Energy Fuels 2010, 24, 6307–6313 Published on Web 11/12/2010
NOx Emission of Fine- and Superfine- Pulverized Coal Combustion in O2/CO2 Atmosphere Xiumin Jiang,* Xiangyong Huang, Jiaxun Liu, and Xiangxin Han Institute of Thermal Energy Engineering, School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai, 200240, P. R. China Received August 6, 2010. Revised Manuscript Received October 25, 2010
The effects of particle size, stoichiometric ratio (λ), atmosphere, temperature, and recycled NO on the emissions of three fractional nitrogens, [N]N2O, [N]NO, and [N]NO2, during the combustion of superfine pulverized coal in O2/CO2 atmosphere were investigated in the present study. NO2 has shown little contribution to NOx compared with N2O and NO in all the cases. As the stoichiometric ratio increases, the trend is much similar in CO2/O2 and N2/O2. However, the conversion ratio from fuel-N to NOx in CO2/O2 atmosphere is less than that in N2/O2 atmosphere especially at λ >1.2. [N]NO makes up the largest portion of [N]NOx at λ>1, and [N]N2O dominates at λ 1, ∼90% at λ > 1.4, which is consistent with the (12) Kramlich, J. C.; Linak, W. P. Prog. Energy Combust. Sci. 1994, 20, 149–202. (13) Goel, S.; Morihara, A.; Tullin, C. J.; Sarofim, A. F. Proc. Combust. Inst. 1994, 25, 1051–1059. (14) Goel, S.; Morihara, A.; Tullin, C. J.; Sarofim, A. F. Symp. (Int.) Combust. 1994, 25 (1), 1051–1059.
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Figure 5. Effect of atmosphere on NOx evolution (NMG coal).
were opened. The condition thus becomes favored for the nitrogen converted to NO but unfavorable for the N2O formation and NOx reduction. Although [N]N2O decreases as R increases, it could be seen that [N]NO increases obviously and its growth rate surpasses the reducing rate of [N]N2O, which results in the conversion ratio increases with the stoichiometric ratio. 3.3. Effects of Atmosphere. The effects of atmosphere at various stoichiometric ratios were studied. It is clearly indicated in Figure 5 that the trend of NOx evolution is much similar in both atmospheres, CO2/O2 and N2/O2, as the stoichiometric ratio increases. [N]NO2 shows little difference in both atmospheres. The conversion ratio in CO2/O2 atmosphere is less than that in N2/O2 atmosphere especially at λ>1.2 which has been reported previously.15 Although the experiments conducted at the furnace temperature of 1100 °C at which thermal NOx seems to not easily form, actually the furnace controlled parameter cannot instantly reflect the complex temperature field in the tube when the coal combustion occurs. In fact, the temperature field in the tube is not uniform due to the strong mixing of reactants and complex reactions. It is likely that the peak temperature at some inner region of the tube exceeds the preheated temperature during the combustion process and it thus provides the possibility for the formation of thermal NOx and prompt NOx in N2/O2 atmosphere. On the other hand, a lower adiabatic flame temperature and milder combustion result from the substitution of N2 with CO2 and thus the nitrogen conversion process has been influenced. However, [N]N2O in CO2/O2 atmosphere is higher than that in N2/O2 atmosphere, especially at λ e 1.2. It should be noted that the CO2-char gasification in CO2/O2 atmosphere is much more intense than that in N2/O2. The conversion of NO to N2O is promoted in the presence of significant CO because char catalyzes reduction of NO by CO.16,17 Another potential reason is the elevated concentration of CO2 influences the radical pool of the combustion process,18,19which has significant effects on the nitrogen conversion mechanism. 3.4. Effects of Particle Size. From Figure 6, it could be concluded that the particle size has a significant effect on
NOx emissions. It is clearly indicated that the contribution of [N]NO2 is relatively small and the value remains stable. For the three coals, [N]N2O increases as the mean particle size increases while [N]NO shows the opposite trend. Researchers have recognized that as the important precursors of NOx, HCN, and NH3 arise from secondary tar cracking reactions after primary devolatilization. Thus it needs time for the fuel nitrogen to release from the solid matrix and react with oxidants to finally form NOx. Smaller particles accelerate this process, and the volatile nitrogen of smaller particles release earlier in the combustion chamber and thus experience longer time in combustion conditions, which increases the possibility of conversion of volatile nitrogen to NO. Moreover, the concentration of volatile matter in the medial or rear part of the quartz tube is lower for coal of smaller particle size due to the intent release and combustion of volatile matter at the front of the tube. Consequently, less NOx is reduced through homogeneous reactions. Another potential reason may be that with longer evolution and distance and greater heating resistance, coarser particles retard the release of volatile matter including the volatile nitrogen. As a result, for coarser particles it is hard to ignite and burn out, accompanied with more nitrogen remaining in the char and less