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Mechanism of N2 formation during coal char oxidation
Proceedings of the Combustion Institute, Volume 28, 2000/pp. 2189–2195
MECHANISM OF N2 FORMATION DURING COAL CHAR OXIDATION TOSHIAKI AIHARA, KOICHI MATSUOKA, TAKASHI KYOTANI and AKIRA TOMITA Institute for Chemical Reaction Science Tohoku University Sendai 980-8577, Japan
In spite of many studies, the reaction mechanism of the evolution of nitrogen-containing gases during coal oxidation is not fully understood yet. One reason is that the nitrogen mass balance has not been well established in most studies. In the present study, we attempted to clarify the N2 formation mechanism during the reaction between coal char and O2 by paying special attention to nitrogen mass balance. The closure of the nitrogen mass balance was 100 Ⳳ 6%. Blair Athol coal char was used as a sample, and the formation of N-containing species was determined during temperature-programmed reaction as well as isothermal reactions at 700 and 850 ⬚C in a packed-bed reactor. In all the cases, N2 was the major product, with a little amount of NO as a minor product. Under the isothermal reaction conditions, the ratio of NO/ N2 was low in the initial stage and increased in the later stage. The effect of bed height on the product gas distribution was examined, and it was found that the shallower the bed, the higher the ratio of NO/ N2. When the O2/He gas mixture was switched to He, the formation of N2 and NO became almost negligible. From these observations, we proposed the following reaction mechanism for the N2 formation: the formation of NO as a primary oxidation product, and the reaction of NO with surface nitrogen species to form N2. The NO capture on carbon surface to form surface nitrogen species is thought to be an important step during the course of this reaction.
Introduction The understanding of NOx formation during coal combustion is one of the most important issues for a cleaner use of coal [1,2]. Nitrogen in coal is converted to volatile nitrogen and char nitrogen upon pyrolysis. The gas phase reaction of volatile nitrogen is well understood, but in spite of many studies, the reaction mechanism of the evolution of nitrogencontaining gases from char nitrogen is not fully understood yet. Boldly speaking, most researchers are interested in the formation of toxic NOx, but not in the non-toxic N2 gas. Thus, the combustion experiments were frequently made with O2/N2 mixture without considering the formation of N2 gas [3–7]. Even when an O2/Ar or O2/He mixture was used as a reactant gas, many researchers did not find it important to consider the nitrogen mass balance [8– 12], except for a recent study by Ashman et al. who made a detailed product analysis including N2 [13]. We have been engaged in the research on the reactions of pure carbon with NO and N2O by putting emphasis on the nitrogen mass balance. We found that the detailed examination of nitrogen mass balance is very important to elucidate the reaction mechanism [14–18]. For example, we could quantify the amount of nitrogen species accumulated during C/NO reaction, and using this information we found the important role of the surface species in the formation of N2 gas [15–17].
In the present study, we attempt to clarify the reaction mechanism of the N2 formation during O2oxidation of char. Although the present reaction conditions are far from the conditions practically used in the pulverized coal combustion system, the fundamental knowledge of the relevant reactions is always important in understanding the practical system. The reaction pathways are discussed by referring to the C/NO reaction mechanism that was revealed in our previous studies. The following three steps are considered: (1) formation of NO as a result of oxidation of char nitrogen, (2) the reaction of NO formed in step 1 with the char generating new nitrogen surface species, and (3) the reaction between the surface nitrogen species and the gas-phase NO, releasing N2 gas as a final product. Experimental Method Sample Blair Athol coal, Australian bituminous coal, was selected for the present investigation. Its analysis on dry basis is C: 75.6, H: 4.3, N: 1.7, O (by diff.): 9.3, ash: 8.8%. Ash is composed of 64% SiO2, 32% Al2O3, 1.6% TiO2, 1.2% Fe2O3, 0.3% CaO, and others. The char was prepared by devolatilizing the coal at 1000 ⬚C for 30 min in a small fluidized bed reactor made of quartz with an inner diameter of 16 mm. Coal
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washed with water. The residue was pyrolyzed as above. Apparatus The reactor used in this study was a fixed bed reactor as shown in Fig. 1. Gaseous products were simultaneously analyzed both by a high-speed gas chromatograph (GC) (Aera M200) and by a mass spectrometer (MS) (Anelva AQA 200). MS was mainly used for NO detection. The GC detector was a thermal conductivity detector (TCD), and it was equipped with a MS-5A column for O2, N2, and CO separation and a PoraPlot Q column for CO2 and N2O separation. Experimental Procedure
Fig. 1. Schematic diagram of apparatus.
Fig. 2. Temperature-programmed gas evolution from coal char in He. Heating rate: 5 ⬚C/min.
particles with size between 70 and 150 lm was first fluidized with He gas at ambient temperature without adding any inert particles as a bed material, and then the reactor was put into the heated furnace. The analysis of char obtained was C: 83.0, H: 0.6, N: 1.5, O (by diff.): 4.1, ash: 10.8% on dry basis. In order to check the effect of mineral matter on NO and N2 formation behavior, demineralization of the above coal was carried out by the conventional HF and HCl treatment method. The coal was first treated with aqueous HF solution for 24 h and washed with deionized water. It was then treated with 6N HCl at 60 ⬚C for 12 h and thoroughly
Under a standard experimental condition, about 100 mg of char sample (75–150 lm) was used, which corresponded to a sample bed height of 4 mm. The gas used for oxidation was 0.55% or 2.0% O2 diluted with He at an atmospheric pressure, and the total gas flow rate was 200 mL/min unless otherwise stated. Experiments were also carried out to check the effects of sample particle size, gas flow rate, and bed height on the oxidation reaction. Prior to all the reactions, the char was again heat-treated in He at 1000 ⬚C for 30 min to remove the surface adsorbed species. This procedure is important to attain a perfect nitrogen balance, because the uptake of N2 gas on carbon has been observed when carbon is stored in an ambient atmosphere [19,20]. After the heat treatment, the sample was cooled down to 300 ⬚C, and temperature-programmed reactions were carried out in an O2/He mixture. The sample was heated at a constant heating rate of 5 ⬚C/min up to 1000 ⬚C, where it was held for 30 min. For isothermal reactions, the pretreated char was heated to 700 or 850 ⬚C in pure He before switching over to the reactant gas. A step response experiment was run at 700 ⬚C by switching O2/He mixture to He, and the outlet gas was analyzed to determine the desorption profile of surface species. Results N2 Uptake at Ambient Atmosphere We recently found that a significant amount of N2 is trapped on coal char from ambient atmosphere during storage, which makes it difficult to obtain an accurate nitrogen mass balance in the subsequent process [19]. Therefore, we first determined the amount of N2 trapped from ambient atmosphere in the present coal char sample. Fig. 2 shows an example of the gas evolution profile during heating in
N2 FORMATION DURING COAL CHAR OXIDATION
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Fig. 3. Temperature-programmed reaction profile in the char/O2 reaction. O2 concentration in He: 0.55%; heating rate: 5 ⬚C/min. The concentration of NO measured by MS and that of the other gases measured by GC are indicated by a bold line and the symbols, respectively.
