DK201600626A1 - A process for the removal of siloxanes from flue gas - Google Patents

Abstract

In a process for the removal of siloxanes from a hot flue gas, the flue gas is passed through a siloxane removal unit containing a bed of a siloxane sorbent material, which has surface hydroxyl groups over the relevant temperature range, preferably alumina, and the siloxane removal unit is located before the deNOx unit, in which selective catalytic reduction (SCR) is performed, in a position where the gas has a temperature in the range from 200 to 600°C.

Description

Title: A process for the removal of siloxanes from flue gas

The present invention relates to a novel process for the removal of siloxanes from flue gases. More specifically, the invention is focused on the importance of removing any siloxanes and also silanols, i.e. fully or partly hydrolyzed siloxanes, from an engine flue gas before it reaches the deNOx system. The importance of removing siloxanes is due to the fact that the application of deNOx to reduce the content of nitrogen oxides (NOx) in flue gases from power generation based on landfill gas (LFG) is prohibited if siloxanes are present in the LFG, since these compounds are known as selective catalytic reduction (SCR) catalyst poisons .

Landfill gas contains siloxanes as one of many types of chemical impurities. The siloxane level in the upgraded renewable natural gas (RNG) must follow strict requirements in order to get the RNG approved for the natural gas grid. SCR is recognized world-wide as the most effective NOx control technology for boilers and combustion turbines when a substantial NOx reduction of 50 to 95% is required. In addition to its proven high performance, it is also an economically viable solution, and the technology has even provided some utilities with the capability to achieve lower heat rates by allowing optimization of burner operation and reduction or omission of flue gas recirculation, further adding to its cost effectiveness.

Chemically, the SCR process reduces the NOx molecule into molecular nitrogen and water vapor. A nitrogen-based reagent such as ammonia or urea is injected into the ductwork, downstream of the combustion unit. The waste gas mixes with the reagent and enters a catalyst-containing reactor. The hot flue gas and reagent diffuse through the catalyst. The reagent reacts selectively with the NOx within a specific temperature range and in the presence of the catalyst and oxygen .

The technology of SCR has been applied to a wide variety of industrial applications for decades. Thus, flue gases generated from refinery off-gas combustion to natural gas, oil or coal fired units have been treated using SCR. More recent SCR applications include reduction of NOx emissions generated from boilers or diesel engines, as well as process gas streams in e.g. nitric acid plants, calcining ovens and gas turbines fired by landfill and/or digester gas.

Siloxanes are organosilicon compounds comprising silicon, carbon, hydrogen and oxygen which have Si-O-Si 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 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 various combinations . 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 negligible content of siloxanes, typically linear or cyclic dimethyl Si-O-Si compounds. Combustion of sulfur containing compounds leads to formation of sulfur trioxide which will react with moisture in the gas to form sulfuric acid. The sulfuric acid formed can condense in cold spots and lead to corrosion. However, particularly siloxanes give rise to problems because they are converted to SiCh 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. Also any catalysts installed to control exhaust gas emissions are sensitive to S1O2 entrained in the gas stream, in fact even more so than the engine itself. For an SCR catalyst, for example, the Si02 tolerance is way below 250 ppb, and it can even be as low as 5 ppb.

It is known in the landfill gas industry that adsorbents, such as activated carbon, silica or alumina, can be used to remove siloxanes present in the gas. However, so far, the siloxane removal by adsorbents has been carried out as a pre-treatment step before the power generator. These adsorbents can be used as scavengers, or they can be used in a regenerative process configuration using temperature swing adsorption. Siloxanes introduce severe issues for boilers, gas engines and gas turbines where they cause excessive wear on the equipment, fouling and frequent oil change-outs . WO 2008/024329 A1 discloses a system comprising an adsorbent 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 adsorbent bed comprises at least two of activated carbon, silica gel and a molecular sieve.

In US 9,039,807 B2, another regenerative adsorption process for siloxane removal is described. This process uses an adsorbent having a neutral surface, and it is used at a temperature of around 35-50°C. When the adsorbent bed is filled to capacity, it is heated to remove the siloxanes and regenerate the bed.

