MXPA97005622A - Catalytic selective reduction of nitrog oxides - Google Patents

Abstract

The selective catalytic reduction of nitrogen oxides is effected by reacting nitric oxide, nitrogen dioxide or a mixture thereof with an aliphatic carboxylic acid reducing agent having from 1 to 5 carbon atoms at a temperature ranging from about 250 to about 600 ° C. , in the presence of a catalyst comprising metal oxide selected from the group consisting of vanadium oxide, copper oxide, nickel oxide and iron oxide, the catalyst being supported on a porous carrier. The method of the invention allows substantially and completely reducing Nox to harmless N2, in an efficient, environmentally condescending and cost effective manner.

Description

SELECTIVE CATALYTIC REDUCTION OF NITROGEN OXIDES V- DESCRIPTION OF THE INVENTION The present invention relates to improvements in the emission control of environmentally hazardous and regulated nitrogen oxides (NOx), which are produced in a variety of processes such as the combustion of fossil fuels, more particularly, the invention relates to a improved process for the selective catalytic reduction of nitrogen oxides to nitrogen. Air pollution caused by NOx emissions has become an increasing global issue in recent years. Nitrogen oxides contribute to the rain of acid and photochemical smog, and can cause respiratory problems. It has now been recognized that ozone at ground level is formed in the atmosphere through a photochemical reaction not only of organic compounds, but also of nitrogen oxides. The main sources of NOx emissions in the countries industrialized are transportation, electrical installations, and industrial kettles. Much of the NOx is a combustion product of fossil fuels, such as coal, oil or gas. At present, severe regulations are being implemented on the control of NOx emission in industrialized countries and the The limit of NOx discharge to the environment is successively being revised to place enormously effective control requirements with the ultimate goal of zero NOx emission. In California, for example, emission limits of 9 ppm or less have been imposed for industrial kettles above approximately 5860 kw (20 million / btuhr.). Due to these severe regulations on NOx emissions, the development of effective NOx control technology has gained importance in recent years. So far, the most effective technology to control NOx emissions is the reduction V 1, selective catalytic (SCR) of NOx. In this method, NOx (NO + NO2) are reduced through NH3 to N and H2O, usually at 250-400 ° C on a catalyst. The following reactions occur: 4NO + 4NH3 + 02? 4N2 + 6HzO 6NO2 + 8NH3? 7N2 + 12H2O 15 2NO2 + 4NH3 + O2? 3N2 + 6H2O Since usually over 80% by volume of NOx is in the form of NO, the first reaction is the most important. The catalytic reduction by NH3 requires an ammonia injection system and an ammonia storage system. A The practical disadvantage of this procedure is that it requires a complex and expensive process to safely handle NH3, which is a dangerous chemical. The known catalytic systems, which are able to effectively catalyze the NOx reduction reactions Above, using NH3, are supported noble metals, supported base metal oxides and zeolites. Noble metal catalysts, such as those based on Pt, Rh, Ru, or Pd supported ^ on AI2O3 or other vehicles, which are widely used in catalytic converters for the reduction of NOx exhaust in automobiles, are usually not considered for the treatment of combustion gas, due to several disadvantages. These disadvantages include high cost, susceptibility to S02 poisoning and substantial reduction of catalytic activity at high temperatures or in the presence of excess oxygen due to the accumulation of the absorbed oxygen. Japanese Patent Document, J PA-06 226 052 relates to catalysts, which comprise a metal (for example, Cu, Ni, Fe), which is in ionic form and catalysts, which comprise metal oxides of Al , Zn, T, and Si. The catalysts based on vanadia or tungsten-vanadia as active components supported on titania of porous anatase type are currently known, since they are very promising for the selective catalytic reduction of NO by N H3 mainly due to its high activity at low temperatures Y good resistance to SO2 poisoning. These catalysts are currently used in many commercial facilities. However, even with these catalysts, there are a number of problems. During the SCR procedure, N H3 may also suffer oxidation to undesirable NOx in accordance with the following reactions: 4N H3 + 302? 2 N2 + 6H20 4N H3 + 5O2? 4N0 + 6H2O 2 N H3 + 2O2? N2 + 3H2O When the oxidation of N H3 proceeds in parallel with the SCR, results in a high consumption of N H3 and a lower NOx removal efficiency. The oxidation reactions of ammonia at higher temperatures (> 425 ° C) are dominant. The usual operating temperature required for the SCR reaction ranges from about 300 to about 425 ° C for a peak NOx conversion efficiency. This temperature restriction limits the flexibility of the location of the SCR reactor in the integrated combustion gas cleaning unit and incurs heat exchanger costs for applications where the temperature of the combustion gas exceeds this temperature limit. From a practical point of view, the selectivity and the activity of the catalysts must be retained on a wide scale of - temperature. Another serious disadvantage with the selective catalytic reduction of NOx through N H3 is the risk of unacceptably high levels of ammonia emission known as "ammonia leakage". The role of ammonia in the pollution of the atmosphere is known. The ammonia leak can, at first, be suppressed by reducing the N H3 / NOx ratio entering the reactor. However, this adversely affects the removal efficiency of 25 NOx.

