Measurement of Gas Species, Temperatures, Coal Burnout, and Wall

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Measurement of Gas Species, Temperatures, Coal Burnout, and Wall Heat Fluxes in a 200 MWe Lignite-Fired Boiler with Different Overfire Air Damper Openings Jianping Jing, Zhengqi Li,* Guangkui Liu, Zhichao Chen, and Chunlong Liu School of Energy Science and Engineering, Harbin Institute of Technology, 92 West Dazhi Street, Harbin 150001, People’s Republic of China ReceiVed March 23, 2009. ReVised Manuscript ReceiVed May 29, 2009

Measurements were performed on a 200 MWe, wall-fired, lignite utility boiler. For different overfire air (OFA) damper openings, the gas temperature, gas species concentration, coal burnout, release rates of components (C, H, and N), furnace temperature, and heat flux and boiler efficiency were measured. Cold air experiments for a single burner were conducted in the laboratory. The double-swirl flow pulverized-coal burner has two ring recirculation zones starting in the secondary air region in the burner. As the secondary air flow increases, the axial velocity of air flow increases, the maxima of radial velocity, tangential velocity and turbulence intensity all increase, and the swirl intensity of air flow and the size of recirculation zones increase slightly. In the central region of the burner, as the OFA damper opening widens, the gas temperature and CO concentration increase, while the O2 concentration, NOx concentration, coal burnout, and release rates of components (C, H, and N) decrease, and coal particles ignite earlier. In the secondary air region of the burner, the O2 concentration, NOx concentration, coal burnout, and release rates of components (C, H, and N) decrease, and the gas temperature and CO concentration vary slightly. In the sidewall region, the gas temperature, O2 concentration, and NOx concentration decrease, while the CO concentration increases and the gas temperature varies slightly. The furnace temperature and heat flux in the main burning region decrease appreciably, but increase slightly in the burnout region. The NOx emission decreases from 1203.6 mg/m3 (6% O2) for a damper opening of 0% to 511.7 mg/m3 (6% O2) for a damper opening of 80% and the boiler efficiency decreases from 92.59 to 91.9%.

1. Introduction Coal is globally used as the energy resource for thermal power plants, mostly as pulverized coal. Lignite is a coal species that readily ignites and slags. Different aerodynamic fields lead to variations in the coal combustion process, ignition position, and NOx formation. The premature ignition of coal particles burns the burner nozzle, and it is easy for serious slagging to result in the burner nozzle region. In recent years, NOx emission control has become stricter worldwide. An overfire air (OFA) system installed on the boiler could greatly influence the combustion characteristics and aerodynamic field in the furnace to control NOx emission, so it is necessary to study the influence of OFA flow on combustion characteristics and NOx formation characteristics in the main burning region. More and more research has been conducted worldwide to reduce pollutant emissions in the combustion zone. Industrial experiments performed on full-scale boilers revealed the coal combustion characteristics and mechanism of NOx formation. Costa et al. measured local mean gas species concentrations (for O2, CO, CO2, and NOx), gas temperatures, and char burnout at several ports of 300 MWe, front-wall-fired, pulverized-coal utility boilers first without and then with OFA.1-3 Li et al. * To whom correspondence should be addressed. Tel.: +86 451 86418854. Fax: +86 451 86412528. E-mail: [email protected]. (1) Costa, M.; Azevedo, J. L. T.; Carvalho, M. G. Combust. Sci. Technol. 1997, 129, 277–293. (2) Costa, M.; Silva, P.; Azevedo, J. L. T. Combust. Sci. Technol. 2003, 175, 271–289. (3) Costa, M.; Azevedo, J. L. T. Combust. Sci. Technol. 2007, 179, 1923– 1935.

