CN1355721A - Method for removing nitrogen oxides from oxygen-containing gas stream - Google Patents

Abstract

A process for removing nitrogen oxides from a gas stream containing nitrogen oxides and oxygen involves bringing the gas stream into contact with a reducing gas stream in the presence of a deNOx catalyst. The reducing gas stream contains CO, H2 and, optionally, NH3; it is produced by reacting hydrocarbons with the oxygen from the above mentioned nitrogen oxide- and oxygen-containing gas stream. A catalyst, such as an oxidising or stream-reforming catalyst may be used to promote the reaction which produces the reducing gas stream.

Description

Method for removing nitrogen oxides from oxygen-containing streams

The present invention relates to the catalytic conversion of nitrogen oxides, which are formed during the combustion of hydrocarbons and/or synthesis gas (H ₂ /CO), to molecular nitrogen. More specifically, the present invention relates to the conversion of nitrogen oxides in the presence of oxygen, which are produced when combustion equipment such as internal combustion Combustion in the presence of oxygen. The invention still further relates to the conversion of nitrogen oxides which may be formed during industrial processes such as the production of nitric acid.

When hydrocarbons are combusted with molecular oxygen (eg, from air), at the temperatures and pressures prevailing in such combustion processes, nitrogen oxides may be formed. Among these nitrogen oxides, NO and NO ₂ (commonly denoted by NOx) are very harmful to the environment. Nitrogen oxides are responsible, among other things, for the formation of acid rain and photochemical smog.

Various methods of reducing NOx emissions are known, many of which are in practice.

In engines operating stoichiometrically, so-called three-way catalyst systems are often used to reduce NOx emissions. In such systems, the NOx conversion catalyst is capable of converting nitrogen oxides into harmless compounds (such as _N2 ) by reacting NOx with reducing combustion products (such as hydrocarbons and carbon monoxide) present in the exhaust.

In general, three-way catalysts known to be effective in reducing nitrogen oxides cannot perform this conversion in the presence of significant amounts of oxygen.

This problem is particularly acute in the removal of nitrogen oxides from the aforementioned lean-burn engines, such as lean-burn gas turbines, diesel engines, gas engine exhaust, and industrial process exhaust gases, since in such gases there is considerable presence in addition to nitrogen oxides. The amount of oxygen; and the absence of hydrocarbons and/or CO, or the presence of insufficient amounts, prevents the successful operation of the above-mentioned three-way catalyst.

Therefore, in the presence of a considerable amount of oxygen in the exhaust gas, it is often necessary to add a certain amount of reducing agent. In the presence of suitable catalysts (deNOx catalysts), nitrogen oxides can be converted by reducing agents. Such a process is known as so-called selective catalytic reduction (SCR).

The reducing agents widely used in SCR reaction are ammonia and urea. However, we know from the literature that hydrocarbons such as ethylene, propylene and propane can also act as reducing agents (see for example "Mechanism of the lean NOx reaction over Cu/ZSM-5" by GP Ansell et al., published in Appl. Catal .B, 2 (1993) pp. 81-100). Other possible reducing agents are CO, _H2 and _CH4 , ethanol, hydrocarbons, especially fuels such as gasoline and diesel.

However, in practical SCR applications, ammonia and urea, or urea aqueous solution, are the most used reducing agents so far. The use of these reducing agents entails several disadvantages. Its dosage is extremely critical. If too much ammonia and urea are injected into the deNOx catalyst (ie more than necessary for nitrogen oxide conversion), this will lead to so-called ammonia slip. From an environmental point of view, the ammonia emitted in this case is even more harmful than the emitted NOx. In addition, the oxidation of excessive ammonia may lead to the formation of NOx, which runs counter to the original goal of reducing NOx emissions. Another disadvantage of using ammonia or urea is that, if they are not produced on site, they must be stored for periodic replenishment. Ammonia, in particular, is very dangerous and harmful, and transporting it carries a great deal of risk in terms of safety and the environment. As a result, the capital and operating costs of this technology are high.

