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Effects of CaO and burnout on the kinetics of no reduction by beulah zap char
Twenty-Sixth Symposium (International) on Combustion/The Combustion Institute, 1996/pp. 2251–2257
EFFECTS OF CaO AND BURNOUT ON THE KINETICS OF NO REDUCTION BY BEULAH ZAP CHAR FENG GUO and WILLIAM C. HECKER Department of Chemical Engineering and Advanced Combustion Engineering Research Center (ACERC), Brigham Young University, 350 CB, Provo, Utah 84604, USA
The heterogeneous reaction of NO with char is important in understanding the formation and reduction of NOx from coal combustion processes. The kinetics of NO reduction by North Dakota lignite char (NDL), its acid-washed char (NDW), and its calcium-reloaded char (NCa) were investigated in a packed-bed reactor at temperatures from 723 to 1073 K. The results show that the reaction rate of NO with char increases significantly as the CaO content of the char increases. They also indicate clearly that the reaction is first order with respect to NO pressure and that there is a sharp increase in the apparent activation energy with increasing temperature. In the low temperature regime, the activation energies for all three char types are essentially the same (22–26 kcal/mol); in the high temperature regime, they are all higher, but decrease from 60 to 45 kcal/mol as the CaO content increases. The temperature at which the shift takes place also decreases as the CaO content increases. Using a series of six NDL chars, the effect of char burnout level on the reaction of NO with char was also studied. The transition temperatures and apparent activation energies were found to be independent of char burnout, but both the reaction rate constant and CaO surface area (determined by CO2 uptake at 573 K) decreased as char burnout level increased from 0 to 80%. When the reaction rates are normalized by CaO surface area, they become essentially independent of burnout level, which suggests the importance that CaO sites play in the reduction process. The correlation of rate with CaO surface area is quantitative and also holds for the three char types (NDL, NDW, and NCa) in the low-temperature regime. It does not hold for the three char types in the high-temperature regime.
Introduction The reduction of NO emissions from combustion processes has become increasingly important in protecting the world’s environment. It has been shown that selective catalytic reduction (SCR) with ammonia is an effective commercial technique to remove NOx from combustion flue gas. However, the implementation of this technique is limited by high investment and operating costs, “ammonia slip,” and SOx poisoning, which motivate the search for alternatives [1]. Carbon (activated carbon or char) is a promising reducing agent for NOx reduction with many potential advantages, such as low cost, easy availability, high efficiency, simplicity of process, and no secondary pollution [1–7]. Moreover, the heterogeneous reaction of NO with char is very important for the understanding and modeling of the formation of NOx from coal combustion processes. The reaction may significantly destroy the NOx formed earlier in coal combustion, which partially contributes to low NO emission from fluidized-bed combustion [2,3]. Therefore, the reaction of NO with char is receiving significant attention in the literature. Previous investigations of the reaction of NO with
char involve the kinetics and mechanism [6–23], the effects of char surface area [24,25], the effects of feed gas composition [10,7,25], and the catalytic effects of metals [5,10,22,26–29]. The reaction of NO with char has generally been reported to be first order with respect to NO partial pressure [7,13–16], but reaction orders between 0.42 and 0.73 have also been reported [30]. A sharp shift in the activation energy has been observed in the temperature range of 873–973 K, which suggests a complex reaction mechanism [6,7,15–18]. Several mechanisms have been proposed [5,6,12,16,18,19,31]. However, questions concerning N2 formation, the surface complexes, the nature of active surface sites, and the effects of minerals in char are still not well understood. In most previous studies, chars were taken to be pure carbon, thus the effects of char ash and its composition on the kinetics and mechanism of the reduction reactions are not well known. Although the catalytic effects of certain metals or metal oxides on the reactions have been investigated [5,10,22,26– 29], little is known about their effects on the kinetics of the reaction. Moreover, the effect of burnout level on the reaction have not been reported previously. Therefore, the objectives of this study are to
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NOx FORMATION AND CONTROL TABLE 1 Properties of North Dakota lignite and chars
Ash content (%) Char burnout (%) CaO content (%) App. density (g/cc) True density (g/cc) Porosity (%) Particle diam. (lm) N2 S.A. (m2/g) CO2 S.A. (m2/g) CaO S.A. (m2/g) CaO dispersion (%)
ND lignite
NDL
NDW
NCa
ND21
ND26
ND24
ND17
ND9
6.0 N/A 1.3 1.41 1.43 1.2 75 0.32 267 9.0 54
13.6 0.0 3.5 0.63 1.88 66 66 190 543 17.8 41
5.4 N/A 1.1 0.62 1.74 64 65 116 526 3.2 24
7.0 N/A 2.4 0.74 1.75 58 64 128 543 4.8 17
14.4 6.5 3.7 0.51 1.93 74 70 134 567 17.2 36
18.8 31.8 4.8 0.59 2.00 70 61 163 540 13.1 24
21.9 43.9 5.6 0.61 2.02 70 57 87 511 12.1 18
27.2 57.8 7.0 0.63 1.80 65 52 118 517 11.1 15
42.8 79.0 11.0 0.82 2.07 61 41 56 401 7.0 6