He. A substantial amount of N2 gas was detected along with CO and CO2 gases. The N2 desorbed in the low temperature range was not from the decomposition of nitrogen surface complex in coal char but due to the release of trapped N2. The total amount of nitrogen evolved as N2 was around 0.35 mg atom/ g char, which corresponds to 30% of nitrogen content determined by the elemental analysis. This implies that the trapped N2 cannot be neglected in the mass balance calculation. After this heat treatment, char was used for the subsequent reaction without exposing to air. Temperature-Programmed Reaction The gas evolution profile during the temperatureprogrammed reaction with O2 is illustrated in Fig. 3, where the symbols indicate the gas concentration determined by GC, while a thick line without any symbol indicates the concentration of NO measured by MS. The beginning of O2 consumption at about 550 ⬚C is marked by the evolution of an equal amount of CO and CO2. As the temperature increased to 650 ⬚C, CO evolution became greater than CO2. Among the nitrogen-containing species, N2 was the predominant one, and it appeared at a temperature higher than the evolution temperature of CO and CO2 by 60 ⬚C. This implies the unbalance between carbon conversion and nitrogen conversion. Under the present conditions, O2 gas was almost completely consumed above 700 ⬚C, and it appeared again after reaching 1000 ⬚C. At this point, the char conversion was around 60%, and it became 85% after 30 min holding at 1000 ⬚C. During this isothermal stage at 1000 ⬚C, the concentration of N2 decreased while that of NO increased. N2O formation was almost negligible throughout the reaction.
Fig. 4. Isothermal reaction of char with 2% O2/He. Temperature: (a) 700 ⬚C, (b) 850 ⬚C.
Isothermal Reactions Figure 4a shows the result of isothermal reactions at 700 ⬚C. The concentration of O2 in the outlet gas sharply decreased to about 0.1% in the first 5 min, and then gradually increased toward the original concentration level. The evolution of CO, CO2, and NO takes place immediately upon O2 introduction. The concentration of CO and CO2 gradually decreased, while that of NO kept more or less constant until the final stage. On the other hand, the initial N2 formation profile was quite different from the above product gases. The N2 formation rate slowly increased and reached the maximum at 20 min. This suggests that the N2 formation mechanism is different from the others. The formation rate of N2 was always higher than that of NO, except in the final stage where they became nearly equal to each other. The result of the isothermal reaction at 850 ⬚C is shown in Fig. 4b. The reaction rate was faster than at 700 ⬚C, and thus O2 was totally consumed in the first 15 min. The principal N-containing product was again N2, with some NO and a very little amount of N2O. The consumption rate of char was so fast that the concentration of unreacted O2 rapidly increased after 30 min, making the atmosphere highly oxidative. It is noteworthy that the gas composition suddenly changed at around 23 min, where the concentration of CO sharply decreased while CO2 and NO
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Fig. 5. Nitrogen conversion versus carbon conversion. Nitrogen evolved during the pre-heat-treatment stage was not included in this balance. O2 concentration: 2%; temperature: 700 ⬚C and 850 ⬚C.
Fig. 6. Effect of bed height on the formation of nitrogen-containing gases. Bed height: (a) 2 mm, (b) 4 mm, (c) 8 mm; O2 concentration: 2%; temperature: 850 ⬚C.
increased. This behavior is observed for all the reactions carried out at 850 ⬚C. In the experiment shown in Fig. 4b, where the total gas flow rate was 200 mL/min, all O2 gas was consumed in the initial 15 min at 850 ⬚C. This means that the reaction rate was controlled by O2 gas supply. In fact, when the gas flow rate was increased, the conversion rate increased until a total flow rate of 600 mL/min. The effect of particle size was examined at a total flow rate of 200 mL/min, and it was found that the effect was very small in the range of 0.04 to 1.0 mm. This is quite natural if the reaction is controlled by O2 gas supply. The detailed mass balance was established during these isothermal reactions. In Fig. 5, the nitrogen conversion is plotted against the carbon conversion. The curves deviate slightly from the 1:1 line. The deviation looks rather small, but the carbon consumption in the initial stage is in some cases twice as much as the nitrogen consumption. This deviation is due to either the smaller reactivity of nitrogen species compared with carbon or the higher accumulation rate of once-formed gaseous nitrogen species on carbon surface compared with those of CO and CO2, as suggested by Ashman et al. [13]. In order to check the possibility of secondary reactions, the gas formation behavior with different bed heights was examined. Fig. 6 shows the gas evolution profile only for nitrogen-containing species for different bed heights. In all the cases, the formation rate of N2 is higher than NO until in the later stages where NO formation increased and became somewhat larger than N2. It is worthwhile to point out that NO concentration increased sharply near the end of reaction; 10, 25, and 60 min for Fig. 6a, 6b, and 6c, respectively. These points roughly correspond to the appearance of O2 in the outlet gas. Interestingly, evolution of a small amount of N2O was also detected in all three cases around these times. For the purpose of modifying the extent of secondary reaction, preliminary experiments were carried out by diluting the char with inert quartz sand. By keeping the bed height constant at 10 mm, the ratio of quartz/char was varied from 4 to 60 by weight. With increasing extent of dilution, the N2/ NO ratio decreased from 3 to 1. The dilution of char particles might suppress the secondary reaction of NO gas with char. The amounts of N2, NO, and N2O evolved during the isothermal reaction stage, together with N2 evolved during the heat-treatment stage, are summarized in Table 1. The amount of total nitrogen was very close to that determined by elemental analysis. In Table 1, the ratio of NO to the total nitrogen obtained in the burn-off stage is also listed. The effect of bed height is clearly seen. When the sample bed was high, relatively more N2 and less NO was formed from char nitrogen.
