REDUCTION OF NITRIC OXIDE ON THE CHAR SURFACE AT PULVERIZED COMBUSTION CONDITIONS

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1 Proceedings of the Combustion Institute, Volume 29, 2002/pp REDUCTION OF NITRIC OXIDE ON THE CHAR SURFACE AT PULVERIZED COMBUSTION CONDITIONS ALEJANDRO MOLINA, ERIC G. EDDINGS, DAVID W. PERSHING and ADEL F. SAROFIM Department of Chemical and Fuel Engineering University of Utah This study presents an experimental evaluation of the rate of nitric oxide reduction on the char surface. It addresses the claim that the rate for the destruction of nitric oxide on the char surface has been underpredicted due to char deactivation in the process of char formation. Experiments conducted with chars produced in situ, char previously produced at pulverized combustion conditions, and char produced with an activated carbon showed the existence of three phenomena during the reduction of nitric oxide: (1) the homogeneous reaction of the volatiles that evolved after the injection of the solid into the reaction with nitric oxide; (2) the accumulation of nitrogen on the char surface, probably through the formation of C(N) complexes, and (3) the heterogeneous reaction of nitric oxide with char. The nitric oxide reaction with char was found to be dominant at pulverized combustion conditions (T 1500 K) with a rate within 1 order of magnitude of that predicted by an expression recommended in previous studies. At fluidized-bed conditions (T 1300 K), the second phenomenon may be important and traditional rate expressions may underpredict the nitric oxide conversion to N 2 when used at combustion conditions when the nitric oxide char reactions begin immediately after char formation and before a pseudo-steady state is reached. For the solid used in this study, the increase in nitric oxide reduction due to formation of C(N) sites was a factor of 2 3. Introduction The three most common nitrogen oxides produced during coal combustion (, 2, and N 2 O) are pollutants. In order to control the production of these oxides, it is fundamental to understand all significant mechanisms for nitrogen oxidation during coal combustion. This study examines one of these mechanisms: the reduction of on the char surface. This reaction has been extensively studied, and several comprehensive reviews can be found in the literature [1,2]. There is relative agreement [3] that the mechanism that better explains the reaction is that described by Tomita and coworkers [4]: 2C(_) r C(N) C(O) (reaction 1) and C(N) r N 2 C(O) or CO (reaction 2). Reaction 1 represents the dissociative chemisorption of. The active surface sites react with incoming molecules to produce N 2 and CO (reaction 2). Even though the mechanism proposed by Tomita and coworkers [4] explains most of the experimental observation on the reduction of in char, it does not provide the kinetic parameters for the reaction. This is probably one of the areas where there are more uncertainties in the study of the conversion of char nitrogen to nitrogen oxides. Most of the controversy originates from the failure of most rate kinetic parameters on predicting the conversion of char nitrogen to at pulverized coal combustion conditions [5 8]. There are numerous papers on the kinetics of the reduction of on the char surface. Aarna and Suuberg [1] reviewed more than 20 different experiments of this reaction for different carbonaceous materials and environmental conditions. They proposed an expression (equation 1) that represented the results in these studies for the rate of destruction on the char surface (k ) at temperatures above 700 K within 1 order of magnitude: 6 k exp( 16,000/T(K)) (g m h atm ) (1) Molina et al. [3] confirmed that equation 1 represented within 1 order of magnitude the rates of /C reactions from different studies [9 13]. However, a recent study by Jensen et al. [6] suggests that most of these kinetics underestimate the rate of reduction on the char surface because of thermal deactivation of chars produced under inert atmospheres. These authors proposed an alternative expression (equation 2) for the temperature range of K that is, according to them, a factor of greater than the range of the literature data: 6 k 6 10 exp( 14,800/T(K)) (m kg C s ) (2) Several authors [13 16] have observed, particularly at low temperatures ( K), an increase 2275

