WO2018149709A1

WO2018149709A1 - A method for the removal of oxygen from an industrial gas

Info

Publication number: WO2018149709A1
Application number: PCT/EP2018/053027
Authority: WO
Inventors: Kresten Egeblad; Niklas Bengt Jakobsson; Rasmus TRANE-RESTRUP; Jacob Hjerrild Zeuthen; Troels Dahlgaard Stummann
Original assignee: Haldor Topsøe A/S
Priority date: 2017-02-16
Filing date: 2018-02-07
Publication date: 2018-08-23

Abstract

Oxygen is removed from a gas feed such as a landfill gas, a digester gas or an industrial CO₂ off-gas by removing sulfur-containing compounds and siloxanes from the feed gas, 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, optionally converting any or all of the volatile organic compounds (VOCs) present in the gas, including sulfur-containing compounds and/or chlorine-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 oxygen conversion in at least one suitable reactor, and cleaning the resulting oxygen-depleted gas. The reactants to be injected comprise one or more of H₂, CO, ammonia, urea, methanol, ethanol and dimethyl ether (DME).

Description

A method for the removal of oxygen from an industrial gas

The present invention relates to a novel method for the re^¬ moval of oxygen from an industrial gas through selective catalytic oxidation via reactant injection.

More specifically, the invention concerns an alternative route to reduce the oxygen content in industrial gases, where the reduction of the content of oxygen is crucial for the valorization of the gas. The method of the invention is especially focused on siloxane/silanol removal and/or sul^¬ fur conversion/removal taking place in the hot loop rather than before it. The method of the invention is focusing on landfill gas, digester gas and industrial CO₂ off-gas. Today, oxygen re^¬ moval is accomplished through PSA (pressure swing adsorp^¬ tion) , membrane or scrubber technologies with very high capital expenditure (CAPEX) and also a substantial loss of valuable components, such as methane in the main gas to the oxygen-containing off-gas. The present invention comprises addition of components, such as ¾, CO, methanol, ammonia or ethanol, to the main gas stream and leading the result^¬ ant gas stream to at least one catalytic reactor. In said reactor (s), the oxygen is converted selectively to CO₂ and water across the catalyst.

Removal of oxygen from fuel gas streams is often a require^¬ ment for distribution of the gas in the natural gas grid, and it is also a requirement when utilizing the gas as a vehicle transportation fuel. In addition, removal of oxygen is also critical for the utilization of other industrial gas streams, such as in producing merchant or industrial grade CO₂ from oxygen-containing off-gases.

US 3.361.531 describes the removal of oxygen from oxygen- containing environments and gas mixtures by absorption in a solid material contact mass. More specifically, a compound selected from copper carbonate, manganese carbonate and iron carbonate is contacted with a hydrogen-containing gas at an elevated temperature below about 500 °C, thereby re- ducing the carbonate to the corresponding oxide compound.

This oxide compound is brought into contact with said oxy^¬ gen-containing environment at around ambient temperature, thereby absorbing the oxygen and oxidizing the oxide compound .

US 2013/0209338 Al discloses an integrated cleaning system to clean biogas from sources such as landfills and digest^¬ ers for heat and power generation systems. Siloxanes, chlo^¬ rine, oxygen and sulfur are removed to ppb levels, and the majority of water and some VOCs is removed as well. The system cools a biogas stream to partially remove contami^¬ nants, blends in a small concentration of hydrogen gas and then combusts the remaining oxygen to heat the biogas and leave sufficient hydrogen suitable for a downstream se- quence of further contaminant conversion and removal in stages using a hydrodesulfurization bed and adsorbent media beds. This may well be a reasonable approach for low sulfur levels, but not when the ¾S level exceeds 200-300 ppm. The approach according to the present invention is different in that methanol or other reducing agents can be used, and the impurities are kept in oxidized form to be removable from the stream by using a scrubber. The technologies dominating the industry today are PSA and membrane based technologies in small and medium sized pro^¬ jects (typically up to 10,000 Nm³/h gas), whereas distilla- tion and cryogenic separation are dominating in larger scale applications.

