WO2015173234A1

WO2015173234A1 - Method and apparatus for purification of biogas

Info

Publication number: WO2015173234A1
Application number: PCT/EP2015/060464
Authority: WO
Inventors: Marco Johannes Gerardus LINDERS; Earl Lawrence Vincent Goetheer; Johannes Wouterus Van Groenestijn
Original assignee: Nederlandse Organisatie voor Toegepast Natuurwetenschappelijk Onderzoek TNO
Current assignee: Nederlandse Organisatie voor Toegepast Natuurwetenschappelijk Onderzoek TNO
Priority date: 2014-05-12
Filing date: 2015-05-12
Publication date: 2015-11-19
Anticipated expiration: 2016-11-12

Abstract

The invention is related to a method for absorbing CO₂ and H₂S from a methane comprising gas stream by contacting said stream with a liquid solution that comprises a non-chelating amine, a metal, a chelating ligand and a stabilizer.

Description

Title: Method and apparatus for purification of biogas

The invention is in the field of biogas purification. In particular the invention is directed to a method for absorption of CO2 and H2S from methane comprising gas streams.

Biogas is produced during anaerobic digestion of organic materials such as manure, sewage sludge, organic fractions of household and industrial waste and energy crops. It is typically produced at

atmospheric pressure. Biogas can be used as a renewable energy source, for example as fuel for vehicles or as a substitute for natural gas. Additional advantages are a lower release of methane into the atmosphere (methane is a known greenhouse gas) compared to traditional manure management and landfills, as well as the simultaneous production of a high quality digestate for applications as fertilizer.

The exact composition of the biogas is amongst others dependent on the type of material used in the anaerobic digestion. Typically it contains 50-70 vol% methane, 30-50 vol% C0₂ and 0-4000 ppm H₂S. These high amounts of CO2 and H2S in biogas result in a relatively low energy content per volume, and therefore needs to be treated prior to use as energy source. Raw biogas, i.e. biogas as directly obtained after the anaerobic digestion, can be cleaned of unwanted substances such as particles, water, H2S and CO2 in a process called upgrading. The resulting upgraded biogas is much higher in methane content than the raw biogas.

Several countries have defined standards with which the upgraded biogas must comply before grid injection or utilization as vehicle fuel. In 2010, the European Commission mandated CEN (M/475) to set European standards for upgraded biogas requirements. This stresses the importance of the biogas upgrading process.

Several commercially available technologies for biogas upgrading exist. One of the most commonly applied technologies for CO2 removal is water scrubbing. This technology is based on the difference in water solubility of CO2 compared to methane. At low temperature, CO2 dissolves much better in water than methane and the gases can accordingly be separated. Drawbacks are the requirement of elevated pressures, significant losses of methane, high amounts of water and the need for H2S removal prior to water scrubbing. The raw biogas can be treated prior to the water scrubbing process in order to first remove H2S from the methane comprising gas stream. This can for instance be effected by microorganisms that oxidize H2S to sulfate in the presence of oxygen. However, due to residual oxygen, the upgraded biogas is not directly suitable for grid injection or vehicle utilization and subsequent procedures are required for oxygen removal. Another alternative to separately remove H2S from biogas is to allow H2S to react with metal chelates such as Fe³⁺EDTA, where EDTA is the chelating ligand ethylenediaminetetraacetic acid, to form elementary sulfur. The elementary sulfur precipitates and is accordingly separated from the biogas. A continuous process wherein H2S is absorbed from a gas stream by

Fe³⁺EDTA and the consumed Fe³⁺EDTA is regenerated by oxidation is for instance described in EP0279667, incorporated herein in its entirety.

A technology to simultaneously remove both CO2 and H2S from raw biogas is chemical scrubbing. According to this process amine- containing solutions react with the CO2 and H2S in the gas stream. The reacted amine-containing solutions can be regenerated by heating to release the separated CO2 and H2S. Drawbacks of the chemical scrubbing

technology are the loss of amine-containing solution by evaporation and the requirement of a subsequent treatment of the H2S in for instance a Claus plant. A Claus plant is due to its high capital expenditure economically unfeasible for small scale biogas production facilities.

