Effect of Air-Staging on Anthracite Combustion and NOx Formation

Energy & Fuels 2009, 23, 111–120

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Effect of Air-Staging on Anthracite Combustion and NOx Formation Weidong Fan,* Zhengchun Lin, Youyi Li, Jinguo Kuang, and Mingchuan Zhang School of Mechanical & Power Engineering, Shanghai Jiao Tong UniVersity, No. 800, Dongchuan Road, Minhang District, Shanghai 20024, People’s Republic of China ReceiVed May 17, 2008. ReVised Manuscript ReceiVed NoVember 2, 2008

Experiments were carried out in a multipath air inlet one-dimensional furnace to assess NOx emission characteristics of the staged combustion of anthracite coal. These experiments allowed us to study the impact of pulverized coal fineness and burnout air position on emission under both deep and shallow air-staged combustion conditions. We also studied the impact of char-nitrogen release on both the burning-out process of the pulverized coal and the corresponding carbon content in fly ash. We found that air-staged combustion affects a pronounced reduction in NOx emissions from the combustion of anthracite coal. The more the air is staged, the more NOx emission is reduced. In shallow air-staged combustion (fM ) 0.85), the fineness of the pulverized coal strongly influences emissions, and finer coals result in lower emissions. Meanwhile, the burnout air position has only a weak effect. In the deep air-staged combustion (fM ) 0.6), the effect of coal fineness is smaller, and the burnout air position has a stronger effect. When the primary combustion air is stable, NOx emissions increase with increasing burnout air. This proves that, in the burnout zone, coal char is responsible for the discharge of fuel-nitrogen that is oxidized to NOx. The measurement of secondary air staging in a burnout zone can help inhibit the oxidization of NO caused by nitrogen release. Air-staged combustion has little effect on the burnout of anthracite coal, which proves to be suitable for air-staged combustion.

1. Introduction In accordance with China’s standards on power coal classification, coal with Vdaf (volatile matter content on dry ashfree basis) of less than 20% is classified as low volatile coal. This type of coal is plentiful in China and has become one of the major sources of power coal. Low volatile coal is difficult to ignite and burn out; therefore, it requires a higher ignition and burnout temperature, as well as a longer burnout time. At present, the W-flame boiler manufactured by Foster Wheeler Corp. (FW, U.S.) is seen as the best furnace for low volatile coal. Therefore, China has put over 60 W-flame boilers into operation, all of capacity greater than or equal to 300 MWe, with dozens more under construction. Judging from operational performance, when anthracite coal with a Vdaf of less than 12% is used, excess carbon content in the fly ash and excess NOx emissions (usually over 1000 mg/m3) are problematic. Therefore, Chinese scholars have carried out many investigations on the W-flame furnace.1-3 However, there has still been little research on how to control NOx emissions when using anthracite coal. With increasing public demand for reduced pollution, many countries have lowered their NOx emission limits. For example, as of 2008, the allowed concentration for power plants over 500 MWe in the EU is 500 mg NO2/N · m3 at 6% O2. After 2016, the limit allowed for power plants over 500 MWe will * To whom correspondence should be addressed. Telephone: +86-2134206049. Fax: +86-21-34206115. E-mail: [email protected]. (1) Fan, J. R.; Jin, J.; Liang, X. H.; Chen, L. H.; Cen, K. F. Chem. Eng. J. 1998, 71, 233–242. (2) Ren, F.; Li, Z. Q.; Zhang, Y. B.; Sun, S. Z.; Zhang, X. H.; Chen, Z. C. Energy Fuels 2007, 21, 668–676. (3) Li, Z. Q.; Ren, F.; Zhang, J.; Zhang, X. H.; Chen, Z. C.; Chen, L. Z. Fuel 2007, 86, 2457–2462.

be 200 mg NO2/N · m3 at 6% O2.4 This makes the need for much more stringent NOx emission control techniques urgent.4,5 Air-staged combustion is the most sophisticated low-NOx combustion technology for reducing NOx emissions, and it is thus the most widely adopted in coal-fired power plants in China and abroad. However, air-staged combustion technology is mainly used with high volatile coal, such as bituminous and lignitous coal. This is mainly because it provides better control of NOx emissions and also guarantees that, after air staging, the burnout of the coal particles will not be too attenuated. Therefore, many scholars have mainly concentrated only on high volatile coal in their research on NOx emissions from air-staged combustion technology.6-17 Comparatively few studies have investigated anthracite coal. (4) Bris, T. L.; Cadavid, F.; Caillat, S.; Pietrzyk, S.; Blondin, J.; Baudoin, B. Fuel 2007, 86, 2213–2220. (5) Li, Z. Q.; Chen, Z. C.; Sun, R.; Wu, S. H. J. Energy Inst. 2007, 80, 123–130. (6) Habib, M. A.; Elshafei, M.; Dajani, M. Comput. Fluids 2008, 37, 12–23. (7) Chaiklangmuang, S.; Jones, J. M.; Pourkashanian, M.; Williams, A. Fuel 2002, 81, 2363–2369. (8) Kim, H. S.; Baek, S. W.; Yu, M. J. Int. J. Heat Mass Transfer 2003, 46, 2993–3008. (9) Man, C. K.; Gibbins, J. R.; Witkamp, J. G.; Zhang, J. Fuel 2005, 84, 2190–2195. (10) Backreedy, R. I.; Jones, J. M.; Ma, L.; Pourkashanian, M.; Williams, A. Fuel 2005, 84, 2196–2203. (11) Li, S.; Xu, T. M.; Sun, P.; Zhou, Q. L.; Tan, H. Z.; Hui, S. E. Fuel 2008, 87, 723–731. (12) Fo¨rtsch, D.; Kluger, F.; Schnell, U.; Spliethoff, H.; Hein, K. R. G. Twenty-Seventh Symposium (International) on Combustion; The Combustion Institute, 1998; pp 3037-3044. (13) Spliethoff, H.; Greul, U.; Ru¨diger, H.; Hein, K. R. G. Fuel 1996, 75, 560–564. (14) Coda, B.; Kluger, F.; Fo¨rtsch, D.; Spliethoff, H.; Hein, K. R. G. Energy Fuels 1998, 12, 1322–1327. (15) Staiger, B.; Unterberger, S.; Berger, R.; Hein, K. R. G. Energy 2005, 30, 1429–1438. (16) Chae, J. O.; Chun, Y. N. Fuel 1991, 70, 703–707.