Although the catalysts based on vanadia and tungsten-vanadia exhibit resistance to S02 poisoning, they catalyze the * Oxidation of SO2 to S03. This last compound (SO3) reacts with NH3 and H2O to form compounds such as NH HSO and (NH4) S2O7. These compounds cause corrosion, clogging the catalytic reactor and the other parts of the system, and more undesirably, clogging the pores of the catalysts. The pore plugging of the catalyst finally results in a deactivation of the catalyst at a fixed ratio of NH3 / NO, and an increase in ammonia leakage. The loss in activity can be restored by increasing the NH3 / NO input ratio. However, the increase in the NH3 / NO ratio has the effect that the ammonia leak also increases. The filling of the pores of the catalyst and the reactor can also occur due to the possible formation of NH3N3 by a homogeneous reaction between NH3, N02 and H2O. Therefore, it is an object of the present invention to overcome the above disadvantages and to provide a method for the direct and substantially complete reduction of nitrogen oxides. It is another object of the invention to provide an improved method for selective catalytic reduction, which avoids the use of a dangerous or toxic gas. In accordance with the present invention, a method for the selective catalytic reduction of oxides of Nitrogen to nitrogen, which comprises the reaction of nitric oxide, nitrogen dioxide or a mixture thereof, with a reducing agent consisting of an aliphatic carboxylic acid having from 1 to 5 carbon atoms at a varying temperature from about 250 to about 600 ° C, in the presence of a catalyst comprising a metal oxide, selected from the group consisting of vanadium oxide, copper oxide, nickel oxide, and iron oxide, the catalyst being supported on a porous vehicle. The applicant has unexpectedly found that using As reducing agent an aliphatic carboxylic acid containing from 1 to 5 carbon atoms, direct and substantially complete reduction of nitrogen oxides can be achieved, provided that the reduction is carried out within the above temperature scale and in the presence of the catalyst above. definite. The carboxylic acids used in accordance with the present invention, besides being environmentally safe, possess a very reactive and labile hydrogen atom in their structure. An agent of > Oxidation such as NO and NO2 can easily extract this labile hydrogen, forming radicals H NO and / or H NO2. These species Once reacted, they undergo a series of reactions to produce N2 and H2O. The corresponding organic radicals generated from the primary decomposition of carboxylic acids easily undergo additional reactions to produce CO2 and H2O. The catalytic reduction of NOx with carboxylic acids ensures a complete destruction of NOx, so that the final products comprise N2, CO2 and H2O, only. Under these conditions, complete oxidation of intermediate products occurs. The * Total reactions are as follows: catalyst NO + RCOOH? N2, C02, H2O (1) NO2 + RCOOH catalyst? N2, CO2, H2O (2) catalyst NO + RCOOH + O2? N2, CO2, H2O (3) 10 catalyst NO2 + RCOOH + O2? N2, CO2, H2O (4) wherein R represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms. 15 The mechanism for the selective catalytic reduction of NOx with RCOOH is believed to be as follows: RCOOH + 2S - RCOOads + H -ads (5) jjr ^ 20 NO + S (NO) ads (6) - (NO) ad £ H ade - »(NO.H) ads + S (7) (NO.H) ads + Had £ - > N ads H2O (8) [o] RCOOad s? C02 and H20 (1 0) ? where S denotes a vacant surface site and the subscription 'ads' refers to a species adsorbed on the catalyst. In addition to the reactions (1) - (4), the undesirable side reaction (11) can also occur, that is, RCOOH can to some degree be oxidized by O 2 (present