ARTICLE pubs.acs.org/EF
Investigation of NOx Emissions for Superfine Pulverized Coal in Air-Staging Combustion Jun Shen,† Xiumin Jiang,*,† Jiaxun Liu,†,‡ Xiangyong Huang,† and Hai Zhang† †
Institute of Thermal Energy Engineering, School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China ‡ School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, People’s Republic of China ABSTRACT: Superfine pulverized coal combustion is a new pulverized coal combustion technology that has better combustion stability, higher combustion efficiency, and comprehensive cost-effective operation. The novelty of this present paper is that fundamental experiments on an electrically heated drop-tube furnace were carried out to understand the NOx emissions of airstaging combustion for two superfine pulverized bituminous coals used in China for the first time. The results indicate that highvolatile-containing Neimenggu (NMG) coal possesses better effectiveness of NOx abatement than low-volatile-containing Shenhua (SH) coal. Interesting saddle-point effects of the highest NOx emissions have been found for both coals around the average particle size of 17.44 μm for SH coal and ∼30 μm for NMG coal. For different stoichiometric ratios and positions of over fire air (OFA) ports, NMG coal provides a higher ability of deNOx efficiency than SH coal. The superfine pulverized coal combustion of the NMG_25.86 μm particle can even reach the highest deNOx efficiency up to 70%. The findings of this paper will provide guidance for further studies on the NOx emission characteristics of superfine pulverized coal combustion.
1. INTRODUCTION The world is gradually facing an increasing need for electrical power, especially in China.1 Despite the emerging renewable resources, the fact that coal, as a secure and competitive type of energy resources, is being used as the primary fossil fuel to generate about 70% of China’s power structure will not be altered in the foreseeable future.2,3 The technology of superfine pulverized coal particle combustion has been focused since the end of last century as an effective way to achieve high combustion efficiency and to optimize comprehensive operation costs.4 Previous studies have indicated that this new promising combustion method possesses many advantages, such as better combustion stability, higher combustion efficiency, and lower ignition and char burnout temperatures than using conventional combustion techniques.58 The problem of nitrogen oxide (NOx) emissions from pulverized coal combustion in conventional coal-fired power plants still continues to be a major environmental issue. Nitrogen monoxide (NO) and nitrogen dioxide (NO2) have been acting as acid rain precursors and participating in the generation of photochemical smog through ozone production.9 Nitrous oxide (N2O) can be neglected in most of the combustion situations unless it is involved in the case of fluidized-bed combustion (FBC). The Large Combustion Plant Directive of the European Union has announced that, starting from Jan 1, 2008 and ending no later than Dec 31, 2015, combustion plants with capacities above 500 MW will be required to limit the emissions of their NOx to values below 500 mg N1 m3 at 6% O2.10 In China, the latest governmental regulation regarding the limitation of NOx emission will be controlled to a range from 200 to 400 mg/m3 depending upon different regions by 2015. Hence, stringent government regulations have acted as a driving force to motivate a vast amount of research. r 2011 American Chemical Society
Figure 1. PSD of SH bituminous coal specimens.7
Air-staging combustion technology has long been a subject aimed to abate the NOx emissions.1113The mechanism is mainly to hinder the formation of NOx by dividing the furnace into two separate burning zones. An air-deficient zone limits the initial supply of oxygen available for fuel to combust. Then, with an addition of over fire air (OFA), the coal dust would finally complete the process of burnout. The novelty of this present paper is that a laboratory-scale experiment was first carried out on the technology of superfine pulverized coal combustion to achieve NOx abatement. In combination with the air-staging combustion method, emissions of NOx were tentatively investigated for the first time. Received: April 19, 2011 Revised: September 21, 2011 Published: September 21, 2011 4999
dx.doi.org/10.1021/ef201023g | Energy Fuels 2011, 25, 4999–5006
Energy & Fuels
ARTICLE
Meanwhile, CO emissions were also introduced to provide an auxiliary understanding about the combustion process, especially in the case of high concentrations in our experiments.
