Reducing no2 emissions in process gases

Abstract

The invention relates to a method for reducing the concentration of NO, NO2 and optionally SO2 in a process gas.

Description

Technical Field

[0001] This invention relates to a method for reducing NO2 emissions in process gas denitrification and desulfurization units to provide purified process gases for use in carbon capture processes or as non-opaque gases emitted into the atmosphere.

[0002] background

[0003] Due to concerns about climate change, there is growing interest in capturing CO2 from process gases involved in the oxidation of carbonaceous feedstocks, such as cement production, power plants burning biomass or fossil fuels, metal production, carbon black production, and refinery operations (e.g., fluidized bed catalytic cracking (FCC)). The CO2 concentration in these process gases typically ranges from 5-30 vol%, thus requiring process equipment to capture and concentrate CO2 into a nearly pure CO2 stream for use in sequestration, enhanced oil recovery, or food / beverage or chemical production.

[0004] The most mature CO2 capture technology is the so-called regenerated amine or thermal potassium carbonate (HPC) process. Both processes share the characteristic that (acidic) CO2 gas is absorbed in an alkaline solution during the absorption process, typically using an irrigated packed bed absorber. The CO2-loaded liquid leaving the absorption process is then transferred to a distillation / stripping process, where CO2 is released as a nearly pure CO2 stream, while the CO2-lean absorbent solution is recovered and returned to the CO2 absorption process. CO2 desorption typically takes place in a stripping column, where heat is provided in a reboiler at the bottom of the column, and the distillate at the top is cooled and partially condensed; the condensate (primarily water) is returned, leaving only pure CO2 leaving the column. The heat provided is typically low-pressure steam, but can also be any fluid with high energy density and a sufficiently low temperature to protect the absorbent from thermal degradation. This can be heat transfer oil, process gas, or hot air.

[0005] The absorbent solution is not specifically designed for CO2, meaning that other acidic compounds such as SO2 and NO2 will also be absorbed into the solution. These two compounds almost irreversibly react with the absorbent solution to form so-called thermally stable salts, which degrade the absorbent solution, leading to reduced CO2 capture efficiency and / or increased energy consumption during distillation / stripping.

[0006] To maintain the efficiency of the CO2 capture process, a portion of the degraded absorbent solution must be removed and replaced with fresh absorbent solution, or the absorbent solution must be chemically regenerated. Both solutions increase the operating costs of the carbon capture process and generate a stream of waste that needs to be treated.

[0007] Therefore, it is advisable to reduce the concentration of SO2 and NO2 in the process gas as much as possible before the process gas enters the carbon capture equipment.

[0008] The most common method for removing SO2 from process gases is to absorb (acidic) SO2 and react it with an alkaline reactant (such as aqueous solutions of NaOH, NH3, Mg(OH)2, and CaCO3). These processes are typically carried out in the range of 40-60°C. These processes have been used for decades and are characterized by their efficiency, relative inexpensiveness, and simplicity. The disadvantages of these processes are the need for a chemical supply and the removal of (waste) products, which requires additional logistics and storage facilities. Low local availability of chemicals or the presence of unusable products can significantly increase the operating costs of this solution.

[0009] Another way to remove SO2 from process gases is to catalytically oxidize SO2 to SO3, which is then combined with water to condense sulfuric acid, which can be sold as a valuable chemical. This process is often referred to as wet sulfuric acid technology, or WSA for short. It requires the presence of H2O and O2 in the process gas, which is usually quite common. However, if their presence is insufficient, they can be added in the form of O2 or air, as well as liquid water or vapor.

[0010] NO x NH3 is typically removed via a process known as selective catalytic reduction (SCR), in which NH3 selectively reacts with NO. x The reaction produces N2 and H2O. This process inevitably produces small amounts of NO, NO2, and / or NH3, and if the process also involves SO2 oxidation, this step will increase the NO2 concentration because it also oxidizes NO and NH3. Since NO2 degrades the amine trapping solution, a NO2 removal step is necessary.