2 2276 FORMATION AND DESTRUCTION OF POLLUTANTS x and Related Species TABLE 1 Coal and char analysis Proximate Analysis (%) Ultimate Analysis (dry, ash free (%) Moist. Ash Vol. Mat. Fixed Carbon C H N S O Coal Char drop tube Char U-furnace N.A. N.A Activated carbon N.A. N.A in the rate of reduction during the initial stages of the reduction of on the char surface. All these authors suggest that the accumulation of nitrogen in the initial stages of char gasification is the reason for this behavior. The nitrogen accumulation tends to disappear as temperature increases [15]. Other studies [13,17,18] have observed nitrogen accumulation during char oxidation. All this suggests the existence of an adsorption-limited process that is more important at low temperatures when the char surface is not clean. At higher temperatures, the population of surface complexes is smaller and the reaction is no longer adsorption limited; therefore, the effect is lower. This study evaluates the rate of reduction on the char surface at conditions that simulate pulverized coal combustion. It evaluates if variations in temperature, char preparation, and extent of reaction can explain the discrepancies observed in the rates of reduction on char. Experimental The experiments were carried out in an electrically heated, laminar-flow, drop tube reactor (DTR) with a diameter of m. The conventional setup of the DTR was modified to allow the implementation of batch experiments with in situ char formation, adapting a methodology developed by Jensen et al. [6] (see Ref. [7] for details). A specified amount of solid (5 15 mg) was introduced to the reactor by means of a custom-built distributor that spreads the coal uniformly over the entire cross section. The solid stream is then collected over an alumina/silica wool placed in the high-temperature section of the DTR. The total gas flow was 4000 sccm for all the experiments. The concentration of CO, CO 2, and was evaluated by Fourier transform infrared (FTIR) (254 cm 3 gas analysis cell, mercury-cadmium-telluride (MCT)/A detector, 0.5 cm 1 resolution) and double-checked by nondispersive infrared (NDIR) (CO and CO 2 ) and chemiluminescence (). 2, HCN, N 2 O, SO 2,C 2 H 2,C 2 H 4, and CH 4 were also evaluated by FTIR. However, the concentration of these species was not corroborated by an additional technique and therefore should be understood to be less precise. The FTIR average response time was 1.2 s. A mixture of /N 2 /He with a specific concentration (varying from 250 to 750 ppm) is added to the reactor until a steady-state concentration is reached. The solid is injected at t 0 into this mixture, begins to react in the gas phase, and continues to react when captured on the wool. After 5 min, oxygen is allowed into the reactor to consume any remaining mass of char. One bituminous coal, one char, and one activated carbon were used in this study. Table 1 presents the ultimate and proximate analysis of the samples. The coal is a raw Illinois number six coal; char drop tube is produced by injection of the coal in a DTR without the presence of the wool. The analysis of this char represents an approximation to the composition of the char formed after the injection of the coal into the reactor. U-furnace char is produced in a selfsustained pulverized Illinois number six coal flame [19]. The activated carbon is a commercial activated carbon produced from coconut shells. The particle size was lm for all the experiments. The Brunaeur, Emmett, and Teller (BET) surface area for the chars prepared in the U-furnace is 105 m 2 /g; that of the activated carbon is 1680 m 2 /g [19]. Results and Discussion Process Affecting the Rate of Reduction Figure 1 presents the concentration profiles for CO 2, CO,, and HCN for the injection of the three solids into the reactor at 1273 K. Analogous plots were obtained at 1415, 1556, and 1698 K, but they are not presented due to space constraints. At all temperatures, there is a rapid increase of CO, CO 2, and HCN concentration as soon as the solids are injected. Simultaneously, the concentration decreases. This increase in the concentration of CO, CO 2, and HCN after the solid injection is more pronounced for coal than for the other two

3 /CHAR REACTION AT PC CONDITIONS 2277 solids, and this difference decreases as the temperature increases. This peak corresponds to the release of volatiles. It is also observed in the char and the activated carbon since these two solids release some residual volatile compounds once injected into the DTR. The CO concentration is high when volatiles are present in the system. However, after that initial period, the CO concentration remains relatively constant throughout the reaction. Given the high ratio of gas flow/sample size used, we consider that the effect of CO in the system during the heterogeneous reactions is low. The concentration of HCN and, to a certain extent, CO and CO 2 in Fig. 1 represents the evolution of volatiles and their secondary pyrolysis products (similar results were obtained at higher temperatures). Detailed analysis of the FTIR spectra just after the injection of coal shows evidence of the presence of some other light hydrocarbons (CH 4,C 2 H 2, and C 2 H 4 ) characteristic of primary devolatilization products. The analysis of the spectra also shows that these species are more evident at the