DK201900886A1 - A process for the preparation of gaseous fuel from a raw gas stream - Google Patents

Abstract

A process for the preparation of gaseous fuel comprising the C1-C5 hydrocarbons, carbon dioxide, water, sulfur dioxide and hydrogen halides.

Description

Title: A process for the preparation of gaseous fuel from a raw gas stream

The present invention relates to a process for the preparation of gaseous fuel from a raw gas stream containing C1-C5 hydrocarbons and carbon dioxide, and impurities of organic and inorganic sulfur compounds, halogenated and non-halogenated volatile organic compounds, and silicon compounds.

Biogas is a waste gas or product gas from sources including landfills and anaerobic digesters. In general, many raw gas streams contain an assortment of impurities, including organic and inorganic sulfur compounds, halogenated and nonhalogenated volatile organic compounds.

Typical raw gas streams are waste gas or product gas from sources including biogas streams. Those raw gas streams contain, beside desired lower hydrocarbons, the above mentioned impurities.

To be useful as gaseous fuel in a reciprocating gas engine or gas turbine, the silicon compounds must be removed and the organic and inorganic sulfur compounds must be oxidized to compounds harmless for the operation of the gas turbine or reciprocating gas engine.

One of the reasons that raw gas streams must be cleaned prior to use is that sulfur compounds can create a corrosive environment inside the gas turbine and gas engine equipment. Furthermore, hydrogen sulfide present in the feed gas to gas engines will cause degradation of the lubricating oil and lead to a need of frequent maintenance.

DK 2019 00886 A1

Another reason to clean biogas is that other impurities, such as siloxanes, can be deposited within heat and power generation equipment and cause significant damage to the internal components.

The heavy hydrocarbons and inorganic sulfur compounds will solidify or condense on the surfaces of the equipment when cooling the process gas. These compounds must therefore be eliminated/modified before the gas is cooled to avoid fouling and plugging of tubes and valves.

Typically, organic and inorganic sulfur compounds, halogenated and non-halogenated volatile organic compounds are reacted by catalytic oxidation to carbon dioxide, water and inorganic halides and sulfur compounds.

Landfill gases often contain small amounts of H2S. During removal of siloxanes, generation of inorganic sulfur can occur and the hydrocarbon species may undergo a variety of different reactions that lower their dew point.

Oxidation of organic sulfur compounds and and H₂S, results in formation of SO₂ and trace amounts of SO₃. The latter will react with water vapor in the gas and form sulfuric acid. The sulfuric acid will lead to corrosion of the equipment making the proccess unfeasable. Hence, SO₃ must be removed before the process gas is cooled below the acid dew point.

Carbon dioxide, water, and small amounts of sulfur dioxide and hydrogen halides are acceptable in the fuel for a gas turbine or reciprocating gas engine power plant.

DK 2019 00886 A1

Thus, expensive cleaning processes for the removal of these compounds can be avoided.

However, as mentioned above, removal of SO3 from the fuel gas is required because of the above reasons.

Removal of sulfur oxides is typically achieved by scrubbing with an alkaline solution or by dry scrubbing, i.e. injection of reactive powders which subsequently must be filtered out.

Commonly used sulfur oxide absorbents for dry scrubbing like Mg(OH)₂, Ca(OH)₂ and Trona are readily available and cheap but are not selective and will also absorb SO₂, resulting in an excessive consumption of absorbent.

Using an SO₃ selective absorbent will eliminate the risk of undesirable generation and condensation of sulfuric acid in the process equipment. The equipment will be able to operate for an extended period due to the absence of corrosive attack by the sulfuric acid on the metals.

U.S. Pat. No. 6,126,910 and U.S. Pat. No. 6,803,025 describe the use of soluble sulfite/bisulfite solutions, such as sodium sulfite (Na2SO3), sodium bisulfite (NaHSO3), potassium sulfite (K2SO3), potassium bisulfite (KHSO3) and mixtures thereof to remove SO3 and other acidic gases from a flue gas without removing or decreasing the amount of sulfur dioxide also present in the flue gas. The process entails dissolving the solid sulfite/bisulfite salts to increase the surface area and ensure a good contact between

DK 2019 00886 A1 the acidic gas and the absorbent, injecting (e.g., spraying) a concentrated solution containing a sulfite/bisulfite into the flue gas stream to react acidic gases (e.g., HCl, HF and/or SO₃) and form a reaction product, without reacting the sulfur dioxide.

