Characterization of Combustion and NOx Emissions with Respect to

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Characterization of Combustion and NOx Emissions with Respect to Overfire Air Damper Opening in a Down-Fired Pulverized-Coal Furnace Min Kuang,*,† Zhengqi Li,† Xinjing Jing,† Xianyang Zeng,‡ Loufeng Zhao,§ and Zhongqian Ling‡ †

School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, P. R. China Institute of Thermal Engineering, China Jiliang University, 258 Xueyuan Street, Xiasha Higher Education District, Hangzhou 310018, P. R. China § Zhejiang Feida Corporation, 37 Qingchun Road, Zhuji 311800, P. R. China ‡

ABSTRACT: The application of deep-air-staging combustion technology to significantly reduce NOx emissions and simultaneously control combustible loss to acceptable levels is already well established in tangential-fired and wall-arranged furnaces. However, this technology has not yet been widely applied to down-fired furnaces, and thus there is a requirement to investigate the potential benefits. This work presents an experimental investigation of the combustion and NOx emission characteristics in a newly operated down-fired 600 MWe furnace equipped with a deep-air-staging combustion configuration. Fullload industrial-size measurements were performed by acquiring various data such as gas temperatures and species concentrations in the furnace, CO and NOx emissions in flue gas, and carbon in fly ash. Overfire air (OFA) damper opening settings of 15%, 30%, 50%, and 70% were tested in turn so as to evaluate the OFA effect, following which the staged-air damper was opened from 30% to 50% to assess its ability to improve the poor burnout associated with low NOx operating conditions. It was found that, as OFA was opened, gas temperatures in the burner zone and along the flame travel path (prior to penetration of the flame into the hopper region) exhibited an initial increase and subsequently decreased. Similar to trends that have been seen in most tangentialfired and wall-arranged furnaces, opening OFA continuously increased the exhaust gas temperature, CO levels in flue gas, and carbon in fly ash, while simultaneously decreasing NOx emissions and boiler efficiency. When opening OFA to generate deep-airstaging conditions, only the 50% setting balanced NOx emissions and combustible loss, although it was still generating high levels of carbon in fly ash. With the OFA damper opening fixed at 50%, increasing the staged-air damper opening from 30% to 50% reduced combustible loss and had a slight influence on NOx emissions. On the basis of the observed high levels of carbon in fly ash, accompanied by reasonably low levels of NOx emissions [9.81% and 878 mg/Nm3 at 6% O2 (dry)], a combustion retrofit based on enlarging the shallow staged-air angle is recommended to improve furnace burnout under deep-air-staging conditions. furnaces at normal full-load operations]4,6,11,16−18 have been widely reported in these furnace operations. Accordingly, lots of research has been reported on causes for these problems and various solutions in dealing with them, such as burning blended fuels (i.e., adding bituminous coal or biomass into anthracite)7,19,20 or retrofitting the combustion configuration (consisting of exchanging the fuel-rich and fuel-lean coal/air flow nozzle locations and inclining downward the F-layer secondary air)21−23 to improve burnout, cutting off the burners close to the side walls to alleviate the serious slagging on the side walls,14 and parametric tuning of operating conditions to reduce NOx emissions (the NOx reduction being no more than 20% by using this method).10,11,18,24,25 However, the still high NOx emissions [above 1100 mg/Nm3 at 6% O2 (dry) even after reducing the initial stoichiometry in the burner zone] hinder the increased adoption of such furnaces in North America and Western Europe, where pollutant emission standards for thermal power generators are particularly strict, even though down-fired furnaces were widely installed in these

1. INTRODUCTION Anthracite and lean coal, characterized by low volatile matter and poor reactive activity, present difficulties in achieving ignition, maintaining stable combustion, and completing burnout when industrially fired in furnaces.1−3 Consequently, effective ignition conditions, high gas temperature levels, and long resistance times for coal particles in the high-temperature furnace zone must be established if good burnout needs to be achieved in industrial firing of these fuels.4−6 Down-fired boilers, designed especially for industry firing anthracite and lean coal, apply various carefully designed strategies. These are: (1) creating a W-shaped flame to prolong residence times of coal particles in the furnace zone; (2) positioning large refractory coverage on furnace walls to attain high gas temperature levels; (3) directing the up-flowing hot gas to mix with primary air in the burner outlet zone, thereby assisting coal ignition and improving flame stability; and (4) organizing air-staging combustion to inhibit NOx production.6−9 However, unfortunately, the actual combustion performance essentially deviates from the designed combustion concept and some severe problems such as poor burnout,10−13 heavy slagging,14,15 and particularly high NOx emissions [reaching levels of 1600 mg/Nm3 at 6% O2 (dry) for large quantities of down-fired © 2013 American Chemical Society

