Regulating Low-NOx and High-Burnout

Article pubs.acs.org/est

Regulating Low-NOx and High-Burnout Deep-Air-Staging Combustion under Real-Furnace Conditions in a 600 MWe DownFired Supercritical Boiler by Strengthening the Staged-Air Effect Min Kuang,† Zhihua Wang,*,† Yanqun Zhu,† Zhongqian Ling,‡ and Zhengqi Li§ †

State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, P. R. China Institute of Thermal Engineering, China Jiliang University, Hangzhou 310018, P.R. China § School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, P.R. China ‡

ABSTRACT: A 600 MWe down-fired pulverized-coal supercritical boiler, which was equipped with a deep-air-staging combustion system for reducing the particularly high NOx emissions, suffered from the well-accepted contradiction between low NOx emissions and high carbon in fly ash, in addition to excessively high gas temperatures in the hopper that jeopardized the boiler’s safe operations. Previous results uncovered that under low-NOx conditions, strengthening the staged-air effect by decreasing the staged-air angle and simultaneously increasing the staged-air damper opening alleviated the aforementioned problems to some extent. To establish low-NOx and high-burnout circumstances and control the aforementioned hopper temperatures, a further staged-air retrofit with horizontally redirecting staged air through an enlarged staged-air slot area was performed to greatly strengthen the staged-air effect. Full-load industrial-size measurements were performed to confirm the availability of this retrofit. The present data were compared with those published results before the retrofit. High NOx emissions, low carbon in fly ah, and high hopper temperatures (i.e., levels of 1036 mg/m3 at 6% O2, 3.72%, and about 1300 °C, respectively) appeared under the original conditions with the stagedair angle of 45° and without overfire air (OFA) application. Applying OFA and reducing the angle to 20° achieved an apparent NOx reduction and a moderate hopper temperature decrease while a sharp increase in carbon in fly ash (i.e., levels of 878 mg/m3 at 6% O2, about 1200 °C, and 9.81%, respectively). Fortunately, the present staged-air retrofit was confirmed to be applicable in regulating low-NOx, high-burnout, and low hopper temperature circumstances (i.e., levels of 867 mg/m3 at 6% O2, 5.40%, and about 1100 °C, respectively).

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INTRODUCTION NOx is mainly responsible for the formation of acid rain and photochemical smog, which has caused continuous deterioration to the ecological environment and human health.1 Coal combustion is a major anthropogenic source of NOx.2 As the typical low-volatile and hard-to-burn fuels, anthracite and lean coal are abundant in reserves and widely fired in thermal power generators in the world.3,4 Down-fired boilers are developed especially for burning these fuels by adopting various carefully designed strategies.5 But unfortunately, particularly high NOx emissions (typically in the range 1400−2100 mg/m3 at 6% O2)6−9 generate, despite the conventional air-staging utilization for inhibiting the NOx production. This is mainly attributed to high gas temperature levels and long pulverized-coal residence times needed for completing fuel combustion, in addition to the essentially high char-volatile ratio in these fuels.7,10 NOx reduction for large-scale pulverized-coal furnaces is generally achieved via two approaches:11 (1) using various primary techniques such as low-NOx burners, fuel/air staging, overfire air (OFA), reburning, and flue gas recirculation to restrain the NOx production in furnaces; and (2) applying the postcombustion treatment (such as the typical secondary technique of the selective catalytic reduction (SCR)) to reduce the existing NOx in flue gas. The strictest pollutant emission © 2014 American Chemical Society