lower temperatures (1273 K) and tend to decrease as the temperature increases, because these species are oxidized to CO 2 in an increasing amount as the temperature is raised. A small amount of O 2 that was occluded between particles or chemisorbed by the coal was injected with the coal into the system. This oxygen was quickly consumed and was responsible for the partial oxidation of the volatiles. Therefore, it did not affect the char/ heterogeneous reaction. It is important to note that due to the large volume in the reactor, there is a relative broad residence time distribution (average residence time of 20 s in reactor and analysis system), that explains the long times of s in Fig. 1, compared with devolatilization times of under 1 s [20]. The reason for the discussion of the detection of volatile compounds just after char addition is because they can influence the apparent rate of reduction on the char surface. It is well known that is reduced by hydrocarbons [21 23]. Thus, it can be expected that at the temperatures at which the production of volatiles was higher (1273 K), the reduction of nitrogen oxide due to gas-phase reaction is important and can affect the evaluation of the rate of reaction with the char surface. As the temperature increases and the volatiles are consumed to form CO 2, the reduction of through homogeneous reactions should decrease. This effect should be more important when coal is injected in the system than when U-furnace char or activated carbon is injected, since those solids produce less volatiles. The trends in concentrations in Fig. 1 (and similar curves obtained at higher temperatures) are consistent with these postulates. Fig. 1. Profiles of CO 2, CO,, and HCN versus time during char reaction with a 750 ppm /He stream. Solids are injected at time 0; T 1273 K; sample size 5 mg. Symbols: s are coal, s are U-furnace char, and s are activated carbon. This effect is easier to appreciate in Fig. 2, where the reduction of (X ) as computed by equation 3 is plotted as a function of time. X ([] in [] out)/[] in (3) Figure 2 shows an increase in conversion when the solids are injected. At 1698 K, the increase in conversion after solid injection is slow and is

4 2278 FORMATION AND DESTRUCTION OF POLLUTANTS x and Related Species Fig. 2. reduction after injection of carbonaceous materials into the drop tube. Solids are injected at time 0. (a) T 1273 K; (b) T 1698 K. Sample size 5 mg; reaction was with 750 ppm in He. Symbols: s are coal, s are U-furnace char, and s are activated carbon. followed by a slow decrease in conversion as the solid is consumed. The consumption of carbon is faster at 1698 K, resulting in a steeper decrease of X as time advances. Although the mass of carbonaceous material injected in the reactor is always the same, the actual mass of the char reacting with the is different in every case, since the amount of material devolatilized varies between solids. A caseby-case consideration of the residual mass is necessary, and when these are similar, as they were for a few cases, a direct comparison of conversion is possible. However, this comparison is not valid for every case. At 1273 K (Fig. 2a), the increase in reduction just after the injection of the solids is steeper when coal is injected than when the other two solids are injected. Two peaks are observed: one in the first 20 s and a broader, second one that goes from 20 to 50 s. The peaks are superimposed. They are easier to observe in Fig. 3, that shows the reduction of for similar experiments when coal and U-furnace char are injected into the reactor with a 250 ppm / He composition and for which the sample size for coal was 15 mg and for char 5 mg. At 1273 K, the steep increase in the conversion of just after coal injection is evident. A broader peak that extends from 20 to 250 s follows the first peak (15 20 s). Fig. 3. reduction after injection of carbonaceous materials into the drop tube. Solids are injected at time 0. (a) T 1273 K; (b) T 1353 K; (c) T 1698 K. Symbols: s are coal (15 mg), s are U-furnace char (5 mg): reaction was with 250 ppm in He. After 250 s, the conversion of stabilizes. For U- furnace char at this same temperature, only the second broad peak is observable, although it only extends to approximately 150 s. For coal at 1353 K, both peaks occur, although the second peak is not so evident. At 1698 K, no peak is observed, as was the case in Fig. 2b. The analysis above suggests three different phenomena occurring during the process of reduction on chars produced in situ. The first corresponds to homogeneous reduction of by the volatiles produced just after the solids are injected in the reactor. The fact that this process is more evident for coal supports this conclusion. This peak is only present at low temperatures since at higher temperatures, most of these volatile species are oxidized to CO 2. The second process corresponds to adsorption of nitrogen oxide complexes on the char surface. This