In conclusion, most of the current biogas upgrading technologies involve multiple processes to remove both CO2 and H2S from the raw biogas, which increases both economic costs and environmental impacts. It is desirable that biogas is upgraded in a single procedure, i.e. a combined absorption of CO2 and H2S, as well as a conversion of absorbed H2S, all in one process.

US-A-4400368 and US-A-4091073 both describe removing H₂S and CO2 from methane comprising gas streams in a single procedure by contacting the gas with a liquid solution comprising a metal, a chelating ligand, a stabilizer as well as an absorbent for CO2. The absorbents are physical absorbents for CO2 and therefore less effective at absorbing CO2 at low (partial) pressures. It is desirable that the absorption of CO2 is more effective at these low (partial) pressures.

It was found that these objectives can be met by contacting the biogas stream with a single solution comprising a non-chelating amine, a metal and a chelating ligand to capture both CO2 and H2S. Thus the present invention is directed to a method for absorbing CO2 and H2S from a methane comprising gas stream by contacting said stream with a liquid solution that comprises a non-chelating amine, a metal, a chelating ligand and optionally a stabilizer. The liquid solution may be referred to as a lean solution prior to absorption and as an enriched solution after absorption of C0₂ and H₂S.

Figure 1 is a schematic representation of an apparatus in accordance with the present invention.

Figure 2 is a schematic representation of another embodiment of an apparatus in accordance with the present invention.

Figure 3 is a schematic representation of yet another embodiment of an apparatus in accordance with the present invention.

Figure 4 is a schematic representation of yet another embodiment of an apparatus in accordance with the present invention.

Figure 5 is a schematic representation of yet another embodiment of an apparatus in accordance with the present invention. Figure 6 is a schematic representation of yet another embodiment of an apparatus in accordance with the present invention.

Although the invention aims at the purification (i.e. upgrading) of biogas, it will be appreciated that the invention is also applicable for removal of CO2 and H2S from other methane comprising gas streams, including gas streams at elevated pressure such as natural gas.

The methane comprising stream may be of atmospheric or of elevated pressure. For example, the stream may have a pressure of 1-80 bar, typically 1-50 bar. Preferred values for elevated pressures are 1 to 10, more preferably 1 to 5 bar, even more preferably 1 to 2 bar.

In the present invention, H2S is absorbed in the liquid solution by a redox reaction with a metal (M^x). A chelating ligand (L) is present to solubilize the metal.

With a chelating ligand is meant a ligand that comprises three or more, typically 4 to 8, functional groups that may simultaneously bind to the metal of the present invention. With a non-chelating amine is meant an amine that comprises two or less functional groups that may simultaneously bind to the metal of the present invention. Typical functional groups in this respect are amines, carboxylic acids, hydroxides, sulfides, phosphines and the like.

The metal and chelating ligand form a metal chelate complex (M^XL) that reacts with the H2S in the gas stream, typically according to the following reaction scheme: 2 M^XL + H₂S → 2 M^(X-DL + S + 2 H⁺ (1) in which the metal chelate complex is reduced to form M^L and

elementary sulfur S, which precipitates. Depending on the metal used, the oxidation state of M^x can be decreased by more than one as well, e.g. to form M^(X-¾L, etc. The elementary sulfur is accordingly separated from the gas stream and can be separated from the liquid solution by a liquid- solid separator. Thus no separate step is needed to convert H2S.

For an economically feasible process, the metal chelate is preferably regenerated. The regeneration limits the required amount of metal.

In the regeneration step, M^XL is regained by a reaction of M^-^L with an oxygen containing gas, preferably air, according to the following reaction scheme: 2 M^(X-DL + ½ 0₂ + H₂0 → 2 M^XL + 2 OH- (2)

The non-chelating amine that is present in the liquid solution absorbs the CO2 in parallel to the reaction of the metal chelate with H2S. Thus in one liquid solution both CO2 and H2S are absorbed from the gas stream.