[0011] KR20220075898 discloses a process in which CO and NO2 react upstream of an SCR process. This process cannot avoid a small amount of NO2 escaping during the SCR process. Summary of the Invention

[0012] Therefore, this application provides a method for reducing the concentrations of NO, NO2, and optionally SO2 in a process gas, the method comprising subjecting the process gas to: a. An SCR step, which catalytically converts NO and NO2 in the process gas into N2 and H2O through reaction with a first NH3 stream. b. A combined step of NO2 reduction and CO oxidation in the presence of a sulfur-resistant dual-purpose catalyst, thereby catalytically reducing NO2 in the process gas to NO by reacting with CO, and oxidizing CO to CO2 by reacting with O2. The characteristic feature is that, after steps a and b, the NO2 / NO in the process gas... x The molar ratio is less than 0.05, preferably less than 0.02, and most preferably less than 0.01.

[0013] Further details of the technology will be presented in the following description, embodiments and patent claims. Attached Figure Description

[0014] Figure 1 The overall method of the present invention is shown.

[0015] Figure 2 The efficiency of the sulfur-resistant dual-purpose catalyst in Example 2 is shown.

[0016] Detailed disclosure

[0017] SO2 and NO in process gases x The conversion of (NO2+NO) is well known in the industry, and there are many mature technologies to achieve this gas-phase conversion, each with its own inherent advantages and disadvantages.

[0018] This invention proposes a novel layout combining SCR reaction and WSA process, and introduces a novel sulfur-resistant dual-purpose catalyst system to reduce NO2 emissions to sub-ppm concentrations while simultaneously eliminating CO emissions. The new layout is ideally suited for process gases containing reducing agents such as CO, where CO can be used as a reactant and subsequently oxidized to CO2. Examples of such process gases might originate from fluidized catalytic cracking processes, carbon black production processes, or metallurgical processes (such as steel mills).

[0019] This invention provides a method for reducing the concentration of NO, NO2, and optionally SO2 in process gases.

[0020] Prior to process step (a), the process gas typically contains 10–5,000 ppmv of CO and 1–1,000 ppmv of NO + NO2, as some NO may be converted into the problematic NO2.

[0021] Process gases typically contain up to 30% CO2, preferably 5-20% CO2.

[0022] Appropriately, prior to step (a), the process gas also contains an excess of CO relative to NO2, i.e., a CO / NO2 molar ratio greater than 1.

[0023] Process gases also have one or more of the following characteristics: - O2 concentration in the range of 2-20 vol%. - H2O concentration in the range of 2-40 vol%. - SO2 concentration in the range of 100-10,000 ppmv.

[0024] The remainder of the process gas is suitably N2, Ar and a small amount of (mnor) impurities.

[0025] Contains SO2 and NO x Process gases containing reducing agents can be heated or cooled to an optimal temperature range of 250-450°C. Before or after this temperature conditioning, the process gas can be passed through a particulate filter (e.g., a bag filter, ceramic candle filter, or electrostatic precipitator) and / or a process gas blower to purify the gas to remove particulate matter (dust) and provide the necessary gas pressure for the process gas to pass through the process equipment. Therefore, appropriately, the process gas is preconditioned by one or more preconditioning steps selected from the following: - Removes particulate matter, for example, in bag filters, electrostatic precipitators, or candle filters. - Temperature is regulated by heating or cooling, for example, through heat exchangers, burners, heaters, and - Regulate the pressure, for example, by means of a compressor or blower (e.g., regulate to approximately atmospheric pressure).

[0026] In its broadest sense, the method includes subjecting the process gas to: a. SCR step, followed by b. Combined steps of NO2 reduction and CO oxidation.

[0027] After steps a and b, the NO2 / NO ratio in the process gas x The molar ratio is less than 0.05, preferably less than 0.02, and most preferably less than 0.01.

[0028] In SCR step (a), NO and NO2 in the process gas are catalytically converted into N2 and H2O by reacting with the first NH3 stream. To make NO in the process gas... x Reduction, also known as selective catalytic reduction (SCR), is currently the most widely used process, capable of achieving very low NO levels. x Concentration. This process requires the addition of NH3 to facilitate the catalytic reaction.

[0029] 4 NO + 4 NH3 + O2 4 N2 + 6 H2O

[0030] NO2 + NO + 2NH3 2 N2 + 3 H2O

[0031] Adding an NH3 source to the process gas increases the NH3 / NO ratio in the process gas. x The ratio is close to 1 in order to achieve NO on SCR catalysts. x High conversion rate to N2. To obtain NH3 and NO. x To ensure uniform distribution, a static mixing device is typically installed between the NH3 injection point and the SCR catalyst.

