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Energy & Fuels 2009, 23, 3558–3564
Experimental Study on NOx Emission and Unburnt Carbon of a Radial Biased Swirl Burner for Coal Combustion Shan Xue,* Shi’en Hui, Qulan Zhou, and Tongmo Xu State Key Laboratory of Multiphase Flow in Power Engineering, School of Energy and Power Engineering, Xi’an Jiaotong UniVersity, Xi’an 710049, China ReceiVed January 23, 2009. ReVised Manuscript ReceiVed May 27, 2009
Pilot tests were carried out on a 1 MW thermal pulverized coal fired testing furnace. Symmetrical combustion was implemented by use of two whirl burners with dual air adjustment. The burnout air device was installed in various places at the top of the main burner, which consists of a primary air pipe with a varying crosssection and an impact ring. In the primary air pipe, the air pulverized coal (PC) stream was separated into a whirling stream that was thick inside and thin outside, thus realizing the thin-thick distribution at the burner nozzle in the radial direction. From the comparative combustion tests of three coals with relatively great characteristic differences, Shaanbei Shenhua high rank bituminous coal (SH coal), Shanxi Hejin low rank bituminous coal (HJ coal), and Shanxi Changzhi meager coal (CZ coal), were obtained such test results as the primary air ratio, inner secondary air ratio, outer secondary air ratio, impact of the change of outer secondary air, change of the relative position for the layout of burnout air, change of the swirling intensity of the primary air and secondary air, etc., on the NOx emission, and unburnt carbon content in fly ash (CFA). At the same time, the relationship between the NOx emission and burnout ratio and affecting factors of the corresponding test items on the combustion stability and economic results were also acquired. The results may provide a vital guiding significance to engineering designs and practical applications. According to the experimental results, the influence of each individual parameter on NOx formation and unburned carbon in fly ash agrees well with the existing literature. In this study, the influences of various combinations of these parameters are also examined, thus providing some reference for the design of the radial biased swirl burner, the configuration of the furnace, and the distribution of the air.
1. Introduction Across the world, and especially in China, coal combustion has induced a serious air pollution problem, which makes it of great significance to seek advanced combustion technologies with low pollution and high efficiency. The mechanisms of NOx emission and the control of NOx emission have been studied since the 1950s, and great progress was made from the 1970s to 1980s. Many kinds of burners with low NOx emission were designed and experimentally studied.1-23 Okamoto et al.1 achieved low NOx combustion by applying circular corner firing in combination with the PM (pollution minimum) burner, the in-furnace NOx removal system, and the high fine pulverized coal (PC). Kimoto et al.2,3 made the swirl of primary air inhibited by a straightener to reduce NOx efficiently and found that with a proper straightener coefficient the unburned carbon in the fly ash was reduced very efficiently with a little increase in the NOx emission at lower load by keeping the coal concentration at a higher value. An et al.4 studied the characteristics of NOx emission reactions in low NOx burners, and conventional impeller less pulverized coal burners experimentally and numerically, resulting in reducing NOx emission by 28.6% with the low NOx burners instead of conventional impeller less pulverized coal burners. Wan et al.5 numerically investigated the parameters controlling the formation of CFA from coal reburned in a coal-fired boiler and claimed that by moving some air from the lower burners to the upper burners to compensate * To whom correspondence should be addressed. E-mail: xindiyzh@ 163.com.
for the shorter residence time significantly, it is possible to reduce the overall LOI (loss on ignition) without adverse impacts on the NOx emission. Stephenson et al.6 developed a burnout predictor code including both coal and plant effects as well as a burnout model, and the predicted C-in-ash agreed well with the plant data. The results showed that the calculated sensitivities to changes under plant conditions can depend enormously on the state of the plant. Man et al.7 researched the NOx emission in air-staged combustion of pulverized coals. With the lower NOx emission technologies and the efficient equipment, the pollution has been greatly reduced. Hesselmann et al.8 studied the fuel combustion in a 160 kWth NOx reduction test facility (NRTF). A significant NOx reduction (60%) can be achieved with the use of natural gas as a reburned fuel. Jun et al.9 presented an experimental study on the influence of coal quality on nitrogen oxide (NOx) emission and unburned carbon in fly ash. Coal quality characteristics, especially the ash content, and operation conditions have been studied on the influence of furnace burnout performance with a direct flow burner. Gu et al.11 investigated the NOx emission in a swirl pulverized coal burner by numerical simulation. The results show that the minimum outlet NOx emission appears at the maximum Ldav (average particle penetration depth) and τeav (average effective time) and the minimum outlet NOx emission, and the maximum Ldav and the maximum τeav occur at the same time. Zhang et al.22 established a stable combustion with a special burner for burning pure petroleum coke and anthracite coal and carried out a series of experiments to test the burning performance.
