Improved NOx Emissions and Combustion Characteristics for a

Apr 29, 2010 - NOx emissions was employed in a 300MWe down-fired boiler as an economical ... on this boiler and measurements taken of gas temperature...
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Environ. Sci. Technol. 2010, 44, 3926–3931

Improved NOx Emissions and Combustion Characteristics for a Retrofitted Down-fired 300-MWe Utility Boiler ZHENGQI LI,1 FENG REN, ZHICHAO CHEN, GUANGKUI LIU, AND ZHENXING XU School of Energy Science and Engineering, Harbin Institute of Technology, 92, West Dazhi Street, Harbin 150001, P.R. China

Received January 22, 2010. Revised manuscript received March 31, 2010. Accepted April 19, 2010.

A new technique combining high boiler efficiency and lowNOx emissions was employed in a 300MWe down-fired boiler as an economical means to reduce NOx emissions in downfired boilers burning low-volatile coals. Experiments were conducted on this boiler after the retrofit with measurements taken of gas temperature distributions along the primary air and coal mixture flows and in the furnace, furnace temperatures along the main axis and gas concentrations such as O2, CO and NOx in the near-wall region. Data were compared with those obtained before the retrofit and verified that by applying the combined technique, gas temperature distributions in the furnace become more reasonable. Peak temperatures were lowered from the upper furnace to the lower furnace and flame stability was improved. Despite burning low-volatile coals, NOx emissions can be lowered by as much as 50% without increasing the levels of unburnt carbon in fly ash and reducing boiler thermal efficiency.

Introduction Nitrogen oxides (NOx) have long been recognized as acid rain precursors that pose a significant threat to the environment. Its main source derives from primary emissions from coal-fired power plants into the air (1-3). Down-fired combustion technology, which is employed in burning lowvolatile coals such as anthracite and lean coal, is developing quickly in China. The technology increases the coal burnout rate by increasing particulate residence times in the furnace. However, in practice, extremely high NOx emissions persist. Ordinarily, NOx emissions exceed 1300 mg/m3 at 6% O2, and is at times as high as 2100 mg/m3 (4-7). The proposed Chinese emission standard for anthracite is a relatively lower 1100 mg/m3. NOx emission reductions are generally achieved using two approaches: either controlling combustion or postcombustion treatment. For primary removal of NOx, modifications of the combustion system are generally less costly (8). Of these methods, air staging or overfire air (OFA) is the more mature and widely used. Many laboratory experiments and industrial applications have confirmed that the introduction of OFA is an effective way to remove NOx for a variety of coals and boilers (9-13). However, the resulting low-NOx com1 Corresponding author: Tel.: +86-451-86413231, ext 806; Fax: +86-451-86412528. Email address: [email protected] (Z.Q. Li).

