DE10261817A1 - Selective catalytic reduction of nitrogen oxides with ammonia and oxygen, useful for purifying stationary or mobile engine exhaust gas and waste gas from nitric acid manufacture, uses added hydrogen and catalyst containing silver - Google Patents

Abstract

In selective catalytic conversion of nitrogen oxides (NOx) with ammonia (NH3) in the presence of oxygen (O2), hydrogen (H2) is added to the gas mixture and reaction is carried out on a catalyst containing silver (Ag). An Independent claim is also included for a catalyst system, comprising an alumina support on which Ag is deposited as metallic Ag or Ag complex in an atmosphere of NOx, NH3, O2 and 0.05-4 vol.% H2.

Description

The invention relates to a process for the catalytic conversion of nitrogen oxides (NO, NO ₂ , hereinafter referred to as NO _x , with 1 ≤ x ≤ 2) in nitrogen, which is a decisive improvement of the established selective catalytic reduction (SCR = Selective Catalytic Reduction) of NO _{x allowed} by ammonia (NH ₃ ).

The SCR of NO _x by NH ₃ was patented by G. Cohn (Engelhard) US-A-2975025 , This first application was carried out to remove nitrogen oxides from the exhaust gases in the production of nitric acid and was based on catalysts containing precious metals.

The underlying reaction can be summarized in the form 4NO + 4NH ₃ + O ₂ → 4N ₂ + 6H ₂ O (1) can be specified.

It is true that the direct decomposition of nitrogen monoxide into the elements 2 NO → N ₂ + O ₂ (2) themodynamically possible and will also z. B. catalyzed by Cu-containing zeolites at temperatures around 400 ° C, but due to the insufficient conversion rates, no economically attractive process has been established so far.

The SCR of NO _x in oxygen-rich exhaust gases from stationary internal combustion engines using NH ₃ on catalysts based on vanadium and titanium oxides in the temperature range of 300–400 ° C is currently the best available technology (BACT = best available control technology).

Commercial SCR catalysts are based on the active component V ₂ O ₅ and a TiO _{2 support} . They also contain SiO ₂ for structural stabilization and WOa to extend the very limited temperature window of activity.

It is known that no SCR catalytic converter is able to economically justify the different operating conditions in which stationary (power plants, combustion plants, stationary gas and diesel engines) and mobile (automobiles, commercial vehicles) plants emit NO _x To guarantee sufficient implementation of nitrogen oxides. There are essentially three classes of catalysts used commercially: precious metal-containing catalysts for low exhaust gas temperatures (180-290 ° C), catalysts with predominantly V ₂ O ₅ as active component for the middle (260-450 ° C) and zeolite-based catalysts for the high temperature range (450–650 ° C).

In general, the operating range of the catalyst with respect to the reaction temperature is determined by the required balance between the desired reduction of nitrogen oxides and the undesirable direct oxidation of ammonia to NO and N ₂ O.

There are a large number of process variants. Leading providers of SCR technology are Babcock Hitachi (BHK) and Mitsubishi Heavy Industries (MHI). Catalysts are offered by Engelhard, Grace and Norton. Engelhard recently announced a ZNX process based on a zeolite catalyst supported on a honeycomb ceramic support. The technology of Grace and Norton is used in the patents US 4,929,586 (1990) and 4735927 (1988). Shell's DeNox system (SDS) works with catalysts based on V / TiO ₂ and is used for the exhaust gas purification of industrial furnaces, gas turbines and waste incineration plants as well as for the treatment of exhaust gases from caprolactam production and nitric acid production. A silver-containing catalyst is not used in any of the processes. Silver-containing catalysts play no role in the SCR of NO _x with NH ₃ due to their low activity. In contrast, silver-containing catalysts are known to be active for the reaction of NO _x with hydrocarbons and oxygen-containing reducing agents such as alcohols, aldehydes and ethers. Thus, the silver-containing catalysts described in patents (eg US Pat. No. 5,710,088 of January 20, 1998) have been used without exception for the reduction of NO _x with hydrocarbons.

The design of exhaust gas cleaning systems essentially depends on the activity of the available catalyst. The V / TiO ₂ SCR catalyst reaches the limits of its performance, especially at temperatures below 250 ° C and at high gas volume loads. This disadvantage can be partially remedied by increasing the installed catalyst volume, which corresponds to a lower gas volume load. In connection with this, the amount of catalyst required to comply with the required emission values increases in some cases up to 100 m ³ .