10.1021/ef900055s CCC: $40.75 2009 American Chemical Society Published on Web 06/08/2009
Study on NOx Emission and Unburnt Carbon
In the existing literature, however, the studies of the influences on the burning performance of a burner are not comprehensive yet. In this Article, a new type of radial biased swirl coal burner mounted on a 1 MW pulverized coal test furnace is utilized to clarify NOx emission and coal combustion characteristics. The burner has a primary air pipe with a continuously changing cross-section and an impact ring. Inside the primary air pipe, a mixture of PC and air is divided into two streams, that is, the inner PC rich annular jet and the outer PC lean annular jet, to provide a biased firing. The inner and outer PC streams can be further divided by the impact ring, and most of the dense PC is concentrated outside of the primary air pipe and flows through the flaring of primary air to strengthen the central recirculation zone and improve the ignition characteristics of the burner. The integral-movable axial spinning swirlers are installed inside the inner and outer secondary air nozzle with flaring areas to form an inverse flow zone, organize the prophase combustion, and blend the air and coal. A gear-shaped ring is installed at the primary air inlet to improve coal ignition and keep the combustion stable. Moreover, the hard-to-burn coal can be well burned with this burner. The burner’s adaptability to various coals has been investigated in the experiments. The influence of different coals and aerodynamic parameters on reduction of NOx emission has been studied in hot state tests too. To reduce both NOx emission and CFA, not only the air staged combustion technology of the burner but also an overfire air (OFA) nozzle was used. 2. Experimental Details 2.1. Experimental System. The thermal tests of the swirl symmetrical combustion were conducted on a 1 MW thermal testing (1) Okamoto, A.; Tokuda, K.; Kaneko, S.; Sato, S.; Gengo, T.; Wakabayashi, Y.; Iida, Y. Fuels Combust. Technol. DiV. 1999, 23, 299– 303. (2) Kimoto, M.; Tsuji, H.; Makino, H.; Kiga, T. J. Jpn. Inst. Energy 1999, 78, 404–415. (3) Kimoto, M.; Ikeda, M.; Makino, H.; Kiga, T. Fuels Combust. Technol. DiV. 1999, 23, 293–298. (4) An, E.; Yu, J.; Zhu, J.; Wang, Y.; Yan, G.; Xiao, B.; Shi, M.; Song, Q.; Zhou, H.; Yang, D.; Chen, Y. Power Eng. 2006, 26, 784–789. (5) Wan, H.; Yang, C.; Adams, B. R.; Chen, S. L. Fuel 2008, 87, 290– 296. (6) Staged Henson, P. Fuel 2007, 86, 2026–2031. (7) Man, C. K.; Gibbins, J. R.; Witkamp, J. G.; Zhang, J. Fuel 2005, 84, 2190–2195. (8) Hesselmann, G. J. Fuel 1997, 76, 1269–1275. (9) Jun, X.; Sun, X.; Hu, S.; Yu, D. Fuel Process. Technol. 2000, 68, 139–151. (10) Liu, Y.; Gupta, R.; Elliott, L.; Wall, T.; Fujimori, T. Fuel Process. Technol. 2007, 88, 1099–1107. (11) Gu, M.; Zhang, M.; Fan, W.; Wang, L.; Tian, F. Fuel 2005, 84, 2093–2101. (12) Bosoaga, A.; Panoiu, N.; Mihaescu, L.; Backreedy, R. I.; Ma, L.; Pourkashanian, M.; Williams, A. Fuel 2006, 85, 1591–1598. (13) Tsumura, T.; Okazaki, H.; Dernjatin, P.; Savolainen, K. Appl. Energy 2003, 74, 415–424. (14) Zhang, H.; Xu, X. China Particuol. 