Ind. Eng. Chem. Res. 51 (48), 15786-15795 (2012), describes an experimental study of an internal combustion engine that is operating on natural gas spiked with siloxanes. The goal of the study was to understand the impact of siloxane impurities on engine performance. These impurities were shown to decompose completely during combustion of the gas in the engine, thereby forming silica microparticulates which coat the internal metal surfaces in the engine, such as piston heads, as well as the engine's oxygen sensors and spark plugs, and they also collect in the engine oil. It was found that a certain fraction of them, furthermore, was carried out of the engine in the flue gas, and they also deposited inside a catalyst monolith bed placed downstream of the engine, resulting in severe catalyst deactivation. These findings indicate that siloxane impurities readily decompose in the gas combustion environment to form silica particulates that will coat exposed metal surfaces. They also point to the critical importance for engine performance to adequately remove such siloxane impurities from the gas prior to use.

The present invention relates to the thorough removal of siloxanes from an engine flue gas before it reaches the deNOx system. This is done by means of passing the hot flue gas through a bed containing a siloxane absorption material, preferably alumina.

It may be advantageous, in addition to SCR, to remove carbon monoxide from the flue gas. For this purpose, a carbon monoxide oxidation catalyst can be used to remove carbon monoxide from the flue gas, either upstream or downstream from the SCR catalyst.

Regarding the temperature of the flue gas, this is between around 200°C and a temperature slightly above the actual engine exhaust temperature. As long as there are OH groups present on the surface of the sorbent, any adsorbed siloxane will react and form a glass.

More specifically, the invention concerns a process for the removal of siloxanes from a hot flue gas from an engine exhaust outlet, wherein - the flue gas is passed through a siloxane removal unit, which has surface hydroxyl groups over the relevant temperature range, and - the siloxane removal unit is located before the deNOx unit, in which selective catalytic reduction (SCR) is performed, in a position where the gas has a temperature in the range from 200 to 600°C.

Preferably the gas has a temperature in the range from 250 to 450 °C.

As mentioned, a preferred siloxane absorption material is alumina. However, the siloxane absorption material can also be hydrated silica, a zeolite or any other material or compound having surface hydroxyl groups in the relevant temperature interval. Further, the siloxane absorption material can be a sorbent comprising V2O5 on a TiC>2 carrier.

The siloxane absorption material can have a "polishing" effect, i.e. being even more sensitive to siloxane poisoning than alumina. Compounds having such polishing effect can e.g. be a zeolite as well as one or more of the compounds traditionally used in deNOx catalysts. SCR catalysts are most often based on a porous Ti02 carrier material, on which the catalytically active components, in the form of V2O5 combined with W and/or Mo oxides, are dispersed .

The siloxane absorption material to be used serves as a catalyst guard, which must be located upstream from the SCR catalyst because this catalyst will decompose any siloxanes present in the flue gas, thereby forming silicon which will cover the catalyst surface and thereby block the catalyst and reduce the oxidation activity. In order to secure a reasonable lifetime of the SCR catalyst, the content of Si in the feed gas should be below 10 ppb. A specially formulated metal-free catalyst, preferably an alumina formulation, is designed as a silica guard catalyst for severe silica poisoning problems. It exhibits a very high surface area to maximize silica pick-up and thus protect downstream catalysts.

The primary benefit of using the process of the invention is that the process enables the use of selective catalytic reduction (SCR) on landfill gas based power production, thereby indeed allowing landfill gas power plants to reach very low NOx emission levels. In addition, by placing the siloxane removal unit after the engine exhaust outlet and before the deNOx SCR system at a position, where the gas has a temperature in the range of 250 to 450°C, the invention presents the further advantage of avoiding the use of heat exchangers, trim heaters and/or air coolers to actively heat up the gas for the siloxane removal and subsequently cool it down.

The use of alumina to capture the siloxanes at a higher temperature provides a sharper absorption front and a higher degree of siloxane removal. Also a higher sorbent capacity for siloxanes is obtained.

Furthermore, by positioning the SCR downstream from the engine, the total siloxane load is reduced due to the fact that a part of the siloxanes is combusted in the engine.

