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Effect of postflame injection of fuel on the destruction of chlorinated hydrocarbons and the oxidation of NO
Effect of Postflame Injection of Fuel on the Destruction of Chlorinated Hydrocarbons and the Oxidation of NO S. C. LEE*
Environmental Engineering Program, Department of Civil and Structural Engineering, Hong Kong Polytechnic University, Hong Kong
and C. P. KOSHLAND, D. LUCAS, R. F. SAWYER
School of Public Health, Energy and Environment Division, Lawrence Berkeley National Laboratory, and Department of Mechanical Engineering, University of California at Berkeley, Berkeley, CA 94720 USA Postflame injection of fuel has been proposed as a means of reducing chlorinated hydrocarbons (CHCs) in a combustion exhaust. In this study, the effects of this strategy on CHCs and NOx are investigated. A small amount of fuel, such as CO or CH3OH, has been injected into the postflame region from a turbulent combustion-driven flow reactor to assess its effect on the destruction of two CHCs (CH3Cl and C2H5Cl) and simultaneously, the oxidation of NO to NO2. The results suggest that this strategy is effective only in certain conditions. There is an optimal temperature ⬃ 1050 K, where NO is most effectively converted to NO2. Adding fuel to the postflame region increases the concentrations of both HO2 and OH radicals, but temperature is the key factor in determining which radical will dominate the reaction pathway. For the destruction of CHCs, attack by OH is the major destruction route, with T ⬎ 1200 K desired. For NO oxidation, the HO2 radical is the key species, and lower temperatures are necessary. © 1999 by The Combustion Institute
INTRODUCTION Many exhaust streams contain trace amounts of chlorinated hydrocarbons (CHCs), as well as NOx, CO, and other substances. Lyon [1– 4] has suggested that the trace organics remaining in the postflame region can be reduced by the addition of small amounts of various chemicals, including hydrocarbons, hydrogen peroxide [5], and ozone [6]. Numerous emission control strategies use chemical additives in the exhaust stream for NOx reduction or oxidation [7]. The focus of this paper is to assess the ability of postflame fuel-injection to reduce emissions of CHCs and to determine whether such strategies can simultaneously control NOx emissions. A recent study [8, 9] of the effect of postflame injection of CO, CH4, or C2H6 on the destruction of CH3Cl, C2H5Cl, or 1,1,1-C2H3Cl3 indicates that such fuel injection enhances, inhibits, or has no effect on the destruction of CHCs in the exhaust gas, with the effect dependent on the nature of both the CHC and the fuel. All three fuels studied previously (CH4, C2H6, and CO) enhance the destruction of CH3Cl. CO is the only effective postflame fuel for the destruction of C2H5Cl, and none of the fuels improve * Corresponding author: E-mail: [email protected] 0010-2180/99/$–see front matter PII S0010-2180(99)00033-4
the destruction of 1,1,1-C2H3Cl3. For compounds that have bimolecular destruction pathways, postflame fuel injection changes the bimolecular reaction rate by altering the radical pool. For compounds whose destruction begins with a unimolecular destruction step, postflame fueladdition does not enhance rates of destruction. Postflame treatment of NO has an extensive history, recently reviewed by Bowman [7]. Strategies for NO removal using homogeneous gasphase reactions can generally be divided into two classes, that of NO reduction and NO oxidation. Further details are shown in Table 1. The reduction of NO, usually termed selective non-catalytic reduction (SNCR), includes among others injectants, ammonia, and urea. The desired product is N2. NO oxidation uses the HO2 radical to convert NO to NO2. The NO2 formed is more soluble and can be removed in conventional scrubbers. The methods using NH3 to reduce NO have a relatively wide temperature window over which the process operates (1100 –1400 K). The window can be shifted by the addition of other species including H2 [10], H2O2 [11], N2H4 [12], natural gas [13], C2H6 [13], and CO [13]. The addition of small amounts of fuel with NH3 reduces the optimal reaction temperature by 150 –200 K [13]. Myerson [14] investigated the COMBUSTION AND FLAME 119:154 –160 (1999) © 1999 by The Combustion Institute Published by Elsevier Science Inc.