2. EXPERIMENTAL SECTION 2.1. Coal Samples. Coal samples used in this experiment, Shenhua (SH) coal and Neimenggu (NMG) coal, belong to two bituminous power coals designed for utility boilers in China. In this experiment, they were pulverized into eight different average particle sizes using a jet mill. The average particle size can be expressed in the following formula: Dv ¼
Figure 2. PSD of NMG bituminous coal specimens.7
Table 1. Ultimate and Proximate Analyses of the SH and NMG Bituminous Coals proximate analysis (wt %) (ad)
SH
NMG
ultimate analysis (wt %) (ad)
moisture
11.50
C
63.13
volatile
24.22
H
3.62
ash fixed carbon
10.7 53.58
O N
9.94 0.70
S
0.41
moisture
14.72
C
54.82
volatile
35.69
H
4.39
ash
10.64
O
14.58
fixed carbon
38.95
N
0.63
S
0.22
∑γi di 43 ∑γi di
where Dv stands for the volume mean diameter, di stands for the mean diameter of a specific narrow range of particle sizes, and γi stands for the corresponding yield (or mass fraction). The volume mean diameter Dv can be calculated from the particle size distribution (PSD) data of Figures 1 and 2. A jet mill was used in this experiment for pulverization of coal particles. The coal gained substantial kinetic energy with the help of supersonic flow, which then entered a comminution chamber, where intense collision, friction, and shearing of the coal took place. The comminuted coal was carried by the uprising gas into the grading wheel, where segregation of coarse and fine particles was performed by the function of centrifugal force and driving force of a fan. Coarse particles were returned to the chamber for further comminution, while qualified fine particles entered a cyclone separator for collection. The PSDs were analyzed by the Malvern MAM5004 Laser Mastersizer (Malvern, U.K.). The average particle sizes were approximately 14.71, 17.44, 21.30, and 44.26 μm for SH samples and 12.56, 15.00, 25.86, and 52.78 μm for the case of NMG coal, whose PSDs were displayed in Figures 1 and 2. Coal properties for each coal type were listed in Table 1. The ultimate and proximate analysis data were acquired on a LECO CHN 600 (St. Joseph, MI) and LECO MAC 500 (St. Joseph, MI), respectively. 2.2. Apparatus. A one-dimensional electrically heated tube reactor was adopted in our experiment. A schematic diagram of the experimental apparatus was shown in Figure 3. The main combustion chamber consisted of a cylindrical quartz glass tube with a diameter of 65 mm, to which four branch quartz tubes were connected along the axial positions of 250, 500, 750, and 1000 mm between the OFA ports and the brackets on the roof of the furnace. The entire system was heated by a
Figure 3. Schematic diagram of the experimental apparatus. 5000
dx.doi.org/10.1021/ef201023g |Energy Fuels 2011, 25, 4999–5006
Energy & Fuels
ARTICLE
Figure 4. Effect of the coal quality for superfine pulverized coal in unstaged combustion with (a) oxygen concentration of 21% and temperature of 1000 °C, (b) stoichiometric ratio of 0.84 and temperature of 1000 °C, and (c) oxygen concentration of 21% and stoichiometric ratio of 0.84. SiC element with a heating length of 2000 mm. The feed gases with different ratios of stoichiometry and oxygen concentration were supplied by gas cylinders and regulated by mass flow controllers. The compositions of exhaust gases (NOx and CO) were measured by an online portable Fourier transform infrared spectrometer GASMET DX-4000 (Temet Instruments Oy, Helsinki, Finland) calibrated before each test. Pulverized coal was continuously fed into the combustor by a screw conveyor at a feed rate of 1 g min1. A gas mixer was used to distribute primary air and OFA, which went through the top of the furnace and into ports along the axial position of the reactor, respectively. A programmed temperature controller was used to adjust the heat rate to an average of 20 °C min1 until 1000 °C and then stabilize the flame temperature. Because of a temperature limit of the quartz glass tube reactor around 1200 °C, a standard of 1000 °C was set as the basis temperature of our experiment. Because nitric oxide was always the dominant part of NOx (i.e., NO, NO2, and N2O) under conditions of this study (>95%), the NOx analyzer was set to measure nitric oxide for all of our tests reported below. To eliminate the dilution effect caused by excess oxygen, the unit of NOx emissions was converted and expressed in the commonly used mg N1 m3 with a 6% O2 basis. The amount of CO emissions was also added in the NMG combustion figure for reference but was excluded from that of SH coal because of its negligible concentration below 200 ppm.