2004, 2, 230–233. (15) Li, Z.; Ren, F.; Zhang, J.; Zhang, X.; Chen, Z.; Chen, L. Fuel 2007, 86, 2457–2462. (16) Kurose, R.; Makino, H.; Suzuki, A. Fuel 2004, 83, 693–703. (17) Bris, T. L.; Cadavid, F.; Caillat, S.; Pietrzyk, S.; Blondin, J.; Baudoin, B. Fuel 2007, 86, 2213–2220. (18) Sara, W.; Tom, S.; Brian, E.; Brian, S.; Jim, S. EnViron. Control DiV. Publ. 1996, 1, 241–248. (19) Jochem, M.; Schreier, W.; Thierbach, H. EnViron. Control DiV. Publ. 1996, 119–149. (20) Wang, J. J. Eng. Therm. Energy Power 2002, 17, 632634 + 656. (21) Qin, Y.; Sun, R.; Li, Z.; Sun, S.; Wu, S. Proc. Chin. Soc. Electrical Eng. 2000, 20, 72–76. (22) Zhang, H.; Yue, G.; Lu, J.; Jia, Z.; Mao, J.; Fujimori, T.; Suko, T.; Kiga, T. Proc. Combust. Inst. 2007, 31, 2779–2785. (23) Mao, J.; Zhao, S. Sci. Press (in Chinese) 1998, 209–210.
Energy & Fuels, Vol. 23, 2009 3559 Table 1. Basic Properties of Testing Coals name
unit
SH coal
HJ coal
CZ coal
Car Har Oar Nar Sar Aar Mar Vdaf Qnet,ar R90
% % % % % % % % kJ/kg %
73.63 4.54 11.38 0.95 0.34 6.56 2.6 32.76 28 370 12.47
56.55 3.14 5.46 0.69 2.35 29.47 2.34 19.26 21 790 9.08
69.83 3.09 1.79 0.85 5.01 18.23 1.2 11.48 26 860 2.44
facility with a testing thermal power of 0.9 MW. The basic characteristics of the three testing coals, SH coal, HJ coal, and CZ coal, are as shown in Table 1. As there are numerous factors affecting combustion, only the main affecting factors are dealt with in the tests, that is, the change of the air stream rate, the relative position of the OFA nozzle, and the change of the swirling intensity of the inner and outer secondary air. The thermal tests aimed to control the combustion temperature in the main combustion area and to reduce as much as possible the NOx emission by use of the air staged combustion technology of the burner, and by means of changing the aerodynamic parameters of various coals and setting the OFA nozzle at the top of the main burner to lower the CFA. The entire testing system is composed of six subsystems: main body of the testing furnace, blast and induced draft system, PC feeder system, water-cooling system, ignition system, and measuring system as shown in Figure 1. The furnace of the test facility is the core body of the test-bed. The blast and induced draft air and the PC feeder system supply the air and PC needed by combustion. The water-cooling system ensures the cooling of all measuring instruments and devices and the temperature reduction of the flue gas. The ignition system ensures the normal ignition and stable combustion of the testing. The measuring system mainly gathers all original data needed by research. Even though the measuring devices for the current test are able to detect O2, CO, CO2, NOx, and the carbon content in fly ash, the main objective of this Article is to obtain the amount of the NOx emission and the carbon content in fly ash. So, a sampling measuring-opening is set at the outlet of the furnace. After samples of the flue gas and fly ash were taken separately by using a watercooled flue gas sampling gun and a fly ash sampling gun according to the principle of isokinetic sampling, the composition of the flue gas was analyzed by a Gasmet flue gas analyzer, and the contents of the combustible substances were analyzed with a weighing method.
Figure 1. Testing system diagram.
