Energy Fuels 2010, 24, 4161–4165 Published on Web 07/19/2010
: DOI:10.1021/ef100728f
Experimental Studies on the Effect of the Pulverized Coal Concentration on Lean-Coal Combustion in a Lateral-Ignition Tiny-Oil Burner Zhengqi Li,* Chunlong Liu, Qunyi Zhu, Weiguang Kong, Yang Zhao, and Zhichao Chen School of Energy Science and Engineering, Harbin Institute of Technology, 92, West Dazhi Street, Harbin 150001, People’s Republic of China Received April 8, 2010
Because of the low volatile content and high ignition temperature of lean coal, it is difficult to employ tinyoil ignition technology in the firing up and partial-load operation of a pulverized-coal-fired boiler. To investigate the influence of the coal concentration on lean-coal combustion in a lateral-ignition tiny-oil burner, full-scale reacting-flow experiments were performed using an experimental setup. As the coal concentration increased from 0.27 to 0.80 kg (corresponding to a coal feed rate increasing from 1.5 to 4.5 tons/h), the gas temperatures at the burner center line and 116.5 mm from the burner center line (10 mm from the burner wall) decreased gradually at equivalent measuring points. The gas temperatures on the side with oil guns were higher than those on the opposite side for the same distances from the burner exit. O2 concentrations at the exit of the burner were 0.41-3.64%. The burner resistance resulting from the flow and combustion of pulverized coal was 1400-2200 Pa. Increasing the coal concentration decreased coal burnout and the release rates of C and H at equivalent points at the burner exit.
expenditure of gas, which could reduce energy consumption. Belosevic et al. conducted a numerical study on pulverizedcoal ignition by means of plasma torches in air-coal dust mixture ducts of utility boiler furnaces.4 Results of the predictions suggested the importance of the mass flow rate of extremely hot air-plasma; in particular, the mass flow rate of the much colder air-coal dust mixture strongly affects processes in the duct. Plasma torches produced less thermal power than oil guns. The highest thermal power produced by a torch in China is 300 kW, which is equivalent to the release of heat of 24 kg of 0 light diesel oil per hour.5 Because of the low volatile content and high ignition temperature of lean coal, more thermal power is needed to ignite lean coal. Therefore, it has been difficult to apply plasma ignition technology to lean coal. The thermal power of an oil gun can be adjusted by the oil pressure and the diameter of the oil-gun nozzle. Therefore, the oil gun is suitable for igniting different ranks of coals, including low-volatile-content lean coal. The author has proposed tiny-oil ignition centrally fuelrich burners for use in a wall-fired pulverized-coal utility boiler.6 The effect of the coal-feed rate (2-5 tons/h) on coal ignition was studied in terms of the gas temperature distribution, char burnout, and release rate of C and H at the exit of the burner, gas compositions, and the burner resistance. Ignition was successful under the different coal-feed rates. The flame was bright. The gas temperature exceeded 1000 °C on the center line and was below 200 °C near the wall, which met the operational requirements. To enhance the coal suitability of tiny-oil ignition centrally fuel-rich burners, the author obtained results for the ignition and combustion
1. Introduction Plasma-chemical preparation and tiny-oil ignition technology are widely used for pulverized-coal ignition and combustion stabilization in pulverized-coal utility boilers, with the aim to save on liquid fuel. Gorokhovski et al. studied pulverized-coal combustion using a plasma-assisted burner applied to many utility boilers.1 It was found that the fuel mixture was hot (1350 K) and contained significant concentrations of highly reactive species. Prompted self-ignition was observed for this kind of plasma-assisted burner. Sugimoto et al. studied the stabilization of pulverized-coal combustion with a 10 kW plasma torch for high-, medium-, and lowvolatile-content coals.2 Volatile components were measured by Fourier-transform infrared spectroscopy at 400, 600, and 800 °C in the experiment. Less volatile matter required