3. RESULTS AND DISCUSSION 3.1. Effect of the Coal Quality. Figure 4 shows the different
performances of NOx emissions between two bituminous coals,
Figure 5. Schematic illustration of coal surface morphology.15
NMG_12.56 μm (average particle size of 12.56 μm) and SH_14.71 μm (average particle size of 14.71 μm), which belong to the finest samples in our test. 5001
dx.doi.org/10.1021/ef201023g |Energy Fuels 2011, 25, 4999–5006
Energy & Fuels
Figure 6. Effect of the coal particle size for SH bituminous coal with (a) axial position of the OFA port of 750 mm and temperature of 1000 °C and (b) primary air zone stoichiometric ratio of 0.84 and temperature of 1000 °C.
Previous studies have revealed that the conversion of volatile nitrogen to NO dominates the process mechanism contributing to NO emissions in the pulverized coal systems, especially in the case of air-staging combustion.14,15 An interesting trend, including the case of all figures below, was observed such that NOx emissions from high-volatile NMG coal turn out to be generated even less than those from low-volatile SH coal. In such a situation, the nitrogen content in each coal has to be taken into consideration. However, Thomas et al.16 found that the conversion ratio of fuel N to NOx increases with the increase of coal quality (i.e., the maturity of these two coals) for bituminous coals and that the impact of reactivity of coal quality on conversion of fuel N to NOx is much higher than that of the nitrogen content in coals. In general, char reactivity toward NO would be expected to increase with the fuel volatility.9 Balek et al.17 reported that the reaction activity of coal char mainly relies on the opportunities of the interaction between reactant gases and active sites within the poor structure of the char. From this theory, in comparison to NMG coal seen in Table 1, the relatively lower volumes of N species volatiles (i.e., 24.22%) from SH coal would result in poor expansibility during the process of pyrolysis. In such a case, the consequent smaller pore volumes and reduced active sites of char
ARTICLE
of SH coal would have a slighter impact on the reduction of NOx. On the other hand, Liu et al.18 also suggested that, with the increase of coal quality (i.e., NMG coal to SH coal), the volatiles decrease as a result of a decline in the amount of organic groups around the coal matrix, which leads to a decrease of interfacial thickness, which acts as a link between the coal matrix and pores. Under this circumstance, the transport rate of O2 would be enhanced, thereby pulling out more organic nitrogen from coal char to form NOx for SH coal.19 The interfacial thickness is calculated by the small-angle X-ray scattering (SAXS) experiment, and the quantity of the organic functional group is detected by the Fourier transform infrared (FTIR) spectroscopy experiment. See Figure 5, and more discussion is available in our previous work.18 3.2. Effect of the Particle Size. Figure 6 gives the effect of the particle size on NOx emissions for SH bituminous coal. It can clearly be seen that there exists a saddle at the point of 17.44 μm for both the stoichiometric ratio and position of OFA port conditions. The results are somewhat interesting that, for the average particle size of 17.44 μm, a decrease in NOx emissions occurs with both an increase and a decrease in the particle size. A probable explanation is that, for the superfine coal particle of 14.71 μm, its lower NOx emissions may thank the co-acting of the homogeneous reduction by volatiles and the heterogeneous reduction by coal char. For the former reaction mechanism mentioned above, the timing of volatiles emerging from the coal char is advanced because of the decreased diameter of