10.1021/ef800343j CCC: $40.75  2009 American Chemical Society Published on Web 12/12/2008

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Figure 1. The air-staged combustion system.

Some information on anthracite behavior in staged-air systems is available. For example, Burdett carried out industrial tests to investigate the effects of air staging on NOx emissions from a 500 MWe down-fired furnace unit,18 and Li investigated how changes in exhaust gas (third air) can affect the amount of NOx emissions on a 300 MWe W-flame furnace.3 For high volatile coal, the latest development in air-staged technology is the vertical staging of the air inlets into the furnace.19 It is named overall air-staged technology. As shown in Figure 1a, the combustion air in the furnace is staged into stage-one air and stage-two air (burnout air). Along the axis of the furnace, the primary combustion zone, reducing zone, and burnout zone are formed. In China’s newly built 600 MWe boiler, when overall air-staged technology is adopted, the NOx emission is no more than 250 mg/m3 for coal with a Vdaf of 30% or greater. This is far lower than the 800 mg/m3 observed when not using stagedair combustion. This shows that it is valuable to explore whether the overall air-staged technology within the furnace is suitable for anthracite coal furnaces. Promotional materials from the Foster Wheeler Corp. (FW) claim that an overall air-staged technology within the W-flame furnace has been developed (as shown in Figure 1b). The exhaust air vents on the furnace are moved to the vertical hearth wall of the upper hearth, and circular burnout air is added outside (shown by label A). In 2002, FW reconstructed the W-flame boilers of several power plants using this technology to reduce their NOx emissions. These reconstructions all achieved very good results, according to FW’s promotional report. For example, for anthracite coal with a Vdaf of 7%, before reconstruction the emission was 1200 mg/m3, but afterward the guaranteed emissions ceiling was ∼510 mg/m3, and the actual minimum value was 250 mg/m3. The NOx emission reductions delivered using the air-staged technology of anthracite coal are of tremendous interest. However, regrettably there is yet no publicly published laboratory data to support these claims. In (17) Costa, M.; Azevedo, J. L. T. Combust. Sci. Technol. 2007, 179, 1923–1935. (18) Burdett, N. A. J. Energy Inst. 1987, 60, 103–107. (19) Richards, G. H.; Maney, C. Q.; Borio, R. W. Power Plant Laboratories, Alstom Power Inc., 2002.

this Article, a one-dimensional droptube furnace system is used to simulate overall air-staged technology within a furnace to study the NOx emissions from a typical anthracite coal under different air-staging conditions, including such key parameters as burnout air position and its ratio in the total air. Pulverized coal fineness may also have an important impact on NOx emission. Sarofim20 holds that more finely pulverized coal generates less NOx in unstaged combustion and more NOx in staged combustion, but his research investigated only singly sized coal particles. The literature21 states that little research has been done on the impact of fineness on NOx emissions. While some researchers have concluded that finer anthracite coal reduces NOx emission, others have concluded that increased fineness has no effect on NOx emission. Many boilers in Chinese power plants use conventional air-staged technology, and it can be seen from the result of this adjusted combustion that more finely pulverized coal slightly increases NOx emissions. Therefore, further research is needed to study the effects of coal fineness on NOx emission in staged combustion, especially in the deep stage. This Article investigates the impact of pulverized coal fineness to find out how staging ratio and pulverized coal fineness influence NOx emissions, and it addresses how to achieve an optimum combination of these two. This study not only demonstrates the reduced effect of overall air-staged technology on anthracite coal, but also provides a basis for developing an overall air-staged technology within the furnace. 2. Experimental Section 2.1. Experimental Instruments. One-dimensional droptube furnace systems are used by many researchers10,12,14,19,22 to study the NOx emissions behavior because it is easier to get regular results from the experiments. Therefore, we also adopted this system. However, to better simulate the effect of burnout air changes on NOx emissions in air-staged technology, the air-tightness of the (20) Song, Y. H.; Pohl, J. H.; Bee´r, J. M.; Sarofim, A. F. Combust. Sci. Technol. 1982, 28, 31–40. (21) Lans, V. D.; Glarborg, R. P.; Dam-Johansen, K. Prog. Energy Combust. Sci. 1997, 23, 349–377. (22) Jin, J.; Zhang, Z. X.; Li, R. Y. Proc. CSEE (Chinese) 2006, 26, 35–39.