POSTFLAME INJECTION OF FUEL
155 TABLE 1
Comparison of NO Reduction with NO Oxidation
Temperature Final product Key radical Typical additive Key reaction
NO Reduction
NO Oxidation
1100–1400 K N2, N2O OH NH3 NH3 ⫹ NO 3 NH2 ⫹ H2O NH2 ⫹ NO 3 N2 ⫹ H2O
900–1100 K NO2 HO2 CH3OH, CnH2n⫹2, CO NO ⫹ HO2 3 NO2 ⫹ OH
reduction of NO by hydrocarbons in the presence of oxygen over the temperature range of 1200 –1700 K and showed a 10 to 45% reduction in NO, depending on temperature, residence time, and stoichiometry. These reductions were found to be sensitive to the ratio of oxygen to hydrogen in the exhaust. Muzio and Arand [10] investigated the gas-phase reduction of NO to N2 in combustion byproducts by injecting a small amount of H2, CH4, C2H6, or CO at 700 to 1930 K in a lean primary mixture. Significant reductions in NO only occurred when the mixture was fuel-rich and the excess oxygen has been effectively consumed. The effect of adding small amounts of fuel on the oxidation of NO to NO2 has been studied by Lyon et al. [2, 4, 15]. They found that methanol (CH3OH) converts SO3 to SO2 and NO to NO2 simultaneously, even in the presence of a large excess of O2. Evans et al. [16] converted NO to NO2 by injecting methanol into the flue gas. The optimal injection temperature for methanol was ⬃1070 K, when NO was more than 80% oxidized with a residence time of 55 ms. Higher oxygen levels increased the oxidation of NO to NO2; excess methanol led to higher levels of CO and CH2O. Hori and Matsunaga [17] measured the concentrations of NO and NO2 in hot combustion gases, to which nine types of fuel (seven hydrocarbons from C1 to C4, H2 and CO) were added. Their results show that the oxidation of NO to NO2 is strongly promoted by the addition of a small amount of fuel, with the effectiveness strongly depending on the type of fuel. The effectiveness increases in the following order: CO ⬍ H2 ⬍ CH4 ⬍ C2H6 ⬍ C2H4 ⬍ C3H8 ⬍ i-C4H10 ⬍ n-C4H10. Chemical kinetic calculations [17] indicate that the oxidation proceeds mainly through the reaction: NO ⫹ HO2 3 NO2 ⫹ OH
even in the presence of fuel. High effectiveness is obtained by fuels that easily decompose and consequently produce large amounts of HO2. Glarborg et al. [18, 19] have also investigated the mutually promoted oxidation of various organic compounds and nitric oxide in the presence of oxygen in a laboratory-scale flow reactor. The efficiency of a given compound in oxidizing NO is dependent on the production of HO2 radicals. The major drawback for methanol injection is the narrow temperature regime for NO oxidation. In this paper, the effect is examined of the injection of CH3OH and CO on the destruction of different chlorinated hydrocarbons (CH3Cl, and C2H5Cl) and on NO oxidation in the postflame region. EXPERIMENT Experiments were performed in the postflame region of a combustion-driven turbulent flow reactor (5.2 cm diameter and 3.9 m length) described in detail previously [20, 21]. A premixed natural gas/air flame was stabilized on a screen with secondary air injected to control the temperature and equivalence ratio. To minimize the temperature drop, the combustor was insulated downstream of the secondary air injection location. A constant flow of nitrogen was doped with hydrocarbons and chlorinated hydrocarbons, and later injected into the hot combustion products through a quartz injector, mounted at 90o to the central axis of the combustor. The temperature at the point of injection could be varied from 800 to over 1300 K. The sampling and analytical methods were those used previously to study the thermal destruction of chlorinated hydrocarbons [22]. Gas samples were withdrawn at various points along the reactor by uncooled quartz probes, and were
156
S. C. LEE ET AL. TABLE 2 Compounds Studied
Compound
Primary Destruction Pathway
Toxic Byproduct
Fuel Added
Methyl chloride CH3Cl Ethyl chloride C2H5Cl
Bimolecular Unimolecular and bimolecular
C2H3Cl C2H3Cl
CO, CH3OH CO, CH3OH
directed either to a 60-cm-long multipass infrared optical cell in a Biorad Diglab model FTS-40 Fourier transform infrared (FTIR) spectrometer or to a chemiluminescence NOx analyzer (Thermo-Electron model 14A). For most measurements, the outlet concentrations were measured at the last sampling port (3.5 m from the injection point), where the temperature was ⬃ 600 K. The background concentration of CO at the injection point was below 50 ppm, which is much lower than the injected concentrations of fuel. NO was formed in the flame in the reactor, at an initial concentration of ⬃ 100 ppm; after dilution the levels ranged from 80 –20 ppm, depending on the amount of secondary air injected. Table 2 shows the compounds studied: CH3Cl and C2H5Cl were selected because their specific destruction pathways differ [20]. The destruction of CH3Cl occurs mainly through bimolecular radical attack, whereas C2H5Cl is destroyed through both unimolecular and bimolecular channels. CO and CH3OH were selected because they have been proposed and studied as additives in the postflame region, and also their reaction pathways differ [20, 23].
were required for a destruction ⬎90%. Figure 1 shows the injection of CH3OH can increase the destruction efficiency at a given temperature. The destruction of CH3Cl increases with increasing amounts of CH3OH. As shown in Fig. 1, the concentrations of CO were significantly higher with CH3OH injected. In fact, the concentrations of CO increased with more CH3OH added. The concentration of formaldehyde (CH2O), which is an intermediate toxic byproduct from the injection of CH3OH, increases with the progressive addition of CH3OH (see Fig. 1), but is below 20 ppm at temperatures larger than
RESULTS AND DISCUSSION CHC Destruction Using CH3OH or CO The experiments were performed in the combustion-driven turbulent flow reactor to determine if the concentration of CH3Cl is reduced by the injection of CH3OH. 450 ppm of CH3Cl was injected to the reactor at various temperatures with 0, 450, 1350, or 2250 ppm of CH3OH. In the case of only CH3Cl, Fig. 1 shows that essentially no CH3Cl was destroyed below 1000 K, and that temperatures in excess of 1180 K were required for a reduction ⬎90%. When 4500 ppm of CH3OH was injected with the CH3Cl, the concentration of CH3Cl began to decrease at 950 K; temperatures above 1100 K
Fig. 1. Outlet concentration of 450 ppm CH3Cl with various concentration of CH3OH injection at residence times ⫽ 0.29 – 0.39 s. (a) CH3Cl. (b) CO. (c) CH2O.
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157
Fig. 3. Effect of CH3OH or CO injection temperature on NO oxidation. [NO]i ⫽ 80 –20 ppm, [Fuel] ⫽ 1200 ppm, residence time ⫽ 0.26 – 0.41 s.
Fig. 2. Effect of adding 1000 ppm CH3OH on the destruction of 140 ppm C2H5Cl and its byproducts (residence time ⫽ 0.26 – 0.41 s).