measured local mean concentrations of O2, CO, CO2, and NOx gas species, gas temperatures, and char burnout in a 300 MWe wall-fired pulverized-coal utility boiler without OFA.4 Experiments have been performed for pulverized-coal, tangentially fired, wall-fired boilers, and down-fired boilers.5-12 However, the influence of OFA flow on combustion characteristics and NOx formation characteristics in the main burning region of a lignite utility boiler requires further research. At different OFA damper openings, local mean concentrations of O2, CO, and NOx, gas temperatures, coal burnout, and releases of C, H, and N from coal were recorded in the region of the burner in a 200 MWe, wall-fired, pulverized-coal utility boiler, and the furnace temperature, heat flux, NOx emissions, and boiler efficiency were measured. A comparison was made between data for the centrally fuel-rich burner recorded by Li et al. and data recorded along the monitoring pipe of the burner in this paper in terms of the concentrations of O2, CO, and NOx, gas (4) Li, Z. Q.; Jing, J. P.; Chen, Z. C.; Ren, F.; Xu, B.; Wei, H. D.; Ge, Z. H. Combust. Sci. Technol. 2008, 180, 1370–1394. (5) Butler, B. W.; Webb, B. M. Fuel 1991, 70, 1457–1464. (6) Butler, B. W.; Wilson, T.; Webb, B. W. Proc. Combust. Inst. 1992, 24, 1333–1339. (7) Bonin, M. P.; Queiroz, M. Combust. Flame 1991, 85, 121–133. (8) Ren, F.; Li, Z. Q.; Zhang, Y. B.; Sun, S. Z.; Zhang, X. H.; Chen, Z. C. Energy Fuels 2007, 21, 668–676. (9) Li, Z. Q.; Yang, L. B.; Qiu, P. H.; Sun, R.; Chen, L. Z.; Sun, S. Z. Int. J. Energy Res. 2004, 28, 511–520. (10) Fan, J. R.; Sun, P.; Zheng, Y. Q.; Ma, Y. L.; Cen, K. F. Fuel 1999, 78, 1387–1394. (11) Black, D. L.; McQuay, M. Q. Combust. Fire 1996, 328, 19–27. (12) Queiroz, M.; Bonin, M. P.; Shirolkar, J. S.; Dawson, R. W. Energy Fuels 1993, 7, 842–851.

10.1021/ef900249k CCC: $40.75  2009 American Chemical Society Published on Web 06/25/2009

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Figure 1. Double-swirl flow burner (dimensions in meters): (1) central igniting ports, (2) central duct, (3) primary air duct, (4) monitoring ports, (5) secondary air bellows, (6) inner secondary air duct, (7) outer secondary air duct, (8) water wall, (9) outer secondary air vanes, and (10) inner secondary air vanes. Table 1. Design Parameters of the Swirl Burner in the Utility Boiler quantity

double-swirl flow burner

exit area of the primary air (m2) exit area of the inner secondary air (m2) exit area of the outer secondary air (m2) temperature of the primary air (°C) temperature of the secondary air (°C) mass flow rate of the primary air (kg s-1) mass flow rate of the inner secondary air (kg s-1) mass flow rate of the outer secondary air (kg s-1) tangential vane angles of inner secondary air (deg) tangential vane angles of outer secondary air (deg)

0.1785 0.1783 0.3962 75 353 4.25 1.61 3.57 80 75

temperatures, and coal burnout.4 The combustion and NOx formation characteristics of different burners have been compared, which is helpful in burner amelioration. Different OFA damper openings could result in different secondary air and thus different aerodynamic fields in the burner region. Therefore, cold air flow experiments in a small-scale burner model were carried out in the present study. 2. Experimental Section 2.1. Experimental Utility Boiler. A 200 MWe, wall-fired, pulverized-coal boiler was designed by Ishikawajima-Harima Heavy Industries Co., Ltd. The opposite-wall-fired, pulverized-coal boiler with a dry-ash furnace is equipped with 20 double-swirl flow pulverized-coal burners. There are 12 burners arranged in 3 rows on the front wall of the furnace. The other eight burners are arranged in two rows on the rear wall, opposite the eight burners in the middle and bottom of the three rows on the front wall. Figure 1 shows a double-swirl flow pulverized-coal burner, which has a primary air duct in the central zone. Pulverized coal carried by the primary air flow enters the burner along the tangential direction and injects into the furnace as a swirl jet flow. The inner secondary air and the outer secondary air around the primary air swirl owing to tangential vanes installed in the inner secondary air duct and the outer secondary air duct, respectively. The swirl intensities of the inner and outer secondary air are controlled by varying the tangential vane angle. The central duct is a pipe located on the chamber axis of the burner with a little air. Table 1 lists the design parameters of the burner. 2.2. Cold Air Experiments for the Burner Model. As the OFA damper opening widens, the amount of secondary air in the burner region decreases. Therefore, the aerodynamic field in the burner region varies. Different aerodynamic fields lead to variations in the coal combustion process, ignition position, and NOx formation. The aerodynamic fields in the burner region for different secondary airs can then be used in analyzing measurements made for a fullscale boiler.

