The present invention relates to catalytic hot gas filtra- tion for combined ¾S and siloxane abatement in landfill gas and biogas applications. More specifically, the inven ^¬ tion relates to a process for the combined removal of si ^¬ loxanes and sulfur-containing compounds from biogas

streams, in particular biogas streams from landfills and anaerobic digesters.

The major challenge for utilizing landfill gas is the need to remove sulfur-containing compounds and siloxanes from the gas. At present, the technology provides separate units to handle sulfur and siloxane removal, and this fact adds up to significant capital cost as well as operational cost. Another significant issue is that the current technology on offer in the market adds to the complexity of operation for these relatively small (5-20 MW) units.

Biogas is typically a waste product from sources including landfills and anaerobic digesters. In general, biogas con ^¬ tains approximately 50-75% methane, 25-50% carbon dioxide, 0-10% nitrogen, 0-1% hydrogen, 0.1-3% sulfur and 0-2% oxy- gen, all by volume. It also contains an assortment of impu ^¬ rities that can include siloxanes as well as chlorine, var ^¬ ious volatile organic compounds (VOCs) and ammonia. Because biogas is typically generated from organic matter, it is generally considered a renewable form of energy.

Since biogas contains methane, it is convertible to a bio ^¬ gas fuel for power or heat generation. However, it needs to be cleaned first. One of the reasons that biogas should be cleaned prior to use is that sulfur impurities in biogas can create a corrosive environment inside power generating equipment or, even worse, poison any catalysts that may be present. Furthermore, hydrogen sulfide present in the feed gas to gas engines will cause degradation of the lubricat ^¬ ing oil and lead to a need of frequent maintenance. A fur ^¬ ther reason to clean biogas is that other impurities, such as siloxanes, can be deposited within heat and power gener- ation equipment and cause significant damage to the inter ^¬ nal components.

Landfill gas is a gas originating from landfills as a re ^¬ sult of various bacterial digestion processes in the land- fill itself. The gas typically contains roughly 45-50% CH ₄ , 45-50% CO ₂ , up to 1% ¾S, some nitrogen and siloxanes along with low levels of organic sulfur components and VOCs .

Landfill gas has a high energy content and is typically used as a fuel for gas engines, although smaller gas tur ^¬ bines and boilers can also work using landfill gas. In some cases, the gas is upgraded and exported to the public gas grid or used as a fuel gas for other industrial processes. The dominating market today is in the US, and reciprocating gas engines are dominating the market for landfill gas uti ^¬ lization .

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 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 land- fills is often heavily contaminated with these compounds.

It is known that siloxanes can be removed by using non-re ^¬ generative 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.

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. Combustion of sulfur containing compounds leads to formation of sulfur trioxide that will react with moisture in the gas to form sulfuric acid, which in turn can condense in cold spots and lead to corrosion. However, particularly siloxanes give rise to problems because they are converted to S1O 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 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 S1O 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 S1O ₂ toler ^¬ ance can be as low as 250 ppb .

For the reasons outlined above it is desirable to remove siloxanes and sulfur containing compounds from gas streams to increase the engine service time and the catalyst life ^¬ time .

According to the present technology in the field, separate units are used to carry out the removal of siloxanes and the removal of sulfur containing compounds. Thus, in WO 2006/104801 A2 a siloxane removal process is described, where biogas released from landfills and sewage treatment plants is freed of siloxane contaminants by passing biogas at a temperature of 35-50°C through a bed containing acti ^¬ vated alumina, which adsorbs the siloxanes. When the acti ^¬ vated alumina becomes saturated with siloxanes, the adsorp ^¬ tion capability of the activated alumina is recovered by passing a regeneration gas through the bed of activated alumina. In a system with two or more beds of activated alumina, one bed is used to remove siloxanes, while one or more of the other beds are being regenerated.

WO 2008/024329 Al discloses a system comprising an adsor- bent bed for removing siloxanes from biogas down to a very low siloxane level, so that the cleaned biogas can be used as intake air for equipment, such as combustion engines or gas turbines. The sole specific example in the description indicates a reaction temperature in the adsorption towers of between -28.9°C and 121°C. The adsorbent bed comprises at least two of activated carbon, silica gel and a molecu ^¬ lar sieve. US 2010/0063343 Al describes the cleaning and recovery of a methane fuel from landfill gas, more particularly a process for concentrating and removing certain commonly occurring pollutants from landfill gas. The harmful constituents treated include water, particulates, sulfur (as hydrogen sulfide) and siloxanes.

US 2012/0301366 discloses a microwave-induced destruction of siloxanes and hydrogen sulfide in biogas, and finally US 2015/0209717 describes a process for the removal of silox ^¬ anes and related compounds from gas streams by adsorption.