investigate the kinetics of the reaction of NO with Beulah Zap chars, to study the effects of CaO on the kinetics, and to determine the effects of char burnout level (or conversion) on the reaction. Experimental Methods Char Properties The parent char used in this study was prepared from 63/74 km particles of North Dakota Beulah Zap lignite in a methane flat-flame burner (FFB) at a high heating rate (104–105 K/s) and high temperature (peak gas temperature was 1900 K). The resulting parent char (dubbed NDL), a portion of the NDL that was washed with HCl to remove mineral matter (dubbed NDW), and a portion of NDW reloaded with calcium oxide (dubbed NCa) were used. In addition, a series of chars made by burning out NDL char to various extents with 3–5% oxygen in a drop tube reactor (DTR) at about 1800 K were also used in this study. All of these chars were made previously in our laboratory, and details of their preparation have been reported elsewhere [32,33]. The properties of these chars, along with those of the lignite, are shown in Table 1. Reactor Operation The reduction of NO by char was carried out for all chars at 5–6 temperatures between 723 and 1073 K in a 10-mm-i.d. VYCOR glass, vertical packed-bed reactor with a fritted quartz disc of medium porosity as a support. For each run, 0.2 g char mixed with 2 g silicon carbide (inert for NO reduction) was packed in the reactor and heated in He to the maximum temperature desired using an electric furnace. NO diluted with He (3130 ppm NO) was then fed downward through the reactor until the outlet NO
concentration reached a quasi-steady-state value. The transient usually lasted about 1 h and was probably due to the build-up of adsorbed species on the char. Once the quasi-steady-state was reached, data were collected at 5–6 flow rate settings (between 100 and 500 ml/min (NTP)) at each of the 5–6 temperatures studied. For each run, it took about 4 h to collect all the data, and the burnout of the char was about 10% during that period of time. Accompanying the slow loss of char, the outlet NO concentration increased very slightly with time, and thus we have used the term quasi–steady state. In the calculation of rates and kinetic parameters (described later), the small loss in char mass was accounted for by normalizing by the char mass available. The inlet gas pressure in the reactor was controlled at 300 kPa (3 atm). The outlet pressure at each run condition was measured to determine the pressure drop cross the packed bed. The average pressure was used in calculation of concentrations. Gas Analysis The composition of the outlet gas was continuously monitored for N2, CO, CO2, N2O, and O2 by a GC (Perkin-Elmer, 3920B) with TCD and two columns (one packed with Chromosorb 106, the other packed with molecular sieve 5A), and for NO and NO2 by a Chemiluminescence NOx analyzer (Thermo Environmental, 42H). Nitrogen and oxygen mass balances were determined between inlet and outlet streams for each run, and variations always fell within 55%. Mass Transfer Influences? The calculation of film mass transfer and pore diffusion resistances for a worst-case scenario (the maximum particle diameter, the minimum flow rate, and the maximum reaction rate observed) indicated that
CaO EFFECT ON NO/CHAR KINETICS
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Fig. 2. The variation of NO conversion with temperature for three char types. Fig. 1. Experimental data for NDL char plotted in form of first-order integrated rate expression.
film mass transfer (maximum MT resistance, (Cg 1 Cs)/Cg 4 2.7%) and pore diffusion (minimum effectiveness factor, g 4 0.99) were negligible in this study. Results Kinetics Because a broad range of NO conversion was observed in this study, the reactor was modeled as an integral plug flow reactor. The reaction rate constants and reaction order were obtained by integral analysis [38] of the experimental data. The experimental data for NDL char plotted as: 1ln(1 1 XNO) vs. C0NO W/F0NO (XNO is NO conversion, F0NO is inlet molar flow rate of NO (mol/s), W is char mass available (g), C0NO is inlet NO concentration (mol/L)) are shown in Fig. 1. The data for all
six temperatures are very linear, which is consistent with the reaction being first order with respect to NO partial pressure. Attempts to fit the data to other orders were made, but straight lines were not obtained for all temperatures. Table 2 lists values for the reaction rate constants, their lower and upper 95% confidence limits, and statistical analysis parameters for NDL char data obtained at six different temperatures. The correlation coefficients, t-test values, and F-test values all show that the experimental data were excellently fit by first-order kinetics under the experimental conditions. The standard error was typically less than 5%. Similar first-order behavior was observed for the other two char types, NDW and NCa. The Effect of CaO The variations of NO conversion with temperature at a flow rate of 303 ml/min (NTP) are shown in Fig. 2 for the three char types. The results for other flow rates are similar. The conversion of NO increases
TABLE 2 Reaction rate constants and statistical anaysis parameters for NO reduction by NDL char
T 8C 650 625 600 600 550 500 470
k1 L/s-gChar 0.0736 0.0353 0.0182 0.0181 0.0070 0.0029 0.0016
k1 Low 95%
k1 Up 95%
Std error
Corr coef r2
t-Test
t-Distrib a 4 0.01
F-Test
F-Distrib a 4 0.01
0.0562 0.0333 0.0161 0.0158 0.0063 0.0026 0.0016
0.0909 0.0373 0.0202 0.0205 0.0078 0.0032 0.0017
4.03E-03 7.29E-04 7.40E-04 8.48E-04 2.66E-04 1.10E-04 2.56E-05
0.994 0.998 0.993 0.991 0.994 0.994 0.999
18.2 48.5 24.6 21.4 26.5 26.2 64.1
9.925 4.604 4.604 4.604 4.604 4.604 4.604
333 2348 457 604 701 686 4103
199 26 26 26 26 26 26
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NOx FORMATION AND CONTROL
Fig. 3. Arrhenius plots of the reaction rate constants for NO reduction with char for three char types.