coal particles. Thus, the early produced intermediates, such as HCN and NH3, would dwell in the air-deficient zone for a longer time to prompt the fuel N to the formation of N2. For the latter heterogeneous reaction mechanism, as the average coal particle size decreases, the heating resistances would be reduced, leading to a promotion of heat- and mass-transfer abilities of reactant gases.20 For this reason, the smaller coal char particle could establish a reduction relationship with NOx in a shorter time. On the other hand, previous studies reported that the specific area and pore volume of the coal particle increase as the average particle size decreases,5 which would result in the emergence of more active sites for smaller particles to enhance the reduction rate between coal char and NOx. As for the case of larger particles, the combustion process takes a longer time to initiate because of its more difficult tendency to the release of volatile. However, once it is established, the combustion process will be more rapid and intense.21 The volatiles are ejected as gaseous jets and dissolve away from the char surface.22 These jets create a local “microzone” of high entrainment to enhance the reduction reaction between volatiles, char, and NOx. However, for the particle size of 17.44 μm, its highest NOx emissions imply that its reduction mechanisms are still not as efficient as its counterparts. As for NMG coal, Figure 7 shows that the NOx emission trend of unstaged combustion is similar to that of SH coal; that is to say, there also exists a saddle point of highest NOx emissions of ∼30 μm. While in the staged combustion condition, the saddlepoint effect is weakened and, finally, the trend for NOx emissions gradually evolves to a near-linear type, especially in the deeper stage. This may be due to the substantial amount of CO up to ∼3%, which occurs in the deep staged condition. The presence of CO has been known to promote the reduction rate between NOx because char catalyzes reduction of NO by CO.23,24 Figure 8 shows the unburned carbon (UBC) content in mass fraction unit in fly ash of different particle sizes for NMG coal. It can be seen 5002
dx.doi.org/10.1021/ef201023g |Energy Fuels 2011, 25, 4999–5006
Energy & Fuels
ARTICLE
Figure 7. Effect of the coal particle size for NMG bituminous coal with (a) axial position of the OFA port of 750 mm and temperature of 1000 °C and (b) primary air zone stoichiometric ratio of 0.84 and temperature of 1000 °C.
Figure 8. Effect of the particle size on UBC content in fly ash for NMG coal.
that, as the average particle size decreases, the fly ash carbon content decreases, which means that better thermal efficiency is acquired using superfine pulverized coal. 3.3. Effect of the Primary Air Zone Stoichiometric Ratio. The trend for both coals is similar; that is to say, the amount of NOx emissions decreases with an increase in the degree of primary air
Figure 9. Effect of the primary air zone stoichiometric ratio for SH bituminous coal at an axial position of the OFA port of 750 mm and temperature of 1000 °C.
zone stoichiometry. SH coal possesses a steady deNOx efficiency of ∼30% in the deep staged combustion (see Figure 9). While in the case of NMG coal, the circumstance is slightly different. Figure 10 indicates that NOx emissions of NMG display a 5003
dx.doi.org/10.1021/ef201023g |Energy Fuels 2011, 25, 4999–5006
Energy & Fuels
ARTICLE
Figure 12. Effect of the axial position of the OFA port for SH bituminous coal at a primary air zone stoichiometric ratio of 0.84 and temperature of 1000 °C.
Figure 10. Effect of the primary air zone stoichiometric ratio for NMG bituminous coal at an axial position of the OFA port of 750 mm and temperature of 1000 °C.
