Energy Fuels 2010, 24, 375–378 Published on Web 11/10/2009
: DOI:10.1021/ef900859q
Influence of the Coal Feed Rate on Lean Coal Ignition in a Full-Scale Tiny-Oil Ignition Burner Zhengqi Li,* Chunlong Liu, Yang Zhao, and Zhichao Chen School of Energy Science and Engineering, Harbin Institute of Technology, 92, West Dazhi Street, Harbin 150001, P.R. China Received August 6, 2009. Revised Manuscript Received October 20, 2009
A tiny-oil ignition burner was proposed to reduce oil consumption in firing up and partial-load operation. To investigate the influence of the feed rate on lean coal ignition in the tiny-oil ignition burner, full-scale reacting-flow experiments were performed using an experimental setup. The ignition burner was identical to the burner used in an 800 MWe utility boiler. As coal feed rates increased from 1 to 5 t/h, the gas temperatures at equivalent measuring points at the exits of the first and second combustion chambers and on the burner center line decreased gradually. O2 concentrations at the exit of the burner were 1.79-7.18%. Increasing the coal feed rate decreased coal burnout and the release rates for C and H at equivalent points at the exits.
heats the mixture of air and pulverized coal. The pulverized coal near the wall of the metal tube ignites after a certain distance, and the flame of the ignited pulverized coal diffuses radial to the center of the burner. All the pulverized coal ultimately ignites. Induction heating can supply a reliable and convenient source of energy to ignite the pulverized coal stream, but the use of this technology in a utility boiler has not been reported. Tiny-oil ignition involves ignition of concentrated pulverized coal in a first combustion chamber using the heat of oil combustion and then ignition of dilute pulverized coal in a second combustion chamber with the heat of oil combustion and the combustion of the concentrated pulverized coal such that the pulverized coal ignites stage by stage. This technology has been successfully used for tangentially fired burners in boilers separately burning bituminous coal and lean coal by Jia et al.5 and Huang.6 Fang et al.7 investigated tiny-oil ignition for a swirl burner. The oil combustion chamber is axially inserted into the burner at its bend. In this design, a single-stage coal concentrator is set on the outside of the oil combustion chamber. The first, second, and third stages are set in sequence after the oil combustion chamber. This arrangement is used for the boilers burning bituminous coal with swirl burners. However, the single concentrator provides poor rich/lean coal separation and is not of benefit to coal ignition or NOx reduction. Because of the low volatile content and the high ignition temperature of lean coal, there has been no report of successful tiny-oil ignition of boilers burning lean coal with swirl burners. Tiny-oil ignition, centrally fuel-rich burners have been proposed (see Figure 1). Using multiplestage concentrations allows for a higher pulverized coal concentration in the first combustion chamber. The ignition temperature of lean coal is reduced in the first combustion chamber, which results in the successful ignition of pulverized lean coal. The process is summarized as follows. Atomized oil
1. Introduction A boiler is usually fired up by preheating with an oil gun with an output of 0.6-1.5 t/h. After the boiler hearth reaches a certain temperature, pulverized coal is input, and as the hearth temperature increases, more and more pulverized coal is input. At the same time, the output from the oil gun is decreased gradually until the pulverized coal can be ignited without oil combustion. For example, in starting up a lean coal-fired 300 MWe utility boiler, about 100 tons of fuel oil is consumed. Concerns over increasing financial costs for pulverized coal-fired power stations arising from oil consumed in the firing-up process and partial-load operation have spurred interest in developing oil-free and tiny-oil ignition burners. There have been various studies on oil-free ignition burners. Sugimoto et al. studied the stabilization of pulverized coal combustion for a plasma-assisted burner.1 Kanilo et al. investigated the ignition and combustion of pulverized coal by a microwave-assisted burner.2 In China, Zhang et al. described the application of plasma ignition technology to bituminous coal-fired boilers.3 However, for both the plasmaassisted and microwave-assisted burners, the main problems are the difficulty in extending the capacity of the burner and the frequent maintenance required during operation. Li et al. investigated induction-heating ignition of a pulverized coal stream.4 Induction-heating oil-free ignition of pulverized coal system comprises a power supply system and induction-heating ignition system. Modulated intermediate-frequency current is the input to the induction-heating ignition system. The heating part of the ignition system is a metal tube inside an induction coil. Acting as the primary air duct, the metal tube *To whom correspondence should be addressed. Telephone: þ86 4518648854. Fax: þ86 45186412528. E-mail:
[email protected]. (1) Masaya, S.; Kaoru, M.; Koichi, T.; Oleg, P. S.; Masao, S.; Masakazu, N. Thin Solid Films 2002, 407, 186–191. (2) Kanilo, P. M.; Kazanesev, V. I.; Rasyuk, N. I.; Schunemann, K.; Vavriv, D. M. Fuel 2003, 82, 187–193. (3) Zhang, X. Y.; Luo, Z. B.; Zhang, S. K.; Zou, G. W.; Jiang, B. H. China Power 2003, 36, 25-29. (In Chinese) (4) Li, W. J.; Cen, K. F.; Zheng, C. G.; Zhou, J. H.; Cao, X. Y. Fuel 2004, 83, 2103–2107. r 2009 American Chemical Society
(5) Jia, Y. C.; Feng, W. X.; Wang, Z. J.; Du, J. W.; Ju, Y. H. Heilongjiang Electr. Power 2005, 27 (4), 263–266. (6) Huang, W. F. Electr. Equip. 2008, 9 (3), 27–30. (7) Fang, L.; Tao, J.; Li, J.; Li, F. R.; Zheng, J. P. Zhejiang Electr. Power 2008, 6, 24–26.
