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Evaluation of amines for the selective catalytic reduction (SCR) of NOx from diesel engine exhaust
Fuel 85 (2006) 1772–1780 www.fuelfirst.com
Evaluation of amines for the selective catalytic reduction (SCR) of NOx from diesel engine exhaust James F. Kelly *, Maria Stanciulescu, Jean-Pierre Charland CANMET Energy Technology Centre—Ottawa, 1 Haanel Drive, Ottawa, Ont., Canada K1A 1M1 Received 22 September 2005; received in revised form 5 January 2006; accepted 23 January 2006 Available online 20 March 2006
Abstract With a view to developing onboard generation of selective reductants for NOx removal from diesel engine exhaust we compared the performance of a primary, secondary and tertiary amine to NH3 using a typical mini core NH3-SCR catalyst. Primary amines with short hydrocarbon chains, e.g. CH3NH2 (maximum NOx conversion, 50%) approached the NOx conversion obtained using NH3 (maximum NOx conversion, 70%). Increasing the amine to NOx ratio greater than 1 results in NOx conversions closer to those of NH3 (maximum NOx conversion increased to 60%). Secondary and tertiary amines had smaller NOx conversions as a function of temperature and the drop in NO and NOx conversion decreased with increasing amine hydrogen substitution. Also, the maximum NOx conversion for each reductant tends to move to a lower temperature as the degree of substitution increases. Unlike NH3, the amines can react in the gas phase at temperatures within the range of diesel engine exhaust. Due to this gas phase reactivity the NOx conversions measured using the mini core SCR catalyst also contain a gas phase conversion component. Gas phase conversions were investigated by replacing the mini core SCR catalyst with an equivalent length of quartz beads. Subtraction of the two results highlighted the differences between the mini core catalytic and gas phase conversions measured in this manner over the temperature range investigated. These differential NOx conversions for the three amines had maxima at about 375 8C. q 2006 Elsevier Ltd. All rights reserved. Keywords: NOx reduction; Amine reductants; SCR of NOx using amines
1. Introduction The reduction of NOx emissions from diesel engine exhaust is a significant technical challenge, which can limit expansion of the diesel engine market especially in North America. In order to meet new US EPA emission standards to be phased in beginning in 2007, more than 90% reductions in NOx emissions will be required. Considerable R&D is being done on NOx traps with associated catalysts and selective catalytic reduction (SCR) using NH3 or urea as reductant. Both these approaches are considered to have good potential of meeting these standards. For heavy duty diesel engines, R&D in the US has emphasized NOx traps while in Europe SCR is receiving more attention. Based on its use in stationary combustion applications the SCR approach has a long history in terms of catalyst * Corresponding author. Tel.: C1 613 996 0396; fax: C1 613 943 8882. E-mail address: [email protected] (J.F. Kelly).
0016-2361/$ - see front matter q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2006.01.024
development and has recently been adapted for vehicle use by including a reductant storage vessel on board. However, mobile SCR systems have a more serious problem related to necessary changes in infrastructure required to make the reductant (urea or NH3) available at filling stations. One way to overcome this problem is to generate suitable SCR reductants in situ on board the vehicle. For a diesel engine vehicle this limits potential feedstocks to generate these reductants to diesel fuel and air. If the N2 in the air can be made to react with the hydrogen in the diesel fuel it may be possible to produce NH3 and amines on board. Plasma systems have been shown to be able to produce NH3 from N2 and H2 at low temperatures and atmospheric pressure [1–3]. In a simulation of Titan’s atmosphere, a CH4/N2 microwave discharge was able to produce NH3, nitriles and a series of primary amines [4]. Recently, Bromberg et al. [5] have suggested that it may be possible to produce NH3 on board using a plasmatron diesel fuel reformer. The use of amines to reduce NOx has been reported by Yoshihara et al. [6–12] who have described the development of a selective non-catalytic technique for NOx removal from diesel engine exhaust using methylamine (CH3NH2) as reductant.
J.F. Kelly et al. / Fuel 85 (2006) 1772–1780
In this paper, we report on the use of a series of amines as reductants using a typical mini core NH3-SCR catalyst. The NOx conversion performance of a primary, secondary, and tertiary amine is compared with NH3. Since the amines can also reduce NOx due to gas phase or non-catalytic reactions over the temperature range investigated, gas phase results were estimated by replacing the mini core SCR catalyst with an equivalent length bed of quartz beads (!80 mesh). Differential conversions were obtained by subtracting the gas phase results from the mini core SCR catalytic results.
2. Experimental 2.1. Catalyst testing unit Fig. 1 shows a schematic of the catalyst testing unit. Twelve electronic mass flow controllers are used to produce a realistic simulated diesel exhaust feed to the catalytic reactor. Water (10% by volume) is added to the feed by a calibrated low flow rate dual syringe infusion/withdrawal pump before the SO2 or NO is added to prevent corrosion of a humidity probe, which is used to monitor the water concentration in the feed on a continuous basis. A primary standard (NIST traceable) flowmeter (Bios DryCal DC-Lite model DCL-L) was used to measure the flows set for each flow meter. Also, to allow for screening of reductant compounds, which are liquids at room temperature a trace source permeation tube unit (KIN-TEK) was installed to directly inject these reductants into the reactor feed at parts per million by volume levels. The KIN-TEK system includes a precision temperature furnace to control the rate of injection along with a separate helium mass flow controller (MFC 10).