N2 FORMATION DURING COAL CHAR OXIDATION TABLE 1 Nitrogen Mass Balance During Char Oxidation with Different Bed Height Bed Height (mm) Evolved Gas (mmol/g char) Heat-treatment stage N as N2 Burn-off stage N as N2 N as NO N as N2O Total N from both stages Total N from elemental analysis N as NO/N from burn-off stage
2
4
9
0.34
0.35
0.37
0.61 0.17 0 1.12 1.2 0.22
0.65 0.15 0 1.15 1.2 0.19
0.67 0.08 0 1.12 1.2 0.11
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Effect of Mineral Matter Since it has been reported that some inorganic species, particularly Fe and Ca compounds, affect the reaction profile of N2 removal from coal char [21], we checked this effect by comparing the present result with that of demineralized Blair Athol char. The demineralization treatment reduced the ash content in the pyrolyzed char to 0.2%. The temperature-programmed reaction profile of this demineralized char was very similar to that presented in Fig. 3. Thus, at least for this particular system, the catalytic effect by mineral matter is not significant. Perhaps mineral matter in Blair Athol coal is not catalytically active.
Discussion Nitrogen Mass Balance
Fig. 7. Step response experiment. O2 concentration: 2%; temperature: 700 ⬚C.
Step Response Experiments In order to clarify the mechanism of N2 formation, a step response experiment was carried out by switching the reactant gas from O2/He mixture to He and then back to O2/He. The result is shown in Fig. 7. Up to 20 min, the gas evolution profile is naturally similar to that in Fig. 4a. At 20 min, the gas was switched from O2/He to He. The formation of N2, NO, CO, and CO2 almost terminated immediately. If we look at the result more closely, we can see a decay profile of the gases. Among all the gases, the largest evolution was seen with CO. At 30 min, the concentration of CO was 0.005%, which corresponds to four-thousandth of the concentration before switching. The evolution of CO2 became negligible at 25 min. Although a slow decay of N2 was observed until 28 min, the total amount of nitrogen observed as N2 during this period was only 0.32 lmol. This is much smaller than the estimated amount of char nitrogen at 20 min, 51 lmol.
As shown in Table 1, the nitrogen mass balance for the reaction at 850 ⬚C was fairly good. The result at 700 ⬚C is not shown in the Table 1, but the agreement was also good. In the present study, the calculation of N2 mass balance was carried out by including the amount of N2 trapped in coal char during the storage in an ambient atmosphere. This amount is assumed to be equal to N2 evolved during the heat-treatment stage, as shown in the first row of Table 1. This is not negligibly small, and the estimation of this value is important to obtain a good nitrogen mass balance. On the basis of this reliable mass balance, we can discuss the relationship between nitrogen and carbon conversions. Fig. 5 indicates that the conversion rate of char nitrogen to gaseous nitrogen is lower than that of carbon in the initial stage. At 700 ⬚C, only 12% of char nitrogen was converted until the 20% carbon conversion. On the other hand, the conversion rate of char nitrogen became higher than that of carbon during the final stage. The nitrogen conversion was 26% in the region of carbon conversion between 80% and 100%. The smaller conversion rate of char nitrogen in the initial stage can be reasonably explained by assuming the secondary reaction. The primary product, NO, might be captured on the carbon surface in the downstream, whereas there is no chance for the carbon in CO and CO2 to be accumulated on the carbon surface. Such nitrogen accumulation was clearly demonstrated in our previous studies on the C/NO reaction [15,16], where a pure carbon with no inherent nitrogen was used as a reactant. Even though the concentration of NO in the present study is smaller than the previous studies by one order of magnitude, the occurrence of such phenomenon is highly probable.
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N2 and N2O Formation Mechanism The first step of the gas evolution from char nitrogen is undoubtedly the formation of NO as expressed in equation 1, where C(N) and C(O) denote nitrogen and oxygen species on char surface, respectively. In the second step, N2 might be formed through the reaction between C(N) and NO (equation 2) and/or through the solid-state recombination of two C(N) species (equation 3). C(N) Ⳮ O2 → NO Ⳮ C(O)
(1)
C(N) Ⳮ NO → N2 Ⳮ C(O)
(2)
2 C(N) → N2 Ⳮ 2 C( ) (3) Fig. 7 indicates that N2 formation stopped almost instantly in the absence of O2. N2 formation via equation 2 would stop, because NO is unavailable. If reaction 3 is very rapid even in the absence of O2, N2 might keep evolving for a while in He. However, this was not the case. Only an imperceptible amount of char nitrogen was evolved as N2 in this region. It is not easy to unequivocally identify the principal N2 formation path only from the present results. However, N2 formation reaction is essentially the same as in the C/NO reaction, which has been investigated in detail. N2 formation through equation 3 is the most popular mechanism, as reviewed by Thomas [2]. This is possible in principle, but this would be rather difficult to take place, because a new N⬅N bond should be made from two separate nitrogen atoms. The average distance of two nitrogen species in char is not short enough. On the other hand, the reaction between solid-phase nitrogen and gas-phase nitrogen (reaction 2) might be easier to take place than the reaction in the solid state. Our previous studies on the C/NO reaction have presented unambiguous evidence that the reaction 2 is the predominant N2 formation route [15,16]. Thus, we presume that this route can also be the main route for N2 formation in the present char oxidation system. Another important reaction in this system is the accumulation of nitrogen by char surface (equation 4). This was clearly evidenced in the previous studies for the C/NO reaction [14–17] as well as for the char oxidation reaction [13]. C( ) Ⳮ NO → C(N) Ⳮ C(O) (4) It is natural to consider the occurrence of this reaction in the present reaction system, because we have both NO and C(). It will be interesting to know whether the inherent C(N) and newly formed C(N) are different in nature. From the present study, it is impossible to differentiate these two species. However, it can be surmised that the reactivity of newly formed C(N) would be higher than the inherent C(N), since the former is located only near char surface while the latter is distributed uniformly in the char matrix [13].