For applications in the digester gas and landfill gas puri^¬ fication industry the gas flows are in the range of 500 to 10,000 Nm³/h, and technologies based on PSA and membranes are dominating. Apart from an often prohibitive CAPEX, PSA and membrane technologies have a high operation cost be^¬ cause of their complexity and gas compression as well as a substantial loss of valuable hydrocarbons, such as methane, from the feed gas stream to the oxygen-containing waste gas stream.

In the method according to the present invention, one or more components suitable for catalytic oxidation are in- jected into the oxygen-containing main gas stream after removal of sulfur-containing compounds and siloxanes 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.

The components to be injected may comprise one or more of i.a. ¾, CO, ammonia, urea, ethanol and dimethyl ether (DME) .

The active catalyst may comprise a metal selected among va^¬ nadium, tungsten, chromium, copper, manganese, molybdenum, platinum, palladium, rhodium and ruthenium in metallic or metal oxide form supported on a carrier selected from alu^¬ mina, titania, silica and ceria and combinations thereof. Sulfur impurities in an industrial gas can create a corro^¬ sive 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, ¾S needs to be removed if the gas is to be sent to gas pipelines or used as fuel in vehicles.

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.

Siloxanes are organosilicon compounds comprising silicon, carbon, hydrogen and oxygen which have Si-O-Si bonds. Si^¬ loxanes can be linear as well as cyclic. They may be pre^¬ sent in digester or landfill gas because they are used in various beauty products, such as e.g. cosmetics and sham^¬ poos 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 com^¬ pounds. It is known that siloxanes can be removed using non-regenerative packed bed adsorption with activated car^¬ bon 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. A silanol is a functional group in silicon chemistry with the connectivity Si-O-H. It is related to the hydroxy func^¬ tional group C-O-H found in all alcohols.

So a major issue in the utilization of raw gas from land- fills 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 Si-O-Si compounds, and silanols. Pipeline specifications for natural gas are even stricter. In this case, ¾S must be removed to a residual concentra^¬ tion below 5 ppm, and CO₂ and 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 S1O₂ during combustion, leading to build-up of abrasive solid deposits inside the engine and causing damage, re^¬ duced 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 ex- haust gas emissions are sensitive to S1O₂ entrained in the gas stream, in fact even more so than the engine itself. For an SCR (selective catalytic reduction) catalyst, for example, the S1O₂ tolerance can be as low as 250 ppb .

For the reasons outlined above it is very desirable to re- move siloxanes, silanols and sulfur-containing compounds from gas streams.

Thus, the present invention relates to a method for the re^¬ moval of oxygen from an industrial gas feed, said process comprising the steps of:

(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) optionally converting any or all of the volatile or^¬ ganic compounds (VOCs) present in the gas, including sul^¬ fur-containing compounds and/or chlorine-containing compounds, in the heated feed gas, (e) injecting one or more reactants for oxygen conversion into the heated feed gas,

(f) carrying out a selective catalytic oxygen conversion in at least one suitable reactor, and

(g) cleaning the resulting oxygen-depleted gas. Preferably the gas feed, from which oxygen is to be re^¬ moved, is a landfill gas, a digester gas or an industrial CO2 off-gas. In a preferred embodiment of the method of the invention, a gas stream, such as a landfill gas containing ¾S and organic sulfur along with siloxanes, CO₂, ¾0, methane and various VOCs (volatile organic carbon compounds) , is treated .

The components to be injected in step (d) comprise one or more of ¾, CO, ammonia, urea, methanol, ethanol and dime^¬ thyl ether (DME) . Landfill gas of low quality, i.e. having a high content of nitrogen and oxygen, is more difficult and expensive to up^¬ grade 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 re^¬ actant injection in between. This approach has the added benefit that the energy recovered after each reactor can be used in a reboiler in the CO₂ 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. The heat coming from the oxidation can be transferred to an oil or steam circuit which is used both to run a reboiler in the amine wash in the subsequent CO2 removal and to pre^¬ heat the feed.