1050 K. Similar results have been observed [8, 9] when CO is injected with CH3Cl. The results of injecting 140 ppm of C2H5Cl together with 0 or 1000 ppm of CH3OH are shown in Fig. 2. In the case of C2H5Cl alone, essentially no C2H5Cl was destroyed at below 950 K, but a temperature above 975 K causes a destruction larger than 90%. However, with CH3OH also injected, C2H5Cl was destroyed at above 980 K and the necessary temperature for the same extent of destruction (90%) is increased to 1075 K. The injection temperature required for the destruction of C2H3Cl (a byproduct of C2H5Cl destruction) is also increased when CH3OH is injected. Similar results have been observed with the addition of CO with C2H5Cl, but the inhibitory effect of CO is less significant than that of CH3OH on C2H5Cl [8, 9]. NO Oxidation Using CH3OH or CO in the Absence of Chlorine-Containing Species The oxidation of NO to NO2 as a function of the temperature at which CH3OH was injected is shown in Fig. 3. Significant conversion begins at
950 K, and increases to a maximum near 1075 K. If the temperature at the injection point is too low for CH3OH to react, then not enough HO2 radicals are formed to convert NO to NO2. Conversely, if the temperature is too high at the point of injection, the HO2 formed decomposes in the reaction: HO2 3 OH ⫹ O, and the reaction between NO and OH is not important. This explains why NO oxidizes in a narrow temperature window. At the temperature (1075 K) of maximum oxidation of NO and a ratio of injected CH3OH to initial NO of 25, 80% conversion was achieved. Similar previous studies [18, 19] have shown that methanol has a high potential for oxidizing NO. Our results confirm that when 1000 ppm of CO was injected, a similar U-shaped curve (see Fig. 3) was observed, but CO is 30% less effective than CH3OH in converting NO to NO2. Figure 4 shows the effect of residence time at four different injection temperatures when injecting CO. At 950 K, the temperature is too low for NO to oxidize, even at extended residence times. At 1024 K, longer residence times give more oxidation of NO. When the temperature was further increased to 1095 K, NO was converted at very short residence times, but at longer residence times NO2 reconverts to NO. This phenomenon is more pronounced at even higher temperatures, when the larger concentrations of OH react with NO2. Alternatively, NO2 reacts with other radicals such as free H atoms in NO2 ⫹ H 3 NO ⫹ OH. A similar trend was observed when injecting CH3OH.
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S. C. LEE ET AL.
Fig. 4. NO oxidation as a function of residence time at four different temperatures. [NO]i ⫽ 80 –20 ppm, [CO] ⫽ 1200 ppm.
NO Oxidation Using CH3OH in the Presence of Chlorine-Containing Species The oxidation of NO on adding CH3OH to two different chlorine-containing species (CH3Cl and C2H5Cl) was examined to determine their combined effect. Figure 5 compares three different injection cases: 1200 ppm of CH3OH, 1200 ppm of CH3OH with 250 ppm CH3Cl, and 250 ppm of CH3Cl. When CH3Cl was injected alone, there was no oxidation of NO. Also CH3Cl destruction was enhanced in the presence of CH3OH. The injection of CH3OH can decrease the temperature required to maintain the same destruction efficiency or increase the destruction efficiency at a given temperature. Figure 6 compares three different injection cases: 1200 ppm of CH3OH, 1200 ppm of CH3OH with 250 ppm C2H5Cl, and 250 ppm of C2H5Cl. Surprisingly, we find that C2H5Cl alone may result in no oxidation of NO, similar to that of the injection of CH3OH. The injection of CH3OH has an inhibitory effect on the destruction of C2H5Cl. The above results indicate that by careful selection of the injected fuel and the temperature of injection, both the destruction of the CHCs and the oxidation of NO to NO2 can be achieved. The presence of C2H5Cl enhances the oxidation of NO; this contrasts with CH3Cl, which does neither of these two things. This is explained by the difference in the temperature at which the species react. At temperatures below 1000 K, CH3Cl is essentially inert. At tempera-
Fig. 5. Effect of CH3Cl on NO oxidation. [CH3Cl] ⫽ 250 ppm, [CH3OH] ⫽ 1200 ppm, residence time ⫽ 0.26 – 0.41 s.
tures above 1000 K, CH3Cl starts to react. However, this temperature is too high to obtain NO oxidation. The destruction of C2H5Cl begins at ⬃ 950 K, either through unimolecular decomposition (C2H5Cl 3 C2H4 ⫹ HCl) or through bimolecular reactions with Cl or OH (C2H5Cl ⫹ Cl 3 C2H4Cl ⫹ HCl or C2H5Cl ⫹ OH 3 C2H4Cl ⫹ H2O) [20]. The oxidation of NO is initiated by the byproducts (C2H4, or C2H4Cl), which readily decompose and consequently generate HO2 radicals for oxidizing NO. The mechanism for mutual oxidation of CH3OH and NO has been discussed by Lyon et al. [15]. The oxidation of NO to NO2 proceeds almost solely through reaction: NO ⫹ HO2 3 NO2 ⫹ OH CH3OH functions as a source of HO2 radicals through the chain-propagating sequence reaction: CH3OH ⫹ OH 3 CH2OH ⫹ H2O CH2OH ⫹ O2 3 CH2O ⫹ HO2
POSTFLAME INJECTION OF FUEL
159
Fig. 7. Calculated OH and HO2 radical concentrations for 100 ppm CH3Cl with or without 1000 ppm CO at residence time ⫽ 1 s.
CO ⫹ OH 3 CO2 ⫹ H H ⫹ O2 3 HO ⫹ O Fig. 6. Effect of C2H5Cl on NO oxidation. [C2H5Cl] ⫽ 250 ppm, [CH3OH] ⫽ 1200 ppm, residence time ⫽ 0.26 – 0.41 s.