in the combustion exhaust of fuel burners). according to the following total reaction: f 10 RCOOH + O? CO2, H2O (1 1) The catalysts defined above used in the selective catalytic reduction based on RCOO H according to the invention are effective in promoting reactions (1) to (4) and suppressing the side reaction (11). The loading of metal oxide on the support can range from about 5 to about 50 mol%, and most preferably from about 8 to 20 mol%, approximately. The total surface area (BET) of the catalyst may vary in scale from about 50 to about 500 m2 / g, and most preferably in the range from 1 00 to 300 m2 / g, approximately. The reaction is preferably carried out at a temperature of about 450 to about 500 ° C. Preferably, the nitrogen or the water vapor is mixed with the reducing agent. The method of the invention substantially and completely reduces NOx to harmless N2, in an efficient, environmentally condescending and cost-effective manner. * Other aspects and advantages of the invention will be more readily apparent from the following description of a preferred embodiment illustrated by way of example in the accompanying drawing, in which: Figure 1 is a flow diagram of a method for selective reduction of nitrogen oxides according to the invention. 10 In the procedure, which is illustrated schematically in the Figure 1, the gaseous mixture containing NOx, produced in the fuel burner 10 by combustion of fuel and discharged via the line 12 is passed to a heat exchanger 14 to recover most of the heat generated by the combustion of the fuel and reducing the temperature of the gas stream to approximately 250-600 ° C, and then it is sent to a catalytic converter 16 containing a fixed bed of a catalyst based on vanadium oxide, copper oxide , nickel oxide or iron oxide. Since the gas stream that contains NOx enters the converter 16, this is mixed with a stream of gas, which is fed via the feed line 18 and contains, as a reducing agent, an aliphatic carboxylic acid having from 1 to 5 carbon atoms in mix with nitrogen and water vapor. The resulting gas mixture is made passed through the catalyst bed maintained at a temperature of 250-600 ° C and reacted with the reducing agent. The effluent stream, which is discharged via line 20 and is free of NOx contaminants, is passed through a heat exchanger 22 to recover useful heat and then through a stack 24 before being discarded at a regulating height towards the natural environment The following non-limiting examples further describe the invention. EXAMPLE 1 A V2O5 /? - Al2? 3 catalyst containing 10 mol% of VO was prepared by impregnating? -AI2O3 (10 g) with a solution of oxalic acid (4.0 g) and ammonium metavanadate (2.34 g) in water distilled (50 ml). Impregnation was performed by adding V2? 5 /? - AI2O3 to the solution followed by mixing and evaporation of water. The impregnated material was further dried in an oven at 120 ° C for 8 hours. =? - hours and calcined in a muffle furnace at 500 ° C for 2 hours. The BET surface area of the catalyst was 175 m2 / g. A quartz micro-reactor was packed with 0.3 g of the catalyst above and was placed in a continuous flow reactor. A gaseous mixture containing nitric oxide and acetic acid was passed through the downflow reactor at a flow rate of 70 ml / min. The molar composition of the gaseous feed mixture was as follows: 0.106% NO, 0.28% acetic acid, 2.15% water vapor and the rest nitrogen. The temperature of the reactor was maintained at 435 ° C. The composition of the reactor effluent was analyzed by means of a chemiluminescence NOx analyzer, and also by gas chromatography. The concentration of nitric oxide at several times in the stream is reported in Table 1.