The invention is illustrated further with reference to the figure, where the present invention is combined with Appli^¬ cant's GECCO™ technology for digester and landfill gas conditioning. The feed gas is heated to 200-450°C and fed to a siloxane or silanol absorption bed comprising alumina, alumina with nickel, silica or combinations thereof. After siloxane removal, the gas is fed to a catalytic reactor containing a catalyst selected from tungsten, vanadium, molybdenum, platinum and palladium in metallic or metal oxide form supported on a T 1 O2 carrier. In this catalytic reac^¬ tor, the catalyst converts the sulfur compounds to S O2 and the VOC compounds (not methane and light [i.e. C3 and lower] hydrocarbons) to CO2 and water and also hydrogen halides if some of the VOCs are halogenated.

One or more components suitable for catalytic oxidation, i.e. ¾, CO, ammonia, urea, methanol, ethanol, DME etc., is/are injected into the main gas stream containing oxygen, and the gas stream is fed to the catalytic reactor contain- ing a catalyst such as 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. In the reactor, the injected compo- nent(s) is/are selectively oxidized to ¾0 and CO2 , while the valuable hydrocarbons, such as methane and light [i.e. C3 and lower] hydrocarbons, are substantially not con^¬ verted. It is preferred that the catalyst comprises tung^¬ sten, vanadium, molybdenum, platinum or palladium in metallic or metal oxide form supported on a T 1 O2 carrier.

The hot reactor exit gas can be utilized to heat the reac^¬ tor inlet gas by using a feed-effluent heat exchanger.

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.

Downstream from the heat exchanger, the S O2 is removed in a wet caustic or H2O2 scrubber or a dry scrubber using a caustic sorbent. After the S O2 removal, CO2 is removed by using amine-based technology, solvent-based CO2 removal technology, water-based CO2 removal technology or alterna^¬ tively PSA and/or membrane technology. 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, alterna^¬ tively in a TSA configuration. Alternatively, the nitrogen removal unit is positioned downstream from the water re- moval unit.

It is further preferred that the catalyst used for sulfur and/or oxygen removal is monolithic to decrease the power consumption for transport of the landfill gas through the cleaning section.

Claims

Claims :

1. A method for the removal of oxygen from an indus^¬ trial gas feed containing VOCs, sulfur compounds and/or chlorine compounds and optionally siloxanes and silanols, said process comprising in sequence the steps of:

(a) heating the feed gas, (b) optionally removing siloxanes and silanols from the heated feed gas,

(c) oxidizing in the heated feed gas any or all of the VOCs to C02, oxidizing the sulfur-containing compounds to S02 and/or oxidizing the chlorine-containing compounds to C02 and HC1,

(d) removing S02 and/or HC1 in a wet caustic or H202 scrubber or in a dry scrubber using a caustic sorbent,

(e) injecting one or more reactants selected from H2, CO, ammonia, urea, methanol, ethanol and dimethyl-ether (DME) for oxygen conversion into the heated feed gas from step

(d) ,

(g) cleaning the resulting oxygen-depleted gas.

2. 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 CO₂ off-gas.

3. Method according to claim 1 or 2, wherein the cleaning in step (g) comprises removal of CO₂ in a separa^¬ tion unit, removal of ₂ and drying of the cleaned gas.

4. Method according to any of the claims 1-3, wherein the gas has a high content of nitrogen and oxygen, and wherein two reactors with cooling in between are used for the selective catalytic oxygen conversion in step (f) .

5. Method according to claim 4, wherein the heat re- covered after each reactor is used in a re-boiler in the

CO₂ separation unit.

6. Method according to claim 1 or 2, wherein the feed gas is heated to a temperature of between 150 and 450°C.

7. Method according to claim 1 or 2, wherein the feed gas is heated to a temperature of between 150 and 450°C and thereafter fed into step (b) and/or step (c) .

8. Method according to claim 7, wherein the feed gas is heated through heat exchange with the effluent gas from the oxygen removal step prior to be fed into step (b) and/or step (c) .

9. Method according to claim 2, wherein the landfill gas contains ¾S and organic sulfur compounds, CO₂, ¾0, me^¬ thane, VOCs and optionally siloxanes and silanols.

10. Method according to any of the preceding claims, wherein the catalyst for the oxygen conversion 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 car^¬ rier selected from alumina, titania, silica and ceria.