Effect of Air-Staging on Anthracite Combustion

Energy & Fuels, Vol. 23, 2009 113 Table 1. Characteristics of Yangquan Coal proximate analysis, wt % (air-dried) volatile matter

ash

moisture

fixed carbon

net heating value (kJ/kg)

7.42

17.29

1.07

74.22

28020

ultimate analysis, wt % (air-dried)

Figure 2. Multipath air inlet one-dimensional furnace system. (1) Oilless air compression engine. (2) Reducing valve. (3) Air distribution box. (4) Glass rotor flow-meter. (5) Micro screw trace feeder. (6) Inner shell of quartz glass. (7) Tube electric resistance furnace. (8) Thermocouple. (9) Temperature regulator. (10) Fly ash collector. (11) Vacuum pump. (12) Drying tube. (13) Dedusting tube. (14) Infrared gas measuring apparatus for SO2. (15) Dedusting tube. (16) Drying tube. (17) Infrared gas measuring apparatus for NOx. (18) Dedusting tank.

experimental system needs to be ensured. Therefore, in our research, the air-tightness of the system is of critical importance. Meanwhile, multipath air inlets are set in the combustion to simulate the impact of burnout-air position. In the multipath air inlet one-dimensional furnace system, research can be done to study one-dimensional pyrolysis, combustion processes, and emissions characteristics, all under controlled atmospheric and temperature conditions. As shown in Figure 2, this system comprises a tube heating furnace, an inner shell of quartz glass, a pulverized coal feeder, a system for air distribution, and a sampling and measurement system. The air from the oil-less air compression engine enters the air distribution box through a reducing valve, and the air distribution is controlled by a flow meter. During the experiment, primary and secondary air is injected into the furnace from the top, and pulverized coal flows downward. The pulverized coal feeder system uses a micro screw trace feeder that regulates the powder flow rate through power frequency control. Pulverized coal is brought into the combustor by the primary air. The electric heating power of the tube-heating furnace is 12 kW. The inner shell of quartz glass is a proxy for a real furnace; the highest temperatures in experiments can reach 1573 K. The height of the furnace is 2300 mm, and the inner diameter is 80 mm. A number of branch pipes connect to it at different heights. The branch pipes and the simulated furnace are connected at different heights to emulate the residence time distribution of combusting coal traveling from the primary combustion zone to the separate burnout air. The inner shell of quartz glass is completely airtight, and, as a result, after combustion, the analysis of the air-gas components is much more accurate. The exit from the simulated furnace has a flute-shaped sample tube on the end, used to withdraw flue gas or

carbon

hydrogen

sulfur

nitrogen

oxygen

75.1

3.18

0.84

1.24

1.28

fly ash. After dedusting and drying, sampled flue gas enters the infrared gas measuring apparatus for measurement of sulfur dioxide and nitrogen oxides content, or it enters the fly ash collector. After going through the dust catcher, the rest of the flue gas is discharged into the air. 2.2. Experimental Coals. The coal used in the experiment was Yangquan anthracite coal from China. The characteristics in Table 1 indicate that the coal is anthracite with a high heating value and low volatile content. In China, this type of coal is used in a W-flame furnace for power generation, and industrial experiments have studied3 the combustion characteristics of this type of coal in the 300 MWe W-flame furnace. By studying the effects of air changing on combustion, this Article also aims to provide a basis for reconstructing furnaces with lower NOx emissions from anthracite coal combustion. Therefore, in our experiments, we adopted coal fineness characteristics that replicate those reported in the literature:3 sieve analysis data were R90 ) 5% and R74 ) 7%. To study the effect of coal fineness on NOx emissions, three kinds of pulverized coal, with finenesses of 2%, 9%, and 13%, were chosen. In this way, the fineness of the experimental pulverized coal is 2%, 5%, 9%, or 13%. The coal was carefully processed by a specialized powder processing company and formulated strictly according to the Rosin-Rammler distribution function. The formulation parameters are calculated from sieve analysis data of R90 ) 5% and R74 ) 7%. These values were provided by a local power plant and were used to derive a uniformity coefficient of n ) 0.609. 2.3. Experimental Methods. Apart from the fineness of pulverized coal, the entering position of burnout air plays a decisive role in maintaining high efficiency combustion.11,13 In our experiment, we sought to study the impact of the burnout air position on NOx emission. As shown in Figure 2, in the multipath air inlet onedimensional furnace system, four quartz branch tubes are connected with the main quartz tube. The ratio between the distances from the aforementioned connection part to the furnace length is defined as the position ratio of the burnout air, MS, which is used to simulate the ratio between the main-burner-to-burnout-air-vent distance to the furnace height. The MS values for the positions where the four quartz branch tubes connect with the main tube are 0.22, 0.43, 0.65, and 0.87, respectively. The primary combustion air rate refers to the ratio of air (by which the feeder brings in pulverized coal) in the total air, fM. The staged air rate refers to the ratio of burnout air to total air, fS. In addition, the total excess air ratio is expressed as RT, so the excess air ratios of the primary combustion air rate and of the burnout air rate are RM ) RT × fM and RS ) RT × fS, respectively; thus, fM + fS ) 1. By adjusting the combination of such parameters as RT, fM, fS, and MS, the staged combustion characteristics of Yangquan coal can be fully understood. During unstaged combustion, the total excess air ratio of unstaged combustion, RT, ranges from 1 to 1.5. Staged combustion takes place under excess air with an air ratio of 1.2 (approximately 3.5% O2 in the flue gas, if burned out). During air-staged trials, 0.6 and 0.85 were adopted for the primary combustion air rate fM. Here, fM ) 0.6 is defined as deep air-staged, and fM ) 0.85 is defined as shallow air-staged. The corresponding excess air ratios in the primary combustion zone, RM, are 0.72 and 1.02, respectively, for deep and shallow air-staged. When MS is 0.22, a residence time of 5.18 or 3.65 s in the fuel-rich primary zone has been chosen, and


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Figure 3. NOx emission law in unstaged combustion.

