Combustion and NOx Emission

Article pubs.acs.org/est

Combustion and NOx Emission Characteristics with Respect to Staged-Air Damper Opening in a 600 MWe Down-Fired PulverizedCoal Furnace under Deep-Air-Staging Conditions Min Kuang,†,§ Zhengqi Li,‡ Zhihua Wang,*,† Xinjing Jing,‡ Chunlong Liu,‡ Qunyi Zhu,‡ and Zhongqian Ling§ †

State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, People’s Republic of China School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, People’s Republic of China § Institute of Thermal Engineering, China Jiliang University, Hangzhou 310018, People’s Republic of China ‡

S Supporting Information *

ABSTRACT: Deep-air-staging combustion conditions, widely used in tangential-fired and wall-arranged furnaces to significantly reduce NOx emissions, are premature up to now in down-fired furnaces that are designed especially for industry firing low-volatile coals such as anthracite and lean coal. To uncover combustion and NOx emission characteristics under deep-air-staging conditions within a newly operated 600 MWe down-fired furnace and simultaneously understand the staged-air effect on the furnace performance, full-load industrial-size measurements taken of gas temperatures and species concentrations in the furnace, CO and NOx emissions in flue gas, and carbon in fly ash were performed at various staged-air damper openings of 10%, 20%, 30%, and 50%. Increasing the staged-air damper opening, gas temperatures along the flame travel (before the flame penetrating the staged-air zone) increased initially but then decreased, while those in the staged-air zone and the upper part of the hopper continuously decreased and increased, respectively. On opening the staged-air damper to further deepen the air-staging conditions, O2 content initially decreased but then increased in both two near-wall regions affected by secondary air and staged air, respectively, whereas CO content in both two regions initially increased but then decreased. In contrast to the conventional understanding about the effects of deepair-staging conditions, here increasing the staged-air damper opening to deepen the air-staging conditions essentially decreased the exhaust gas temperature and carbon in fly ash and simultaneously increased both NOx emissions and boiler efficiency. In light of apparently low NOx emissions and high carbon in fly ash (i.e., 696−878 mg/m3 at 6% O2 and 9.81−13.05%, respectively) developing in the down-fired furnace under the present deep-air-staging conditions, further adjustments such as enlarging the staged-air declination angle to prolong pulverized-coal residence times in the furnace should be considered to improve the deepair-staging combustion configuration.

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furnaces at normal full-load operations)3,4,8,11−13 appear similarly in down-fired furnace operations. Accordingly, various solutions have been reported on dealing with these problems, such as burning blended fuels (i.e., adding bituminous coal or biomass into anthracite)5,14,15 or retrofitting the combustion configuration to improve burnout,16−18 changing burner operation models to alleviate the serious slagging on walls,10 and parametric tuning of air-staging conditions to reduce NOx emissions.13,19,20 However, the still high NOx emissions (above 1100 mg/m3 at 6% O2 after the mentioned parametric tuning) hinder the further application of down-fired furnaces in developed countries where pollutant emission standards are particularly strict.11,13,14 Although the number of down-fired furnaces has popularized well in China in the past 25

INTRODUCTION 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.1−3 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.4,5 Down-fired boilers, designed especially for industry firing anthracite and lean coal,3−6 attempt various carefully designed strategies to attain good firing of these fuels, such as 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 fuelburning zone, and supplying air in various staging patterns so as to inhibit NOx production. However, the actual combustion performance essentially deviates from the designed combustion concept and various problems such as poor burnout,7−9 heavy slagging,10 and particularly high NOx emissions (reaching levels of 1600 mg/m3 at 6% O2 for large quantities of down-fired © 2013 American Chemical Society

Received: Revised: Accepted: Published: 837

July 17, 2013 November 14, 2013 November 25, 2013 November 25, 2013 dx.doi.org/10.1021/es403165f | Environ. Sci. Technol. 2014, 48, 837−844

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 furnace.

years,1,11,18 reducing further the still high NOx emissions and simultaneously maintaining acceptable burnout levels are urgent matters for boiler manufacturers to further popularize down-fired furnaces, with confronting the pressure from increasingly stringent NOx emission standards for thermal power generators in the country. Similarly to circumstances within tangential-fired and wall-arranged furnaces, the concept by regulating deep-air-staging combustion is currently preferred as the low-cost and effective method21,22 to sharply reduce NOx emissions of down-fired furnaces in China.