Fig. 4. Arrhenius plots of the reaction rate constants normalized by CaO surface area for three char types.
noticeably in the order NDL . NCa . NDW. For example, the temperatures for 50% conversion are 870, 940, and 980 K for NDL, NCa, and NDW, respectively. This is in the same order as the CaO content of the three chars, 3.5% . 2.4% . 1.1%. Therefore, the effect of CaO on the reaction of NO with char appears to be significant. Figure 3 is an Arrhenius plot of the rate data for the NDL, NDW, and NCa chars. The bars around points in the figure show the variation of the reaction rate constants within a 95% confidence interval. A sharp shift in the apparent activation energy with increasing temperatures was observed for three char types, as has also been reported in the literature [6,7,15–18]. The temperature at which the transition takes place increases from 823 to 973 K as the CaO content decreases. This shift to higher activation energy with increasing temperature is opposite to that expected if a reaction is changing from chemical rate control to mass transfer control, and suggests different mechanisms or rate-determining steps at high and low temperatures. It is also noted that at low temperatures the apparent activation energies for all three char types are essentially the same (22–26 kcal/ mol); at high temperatures, however, the activation energies vary from 45 to 60 kcal/mol, increasing as the CaO content of the chars decreases. In most heterogeneous and catalytic reactions, it is not the gross amount or mass of catalyst or solid that is proportional to the reaction rate, but rather the amount of active surface area that the given material contains. CaO surface area can be measured by selective CO2 chemisorption at 573 K using a thermogravimetric analyzer [32,33]. This was done for the NDL, NDW, and NCa chars, and the resulting values were 17.8, 3.2, and 4.8 m2/g of char, respectively (see also Table 1). If the rate constants shown in Fig. 3 are then normalized by the appropriate CaO surface area, the Arrhenius plot shown in Fig. 4 results. This indicates quite definitively that for the low-temperature regime the rate constant is independent of char type, while in the high-temperature regime it still varies as a function of char type with the high-CaO-containing chars still showing higher values. This seems to indicate a direct firstorder dependence on CaO surface area in the lowtemperature regime, and possibly an even higher order dependence on CaO surface area in the high-temperature regime. The Effect of Char Burnout
Fig. 5. Arrhenius plots of the reactions rate constants for NO reduction by NDL chars with different burnout levels.
The Arrhenius plot of rate data for six NDL chars with different burnout levels is shown in Fig. 5. The transition temperatures and apparent activation energies in both temperature regimes are apparently independent of char burnout level. The reaction rate constants decrease (by about a factor of 3) as char burnout level increases from 0 to 80% over the en-
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Fig. 8. Arrhenius plot of the rate constants normalized by CaO surface area for NO reduction by NDL chars with six different burnout levels. Fig. 6. The dependence of the reaction rate constants for NO reduction by NDL char on CaO surface area.
level for all five measured temperatures as shown in Fig. 7. When the data of Fig. 5 are replotted using the CaO-normalized rate constants instead of the mass-based rate constants, all points for six different burnout levels fall on one line (Fig. 8), again confirming that the reaction rate of NO reduction by NDL chars normalized by CaO surface area is independent of char burnout level. These results suggest strongly that CaO is involved directly in the reaction process, probably as a catalyst. Discussion
Fig. 7. The effect of char burnout level on the rate constants normalized by CaO surface area for NO reduction by NDL chars.