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A three zone furnace is used to heat the quartz reactor tube (6.12 mm nominal ID by 40.64 cm L). The catalyst used for all the tests reported in this paper is a 12 cell mini core cut from a typical SCR catalyst block with cordierite substrate of 400 cpsi. The monolith catalyst containing V2O5 plus WO3 and TiO2 was obtained from a large catalyst company and had a specific surface area of 13.8 m2/g monolith. The total reactor volume occupied by the mini core is 0.50 mL. Experiments are done at GHSV 60,000 hK1, measured at STP (20 8C and 1 atm), which equates to a total feed flow rate of 500 mL/min at STP. The setpoints of the three zones of the reactor furnace are adjusted so that the vertical temperature profile measured over the length of the mini core is constant. The product gases exit the reactor and pass through a Nafion dryer (Perma Pure Model PD-625-24SS) to selectively remove water before slip streams are analyzed by a NOx analyzer (Thermo Environmental model 42H), gas chromatographs (MTI M200 and M200H) and a FTIR unit (Innova type 1301 gas analyzer). Initial experiments using amines as reductants clearly showed that there was significant interference in the operation of the NO2 converter section of the NOx analyzer when amines are used as reductants. Therefore, reliable NOx measurements using the chemiluminescence analyzer were not possible although NO concentrations measured by the NOx analyzer normally agreed well with those measured by the FTIR unit. Therefore, unless stated otherwise all NO and NOx conversion results for all reductants except NH3 are based on FTIR measurements. The dryer also removes alcohols and aldehydes as well as ammonia and amines so measurement of unreacted reductants exiting the reactor was not possible. Also, since the effluent gases cool down as they pass through the dryer, formation and precipitation of ammonium salts like ammonium sulphate or
Fig. 1. Catalyst testing unit.
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Table 1 Reductant screening operating conditions
2.2. Operating conditions
SCR catalyst
Table 1 shows normal operating conditions used to test each reductant with the SCR mini core catalyst. The reactor temperature is increased linearly from 200 to 600 8C at 3 8C/ min while the feed rates are kept constant. This allows measurement of NOx conversions as a function of temperature over a wide range of interest for diesel engine exhaust. Unless indicated otherwise the reductant feed concentration is kept equal to the NO concentration as shown in Table 1. The balance gas is helium rather than N2 so that NOx conversions can be compared with N2 production. No CO2 is used in the feed because it absorbs IR so strongly that accurate FTIR measurements of other components would not be possible. A small amount of SO2 (5 ppmv) is added to the feed to simulate a low sulphur diesel fuel exhaust stream. The pressure drop across the reactor during experiments is typically less than 4 psig.
Reactor temperature Reactor GHSV at STPa Feed component CO O2 NO NO2 C3H6 H2O SO2 Reductant He a
12 cell mini core from 400 cpsi block V2O5CWO3 and TiO2 Linear ramp from 200 to 600 8C at 3 8C/min 60,000 hK1 100 ppmv 120,000 ppmv (12 vol%) 500 ppmv Traces 100 ppmv 100,000 ppmv (10 vol%) 5 ppmv 500 ppmv Balance
STPZ20 8C, 1 atm.
nitrate can occur. Therefore, the dryer was reconditioned after each run by cleaning in a warm acid bath (10% HNO3) after flushing with de-ionized water. Process lines are heat traced and kept at 220 8C to prevent water condensation or formation of ammonium salts from the inlet side of the water injection block to the inlet of the Nafion dryer. There is provision for online calibration of each analyzer or gas chromatograph. The overall operation of the unit is controlled by a Honeywell integrated SCAN 3000/S9000 control system with provision for 100 data/control points. The S9000 controller consists of analog and digital inputs and outputs, which maintain control of the 12 mass flow controllers and 14 heater zones. It monitors seven temperatures throughout the system as well as outputs from the humidity probe and chemiluminescence analyzer. The SCAN 3000 software provides a userfriendly interface to the S9000 allowing for real time monitoring and control. The software provides a graphical representation of the catalyst screening unit indicating all process variables. It allows the operator to monitor and manipulate the various process conditions. The software also functions as a data logger enabling a permanent record of process variables for each experiment.
2.3. Hysteresis and experimental repeatability The degree of conversion hysteresis with the mini core SCR catalyst was investigated at the start of experimental testing. Fig. 2 compares the NOx analyzer up ramp results with the down ramp results, i.e. decreasing the reactor temperature from 600 to 200 8C at 3 8C/min. A small amount of hysteresis is observed between the up and down ramps especially for temperatures below about 350 8C but it is not excessive. Based on these results all further reductant testing was done using the up ramp only. Experimental repeatability and catalyst deactivation were checked by repeating runs over a number of months using NH3 as reductant as shown in Fig. 3. The run dates are indicated in the legend (year/month/day). Experimental repeatability is about G5 units as shown by the comparison of results from three runs performed at about the same time (010925, 010927, and 011002). The last repeat data (020116) show that the mini core catalyst may have slightly deactivated for NO x
Fig. 2. Conversion hysteresis between increasing and decreasing temperature ramps.
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Fig. 3. Comparison of original and repeat results for mini core SCR catalyst using NH3 as reductant.