With use of isothermal modeling technology, a cold experiment was carried out in the laboratory. The burner model is one-third of a prototype burner. The test facility is illustrated in Figure 2, where x is the distance to the exit of the burner along the jet flow direction, r the distance to the chamber axis along the radial direction, and d the burner outer diameter of the outer secondary air duct (d ) 0.338 m). The origin is the intersection of the central axis and outer secondary air duct nozzle section of the burner (see Figure 1). If x is negative, the measuring point is within the burner. An IFA300 constant-temperature anemometer system was used to measure the air velocity at measurement points. Using a probe with hot-film sensors, we measured the three-dimensional flow field at the exit of the burner.13 The error for velocity measurements was less than 5%. Figures 3, 4, and 5 show the mean axial velocity, mean radial velocity, and mean tangential velocity profiles, respectively, when the secondary air flow is 0.49, 0.57, and 0.66 kg s-1. As observed in Figure 3, for the three cases, the mean axial velocities along the radial direction are negative in the range of 0.3d-0.6d from the burner jet (x/d ) 0) to the x/d ) 0.25 cross section, and those for the cross sections of x/d g 0.5 are all positive. Because it is a swirl jet flow, the primary air with high velocity diffuses into the secondary air region. Mixing occurs quickly, owing to the high turbulent mass-exchange rate caused by the high-velocity gradients between primary air and secondary air, so the mean axial velocity decreases quickly along the radial direction. With the secondary air flow increasing, the mean axial velocity at the nozzle increases. As the mass flow rate of the secondary air increases from 0.49 to 0.66, swirl numbers increase from 0.25 to 0.261. The swirl capability of the whole flow increases. The central recirculation zone then increases slightly. As shown in Figure 4, there are two peaks in the profiles of mean radial velocity from the burner jet (x/d ) 0) to the x/d ) 1.0 cross section for the three different secondary air flows: an inner peak near the burner center in the primary air flow zone and an outer peak near the wall in the secondary air flow zone. The maximum value of the outer peak is higher than that of the inner peak in all cases. Both maxima increase with increasing secondary air flow, and following further development of air flow, the radial velocity decreases. There are small differences among the three radial velocities for different secondary air flows at cross sections of x/d > 1.0. The radial velocities are negative near the chamber axis from the burner jet (x/d ) 0) to the x/d ) 1.0 cross section for the three secondary air flows. The primary air shifts toward the chamber axis. This is advantageous in increasing the pulverized coal concentration in the central region of the burner. Figure 5 shows that the mean tangential velocity has a single peak in the primary air nozzle region for the three secondary air flows. Because secondary air swirls less and the recirculation zone is axial backflow, the tangential velocities in the central recirculation zone and secondary air outlet are small. With the secondary air flow increasing, the peak value of tangential velocity increases, the swirl intensity and mixing of air flow strengthen, and the rate of the decrease in tangential velocity increases. The tangential velocity decays faster than the axial velocity, and thus the tangential swirl characteristic disappears rapidly in a strong swirl jet because of turbulent mixing. As seen in cross sections at x/d g 1.5, the tangential velocity is small and thus axial flow is the main mode of flow. Figure 6 shows profiles of turbulence intensity for three secondary air flows of 0.49, 0.57, and 0.66 kg s-1. The turbulence intensity T is defined as

T)

√U′2 + V′2 + W′2 U0

(1)

where U′, V′, and W′ are the axial, radial, and tangential fluctuation velocities respectively and U0 is the mean velocity of the burner (13) Acrivlellis, M. DISA information 1978, 23, 11–16.

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Figure 2. Cold air experimental setup.

Figure 3. Mean axial velocity profiles for secondary air flow of 0.49 kg s-1 (-0-), 0.57 kg s-1 (-O-), and 0.66 kg s-1 (-×-).