tire temperature range. Note that these rate constants are already per gram of available (or residual) char, and thus the decrease is real. (If they were per gram of initial char, the differences would be even greater, by an additional factor of 5.) It has also been shown by previous work in this laboratory [32,33] that the CaO surface area for these same chars decreases with burnout level. A cross plot (Fig. 6) shows that the rate constants increase as CaO surface area increases. When the rate constants are normalized by CaO surface area, they become essentially independent of char burnout
The fact that the CaO surface area correlates the data in both the low- and high-temperature regimes for this NDL series of chars is in contrast to the behavior of the three char types shown in Figs. 3 and 4, in which the low-temperature regime was correlated but the high-temperature regime was not. The reasons for this difference are not clear at this time. The behavior suggests that there is a difference between the three char types that does not exist between the members of the NDL series that is important for determining reactivity in the high-temperature regime, but not in the low-temperature regime. Possibilities would include char organic structure or other inorganic constituents. Other inorganic constituents besides CaO were removed in the washing process (see ash content values in Table 1), while in the NDL series, the ratio of inorganic constituents would have remained fairly constant. One crude measure of organic structure would be N2 BET surface area. Attempts to correlate the rate constant data for the NDL chars with N2 BET surface areas of the chars (see Table 1) have been made. For the NDL series, a moderate correlation was observed [34], albeit less definitive than that with CaO surface area. However, for the three char types no
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clear correlation was observed at either high or low temperature. Thus, BET surface area does not appear to explain the differences between the two series. Conclusions The kinetics of the reaction of NO with char have been determined for three kinds of char with different CaO contents. The results show that the reaction rate increases significantly as the CaO content of the char increases. The reaction is first order with respect to NO partial pressure and has a sharp shift in the activation energy with temperature for all char types studied. In the low-temperature regime, the activation energies for all three char types are essentially the same (22–26 kcal/mol); in the high-temperature regime, they are all higher but decrease from 60 to 45 kcal/mol as the CaO content increases. The temperature at which the shift takes place also decreases as the CaO content increases. The effect of char burnout level on the reaction was also studied. The transition temperatures and apparent activation energies were found to be independent of char burnout, but both the reaction rate constant and CaO surface area decreased as char burnout level increased from 0 to 80%. When the reaction rates are normalized by CaO surface area, they become essentially independent of burnout level, which suggests the important part that CaO sites play in the reduction process. Finally, the data in the low-temperature regime for both char series were correlated very well by CaO surface area. Data in the high-temperature regime for the burnout series of similar NDL chars were again correlated well by CaO surface area, but data for the three char types were not. The difference is not yet understood. Acknowledgments This work was sponsored by the Advanced Combustion Engineering Research Center at Brigham Young University. Funds for this center are received from the National Science Foundation, the State of Utah, the U.S. Department of Energy, and a number of industrial participants. We also thank Richard F. Cope for char preparation and Troy Ness for technical assistance.
REFERENCES 1. Bosch, H., Janssen, F., Catalysis Today 2:369–521 (1988). 2. Pereira, F. J., Beer, J. M., Gibbs, B., Hedley, A. B., Fifteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1975, pp. 1149– 1155.
3. Johnson, J. E., Fuels 73:1398–1415 (1994). 4. Illan-Gomez, M. J., Linares-Solano, A., Salinas-Martinez de Lecea, C., Energy Fuels 7:146–154 (1993). 5. Yamashita, H., Tomita, A., Energy Fuels 7:85–89 (1993). 6. Teng, H., Suuberg, E. M., Calo, J. M., Energy Fuels 6:398–406 (1992). 7. Furusawa, T., Tsunoda, M., Tsujimura, M., Adschiri, T., Fuel 64:1306–1309 (1985). 8. Furusawa, T., Kunii, D., Osuma, A., Yamada, N., Int. Chem. Eng. 20:239–289 (1980). 9. Wendt, J. O. L., Pershing, D. W., Lee, J. W., Glass, J. W., Seventeenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1979, pp. 77–85. 10. Levy, J., Chan, L. K., Sarofim, A. F., Beer, J. M., Eighteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1981, pp. 111–120. 11. Watts, H., Trans. Faraday Soc. 54:93–105 (1958). 12. Smith, R. N., Swinehart, J., Lesnini, D., J. Phys. Chem. 63:544–547 (1959). 13. Richthofen, A. V., Wendel, E., Neuschiitz, D., Fresenius’ J. Anal. Chem. 346:261–264 (1993). 14. Edwards, H. W., AIChE Symp. Ser. No. 126 68:91–104 (1972). 15. Song, Y. H., Beer, J. M., Sarofim, A. F., Combust. Sci. Technol. 25:237–240 (1981). 16. Chan, L. K., Sarofim, A. F., Beer, J. M., Combust. Flame 52:37–45 (1983). 17. Radovic, L. R., Walker, P. L., Jr., Fuel Process Technol. 8:149–154 (1984). 18. Suuberg, E. M., Teng, H., Calo, J. M., Twenty-Third Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1990, pp. 1199–1205. 19. Teng, H., Suuberg, E. M., J. Phys. Chem. 97:478–483 (1993). 20. Teng, H., Suuberg, E. M., Ind. Eng. Chem. Res. 32:416–423 (1993). 21. Chu, X., Schmidt, L. D., Ind. Eng. Chem. Res. 32:1359–1366 (1993). 22. Lai, C.-K. S., Peters, W. A., Longwell, J. P., Energy Fuels 2:586–588 (1988). 23. Matos, M. A. A., Pereira, F. J. M. A., Ventura, J. M. P., Fuel 69:1435–1439 (1990). 24. Shimizu, T., Sazawa, Y., Adschiri, T., Furusawa, T., Fuel 71:361–365 (1992). 25. Yamashita, H., Yamada, H., Tomita, A., Appl. Catalysis 78:L1–L6 (1991). 26. Furusawa, T., Koyama, M., Tsjimura, M., Fuel 64:413– 414 (1985). 27. Illan-Gomez, M. J., Linares-Solano, A., Radovic, L., Salinas-Martinez de Lecea, C., Energy Fuels 9:97–103 (1995). 28. Illan-Gomez, M. J., Linares-Solano, A., Radovic, L., Salinas-Martinez de Lecea, C., Energy Fuels 9:104– 111 (1995). 29. Illan-Gomez, M. J., Linares-Solano, A., Radovic, L., Salinas-Martinez de Lecea, C., Energy Fuels 9:112– 119 (1995).
