- Email: [email protected]
NOX abatement catalyst with low SO2 oxidation activity
T.S.R. Prasada Rao and G. Murali Dhar (Editors) Recent Advances in Basic and Applied Aspects of Industrial Catalysis Studies in Surface Science and Catalysis, Vol. 113 9 1998 Elsevier Science B.V. All rights reserved
NO~ abatement catalyst with low S02 oxidation activity. Uma Parameswaran, Amiya kumar Nandi, D.S.Sawant, D. Venkateswaran, Sumit Bhaduri, R.M.Cursetji The Associated Cement Cos. Ltd., Research & Consultancy Directorate, L.B.S. Marg, Thane 400 604, India. ABSTRACT The commercially available catalytic processes for NOx abatement involve the use of transition metal ions such as V 4+/5+, W6+, Cu2+ etc. on a titania (anatase) or zeolite support. The metal ions catalyse the reaction between nitric oxide, ammonia and oxygen to give nitrogen and water. Using Indian ilmenite, high surface area titania (anatase) has been obtained which when doped with optimum amounts of transition metal ions, yields a catalyst capable of converting NOx with more than 90% efficiency within a temperature window of 250-450~ Moreover this catalyst has minimal SO2 oxidation activity even at high temperatures (-450 ~ This catalyst therefore has the potential of commercial use in coal fired thermal power plants where the NOx has a high level of SO2 impurity. Accelerated ageing tests have also been carded out on this catalyst and these studies indicate that the expected lifetime would be greater than or equal to five years. Spectroscopic and other physico-chemical data on the fresh and used catalysts are also presented. I. INTRODUCTION The burning of coal in thermal power plants results in major pollutants such as suspended particulate matter (SPM), sulphur dioxide (SO2), oxides of nitrogen (NOx) etc, of which NOx is believed to be a key component responsible for several hazards associated with ecology and human health(l). Given the relative abundance of coal in India, coal-based thermal power plants will continue to play a dominant role in the power sector. Therefore NOx abatement through primary and secondary measures assumes great importance. Post combustion techniques such as selective catalytic reduction (SCR) can reduce NOx emissions by >95% (2). In the SCR process, stochiometric quantities of ammonia (NH3) is injected along with the flue gas over a catalyst at temperatures between 300 and 400 ~ to reduce NOx to harmless nitrogen (N2) and water (1-120). It may be emphasized that Indian coals have a high sulphur content and as already mentioned, this leads to the formation of sulphur dioxide during combustion. In presence of the catalyst, this SO2 may get oxidised to sulphur trioxide (SO3) which in turn may react with excess NH3 resulting in the formation of ammonium salts causing
384 clogging of catalyst pores and corrosion of the downstream equipment. It is therefore necessary for commercial catalysts to have high NOx reduction but minimum SO2 oxidation activities. The commercially available catalysts for this application are based on transition metal ions such as vanadium (V4+/5+) and tungsten (W6+) on titania (TiO2) anatase support, or copper (Cu2+) on a zeolite support.It is the titania based SCR catalyst which is in operation in most of the plants worldwide (3). As reported in the literature, the role of each of these catalyst components is supposed to be very specific. Titania-tungsten oxide offer durability and low SO2 oxidation capacity. V205 is added to lower the temperature window for NOx reduction. In this paper, TiO2 obtained from Indian ilmenite has been used to synthesize high surface area titania anatase into which tungsten oxide (WO3) and vanadium pentoxide (V205) have been incorporated. Zeolite based catalysts have also been extensively studied for SCR reaction. Zeolite based catalysts are in operation in several chemical process industries (4). Copper loaded zeolites have been reported as excellent SCR catalysts with a wide temperature window particularly for nitric acid tail gas plants. This paper describes the performance evaluation studies and ESR characterisation data on the conventional titania anatase based catalyst and a less common system, viz. vanadyl exchanged mordenite.
2. EXPERIMENTAL The titanium dioxide obtained from ilmenite ore was suitably processed and calcined to yield high surface area TiO2 anatase which was used in the catalyst formulations synthesized in the laboratory. The synthesized samples were characterized using techniques such as X-ray powder diffraction (XRD), and FTIR and ESR spectral techniques.