TABLE 1 Time in the Conc. Of NOx Conc. Of N2O% Molar of current, min. ppm ppm conversion of NOx 0 1062 + 11 N.D. 0 15 26 + 0.3 N.D. 97.6 35 10 + 0.1 N.D. 99.1 60 5.5 + 0.06 N.D 99.5 80 3.9 _ + 0.04 N.D. 99.6 N.D. = Not detected 10 NOx detection limit = 50 ppb i- As is evident from Table 1, under a stable state, the concentration of nitric oxide was reduced from 1060 ppm to 3.9 ppm, indicating a conversion of 99.6%. The formation of other nitrogen oxides, such as N02, was not detected.

EXAMPLE 2 The same feed mixture of Example 1 was passed through a micro-reactor packed with 0.3 g of a V2O5 /? - Al2? 3 catalyst containing 10 mol% of V2Os, at a rate of ^ flow of 70 ml / min. The temperature of the reactor was maintained at 445 ° C. The composition of the reactor effluent was analyzed in the same manner as in Example 1. The concentration of NOx in the reactor effluent is reported in Table 2. TABLE 2 Time in the Conc. Of NOx Conc. Of N20% Molar of current, min. ppm ppm conversion of NOx 1 80 1 .5 N. D. 99.86 N. D. = Not detected 10 As is evident from Table 2, the concentration of nitric oxide in the reactor effluent was 1.5 ppm, indicating a NO conversion of 99.86%. No other nitrogen oxides such as NO2 or N2O were detected in the reactor effluent. ß 15 EXAMPLE 3 A gaseous mixture containing 0.62 mole% of nitric oxide, 0.65 mole% of acetic acid, 3.09 mole% of water vapor and 95.64 mole%, was passed through a packed micro-reactor with 0.3 of a V2? 5 /? - AI2O3 containing 10 mol% of V205, at a flow rate of 100 ml / min. The effluent from the reactor was analyzed under steady-state conditions. The concentration of nitric oxide in the reactor effluent at various reaction temperatures is reported in Table 3. ^ TABLE 3 Temp. of Conc. of No in Conc. of N2 in% Molar of reactor ° C the effluent of the reactor conversion effluent, ppm reactor, ppm NO 375 3200 1375 48.4 450 738 2725 88.1 > 480 198 2993 96.8 490 73 3058 98.8 520 0 3096 100.0 As is evident from Table 3, on the temperature scale of 375-520 ° C, the conversion of NO varies in the scale of 48% to 100%. A corresponding generation of N2 was observed, as shown by the concentration of 2 in the effluent of the reactor. The conversion of NO in the absence of acetic acid was zero on the temperature scale of 375-520 ° C.

EXAMPLE 4 A CuO-NiO /?-AI2O3 catalyst containing 5% by weight of Cu and 5% by weight of Ni, calculated as metallic elements, was prepared by impregnating? -AI203 (10 g) with a solution of cupric nitrate [ Cu (N03) 2 »3H2O] (1901 g) and nickel nitrate [Ni (NO3) 2-6H2O] (2.477 g) in distilled water (50 ml). The impregnated material was dried in an oven at 120 ° C for 18 hours and ^ f calcined in a muffle furnace at 500 ° C for 2 hours. The BET surface area of the catalyst was 175 m2 / g. A gaseous mixture containing 0.058 mol% (or 580 ppm) of nitrogen oxides, 0.1 mol% of acetic acid, 2.5 mol% of oxygen, 16.1 mol% of carbon dioxide in nitrogen was passed through a micro- quartz reactor packed with 1.0 μm of a CuO-N iO /? - AI2O3 catalyst at a flow rate of 100 ml / min. The concentration of ß 10 nitrogen oxides in the reactor effluent under a stable state was verified at various reaction temperatures, and is reported in Table 4.

TABLE 4 15 As is evident from Table 4, with oxygen present in the gaseous feed mixture, the NOx conversion passes through a maximum on the temperature scale of 230-460 ° C. By eiemDlo. at an intermediate temperature of 270 ° C. the concentration of NOx in the reactor effluent was as low as 6 ppm, representing a conversion of 99.0 mol%. ^ COMPARATIVE EXAMPLE 5 A catalyst was prepared consisting of a ZSM-5 type zeolite in protonated form having a SiO2 / AI O3 ratio of 36, by crystallization of silica rich gels containing tetrapropyl ammonium bromide as a template, following the * 10 procedure underlined in the patent of E. U.A. No. 3,702,886. The BET surface area of this catalyst was 376 m2 / g. A gaseous mixture containing 0.15 mol% nitric oxide, 0.31 mol% acetic acid, 0.95 mol% steam and 98.58 mol% nitrogen was passed through a micro reactor. packed with 0.15 g of the zeolite catalyst at a flow rate of 45 ml / ml. The temperature of the reactor was maintained at 500 ° C. The effluent from the reactor was analyzed under steady-state conditions.