Received: June 27, 2013 Revised: August 23, 2013 Published: August 26, 2013 5518

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Figure 1. Schematics of furnace configuration, burner layout pattern, combustion partition, and measurement port layout used in industrial-size experiments for the 600 MWe down-fired boiler.

regions in the 1960s to 1980s.4,16,18,19 Down-fired furnaces have, however, become very popular in China over the past 25 years; the country contains approximately 80% of the world’s down-fired furnaces. This is largely because China has both the largest reserves and highest consumption rates of anthracite and lean coal in the world, generates about 30% of its electricity by burning these fuels, and has relatively relaxed pollutant emission standards for thermal power generators.2,5,22 Even so, significantly reducing the particularly high NOx emissions associated with these furnaces while simultaneously maintaining acceptable levels of carbon in fly ash are urgent priorities for

boiler managers and manufacturers, since these improvements will allow down-fired furnaces to operate both economically and with minimal pollutant emissions and thus further popularize the use of such furnaces in China. Currently, comprehensive combustion retrofits to in-service down-fired furnaces as well as the use of carefully designed combustion configurations in new designs to establish deep-air-staging conditions are seen as the preferred approaches that will be applied over the next several years to improve down-fired furnaces in China.26,27 5519

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Mitsui Babcock Energy Limited (MBEL) down-fired furnaces. As noted above, investigations into deep-air-staging combustion within down-fired furnaces indicate that further improvements are required, owing to reports of relatively poor burnout and NOx emissions which remain high. In addition, reports to date27,35,36 have focused exclusively on FW and B&W downfired furnaces, with no work related to the Stein and MBEL types. To improve our understanding of down-fired furnace performance under deep-air-staging conditions and provide information useful for perfecting deep-air-staging combustion in down-fired furnaces, this paper presents an experimental evaluation of the combustion characteristics and NOx emissions in a newly operated down-fired 600-MWe supercritical furnace. This furnace is equipped with a deep-air-staging combustion configuration based on the concept of multiple injection and multiple staging (i.e., the MIMSC technology in the literature37). The furnace configuration was initially designed and manufactured following MBEL standards, although the combustion system incorporates the proposed MIMSC technology so as to improve coal combustion and reduce NOx emissions. Therefore, to some extent, this furnace can be considered as a retrofitted version of an MBEL type. Since the application of OFA typically has a significant effect on coal combustion and NOx production under deep-air-staging conditions,32−36 this paper focuses on the effect of the OFA damper opening on the furnace performance. Accordingly, fullload industrial-size measurements were performed, and our focus was primarily on comparing data from several OFA damper opening settings.

The deep-air-staging combustion technology used in largescale pulverized-coal furnaces is generally characterized by substoichiometric air conditions in both at the coal ignition stage and the primary combustion zone.25,28−31 To achieve these effects, the combustion air is supplied into the furnace in several stages as coal combustion proceeds. Carefully applying deep-air-staging combustion to tangential-fired and wallarranged furnaces burning coals with relatively high volatile matter (such as bituminous coal and lignite) can produce good furnace performance, and ultralow NOx emissions and relatively high burnout have been reported to result in many industrial applications. As an example, Costa et al. retrofitted a 300-MWe front-wall-fired boiler with low-NOx burners and also applied OFA, with the result that NOx emissions were reduced from 997 to 620 mg/Nm3 at 6% O2 (dry).32 Furthermore, applying boosted overfire air (OFA) ports to replace the conventional OFA ports further reduced NOx emissions to 469 mg/Nm3 at 6% O2 (dry). In addition, carbon in fly ash of 5.8% observed after these two retrofits, when compared with the value of 3.2% before the retrofits, demonstrated that coal burnout was still acceptable, despite deep-air-staging conditions forming in the furnace.32,33 By using two kinds of OFA ports (close-coupled OFA introduced immediately above the top coal nozzle using the main windbox and separated OFA introduced through a windbox separated from the main windbox supplying the bulk of the combustion air) to form deep-air-staging conditions within a 300-MWe tangential-fired boiler, Li et al. found that NOx emissions could be reduced by 44% with an associated boiler efficiency decrease of only 0.21%.34 Presently, when China’s newly built 600 MWe pulverized-coal boilers are used with the application of deep-air staging combustion technology, NOx emissions are no more than 250 mg/Nm3 at 6% O2 (dry) when burning coal with volatile matter of 30% or greater.2 When compared with tangential-fired and wall-arranged furnaces, however, the application of deep-air staging combustion to down-fired furnaces burning anthracite and lean coal is much less developed, and very few reports on this subject have been published, other than those by Li et al.,27 Garcia-Mallol et al.,35 and Leisse et al.36 Li et al.27 retrofitted a 300-MWe down-fired furnace with so-called combined high efficiency and low-NOx technology and found that NOx emissions could be lowered by as much as 50% [from 2101 to 1057 mg/Nm3 at 6% O2 (dry)]. Although this retrofit actually resulted in a slight decrease in carbon in fly ash (from 7.84% to 7.54%), levels of carbon in fly ash were still high. By using a newly developed approach consisting of a “fuel preheat nozzle” and “vent-to-OFA”, Garcia-Mallol et al.35 achieved improvements in coal ignition and combustion stability, along with sharp reductions in NOx emissions to low levels below 510 mg/Nm3 at 6% O2 (dry) (equivalent to reducing NOx by over 50%) within several down-fired furnaces with capacities of 50− 350 MWe. Simultaneously, because of the significant increase observed in burnout loss, static-adjustable pulverized-coal classifiers were combined with ball mills to lower the particle size of the coal introduced into the furnace, thereby improving the burnout rate. By retrofitting swirl burners and applying OFA within a 350-MWe down-fired furnace, Leisse et al.36 achieved significant NOx reductions [from 1700 to 1060 mg/ Nm3 at 6% O2 (dry)], accompanied by an increase in carbon in fly ash (from 3.5% to 5.7%). According to the published literature,37 down-fired furnaces can be divided into four types according to their manufacturers: Foster Wheeler (FW), Babcock & Wilcox (B&W), Stein, and