2.1 Catalyst with Titania support Titania anatase (25 gms) was blended with tungstic acid (2 gms) and silica sol (6 gm containing 30% silica) along with water (5 co) to make a paste which was kneaded and extruded in the form of 4 mm extrudates. This sample was then dried and calcined in air at 550~ for 4 hrs. It was then soaked in a solution ofvanadyl oxalate (1.0 gm V205 dissolved in 5 gms Oxalic acid), for 40 minutes followed by drying and calcination in air at 550~ The concentration of V205 as analysed by wet chemical methods was 0.48% by weight.
2.2 Catalyst with Mordenite support 25 gins sodium mordenite powder with SiO2 to A[203 molar ratio > 10 was stirred with 75 ml vanadyl oxalate for 12 hrs. A part of the sample was washed with oxalic acid for 18 hours followed by water wash, filtered, dried and calcined at 550~ The V205 content was found to be 0.32% by weight. The titania and mordenite based samples were tested for their performance by subjecting them to NOx reduction and SO2 oxidation in a microreactor (Chemical Data System's Dual catalytic reactor system Model 810-CD-HP). Technovation make NO & SO2 analysers were used for monitoring NOx and SO2 at the inlet and outlet.
385 3. RESULTS & DISCUSSION 3.1.
Performance data
The activity of the above catalysts was tested for NOx reduction and SO2 oxidation sequentially under test conditions listed in table 1. Table 1 Performance test conditions Catalyst volume (cc.) Space velocity (hr~) NH3/NOx ratio Inlet NOx cone. (ppm) Inlet SO2 cone. (ppm)
2 10000 1.4 1000 1200
a) Titania-vanadia catalyst The titania-vanadia system showed a high reduction in NOx (> 90%) at temperatures between 250~ and 450~ The SO2 oxidation was nil at plant operating conditions of 300~ to
100 o= 8 z
~.
-
A
10
80
8
60
6
r , 4 cr
40 20 . , J
150 200 250 350 450 Temperature o C -
% N0x conversion - - 4 -
% S02 oxidation]
Fig. 1 Performance data on titania based catalyst 350~ At temperatures < 250~ (Figure 1).
the NOx conversion showed high sensitivity to temperature
386 The activation energy (Ea) calculated in the region 150~ to 250~ using Arrhenius plot is approximately 42.0 KJ/mole. (Figure 2) which is close to the reported value. (5)
O
"~ 5 3 Zl i
i
i
1
1.912 2.008 2.1142 2.2321 2.3641 (I/T)
x 10 3
(K"l)
Fig.2. Arrhenius plot b) Ageing studies with TiO2 based catalyst Ageing studies were conducted by exposing a small quantity of the catalyst to 100 times the experimental concentration of NOx and SO2 at 400~ for 24 hrs. After each exposure, the NOx reduction efficiency was checked. Subsequently, SO2 (100 times the experimental concentration) was saturated with water and passed over the catalyst at 400~ for 24 hrs. On evaluation of NOx reduction activity it was found that there was no fall in catalytic activity even after exposing to acidic atmosphere thereby implying a long catalyst lifetime.
y
100
..~
80
r,~
=~
..-i------
--~
m,,,,,,,- am.
60
0
0 Z 0~
40 20 r
0
100
200
300
400
500
Temperature o C = "
Fresh catalyst-NOx Fresh catalyst-SO2
-- I - -Aged catalyst-NOx " ~ - Aged catalyst-SO2
Fig. 3 Comparative performance data on fresh and aged catalyst
387 c) Mordenite supported catalyst Performance test conducted with vanadyl-mordenite sample gave satisfactory results (Fig.4). Higher temperatures were required for NOx conversion as compared to titania based catalyst but SO2 oxidation was very low at these temperatures. 100 O .N
10
--
80-
60o= r o 40z 20-
4 c3 raO
9
0
2
,
100
0
0~
0
200
300
400
500
Temperature o C % NOx conversion
.-.A... % SO2 oxidation J
Fig. 4 Performance data on vanadyl mordenite 3.2 Charaeterisation
data
The ESR and FTIR spectra were recorded on the titania based catalyst formulation. The ESR spectrum of the sample recorded at 77~ showed hyperfine splittings characteristic of V 4+ oxidation state. (Fig. 5)
200G
200G
Fresh catalyst
Aged catalyst
Fig. 5 ESR speemmaof titania based formulation
The FTIR spectrum showed bands at -~1100 cm1 and 1000 crnq probably corresponding to V - O and W - O bonds (6). The ESR spectrum of the aged sample was very similar to that of the l~esh sample which indicates tittle change in bulk concentration of V 4+ ion.