Hjuler et al. [18] identified two key steps: CH3OH ⫹ OH 3 CH2OH ⫹ H2O Where CH2OH reacts preferably with O2 leading to HO2 radical, and CH3OH ⫹ OH 3 CH3O ⫹ H2O Where subsequently CH3O easily dissociates, forming H atoms. Therefore, CH2OH, not CH3O acts to promote the oxidation of NO. When CO is injected at low temperatures, it also functions as a source of HO2 radicals through the following reactions: CO ⫹ OH 3 CO2 ⫹ H H ⫹ O2 ⫹ M 3 HO2 ⫹ M For which the net reaction is: CO ⫹ OH ⫹ O2 3 CO2 ⫹ HO2. Thus one HO2 is generated for every CO molecule oxidized. However, when CO is injected at high temperatures, the CO produces OH through:
O ⫹ H2O 3 2 OH Because the overall reaction is: CO ⫹ H2O ⫹ O2 3 CO2 ⫹ 2 OH For every CO molecule oxidized, two OH radicals are generated; these can then reconvert NO2 back to NO. The injection temperatures must thus be carefully chosen to promote the oxidation of NO. It is known [1, 3] that all the radicals have coupled concentrations, so adding fuel to the postflame region will increase concentrations of HO2 and OH. Temperature is the key factor in determining which radical (HO2 or OH) will dominate. Numerical modeling of the chemical processes in the flow reactor was performed using the CHEMKIN package with a driver program for a plug flow reactor (PFR). The chemical kinetic mechanism (218 reactions with 43 species) was originally developed by Karra et al. [24] and modified by Fisher and Koshland [23]. Figure 7 shows the concentrations of OH and HO2 at various temperatures when 100 ppm of CH3Cl was injected either by itself or with 1000 ppm of CO. The dominant species is HO2 at temperatures less than 900 K, but OH dominates above 1000 K. For CHC destruction, OH attack is the major destruction
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route, with T ⬎ 1200 K desired. For NO oxidation, the HO2 radical is the key species, and lower temperatures are necessary.
2. 3. 4. 5.
CONCLUSIONS
6.
Postflame injection of CO or CH3OH enhances, inhibits, or has no effect on the destruction of CHCs present in an exhaust gas. The effect depends on the nature of both the CHC and the fuel. Postflame injection of fuel may also result in NO being converted to NO2. The results indicate that by careful selection of the injected fuel species and the temperature of injection, both the destruction of CHC and the oxidation of NO to NO2 can be achieved. When injecting compounds to enhance the destruction of CHCs, one should also consider the potential impact on NOx emissions. The optimal temperature for the oxidation of NO is ⬃ 1050 K. At low temperatures (⬍ 1000 K), the temperature is too low to generate enough HO2 radicals to oxidize NO. At high temperatures (⬎ 1100 K), HO2 radicals form OH, thus reconverting the NO2 back to NO. To oxidize NO, HO2 is the key species and postflame fuel injection at 1050 K is required to generate optimal HO2 concentrations. For existing incinerators not meeting standards for the destruction of toxics, careful selection of the temperature window for the injection of additives into the postflame gases may be an economical way to achieve higher destruction efficiencies of CHCs and simultaneously converting NO to NO2. The effectiveness will depend on the nature of both the fuel and the CHCs present.
7.
This research was supported by the National Institutes of Environmental Health Sciences (NIEHS) Superfund Research Program, Grant P42 ES047050-01, and by the University of California Toxic Substances Health Effects Teaching and Research Program. The first author would like to thank Miss Gloria Chiu for preparing the figures. Special thanks are given to the Research Council of the Hong Kong Polytechnic University for this project.
8. 9. 10.
11.
12. 13. 14.
15. 16.
17.
18. 19.
20.
21. 22. 23. 24.
Lyon, R. K., U.S. Patent 3,900,559 (1975). Lyon, R. K., U.S. Patent 4,974,530 (1990). Lyon, R. K., U.S. Patent 4,849,192 (1989). Cooper, C. D., Clausen, C. A., Tomlin, D., Hewett, M., and Martinez, A., J. Hazard. Mater. 27:273 (1991). Cooper, C. D., Clausen, C. A., Hewett, M., and Martinez, A., J. Hazard. Mater. 31:75 (1992). Bowman, C. T., Twenty-Fourth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1992, p. 859. Koshland, C. P., Lee, S., and Lucas, D., Combust. Flame 92:106 (1993). Lee, S., Koshland, C. P., Lucas, D., and Sawyer, R. F., Combust. Sci. Technol. 101:247 (1994). Muzio, L. J., and Arand, J. K., Sixteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1976, p. 199. Azuhata, S., Kaji, R., Akimoto, H., and Hishinuma, Y., Eighteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1981, p. 845. Azuhata, S., Akimoto, H., and Hishinuma, Y., AIChE J. 31:1223 (1985). Lodder, P., and Lefers, J. B. (1985). Chem. Eng. J. 30:161 (1985). Myerson, A. L., Fifteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1975, p. 1085. Lyon, R. K., Cole, J. A., Kramlich, J. C., and Chen, S. L., Combust. Flame 81:30 (1990). Evans, J., Newhall, J., England, G., and Seeker, W. R. (1992). Development of Advanced NOx Control Concepts for Coalfired Utility Boilers. DOE/PC/90363T5. Hori, M., and Matsunaga, N., Twenty-Fourth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1992, p. 909. Hjuler, K., Glarborg, P., and Dam-Johansen, K., Ind. Eng. Chem. Res. 34:1882 (1995). Glarborg, P., Kubel, D., Kristensen, P. G., Hansen, J., and Dam-Johansen, K., Combust. Sci. Technol. 461:110 (1995). Fisher, E. M., Koshland, C. P., Hall, M. J., Sawyer, R. F., and Lucas D., Twenty-Third Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1990, p. 895. Koshland, C. P., Fisher, E. M., and Lucas, D., Combust. Sci. Technol. 82:49 (1992). Hall, M. J., Lucas, D., and Koshland, C. P., Environ. Sci. Technol. 25:260 (1991). Fisher, E. M., and Koshland C. P., J. Air Waste Management Assoc. 40:1384 (1990). Karra, S. B., Gutman, D., and Senkan S. M., Combust. Sci. Technol. 60:45 (1988).
