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/20—Metals or compounds thereof
    • B01D2255/207—Transition metals
    • B01D2255/20707—Titanium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/20—Metals or compounds thereof
    • B01D2255/207—Transition metals
    • B01D2255/20723—Vanadium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2257/00—Components to be removed
    • B01D2257/55—Compounds of silicon, phosphorus, germanium or arsenic
    • B01D2257/556—Organic compounds
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2258/00—Sources of waste gases
    • B01D2258/05—Biogas
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/26—Drying gases or vapours
    • B01D53/265—Drying gases or vapours by refrigeration (condensation)
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02C—CAPTURE, STORAGE, SEQUESTRATION OR DISPOSAL OF GREENHOUSE GASES [GHG]
    • Y02C20/00—Capture or disposal of greenhouse gases
    • Y02C20/40—Capture or disposal of greenhouse gases of CO2

Definitions

  • the present invention relates to a novel Process for up to 99.9 % removal of H 2 S and other S-compounds and up to 99 % removal of siloxanes in biogas extracted from landfills or produced by anaerobic digestion of organic materials such as manure, food and household waste.
  • the content of C0 2 , CH 4 , N 2 and 0 2 in the biogas pass unaffected through the Process.
  • the biogas purified in the Process may be utililized directly as fuel gas for gas turbines or gas engines, or it may be passed to a plant for catalytic conversion of the content of C0 2 in the gas to CH 4 , higher hydrocarbons or methanol by catalytic reduction with appropriate catalysts and H 2 supplied from an external source and from the electrolysis of H 2 S0 4 in the Process.
  • the Process can as well be used for purification of other gases comprising CH 4 , C0 2 , 0 2 and N 2 .
  • Catalysts used for C0 2 -reduction processes such as methanation, methanol synthesis or Fischer-Tropsch synthesis are extremely sensitive to sulfur poisoning, requiring sulfur to be removed down to 10-20 ppb for stable long-term operation.
  • This final ultra-purification which is not part of the present invention, is typically accomplished by passing the gas though an active carbon filter bed followed by a bed of hot zink- oxide pellets upstream of the C0 2 -reduction reactor.
  • the Process uses only biogas and electricity as input and generates no secondary output apart from a stream of condensed water, comprising also the content of NH 3 and basic compounds in the biogas, and a useful stream of concentrated sulfuric acid and H 2 from the H 2 S0 4 -electrolyzer equivalent to the amount of sulfur in the biogas.
  • the electrolyzer may be operated with additional H 2 -production and 0 2 generated at the cathode in place of H 2 S 2 0 8 .
  • FIG. 1 shows Process principles without catalytic oxidation of H 2 S to S0 2 upstream of the sorption tower.
  • Figs 2 and 3 show examples of embodiment with and without said oxidation of H 2 S to S0 2 , respectively.
  • the numbers in cursive refer to the numbers seen on the figures.
  • the raw biogas in line 3 from anaerobic digestion of organic materials in the digester 2 is typically saturated with H 2 0 at 30-35 °C at slightly above atmospheric pressure and contains on dry basis typically about 60% CH 4 40% C0 2 , 0,05 - 1 % H 2 S and up to 100 mg siloxanes per Nm 3 .
  • the digestor should be operated without admission of air and at conditions giving highest possible yield of CH 4 and H 2 S.
  • the biogas is then compressed to about 12 bar abs in two compressors 4 and 7 with condensation of water in the intercooler 5. NH 3 and basic compounds in the gas are extracted as carbonates in the condensate.
  • the biogas is heated in the heat exchanger 9 to a temperature in the range of 180 - 300 °C and passed through an oxidation reactor 11 loaded with a catalyst by which H 2 S and other combustible S-compounds are oxidized selectively to S0 2 according to the reaction:
  • the amount of 0 2 needed for complete oxidation to S0 2 of H 2 S and other S-compounds is added to the gas upstream of the oxidation reactor in an amount corresponding to 0.2 - 0.8 % excess 0 2 remaining in the gas after complete oxidation of H 2 S and other combustible S-compounds to S0 2 .