CaO EFFECT ON NO/CHAR KINETICS 30. Johnson, J. E., Dam-Johansen, K., Eleventh International Conference on Fluidized-Bed Combustion, American Soc. Mech. Eng., New York, 1991, pp. 1389– 1396. 31. De Soete, G. C., Twenty-Third Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1990, pp. 1257–1264. 32. Cope, R. F., Arrington, C. B., Hecker, W. C., Energy Fuels 8:1095–1099 (1994). 33. Cope, R. F., Ph.D. Dissertation, Brigham Young University, Provo, Utah, 1995.
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34. Guo, F., Hecker, W. C., Western States Section/The Combustion Institute, Stanford, California, Oct. 1995, Paper WSS/CI 95F-178. 35. Illan-Gomez, M. J., Linares-Solano, A., Radovic, L., Salinas-Martinez de Lecea, C., Coal Science (Pajaves, J. A. and Tascon, J. M. D., Eds.), Coal Sci. Technol., 24, Elsevier, Amsterdam, 1995, pp. 1911–1914. 36. Yang, R. T., Wong, C., J. Chem. Phys. 75:4471–4476 (1981). 37. Ben-Aim, R. I., Int. Chem. Eng. 27:70–75 (1987). 38. Levenspiel, O., Chemical Reaction Engineering, 2nd ed., Wiley and Sons, New York, 1972.
EFFECTS OF CaO AND BURNOUT ON THE KINETICS OF NO REDUCTION BY BEULAH ZAP CHAR FENG GUO and WILLIAM C. HECKER Department of Chemical Engineering and Advanced Combustion Engineering Research Center (ACERC), Brigham Young University, 350 CB, Provo, Utah 84604, USA
The heterogeneous reaction of NO with char is important in understanding the formation and reduction of NOx from coal combustion processes. The kinetics of NO reduction by North Dakota lignite char (NDL), its acid-washed char (NDW), and its calcium-reloaded char (NCa) were investigated in a packed-bed reactor at temperatures from 723 to 1073 K. The results show that the reaction rate of NO with char increases significantly as the CaO content of the char increases. They also indicate clearly that the reaction is first order with respect to NO pressure and that there is a sharp increase in the apparent activation energy with increasing temperature. In the low temperature regime, the activation energies for all three char types are essentially the same (22–26 kcal/mol); in the high temperature regime, they are all higher, but decrease from 60 to 45 kcal/mol as the CaO content increases. The temperature at which the shift takes place also decreases as the CaO content increases. Using a series of six NDL chars, the effect of char burnout level on the reaction of NO with char was also studied. The transition temperatures and apparent activation energies were found to be independent of char burnout, but both the reaction rate constant and CaO surface area (determined by CO2 uptake at 573 K) decreased as char burnout level increased from 0 to 80%. When the reaction rates are normalized by CaO surface area, they become essentially independent of burnout level, which suggests the importance that CaO sites play in the reduction process. The correlation of rate with CaO surface area is quantitative and also holds for the three char types (NDL, NDW, and NCa) in the low-temperature regime. It does not hold for the three char types in the high-temperature regime.