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Energy Fuels 2010, 24, 375–378
: DOI:10.1021/ef900859q
Li et al. Table 1. Equipment Used and Technical Characteristics 1.2 m3 0.5-2.0 MPa 30-50 kg/h 0-150 kg/h 0-0.8 MPa/ 0.9 N m3/min 5000 Pa/ 0-20 000 m3/h 1-5 t/h
oil tank oil pump main oil gun auxiliary oil gun air compressor primary air blower coal feeder
Table 2. Operation Parameters Figure 1. Experimental setup (all dimensions in millimeters).
oil type oil pressure oil flow rate of main oil gun oil flow rate of auxiliary oil gun compressed air pressure compressed air flow rate primary air velocity primary air temperature coal feed rates
from the main oil gun (an oil gun with a high-energy igniter) is ignited by the high-energy igniter and burns in an adiabatic chamber. An oil flame forms and subsequently ignites the atomized oil from the auxiliary oil-gun (an oil gun without a high-energy igniter). Afterward, the igniter is turned off, and the oil flame is maintained by the two oil guns and burns steadily. The two oil guns are arranged in the central pipe. Cone separators are installed in the primary air-coal mixture duct to concentrate the pulverized coal into the central zone of the burner. The fuel-rich primary air-coal mixture passes into the first combustion chamber whereby the fuel-rich primary air-coal mixture is ignited by a high-temperature oil flame formed by the main and auxiliary oil guns. Next, the burning pulverized coal and oil flame from the first combustion chamber are directed into the second combustion chamber, where the coal is ignited. During firing-up using the two oil guns in the presence of coal, instantaneous ignition is achieved by the oil flame and a steady burn of the pulverized coal develops. The flame produced by the two oil guns and pulverized coal is bright and steady during the entire process. After the boiler has fired up, the main and auxiliary oil guns are shut down and the burner switches operation to become a centrally fuel-rich burner, characterized by high combustion efficiency and low NOx emission.5 The influence of the coalfeed rate on the lean coal ignition in the full-scale tiny-oil ignition burner was investigated in the present work.
diesel oil 1.0 MPa 45 kg/h 115 kg/h 0.4 MPa 0.9 N m3/min 17 m/s 15 °C 1-5 t/h
Table 3. Ultimate Analysis Results and Other Characteristics of 0# Light Diesel Oil Used in the Experiments carbon (%) 85.33 ash (%) 0.38
hydrogen (%)
sulfur (%)
nitrogen (%)
oxygen (%)
13.29
0.25
0.04
0.66
moisture (%)
gross calorific value (kJ/kg)
flash point (°C)
density (kg/m3)
0.05
41 320
62
870
Table 4. Characteristics of Lean Pulverized Coal Used in the Experiments proximate analysis (as air-dry basis, wt. %) volatile matter 12.32
ash
moisture
fixed carbon
gross calorific value (kJ/kg)
20.94
1.02
65.72
25 810
ultimate analysis (as air-dry basis, wt. %) carbon 67.35
2. Experimental Setup
hydrogen
sulfur
nitrogen
oxygen
3.57
1.54
0.97
4.61
sampled at the exit of the tiny-oil ignition burner. Gases were sampled using a water-cooled stainless steel probe and analyzed online using a Testo 350 M instrument.8 The water-cooled stainless steel probe was held at the exit of the burner by a bracket and used to cool the high temperature gas. The probe consisted primarily of a water-inlet pipe, water-outlet pipe, sampling tube, outer pipe and supporting components. The high-pressure cool water coming from the water-inlet pipe cooled the sampling tube and then flowed out via the water-outlet pipe. Water was circulated by a water pump. The gas was sampled in the sampling tube. If gas enters this tube, the temperature deceases rapidly and the pulverized coal stops burning. Samples passed through filtrating devices and sucked into a Testo 350 M gas analyzer by a pump for subsequent analysis. The accuracy of the Testo 350 M gas analyzer was 1% for O2 and 5% for CO. Each sensor was calibrated before measurement. The maximum CO concentration was 10 000 ppm in this equipment. Table 1 lists the equipment used and their technical characteristics. Table 2 lists operating parameters. Table 3 presents ultimate analysis results and other characteristics of the 0# light diesel oil used in the experiments. Table 4 gives the characteristics of the lean pulverized coal used in the experiments. The pulverized coal fineness was R90 = 7.5%, that is, 92.5% of the particles passed through a sieve with 90 μm holes.