In case non-hindered primary and secondary amines are used, the reaction of the CO2 in the gas stream with the non-chelating amine in the liquid solution typically proceeds according to the following reaction scheme: 2 RNH2 + CO2 → RNHCO2- + RNH₃ ⁺ (3) in which half of the non-chelating amine reacts with CO2 to form carbamate anion. The carbamate ion formed can undergo hydrolysis to form hydrogen carbonate and amine according to:

RNHCO2- + H₂O→ HCO3- + RNH₂ (4)

In case tertiary amines or hindered primary or secondary amines are used, the reaction typically proceeds according to: R1R2R3N + C0₂ + H₂0→ RiR₂R₃NH⁺ + HCO3- (5) in which the CO2 is hydrolyzed towards carbonic acid. Carbonic acid is neutralized with the non-chelating amine, leading to protonated amine and bicarbonate.

It is known from the prior art (see e.g. GB999799, incorporated herein by reference), that for a method wherein H2S is absorbed from a gas stream by a reaction with metal chelates, there is an optimum pH for any given chelating agent. This optimum pH lies typically between 2.5 and 9. At low pH, the metal chelate shows no reactivity. At high pH, the bond between the metal and the chelate ligand is weakened, resulting in the precipitation of ferric oxide and thus loss of metal from the process.

It is also known from the prior art, that for a method wherein CO2 is absorbed from a gas stream by a reaction with amine, there is an optimum pH, which is typically in the range of 9-11.

Using the present invention it is possible, by careful selection of the parameters that govern the absorption process, i.e. selection of the non- chelating amine, the metal chelate, pH of the liquid solution and the temperature to combine both H2S and CO2 absorption.

The metal can be any metal or mixture thereof capable of oxidizing H2S.

The metal can be added to the process in a reactive form, i.e.

being capable of oxidizing H2S, or in a preliminary unreactive form.

Moreover, it can be added as a complex with the chelating agents or as a salt with another counter ion. Furthermore, it can be added as a charged or as an uncharged compound. In a particular embodiment, the metal first has to be activated before it can oxidize H2S.

In a preferred embodiment of the present invention, the metal is a cation selected from the group consisting of Fe, V, Cu, Zn, Mn, Mg and combinations thereof. Depending on the metal chosen, different oxidation states can be used, e.g. Fe²⁺/Fe³⁺ or V⁴⁺/V⁵⁺.

According to the present invention, a chelating ligand is defined as an organic compound containing more than one functional groups that are capable of forming a bond with the metal in such a way that more than one coordination bonds are formed between the said organic compound and the metal.

In a preferred embodiment, the chelating ligand contains one or more amine groups and one or more oxygen-containing groups such as alcohols and carboxylic acids.

The chelating ligand can be added as purely organic compound or as a salt thereof. Moreover, it can be added as a complex formed with the metal.

In a preferred embodiment, the chelating ligand is selected from the group consisting of nitrilotriacetic acid (NTA),

ethylenediaminetetraacetic acid (EDTA), (hydroxyethyl)ethylenediamine triacetic acid (HEDTA), pentetic acid (DTPA), triethanolamine (TEA), 1,2- cyclohexylenedinitrilotetraacetic acid (CyDTA), N-methyliminodiacetic acid (MIDA), 1,3-propanediaminetetraacetic acid (PDTA), 1,4- butanediaminetetraactetate (BDTA), etidronic acid (HEDP), and

combinations thereof.

Degradation of the chelating ligand can occur due to reaction with oxygen during the regeneration. Hence, to avoid degradation of the chelating ligand by oxidation in the regeneration, in a preferred

embodiment a stabilizer is present. Preferably the stabilizer is a radical scavenger. Such a radical scavenger is typically capable of reacting with and de-activating radicals.

In a more preferred embodiment the stabilizer is selected from the group consisting of thiodiglycolic acid, 3,3-thiodipropionic acid, sodium thiocyanate, sodium dithionite, ammonium thiosulfate, sodium thiosulfate, Ν,Ν-diethylhydroxylamine, thiourea, thiosemicarbazide, bisulphite, sodium benzoate, para-toluenesulfonic acid, potassium iodide, potassium bromide, potassium chloride, 2-propanol, 1-butanol, ethylene glycol, sodium formate, sucrose, sorbitol, nitrites, amino acids, aliphatic aldehydes, aryl sulfonic acids and combinations thereof.