Introduction The reduction of NO emissions from combustion processes has become increasingly important in protecting the world’s environment. It has been shown that selective catalytic reduction (SCR) with ammonia is an effective commercial technique to remove NOx from combustion flue gas. However, the implementation of this technique is limited by high investment and operating costs, “ammonia slip,” and SOx poisoning, which motivate the search for alternatives [1]. Carbon (activated carbon or char) is a promising reducing agent for NOx reduction with many potential advantages, such as low cost, easy availability, high efficiency, simplicity of process, and no secondary pollution [1–7]. Moreover, the heterogeneous reaction of NO with char is very important for the understanding and modeling of the formation of NOx from coal combustion processes. The reaction may significantly destroy the NOx formed earlier in coal combustion, which partially contributes to low NO emission from fluidized-bed combustion [2,3]. Therefore, the reaction of NO with char is receiving significant attention in the literature. Previous investigations of the reaction of NO with
char involve the kinetics and mechanism [6–23], the effects of char surface area [24,25], the effects of feed gas composition [10,7,25], and the catalytic effects of metals [5,10,22,26–29]. The reaction of NO with char has generally been reported to be first order with respect to NO partial pressure [7,13–16], but reaction orders between 0.42 and 0.73 have also been reported [30]. A sharp shift in the activation energy has been observed in the temperature range of 873–973 K, which suggests a complex reaction mechanism [6,7,15–18]. Several mechanisms have been proposed [5,6,12,16,18,19,31]. However, questions concerning N2 formation, the surface complexes, the nature of active surface sites, and the effects of minerals in char are still not well understood. In most previous studies, chars were taken to be pure carbon, thus the effects of char ash and its composition on the kinetics and mechanism of the reduction reactions are not well known. Although the catalytic effects of certain metals or metal oxides on the reactions have been investigated [5,10,22,26– 29], little is known about their effects on the kinetics of the reaction. Moreover, the effect of burnout level on the reaction have not been reported previously. Therefore, the objectives of this study are to
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NOx FORMATION AND CONTROL TABLE 1 Properties of North Dakota lignite and chars
Ash content (%) Char burnout (%) CaO content (%) App. density (g/cc) True density (g/cc) Porosity (%) Particle diam. (lm) N2 S.A. (m2/g) CO2 S.A. (m2/g) CaO S.A. (m2/g) CaO dispersion (%)
ND lignite
NDL
NDW
NCa
ND21
ND26
ND24
ND17
ND9
6.0 N/A 1.3 1.41 1.43 1.2 75 0.32 267 9.0 54
13.6 0.0 3.5 0.63 1.88 66 66 190 543 17.8 41
5.4 N/A 1.1 0.62 1.74 64 65 116 526 3.2 24
7.0 N/A 2.4 0.74 1.75 58 64 128 543 4.8 17
14.4 6.5 3.7 0.51 1.93 74 70 134 567 17.2 36
18.8 31.8 4.8 0.59 2.00 70 61 163 540 13.1 24
21.9 43.9 5.6 0.61 2.02 70 57 87 511 12.1 18
27.2 57.8 7.0 0.63 1.80 65 52 118 517 11.1 15
42.8 79.0 11.0 0.82 2.07 61 41 56 401 7.0 6
investigate the kinetics of the reaction of NO with Beulah Zap chars, to study the effects of CaO on the kinetics, and to determine the effects of char burnout level (or conversion) on the reaction. Experimental Methods Char Properties The parent char used in this study was prepared from 63/74 km particles of North Dakota Beulah Zap lignite in a methane flat-flame burner (FFB) at a high heating rate (104–105 K/s) and high temperature (peak gas temperature was 1900 K). The resulting parent char (dubbed NDL), a portion of the NDL that was washed with HCl to remove mineral matter (dubbed NDW), and a portion of NDW reloaded with calcium oxide (dubbed NCa) were used. In addition, a series of chars made by burning out NDL char to various extents with 3–5% oxygen in a drop tube reactor (DTR) at about 1800 K were also used in this study. All of these chars were made previously in our laboratory, and details of their preparation have been reported elsewhere [32,33]. The properties of these chars, along with those of the lignite, are shown in Table 1. Reactor Operation The reduction of NO by char was carried out for all chars at 5–6 temperatures between 723 and 1073 K in a 10-mm-i.d. VYCOR glass, vertical packed-bed reactor with a fritted quartz disc of medium porosity as a support. For each run, 0.2 g char mixed with 2 g silicon carbide (inert for NO reduction) was packed in the reactor and heated in He to the maximum temperature desired using an electric furnace. NO diluted with He (3130 ppm NO) was then fed downward through the reactor until the outlet NO
concentration reached a quasi-steady-state value. The transient usually lasted about 1 h and was probably due to the build-up of adsorbed species on the char. Once the quasi-steady-state was reached, data were collected at 5–6 flow rate settings (between 100 and 500 ml/min (NTP)) at each of the 5–6 temperatures studied. For each run, it took about 4 h to collect all the data, and the burnout of the char was about 10% during that period of time. Accompanying the slow loss of char, the outlet NO concentration increased very slightly with time, and thus we have used the term quasi–steady state. In the calculation of rates and kinetic parameters (described later), the small loss in char mass was accounted for by normalizing by the char mass available. The inlet gas pressure in the reactor was controlled at 300 kPa (3 atm). The outlet pressure at each run condition was measured to determine the pressure drop cross the packed bed. The average pressure was used in calculation of concentrations. Gas Analysis The composition of the outlet gas was continuously monitored for N2, CO, CO2, N2O, and O2 by a GC (Perkin-Elmer, 3920B) with TCD and two columns (one packed with Chromosorb 106, the other packed with molecular sieve 5A), and for NO and NO2 by a Chemiluminescence NOx analyzer (Thermo Environmental, 42H). Nitrogen and oxygen mass balances were determined between inlet and outlet streams for each run, and variations always fell within 55%. Mass Transfer Influences? The calculation of film mass transfer and pore diffusion resistances for a worst-case scenario (the maximum particle diameter, the minimum flow rate, and the maximum reaction rate observed) indicated that
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Fig. 2. The variation of NO conversion with temperature for three char types. Fig. 1. Experimental data for NDL char plotted in form of first-order integrated rate expression.