  • Known catalysts comprising oxides of V and Ti (ref 5) are very active for oxidation of H 2 S down to below 200 °C and do not affect CH 4 or C0 2 at temperatures up to 350 - 400 °C at the actual operating conditions.
  • H 2 S 2 0 4 peroxy disulfuric acid
  • both peroxy acids are calculated together as H 2 S 2 0 8 .
  • H 2 S 2 0 4 may instead be generated by adding H 2 0 2 in aqueous solution to the circulating acid. Then the Process would produce excess H20 and make the maintenance of a high acid strength impossible and necessitate operation with more dilute acid down to 80 % H2S04.
  • siloxanes which are insoluble in water and dilute sulfuric acid, are soluble in sulfuric acid stronger than 50% H 2 S0 4 , the solubility increasing infinitely with acid strength. Acid of 90-98% strength dissolves at least 20 % siloxanes which may separate out again, apparently when the solution is diluted with a lot of water.
  • the strength of the circulating acid in line 32 is controlled by controlling the flow of make-up water added in line 40 to the circulating acid in order to make-up for the deficit of w kg/h of H 2 0 in the H 2 0 balance calculated by the over-all reaction of sorption of S0 2 and the formation of H 2 S 2 0 8 in the electrolyzerr:
  • w is controlled by the control valve 42 (or a volumetric pump) controlled by the continuous acid strength analyzer 41 being connected to recirculating acid flowing in line 32.
  • H 2 0 vapor pressure and H 2 S0 4 vapor pressure
  • H 2 S0 4 vapor pressure becomes significant at 10 bar with above 98.5% acid strength at 100 °C acid temperature.
  • H 2 S0 4 mist is removed in the mist filter 16.
  • w should be at least 0,5-1 kg H 2 0/h per 1000 Nm3/h In order to have robust control of acid strength.
  • the absorption process should be operated with 95 - 99 % (minimum 90%) acid strength and about 100 °C temperature (max 120 0 C) of the Tower exit gas in line 17. Maintenance of high strength of the acid in the circulating loop is easier, the higher the S0 2 -content in the gas because 2 mol of H 2 0 is removed per mol of S0 2 absorbed in the absorption process (2).
  • the stream of condensate in line 52 may be mixed with the stream in line 53 of concentrated sulfuric acid generated by the oxidation of the H 2 S in the biogas and added to the digested slurry from the digester. Remaining trace of H 2 S 2 0 8 will immediately be reduced to H 2 S0 4 in contact with the slurry.
  • the only input for the process is electricity and biogas.
  • Ref (6) reports 95% removal of H 2 S by absorption in dilute H 2 0 2 dissolved in water at pH above 3-4, but the absorption stops when the liquid becomes more acidic.
  • the solubility of H 2 S in water is about 8 times lower than that of S0 2 .
  • the rate of absorption of H 2 S in hot, concentrated acid with H 2 S 2 0 8 may be higher than that of S0 2 because very fast oxidation of H 2 S by H 2 S 2 0 8 in the liquid with no equilibrium limitation will decrease the liquid film restriction of the absorption of H 2 S.