The catalyst composition is largely optimized, so that only small activity reserves can be expected here. The technological implementation, including the supply of ammonia, is mature. Attempts to replace the V / TiO ₂ SCR catalyst with another on a commercial scale have so far been unsuccessful.

Silver-containing catalysts were not considered for the NH ₃ -SCR of NO _x due to insufficient activity.

The invention has for its object to provide an improved process for the catalytic conversion of nitrogen oxides into nitrogen in the presence of ammonia. Surprisingly, it was found that the additional supply of hydrogen to the continuous gas stream, which contains NOx, oxygen, water and ammonia, the conversion of NO _x on catalysts containing silver increased several _times compared to gas flow without hydrogen at the same temperature. Especially at low temperatures, e.g. B. 150 ° C, at which no NOx conversion is observed in the presence of NH ₃ , O ₂ , H ₂ O without the addition of hydrogen, the addition of less than one vol.% Hydrogen provides an increase in the NOx conversion to 60%. This makes a process for NH ₃ -based SCR of NO _x on silver-containing catalysts competitive with the previous process on V-TiO ₂ and, in addition to replacing the toxic catalyst system, offers the technological advantage of drastically reducing the amount of catalyst required for a desired degree of conversion of NO _x , This saves costs and significantly reduces the requirements for the spatial design of the exhaust gas cleaning system.

The object of the invention is therefore a process for the selective catalytic conversion of nitrogen oxides by ammonia in the presence of oxygen, which is characterized is that hydrogen is added to the gas mixture and the reaction on a silver-containing catalyst.

A gas mixture is preferred used, the hydrogen content immediately before contact with the Catalyst in the range of 0.05-5 % By volume, more preferably 0.05-4 Vol% lies.

According to a more preferred embodiment a gas mixture is used, the hydrogen content immediately before contact with the catalyst in the range of 0.8-2.5 vol.% lies.

The hydrogen can be permanent or be added in pulses, preferably permanently. hydrogen can be added as a gas or by an upstream or simultaneous ongoing reaction during the Process. The addition as a gas is preferred.

As a silver-containing catalyst one can be used, the silver in the form of metallic silver, Contains silver complexes or mixtures thereof. Metallic silver is prefers.

The particle size of the silver on a support is 1 to 80 nm, preferably 1-20 nm and most preferably it is equal to or less than 5 nm.

The catalyst is applied to a support being the carrier selected from the group is composed of crystalline aluminum oxide, amorphous aluminum oxide, crystalline aluminum silicate, amorphous aluminum silicate, titanium oxide, Zirconium oxide, gallium oxide, cerium oxide, silicon dioxide as silica gel and mixtures thereof.

Supported catalysts are preferred whose silver content is in the range from 0.1 to 20% by weight, preferably 1 to 5% by weight, based on the total weight of the catalyst.

A particularly suitable catalyst for the The reaction is silver-containing aluminum oxide.

A further preferred embodiment consists in that an aluminum oxide carrier is used which contains silicon in a proportion of up to 50% by weight, provided that a surface area of at least 10 m ² / g determined by the BET method remains.

Supported catalysts with a BET surface area in the range from 30 to 140 m ² / g are generally preferred.

The modification of the catalyst with silver can by impregnation, Precipitation, Evaporation, sol-gel processes and mechanical admixture based of silver salts or silver complexes.

The catalytic conversion will carried out in a fixed bed reactor or in a monolithic reactor.

Compared to conventional processes, the advantage of the method according to the invention, as described below, is that (i) sufficient activity is achieved even at low temperatures of approximately 200 ° C., (ii) inexpensive Ag / Al ₂ O as catalysts ₃ materials can be used without complex production, (iii) the NaO formation occurring on noble metal-containing catalysts is avoided, (iv) the use of toxic vanadium compounds as a catalyst component can be dispensed with and (v) investment costs due to the possible reduction of the necessary Catalyst volumes can be reduced for a desired degree of conversion of the NOx.

The process does not require any complex conversions. Under normal exhaust gas conditions, hydrogen is also added to the gas stream in addition to ammonia as a reducing agent. For the promotional effect of hydrogen on the conversion of nitrogen oxides, a silver-containing catalyst is required, which partially or completely replaces the conventional V / TiO ₂ material. Silver / alumina catalysts are e.g. B. able to selectively convert NO _x in the presence of oxygen and ammonia by adding hydrogen in excess to the NO _x and NH ₃ content, nitrogen oxides with significantly higher degrees of conversion than without the addition of hydrogen to nitrogen. The advantageous temperature range of the implementation is between 150 and 550 ° C and corresponds to the prevailing requirements for stationary and mobile exhaust gas swell. With regard to the catalyst composition, silver contents of 0.1-20% by mass, preferably 1-5% by mass, are sufficient. The choice of aluminum oxides is not decisive, provided that a surface, determined by the BET method of nitrogen adsorption, of more than 10 m ² / g is available.