5 /CHAR REACTION AT PC CONDITIONS 2279 process is characteristic of the -char reaction at low temperatures and has a first-order dependence on the partial pressure [13 16]. The data in Fig. 2 and Fig. 3 show that the appearance of the second broad peak only occurs at low temperatures, and it is more evident at low pressure. A lower pressure facilitates the measurement of the accumulation of C(N) from the heterogeneous reaction of nitric oxide with char, since the accumulation process will be slower at low partial pressures. Also evident in Fig. 2a and Fig. 3a is that after this process occurs, the rate of reduction on the char surface resumes at pseudo-steady-state conditions. The final process is the reaction of on the char surface. This occurs once the char surface has reached equilibrium between adsorption of to form nitrogen complexes on the char C(N) (reaction 1) and the reaction of these complexes with incoming to form N 2 (reaction 2). At high temperatures, reaction 2 occurs at high enough rates that there is no nitrogen accumulation through reaction 1. The decrease in the extent of the reduction of once the third process begins is only due to the reduction in the char mass available for reduction and is not a consequence of char deactivation. Evaluation of the Rate of Reduction on the Char Surface The detection of these three phenomena is important in the understanding of the mechanism of reduction on the char surface. However, from the point of view of how much nitrogen oxides are actually produced during char combustion, it is more relevant to evaluate their magnitude. The influence of reduction by volatiles depends on the amount and composition of the volatiles released and the reactions occurring in the gaseous phase. It is not the objective of this study to explore these two fields; detailed studies are available in the literature [24 28]. Nevertheless, it is important to recognize that reaction of volatiles with can have an effect on the apparent conversion of char nitrogen to under specific conditions. The results in this study, as well as in others [13,15], suggest that C(N) accumulation on char is important at temperatures below 1300 K, in the range of fluidized-bed combustion, but may not be important at higher temperatures. Although it is difficult to evaluate the exact magnitude for the rate of reduction by this process, it is possible to compare the relative reduction of by C(N) accumulation on the char surface to the total reaction once pseudo-steady state is reached. If the rate of reduction on the char surface is evaluated during the process of formation of C(N) complexes ( 10 s in Fig. 1a) and when the reaction is at pseudo-steady state ( 150 s), it is possible to obtain an approximation of the relative contribution of the process of C(N) formation on the reduction of on the char surface. Equation 4 presents the ratio of the rate of reduction for both processes. I I II in in ln ln II II I I II out out II W [] in [] in I I II W [] out [] out k m [] m [] k W [] W [] ln ln (4) In equation 4, the expression for k is taken from Ref. [6]; the superscript I represents the process of C(N) accumulation, and the superscript II the process when pseudo-steady state is reached. Rather than describing the elementary steps of reactions 1 and 2, equation 4 represents the ratio between the global rates for the /char reaction when nitrogen accumulation is the controlling process and when pseudo-steady state is obtained. Here, m I m II are the volumetric flow rates, and W is the mass of carbon. Since the values for concentrations are known, the only unknown is the ratio between the masses of char at both times, W II /W I. Since the reaction is more advanced at time II, W II /W I is always less than 1. Therefore, W II /W I 1 in equation 4 I II represents a maximum for k /k. This discussion assumes that the rate for the /char reaction is proportional to the internal surface area of the particle. The slow reactivity of this reaction justifies this assumption. When the data in Fig. 1 are applied to equation I 4, with W II /W I 1, k /k 2.6. For the data I II in Fig. 3, k /k 2.7. This implies that the process of C(N) formation produces an increase of the order of at most 2 3 times the rate at pseudo-steadystate conditions. To evaluate the rate for the /carbon reaction, the experiment in Fig. 1 was perturbed by injection of oxygen at a time when X versus t was linear. It is possible to evaluate the total carbon mass in the reactor from the CO and CO 2 concentration versus time. With W, k can be computed from an equation similar to the one used in the numerator in equation 4. Table 2 summarizes the results of this calculation for the three solids and compares these values to the one predicted by equations 1 and 2. Table 2 shows that the rates of reduction by the coal and the U-furnace char are almost identical. The CO 2 profiles after O 2 injection for both solids (not presented) showed that the mass inside the reactor is similar. At the same time, the concentration for both solids is also the same (Fig. 1) and both rates should be equal. The rate for activated carbon is higher, since the mass is lower (from the CO 2 profile not presented), but the concentration is lower than for coal and U-furnace char. Assuming that the surface areas of the different materials after injection in the DTR are similar to those obtained before injection, the surface area for II