a higher temperature for the sufficient generation of volatile emissions. This result implied that a high-temperature field and high energy density of plasma are appropriate for encouraging the combustion of inferior coal. Kanilo et al. investigated the ignition and combustion of pulverized coal using a microwave-assisted burner with a 4.5 kW plasma torch for coal feed rates from 3.0 to 50 kg/h.3 On the basis of the experiment, a burner with a capacity of 6 tons/h was designed using this technology and a single burner was found to consume 220 kW. In the startup of a 200 MW boiler using this kind of burner, the electric energy expenditure would be about 15 800 kWh, which is only 10% of the required *To whom correspondence should be addressed. Telephone: þ864518648854. Fax: þ86-45186412528. E-mail:
[email protected]. (1) Gorokhovski, M. A.; Jankoski, Z.; Lockwood, F. C.; Karpenko, E. I.; Messerle, V. E.; Ustimenko, A. B. Combust. Sci. Technol. 2007, 179, 2065–2090. (2) Sugimoto, M.; Maruta, K.; Takeda, K.; Solonenko, O. P.; Sakashita, M.; Nakamura, M. Thin Solid Films 2002, 407, 186–191. (3) Kanilo, P. M.; Kazanesev, V. I.; Rasyuk, N. I.; Schunemann, K.; Vavriv, D. M. Fuel 2003, 82, 187–193. r 2010 American Chemical Society
(4) Belosevic, S.; Sijercic, M.; Stefanovic, P. Int. J. Heat Mass Transfer 2008, 51, 1970–1978. (5) Yao, W. D.; Li, S.; Guo, X. F. Huadian Technol. 2008, 30 (1), 14–18 (in Chinese). (6) Liu, C. L.; Li, Z. Q.; Zhao, Y.; Chen, Z. C. Fuel 2009, 89, 1690– 1694.
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Energy Fuels 2010, 24, 4161–4165
: DOI:10.1021/ef100728f
Li et al. Table 1. Technical Characteristics of Equipment Used oil tank (m3) oil pump (MPa) main oil gun (kg/h) auxiliary oil gun (kg/h) air compressor primary air blower coal feeder (ton/h)
1.2 0.5-2.0 30-100 0-150 0-0.8 MPa/0.9 N m3 min-1 5000 Pa/0-20000 m3/h 1-5
Table 2. Operation Parameters oil type oil pressure (MPa) oil flow rate of main oil gun (kg/h) each oil flow rate of auxiliary oil gun (kg/h) compressed air pressure (MPa) compressed air flow rate (N m3 min-1) primary air velocity (m/s) primary air temperature (°C) coal feed rates (ton/h)
Figure 1. Experimental setup.
of lean coal.7 Although tiny-oil ignition has been widely employed in tangentially fired and wall-fired boilers, it has received little theoretical study. There are some advantages in a W-shaped boiler burning lean coal and anthracite. However, it is difficult to ignite lean coal and anthracite in the process of tiny-oil ignition in a W-shaped boiler. Therefore, this paper focuses on the influence of the pulverized coal concentration on lean-coal combustion in a lateral-ignition tiny-oil burner used in a W-shaped boiler. In the case of a burner with a small diameter, there is no space to set an oil combustion chamber. This paper puts forward a new setting of oil guns, and the resulting burner is referred to as a lateral-ignition tiny-oil burner. In the lateral ignition scheme for the tiny-oil burner, there is a main oil gun (an oil gun with a high-energy igniter) and auxiliary oil guns (oil guns without a high-energy igniter) on one side of the ignition burner. The oil guns are at a certain angle to the flow direction of primary air (Figure 1). The process of oil ignition is summarized as follows. Atomized oil from the main oil gun is ignited by the high-energy igniter and burns in an adiabatic chamber. An oil flame forms and subsequently ignites atomized oil from two auxiliary oil guns. Afterward, the igniter is turned off, and the oil flame is maintained by the oil guns and burns steadily. During firing up using the three oil guns in the presence of coal, instantaneous ignition is achieved by the oil flame and a steady burn of the pulverized coal develops. With the flow of air-coal dust, the flame gradually diffuses from the side with the oil guns to the opposite side. Finally, the majority of the pulverized coal is ignited. In the firing up of the pulverized-coal-fired boiler, the coal concentration (corresponding to the coal feed rate) increases. The effect of the coal concentration on lean-coal combustion in a lateral-ignition tiny-oil burner is experimentally investigated in the present work.