388 In the vanadyl exchanged mordenite sample it was important to establish that VO 2+ was exchanged with the sodium cation of mordenite and not surface adsorbed.To investigate this, the ESR spectrum of thoroughly washed vanadyl mordenite was recorded at 77~ (Fig 6).
200G Fig. 6. ESR spectrum of Vanadyl mordenite. The vanadyl mordenite sample exhibited eight equally spaced hyperfine splittings indicating the presence of atomically dispersed paramagnetic V 4+ ions(6). Both mordenite and vanadyl mordenite samples have very strong infrared absorption at 1100 cm -1 for "Si-O' functionalities. Thus as reported by other workers for similar sample direct infrared evidence for "V=O ' functionality in this region could not be obtained. 4. CONCLUSION The activities of titania and mordenite supported catalysts were evaluated for NOx reduction and SO2 oxidation. The following conclusions can be drawn from the experimental results: * * *
TiO2 - V 2 0 5 - W O 3 formulation gives a wide temperature window for NOx reduction while keeping SO2 oxidation very low at operating plant conditions. Accelerated ageing studies on the titania based catalyst confirm that the catalyst is expected to have a long life even in the presence of acidic atmospheres. Vanadyl mordenite has a small temperature window for NOx reduction while SO2 oxidation is very low even at high temperatures.
ACKNOWLEDGEMENTS We acknowledge financial assistance from Technology Information Forecasting & Assessment Council (TIFAC) of Dept. of Science & Technology (DST), Govt. of India. Our thanks to RSIC personnel at IIT, Bombay for assistance in recording ESR and FTIR spectra.
389 REFERENCES
~
2. 3. 4. .
6. 7.
Catal. Today, 2, No.4 (1988) 369. A. Garg, CEP, Jan (1994) 46. N. Nojiri, Catal. Rev.- Sci. Eng., 37, No.1 (1995) 145. J.R. Kiovsky, Presented at Inst.of Chem. Engineers, "Control of sulphur and other gaseous emissions", Third Intl. Symp.,UK pql (18). Chem. Ing. Tech. 62, No. 1 (1990) 60. G. Ramis, Appl. Catal B, No. 1 (1992) L9. P. Ratnasamy, J. Chem. Soc.,Chem. Commun. (1992) 1613.
NO~ abatement catalyst with low S02 oxidation activity. Uma Parameswaran, Amiya kumar Nandi, D.S.Sawant, D. Venkateswaran, Sumit Bhaduri, R.M.Cursetji The Associated Cement Cos. Ltd., Research & Consultancy Directorate, L.B.S. Marg, Thane 400 604, India. ABSTRACT The commercially available catalytic processes for NOx abatement involve the use of transition metal ions such as V 4+/5+, W6+, Cu2+ etc. on a titania (anatase) or zeolite support. The metal ions catalyse the reaction between nitric oxide, ammonia and oxygen to give nitrogen and water. Using Indian ilmenite, high surface area titania (anatase) has been obtained which when doped with optimum amounts of transition metal ions, yields a catalyst capable of converting NOx with more than 90% efficiency within a temperature window of 250-450~ Moreover this catalyst has minimal SO2 oxidation activity even at high temperatures (-450 ~ This catalyst therefore has the potential of commercial use in coal fired thermal power plants where the NOx has a high level of SO2 impurity. Accelerated ageing tests have also been carded out on this catalyst and these studies indicate that the expected lifetime would be greater than or equal to five years. Spectroscopic and other physico-chemical data on the fresh and used catalysts are also presented. I. INTRODUCTION The burning of coal in thermal power plants results in major pollutants such as suspended particulate matter (SPM), sulphur dioxide (SO2), oxides of nitrogen (NOx) etc, of which NOx is believed to be a key component responsible for several hazards associated with ecology and human health(l). Given the relative abundance of coal in India, coal-based thermal power plants will continue to play a dominant role in the power sector. Therefore NOx abatement through primary and secondary measures assumes great importance. Post combustion techniques such as selective catalytic reduction (SCR) can reduce NOx emissions by >95% (2). In the SCR process, stochiometric quantities of ammonia (NH3) is injected along with the flue gas over a catalyst at temperatures between 300 and 400 ~ to reduce NOx to harmless nitrogen (N2) and water (1-120). It may be emphasized that Indian coals have a high sulphur content and as already mentioned, this leads to the formation of sulphur dioxide during combustion. In presence of the catalyst, this SO2 may get oxidised to sulphur trioxide (SO3) which in turn may react with excess NH3 resulting in the formation of ammonium salts causing