6 2280 FORMATION AND DESTRUCTION OF POLLUTANTS x and Related Species TABLE 2 Comparison of kinetic constants for the reaction of with char at 1273 K Solid Present Study k (mol m 3 s 1 ) Aarna and Suuberg [1] Jensen et al. [6] Coal U-Furnace char Activated carbon Note: Results are from experiments in this study and from the application of two kinetic expressions found in the literature. the activated carbon is 1 order of magnitude higher than that for the char. Therefore, the higher reduction rate observed for the activated carbon is an expected result. Nevertheless, the difference between surface rates is not proportional to surface area, suggesting that not all the area is active for the reduction of. The comparison of the rate constants found by the experiments in the present study with the one obtained after applying equations 1 and 2 to the conditions of the experiments in the DTR shows that equation 1 predicts kinetic reaction rates that differ by less than 1 order of magnitude from the results of the present study. The prediction for the coal and U-furnace char are the same, since in the calculation it was assumed that both had similar surface areas. The predictions for the activated carbon are higher in proportion to the difference in surface areas. Equation 2 gives rates that are higher than the present values by more than 1 order of magnitude. This expression does not predict any difference between the reactivity of coal, U-Furnace char, or activated carbon since it does not consider the surface area available for reaction. The overestimation on the experimental results can be related to the different coal used in the experiments by Jensen et al. [6] and to the fact that the rate expression they reported was not normalized by char surface area, which makes it difficult to extend their results to different solids. Conclusions The experiments performed in this study suggest that three processes govern the reduction of by char. The first is the homogeneous reaction of the volatiles evolved with. The second is the accumulation of nitrogen on the char surface, probably through the formation of C(N) complexes. The third is the heterogeneous reaction of with char. The reduction of by char is found to dominate at pulverized combustion conditions (T 1500 K). The rate for reduction for this process was found to be within 1 order of magnitude of the value predicted by the expression recommended by Aarna and Suuberg [1]. Nomenclature k rate of reduction on the char surface (units as specified in the text) [] in concentration at reactor inlet [] out concentration at reactor outlet T temperature (K) v flow rate (m 3 s 1 ) W sample weight (kg of C) X reduction Acknowledgments This work was sponsored by the DOE (grant DE-FG26-97FT97275). REFERENCES 1. Aarna, I., and Suuberg, E., Fuel 76:475 (1997). 2. Li, Y. H., Lu, G. Q., and Rudolph, V., Chem. Eng. Sci. 53:1 (1998). 3. Molina, A., Eddings, E. G., Pershing, D. W., and Sarofim, A. F., Prog. Energy Combust. Sci. 26:507 (2000). 4. Tomita, A., Fuel Process. Technol. 71:53 (2001). 5. Visona, S., and Stanmore, B., Combust. Flame 106:207 (1996). 6. Jensen, L. S., Jannerup, H. E., Glarborg, P., Jensen, A., and Dam-Johansen, K., Proc. Combust. Inst. 28:2271 (2000). 7. Molina, A., Eddings, E. G., Pershing, D. W., and Sarofim, A. F., Production of Nitrogen Oxide during Char Oxidation at Pulverized Coal Combustion Conditions, paper 210, U.S. Sections Second Joint Meeting of the Combustion Institute, Oakland, CA, March 25 28, Commandré, J.-M., Stanmore, B. R., and Salvador, S., Combust. Flame 128:211 (2002). 9. Chan, L., Sarofim, A. F., and Beér, J. M., Combust. Flame 52:37 (1983). 10. Levy, J., Chan, A., Sarofim, A. F., and Beér, J. M., Proc. Combust. Inst. 18:111 (1980). 11. Song, Y., Beér, J. M., and Sarofim, A. F., Combust. Sci. Technol. 25:237 (1981). 12. de Soete, G., Proc. Combust. Inst. 23:1257 (1990). 13. Guo, F., and Hecker, W., Proc. Combust. Inst. 27:305 (1998). 14. Illán-Gómez, M., Linares-Solano, A., Radovic, L., and Salinas-Martínez de Lecea, C., Energy Fuels 9:97 (1995). 15. Aarna, I., and Suuberg, E., Proc. Combust. Inst. 27:3061 (1998).