Figure 4. NOx emission law in staged combustion (RT ) 1.2).

the air-to-fuel equivalence ratios are 0.75 and 0.85. The coal feed rate was approximately 1 g/min for all of the tests.

3. Results and Discussion 3.1. Unstaged Combustion. Figure 3 shows a typical trend for NOx concentration during unstaged combustion. The NOx concentration is converted to a 6% O2 basis, both here and below. It can be seen that the excess air ratio greatly impacts the generation of NOx. NOx emissions increase monotonically with increasing excess air ratios. To ensure the burnout of anthracite coal in the W-flame furnace, the excess air ratio at the exit of the furnace should not be less than 1.2, and the concentration of NOx emission should be above 1000 mg/m3. However, with the excess air ratio below 1.2, the concentration of NOx emissions is below 800 mg/m3, which shows that if other combustion parameters of the furnace are kept optimal, a hypoxic regime should be maintained. Figure 3b explicitly shows NOx emission levels as a function of coal fineness. It can be clearly seen that, regardless of how RT changes, the rate of NOx emissions for coal with R90 ) 5% remains the lowest. Research shows that in unstaged combustion, more finely pulverized coal generates more NOx emissions. Therefore, it is not that finer pulverized coal generates less NOx emissions, but there exists a critical fineness. A fineness of R90 ) 5 % is an option for industrial operation because it not only helps reduce NOx emissions, it is also satisfactory for the burnout rate. 3.2. Comparison of Air-Staged and Unstaged Combustion and the Location of OFA Injection Ports on NOx Emissions. Figure 4 shows the influences of fS and the location of OFA injection ports on NOx emissions. fS has a significant influence on NOx emissions.11,13 Figure 4 demonstrates the comparison between unstaged air combustion, deep-staged air combustion,

and shallow-staged air combustion. As shown in Figure 4, the NOx emission in staged combustion is obviously lower than that in unstaged combustion. Further, deep staging gave lower emissions than shallow staging. The effect of coal fineness on emissions depended on the location of air staging. As shown in Figure 4, in shallow-staged combustion with an fM ) 0.85 (corresponding to RM ) 1.02), pulverized coal fineness has a significant impact on NOx emissions. The overall trend is that a smaller fineness indicator R90 results in lower NOx emission (shown in the figure by the dotted line). When the fineness indicator is R90 ) 2% in relatively unstaged combustion, the NOx emissions reduction is the largest. This trend is in line with the one observed by Sarofim.20 However, in deep staged combustion with fM ) 0.6 (corresponding to RM ) 0.72), the importance of coal fineness as a determinant of NOx emissions decreases. Except for the higher emissions seen when R90 ) 12%, the NOx levels for R90 values between 2% and 9% were similar. These findings are in accord with our previous study on bituminous coal.23 In the primary combustion zone, the decrease of fM influences the amount of reducing species (HCN, NH3, NCO, etc.) in the combustion zone. This is conducive to NOx destruction.11,13 Figure 5 shows Coda’s component distribution obtained from the staging experiment on a one-dimensional droptube furnace similar to ours.14 In the figure, Coda used bituminous coal to carry out a comparison between unstaged combustion and deep air-staged combustion under the conditions of RT ) 1.15 and RM ) 0.75. It can be seen from the figure that the peak value of NO in the staged combustion is lower than that in the unstaged one because the staged combustion restricts the generation of NO to the early period of coal burning. Mean(23) Lu, J.; Xie, J. W.; Fan, W. D.; Zhang, M. C. Power Eng. (Chinese) 2007, 27, 949–953.


Figure 5. Gas components distribution law of air-staged combustion.14

while, the generated NO is reduced in the reducing zone before the injection of burnout air, and the peak value formed in the primary combustion zone drops off rapidly. At the same time, the oxygen concentration in the reducing zone is much lower than that in the unstaged combustion. Oxygen concentrations of nearly zero are reached at the later stages of the reducing zone, nearly before the injection of the burnout air. This shows that in staged combustion, oxygen concentration in the reducing zone should remain low enough that a great amount of NO generated in the primary combustion zone can be reduced. Therefore, for the anthracite coal tested in this Article, when the air is staged to a greater degree, the generation of NO is more suppressed, and the oxygen concentration of the reducing zone remains low, facilitating the reduction of reducing species. Therefore, when fM is lower, NOx emissions are lower. Although ignition and burnout are difficult with anthracite coal, when the air is staged to a smaller degree (fM ) 0.85, RM ) 1.02), a smaller R90 can quickly combust and consume oxygen. Thus, more oxygen is needed for NO generation in the primary combustion zone. This way, the peak value of NO is lower and the fuel-nitrogen will be more early and abundantly released to form a great amount of reducing species (HCN, NH3, NCO, etc.). Meanwhile, the oxygen concentration of the reducing zone is lower, resulting in the formation of more highly reducing species. As a result, NO in the primary combustion zone can be further reduced, so less NO is discharged. Bigger R90 values monotonically lead to a slower combustion process. Not only will strong reducing conditions be formed due to high oxygen concentrations in the reducing zone, but also the fuel-nitrogen will release abundantly. This fuel-nitrogen will then be oxidized into NO14 after the injection of burnout air together with more unburned char whose combustion is delayed. This process leads to higher NO emission. Therefore, for the shallow staged combustion, smaller R90 values lead to lower NO emissions. By comparison of Figure 3a and Figure 4a, it is also found that the NOx emissions from staged combustion with fM ) 0.85 (corresponding to RM ) 1.02) are less than those for unstaged combustion with the same burner stoichiometry of 1.02 (corresponding to RT ) 1.02) when R90 ) 2%. This result is confusing. It seemed to us more likely that NOx emissions from staged combustion would be the same as or greater than those for unstaged combustion with the same burner stoichiometry, such as when R90 ) 5%, 9%, or 13%. Yet, although these results may be counterintuitive, we have obtained similar results in the past, multiple times, when conducting experiments on several other coals.23 However, this result was never seen for the

