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Energy & Fuels 2007, 21, 1872-1877
The NO-Carbon Reaction: The Influence of Potassium and CO on Reactivity and Populations of Oxygen Surface Complexes Diana Lo´pez* and Joseph Calo DiVision of Engineering, Brown UniVersity, ProVidence, Rhode Island 02912 ReceiVed January 22, 2007. ReVised Manuscript ReceiVed April 19, 2007
Results on the effects of a metal catalyst and the role of CO as a reducing agent are reported for a resin char and a Wyodak coal char, as well as demineralized samples of the latter. The effect of an active metal catalyst, such as potassium in the current work, is to significantly increase the reactivity both by increasing the number of reaction sites via the catalyst dispersion and reducing the activation energy and by increasing CO2 production. The latter is a beneficial result because it means that less carbon is consumed per molecule of NO reduced. Additional CO in the gas phase “catalyzes” NO reduction via the creation of more labile surface complexes and facilitation of desorption of other oxygen complexes. This effect decreases with an increasing temperature and disappears by about 1123 K. The activation energy of this reaction is comparable to that induced by the metal catalyst.
1. Introduction A considerable amount of work has been conducted regarding the reaction of nitric oxide with carbon, because of the interest in this system as a means of reducing NOx emissions, as well as in elucidating their role in heterogeneous combustion systems. Promoted carbons offer some potentially significant advantages for heterogeneous NOx reduction. These include low cost, high activity at low temperatures, which minimizes the carbon loss and improves oxygen resistance, and a support material that can be engineered with respect to porosity, transport, and catalyst dispersion characteristics. Dependent upon the specific nature of the carbonaceous material and the operating conditions, carbon can simultaneously behave like a reactant, a catalyst, and a catalyst support. Considerable work has been done on the catalytic effects of various species on carbon gasification reactions. A number of workers have observed the catalytic effects of certain mineral matter impurities on the reaction of NO with carbonaceous materials.1,2 Illa´n-Go´mez et al.3 studied the catalytic effects of a series of transition metals (Cr, Co, Fe, Ni, and Cu) on NO reduction. It was found that all of these metals catalyzed the reaction to some degree and resulted in a decrease in the apparent activation energy and a substantial shift of the NO consumption curves during temperature-programmed reduction (TPR) to lower temperatures. It is also known that certain gases, such as NH3, CO, and H2, can act as reducing agents for NO in carbonaceous systems, in which carbon behaves somewhat like a heterogeneous catalyst. One important issue in the NO-carbon reaction mechanism is the enhancement of NO reduction in the presence of carbon monoxide. It has been established by several workers that surface catalysis by carbon is involved in the NO-CO
reaction.4-6 This may be due to either a direct reaction of NO with CO, catalyzed by the carbon surface7,8
* To whom correspondence should be addressed: Institute of Chemistry, University of Antioquia, A.A. 1226, Medellin, Colombia. Telephone: +574210-6613. Fax: +574-210-6565. E-mail:
[email protected]. (1) Rodriguez-Mirasol, J.; Ooms, A.; Pels, J.; Kapteijn, F.; Moulijn, J. Combust. Flame 1994, 99, 499. (2) Chan, L. K.; Sarofim, A. F.; Beer, J. M. Combust. Flame 1983, 52, 37. (3) Illa´n-Go´mez, M.; Linares-Solano, A.; Salinas-Martı´nez de Lecea, C. Energy Fuels 1995, 9, 976.
(4) Furusawa, T.; Tsunoda, M.; Tsujimura, M.; Adschiri, T. Fuel 1985, 64, 1306. (5) Aarna, I.; Suuberg, E. M., In 27th International Symposium on Combustion, The Combustion Institute, Pittsburgh, PA, 1997. (6) Zevenhoven, R.; Hupa, M. Fuel 1998, 77 (11), 1169. (7) Aarna, I.; Suuberg, E. M. Energy Fuels 1999, 13, 1145. (8) Wu, S. L.; Iisa, K. Energy Fuels 1998, 12 (3), 457. (9) Johnsson, J. E.; Dam-Johansson, K. In 11th International Conference on Fluidized Bed Combustion, New York, 1991; p 1389.