Deep-air-staging combustion, regulated in furnaces by supplying the combustion air in several stages as combustion proceeds, is generally characterized as substoichiometric air conditions formed both at the coal ignited stage and in the primary combustion zone.23−25 Admittedly, regulating deep-airstaging combustion in tangential-fired and wall-arranged furnaces (firing bituminous coal and lignite) has been widely reported to attain ultralow NO x emissions and well burnout.26−28 On the contrary, investigations referring to down-fired furnaces have seldom reported on (i) deep-airstaging combustion application and (ii) detailed combustion 838

dx.doi.org/10.1021/es403165f | Environ. Sci. Technol. 2014, 48, 837−844


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For coals used in down-fired furnaces, particularly poor coal generally raises problems of late coal ignition and poor combustion stability, whereas high grade coals (such as bituminous coal) maybe produce the potential nozzle distortion and slagging in burners. Here, the operational conditions not deviating apparently from those designed (i.e., coal feeding rate, primary-air temperature, and secondary/staged-air temperature controlled at 240−300 t/h, 90−110 °C, and 350−370 °C, respectively), enabled the present furnace to establish well combustion performance at the normal full load. Industrial-Size Measurements. Full-load industrial-size measurements were performed in turn at staged-air damper openings of 10%, 20%, 30%, and 50%, respectively, accompanied by fixing secondary-air and OFA damper openings to maintain deep-air-staging conditions. In light of coal characteristics varying from one day to the next, these four settings were thus performed successively within 1 day so as to ensure the measuring results to be comparable using similar coal. Again, considerable efforts for allowing minimum variation in boiler operating conditions needed to be ensured. The Supporting Information provides coal characteristics and the mean operation parameters over the 5 h duration of each experimental run. Coal properties (consisting of volatile matter, moisture, ash, fixed carbon, and net heating value of 7.26%, 7.76%, 28.71%, 56.27%, and 21.25 MJ/kg, respectively, as received) indicate that the used coal actually belongs to lowvolatile and high-ash lean coal. Considering that the previously published work had uncovered that symmetric combustion developed in the furnace under various operation conditions,30,31 the attention of this paper was no longer focused on the symmetric combustion topic and thus only the front-half part of the furnace was selected to acquire real-furnace data about the coal combustion process. Detailed data-acquiring methods are as follows. Gas Temperatures along the Flame Travel. As graphed in Figure 1, ports 1−4 are respectively located just below the fuelrich coal/air flow nozzle outlet and at the transition zone where the flame turns around to be redirected upward. Apparently, a smooth curve through, in turn, the furnace regions corresponding to ports 1−4 can be acceptable to denote the downward flame travel. The Supporting Information provides a detailed explanation about the representative locations of the selected ports 1−4 in all observation ports for coal combustion data acquisitions. A 3i hand-held pyrometer (a type of noncontact infrared thermometer), with a measurement range from 600 to 3000 °C, accurate to within 1 °C and with an error of ±30 °C, was inserted, in turn, through ports 1−4 (Figure 1) to measure gas temperatures as the downward flame proceeds. Consequently, gas temperatures acquired through ports 1−4 by the hand-held pyrometer (as shown in Figure 2) are taken as the downward flame temperatures at different stages. The local combustion status, clearly affecting the temperature data reading in the pyrometer, was taken as the major source of uncertainty in these measurements. Consequently, at each port, the measurement was taken 10 times within 5 min and average values were adopted. Gas Temperatures and Species Concentrations in the near-Wall Region. A K-type 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 water-cooled stainless steel tube to form a thermocouple device. The thermocouple device and a 3 m water-cooled stainless steel probe (i.e., a centrally located 10

and NOx emission characteristics under deep-air-staging conditions, because of good ignition conditions, high gas temperature levels and long pulverized-coal residence times needed to be simultaneously maintained for good burnout of anthracite and lean coal. But unfortunately, establishing these circumstances always produces large quantities of NOx. This means that it is very difficult to effectively regulate deep-airstaging conditions in down-fired furnaces to significantly reduce NOx emissions if acceptable burnout rates need to be maintained. After several years of a collaborative effort among our group, boiler manufacturers, and power plant managers, a deep-air-staging down-fired combustion technology based on the concept of multiple injection and multiple staging (i.e., the MIMSC technology in the literature29) was developed and finally trialed in two newly operated down-fired 600 MWe supercritical furnaces.30 The deep-air-staging combustion configuration in the two down-fired furnaces consists of fuel rich/lean combustion and two layers of secondary air supplying in the burner zone, staged-air supplying in the middle period of coal combustion, and overfire air (OFA) supplying in the burnout zone. Under deep-air-staging conditions, changing the staged-air damper opening is usually as the preferred method for boiler managers to adjust the furnace combustion status. Therefore, to deepen the understanding about combustion and NOx emission characteristics under deep-air-staging conditions within down-fired furnaces and simultaneously uncover the staged-air effect on the furnace performance, this paper presents an industrial-size experimental investigation at various staged-air damper openings within one of the two furnaces.

