Combustion Flexibility of a Large-Scale Down-Fired Furnace with

Article pubs.acs.org/EF

Combustion Flexibility of a Large-Scale Down-Fired Furnace with Respect to Boiler Load and Staging Conditions at Partial Loads Zhongqian Ling, Min Kuang,* Xianyang Zeng, and Guangxue Zhang Institute of Thermal Engineering, China Jiliang University, Hangzhou 310018, P. R. China ABSTRACT: Down-fired boilers, designed especially for industry-firing low-volatile coals such as anthracite and lean coal, suffer similarly from various problems, such as poor burnout, high NOx emissions, and asymmetric combustion. A new down-fired combustion technology was developed specially for these problems and finally trialed in a 600-MWe supercritical boiler. To evaluate the flexibility of this technology application with respect to boiler load and air-staging conditions at partial loads, industrial-size measurements (taken of gas temperatures and species concentrations in the furnace, CO and NOx emissions in the flue gas, and carbon in fly ash) were performed within the furnace at different boiler loads (i.e., 370, 450, and 600 MWe) and staged-air damper openings (i.e., 10, 30, and 45% at 450 MWe), respectively. It was found that a relatively symmetric gas temperature distribution pattern developed for all settings, despite asymmetric burner operation models at 450 and 370 MWe. Decreasing boiler load reduced gas temperature levels in the furnace to weaken coal combustion, thereby increasing combustible loss and reducing both levels of NOx emissions and exhaust gas temperature. On increasing the staged-air damper opening at 450 MWe, both gas temperatures in the furnace and NOx emissions increased initially but then decreased, while carbon in fly ash and exhaust gas temperature respectively decreased and increased. A comprehensive evaluation of results in this work suggested that (i) the boiler flexibility was poorer at 370 MWe than at 450 and 600 MWe, (ii) relatively high-O2 operation conditions needed to be set for improving the poor burnout at the low 370-MWe load, and (iii) increasing the staged-air damper opening was favorable for the furnace performance at the moderate 450-MWe load and the optimal damper opening at the load was 45%. Again, countermeasures were recommend respectively for improving the poor furnace performance at the low 370-MWe load and for reducing the still high NOx emissions at moderate and full loads.

1. INTRODUCTION Anthracite and lean coal are abundant in reserves and widely fired in thermal power generators in the world. However, because of their low volatile matter and poor reactive activity, these fuels present difficulties in achieving ignition, maintaining stable combustion, and completing burnout when industrially fired in furnaces.1−4 Down-fired furnaces, designed especially for industry-firing anthracite and lean coal, attempt various carefully designed strategies to attain well firing of these fuels. These mainly consist of creating a W-shaped flame to prolong pulverized-coal residence times in the furnace, positioning large refractory coverage on furnace walls to attain high gas temperature levels in the fuel-burning zone, and supplying air in various staging patterns so as to inhibit NOx production.5−7 However, the actual operation performance essentially deviates from the designed concepts. Various problems, such as poor burnout,8−10 heavy slagging,11 particularly high NOx emissions (reaching levels up to 1600 mg/m3 at 6% O2),6,9,12−14 and asymmetric combustion,10,15,16 have been reported widely in down-fired furnace operations. The current fundamental understanding of NOx formation processes in coal combustion system, described in detail in a previously published work,17 needs to be listed here so as to understand well the causes of particularly high NOx emissions in down-fired furnaces. NO, accounting for a 90−95% share of the total NOx emissions in coal-fired furnaces, forms via three principal sources (i.e., fuel NO, thermal NO, and prompt NO): (i) Fuel NO is formed from nitrogen bound in the fuel. During the devolatilization process, nitrogen bound in the coal is released to form nitrogen-containing species (such as HCN and NH3), which © 2013 American Chemical Society

are generally oxidized to form NO in fuel-lean regions. Generally, fuel NO is the dominant NO formation mechanism in flames which contain nitrogen in the fuel and typically accounts for more than 80% of the NO formed in coal-fired systems. Controlling the local environment in which nitrogen is released from the fuel is a primary means of controlling NO emissions. (ii) Thermal NO is formed from oxidation of atmospheric nitrogen at relatively high temperatures in fuellean environments. The thermal-NO formation is highly dependent on temperature, residence time, and atomic oxygen concentration. (iii) Prompt NO is formed by the reaction of atmospheric nitrogen with hydrocarbon radicals in fuel-rich regions of flames, which is subsequently oxidized to form NO. Prompt NO is only significant in very fuel-rich systems and is a small portion of the total NO formed in most combustion systems. The total NOx emissions in coal combustion are influenced not only by furnace design parameters and firing conditions but also coal characteristics. According to the knowledge of NOx formation in coal-fired systems and combustion-regulating principles in down-fired furnaces, explanation of particularly high NOx emissions within downfired furnaces lies within two aspects: (i) high gas temperature levels and (ii) long pulverized-coal residence times in the hightemperature lower furnace. With respect to gas temperature, in comparison with tangential-fired and wall-arranged furnaces Received: October 31, 2013 Revised: November 30, 2013 Published: December 4, 2013 725

dx.doi.org/10.1021/ef402161u | Energy Fuels 2014, 28, 725−734

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Figure 1. Schematics of furnace configuration, burner and staged-air slot layout patterns, and measuring port layout in industrial-size experiments for the 600-MWe down-fired furnace.