Figure 11. Effect of the primary air zone stoichiometric ratio on the UBC content in fly ash for NMG_15.00 μm coal.
steeper curve, which means that a more efficient deNOx process is established. The smaller three particle sizes could obtain an average efficiency of ∼65%. For the 25.86 μm particles alone, the
Figure 13. Effect of the axial position of the OFA port for NMG bituminous coal at a primary air zone stoichiometric ratio of 0.84 and temperature of 1000 °C.
deNOx efficiency reaches the highest point up to 72% at a stoichiometric ratio of 0.72. However, for the 52.78 μm particles, 5004
dx.doi.org/10.1021/ef201023g |Energy Fuels 2011, 25, 4999–5006
Energy & Fuels
ARTICLE
prevents the production mechanism of NOx, is able to provide less NOx emissions than low-volatile SH coal. (2) Interesting saddle points of the highest NOx emissions for both SH and NMG bituminous coals are found at 17.44 and ∼30 μm, respectively. The unique physical and chemical structure of superfine pulverized coal plays an important role in the homo- and heterogeneous reduction mechanism with NOx. The mechanism of the “saddlepoint effect” is still not well-understood and remains to be a subject in the future. (3) SH bituminous coal possesses a steady deNOx efficiency of ∼30% in the staged combustion, while NMG bituminous coal exhibits a higher ability of NOx abatement during staged combustion. The combustion of NMG_25.86 μm average particles can reach deNOx efficiency up to 70% in each staged condition. Despite the low NOx emissions of NMG_52.78 μm average particles, enormous CO emissions produced indicate that this combustion process is not complete enough. Figure 14. Effect of the axial position of the OFA port on the UBC content in fly ash for NMG_15.00 μm coal.
the deep staged combustion is not that efficient to remove NOx because of its lower unstaged combustion basis index of NOx emissions. It is important to note that the high concentration of CO up to ∼2% indicates that the overall combustion process takes place in an oxygen-deficient environment, where carbon in char could easily grab the scarce oxygen, leading to no supply of oxygen for nitrogen. A substantial amount of CO also implies an incomplete combustion process, which can be harmful to the overall efficiency and economics. Carbon in fly ash can be seen in Figure 11. A compromise can be reached that the stoichiometric ratio in the first primary zone at 0.96 is able to acquire a relatively lower CO concentration at ∼0.75% to achieve a better combustion efficiency, where an UBC content of 3.3% can be reached. 3.4. Effect of the Axial Position of the OFA Port. With an increase in the axial positions between the roof and the OFA ports, there is a decrease in NOx emissions for both coals. As the axial positions of OFA ports increase, the residence time of coal particles in the air-deficient primary air zone would be prolonged, thereby encouraging the conversion of fuel N to nitrogen.25 Figure 12 shows that the SH coal combustion process provides a modest and steady deNOx efficiency of ∼30% with an increased position of the OFA port. In comparison to SH coal, NMG coal has a better sensitivity of the deNOx efficiency with a delayed injection of OFA (see Figure 13). For the 25.86 μm particles again, they can reach deNOx efficiency up to 70% at the last injection port (1000 mm). However, it is necessary to realize that there exists a flat zone covering the positions of OFA ranging from 500 to 750 mm, which means that the delayed injection of OFA will not enhance the abatement of NOx to a great degree. While in the range between 750 and 1000 mm, drastic NOx reduction efficiency can be attained. However, Figure 14 implies a greater combustion efficiency loss for this case, where the UBC content reaches 5.5%. Hence, it is recommended that the optimum position of the OFA port can be located at 750 mm.