The results presented in Figs. 3, 4, and 6 can be well understood by the above mechanism. In both the temperature-programmed reaction and isothermal reaction, N2 formation rate was much higher than NO, and N2 formation starts a little after CO and CO2 formation begins. The observation can be understood, if we assume that a significant portion of NO produced via equation 1 is trapped by carbon in the downstream and the trapped nitrogen is converted to N2 via equation 2. In the isothermal reactions, the ratio of NO/N2 increased in the later stage and this ratio decreased with increasing bed height. The amount of char becomes small if the bed is shallow or if the reaction time is long. Under such a condition, the reaction between C(N) and NO becomes difficult to take place and, as a result, NO can escape from the bed without being trapped or reduced. A sudden increase in NO concentration shown in Fig. 6 near the end of reaction might be due to some sudden change of reaction atmosphere. The char ability to trap NO is likely to decrease because of carbon exhaustion. Although the formation of N2O was very little, it is important to point out the occurrence of N2O formation in the vicinities where the NO formation rate suddenly increases (Fig. 6). It has been generally accepted that the C/NO reaction produces N2O in the presence of O2 [2]. However, in the present study, N2O cannot be seen in the early stage of Fig. 6. The reason for this may be that the N2O could have been formed,but it might be quickly consumed by the reaction with char, because there is no O2 anymore in the downstream of the bed. If O2 was not completely consumed, then some N2O would appear in the product gas stream. This is what has been observed in Fig. 6. On the other hand, in the very final stage, the amount of C(N) became less and less, and therefore N2O formation rate decreased to a negligible level. This gives the reason why N2O was formed only in the limited region, as in Fig. 6.
Conclusion Almost perfect nitrogen balance was obtained during the isothermal reaction between Blair Athol char and O2. N2 was the principal product and the production of NO was rather small, while almost no N2O was observed under the present condition. The ratio of NO/N2 increased with increasing char conversion as well as with decreasing bed height due to the secondary reaction. The three-step reaction mechanism for N2 formation was proposed: (1) NO formation by the oxidation of char nitrogen; (2) NO reaction with char leading to the formation of surface nitrogen species, C(N); and (3) N2 formation by the reaction of NO with either inherent C(N) or newly formed C(N).
N2 FORMATION DURING COAL CHAR OXIDATION REFERENCES 1. Mitchell, S. C., NOx in Pulverized Coal Combustion, IEA Coal Research, London, 1998. 2. Thomas, K. M., Fuel 76:457–473 (1997). 3. Chen, S. L., Heap, M. P., Pershing, D. W., and Martin, G. B., Fuel 61:1218–1224 (1982). 4. Kasaoka, S., Sasaoka, E., and Ozaki, T., J. Fuel Soc. Jpn. 62:53–62 (1983). 5. Moritomi, H., Suzuki, Y., Kido, N., and Ogisu, Y., Proceedings of the Eleventh International Conference on FBC, 1991, pp. 1005–1011. 6. Pels, J. R., Wo´jtowicz, M. A., and Moulijn, J. A., Fuel 72:373–379 (1993). 7. Goel, S., Zhang, B., and Sarofim, A. F., Combust. Flame 104:213–217 (1996). 8. De Soete, G. G., Proc. Combust. Inst. 23:1257–1264 (1990). 9. Shimizu, T., Sawada, Y., Adschiri, T., and Furusawa, T., Fuel 71:361–365 (1992). 10. Tullin, C. J., Goel, S., Morihara, A., Sarofim, A. F., and Beer, J. M., Energy Fuels 7:796–802 (1993). 11. Krammer, G. F., and Sarofim, A. F., Combust. Flame 97:118–124 (1994).
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12. Feng, B., Liu, H., Yuan, J., Lin, Z., Liu, D., and Leckner, B., Energy Fuels 10:203–208 (1996). 13. Ashman, P. J., Haynes, B. S., Buckley, A. N., and Nelson, P. F., Proc. Combust. Inst. 27:3069–3075 (1998). 14. Suzuki, T., Kyotani, T., and Tomita, A., Ind. Eng. Chem. Res. 33:2840–2845 (1994). 15. Chambrion, Ph., Orikasa, H., Kyotani, T., and Tomita, A., Fuel 76:493–498 (1997). 16. Chambrion, Ph., Kyotani, T., and Tomita, A., Energy Fuels 12:416–421 (1998). 17. Chambrion, Ph., Orikasa, H., Suzuki, T., Kyotani, T., and Tomita, A., Proc. Combust. Inst. 27:3053–3059 (1998). 18. Noda, K., Chambrion, Ph., Kyotani, T., and Tomita, A., Energy Fuels 13:941–946 (1999). 19. Matsuoka, K., Horii, T., Chambrion, Ph., Kyotani, T., and Tomita, A., in Prospects for Coal Science in the 21st Century: Proceedings of the Tenth International Conference on Coal Science, Vol. 1 (B. Q. Li and Z. Y. Liu, eds.), Shanxi Science and Technology Press, Taiyuan, China, 1999, pp. 13–16. 20. Matsuoka, K., Horii, T., Chambrion, Ph., Kyotani, T., and Tomita, A., Carbon 38:775–778 (2000). 21. Ohtsuka, Y., Sekiyu Gakkaishi 41:182–192 (1998).
COMMENTS Lars Skaarup Jensen, F. I. Smidth & Co., A/S, Denmark. You presented three reactions (1–3) accounting mechanistically for NO formation and reduction during char combustion. Have you determined kinetic rate expressions for these reactions? Rate expressions would indicate the reactions’ relative importance and would be necessary in order to extrapolate your results to combustion conditions that are different from your experimental conditions. Author’s Reply. We have checked the effect of reactant gas flow rate on the char conversion rate and found that the O2 supply is not enough under most of our experimental conditions. Oxygen gas is completely consumed except in the very final stage. Therefore, it was impossible to determine the true chemical reaction rate. However, I completely agree with you, and the determination of kinetic rate expression should be our next subject.
● Franz Winter, Vienna University of Technology, Austria. Did you also look for HCN and HNCO? What is their relative importance? Author’s Reply. We have checked for the presence of HCN and HNCO by mass spectroscopy. We observed some HCN, but there was no evidence of HNCO. The amount of HCN, of course, depends on the reaction conditions. At 850 ⬚C, usually the ratio of N in HCN to the total N was less than 3%. Although the reliability of data for HCN by mass spectroscopy is not high enough, we think the contribution of HCN and HNCO would be no more than the above value, judging from a good nitrogen balance obtained without considering the contribution of HCN and HNCO.