that are frequently used to fire high-volatile fuels such as lignite and bituminous coal, higher gas temperature levels (with flame kernel temperatures even up to 1800 °C) need to be established within down-fired furnaces. Admittedly, the hightemperature circumstances in down-fired furnaces greatly favor the thermal-NO production when gas temperatures reach up to 1500 °C in the primary combustion zone with sufficient oxygen, resulting in the thermal-NO production sharply exceeding its conventional share (below 20%) in the total

NO. For example, a published work reported that, calculated from the nitrogen content in the coal used (i.e., selecting 0.28 as the conversion rate of fuel N to NO) and industrial-size NOx emission data (i.e., 1725.9−2088 mg/m3 at 6% O2) in a 300MWe down-fired furnace, the fuel-NO share was determined as 45−54%, instead of the conventional levels above 80%.18 Concerning residence times, to extend further the pulverizedcoal residence times in a down-fired furnace, high momentum flow rates of arch-air flows (consisting of primary air and 726


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Table 1. Coal Characteristics and Major Operational Parameters in Real-Furnace Experimentsa Proximate Analysis, wt % (as received) volatile matter

ash

8.95

32.61

fixed carbon

moisture

net heating value (MJ/kg)

7.60 50.84 Ultimate Analysis, wt % (as received)

carbon

hydrogen

sulfur

51.50

2.12

3.83

19.46

nitrogen

oxygen

0.85 operational parameters boiler load

a

1.49 staged-air damper opening

quantity

600 MWe

450 MWe

370 MWe

10%

30%

45%

flow rate of main steam (t/h) pressure of the main steam (MPa) total rate of primary air (kg/s) temperature of primary air (°C) total rate of secondary air (kg/s) temperature of secondary air (°C) coal feeding rate (t/h) pulverized-coal fineness (R90, %) O2 at the furnace outlet (%)

1787.5 23.66 118.0 95 554.2 360 261.2 8.6 3.10

1367 18.56 96.4 102 424.3 368 203.2 7.4 2.91

1143 18.00 78.6 108 330.7 369 168.0 6.5 2.72

1378 18.79 100.2 98 427.8 365 207.1 7.8 2.87

1367 18.56 96.4 102 424.3 368 203.2 7.4 2.91

1371 19.03 97.6 101 421.9 370 201.7 7.1 3.10

Note that settings “450 MWe” and “30%” are indeed one case used twice in columns listing boiler load and staged-air damper opening, respectively.

boiler manufacturers so as to popularize further the down-fired furnace application. After about 5 years of collaborative efforts of our group and boiler manufacturers, a new down-fired combustion technology based on the multiple-injection and multiple-staging concept (i.e., the MIMSC technology in the literature28) was developed and finally trialed in a 600-MWe down-fired supercritical boiler.29 To uncover the boiler flexibility of this new down-fired combustion technology application with respect to boiler load and air-staging conditions, it is necessary to perform a series of industrialsize investigations into coal combustion characteristics within the 600-MWe down-fired furnace. Compared with studies focusing on combustion status at full load for conventional tangential-fired and wall-arranged furnaces, more attention needs to be addressed to industrial-size investigations at different loads for down-fired furnaces. This is because, up to now, down-fired furnaces in China, accounting for about 80% of the market share of the down-fired furnace application in the world,4,11,12 are mainly located in the southwest of the country. In this region, hydropower resources for hydroelectric stations and anthracite reserve are particularly abundant. In the rainy season (from March to August) and other times with relatively low electric power demands, down-fired furnaces must concede to hydroelectric stations by dropping their boiler loads to about 80% or even 50% of the design electrical outputs. Accordingly, during the past 1.5 years’ operations of the mentioned 600MWe down-fired furnace, boiler loads ranking at approximately 350 and 450 MWe have appeared widely in night and daytime operations of the present furnace, respectively, except for fullload operations at the peak period of electric power demands. This means that the circumstances with full-load operations are apparently fewer that those near the two mentioned load grades. In addition, adjusting the staged-air damper opening is usually the preferred method for boiler managers to optimize the furnace combustion status. Consequently, in this work industrial-size experiments were performed within the 600MWe down-fired furnace at various boiler loads (i.e., 370, 450, and 600 MWe) and staged-air damper openings (i.e., 10, 30, and 45% at a moderate 450-MWe load), so as to evaluate the