[0032] The NH3 source can be liquid NH3, NH3 aqueous solution, NH3 vapor, or urea. It can be added to the process gas in liquid form or vapor form through multiple nozzles. It is preferred to add it together with a certain amount of carrier gas to ensure thorough mixing with the process gas.

[0033] The NH3 used in step a is added to the process gas so that the obtained NH3 / (NO+NO2) ratio is between 0.9 and 1.1.

[0034] The reaction is usually carried out in a temperature range of 250-450°C and a pressure of about 1 atm.

[0035] The so-called SNCR process is a non-catalytic version of SCR and requires temperatures in the range of 700-1000℃. This process is less efficient than the catalytic version. In low-temperature applications, ozone (O3) can be added to the process gas to oxidize NO to NO2 / HNO3, which can then be washed out during the absorption process.

[0036] On SCR catalysts, most NO x It will react with NH3 to produce NO. x Both the NO and NH3 concentrations of the SCR process gases are reduced. Typically, the design target for SCR catalysts is greater than 90% NO. x Conversion rate as high as 98-99%.

[0037] If NO x If a very high conversion rate is required, then (local) NH3 / NO x The ratio is often the decisive factor, therefore a second SCR catalyst may be required, and a mixing device should be installed between the first and second SCR catalysts to ensure a uniform NH3 / NO ratio before the second catalyst. x Compared to that, additional amounts of NH3 may need to be added.

[0038] SCR catalysts have been used for decades, and many chemical formulations are applicable to the purposes of this invention. Vanadium / titanium dioxide / tungsten-based catalysts on a monolithic support are the most well-known type of SCR catalyst.

[0039] Downstream of the SCR catalyst, the process gas will have very low concentrations of NO, NO2, and unconverted NH3. The SCR catalyst does not convert CO, therefore the CO concentration remains constant.

[0040] The combined NO2 reduction and CO oxidation step (b) is carried out in the presence of a sulfur-resistant dual-purpose catalyst, thereby catalytically reducing NO2 in the process gas to NO by reacting with CO and oxidizing CO to CO2 by reacting with O2.

[0041] The CO and O2 required for this step are typically present in the process gas itself at the necessary concentrations. However, CO and / or O2 can be added from external sources as needed.

[0042] This disclosure describes the use of a sulfur-resistant dual-purpose catalyst system that, according to the following reaction, firstly reduces NO2 to NO using CO, and then oxidizes the remaining CO to CO2: NO2 + CO NO + CO2

[0043] 2 CO + O2 CO2

[0044] This catalyst oxidizes problematic NO2 to NO, which is not absorbed into downstream carbon capture equipment, thus significantly extending the lifespan of the carbon capture absorbent. Furthermore, the catalyst oxidizes toxic CO gas into the much less problematic CO2, which is captured in downstream carbon capture equipment. The reaction rate of NO2 with CO must be higher than that of CO with O2; otherwise, the NO2 reduction reaction will not occur. The reaction rates of these reactions can be controlled by catalyst selection, reaction temperature, or a combination thereof.

[0045] This dual-purpose catalyst system may comprise a combination of a platinum group metal (preferably palladium or platinum) supported on a support and an oxide of one or more metals selected from vanadium, chromium, tungsten, molybdenum, cerium, niobium, manganese, and copper, wherein the support comprises an oxide of one or more metals selected from aluminum, silicon, and titanium. Similar effects can be observed when gold is used as the active metal. The catalyst is preferably monolithic, but granular form can also be used.

[0046] In the development of the method of the present invention, SO2 can be removed from the process gas. In this regard, the method further includes subjecting the process gas to: a1. An SO2 oxidation step between steps a and b (above), which catalytically oxidizes SO2 in the process gas to SO3 by reacting with O2; c. A cooling and condensation step following step b, wherein sulfuric acid vapor is condensed and removed from the process gas. After steps a, a1, b, and c, the NO2 / NOx molar ratio in the process gas is less than 0.05, preferably less than 0.02, and most preferably less than 0.01.

[0047] As an alternative, SO2 can also be removed by a scrubber or similar method after step (b).