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bustion environment usually leads to high carbon levels in fly ash that is financially disadvantageous for power plant management (9-12). For a down-fired boiler, the only reported retrofit plan is one called “vent-to-OFA” proposed by Foster Wheeler Power Group. They also admit that an increase in unburnt fuel is inevitable (14). Thus, a new method needs to be developed to retrofit down-fired boilers so that NOx emissions can be reduced without raising carbon levels in fly ash and lowering thermal efficiencies of the boiler itself. To date, no literature has reported the application of low NOx emission retrofits to current down-fired boilers. Here, we propose and employ a new combined highefficiency and low-NOx (CHELNO) technique on a 300MWe down-fired utility boiler. In situ experiments were conducted on this boiler and measurements taken of gas temperature distributions, ignition of primary air/fuel mixture flow and gas component concentrations in the furnace. Data were compared with those taken preretrofit. These will be of benefit in the design and operation of similar boilers as well as having comparative value in theoretical and numerical calculations. Utility Boiler and the CHELNO Technique. Figure 1 depicts a cross-sectional schematic view of the 300MWe furnace. The arches divide the furnace into two parts, the lower furnace below the arches and the upper furnace above the arches. Figure 1 also depicts the original combustion system of the boiler. Cyclone burners are set on the arches to form a W-shaped flame. The cyclone burner centrifugally separates the primary air/coal stream into two flows: the fuel-rich and the fuel-lean flows. An oil igniter is provided near each burner assembly with the tip positioned near the burner nozzle. Oil air from the C boxes is sent to the furnace through a port surrounding the oil igniter (Figure 1). Under the arch there are three tiers of slots (labeled D, E and F) between the vertical waterwall tubes in the front and rear walls for the secondary air to be horizontally fed into the furnace. More details are described elsewhere (4-6). As for the design concept, this downshot firing arrangement should give longer residence times for the flame so that most of the burning of the char can take place in the lower furnace. This is where high temperatures of up-flowing recirculating gases prevail to support ignition of primary air/ coal and staged secondary air admission under the arch can lower NOx emissions. However, in practice, the situation is quite the contrary. In Li et al (4-6) described the horizontal fed secondary air was found to prevent primary air/fuel from reaching a low position in the lower furnace and peak temperatures appeared in the upper furnace sustaining high levels of unburnt carbon in fly ash. The ignition zone of pulverized coal is far from the nozzle as the fuel lean flow inhibits up-flowing hot gases heating the fuel-rich flow (see Figure 2(a)). NOx emissions are high at around 2000 mg/m3 (at 6% O2) because all coal and air supplied to the furnace are concentrated in the lower furnace (4-6). To lower NOx emission levels of a down-fired boiler to acceptable, but affordable levels, we chose here the OFA method. In implementing OFA, with some the air introduced to the upper furnace, stoichiometric values within the lower furnace are lower than 0.9, thus a reductive atmosphere was built up in the lower furnace. The coal burns incompletely in this reductive atmosphere, which is disadvantageous for the burnout. Moreover, the incomplete combustion of the coal produces less heat than the complete combustion that has a stoichiometry close to 1.0. Thus a low-temperature zone is built up together with the reductive atmosphere. The low temperature gas supplies less heat to the burner zone and creates in theory a deteriorating environment for fuel 10.1021/es1002378

 2010 American Chemical Society

Published on Web 04/29/2010

FIGURE 1. Furnace and combustion system of the 300 MWe down-fired boiler.

FIGURE 2. Combustion system for the down-fired boiler. ignition. Thus, extra measures are necessary to assist the principal low NOx emission retrofit. In 2006, Li et al. had retrofitted the combustion system of a 300MWe furnace by declining the F-tier secondary air (see Figure 2(b) for the configuration of the combustion system; there, DSA is short for declined secondary air). Its effect is largely to lower the carbon content in fly ash. Ignition of the fuel-rich flow was also improved (15), and thus this measure should be retained to assist the low NOx emission retrofit here. Another measure selected has been to feed the fuel-lean flow into the furnace through the C ports instead of the original fuel lean nozzles so that the fuel-rich flow can be heated directly by upflowing gases and not be obstructed by the fuel lean flow. This is advantageous in the timely ignition of the fuel-rich flow. In Foster Wheeler’s “vent-to-OFA” plan, fuel-lean flow is fed into the upper furnace through pipes inside the OFA ducts (14). We believe this plan will shorten residence times of pulverized coal from the fuel-lean flow nozzle and hinders burnout. These above measures comprise what we here call the combined high efficient and low NOx emission technique (CHELNO for short; see Figure 2(c) for the configuration of this combustion system) and were applied in a 300MWe