  • the biggest advantage of direct absorption of H 2 S is that the cost and complications of the step of oxidation of H 2 S to S0 2 and the risk of deactivation of the oxidation catalyst are avoided. Furthermore, it is easier to maintain high concentration of H 2 S0 4 in the acid loop at low concentrations of H 2 S in the gas, because one mol of H 2 S takes out double as much H 2 0 as one mol of S0 2
  • step (1) heating the gas to typically 200-250 °C and oxidizing its content of S-compounds selectively to S0 2 by a catalyst comprising oxides of V and Ti with a small excess of 0 2 added to the gas in step (1),

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Abstract

A Process for removal of up to 99,9 % of H2S and other S-compounds and up to 99% of siloxanes in biogas and other gases comprising CO2 and CH4, based on selective absorption in an absorption tower of SO2 or H2S in concentrated sulfuric acid comprising peroxy disulfuric acid (H2S2O8) and peroxy sulfuric acid (H2SO5} oxidizing H2S and SO2 to H2SO4 comprising the consecutive steps of : (1) compressing the gas to 3 pressure above 3 bar, typically 5-40 bar, and condensing its content of H2O typically at 30 - 40 °C (2) heating the gas to typically 200-250 °C and oxidizing its content of S-compounds selectively to SO2 by a catalyst comprising oxides of V and Ti with a small excess of O2 added to the gas in step (1), (3) cooling and condensing H2O at 0 -50 °C, typically at 20 - 40 °C (4) oxidizing the SO2 and H2S in the biogas to H2SO4 by contacting the gas at about 100 °C in an absorption tower with recirculating hot concentrated sulfuric acid comprising H2S2O8 generated preferably by electrolysis of H2SO4 in an electrolyzer inserted in said recirculation of concentrated sulfuric acid. The formation of the H2S2O8 in the electrolyzer also ensures maintenance of high concentration of the circulating acid as H2O is consumed in the over-all process. Siloxanes in the gas are absorbed and dissolved in the concentrated sulfuric acid while CH4, CO2 and O2 in the gas pass unaffected through the sorption tower. In a potential!y much more advantageous but not so well validated simplified version of the Process, seen in fig 1 and 3, the gas is passed directly from step (1) to step (4) for direct oxidation of H2S to H2SO4 in the sorption tower. The effluent gas from the sorption tower can be passed to applications such as combustion in gas turbines or engines, or to further steps of ultra purification upstream of catalytic reduction with H2 of the C02 in the gas to CH4, methanol or higher hydrocarbons. The Process consumes only biogas and electricity and produces no byproducts apart from condensed water, a stream of useful concentrated sulfuric acid equivalent to the amount of sulfur in the biogas and a stream of useful H2 from the electrolysis of H2SO4. The digester should be operated with highest possible content of H2S, 1-2 % or more, in the gas passed to the process of the invention.

Description

Process for purification of biogas without removing C02 or CH4 from the g
Description of the process of the invention
The present invention relates to a novel Process for up to 99.9 % removal of H2S and other S-compounds and up to 99 % removal of siloxanes in biogas extracted from landfills or produced by anaerobic digestion of organic materials such as manure, food and household waste. The content of C02, CH4, N2 and 02 in the biogas pass unaffected through the Process. The biogas purified in the Process may be utililized directly as fuel gas for gas turbines or gas engines, or it may be passed to a plant for catalytic conversion of the content of C02 in the gas to CH4, higher hydrocarbons or methanol by catalytic reduction with appropriate catalysts and H2 supplied from an external source and from the electrolysis of H2S04 in the Process.
The Process can as well be used for purification of other gases comprising CH4, C02, 02 and N2.
Catalysts used for C02-reduction processes such as methanation, methanol synthesis or Fischer-Tropsch synthesis are extremely sensitive to sulfur poisoning, requiring sulfur to be removed down to 10-20 ppb for stable long-term operation. This final ultra-purification, which is not part of the present invention, is typically accomplished by passing the gas though an active carbon filter bed followed by a bed of hot zink- oxide pellets upstream of the C02-reduction reactor.
The Process uses only biogas and electricity as input and generates no secondary output apart from a stream of condensed water, comprising also the content of NH3 and basic compounds in the biogas, and a useful stream of concentrated sulfuric acid and H2 from the H2S04-electrolyzer equivalent to the amount of sulfur in the biogas. The Process in which the H2S is absorbed directly in the sorption tower, as seen in fig 1 and 3, generates 4 mol H2 per mol of H2S while the Process in which H2S is oxidized to S02 upstream of the sorption tower generates 1 mol H2 per mol H2S in the inlet gas. The electrolyzer may be operated with additional H2-production and 02 generated at the cathode in place of H2S208.