REFERENCES 1.
Lyon, R. K., Twenty-Third Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1990, p. 903.
Received 30 October 1997; revised 29 January 1999; accepted 12 February 1999
Environmental Engineering Program, Department of Civil and Structural Engineering, Hong Kong Polytechnic University, Hong Kong
and C. P. KOSHLAND, D. LUCAS, R. F. SAWYER
School of Public Health, Energy and Environment Division, Lawrence Berkeley National Laboratory, and Department of Mechanical Engineering, University of California at Berkeley, Berkeley, CA 94720 USA Postflame injection of fuel has been proposed as a means of reducing chlorinated hydrocarbons (CHCs) in a combustion exhaust. In this study, the effects of this strategy on CHCs and NOx are investigated. A small amount of fuel, such as CO or CH3OH, has been injected into the postflame region from a turbulent combustion-driven flow reactor to assess its effect on the destruction of two CHCs (CH3Cl and C2H5Cl) and simultaneously, the oxidation of NO to NO2. The results suggest that this strategy is effective only in certain conditions. There is an optimal temperature ⬃ 1050 K, where NO is most effectively converted to NO2. Adding fuel to the postflame region increases the concentrations of both HO2 and OH radicals, but temperature is the key factor in determining which radical will dominate the reaction pathway. For the destruction of CHCs, attack by OH is the major destruction route, with T ⬎ 1200 K desired. For NO oxidation, the HO2 radical is the key species, and lower temperatures are necessary. © 1999 by The Combustion Institute
INTRODUCTION Many exhaust streams contain trace amounts of chlorinated hydrocarbons (CHCs), as well as NOx, CO, and other substances. Lyon [1– 4] has suggested that the trace organics remaining in the postflame region can be reduced by the addition of small amounts of various chemicals, including hydrocarbons, hydrogen peroxide [5], and ozone [6]. Numerous emission control strategies use chemical additives in the exhaust stream for NOx reduction or oxidation [7]. The focus of this paper is to assess the ability of postflame fuel-injection to reduce emissions of CHCs and to determine whether such strategies can simultaneously control NOx emissions. A recent study [8, 9] of the effect of postflame injection of CO, CH4, or C2H6 on the destruction of CH3Cl, C2H5Cl, or 1,1,1-C2H3Cl3 indicates that such fuel injection enhances, inhibits, or has no effect on the destruction of CHCs in the exhaust gas, with the effect dependent on the nature of both the CHC and the fuel. All three fuels studied previously (CH4, C2H6, and CO) enhance the destruction of CH3Cl. CO is the only effective postflame fuel for the destruction of C2H5Cl, and none of the fuels improve * Corresponding author: E-mail: [email protected] 0010-2180/99/$–see front matter PII S0010-2180(99)00033-4
the destruction of 1,1,1-C2H3Cl3. For compounds that have bimolecular destruction pathways, postflame fuel injection changes the bimolecular reaction rate by altering the radical pool. For compounds whose destruction begins with a unimolecular destruction step, postflame fueladdition does not enhance rates of destruction. Postflame treatment of NO has an extensive history, recently reviewed by Bowman [7]. Strategies for NO removal using homogeneous gasphase reactions can generally be divided into two classes, that of NO reduction and NO oxidation. Further details are shown in Table 1. The reduction of NO, usually termed selective non-catalytic reduction (SNCR), includes among others injectants, ammonia, and urea. The desired product is N2. NO oxidation uses the HO2 radical to convert NO to NO2. The NO2 formed is more soluble and can be removed in conventional scrubbers. The methods using NH3 to reduce NO have a relatively wide temperature window over which the process operates (1100 –1400 K). The window can be shifted by the addition of other species including H2 [10], H2O2 [11], N2H4 [12], natural gas [13], C2H6 [13], and CO [13]. The addition of small amounts of fuel with NH3 reduces the optimal reaction temperature by 150 –200 K [13]. Myerson [14] investigated the COMBUSTION AND FLAME 119:154 –160 (1999) © 1999 by The Combustion Institute Published by Elsevier Science Inc.
POSTFLAME INJECTION OF FUEL
155 TABLE 1
Comparison of NO Reduction with NO Oxidation
Temperature Final product Key radical Typical additive Key reaction
NO Reduction
NO Oxidation
1100–1400 K N2, N2O OH NH3 NH3 ⫹ NO 3 NH2 ⫹ H2O NH2 ⫹ NO 3 N2 ⫹ H2O
900–1100 K NO2 HO2 CH3OH, CnH2n⫹2, CO NO ⫹ HO2 3 NO2 ⫹ OH
reduction of NO by hydrocarbons in the presence of oxygen over the temperature range of 1200 –1700 K and showed a 10 to 45% reduction in NO, depending on temperature, residence time, and stoichiometry. These reductions were found to be sensitive to the ratio of oxygen to hydrogen in the exhaust. Muzio and Arand [10] investigated the gas-phase reduction of NO to N2 in combustion byproducts by injecting a small amount of H2, CH4, C2H6, or CO at 700 to 1930 K in a lean primary mixture. Significant reductions in NO only occurred when the mixture was fuel-rich and the excess oxygen has been effectively consumed. The effect of adding small amounts of fuel on the oxidation of NO to NO2 has been studied by Lyon et al. [2, 4, 15]. They found that methanol (CH3OH) converts SO3 to SO2 and NO to NO2 simultaneously, even in the presence of a large excess of O2. Evans et al. [16] converted NO to NO2 by injecting methanol into the flue gas. The optimal injection temperature for methanol was ⬃1070 K, when NO was more than 80% oxidized with a residence time of 55 ms. Higher oxygen levels increased the oxidation of NO to NO2; excess methanol led to higher levels of CO and CH2O. Hori and Matsunaga [17] measured the concentrations of NO and NO2 in hot combustion gases, to which nine types of fuel (seven hydrocarbons from C1 to C4, H2 and CO) were added. Their results show that the oxidation of NO to NO2 is strongly promoted by the addition of a small amount of fuel, with the effectiveness strongly depending on the type of fuel. The effectiveness increases in the following order: CO ⬍ H2 ⬍ CH4 ⬍ C2H6 ⬍ C2H4 ⬍ C3H8 ⬍ i-C4H10 ⬍ n-C4H10. Chemical kinetic calculations [17] indicate that the oxidation proceeds mainly through the reaction: NO ⫹ HO2 3 NO2 ⫹ OH