Energy & Fuels, Vol. 23, 2009 115

pulverized coal sample composed of large sized particles or with a large fineness R90. Here, we discuss a possible explanation for this interesting result. Although the excess air ratio in the primary combustion zone is 1.02, close to the theoretical stoichiometric ratio, its burning rate is still very high and the combustion is very intense because the pulverized coal sample is composed of small particles (R90 ) 2%). In the meantime, because the excess air ratio in the primary combustion zone is close to 1, the combustion temperature will reach the highest values. This will further promote coal combustion and improve its burnout rate. It may also cause a great deal of thermal NOx formation because the combustion temperature is very high after peak formation of fuel-derived NOx in the primary combustion zone. We calculated that the theoretical combustion temperature can reach above 1700 °C for the unstaged combustion with high burnout rates when R90 ) 2%. Because the furnace is adiabatic, the actual temperature in the furnace must also exceed 1500-1600 °C, a critical temperature for thermal NOx formation at exponential growth speeds. Therefore, a great deal of thermal NOx formed at its late stage for the unstaged combustion. This resulted in much higher NOx emissions. Yet for staged combustion with RM ) 1.02 and RT ) 1.2, the injection of burnout air will greatly lower temperature in the combustion zone. In this case, the theoretical combustion temperature will be below the critical temperature for thermal NOx formation at exponential growth speeds. It can also be concluded that the extent of thermal NOx formation is lowered at exponential speed after the injection of burnout air. As a result, the injection of the burnout air causes the extent of total NOx formation to significantly decrease. Therefore, when R90 ) 2%, the NOx emission in the staged combustion is less than that of unstaged combustion with the same burner stoichiometry. Because many other mechanisms are also involved, such as microscopic surface physicochemical reactions, reduction, etc., it is also possible that the above explanations are incomplete. Further research on this problem will be done in the future. When the air is staged to a higher degree, for example, deeply air-staged (fM ) 0.6, RM ) 0.72), the air for transporting the pulverized coal in the primary combustion zone decreases, which is helpful for the ignition and initial burning.24 Pulverized coal with larger R90 values can ignite more easily and burn more quickly. It also consumes more oxygen in the primary combustion zone. These facts, when coupled with the fact that the oxygen supply in deep air staging is limited, indicate that the peak value of NO will be smaller, and the low oxygen concentration in the reducing zone will form strong reducing conditions. In this way, more NO can be reduced, and the final emission will be relatively small. In Figure 4b, this is the very reason why NOx emission in R90 ) 2-9% changes little, but for the pulverized coal composed of large size particles with R90 ) 13%, it still burns relatively slowly. There is a relative oversupply of oxygen in the primary combustion zone, and the oxygen concentration in the reducing zone is still fairly high. All of these factors lead to a high peak value of NO produced in the primary combustion zone, and they lead to a relatively small reducing effect in the reducing zone. A great amount of char is concentrated, released, and oxygenized in the burnout zone, which makes the NO emission bigger than for values of R90 between 2% and 9%. Figure 4 also shows the relationship between the location of OFA injection ports and NOx emission using Ms. To demonstrate (24) Xu, X. C. Combustion Theory and Combustion Equipment (Chinese); China Machine Press: Beijing, 1990.


Fan et al.

Figure 6. The impact of burnout position on NOx emission law (RT ) 1.2). Fill pattern: fM ) 0.6. Hollow pattern: fM ) 0.85.

the impact more vividly, Figure 6 shows the effect of Ms as a horizontal ordinate on emissions. It indicates that NOx emissions monotonically decrease with increasing the relative location of OFA injection ports in deep air-staging. This phenomenon may be explained as follows: increasing the location of OFA injection ports makes the heights of the reduction zone expand, also increasing the residence time of flue gas in reduction zone. Increasing the residence time in the reduction zone brings about a drastic reduction of NOx emissions by gaseous N species (NO, NH3, HCN) and char. However, by increasing the location of OFA injection ports at fM ) 0.85, the NOx emission remains nearly constant. The phenomenon may be explained as follows: increasing the location of OFA injection ports makes the residence time of flue gas in the reduction zone increase. However, the previously mentioned combustion characteristics of anthracite coal are detrimental to the initial and later period of burning in minor air staging because, as compared to deep air-staging, the initial combustion is fed too much air too early. In this way, the burning rate of the pulverized coal is low; the burning process lags a lot; the strong reducing conditions from low oxygen concentrations are not yet generated; the actual length of the reducing zone is short; and the independent reducing zone is not evenly formed. The offend result is a weakened capacity for NOx reduction.9,11 The lagging of the combustion process leads to more fixed carbon in the coal being burned in the burnout zone, and thus much of the fuel-nitrogen is released to generate NO. It can be seen in Figure 5 that, after the injection of burnout air, NOx rises to some extent,14 making it impossible to reduce part of the generated NO in the burnout zone before leaving the furnace. The curve in Figure 6 shows the ratio O2as/ O2baseline of the remaining oxygen concentrations O2as in staged combustion to unstaged combustion O2baseline. This figure demonstrates the burning characteristics of anthracite coal: less air at the initial combustion will be conducive to its burning and burning-out. This figure shows that the remaining oxygen concentration in staged combustion in unstaged combustion is lower in deep air-staged combustion than in shallow stage combustion (except for very finely pulverized coal). Therefore, combining the analysis above, when the distance of the reducing zone increases, a bigger lag between the shallow air-staged combustion and the deep air-staged combustion leads to a decrease in the effect of the reducing zone, and the oxygenation of fuel-nitrogen makes the NOx emission remain stable (as shown in Figure 4). For anthracite coal, the release of char-nitrogen in the burnout zone caused by the lagging burning might eventually have more of an effect on NOx emission than with any other coal. One reason for this is that anthracite coal has far less volatile content