A wide range of processes for removal of H2S or other combustible sulfur-compounds are known but no one mention use of peroxides dissolved in concentrate sulfuric acid for desulfurization of the gas.
Detailed description of the Process of the invention.
The diagram in fig 1 shows Process principles without catalytic oxidation of H2S to S02 upstream of the sorption tower. Figs 2 and 3 show examples of embodiment with and without said oxidation of H2S to S02, respectively. The numbers in cursive refer to the numbers seen on the figures.
The raw biogas in line 3 from anaerobic digestion of organic materials in the digester 2 is typically saturated with H20 at 30-35 °C at slightly above atmospheric pressure and contains on dry basis typically about 60% CH4 40% C02, 0,05 - 1 % H2S and up to 100 mg siloxanes per Nm3. The digestor should be operated without admission of air and at conditions giving highest possible yield of CH4 and H2S. The biogas is then compressed to about 12 bar abs in two compressors 4 and 7 with condensation of water in the intercooler 5. NH3 and basic compounds in the gas are extracted as carbonates in the condensate.
The Process (fig 2) with oxidation of H2S to S02 upstream of the absorption tower. Here, the biogas is heated in the heat exchanger 9 to a temperature in the range of 180 - 300 °C and passed through an oxidation reactor 11 loaded with a catalyst by which H2S and other combustible S-compounds are oxidized selectively to S02 according to the reaction:
(1) H2S + 1½ 02 => S02 + H20 + 530 kJ/mol
The amount of 02 needed for complete oxidation to S02 of H2S and other S-compounds is added to the gas upstream of the oxidation reactor in an amount corresponding to 0.2 - 0.8 % excess 02 remaining in the gas after complete oxidation of H2S and other combustible S-compounds to S02. Known catalysts comprising oxides of V and Ti (ref 5) are very active for oxidation of H2S down to below 200 °C and do not affect CH4 or C02 at temperatures up to 350 - 400 °C at the actual operating conditions.
Most of the remaining H20 is then condensed by cooling the gas to typically 25-30 °C in 13 before the gas is passed to the S02 absorption tower 15 in which the S02 is absorbed by H2S204 (peroxy disulfuric acid) dissolved in 100 0 C hot circulating, 95-99 % concentrated sulfuric acid, according to the reaction:
(2) S02 + H2S208 + 2H20 => 3H2S04 + 150 kJ/mol
The H2S204 is generated continuously according to reaction
(3) 2H2S04 + power (approx. 0.1 kWh/mol H2) => H2S208 + H2 + heat (approx. 90 kJ/mol) by high current density electrolysis of strong and hot sulfuric acid in a special electrolyzer 36 inserted in the sulfuric acid circulating loop, as seen in the flow schemes in fig 2 and 3.
(An unknown fraction of the H2S208 in concentrated sulfuric acid is actually present as H2S05 (Cam's acid) formed by the reaction H2S208 = H2S05 + H2S04. In the present script both peroxy acids are calculated together as H2S208.)
Generation of H2S208 by high current density electrolysis of hot, highly concentrated sulfuric acid is well known and proven technology (ref 8 and 9) which could easily be adapted to the process of the invention in cooperation with established suppliers of such electrolyzers.
Hence, the over-all reaction (1) + (2) + (3) is
(4) H2S + 1 ½ 02 + H20 => H2S04 + H2
H2S204 may instead be generated by adding H202 in aqueous solution to the circulating acid. Then the Process would produce excess H20 and make the maintenance of a high acid strength impossible and necessitate operation with more dilute acid down to 80 % H2S04.