conversions above about 400 8C as the drop is about 10 units compared with the original results. 3. Results and discussion 3.1. Gas phase versus catalytic NOx reduction Given high enough gas phase reaction temperatures NH3 is capable of reducing NOx without the use of a catalyst. Exxon developed a selective non-catalytic NOx reduction process using NH3 reductant, which operates between 850 and 1050 8C [13]. In our experiments, between 200 and 600 8C one would expect no significant gas phase reduction of NOx using NH3. This was confirmed by an early experiment using NH3 as reductant where the mini core SCR catalyst was replaced by an equivalent length of quartz beads in the reactor. Fig. 4 verifies that no gas phase NOx reduction occurs when using NH3 as reductant over our temperature range. However, for amines the situation is quite different as described by Yoshihara et al. [6–12] using methylamine (CH3NH2) as reductant. NOx conversions up to about 80% were reported at lower GHSV!17,150 hK1 for temperatures
about 400 8C. Measured NOx conversions increased with O2 concentration in the exhaust. For 0% O2, no NOx conversion was observed, while for O2 concentrations O5%, which corresponds to most diesel engine exhausts NOx conversion differences of 5% or less were measured (10). Eighty percent NOx conversion required about 10% O2 content in the exhaust and good gas phase mixing between the CH3NH2 and exhaust gases. Other exhaust gases like CO, CO2, hydrocarbons and H2O had no effect on NOx conversions [10]. Therefore for the amine reductants we investigated we would expect significant gas phase reduction of NOx to occur even when running tests using the mini core SCR catalyst. During such tests the temperature profile along the length of the quartz reactor results in high enough gas phase temperatures before and after the catalyst monolith to allow gas phase reactions. This was verified by testing methylamine reductant with the mini core SCR catalyst replaced by an equivalent length of quartz beads. The results are shown in Fig. 5. The GHSV was estimated as 10,000 hK1 based on the net gas phase volume of the quartz tube over which the axial temperature profile was estimated to be reasonably flat. This was obtained from independent measurements of the axial temperature
Fig. 4. Conversions for Run 001215: NH3 reductant, no catalyst, gas phase reaction only.
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Fig. 5. Conversions of Run 011019: CH3NH2 reductant, no catalyst, estimated GHSV 10,000 hK1.
profile from the bottom to the top of the reactor tube at different nominal mid catalyst bed temperatures over the range (200– 600 8C) of the temperature ramp. Therefore, the NOx conversions measured when using the mini core SCR catalyst and amine reductants are a combination of gas phase and catalytic reactions. As shown in Fig. 5 for temperatures below about 400 8C catalytic reactions will be predominant whereas for higher temperatures gas phase reactions may become more predominant. The difference between NO and NOx conversions is due to the formation of larger amounts of NO2 in the gas phase reactions. For CH3NH2 reductant a possible reaction path for these gas phase reactions based on oxidative-reduction reactions with a nitroso-methylamine intermediate can be written as:
or the overall reaction as:
2CH3 NH2 C 2NO C 3:5O2 / 2HCN C 2HNO3 C 3H2 O
(5)
4CH3 NH2 C 4NO C 5O2 / 4N2 C 4CO C 10H2 O
CH3 NH2 C 2NO C 3O2 / CH3 NO2 C 2HNO3
(6)
2CH3 NH2 C 4NO C 7O2 / 2CH3 NO3 C 4HNO3
(7)
(2)
CH3 NH2 C 2NO C 3O2 / HOCN C 2HNO3 C H2 O
(8)
(3)
CH3 NH2 C 2NO C 3O2 / HNCO C 2HNO3 C H2 O
(9)
(1)
2CH3 NH2 C 3NO C O2 / NO2 C 2NO–CH2 –NH2 C H2 O 2NO–CH2 –NH2 C 7O2 / 4NO2 C 2CO2 C 4H2 O
6CH3 NH2 C 7NO C 13O2 / 4N2 C 5NO2 C 4CO C 2CO2 C 15H2 O
(4)
Other gas phase reactions with NO are possible and can produce a number of other compounds such as hydrogen cyanide (HCN), nitric acid (HNO3), methyl nitrite (CH3NO2 unstable), methyl nitrate (CH3NO3 unstable), cyanic acid (HOCN, unstable), isocyanic acid (HNCO), methylammonium nitrite ([CH3NH3]NO2, unstable) and methylammonium nitrate ([CH3NH3]NO3, unstable) as follows:
Fig. 6. Measured FTIR and GC results for CH3NH2 reductant using mini core SCR catalyst.
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Fig. 7. Comparison of NOx conversions with ammonia as a function of amine type or amine substitution using mini core SCR catalyst.
CH3 NH2 C 2NO C H2 O C O2 / ½CH3 NH3 NO2 C HNO3
(10)
CH3 NH2 C 2NO C H2 O C O2 / ½CH3 NH3 NO3 C HNO2
(11)
Some HCN formation is evident from the FTIR measurements. There may be other compounds produced, which are trapped or removed by the Nafion dryer. This makes identification of all possible compounds extremely difficult or impossible. NOx conversion results will show data for both the gas phase only and the mini core SCR catalytic results. The difference between these two (differential conversion) will also be discussed since it serves to highlight differences between the mini core SCR catalytic results and the gas phase results. 3.2. Mini core SCR results for primary, secondary and tertiary amine reductants Methylamine (CH3NH2), dimethylamine [(CH3)2NH], and trimethylamine [(CH3)3N] were used to investigate the effect of
amine type on NOx conversions over the mini core SCR catalyst. Fig. 6 shows the measured FTIR and GC results for CH3NH2 as a function of temperature. The drop in NOx concentration agrees fairly well with the increase in N2 production. However, we cannot calculate actual N balances since any unreacted CH3NH2 is removed from the product gases by the Nafion dryer before the FTIR measurements. Similar N2 production results were also obtained for (CH3)2NH and (CH3)3N. There is a small amount of N2O production (maximum about 20–25 ppmv at 600 8C) starting at about 450 8C, which was also observed with the other amines. Also, there is an increase in CO and CO2 production and a decrease in C3H6 concentration due to oxidation as the temperature increases. The measured NOx conversions for the different amine types are compared with NH3 in Fig. 7. Clearly there is a drop in NOx conversion from NH3 to the amines and the more substituted the amine is the lower the NOx conversion. Therefore, primary amines are more effective NOx reductants than secondary or tertiary amines with this type of catalyst. The amines can react with NOx in the gas phase at hotter exhaust temperatures while NH3 cannot giving them an advantage as a NOx reductant. Also, as shown in Fig. 7 the maximum NOx
Fig. 8. Effect of increasing methylamine concentration on NOx conversion using mini core SCR catalyst.