[0048] In this respect, NO from step a. x The depleted process gas is directed to an SO2 oxidation catalyst, a well-known vanadium-based molten base catalyst. On the catalyst, the main reaction is the oxidation of SO2 to SO3 using O2 present in the process gas. SO2 oxidation catalyst reactors are typically designed to achieve SO2 conversions above 98%, with the maximum conversion limited by the thermodynamic equilibrium between SO2 and SO3. The SO2 oxidation catalyst also converts any NH3 escaping from the SCR catalyst to N2 or NO. x Depending on the gas composition and catalyst temperature, typically 40-70% of NH3 is converted to N2, and the remaining NH3 is converted to NO. x .

[0049] SO2 oxidation catalysts can also balance the reaction between NO and NO2: 2 NO + O2 NO2

[0050] The reaction is exothermic, therefore more NO2 will be formed at lower temperatures and higher O2 concentrations. The O2 concentration in the process gas is constant, and the SO2 conversion rate typically determines the catalyst's operating temperature; meaning there are few ways to reduce NO2 formation. At 400°C, the total NO... x Approximately 30-50% of the concentration will exist as NO2. Within a temperature range of 350-450℃, the SO2 oxidation catalyst will not oxidize CO.

[0051] Following SO2 oxidation (step a1), NO2 concentrations typically range from 2 to 20 ppm, which can have a very adverse effect on downstream carbon capture equipment and, if released into the atmosphere, may result in brown / red plumes from the chimney. Furthermore, CO concentrations show almost no change compared to the concentrations in the process gas entering the equipment.

[0052] For sulfuric acid production desulfurization equipment, the dual-purpose catalyst system is usually located downstream of the SO2 oxidation catalyst, used to reduce NO2 to NO and oxidize the remaining CO to CO2.

[0053] A dual-purpose catalyst system may contain a single dual-purpose catalyst for performing both NO2 reduction and CO oxidation, or it may comprise one or more catalyst beds, each arranged for NO2 reduction and CO oxidation, respectively. One or more heat exchangers may be arranged between the catalyst beds.

[0054] Downstream of the dual-purpose catalyst system, SO3 formed in the SO2 oxidation catalyst reacts with H2O present in the gas to form H2SO4 vapor. By cooling the process gas, for example in an acid-resistant glass tube heat exchanger, the H2SO4 is condensed and removed as a commercial-grade sulfuric acid product. NO and NO2 exhibit very low solubility in sulfuric acid, with only a few percent of NO being soluble in sulfuric acid. x It will be absorbed into the sulfuric acid product.

[0055] After steps a and b, and optionally steps a, a1, b and c, the NO2 concentration of the process gas is suitably below 2 ppmv, preferably below 1 ppmv.

[0056] In one aspect of the method of the present invention, CO can be added to the process gas at any point upstream of step b. As mentioned above, the process gas ideally contains 10-5,000 ppmv of CO.

[0057] In another aspect of the method, at least one of catalytic steps a, a1, and b, preferably all of catalytic steps a, a1, and b, is carried out independently within a temperature range of 250-450°C. More specifically, at least catalytic step a1 is carried out within a temperature range of 350-450°C.

[0058] Suitably, at least one of catalytic steps a, a1 and b, preferably all of them, is carried out without active temperature control between steps, thereby avoiding the need for heat exchangers between catalytic steps.

[0059] Therefore, the concentrations of SO2 and NO2 in the purified process gas are significantly reduced, allowing it to be sent to a carbon capture device or released into the atmosphere. Thus, after steps a and b, and optionally after steps a, a1, b, and c, the process gas can be fed to a carbon capture device, where a CO2-rich stream is separated from the process gas. The carbon capture device is suitably based on the absorption of CO2 into a mixture of amines (primary, secondary, and / or tertiary amines), a hot (promoted or unpromoted) potassium carbonate solution, or a combination thereof. To improve the carbon capture and energy efficiency of the hot potassium carbonate process, so-called promoters can be added to the carbonate solution. Such promoters can be amines, amino acids, and compounds containing V, B, and As. Optionally, the purified process gas can be sent to a final SO2 purification unit to further reduce the SO2 content.

[0060] Attached Figure

[0061] Figure 1 The layout of the process equipment used to reduce NO2 concentration to sub-ppm using a dual-purpose catalyst system is shown.

[0062] NO content originating from upstream processes such as carbon black processes or fluidized catalytic cracking units x (NO+NO2), SO x Process gases (2) of SO2+SO3, O2, H2O, and CO are directed to the process gas conditioning section (4). In the process gas conditioning section (4), particulate matter can be removed, for example, by a bag filter, candle filter, or electrostatic precipitator; pressure can be increased by a blower / compressor; and temperature can be regulated by a heater / cooler / burner or other means to provide a regulated atmosphere ready for SCR reaction and NO2 reaction. x Reducing process gas (6).