secondary air) are needed to deepen the downward coal/ airflow penetration depth in the lower furnace, in addition to a relatively low wall-air supply to weaken the wall-air block on the downward coal/airflow penetration. But, unfortunately, this airsupplying model usually develops an oxygen-rich atmosphere in the burner region for the devolatilization process and coal ignition, thereby favoring the fuel-NO generation. To improve the poor down-fired furnace performance, various solutions have been reported to deal with the aforementioned problems, such as burning blended fuels7,13,19 or retrofitting the combustion configuration20,21 to improve burnout, changing burner operation models to alleviate slagging,11 and adjusting air distribution,9,14,22,23 or performing comprehensive combustion system retrofits to reduce NOx emissions.12,20 In addition, severely asymmetric combustion may result in large differences in wall temperatures and limits being exceeded. Under these circumstances, problems such as fine cracking and tube bursting of water-cooled walls are likely to develop and thus jeopardize safe boiler operations. Therefore, asymmetric combustion in down-fired furnaces should be eliminated or mitigated by the greatest extent. After determining the formation of a deflected flow field as the major reason given for asymmetric combustion, Kuang et al. found that establishing asymmetric air-distribution models could improve the flow-field symmetries.10,24 However, little research has been reported on comprehensive methods that can simultaneously deal with all these problems. This can be attributed to the fact that good ignition conditions, high gas temperature levels, and long pulverized-coal residence times needed to be simultaneously established for well-firing anthracite and lean coal in down-fired furnaces.5−7,12,14 But unfortunately, establishing all these circumstances always produces large quantities of NOx.14,22,25−27 Apparently, it is difficult to regulate air-staging conditions in down-fired furnaces to significantly reduce NOx emissions under good burnout circumstances. Again, the coexistent heavy slagging and asymmetric combustion apparently compound difficulties in NOx reduction.10−12 Developing a stable and symmetric combustion pattern (accompanied by good burnout, weak slagging tendency, and low NOx emissions) is thus urgent for 727


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flexibility of the MIMSC technology with respect to boiler load and air-staging conditions at partial loads.

At 450 MWe, the coal supplying models were the same as those at 600 MWe for all six millers except for the decommissioned mill A. While at 370 MWe, a decreasing pattern in the coal feeding from the central part to the two sides (i.e., the rated coal-feeding rate for both millers B and C, 75% for both millers D and E, and 50% for the miller F) was established to centralize further the flame in the furnace central part zone. Additional material needs to be provided to disclose the furnace air distribution for the two conditions of industrial-size experiments. Before the industrial-size measurements, cold flow experiments were performed in the real furnace by measuring secondary-air and stagedair velocities at their port outlets, aimed at acquiring the secondary-air and staged-air ratios (i.e., ratios of the secondary-air and staged-air flow rates to the overall airflow rate into the furnace, respectively) at a series of damper opening setups. Results of the cold flow experiments suggested that with the secondary-air damper opening fixed at 60% in these industrial-size experiments, the secondary-air and staged-air ratios were approximately 67.4%:7.5%, 64.4%:10.5%, and 60.9%:14.9% for the three staged-air damper openings of 15, 30, and 45%, respectively. At the lowest 370-MWe load, because of the relatively higher primary air ratio needed to be established for the pulverizedcoal feeding, the corresponding secondary-air and staged-air ratios were a little lower than those at 450 and 600 MWe, despite no change appearing at the secondary-air and staged-air shares in the total secondary air. 2.3. Methods in Industrial-Size Data Acquiring. In view of coal characteristics varying from one day to the next, these five settings in the two mentioned experiments were performed successively so as to ensure that the measured results were comparable using similar coal. Again, during each experimental run, considerable efforts for allowing minimum variation in boiler operating conditions need to be ensured. Table 1 lists coal characteristics and the mean operational parameters over the 5 h duration of each experimental run. Detailed information about data acquiring methods in each experimental run is as follows. (1) To determine gas temperature distributions in the front- and real-half parts of the lower furnace, a 3i hand-held pyrometer (a type of noncontact infrared thermometer), with a measurement range from 600 to 3000 °C, was inserted in turn through observation ports (located on the two wing walls connected with the right-side wall, Figure 1) to measure gas temperatures. These gas temperature data were used to (i) evaluate the combustion symmetry extent at various settings and (ii) determine effects of boiler load and staged-air damper opening on the general gas temperature levels in the lower furnace. The local combustion status, affecting clearly the temperature data reading in the hand-held pyrometer, was taken as the major source of uncertainties in this type of temperature measurements. Consequently, at each port the measurement was taken for ten times within 5 min and average values were adopted. (2) For gas temperatures and species concentrations in the near wing-wall region, upon considering that only the front-arch burner groups were in service in the near wing-wall region at 370 and 450 MWe, the front-half part of the lower furnace was selected to perform these measurements so as to obtain comparable data at different settings. A thermocouple with a 0.3-mm-i.d. and 8-m-length nickel− chromium/nickel−silicon fine wire, located in a 4-mm-i.d. twin-bore stainless steel sheath, was placed inside a 6-m-length water-cooled stainless steel tube to form a thermocouple device. The thermocouple device and a 3-m-length water-cooled stainless steel probe (i.e., a centrally located 10-mm-i.d. sampling pipe surrounded by a 60-mm-i.d. stainless steel tube to allow the high-pressure cooling water for probe cooling, with a gas sampling flow rate of 1 L/min and high-pressure cooling water flow rate of 60 L/min, respectively) were inserted in turn into the furnace through several observation ports (i.e., ports 1−4 for the thermocouple and ports 2 and 3 for the probe, respectively). After quickly quenching the chemical reactions and cooling by the high-pressure 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. Sources of uncertainty in the thermocouple measurements originated mainly from radiation losses (i.e., radiation from the gas to the thermocouple and from the