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Fig. 9. Comparison of gas phase conversions for primary, secondary, and tertiary amines.
conversion for each reductant tends to move to a lower temperature as the degree of substitution increases. The effect of increasing amine concentration was also investigated by testing CH3NH2 at double the normal concentration or 1000 ppmv. Fig. 8 indicates that higher NOx conversions can be obtained by using more CH3NH2 with the largest increase occurring between 350 and 475 8C. It may also be possible to increase NOx conversions by slightly modifying the SCR catalyst to make better use of amine reductants especially primary amines.
Fig. 9. For these experiments the GHSV was estimated as 10,000 hK1 based on the gas volume of the quartz reactor tube over which the temperature profile was estimated to be reasonably constant and equal to that at the catalyst bed. For all three amines there is a large difference between the NO conversion and the NOx conversion due to NO2 production, which increases with temperature. The amount of NO2 produced at higher temperatures is similar for all three amines. However, we cannot determine how much of this NO2 comes from NO oxidation versus reductant or amine oxidation.
3.3. Gas phase results for primary, secondary and tertiary amine reductants
3.4. Differential conversions
As discussed earlier NH3 shows no gas phase reactivity over the temperature range investigated (see Fig. 4) but the amine reductants are reactive at temperatures above about 400 8C (see Fig. 5). The effect of amine substitution on gas phase reactions was investigated by replacing the mini core SCR catalyst with an equivalent length of quartz beads. The results are shown in
The gas phase conversions were measured by replacing the mini core SCR catalyst with an equivalent length of quartz beads. Subtracting the gas phase conversions from the mini core SCR results gives a differential conversion as a function of temperature. This differential conversion cannot be interpreted as a ‘net catalytic conversion’ because the gas phase conversions measured in this manner will be different
Fig. 10. Gas phase and mini core SCR catalyst conversions for CH3NH2 reductant.
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Fig. 11. Differential NO and NOx conversions compared with the mini core SCR catalyst NOx results for CH3NH2 reductant.
Fig. 12. Differential NOx conversions as a function of amine type.
depending whether or not the mini core is present, i.e. the presence of the mini core will change the reactant gas phase concentrations inside the channels of the mini core and in the quartz reactor tube after the mini core. However, in the limit as the mini core SCR reactions approach very small values the measured gas phase reactions would approach those occurring in the gas phase spaces of the mini core catalyst and in the quartz tube after the mini core. This implies that these gas phase results may be used to approximate net catalytic conversions for very small NO or NOx conversions. However, as shown in Fig. 9 there are no significant gas phase reactions until about 400 8C at which temperature the mini core results are near maximum NOx conversions making estimates of net catalytic conversions impossible. However, the use of a differential conversion allows for easier comparison of the measured results. For CH3NH2 the mini core SCR catalyst and gas phase results are shown in Fig. 10. Fig. 11 shows the differential conversions along with the mini core or catalyst results for comparison. All three curves are the same up to about 400 8C since there are no significant gas phase reactions
until this temperature. The zero percent conversion or zero crossover occurs at a temperature where the gas phase conversions are greater than those measured with the mini core SCR catalyst. As shown in Fig. 11 the zero crossover for NOx conversion occurs at about 560 8C while the same point for the NO conversion is about 430 8C. The differential NOx conversions for all three amines are shown in Fig. 12. For all three amines the maximum differential NOx conversion occurs at the same temperature of about 375 8C. The zero crossover temperatures decrease in moving from the primary to the tertiary amine. The largest decrease occurs with (CH3)2NH and (CH3)3N since for CH3NH2 the mini core catalyst NOx conversions for temperatures greater than 400 8C (see Fig. 7) are significantly higher over a much wider temperature window than the mini core catalyst conversions of the other amines. 4. Conclusions Based on results using the mini core SCR catalyst, methylamine is clearly the best amine reductant and can
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approach the NOx conversions obtained using NH3. NOx conversions are lower for the other amines, decreasing with the degree of amine substitution. This implies that any on board synthesis should aim at producing primary rather than secondary and tertiary amines. It also raises the prospect of modifying typical SCR catalysts to improve NOx conversions when using amines versus ammonia. The results also indicate that care must be taken when measuring catalytic NOx conversions using reductants that can react with NO and NOx in the gas phase over the temperature range investigated. For these cases, the measured conversions are a combination of catalytic and gas phase reactions. Replacing the catalyst with an inert material allows measurement of gas phase conversions. The differences between mini core SCR results and these gas phase results (differential conversions) cannot be interpreted as net catalytic conversions except in the limiting case of very small catalytic reactions or conversions. However, these differential conversions make comparisons easier and serve to highlight differences between the measured mini core SCR and gas phase results. Acknowledgements The authors would like to thank the Government of Canada Panel on Energy Research and Development (PERD) program for financial support under the Advanced Fuels and Transportation Emissions Reduction (AFTER) program.