standards for coal-fired power generation units have been brought into effect in China since July 1, 2014. The allowed NOx levels are 200 mg/m3 at 6% O2 for down-fired furnaces (fueled with anthracite and lean coal) and 100 mg/m3 at 6% O2 for tangential-fired and wall-arranged furnaces (mainly fueled with bituminous coal and lignite), respectively.12,13 In light of the particularly high NOx production in down-fired furnaces, a combination of primary and secondary techniques must be equipped if minimum NOx emissions, safe boiler’s operations, and relatively low NOx-reduction cost need to be simultaneously achieved. The expected aim is that the NOx production is initially control to be about 800 mg/m3 at 6% O2 in furnaces by primary techniques and then the final NOx emissions are reduced to 200 mg/m3 at 6% O2 via the high-cost flue gas SCR treatment. Of primary techniques, a combined scheme characterized by applying low-NOx burners and regulating deep-air-staging conditions along the furnace height (usually including air staging in the primary combustion zone and OFA in the burnout zone) is taken as a low-cost and effective Received: Revised: Accepted: Published: 12419

July 18, 2014 September 20, 2014 September 25, 2014 September 25, 2014 dx.doi.org/10.1021/es503477q | Environ. Sci. Technol. 2014, 48, 12419−12426

Environmental Science & Technology

Article

Figure 1. Schematics of furnace configuration, burner layout pattern, combustion partition, and measuring port layout in industrial-size experiments for the 600-MWe down-fired boiler. a. general gas temperature distributions in the front and rear parts of the furnace. b. the selected gas temperature comparison along the flame travel in the front part of the furnace.

method to reduce NOx emissions.14−16 However, the present status is that the deep-air-staging combustion, which has been widely reported in tangential-fired and wall-arranged furnaces to obtain ultralow NOx emissions and good burnout,12,17−19 is still far premature in down-fired furnaces. The published investigations related with the NOx reduction in down-fired furnaces mainly focus on (i) parametric tuning of air staging conditions to reduce NOx emissions6,10,20,21 and (ii) combustion retrofits consisting of OFA application to deepen air staging and burner modifications to improve combustion perfromance.7,9,22,23 In the parametric tuning aspect, industrialsize and numerically calculated results from Cañadas et al.,6 Burdett,20 and Fueyo et al.21 uncovered that within several

300−500 MWe down-fired furnaces, decreasing the stoichiometric air in the burner zone by solely strengthening the secondary air distribution along the down-shot flame could reduce NOx emissions by approximately 20%, with the unburnt loss controlled within acceptable levels. A further low-cost NOx reduction by controlling combustion must rely on the combustion retrofit aspect. By positioning OFA and retrofitting burners in 50−350 MWe down-fired furnaces, Garcia-Mallol et al.,7 Li et al.,9 and Leisse et al.22 found that a sharp NOx reduction up to about 50% could achieve under low-NOx operational conditions, but unfortunately, relatively high levels of carbon in fly ash (typically reaching 7.5−10%) were almost inevitable. These aforementioned results suggest that great 12420