4. CONCLUSION On the basis of the data acquired from our experiment, the following conclusions can be drawn: (1) The coal qualities can noticeably affect the reduction mechanism of NOx. The highvolatile NMG coal, because of more active sites in its coal char for the reduction mechanism and its thicker interfacial layer, which
’ AUTHOR INFORMATION Corresponding Author
*Telephone: +86-21-34205681. Fax: +86-21-34205681. E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grant 50876060). ’ REFERENCES (1) Toftegaard, M. B.; Brix, J.; Jensen, P. A.; Glarborg, P.; Jensen, A. D. Prog. Energ. Combust. Sci. 2010, 36, 581–625. (2) Li, Y. W.; Zhao, C. S.; Wu, X.; Lu, D. F.; Han, S. Korean J. Chem. Eng. 2007, 24, 319–327. (3) Kavouridis, K.; Koukouzas, N. Energy Policy 2008, 36 (2), 693–703. (4) Nakamura, M.; Takashi, K.; Kuwahara, M.; Watanabe, H.; Kitamura, R.; Tanaka, T. Proceedings of the International Conference on Power Engeering-97, ICOPE-97; Tokyo, Japan, 1997; Vol. 2, pp 453458. (5) Jiang, X. M.; Zheng, C. G.; Yan, C.; Liu, D. C.; Qiu, J. R.; Li, J. B. Fuel 2002, 81, 793–797. (6) Jiang, X. M.; Zheng, C. G.; Qiu, J. R.; Li, J. B.; Liu, D. C. Energy Fuels 2001, 15, 1100–1102. (7) Liu, J. X.; Jiang, X. M.; Huang, X. Y.; Wu, S. Y. Energy Fuels 2010, 24, 844–855. (8) Jiang, X. M.; Huang, X. Y.; Liu, J. X.; Han, X. X. Energy Fuels 2010, 24, 6307–6313. (9) Glarborg, P.; Jensen, A. D.; Johnsson, J. E. Prog. Energy Combust. Sci. 2003, 29, 89–113. (10) Ribeirete, A.; Coasta, M. Proc. Combust. Inst. 2009, 32, 2667–2673. (11) United States Department of Energy and National Energy Technology Laboratory (DOE/NETL). Micronized Coal Reburning Demonstration for NOx Control; DOE/NETL: Washington, D.C., 2001; DOE/NETL-2001/1148. (12) Sen, L.; Xu, T. M.; Hui, S.; Zhou, Q. L.; Tan, H. Z. Fuel Process. Technol. 2009, 90 (1), 99–106. (13) Backreedy, R. I.; Jones, J. M.; Ma, L.; Pourkashanian, M.; Williams, A. Fuel 2005, 84, 2196–2203. (14) Chen, S. L.; Heap, M. P.; Pershing, D. W.; Martin, G. B. Proceedings of the 19th International Symposium on Combustion; The Combustion Institute: Pittsburgh, PA, 1982; pp 12711280. (15) Coda, B.; Kluger, F.; F€ ortsch, D.; Spliethoff, H.; Hein, K. R. G. Energy Fuels 1998, 12, 1322–1327. (16) Wang, W. X.; Brown, S. D.; Thomas, K. M.; Crelling, J. C. Fuel 1994, 73, 341–347. (17) Balek, V.; de Koranyi, A. Fuel 1990, 69, 1502–1506. 5005
dx.doi.org/10.1021/ef201023g |Energy Fuels 2011, 25, 4999–5006
Energy & Fuels
ARTICLE
(18) Liu, J. X.; Jiang, X. M.; Huang, X. Y.; Shen, J.; Wu, S. H. Energy Fuels 2010, 25, 684–693. (19) Van Der Lans, R. P.; Glarborg, P.; Dam-Johansen, K. Prog. Energy Combust. Sci. 1997, 23, 349–377. (20) Jiang, X. M.; Li, J. B.; Qiu, J. R. J. China Coal Soc. 1999, 24, 633–647 (in Chinese). (21) Abbas, T.; Costen, P.; Lockwood, F. C.; Romo-Millares, C. A. Combust. Flame 1993, 93, 316–326. (22) Kramlich, J. C.; Seeker, W. R.; Samuelsen, G. S. Fuel 1988, 67, 1182–1189. (23) Aarna, I.; Suuberg, E. M. Fuel 1997, 76, 475–495. (24) Aarna, I.; Suuberg, E. M. Energy Fuels 1999, 13, 1145–1153. (25) Spliethoff, H.; Greul, U.; R€udiger, H.; Hein, K. R. G. Fuel 1996, 75 (5), 560–565.
5006
dx.doi.org/10.1021/ef201023g |Energy Fuels 2011, 25, 4999–5006