384 clogging of catalyst pores and corrosion of the downstream equipment. It is therefore necessary for commercial catalysts to have high NOx reduction but minimum SO2 oxidation activities. The commercially available catalysts for this application are based on transition metal ions such as vanadium (V4+/5+) and tungsten (W6+) on titania (TiO2) anatase support, or copper (Cu2+) on a zeolite support.It is the titania based SCR catalyst which is in operation in most of the plants worldwide (3). As reported in the literature, the role of each of these catalyst components is supposed to be very specific. Titania-tungsten oxide offer durability and low SO2 oxidation capacity. V205 is added to lower the temperature window for NOx reduction. In this paper, TiO2 obtained from Indian ilmenite has been used to synthesize high surface area titania anatase into which tungsten oxide (WO3) and vanadium pentoxide (V205) have been incorporated. Zeolite based catalysts have also been extensively studied for SCR reaction. Zeolite based catalysts are in operation in several chemical process industries (4). Copper loaded zeolites have been reported as excellent SCR catalysts with a wide temperature window particularly for nitric acid tail gas plants. This paper describes the performance evaluation studies and ESR characterisation data on the conventional titania anatase based catalyst and a less common system, viz. vanadyl exchanged mordenite.
2. EXPERIMENTAL The titanium dioxide obtained from ilmenite ore was suitably processed and calcined to yield high surface area TiO2 anatase which was used in the catalyst formulations synthesized in the laboratory. The synthesized samples were characterized using techniques such as X-ray powder diffraction (XRD), and FTIR and ESR spectral techniques.
2.1 Catalyst with Titania support Titania anatase (25 gms) was blended with tungstic acid (2 gms) and silica sol (6 gm containing 30% silica) along with water (5 co) to make a paste which was kneaded and extruded in the form of 4 mm extrudates. This sample was then dried and calcined in air at 550~ for 4 hrs. It was then soaked in a solution ofvanadyl oxalate (1.0 gm V205 dissolved in 5 gms Oxalic acid), for 40 minutes followed by drying and calcination in air at 550~ The concentration of V205 as analysed by wet chemical methods was 0.48% by weight.
2.2 Catalyst with Mordenite support 25 gins sodium mordenite powder with SiO2 to A[203 molar ratio > 10 was stirred with 75 ml vanadyl oxalate for 12 hrs. A part of the sample was washed with oxalic acid for 18 hours followed by water wash, filtered, dried and calcined at 550~ The V205 content was found to be 0.32% by weight. The titania and mordenite based samples were tested for their performance by subjecting them to NOx reduction and SO2 oxidation in a microreactor (Chemical Data System's Dual catalytic reactor system Model 810-CD-HP). Technovation make NO & SO2 analysers were used for monitoring NOx and SO2 at the inlet and outlet.
385 3. RESULTS & DISCUSSION 3.1.
Performance data
The activity of the above catalysts was tested for NOx reduction and SO2 oxidation sequentially under test conditions listed in table 1. Table 1 Performance test conditions Catalyst volume (cc.) Space velocity (hr~) NH3/NOx ratio Inlet NOx cone. (ppm) Inlet SO2 cone. (ppm)
2 10000 1.4 1000 1200
a) Titania-vanadia catalyst The titania-vanadia system showed a high reduction in NOx (> 90%) at temperatures between 250~ and 450~ The SO2 oxidation was nil at plant operating conditions of 300~ to
100 o= 8 z
~.
-
A
10
80
8
60
6
r , 4 cr
40 20 . , J
150 200 250 350 450 Temperature o C -
% N0x conversion - - 4 -
% S02 oxidation]
Fig. 1 Performance data on titania based catalyst 350~ At temperatures < 250~ (Figure 1).
the NOx conversion showed high sensitivity to temperature
386 The activation energy (Ea) calculated in the region 150~ to 250~ using Arrhenius plot is approximately 42.0 KJ/mole. (Figure 2) which is close to the reported value. (5)
O
"~ 5 3 Zl i
i
i
1
1.912 2.008 2.1142 2.2321 2.3641 (I/T)
x 10 3
(K"l)
Fig.2. Arrhenius plot b) Ageing studies with TiO2 based catalyst Ageing studies were conducted by exposing a small quantity of the catalyst to 100 times the experimental concentration of NOx and SO2 at 400~ for 24 hrs. After each exposure, the NOx reduction efficiency was checked. Subsequently, SO2 (100 times the experimental concentration) was saturated with water and passed over the catalyst at 400~ for 24 hrs. On evaluation of NOx reduction activity it was found that there was no fall in catalytic activity even after exposing to acidic atmosphere thereby implying a long catalyst lifetime.
y
100
..~
80
r,~
=~
..-i------
--~
m,,,,,,,- am.