I calculate that the rate of absorption of S02 in the circulating acid in the sorption tower 15 is liquid film controlled which means that the rate increases with increasing solubility of S02 in sulfuric acid. This seems confirmed by the experience in ref 7 with removal of small concentrations of S02 in tail gas from sulfuric acid plants by scrubbing the tail gas at atmospheric pressure with hot concentrated sulfuric acid containing a small concentration of H2S208 generated by electrolysis of H2S04 or by adding H202 to the sulfuric acid. The solubility of S02 in sulfuric acid, and thereby also the rate of absorption, increases strongly with gas pressure and with acid strength above 85% H2S04 in the acid (ref 8). Furthermore, I have observed that siloxanes, which are insoluble in water and dilute sulfuric acid, are soluble in sulfuric acid stronger than 50% H2S04, the solubility increasing infinitely with acid strength. Acid of 90-98% strength dissolves at least 20 % siloxanes which may separate out again, apparently when the solution is diluted with a lot of water.
I see no signs that the siloxanes dissolved in concentrated sulfuric acid are oxidized by H2S208 dissolved in the acid under formation of Si02 or other precipitates, even not after weeks of residence time of siloxanes dissolved in 96% H2S04 with 1-2 % H2S208.
Maintenance of minimum 90 % strength, preferably 98-99% strength of the circulating sulfuric acid is crucial for optimal removal of S02 as well as siloxanes in the sorption tower.
The strength of the circulating acid in line 32 is controlled by controlling the flow of make-up water added in line 40 to the circulating acid in order to make-up for the deficit of w kg/h of H20 in the H20 balance calculated by the over-all reaction of sorption of S02 and the formation of H2S208 in the electrolyzerr:
(2) + (3) S02 + 2 H20 H2S04 + H2.
Thus the flow w (kg water/h) of make-up water in line 40 being added in order to maintain the strength of the circulating acid at a desired constant level is calculated from the H20-balance:
(5) w kgH20/h = (a) H20 consumed by reaction (2)+(3) - (b) H20 in the tower exit gas in line 17
+ (c)H20 in the tower exit gas in line 17 + (d)the content of H20 in the products stream in line 50.
In order to maintain the acid strength in the circulation loop at a desired set-point, w is controlled by the control valve 42 (or a volumetric pump) controlled by the continuous acid strength analyzer 41 being connected to recirculating acid flowing in line 32.
For calculations of water balances, I use H20 vapor pressure (and H2S04 vapor pressure) in equilibrium with sulfuric acid at various values of temperature and acid strength seen in table 1. H2S04 vapor pressure becomes significant at 10 bar with above 98.5% acid strength at 100 °C acid temperature. H2S04 mist is removed in the mist filter 16.
w should be at least 0,5-1 kg H20/h per 1000 Nm3/h In order to have robust control of acid strength. The absorption process should be operated with 95 - 99 % (minimum 90%) acid strength and about 100 °C temperature (max 120 0 C) of the Tower exit gas in line 17. Maintenance of high strength of the acid in the circulating loop is easier, the higher the S02-content in the gas because 2 mol of H20 is removed per mol of S02 absorbed in the absorption process (2). This means that at below 600 ppm S02 in the gas, minimum 90- 92% strength of the circulating acid can only be maintained at above 40 bar total pressure (requiring 3 compressors in series), condensation of H20 at 10 0 C in the gas inlet cooler 13, and heating of the circulating acid to 110 0 C in 33. However, with 0.6 % S02, 95-98% acid strength is easily maintained even at 10 bar pressure and condensation at 30 -40 °C.
At 11-12 bar pressure, 100 0 C and above 95 % acid strength, up to 99,9 % removal of S02 and 99 % removal of siloxanes can be achieved with sufficient size of the absorption tower, as S02 is absorbed without equilibrium limitations. Higher pressure will increase absorption efficiency and improve the H20-balace on the expense of higher investment costs. An active carbon guard filter downstream of the absorption tower may insure desired removal efficiencies. Final ultra-purification to 10-20 ppb S may by a bed of ZnO adsorbent upstream of C02 hydrogenation reactor. It is not important to minimize the consumption of H2S208 insidenummere the process according to the invention, or to minimize parallel formation in the electrolyzer of 02 (by electrolysis of H20) in addition to the formation of H2S208, because all formation of H2S208 and 02 is accompanied with formation of the equivalent amount of useful H2.