fuel nitrogen converted. The delayed volatile matter then releases at the rear part of the tube to form a reductive region, similar to the “reburning” scheme. As a result of insufficient oxidation, less evolved HCN and NH3 seems be successfully converted to NO. In addition, it has been found that the nitrogen in the char is largely oxidized to NO for small particles and as the particle size increases, reaction of char with NO may serve to efficiently remove NO and reduce the char-N to NO conversion efficiency.20 Under the competition of N2O and NO emissions, the conversion ratio has a special trend of descend first then ascend, reaching the minimum at the particle size range of 15-25 μm, though the conversion ratio at a particle size below 15 μm is much higher than that at a particle size above 20 μm for TF and SH coals. An obvious difference can be observed between NMG and the other two coals on N2O. It can be seen from Figure6a that the [N]N2O of NMG is more significant than that of TF and SH, especially for coarser coal particles. For NMG, [N]NO predominates when the particle size is less than 20 μm while [N]N2O becomes close to or even greater than [N]NO as the
(15) Buhre, B. J. P.; Elliott, L. K.; Sheng, C. D.; Gupta, R. P.; Wall, T. F. Prog. Energy Combust. Sci. 2005, 31, 283–307. (16) Aarna, I.; Suuberg, E. M. Fuels 1997, 76 (6), 475–491. (17) Aarna, I.; Suuberg, E. M. Energy Fuels 1999, 13 (6), 1145–1153. (18) Normann, F.; Andersson, K.; Leckner, B.; Johnsson, F. Proc. Combust. Inst. 2009, 35 (5), 385–397. (19) Liu, F.; Guo, H.; Smallwood, J. S. Combust. Flame 2003, 133 (4), 495–498.
(20) Glarborg, P.; Jensen, A. D.; Johnsson, J. E. Proc. Combust. Inst. 2003, 29 (2), 89–113.
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Figure 6. Effect of particle size on NOx evolution.
Figure 7. Variations of CO and CH4 at different particle sizes.
particle size is greater than 20 μm. The contents of CO and CH4 shown in Figure 7 provide a valuable clue. The concentrations of CO of NMG are almost one order higher than that of TF and SH, and its CH4 concentrations is also obviously higher. Interestingly, the trends of CO and CH4 are exactly opposite to that of NO but in accordance with that of N2O. The major source of N2O appears to be oxidation in the gas phase of HCN through eqs 5 and 6.3 The underlying reason can be roughly deduced from the coincidence that the NO reduction is greatly enhanced at the conditions, and as the intermediate product in the NO transformation routes, N2O failed to be finally reduced to N2 within the limited residue time, although the exact
mechanism still remains an unrevealed research subject. Moreover, Payne et al.21 believe that fuel volatility influences the evolution of radical species. For NMG, a higher content of volatile matter may create different atmospheres for nitrogen conversion to N2O. 3.5. Effects of Temperature. An increase in temperature has complex impacts on NOx conversion.2 Under higher temperature, more volatiles are produced which increases nitrogen evolution and the oxidation of nitrogen into NO but char reduction are also accelerated. On the other hand, more volatiles evolve more rapidly in the early stage of combustion and may advance the reduction of NOx. From Figure 8 it can be seen that as the temperature goes up, [N]NO shows an opposite trend as [N]N2O, the former increases and the latter decreases, which results in that conversion ratio increases
(21) Payne, R.; Moyeda, D. K.; Maly, P.; Glavicic, T.; Weber, B. Proc. EPRI/EPA Joint Symp. Stationary Combust. NOx Control 1995, 4, 16–19.
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Figure 8. Effect of temperature on NOx evolution.
Figure 9. Effect of recycled NO on NOx evolution and reduction rate.
slightly. It is generally accepted that nitrous oxide emissions can be significant in fluidized bed combustion largely due to the low or moderate temperature during combustion but are negligible in most conventional combustion systems. It is found that at low temperatures, copious pyrolysis gases, including hydrocarbon and nitrogen containing species such as HCN, NH3, and HNCO, evolved, in particular for low rank coals. However, the species cannot be rapidly consumed by the oxidant due to the relative slow reaction rate at the low temperature. Therefore, the significant conversion of fuel nitrogen to N2O occurs at the reducing atmosphere.12,22 On the other hand, the decomposition and the
oxidation by O2 of N2O are intensified by the increase in temperature. Therefore, it can be easily observed that the conversion of N2O decreases with increasing bed temperatures while the reverse trend was obtained for NO conversion, similar to previous studies.23-26 It is interesting that the [N]N2O and conversion ratio of NMG are less than that of TF (23) Tullin, C. J.; Goel, S.; Morihara, A.; Sarofim, A. F.; Beer, J. M. Energy Fuels 1993, 7 (6), 796–802. (24) Hayhurst, A. N.; Lawrence, A. D. Combust. Flame 1996, 105 (3), 341–357. (25) W ojtowicz, M. A.; Pels, J. R.; Moulijn, J. A. Fuel 1994, 73 (9), 1461–1421. (26) Pels, J. R.; W ojtowicz, M. A.; Moulijn, J. A. Fuel 1993, 72 (3), 373–378.
