Method for the removal of oxygen from an industrial gas feed

Abstract

Oxygen is removed from a gas feed such as a landfill gas, a digester gas or an industrial CO.sub.2 off-gas by heating the feed gas, optionally removing siloxanes and silanols from the heated feed gas, optionally removing part of the sulfur-containing compounds in the heated feed gas, injecting one or more reactants for oxygen conversion into the heated feed gas, carrying out a selective catalytic conversion of any or all of the volatile organic compounds (VOCs) present in the gas, including sulfur-containing compounds, chlorine-containing compounds and any of the reactants injected, in at least one suitable reactor, and cleaning the resulting oxygen-depleted gas. The reactants to be injected comprise one or more of H.sub.2, CO, ammonia, urea, methanol, ethanol and dimethyl ether (DME).

Claims

1. A method for the removal of oxygen from an industrial gas feed, said method comprising the following steps: (a) heating the feed gas, (b) optionally removing siloxanes and silanols from the heated feed gas, (c) optionally removing part of the sulfur-containing compounds in the heated feed gas, (d) injecting one or more reactants for oxygen conversion into the heated feed gas, (e) carrying out a selective catalytic conversion of any or all of the volatile organic compounds (VOCs) present in the gas, including sulfur-containing compounds, chlorine-containing compounds and any of the reactants injected in step (d), in at least one reactor, and (f) cleaning the resulting oxygen-depleted gas.

2. Method according to claim 1, wherein the reactor in step (e) is divided into two or more reactors with inter-bed cooling in between.

3. Method according to claim 1, wherein the gas feed, from which oxygen is to be removed, is a landfill gas, a digester gas or an industrial CO.sub.2 off-gas.

4. Method according to claim 1, wherein the cleaning in step (f) comprises removal of CO.sub.2 in a separation unit, removal of N.sub.2 and drying of the cleaned gas.

5. Method according to claim 1, wherein the gas comprises nitrogen and oxygen.

6. Method according to claim 2, wherein the energy recovered after each reactor is used in a re-boiler in the CO.sub.2 separation unit.

7. Method according to claim 1, wherein the feed gas is heated to a temperature of between 150 and 450 C.

8. Method according to claim 1, wherein the feed gas is heated to a temperature of between 150 and 450 C. and thereafter fed to the units for sulfur, siloxane and silanol removal.

9. Method according to claim 8, wherein the feed gas to the sulfur, siloxane and silanol removal units is heated through heat exchange with the effluent gas from the oxygen removal step.

10. Method according to claim 1, wherein the components to be injected comprise one or more of H.sub.2, CO, ammonia, urea, methanol, ethanol and dimethyl-ether (DME).

11. Method according to claim 3, wherein the landfill gas contains H.sub.2S and organic sulfur along with siloxanes, silanols, CO.sub.2, H.sub.2O, methane, chlorinated compounds, freon compounds and various VOC (volatile organic carbon) compounds.

12. Method according to claim 1, wherein the catalyst comprises a metal selected among vanadium, tungsten, chromium, copper, manganese, molybdenum, platinum, palladium, rhodium and ruthenium in metallic or metal oxide form supported on a carrier selected from alumina, titania, silica and ceria.

13. Method according to claim 1, wherein the sulfur components are converted to SO.sub.2 through selective catalytic conversion and the SO.sub.2 is removed in a scrubber.

14. Method according to claim 13, wherein the SO.sub.2 is removed in a wet caustic or H.sub.2O.sub.2 scrubber or in a dry scrubber using a caustic sorbent.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) The sole FIGURE shows the method of the present invention, combined with digester and landfill gas conditioning steps.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(2) In the method according to the present invention, one or more components suitable for catalytic oxidation are injected into the oxygen-containing main gas stream after removal of siloxanes and silanols from the gas. The components and the catalyst are chosen so that the catalyst oxidizes the injected components using the oxygen in the stream without substantially oxidizing the valuable components, such as methane, in the gas stream.

(3) The components to be injected may comprise one or more of i.a. H.sub.2, CO, ammonia, urea, ethanol and dimethyl ether (DME).

(4) The active catalyst may comprise a metal selected among vanadium, tungsten, chromium, copper, manganese, molybdenum, platinum, palladium, rhodium and ruthenium in metallic or metal oxide form supported on a carrier selected from alumina, titania, silica and ceria and combinations thereof. Sulfur impurities in an industrial gas can create a corrosive environment inside power generating equipment or even poison catalysts that may be present. In addition, 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. Furthermore, H.sub.2S needs to be removed if the gas is to be sent to gas pipelines or used as fuel in vehicles.

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

(6) Siloxanes are organosilicon compounds comprising silicon, carbon, hydrogen and oxygen which have SiOSi bonds. Siloxanes can be linear as well as cyclic. They may be present in biogas because they are used in various beauty products, such as e.g. cosmetics and shampoos that are washed down drains or otherwise disposed of, so that they end up in municipal wastewater and landfills. Siloxanes are not broken down during anaerobic digestion, and as a result, waste gas captured from treatment plants and landfills is often heavily contaminated with these compounds. It is known that siloxanes can be removed using non-regenerative packed bed adsorption with activated carbon or porous silica 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.

(7) A silanol is a functional group in silicon chemistry with the connectivity SiOH. It is related to the hydroxy functional group COH found in all alcohols.