60
0
0 Z 0~
40 20 r
0
100
200
300
400
500
Temperature o C = "
Fresh catalyst-NOx Fresh catalyst-SO2
-- I - -Aged catalyst-NOx " ~ - Aged catalyst-SO2
Fig. 3 Comparative performance data on fresh and aged catalyst
387 c) Mordenite supported catalyst Performance test conducted with vanadyl-mordenite sample gave satisfactory results (Fig.4). Higher temperatures were required for NOx conversion as compared to titania based catalyst but SO2 oxidation was very low at these temperatures. 100 O .N
10
--
80-
60o= r o 40z 20-
4 c3 raO
9
0
2
,
100
0
0~
0
200
300
400
500
Temperature o C % NOx conversion
.-.A... % SO2 oxidation J
Fig. 4 Performance data on vanadyl mordenite 3.2 Charaeterisation
data
The ESR and FTIR spectra were recorded on the titania based catalyst formulation. The ESR spectrum of the sample recorded at 77~ showed hyperfine splittings characteristic of V 4+ oxidation state. (Fig. 5)
200G
200G
Fresh catalyst
Aged catalyst
Fig. 5 ESR speemmaof titania based formulation
The FTIR spectrum showed bands at -~1100 cm1 and 1000 crnq probably corresponding to V - O and W - O bonds (6). The ESR spectrum of the aged sample was very similar to that of the l~esh sample which indicates tittle change in bulk concentration of V 4+ ion.
388 In the vanadyl exchanged mordenite sample it was important to establish that VO 2+ was exchanged with the sodium cation of mordenite and not surface adsorbed.To investigate this, the ESR spectrum of thoroughly washed vanadyl mordenite was recorded at 77~ (Fig 6).
200G Fig. 6. ESR spectrum of Vanadyl mordenite. The vanadyl mordenite sample exhibited eight equally spaced hyperfine splittings indicating the presence of atomically dispersed paramagnetic V 4+ ions(6). Both mordenite and vanadyl mordenite samples have very strong infrared absorption at 1100 cm -1 for "Si-O' functionalities. Thus as reported by other workers for similar sample direct infrared evidence for "V=O ' functionality in this region could not be obtained. 4. CONCLUSION The activities of titania and mordenite supported catalysts were evaluated for NOx reduction and SO2 oxidation. The following conclusions can be drawn from the experimental results: * * *
TiO2 - V 2 0 5 - W O 3 formulation gives a wide temperature window for NOx reduction while keeping SO2 oxidation very low at operating plant conditions. Accelerated ageing studies on the titania based catalyst confirm that the catalyst is expected to have a long life even in the presence of acidic atmospheres. Vanadyl mordenite has a small temperature window for NOx reduction while SO2 oxidation is very low even at high temperatures.
ACKNOWLEDGEMENTS We acknowledge financial assistance from Technology Information Forecasting & Assessment Council (TIFAC) of Dept. of Science & Technology (DST), Govt. of India. Our thanks to RSIC personnel at IIT, Bombay for assistance in recording ESR and FTIR spectra.
389 REFERENCES
~
2. 3. 4. .
6. 7.
Catal. Today, 2, No.4 (1988) 369. A. Garg, CEP, Jan (1994) 46. N. Nojiri, Catal. Rev.- Sci. Eng., 37, No.1 (1995) 145. J.R. Kiovsky, Presented at Inst.of Chem. Engineers, "Control of sulphur and other gaseous emissions", Third Intl. Symp.,UK pql (18). Chem. Ing. Tech. 62, No. 1 (1990) 60. G. Ramis, Appl. Catal B, No. 1 (1992) L9. P. Ratnasamy, J. Chem. Soc.,Chem. Commun. (1992) 1613.