References [1] Nomura O, Oyama H, Sakamoto Y. Ammonia synthesis exploiting a plasma-wall interaction. In: Takagi T, editor, Proceedings of the international ion engineering congress, Kyoto, Japan; 1983. p. 1481–4. [2] Cao GJ. Jishou University; 1998;19:1–5. [3] Bai M, Wang Y, Han H, Chu Q, Bai X, Li S. Synthesis of ammonia and hydrazine in a dielectric barrier discharge at ambient pressure. In: Proceedings of the fourth international conference applied electrostatics, Dalian, China; 2001. p. 472–5. [4] Fujii T, Arai N. Astrophys J 1999;519:858–63. [5] Bromberg L, Cohen DR, Heywood J, Rabinovich A. Onboard plasmatron generation of hydrogen rich gas for diesel aftertreatment and other applications. In: Eighth diesel engine emissions reduction (DEER) conference, San Diego, CA; 2002. [6] Yoshihara Y, Nishiwaki K, Funaki K. Nippon Kikai Gakkai Zenkoku Taikai Keon Ronbunshu 1994;72:374–6. [7] Yoshihara Y, Nishiwaki K, Nakanishi Y. Non-catalytic reduction of nitrogen oxide in a diesel exhaust. In: Proceedings of the 73rd annual meeting. J Soc Mech Eng 1995;3:212–3. [8] Yoshihara Y, Tanaka T. Nippon Kikai Gakkai Ronbunshu B hen 1995;61: 766–71. [9] Nakanishi Y, Yoshihara Y, Nishiwaki K. Nippon Kikai Gakkai Ronbunshu B hen 1998;64:1262–7. [10] Nakanishi Y, Yoshihara Y, Nishiwaki K. J Eng Gas Turb Power Trans ASME 1999;121:563–8. [11] Yoshihara Y, Tanaka A, Nishiwaki K. Method for direct desulfurization of exhaust gas. Japan Patent JP6126131; 1994. [12] Nakanishi Y, Gone JD, Yoshihara Y, Nishiwaki K. Application of a new selective non-catalytic NO reduction system to diesel exhaust. In: Proceedings of the fifth international symposium on diagonistics and modelling of combustion in internal combustion engines, Nagoya, Japan; 2001. [13] Bosch H, Janssen F. Catal Today 1988;2:391–2.
Evaluation of amines for the selective catalytic reduction (SCR) of NOx from diesel engine exhaust James F. Kelly *, Maria Stanciulescu, Jean-Pierre Charland CANMET Energy Technology Centre—Ottawa, 1 Haanel Drive, Ottawa, Ont., Canada K1A 1M1 Received 22 September 2005; received in revised form 5 January 2006; accepted 23 January 2006 Available online 20 March 2006
Abstract With a view to developing onboard generation of selective reductants for NOx removal from diesel engine exhaust we compared the performance of a primary, secondary and tertiary amine to NH3 using a typical mini core NH3-SCR catalyst. Primary amines with short hydrocarbon chains, e.g. CH3NH2 (maximum NOx conversion, 50%) approached the NOx conversion obtained using NH3 (maximum NOx conversion, 70%). Increasing the amine to NOx ratio greater than 1 results in NOx conversions closer to those of NH3 (maximum NOx conversion increased to 60%). Secondary and tertiary amines had smaller NOx conversions as a function of temperature and the drop in NO and NOx conversion decreased with increasing amine hydrogen substitution. Also, the maximum NOx conversion for each reductant tends to move to a lower temperature as the degree of substitution increases. Unlike NH3, the amines can react in the gas phase at temperatures within the range of diesel engine exhaust. Due to this gas phase reactivity the NOx conversions measured using the mini core SCR catalyst also contain a gas phase conversion component. Gas phase conversions were investigated by replacing the mini core SCR catalyst with an equivalent length of quartz beads. Subtraction of the two results highlighted the differences between the mini core catalytic and gas phase conversions measured in this manner over the temperature range investigated. These differential NOx conversions for the three amines had maxima at about 375 8C. q 2006 Elsevier Ltd. All rights reserved. Keywords: NOx reduction; Amine reductants; SCR of NOx using amines
1. Introduction The reduction of NOx emissions from diesel engine exhaust is a significant technical challenge, which can limit expansion of the diesel engine market especially in North America. In order to meet new US EPA emission standards to be phased in beginning in 2007, more than 90% reductions in NOx emissions will be required. Considerable R&D is being done on NOx traps with associated catalysts and selective catalytic reduction (SCR) using NH3 or urea as reductant. Both these approaches are considered to have good potential of meeting these standards. For heavy duty diesel engines, R&D in the US has emphasized NOx traps while in Europe SCR is receiving more attention. Based on its use in stationary combustion applications the SCR approach has a long history in terms of catalyst * Corresponding author. Tel.: C1 613 996 0396; fax: C1 613 943 8882. E-mail address: [email protected] (J.F. Kelly).
0016-2361/$ - see front matter q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2006.01.024
development and has recently been adapted for vehicle use by including a reductant storage vessel on board. However, mobile SCR systems have a more serious problem related to necessary changes in infrastructure required to make the reductant (urea or NH3) available at filling stations. One way to overcome this problem is to generate suitable SCR reductants in situ on board the vehicle. For a diesel engine vehicle this limits potential feedstocks to generate these reductants to diesel fuel and air. If the N2 in the air can be made to react with the hydrogen in the diesel fuel it may be possible to produce NH3 and amines on board. Plasma systems have been shown to be able to produce NH3 from N2 and H2 at low temperatures and atmospheric pressure [1–3]. In a simulation of Titan’s atmosphere, a CH4/N2 microwave discharge was able to produce NH3, nitriles and a series of primary amines [4]. Recently, Bromberg et al. [5] have suggested that it may be possible to produce NH3 on board using a plasmatron diesel fuel reformer. The use of amines to reduce NOx has been reported by Yoshihara et al. [6–12] who have described the development of a selective non-catalytic technique for NOx removal from diesel engine exhaust using methylamine (CH3NH2) as reductant.
J.F. Kelly et al. / Fuel 85 (2006) 1772–1780
In this paper, we report on the use of a series of amines as reductants using a typical mini core NH3-SCR catalyst. The NOx conversion performance of a primary, secondary, and tertiary amine is compared with NH3. Since the amines can also reduce NOx due to gas phase or non-catalytic reactions over the temperature range investigated, gas phase results were estimated by replacing the mini core SCR catalyst with an equivalent length bed of quartz beads (!80 mesh). Differential conversions were obtained by subtracting the gas phase results from the mini core SCR catalytic results.