Although the selection of other reducing agents, such as the above-mentioned hydrocarbons, may partly solve these problems, there are still disadvantages, such as the need for separate transportation and storage. The accompanying safety and environmental hazards are also often unacceptable.

Using the same fuel as the reductant that is combusted in engines, gas turbines or industrial processes could potentially solve this problem. However, hydrocarbons present in, for example, gasoline and diesel fuel have not been shown to be sufficiently active for NOx conversion under process conditions with acceptable rates and selectivities.

It is known that starting from SCR catalysts, the required reducing agents are produced from available hydrocarbons such as methanol, LPG and natural gas, optionally under hydrogenation conditions, the hydrogen coming from electrolysis or storage tanks.

The technology described in DE-A-44 04 617 is to use an electrically heated reactor, catalytic cracking of hydrocarbon-containing fuels at 200-700 ° C, and the cracked products are further activated with air before being injected into the tail gas as a reducing agent, The entire gas flow is then passed through the SCR catalyst.

According to DE-A-196 00 558, cracked hydrocarbons are also used as reducing agents. These hydrocarbons come from diesel fuel. According to this known method, under SCR conditions, when the cracked hydrocarbons are not sufficiently active, hydrogen can be added to the tail gas to reduce NOx on the SCR catalyst. This hydrogen comes from storage tanks, or is produced by electrolysis or methanol conversion.

In DE-A-42 30 408 it is known to add hydrogen as reducing agent to the exhaust gas of a continuous combustion process to reduce NOx in the exhaust gas. The hydrogen here can also be obtained by electrolysis, or by steam reforming or partial oxidation (PO) of hydrocarbon-containing fuels. In order to obtain hydrogen with as little CO as possible, two shift reactors are installed after the reformer to convert most of the CO with steam to form hydrogen and CO ₂ .

In EP-A-0 537 968 the use of hydrogen produced on site as reducing agent for the reduction of NOx in the exhaust gas of internal combustion engines is disclosed. This document describes reforming technologies (steam reforming or partial oxidation) of hydrocarbon-containing fuels. According to this document, the conditions must be chosen such that the CO content in the hydrogen is so low that no emission problems arise. The temperature required for the conversion is achieved using off-gas heat. NOx reduction is performed on the SCR catalyst. The oxidant used for its partial oxidation includes air.

According to the present invention, in the reducing agent forming step, hydrocarbons are converted to reducing agents under suitable conditions by optionally contacting with a reducing agent forming catalyst. In the reducing agent forming step, the product contains hydrocarbons which may be unreacted hydrocarbons in the feed to that step, but may also be lower hydrocarbons formed by cracking reactions in this step.

The reducing agent can be prepared starting from residual hydrocarbons contained in the exhaust gas from the combustion plant using a catalyst capable of forming the reducing agent; these hydrocarbons can also be obtained from a different source, for example from fuel already on site at the combustion plant . Combinations of effluents with such different sources are naturally also possible. The oxygen required in this step comes at least partly from the exhaust gas to be treated, ie the gas containing nitrogen oxides and oxygen. Preferably, almost all of the oxygen present in the portion of the tail gas used to form the reducing agent is used to form the reducing gas. Hydrocarbons present in this part of the tail gas can also be converted into reducing compounds in this step.

By employing an appropriate process for forming the reducing agent, optionally using a catalyst, the reducing agent necessary for the catalytic reduction of NOx in the form of CO and/or H ₂ , optionally supplemented with hydrocarbons, can be obtained in situ (in situ) are prepared from hydrocarbons in order to at least partially eliminate the above-mentioned disadvantages of NOx removal under oxygen-enriched conditions.

In addition to the above CO and/or _H2 reducing agents optionally supplemented with hydrocarbons, it is also possible under suitable process conditions due to At a position of chemical equilibrium, ammonia (NH ₃ ) is formed in the presence of hydrogen and nitrogen. As mentioned earlier, ammonia is a good reducing agent.