down-fired pulverized-coal boiler. Corresponding in situ industrial experiments were repeated to investigate combustion characteristics of the boiler after this retrofit. The data obtained were compared with those from the original boiler before any retrofit as well as those after retrofitting with simply declined F-tier secondary air. Data Acquisition Methods and Experimental Conditions. In situ experiments were performed in the 300MWe downfired pulverized-coal boiler retrofitted by CHELNO to investigate changes in the combustion process and NOx formation in the furnace. It should be emphasized that, during experimental procedures, the boiler operators made considerable effort to ensure minimum variation in boiler operating conditions and chemical and particle-size characteristics of the coal. Use of soot blowers in the furnace was avoided during measurements. Coal used in all experiments was Yangquan anthracite. Sample characteristics, which are listed in Table 1, indicate that the coal used in this experiment is similar to that used in the original boiler and the DSAretrofitted boiler; all anthracite samples had a high heating value and a low volatile content. Table 1 also lists averaged operating parameters over the duration of each experimental run. These indicate that the data obtained in the CHELNOVOL. 44, NO. 10, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Coal Characteristics, Boiler Operation Parameters Proximate analysis, wt.% (as received)

Original* DSA* CHELNO

Ultimate analysis, wt.% (air-dried wt %)

Volatile

Ash

Moisture

Fixed carbon

Net heating value (kJ/kg)

8.17 9.22 8.02

29.4 25.81 23.66

0.7 0.42 1.88

61.73 64.55 66.44

23390 22430 23200

C

H

63.65 64.76 66.58

2.36 2.83 2.63

S 1.45 1.35 1.42

N 0.86 1.06 0.98

O 1.58 3.54 2.85

Operation parameters in experiments Quantity

Original*

DSA*

CHELNO

Boiler load (MWe) Total flux of the primary air (104Nm3/h) Temperature of the primary air (°C) Total flux of the secondary air (104Nm3/h) Flux ratio of OFA air to the total air Stoichiometry of the lower furnace Temperature of the secondary air (°C) O2 at the furnace exit (dry volume %) O2 in flue gases (dry volume %) CO in flue gases (dry volume %) NOx in flue-gas (mg/m3 at 6% O2 dry) Carbon in fly ash (%) Thermal efficiency of the boiler (%)

300 13.2 105 56.6 0 1.03 320 2.82 ( 0.12 4.17 ( 0.10 14 ( 3 2101 ( 35 7.84 91.08

300 14.1 105 58.7 0 1.02 320 2.72 ( 0.21 3.01 ( 0.15 16 ( 4 1926 ( 27 4.91 93.25

300 15.2 105 55.4 0.25 0.77 320 2.69 ( 0.18 3.47 ( 0.13 27 ( 3 1057 ( 25 7.54 91.70

* data are from literature (15).

retrofitted boiler are comparable with the other two set-ups. The OFA ratio is set at 25%. Here the OFA ratio is defined as the mass flux ratio of the OFA to the total combustion air fed into the furnace. During the experiments, the following parameters were measured. (a) Furnace gas. These were measured with a 3i hand-held pyrometer (manufactured by Raytek, America) inserted through monitoring ports in the front, rear and side walls. Measurement information including specific measuring locations and procedures has been discussed in a previous report (15). (b) Gas temperatures in the burner region. As shown in Figure 2, a fine-wire thermocouple with a 0.3-mmdiameter nickel-chromium/nickel-silicon thermistor was inserted along the line parallel to the axis of the cyclone to measure primary airflow/temperature distributions. Descriptions and error estimations have been given in (15). (c) Gas species compositions. Gases were sampled using a 2.5m-long water-cooled stainless steel probe for analysis of local mean concentrations of O2, CO and NOx. The probe comprises a centrally located 10-mm-i.d. tube surrounded by a tube for probe cooling through which quenched samples were evacuated. The water-cooled probe was inserted into the furnace through monitoring ports 1-3 (see Figure 1). Gases were withdrawn and analyzed online using a Testo 350 M system. The flue gas was also analyzed online. Calibrations with standard mixtures including zero concentrations were performed before each measurement session. The measurement accuracy of this system for O2 concentrations was 0.8%, while those for CO, NOx were 50 ppm. Each test run was finished in 8 h to ensure the relative consistency of the coal quality. When performing the measurement, after the reading is relatively stable at a certain measurement point, the thermocouple and water-cooled probe are kept in the furnace for about 60 s, every 10 s a reading recorded, with the average as the final value of this point. The reason for not taking a longer time as the measuring time is to avoid heavy deposition of soot or ash on the thermocouple or water-cooled probe. It is especially difficult to establish and maintain constant operating conditions over a long time. The coals vary almost every day, even on the condition that they were transported from the same mine. Thus, it is impossible to completely repeat the industrial experiment. However, we still try carrying out several repeated runs for each configuration of the 3928