diesel oil 1.0 60 130 0.4 0.9 25 10 1.5-4.5
Table 3. Ultimate Analysis Results and Other Characteristics of 0 Light Diesel Oil Used in the Experiments carbon (%)
hydrogen (%)
sulfur (%)
nitrogen (%)
oxygen (%)
85.33
13.29
0.25
0.04
0.66
ash (%)
moisture (%)
gross calorific value (kJ/kg)
flash point (°C)
density (kg/m3)
0.38
0.05
41320
62
870
Table 4. Characteristics of Lean Pulverized Coal Used in the Experiments proximate analysis (on an air-dry basis, wt %) volatile matter
ash
moisture
fixed carbon
gross calorific value (kJ/kg)
12.94
24.10
1.67
61.29
24930
ultimate analysis (on an air-dry basis, wt %) carbon
hydrogen
sulfur
nitrogen
oxygen
65.23
3.14
0.36
1.18
4.32
from the oil tank and sent to the main gun and two 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 ignited and combusted in the primary air duct. 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 burner center line and 10 mm from each side wall. Ash samples were sampled at the exit of the tinyoil ignition burner. Gases were sampled using a water-cooled stainless-steel probe and analyzed online using a Testo 350M instrument. The water-cooled stainless-steel probe was introduced in a former paper.6 The accuracy of the Testo 350M 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 gives the technical characteristics of the equipment used. 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 fineness of the pulverized coal was R90 = 5.5%; i.e., 94.5% of particles passed through a sieve with 90 μm holes.
2. Experimental Section Figure 1 shows the tiny-oil ignition apparatus. The measurement and control of the coal feed rate was 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 downpipe 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 (7) Li, Z. Q.; Liu, C. L.; Zhao, Y.; Chen, Z. C. Energy Fuels 2010, 24 (1), 375–378.
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: DOI:10.1021/ef100728f
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Figure 2. Profiles of gas temperatures measured along the burner center line for the burner with three oil guns at different pulverizedcoal concentrations.
3. Results and Discussion 3.1. Gas Temperature Distribution. Figure 2 shows profiles of the gas temperature measured along the burner center line; here, x is the measured distance to plane A (see Figure 1). During firing up using the three oil guns in the absence of coal, the gas temperature measured along the burner center line increased rapidly with increasing x. This indicates that the flame of oil combustion diffused to the center continuously. The oil burned continuously in the combustion chamber; the released heat increased; and the temperature increased gradually. During firing up with the three oil guns in the presence of coal, the pulverized coal burned adequately by being in contact with the flame of oil combustion. The pulverized coal burned and released heat continuously. Along the flow of primary air, the gas temperature along the center line increased rapidly first and then remained basically invariant after x = 1050 mm. When the pulverizedcoal concentration increased from 0.27 kg (0.27 kg of coal in 1 kg of air) to 0.80 kg, which corresponded to coal feed rates from 1.5 to 4.5 tons/h, the absorption heat of pulverized coal increased gradually and some of the pulverized coal could not ignite and release heat. The gas temperature at the same measuring point decreased with the increase in the pulverized coal concentration. Comparing temperatures for the input of only oil and the input of pulverized coal, it is found that the temperature increased rapidly with increasing x in the case of burning oil only. In addition, the gas temperatures were higher with the input of oil only than with the input of coal because only the air absorbed heat when the coal had not been delivered. After the pulverized coal was input, it came into contact with the hightemperature oil flame, was heated, then released volatile matter, and was burned. The coal burned and released heat continuously. The oil and coal also absorbed much heat at the moment of burning and releasing heat. The content of volatile matter in the lean coal was low. As a result, the heat released by the volatile matter was limited. A high ignition temperature and long period of burning out are required for the burning of coke. The overall effect is that the temperature was lower than when burning the oil only. The temperature at the exit of the burner center line was 1089 °C for only the input of oil. When the concentration of the pulverized coal was 0.80 kg, the temperature at this point decreased to 931 °C. Figure 3 shows the profiles of gas temperature on the side with the oil guns (r1 = þ116.5 mm) and the opposite side (r1 = -116.5 mm) in the plane where the three oil guns lie
Figure 3. Profiles of gas temperatures (a) on the side with the oil guns and (b) on the opposite side for the burner with three oil guns at different pulverized-coal concentrations.