than bituminous coal, and most of its fuel-nitrogen must be stored in the char. Another reason is that it is more difficult for anthracite coal to ignite and burn. Therefore, greater lag in the burning of anthracite coal will lead to greater release of the remaining char-nitrogen in burnout zone, which will contribute greatly to NOx emissions. Coal combustion theory tells us that the combustion stage of anthracite coal does not need fixing with too much air, and it will mix with the remaining needed air in later stages. From this perspective, it can be concluded that deep air-staged combustion might be more conducive than shallow staging to the initial ignition and later combustion of anthracite coal. In shallow staging combustion, more charnitrogen is released to generate NOx. That is, the reducing zone is lengthened. Definite research findings have concluded that char-nitrogen gradually releases and gets oxygenized in the burnout process, which is of great help in controlling the ultimate NOx emission in the combustion system. For example, Spliethoff13 holds that the release of char-nitrogen from the fuel-rich zone is also of great importance. Too much remaining char-nitrogen will sharply increase the ultimate NOx emission. Coda14 also carried out extensive research into this aspect. In the present Article, an experiment is designed to demonstrate the impact of the afterrelease of char-nitrogen on the ultimate NOx emission, and it is very important to realize this impact. 3.3. Contribution Analysis of Char Burnout Process on the Ultimate NOx Emission in the Burnout Zone. The results above show that the release of fuel-nitrogen in anthracite coal may have an important impact on the ultimate NOx emissions. Coda’s experiment14 showed that in the process of air-staged combustion, abundant char-nitrogen will be released and oxidized after the injection of the burnout air. This will lead to a rise in NO emissions, as shown in Figure 5, where calculations based on a mass balance basis show that the NO can theoretically rise by 100-150 ppm (0% O2). He also used a mathematical model proposed in the literature12 to simulate the trends between various parameters on the oxygenation of nitrogen into NO in the burnout process of char. In staged combustion, as the char burns out, abundant fuel-nitrogen will be released along with the loss of total char mass, even in the reducing zone. In accordance with (dN/N)/(dC/C) trends raised in the literature,25 Coda further determined the reaction law of carbon and nitrogen in one-dimensional combustion from his experimental data. For high volatile coal, this law shows that air-staged combustion makes (dN/N)/(dC/C) bigger than 1 from the primary combustion zone to the burnout zone. The highest peak values appear (25) Baxter, L. L.; Mitchell, R. E.; Fletcher, T. H.; Hurt, R. H. Energy Fuels 1996, 10, 188–196.


Figure 7. With a stable main combustion air (RM ) 0.72), the NOx emission law is shown when staged air increases.

and then become less than 1 in the burnout zone. Furthermore, it is less than 1 in the whole process in unstaged combustion, which shows that air staging is conducive to a more rapid release of fuel-nitrogen in the initial combustion. However, for low volatile coal, (dN/N)/(dC/C) is bigger than 1 only for a certain section. No maximum appears, but (dN/N)/(dC/C) is bigger than that for high volatile coal in the burnout zone. In staged combustion for low volatile coal, the fuel-nitrogen release of char in the burnout zone is greater than that for high volatile coal. The analysis of fuel-nitrogen release and NO generation is beneficial to research on optimizing air-staging conditions. In this Article, an experiment was designed to study how fuel-nitrogen release in the burnout zone affects ultimate NOx emissions by increasing staged air with the same primary combustion air. The results of the above experiment show that the NOx emission is closely related to fM. It decides not only the residence time of NOx in the reducing zone, but also that of pulverized coal in the furnace. With RT and MS remaining stable, when the fM is smaller, the actual reducing section is longer and the NOx is better reduced. For this experiment, let us first presume that after the burnout air is injected into the onedimensional furnace, it is expanded when heated. Next, it is carried by the upper flow and then forced to flow downward, one-dimensionally. On the basis of this presumption, we fix MS ) 0.42, RT ) 1.2, and RM ) 0.72, and we gradually increase RS. This means that when both the generation of NOx in the primary combustion zone and the reducing time in the reducing zone are fixed, the impact of burnout air changes on the release and oxygenation of the fuel-nitrogen when the char in burned out. Therefore, it can also be presumed that the change of burnout air has no effect on the NOx value before its injection, and it can be concluded that the change of the ultimate NOx emissions is caused by the burnout air. In Figure 7, the abscissa shows Rs/RM, which is the ratio between burnout air and primary combustion air. The top coordinate shows RT, which is the corresponding excess air of burnout air increases. Vertical ordinates show KNOx, which is the ratio between NOx emission in each burnout air increase case and that in base case (RM ) 0.72, MS ) 0.42). As the burnout air increases, the NOx emission obviously increases. Therefore, it can be concluded that the amount of NOx emission does not increase before the injection of the burnout air. This is because the increased burnout air causes the rise of NOx in the burning process of char, which shows that the char is still releasing a lot of fuel-nitrogen in the burnout zone. The oxidizing conditions formed in the burnout zone after the injection of burnout air cause these sources of fuel-nitrogen to be oxidized into NOx, and it also causes the oxygen concentration in the atmosphere to be high enough to


