WO2018189027A1

WO2018189027A1 - A process for removal of fully or partly hydrolyzed organosilicon compounds from flue gases upstream of a catalytic emissions control unit

Info

Publication number: WO2018189027A1
Application number: PCT/EP2018/058730
Authority: WO
Inventors: Niklas Bengt Jakobsson; Kresten Egeblad; Francesco Castellino; Troels Dahlgaard Stummann; Thomas Nathan WHITE
Original assignee: Haldor Topsoe AS
Current assignee: Topsoe AS
Priority date: 2017-04-11
Filing date: 2018-04-05
Publication date: 2018-10-18
Anticipated expiration: 2019-10-11

Abstract

In a process for the removal of fully or partly hydrolyzed organosilicon compounds including siloxanes, silazanes and silanols from a hot flue gas upstream of a catalytic emis- sions control system, the flue gas is passed through a si- loxane removal unit containing a siloxane sorbent material dispersed on a monolithic support. The siloxane removal unit is located upstream any emissions control catalyst unit, such as a 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 800°C.

Description

A PROCESS FOR REMOVAL OF FULLY OR PARTLY HYDROLYZED ORGANOSILICON COMPOUNDS FROM

FLUE

GASES UPSTREAM OF A CATALYTIC EMISSIONS CONTROL UNIT

5 The present invention relates to a novel process for the removal of fully or partly hydrolyzed organosilicon com^¬ pounds from flue gases by absorption in an absorption unit installed upstream of a catalytic emissions control unit or integrated therewith.

10

More specifically, the invention is focused on the im^¬ portance of removing any siloxanes, silazanes and silanols, i.e. fully or partly hydrolyzed silicon-containing organic compounds, from the flue gas resulting from power genera-

15 tion and incineration, i.e. in equipment such as engines, turbines, boilers and thermal oxidizers, from fuels or waste off-gases containing said compounds, i.e. fuel gases such as landfill gas, digester gas and off-gases from in^¬ dustrial operations, before the gas reaches the catalytic

20 emissions control system consisting e.g. of deNOx or CatOx.

The importance of removing silicon-containing compounds can be exemplified by considering the application of deNOx to reduce the content of nitrogen oxides (NOx) in flue gases from power generation based on landfill gas (LFG) and waste

25 water treatment off-gas which is prohibited or not cost ef^¬ fective if siloxanes are present in the LFG, since these compounds are known as poisons to emissions control cata^¬ lysts including, but not limited to selective catalytic re^¬ duction (SCR) catalysts.

30

The term "CatOX" as used herein, refers to catalytic oxida^¬ tion. SCR is recognized world-wide as the most effective NOx con^¬ trol technology for boilers, engines and combustion tur^¬ bines when a substantial NOx reduction of 50 to 95~6 is re quired. In addition to its proven high performance, it is also an economically viable solution, at least when the gas to be treated is free from catalyst poisons, and the tech^¬ nology 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 re- circulation, further adding to its cost effectiveness.

Chemically, in the SCR process, the NO and NO₂ molecules (NOx) are reduced over a catalyst into molecular nitrogen and water vapor by reaction with a reductant pre-mixed into the flue gas. The reductant is typically a nitrogen-based reagent, such as ammonia or urea, injected into the duct^¬ work, downstream of the combustion unit. The waste gas mixes with the reagent and enters a catalyst-containing re^¬ actor. 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 gen^¬ erated from refinery off-gas combustion to natural gas, oil or coal fired units have been treated using SCR. More re^¬ cent SCR applications include reduction of NOx emissions generated from boilers or diesel engines, as well as pro- cess gas streams in e.g. nitric acid plants, in calcining ovens and in gas turbines fired by landfill and/or digester gas . SCR based systems for NOx removal may include one or more layers of an oxidation catalyst, which also makes it possi^¬ ble to efficiently remove CO and a significant portion of any un-burned hydrocarbons. CatOx installed downstream of the SCR is also useful in minimizing any ammonia slip. NOx SCR systems employ deNOx and CatOx catalysts based on e.g. a porous ceramic fiber support reinforced with T1O₂ and ho^¬ mogeneously impregnated with the active components WO₃ and V₂0₅, respectively.

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 landfill gas and biogas because they, or their pre- cursors, are used in various beauty products, such as e.g. cosmetics and shampoos that are washed down drains or oth^¬ erwise disposed of, so that they end up in municipal wastewater and landfills. Siloxanes are not completely bro^¬ ken down during anaerobic digestion, and as a result, waste gas captured from treatment plants and landfills is often heavily contaminated with these compounds.

A silazane is any hydride of silicon and nitrogen having a straight or branched chain of silicon and nitrogen atoms joined by covalent bonds. By extension, the word is also used for any organic derivative of such compounds. They are analogous to siloxanes, with -NH- replacing -0- . Their in^¬ dividual name is dependent on the number of silicon atoms in the chemical structure. Hexamethyldisilazane, for exam- pie, contains two silicon atoms bonded to the nitrogen atom. The majority of silazanes are moisture sensitive. 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 tem- peratures to precipitate the siloxanes out from the gas.

Further, liquid extraction technologies are used. In addi^¬ tion, these technologies can be used in various combina^¬ tions .

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. Particularly, si^¬ loxanes 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. Also any catalysts installed to con^¬ trol exhaust gas emissions are sensitive to siloxanes en^¬ trained in the gas stream, in fact even more so than the engine itself. For an SCR catalyst, for example, the silox- ane 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 only as a pre-treatment step before the power generator. These ad- sorbents 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 ex^¬ cessive wear on the equipment, fouling and frequent oil change-outs . WO 2008/024329 Al 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 ad^¬ sorbent having a neutral surface, and it is used at a tem- perature 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 impu^¬ rities 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 perfor- mance 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 passing the hot flue gas through a monolithic siloxane removal unit, i.e. a monolith coated with a material capable of absorbing siloxanes.