dx.doi.org/10.1021/es503477q | Environ. Sci. Technol. 2014, 48, 12419−12426


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whose detailed description has already appeared in the literature13 and is not repeated here. To clarify the understanding about the combustion modifications through the aforementioned three steps, configuration differences and the reported combustion performance need to be presented here: (1) At the first step, a large staged-air declination angle of 45°, aiming at establishing a deep flame penetration depth and a postponed staged-air mixing behavior with the downward flame, was equipped for achieving high burnout and relatively low NOx emissions under the circumstances without an OFA system. Again, 8 tiny-oil ignition burners (designed to improve combustion stability at low loads and already confirmed to be invalid) were symmetrically positioned at the central zone of 8 groups of staged-air slots on the front and rear walls, occupying about one-third of the designed stagedair slot area and resulting in the staged-air flux being clearly lower than the designed value. These circumstances greatly weakened the staged-air block on the downward flame, attenuated air staging conditions, and simultaneously strengthened a lot the essentially strong secondary-air rigidity. In consequence, excessively high levels (about 1300 °C) of gas temperatures in the hopper region (called “hopper temperatures” in the following text) appeared because of a particularly large flame penetration depth, in addition to the averaged NOx emissions of about 1200 mg/m3 at 6% O2.24 After nearly 1 year of furnace operations, it was found that a thermal fatigue problem and flame washing marks appeared at the hopper wall-cooled tubes in the zones just below burners on the arches, confirming that the downward long flame actually washed over the hopper walls. (2) To resolve the above problems, at the second step OFA was equipped to greatly deepen air staging and the staged-air angle was sharply reduced to 20° (used to strengthen the staged-air block effect for a shortened flame penetration depth and lowered hopper temperatures). These modifications were confirmed to be applicable in sharply reducing NOx emissions and moderately lowering hopper temperatures at an optimized setting, with the exception of high carbon in fly ash because of the limited staged-air flux supplying in the oxygen-lean primary combustion zone at high OFA settings.13 As mentioned previously in the Introduction section, opening the staged-air damper opening under deep-air-staging conditions improved the poor burnout and still high hopper temperature levels but varied slightly NOx emissions, despite the best performance only obtaining levels of 9.81%, about 1200 °C, and 878 mg/m3 at 6% O2, respectively.13 These findings thus induced the third step application with a sharply strengthened staged-air effect by greatly increasing the staged-air slot area and flatting further the staged-air angle. (3) To sharply strengthen the staged-air effect at the third step, all tiny-oil ignition burners were removed to recover the designed staged-air slot area (Figure 1d) and the staged-air angle was redirected to 0°. The greatly increased staged-air flux and its horizontal airflow supplying direction were expected to allow more air supplying into the oxygen-lean primary combustion zone and bottom hopper, thereby improving the poor burnout

efforts must be paid on perfecting the deep-air-staging combustion application in down-fired furnaces, so as to resolve the contradiction between low NOx production and high burnout loss. In response to this subject, a deep-air-staging down-fired combustion technology was developed by our group and trialed in a 600 MWe down-fired supercritical furnace.13,24 The deep-air-staging combustion system was designed to contain fuel rich/lean combustion and two layers of secondary air supplying in the burner zone, staged-air supplying in the primary combustion zone, and OFA supplying in the burnout zone. However, the final application of the combustion system actually underwent three steps in establishing a perfect combination of low NOx production, high burnout, and safe boiler’s operational conditions. Industrial-size results uncovered that at the first step (applying the deep-air-staging combustion system without an OFA system),24 high NOx emissions, low carbon in fly ash, and high hopper temperature levels appeared in the furnace. While at the second step with OFA equipped and the staged-air declination angle sharply reduced from 45° to 20°,13 low NOx emissions, poor burnout, and moderately lowered hopper temperatures developed at an optimized setting. Again, it was found that under deep-air-staging conditions, increasing the staged-air flux improved the poor burnout and still high hopper temperature levels but varied slightly NOx emissions.13 Based on experiences gained from the two steps, a third step characterized by (i) redirecting staged air to a horizontal direction and (ii) enlarging the staged-air slot area to apparently increase the staged-air flux, was recently performed for resolving the poor burnout and still high hopper temperature levels while maintaining low-NOx conditions. The present work is to confirm the availability of the third step by reporting industrial-size results after the third step and comparing operation results among the three steps. The information gained from this study is useful to deepen the understanding about how to establish deep-air-staging combustion in down-fired furnaces and to resolve various potential combustion problems.

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EXPERIMENTAL SECTION Utility Boiler. Figure 1 shows schematics of the furnace configuration and combustion system of the aforementioned 600 MWe down-fired boiler. As illustrated in Figure 1a, the furnace arches divide the furnace into two zones: the octagonal lower furnace with four wing walls (see the horizontal crosssection in Figure 1b) and the rectangular upper furnace, which act as the fuel-burning zone and fuel-burnout zone, respectively. A total of 24 louver concentrators (symmetrically arranged on the arches) divide the primary air/fuel mixture into fuel-rich and fuel-lean coal/air flows needed to regulate fuel rich/lean combustion. The burner layout pattern with 12 burner groups symmetrically lining the front and rear arches and uniformly positioned along the furnace breadth, is clearly illustrated in Figure 1b. As shown in Figure 1c, a total of eight fuel-rich coal/ air flow nozzles, eight fuel-lean coal/air flow nozzles, 14 inner secondary-air ports, and 12 outer secondary-air ports feed each burner group. Corresponding to burner groups on the arches, there are 12 groups of staged-air slots symmetrically located at the lower parts of the front and rear walls in the lower furnace. As a schematic of staged air, Figure 1d only graphs the stagedair slot layout along a half of the front (or rear) wall. Staged air, used to organize air-staging conditions and adjust the flame penetration, is partitioned hot air from the secondary-air box. Figure 1e graphs the deep-air-staging combustion configuration, 12421