The main advantage of using part of the tail gas as a source of hydrocarbons, and of course as a source of oxidation for (catalytic) partial oxidation and/or steam reforming, over the prior art is that it minimizes the emission of unburned hydrocarbons, the entire tail gas The oxygen content in the tail gas decreases, so that the conditions for deNOx are improved, and the energy in the form of heat in the tail gas can be directly used in the catalytic process without using an additional heat exchange area.

Another advantage is that there is no need to produce pure hydrogen, or hydrogen with a small amount of CO. As a result, it is possible to dispense with shift reactors and membrane technology.

The step of forming the reducing agent may be a partial oxidation step, in which case a partial oxidation catalyst may be used. Alternatively, it is possible to perform partial oxidation without a catalyst, for example by controlling the energy supplied to the fuel stream, such as by using electrical discharge techniques. The reducing agent forming step may also be a steam reforming step, in which case a steam reforming catalyst is used. A combined partial oxidation and steam reforming step may also be used.

Suitable reducing agent-forming catalysts are, for example, partial oxidation catalysts. In the presence of this catalyst, partial oxidation of hydrocarbons takes place. As stated above, according to the present invention, the oxygen required for the partial oxidation comes from the exhaust gas from the combustion plant, optionally supplemented with oxygen obtained from other sources, such as external air. Partially oxidized products are most suitable as reducing agents.

Another possibility for the preparation of gas streams comprising CO and/or _H2 and optionally hydrocarbons from hydrocarbon-containing streams is the use of so-called steam reforming. In steam reforming, water is added in this step in addition to hydrocarbons. The water can come from the engine combustion effluent, from a separate storage tank, or from a combination of both. In steam reforming, water (steam) is used to convert hydrocarbons into hydrocarbon mixtures such as methane and/or _H2 and _CO2 . In addition to these components, CO may also be present as a result of chemical equilibrium. The resulting mixture is very suitable as a reducing agent.

Next, the reducing agent contacts the deNOx catalyst along with the exhaust gas from the combustion equipment, causing the desired nitrogen oxide conversion to occur.

On-site production of the reducing agent offers a number of important advantages. According to the present invention, in the case of natural gas, the reducing agent can be supplied continuously, or in any case fuel and reducing agent can be supplied simultaneously to the combustion equipment, so that a separate storage tank is no longer required. This is practical when used in mobile combustion equipment such as trucks or cars because there is no separate tank for the reductant required. For stationary equipment, there may also be important advantages. The fact that ammonia and urea (whether urea solution or not) is not used, at least not obtained elsewhere, is a great advantage, since there are no longer the disadvantages associated with the use of the above-mentioned reducing agents.

Another advantage of the present invention is that, if the step of forming the reducing agent uses exhaust gases from combustion plants as raw material, as a result, the content of hydrocarbons in the exhaust gases is reduced after their reaction with NOx. Such a reduction is advantageous since the emission of hydrocarbons is undesirable from an environmental point of view. Another advantage of using exhaust gas from combustion equipment to form the reducing gas is that it reduces the amount of hydrocarbons required to form the reducing agent.

Therefore, the present invention is characterized in that the nitrogen oxide content in the gas stream is reduced by contacting the gas stream containing nitrogen oxides and oxygen in the presence of a deNOx catalyst with a reducing gas stream; said reducing gas stream contains CO, H ₂ and possibly NH ₃ , and the reducing gas stream is formed by converting oxygen with hydrocarbons in a gas stream containing nitrogen oxides and oxygen, optionally in the presence of a reducing agent forming catalyst.

According to a preferred embodiment, the gas stream containing nitrogen oxides and oxygen is an exhaust gas from a fuel combustion step comprising at least a) a stream comprising one or more fuels and b) a stream containing excess oxygen relative to the fuel In the step of feeding, the stream b) may further include nitrogen; wherein said waste gas is contacted with a deNOx catalyst together with c) a reducing gas stream which may additionally optionally include one or more hydrocarbons ; wherein, stream c) does not add ammonia, does not add urea, and is substantially obtained by contacting streams d) and e) with a catalyst forming a reducing agent, stream d) comprising one or more hydrocarbons, stream e) including oxygen and water.

