NZ298726A - Selective catalytic reduction of nitrogen oxides - Google Patents

Description

New Zealand No. 298726 International No. PCT/CA96/00027 TO BE ENTERED AFTER ACCEPTANCE AND PUBLICATION Priority dates: 25.01.1995; Complete Specification Filed: 19.01.1996 Classification:^) B01 D53/56,94,86 Publication date: 25 November 1998 Journal No.: 1434 NEW ZEALAND PATENTS ACT 1953 COMPLETE SPECIFICATION Title of Invention: Selective catalytic reduction of nitrogen oxides Name, address and nationality of applicant(s) as in international application form: GAZ METROPOLITAIN AND COMPANY, LIMITED PARTNERSHIP, 1717 Du Havre, Montreal, Quebec H2K 2X3, Canada; RAJ NARAIN PANDEY, 34 Old Colony Trail, Guelph, Ontario N1G 4A9, Canada 2987 WO 96/22828 PCT/CA96/00027 SELECTIVE CATALYTIC REDUCTION OF NITROGEN OXIDES The present invention pertains to improvements in the emission control of environmentally harmful and regulated nitrogen oxides (NOx) which are produced in a variety of processes such as the combustion of fossil 5 fuels. More particularly, the invention relates to an improved process for the selective catalytic reduction of nitrogen oxides to nitrogen.

Atmospheric pollution caused by N0X emissions has become a matter of growing global concern in recent 10 years. Nitrogen oxides contribute to acid rain and photochemical smog, and can cause respiratory problems. It is now recognized that the ground-level ozone is formed in the atmosphere through a photochemical reaction not only from volatile organic compounds but 15 also from oxides of nitrogen.

The main sources of NOx emissions in industrialized countries are transportation,- electric utilities, and industrial boilers. Much of the NOx is a product of combustion of fossil fuels such as co£i,1, oil 2 0 or gas.

Stringent regulations on NOx emission control are currently being implemented in industrialized countries and the limit of NOx discharge into the environment is successively being revised to place 2 5 increasingly effective control requirements with an ultimate goal of zero NOx emission. In California, for instance, emission limits of 9' ppm or less have been imposed for industrial boilers above approximately 5B60 kw (20 million btu/hr.) 3 0 Due to these stringent regulations on NOx emissions, the development of an effective NOx control technology has gained importance in recent years. To date, the most effective technology for controlling NOx SUBSTITUTE SHEET (RULE 26) 29 8 72 emissions is the selective catalytic reduction (SCR) of N0X. In this method, NOx (NO + N02) are reduced by NH 3 to N2 and HjO, usually at 250-400°C over a catalyst. The following reactions occur: 4N0 + 4HN3 + 02 * 4N2 + 6Ha0 6N02 + 8NH3 * 7N2 + 12H20 2N0, + 4NH3 + 02 > 3N2 + 6H20 Since usually over 80 vol. % of the Nox is in the form of NO, the first reaction is the most important. 10 Selective catalytic reduction by NH3 requires an ammonia injection system and an ammonia storage system. A practical disadvantage of this process is that it requires a complex and expensive set-up for safely handling NH3 which is a hazardous chemical. 15 Known catalytic systems which are able to catalyze effectively the above NOx reduction reactions using NK 3 are supported noble metals, supported base metal oxides and zeolites. Noble metal catalysts such as those based or. ?t, Rh, Ru or Pa supported on A1203 or other carriers, 20 which are used widely in catalytic converters for automobile-exhaust' Nox reduction, are usually not considered for flue gas treatment due to several drawbacks. These drawbacks include high cost, susceptibility to S02 poisoning and substantial reduction 25 of the catalytic activity at high temperatures or in the presence of excess oxygen due to accumulation of absorbed oxygen.

Japanese document, JP-A-06 226 052 relates to catalysts which comprise a metal( e.g., Cu, Ni, Fe) which 30 is in ionic form and catalysts which comprise metal oxides of Al, Zn, Ti and Si.