2. EXPERIMENTAL SECTION 2.1. Utility Boiler. Figure 1 presents the transverse and vertical cross sections through the furnace, burner layout pattern on furnace arches, air distribution model along the furnace height, and combustion configuration with the deep-air-staging MIMSC technology. From panels a and b of Figure 1, it can be seen that the arches divide the furnace into two: the octagonal lower furnace (i.e., the fuelburning 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 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 corresponding to a pair of concentrators. Panel c of Figure 1 graphs the positioned pattern of various nozzles and ports corresponding to each burner. As illustrated in panel d of Figure 1, the combustion configuration with the deep-airstaging MIMSC technology consists of four sections: (1) regulating the 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 in the zone below arches; (3) feeding the high-speed staged air (with a declination angle of 20°) into the lower furnace through the lower part of the front and rear walls to establish a second combustion stage along the flame travel; (4) positioning OFA ports (also with a declination angle of 20°) on the front and near arches but close to the furnace center, thereby supplying OFA into the furnace throat zone to develop a third combustion stage. Introductory material about technical principles of the MIMSC technology can be found elsewhere.37 2.2. Industrial-Size Measurements. To determine the effect of OFA on coal combustion and NOx emissions, normal full-load industrial-size measurements were performed in turn at OFA damper 5520

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openings of 15%, 30%, 50%, and 70%, respectively, using fixed secondary-air and staged-air damper openings. During this series of experiments, it was found that high OFA settings (such as 50% and 70%) resulted in low levels of NOx emissions [about 500−800 mg/ Nm3 at 6% O2 (dry)] while, unfortunately, generating high levels of carbon in fly ash (11−15%) and CO emission of 300−600 ppm (dry volume), as summarized in Table 1. To control both levels of NOx

a much lower pressure drop in secondary-air boxes (from 1120 to 1046 Pa, Table 1) because of the high secondary-air and OFA damper openings (60% and 50%, respectively) and the direct connection of staged-air ducts to secondary-air boxes rather than OFA boxes, as shown in Figure 1. The mentioned cold-flow experiments demonstrated that a significantly greater reduction in mass flow rate occurs in the case of secondary air (about 30 kg/s) as compared with OFA (reducing slightly from 101.6 to 96.8 kg/s), corresponding to an increase in staged air. Compared with the observed changes in secondary air and staged air, the negative effect of the slight reduction in OFA on the experimental results is marginal. To ensure that the measured results were comparable, these five carefully designed settings were tested successively over a span of 2 days. During each experimental run, considerable effort was put into minimizing both differences in coal characteristics and variations in boiler operating conditions. Table 1 lists coal characteristics and the mean operational parameters over the 5-h duration of each experimental run. The details of data acquisition methods applied in each experimental run are as follows: (1) Gas temperatures acquired along the flame travel in the lower furnace. 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 observation ports 1−4 (located on the wing wall connected between the front wall and rightside wall, Figure 1) to measure the highest gas temperatures at different stages along the downward flame travel. 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 observation port the temperature measurement was taken for 10 times within 5 min, and the average value was adopted. (2) Gas temperature distributions acquired in the burner zone and in the near wing-wall region. Similarly to that performed by Costa et al.,32,33 a fine-wire thermocouple technique was also applied in these measurements in this work. Here, a K-type thermocouple with a 0.3-mmdiameter and 8-m-length nickel−chromium/nickel−silicon fine wire, located in a 4-mm-diameter twin-bore stainless steel sheath, was placed inside a 6 m water-cooled stainless steel probe to form a thermocouple device. By using the same method in the literature,38 the thermocouple device was inserted into the furnace through a selected burner nozzle to acquire gas-temperature change trends with the thermocouple proceeding in the burner region. Again, the same thermocouple device was inserted into the furnace through ports 1 and 3 (Figure 1) to perform gas temperature measurements in the near wing-wall region. Sources of uncertainty in the fine-wire thermocouple temperature 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.32,33 and De,39 the deviation of the 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%. (3) Gas species concentration distribution in the nearwall region. A 3-m-long 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) was inserted into the furnace through ports 1 and 3 to sample flue gas in the near wing-wall region. 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 (dry volume) for NOx. The major sources of uncertainty in gas species concentration measurements 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. Introductory material about operation principles of the water-cooled steel probe, configuration of the gas sampling system,