Figure 1 shows the tiny-oil ignition apparatus. The ignition burner was identical to a burner used in an 800 MWe utility boiler. The measurement and control of the feed rate were as follows. Pulverized coal was stored in the coal hopper. A sensor in the coal hopper weighed the pulverized coal online. The feeder transported pulverized coal from the hopper. The rotational speed of the feeder was adjusted by a frequency converter to control the coal feed rate. The coal feed rate was equal to the reduction in weight in the coal hopper divided by experimental time. The feeder rate was calibrated multiple times and the magnitude of the feed rate variation was 5%. The down pipe of the feeder was connected with the primary air pipe. The pulverized coal was then carried to the tiny-oil ignition burner by the primary air. Oil was drawn from the oil tank and sent to the main and auxiliary guns. The oil guns employed mechanical and air atomization. Administered as atomized air, compressed air entered the oil guns. A small fraction of the atomized air was also consumed in oil combustion. The main body of air consumed in oil combustion was supplied by another blower. The pulverized coal was ignited in the primary air duct. In the experimental setup, there was no separation into inner and outer secondary air. A nickel chromium-nickel silicon thermocouple was placed inside a stainless steel probe. The end of the sheath was exposed in the burner while taking temperature measurements, and the measurement error was less than 8%. The temperatures of all gases were measured at the center of the burner and at the exits of the first and second combustion chambers. Ash samples were
(8) Li, Z. Q.; Jing, J. P.; Chen, Z. C.; Ren, F.; Xu, B.; Wei, H. D.; Ge, Z. H. Combust. Sci. Technol. 2008, 180 (7), 1370–1394.
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Energy Fuels 2010, 24, 375–378
: DOI:10.1021/ef900859q
Li et al.
Figure 2. Profiles of gas temperatures measured along the burner center line for the burner with two oil guns at different coal feed rates.
3. Results and Discussion 3.1. Gas Temperature Distribution. Figure 2 depicts the profiles of the gas temperature measured along the burner center line; here, x is the measured distance from the central pipe exit (see Figure 1). During firing up using the two oil guns in the absence of coal, gas temperatures first decreased and then increased with increasing distance. Some of the oil from the main and auxiliary oil guns burnt out in the central pipe. High-temperature gas then formed. Cold air diffused into the gas, resulting in a gradual decrease in gas temperature. The cold air then diffused into the center region of the burner, and it was supplied as part of the air consumed in oil combustion. As the gas flowed toward the burner nozzle, the oil combustion released more heat and the gas temperature increased. During firing up with the two oil guns in the presence of coal, gas temperatures again first decreased and then increased with increasing distance. The high-temperature oil flame heated the pulverized coal. As the pulverized coal and cold air absorbed the heat, the gas temperature decreased gradually. After the pulverized coal flowed some distance, it ignited. The gas temperature increased with the coal combustion. Figure 3 shows the profiles of gas temperature measured at the exits of the first and second combustion chambers, at radii r1 and r2, respectively, from the center line of the burner (see Figure 1). During firing up with the two oil guns in the absence of coal, the gas temperatures on the side of the auxiliary oil gun (r1 < 0, r2 < 0) were higher than those on the side of the main oil gun (r1 > 0, r2 > 0) because of the higher flow rate of the auxiliary oil gun. For example, at the first combustion chamber exit, gas temperatures at r1 = -57 and -114 mm, where the auxiliary oil gun was mounted, were 1002 and 1005 °C, respectively, and temperatures at r1 = 57 and 114 mm, where the main oil gun was mounted, were 657 and 339 °C, respectively. Gas temperatures at the second combustion chamber exit were higher than those at the first combustion chamber exit because more heat was released through oil combustion than heat was absorbed by cold air. During firing up with the two oil guns in the presence of coal, the pulverized coal burned adequately, releasing heat in the process. Gas temperatures at the second combustion chamber exit were higher than those at the first combustion chamber exit. By increasing the coal feed rates, more heat was absorbed by the pulverized coal, thereby decreasing the temperature. At the same time, much more coal ignited, and the heat released from coal combustion increased. The temperature of the pulverized coal then
Figure 3. Profiles of gas temperatures at exits of (a) the first and (b) second combustion chambers for the burner with two oil guns at different coal feed rates.