It has been found that amines can be lost from the process by evaporation. Therefore, amines with low or no vapour pressures are preferred, e.g. vapor pressures lower than 900 Pa at 25°C, preferably lower than 150 Pa at 25°C. Furthermore amines can be lost by oxidation during the regeneration of the metal. Loss of amines should be avoided. Therefore, in a preferred embodiment of the present invention, the non-chelating amine is added as a salt thereof, thereby having low or no vapor pressure to minimize e.g. evaporation. Amine salts were also found to be effective over a broad pH range.

The non-chelating amine salt is preferably a salt with potassium or sodium, potassium being preferred.

The non-chelating amine is preferably an amino acid. Herein amino acid is defined as an organic compound which comprises at least one amine group and at least one acid group selected from carboxylic acid, phosphoric acid and sulfonic acid. The acid groups can be bound to the same atom of the organic compound (as is the case with the natural occurring amino acids) or to a different atom.

More preferably, the amino acid is selected from a group consisting of taurine, glycine, alanine, asparagine, glutamine, lysine, histidine and derivatives of these such as N-methylglycine, N-methylalanine and combinations thereof.

Alternatively, the non-chelating amine is a non-chelating amine other than an amino acid. Such a non-chelating amine may be a monoamine or polyamine and is preferably selected from the group consisting of the monoamines monoethanolamine (MEA) and 2 piperidine ethanol (PE), and the polyamines piperazine and aminoethylpiperazine (AEP), and

combinations thereof.

In a preferred embodiment the non-chelating amine is regenerated by heating the liquid solution containing CO2. Such a heating step is preferably conducted after regenerating the metal by oxidation.

In a preferred embodiment of the present invention, the lean liquid solution (i.e. the liquid solution prior to the absorption of CO2 and H2S) has a pH of 8-13, preferably a pH of 9-12, more preferably a pH of 10- 11.

In a preferred embodiment of the present invention, the absorption temperature, i.e. the temperature whereby the absorption is taking place, is between 10 and 50 °C, preferable between 20 and 45 °C, more preferable between 30 and 45 °C.

The content of the non-chelating amine in the lean solution may be 10 to 60 wt%, preferably 20 to 50 wt%.

The present invention allows for a simplified biogas upgrading apparatus, viz. only one absorption column is required for the absorption of both CO2 and H2S from the gas stream.

Such an apparatus (see figure 1) comprises an absorption column. The absorption column comprises a first inlet (1), preferably located at the bottom part of the absorption column, through which the gas stream may be introduced and it comprises a second inlet (2), preferably located at the top part of the absorption column, through which the liquid solution may be introduced. It furthermore contains a volume (3) where said gas stream contacts the lean liquid solution. The absorption column furthermore comprises a first outlet (4), preferably located at the top part of the absorption column, through which the purified (or upgraded) gas stream, i.e. a gas stream containing less CO2 and H2S compared to the gas stream that was introduced into the absorption column, may leave and it comprises a second outlet (5), preferably located at the bottom part of the absorption column through which the liquid solution rich in CO2 and sulfur, containing reduced metal complex M^L may leave.

The apparatus may further comprise a gas injection vessel such as a flotation vessel, a flocculation vessel or a bubble column (for

regeneration of the metal), a liquid- solid separator such as a plate settler (for sulfur removal) and a stripper (for CO2 removal). Typically, the absorption column is connected to the gas injection vessel, the gas injection vessel is connected to the liquid-solid separator and the liquid-solid separator is connected to the stripper. In this configuration, the gas injection vessel is preferably a flocculation vessel to enhance sulfur removal. Depending on the extent of sulfur precipitation, the liquid- solid separator may also change places with the gas injection vessel (such that the liquid- solid separator will be connected to the absorption column and the gas injection vessel will be connected to the stripper.