MECHANISM OF N2 FORMATION DURING COAL CHAR OXIDATION TOSHIAKI AIHARA, KOICHI MATSUOKA, TAKASHI KYOTANI and AKIRA TOMITA Institute for Chemical Reaction Science Tohoku University Sendai 980-8577, Japan
In spite of many studies, the reaction mechanism of the evolution of nitrogen-containing gases during coal oxidation is not fully understood yet. One reason is that the nitrogen mass balance has not been well established in most studies. In the present study, we attempted to clarify the N2 formation mechanism during the reaction between coal char and O2 by paying special attention to nitrogen mass balance. The closure of the nitrogen mass balance was 100 Ⳳ 6%. Blair Athol coal char was used as a sample, and the formation of N-containing species was determined during temperature-programmed reaction as well as isothermal reactions at 700 and 850 ⬚C in a packed-bed reactor. In all the cases, N2 was the major product, with a little amount of NO as a minor product. Under the isothermal reaction conditions, the ratio of NO/ N2 was low in the initial stage and increased in the later stage. The effect of bed height on the product gas distribution was examined, and it was found that the shallower the bed, the higher the ratio of NO/ N2. When the O2/He gas mixture was switched to He, the formation of N2 and NO became almost negligible. From these observations, we proposed the following reaction mechanism for the N2 formation: the formation of NO as a primary oxidation product, and the reaction of NO with surface nitrogen species to form N2. The NO capture on carbon surface to form surface nitrogen species is thought to be an important step during the course of this reaction.
Introduction The understanding of NOx formation during coal combustion is one of the most important issues for a cleaner use of coal [1,2]. Nitrogen in coal is converted to volatile nitrogen and char nitrogen upon pyrolysis. The gas phase reaction of volatile nitrogen is well understood, but in spite of many studies, the reaction mechanism of the evolution of nitrogencontaining gases from char nitrogen is not fully understood yet. Boldly speaking, most researchers are interested in the formation of toxic NOx, but not in the non-toxic N2 gas. Thus, the combustion experiments were frequently made with O2/N2 mixture without considering the formation of N2 gas [3–7]. Even when an O2/Ar or O2/He mixture was used as a reactant gas, many researchers did not find it important to consider the nitrogen mass balance [8– 12], except for a recent study by Ashman et al. who made a detailed product analysis including N2 [13]. We have been engaged in the research on the reactions of pure carbon with NO and N2O by putting emphasis on the nitrogen mass balance. We found that the detailed examination of nitrogen mass balance is very important to elucidate the reaction mechanism [14–18]. For example, we could quantify the amount of nitrogen species accumulated during C/NO reaction, and using this information we found the important role of the surface species in the formation of N2 gas [15–17].
In the present study, we attempt to clarify the reaction mechanism of the N2 formation during O2oxidation of char. Although the present reaction conditions are far from the conditions practically used in the pulverized coal combustion system, the fundamental knowledge of the relevant reactions is always important in understanding the practical system. The reaction pathways are discussed by referring to the C/NO reaction mechanism that was revealed in our previous studies. The following three steps are considered: (1) formation of NO as a result of oxidation of char nitrogen, (2) the reaction of NO formed in step 1 with the char generating new nitrogen surface species, and (3) the reaction between the surface nitrogen species and the gas-phase NO, releasing N2 gas as a final product. Experimental Method Sample Blair Athol coal, Australian bituminous coal, was selected for the present investigation. Its analysis on dry basis is C: 75.6, H: 4.3, N: 1.7, O (by diff.): 9.3, ash: 8.8%. Ash is composed of 64% SiO2, 32% Al2O3, 1.6% TiO2, 1.2% Fe2O3, 0.3% CaO, and others. The char was prepared by devolatilizing the coal at 1000 ⬚C for 30 min in a small fluidized bed reactor made of quartz with an inner diameter of 16 mm. Coal
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washed with water. The residue was pyrolyzed as above. Apparatus The reactor used in this study was a fixed bed reactor as shown in Fig. 1. Gaseous products were simultaneously analyzed both by a high-speed gas chromatograph (GC) (Aera M200) and by a mass spectrometer (MS) (Anelva AQA 200). MS was mainly used for NO detection. The GC detector was a thermal conductivity detector (TCD), and it was equipped with a MS-5A column for O2, N2, and CO separation and a PoraPlot Q column for CO2 and N2O separation. Experimental Procedure
Fig. 1. Schematic diagram of apparatus.
Fig. 2. Temperature-programmed gas evolution from coal char in He. Heating rate: 5 ⬚C/min.
particles with size between 70 and 150 lm was first fluidized with He gas at ambient temperature without adding any inert particles as a bed material, and then the reactor was put into the heated furnace. The analysis of char obtained was C: 83.0, H: 0.6, N: 1.5, O (by diff.): 4.1, ash: 10.8% on dry basis. In order to check the effect of mineral matter on NO and N2 formation behavior, demineralization of the above coal was carried out by the conventional HF and HCl treatment method. The coal was first treated with aqueous HF solution for 24 h and washed with deionized water. It was then treated with 6N HCl at 60 ⬚C for 12 h and thoroughly
Under a standard experimental condition, about 100 mg of char sample (75–150 lm) was used, which corresponded to a sample bed height of 4 mm. The gas used for oxidation was 0.55% or 2.0% O2 diluted with He at an atmospheric pressure, and the total gas flow rate was 200 mL/min unless otherwise stated. Experiments were also carried out to check the effects of sample particle size, gas flow rate, and bed height on the oxidation reaction. Prior to all the reactions, the char was again heat-treated in He at 1000 ⬚C for 30 min to remove the surface adsorbed species. This procedure is important to attain a perfect nitrogen balance, because the uptake of N2 gas on carbon has been observed when carbon is stored in an ambient atmosphere [19,20]. After the heat treatment, the sample was cooled down to 300 ⬚C, and temperature-programmed reactions were carried out in an O2/He mixture. The sample was heated at a constant heating rate of 5 ⬚C/min up to 1000 ⬚C, where it was held for 30 min. For isothermal reactions, the pretreated char was heated to 700 or 850 ⬚C in pure He before switching over to the reactant gas. A step response experiment was run at 700 ⬚C by switching O2/He mixture to He, and the outlet gas was analyzed to determine the desorption profile of surface species. Results N2 Uptake at Ambient Atmosphere We recently found that a significant amount of N2 is trapped on coal char from ambient atmosphere during storage, which makes it difficult to obtain an accurate nitrogen mass balance in the subsequent process [19]. Therefore, we first determined the amount of N2 trapped from ambient atmosphere in the present coal char sample. Fig. 2 shows an example of the gas evolution profile during heating in
N2 FORMATION DURING COAL CHAR OXIDATION
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Fig. 3. Temperature-programmed reaction profile in the char/O2 reaction. O2 concentration in He: 0.55%; heating rate: 5 ⬚C/min. The concentration of NO measured by MS and that of the other gases measured by GC are indicated by a bold line and the symbols, respectively.