Figure 4. The flame produced while operating the two oil guns (a) in the absence of coal and (b) with coal at a feed rate of 3 t/h.
increased. As coal feed rates increased from 1 to 5 t/h, the heat released from coal combustion was less than the absorbed heat. Thus, gas temperatures at equivalent measuring points at the exits of the first and second combustion chambers and on the burner center line (see Figure 2) decreased gradually. As the radius increased, gas temperatures decreased gradually. Wall temperatures of the second combustion chamber were less than 313 °C. At low temperature, the burner wall could withstand ignition. Figure 4 shows photographs of the oil and coal flame. 377
Energy Fuels 2010, 24, 375–378
: DOI:10.1021/ef900859q
Li et al. Table 5. O2 concentration at the center point of the burner exit coal feed rate (t/h) O2 (%)
1 1.79
2 3.05
3 3.94
β ¼ 1 -½ðwix =wik Þðwk =wx Þ
4 4.61
5 7.18
ð2Þ
where wix is the weight percentage of the species of interest in the char sample, wik is the weight percentage of the species of interest in the input coal.9 The distributions of coal burnout and the release rates for C and H were similar for different coal feed rates. The coal burnout and release rates for C and H on the side of the auxiliary oil gun were greater than those on the side of the main oil gun. They decreased with an increase in the coalfeed rate. For example, at the center of the burner (r2 = 0), the coal burnout and release rates for C and H decreased from 33, 28, and 98% to 9, 9, and 23% as the coal-feed rate increased from 1 to 5 t/h. 3.3. Gas Compositions. Table 5 lists O2 concentrations at the center point of the burner exit. O2 concentrations ranged 1.79-7.18% for coal-feed rates of 1, 2, 3, 4, 5 and t/h. 4. Conclusion (1) When the primary temperature and air velocity were 15 °C and 17 ms-1 respectively, the oil flow rate was 160 kg per hour and ignition was successful when the lean coal-feed rate was increased from 1 to 5 t/h. Wall temperatures of the second combustion chamber were less than 313 °C. With the subsequent low temperature, the burner wall could withstand ignition. O2 concentrations at the exit of the burner were 1.79-7.18%. (2) During firing up using the two oil guns in the absence and presence of coal, gas temperatures decreased first and then increased with increasing distance on the center line. As coal feed rates increased from 1 to 5 t/h, the gas temperatures at equivalent measuring points at the exits of the first and second combustion chambers and on the burner center line decreased gradually. (3) Distributions of coal burnout and release rates for C and H were similar for different coal-feed rates. Increasing the coal feed rate decreased coal burnout and the release rates for C and H at equivalent points at the exits.
Figure 5. Coal burnout and release rates for C and H at the exit of the burner.
3.2. Coal Burnout and Rate of Release of C and H at the Exit of the Burner. Figure 5 shows the coal burnout and rate of release of C and H at the exit of the tiny-oil ignition burner. Coal burnout was calculated using ψ ¼ ½1 -ðwk =wx Þ=ð1 -wk Þ
Acknowledgment. This work was supported by the Hi-Tech Research and Development Program of China (contract No. 2007AA05Z301), Postdoctoral Foundation of Heilongjiang Province (LRB07-216), Heilongjiang Province via 2005 Key Projects (contract No. GC05A314), and Hi-Tech Research and Development Program of China (863 program) (contract No. 2006AA05Z321).
ð1Þ
where ψ is the coal burnout factor, wk is the ash weight fraction in the input coal, and wx is the ash weight fraction in the char sample. β is the percentage release of components (C and H), which was calculated using
(9) Costa, M.; Silva, P.; Azevedo, J. L. T. Combust. Sci. Technol. 2003, 175, 271–289.
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