[0063] The ammonia-containing stream (5) can optionally be split into a first ammonia-containing stream (7) and a second ammonia-containing stream (13). The majority of the ammonia is present in stream (7), which is added to and mixed with the conditioned process gas (6) to provide NH3 / NO. x The process gas concentration is close to 1. In the (first) SCR reactor (10), most of the NO... x It reacts with NH3 to form N2 and H2O. NO x The depleted process gas (12) exits the SCR reactor and is optionally mixed with a small amount of NH3-containing stream (13) to form process gas (14), which is prepared for use in an optional second SCR reactor (16), in which residual NO is generated. x The reduction of SO2. The SCR tail gas (18) contains low concentrations of NO, NO2 and NH3, which is optionally directed to the SO2 oxidation reactor (20), where SO2 is oxidized to SO3 and NH3 is oxidized to N2 and NO. x Furthermore, NO is partially oxidized to NO2.

[0064] The converted process gases (18, 22) are fed to a dual-purpose catalyst reactor (24), where NO2 is reduced to NO by reacting with CO, and CO is oxidized to CO2 by reacting with O2. The converted process gas (26), containing a very low NO2 concentration, is cooled to 220-290°C in a heat exchanger (28), and the cooled converted process gas is directed via line (30) to a sulfuric acid condenser (32). The cooled converted process gas is further cooled to about 70-100°C by indirect heat exchange with a cooler cooling stream (42) (usually air), the sulfuric acid is condensed, and it leaves the corrosion-resistant condenser (32) via a bottom line (36). The cooled process gas (34), which contains almost no sulfuric acid, SO2, and NO2, can then be directed to a carbon capture unit (38), where CO2 is captured and concentrated, and it leaves the unit as a nearly pure CO2 stream (40). Then, the CO2-depleted process gas (42) is guided into the atmosphere through the chimney.

[0065] Another advantage of integrating WSA with carbon capture is that the heated cooling stream (44) exiting the sulfuric acid condenser (32) can be used for heating purposes, such as meeting the high energy requirements of the stripping section of the carbon capture unit, and / or as preheated combustion air in other processes and / or as carrier gas for NH3 in SCR processes.

[0066] The technology of the present invention has been described in conjunction with several embodiments and accompanying drawings. Those skilled in the art can combine elements of these embodiments and drawings as needed, within the scope of the invention as defined by the appended claims. All documents mentioned herein are incorporated by reference.

[0067] Example 1

[0068] This embodiment describes the process gas passing through a process layout (such as...) Figure 1 (As shown) NO x The changes in concentration, and indicate the effect of the outlet NO2 concentration on the NO content. x The operation of the reduced SCR system is insensitive.

[0069] The base is 350 ppm NO from upstream processes such as FCC units or carbon black production equipment. x Process gases.

[0070] The design goal of SCR is to reduce NH3 / NO3. x Achieving 98% NO reduction at a ratio of 1.0 x Conversion rate. Actual NH3 / NO x The ratio will determine the actual NO x Conversion rate; increasing NH3 concentration will increase NO conversion rate.x While increasing the conversion rate, it also increases the escape of NH3 from the reactor. Reducing the NH3 concentration will reduce NO. x Conversion rate, and resulting in higher NO levels in the reactor. x Escape.

[0071] SO2 oxidation catalysts exhibit high efficiency in converting NH3, but low selectivity for N2. A 100% NH3 conversion was achieved, with an estimated N2 selectivity of 40%, meaning the remaining 60% becomes NO. x The NO / NO2 ratio depends on the chemical equilibrium in the SO2 oxidation catalyst. Under operating conditions, the NO2 / NO2 ratio leaving the catalyst... x The ratio is approximately 0.4. The sulfur-resistant dual-purpose NO2 reduction catalyst is located downstream of the SO2 oxidation catalyst, and its NO2 reduction efficiency is 99% when the molar ratio of CO to NO2 exceeds 1.

[0072] All three catalyst systems operate within the same temperature range of 350–450 °C, and it is desirable for them to operate at the same temperature, thus eliminating the need for heat exchangers between the catalysts. This simplifies process layout and control, and reduces the cost of process equipment.