even in the presence of fuel. High effectiveness is obtained by fuels that easily decompose and consequently produce large amounts of HO2. Glarborg et al. [18, 19] have also investigated the mutually promoted oxidation of various organic compounds and nitric oxide in the presence of oxygen in a laboratory-scale flow reactor. The efficiency of a given compound in oxidizing NO is dependent on the production of HO2 radicals. The major drawback for methanol injection is the narrow temperature regime for NO oxidation. In this paper, the effect is examined of the injection of CH3OH and CO on the destruction of different chlorinated hydrocarbons (CH3Cl, and C2H5Cl) and on NO oxidation in the postflame region. EXPERIMENT Experiments were performed in the postflame region of a combustion-driven turbulent flow reactor (5.2 cm diameter and 3.9 m length) described in detail previously [20, 21]. A premixed natural gas/air flame was stabilized on a screen with secondary air injected to control the temperature and equivalence ratio. To minimize the temperature drop, the combustor was insulated downstream of the secondary air injection location. A constant flow of nitrogen was doped with hydrocarbons and chlorinated hydrocarbons, and later injected into the hot combustion products through a quartz injector, mounted at 90o to the central axis of the combustor. The temperature at the point of injection could be varied from 800 to over 1300 K. The sampling and analytical methods were those used previously to study the thermal destruction of chlorinated hydrocarbons [22]. Gas samples were withdrawn at various points along the reactor by uncooled quartz probes, and were
156
S. C. LEE ET AL. TABLE 2 Compounds Studied
Compound
Primary Destruction Pathway
Toxic Byproduct
Fuel Added
Methyl chloride CH3Cl Ethyl chloride C2H5Cl
Bimolecular Unimolecular and bimolecular
C2H3Cl C2H3Cl
CO, CH3OH CO, CH3OH
directed either to a 60-cm-long multipass infrared optical cell in a Biorad Diglab model FTS-40 Fourier transform infrared (FTIR) spectrometer or to a chemiluminescence NOx analyzer (Thermo-Electron model 14A). For most measurements, the outlet concentrations were measured at the last sampling port (3.5 m from the injection point), where the temperature was ⬃ 600 K. The background concentration of CO at the injection point was below 50 ppm, which is much lower than the injected concentrations of fuel. NO was formed in the flame in the reactor, at an initial concentration of ⬃ 100 ppm; after dilution the levels ranged from 80 –20 ppm, depending on the amount of secondary air injected. Table 2 shows the compounds studied: CH3Cl and C2H5Cl were selected because their specific destruction pathways differ [20]. The destruction of CH3Cl occurs mainly through bimolecular radical attack, whereas C2H5Cl is destroyed through both unimolecular and bimolecular channels. CO and CH3OH were selected because they have been proposed and studied as additives in the postflame region, and also their reaction pathways differ [20, 23].
were required for a destruction ⬎90%. Figure 1 shows the injection of CH3OH can increase the destruction efficiency at a given temperature. The destruction of CH3Cl increases with increasing amounts of CH3OH. As shown in Fig. 1, the concentrations of CO were significantly higher with CH3OH injected. In fact, the concentrations of CO increased with more CH3OH added. The concentration of formaldehyde (CH2O), which is an intermediate toxic byproduct from the injection of CH3OH, increases with the progressive addition of CH3OH (see Fig. 1), but is below 20 ppm at temperatures larger than
RESULTS AND DISCUSSION CHC Destruction Using CH3OH or CO The experiments were performed in the combustion-driven turbulent flow reactor to determine if the concentration of CH3Cl is reduced by the injection of CH3OH. 450 ppm of CH3Cl was injected to the reactor at various temperatures with 0, 450, 1350, or 2250 ppm of CH3OH. In the case of only CH3Cl, Fig. 1 shows that essentially no CH3Cl was destroyed below 1000 K, and that temperatures in excess of 1180 K were required for a reduction ⬎90%. When 4500 ppm of CH3OH was injected with the CH3Cl, the concentration of CH3Cl began to decrease at 950 K; temperatures above 1100 K
Fig. 1. Outlet concentration of 450 ppm CH3Cl with various concentration of CH3OH injection at residence times ⫽ 0.29 – 0.39 s. (a) CH3Cl. (b) CO. (c) CH2O.
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Fig. 3. Effect of CH3OH or CO injection temperature on NO oxidation. [NO]i ⫽ 80 –20 ppm, [Fuel] ⫽ 1200 ppm, residence time ⫽ 0.26 – 0.41 s.
Fig. 2. Effect of adding 1000 ppm CH3OH on the destruction of 140 ppm C2H5Cl and its byproducts (residence time ⫽ 0.26 – 0.41 s).