Below, a number of embodiments of the apparatus is described in order to illustrate in more detail how the different parts of the apparatus may be connected.

In a preferred embodiment of the apparatus (see figure 2), said second outlet (5) is connected to a gas injection vessel which acts as a regenerator of the reduced metal complex M^L. A gas injection vessel is a vessel that is suitable for injecting a gas such as oxygen into the vessel, e.g. a flotation vessel or bubble column. Said gas injection vessel comprises a first inlet (6), preferably located at the top part of the gas injection vessel, through which the liquid solution rich in sulfur may be introduced. It furthermore comprises a second inlet (7), preferably located at the bottom part, through which oxygen, preferably oxygen in air may be introduced. It furthermore comprises a volume of oxidation (10) wherein said oxygen may contact said liquid solution rich in sulfur. Moreover, said gas injection vessel comprises a first outlet (8), preferably located at the top part through which unreacted oxygen, preferably unreacted oxygen in air may leave and a second outlet (9), preferably located at the bottom through which said liquid solution rich in CO2 and sulfur containing the oxidized metal (i.e.

regenerated metal) may leave.

In a more preferred embodiment (see figure 3), said second outlet (9) of the gas injection vessel, is connected to a liquid-solid separator, where the sulfur may be removed from the liquid solution. The liquid-solid separator comprises a first inlet (11) through which the liquid solution rich in CO2 and sulfur may be introduced. It furthermore comprises a first outlet

(12) through which the sulfur may leave and it comprises a second outlet (13) through which the liquid solution rich in CO2 may leave. In a variant of this embodiment, the liquid-solid separator and the gas injection vessel have changed places, such that the liquid-solid separator is placed between and connected to the absorption column (or flash vessel if present) and the gas injection vessel.

In a more preferred embodiment (see figure 4), said second outlet

(13) of the liquid-solid separator is connected to a stripper by a first inlet

(14) of said stripper, preferably located at the top part, through which the liquid solution rich in CO2, an oxidized metal, a non-chelating amine, a chelating ligand and a stabilizer may be introduced. The stripper

furthermore comprises a volume (15) wherein said liquid solution rich in CO2 is heated. As a consequence the CO2 leaves the liquid solution.

Moreover said stripper comprises a first outlet (16), preferably located at the top part through which CO2 may leave and a second outlet (17), preferably located at the bottom through which said liquid solution stripped of CO2 may leave. In a variant of this embodiment, the liquid-solid separator and the gas injection vessel have changed places (such that the liquid-solid separator is placed between and connected to the absorption column (or flash vessel if present) and the gas injection vessel) and the stripper will be connected to the gas injection vessel. In a more preferred embodiment (see figure 5) said second outlet (17) of the stripper is connected to said second inlet (2) of said absorption column in such a way that the apparatus allows for a continuous absorption and regeneration process.

The apparatus may further comprise one or more flash vessels, which are typically connected in series with the absorption vessel. The one or more flash vessels are placed directly after the absorption vessel and before the solid/liquid separator and/or gas injection vessel. Accordingly, said second outlet (5), may be connected to a first flash vessel, wherein the pressure of the enriched liquid solution may be reduced. The flash vessel comprises an inlet, through which the enriched liquid solution may be introduced. It furthermore comprises a first outlet, preferably located at the bottom, through which the enriched liquid solution at a reduced pressure may leave and a second outlet, preferably located at the top, through which vapor and gases such as methane and CO2 may leave. The outlet of the last flash vessel is connected to the liquid-solid separator or gas injection vessel. This configuration is desirable when the gas to be treated has an elevated pressure.