Table 1. Coal Characteristics and Operational Parameters in Full-Load Industrial-Size Experiments

a

Coal for these four settings with adjusting the OFA damper opening (the first day of the whole experimental period). bCoal for the left setting with adjusting the staged-air damper opening (the second day of the whole experimental period). emissions and combustible loss when applying an OFA damper opening of 50%, simultaneously changing the staged-air damper opening from the previously fixed 30% to a higher level of 50% was tested to assess the feasibility of using this adjustment to improve the poor burnout under low NOx operating conditions. For convenience, the four OFA damper opening settings are hereafter referred as 15%, 30%, 50%, and 70%, whereas the condition characterized by adjusting the staged-air damper opening is termed SA50%. Before the furnace was entered into service, the air flow rates and velocities of various jets (the primary air, secondary air, staged air, and OFA) into the furnace at various common damper opening settings were determined by performing real-furnace cold-flow experiments similar to those described in the literature.38 These cold-flow experimental results suggested that the OFA mass flow rates were approximately 36.3, 59.2, 101.6, and 130.4 kg/s when employing OFA openings of 15%, 30%, 50%, and 70%. Accordingly, the OFA ratios (the ratio of OFA mass flow rate to the total air mass flow rate into the furnace) were about 5.3%, 8.6%, 14.9%, and 19.0%, respectively. When controlling the total combustion air into the furnace with fixed secondary-air and staged-air damper openings, opening the OFA damper reduced the secondary-air box pressure (Table 1) as well as the mass flow rates of secondary air and staged air. In contrast to the results obtained when opening OFA, moving from the OFA 50% setting to the SA50% condition resulted in 5521

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parameters for the high-pressure water, and methods for calculating the quenching rates in the probe can be found elsewhere.38

temperatures at different settings. These data show that, as the flame proceeds, a similar increase pattern is exhibited at all five settings. This pattern consists of relatively slow increase rates from port 1 to port 3 followed by more rapid increase from port 3 to port 4. The exception is the decrease in port 3 at the SA50% setting. This pattern of increase may be explained as follows. First, the furnace region corresponding to port 1 is just below the furnace arch and not far from the burner outlets. In this region, the coal/air flow ejected through the fuel-rich coal/ air flow nozzle is ignited and little two-stage secondary air can mix with the ignited coal/air flow, thereby generating a relatively fuel-rich, oxygen-deficient, and low-temperature (940−1030 °C) chemical atmosphere at this stage. Second, the furnace region corresponding to port 2 is sufficiently far from the burner nozzle outlet to allow secondary air to gradually mix with the ignited coal/air flow, thereby advancing coal combustion in this region and raising gas temperatures. Third, in the furnace region corresponding to port 3, staged air readily mixes with the downward flame because of the shallower staged-air declination angle of 20° (Figure 1). The low-temperature staged air actually plays dual roles: initially cooling the coal/air flame and then supplying air to aid in further coal combustion, resulting in gradually increasing gas temperatures in this region as compared with those at port 2, with the exception of the temperature decrease observed at the SA50% setting due to the much greater supply of staged air. Last, in the region near port 4 where staged air mixes efficiently with the downward coal/air flame to produce intense combustion, gas temperatures increase sharply. Figure 2 also shows that, when opening OFA, gas temperatures measured through ports 1−3 initially increase but then decrease, whereas the port 4 readings steadily decrease. The effect of OFA damper opening on gas