jet. As seen in Figure 6, the turbulence intensity distributions from the burner jet (x/d ) 0) to the x/d ) 1.0 cross section are doublepeak structures. Both peaks appear at the boundary of the recirculation zone, and values are small in the primary air region, in the recirculation zone, and at the boundary of the jet. With the air spreading downward, turbulent energy is continually generated. In the region of x/d ) 0.25 to x/d ) 0.5, the turbulence intensity reaches a peak value and then decreases. Thus, pulverized coal burns strongly in the region with high turbulence intensity. With the secondary air flow increasing, the turbulence intensity at the outlet is at a high level and turbulent mixing is strong. Because of the intense mixing of air flow in the early stage, the dissipation rate for turbulence energy increases. Figure 7 shows the aerodynamic field profiles for the three secondary air flows and tangential vane angles for the inner and outer secondary air of 80° and 75°, respectively. In the cold flow experiments, a coordinate frame was set at the outlet of the burner. A thin cloth was fixed for each grid of the frame. The traverse distance between two measurements was 0.05 m. We estimated that the uncertainty in establishing the location of the central recirculation zone border was 0.05 m. From the flow direction of

the cloth, the jet borders and central recirculation zone boundary of the burner were determined.14 There are no recirculation zones in the central region of the burner in the three cases, but there are two ring recirculation zones in the outlet region of the burner. With the secondary air flow increasing, the divergence angle and the size of the recirculation zone increase. The largest recirculation zone starts at the secondary air region in the burner, and its length and diameter are 0.82d and 0.33d, respectively. These data agree well with measurements for the IFA300 constant-temperature anemometer system and the design of the double-swirl flow burner. 2.3. Flue Gas Measurement for a Full-Scale Boiler. 2.3.1. Data Acquisition Techniques. Because the burners in a wallfired boiler work largely independently and there is little interference between them, data were obtained for the gas temperature, gas species concentrations, and coal burnout in the region of the No. 1 and No. 2 burners, which were at the bottom on the rear wall (see Figure 8). The initial measurement position is the intersection of the burner axis and rear water wall. X is the distance between the (14) Li, Z. Q.; Chen, Z. C.; Sun, R.; Wu, S. H. J. Institute Energy 2007, 80, 123–130.

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Figure 4. Mean radial velocity profiles for secondary air flow of 0.49 kg s-1 (-0-), 0.57 kg s-1 (-O-), and 0.66 kg s-1 (-×-).

Figure 5. Mean tangential velocity profiles for secondary air flow of 0.49 kg s-1 (-0-), 0.57 kg s-1 (-O-), and 0.66 kg s-1 (-×-).

measurement point and the initial position and L is the projection of the distance between the initial measurement position and the point measured through the monitoring ports in the burner axis direction. If X and L are negative, the measurement point is within the burner (see Figure 1). Measurements were made at positions in the direction from the sidewall to the burner by employing viewing ports in the sidewall, as shown in Figure 8. To avoid the water-cooled stainless steel probe being distorted by hot gas, the distance between the measurement point and sidewall was set at less than 1.5 m. Thus, the measured point is not at the center of the burner. Gases were sampled using a water-cooled stainless steel probe and analyzed online using a Testo 335 instrument. The gas temperature was measured using a nickel chromium-nickel silicon thermocouple placed inside a water-cooled stainless steel probe. Char sampling was also performed using a water-cooled stainless steel probe.

The flow field in the present study is heterogeneous and, therefore, the requirement of isokinetic sampling is of some importance in obtaining representative local char samples. However, in swirling, recirculating and highly turbulent flows, the concept of isokinetic sampling has little meaning. Nevertheless, with less suction velocity, aerodynamic disturbances were minimized. The measurement range of the Testo 335 gas analyzer for the species measurements was 0-22% for O2 and 0-5000 ppm for both CO and NOx. The accuracy was 1% for O2, 5% for CO, and 5 ppm for NO and NO2. Each sensor was calibrated before measurement. 2.3.2. Coal Burnout. Coal burnout was calculated using

ψ ) [1 - (ωk /ωx)]/(1 - ωk)

(2)

where Ψ is the coal burnout, ω is the ash weight fraction, and the subscripts k and x refer to the ash contents in the input coal and char sample, respectively.2



Figure 6. Turbulence intensity profiles for secondary air flow of 0.49 kg s-1 (-0-), 0.57 kg s-1 (-O-), and 0.66 kg s-1 (-×-).

values given in Table 3 are averaged over the duration of the experimental campaign.

3. Results and Discussion

Figure 7. Aerodynamic field profiles for secondary air flow of 0.49 kg s-1 (-0-), 0.57 kg s-1 (-O-), and 0.66 kg s-1 (-×-).