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EXPERIMENTAL SECTION Utility Boiler. Figure 1 presents the vertical and transverse cross sections through the furnace, burner layout pattern on furnace arches, and combustion configuration with the deep-airstaging MIMSC technology. The furnace 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). A total of 24 louver concentrators symmetrically arranged on the arches divide the primary air/fuel mixture of 12 groups of burners into fuel-rich and fuel-lean coal/air flows needed to regulate fuel rich/lean combustion. The combustion configuration with the deep-airstaging MIMSC technology consists of four sections: (1) regulating fuel rich/lean combustion in the burner zone to enrich the pulverized-coal concentration, lower the coal/air flow velocity, and establish a relatively oxygen-lean atmosphere before 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; (3) feeding the high-speed staged air (with a declination angle of 20°) through the lower part of the front and rear walls to establish a second combustion stage along the flame travel; (4) supplying OFA (also with a declination angle of 20°) into the furnace throat to form a third combustion stage. Introductory material about MIMSC technical principles can be found elsewhere.29 After nearly 1 year of furnace operations with frequently changed coal properties, it was found that the deep-air-staging down-fired technology displayed good coal flexibility in the present furnace, when firing anthracite and lean coal with volatile matter ranging from 6−10%, ash content not exceeding 35% (as received), and the net heating value above 16 MJ/kg. 839



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RESULTS AND DISCUSSION

Acquired Gas Temperatures in Selected Locations. Figure 2 shows that as the flame proceeds, a similar temperature change pattern (i.e., two increase stages respectively appearing between ports 1 and 2 and between ports 3 and 4, accompanied by a decrease trend between ports 2 and 3) appears for all four settings, with the exception of (i) the slight changes between ports 2 and 3 at openings of 10% and 30% and between ports 3 and 4 at the 20% setting and (ii) the decrease between ports 3 and 4 at the 10% opening. Explanation of this pattern is as follows: (1) in port 1, where the ejected coal/air flow only leaves from the burner nozzle outlet about 3 m, the pulverized-coal is ignited and secondary air can seldom mix with the ignited coal/air flow, thereby developing the relatively fuel-rich, oxygen-deficient, and lowtemperature (960−1050 °C) atmosphere at this stage; (2) in port 2, the flame is sufficiently far from the burner nozzle outlet to allow secondary air mixing with the ignited coal/air flow, thereby advancing coal combustion in this region to apparently raise gas temperatures; (3) in port 3, where the shallowly inclined staged air (Figure 1) can easily mix with the downward flame, staged air actually plays dual roles (whose cooperation is affected greatly by the staged-air penetrating ability into the flame), i.e., cooling the flame and supplying air to aid the further coal combustion. Because of supplying only a little staged-air flux to hardly aid coal combustion, gas temperatures vary slightly or present a decrease pattern from port 2 to port 3 at 10% and 20%. The highest staged-air setting of 50%, with its cooling effect being greater than the combustion abidance, produces the temperature decrease from port 2 to port 3. For the moderate opening of 30%, the slight temperature change between ports 2 and 3 can be attributed to the behavior that the mentioned staged-air dual roles maybe cancel each other; (4) as the coal/air flame proceeds near port 4, where staged air mixes well with the downward flame to develop intense combustion, gas temperatures thus increase sharply at higher settings of 30% and 50% and decrease (or vary slightly) at the two lower settings. Figure 2 also shows that on opening staged air, gas temperatures in ports 1 and 2 initially increase but then decrease, whereas those in ports 3 and 4, respectively, decrease and increase. Explanation of these change trends is as follows. Increasing the staged-air damper opening decreases and increases the secondary-air and staged-air fluxes, respectively. In ports 1 and 2, moderately decreasing secondary air by opening staged air from 10% to 20% can weaken the lowtemperature secondary air mixing with the coal/air flow, thereby acting positively on gas temperatures before the flame penetrating the staged-air zone. When the circumstances turn to high staged-air settings such as 30% and 50%, the resulted sharp secondary-air reduction greatly deteriorates the oxygendeficient atmosphere below arches before the flame penetrating the staged-air zone, resulting in both gas temperatures in ports 1 and 2 decreasing. In port 3, the weakening in secondary air aiding coal combustion and the strengthening in the staged-air cooling effect develop a temperature decrease pattern with the staged-air damper opening. The temperature increase pattern in port 4 is attributed to the fact that in port 4 the continuously increased staged air can mix well with the downward coal/air flame, thereby clearly aiding the further coal combustion process to raise gas temperature levels. Generally, good burnout under deep-air-staging conditions needs a higher gas temperature for completing a majority of