[0073] NH3 / NO in operation x The ratio is not constant because it depends on the NO content in the process gas. x Changes in NO concentration and analysis x Concentration and the rate of NH3 flow adjustment. Therefore, the NH3 / NO ratio must be anticipated. x The changes in both space and time are compared. Table 1 shows the NO and NO2 concentrations, indicating that when operating near the design conversion efficiency, NO from the SCR... x Minimize escape; regardless of the direction of deviation from this optimal operating point, NO will increase. x Escape. NH3 / NO x When the ratio is too low, unconverted NO x It will leave SCR; while NH3 / NO x When the ratio is too high, NO x The emission rate is very low, but the emission rate of NH3 is very high. NH3 will be partially converted to NO on the downstream SO2 oxidation catalyst. x .

[0074] This sulfur-resistant dual-purpose catalyst system effectively reduces NO2 concentration, enabling operation with NO2 concentrations below 0.5 ppm over a wide operating range of the SCR reactor.

[0075] Without a dual-purpose catalyst system, NO2 emissions from the SO2 oxidation catalyst would be higher than 3-4 ppm, even under optimal operating conditions.

[0076] A layout employing two SCR reactors in series, with process gas mixed between the reactors, can achieve a higher designed SCR conversion rate, i.e., lower emissions under optimal operating conditions. However, insufficient or excessive NH3 feed will still lead to NO emissions. x Increased emissions, as shown in Table 1 (NH3 / NO) x The cases with ratios of 0.9 and 1.10 are shown.

[0077] So-called ammonia escaping catalysts (ASCs) can convert NH3 escaping from an SCR reactor into N2, with a much higher selectivity than SO2 oxidation catalysts, i.e., 90% instead of 40%. They are typically located immediately downstream of the SCR reactor and ensure that, even with an excess of NH3 feed, the total NO reaching the SO2 oxidation catalyst is minimized. x The concentration decreases. However, in this embodiment, it cannot guarantee that the NO2 concentration in the outlet gas of the SO2 oxidation catalyst is below 2-3 ppm.

[0078] ASC may also have CO oxidation capabilities, thus limiting the use of dual-purpose catalyst systems downstream of SO2 oxidation catalysts, because there is too little or even no CO needed to reduce NO2 to NO in this case.

[0079] Similarly, installing a dual-purpose catalyst system between the SCR and SO2 oxidation catalysts will reduce both NO2 and CO reaching the SO2 oxidation catalyst, but NO will be re-oxidized to NO2 in the SO2 oxidation catalyst.

[0080] Table 1:

[0081] Example 2: This example demonstrates the efficiency of a sulfur-resistant dual-purpose catalyst, which can first use CO to reduce NO2 to NO, and then use O2 to oxidize CO to CO2.

[0082] The data were measured in a laboratory-scale reactor using simulated process gases (i.e., containing NO, NO2, O2, CO, and SO2). SO2 does not participate in the reaction but is a common poisoning agent in many catalyst systems.

[0083] The composition of the simulated process gas is as follows: 3 vol% O2 5 vol% H2O 1000 ppm CO 50 ppm NO x (25 ppm NO / 25 ppm NO2) 500 ppm SO2 The margin is N2 The pressure is close to atmospheric pressure.

[0084] Data from different operating temperatures indicate that, within the 220-400℃ range, the reduction of NO2 to NO using CO as a reducing agent is almost complete. However, the oxidation of CO to CO2 using O2 as an oxidizing agent is slower, requiring temperatures above 350℃ to achieve a similar conversion rate. (See [reference needed]). Figure 2 .

[0085] This means that the dual-purpose catalyst exhibits optimal performance at around 350-450°C, which is the normal operating temperature for SO2 oxidation catalysts. Therefore, the dual-purpose catalyst system can be advantageously connected and installed directly in the SO2 oxidation reactor.

[0086] High NO2 reduction and CO conversion rates can be achieved at a lower temperature of 250°C, but a larger catalyst volume is required to achieve very high CO conversion rates. Figure 1 In the process layout shown, an alternative is to place the sulfur-resistant dual-purpose catalyst between the process gas cooler (28) and the sulfuric acid condenser (32).