For reason of a difference in the reaction kinetics of the reaction of the metal chelate with the H2S compared to the reaction of the non- chelating amine with the CO2, in a preferred embodiment the apparatus (see figure 6) comprises an absorption column that is divided into two sections or two columns. Typically, the first section or column comprises the first inlet (1; for gas to enter) and second outlet (5; for liquid to leave), while the second section or column comprises the second inlet (2; for liquid to enter) and first outlet (4; for gas to leave). The two sections or columns are connected via an outlet (18) (typically located at the top of the first section or column) and an inlet (19) (typically located at the bottom of the second section or column) through which the methane comprising gas stripped from H2S may pass. After treatment of the gas in the first section or column, all H2S may already have been removed and converted to sulfur (due to the fast kinetics), such that the second section or column is effectively only used to remove CO2. Accordingly, only part of the liquid solution has to be subjected to the solid-liquid separator and metal regeneration. The apparatus in this embodiment may thus comprise a first section or column which comprises a volume (3a) where the H2S may be absorbed and part of the CO2 may be absorbed and a second section or column which comprises a volume (3b) where the remaining part of the CO2 may be absorbed.

More specifically, the absorption column may comprise a third outlet (20) located at the bottom part of the second section or column and a third inlet (21) located at the top part of the first section or column. The third outlet (20) is connected to the third inlet (21) and to the inlet (14) of the stripper. Thus, part of the liquid solution rich in CO2 may go to the second section or column via the third inlet (21) and another part of the liquid solution rich in CO2 may go to stripper. The apparatus in this embodiment may comprise an additional flash vessel in between the outlet (20) of the second section or column and the inlet (14) of the stripper, in particular when the methane comprising gas is treated at elevated pressures.

For the purpose of clarity and a concise description, features are described herein as part of the same or separate embodiments. However, it will be appreciated that the scope of the invention may include

embodiments having combinations of all or some of the features described.

The invention is further illustrated by the following experimental examples.

Example 1:

The liquid solution to capture both CO2 and H2S was prepared in the following way. First, a solution of the potassium salt of N-methylalanine was prepared with a molarity M of 4.0 (moles per volume of solution). In a next step CO2 was added to this solution in order to decrease the pH to below 10.6. Subsequently, the liquid solution was prepared in such a way that the final solution contains the following substances and concentrations:

a) Potassium salt of N-methylalanine, 2.0 M K⁺C4H₈N0₂- b) Iron(II) sulfate heptahydrate, 0.025 M FeS0₄.7H₂0 c) Ethylenediaminetetraacetic acid disodium salt dihydrate, 0.05 M CioHi4N2Na208.2H₂0

d) Sodium thiosulfate pentahydrate, 0.15 M Na2S203.5H20 This solution was stable (no precipitation occurred) at room temperature, 20°C, and the solution remained stable up to a temperature of about 100°C, at which temperature part of the CO2 was removed from the solution.

A fresh batch of 250 ml solution was prepared according to above recipe. With this solution a cyclic test was performed, in which the solution was exposed to both CO2 and H2S, comprising of the following steps:

1) Air was passed through the solution during four hours to activate the iron; in this step the iron was oxidized to Fe³⁺.

2) The solution was heated under reflux to about 105°C during two hours, after which it was cooled to 20°C, resulting in a lean CO2 concentration of about 0.3 mol/1.

3) The solution was exposed during about four hours to a gas flow comprising 40 vol% CO2, 2000 ppm H2S, balanced with nitrogen to 100%. During more than three hours no H2S was detected in the outgoing gas flow and a solid substance (sulfur) was formed. This resulted in a rich solution with a CO2 concentration of about 1.15 mol/1.

4) The solid substance was recovered from the solution by filtration. Analysis of the solid substance confirmed the formation of sulfur.

5) Steps 1-4 were repeated with similar results.

Claims

1. A method for absorbing CO2 and H2S from a methane comprising gas stream by contacting said stream with a liquid solution that comprises a non-chelating amine, a metal, a chelating ligand and a stabilizer.

2. A method according to claim 1 wherein said gas stream comprises biogas, and wherein said gas stream is preferably at atmospheric pressure or at elevated pressure of 1 to 80, preferably 1 to 5 bar, more preferably 1 to 2 bar.

3. A method according to any of the previous claims, further comprising a metal regeneration step, wherein said metal is oxidized, preferably using oxygen, more preferably oxygen from air.

4. A method according to any of the previous claims, further comprising a non-chelating amine regeneration step by heating the liquid solution.