film mass transfer (maximum MT resistance, (Cg 1 Cs)/Cg 4 2.7%) and pore diffusion (minimum effectiveness factor, g 4 0.99) were negligible in this study. Results Kinetics Because a broad range of NO conversion was observed in this study, the reactor was modeled as an integral plug flow reactor. The reaction rate constants and reaction order were obtained by integral analysis [38] of the experimental data. The experimental data for NDL char plotted as: 1ln(1 1 XNO) vs. C0NO W/F0NO (XNO is NO conversion, F0NO is inlet molar flow rate of NO (mol/s), W is char mass available (g), C0NO is inlet NO concentration (mol/L)) are shown in Fig. 1. The data for all
six temperatures are very linear, which is consistent with the reaction being first order with respect to NO partial pressure. Attempts to fit the data to other orders were made, but straight lines were not obtained for all temperatures. Table 2 lists values for the reaction rate constants, their lower and upper 95% confidence limits, and statistical analysis parameters for NDL char data obtained at six different temperatures. The correlation coefficients, t-test values, and F-test values all show that the experimental data were excellently fit by first-order kinetics under the experimental conditions. The standard error was typically less than 5%. Similar first-order behavior was observed for the other two char types, NDW and NCa. The Effect of CaO The variations of NO conversion with temperature at a flow rate of 303 ml/min (NTP) are shown in Fig. 2 for the three char types. The results for other flow rates are similar. The conversion of NO increases
TABLE 2 Reaction rate constants and statistical anaysis parameters for NO reduction by NDL char
T 8C 650 625 600 600 550 500 470
k1 L/s-gChar 0.0736 0.0353 0.0182 0.0181 0.0070 0.0029 0.0016
k1 Low 95%
k1 Up 95%
Std error
Corr coef r2
t-Test
t-Distrib a 4 0.01
F-Test
F-Distrib a 4 0.01
0.0562 0.0333 0.0161 0.0158 0.0063 0.0026 0.0016
0.0909 0.0373 0.0202 0.0205 0.0078 0.0032 0.0017
4.03E-03 7.29E-04 7.40E-04 8.48E-04 2.66E-04 1.10E-04 2.56E-05
0.994 0.998 0.993 0.991 0.994 0.994 0.999
18.2 48.5 24.6 21.4 26.5 26.2 64.1
9.925 4.604 4.604 4.604 4.604 4.604 4.604
333 2348 457 604 701 686 4103
199 26 26 26 26 26 26
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Fig. 3. Arrhenius plots of the reaction rate constants for NO reduction with char for three char types.
Fig. 4. Arrhenius plots of the reaction rate constants normalized by CaO surface area for three char types.
noticeably in the order NDL . NCa . NDW. For example, the temperatures for 50% conversion are 870, 940, and 980 K for NDL, NCa, and NDW, respectively. This is in the same order as the CaO content of the three chars, 3.5% . 2.4% . 1.1%. Therefore, the effect of CaO on the reaction of NO with char appears to be significant. Figure 3 is an Arrhenius plot of the rate data for the NDL, NDW, and NCa chars. The bars around points in the figure show the variation of the reaction rate constants within a 95% confidence interval. A sharp shift in the apparent activation energy with increasing temperatures was observed for three char types, as has also been reported in the literature [6,7,15–18]. The temperature at which the transition takes place increases from 823 to 973 K as the CaO content decreases. This shift to higher activation energy with increasing temperature is opposite to that expected if a reaction is changing from chemical rate control to mass transfer control, and suggests different mechanisms or rate-determining steps at high and low temperatures. It is also noted that at low temperatures the apparent activation energies for all three char types are essentially the same (22–26 kcal/ mol); at high temperatures, however, the activation energies vary from 45 to 60 kcal/mol, increasing as the CaO content of the chars decreases. In most heterogeneous and catalytic reactions, it is not the gross amount or mass of catalyst or solid that is proportional to the reaction rate, but rather the amount of active surface area that the given material contains. CaO surface area can be measured by selective CO2 chemisorption at 573 K using a thermogravimetric analyzer [32,33]. This was done for the NDL, NDW, and NCa chars, and the resulting values were 17.8, 3.2, and 4.8 m2/g of char, respectively (see also Table 1). If the rate constants shown in Fig. 3 are then normalized by the appropriate CaO surface area, the Arrhenius plot shown in Fig. 4 results. This indicates quite definitively that for the low-temperature regime the rate constant is independent of char type, while in the high-temperature regime it still varies as a function of char type with the high-CaO-containing chars still showing higher values. This seems to indicate a direct firstorder dependence on CaO surface area in the lowtemperature regime, and possibly an even higher order dependence on CaO surface area in the high-temperature regime. The Effect of Char Burnout
Fig. 5. Arrhenius plots of the reactions rate constants for NO reduction by NDL chars with different burnout levels.