The stream of condensate in line 52 may be mixed with the stream in line 53 of concentrated sulfuric acid generated by the oxidation of the H2S in the biogas and added to the digested slurry from the digester. Remaining trace of H2S208 will immediately be reduced to H2S04 in contact with the slurry.
The only input for the process is electricity and biogas.
The process (figs 1 and 3) without the step of oxidation of H2S to S02 upstream of the sorption tower Here, H2S is adsorbed directly in the sorption tower according to the reaction
(6) H2S + 4H2S208+ 4H20 => 9H2S04 + approx. 7200 kJ/mol,
as is seen in fig 1 and 3. The system for generation of peroxy sulfuric acid and control of acid strength is identical to that seen in fig 2.
This simplified version of the process is clearly superior to the version with oxidation of H2S to S02 and would be preferred immediately if there were similar experience with absorption of H2S as with absorption of S02 by H2S208 in strong sulfuric acid. The control of acid strength is more robust as the absorption of 1 mol H2S consumes 4 mol of H20 including the electrolysis; the 4 times higher power consumption per mol H2S is compensated by the production of 4 mol H2 per mol of H2S.
Furthermore, the risk of deactivation of the oxidation catalyst by siloxanes in the gas is avoided.
Ref (6) reports 95% removal of H2S by absorption in dilute H202 dissolved in water at pH above 3-4, but the absorption stops when the liquid becomes more acidic. The solubility of H2S in water is about 8 times lower than that of S02.
No references are found in literature to experiments with absorption of H2S by H2S208 or H202 dissolved in concentrated sulfuric acid. However, one single reference ("H2S solubility in sulfuric acid" by Alexendrova et al in J. Applied Chem (USSR) 1978, 57, 1221-23) indicate that the solubility of H2S in concentrated sulfuric acid increases strongly with acid strength, like that of S02 in concentrated sulfuric acid, indicating that absorption of H2S by H2S208 in hot circulating sulfuric acid could be an attractive alternative at sufficiently high total pressure.
However, arguments for the feasibility of absorption and oxidation of H2S to H2S04 in H2S208 dissolved in hot, high strength sulfuric acid according to (6) are very convincing from another angle of approach: It is known that H2S cannot be dried by bubbling it through 95-98 % H2S04 because even at room temperature, some of the H2S is oxidized to sulfur and S02 by the acid according to the reaction
(7) H2S + H2S04 => 2S02 + S + 2 H20.
Use of this reaction for industrial production of sulfur from H2S reacting with H2S04 is known (ref 8).
A simple, indicative laboratory experiment at atmospheric pressure with bubbling dilute H2S-gas through a flask A with 96% H2S04 and another flask B with NH4HS208 dissolved in 96% H2S04, both at 80-100 °C, was very convincing: In flask A, there was a lot of precipitation of sulfur and smell of S02 from the flask. In flask B, the solution remained clear with no smell of S02 or H2S and with significant heating up of the liquid during the experiment.
The rate of absorption of H2S in hot, concentrated acid with H2S208 may be higher than that of S02 because very fast oxidation of H2S by H2S208 in the liquid with no equilibrium limitation will decrease the liquid film restriction of the absorption of H2S.
The biggest advantage of direct absorption of H2S is that the cost and complications of the step of oxidation of H2S to S02 and the risk of deactivation of the oxidation catalyst are avoided. Furthermore, it is easier to maintain high concentration of H2S04 in the acid loop at low concentrations of H2S in the gas, because one mol of H2S takes out double as much H20 as one mol of S02
It is no disadvantage that the absorption of H2S consumes 4 times more H2S208 than absorption of S02, as it also generates 4 times more useful H2 per mol of H2S in the inlet gas.