2. Experimental 2.1. Catalyst testing unit Fig. 1 shows a schematic of the catalyst testing unit. Twelve electronic mass flow controllers are used to produce a realistic simulated diesel exhaust feed to the catalytic reactor. Water (10% by volume) is added to the feed by a calibrated low flow rate dual syringe infusion/withdrawal pump before the SO2 or NO is added to prevent corrosion of a humidity probe, which is used to monitor the water concentration in the feed on a continuous basis. A primary standard (NIST traceable) flowmeter (Bios DryCal DC-Lite model DCL-L) was used to measure the flows set for each flow meter. Also, to allow for screening of reductant compounds, which are liquids at room temperature a trace source permeation tube unit (KIN-TEK) was installed to directly inject these reductants into the reactor feed at parts per million by volume levels. The KIN-TEK system includes a precision temperature furnace to control the rate of injection along with a separate helium mass flow controller (MFC 10).
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A three zone furnace is used to heat the quartz reactor tube (6.12 mm nominal ID by 40.64 cm L). The catalyst used for all the tests reported in this paper is a 12 cell mini core cut from a typical SCR catalyst block with cordierite substrate of 400 cpsi. The monolith catalyst containing V2O5 plus WO3 and TiO2 was obtained from a large catalyst company and had a specific surface area of 13.8 m2/g monolith. The total reactor volume occupied by the mini core is 0.50 mL. Experiments are done at GHSV 60,000 hK1, measured at STP (20 8C and 1 atm), which equates to a total feed flow rate of 500 mL/min at STP. The setpoints of the three zones of the reactor furnace are adjusted so that the vertical temperature profile measured over the length of the mini core is constant. The product gases exit the reactor and pass through a Nafion dryer (Perma Pure Model PD-625-24SS) to selectively remove water before slip streams are analyzed by a NOx analyzer (Thermo Environmental model 42H), gas chromatographs (MTI M200 and M200H) and a FTIR unit (Innova type 1301 gas analyzer). Initial experiments using amines as reductants clearly showed that there was significant interference in the operation of the NO2 converter section of the NOx analyzer when amines are used as reductants. Therefore, reliable NOx measurements using the chemiluminescence analyzer were not possible although NO concentrations measured by the NOx analyzer normally agreed well with those measured by the FTIR unit. Therefore, unless stated otherwise all NO and NOx conversion results for all reductants except NH3 are based on FTIR measurements. The dryer also removes alcohols and aldehydes as well as ammonia and amines so measurement of unreacted reductants exiting the reactor was not possible. Also, since the effluent gases cool down as they pass through the dryer, formation and precipitation of ammonium salts like ammonium sulphate or
Fig. 1. Catalyst testing unit.
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Table 1 Reductant screening operating conditions
2.2. Operating conditions
SCR catalyst
Table 1 shows normal operating conditions used to test each reductant with the SCR mini core catalyst. The reactor temperature is increased linearly from 200 to 600 8C at 3 8C/ min while the feed rates are kept constant. This allows measurement of NOx conversions as a function of temperature over a wide range of interest for diesel engine exhaust. Unless indicated otherwise the reductant feed concentration is kept equal to the NO concentration as shown in Table 1. The balance gas is helium rather than N2 so that NOx conversions can be compared with N2 production. No CO2 is used in the feed because it absorbs IR so strongly that accurate FTIR measurements of other components would not be possible. A small amount of SO2 (5 ppmv) is added to the feed to simulate a low sulphur diesel fuel exhaust stream. The pressure drop across the reactor during experiments is typically less than 4 psig.
Reactor temperature Reactor GHSV at STPa Feed component CO O2 NO NO2 C3H6 H2O SO2 Reductant He a
12 cell mini core from 400 cpsi block V2O5CWO3 and TiO2 Linear ramp from 200 to 600 8C at 3 8C/min 60,000 hK1 100 ppmv 120,000 ppmv (12 vol%) 500 ppmv Traces 100 ppmv 100,000 ppmv (10 vol%) 5 ppmv 500 ppmv Balance
STPZ20 8C, 1 atm.
nitrate can occur. Therefore, the dryer was reconditioned after each run by cleaning in a warm acid bath (10% HNO3) after flushing with de-ionized water. Process lines are heat traced and kept at 220 8C to prevent water condensation or formation of ammonium salts from the inlet side of the water injection block to the inlet of the Nafion dryer. There is provision for online calibration of each analyzer or gas chromatograph. The overall operation of the unit is controlled by a Honeywell integrated SCAN 3000/S9000 control system with provision for 100 data/control points. The S9000 controller consists of analog and digital inputs and outputs, which maintain control of the 12 mass flow controllers and 14 heater zones. It monitors seven temperatures throughout the system as well as outputs from the humidity probe and chemiluminescence analyzer. The SCAN 3000 software provides a userfriendly interface to the S9000 allowing for real time monitoring and control. The software provides a graphical representation of the catalyst screening unit indicating all process variables. It allows the operator to monitor and manipulate the various process conditions. The software also functions as a data logger enabling a permanent record of process variables for each experiment.
2.3. Hysteresis and experimental repeatability The degree of conversion hysteresis with the mini core SCR catalyst was investigated at the start of experimental testing. Fig. 2 compares the NOx analyzer up ramp results with the down ramp results, i.e. decreasing the reactor temperature from 600 to 200 8C at 3 8C/min. A small amount of hysteresis is observed between the up and down ramps especially for temperatures below about 350 8C but it is not excessive. Based on these results all further reductant testing was done using the up ramp only. Experimental repeatability and catalyst deactivation were checked by repeating runs over a number of months using NH3 as reductant as shown in Fig. 3. The run dates are indicated in the legend (year/month/day). Experimental repeatability is about G5 units as shown by the comparison of results from three runs performed at about the same time (010925, 010927, and 011002). The last repeat data (020116) show that the mini core catalyst may have slightly deactivated for NO x
Fig. 2. Conversion hysteresis between increasing and decreasing temperature ramps.