Figure 2. Gas temperatures along the flame travel in the furnace.

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 highpressure cooling water flow rate of 60 L/min, respectively) were inserted, in turn, into the furnace through ports 1 and 3 (Figure 1) parallel to the front wall. After the samples were quickly quenched and the chemical reactions were cooled 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 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 Costa et al.26,27 and De,32 the deviation of measuring 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 probe could quickly quench chemical reactions and effectively avoid aerodynamic disturbances of the gas flow in the sampling pipe. Because of the low gas velocity and gas flow rate in the thin sampling pipe and high flow rate of the cooling water in the external tube, the cooling rate was particularly high and the quenching was rapidly achieved upon samples being drawn into the probe. Calculated from the inlet and outlet temperatures of the captured gas samples and the required residence times for cooling gas samples in the sampling pipe, the estimated quenching rates were approximately 106 K/sec. A more detailed explanation about methods used to estimate quenching rates is provided in the Supporting Information. Quantifying probe flow disturbances was not attempted. 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 thermocouple device (without the water-cooling effect) 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. These samples were also analyzed online by the Testo 350 M instrument. 840



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settings show two gas temperature patterns for ports 1 and 3. That is, gas temperatures in port 1 increase initially but then decrease, whereas those in port 3 increase initially but then vary slightly or decrease a little with distance. Explanation of these observations lies on two aspects: (1) the high-temperature gas accumulating in the near-wall region develops the initial increase stage in these profiles; (2) port 1 is not far from the burner nozzle outlets and that the zone near port 3 is affected directly by staged air (Figure 1). After the thermocouple reaches distances beyond 1.2 m, measuring points gradually approach the zones affected by secondary air in port 1 and staged air in port 3. Because of the flux of secondary air being much higher than that of staged air in the MIMSC technology, the apparently stronger secondary-air cooling effect develops the temperature decrease stage beyond 1.2 m in port 1, whereas in port 3, the staged-air dual roles (i.e., the relatively weaker cooling effect and supplying combustion air to advance further coal combustion) enable gas temperatures beyond 1.2 m to vary slightly or decrease a little. Additionally, Figure 3 also reveals that with opening staged air, gas temperatures in port 1 initially increase but then decrease, whereas those in port 3 decrease continuously. Considering that these change trends with respect to opening staged air are similar to those depicted in Figure 2, the related explanation is not repeated here because it can be found in the discussion about Figure 2. Local Gas Species Concentrations in the Furnace. Figure 4 presents gas species concentrations in the near-wall region. For all four settings, O2 content in port 1 generally presents a decrease pattern with distance, whereas that in port 3 generally decreases initially but then increases. This O2 content change trend with respect to distance is attributed to three aspects: (1) relatively lower secondary-air and staged-air fluxes are fed into the furnace under deep-air-staging conditions, in comparison with conventional operation models; (2) as the probe gradually approaches the zone where coal combustion proceeds, the continuous coal combustion process consumes

combustion share in the stage before the unburnt coal particles entering into the fuel-burnout zone. Apparently, before the flame enters into the OFA zone, higher gas temperature levels in port 4 at high staged-air settings of 30% and 50% should be more favorable in this work, compared with lower settings of 10% and 20%. Figure 3 presents gas temperatures in the near-wing wall region (temperatures acquired by the thermocouple device).