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Table 1. Operational Parameters and Coal Characteristics in Full-Load Industrial-Size Measurements for Steps (i)−(iii)a

and lowering further hopper temperatures while maintaining low NOx emissions. Industrial-Size Measurements. To confirm the availability of the third step, full-load industrial-size measurements were performed in detail at a carefully selected deep-air-staging operation setting with a high staged-air and OFA fluxes. It should be noted that this real-furnace setting were selected by comparing major operational parameters (i.e., NOx and CO emissions, carbon in fly ash, hopper temperatures, and exhaust gas temperature) of a series of settings with relatively simpler industrial-size measurements after the third step. Real-furnace data of the previous two steps under similar high staged-air circumstances13,24 were repeated here to compare with the present results and thus highlight the progress in perfecting deep-air-staging combustion in the down-fired furnace. For convenience, the three settings used for data comparisons are hereafter referred as Step (i), Step (ii), and Step (iii), respectively. Considerable efforts were paid for allowing minimum variation in boiler operational conditions over the 5 h duration in the present experimental run. As shown in Table 1, similar primary operational parameters including coal characteristics, fuel consumption, total air and primary supplying, various air temperatures, boiler load, and pulverized-coal fineness were kept for the three settings so as to ensure the data to be comparable, with the exception of large differences in (i) the total secondary air distribution among secondary air, staged air, and OFA and (ii) the staged-air angle and slot area because of the continuously strengthened stagedair effect. It should be noted that the effective staged-air slot area is the projected area (perpendicular to the staged-air supplying direction) of the total slot area on the front and rear walls. The two staged-air angle reduction stages and final tinyoil ignition burner removal not only increased apparently the effective staged-air slot area but also weakened the staged-air resistance into these narrow slots. Consequently, the staged-air mass flow rate increased continuously from Step (i) to Step (iii), despite high OFA ratios appearing at the latter two steps. The mass flow rate and airflow ratio of each jet (Table 1) in the total secondary air were determined by performing a series of real-furnace cold airflow experiments at various damper opening combinations before the furnace entering into service at each step, following the cold airflow experiment methods provided in the published work.24 In light of industrial-size measurements in Step (iii) being almost the same as those in Steps (i) and (ii), only a simple introduction about dataacquiring methods in Step (iii) is provided below and details about errors, uncertainty analysis, and instrument configurations and technical parameters can be viewed in the literature:13,24 (1) General gas temperature distribution in the furnace. A noncontact infrared thermometer, with a measurement range from 600 to 3000 °C and accurate to within 1 °C, was inserted in turn through a series of observation ports (Figure 1a) to measure the general gas temperature distribution pattern in the furnace. Aside from the general gas temperatures distribution comparison, temperature profiles of ports 1−4 (Figure 1a) were graphed to highlight the temperature change pattern with respect to the staged-air effect as the downward flame proceeds. At each port the measurement was taken for ten times within 5 min and average values were adopted

quantity primary air rate (kg/s) primary air temperature (°C) rate of total secondary air (kg/s) temperature of total secondary air (°C) coal feeding rate (t/h) pulverized-coal fineness (R90, %) Details of Jets Contained in Total Secondary secondary air mass flow rate (kg/s) ratio to the total air (%) staged air declination angle θ (deg) effective slot area (m2) mass flow rate (kg/s) ratio to the total air (%) OFA mass flow rate (kg/s) ratio to the total air (%)

step (i)

step (ii)

step (iii)