Figure 8. With a stable main combustion air (RT ) 1.2, RM ) 0.72), the NOx emission law is shown when burnout air is staged.

oxidize these fuel-nitrogens more extensively. It can also be seen from the figure that the NOx emissions rise for smaller R90 more quickly as the burnout air increases; that is, kNOx is bigger. In part, this is because the base NOx with a smaller R90 is lower. Another reason is that when R90 is smaller, an increase in burnout air can more easily raise its burnout degree, which correspondingly leads to greater release of nitrogen from the char, and so more NOx is generated. Meanwhile, higher burnout degrees will decrease the heterogeneous reducing ability of char in the burnout process. It can be seen that injecting burnout air in staged combustion will form various oxygen concentration conditions in the burnout zone. This heterogeneity can influence the burnout process of the char, and it can also the influence the release and oxygenation of char-nitrogen in the burnout process. The increase of oxygen in the burnout zone will surely lead to secondary generation of NO, which will cause rises in the ultimate NOx emissions. Thus, by drawing on the principle that air-staging combustion lowers NOx emission in the primary combustion zone, this Article raises the concept of secondary air staging. That is, the burnout air is injected two times, and a new injection point for remaining burnout air is added at some distance away from the original injection point. With a secondary air staging, the burnout zone can be considered as divided into two sections. In one section, the oxygen is controlled so that the NO generation after the release of nitrogen gets under control. Meanwhile, the control of oxygen will strengthen the reduction of NO by the char, so the ultimate NOx will decrease. To test the effect of secondary air staging in the burnout zone, the experiment still uses the above-mentioned RT ) 1.2, RM ) 0.72, and MS ) 0.42 as the operation modes. With MS ) 0.54, another vent is set up for the burnout air, whose ratio in the total air is marked by fR. The experimental result can be seen in Figure 8. In the figure, kNOx shows the ratio of the NOx emissions from R90 pulverized coal in fR operation mode versus datum from the operation mode. As fR increases, kNOx is smaller than 1 and nearly shows a decreasing trend. This proves that the secondary air staging in the burnout zone can indeed restrict the release of nitrogen and the generation of NO. It can also be seen that secondary air staging has a better effect on pulverized coal with smaller R90’s, which is in line with the trends on air staging in the primary combustion zone. The secondary air staging in the burnout zone has a smaller role in reducing NOx emission than does the primary combustion zone. In the figure, kNOx is never lower than 0.7, which means that no more than 30% of the NOx emission is decreased. This result is in line with the general principles of air-staged combustion (the effects are more obvious for higher volatile


Fan et al.

Figure 9. Comparison of UBC in staged combustion and unstaged combustion (RT ) 1.2).

coal).13,26 Another reason is that the char in the burnout zone gradually increases in its burnout degree, and also the NO reducing conditions in the burnout zone are inferior to the reducing conditions formed by air staging in the primary combustion zone. Of course, the secondary air staging is still a good way to restrict the release of nitrogen and the oxygenation of too much NO in the burnout zone in the air-staged combustion of low volatile coal. For anthracite coal with a stable RM and increased RS, increasing RT still has a small impact on NOx emission in unstaged combustion. According to the data in Figures 3 and 7, in the condition of RT ) 1.2-1.5 and an unstaged constant value for RM ) 0.72, when RS is gradually increased, the ratio of average increase rate of NOx is 3.50, 3.47, 9.38, and 1.93, with R90 ) 2%, 5%, 9%, and 12%, respectively. Therefore, staged combustion is conducive to the burnout of char in the primary combustion, and it will foster an earlier release of nitrogen so that the NOx can be reduced before the injection of the burnout air. These are clearly the factors causing lower NOx emissions, which justifies why the RM at the initial stage is the main influencing factor of NOx emission. Through the above analysis, we can see what factors will reduce NOx emission when using anthracite coal. Timely ignition must be guaranteed, combustion must be strengthened, the combustion process must be shortened, and nitrogen must be released as soon as possible. It is generally believed that airstaged combustion is not suitable for anthracite coal. On the contrary, appropriate air staging not only encourages initially strengthened burning, but also lowers the release of charnitrogen in later burning. This also shows that, in the reconstruction of air-staged combustion, the W-flame boiler of FW Co. can achieve good results, which is further shown in the NOx emission effects of this experiment. 3.4. The Impact of Air-Staged Combustion on the Carbon Content in Fly Ash. The air staging in engineering practice can reduce NOx emission, but at the same time it may have a negative impact on the burnout of pulverized coal. This is shown by the fact that unburnt carbon (marked by UBC) rises after air staging. Therefore, the corresponding carbon content in fly ash is analyzed for the staged combustion of Yangquan anthracite coal, as shown in Figure 9. As compared to the unstaged operation mode, UBC in most staged operation modes did not increase. This was especially true for pulverized coal with smaller R90 fineness values, such as R90 ) 2%, which dropped after staging. Only for pulverized coal with large R90 values, R90 ) 12% for example, did UBC increase after staging, (26) Richards, G. H.; Marion, J. L.; Maney, C. Q. Presented at the 27th International Technical Conference on Coal Utilization & Fuel Systems, March 4-7, 2002.

and the decrease was only to a small extent with only a minor impact. In addition, burnout air position has an impact on UBC. Ms ) 0.87 especially can significantly increase the UBC. Therefore, a burnout air position that is appropriately staged needs to be chosen. From the air-staged level, the UBC of deep air-staged combustion is higher than that of shallow air-staged, but it is still lower than that of unstaged combustion. Therefore, from the point of view of UBC, shallow staging should be chosen. The principle that air staging of Yangquan anthracite coal has little effect on UBC can be analyzed as follows. According to the principle of pulverized-coal combustion, the total rate of combustion is determined not only by the rate of chemical reactions, but also by the intensity of oxygen supply to the reaction zone. The total reaction rate of coal particle (according to the diffusive-kinetic theory of fuel combustion) is given by the expression:27 Ks )