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Fig. 3. Comparison of original and repeat results for mini core SCR catalyst using NH3 as reductant.
conversions above about 400 8C as the drop is about 10 units compared with the original results. 3. Results and discussion 3.1. Gas phase versus catalytic NOx reduction Given high enough gas phase reaction temperatures NH3 is capable of reducing NOx without the use of a catalyst. Exxon developed a selective non-catalytic NOx reduction process using NH3 reductant, which operates between 850 and 1050 8C [13]. In our experiments, between 200 and 600 8C one would expect no significant gas phase reduction of NOx using NH3. This was confirmed by an early experiment using NH3 as reductant where the mini core SCR catalyst was replaced by an equivalent length of quartz beads in the reactor. Fig. 4 verifies that no gas phase NOx reduction occurs when using NH3 as reductant over our temperature range. However, for amines the situation is quite different as described by Yoshihara et al. [6–12] using methylamine (CH3NH2) as reductant. NOx conversions up to about 80% were reported at lower GHSV!17,150 hK1 for temperatures
about 400 8C. Measured NOx conversions increased with O2 concentration in the exhaust. For 0% O2, no NOx conversion was observed, while for O2 concentrations O5%, which corresponds to most diesel engine exhausts NOx conversion differences of 5% or less were measured (10). Eighty percent NOx conversion required about 10% O2 content in the exhaust and good gas phase mixing between the CH3NH2 and exhaust gases. Other exhaust gases like CO, CO2, hydrocarbons and H2O had no effect on NOx conversions [10]. Therefore for the amine reductants we investigated we would expect significant gas phase reduction of NOx to occur even when running tests using the mini core SCR catalyst. During such tests the temperature profile along the length of the quartz reactor results in high enough gas phase temperatures before and after the catalyst monolith to allow gas phase reactions. This was verified by testing methylamine reductant with the mini core SCR catalyst replaced by an equivalent length of quartz beads. The results are shown in Fig. 5. The GHSV was estimated as 10,000 hK1 based on the net gas phase volume of the quartz tube over which the axial temperature profile was estimated to be reasonably flat. This was obtained from independent measurements of the axial temperature
Fig. 4. Conversions for Run 001215: NH3 reductant, no catalyst, gas phase reaction only.
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Fig. 5. Conversions of Run 011019: CH3NH2 reductant, no catalyst, estimated GHSV 10,000 hK1.
profile from the bottom to the top of the reactor tube at different nominal mid catalyst bed temperatures over the range (200– 600 8C) of the temperature ramp. Therefore, the NOx conversions measured when using the mini core SCR catalyst and amine reductants are a combination of gas phase and catalytic reactions. As shown in Fig. 5 for temperatures below about 400 8C catalytic reactions will be predominant whereas for higher temperatures gas phase reactions may become more predominant. The difference between NO and NOx conversions is due to the formation of larger amounts of NO2 in the gas phase reactions. For CH3NH2 reductant a possible reaction path for these gas phase reactions based on oxidative-reduction reactions with a nitroso-methylamine intermediate can be written as:
or the overall reaction as:
2CH3 NH2 C 2NO C 3:5O2 / 2HCN C 2HNO3 C 3H2 O
(5)
4CH3 NH2 C 4NO C 5O2 / 4N2 C 4CO C 10H2 O
CH3 NH2 C 2NO C 3O2 / CH3 NO2 C 2HNO3
(6)
2CH3 NH2 C 4NO C 7O2 / 2CH3 NO3 C 4HNO3
(7)
(2)
CH3 NH2 C 2NO C 3O2 / HOCN C 2HNO3 C H2 O
(8)
(3)
CH3 NH2 C 2NO C 3O2 / HNCO C 2HNO3 C H2 O
(9)
(1)
2CH3 NH2 C 3NO C O2 / NO2 C 2NO–CH2 –NH2 C H2 O 2NO–CH2 –NH2 C 7O2 / 4NO2 C 2CO2 C 4H2 O
6CH3 NH2 C 7NO C 13O2 / 4N2 C 5NO2 C 4CO C 2CO2 C 15H2 O
(4)
Other gas phase reactions with NO are possible and can produce a number of other compounds such as hydrogen cyanide (HCN), nitric acid (HNO3), methyl nitrite (CH3NO2 unstable), methyl nitrate (CH3NO3 unstable), cyanic acid (HOCN, unstable), isocyanic acid (HNCO), methylammonium nitrite ([CH3NH3]NO2, unstable) and methylammonium nitrate ([CH3NH3]NO3, unstable) as follows:
Fig. 6. Measured FTIR and GC results for CH3NH2 reductant using mini core SCR catalyst.
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Fig. 7. Comparison of NOx conversions with ammonia as a function of amine type or amine substitution using mini core SCR catalyst.
CH3 NH2 C 2NO C H2 O C O2 / ½CH3 NH3 NO2 C HNO3
(10)
CH3 NH2 C 2NO C H2 O C O2 / ½CH3 NH3 NO3 C HNO2
(11)
Some HCN formation is evident from the FTIR measurements. There may be other compounds produced, which are trapped or removed by the Nafion dryer. This makes identification of all possible compounds extremely difficult or impossible. NOx conversion results will show data for both the gas phase only and the mini core SCR catalytic results. The difference between these two (differential conversion) will also be discussed since it serves to highlight differences between the mini core SCR catalytic results and the gas phase results. 3.2. Mini core SCR results for primary, secondary and tertiary amine reductants Methylamine (CH3NH2), dimethylamine [(CH3)2NH], and trimethylamine [(CH3)3N] were used to investigate the effect of
amine type on NOx conversions over the mini core SCR catalyst. Fig. 6 shows the measured FTIR and GC results for CH3NH2 as a function of temperature. The drop in NOx concentration agrees fairly well with the increase in N2 production. However, we cannot calculate actual N balances since any unreacted CH3NH2 is removed from the product gases by the Nafion dryer before the FTIR measurements. Similar N2 production results were also obtained for (CH3)2NH and (CH3)3N. There is a small amount of N2O production (maximum about 20–25 ppmv at 600 8C) starting at about 450 8C, which was also observed with the other amines. Also, there is an increase in CO and CO2 production and a decrease in C3H6 concentration due to oxidation as the temperature increases. The measured NOx conversions for the different amine types are compared with NH3 in Fig. 7. Clearly there is a drop in NOx conversion from NH3 to the amines and the more substituted the amine is the lower the NOx conversion. Therefore, primary amines are more effective NOx reductants than secondary or tertiary amines with this type of catalyst. The amines can react with NOx in the gas phase at hotter exhaust temperatures while NH3 cannot giving them an advantage as a NOx reductant. Also, as shown in Fig. 7 the maximum NOx
Fig. 8. Effect of increasing methylamine concentration on NOx conversion using mini core SCR catalyst.