The percentage release of components (C, H, and N) was calculated using

β ) 1 - [(ωix /ωik)(ωRk /ωRx)]

(3)

where ωi is the weight percentage of the species of interest, ωR is the ash weight percentage, and the subscripts k and x refer to different contents in the input coal and char sample, respectively.2 2.3.3. Measurements of Furnace Temperature and Heat Flux. An optical pyrometer with a measurement range of 500-2000 °C, accuracy of 1 °C, and error range of (30 °C was used to measure the furnace temperature. A radiation heat flux meter with measurement range of 0-1200 kW/m2, wavelength range of 0.3-50 µm, accuracy of 0.05 kW/m2, and error range of (2% was used to measure the heat flux in the furnace. The measurements of furnace temperature and heat flux were conducted through the viewing ports on the sidewall. During the experimental campaign, the utility boiler was operated stably at a rated load. OFA flow was controlled by the OFA damper opening, and measurements were taken for four OFA damper openings. Table 2 lists the characteristics of the coal used in the experiments. The coal was lignite. Table 3 summarizes the boiler operation parameters and results for the four damper openings. The

Figure 9 shows profiles of the gas temperature and gas species concentrations (O2, CO, and NOx) measured through centraligniting ports of the two burners for different OFA damper openings. The turning points of O2 and CO concentrations indicate the ignition position to a certain extent. Therefore, from the turning points of O2 and CO concentrations, the distances between the ignition point and burner nozzle are determined as being in the range from 0.85 to 1.05 m for both burners. The coal ignites late. From the results of cold air experiments, there is no high-temperature recirculation zone in the central region of the burner that has sufficient heat to ignite the coal. When the damper opening is widened, the flux of pulverized coal and primary air flow in the burner region varies slightly, but the secondary air flow decreases. It is beneficial to ignite the coal earlier and have a higher gas temperature in the central region of the burner. The gas temperature in the No. 2 burner region is higher than that in the No. 1 burner region for the same damper opening because the No. 2 burner is far from the lowtemperature sidewall. As seen in the O2 concentration profiles, O2 concentrations in the central region of the burner begin decreasing 0.95 m from the burner nozzle. With the damper openings widening, the O2 concentration decreases more quickly because secondary air flow in the bottom row burners decreases. The reason for the sharp decrease in O2 concentrations is that the pulverized coal combusts rapidly and consumes a great deal of oxygen. O2 concentration in the No. 2 burner region decreases earlier than that in the No. 1 burner region because the No. 2 burner is located at the center of the furnace wall, where there is a higher gas temperature, and thus coal particles ignite earlier. As seen in the CO concentration profiles, CO concentrations increase sharply near the ignition point and quickly exceed 5000 ppm because the gas temperature increases and O2 concentrations decrease after most coal particles combust in the central region of the burner. The upper limit of the CO measurement

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Figure 8. Schematic view of the boiler showing burners and viewing ports (all dimensions in meters). Table 2. Characteristics of the Coal Used in the Experiments OFA damper opening 0%

20%

50%

80%

Proximate Analysis (As Received, wt %) ash 15.15 13.58 14.28 volatiles 37.52 37.38 38.13 fixed carbon 39.7 39.58 40.03 moisture 21.3 23.2 19.9 -1 18460 18250 19310 net heating value (kJ kg )

13.07 38.89 41.19 18.9 20420

Ultimate Analysis (As Received, wt %) 49.93 49.51 52.27 2.91 2.87 3.47 0.57 0.56 0.69 0.49 0.55 0.52 9.65 9.73 9.87

53.20 3.32 0.71 0.59 9.78

carbon hydrogen nitrogen sulfur oxygen

Table 3. Operation Parameters and Results for Four Different OFA Damper Openings OFA damper opening flow rate of the main steam (ton/h) pressure of the main steam (MPa) temperature of the main steam (°C) desuperheat water quantity of superheating steam (ton/h) negative pressure of the furnace (Pa) primary air flow rate (ton/h) primary air/fuel temperature (°C) secondary air temperature (°C) total secondary air flow rate (ton/h) total coal feed rate (ton/h) exhaust gas temperature (°C) carbon in ash (%) NOx emission (mg/m3 (6% O2)) boiler efficiency (%)