Claims

1. A method for reducing the concentrations of NO, NO2, and optionally SO2 in a process gas (2), the method comprising subjecting the process gas to: a. SCR step (10), which catalytically converts NO and NO2 in the process gas into N2 and H2O by reacting with the first NH3 stream (7). b. A combined step (24) of NO2 reduction and CO oxidation in the presence of a sulfur-resistant dual-purpose catalyst system, thereby catalytically reducing NO2 in the process gas to NO by reacting with CO, and oxidizing CO to CO2 by reacting with O2. Its features are, After steps a and b, the NO2 / NO ratio in the process gas x The molar ratio is less than 0.05, preferably less than 0.02, and most preferably less than 0.

01.

2. The method according to claim 1, further comprising subjecting the process gas to: a1. The SO2 oxidation step (20) between steps a and b, which catalytically oxidizes SO2 in the process gas to SO3 by reacting with O2; c. A cooling and condensation step (32) following step b, wherein sulfuric acid vapor is condensed and removed from the process gas. After steps a, a1, b, and c, the NO2 / NO ratio in the process gas... x The molar ratio is less than 0.05, preferably less than 0.02, and most preferably less than 0.

01.

3. The method according to any one of the preceding claims, wherein prior to step (a), the process gas comprises 10-5,000 ppmv of CO and 0-1,000 ppmv of NO + NO2.

4. The method according to any one of the preceding claims, wherein prior to step (a), the CO:NO2 molar ratio of the process gas is greater than 1:

1.

5. The method according to any one of the preceding claims, wherein after steps a and b, steps a, a1, b and c, or steps a, b, a1 and c, the NO2 concentration of the process gas is less than 2 ppmv, preferably less than 1 ppmv.

6. The method according to any one of the preceding claims, wherein the process gas originates from a fluidized bed catalytic cracking process, a carbon black production process, or a metallurgical process, such as a steel mill.

7. The method according to any one of the preceding claims, wherein prior to step (a), the process gas satisfies at least one of the following conditions: - O2 concentration in the range of 2-20 vol%. - H2O concentration in the range of 2-40 vol%. - SO2 concentration in the range of 100-10,000 ppmv.

8. The method according to any one of the preceding claims, wherein CO is added to the process gas at any point upstream of step b.

9. The method according to any one of the preceding claims, wherein at least one of catalytic steps a, a1 and b, preferably all of catalytic steps a, a1 and b, is carried out in a temperature range of 250-450°C.

10. The method according to any one of the preceding claims, wherein at least the catalytic step a1 is carried out in a temperature range of 350-450°C.

11. The method according to any one of the preceding claims, wherein at least one, preferably all, of the catalytic steps a, a1, and b is carried out without active temperature control between steps.

12. The method according to any one of the preceding claims, wherein the NH3 used in step a is added to the process gas such that the obtained NH3 / (NO+NO2) ratio is between 0.9 and 1.

1.

13. The method according to any one of the preceding claims, prior to step (a), the process gas is pre-conditioned by one or more pre-conditioning steps (4) selected from: - Removes particulate matter, for example, in bag filters, electrostatic precipitators, or candle filters. - Temperature is regulated by heating or cooling, for example, through heat exchangers, burners, heaters, and - Regulate the pressure, for example, by means of a compressor or blower (e.g., regulate to approximately atmospheric pressure).

14. The method according to any one of the preceding claims, wherein the sulfur-resistant dual-purpose catalyst system in step b. is a sulfur-resistant dual-purpose catalyst comprising a combination of platinum group metals (e.g., Pt and / or Pd) supported on a support and one or more oxides of vanadium, chromium, tungsten, molybdenum, cerium, niobium, manganese and copper, said support being selected from aluminum, silicon and titanium.

15. The method according to claim 14, wherein the sulfur-resistant dual-purpose catalyst system used in step b is a monolithic structure.

16. The method according to any one of the preceding claims, wherein in step b., the reaction rate of NO2 with CO is higher than that of CO with O2.

17. The method according to any one of the preceding claims, wherein after steps a and b, steps a, a1, b and c, or steps a, b, a1 and c, the process gas is fed to a carbon capture device, where a CO2-rich stream (40) is separated from the process gas.

18. The method of claim 17, wherein the carbon capture device is based on absorbing CO2 into a mixture of amines (primary, secondary, and / or tertiary amines), a hot (promoted or unpromoted) potassium carbonate solution, or a combination thereof.

19. The method according to any one of the preceding claims, wherein in step c, sulfuric acid vapor is condensed in an indirectly cooled corrosion-resistant heat exchanger.

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