We found that subjecting a raw gas stream to a catalytic oxidation, in which organic and inorganic sulfur compounds, halogenated and non-halogenated volatile organic compounds are selectively oxidized, without substantially oxidizing, the lower hydrocarbons and the sulfur containing compounds present in the gas to sulfur dioxide and sulfur trioxide, and solely removing the sulfur trioxide by selectively absorbing from the thus prepared gaseous fuel results in a significantly simpler and less costly process for preparation of gaseous fuel.

Pursuant to this finding, this invention provides a process for the preparation of gaseous fuel from a raw gas stream containing higher hydrocarbons, C1-C5 hydrocar-bons and carbon dioxide, and impurities of organic and inorganic sulfur compounds, halogenated and non-halogenated volatile organic compounds, and silicon com-pounds, comprising the steps of (a) removing the silicon compounds contained in the raw gas stream by passing the raw gas stream through a sili-con compound absorbent;

(b) contacting a silicon compounds depleted raw gas from step (a) in one or more oxidation steps with a selective oxidation catalyst and oxidizing selectively the organic

DK 2019 00886 A1 and inorganic sulfur compounds and the halogenated and nonhalogenated volatile organic compounds to carbon dioxide, water, sulfur dioxide, hydrogen halides, and amounts of sulfur trioxide;

(c) decreasing or removing the amounts of sulfur trioxide in the effluent gas from step (b) by selective ab-sorption in a fixed bed with solid absorbent; and (d) withdrawing a sulfur trioxide depleted effluent gas from step (c) comprising the C1-C5 hydrocarbons, carbon dioxide, water, sulfur dioxide and hydrogen halides to provide the gaseous fuel.

In an embodiment of the invention, steps a, b, c and d are performed at a temperature between 200 and 450^o C.

In further an embodiment, the sulfur trioxide depleted effluent gas from step (c) is cooled to a temperature of between 0 and 100^o C.

Preferably, the sulfur trioxide concentration in the sulfur trioxide depleted effluent gas from step (c) is less than 15 mg/Nm3, preferably less than 5 mg/Nm3.

Traces of silanols, together with volatile siloxanes, are present particularly in some biogases and landfill gas, resulting from the degradation of silicones and siloxanes released from consumer products. As their combustion forms particles of silicates and microcrystalline quartz, which

DK 2019 00886 A1 cause abrasion of combustion engine parts, they pose problems for the use of such gases in combustion engines and must be removed.

The step of removing siloxanes is preferably carried out by heating the raw gas stream and passing the heated raw gas stream through a siloxane absorption bed prior to the oxidation step(s).

It is known that siloxanes can be removed using non-regenerative packed bed adsorption with activated carbon, porous silica or alumina as sorbent. Regenerative sorbents can also be used as well as units based on gas cooling to very low temperatures to precipitate the siloxanes out from the gas. Further, liquid extraction technologies are used. In addition, these technologies can be used in combination.

Regenerative systems using activated alumina, activated alumina plus silica and activated carbon adsorbents to capture siloxanes and silanols have been reported. After saturation of the adsorbent with siloxane impurities, the adsorbed siloxanes and silanols are removed in situ using pressure swing adsorption (PSA) or thermal swing adsorption (TSA) to enable the bed to be re-used.

However, in the process according to the invention, it is preferred to use a non-regenerative absorbent technology at elevated temperature.

Alumina is highly absorptive for siloxanes and/or silanols. Thus, the silicon compound absorbent for use in the process according to invention comprises preferably alumina.

DK 2019 00886 A1

Evidently, it is required in the process according to the invention that no or only minor amounts of the desired C1C5 hydrocarbons are oxidized in the catalytic oxidation step .

It is known that vanadium, tungsten and metallic or oxidic palladium and/or platinum supported on titania have sufficient selectivity in the oxidation with respect to higher hydrocarbons, such as halogenated and non-halogenated volatile organic compounds and in the oxidation of organic and inorganic sulfur compounds at temperatures between 200 and

450^oC.

Consequently, a preferred selective oxidation catalyst in the one or more oxidation steps is at least one of oxides of vanadium, tungsten, titanium and metallic or oxidic palladium and/or platinum.

Prefereably, the selective oxidation catalyst comprises copper and/or manganese.

To reduce pressure drop over the selective oxidation catalyst, the catalyst may be supported on a monolithic substrate.

In an embodiment of the invention, the one or more oxidation steps are performed in parallel.