3. RESULTS AND DISCUSSION As shown in Figure 1, ports 1−3 are generally located along the 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. Using a smooth curve running through ports 1−4 to denote the flame travel path through the furnace prior to the upward redirection of the downward coal/air flow appears to be an acceptable assumption. Consequently, the gas temperature data acquired at ports 1−4 by use of the hand-held pyrometer are used to denote the downward flame temperatures at different stages along the flame travel path. Figure 2 presents these gas

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

Figure 3. Gas temperature distributions in the burner zone and in the near wing-wall region through ports 1 and 3. 5522

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early stages of these temperature increase curves, opening OFA results in an initial increase followed by a subsequent decrease in gas temperatures in the burner zone, such that the 50% setting generates the highest temperatures among the four OFA settings. Moreover, increasing the staged-air damper opening from 30% to 50% generally decreases temperatures in this zone. Temperature changes in this zone associated with various OFA and staged-air damper openings are similar to the trends observed along the flame travel path (i.e., the temperatures acquired through ports 1 and 2, shown in Figure 2) and occur for the same reasons noted previously when discussing Figure 2. Figure 3a also shows that when following the same approach used prior in the literature38 (taking 850 °C as the ignition point for anthracite and lean coal burning in a down-fired furnace), the ignition distances are 1.88, 1.80, 1.65, 2.01, and 1.73 m for the five settings of 15%, 30%, 50%, 70%, and SA50%, respectively. In comparison with conventional down-fired furnaces, which suffer from late ignition with ignition distances typically greater than 2.5 m,11,13,17,26 the MIMSC technology is evidently able to advance coal ignition in the present furnace, regardless of the OFA and staged-air damper openings applied. The advanced ignition observed in this study is the result of employing a variety of carefully designed strategies in the deepair-staging MIMSC technology. These include regulating the fuel rich/lean combustion to develop a high-particle-concentration and low-velocity zone below the burner nozzle outlets, supplying the fuel-rich coal/air flow through nozzles centered over the furnace to allow the hot recirculating gas to directly heat the coal/air mixture, adopting a centralized layout pattern for the fuel-rich coal/air flow nozzles to maintain high gas temperature levels in the burner zone, and significantly delaying the mixing of secondary air and the fuel-rich coal/air flow prior to ignition. The ignition distance is initially reduced but then lengthened with opening OFA and again lengthened when the circumstances turned to SA50%. This general trend matches the observed variations in gas temperature changes at ports 1 and 2 presented in Figure 2 and in the burner zone graphed in Figure 3a and is primarily attributed to the above-mentioned dual effects of secondary-air reduction resulting from opening OFA or staged air. In Figure 3b, the profiles corresponding to ports 1 and 3 indicate gas temperatures at an early stage of coal ignition and in the staged-air zone, respectively. In the near wing-wall region, gas temperature levels are a little higher near port 3 than in the vicinity of port 1 for all five settings except for SA50%. This may be explained by differences in the data-acquisition locations and the combustion air (i.e., secondary air and staged air) already having mixed with the ignited coal/air mixture to advance the coal combustion as the flame proceeds into the region near port 3. This phenomenon is also in accordance with the observed variations in gas temperatures near ports 1 and 3 in Figure 2. The exception at the SA50% setting is attributed to the much stronger cooling effect of the low-temperature staged air in the zone near port 3. As the distance from the wing wall increases, gas temperatures acquired through port 1 for all five settings and those acquired through port 3 at SA50% initially increase but then decrease, whereas those measured through port 3 for the four OFA settings exhibit a rapid initial increase and then show only slight variation. Explanation of these observations lies on two aspects: (i) High-temperature gas accumulating in the near-wall region produces the initial increases in these profiles. (ii) The zone corresponding to port 1 is not far from the burner nozzle outlets, and the region near