The combustion apparatus described is suitable for generating heat and optionally other forms of energy. The combustion device can work on the basis of flames, but the combustion in the combustion device can also take place via catalytic routes. Preferred combustion devices are gas engines, gas turbines, diesel engines or gasoline engines.

A stream c) substantially free of added ammonia and urea is understood to mean a _stream in which no addition of such reducing agents is required according to the _invention , but ammonia and Possible derivatives are eg urea.

According to the invention, NOx is reduced using a mixture of H ₂ , CO and possibly NH ₃ . In addition, incompletely (completely) converted hydrocarbons may be present in the reducing gas stream. These hydrocarbons are also used as reducing agents. the reducing gas mixture is produced by converting a portion of the hydrocarbons in the tail gas with optionally additional hydrocarbons by utilizing oxygen and water vapor or optionally additional air and/or water vapor in the portion of the tail gas stream, By (catalytic) partial oxidation, steam reforming or a combination of both techniques. A reducing agent such as hydrogen may optionally be added to the obtained reducing gas mixture.

Particularly preferred are methods and apparatus adapted according to the invention in which the engine includes a heat exchanger so that at least part of the heat released by the combustion can be used efficiently, for example for heating a greenhouse or other space. Such equipment is a device that generates heat and energy at the same time, and the energy is generally in the form of electricity, or it can be a device that co-produces heat and electricity or a separate production device.

The present invention is also applicable to various means of transportation, such as boats, planes, trucks, cars and trains, as long as it uses a hydrocarbon combustion propulsion engine.

The method and plant according to the invention are particularly suitable for combustion plants operating in so-called lean conditions, ie the ratio of streams a) and b) is selected so that the oxygen content is at least that required for complete combustion of the fuel in stream a). In this state, there is oxygen in the exhaust gas discharged from the combustion equipment, which can effectively perform deNOx reaction with the exhaust gas discharged from the catalytic partial oxidation step.

Fuels suitable as feed a) are hydrocarbons and/or synthesis gas (CO/ _H2 mixture).

The hydrocarbons in the step of forming the reducing agent are preferably at least partially homologous to the combustion plant fuel. In this case, streams a) and d) comprise the same components.

In order to reduce the hydrocarbon content in the exhaust gas of the plant according to the method according to the invention, the hydrocarbons present in the exhaust gas of the combustion plant are at least partly used as feed for the step of forming the reducing agent, which may or may not be supplemented with hydrocarbons from other sources.

In order to keep the oxygen content low when the stream contacts the deNOx catalyst, the exhaust gas from the combustion equipment is used as the oxygen source, which may or may not be supplemented with oxygen from other sources.

As fuel for the combustion plant and/or feed for the reducing agent formation step, in principle all hydrocarbons are suitable, except that synthesis gas can be used. In practice it is particularly preferred that the hydrocarbons of streams a) and d) are independently selected from natural gas (mainly consisting of methane), methane, diesel, gasoline, fuel oil, methanol, ethanol, naphtha, kerosene, ethane, Propane, butane, LPG and their derivatives or mixtures.

The nitrogen oxide conversion catalyst may be selected from those catalysts capable of catalyzing the reduction of NOx, such as conventional catalysts for NOx removal. Preferably these catalysts are selected from: zeolites; metal-exchanged zeolites, for example Co-, Cu- and/or Ce-exchanged zeolites; Pt, Rh and/or Ir catalysts, which may optionally be supported on supports, such as coated catalysts, And may further include such as Ba, La, Y, Sr, Pr, Ce, Si, Ti, Al and/or Zr.

The hydrocarbon partial oxidation catalyst can be selected from Pt, Rh, Ru, Pd, Co and Ni, and if necessary, it can be supported on a suitable carrier, such as _Al2O3 , _SiO2 , _{TiO2 , ZrO2} , silica/ Alumina-zeolites and mixtures thereof, optionally stabilized with Si, La, Ba, Y, mixtures thereof, etc.