1050 K. Similar results have been observed [8, 9] when CO is injected with CH3Cl. The results of injecting 140 ppm of C2H5Cl together with 0 or 1000 ppm of CH3OH are shown in Fig. 2. In the case of C2H5Cl alone, essentially no C2H5Cl was destroyed at below 950 K, but a temperature above 975 K causes a destruction larger than 90%. However, with CH3OH also injected, C2H5Cl was destroyed at above 980 K and the necessary temperature for the same extent of destruction (90%) is increased to 1075 K. The injection temperature required for the destruction of C2H3Cl (a byproduct of C2H5Cl destruction) is also increased when CH3OH is injected. Similar results have been observed with the addition of CO with C2H5Cl, but the inhibitory effect of CO is less significant than that of CH3OH on C2H5Cl [8, 9]. NO Oxidation Using CH3OH or CO in the Absence of Chlorine-Containing Species The oxidation of NO to NO2 as a function of the temperature at which CH3OH was injected is shown in Fig. 3. Significant conversion begins at
950 K, and increases to a maximum near 1075 K. If the temperature at the injection point is too low for CH3OH to react, then not enough HO2 radicals are formed to convert NO to NO2. Conversely, if the temperature is too high at the point of injection, the HO2 formed decomposes in the reaction: HO2 3 OH ⫹ O, and the reaction between NO and OH is not important. This explains why NO oxidizes in a narrow temperature window. At the temperature (1075 K) of maximum oxidation of NO and a ratio of injected CH3OH to initial NO of 25, 80% conversion was achieved. Similar previous studies [18, 19] have shown that methanol has a high potential for oxidizing NO. Our results confirm that when 1000 ppm of CO was injected, a similar U-shaped curve (see Fig. 3) was observed, but CO is 30% less effective than CH3OH in converting NO to NO2. Figure 4 shows the effect of residence time at four different injection temperatures when injecting CO. At 950 K, the temperature is too low for NO to oxidize, even at extended residence times. At 1024 K, longer residence times give more oxidation of NO. When the temperature was further increased to 1095 K, NO was converted at very short residence times, but at longer residence times NO2 reconverts to NO. This phenomenon is more pronounced at even higher temperatures, when the larger concentrations of OH react with NO2. Alternatively, NO2 reacts with other radicals such as free H atoms in NO2 ⫹ H 3 NO ⫹ OH. A similar trend was observed when injecting CH3OH.
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Fig. 4. NO oxidation as a function of residence time at four different temperatures. [NO]i ⫽ 80 –20 ppm, [CO] ⫽ 1200 ppm.
NO Oxidation Using CH3OH in the Presence of Chlorine-Containing Species The oxidation of NO on adding CH3OH to two different chlorine-containing species (CH3Cl and C2H5Cl) was examined to determine their combined effect. Figure 5 compares three different injection cases: 1200 ppm of CH3OH, 1200 ppm of CH3OH with 250 ppm CH3Cl, and 250 ppm of CH3Cl. When CH3Cl was injected alone, there was no oxidation of NO. Also CH3Cl destruction was enhanced in the presence of CH3OH. The injection of CH3OH can decrease the temperature required to maintain the same destruction efficiency or increase the destruction efficiency at a given temperature. Figure 6 compares three different injection cases: 1200 ppm of CH3OH, 1200 ppm of CH3OH with 250 ppm C2H5Cl, and 250 ppm of C2H5Cl. Surprisingly, we find that C2H5Cl alone may result in no oxidation of NO, similar to that of the injection of CH3OH. The injection of CH3OH has an inhibitory effect on the destruction of C2H5Cl. The above results indicate that by careful selection of the injected fuel and the temperature of injection, both the destruction of the CHCs and the oxidation of NO to NO2 can be achieved. The presence of C2H5Cl enhances the oxidation of NO; this contrasts with CH3Cl, which does neither of these two things. This is explained by the difference in the temperature at which the species react. At temperatures below 1000 K, CH3Cl is essentially inert. At tempera-
Fig. 5. Effect of CH3Cl on NO oxidation. [CH3Cl] ⫽ 250 ppm, [CH3OH] ⫽ 1200 ppm, residence time ⫽ 0.26 – 0.41 s.
tures above 1000 K, CH3Cl starts to react. However, this temperature is too high to obtain NO oxidation. The destruction of C2H5Cl begins at ⬃ 950 K, either through unimolecular decomposition (C2H5Cl 3 C2H4 ⫹ HCl) or through bimolecular reactions with Cl or OH (C2H5Cl ⫹ Cl 3 C2H4Cl ⫹ HCl or C2H5Cl ⫹ OH 3 C2H4Cl ⫹ H2O) [20]. The oxidation of NO is initiated by the byproducts (C2H4, or C2H4Cl), which readily decompose and consequently generate HO2 radicals for oxidizing NO. The mechanism for mutual oxidation of CH3OH and NO has been discussed by Lyon et al. [15]. The oxidation of NO to NO2 proceeds almost solely through reaction: NO ⫹ HO2 3 NO2 ⫹ OH CH3OH functions as a source of HO2 radicals through the chain-propagating sequence reaction: CH3OH ⫹ OH 3 CH2OH ⫹ H2O CH2OH ⫹ O2 3 CH2O ⫹ HO2
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Fig. 7. Calculated OH and HO2 radical concentrations for 100 ppm CH3Cl with or without 1000 ppm CO at residence time ⫽ 1 s.
CO ⫹ OH 3 CO2 ⫹ H H ⫹ O2 3 HO ⫹ O Fig. 6. Effect of C2H5Cl on NO oxidation. [C2H5Cl] ⫽ 250 ppm, [CH3OH] ⫽ 1200 ppm, residence time ⫽ 0.26 – 0.41 s.