Catalysts based on vanadia or tungsten-vanadia as active components supported on porous anatase-type titania are currently known to be most promising for the 35 selective catalytic reduction of NO by NH3 mainly because of their high activity at low temperatures and good resistance to SO2 poisoning. These catalysts are presently used in many commercial installations. However, even with these catalysts, a number of problems are encountered. During the SCR process, NH3 5 can also undergo oxidation to undesirable NOx according to the following reactions: 4NH3 + 302 2N2 + 6H20 4NH3 + 502 4N0 + 6H20 2NH3 + 202 N20 + 3H20 When the NH3 oxidation proceeds in parallel with SCR, it results in a greater NH3 consumption and a lower N0X removal efficiency. Ammonia oxidation reactions are dominant at higher temperatures (>425°C). The usual operating temperature required for SCR 15 reaction ranges from about 300 to about 425°C for peak NOx conversion efficiency. This temperature constraint limits the flexibility of the SCR reactor location in the integrated flue gas clean-up unit and incurs a heat exchanger cost for applications where the flue gas 20 temperature exceeds this temperature limit. From a practical view point, the selectively and activity of the catalysts should be retained over a broad temperature range.

Another serious disadvantage with the selective 25 catalytic reduction of NOx by NH3 is the risk of unacceptably high levels of ammonia emission known as "ammonia slip". The role of ammonia in polluting the atmosphere is well known. Ammonia slip can, in principle, be suppressed by lowering the reactor inlet 30 NH3/NOx ratio. This however, adversely affects the NOx removal efficiency.

Although vanadia and tungsten-vanadia based catalysts exhibit resistance to S02 poisoning, they WO 96/22828 PCT/CA96/00027 catalyze oxidation of SO2 to SO3. This latter compound (SO3) reacts with NH3 and H2O to form compounds such as NH4HSO4 and (NH4)2S207. These compounds cause corrosion, plugging of the catalytic reactor and other 5 parts of the system, and more undesirably, plugging of the pores of the catalysts. Pore plugging of the catalyst eventually results in a deactivation of the catalyst at a fixed NH3/NO ratio and an increase of ammonia slip. The loss in activity can be restored by 10 increasing the inlet NH3/NO ratio. However, increasing the NH3/NO ratio has the effect that ammonia slip also increases. Plugging of the catalyst pores and the reactor can also occur due to possible formation of NH4NO3 by homogeneous reaction between NH3, NO2 and 15 H20.

It is therefore an object of the present invention to overcome the above drawbacks and to provide a process for the direct and substantially complete reduction of nitrogen oxides.

It is another object of the invention to provide an improved process for the selective catalytic reduction of nitrogen oxides, which avoids the use of a hazardous or toxic gas.

In accordance with the present invention, there 25 is thus provided 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 having 1 to 5 carbon atoms at 30 a temperature ranging 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, iron oxide, and mixtures thereof, the catalyst being supported on a porous carrier. In one embodiment the carrier is selected from the group consisting of alumina, silica and titania. In a preferred embodiment the catalyst comprises vanadium oxide supported on y-alumina. In a further embodiment the catalyst comprises a mixture of copper oxide and nickel oxide supporter on Y-alumina• Applicant has found quite unexpectedly that-by using as a reducing agent an aliphatic carboxylic acid containing 1 to 5 carbon atoms the direct and substantially complete reduction of nitrogen oxides can be achieved, provided that the reduction be carried out within the above tenperature range and in the presence of the above defined catalyst. In one embodiment of the invention the reduction process reaction is carried out at a temperature ranging from about 450 to about 550°C.

The carboxylic acids used in accordance vith ihe present invention, in addition to being environmentally safe, possess a very reactive or labile hydrogen, atom in their structure. An oxidizing agent such as NO and NOj can easily abstract this labile hydrogen, forming HNO and/or HNO; radicals. These reactive species once formed undergo a series of reactions to produce N: and H20. The corresponding organic radicals generated from the primary decomposition of carboxylic acids readily undergo further reactions to produce C0; and HjO. The catalytic reduction cf No, with carboxylic-acids ensures complete destruction cf No,, so that the final products comprise Ni( CO; and K ;0 only. Under these conditions, complete oxidation of intermediate products occurs. The overall reactions are as follows : Ni( CO., H;.0 (1) Na, CO;, HjO (2) Na, C0Jf HjO (3) Na, C02, HjO (4) NO + RCOOH > NO* + RCOOH NO + RCOOH + Oj > NO> + RCOOH + 02 ^ INTELLECTUAL PROPERTY OFFICII OF N.Z. 1 '» S£P 1998 where R represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms.