temperatures is evidently greater at port 1 than at the other three ports. With the OFA damper opening fixed at 50%, increasing the staged-air damper opening decreases the temperatures measured at ports 1−3 but increases the port 4 temperature (see the comparison of settings of 50% and SA50%). These observations may be explained as the result of two factors. The first factor involves opening OFA, which decreases secondary air and has dual effects: reducing the mixing of the low-temperature secondary air with the coal/air flow to increases temperatures and reducing the coal/air flow penetration into the lower furnace to decrease temperatures. This second effect occurs because, as the result of the MIMSC technology employed in this furnace, the coal/air flow is carried by the high-speed secondary air to penetrate into the lower furnace. Moderately increasing the OFA opening reduces the low-temperature secondary air mixing with the coal/air flow, thereby increasing gas temperatures before the intense combustion stage in the vicinity of port 4. When the OFA opening is increased significantly (especially going from 50% to 70%), a large decrease in the secondary-air velocity greatly reduces the coal/air flow penetration depth, thereby decreasing gas temperatures along the flame travel path. There is no doubt that increasing the OFA opening deteriorates the oxygendeficient atmosphere in the primary combustion zone. Consequently, combustion intensity is weakened in the primary combustion zone and temperatures near port 4 decrease. The second factor explaining our experimental observation is related to fixing the OFA damper at a high opening value of 50% while increasing the staged-air damper opening from 30% to 50%. These circumstances shorten the downward coal/air flow penetration depth because of the strengthened force of the staged air while simultaneously deteriorating the oxygendeficient atmosphere in the lower furnace and enhancing the mixing of staged air in the region near port 3. As a result, gas temperatures in the region corresponding to ports 1−3 decrease, especially in the vicinity of port 3. In contrast, the gas temperature increase in the region near port 4 is attributed to the enhanced staged air, which aids the combustion of unburned coal particles. Generally, higher gas temperature levels in the primary combustion zone (the region corresponding to port 4 in this work) facilitate relatively high burnout in down-fired furnaces. Gas temperature distributions measured by the thermocouple device in the burner zone and in the near-wing wall regions near ports 1 and 3 are summarized in panels a and b of Figure 3, respectively. From Figure 3a, it can be seen that, as the thermocouple proceeds, gas temperatures in the burner zone at all five settings present a similar pattern of increase. In the zone close to the burner nozzle outlet (within 0 to 0.9 m), gas temperatures rise slowly to within 360−550 °C, and there are no apparent differences between settings. These results are obtained because the thermocouple is inserted into the furnace through an inner secondary air port adjacent to the selected fuel-rich coal/air flow nozzle and gas temperatures in this zone are affected primarily by the low-temperature secondary air. Subsequently, the rate of temperature rise is seen to increase, and differences between the various settings become more apparent with increasing distance. These observations may be explained as the result of a combination of (i) the effect of radiant heat from the surrounding walls and (ii) mixing of the fuel-rich coal/air flow with the hot recirculating gas below the furnace arches and with secondary air (greatly affected by OFA and staged-air damper openings). With the exception of the 5523

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Figure 4. Gas species concentrations in the near wing-wall region through ports 1 and 3.

condition. This is due to less secondary air and staged air being fed into the furnace at high OFA setting above 50% as well as gradual reductions in O2 levels associated with continued coal combustion. When the SA50% condition is applied, the trend of increasing O2 levels beyond 1.6 m seen at port 3 is attributed to the much greater supply of staged air associated with the high staged-air damper opening of 50%, in comparison with the 30% opening applied in conjunction with the other four OFA settings. The variations in CO levels generally follow a trend in which low O2 concentrations are associated with elevated CO. The data obtained at both ports 1 and 3 show that CO levels at high OFA settings of 50% and 70% are significantly elevated compared with the levels at the other three settings and generally increase with distance. This occurs partly because coal particles are being burned in a very oxygen-deficient atmosphere, in which the O2 content starts below 6% and decreases with distance. It is also observed that CO levels measured at settings of 15%, 30%, and SA50% are much lower and exhibit only slight variation, because of the relatively high O2 levels in the vicinities of ports 1 and 3. Increasing the OFA damper opening increases the supply of OFA and simultaneously reduces the secondary-air and staged-air fluxes, thereby continuously reducing O2 levels around ports 1 and 3 and thus increasing the air-staging conditions in the lower furnace. Accordingly, CO levels sharply increase as the OFA damper opens, although there is an exception in which no apparent difference is seen between settings of 15% and 30% because of the relatively high O2 levels appearing under much shallower staging conditions. Additional factors, however, are required to explain the differences observed between the 50% and SA50% settings. Increasing the staged-air damper opening from 30% to 50% while maintaining OFA at the 50% opening increases the O2 levels in the vicinities of both ports compared with the concentrations obtained at the 50% OFA setting. This occurs because the SA50% setting results in relatively slow coal combustion (as can be seen by comparing gas temperatures at the two settings in Figures 2 and 3b, respectively) and thus reduces O2 consumption in the zone near port 1, while simultaneously supplying more staged air into the zone near port 3. This occurs even though opening the staged air reduces the secondary-air supply.