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Fig. 9. Comparison of gas phase conversions for primary, secondary, and tertiary amines.
conversion for each reductant tends to move to a lower temperature as the degree of substitution increases. The effect of increasing amine concentration was also investigated by testing CH3NH2 at double the normal concentration or 1000 ppmv. Fig. 8 indicates that higher NOx conversions can be obtained by using more CH3NH2 with the largest increase occurring between 350 and 475 8C. It may also be possible to increase NOx conversions by slightly modifying the SCR catalyst to make better use of amine reductants especially primary amines.
Fig. 9. For these experiments the GHSV was estimated as 10,000 hK1 based on the gas volume of the quartz reactor tube over which the temperature profile was estimated to be reasonably constant and equal to that at the catalyst bed. For all three amines there is a large difference between the NO conversion and the NOx conversion due to NO2 production, which increases with temperature. The amount of NO2 produced at higher temperatures is similar for all three amines. However, we cannot determine how much of this NO2 comes from NO oxidation versus reductant or amine oxidation.
3.3. Gas phase results for primary, secondary and tertiary amine reductants
3.4. Differential conversions
As discussed earlier NH3 shows no gas phase reactivity over the temperature range investigated (see Fig. 4) but the amine reductants are reactive at temperatures above about 400 8C (see Fig. 5). The effect of amine substitution on gas phase reactions was investigated by replacing the mini core SCR catalyst with an equivalent length of quartz beads. The results are shown in
The gas phase conversions were measured by replacing the mini core SCR catalyst with an equivalent length of quartz beads. Subtracting the gas phase conversions from the mini core SCR results gives a differential conversion as a function of temperature. This differential conversion cannot be interpreted as a ‘net catalytic conversion’ because the gas phase conversions measured in this manner will be different
Fig. 10. Gas phase and mini core SCR catalyst conversions for CH3NH2 reductant.
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Fig. 11. Differential NO and NOx conversions compared with the mini core SCR catalyst NOx results for CH3NH2 reductant.
Fig. 12. Differential NOx conversions as a function of amine type.
depending whether or not the mini core is present, i.e. the presence of the mini core will change the reactant gas phase concentrations inside the channels of the mini core and in the quartz reactor tube after the mini core. However, in the limit as the mini core SCR reactions approach very small values the measured gas phase reactions would approach those occurring in the gas phase spaces of the mini core catalyst and in the quartz tube after the mini core. This implies that these gas phase results may be used to approximate net catalytic conversions for very small NO or NOx conversions. However, as shown in Fig. 9 there are no significant gas phase reactions until about 400 8C at which temperature the mini core results are near maximum NOx conversions making estimates of net catalytic conversions impossible. However, the use of a differential conversion allows for easier comparison of the measured results. For CH3NH2 the mini core SCR catalyst and gas phase results are shown in Fig. 10. Fig. 11 shows the differential conversions along with the mini core or catalyst results for comparison. All three curves are the same up to about 400 8C since there are no significant gas phase reactions
until this temperature. The zero percent conversion or zero crossover occurs at a temperature where the gas phase conversions are greater than those measured with the mini core SCR catalyst. As shown in Fig. 11 the zero crossover for NOx conversion occurs at about 560 8C while the same point for the NO conversion is about 430 8C. The differential NOx conversions for all three amines are shown in Fig. 12. For all three amines the maximum differential NOx conversion occurs at the same temperature of about 375 8C. The zero crossover temperatures decrease in moving from the primary to the tertiary amine. The largest decrease occurs with (CH3)2NH and (CH3)3N since for CH3NH2 the mini core catalyst NOx conversions for temperatures greater than 400 8C (see Fig. 7) are significantly higher over a much wider temperature window than the mini core catalyst conversions of the other amines. 4. Conclusions Based on results using the mini core SCR catalyst, methylamine is clearly the best amine reductant and can
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approach the NOx conversions obtained using NH3. NOx conversions are lower for the other amines, decreasing with the degree of amine substitution. This implies that any on board synthesis should aim at producing primary rather than secondary and tertiary amines. It also raises the prospect of modifying typical SCR catalysts to improve NOx conversions when using amines versus ammonia. The results also indicate that care must be taken when measuring catalytic NOx conversions using reductants that can react with NO and NOx in the gas phase over the temperature range investigated. For these cases, the measured conversions are a combination of catalytic and gas phase reactions. Replacing the catalyst with an inert material allows measurement of gas phase conversions. The differences between mini core SCR results and these gas phase results (differential conversions) cannot be interpreted as net catalytic conversions except in the limiting case of very small catalytic reactions or conversions. However, these differential conversions make comparisons easier and serve to highlight differences between the measured mini core SCR and gas phase results. Acknowledgements The authors would like to thank the Government of Canada Panel on Energy Research and Development (PERD) program for financial support under the Advanced Fuels and Transportation Emissions Reduction (AFTER) program.
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