A reduction of the NO _x by H ₂ without the additional presence of NH ₃ only leads to insufficient degrees of conversion of the NO _x . Only the joint action of NH ₃ and H ₂ leads to the described promotion of the reaction.

With regard to the causes of this "hydrogen effect", experimental investigations allow the following interpretation: The prevailing excess of oxygen in the exhaust gases (2–10 vol.% Compared to 500–5000 ppm NO _x ) leads to the oxidation of existing silver clusters of low nuclearity (Ag _n - Detect clusters of n = 5–8 atoms), which block the necessary sub-step of NO activation.The ammonia is not able to reduce the oxidation of the silver clusters sufficiently, but hydrogen, although mainly oxidized to water by oxygen , is apparently able to achieve an intermediate reduction of the silver clusters, which can activate the NO _x and thus accelerate the reaction, possibly undertaken by the catalyzed oxyhydrogen reaction between H ₂ and O ₂ H ₂ + ½O ₂ → H ₂ O (3) at which runs simultaneously. The exact causes must be clarified in scientific studies. The effect is clearly not associated with a restructuring of the active phase, since a change between hydrogen-containing electricity and hydrogen-free electricity leads to an instantaneous change in the achievable degree of NO _x conversion.

The catalytic oxidation of NH ₃ to N ₂ , NO and N ₂ O is not significant. The oxidation of NH ₃ to NO (and N ₂ O) takes place at higher temperatures and is the basis for the nitric acid production according to the Oswald process, in which N ₂ O occurs as an undesirable by-product.

A comparison of the achievable degrees of NO _x conversion is given in Table 1 for the method according to the invention when using an aluminum oxide containing 1% by weight Ag as catalyst in comparison to reference samples. The decisive factor for the comparability is the conversion of NO _x to N ₂ achieved at a temperature under identical gas volume loads and almost identical starting material composition. Where this comparability is not possible, it can be estimated that, with the same degree of conversion, the catalyst that achieves this value at the higher gas volume load is the more active.

The indication of the selectivity of the implementation takes into account the fact that conversion of NO _{x can} also lead to significant N ₂ O formation. Gas volumes are generally standard volumes, whereby the standard temperature is assumed to be 25 ° C. The ratio of NO _x to NH ₃ is determined according to the stoichiometry of the summary implementation according to Eq. (1) generally set to 1: 1 (molar) or 1: 0.9 to avoid ammonia slip as much as possible.

The invention further relates to a catalytic system consisting of an aluminum oxide support with silver deposited thereon as metallic silver or silver complexes in an atmosphere which consists at least of nitrogen oxides, ammonia, oxygen and 0.05-4% by volume hydrogen. Other gases such as CO, CO ₂ , hydrocarbons and others, which can also be components of exhaust gases, can also be present in this atmosphere.

Table 1 Conversion of NO _x by NH ₃ on a commercial SCR catalyst compared to the process according to the invention

The invention is intended to be used in exemplary embodiments explained using a drawing become. It shows

1 the on an Ag / A ₂ O ₃ catalyst with an Ag content of 1% by weight and an Al ₂ O _{3 support} , which has a surface area of 235 m ² / g, a pore volume of 0.4 cm ³ / g and has an average pore diameter of 0.5 nm and is X-ray amorphous, with a simulated exhaust gas mixture consisting of 1000 ppm NO, 1000 ppm NH ₃ , 6% O ₂ , 7% H ₂ O, with and without hydrogen addition of 1% by volume in the temperature range of 150-550 ° C achieved degrees of conversion of NO _x to nitrogen, with the gas volume load of the catalyst is 30 Nm ³ / kg _Kat h.