port 3 is directly affected by staged air (see Figure 1). As the thermocouple is inserted further, the measuring points gradually approach the zones affected by either the lowtemperature secondary air (for port 1) or staged air (for port 3). Because the flux of secondary air is much higher than that of staged air, the apparently stronger secondary-air cooling effect leads to the decreased temperatures seen past 1.2 m in the data for port 1, whereas the dual effects of staged air (slightly cooling the downward flame and simultaneously raising gas temperatures by supplying combustion air into the flame to improve coal combustion) maintain gas temperature levels near port 3 after 0.9 m. Figure 3b also reveals that opening OFA initially increases but subsequently decreases gas temperatures measured through ports 1 and 3 and that increasing the staged-air damper opening from 30% to 50% generally decreases temperatures in the two zones, with the exception of the increase beyond 1.2 m seen at port 1 at settings of 70% and SA50%. As was discussed with regard to the temperature change trends seen in Figure 2, the dual effects of secondary-air and staged-air reductions associated with opening OFA, along with the appearance of two phenomena (strengthening the oxygen-lean atmosphere in the zone below the arches and simultaneously supplying more low-temperature air to the staged-air zone to lower gas temperatures near ports 1 and 3) associated with enlarging the staged-air damper opening at this high OFA setting of 50%, account for the trends seen in Figure 3b. The abnormal variations beyond 1.2 m at port 1 at settings of 70% and SA50% are attributed to an apparent reduction in the low-temperature secondary air cooling effect at these high OFA and staged-air damper openings. Figure 4 presents plots showing the concentrations of O2, CO, and NOx in the near-wall regions near ports 1 and 3. When the two lower OFA settings of 15% and 30% are applied, both of which allow much more secondary air and staged air into the furnace, the O2 levels are initially constant at relatively higher levels and then increase with distance as the probe approaches the zones at distances beyond 1.6 m which are affected by secondary air (port 1) or staged air (port 3). In contrast, at the three higher OFA settings of 50%, 70%, and SA50%, similar patterns of continuous decrease with distance are evident, with the exception of the port 3 data for the SA50% 5524

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combustion in the burner zone, a unique burner configuration that avoids premature mixing between secondary air and fuelrich coal/air flow prior to ignition, and the three-layer airstaging combustion configuration. Only when opening the OFA damper up to 50% to produce deep-air-staging conditions are NOx emissions reduced to levels close to the value of 800 mg/ Nm3 at 6% O2 (dry), which will be considered acceptable in the near future. Unfortunately, under these conditions, carbon content in fly ash surges to high levels exceeding 10%. Additional reductions in NOx emissions to 503 mg/Nm3 at 6% O2 (dry), an ultralow level for a down-fired furnace, can be attained by further opening OFA to the 70% setting if the associated significant increase in combustible loss can be tolerated. Increasing the staged-air damper opening from 30% to 50% (accompanied by fixing the OFA damper opening at 50%) improves combustible loss and simultaneously maintains the deep-air-staging conditions that inhibit NOx production, although carbon in fly ash remains high at 9.81% and NOx emissions increase slightly. These observations may be explained as the result of several factors. (i) Intense coal combustion develops following mixing of the staged air with the downward coal/air flame, which typically occurs in the zone corresponding to port 4 as shown by the elevated gas temperature levels along the flame travel path in Figure 2. (ii) Supplying more staged air in this zone strengthens the intense coal combustion process and simultaneously favors the thermal formation of NOx in the resulting high temperature atmosphere. And, (iii) the shallower staged-air declination angle of 20° (Figure 1), which reduces the coal/air flame penetration depth and limits the residence times of coal particles in the lower furnace because of the high staged-air transverse impulse on the downward flame, may be the major cause of the still high levels of carbon in fly ash (about 10%), despite the negative effects of deep-air-staging conditions on burnout. With regard to point i, F-layer secondary air declination has been shown to improve burnout in two 300-MWe down-fired furnaces by extending the coal/air flame penetration.22,23 With respect to point ii, good burnout (carbon levels in fly ash of 2.54−3.72%) has been reported in a down-fired furnace incorporating a previous version of MIMSC technology with a much deeper staged-air declination angle of 45° and without OFA,38 and so retrofitting the staged-air angle from 20° to a higher value such as 30° or even the original 45° is recommended to achieve acceptably low levels of carbon in fly ash and NOx emissions of 5% and 800 mg/Nm3 at 6% O2 (dry), respectively. The optimal staged-air angle in a given system can be determined by performing comprehensive CFD simulations concerning coal combustion and NOx formation with respect to the angle in the furnace.