7 /CHAR REACTION AT PC CONDITIONS Yamashita, H., Tomita, A., Yamada, H., Kyotani, T., and Radovic, L., Energy Fuels 7:85 (1993). 17. Ashman, P. J., Haynes, B. S., Buckley, A. N., and Nelson, P. F., Proc. Combust. Inst. 27:3069 (1998). 18. Abbasi, M., Mechanistic and Kinetic Aspects of Nitrogen Oxides Formation in Coal Char and Model Char Oxidation Processes, Ph.D. thesis, University of Utah, Salt Lake City, Spinti, J. P., An Experimental Study of the Fate of Char Nitrogen in Pulverized Coal Flames, Ph.D. thesis, University of Utah, Salt Lake City, Kobayashi, H., Howard, J. B., and Sarofim, A. F., Proc. Combust. Inst. 16:411 (1976). 21. Bilbao, R., Alzuata, M. U., Millera, A., and Duarte, M., Ind. Eng. Chem. Res. 34:4540 (1995). 22. Alzueta, M., Bilbao, R., Millera, A., Glarborg, P., Østberg, M., and Dam-Johansen, K., Energy Fuels 12:329 (1998). 23. Xu, H., Smoot, L. D., and Hill, S. C., Energy Fuels 13:411 (1999). 24. Miller, J., and Bowman, C., Prog. Energy Combust. Sci. 15:287 (1989). 25. Kilpinen, P., Glarborg, P., and Hupa, M., Ind. Eng. Chem. Res. 31:1477 (1992). 26. Perry, S. T., Fletcher, T. H., Solum, M. S., and Pugmire, R. J., Energy Fuels 14:1094 (2000). 27. Niksa, S., and Kerstein, A. R., Energy Fuels 5:647 (1991). 28. Solomon, P. R., Hamblen, D. G., Carangelo, R. M., Serio, M. A., and Deshpande, G. V., Energy Fuels 2:405 (1988). COMMENTS Jost Wendt, University of Arizona, USA. Did you investigate the effects of char porosity on your kinetics? Did the use of helium as a diluent change the appropriate effectiveness factors for the char particles and lead to regimes of kinetics control that may not be the case for combustion with air? This would thus affect the net conversion to reduction of. Author s Reply. This paper aimed to study the rate of reduction on the char surface under a kineticcontrolled regime. Thus, we consider that the use of helium as inert gas an advantage, not a drawback. However, the application of the kinetic expression recommended in this paper to a combustion system needs to consider any mass transfer limitations, as this question suggests. Peter Glarborg, Technical University of Denmark, Denmark. Soot may be formed in significant quantities from secondary pyrolysis of tar in the volatiles. Did you see any indications that soot participated in reducing in the coal experiments? Author s Reply. The issue of soot formation is a very interesting one since soot, due to its high surface area, could contribute to the higher reactivity of the char prepared in situ. However, during the experiments reported in this study, soot was not detected. Nevertheless, it is important to note that the experimental setup was not designed to detect the presence of soot in the system. Akina Tomita, Tohoku University, Japan. The difference between the mechanisms II and III is not clear to me. Is this related to the concentration of C(N)? The concentration of C(N) would be relatively low at high temperatures, but I believe that the mechanism C(N) r N 2 would hold true both for low temperature reaction and for high temperature reaction. Could you comment on this? Author s Reply. This paper does not question the mechanism described by Tomita et al. [1] but uses it to understand the difference on conversion between a char prepared in situ and a char prepared ex situ. The mechanism proposed by Tomita can be used to explain a higher apparent rate of reduction due to nitrogen accumulation (C(N) r C(N) C(O)) in the first seconds after char exposure to. This is considered as mechanism II. Mechanism III refers to the global process of reduction (C(N) r N 2 ) that occurs once the equilibrium is reached. REFERENCE 1. Tomita, A., Fuel Process. Technol. 71:53 (2001).