(22) Aemand, L. E.; Leckner, B. Energy Fuels 1993, 7 (6), 1097–1107.
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at lower temperature (below 900 °C) but more at higher temperature (above 900 °C). This is due to the fact that preferential retention of nitrogen at low temperatures is greater for low rank coals than for coals of higher rank.27 However, as the temperature increases the nitrogen in low rank coals is easier to evolve in the form of light nitrogen containing species than that in high rank coals because their volatile-N consists of significant tarry compounds. 3.6. Effects of Concentration of Recycled NO. The formula of the conversion ratio is a little different in this section from the previous definition, considering the existence of recycled NO. It is calculated from the net converted nitrogen (subtracting recycled nitrogen of recycled NO from the nitrogen of NOx in exhaust gas) divided by the input coalN, i.e., ([N]NOx - [N]recyled NO)/coal-N. The results presented in Figure 9 clearly indicate that the nitrogen of NOx increases as recycled NO increases. However, the conversion ratio decreases as recycled NO increases. With consistent coal samples and experimental conditions, it could be deduced some NOx may be reduced or fixed in char at high NOx concentration with the existence of the recycled NO. The reduction rate is calculated from the difference of nitrogen in exhaust NOx between with and without recycled NO conditions divided by the nitrogen of recycled NO, i.e., ([N]NOx,CO2/O2/NO - [N]NOx,CO2/O2)/[N]recyled NO. From Figure 9c, it can be seen that the reduction rate increases as the concentration of recycled NO increases for both coals. This is consistent with others’ reports.2 It was believed that recycled NO is destroyed in the flame through its reactions with hydrocarbon radicals in the form of CHi, and reduction reactions occur between recycled NOx and fuel-N. However, [N]N2O increases as recycled NO increases. From the view of reaction kinetics, the increase in NO concentration accelerates eqs 6 and 7 and also promotes the conversion of char-N to N2O.28 It is noteworthy that the reduction rates of NMG’s cases are higher than that of TF’s cases, above 12% vs below 12% due to their difference in volatility and char reactivity.
superfine-pulverized coal under oxy-fuel condition. On the basis of the experiments, the following conclusions are drawn: (1) Low yield of NO2 was detected in the exhaust gas and N2O and NO were the major contributor of NOx in all the cases in CO2/O2 atmosphere. [N]NO makes up the largest portion of [N]NOx at λ >1 and [N]N2O dominates at λ1.2 due to the lower adiabatic flame temperature and the absence of thermal NOx and prompt NOx in the oxy-fuel condition. However, [N]N2O in CO2/O2 atmosphere is higher than that in N2/O2 atmosphere especially at λ e 1.2 resulting from the presence of significant CO and a high concentration of CO2. (3) For the three coals, [N]N2O increases as the mean particle size increases while [N]NO shows the opposite trend. As a result, there exist a minimum for conversion ratio at the particle size range of 15-25 μm under the combined effects of [N]N2O and [N]NO. [N]N2O of NMG is more significant than that of TF and SH, especially for coarser coal particles, which is explained by enhanced NO reduction, and N2O failed to be finally reduced to N2 within the limited residue time. (4) As temperature goes up, [N]NO and the conversion ratio increase because at higher temperature more volatiles are produced which increases nitrogen evolution and the oxidation of nitrogen into NO. However, [N]N2O decreases obviously, which could be explained by the existence of copious hydrocarbon and nitrogen containing species that are favored for the conversion of fuel nitrogen to N2O at low temperature while the decomposition and the oxidation by O2 of N2O are intensified by the increase in temperature. (5) The conversion ratio decreases while the reduction rate increases as recycled NO increases. Recycled NO is destroyed in the flame through its reactions with hydrocarbon radicals in the form of CHi, and reduction reactions occur between recycled NOx and fuel-N. From the view of reaction kinetics, the increase in NO concentration accelerates the formation reactions of N2O and also promotes the conversion of char-N to N2O.
4. Conclusions The fractional nitrogen in the forms of N2O, NO, and NO2 were evaluated in detail during the combustion of fine- and (27) Baxter, L. L.; Mitchell, R. E.; Fletcher, T. H.; Hurt, R. H. Energy Fuels 1996, 10 (1), 188–196. (28) Krammer, G. F.; Sarofim, A. F. Combust. Flame 1994, 97 (1), 118–124.
Acknowledgment. The authors gratefully acknowledge the financial supports by the National Natural Science Foundation of China (Grant No. 50876060).