The mechanism for the selective catalytic reduction of N0X with RCOOH is believed to be as 5 follows: RCOOH .+ 2S — RCOOads + Hads (5) NO + S (NO) acis (6) (NO) ads + Hads (NO.H) ads + S (7) (NO.H) ads + Hads Nads + H20 + S (8) Nads + Nads N2 (9) [0] RCOOgds c°2 and H20 (10) where S denotes a vacant surface site and subscript 'ads' refers to an adsorbed species on the catalyst.

In addition to reactions (l)-(4), undesirable side reaction (11) could also occur, that is, RCOOH could to some extent be oxidized by 02 (present in 20 combustion exhaust of fuel burners) according to the' following overall reaction: RCOOH + 02 ** C02, H20 (11) The above defined catalysts used in the RCOOH- based selective catalytic reduction 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 may 30 vary in the range of about 5 to about 50 mole %, and more preferably in the range of about 8 to about 20 mole %. The total (BET) surface area of the catalyst may vary in the range of about 50 to about 500 m2/g, and more preferably in the range of about 100 to about 300 m^/g.

The reaction is preferably carried out at a temperature of about 450 to about 500°C. Preferably, 5 nitrogen or water vapor is admixed with the reducing agent.

The process of the invention enables one to substantially completely reduce N0X to harmless N2 in an efficient, environmentally friendly and cost-10 effective manner.

Further features and advantages of the invention will become more readily apparent from the following description of a preferred embodiment as illustrated by way of example in the accompanying, drawing, in which: 15 Figure 1 is a flow diagram of a process for the selective reduction of nitrogen oxides according to the invention.

In the process which is schematically illustrated in Fig. 1, the NOx containing gaseous 20 mixture produced in the fuel burner 10 by the combustion of fuel and discharged via line 12 is passed through a heat exchanger 14 for recovering most of the heat generated by the fuel combustion and lowering the temperature of the gas stream to about 250-600°C, and 2 5 then sent to a catalytic converter 16 containing a fixed bed of a vanadium oxide, copper oxide, nickel oxide or iron oxide based catalyst. As the NOx containing gas stream enters into the converter 16, it is mixed with a reducing gas stream which is fed via 3 0 feed line 18 and contains, as a reducing agent, an aliphatic carboxylic acid having 1 to 5 carbon atoms in admixture with nitrogen and water vapor. The resulting gaseous mixture is passed through the catalyst bed WO 96/22828 PCT/CA96/00027 maintained at a temperature of 250-600°C and reacted with the reducing agent. The effluent strewn which is discharged via line 20 and is free of N0X contaminants is passed through a heat exchanger 22 for recovering 5 useful heat and then through a stack 24 before being discharged at a regulatory height to the natural environment.

The following non-limiting examples further illustrate the invention.

EXAMPLE 1 A V2O5/7-AI2O3 catalyst containing 10 mole % V2O5 was prepared by impregnating 7-AI2O3 (10 g) with a solution of oxalic acid (4.0 g) and ammonium metavanadate (2.34 g) in distilled water (50 ml). The 15 impregnation was carried out by adding the V2O5/7-AI2O3 to the solution followed by mixing and water evaporation. The impregnated material was further dried in an oven at 120°C for 8 hours and calcined in a muffle furnace at 500°C for 2 hours. The BET surface 20 area of the catalyst was 175 m^/g.

A quartz microreactor was packed with 0.3 g of the above catalyst and placed in a continuous flow reactor. A gaseous mixture containing nitric oxide and acetic acid was passed through the downflow reactor at 25 a flow rate of 70 ml/min. The molar composition of the feed gaseous mixture was as follows: 0.106% NO, 0.28% acetic acid, 2.15% water vapor and balance nitrogen. The reactor temperature was maintained at 435°C.

The composition of the reactor effluent was 30 analyzed by a chemiluminescence NOx analyzer, and also by gas chromatography. The concentration of nitric oxide at various times on-stream is reported in Table 1.

TABLE 1 Time on NOx Cone.

N20 Cone.

NOx Stream, ppm ppm Conversion min. mole % 0 1062 + 11 N.D. 0 26 + 0.3 N.D. 97.6 + 0.1 N.D. 99.1 60 .5 + 0.06 N.D. 99.5 80 3.9 + 0.04 N.D. 99.6 N.D. = Not Detected N0X Detection Limit = 50 ppb 5 As it is apparent from Table 1, under a steady 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 oxides of nitrogen such as N02 or N20 was not detected.