He. A substantial amount of N2 gas was detected along with CO and CO2 gases. The N2 desorbed in the low temperature range was not from the decomposition of nitrogen surface complex in coal char but due to the release of trapped N2. The total amount of nitrogen evolved as N2 was around 0.35 mg atom/ g char, which corresponds to 30% of nitrogen content determined by the elemental analysis. This implies that the trapped N2 cannot be neglected in the mass balance calculation. After this heat treatment, char was used for the subsequent reaction without exposing to air. Temperature-Programmed Reaction The gas evolution profile during the temperatureprogrammed reaction with O2 is illustrated in Fig. 3, where the symbols indicate the gas concentration determined by GC, while a thick line without any symbol indicates the concentration of NO measured by MS. The beginning of O2 consumption at about 550 ⬚C is marked by the evolution of an equal amount of CO and CO2. As the temperature increased to 650 ⬚C, CO evolution became greater than CO2. Among the nitrogen-containing species, N2 was the predominant one, and it appeared at a temperature higher than the evolution temperature of CO and CO2 by 60 ⬚C. This implies the unbalance between carbon conversion and nitrogen conversion. Under the present conditions, O2 gas was almost completely consumed above 700 ⬚C, and it appeared again after reaching 1000 ⬚C. At this point, the char conversion was around 60%, and it became 85% after 30 min holding at 1000 ⬚C. During this isothermal stage at 1000 ⬚C, the concentration of N2 decreased while that of NO increased. N2O formation was almost negligible throughout the reaction.
Fig. 4. Isothermal reaction of char with 2% O2/He. Temperature: (a) 700 ⬚C, (b) 850 ⬚C.
Isothermal Reactions Figure 4a shows the result of isothermal reactions at 700 ⬚C. The concentration of O2 in the outlet gas sharply decreased to about 0.1% in the first 5 min, and then gradually increased toward the original concentration level. The evolution of CO, CO2, and NO takes place immediately upon O2 introduction. The concentration of CO and CO2 gradually decreased, while that of NO kept more or less constant until the final stage. On the other hand, the initial N2 formation profile was quite different from the above product gases. The N2 formation rate slowly increased and reached the maximum at 20 min. This suggests that the N2 formation mechanism is different from the others. The formation rate of N2 was always higher than that of NO, except in the final stage where they became nearly equal to each other. The result of the isothermal reaction at 850 ⬚C is shown in Fig. 4b. The reaction rate was faster than at 700 ⬚C, and thus O2 was totally consumed in the first 15 min. The principal N-containing product was again N2, with some NO and a very little amount of N2O. The consumption rate of char was so fast that the concentration of unreacted O2 rapidly increased after 30 min, making the atmosphere highly oxidative. It is noteworthy that the gas composition suddenly changed at around 23 min, where the concentration of CO sharply decreased while CO2 and NO
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Fig. 5. Nitrogen conversion versus carbon conversion. Nitrogen evolved during the pre-heat-treatment stage was not included in this balance. O2 concentration: 2%; temperature: 700 ⬚C and 850 ⬚C.
Fig. 6. Effect of bed height on the formation of nitrogen-containing gases. Bed height: (a) 2 mm, (b) 4 mm, (c) 8 mm; O2 concentration: 2%; temperature: 850 ⬚C.
increased. This behavior is observed for all the reactions carried out at 850 ⬚C. In the experiment shown in Fig. 4b, where the total gas flow rate was 200 mL/min, all O2 gas was consumed in the initial 15 min at 850 ⬚C. This means that the reaction rate was controlled by O2 gas supply. In fact, when the gas flow rate was increased, the conversion rate increased until a total flow rate of 600 mL/min. The effect of particle size was examined at a total flow rate of 200 mL/min, and it was found that the effect was very small in the range of 0.04 to 1.0 mm. This is quite natural if the reaction is controlled by O2 gas supply. The detailed mass balance was established during these isothermal reactions. In Fig. 5, the nitrogen conversion is plotted against the carbon conversion. The curves deviate slightly from the 1:1 line. The deviation looks rather small, but the carbon consumption in the initial stage is in some cases twice as much as the nitrogen consumption. This deviation is due to either the smaller reactivity of nitrogen species compared with carbon or the higher accumulation rate of once-formed gaseous nitrogen species on carbon surface compared with those of CO and CO2, as suggested by Ashman et al. [13]. In order to check the possibility of secondary reactions, the gas formation behavior with different bed heights was examined. Fig. 6 shows the gas evolution profile only for nitrogen-containing species for different bed heights. In all the cases, the formation rate of N2 is higher than NO until in the later stages where NO formation increased and became somewhat larger than N2. It is worthwhile to point out that NO concentration increased sharply near the end of reaction; 10, 25, and 60 min for Fig. 6a, 6b, and 6c, respectively. These points roughly correspond to the appearance of O2 in the outlet gas. Interestingly, evolution of a small amount of N2O was also detected in all three cases around these times. For the purpose of modifying the extent of secondary reaction, preliminary experiments were carried out by diluting the char with inert quartz sand. By keeping the bed height constant at 10 mm, the ratio of quartz/char was varied from 4 to 60 by weight. With increasing extent of dilution, the N2/ NO ratio decreased from 3 to 1. The dilution of char particles might suppress the secondary reaction of NO gas with char. The amounts of N2, NO, and N2O evolved during the isothermal reaction stage, together with N2 evolved during the heat-treatment stage, are summarized in Table 1. The amount of total nitrogen was very close to that determined by elemental analysis. In Table 1, the ratio of NO to the total nitrogen obtained in the burn-off stage is also listed. The effect of bed height is clearly seen. When the sample bed was high, relatively more N2 and less NO was formed from char nitrogen.