(see Figure 1). During firing up with the three oil guns in the absence of coal, the gas temperatures on the side of the oil guns (r1 = þ116.5 mm) were higher than 1000 °C at all positions. At r1 = -116.5 mm, the initial temperature was low. With the flame of oil combustion diffusing gradually, the gas temperature increased from 95 to 838 °C rapidly as x increased from 450 to 1050 mm. As x continued to increase to 1650 mm, the heating rate started to slow and the gas temperature climbed slowly to 934 °C. Because of combustion and the release of heat of oil concentrated on the oil-gun side, gas temperatures on the oil-gun side were significantly higher than temperatures on the opposite side. During firing up with the three oil guns in the presence of coal, gas temperatures were lower than those for only the burning of oil. In addition, the gas temperatures on the oil-gun side were higher than temperatures on the opposite side at the same distance x. With the increase of the pulverized coal concentration, more heat was absorbed by the pulverized coal and, thus, the temperatures decreased at the corresponding measurement points of r1 = ( 116.5 mm. At r1 = þ116.5 mm, gas temperatures decreased slightly with x increasing from 450 to 750 mm because the pulverized coal absorbed heat but did not begin to combust and release heat at this moment. For x greater than 750 mm and a pulverized coal concentration between 0.27 and 0.44 kg, gas temperatures quickly increased, which indicates that pulverized coal had ignited, combusted, and thus, released heat. When the pulverized coal concentration was between 0.62 and 0.80 kg, the gas temperatures increased slowly, indicating that pulverized coal did not burn at this position for these two pulverized coal concentrations. At x = 1650 mm, with pulverized-coal concentrations of 0.27, 0.44, 0.62, and 0.80 kg, gas temperatures were 1104, 1147, 485, 4163
Energy Fuels 2010, 24, 4161–4165
: DOI:10.1021/ef100728f
Li et al.
and 321 °C, respectively. At r1 = -116.5 mm, the gas temperatures increased as x increased and the heating rate reduced as the pulverized coal concentration increased. For pulverized coal concentrations of 0.27 0.44, 0.62, and 0.80 kg, gas temperatures were 1051, 693, 423, and 345 °C, respectively, at x = 1650 mm. In comparison of the temperatures of the left and right sides of the nearby burner wall, the temperature field was found to be asymmetric in the burner. Temperatures were obviously higher on the oil-gun side. At coal concentrations of 0.27 and 0.44 kg, the gas temperature near the wall was higher, up to 1200 °C. At coal concentrations of 0.62 and 0.80 kg, the gas temperature near the burner wall was below 600 °C. Therefore, there was a low possibility of slagging in the burner at low gas temperatures. Meanwhile, in practical operation, there is a cooling duct on the outside of the primary air duct. Cooling air could cool the wall and reduce the gas temperature near the wall to below 400 °C, preventing slagging. Slagging was not observed in the experiment. As the pulverized-coal concentration increased, the temperatures near the burner wall all decreased. Figure 4 shows photographs of the oil and coal flame at different coal concentrations. As the coal concentration increased, the length and diameter of the flame enhanced. When the coal concentration was 0.80 kg, there was a little unburned pulverized coal at the edge of the burner exit. How to make the gas temperature distribution more symmetric will be the focus of future work. The first step is to adjust the angle between the oil guns and the primary air duct to 90°. At this angle, the first consideration must be successful and stable ignition. Following this principle, adjustments are required to make the gas temperature distribution more symmetric through experimental study and numerical simulation to solve the problem of the asymmetric temperature distribution in burners. 3.2. Coal Burnout and the 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 lateral-ignition tiny-oil burner. Coal burnout was calculated using ψ ¼ ½1 - ðwk =wx Þ=ð1 - wk Þ
ð1Þ
where ψ is the coal burnout factor, wk is the ash weight fraction of the input coal, and wx is the ash weight fraction of the char sample. β is the percentage release of components (C and H), which was calculated using β ¼ 1 - ½ðwix =wik Þðwk =wx Þ
ð2Þ
where wix is the weight percentage of the species of interest in the char sample and wik is the weight percentage of the species of interest in the input coal.8 The coal burnout and release rates for C and H on the side of the oil guns were higher than those on the opposite side. The flame of the oil and pulverized coal mainly concentrated on the side of the oil guns. The largest value of the coal burnout was at r2 = þ42 mm. For the pulverized coal concentrations of 0.27, 0.44, 0.62, and 0.80 kg, the largest values of coal burnout were 70.2, 43.4, 39.9, and 38.1%, respectively. In common burners, primary air only provides oxygen for the burning of volatile content. In tiny-oil ignition burners, however, char as well as volatile content burns. As a result, pulverized coal burned under an oxygen-poor condition. According to experimental results, the oxygen was almost
Figure 4. Flame produced while operating the three oil guns (a) in the absence of coal and (b) with coal at coal concentrations of 0.27 kg (1.5 tons/h), 0.44 kg (2.5 tons/h), 0.62 kg (3.5 tons/h), and 0.80 kg (4.5 tons/h).