NO + CO f 1/2N2 + CO2
(1)
or an indirect reaction of CO with chemisorbed oxygen produced from NO on the surface, creating a free active site for a further reaction with NO2
CO + C(O) f C( ) + CO2
(2)
NO + C( ) f 1/2N2 + C(O)
(3)
In the temperature range considered here (i.e., 923 K. In the presence of potassium, the reactivity increased from about a factor of 2 at the highest temperature to a factor of 10 at the lowest temperature in comparison to that for the unpromoted resin. The presence of potassium decreased the activation energy to 89 ( 3.0 and 85 ( 5.6 kJ/mol for the 2000 and 4000 ppm of NO cases, respectively. Thus, the presence of potassium catalyzes the NOcarbon reaction both by increasing the number of active sites and/or the turnover rate on these sites as well as by significantly decreasing the apparent activation energy. Illa´n-Go´mez et al.12 reported an activation energy ranging between 30 and 86 kJ/ mol in the low-temperature regime (573-873 K) for the NOcarbon reaction in the presence of different metal catalysts. However, values of the activation energy for the NO-carbon reaction catalyzed with potassium at reaction temperatures >923 K have not been reported heretofore. However, the current values agree with the values reported by Illa´n-Go´mez et al.13 at their highest reported temperature (873 K). The activation energy of the Wyodak coal char (WY) was ca. 68 ( 1.8 kJ/mol at temperatures >823 K. This low activation energy is presumably due to the presence of catalytic mineral matter impurities, particularly, calcium, which has been reported to have catalytic activity for the reduction of NO.14,15 A summary of the activation energies for the NO-carbon reaction on demineralized Wyodak coal char (DWY), catalyzed by potassium, in the temperature range of 773-1073 K, is presented in Table 1. As shown, the activation energy of the demineralized char is not constant over the entire temperature range. As indicated in Figure 1, there appears to be two different temperature regimes, with a “break” located at about 873 K. This “two-regime” Arrhenius behavior has been observed by (12) Illa´n-Go´mez, M. J.; Linares-Solano, A.; Salinas-Martı´nez de Lecea, C.; Calo, J. M. Energy Fuels 1993, 7, 146. (13) Illa´n-Go´mez, M.; Linares-Solano, A.; Radovic, L.; Salinas-Martı´nez de Lecea, C. Energy Fuels 1995, 9, 97. (14) Illa´n-Go´mez, M.; Linares-Solano, A.; Radovic, L.; Salinas-Martı´nez de Lecea, C. Energy Fuels 1995, 9, 112. (15) Zhao, Z.; Li, W.; Li, B. Fuel 2002, 81, 1559.
1874 Energy & Fuels, Vol. 21, No. 4, 2007
Figure 2. Comparison of CO and CO2 desorption from the unpromoted resin char (PRC) and potassium-promoted resin char (K-PRC) following the steady-state reaction in 2000 ppm of NO at 1073 K.
others.16,17 It is generally accepted that this behavior is due to a change in the mechanism from desorption rate control (low temperature) to formation rate control (high temperature) of oxygen surface complex intermediates. The CO/CO2 product ratios increase with temperature and become asymptotically constant at higher temperatures. It is also noted that the CO/CO2 product ratios during the reaction in 4000 ppm of NO actually decrease somewhat at high temperatures because of an increase in the CO2 production. This is most likely due to the oxidation of C(O) surface complexes by NO to produce more CO2 gas product.18 The reactivities were close to first order in NO for all of the char samples, which is consistent with our previous results on unpromoted PRC.19 TPD analysis for the unreacted samples, after they were thermally cleaned, showed an insignificant amount of CO and CO2. CO and CO2 thermal desorption spectra from PRC samples following the steady-state reaction at 1073 K are shown in Figure 2. These spectra indicated that there were considerably more oxygen surface complexes of all types produced on the potassium-promoted char (K-PRC) than on the unpromoted resin char (for example, ∼6× at 1073 K) and that this relative discrepancy increased with a decreasing reaction temperature (for example, ∼12× at 923 K). This is consistent with the data in Figure 1 for the KPRC sample, where the reactivity increased in a higher factor for the lowest temperature range. However, once again, different behavior was observed for the demineralized Wyodak chars. Figure 3 presents the CO and CO2 desorption yields from the unpromoted and potassiumpromoted, demineralized Wyodak chars. As shown, the unpromoted char exhibits slightly greater amounts of oxygen surface complexes of all types than the potassium-promoted char, especially at the lower temperatures. However, the potassiumpromoted char exhibits greater reactivity and CO/CO2 product ratios than the unpromoted char, where the presence of potassium is possibly increasing the average turnover rate on active sites. CO2 TPD spectra from the potassium-promoted resin char and potassium-promoted, demineralized char are presented in (16) Aarna, I.; Suuberg, E. M. Fuel 1997, 76, 475. (17) Suuberg, E. M.; Teng, H.; Calo, J. M. In 23rd International Symposium on Combustion, The Combustion Institute, Pittsburgh, PA, 1990; p 1199. (18) Teng, H.; Suuberg, E. M.; Calo, J. M. Energy Fuels 1992, 6, 398. (19) Calo, J. M.; Lopez, D.; Burnett, A.; Aarna, I.; Suuberg, E. M. In Carbon ’01, Lexington, KY, 2001; paper number 6.1.