There are two major challenges in the utilization of land- fill gas:

(i) The fact that siloxanes in the gas precipitate on the engine parts causes scaling, wear and maintenance is ^¬ sues, and

(ii) ¾S reacts and interferes with the lubrication oil in the gas engine, which causes frequent maintenance stops and oil replacements. In addition, ¾S is combusted in the gas engine and emitted as SOx. Depending on local SOx emission regulations, the use of a flue gas scrubber may become necessary, which will add significantly to the capital cost of the treatment unit. Further, siloxanes may poison and foul the SCR (se- lective catalytic reduction) and CO oxidation catalyst downstream from the gas engine, making siloxane removal ab ^¬ solutely necessary when the engine is equipped with flue gas cleaning using an SCR catalyst for NOx removal and/or a CO oxidation catalyst.

Siloxanes can be removed using non-regenerative packed-bed adsorption with a sorbent such as e.g. activated carbon or porous silica. Regenerative sorbents are also used as well as units, which are based on gas cooling to very low temperatures, to precipitate the siloxanes from the gas. In addition, liquid extraction technologies are used. These various technologies can moreover be used in combination.

As regards the removal of ¾S, there are several alterna ^¬ tives in existence, for example solid sorbents based on ac ^¬ tivated carbon, supported iron-based sorbents, Lo-Cat type technology, regenerative sorbents and biological based sul ^¬ fur removal technology, such as ThioPac.

The major issue for these technologies to remove sulfur and siloxanes is that they add up to significant capital cost as well as operational cost. Another significant issue is that it adds to the operation complexity for these rela ^¬ tively small units (5-20 MW) .

The present invention represents the combined knowledge of the Applicant within SMC ¾S oxidation technology and cata ^¬ lytic filtration.

More specifically, the present invention relates to a pro ^¬ cess for the combined removal of siloxanes and sulfur-con- taining compounds from biogas streams, in particular biogas streams from landfills and anaerobic digesters, said process comprising the steps of: heating the biogas stream and if necessary, to provide a sufficient oxygen level, mixing it with air, feeding the gas mixture to a filter unit provided with cat ^¬ alytic filter candles or filter bags with high temperature resistance, injecting a dry sorbent into the filter unit to capture si- loxanes present in the gas mixture, recycling part of the exit gas from the filter unit to the filter house inlet for the sulfur-containing compounds to be captured by the dry sorbent and recovering the clean gas from the filter unit.

The gas from the landfill is heated and mixed with air, so that there is at least 0.1% residual air when the oxygen is consumed in the reaction

2 H ₂ S + 3 0 ₂ -> 2 S0 ₂ + 2 H ₂ 0

The mixed gas is heated to at least 180°C before entering a filter house unit with catalytic filter candles or filter bags with high temperature resistance. The catalytic fil ^¬ ters are impregnated with a Ti0 ₂ -based catalyst supported by vanadia or a combination of vanadia and palladium. In the filter, H ₂ S is converted to S0 ₂ and low levels of SO ₃ .

The purpose of recirculating part of the exit gas from the first filter unit to the inlet thereof is to minimize the investment costs of the process by utilizing the first fil ^¬ ter for removal of both siloxanes and S O2/SO ₃ .

The gas stream to be treated is mixed with the recircula- tion gas, which contains S O2/SO ₃ . This creates a larger gas flow than the original gas stream. Dry sorbent(s) is/are injected into this mixed gas to absorb siloxanes from the original gas and S O2/SO ₃ from the recirculated gas. The gas then passes the filters, thereby removing the dry sorb- ent(s) from the gas. The catalyst located inside the filter is not exposed to the siloxanes, which otherwise may cause poisoning of the catalyst. The catalyst oxidizes the ¾S, present in the original gas stream, to S O2 and low levels of SO ₃ . The gas exits the filter unit.

A part of the gas exiting the filter unit is recirculated and mixed into the original gas stream. The purpose of this recirculation is to remove some of the S O2/SO ₃ that was formed over the catalyst by oxidizing the ¾S, thereby re- ducing the total sulfur content of the gas leaving the sys ^¬ tem. The amount of recirculated gas depends on the desired level of sulfur content in the gas leaving the system.

The filter house unit is equipped with means for injection of a dry sorbent, where sorbents like limestone, Ca(OH) 2 , Trona (trisodium hydrogen dicarbonate dehydrate, also called sodium sesquicarbonate dihydrate, a ₂ C03 -NaHC03 ·2¾0) and others can be used. These sorbents will catch the si ^¬ loxanes, but specific sorbents targeted to catch siloxanes can be added to the dry sorbent injection step. The exit gas from the filter unit is recycled back to the filter house inlet duct and led to the filter house, where SO ₂ and SO ₃ are captured by the dry sorbent. This way, the catalyst is protected from siloxanes since it is situated inside the filter, and one unit can handle both siloxane and sulfur removal.

Another valid alternative is to divide the filter house unit into two compartments, where siloxane removal is ac- complished in the first compartment with catalytic filters and sulfur removal is accomplished in the second compart ^¬ ment with non-catalytic filters. In this case, the siloxane specific sorbent can be injected into the first unit, while sulfur-specific sorbents can be injected into the second unit.

The clean gas from the filter house unit(s) is used to pre ^¬ heat the feed gas using a feed/effluent heat exchanger.