(8) So a major issue in the utilization of raw gas from landfills and anaerobic digesters is to provide a gas stream with a low sulfur content, i.e. less than a few hundred ppm, and with a very low content of siloxanes, typically linear or cyclic dimethyl SiOSi compounds, and silanols. Pipeline specifications for natural gas are even stricter. In this case, H.sub.2S must be removed to a residual concentration below 5 ppm, and CO.sub.2 and N.sub.2 need to be removed as well. Combustion of sulfur containing compounds leads to formation of sulfur trioxide which will react with moisture in the gas to form sulfuric acid, which can condense in cold spots and lead to corrosion. However, particularly siloxanes give rise to problems because they are converted to SiO.sub.2 during combustion, leading to build-up of abrasive solid deposits inside the engine and causing damage, reduced service time and increased maintenance requirements for many components such as compressors, fans, blowers, burner nozzles, heat recovery surfaces in boilers and for gas engine components such as spark plugs, valves, pistons etc. In addition to causing damage and reduced service time to the engine, also any catalysts installed to control exhaust gas emissions are sensitive to SiO.sub.2 entrained in the gas stream, in fact even more so than the engine itself. For an SCR (selective catalytic reduction) catalyst, for example, the SiO.sub.2 tolerance can be as low as 250 ppb.

(9) For the reasons outlined above it is very desirable to remove siloxanes, silanols and sulfur-containing compounds from gas streams.

(10) Preferably the gas feed, from which oxygen is to be removed, is a landfill gas, a digester gas or an industrial CO.sub.2 off-gas.

(11) In a preferred embodiment of the method of the invention, a gas stream, such as a landfill gas containing H.sub.2S and organic sulfur along with siloxanes, CO.sub.2, H.sub.2O, methane, chlorinated compounds, freon compounds and various VOC (volatile organic carbon) compounds, is treated.

(12) The components to be injected in step (d) comprise one or more of H.sub.2, CO, ammonia, urea, methanol, ethanol and dimethylether (DME).

(13) Landfill gas of low quality, i.e. having a high content of nitrogen and oxygen, is more difficult and expensive to upgrade to pipeline quality than gases with a lower content of nitrogen and oxygen. Using the reactant injection to remove the oxygen from low quality landfill gases will lead to a high temperature increase in the reactor, which in turn will damage the catalyst. If, however, the reactant is dosed at two different points instead of one point, it is possible to use two reactors in series with cooling and reactant injection in between. This approach has the added benefit that the energy recovered after each reactor can be used in a reboiler in the CO.sub.2 separation unit (amine wash) to regenerate the amine, and it can also be used as a feed preheater. The energy for the reboiler and for preheating of the feed would otherwise have to come from electricity or from combustion of landfill gas or natural gas.

(14) The heat coming from the oxidation can be transferred to an oil circuit which is used both to run a reboiler in the amine wash in the subsequent CO.sub.2 removal and to preheat the feed.

(15) The invention is illustrated further with reference to the FIGURE, where the present invention is combined with Applicant's GECCO technology for digester and landfill gas conditioning. The feed gas is heated to 200-450 C. and fed to a siloxane and silanol absorption bed comprising alumina, alumina with nickel, silica or combinations thereof. After siloxane and silanol removal, one or more components suitable for catalytic oxidation, i.e. H.sub.2, CO, ammonia, urea, methanol, ethanol, DME etc., is/are injected into the main gas stream containing oxygen. Then the gas is fed to a catalytic reactor for both selective oxidation of sulfur components and selective catalytic oxygen conversion. Said catalytic reactor contains one or more catalysts converting the sulfur compounds to SO.sub.2 and the VOC compounds (not methane and light [i.e. C3 and lower] hydrocarbons) to CO.sub.2 and water and also hydrogen halides if some of the VOCs are halogenated. The catalyst(s) also effect(s) selective oxidation to H.sub.2O and CO.sub.2, while the valuable hydrocarbons, such as methane and light [i.e. C3 and lower] hydrocarbons, are substantially not converted. It is preferred that the catalyst comprises tungsten, vanadium, molybdenum, platinum or palladium in metallic or metal oxide form supported on a TiO.sub.2 carrier.

(16) The catalyst(s) can be selected from tungsten, vanadium, molybdenum, platinum and palladium in metallic or in metal oxide form supported on a TiO.sub.2 carrier or from vanadium, tungsten, chromium, copper, manganese, molybdenum, platinum, palladium, rhodium or ruthenium in metallic or metal oxide form supported on a carrier selected from alumina, titania, silica and ceria or combinations thereof.

(17) The hot reactor exit gas can be utilized to heat the reactor inlet gas by using a feed-effluent heat exchanger.

(18) The additional heat generated in the oxygen removal step will provide a higher temperature difference for the feed-effluent heat exchanger, which reduces the CAPEX.

(19) Downstream from the heat exchanger, the SO.sub.2 is removed in a wet caustic or H.sub.2O.sub.2 scrubber or a dry scrubber using a caustic sorbent. After the SO.sub.2 removal, CO.sub.2 is removed by using amine-based technology, solvent-based CO.sub.2 removal technology, water-based CO.sub.2 removal technology or alternatively PSA and/or membrane technology.

(20) Nitrogen removal can be accomplished using membrane or PSA based technology. Then water is removed by using cooling and condensation followed by a molecular sieve, alternatively in a TSA configuration. Alternatively, the nitrogen removal unit is positioned downstream from the water removal unit.

(21) It is further preferred that the catalyst is monolithic to decrease the power consumption for transport of the landfill gas through the cleaning section.