COg 1 1 + Rd kr

(1)

Here, KS is the total reaction rate of a coal particle, g/(m2 · s); COg is the oxygen concentration near the coal particle surface, g/m3; Rd is the mass-transfer coefficient, m/s; and kr is the reaction rate constant, m/s. The relative size of Rd and kr determines the burning mode of the coal particles. When kr . Rd, the diffusion resistance is significantly greater than the chemical reaction resistance. This takes place in combustion environments of high temperature. The burning rate of the coal depends on how fast the oxygen spreads to the carbon surface, and the combustion temperature has little effect. This regime is known as diffusion combustion. However, when Rd . kr, the diffusion resistance is significantly smaller than the chemical reaction resistance. The burning rate is decided by the chemical reaction itself and has little to do with diffusional factors. In this case, the burning rate is decided by chemical reaction dynamical factors, such as the combustion temperature, and this regime is known as dynamic combustion. The combustion model of a coal particle can be judged according to the CeMe¨HOB number:28 Sm )

Rd kr

(2)

Here, kr is decided by the Arrhenius law: (27) Smoot, L. D.; Smith, P. J. Coal Combustion and Gasification; Plenum Press: New York, 1985. (28) Cen, K. F.; Yao, Q.; Luo, Z. Y.; Li, X. T. AdVanced Combustion Principle (Chinese); Zhe Jiang University Press: Huangzhou, 2000.


( )

kr ) k0 exp -

E RTp


(3)

Here, E is the activation energy of the coal, J/mol; R is the universal gas constant, J/(mol · K); and Tp is combustion temperature of coal particle, K. Rd is decided by Nusselt criterion. Rddp (4) D Here, D is the diffusion coefficient, and dp is the diameter of the coal particle, m. When dp is small, for example, when the slip velocity of pulverized coal and carried air flow is very small, then Nu is approximately 2. Nu )

Sm )

2D krdp

Figure 10. The effect of primary combustion air rate on theoretical combustion temperatures and relative ratio of burning rate for Yangquan anthracite coal with different burnout degree (RT ) 1.2).

(5)

According to the CeMe¨HOB principle, when Sm is larger than 9, coal combustion is a dynamic combustion.28 When the reaction parameters of coal particles response, that is, E and k0, are certain, then smaller coal particles lead to combustion that is closer to dynamic combustion. The particle size of pulverized coal in power plants is very small, and in most cases the combustion is thus dynamic. Especially in the initial combustion stage, Yangquan anthracite pulverized coal combustion can be considered to be in the dynamic combustion regime. During this period, the pulverized coal stays in the ignition or early combustion stage. The combustion environment temperature is still low. Obviously, it is more important to enhance the temperature quickly to accelerate the ignition of pulverized coal. Here, we analyze this judgment according to the CeMe¨HOB principle. By analyzing the largest R90 ) 12%, which is chosen for the Yangquan anthracite coal, we decide that dp is 100 µm. For the bituminous coal with very good combustion characteristics, E is 75 kJ/mol, D is 3.5 × 10-4 m2/s,27 the value of Tg ) 1600 K, and it is judged to be the combustion mode with a combustion temperature of Tg ) 2000 K. Note that the data above were selected as a more conservative judgment for the dynamic combustion regime. The combustion temperature of pulverized coal (particle surface temperature) is higher than the combustion temperature of the environment.29 However, it is very difficult to obtain accurate Tp by theory or experiment.30 Here, suppose that the temperature difference between Tp and Tg is an invariant, which is 500 K, then Tp ) 2500 K. According to the calculation in formula 5, Sm ) 13.6 > 9, so coal combustion is in dynamic combustion mode. For Yangquan anthracite coal, the value of E is even bigger, and when Tg ) 2000 K, D is slightly bigger. Therefore, the Sm of Yangquan anthracite coal is bigger than 13.6. It is in a dynamic combustion regime, and, therefore, the temperature of the combustion environment determines its combustion rate. In the 1960s, Bee´r and Marsden, and Mironov, carried out research into burning tests of different anthracite pulverized coal by one-dimensional droptube furnace. The results showed that the burning rate of anthracite coal is much lower than that with the pure diffusion control. The results also proved that anthracite coal combustion in the usual furnace of a boiler is a dynamic combustion regime.30 Figure 10 shows that calculating the quality parameters of Yangquan anthracite coal gives us the relationship between theoretical combustion temperatures of different burnout degrees and fM. It can be seen that when the rate of the primary combustion air is smaller, the theoretical combustion temperature (29) Solomon, P. R.; Carangelo, R. M.; Best, P. E.; Markham, J. R.; Hamblen; et al. Fuel 1987, 66, 897–908. (30) Field, M. A.; Gill, D. W.; Morgan, B. B.; Hawksley, P. G. W. Combustion of PulVerized Coal; Institute of Energy: London, 1983.

is higher. When the burnout degree is smaller, the theoretical combustion temperature is very low, which is adverse for combustion. Air staging can raise the theoretical combustion temperature and increase the temperature conducive to coal combustion. The figure also shows the relative changes of kr caused by the change in the theoretical combustion temperature. The kr of fM ) 1 is taken as the gauge, and Sr is the ratio of the value of fM < 1 and kr. Suppose we use the value of E (149.9 kJ/mol) obtained in low temperature (

Effect of Air-Staging on Anthracite Combustion and NOx Formation

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