2. EXPERIMENTAL SECTION 2.1. Utility Boiler. Figure 1 presents the vertical and transverse cross sections through the furnace, concentrator and burner layout patterns on furnace arches, and the staged-air slot configuration. The arches divide the furnace into two sections: the octagonal lower furnace (i.e., the fuel-burning zone) with four wing walls and the rectangular upper furnace (i.e., the fuel-burnout zone). As shown in Figure 1b, a total of 24 louver concentrators, symmetrically arranged on the two arches to connect with six millers labeled as A−F, divide the primary air/fuel mixture into fuel-rich and fuel-lean coal/air flows needed to regulate the fuel rich/lean combustion. There are 12 burners symmetrically lining the front and rear arches and uniformly positioned along the furnace breadth, with each burner group corresponding to a pair of concentrators on arches and a staged-air slot group on the front and rear walls, respectively (panels c and d of Figure 1). The combustion configuration with the MIMSC technology consists of three sections: (1) regulating the fuel rich/lean combustion in the burner region to enrich the pulverized-coal concentration, lower the coal/air flow velocity, and establish a relatively oxygen-lean atmosphere prior to coal ignition; (2) supplying secondary air through arches in a two-stage manner (i.e., the high-speed inner and outer secondary-air jets parallel to the fuel-rich coal/air flow) to postpone the mixing of secondary air and the ignited coal/air flow, thereby forming the first combustion stage; and (3) feeding the high-speed staged air (with a declination angle of 45°) through the lower part of the front and rear walls to establish a second combustion stage along the flame travel path. Introductory material about technical principles of the MIMSC technology can be found elsewhere.28 2.2. Industrial-Size Experiment Setup and Operating Models for the Combustion System. As mentioned in the Introduction, industrial-size experiments were performed at settings divided into two panels, i.e., the boiler load panel containing 370, 450, and 600 MWe and the staged-air panel at a moderate 450-MWe load including stagedair damper openings of 10, 30, and 45%. When varying boiler load to establish the three load settings, all damper openings for the combustion air into the furnace were fixed, and the secondary-air and staged-air damper openings were 60% and 30%, respectively. For the staged-air panel at 450 MWe, only the staged-air damper opening was adjusted. This means that the 450 MWe setting in the boiler load panel and the 30% one in the staged-air panel are actually one case used twice but with different setting names. For the convenience of data comparison in both two panels, “450 MWe (30%)” is used to denote both two settings in those profiles in the Results and Discussion. In combination with the corresponding relation between millers and burner groups in Figure 1c, introductory material about the furnace operation models at different loads is as follows: (i) At fullload operation with supercritical steam parameters (listed in Table 1), all 12 burner groups and six millers were in service. Each of millers B− E (corresponding to the eight burner groups in the middle part of the arches) supplied pulverized-coal at a rated coal-feeding rate of 50 t/h, while both millers A and F (connected with burner groups near the four wing walls) supplied about 75% of the rated coal-feeding rate. This coal-supplying model can allow the flame to be centralized in the furnace central part zone along the furnace breadth and weaken the heat load distribution near the waling-wall and side-wall zones under supercritical operation conditions. (ii) At both 370 and 450 MWe, the boiler operation fell to a subcritical status (steam parameters listed in Table 1), with miller A being out of service and the two related burner groups at the corner of the rear arch shut down. Consequently, a symmetric coal/air supply (designed to establish a W-shaped coal/air flame) formed in the furnace central part zone, whereas an asymmetric pattern (a U-shaped coal/air flame emanating from the front arch) developed along vertical furnace cross sections close to the four will walls. For the decommissioned burner group, primary air/coal was not fed, although the associated secondary air and staged air were used for port-cooling, albeit functioning at 15% capacity of the active burners. 728