EXAMPLE 2 The same feed mixture as in Example 1 was passed through a microreactor packed with 0.3 g of a V2O5/7-AI2O3 catalyst containing 10 mole % V2O5, at 70 ml/min. flow rate. The reactor temperature was maintained at 15 445°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 on Stream, min.

N0X Cone, ppm N2O Cone, ppm N0X Conversion mole % 180 1.5 N.D. 99.86 N.D. = Not Detected As it is apparent from Table 2, the 5 concentration of nitric oxide in the reactor effluent was 1.5 ppm, indicating a NO conversion of 99.86%. No other oxides of nitrogen such as N02 or N20 were detected in the reactor effluent.

EXAMPLE 3 A gaseous mixture containing 0.62 mole % nitric oxide, 0.65 mole % acetic acid, 3.09 mole % water vapor and 95.64 mole % helium was passed through a microreactor packed with 0.3 g of a V205/7~A1203 catalyst containing 10 mole % V205, at a flow rate of 15 100 rnl/min. The reactor effluent was analyzed under steady state conditions. The concentration of nitric oxide in the reactor effluent at various reactir 1 temperatures is reported in Table 3.

TABLE 3 Reactor Cone, of NO Temp. in Reactor °C Effluent, ppm 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 it is apparent from Table 3, in the 5 temperature range of 375-520°C, the NO conversion varies in the range 48% to 100%. A corresponding generation of N2 was observed, as shown by the N2 concentration in the reactor effluent. The conversion of NO in the absence of acetic acid was zero in the 10 temperature range 375-520°C.

EXAMPLE 4 A Cu0-Ni0/7~A1203 catalyst containing. 5 wt.% Cu and 5 wt.% Ni, calculated as metallic elements, was prepared by impregnating 7-Al203 (10 g) with a solution 15 of cupric nitrate [Cu(N03)2.3H20] (1.901 g) and nickel nitrate [Ni(NO3)2•6H20] (2.477 g) in distilled water (50 ml). The impregnated material was dried in an oven at 120°C for 8 hours and calcined in a muffle furnace at 500°C for 2 hours. The BET surface area of the 20 catalyst was 175 m^/g. A gaseous mixture containing 0.058 mol % (or 580 ppm)' nitrogen oxides, 0.1 mol % acetic acid, 2.5 mol % oxygen, 16.1 mol % carbon Cone, of N2 Conversion in Reactor NO Effluent, mole % ppm ( dioxide in nitrogen was passed through a quartz microreactor packed with 1.0 g of CuO-NiO/7*-Al203 catalyst at a flow rate of 100 ml/min. The concentration of nitrogen oxides in the reactor 5 effluent under steady state was monitored at various reaction temperatures, and is reported in Table 4.

TABLE 4 Reactor Cone, of N0X Conversion of Temp. °C in Reactor Effluent, NOx mole % ppm 230 55 90.5 270 6 99.0 350 190 67.2 400 312 46.2 460 391 32.6 As it is apparent from Table 4, with oxygen present in the gaseous feed mixture, the conversion of. N0X passes through a maximum in the temperature range of 230-460°C. For example, at an intermediate temperature of 270°C, the concentration of N0X in the 15 reactor effluent was as low as 6 ppm, representing a N0X conversion of 99.0 mole %.

COMPARATIVE EXAMPLE A catalyst consisting of a ZSM-5 type zeolite in protonated form having a Si02/Al203 ratio of 36 was 20 prepared by crystallizing silica rich gels containing tetrapropyl ammonium bromide as template, following the procedure outlined' in US Patent N° 3,702,886. The BET surface area of this catalyst was 37 6 m2/g. A gaseous mixture containing 0.15 mole % nitric oxide# 0.31 mole % acetic acid, 0.95 mole % water vapor and 98.59 mole % nitrogen was passed through a microreactor packed with 0.15 g of the zeolite catalyst at a flow 5 rate of 45 ml/min. The reactor temperature was maintained at 500°C. The reactor effluent was analyzed under s.teady state conditions. The concentration of nitric oxide in the reactor effluent ' was 0.14%, indicating a NO conversion of only 4.7%. This is much 10 lower compared to 99% conversion obtained using V2O5/7-AI2O3 and CuO-NiO/7~Al203 catalysts under similar conditions.