116.9 93 551.7 363 269.4 8.1

115.3 103 563.2 358 264.1 7.9

114.3 98 560.7 359 272.3 7.6

288.4 42.1 20 4.95 124.8 18.4 96.8 14.3

234.9 34.8 0 9.00 182.9 27.1 87.8 13.0

Air 379.1 56.7 45 3.47 121.7 18.2

Operational Results Reflecting the Furnace Performance O2 at the furnace exit (%) 2.90 3.20 exhaust gas temperature (°C) 151.2 153.5 CO in flue gas (ppm) 18 65 carbon in fly ash (%) 3.72 9.81 NOx emissions (mg/m3, 6% O2) 1036 878

2.90 156.0 33 5.40 867

Proximate Analysis, Wt % (As Received) moisture 7.60 7.76 volatile matter 8.95 7.26 ash 32.61 28.71 fixed carbon 50.84 56.27 net heating value (MJ/kg) 19.46 21.25

7.40 6.59 33.29 52.72 19.28

Ultimate Analysis, Wt % (As Received) 51.50 56.38 2.12 2.02 1.49 1.01 0.85 0.78 3.83 3.34

51.89 1.88 1.38 0.80 3.36

carbon hydrogen oxygen nitrogen sulfur a

Note: Data of Steps (i) and (ii) are mainly from refs 24 and13, respectively. Here, the total air contains primary air and total secondary air.

so as to eliminate the potential negative effect of local combustion fluctuations. (2) Local gas temperatures and species concentrations in the near-wall region. A 6-m-length thermocouple device and a 3-m-length water-cooled stainless steel probe, whose configurations and major technical parameters had been described in detail in the literature,13,24 were inserted in turn into the furnace through selected ports (i.e., ports 1, 3, and 4 for the thermocouple, port 2 for the probe; Figure 1) so as to perform the temperature measurements and suck gas samples, respectively. After quickly quenched the chemical reactions and cooled by the highpressure cooling water, the captured gas samples were analyzed online by a Testo 350 M instrument, with measurement errors of 1% for O2, 5% for CO, and 50 ppm for NOx. The gas temperature measurement error by the fine-wire thermocouple technique, aroused mainly 12422



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Figure 2. Comparisons of general gas temperature distribution patterns among Steps (i)−(iii) (data acquired by the infrared thermometer). a. gas temperature distribution patterns acquired through ports 1, 3, and 4 in the near wing-wall region. b. gas species concentrations acquired through port 2 (located at the preceding combustion stage).

quently, intense combustion and high gas temperatures usually appear in ports 3 and 4. Thereafter, under the circumstances with OFA, the burnout stage is located at the furnace throat zone where the upward unburnt particles mix with OFA to complete the final combustion share. Figure 2 presents two gas temperature comparison patterns among Steps (i)−(iii), where the temperature data were acquired by the infrared thermometer. Panel a of Figure 2 shows the general gas temperature distribution patterns at three steps, with their temperature data in ports 1−4 selected for a simpler and clearer temperature comparison along the downward flame travel (panel b of Figure 2). Patently, both the two temperature comparison patterns uncover that for all three steps, a continuous gas temperature increase generally appears as the downward flame proceeds in the lower furnace before reversing upward, with the exception of a temperature decrease stage from the staged-air zone to the hopper region at Step (iii). The aforementioned combustion zone partition explains the general temperature increase stages as the downward flame penetrates and coal combustion proceeds. At Step (iii), the modified staged air (with a much higher staged-air flux and horizontal supplying direction, Table 1) generates a sharply strengthened staged-air effect on the downward coal/air flame as compared with the previous two steps. In consequence, the present staged air affects the combustion performance with dual roles of (i) favoring coal combustion in the staged-air zone by greatly strengthening the

by the radiation loss uncertainty, had been calculated to be below 8% by De.25 (3) Exhaust gas temperature, CO content, and NO x emissions in flue gas. By using a grid method to position monitoring ports at the air preheater exit, a 4-m-length K-type nickel−chromium/nickel−silicon thermocouple device and an 8 mm-i.d. stainless steel pipe (connected by a vacuum pump at the pipe outlet) were inserted in turn to acquire the exhaust gas temperature and to capture gas samples. These samples were also analyzed online by the Testo 350 M instrument. Additionally, sampling fly ash in flue gas at the air preheater exit was used to determine carbon in fly ash.