N2 FORMATION DURING COAL CHAR OXIDATION TABLE 1 Nitrogen Mass Balance During Char Oxidation with Different Bed Height Bed Height (mm) Evolved Gas (mmol/g char) Heat-treatment stage N as N2 Burn-off stage N as N2 N as NO N as N2O Total N from both stages Total N from elemental analysis N as NO/N from burn-off stage
2
4
9
0.34
0.35
0.37
0.61 0.17 0 1.12 1.2 0.22
0.65 0.15 0 1.15 1.2 0.19
0.67 0.08 0 1.12 1.2 0.11
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Effect of Mineral Matter Since it has been reported that some inorganic species, particularly Fe and Ca compounds, affect the reaction profile of N2 removal from coal char [21], we checked this effect by comparing the present result with that of demineralized Blair Athol char. The demineralization treatment reduced the ash content in the pyrolyzed char to 0.2%. The temperature-programmed reaction profile of this demineralized char was very similar to that presented in Fig. 3. Thus, at least for this particular system, the catalytic effect by mineral matter is not significant. Perhaps mineral matter in Blair Athol coal is not catalytically active.
Discussion Nitrogen Mass Balance
Fig. 7. Step response experiment. O2 concentration: 2%; temperature: 700 ⬚C.
Step Response Experiments In order to clarify the mechanism of N2 formation, a step response experiment was carried out by switching the reactant gas from O2/He mixture to He and then back to O2/He. The result is shown in Fig. 7. Up to 20 min, the gas evolution profile is naturally similar to that in Fig. 4a. At 20 min, the gas was switched from O2/He to He. The formation of N2, NO, CO, and CO2 almost terminated immediately. If we look at the result more closely, we can see a decay profile of the gases. Among all the gases, the largest evolution was seen with CO. At 30 min, the concentration of CO was 0.005%, which corresponds to four-thousandth of the concentration before switching. The evolution of CO2 became negligible at 25 min. Although a slow decay of N2 was observed until 28 min, the total amount of nitrogen observed as N2 during this period was only 0.32 lmol. This is much smaller than the estimated amount of char nitrogen at 20 min, 51 lmol.
As shown in Table 1, the nitrogen mass balance for the reaction at 850 ⬚C was fairly good. The result at 700 ⬚C is not shown in the Table 1, but the agreement was also good. In the present study, the calculation of N2 mass balance was carried out by including the amount of N2 trapped in coal char during the storage in an ambient atmosphere. This amount is assumed to be equal to N2 evolved during the heat-treatment stage, as shown in the first row of Table 1. This is not negligibly small, and the estimation of this value is important to obtain a good nitrogen mass balance. On the basis of this reliable mass balance, we can discuss the relationship between nitrogen and carbon conversions. Fig. 5 indicates that the conversion rate of char nitrogen to gaseous nitrogen is lower than that of carbon in the initial stage. At 700 ⬚C, only 12% of char nitrogen was converted until the 20% carbon conversion. On the other hand, the conversion rate of char nitrogen became higher than that of carbon during the final stage. The nitrogen conversion was 26% in the region of carbon conversion between 80% and 100%. The smaller conversion rate of char nitrogen in the initial stage can be reasonably explained by assuming the secondary reaction. The primary product, NO, might be captured on the carbon surface in the downstream, whereas there is no chance for the carbon in CO and CO2 to be accumulated on the carbon surface. Such nitrogen accumulation was clearly demonstrated in our previous studies on the C/NO reaction [15,16], where a pure carbon with no inherent nitrogen was used as a reactant. Even though the concentration of NO in the present study is smaller than the previous studies by one order of magnitude, the occurrence of such phenomenon is highly probable.
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N2 and N2O Formation Mechanism The first step of the gas evolution from char nitrogen is undoubtedly the formation of NO as expressed in equation 1, where C(N) and C(O) denote nitrogen and oxygen species on char surface, respectively. In the second step, N2 might be formed through the reaction between C(N) and NO (equation 2) and/or through the solid-state recombination of two C(N) species (equation 3). C(N) Ⳮ O2 → NO Ⳮ C(O)
(1)
C(N) Ⳮ NO → N2 Ⳮ C(O)
(2)
2 C(N) → N2 Ⳮ 2 C( ) (3) Fig. 7 indicates that N2 formation stopped almost instantly in the absence of O2. N2 formation via equation 2 would stop, because NO is unavailable. If reaction 3 is very rapid even in the absence of O2, N2 might keep evolving for a while in He. However, this was not the case. Only an imperceptible amount of char nitrogen was evolved as N2 in this region. It is not easy to unequivocally identify the principal N2 formation path only from the present results. However, N2 formation reaction is essentially the same as in the C/NO reaction, which has been investigated in detail. N2 formation through equation 3 is the most popular mechanism, as reviewed by Thomas [2]. This is possible in principle, but this would be rather difficult to take place, because a new N⬅N bond should be made from two separate nitrogen atoms. The average distance of two nitrogen species in char is not short enough. On the other hand, the reaction between solid-phase nitrogen and gas-phase nitrogen (reaction 2) might be easier to take place than the reaction in the solid state. Our previous studies on the C/NO reaction have presented unambiguous evidence that the reaction 2 is the predominant N2 formation route [15,16]. Thus, we presume that this route can also be the main route for N2 formation in the present char oxidation system. Another important reaction in this system is the accumulation of nitrogen by char surface (equation 4). This was clearly evidenced in the previous studies for the C/NO reaction [14–17] as well as for the char oxidation reaction [13]. C( ) Ⳮ NO → C(N) Ⳮ C(O) (4) It is natural to consider the occurrence of this reaction in the present reaction system, because we have both NO and C(). It will be interesting to know whether the inherent C(N) and newly formed C(N) are different in nature. From the present study, it is impossible to differentiate these two species. However, it can be surmised that the reactivity of newly formed C(N) would be higher than the inherent C(N), since the former is located only near char surface while the latter is distributed uniformly in the char matrix [13].