0%

20%

50%

80%

660.8 13.2 537.3 35.0

659.6 13.41 535.4 38.6

652.9 13.44 536.3 39.4

623.4 13.71 532.9 41.8

-66.2 247.8 59.7 360.5 467.3 124.26 144.6 0.18 1203.6 92.59

-47.2 239.8 70.5 367.6 475.4 122.81 149.0 0.40 794.6 92.45

-69.1 247.0 60 356.4 477.9 124.02 154.4 0.75 515.9 91.97

-82.5 247.7 72 357.8 474.4 119.02 156.3 0.24 511.7 91.9

was 5000 ppm; that is, when the CO concentration exceeded 5000 ppm, the gas analyzer displayed 5000 ppm. With the damper openings widening, CO concentrations in the central region of the burner increase more rapidly because of the earlier ignition of coal particles, the higher gas temperature, and the lower O2 concentration. As seen in the NOx concentration profiles, NOx concentrations increase rapidly near the ignition point, with the gas temperature increasing and O2 concentrations decreasing after most coal particles combust in the central region of the burner. With the

damper openings widening, NOx concentrations in the central region of the burner decrease because the secondary air flow and O2 concentrations in the main burning region decrease and the reducing nature of the atmosphere strengthens, which is favorable for controlling NOx emissions. NOx concentrations in the central region of the No. 2 burner are higher than those in the central region of the No. 1 burner because the No. 2 burner is located at the center of the furnace wall, where the gas temperature is higher. Figure 10 shows gas temperature and gas species (O2, CO, and NOx) concentration profiles measured through monitoring ports of the two burners for different damper openings. From the gas temperature profiles, we see that ignition points are located -0.1 m to +0.1 m from the burner nozzle section in the four different cases. The ignition points measured through the monitoring ports are near the burner nozzle and even within the burners. The coal particles in the swirling primary air shift to the secondary air zone under centrifugation. The distance between the exit of the primary air duct and the burner exit is 0.55 m (see Figure 1). It is sufficiently long for the primary air and secondary air to readily mix. The results of the cold air experiments show there are two symmetrical ring recirculation zones in the secondary air nozzle region, and their starting points are in the secondary air region in the burner. Thus, coal particles in the secondary air ignite, owing to the high-temperature gas near the burner nozzle. Ignition points measured through the monitoring ports of the two burners vary slightly as the damper opening widens, but the rise in the gas temperature accelerates. The reducing nature of the atmosphere in the main burning region strengthens as the damper opening widens, and it is easier for slag to accumulate on the burner nozzle.15 The gas temperature profiles show that the gas temperature in the region near the burner nozzle is high because of the premature ignition of coal particles in the secondary air region. Thus, the temperatures measured through the monitoring ports for a damper opening of 0% are higher than those measured by Li et al.4 As seen in the O2 concentration profiles, most coal particles ignite early in the burner and the O2 concentrations decrease rapidly. With further combustion, most coal particles burn out (15) You, C. F.; Zhou, Y. Energy Fuels 2006, 20, 1855–1861.



Figure 9. Profiles of gas temperature and gas species concentrations (O2, CO, and NOx) measured through the central lines of two burners at four different OFA damper openings.

and the O2 concentrations decrease slowly. With the damper opening widening, the secondary air flow in the main burning region decreases. Hence, the O2 concentrations also decrease. Because of the premature ignition of coal particles in the secondary air region, O2 concentrations in the region near the burner nozzle for a damper opening of 0% are lower than those measured by Li et al. Because the primary air and secondary air mix early and sufficiently, and most coal particles ignite in the early stage, O2 concentrations measured through the monitoring ports in the region away from the burner nozzle for a damper opening of 0% are higher than those measured by Li et al.4

As seen in the CO concentration profiles, the CO concentrations quickly exceed 5000 ppm because of the increase in gas temperature and decreases in O2 concentrations after most coal particles combust in the secondary air region. The damper opening has little influence on the CO concentrations. The early ignition of coal particles consumes much oxygen in the secondary air region; thus, compared with the observations made by Li et al., the CO concentrations measured through the monitoring ports for a damper opening of 0% reach their peak values earlier.4 As seen in the NOx concentrations profiles, with the gas temperature increasing and O2 concentrations decreasing after

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Figure 10. Profiles of gas temperature and gas species concentrations (O2, CO, and NOx) measured through the monitoring ports of two burners at four different OFA damper openings.