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Figure 2. Gas temperatures in the front- and rear-half parts of the furnace at various settings. thermocouple to the surrounding wall, respectively) and ash deposition on the thermocouple. The negative influence of ash deposition could be avoided by frequently retracting the thermocouple from the furnace to remove any deposition on the thermocouple, whereas the mentioned radiation losses were unavoidable during temperature measurements. According to De30 and Costa et al.,31 the deviation of the measured gas temperature from the true value by a fine-wire thermocouple technique, aroused mainly by the uncertainty in the radiation losses, was below 8%. For gas species concentration measurements, the major sources of uncertainty were associated with whether the water-cooled probe could quickly quench chemical reactions and effectively avoid aerodynamic disturbances of the gas flow in the sampling pipe. Because of the low gas flow rate in the sampling pipe and high flow rate of the cooling water in the external tube, the water-cooling rate was particularly high and quenching of the chemical reactions was rapidly achieved upon samples being drawn into the probe. (3) With regard to the exhaust gas temperature, CO concentration, and NOx emissions in flue gas, by using a grid method to arrange monitoring ports at the air preheater exit, a 4-m-length thermocouple device (also with a twin-bore stainless steel sheath but without a watercooled external tube) and an 8-mm-i.d. stainless steel pipe (connected by a vacuum pump at the pipe outlet) were inserted in turn to acquire exhaust gas temperatures and capture gas samples in flue gas. These samples were also analyzed online by the Testo 350 M instrument. In addition, fly ash was also sampled at the air preheater exit and then analyzed to obtain the unburnt carbon content.

3. RESULTS AND DISCUSSION 3.1. Impacts on Local Gas Temperature Distribution. Figure 2 presents gas temperatures in the front- and real-half parts of the lower furnace (temperature data acquired by the hand-held pyrometer). It can be seen that with the MIMSC technology, the furnace generally accomplishes a symmetric gas temperature distribution pattern for all five settings. That is, symmetric combustion develops within the furnace, despite asymmetric burner operation models at 370 and 450 MWe. Here, gas temperature gaps between the front- and real-half parts of the lower furnace are within 70 °C, which apparently contrast with those reaching 400−600 °C in down-fired furnaces suffering from severely asymmetric combustion because of the formation of a strongly deflected flow field.10,28 The information in a published work10,29 explains the present symmetric combustion formation: (1) At full load, the burner configuration and air distribution with the MIMSC technology develop an essentially symmetric gas/particle flow field within the present furnace,29 thereby forming symmetric combustion. (2) Considering that the burner operation model at 370 and 450 MWe in the present 600 MWe supercritical furnace is similar to that at 150 MWe in a 300 MWe down-fired subcritical furnace,10 the experience at 150 MWe in the 300 MWe furnace can be referenced to explain the symmetric gas 729


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Figure 3. Gas temperature distribution patterns in the near wing-wall region.

in those ports close to the furnace center) appear in the fourth row of ports that are located in the upper part of the hopper region. Aside from these above observations, Figure 2 also shows that as boiler load decreases, the reduction in coal consumption decreases continuously the whole gas temperatures in the lower furnace. While at the moderate 450 MWe, increasing the stagedair damper opening enables gas temperature levels to initially increase but then decrease, with the exception of a continuous increase in the hopper region. These observations are attributed to the fact that increasing the staged-air damper opening increases the staged-air flux, along with a decrease in secondary air. Moderately increasing the staged-air damper opening from 10 to 30% weakens the low-temperature secondary air mixing with the coal/air flow and simultaneously strengthens the staged-air effect in aiding coal combustion, thereby acting positively on gas temperatures as the flame proceeds. With the circumstances turning to sharply increase the staged-air damper opening to 45%, on one hand, the sharp secondary-air reduction can greatly deteriorate the oxygen-deficient atmosphere in the zone below the arches; on the other hand, the highest staged-air flux favors intense combustion in the hopper region. As a result, gas temperatures decrease before the flame penetrating the staged-air zone, whereas those in the hopper region increase continuously. Generally, in order to attain good burnout and low NOx production in down-fired furnaces under air-staging conditions, it is better to achieve higher gas temperature for completing a large combustion share prior to the unburnt coal particles entering into the fuel-burnout zone. From this point onward, the highest gas temperature levels in the hopper region and in the furnace center at the 45% setting should be more favorable at 450 MWe in this work, in comparison with the other two low staged-air settings of 10 and 30%. Figure 3 presents gas temperatures in the near-wing wall region (temperatures acquired by the thermocouple device). As graphed in Figure 1, ports 1−3 are generally located along the

temperature patterns at 370 and 450 MWe in the present study. That is, a symmetric flow field for a W-shaped flame formation develops in the furnace central zone where burner groups operate symmetrically, whereas a U-shaped coal/air flame forms along the vertical furnace cross sections close to will walls, where only burner groups on the front arch are in service.10 The high-temperature gas accumulating in the rear-half furnace, because of coal combustion in the late stage of the U-shaped coal/air flame and the transverse diffusion of the ignited downward coal/air mixture ejected through the rear arch, results in gas-temperature levels in the rear-half furnace approaching those in the front-half furnace. As a result, a symmetric combustion pattern also develops at 370 and 450 MWe, despite the asymmetric burner operation model appearing in the near wing-wall zone. In Figure 2, it can be seen that as the ignited coal/air mixture proceeds, temperatures generally increase, with the exception of data in some ports in the rear-half furnace at 370 and 450 MWe (because of no coal supply in this region to provide heat release). The explanation of the temperature increase pattern is as follows: (i) In the first row of ports (from the arches to the hopper region), where the ejected coal/air flow only leaves from the fuel-rich coal/air flow nozzles at about 3 m, the pulverized coal is ignited and only a little secondary air can mix with the ignited coal/air flow. In consequence, a relatively fuelrich, oxygen-deficient, and low-temperature chemical atmosphere develops at this stage. (ii) The second row of ports are sufficiently far from the burner nozzle outlets to allow secondary air to mix gradually with the ignited coal/air flow. Again, a portion of staged air can spread to the vicinity of the third row of ports, despite the sharp staged-air declination being used. Therefore, the timely air supply continuously aids coal combustion to raise gas temperatures. (iii) Because of the sharp staged-air declination of 45° (Figure 1), staged air actually mixes well with the downward coal/air flame in the upper part of the hopper region to develop intense combustion. Accordingly, the highest gas temperature levels (except for data 730