5. A method according to any of the previous claims, wherein the non-chelating amine is an amino acid or salt thereof, wherein the amino acid is selected from the group consisting of taurine, glycine, alanine, asparagine, glutamine, lysine, histidine, derivatives of these such as N- methylglycine, and N-methylalanine, and combinations thereof; and wherein the non-chelating amine is preferably a sodium or potassium salt of said amino acid, more preferably a potassium salt.

6. A method according to any of the previous claims, wherein the non-chelating amine is a non-chelating amine other than an amino acid, preferably selected from the group consisting of monoamines such as monoethanolamine (MEA), 2 piperidine ethanol (PE), and polyamines such as piperazine, aminoethylpiperazine (AEP), and combinations thereof.

7. A method according to any of the previous claims, wherein the metal is a cation selected from the group consisting of Fe, V, Cu, Zn, Mn, Mg and combinations thereof, preferably Fe³⁺ or V⁵⁺ and combinations thereof.

8. A method according to any of the previous claims, wherein the chelating ligand is selected from the group consisting of nitrilotriacetic acid (NTA), ethylenediaminetetraacetic acid (EDTA), hydroxyethyl

ethylenediamine triacetic acid (HEDTA), pentetic acid (DTP A),

triethanolamine (TEA), 1,2-cyclohexylenedinitrilotetraacetic acid (CyDTA), N-methyliminodiacetic acid (MIDA), 1,3-propanediaminetetraacetic acid (PDTA), 1,4-butanediaminetetraactetate (BDTA), etidronic acid (HEDP), and combinations thereof.

9. A method according to any of the previous claims wherein the stabilizer is selected from the group consisting of thiodiglycolic acid, 3,3- thiodipropionic acid, sodium thiocyanate, sodium dithionite, ammonium thiosulfate, sodium thiosulfate, Ν,Ν-diethylhydroxylamine, thiourea, thiosemicarbazide, bisulphite, sodium benzoate, para-toluenesulfonic acid, potassium iodide, potassium bromide, potassium chloride, 2-propanol, 1- butanol, ethylene glycol, sodium formate, sucrose, sorbitol, nitrites, amino acids, aliphatic aldehydes, aryl sulfonic acids and combinations thereof.

10. A method according to any of the previous claims, wherein the liquid solution has a pH of 8-13, preferably a pH of 9-12, more preferably a pH of 10-11.

11. A method according to any of the previous claims, wherein said absorption is carried out at a temperature of between 10 and 50 °C, preferably between 20 and 45 °C, more preferably between 30 and 45 °C.

12. An apparatus for purifying a gas stream comprising an absorption column comprising:

- a first inlet (1), preferably located at the bottom part, through which said gas stream may be introduced;

- a second inlet (2), preferably located at the top part, through which a liquid solution comprising a non-chelating amine, a metal, a chelating ligand and a stabilizer may be introduced; - a volume of contact (3) wherein said gas stream may be contacted with said liquid solution;

- a first outlet (4), preferably located at the top part, through which said gas stream containing less CO2 and H2S than upon introduction may leave; - a second outlet (5), preferably located at the bottom part, through which said liquid solution containing more CO2 and sulfur than upon introduction and containing a reduced metal may leave;

13. An apparatus according to claim 12, further comprising

- optionally one or more flash vessels connected to the absorption column;

- a gas injection vessel, which is connected to the absorption column or to said one or more flash vessels if present; and

- a liquid-solid separator connected to said gas injection vessel; and

- a stripper connected to said liquid-solid separator.

14. An apparatus according to claim 13, wherein the gas injection vessel is a flocculation vessel.

15. An apparatus according to claim 13 or 14, wherein said absorption column is divided into two columns or two column sections, wherein the first section or column comprises the first inlet (1) and second outlet (5), while the second section or column comprises the second inlet (2) and first outlet (4); and

wherein the two sections or columns are connected via an outlet (18) and an inlet (19) through which the methane comprising gas stripped from H2S may pass; and

wherein the absorption column comprises a third outlet (20) located at the bottom part of the second section or column and a third inlet (21) located at the top part of the first section or column; and

wherein the third outlet (20) is connected to said stripper and to said third inlet (21).