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RESULTS AND DISCUSSION According to the combustion zone partition in the published work13 and several typical ports (Figure 1) used for data acquiring, coal combustion in ports 1 and 2 is located at the preceding combustion stage, wherein a relatively low-temperature and fuel-rich chemical atmosphere appears and the combustion status is affected apparently by the secondary-air supplying through the furnace arches. Coal combustion in the furnace zone corresponding to ports 3 and 4 is located at the primary combustion stage, where secondary air has already mixed well with the ignited coal/air mixture and the timely staged-air supplying feeds the left combustion air. Conse12423



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Figure 3. Local gas temperature and species concentration distributions in several selected zones in the near wing-wall region (temperature data acquired by the thermocouple device).

chemical atmosphere in the primary combustion zone, as reported in the literature.13 Again, the sole staged-air angle reduction without an effective staged-air flux increase (Table 1, compared with Step (i)) attains a limited increase in the stagedair block effect (shortening the flame penetration to some extent), but unfortunately, establishes a still weak mixing of the staged-air jet with the downward coal/air flame. Consequently, the poorest combustion performance with lowest gas temperature levels and highest burnout loss (see carbon in fly ash and CO emission in Table 1) develops, despite NOx emissions reducing from 1036 to 878 mg/m3 at 6% O2 and hopper temperatures falling moderately to about 1200 °C. Based on the results gained from Step (ii) (i.e., the findings of a higher staged-air flux favoring coal combustion under deep-air-staging conditions13 and a shallower staged-air angle favoring lower hopper temperatures), sharply increasing the staged-air flux by recovering the designed staged-air slot area and redirecting staged air to a horizontal direction (Table 1) were set at Step (iii). At the latest step, the aforementioned dual roles of the present staged air improve the previous poor combustion performance (i.e., low furnace gas temperature levels and high burnout loss) and lower further hopper temperatures to sufficiently low levels of about 1100 °C, in addition to a slight decrease in NOx emissions. These mean that after applying OFA and performing two steps of staged-air modifications, the down-fired furnace finally achieves a perfect combination of low NOx emissions, low carbon in fly ash and safe hopper temperatures (levels of 867 mg/m3 at 6% O2, 5.40%, and about 1100 °C, respectively, Table 1) under deep-air-staging conditions. In short, data in Figure 2 and Table 1 suggest that with the combustion configuration retrofitted from Step (i) to Step (iii), general gas temperature levels decrease initially but then increase while hopper temperatures decrease continuously, the burnout loss increases initially but then decreases,

later staged-air mixing with the unburnt coal particles and (ii) shortening the flame travel (the downward flame hardly penetrating the staged-air zone) by the strong staged-air block. The cooperation of the dual roles of staged air and the findings of a higher staged-air flux favoring coal combustion under deep-air-staging conditions, finally results in the hightemperature zone terminating at the middle and lower parts of the lower furnace (instead of those spreading to the hopper region at the previous steps) and sufficiently low hopper temperatures (levels of about 1100 °C) appearing. Figure 2 also uncovers gas temperature change trends related with the two stages of staged-air modifications. These temperature change trends, combined with operational results listed in Table 1, disclose the general combustion performance associated with each step. Among three steps, Step (i), which is characterized by the sharply inclined staged air with the lowest staged-air ratio and largest secondary air flux (Table 1), has the strongest secondary air assistance favoring the coal combustion in the primary combustion stage and the weakest staged-air block on the downward flame. The resulted longest flame penetration depth extends a lot the residence times of coal particles in the lower furnace. These circumstances finally develop the highest furnace gas temperature levels, lowest carbon in fly ash, and highest NOx emissions (Table 1). Simultaneously, the weakest staged-air block effect and excessively long flame penetration (washing over the hopper walls, as mentioned previously in the Introduction section) generate the highest hopper temperature levels of above 1300 °C and finally result in the occurrence of the aforementioned thermal fatigue problem, which must be resolved for boiler’s safe operations. At Step (ii) which applies OFA for NOx reduction and decreases the staged-air angle (from 45° to 20°) for a strengthened staged-air effect, the regulated deep-airstaging conditions actually develop an oxygen-deficient 12424