Claims (5)

2987 26 The embodiments of the invention, in which an exclusive property or privilege is claimed are defined as follows:

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 having 1 to 5 carbon atoms at a temperature ranging from about 250 to about S00°C, in the presence of a catalyst comprising a metal oxide 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 as claimed in claim 1, wherein said carboxylic acid is selected from the group consisting of formic acid, acetic acid, propionic acid and butyric-acid . 20

3. A process as claimed in claim 2, wherein said carboxvlic acid is acetic acid.

4. A process as claimed in claim 1, wherein said catalyst comprises about 5 to about 50 mole % of said metal cxide. 30

5. A process as claimed in claim 4, wherein said catalyst comprises about 8 to about 20 mole % of said metal cxide. N.Z. PATENT QFFfC-2 3 MAY 19 97 Received - 14 - AN'rr^D SHEET f; "WO 96/22828 PCT/CAS6/00027 293726

6. A process as xlaimed In claim 1, wherein said catalyst nas a total surface area xanging from about 50 to about 500 m2/g. I. A process as claimed In claim 6, swherein ~the total surface area of said catalyst ranges "from about 100 to about 300 m2/g. G. A process as claimed in claim lr -wherein said carrier Is selected from the grotip consisting of alumina,, silica and titania.

9. A process as claimed in claim 2, wherein said catalyst comprises vanadium oxide supported on -y-alumina.

10. A process as claimed in claim 9, wherein said catalyst comprises about 10 mole ft of vanadium oxide. II. A process as claimed in claim lf wherein said catalyst comprises a mixture of copper oxide and nickel oxide supported on -y-alumina.

12. A process as claimed in claim 11, wherein said catalyst comprises about 5 wt.% Cu and about 5 wt.% Ni, calculated as metallic elements.

13. A process as claimed in claim 1, 2, 3, A, 5, €, 7, 8, 9, 10, 11 or 12, wherein nitrogen or water vapor is admixed with said reducing agent.

14. A process as claimed in claim 1, 2, 3, A, 5, 6, 1, 8, 9, 10, 11 or 12, wherein said reaction is carried out at a temperature ranging from about 4 50 to about 550°C. - 25 — INTELLECTUAL PROPERTY OFFICE" OF N.Z. 1 'i SiP 1398 RECEivrn I ° 298726

15. A process as defined in claim 1 -which 6onprises reacting nitric oxide, nitrogen dioxide or a mixture thereof with said reducing agent, in "the presence of molecular oxygen and a -catalyst 5 consisting of a metal oxide 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. 10

16. A process as claimed in claim 15, wherein said carboxylic acid is selected from the .group consisting of formic acid, acetic acid, propionic acid and butyric acid. 15 A process as claimed in claim 16, wherein said carboxylic acid is acetic acid. IE. A process as claimed in claim 15, wherein said 20 catalyst comprises about 5 to about 50 mole % of saic metal oxide.

19. A process as claimed in claim 18, wherein said catalyst comprises about 8 to about 20 mole % of 25 said metal oxide. 2D. A process as claimed in claim 15, wherein said catalyst has a total surface area ranging from about 50 to .about 500 mJ/g. 30

21. A process as claimed in claim 20, wherein the total surface area of said catalyst ranges from about 100 to about 300 tnVg- 35 22. A process as claimed in claim 15, wherein said carrier is selected from the group consisting of alumina, silica, and titania. - 16 - * I:.' 29872 f

23. A process as claimed in claim 15, . wherein said metal oxide comprises vanadium oxide. 5 24. A process as claimed in claim 23, wherein said catalyst comprises about 10 mole % of vanadium oxide.

25. A process as claimed in claim 25, wherein said 10 metal oxide comprises a mixture of copper oxide and nickel oxide.

26. A process as claimed in claim 25, wherein said catalyst comprises about 5 wt. % Cu and about 5 wt. 15 k Ni, calculated as metallic elements.

27. A process as claimed in claim 15, wherein nitrogen cr water vapour is admixed with said reducing agent. 20

21. A process as claimed in claim 15, wherein said reaction is carried out at a temperature ranging from about 4 50 to about 550° C. 25 2 5. A process as claimed in claim 17, wherein the metal oxide comprises vanadium oxide.

30. A process for the selectable catalytic reduction of nitrogen oxides to nitrogen as claimed in claim 1 substantially as herein described with reference to any example thereof and/or the accompanying drawings. 35 - 17 - END OF CLAIMS mreuuilML ttWEBTY OFF® OF N.Z. 1 "•••••-- SWFFT I4 S£P J998 -FECEIV Fn