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Table 2. Experimental Operating Conditions name
unit
SH coal
HJ coal
CZ coal
primary air ratio inner secondary air ratio outer secondary air ratio inner secondary air swirling intensity outer secondary air swirling intensity relative position of the OFA
% % %
17.56 12.43 53.85 1.6
17.56 14.93 51.35 1.79
17.56 17.43 48.85 1.79
1.46
1.38
1.21
0.32
0.32
0.32
The flow rates of the primary air, inner and outer secondary air, and OFA were measured by calibrated back to back pitots, the measuring accuracy being (4.5%. Two PC feeders were calibrated by the weighing method with an accuracy of (3.5%. The flue gas analyzer (GASMET FTIR Dx4000) can measure gas concentration as low as 0.2-2 ppm. The instrument is of the multirange calibration type, and the measurement error should be (2%. The total excessive air coefficient selected during the testing was 1.2. The primary air ratio was 12.56%, 17.56%, or 22.56%. The velocity of the primary air was 11.55, 16.15, or 20.75 m/s. The inner secondary air ratio was 7.43%, 9.93%, 12.43%, 14.93%, or 17.43%. The velocity of the inner secondary air was 7.66, 10.24, 12.82, 15.40, or 17.98 m/s. The outer secondary air ratio was 43.85%, 46.35%, 48.85%, 51.35%, or 53.85%. The velocity of the outer secondary air was 24.92, 26.34, 27.76, 29.18, or 30.60 m/s. The swirling intensity of the inner secondary air was 1.38, 1.60, or 1.79. The swirling intensity of the outer secondary air was 1.21, 1.38, or 1.46. To complete the air staged combustion and lower the NOx emission and CFA, the OFA nozzle was mounted above the main burner. The OFA consists of straight air and swirling air. The OFA ratio was 14.5%. The OFA velocity was 59.92/24.88 m/s (straight-flow air velocity/swirling air velocity). The relative position of the OFA nozzle (referred to as the ratio between the distance from the OFA burner to the main burner and the length of the testing furnace) was 0.22, 0.32, or 0.42, respectively. The experimental operating conditions with relatively ideal effects in the thermal test are as shown in Table 2. The aerodynamic parameter for determining the strength of the air stream, called swirling intensity n, is the ratio between the rotational moment of momentum M and axial moment of momentum KL.
n ) M/(KL)
(1)
in which M ) F0Qωqr (N · m); K ) F0Qωz (N); qualitative dimension L ) πr/4 (m); F0 is the air stream density (kg/m3); Q is the volumetric flow of gas (m3/s); ωq is the tangential velocity of air stream (m/s); ωz is the axial velocity of air stream (m/s); and r is the rotational radius of air stream (m). The swirling intensity of the air in each pipe can be changed by adjusting the position of each whirl cone (see Figure 2). The dimension of the model of the testing burner is geometrically similar to the actual burner, the similarity ratio being 1:10. The section of the furnace is depth × width ) a × b ) 1000 × 800 mm2. The furnace height is 3200 mm. During the testing, the
Figure 2. Diagram of the burner structure.
Reynolds number was introduced into the second self-modeling area. The concentration, density, and fineness of the fuel used in the test were the same as those employed in practice. The sectional thermal load of the furnace in the testing qF ) 1.125 MW/m2. As it is difficult for the Stokes criterion number Stk and the Froude criterion number Fr of the grains to satisfy the complete similarity, in the current research, the similarity for the ratio between the momentum and flow rate of the jet, and that for the concentration, density, and fineness of the fuel were first satisfied. The Stk number is a dimensionless number, which describes the behavior of the grains suspended in the liquid.
Stk ) τV/L
(2)
The Fr number is a criterion number denoting the effect of gravity on the flow.