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

Profiles in ports 1 and 3 indicate gas temperatures at an early stage of coal ignition and in the staged-air zone, respectively. The difference in the data-acquiring location, secondary air already mixing with the ignited coal/air mixture to advance coal combustion when the flame reaching port 3, and staged-air cooling effect depended greatly on the staged-air damper opening explain the phenomenon that the whole temperature levels are higher in port 3 than in port 1 for all four settings, with temperature gaps between the two ports greatly differing among the four settings. These comparisons between ports 1 and 3 are generally in accordance with those displayed in Figure 2. As the thermocouple is inserted into the furnace, all four

Figure 4. Gas species concentration distribution patterns in the near wing-wall region. 841



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Figure 5. Levels of exhaust gas temperature, CO emission, carbon in fly ash, NOx emissions, and boiler efficiency with respect to the staged-air damper opening.

staged-air mixing and the dual effects of the increasing stagedair flux on O2 content (i.e., supplying more O2 and raising the mixing behavior of staged air and the coal/air flame to consume more O2) enables the resulted O2 content pattern in this zone to decrease initially but then increase; (2) in light of low O2 content in coal combustion processes allowing high CO production and the opposite circumstances corresponding to high O2 content, CO content thus presents an opposite pattern to that of O2 content, i.e., generally increasing initially but then decreasing with opening staged air; (3) in port 1, the cooperation of gas temperature change trends (i.e., initially increasing but then decreasing with opening staged air) and the continuously strengthened oxygen-deficient atmosphere with opening staged air develops the mentioned NOx content change pattern, despite relatively higher O2 content being left in port 1 at the highest staged-air setting (because of less O2 consumption for the relatively weaker coal combustion, Figures 2 and 3). In port 3, because gas temperatures decrease continuously and O2 content decreases initially but then increases with opening staged air (Figures 3 and 4), NOx content thus decreases initially but then increases in the zone where the probe approaches the coal/air flame (i.e., at distances beyond 1.0 m). Key Operational Results Depended on the Staged-Air Damper Opening. Aside from the above findings in gas temperatures and species concentrations, this work also provides key operation results such as exhaust gas temperature, CO and NOx emissions in flue gas, carbon in fly ash, and boiler efficiency. As illustrated in Figure 5, when staged air is opened to further deepen air-staging conditions, both exhaust gas temperature and carbon in fly ash decrease continuously and CO emission increases initially but then decreases, while both NOx emissions and boiler efficiency increase continuously. Apparently, all these change trends essentially conflict greatly with the well-accepted knowledge (i.e., deepening air-staging conditions for sharp NOx reduction always raises the levels of exhaust gas temperatures and unburnt combustible loss, which decreases boiler efficiency)1,23,25−28 found widely in conventional furnaces with deep-air-staging conditions. The gas temperature change trends along the downward flame travel (Figure 2) explain these unique findings in the present downfired furnace. A detailed explanation is as follows: after the coal/air flame enters into the upper part of the hopper region through the staged-air zone (Figure 1), the apparent gas temperature increase patterns for all four settings except for both 10% and 20% with low staged-air supplying (see the temperature comparisons between ports 3 and 4 in Figure 2)

the originally low-concentration O2 and thus develops the O2 content decrease stage with distance in ports 1 and 3; (3) the cooperation of the staged-air supplying and the existing O2 in the downward flame (supplied mainly by secondary air) produces the O2 content increase pattern at distances beyond 1.4 m in port 3. The CO content pattern generally follows the characteristics of low O2 content corresponding to high CO content. Again, CO content initially maintains low levels and varies slightly with distance but then generally increases in the zone beyond 1.2 m, with the exception of some fluctuations and the continuously slight changes at settings of 10% and 50% in port 3 (because of their relatively higher O2 content in port 3). For the aspect referring to NOx content, different change trends with respect to distance appear between ports 1 and 3. The gas diffusion from the downward coal/air flame to the near-wall region and relatively weaker mixing of secondary air and coal/air flow in the zone not far from the burner outlets enable NOx content in port 1 to present a generally increasing pattern with distance except for some fluctuations. In port 3, the similar gas diffusion behavior and the staged-air dual roles (i.e., mixing with the gas to lower NOx content and in contrast, aiding combustion to produce more NOx) make the resulted NOx content initially increase but then decrease or vary slightly, with the exception of the further increase stage after 1.2 m at the 50% setting (because of a large staged-air flux fed into the coal/air flame entraining large quantities of unburnt coal particles under deep-air-staging conditions). Figure 4 also shows that O2 content in ports 1 and 3 generally decreases initially but then increases with opening staged air. Accordingly, CO content generally increases initially but then decreases at distances beyond 1.2 m. As the staged-air damper opening increases, NOx content in port 1 increases initially but then decreases, whereas that in port 3 decreases initially but then increases at distances beyond 1.0 m, with the exception the initial fluctuations at the 50% setting. Explanation of these observations is as follows: (1) increasing the staged-air damper opening essentially allows for supplying more staged air and simultaneously cuts the secondary-air flux. Under deep-airstaging conditions, a continuous secondary air reduction can deteriorate the oxygen-deficient atmosphere in the zone below the arches. Consequently, relatively weaker coal combustion (see gas temperatures in port 1 shown in Figures 2 and 3, respectively) develops below the arches at high staged-air settings, resulting in less O2 being consumed and thus leaving relatively higher O2 content to appear in port 1. In port 3, the cooperation of the downward oxygen-deficient atmosphere deteriorating continuously with opening staged air before the 842