Hjuler et al. [18] identified two key steps: CH3OH ⫹ OH 3 CH2OH ⫹ H2O Where CH2OH reacts preferably with O2 leading to HO2 radical, and CH3OH ⫹ OH 3 CH3O ⫹ H2O Where subsequently CH3O easily dissociates, forming H atoms. Therefore, CH2OH, not CH3O acts to promote the oxidation of NO. When CO is injected at low temperatures, it also functions as a source of HO2 radicals through the following reactions: CO ⫹ OH 3 CO2 ⫹ H H ⫹ O2 ⫹ M 3 HO2 ⫹ M For which the net reaction is: CO ⫹ OH ⫹ O2 3 CO2 ⫹ HO2. Thus one HO2 is generated for every CO molecule oxidized. However, when CO is injected at high temperatures, the CO produces OH through:
O ⫹ H2O 3 2 OH Because the overall reaction is: CO ⫹ H2O ⫹ O2 3 CO2 ⫹ 2 OH For every CO molecule oxidized, two OH radicals are generated; these can then reconvert NO2 back to NO. The injection temperatures must thus be carefully chosen to promote the oxidation of NO. It is known [1, 3] that all the radicals have coupled concentrations, so adding fuel to the postflame region will increase concentrations of HO2 and OH. Temperature is the key factor in determining which radical (HO2 or OH) will dominate. Numerical modeling of the chemical processes in the flow reactor was performed using the CHEMKIN package with a driver program for a plug flow reactor (PFR). The chemical kinetic mechanism (218 reactions with 43 species) was originally developed by Karra et al. [24] and modified by Fisher and Koshland [23]. Figure 7 shows the concentrations of OH and HO2 at various temperatures when 100 ppm of CH3Cl was injected either by itself or with 1000 ppm of CO. The dominant species is HO2 at temperatures less than 900 K, but OH dominates above 1000 K. For CHC destruction, OH attack is the major destruction
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route, with T ⬎ 1200 K desired. For NO oxidation, the HO2 radical is the key species, and lower temperatures are necessary.
2. 3. 4. 5.
CONCLUSIONS
6.
Postflame injection of CO or CH3OH enhances, inhibits, or has no effect on the destruction of CHCs present in an exhaust gas. The effect depends on the nature of both the CHC and the fuel. Postflame injection of fuel may also result in NO being converted to NO2. The results indicate that by careful selection of the injected fuel species and the temperature of injection, both the destruction of CHC and the oxidation of NO to NO2 can be achieved. When injecting compounds to enhance the destruction of CHCs, one should also consider the potential impact on NOx emissions. The optimal temperature for the oxidation of NO is ⬃ 1050 K. At low temperatures (⬍ 1000 K), the temperature is too low to generate enough HO2 radicals to oxidize NO. At high temperatures (⬎ 1100 K), HO2 radicals form OH, thus reconverting the NO2 back to NO. To oxidize NO, HO2 is the key species and postflame fuel injection at 1050 K is required to generate optimal HO2 concentrations. For existing incinerators not meeting standards for the destruction of toxics, careful selection of the temperature window for the injection of additives into the postflame gases may be an economical way to achieve higher destruction efficiencies of CHCs and simultaneously converting NO to NO2. The effectiveness will depend on the nature of both the fuel and the CHCs present.
7.
This research was supported by the National Institutes of Environmental Health Sciences (NIEHS) Superfund Research Program, Grant P42 ES047050-01, and by the University of California Toxic Substances Health Effects Teaching and Research Program. The first author would like to thank Miss Gloria Chiu for preparing the figures. Special thanks are given to the Research Council of the Hong Kong Polytechnic University for this project.
8. 9. 10.
11.
12. 13. 14.
15. 16.
17.
18. 19.
20.
21. 22. 23. 24.
Lyon, R. K., U.S. Patent 3,900,559 (1975). Lyon, R. K., U.S. Patent 4,974,530 (1990). Lyon, R. K., U.S. Patent 4,849,192 (1989). Cooper, C. D., Clausen, C. A., Tomlin, D., Hewett, M., and Martinez, A., J. Hazard. Mater. 27:273 (1991). Cooper, C. D., Clausen, C. A., Hewett, M., and Martinez, A., J. Hazard. Mater. 31:75 (1992). Bowman, C. T., Twenty-Fourth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1992, p. 859. Koshland, C. P., Lee, S., and Lucas, D., Combust. Flame 92:106 (1993). Lee, S., Koshland, C. P., Lucas, D., and Sawyer, R. F., Combust. Sci. Technol. 101:247 (1994). Muzio, L. J., and Arand, J. K., Sixteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1976, p. 199. Azuhata, S., Kaji, R., Akimoto, H., and Hishinuma, Y., Eighteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1981, p. 845. Azuhata, S., Akimoto, H., and Hishinuma, Y., AIChE J. 31:1223 (1985). Lodder, P., and Lefers, J. B. (1985). Chem. Eng. J. 30:161 (1985). Myerson, A. L., Fifteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1975, p. 1085. Lyon, R. K., Cole, J. A., Kramlich, J. C., and Chen, S. L., Combust. Flame 81:30 (1990). Evans, J., Newhall, J., England, G., and Seeker, W. R. (1992). Development of Advanced NOx Control Concepts for Coalfired Utility Boilers. DOE/PC/90363T5. Hori, M., and Matsunaga, N., Twenty-Fourth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1992, p. 909. Hjuler, K., Glarborg, P., and Dam-Johansen, K., Ind. Eng. Chem. Res. 34:1882 (1995). Glarborg, P., Kubel, D., Kristensen, P. G., Hansen, J., and Dam-Johansen, K., Combust. Sci. Technol. 461:110 (1995). Fisher, E. M., Koshland, C. P., Hall, M. J., Sawyer, R. F., and Lucas D., Twenty-Third Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1990, p. 895. Koshland, C. P., Fisher, E. M., and Lucas, D., Combust. Sci. Technol. 82:49 (1992). Hall, M. J., Lucas, D., and Koshland, C. P., Environ. Sci. Technol. 25:260 (1991). Fisher, E. M., and Koshland C. P., J. Air Waste Management Assoc. 40:1384 (1990). Karra, S. B., Gutman, D., and Senkan S. M., Combust. Sci. Technol. 60:45 (1988).
REFERENCES 1.
Lyon, R. K., Twenty-Third Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1990, p. 903.
Received 30 October 1997; revised 29 January 1999; accepted 12 February 1999