Fr ) V2/Lg
(3)
where τ is characteristic time (s); V is characteristic velocity (m/s); and L is characteristic dimension (m). 2.2. Features of Burner. The PC condensed structure consists of the primary air pipe with a varying section and the impact ring (see Figure 2). By use of inertia, the PC air stream is separated into a mixed stream thick inside and thin outside, thus implementing the thin-thick combustion in the radial direction. The air stream thick inside and thin outside is further subjected to separation during colliding the impact ring, thereby enabling PC to concentrate at the inner side of the primary air stream pipe. The idea of the air stream thick inside and thin outside is aimed at bringing into play the ignition advantage of the central recirculation zone. This point has used the features of the horizontal thin-thick combustion for reference. The inner secondary air stream uses a straight-flow nozzle. The nozzle employs solid movable axial blades to adjust the swirling intensity of the inner secondary air stream and organize the ignition and combustion in the recirculation zone at the first stage. The whirl cone for the outer secondary air stream coupled with the solid movable axial blades is able to adjust the swirling intensity of the outer secondary air stream and organize the rational recirculation zone to intensify the mixture in the last stage, paying equal attention to both ignition and stable combustion. The combustion-stabilizing ring was mounted at the opening of the primary air stream to increase the adaptability of coals in anticipation for this type burner to perform stable combustion of the hard-to-burn coals in a better way.
3. Results and Discussion 3.1. Impact of the Change of the Primary Air Flow to NOx Emission and CFA. The tests were conducted with the total air flow rate kept constant, the primary air ratio increasing by 5%, the inner and outer secondary air ratio decreasing by 2.5% separately, the inner secondary air ratio increasing by 5%, the outer secondary air ratio decreasing by 5%, and the primary air ratio kept constant. Figure 3 shows the test results of the impact of the change of the primary air velocity on NOx emission. As the air-PC flow has realized the distribution that is thick inside and thin outside in the radial direction, high-concentration PC grains are directly injected into the high-temperature recirculation zone when the air flow is sprayed out of the outlet of the burner. When the primary air ratio is increased, the oxygen concentration rises as well. Timely adequate oxygen supply creates a good condition for PC combustion, allowing the local temperature to enhance. The high-temperature and high-oxygen concentration will impel the emission of NOx, resulting in the increase in NOx emission. When the primary air ratio is increased after augmenting to a certain value, the high-temperature recirculation zone will be
Study on NOx Emission and Unburnt Carbon
Figure 3. Primary air ratio change relative to NOx.
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Figure 5. Inner secondary air ratio change relative to NOx.
Figure 4. Primary air ratio change relative to CFA.
Figure 6. Inner secondary air ratio change relative to CFA.
caused to move backward, influencing the ignition and combustion of PC so that the NOx emission begins to dwindle. In light of the same principle, along with the increase in the primary air ratio, the environment of higher temperature and PC concentration will propel the acceleration of the reaction speed of combustion, leading to a drop of the carbon content in fly ash (see Figure 4). When the primary air ratio is increased to a certain value, along with the increase in the primary air ratio, the drawing-in of a great amount of cool air and the back movement of the recirculation zone, the stable combustion, and burnout will be influenced so that the carbon content in fly ash begins to rise.5,6,8,11 3.2. Impact of the Change of the Inner Secondary on the NOx Emission and CFA. Because of the unique structure of the burner, the high-concentration PC grains are fully mixed with the inner secondary air to intensify the ignition and combustion in the main combustion area at the outlet of the burner, thus giving rise to the NOx emission and the decrease in the CFA (see Figures 5 and 6) when increasing the inner secondary air ratio. When the inner secondary air ratio has increased to a certain value, on account of the drawing-in of a large amount of cold air, the high temperature level in the recirculation zone is lowered, enabling for combustion to be slowed down, which is favorable to the curtailment of NOx emission, but results in the increase in the CFA simultaneously.5,6,8,11
3.3. Impact of the Change of the Outer Secondary Air Ratio on NOx Emission and CFA. Similar to the impact results of the inner secondary air ratio, when increasing the outer secondary air ratio, the inner secondary air ratio is lowered at the same time, and the high-concentration PC grains are fully mixed with the inner and outer secondary air by the swirling suction in the central recirculation zone, thus raising the total temperature level in the main combustion area, intensifying the ignition and combustion in the first stage, and causing the rise of NOx emission and the drop of the CFA (see Figures 7 and 8). When the outer secondary air ratio has increased to a certain value, the back movement of the central recirculation zone will take place and the temperature level in the main combustion area will drop, thereby allowing the NOx emission to be reduced to a certain degree and increasing the CFA accordingly.5,6,8,11 3.4. Impact of the Relative Setting Position of the OFA Nozzle on NOx Emission and CFA. A great deal of research has indicated that in the combustion course, the formation of NOx stems mainly from the fuel type NOx. The NOx formed of the volatiles N makes up 60-80%.23 So the key to control NOx emission consists of the control of the pyrogenation of volatiles, and the process and temperature of combustion. The objective of the air-staged combustion is to control the separating-out of volatiles and the amount of NOx emission during combustion. As oxygen-short combustion is realized in the main combustion area, to enable oxygen-combustion not to raise the CFA, an OFA
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Figure 7. Outer secondary air ratio change relative to NOx.