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Figure 4. Gas species concentration distribution patterns in the near wing-wall region.

line just below the fuel-rich coal/air flow nozzle outlet, while port 4 is located at the transition zone where the downward coal/air flame turns around to be redirected upward. Data comparisons of ports 1−4 suggest that gas temperature levels increase continuously for all five settings, with the exception of the low-temperature and high-speed increase stage (at distances within 0.15−0.90 m for all four ports). These observations are explained as the result of two factors: (i) secondary air gradually mixing with the ignited coal/air mixture to advance coal combustion as the downward coal/air mixture proceeds through the vicinity of ports 1−3 and (ii) the staged-air assistance of coal combustion in the hopper region. Here, the gas temperature increase pattern with the penetration depth is in accordance with that displayed in Figure 2. As the distance to the wing wall increases, a similar changing pattern appears for all five settings. That is, temperatures at ports 1 and 2 initially increase at a high speed within distances 0.15−0.90 m but then decrease, whereas those at ports 3 and 4 rapidly increase all the while. Three aspects explain these observations: (i) The hightemperature gas accumulating in the near-wall region develops the initial increase stage in these profiles. (ii) Ports 1 and 2 are located in the relatively low-temperature zone that is not far from the burner nozzle outlets (Figures 1 and 2). After the thermocouple gradually approaching the zone just below the burner group close to the wing wall, the strong secondary-air cooling effect develops the temperature decrease stage at distances beyond 0.90 m. (iii) At ports 3 and 4, secondary air and staged air have in turn mixed well with the downward coal/ air flame to develop intense combustion and raise gas temperature levels. Again, to protect the thermocouple during temperature measurements, the thermocouple was removed from the furnace when readings were close to 1300 °C. Consequently, temperature curves at ports 3 and 4 terminate at short distances from the wing wall after a fast increase to levels approaching 1300 °C. Additionally, Figure 3 also reveals that decreasing boiler load reduces gas temperatures at all four ports. In contrast, on increasing the staged-air damper opening at 450 MWe, gas temperatures at ports 1−3 initially increase but then decrease, whereas those at port 4 increase

continuously. Here, these temperature-changing trends at ports 1−4 with respect to boiler load and staged air are similar to those presented in Figure 2 and occur for the same reasons noted previously when discussing Figure 2. In comparison with openings of 10 and 30%, the lower coal/air flame temperatures at ports 2 and 3 (the relatively weaker combustion stage) and higher at port 4 (the intense combustion stage) at the 45% setting should favor relatively higher burnout and lower NOx production. 3.2. Impacts on Local Gas Species Concentration Distribution. Figure 4 presents gas species concentrations in the near-wall region. Two typical zones (i.e., ports 2 and 3), respectively affected by secondary air and staged air, were selected to acquire gas species concentrations. For all five settings, because of high-temperature gas accumulating in the near-wall region and the spread of secondary air and staged air, O2 levels in both ports generally fluctuate initially but then increase with distance, with the exception of the continuous fluctuations at settings of 10 and 30% and 370 MWe. The exception at port 3 at settings of 10 and 30% and 370 MWe is attributed to two factors: (i) the low flux and weak rigidity of staged air at moderate and low loads with a low staged-air damper opening of 10−30% and (ii) the continuous O2 consumption for coal combustion to lower O2 levels in the downward coal/air flame. The variations in CO levels generally follow a trend in which low O2 concentrations are associated with elevated CO. As the distance to the wing wall increases, for all five settings CO levels initially maintain low values but then increase prior to the latter decrease stage, with the exception of the flat pattern at 600 MWe at port 2 and at 45% at port 3 (because of their highest O2 levels). Compared with those at port 2, CO levels at port 3 are clearly higher at distances beyond 1.20 m for all five settings except for 45%. These observations are explained by (i) intense combustion proceeding at relatively lower O2 content to produce more CO at port 3 and (ii) high staged-air flux supplied into the vicinity of port 3 at the highest staged-air opening of 45%. For the aspect referring to NOx levels, a generally similar changing trend with distance appears at both ports for all five settings. 731