example 1

0.24 g of a catalyst consisting of 5% by weight Ag containing aluminum oxide (surface area 235 m ² / g, pore volume 0.4 cm ³ / g, average pore diameter approx. 5 nm) and produced by impregnation of the granulated aluminum oxide with a 1 M silver nitrate solution after drying and calcining at 550 ° C in air, in a quartz flow reactor after a 2-hour conditioning phase at 550 ° C in O ₂ (6%) / carrier gas stream (helium) with a simulated exhaust gas stream of composition 1000 ppm NO, 1000 ppm NH ₃ , 6 vol% O ₂ , 7 vol.% H ₂ O (carrier gas: helium) and a volume velocity of 120 cm ³ / min total flow (this corresponds to a gas load of 30 Nm ³ / kg _cat h) , tested at a reaction temperature of 550 ° C and then in 50-degree steps up to a temperature of 150 ° C for about 30 minutes for the achievable degree of conversion of NO to N ₂ . Then 1 vol.% H _{2 is} added to the volume flow and the test series is repeated as described. While an average degree of conversion of 10% of the NO _{x used is} achieved in the investigated temperature range of 150-550 ° C without the addition of hydrogen, the value has increased to more than 90% after the addition of hydrogen. In a relatively wide temperature range of 200–500 ° C, the degree of NO _x conversion is almost 100% and only drops slightly to values of around 90% up to 550 ° C. The selectivity of the reduction of the NO _x to nitrogen remains at almost 90-100%.

Example 2

This example differs from example 1 in that the reaction on the catalyst according to example 1 was increased to 250 Nm ³ / kg _cat h with an otherwise unchanged composition of the simulated exhaust gas. Even with this very high gas volume load, the catalyst in the presence of 1% by volume H _{2 achieved} degrees of conversion of NO _x of 70% at a temperature of 350 ° C.

Example 3

This example differs from Examples 1 and 2 in that water gas (average composition 50% H ₂ , 40% CO, 5% CO ₂ , 4-5% N ₂ ) from coke is present in a catalyst of suitable composition upstream of the actual SCR reactor and water vapor according to Eq. (5) and (6) C + H ₂ O ↔ H ₂ + CO (5) C + 2H ₂ O ↔ H ₂ + CO ₂ (6) formed and the hydrogen is fed into the SCR reactor for promotion.

Example 4

This example differs of Examples 1 and 2 in that the hydrogen production by electrolytic water decomposition is carried out.

Claims (16)

Process for the selective catalytic conversion of nitrogen oxides by ammonia in the presence of oxygen, characterized in that hydrogen is added to the gas mixture and the reaction takes place on a silver-containing catalyst. A method according to claim 1, characterized in that a gas mixture is used whose hydrogen content is immediate before contact with the catalyst in the range of 0.05-5 vol.% lies. A method according to claim 2, characterized in that a gas mixture is used whose hydrogen content is immediate before contact with the catalyst in the range of 0.8-2.5 vol.% lies. A method according to claim 1, characterized in that the hydrogen is added permanently or in pulses. Method according to one of claims 1-4, characterized in that that hydrogen is added as a gas or by an upstream or simultaneous reaction is formed during the process. A method according to claim 1, characterized in that a silver-containing catalyst is used, the silver in the form of metallic silver, silver complexes or mixtures thereof contains. A method according to claim 1, characterized in that a catalyst is used in which the particle size of the silver on a support is equal to or less than 5 nm. A method according to claim 1, characterized in that a supported Catalyst is used, the carrier being selected from the group consisting of crystalline aluminum oxide, amorphous aluminum oxide, crystalline aluminum silicate, amorphous aluminum silicate, titanium oxide, Zirconium oxide, gallium oxide, cerium oxide, silicon dioxide as silica gel and mixtures thereof. Method according to one of claims 1-8, characterized in that a supported Catalyst is used, the silver content in the range of 0.1 to 20% by weight, preferably 1 to 5% by weight, based on the total weight of the catalyst. Method according to one of claims 1-9, characterized in that as a catalyst for the reaction uses silver-containing aluminum oxide. A method according to claim 10, characterized in that an alumina carrier is used which contains silicon up to a proportion of 50% by weight, provided that a surface of at least 10 m ² / g determined by the BET method remains by nitrogen adsorption. A method according to claim 8, characterized in that a supported catalyst is used, the surface area of which is in the range from 30 to 140 m ² / g. Method according to one of claims 1 to 12, characterized in that that a catalyst is used, its modification with silver through watering, Precipitation, Evaporation, sol-gel processes and mechanical admixture based of silver salts or silver complexes. A method according to claim 1, characterized in that the catalytic conversion is carried out in a fixed bed reactor. A method according to claim 1, characterized in that catalytic conversion in a monolithic reactor carried out becomes. Catalytic system consisting of an alumina carrier with then deposited as metallic silver or silver complexes Silver in an atmosphere which at least consist of nitrogen oxides, ammonia, oxygen and 0.05–4 vol% There is hydrogen.