The invention is illustrated further in the examples which follow.

In the examples, the above-mentioned sorbent has been tested for its ability to decompose and deposit siloxanes from landfill gas. The testing took place in a test unit consisting of thin parallel reactors placed in temperature-controlled chambers. Temperatures are fixed while running a test, and the flow of feed gas is controlled by mass flow controllers .

The siloxane feed used was hexamethyldisiloxane (HMDS), an organosilicon compound with the formula 0[Si(CH3) 3] 2· It was dosed by pumping a solution in toluene to a heated prechamber, where it was evaporated. Siloxanes that are not removed on the catalyst can be absorbed in a bubbling flask after the reactor. For this purpose, 100 ml xylene was used, and bubbling was done for 60 minutes. The xylene was analyzed for Si content using inductively coupled plasma mass spectrometry (ICP-MS).

Example 1

Three reactors were filled with various amounts of a specially formulated metal-free alumina catalyst.

More specifically 16 g, 9.3 g and 4.6 g catalyst, respectively, were used. These samples were all tested at 300°C. Further, at 350°C, a test was run with 14 g of catalyst and another test was run without catalyst. This is used to determine the inlet concentration of Si.

The inlet concentration, determined by the results without catalyst, was 94.3 ppmv Si. Absorption in xylene, by bubbling through 100 ml of xylene, was done after 1, 2, 3 and 4 days of testing. The results, shown in Fig. 1, demonstrate the siloxane removal as a percentage of the inlet Si concentration.

It appears clearly from the results that the presence of more catalyst leads to a higher removal efficiency. The single data point with removal at 350°C shows a higher removal efficiency than the corresponding test at 300°C with 14 g of catalyst.

Example 2

The results found in Example 1 did show an effective removal of siloxane, but due to increasingly stricter requirements and blocking of the catalyst, an even higher removal efficiency is required.

It was clearly shown in Example 1 that more catalyst resulted in a better siloxane removal, and it was also seen that a temperature of 350°C instead of 300°C leads to an improved removal. For this reason, a new testing was made with an increased amount of catalyst and more tests done at 350 °C.

To determine whether channeling and bypass of the catalyst could account for some of the Si present in the effluent, some of the tests were performed with crushed catalyst.

More specifically, the tests conducted were as follows:

(a) 22.5 g crushed catalyst, temp. 350°C

(b) 17.5 g crushed catalyst, temp. 350°C

(c) 17.5 g crushed catalyst, temp. 300°C

(d) 22.5 g crushed catalyst, temp. 300°C

(e) 22.5 g extrudated catalyst, temp. 300°C

The results, shown in Fig. 2, demonstrate the removal of Si in catalytic reactors containing alumina. The testing was done over 4 days with a sample being taken each day. The Si was absorbed in the reactor effluent by bubbling through 100 ml xylene.

In each test, the figure shows the Si removal as percentage of the inlet 29 ppmv Si that was determined by a test without catalyst.

Claims (7)

1. Claims :

   1. A process for the removal of siloxanes from a hot flue gas, wherein - the flue gas is passed through a siloxane removal unit containing a bed of a siloxane sorbent material, which has surface hydroxyl groups over the relevant temperature range, and - the siloxane removal unit is located before the deNOx unit, in which selective catalytic reduction (SCR) is performed, in a position where the gas has a temperature in the range from 200 to 600°C.
2. 2. Process according to claim 1, wherein the gas has a temperature in the range from 250 to 450°C.
3. 3. Process according to claim 1 or 2, wherein the siloxane absorption material is alumina.
4. 4. Process according to claim 1 or 2, wherein the siloxane absorption material is hydrated silica.
5. 5. Process according to claim 1 or 2, wherein the siloxane absorption material is a zeolite.
6. 6. Process according to claim 1 or 2, wherein the si-loxane absorption material is a sorbent comprising V2O5 on a TiCh carrier.
7. 7. Process according to any of the preceding claims, wherein the SCR is combined with a catalytic oxidation of CO upstream or downstream from the SCR catalyst.