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Figure 5. Levels of exhaust gas temperature, CO emission, carbon in fly ash, and NOx emissions with respect to boiler load and staged-air damper opening at 450 MWe, respectively.

increase, while both exhaust gas temperature and NO x emissions decrease. These changing trends occur because of continuous reductions in both gas temperatures and O2 levels in the lower furnace (Figures 2−4), despite the reduction in the pulverized-coal fineness and the extension in residence times of coal particles in the furnace favoring high burnout. On increasing the staged-air damper opening to deepen air-staging conditions at 450 MWe, both exhaust gas temperature and CO emission increase and carbon in fly ash decreases, while NOx emissions increase initially but then decrease. The changing trends in NOx emissions and carbon in fly ash essentially conflict with the well-accepted knowledge (i.e., reducing NOx emissions and raising carbon in fly ash with deepening airstaging conditions12,14,22,23,27) found widely on conventional furnaces. These unique findings in the present furnace are the result of a combination of two aspects: (i) gas temperature distribution pattern in the lower furnace and (ii) gas temperature changing trend. With regard to the distribution pattern, as the downward coal/air flame penetrates in the lower furnace, gas temperature increases continuously. Prior to the flame being redirected upward, the highest temperatures appear in the upper part of the hopper region (Figure 2), where the sharply inclined staged air can mix well with the coal/air flame to aid coal combustion. Concerning the gas temperature changing trend, with opening staged air, gas temperatures prior to the flame penetrating the staged-air zone initially increase but then decrease, whereas those in the hopper region increase continuously (Figure 2). Because of relatively higher gas temperature levels and deeper air-staging conditions at settings of 30 and 45% (compared with 10%) as well as the strengthened combustion in the hopper region from 30 to 45%, carbon in fly ash and exhaust gas temperature thus decrease and increase, respectively, while NOx emissions increase initially but then decrease. Figure 5 also shows that with applying the MIMSC technology, the furnace can attain clearly higher burnout rate and relatively lower NOx emissions at 450 and 600 MWe (i.e., carbon in fly ash and NOx emissions of 2.54−3.87% and 923− 1292 mg/m3 at 6% O2, respectively), in comparison with conventional down-fired furnaces with NOx emissions being up to levels of 1600 mg/m3 at 6% O2 and carbon in fly ash ranging from 5.00 to 15.00%.9,12,13,20 In view of NOx emissions also depending to a high extent on the fuel properties such as nitrogen content and volatiles (see the explanation of NOx formation listed in the Introduction section), it is necessary to exclude the influence of coal used, so as to highlight the contribution of the present down-fired combustion system in NOx reduction as compared with those conventional down-

That is, NOx levels initially increase within 0.40−1.20 m but then vary slightly or decrease at distances beyond 1.20 m, with the exception of the continuous increase trend at 10% and 370 MWe at port 3. These observations for NOx levels may be explained as the result of a combination of three aspects: (i) high-temperature gas accumulating in the near-wall region, (ii) coal combustion in the downward coal/air flame continuously generating NOx, and (iii) the diluting effects of secondary air and staged air to lower NOx concentration. At port 3, the high staged-air supply at the highest 45% setting enables NOx levels to decrease at distances beyond 1.20 m, whereas NOx levels at settings of 10% and 370 MWe generally increase with distance because of their low staged-air dilution. Figure 4 also shows that as boiler load decreases, both O2 and NOx levels decrease, whereas CO levels increase. These observations occur because both the combustion air supply and gas temperature levels (Figures 2 and 3) decrease upon reducing the boiler load. The O2 reduction and occurrence of relatively intense coal combustion in both ports at all three loads (see gas temperatures at ports 2 and 3 in Figure 3) thus enable CO levels to increase with decreasing boiler load. But, unfortunately, the severely oxygen-deficient chemical atmosphere in both ports and the lowest O2 at the furnace outlet (Table 1) develop the particularly high CO levels as coal combusted at 370 MWe. On increasing the staged-air damper opening at 450 MWe, because of the increase in staged air (along with a decrease in secondary air), O2 levels generally decrease at port 2 and increase at port 3, resulting in CO levels generally increasing at port 2 and decreasing at port 3. At 450 MWe, NOx levels in both ports initially increase but then decrease with opening staged air, with the exception of distances within 0.40−1.20 m at port 3 at the 45% setting. These observations on NOx levels may be explained as the result of a combination of gas temperatures and O2 levels: (i) At port 2, the combination of gas temperature changing trends and the continuously decreasing oxygen concentration develops the mentioned changing pattern in NOx levels with opening staged air, and (ii) at port 3, because of the dual roles of staged air on NOx levels (i.e., aiding coal combustion to favor NOx production by supplying oxygen and diluting the gas to lower NOx concentration) and the gas temperature changing pattern shown in Figure 2, NOx levels also increase initially but then decreases with opening staged air. 3.3. Impacts on Burnout and NOx Emissions. Figure 5 provides some key operation results such as exhaust gas temperature, CO and NOx emissions in flue gas, and carbon in fly ash at various settings. It can be seen that as boiler load decreases, the levels of carbon in fly ash and CO emission 732