As already discussed hereinbefore, oxidation of organic sulfur compounds and H₂S, results in formation of SO₂ and

DK 2019 00886 A1 trace amounts of SO₃. SO₃ leads to formation of detrimental sulfuric acid and must be removed prior the raw gas stream from the selective catalytic oxidation stage is cooled.

In the process according to the invention SO₃ is selectively absorbed on the solid absorbent, comprising alumina, zinc oxide, sodium bisulfite, potassium sulfite, calcium sulfite, magnesium sulfite, ammonium bisulfite, sodium thiosulfate, potassium thiosulfate, calcium thiosulfate, magnesium thiosulfate, ammonium thiosulfate and mixtures thereof.

Preferably, the solid absorbent in step (c) comprises Al₂O₃, ZnO or mixtures thereof.

Highly porous materials are prone to a higher degree of utilization (percentage of the active substance in the material that can be accessed by SO₃) of the absorbent because the porous structure allows SO₃ not only to be absorbed on the surface.

Alumina is an example of a naturally occurring mineral, and present in many types of mixed metal oxides commonly referred to as clays, rocks and feldspars, examples include sepiolit, bentonite, bauxite, kaolinite.

Many of these solid materials possess an ordered structure that may improve the accessibility to the interior part of the absorbent and thereby enable a higher degree of utilization of the alumina.

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The morphology and surface activity of the solid absorbent may also be altered by thermal, chemical treatment or different preparation routes to increase its SO3 absorption capacity and selectivity.

Despite the limited possibilities for mass transfer between the acidic gas and the solid absorbent due to the absorbent being located in a fixed bed. Solid thiosulfate and sulfite salts are believed to react efficiently with the targeted SO₃ and H₂SO₄ acid gases following contact with the flue gas without reacting with the SO₂ also present due to the limited SO3 concentration.

The chemistries of the reactions occurring with the present invention are summarized in the following equations in which M2 denotes either sodium, potassium and ammonium and M calcium or magnesium.

M SO - SO .M SO.-SO (1) and,

M S O - SO .M SO.-SO -S (2) or,

M SO -H SO..M SO.-H O-SO (3) and,

M S O -H SO..M SO.-H O-SO -S (4)

MSO -SO .MSO.-SO (5) and,

MS O - SO .MSO.-SO -S (6)

DK 2019 00886 A1 or,

MSO3+H2SO4NMSO4+H2O+SO2 (7) and, .t.cyy.;; t.T - S (g)

All embodiments of the inventions are useful for the purification of a raw gas stream of biogas from landfills or anaerobic digesters.

Example 1:

The absorption of SO₂ and SO₃ was investigated in a laboratory scale setup in a tubular absorber reactor made of quartz glass fitted in a temperature controlled tubular oven. A number of materials were tested for absorbing gaseous SO₂ and SO₃ as determined by analysis of outlet and inlet gas flows.

In one set of experiments, the tested materials were exposed to a gas consisting of H₂O, O₂, SO₂, SO₃ and N₂ to evaluate the SO₂ and SO₃ absorption efficiency.

SO₂ was supplied from a gas cylinder and diluted with N₂ and O2 (in air) supplied from a central pressurized system and were dosed through mass flow controllers. SO₃ was supplied by oxidizing a part of the SO₂ with O₂ in the flow over a pre-oxidation catalyst. A second stream of N2 was bubbled through liquid water to evaporate the H2O to the process gas. The two streams were mixed before being added to the absorption reaction chamber, which contained 25 g of absorbent material.

DK 2019 00886 A1

Table 1 shows the experiments the experiments with SO₂ and SO₃ absorption on pure Al₂O₃ and K₂CO₃/Al₂O₃ residence times around 2 seconds. The inlet gas to and outlet gas from the absorption reaction chamber was analyzed for SO₂ and SO₃.

The results show that both Al₂O₃ and K₂CO₃/Al₂O₃ efficiently captures SO₃ and that very little SO₂ is being absorbed over pure Al₂O₃.

Table1:

Material	Al2O3	K2CO3/AUO3
Gas normal residence time [s]	2	2
O2 inlet concentration [vol%]	1	1
Absorber temperature [°C]	300	300
Inlet SO2 concentration [ppmv]	360	405
Inlet SO3 concentration [ppmv]	1.5	6.1
Outlet SO2 concentration [ppmv]	358	296
Outlet SO3 concentration [ppmv]	0.0	0.0
SO2 capture [%]	0.8	27
SO3 capture [%]	~100	~100

In another set of experiments, the tested materials were exposed to a gas consisting of H₂O, O₂, SO₂ and N₂ in order to evaluate the SO₂ absorption efficiency.