It shall be noticed that the absorption of S02 and H2S in concentrated sulfuric acid containing peroxy (di)sulfuric acid according to Claim 1 a), b) and c) may as well be applied to desulfurization of any gas with H2S, S02 or other S-compounds in gas streams comprising C02, H20, 02, N2 and CH4 and, possibly, also H2 and lower aliphatic hydrocarbons which are not absorbed in the circulating acid.
Prior Art
In extensive search in Google and in recent, exhaustive surveys (ref 1,2,3) of technology for purification of biogas or gases with H2S and combustible S-compounds comprised in gases comprising C02, 02, CH4, I have found no relevant prior art to the Process of my invention in literature, apart from partly relevant prior art in refs 7, Hand 12.
It is relevant prior art to remove S02 in off gas from sulfuric acid plants by scrubbing it with concentrated sulfuric acid containing peroxy (di)sulfuric acid generated either by electrolysis of hot, concentrated sulfuric acid or by adding H202 to the acid (ref 7, Hamond/DuPont US 3760061 A, filed 02.03.1999).
It is not relevant prior art to remove H2S from biogas by oxidizing H2S with H202 to sulfates in alkaline or neutral aqueous solutions at above pH 3-4 (ref 6).
The most relevant prior art found in patent literature are:
Ref 9, Bowe, US 2007/0029264 filed 08 february 2008, claims 1, 3 and 4. Describes a process for purifying biogas comprising C02 and CH4 for use in a reformer generating feed gas for a subsequent Fischer-Tropsch process. The biogas is desulfurized by liquid scrubbing absorption without mentioning use of peroxides or concentrated sulfuric acid.
Ref 10, US 2013/00267614 Al(Corey et al.) 10 October 2013. describes a process for converting biogas to liquid fuels comprising the steps of removing moisture from the gas by compression and cooling, removing siloxanes and other contaminants, such as H2S, by filtration with activated carbon before feeding the biogas to a syngas reactor.
Ref 11, US 2013/034616 (Iyer) 26 december 2013 and (12) CN 106345232 A (Changzhou Vocational Inst. Eng.) 25 Januar 2017, WPI Abstract AN-2017-093073, describe removal of siloxanes in biogas by scrubbing with concentrated sulfuric acid but do not mention use of peroxides for removal of H2S or other S- compounds in the gas.
Ref 12, Chanzhou Voc. Inst. Eng, CN 106345232 filed 25.1.2017, WPI Abstract AN-2017-093073. None of the latter 4 references can be seen as prior art of my invention
References:
(1) "Biogas upgrading technologies - developments and innovations", Petersson and Wellnger, I EA report from 2009, wwwJnfotek-biomasse.ch//175 2009 IEA.
(2) "Biogas Upgrading and Global Markets", Susan Haft, bcc report February 2014 (337 pages)
(3) "Biogas upgrading. Evaluation of methods for H2S removal" Danish Technological Institute, 2014 (31 pages)
(4) "F0reningar I biogas: Validering av analys H2S04metodik for siloxaner", Svensk Gasteknologisk Center, November 2011 (25 pages)
(5) Catalysts for low temperature oxydation of H2S. HTAS patentapplication WO 2015/082352 Al.
(6) "Process for Removing H2S from Gas". Patent US200901130008 Al. Michael N. Funk. Priority
19.11.2007.
(7) "High-strength acid containing H202 to scrub S02", Hamond/DuPont, US 3760061 A, filed 02.03.1999
(8) "Reactions between H2S and sulfuric acid: A Novel Pocess for sulfur removal and recovery", Qjnglin Zang et al, Ind. Eng. Chem. Res., 2000, 39, 39(7), pp 2505-2509.
(9) US 2007/0029264 (Bowe) 08 february 2008.
(10) US 2013/00267614 Al(Corey et al.) 10 October 2013.
(11) US 2013/034616 (Iyer) 26 december 2013.