Figure 4 also summarizes variations in NOx emissions. In the zone near port 1, gas diffusion from the downward coal/air flame to the near-wall region results in a trend in which NOx levels generally increase with distance, although there are some fluctuations. In the zone near port 3, however, variations in NOx levels exhibit two trends with distance. In one trend, seen with low OFA settings of 15% and 30%, the levels initially increase but then undergo slight changes. In the other trend, associated with high OFA settings of 50% and 70% as well as SA50%, NOx levels initially increase and then exhibit slight variations before decreasing, with the exception of regions beyond 1.2 m at SA50%. This occurs because increasing both the OFA and staged-air damper openings affects the interaction of the dual roles of staged air: diluting the gas to lower NOx levels and increasing the total NOx production by supplying air to aid coal combustion. With relatively large differences in gas temperatures and O2 levels at port 1 among different OFA settings (Figures 2 and 4), the interaction between gas temperatures and O2 concentration results in NOx levels at this port that initially increase and subsequently decrease as the OFA damper opening is increased. At port 3, increasing the OFA damper opening consistently decreases NOx levels, because of the relatively small differences between gas temperatures among different OFA settings and the continuously decreasing O2 concentrations which occur at this port as the OFA damper is opened. Increasing the staged-air damper opening from 30% to 50% results in a small increase in NOx levels in the zone most affected by secondary air and staged air (i.e., the region beyond 1.4 m), owing to NOx generation by the high levels of O2 which are present. In essence, regulating deep-air-staging conditions in the furnace by significantly opening OFA clearly reduces NOx levels, especially in the zone near port 3, demonstrating that the MIMSC technology may be used in this type of furnace to inhibit the formation of NOx. Table 1 lists key operational results at various OFA and staged-air damper openings. From these data it can be seen that when the OFA damper is opened to produce deeper air-staging conditions, levels of exhaust gas temperature, CO emission, and carbon content in fly ash all increase while both NOx emissions and boiler efficiency decrease. These results are obtained because greatly strengthening air-staging conditions postpones a large share of coal combustion to occur in the burnout zone (which is located in the furnace throat and not far from the upper furnace outlet, Figure 1). Consequently, relatively short burnout times and intense combustion appears in the burnout zone, generating high exhaust gas temperatures and combustible loss. This in turn yields low boiler efficiency despite a significant reduction in NOx emissions. Admittedly, the trends in operational results seen in this work with changing airstaging conditions are similar to those reported in tangentialfired and wall-arranged furnaces.30,32−34 Table 1 also shows that, even under shallower air-staging conditions associated with low OFA damper openings (such as 15% and 30%), the MIMSC technology produces relatively low NOx emissions of 1077−1244 mg/Nm3 at 6% O2 (dry) and acceptable carbon levels in fly ash. This can be demonstrated by comparing the performance of conventional down-fired furnaces, which can generate NOx emissions up to 1600 mg/Nm3 at 6% O2 (dry) while simultaneously producing the same levels of carbon in fly ash.4,6,11,16−18 As noted in the Experimental Section, the improvements seen here with using the MIMSC technology are attributed to a synergistic effect between the fuel rich/lean

4. CONCLUSION This work presents an experimental investigation into the combustion and NOx emission characteristics within a deep-airstaging down-fired 600 MWe furnace. Normal full-load industrial-size measurements were performed at OFA damper opening settings of 15%, 30%, 50%, and 70% to evaluate the OFA effect on combustion and NOx emissions. An additional setting that involved increasing the staged-air damper opening from 30% to 50%, accompanied by fixing OFA at a 50% opening, was tested so as to assess the feasibility of reducing combustible loss under low-NOx conditions. Data obtained at each of the five settings were compared to ascertain the OFA 5525

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and staged-air effects on various furnace parameters. The parameters studied included differences in gas temperatures along the flame travel path in the lower furnace, gas temperature increase trends as the coal/air flow proceeded in the burner zone, gas species concentrations in the near-wall region, carbon in fly ash, and NOx emissions. The results demonstrated that, with opening OFA, gas temperatures in the burner zone and along the flame travel path prior to entry of the flame into the hopper region initially increased but then subsequently decreased. In the near wing-wall regions affected by secondary air and staged air, O2 levels continually decreased while CO levels were initially low (below 300 ppm dry volume) at low OFA settings of 15% and 30% but then surged to especially high levels (above 2000 ppm dry volume) at deep-airstaging conditions (settings of 50% and 70%). NOx emissions decreased continuously, whereas both exhaust gas temperature and combustible loss increased. Deep-air-staging conditions in the furnace lowered NOx emissions to levels of 503−823 mg/ Nm3 at 6% O2 (dry) but unfortunately resulted in high levels of carbon in fly ash ranging 11.47−15.10%. Under deep-airstaging conditions with an OFA opening of 50%, the staged-air increase improved coal combustion in the primary combustion zone, thereby reducing combustible loss while only slightly increasing NOx emissions. Increasing the shallow staged-air angle of the furnace is recommended to improve the still high combustion loss under deep-air-staging conditions, so as to achieve acceptable levels of carbon in fly ash of 5% and NOx emissions of 800 mg/Nm3 at 6% O2 (dry).

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86 451 86418854. Fax: +86 451 86412528. E-mail: [email protected]; [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the National Natural Science Foundation of China (Contract 51306167) and the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Contract 51121004).

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