most coal particles combust in the secondary air region, the NOx concentrations increase rapidly and have maxima near the ignition point. As the O2 concentrations decrease after coal ignition, the reducing nature of the atmosphere strengthens. NOx concentrations then decrease and reach a minimum. NOx concentrations decrease with widening of the damper opening because the secondary air flow and O2 concentrations in the main burning region decrease and the reducing nature of the atmosphere strengthens. It is favorable for there to be a decrease in NOx formation. Because of the early ignition of coal particles in the secondary air region of the double-swirl flow burner, NOx concentrations for a damper opening of 0% reach peak values

earlier than those seen in the observations made by Li et al. Because of the higher gas temperature in the region of doubleswirl flow burner, NOx concentrations for a damper opening of 0% were higher than those seen in the observations made by Li et al.4 Figure 11 shows gas temperature and gas species (O2, CO, and NOx) concentration profiles measured through viewing ports on the sidewall for different damper openings. As seen in the gas temperature profiles, the gas temperature increases rapidly to about 1100 °C in the early stage for the four cases and increases more slowly later on the basis of the high temperature. The gas temperature measured through the viewing ports on



Figure 11. Profiles of gas temperature and gas species concentrations (O2, CO, and NOx) measured through viewing ports on the sidewall at different OFA damper openings.

Figure 12. Coal burnout profiles measured through the central lines of two burners at four different OFA damper openings.

the sidewall varies slightly with widening of the damper opening. As seen in the O2 concentration profiles, the O2 concentrations decrease rapidly with increasing measurement depth in the four cases because the measurement points approach the high-temperature gas gradually. With the damper opening widening, O2 concentrations decrease because of less air flow in the main burning region. As seen in the CO concentration profiles, the CO concentrations increase rapidly while the O2 concentrations decrease with increasing measurement depth in the four cases because the measurement points approach the high-temperature recirculation zone gradually. CO concentrations increase as the damper opening widens because air flow and O2 concentrations in the main burning region decrease. As seen in the NOx concentration profiles, the NOx concentrations increase first rapidly and then slowly for the four damper openings. NOx concentrations decrease as the damper opening widens because air flow and O2 concentrations in the main

burning region decrease and the reducing nature of the atmosphere strengthens. Figure 12 shows coal burnout profiles measured through the central-igniting ports of two burners for different damper openings. At a position 1.45 m from the burner nozzle, the coal burnouts for the four damper openings are 40-75%. This shows that coal particles combust incompletely at that position and coal particles in the central region of the burner ignite late. The results agree well with the measurements of gas temperature and gas species concentrations. Furthermore, as the damper opening widens, the coal burnout decreases. Compared with the No. 1 burner, the coal burnout of the No. 2 burner is greater because the gas temperature in the No. 2 burner region is higher than that in the No. 1 burner region. Figure 13 shows the profiles for the release rates of components (C, H, and N) measured through the central-igniting ports of the No. 1 and No. 2 burners. At a position 1.45 m

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Figure 13. Profiles of release rates of components (C, H, and N) measured through the central lines of two burners at four different OFA damper openings.

from the burner nozzle, the release rates of components (C, H, and N) for the four different damper openings are in the range of 40-85%. The reason for the low release rates of components (C, H, and N) is that coal particles in the central region of the burner ignite late and the gas temperature is low. Hydrogen release was the fastest and carbon release the slowest, which agrees with the laboratory results of Smoot et al.16 and the industrial results of Costa et al.2 In addition, as the damper opening widens, the release rates of components (C, H, and N) decrease. Compared with those for the No. 1 burner, the release rates of components (C, H, and N) for the No. 2 burner are higher because the gas temperature in the No. 2 burner region is higher than that in the No. 1 burner region. Figure 14 shows the coal burnout profiles measured through the monitoring ports of the No. 1 and No. 2 burners. At a position -0.038 m from the burner nozzle in the secondary air region, the coal burnouts for the four damper openings are more than 82%. This shows that coal particles ignite in the burner and the ignition point is in the secondary air region near the burner nozzle. The results agree well with the measurements of the gas temperature and gas species concentrations. In addition, as the damper opening widens, the secondary air flow in the main burning region and thus the coal burnout decrease. Because of the premature ignition of coal particles in the secondary air region, coal particles combust better; therefore, (16) Smoot, L. D.; Hedman, P. O.; Smith, P. J. Prog. Energy Combust. Sci. 1984, 10, 359–441.