SO₂ was supplied from a gas cylinder and diluted with N₂ and O₂ (in air) supplied from a central pressurized system and were dosed through mass flow controllers. A second stream of N₂ was bubbled through liquid water to evaporate the H₂O to the process gas. The two streams were mixed before being added to the absorption reaction chamber.

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Table 2 shows the experiments with SO₂ absorption between various materials at residence times around 2 seconds. The inlet gas to and outlet gas from the absorption reaction chamber was analyzed for SO₂. Table 2 show again that very little SO₂ is being absorbed over pure Al₂O₃, while ZnO and

K₂CO₃/Al₂O₃ absorbed 10 and 24%, respectively. The more alkaline combination of CaO/NaOH absorbed >60% of the SO₂.

This demonstrates that alumina and ZnO possesses a unique selectivity towards selective SO₃ absorption in an SO₂ con10 taining gas.

Table 2:

Material	Al2O3	K2CO3/AUO3	ZnO	CaO/NaOH
Residence time [s]	0.5	0.5	0.5	0.5
Absorber temperature [°C]	300	300	300	300
Inlet O2 concentration [vol%]	1	1	1	1
Inlet SO2 concentration [ppmv]	2005	1950	1955	1980
Outlet SO2 concentration [ppmv]	1700	1460	1765	775
Uptake of SO2 as Al2(SO4)3 [%]	15	25	10	60

DK 2019 00886 A1

Claims (13)

1. A process for the preparation of gaseous fuel from a raw gas stream containing higher hydrocarbons, C1-C5 hydrocarbons and carbon dioxide, and impurities of organic and inorganic sulfur compounds, halogenated and non-halogenated volatile organic compounds, and silicon compounds, comprising the steps of (a) removing the silicon compounds contained in the raw gas stream by passing the raw gas stream through a silicon compound absorbent;

(b) contacting a silicon compounds depleted raw gas from step (a) in one or more oxidation steps with a selective oxidation catalyst and oxidizing selectively the organic and inorganic sulfur compounds and the halogenated and nonhalogenated volatile organic compounds to carbon dioxide, water, sulfur dioxide, hydrogen halides, and amounts of sulfur trioxide;

(c) decreasing or removing the amounts of sulfur trioxide in the effluent gas from step (b) by selective absorption in a fixed bed with solid absorbent; and (d) withdrawing a sulfur trioxide depleted effluent gas from step (c) comprising the C1-C5 hydrocarbons, carbon dioxide, water, sulfur dioxide and hydrogen halides to provide the gaseous fuel.

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2. A process according to claim 1, wherein steps a, b, c and d are performed at a temperature between 200 and

450^o C.

3. Process according to claim 1 or 2, wherein the sul- fur trioxide depleted effluent gas from step (c) is cooled to a temperature of between 0 and 100^o C.

4. Process according to claim 1 or 3, wherein the sul- fur trioxide concentration in the sulfur trioxide depleted effluent gas from step (c) is less than 15 mg/Nm3, preferably less than 5 mg/Nm3.

5. Process of any one of claims 1 to 4, wherein the silicon compound absorbent comprises alumina.

6. Process of any one of claims 1 to 5, wherein the silicon compounds comprise siloxanes and/ or silanols.

7. Process of any one of claims 1 to 6, wherein the selective oxidation catalyst in the one or more oxidation steps is at least one of oxides of vanadium, tungsten, titanium and metallic or oxidic palladium and/or platinum.

8. Process of any one of claims 1 to 6, wherein the selective oxidation catalyst in the one or more oxidation steps comprises copper and/or manganese.

9. Process according to any one of claims 1 to 8, wherein the selective oxidation catalyst is supported on a monolithic substrate.

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10. Process of any one of claims 1 to 9, wherein the one or more oxidation steps are performed in parallel.

11. Process according to any one of claims 1 to 10,

5 wherein the raw gas stream is a biogas stream from landfills or anaerobic digesters.

12. Process according to any one of claims 1 to 11, wherein the solid absorbent in step (c) comprises Al₂O₃,

10 ZnO, sodium bisulfite, potassium bisulfite, calcium sulfite, magnesium sulfite, ammonium bisulfite, sodium thiosulfate, potassium thiosulfate, calcium thiosulfate, magnesium thiosulfate, ammonium thiosulfate or mixtures thereof.

15 13. Process according to claim 12, wherein the solid absorbent in step (c) comprises Al2O3, ZnO or mixtures thereof.