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combustion system with the coal property of a little difference from those displayed in Table 1. On average, the data for major gas species concentrations and temperature could be reproduced in 10%. This little fluctuation of the data does not effect the conclusion we obtained from these three setups.

Results and Discussion Figure 3 shows furnace temperatures as measured using the pyrometer inserted through monitoring ports of the furnace. In general, gas temperatures peak in the lower furnace of the operating boiler; distances from the position of peak gas temperature to the exiting point of the furnace is relatively large. Coal combustion times in the higher temperature zone are longer, a fact that is advantageous in fuel burnout. If peak gas temperatures are in the upper furnace, distances from the positions of peaks to the exiting point of the furnace is relatively shorter. Given this situation, coal combustion times in the higher temperature zone are shorter, and thus disadvantageous to fuel burnout. Moreover, peak temperature positions can result in high gas temperatures at the furnace outlet that may lower the boiler thermal efficiency.

FIGURE 3. Gas temperature variations along the furnace altitude.

FIGURE 4. Gas temperature distributions in the burner zone. Thus, peak gas temperatures should occur in the lower furnace. It can be observed in Figure 4 that for the original and DSA furnaces, both clearly without OFA, the gas temperature measured directly above the arch is either larger than or very close to those in the lower furnace, contrary to the design concept. After CHELNO-retrofitting with air staging, the temperatures in the lower furnace are as high as 1500 °C, about 200 °C higher than in the upper furnace, which seems more practical for good boiler operations. Lower-furnace temperatures are also over 200 °C higher on average than those measured at the same furnace depth in the other two set-ups without OFA, showing a contradictory result. In general, combustion occurs most intensely in that part of the furnace where the stoichiometry is close to 1.0. While under air staging, gas temperatures in the primary combustion zone would fall after some of the air has flowed to the burnout zone and stoichiometric values within this zone are lower than 0.9. Various experiments and numerical simulations have validated this situation for coals with high volatility (9, 10, 16). Here, for low-volatile coals, the situation seems different. With low volatility and reactivity, low-ranked coals usually need higher temperatures to initiate combustion than other coals. Thus, in the initial combustion stage, air should not be supplied in too great a quantity so that the temperature of the coal rises as quickly as possible. For the down-fired furnace without OFA, all air was fed into the lower furnace and quickly mixed with coal before it reaches a high temperature. This is why temperatures of a down-fired furnace are low. After the CHELNO-retrofit, this problem is solved with some of the air flowing to the upper furnace. Figure 4 shows gas temperature distributions in the burner zone along the cyclone axes of the burners. All the measured temperature fluctuated within (10 °C. All distances were measured from the ends of the fuel-rich nozzle in the furnace. The three distributions show that at the measurement point 1200 mm away temperatures cannot reach 600 °C. The significance is that the ignition positions of primary air and pulverized coal mixtures were far away from the fuel-rich nozzle irrespective of a retrofit or not. This is due to the low reactivity of low-volatile coal. Comparison of fuel-rich flow temperature gradients at measurement points less than 600 mm away follow the decreasing sequence DSA > CHELNO > original. The lowest temperature gradients occur in the original boiler due to overall lower temperatures in the lower furnace. However, temperature gradients of the fuel-rich flow with CHELNO-retrofitting are greater than with DSAretrofitting, though the former has obviously higher temperatures in the lower furnace. After channeling part of the air to the upper furnace, it is possible the total gas flux in the