Figure 9. Relative position of OFA nozzle relative to NOx.
Figure 8. Outer secondary air ratio change relative to CFA.
Figure 10. Relative position of OFA nozzle relative to CFA.
burner is set at the top of the main combustion area so that the oxygen supply, turbulent movement, and mixing in the last stage can be intensified and unburned gas and solid grains can undergo complete combustion. Nevertheless, the factors like the mounting position of the OFA nozzle, air ratio, air velocity, etc., affect NOx emission and the burnout of carbon grains as well. It can be seen from Figure 9 that when the relative position of the OFA nozzle and the main burner is moved away from the adjacent place to a distant one, the amount of NOx (converted to O2 ) 6%) formed after the combustion of three coals is all augmented. When continuing to increase the distance between the OFA nozzle and the main burner, it was disclosed that the amount of NOx formed after the combustion of three coals all has a tendency to drop. This lies in that when the OFA nozzle is closer to the main burner, PC does not achieve full ignition and combustion and the OFA mixes related components into the main air stream too early resulting in the reduction of the entire temperature level of the air stream and the combustion velocity and the inhibition of NOx emission. Along with the upward movement of the OFA nozzle, the thermal decomposition of volatiles is completed gradually. At this time, at the stage when the volatiles are fully burnt and the coke starts to ignite, an oxidation temperature and atmosphere environment suitable for combustion and N transformation is formed. When the coke is fully ignited and burnt, the transformation rate of N in the coke is enhanced at the same time. When continuously increas-
ing the distance between the OFA nozzle and the main burner, because of the delayed drawing-in of the OFA air, the amount of oxygen in the main combustion area will be relatively deficient, thus leading to oxygen-short combustion. In this way, NOx emission will be efficiently inhibited. When analyzing the fuel characteristics, even though the N content of the SH coal is the highest (N ) 0.95%) among three coals, the amount of NOx emission is clearly lower than two other coals. This lies in that as the content of volatiles of the SH coal is enormously higher than two other coals (VdaF ) 32.76%), the volatiles will be rapidly ignited and burnt in the main combustion area and consume a large amount of oxygen. As the OFA nozzle is set, the amount of oxygen is subject to greater oxygen deficiency in the main combustion area, which is previously short of oxygen, thus efficiently inhibiting NOx emission. On the contrary, when the low-volatility coal burns, due to the relative delay of ignition and combustion, the oxygen consumption at the first stage is less, and an external environment prone to NOx emission is readily formed, thus forming an external environment favorable for NOx emission. The impact of the relative position of the OFA nozzle on the carbon content in fly ash is shown in Figure 10. The changing tendency for the CFA after the combustion of three different coals is consistent. When the OFA nozzle is very close to the main burner, because of the too early drawing of the OFA air into the main air stream, the overall temperature and combustion
Study on NOx Emission and Unburnt Carbon
Figure 11. Inner secondary air swirling intensity relative to NOx.
Energy & Fuels, Vol. 23, 2009 3563
Figure 13. Outer secondary air swirling intensity relative to NOx.