* The concentration of nitric oxide in the effluent of the reactor was 0.14%, indicating a NO conversion of only 4.7%. This is much lower compared to the 99% conversion obtained using AI2O3 and CuO / NiO /? - AI2O3 catalysts under similar conditions.

Claims (10)

1 .- A process for the selective catalytic reduction of nitrogen oxides to nitrogen, which comprises reacting nitric oxide, nitrogen dioxide, or a mixture thereof with a reducing agent, consisting of an aliphatic carboxylic acid which has from 1 to 5 carbon atoms, at a temperature ranging from about 250 to about 600 ° C, in the presence of a catalyst comprising a metal oxide KR 10 selected from the group consisting of vanadium oxide, copper oxide, nickel oxide, iron oxide and a mixture thereof, said catalyst being supported on a porous carrier.

2. A process according to claim 1, wherein said carboxylic acid is selected from the group consisting of formic acid, acetic acid, propionic acid and butyric acid.

3. A process according to claim 2, wherein said carboxylic acid is acetic acid.

4. A process according to claim 1, wherein said catalyst comprises from about 5 to about 20 mole% of said metal oxide.

5. A process according to claim 4, wherein said catalyst comprises from about 8 to about 20 mol% of said metal oxide.

6. A process according to claim 1, wherein the catalyst has a total surface area ranging from about 50 to about 500m / g.

7. A process according to claim 6, wherein the total surface area of said catalyst ranges from about 100 to about 300 m2 / g.

8. A method according to claim 1, wherein the vehicle is selected from the group consisting of alumina, silica, and titania.

9. A process according to claim 1, wherein the catalyst comprises vanadium oxide supported on α-alumina.

10. A process according to claim 9, wherein the catalyst comprises approximately 10 mol% of vanadium oxide. 1 - A process according to claim 1, wherein said catalyst comprises a mixture of copper oxide and nickel oxide supported on α-alumina. 12. A process according to claim 1, wherein said catalyst comprises about 5% by weight of Cu and about 5% by weight of N i, calculated as metallic elements. 13. A method according to claim 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, or 12, wherein the nitrogen or water vapor is mixed with said agent of reduction. 14. A method according to claim 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, or 12, wherein said reaction is carried out at a temperature ranging from about 450 at approximately 550 ° C. 15. A process according to claim 1, which comprises reacting nitric oxide, nitrogen dioxide, or a mixture thereof with said reducing agent, in the presence of molecular oxygen and a catalyst consisting of an oxide of metal selected from the group consisting of vanadium oxide, copper oxide, nickel oxide, iron oxide, and a mixture thereof, said catalyst being supported on a porous carrier consisting essentially of alumina. 16. A process according to claim 15, wherein said carboxylic acid is selected from the group consisting of formic acid, acetic acid, propionic acid and butyric acid. 17. A process according to claim 16, wherein said carboxylic acid is acetic acid. 18. A process according to claim 15, wherein said catalyst comprises from about 5 to about 50 mol% of said metal oxide. 19. A process according to claim 18, wherein said catalyst comprises from about 8 to about 20 mol% of said metal oxide. 20. A method according to claim 15, wherein the catalyst has a total surface area ranging from about 50 to about 500m2 / g. 21. A process according to claim 20, wherein the total surface area of said catalyst ranges from about 100 to about 300 m2 / g. 22. A process according to claim 15, wherein the vehicle is selected from the group consisting of alumina, silica, and titania. 23. A process according to claim 15, wherein the catalyst comprises vanadium oxide. 24. A process according to claim 23, wherein the catalyst comprises approximately 10 mol% of vanadium oxide. 25. A process according to claim 15, wherein said catalyst comprises a mixture of copper oxide and nickel oxide. 26. A process according to claim 25, wherein said catalyst comprises about 5% by weight of Cu and about 5% by weight of N i, calculated as metallic elements. * 27. A process according to claim 15, wherein nitrogen or water vapor is mixed with said reducing agent. 28. A process according to claim 15, said reaction is carried out at a temperature ranging from about 450 to about 550 ° C. 29. A process according to claim 17, wherein the metal oxide comprises vanadium oxide.