The steam reforming catalyst capable of converting the mixture of hydrocarbons and water into H ₂ , CO, CO ₂ and/or hydrocarbons can be any conventional steam reforming catalyst, as known to those skilled in the art, with or without carrier, or carrierless. The steam reforming catalyst is preferably a supported catalyst comprising Ni, Rh and/or Pt.

When operating a plant according to the invention, in the case of partial oxidation of hydrocarbons, factors such as the hydrocarbon/oxygen ratio, temperature, pressure, residence time and/or amount of catalyst should be selected such that complete oxidation does not occur. The hydrocarbon/oxygen molar ratio is expressed as λpo, such that λpo=1 at the stoichiometric ratio (ie the exact oxygen demand for complete combustion of the fuel). According to the invention, however, λpo<1, preferably 0.2<λpo<0.7. According to the hydrocarbons used, the air-fuel ratio can be adjusted to control λpo.

The temperature of the reducing agent forming step is generally 250-1100°C; the residence time is generally 200-150000h ^-1 (200en 150000h ^-1 ). Although pressure also has an effect, it is generally determined by other process conditions. Typically, the pressure is atmospheric, or slightly higher or not higher than 50 bar.

The use of SCR catalysts to convert NOx to _N2 with reductants is often limited in that sufficient NOx conversion can only be achieved at a certain combustion temperature. Thus, DE-A-196 00 558 gives as an example a curve of the NOx conversion as a function of temperature, where a conversion of up to 40% is possible. This is typical for SCR systems that do not use _NH3 and urea as reductants. The process conditions of the step of forming the reducing agent are selected so that NH ₃ can also be formed, and the benefit is that a higher NOx conversion rate can be achieved.

The method of the present invention is better performed with NOx storage systems rather than SCR systems when high NOx conversion is required, for example when the tail gas is used as horticultural fertilizer gas. If such a NOx storage system is used for the deNOx step (also known as NOx storage and reduction catalyst, NSR), for example as described in the article published by N. Takhashi et al. in Environmental Catalysis, p45, (1995), very High NOx conversion rate. According to this method, nitrogen oxides are adsorbed from a gas stream containing nitrogen oxides and oxygen onto a suitable adsorbent, which is subsequently, for example switched, brought into contact with the reducing gas. Therefore, its deNOx step is a discontinuous operation. This allows for very efficient removal of NOx.

With the NOx storage system, NOx can be adsorbed on an oxidizing medium (λ<1) and a reducing medium (λ<1), and both the NOx in the tail gas and the adsorbed NOx can be converted into nitrogen. In NOx storage systems, a very suitable catalyst composition is platinum on an alumina coating containing barium and/or zeolites. Barium can react with NOx to form barium nitrate. This nitrate decomposes to barium and _N2 on reducing media.

The NOx storage system can be operated in accordance with the present invention by passing exhaust gas through the NOx storage system until the system is saturated with NOx. Thereafter, regeneration is carried out using a reducing agent prepared as previously described. The reducing agent may optionally be supplemented with reducing agent obtained from other places.

The NOx storage system is preferably designed as at least two parallel beds. One bed is used for NOx adsorption, while the other bed is used for regeneration. Once the previous bed is saturated and/or the second bed is fully regenerated, the gas flow is switched so that the regenerated bed continues to adsorb NOx, while the bed saturated with NOx is regenerated, during which the adsorbed NOx is converted into nitrogen.

The effluent from the regeneration of the NOx storage bed may advantageously be recycled to the inlet of the combustion device, such as a gas engine, together with the inlet air. This provides at least two advantages: first, in this way, there is no need to discharge CO-containing gas; second, according to this embodiment, whether the hydrocarbons are completely Conversion to CO/ _H2 is less important.