Figure 12. Inner secondary air swirling intensity relative to CFA.
velocity of the main burner are lowered, which is not beneficial to ignition and stable combustion. As a consequence, the increase in the CFA results. When increasing the distance between the OFA nozzle and the main burner, the full ignition and stable combustion are achieved step by step, the OFA air is complemented in time, and the combustion at the last stage is further intensified, causing a gradual drop of the carbon content in fly ash. Yet when the OFA nozzle is too far away from the main burner, due to the fact that oxygen is not complemented in time leading to oxygen-short combustion, the overall temperature level and combustion reaction velocity get lower at the last stage. The oxygen supply at this time fails to play a too great role in the stable combustion and burnout so that the CFA has a rising tendency. It can be seen from Figure 10 that when burning the CZ coal with the lowest content of volatiles (Vdaf ) 11.48%), its carbon content in fly ash is always lower than two other coals. This stems from the fact that as the OFA air-staged combustion technology is adopted, the excessive air coefficient R in the main combustion area is less than 1; the coal with high content of volatiles is more rapidly ignited and burnt than the coal with low content of volatiles, thus producing carbon black that is very hard to burn out and finally leading to an increase in the CFA.5,6 Summing up the above tests and analyses, it is evident that there is a significant impact of the relative position of the mounting of the OFA nozzle on the emission of NOx and the
Figure 14. Outer secondary air swirling intensity relative to CFA.
CFA. Moreover, the impact tendency for three coals is identical, and only the tendency for the curve of NOx emission and for the curve of the CFA is converse. This means that only by rationally designing and setting the OFA nozzle, can the CFA be reduced as much as possible during lowering the amount of NOx emission simultaneously. The experimental research has also denoted that the adoption of the OFA air-staged combustion technology is very conducive to lowering the NOx emission for high-volatile fuels and to reducing the CFA for low-volatility fuels. 3.5. Impact of Change of Swirling Intensity n on NOx Emission and CFA. From the testing results as shown in Figures 11-14, it is obvious that by increasing the swirling intensity of the inner or outer secondary air, the amount of NOx emission can be raised considerably and the CFA has the dropping tendency, however. This is because the enhancement of the swirling intensity of the inner or outer secondary air can intensify both the recirculation and the swirling suction in the main combustion area and the turbulent mixing at the first stage is strengthened, allowing the high-concentration PC particles to be mixed with the high-temperature flue gas in time so that the ignition and the pyrogenation and combustion of volatiles are intensified. As a consequence, the content of NOx formed during
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combustion is increased with the enhancement of the swirling stream, and the CFA is somewhat lowered. 4. Conclusion There is a significant impact of the change of the primary air ratio on NOx emission and the CFA. When increasing the primary air ratio, the amount of NOx formed during the combustion of three coals is all increased first and decreased second, and the CFA is decreased first and increased second. So, it is needed to set the primary air ratio reasonably and practically. Along with the increase in the inner and outer secondary air ratio, the amount of NOx formed during the combustion of three coals is all increased first and decreased second, and the CFA is decreased first and increased second as well. The burner has relatively fine fuel adaptability. When the inner and outer secondary air ratio and the OFA nozzle are designed and set in a rational way, the amount of the NOx emission can be lowered to 400-700 mg/m3 and possess better economic results. PC thin-thick combustion with the counter impact of the swirling stream coupled with the OFA staged combustion technology is very conducive to the lowering of NOx emission and the CFA. There is a significant impact of the relative position of the OFA nozzle on NOx emission and the CFA. When the relative position of the OFA nozzle and the main burner is fixed from an adjacent place to a farther one, the impact tendency for the three coals is identical. This implies that the curve for
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NOx emission is increased first and lowered second and the curve for the CFA is lowered first and increased second. Only by designing and mounting the OFA nozzle in a rational way, can the CFA be decreased as much as possible while lowering the amount of the NOx emission at the same time. By increasing the swirling intensity of the inner and outer secondary air, the ignition speed and stable combustion in the furnace are conspicuously enhanced, thereby allowing the amount of the NOx emission to rise continuously, and the CFA has a lowering tendency however. In the current tests was obtained the relative position of the OFA nozzle for the first time in an overall way. Test results also covered the aerodynamic parameters of the primary and secondary air, the impact of single factor and comprehensive factors of the swirling intensity of the inner and outer secondary air on the NOx emission and the CFA after burning three different coals, and their impact relationship on the stability and economic results of combustion. The results are of a vital guiding significance to engineering designs and applications for the future. Acknowledgment. The present work is supported by the Major State Basic Research Development Program (No. 2005CB 221206) and National Key Technology R&D Program in the 11th Five Year Plan of China (No. 2006BAK02B03). EF900055S