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affected by the flame penetration and staged-air spread. The excessively deep flame penetration with the flame washing over the hopper walls at Step (i) corresponds to the rapid temperature increase with distance and highest temperature levels among the three steps. At the latter two steps with staged-air modifications, the staged-air cooling effect and coal combustion result in gas temperatures raising continuously with a lowered increase rate. Again, the continuously strengthened staged-air impact (sharply reducing angle and meanwhile increasing air flux) results in the continuous temperature decrease from Step (i) to Step (iii). From panel b of Figure 3, it can be seen that at port 2, gas species concentrations for all three steps generally present similar change patterns with distance. That is, both O2 and CO levels generally increase with distance except for the continuous O2 fluctuations within low levels of 4.5−6% at Step (iii), whereas the NOx concentration increases initially but then decreases at distances beyond 1.2 m from the wing wall. With modifying the combustion configuration from Step (i) to Step (iii), both O2 and NOx levels decrease continuously, whereas the CO concentration decreases initially but then increases (with the Step (iii) setting attaining the highest levels). These change trends with distance and comparisons are explained as follows. As mentioned previously, coal combustion at port 2 is located at the preceding combustion stage. Generally, the combination of coal combustion (consuming O2 and generating CO and NOx) and secondary air spreading from the zone below burners to the surroundings (raising O2 levels) results in (i) O2 increasing all the while and (ii) NOx increasing initially but then decreasing with distance. While at Step (iii), the furnace regulates the deepest air-staging conditions with the least secondary air flux among the three steps (Table 1), actually develops relatively intense combustion at port 2 (Figure 2). The intense coal combustion consuming lots of O2 and the least secondary-air supplying at Step (iii) thus enable the O2 concentration to fluctuate with distance and also result in the lowest O2 and NOx levels and highest CO concentration appearing at port 2. The CO increase pattern with distance is mainly attributed to (i) the postponed secondary air mixing with the ignited coal/air mixture at the preceding combustion stage (because of the special burner nozzle configuration, Figure 1c) and (ii) the relatively oxygen-deficient coal combustion generating lots of CO at this stage. The continuous secondary-air flux reduction, combustion, and strengthened airstaging conditions account for the above comparison patterns of gas species concentrations among the three steps. In summary, the application of deep-air-staging combustion in the 600 MWe down-fired furnace was imperfect with various problems at the former two steps. Both NOx emissions and hopper temperature achieved high levels at Step (i) with the staged-air angle of 45° and without an OFA system, despite high burnout obtaining. At Step (ii) characterized by applying OFA and reducing the staged-air angle to 20° so as to moderately strengthen the staged-air effect, the furnace obtained an apparent NOx reduction and a moderate hopper temperature decrease while a sharp increase in burnout loss. At the latest Step (iii) with sharply strengthening the staged-air effect by apparently increasing the staged-air flux and horizontally redirecting staged air, prefect operational circumstances with low NOx, high burnout, and low hopper temperatures (i.e., levels of 867 mg/m3 at 6% O2, 5.40%, and about 1100 °C, respectively) developed. When extrapolating these results to other down-fired boilers with different