The results presented in Figs. 3, 4, and 6 can be well understood by the above mechanism. In both the temperature-programmed reaction and isothermal reaction, N2 formation rate was much higher than NO, and N2 formation starts a little after CO and CO2 formation begins. The observation can be understood, if we assume that a significant portion of NO produced via equation 1 is trapped by carbon in the downstream and the trapped nitrogen is converted to N2 via equation 2. In the isothermal reactions, the ratio of NO/N2 increased in the later stage and this ratio decreased with increasing bed height. The amount of char becomes small if the bed is shallow or if the reaction time is long. Under such a condition, the reaction between C(N) and NO becomes difficult to take place and, as a result, NO can escape from the bed without being trapped or reduced. A sudden increase in NO concentration shown in Fig. 6 near the end of reaction might be due to some sudden change of reaction atmosphere. The char ability to trap NO is likely to decrease because of carbon exhaustion. Although the formation of N2O was very little, it is important to point out the occurrence of N2O formation in the vicinities where the NO formation rate suddenly increases (Fig. 6). It has been generally accepted that the C/NO reaction produces N2O in the presence of O2 [2]. However, in the present study, N2O cannot be seen in the early stage of Fig. 6. The reason for this may be that the N2O could have been formed,but it might be quickly consumed by the reaction with char, because there is no O2 anymore in the downstream of the bed. If O2 was not completely consumed, then some N2O would appear in the product gas stream. This is what has been observed in Fig. 6. On the other hand, in the very final stage, the amount of C(N) became less and less, and therefore N2O formation rate decreased to a negligible level. This gives the reason why N2O was formed only in the limited region, as in Fig. 6.
Conclusion Almost perfect nitrogen balance was obtained during the isothermal reaction between Blair Athol char and O2. N2 was the principal product and the production of NO was rather small, while almost no N2O was observed under the present condition. The ratio of NO/N2 increased with increasing char conversion as well as with decreasing bed height due to the secondary reaction. The three-step reaction mechanism for N2 formation was proposed: (1) NO formation by the oxidation of char nitrogen; (2) NO reaction with char leading to the formation of surface nitrogen species, C(N); and (3) N2 formation by the reaction of NO with either inherent C(N) or newly formed C(N).
N2 FORMATION DURING COAL CHAR OXIDATION REFERENCES 1. Mitchell, S. C., NOx in Pulverized Coal Combustion, IEA Coal Research, London, 1998. 2. Thomas, K. M., Fuel 76:457–473 (1997). 3. Chen, S. L., Heap, M. P., Pershing, D. W., and Martin, G. B., Fuel 61:1218–1224 (1982). 4. Kasaoka, S., Sasaoka, E., and Ozaki, T., J. Fuel Soc. Jpn. 62:53–62 (1983). 5. Moritomi, H., Suzuki, Y., Kido, N., and Ogisu, Y., Proceedings of the Eleventh International Conference on FBC, 1991, pp. 1005–1011. 6. Pels, J. R., Wo´jtowicz, M. A., and Moulijn, J. A., Fuel 72:373–379 (1993). 7. Goel, S., Zhang, B., and Sarofim, A. F., Combust. Flame 104:213–217 (1996). 8. De Soete, G. G., Proc. Combust. Inst. 23:1257–1264 (1990). 9. Shimizu, T., Sawada, Y., Adschiri, T., and Furusawa, T., Fuel 71:361–365 (1992). 10. Tullin, C. J., Goel, S., Morihara, A., Sarofim, A. F., and Beer, J. M., Energy Fuels 7:796–802 (1993). 11. Krammer, G. F., and Sarofim, A. F., Combust. Flame 97:118–124 (1994).
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12. Feng, B., Liu, H., Yuan, J., Lin, Z., Liu, D., and Leckner, B., Energy Fuels 10:203–208 (1996). 13. Ashman, P. J., Haynes, B. S., Buckley, A. N., and Nelson, P. F., Proc. Combust. Inst. 27:3069–3075 (1998). 14. Suzuki, T., Kyotani, T., and Tomita, A., Ind. Eng. Chem. Res. 33:2840–2845 (1994). 15. Chambrion, Ph., Orikasa, H., Kyotani, T., and Tomita, A., Fuel 76:493–498 (1997). 16. Chambrion, Ph., Kyotani, T., and Tomita, A., Energy Fuels 12:416–421 (1998). 17. Chambrion, Ph., Orikasa, H., Suzuki, T., Kyotani, T., and Tomita, A., Proc. Combust. Inst. 27:3053–3059 (1998). 18. Noda, K., Chambrion, Ph., Kyotani, T., and Tomita, A., Energy Fuels 13:941–946 (1999). 19. Matsuoka, K., Horii, T., Chambrion, Ph., Kyotani, T., and Tomita, A., in Prospects for Coal Science in the 21st Century: Proceedings of the Tenth International Conference on Coal Science, Vol. 1 (B. Q. Li and Z. Y. Liu, eds.), Shanxi Science and Technology Press, Taiyuan, China, 1999, pp. 13–16. 20. Matsuoka, K., Horii, T., Chambrion, Ph., Kyotani, T., and Tomita, A., Carbon 38:775–778 (2000). 21. Ohtsuka, Y., Sekiyu Gakkaishi 41:182–192 (1998).
COMMENTS Lars Skaarup Jensen, F. I. Smidth & Co., A/S, Denmark. You presented three reactions (1–3) accounting mechanistically for NO formation and reduction during char combustion. Have you determined kinetic rate expressions for these reactions? Rate expressions would indicate the reactions’ relative importance and would be necessary in order to extrapolate your results to combustion conditions that are different from your experimental conditions. Author’s Reply. We have checked the effect of reactant gas flow rate on the char conversion rate and found that the O2 supply is not enough under most of our experimental conditions. Oxygen gas is completely consumed except in the very final stage. Therefore, it was impossible to determine the true chemical reaction rate. However, I completely agree with you, and the determination of kinetic rate expression should be our next subject.
● Franz Winter, Vienna University of Technology, Austria. Did you also look for HCN and HNCO? What is their relative importance? Author’s Reply. We have checked for the presence of HCN and HNCO by mass spectroscopy. We observed some HCN, but there was no evidence of HNCO. The amount of HCN, of course, depends on the reaction conditions. At 850 ⬚C, usually the ratio of N in HCN to the total N was less than 3%. Although the reliability of data for HCN by mass spectroscopy is not high enough, we think the contribution of HCN and HNCO would be no more than the above value, judging from a good nitrogen balance obtained without considering the contribution of HCN and HNCO.