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those for conventional tangential-fired and wall-arranged furnaces under deep-air-staging conditions. These new findings enrich the limited understanding about combustion and NOx emission characteristics within down-fired furnaces under deepair-staging conditions and also provide reference materials for modifications of down-fired furnaces already in service and for new designs with deep-air-staging conditions.

uncover that under the deep-air-staging conditions, an intense coal combustion process is actually postponed and appears only after staged air mixes well with the coal/air flame, if a high staged-air flux is supplied. Consequently, a great portion of combustion share is completed in the stage before the unburnt coal particles enter into the fuel-burnout zone (i.e., mainly the OFA zone in the furnace throat) and the combustion share in the fuel-burnout zone reduces greatly with opening further staged air. Accordingly, both exhaust gas temperature and carbon in fly ash decrease, yielding an increase in boiler efficiency. With an increase in the staged-air damper opening, because of more staged air entering into the intense combustion zone to aid coal combustion and greatly favor NOx production, levels of NOx emissions thus increase continuously. The CO emission pattern may be attributed to the aforementioned CO content change trends in two representative zones (i.e., port 1 below arches and port 3 in the staged-air zone, Figure 4) with opening staged air. Figure 5 also shows that with the deep-air-staging MIMSC technology, the down-fired furnace can attain much lower NOx emissions of 696−878 mg/m3 at 6% O2 within a wide staged-air damper opening range of 10−50%, in comparison with conventional down-fired furnaces with NOx emissions being up to levels of 1600 mg/m3 at 6% O2.4,8,11−13,15−17 As mentioned in the Experimental Section, the great NO x reduction is attributed to the cooperation of the fuel rich/ lean combustion formation in the burner zone, unique burner configuration to avoid the premature mixing between secondary air and the fuel-rich coal/air flow before the ignition occurrence, and the three-layer air-staging combustion configuration in the furnace. But unfortunately, carbon in fly ash attains high levels of 9.81−13.05%. Even at the highest staged-air damper opening setting to achieve the lowest burnout loss, carbon in fly ash still approaches 10%. In light of (i) a F-layer secondary air declination apparently improving burnout in two 300 MWe down-fired furnaces by extending the coal/air flame penetration17,18 and (ii) a previous version of the MIMSC technology application (with a much deeper staged-air declination angle of 45° and without OFA)30 in the down-fired furnace attaining good burnout (carbon in fly ash of 2.54− 3.72%), the cause for the present high carbon in fly ash in the furnace should be the apparently shallower staged-air declination angle of 20° (Figure 1). The shallow staged-air angle can apparently shorten the coal/air flame penetration depth and residence times of coal particles in the furnace because of the high staged-air transverse impulse on the downward flame, despite deep-air-staging conditions also causing negative effects on burnout. In light of the successful experience in achieving good burnout in the literature,17,18,30 retrofitting the present staged-air angle from 20° to a larger one (such as an angle in 30−45°) should be a low-cost measure for the furnace to improve burnout under deep-air-staging conditions. And the final staged-air angle can be determined by comprehensive CFD simulations or real-furnace trials about coal combustion and NOx formation with respect to the angle in the furnace. In summary, the deep-air-staging MIMSC technology application develops much lower NOx emissions but with high carbon in fly ash in the down-fired furnace. Deepening further air-staging conditions by opening staged air results in unique changes consisting of decreasing both exhaust gas temperature and carbon in fly ash to raise boiler efficiency and increasing a little NOx emissions, which differ greatly from

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ASSOCIATED CONTENT

S Supporting Information *

Table S1 provides coal properties and major operation parameters in industrial-size experiments, a note explaining the representative locations of ports 1−4 in all observation ports for coal combustion data acquisitions, and explanation material about methods used to estimate quenching rates in the water-cooling probe. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Z. Wang. Tel.: +86 571 87953162. Fax: +86 571 87951616. Email address: [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) and the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant No. 51121004).

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REFERENCES

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Combustion and NOx Emission

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