and NOx emissions decrease continuously, while the exhaust gas temperature increase all the while because of the continuously strengthened air-staging conditions. As complementary materials of findings obtained from Figure 2 and Table 1, gas temperatures (data acquired by the thermocouple device) and species concentrations in the nearwing wall region are graphed in Figure 3 so as to deepen the understanding about changes of the real-furnace combustion characteristics associated with these combustion retrofit steps. Here, temperature profiles in ports 1, 3, and 4 indicate typical gas temperature distribution patterns along the thermocouple inserted distance at the preceding combustion and primary combustion stages and in the upper part of the hopper region, respectively. Gas species concentration profiles only for port 2 are used to uncover gas species concentration varieties with respect to the continuously strengthened staged-air effect and air-staging conditions at the preceding combustion stage. Gas species concentrations at this stage are crucial to control NOx emissions because of fuel NOx (mainly producing at this stage) accounting for the majority of total NOx production in pulverized-coal furnaces. As shown in panel a of Figure 3, gas temperature orders of ports 1, 3, and 4 at each step and temperature comparisons among the three steps at each port are generally in accordance with those displayed in Figure 2. Considering that in the discussion about Figure 2, detailed analysis and explanation have already given out for gas temperature orders and comparisons among different steps and ports, additional discussion about these aspects is therefore not provided here. As the thermocouple penetrates further, all three steps generally show two temperature distribution patterns. That is, gas temperatures in port 1 increase initially but then decrease, whereas those in port 3 and 4 raise continuously with different increase rates, with the exception of slight fluctuations beyond 0.9 m in port 3 at Step (ii). These observations are attributed to the combination of several aspects: (1) The high-temperature gas accumulating in the near-wall region develops the initial temperature increase stage; (2) Port 1 is not far from the burner nozzle outlets, resulting in the cold burner’s jets rapidly cooling the thermocouple when the measuring point approaching the zone below the burner. Obviously, higher furnace gas temperature levels generally correspond to higher temperature values at these three ports. While for the distance beyond 1.5 m at port 1, because Step (iii) has the deepest airstaging conditions and smallest secondary-air flux among the three steps (Table 1) to produce the weakest secondary-air cooling effect, the highest gas temperatures, on the contrary, appear at Step (iii); (3) The furnace zone corresponding to port 3 is affected directly by staged air when a much shallower or even 0° staged-air angle equipped, instead of the hightemperature circumstances filled with the downward flame at Step (i). At Step (ii), the aforementioned weak mixing of staged air with the oxygen-deficient coal/air flame,13 shortened flame penetration depth, and staged-air cooling effect generate the poorest combustion status with lowest gas temperature levels among the three steps, accompanied by the temperature fluctuating stage beyond 0.9 m at port 3. At Step (iii), because of both the increased staged-air flux and strengthened staged-air mixing with the coal/air flame favoring coal combustion in the primary combustion stage, gas temperatures at port 3 increase with distance all the while and temperature levels raise clearly as compared with those at Step (ii); (4) Port 4 is located at the upper part of hopper region where gas temperatures are 12425



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combustion configurations, the limitation of the present findings needs to be discussed. According to the division from Kuang and Li,26 there mainly exist five types of down-fired boilers and the present 600 MWe supercritical boiler belongs to a MIMSC (multiple-injection multiple-staging combustion) type. As reported in the published work,26 different types varied apparently in the gas/particle flow characteristics, pulverizedcoal residence times and gas temperature profiles in the combustion chamber, and NOx emissions, which mainly depended on the furnace configuration, burner type, and air distribution model. In light of these inherent differences (referring to the combustion configuration and combustion performance) between the boiler here and other four types,26 the present findings are therefore specific to the MIMSC type and an extrapolated application into other down-fired boilers varying greatly from the present type needs considerate designs. However, it is no doubt that the experiences gained in this work enrich the limited understanding about deep-air-staging combustion within down-fired furnaces and also provide useful reference materials for deep-air-staging modifications of downfired furnaces already in service and for new designs.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86 571 87953162; fax: +86 571 87951616; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Contract No. 51422605 and 51306167) and the Project funded by the China Postdoctoral Science Foundation (Contract No. 2014M551733).

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REFERENCES

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