coal burnout for a damper opening of 0% is much higher than that measured by Li et al.4 Figure 15 shows the profiles for the release rates of components (C, H, and N) measured through the monitoring ports of the No. 1 and No. 2 burners. At a position -0.038 m from the burner nozzle in the secondary air region, the release rates for the components (C, H, and N) in the four cases are more than 82%. Because of the premature ignition of coal particles, the components (C, H, and N) release rapidly, with the H release rate being the highest and the C release rate the lowest. As the damper opening widens, the release rates of components (C, H, and N) decrease. Figure 16 shows the furnace temperature profiles measured through viewing ports on the sidewall. The maxima for gas temperature in the four cases are in the middle row burner region. As the damper opening widens, the furnace temperature in the OFA region increases slightly because the secondary air flow in the main burning region decreases and more coal particles combust in the OFA region, and the flame center shifts upward. Figure 17 shows the furnace heat flux profiles measured through viewing ports on the sidewall. The maxima of the heat flux are in the middle row burner region, and the heat flux is distributed symmetrically. As the damper opening widens, the heat flux decreases in the main burning region and increases in the OFA region because the secondary air flow in the main



Figure 14. Coal burnout profiles measured through the monitoring ports of two burners at four different OFA damper openings.

Figure 15. Profiles of release rates of components (C, H, and N) measured through the monitoring ports of two burners at four different OFA damper openings.

burning region decreases and more coal particles burn out in the OFA region, and the flame center shifts upward. Boiler efficiencies and NOx emissions at the exits of the furnace were measured for the four damper openings. The NOx emission decreases from 1203.6 mg/m3 (6% O2) for a damper opening of 0% to 511.7 mg/m3 (6% O2) for a damper opening of 80% and the boiler efficiency decreases from 92.59 to 91.9% (see Table 3). Table 3 shows that all values of the temperature and pressure of steam are within the rated range for the different damper openings, and the desuperheat water quantity for

superheating steam increases as the damper opening widens. The damper opening has little influence on the carbon in ash. The exhaust gas temperature increases as the damper opening widens. This is the main reason for the decrease in boiler efficiency. 4. Conclusion The double-swirl flow pulverized-coal burner has two ring recirculation zones that start in the secondary air region in the

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Figure 16. Profiles of furnace temperature (°C) measured through the viewing ports on the sidewall at different OFA damper openings: (a) 0% opening, (b) 20% opening, (c) 50% opening, and (d) 80% opening.

Figure 17. Profiles of heat flux (kW/m2) measured through the viewing ports on the sidewall at different OFA damper openings: (a) 0% opening, (b) 20% opening, (c) 50% opening, and (d) 80% opening.

burner. As the secondary air flow increases, the axial velocity of the air flow increases, the maxima of the radial velocity, tangential velocity, and turbulence intensity all increase, and the swirl intensity of the air flow and the size of recirculation zones increase slightly. For four different OFA damper openings, all ignition points are 0.85-1.05 m from the burner nozzle in the central region of the burner and -0.1 to +0.1 m from the burner nozzle section.

In the central region of the burner, as the OFA damper opening increases, the gas temperature and CO concentration increase while the O2 concentration, NOx concentration, coal burnout, and release rates of components (C, H, and N) decrease, and coal particles ignite earlier. In the secondary air region of the burner, the O2 concentration, NOx concentration, coal burnout, and release rates of components (C, H, and N) decrease, and the gas temperature and CO concentra-


tion vary slightly. In the sidewall region, the gas temperature, O2 concentration, and NOx concentration decrease, while the CO concentration increases and the gas temperature varies slightly. The furnace temperature and heat flux in the main burning region decrease appreciably, but increase slightly in the burnout region. The OFA damper opening has little influence on the temperature and pressure of steam or the carbon in ash. The desuperheat water quantity for reheating steam and the exhaust gas temperature increase as the damper opening widens. The NOx emission decreases from 1203.6 mg/m3 (6% O2) for a


damper opening of 0% to 511.7 mg/m3 (6% O2) for a damper opening of 80% and the boiler efficiency decreases from 92.59 to 91.9%. Acknowledgment. This work was supported by the Hi-Tech Research and Development Program of China (Contract No.: 2007AA05Z301), Key Project of the National Eleventh Five-Year Research Program of China (Contract No.: 2006BAA01B01), Heilongjiang Province via 2005 Key Projects (Contract No.: GC05A314), and the Postdoctoral Foundation of Heilongjiang Province (LRB07-216). EF900249K

Measurement of Gas Species, Temperatures, Coal Burnout, and Wall

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