(12) CN 106345232 A (Changzhou Vocational Inst. Eng.) 25 Januar 2017, WPI Abstract AN-2017-093073.
Figures and table.
Fig 1. Process principles without oxidation of H2S to S02 upstream sorption tower
Fig 2. Process FS at 11-12 bar abs with catalytic oxidation of H2S to S02 upstream sorption tower.
Fig 3. Process FS at 11-12 bar abs with no oxidation of H2S upstream sorption tower.
Table 1. H20 and H2S04 vapor pressures over sulfuric acid at various temperatures and acid strength
(2) heating the gas to typically 200-250 °C and oxidizing its content of S-compounds selectively to S02 by a catalyst comprising oxides of V and Ti with a small excess of 02 added to the gas in step (1),
(3) cooling and condensing H20 at 0 -50 °C, typically at 20 - 40 °C
(4) oxidizing the S02 and H2S in the biogas to H2S04 by contacting the gas at about 100 °C in an absorption tower with recirculating hot concentrated sulfuric acid comprising H2S208 generated preferably by electrolysis of H2S04 in an electrolyzer inserted in said recirculation of concentrated sulfuric acid. The

Claims

Patent Claims
Claim 1:
A process for removal of H2S and other sulfur-compounds and siloxanes in biogas and other gases comprising CH4 and C02 without removing CH4 and C02 in the gas comprising the subsequent process steps of:
a) compression of the gas to above 3 bar abs. in one or several steps with intercooling and condensation of H20 in the gas at 0-50 °C;
b) contacting the gas in an absorption tower with recirculating concentrated sulfuric acid, with a concentration of 80-99 % at a temperature in the range of 30 - 130 °C measured in the gas exiting the absorption tower, said acid comprising H2S208 generated by electrolysis of H2S04 in an electrolyzer inserted in said recirculation of sulfuric acid or by adding H202 to the circulating acid; and
c) controlling the strength of the recirculating acid to a desired value between 80 and 99 % H2S04, by adding to the circulating sulfuric acid a stream of water controlled by continuous measurement of the acid strength.
Claim 2:
A process according to Claim 1, further comprising the subsequent steps of:
(al) Adding an amount of oxygen corresponding to 0,1 - 1 vol % 02 remaining in the gas after calculated complete oxidation of H2S and other S-compounds to in the biogas to S02; and
(a2) Passing the gas through a reactor with catalyst for oxidation of H2S and other S- compounds to S02 at a temperatures in the range of 180 to 400 °C.
between step a) and b).
PCT/DK2018/000158 2017-11-10 2018-11-07 Process for purification of biogas without removing co2 or ch4 from the gas WO2019091521A1 (en)

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US20070029264A1 (en) * 2004-06-15 2007-02-08 Bowe Michael J Processing biological waste materials to provide energy
US20130267614A1 (en) * 2012-04-09 2013-10-10 Peter Coorey Biogas to liquid fuel converter
CN106345232A (en) * 2016-09-26 2017-01-25 常州工程职业技术学院 Method and device for removing siloxane in biomass gas through two-stage mode
WO2017105245A2 (en) * 2015-12-18 2017-06-22 Procede Holding B.V. Removal of sulfur compounds from gas streams via precipitation

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GB1397256A (en) * 1971-11-24 1975-06-11 Metallgesellschaft Ag Process for removing gaseous impurities from waste gases
US20070029264A1 (en) * 2004-06-15 2007-02-08 Bowe Michael J Processing biological waste materials to provide energy
US20130267614A1 (en) * 2012-04-09 2013-10-10 Peter Coorey Biogas to liquid fuel converter
WO2017105245A2 (en) * 2015-12-18 2017-06-22 Procede Holding B.V. Removal of sulfur compounds from gas streams via precipitation
CN106345232A (en) * 2016-09-26 2017-01-25 常州工程职业技术学院 Method and device for removing siloxane in biomass gas through two-stage mode

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