(8) Costa, M.; Silva, P.; Azevedo, J. L. T. Combust. Sci. Technol. 2003, 175, 271–289.
exhausted; the oxygen content was 0.41-3.64% at the center of the burner exit for the different coal concentrations. 4164
Energy Fuels 2010, 24, 4161–4165
: DOI:10.1021/ef100728f
Li et al. Table 5. Gas Compositions at the Center Point of the Burner Exit and the Burner Resistance pulverized coal concentration (kg) O2 (%) CO (ppm) burner resistance (Pa)
0
0.27
0.62
0.80
0.86
0.41 2.73 3.34 more than 10000 2200 1900 1700
3.64
2100
0.44
1400
As the coal concentration increased, the extra coal absorbed more heat and the gas temperature decreased, which is similar to the case of bituminous coal ignition.6 3.3. Gas Compositions at the Center Point of the Burner Exit and the Burner Resistance. Table 5 lists gas compositions at the center point of the burner exit and the burner resistance. For pulverized coal concentrations of 0, 0.27, 0.44, 0.62, and 0.80 kg, O2 concentrations were 0.86, 0.41, 2.73, 3.34, and 3.64%, respectively, and CO concentrations were more than 10 000 ppm for all concentrations. When pulverized coal concentrations were 0 and 0.27 kg, the O2 at the center point of the burner exit was almost exhausted. With an increase in the pulverized coal concentration, the combustion conditions worsened and the O2 concentration increased slightly. The burner resistance while the three oil guns were in operation increased 2100 Pa in the absence of coal and was 2200, 1900, 1700, and 1400 Pa in the presence of coal for pulverized coal concentrations of 0.27, 0.44, 0.62, and 0.80 kg, respectively. 4. Conclusion (1) When the pulverized coal concentrations were 0.27, 0.44, 0.62, and 0.80 kg, the lateral-ignition tiny-oil burner could ignite the experimental lean coal successfully. The flames of the pulverized coal at the exit of the ignition burner were bright. With an increase in the pulverized coal concentration, the coal burnout and release rates for C and H decreased gradually. (2) The combustion of pulverized coal was mainly concentrated on the side of the oil guns. Therefore, the gas temperatures were higher on this side than on the opposite side. When only oil was sent and pulverized coal concentrations were less than 0.44 kg, the gas temperatures on the side of the oil guns were 1000-1200 °C at some measurement points near the burner wall. At this condition, the possibility of slagging in the burner was low. (3) O2 concentrations at the exit of the burner were 0.41-3.64%, and the burner resistance decreased from 2200 to 1400 Pa when the coal concentration increased from 0.27 to 0.80 kg.
Figure 5. Coal burnout and release rates for C and H at the exit of the burner.
Therefore, the limited air quantity limited the amount of pulverized coal ignited. According to calculations, the primary air provided only enough oxygen to burn 320 kg/h of oil and 0.2 ton/h of pulverized coal. As the coal concentration increased from 0.27 to 0.80 kg (i.e., as the coalfeed rate increased from 1.5 to 4.5 tons/h), the quantity of coal burned was almost the same. Therefore, the heat released by the burning of coal was almost the same.
Acknowledgment. This work was supported by the Hi-Tech Research and Development Program of China (Contract 2007AA05Z301), Postdoctoral Foundation of Heilongjiang Province (LRB07-216), Heilongjiang Province via 2005 Key Projects (Contract GC05A314), and Hi-Tech Research and Development Program of China (863 Program) (Contract 2006AA05Z321).
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