The Arrhenius plot of rate data for six NDL chars with different burnout levels is shown in Fig. 5. The transition temperatures and apparent activation energies in both temperature regimes are apparently independent of char burnout level. The reaction rate constants decrease (by about a factor of 3) as char burnout level increases from 0 to 80% over the en-
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Fig. 8. Arrhenius plot of the rate constants normalized by CaO surface area for NO reduction by NDL chars with six different burnout levels. Fig. 6. The dependence of the reaction rate constants for NO reduction by NDL char on CaO surface area.
level for all five measured temperatures as shown in Fig. 7. When the data of Fig. 5 are replotted using the CaO-normalized rate constants instead of the mass-based rate constants, all points for six different burnout levels fall on one line (Fig. 8), again confirming that the reaction rate of NO reduction by NDL chars normalized by CaO surface area is independent of char burnout level. These results suggest strongly that CaO is involved directly in the reaction process, probably as a catalyst. Discussion
Fig. 7. The effect of char burnout level on the rate constants normalized by CaO surface area for NO reduction by NDL chars.
tire temperature range. Note that these rate constants are already per gram of available (or residual) char, and thus the decrease is real. (If they were per gram of initial char, the differences would be even greater, by an additional factor of 5.) It has also been shown by previous work in this laboratory [32,33] that the CaO surface area for these same chars decreases with burnout level. A cross plot (Fig. 6) shows that the rate constants increase as CaO surface area increases. When the rate constants are normalized by CaO surface area, they become essentially independent of char burnout
The fact that the CaO surface area correlates the data in both the low- and high-temperature regimes for this NDL series of chars is in contrast to the behavior of the three char types shown in Figs. 3 and 4, in which the low-temperature regime was correlated but the high-temperature regime was not. The reasons for this difference are not clear at this time. The behavior suggests that there is a difference between the three char types that does not exist between the members of the NDL series that is important for determining reactivity in the high-temperature regime, but not in the low-temperature regime. Possibilities would include char organic structure or other inorganic constituents. Other inorganic constituents besides CaO were removed in the washing process (see ash content values in Table 1), while in the NDL series, the ratio of inorganic constituents would have remained fairly constant. One crude measure of organic structure would be N2 BET surface area. Attempts to correlate the rate constant data for the NDL chars with N2 BET surface areas of the chars (see Table 1) have been made. For the NDL series, a moderate correlation was observed [34], albeit less definitive than that with CaO surface area. However, for the three char types no
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clear correlation was observed at either high or low temperature. Thus, BET surface area does not appear to explain the differences between the two series. Conclusions The kinetics of the reaction of NO with char have been determined for three kinds of char with different CaO contents. The results show that the reaction rate increases significantly as the CaO content of the char increases. The reaction is first order with respect to NO partial pressure and has a sharp shift in the activation energy with temperature for all char types studied. In the low-temperature regime, the activation energies for all three char types are essentially the same (22–26 kcal/mol); in the high-temperature regime, they are all higher but decrease from 60 to 45 kcal/mol as the CaO content increases. The temperature at which the shift takes place also decreases as the CaO content increases. The effect of char burnout level on the reaction was also studied. The transition temperatures and apparent activation energies were found to be independent of char burnout, but both the reaction rate constant and CaO surface area decreased as char burnout level increased from 0 to 80%. When the reaction rates are normalized by CaO surface area, they become essentially independent of burnout level, which suggests the important part that CaO sites play in the reduction process. Finally, the data in the low-temperature regime for both char series were correlated very well by CaO surface area. Data in the high-temperature regime for the burnout series of similar NDL chars were again correlated well by CaO surface area, but data for the three char types were not. The difference is not yet understood. Acknowledgments This work was sponsored by the Advanced Combustion Engineering Research Center at Brigham Young University. Funds for this center are received from the National Science Foundation, the State of Utah, the U.S. Department of Energy, and a number of industrial participants. We also thank Richard F. Cope for char preparation and Troy Ness for technical assistance.
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