According to the process of the present invention, both the deNOx catalyst and the catalyst forming the reducing agent may be present in forms known in the art, such as beds of granules, extrudates, granules and/or pellets, or ceramic or so-called metal Integral, or other structural forms.

The use of structured catalysts is preferred as this allows other relevant factors of the process, such as pressure drop, mixing, contact time, thermal management, mechanical strength and lifetime, etc., to be adapted to the prevailing conditions by proper selection, thus making the process be optimized.

A feature of the present invention is therefore the use of a catalyst suitable for converting nitrogen oxides in combination with a catalyst suitable for partial oxidation or a catalyst suitable for steam reforming to convert nitrogen oxides while generating heat and optionally energy from hydrocarbons substances without adding ammonia or urea.

Claims (14)

1. A method for reducing nitrogen oxides in a gas stream containing nitrogen oxides and oxygen, by contacting the gas stream with a reducing gas stream in the presence of a deNOx catalyst; the reducing gas stream comprises CO, H _and possible NH ₃ , and the reducing gas stream is obtained by converting the oxygen and/or water present in the gas stream containing nitrogen oxides and oxygen with hydrocarbons, optionally with the presence of a reducing agent catalyst. 2. The method of claim 1, wherein substantially all of the oxygen contained in a portion of the nitrogen oxide and oxygen containing gas stream in the step for forming the reducing agent is reacted. 3. A method as claimed in any one of the preceding claims, wherein the gaseous stream comprising nitrogen oxides and oxygen is an exhaust gas from a fuel combustion step comprising combining at least a) a stream comprising one or more fuels and b) a step of feeding a stream containing an excess of oxygen relative to the fuel; wherein said off-gas and c) a reducing gas stream optionally comprising one or more hydrocarbons are combined with said deNOx catalyst; wherein, stream c) does not add ammonia, does not add urea, and is basically obtained from streams d) and e) through the step of forming a reducing agent, and stream d) includes one or more hydrocarbons. 4. A method as claimed in any preceding claim, wherein the gaseous stream comprising nitrogen oxides and oxygen comes from a combined heat and power plant. 5. A method as claimed in any preceding claim, wherein the reducing gas stream is obtained by a partial oxidation step using a partial oxidation catalyst or a steam reforming step using a steam reforming catalyst. 6. A process as claimed in any one of claims 3 to 5, wherein the fuel in stream a) comprises synthesis gas and/or one or more hydrocarbons. 7. A process as claimed in any one of claims 3 to 6, wherein stream a) and stream d) comprise at least partly the same compounds. 8. A process as claimed in any one of claims 3 to 7, wherein the fuel in stream a) comprises hydrocarbons similar to those in stream d), independently selected from natural gas, methane, Diesel, gasoline, fuel oil, methanol, ethanol, naphtha, kerosene, ethane, propane, butane, LPG and their mixtures. 9. A process as claimed in any one of the preceding claims, wherein the catalyst used to form the reducing agent is, for the partial oxidation of hydrocarbons, a partial oxidation catalyst selected from the group consisting of Pt, Rh, Ru, Pd, Co, Ni and mixtures thereof, optionally supported on a support optionally stabilized with Si, La, Ba and/or Y. 10. A method as claimed in any preceding claim, wherein the reducing agent forming catalyst is a steam reforming catalyst comprising Ni, Rh and/or Pt. 11. A process as claimed in any preceding claim wherein the step of forming the reducing agent is a partial oxidation step in which the ratio of hydrocarbon to oxygen is selected such that complete oxidation does not occur. 12. A method as claimed in any one of the preceding claims, wherein the nitrogen oxides in the gaseous stream containing nitrogen oxides and oxygen are adsorbed on a suitable adsorbent and the reducing gas stream is subsequently combined with the Adsorbent contact. 13. A method as claimed in claim 12, wherein said adsorbent is a barium and/or zeolite-containing alumina coating on a structured support. 14. The method as claimed in claim 12 or 13, wherein the product formed in the step of contacting the stream c) with the structured support is used as a fuel for a gas engine.