Lo´ pez and Calo
Figure 3. CO and CO2 desorption yields from unpromoted and potassium-promoted, demineralized Wyodak coal char following the reaction in 2000 ppm of NO in the packed bed reactor.
Figure 4. CO2 desorption rates from K-promoted PRC and Kpromoted, demineralized Wyodak coal char following the reaction in 2000 and 4000 ppm of NO at 1073 K.
Figure 4. As shown, under these conditions, the NO concentration significantly affects the surface population of CO2producing complexes in both chars. The contributions of CO2 signals at 2000 ppm of NO indicate the presence of several active sites with different thermal stabilities, and the population of CO2-producing complexes that desorb at higher temperatures decreases with an increasing NO concentration. This same effect was noted at all temperatures. A mechanism has been formulated that explains this behavior by incorporating the following mechanistic steps in the NO-carbon reaction mechanism:
NO + C(O) f CO2 + C(N)
(4)
NO + C(O2) f CO2 + C(O) + C(N)
(5)
The former has been hypothesized in the literature,17 and the latter is proposed on the basis of our work, where C(O2) represents an oxidized C(O) complex. 3.2. Role of Additional CO in the Feed Gas. One important issue in the NO-carbon reaction mechanism is the enhancement of NO reduction in the presence of carbon monoxide. There is relatively little literature data available on the NO-CO-carbon reaction in the presence of potassium. Kapteijn et al.20 inves(20) Kapteijn, F.; Mierop, A.; Abbel, G.; Moulijn, J. A. J. Chem. Soc., Chem Commun. 1984, 1085.
NO-Carbon Reaction
Energy & Fuels, Vol. 21, No. 4, 2007 1875
Figure 6. CO evolution from potassium-promoted resin char (K-PRC) following the steady-state reaction in 2000 ppm of NO as a function of the reaction temperature.
Figure 5. Summary of reactivities for the unpromoted and potassiumpromoted char samples in 2000 ppm of NO, with and without additional CO in the reactor feed gas.
increasing the number of active sites and/or the turnover rate on these sites as well as by decreasing the activation energy. However, the most significant effect was observed for the potassium-promoted, demineralized Wyodak char (K-DWY) with additional CO. In this case, the reactivity increased by about 50% in the high-temperature regime to about a factor of 3 in the low-temperature regime in comparison to that for the potassium-promoted, demineralized char. The activation energies also decreased in both temperature regimes. A summary of the activation energies for all of the chars in the absence and presence of the additional CO is presented in Table 2. It was also found that, whenever there is an enhancement of reactivity because of catalysis by potassium or CO, the CO/ CO2 product ratio from the PRC decreases; i.e., there is enhanced CO2 production. However, the CO/CO2 product ratio from the demineralized Wyodak char increased in the presence of potassium and decreased in the presence of additional CO. The reactivities were close to first order in NO for all of the char samples, which is consistent with our previous results on unpromoted PRC.19 The CO evolution spectra following the reaction in NO/He and NO/CO/He mixtures are presented in Figures 6 and 7, respectively. These spectra are obviously quite different. As shown in Figure 6, the amount of evolved CO increases with an increasing reaction temperature. However, in the presence of additional CO, the total CO evolution decreases with the reaction temperature and occurs at significantly lower temperatures, as presented in Figure 7. These spectra are actually more similar to those obtained following exposure of the thermally cleaned samples to just CO,
tigated the reduction of NO in a potassium-carbon system in the presence of CO at 673 K. They concluded that NO reacted in about equal amounts with CO and carbon, and the presence of CO decreased the rate of consumption of the carbon support. A summary of reactivity data is presented in Figure 5, along with the data presented in Figure 1 for 2000 ppm of NO for a comparison. For the unpromoted resin char (PRC) in the presence of additional CO, the reactivity increased by more than a factor of 4 at the lowest temperature, while having no discernible effect at the highest temperature, and the activation energy decreased to about 97 ( 2.1 kJ/mol, consistent with other values reported in the literature, e.g., 1167 and 56-132 kJ/mol.6 In the presence of potassium, the reactivity increased again from about a factor of 2 at the highest temperature to a factor of 10 at the lowest temperature in comparison to that for the unpromoted resin char in the absence of additional CO. The presence of potassium decreased the activation energy to 89 ( 3 kJ/mol. However, the presence of significant added amounts of CO in the feed gas did not appear to have any appreciable effect on the reactivity of the promoted resin char like it did for the unpromoted resin char. In the presence of additional CO in the feed gas, the reactivity of the unpromoted, demineralized Wyodak char (DWY) increased by about a factor of 2 at 773 K, diminishing to practically no effect at the highest temperature. The presence of potassium increased the reactivity at temperatures