Thus, the siloxane sorbent material is dispersed on a mono^¬ lithic support, the siloxane removal unit being located up- stream the deNox unit.

It may be advantageous, in addition to SCR, to remove car^¬ bon 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 silox^¬ ane will react and form a glass.

More specifically, the invention concerns a process for the removal of fully or partly hydrolyzed organosilicon com^¬ pounds including, but not limited to siloxanes, silazanes and silanols from a hot flue gas upstream of a catalytic emissions control system, wherein

- the flue gas is passed through a siloxane removal unit containing a siloxane sorbent material dispersed on a mono^¬ lithic support, said material having surface hydroxyl groups over the relevant temperature range, and

- the siloxane removal unit is located upstream any emis- sions control catalyst unit, such as a deNOx unit, in which selective catalytic reduction (SCR) is performed, in a po^¬ sition where the gas has a temperature in the range from 200 to 800°C. Preferably the gas has a temperature in the range from 200 to 600°C, most preferred from 250 to 450°C.

The terms "siloxane sorbent material" and "siloxane removal unit" as used herein are intended to cover not only silox- anes, but also silazanes, silanols and other fully or part^¬ ly hydrolyzed organosilicon compounds.

Preferably the catalytic emissions control system includes either or both of

(a) a deNOx unit, in which selective catalytic reduction (SCR) is performed, and

(b) a catox unit, in which catalytic oxidation is per- formed. The sorbent material preferably has a surface area higher than 50 m²/g.

Using a siloxane removal material, which is dispersed on a monolithic support instead of being a bulk material in a fixed bed, leads to at least three distinct and important advantages: (1) an insignificant pressure drop, (2) only negligible problems with silica dust generated by the pre^¬ ceding combustion of the siloxanes and entrained in the flue gas, and (3) the possibility of using a conventional flue gas "duct" design.

As mentioned, the preferred siloxane absorption material is alumina, more specifically porous alumina, i.e. high sur- face area alumina. However, the siloxane absorption mate^¬ rial 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 ab^¬ sorption material can be a sorbent comprising V₂O₅ on a T1O₂ carrier.

The siloxane absorption unit can have a "polishing" effect, i.e. being a secondary siloxane removal unit installed downstream of a primary siloxane removal unit, where said secondary siloxane removal unit may or may not be installed upstream of the power generating equipment or incinerator equipment .

Preferably the monolithic support is selected from corru- gated fibrous monoliths and extruded cordierite-type mono^¬ liths . The catalytic emissions control system includes a catalytic conversion step upstream or downstream from the emission control catalyst. It can include an SCR catalyst and/or a CatOx catalyst.

The SCR can be combined with catalytic oxidation of CO ei^¬ ther upstream or downstream from the SCR catalyst. The emission control catalyst is preferably a catalyst capable of performing the SCR reaction, CO oxidation and VOC oxida- tion simultaneously.

Advantageously the process according to the invention can be used in combination with a siloxane removal system lo^¬ cated upstream from the gas combustion step, so that the process constitutes a polishing step to protect air emis^¬ sion catalysts downstream from the gas combustion unit.

SCR catalysts are most often based on a porous T1O₂ carrier material, on which the catalytically active components, in the form of V₂O₅ combined with W and/or Mo oxides, are dis^¬ persed .

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 silica 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 material is designed as a catalyst guard to protect any catalysts from severe silica poisoning problems. It exhibits a very high surface area to maximize silica pick-up and thus protect downstream cata- lysts.

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 inven- tion presents the further advantage of avoiding the use of heat exchangers, trim heaters and/or air coolers to ac^¬ tively heat up the gas for the siloxane removal and subse^¬ quently cool it down. The use of a higher temperature in the absorption process provides a sharper absorption front and a higher degree of siloxane removal. Also a higher sorbent capacity for silox- anes 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.

Claims

Claims :

1. A process for the removal of fully or partly hydro- lyzed organosilicon compounds including, but not limited to siloxanes, silazanes and silanols from a hot flue gas up^¬ stream of a catalytic emissions control system, wherein

- the siloxane removal unit is located upstream any emis- sions control catalyst unit, such as a deNOx unit, in which selective catalytic reduction (SCR) is performed, in a po^¬ sition where the gas has a temperature in the range from 200 to 800°C.

2. Process according to claim 1, wherein the catalytic emissions control system includes either or both of

(b) a catox unit, in which catalytic oxidation is per^¬ formed .

3. Process according to claim 1, wherein the sorbent material has a surface area higher than 50 m²/g.

4. Process according to any of the claims 1-3, wherein the monolithic support is selected from corrugated fibrous monoliths and extruded cordierite-type monoliths.

5. Process according to claim 1, wherein the gas has a temperature in the range from 200 to 600°C.

6. Process according to claim 5, wherein the gas has a temperature in the range from 250 to 450°C.

7. Process according to any of the claims 1-6, wherein the catalytic emissions control system includes a catalytic conversion step upstream or downstream from the emission control catalyst.

8. Process according to claim 7, wherein the catalytic emissions control system includes an SCR catalyst.

9. Process according to claim 7, wherein the catalytic emissions control system includes a CatOx catalyst.

10. Process according to claim 8, wherein the SCR is combined with catalytic oxidation of CO upstream or down- stream from the SCR catalyst.

11. Process according to claim 7, wherein the emission control catalyst is a catalyst capable of performing the SCR reaction, CO oxidation and VOC oxidation simultaneously.

12. Process according to claim 1 used in combination with a siloxane removal system located upstream from the gas combustion step, wherein the process constitutes a pol ishing step to protect air emission catalysts downstream from the gas combustion unit.