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fired furnaces.9,12,13,20 Coals in the literature9,12,13,20 had 8.24− 9.44% volatiles and 0.62−0.96% nitrogen content by weight (as received); these levels only differ a little from those for the present furnace (i.e., 8.95 and 0.85% listed in Table 1, respectively). Because the similar nitrogen content and volatiles generally contribute an equivalent effect on NOx formation, the low-NOx superiority of the present down-fired combustion technology over conventional systems is thus confirmed. As noted in the Experimental Section, the NOx reduction and burnout improvement seen here are attributed to a synergistic effect of three aspects: (i) fuel rich/lean combustion formation in the burner region, (ii) unique burner configuration to ensure adequate flame penetration into the hopper region and simultaneously to avoid the premature mixing between secondary air and the fuel-rich coal/air flow before the ignition occurrence, and (iii) air-staging combustion conditions in the furnace. Unfortunately, the furnace suffers from relatively poor burnout and low-oxygen operation conditions at the low 370MWe load (carbon in fly ash of 5.64%, CO concentration reaching up to 2000 ppm in the furnace, and O2 concentration of 3.00−4.00 and 2.72% in the lower furnace and at the furnace outlet; see Table 1 and Figure 4). Moreover, at the moderate and full loads NOx emissions are still high at 923−1292 mg/m3 at 6% O2. At 370 MWe with the burner and air supplying models provided previously in the Experimental Section, it was found that there was almost no room for establishing highoxygen operation conditions in the furnace, because of an apparent decrease in the whole gas temperature levels and the appearance of unsteady combustion circumstances. These observations mean that the boiler flexibility is poorer at 370 MWe than at 450 and 600 MWe. Comprehensive consideration of burnout and NOx emissions suggests that, at 450 MWe, the optimal setting among the three staged-air damper openings is 45%. To improve the poor burnout and low-oxygen combustion status at low loads such as 370 MWe, further shutting down miller F to increase the output of burner groups corresponding to millers D and E (Figure 1b) may be attempted to centralize further the coal/air flame in the central part of the furnace along the furnace breadth. In this way, high gas temperature levels and relatively stable combustion circumstances may be established along the vertical furnace cross sections through the arch central zone so as to establish high-oxygen operation conditions. Considering that the high NOx production of about 500 ppm appears at port 2 (which is not far from the burner nozzle outlets and is characterized by relatively low gas temperature levels and high O2 concentration; see Figures 2 and 4), the still high NOx emissions at 450 and 600 MWe can be significantly reduced by considerate combustion retrofits to sharply reduce the air stoichiometric ratio in the burner region (such as adding a reasonable overfire air system).

all settings, regardless of the asymmetric burner operation models applied at 450 and 370 MWe. Decreasing boiler load reduced gas temperature levels in the lower furnace and levels of O2 and NOx in the near wing-wall region (along with an increase in the CO production), thereby increasing combustible loss and reducing both NOx emissions and exhaust gas temperature. Upon increasing the staged-air damper opening at 450 MWe, carbon in fly ash decreased and exhaust gas temperature increased, while NOx emissions increased initially but then decreased. Aside from symmetric combustion, the furnace attained higher burnout rate and relatively lower NOx emissions at 450 and 600 MWe (i.e., carbon in fly ash and NOx emissions of 2.54−3.87% and 923−1292 mg/m3 at 6% O2, respectively), compared with conventional down-fired furnaces. The relatively poor burnout and low-oxygen operation conditions at the low 370-MWe load uncovered that the boiler flexibility was poorer at 370 MWe than at 450 and 600 MWe. At 450 MWe, the optimal setting among the three staged-air damper openings was found at 45%, if relatively low levels of carbon in fly ash and NOx emissions needed to be achieved. Centralizing further the coal/air flame in the furnace by adjusting operation models of burner groups in service was recommend to maintain high gas temperature levels and relatively stable combustion, so as to establish high-oxygen operation conditions to improve the poor furnace performance at low loads. In addition, reducing further the air stoichiometric ratio in the burner region by considerate combustion retrofits (such as adding an reasonable overfire air system) was put forward to significantly reduce the still high NOx emissions at moderate and full loads.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 571 86914542. Fax: +86 571 86835763. E-mail: [email protected]; [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.: 51306167).

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REFERENCES

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4. CONCLUSION This work presents an experimental investigation into combustion and NOx emission characteristics with respect to boiler load and staged-air damper opening within a down-fired 600 MWe supercritical boiler under multiple-injection and multiple-staging combustion circumstances. Industrial-size measurements (taken of gas temperatures and species concentrations in the furnace, NOx emissions, and carbon in fly ash) were performed at boiler loads of 370, 450, and 600 MWe and at various staged-air damper openings (i.e., 10, 30, and 45%) at 450 MWe. Symmetric combustion developed for 733


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Combustion Flexibility of a Large-Scale Down-Fired Furnace with

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