lower furnace drops. Thus, the quantity of recirculating gas which flows back to the arch also decreases. Even at higher temperatures, the total heat supplied to the fuel-rich flow by the recirculating gas supplies falls. Due to mixing of larger quantities of secondary air, temperature gradients of the fuelrich flow drop under DSA-retrofitting at distances further than 600 mm away and was exceeded by that under CHELNOretrofitting. On comparing flame stability among the three set-ups, we consider the performance with CHELNOretrofitting to be the best because temperature gradients in this case were constant in the primary combustion even after mixing with secondary air, indicating that secondary air is promoting combustion rather than extinguishing it. Figure 5 shows distributions of O2, CO and NOx concentrations in zones near monitoring ports 1, 2, and 3. All the measured O2 fluctuated within (0.6%, CO within (100 ppm in case CHELNO and (15 ppm in the other two cases, NOx within (30 ppm. For the original and DSA-retrofitted boilers, the O2 content both show a concentration sequence of port 1 > port 2 > port 3 that describes well the flow of coal as illustrated in each panel of Figure 2. As coal passes through areas near monitoring ports 1, 2, and 3 in this sequence, O2 is consumed gradually. With CHELNO, O2 concentrations are a little higher near port 3 than near port 2 as some of the air was fed into the furnace through the OFA nozzles mounted between the arches and monitoring port 3. The figure also indicates that near monitoring ports 1 and 2, the O2 is at its lowest levels with CHELNO. One reason for this is that secondary air in these two airflow zones abates some fraction of the air has moved to the upper furnace through the OFA nozzle. In addition, with proper air supply, combustion occurs most intensely in these two zones with CHELNO, and thus exhausts higher levels of O2. In the zone near port 3 above the arch, O2 concentrations for the original and CHELNO furnace were very similar but both higher than with DSA. This may indicate that combustion with DSA is the more complete among the three furnaces with the other two at similar levels. CO concentrations for the original and DSA furnaces are very low in all the three areas, indicating that oxygen is abundant during the combustion process given these two setups. With CHELNO retrofitting, CO concentrations kept rising with highest value attaining 4000 ppm as combustion progressed. This is because introducing OFA forces combustion to occur in a lean-oxygen atmosphere in the lower furnace. From oxygen and CO levels of the three setups, combustion with DSA can be concluded to be have finished the earliest and more efficiently. For the other two set-ups, final combustion efficiencies are difficult to predict from current mentioned data as combustion has not yet finished in the zones near monitoring ports 3. Contrary to the variation in O2 concentrations, NOx concentrations increase during combustion for all three setups. In the zone near monitoring port 1, NOx production is least in the original boiler since ignition of the fuel rich flow is at its latest. Restricted by the lean-oxygen atmosphere in the airflow zone of D and E tiers, NOx production with CHELNO is less than that with DSA in this area, although combustion with the latter is more intense. In the zones near monitoring ports 2 and 3, NOx concentrations quickly increases to high values as the combustion continues in the original and DSA boilers while in the CHELNO-retrofitted boiler, any increase was limited by a lean-oxygen atmosphere in the lower furnace. In analyzing NOx formation, NOx was found to be produced mostly (increasing from 0 to 490 ppm on average) in the stage when the fuel-rich streams flow from the nozzles to the area near monitoring port 1. The reason is that in this stage, gas temperatures of this flow rise very quickly. After reaching the D and E airflow zones, temperatures were as high as 1500 °C. Such high temperatures force the release of volatile-N and some active char-N largely VOL. 44, NO. 10, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 5. Local mean gas species concentrations in the zone near the water-cooled wall. during volatilization and char cracking. Because of the low reactivity of the coal, the reductive atmosphere is not very strong, providing a conducive environment for N-containing compounds to react with O2 supplied from the secondary air ports and form fuel NOx. In the stage when the fuel-rich flow from the area near monitoring port 1 to that near monitoring port 2, NOx production rises from 490 ppm to 596 ppm on average at a much slower rate of increase. This may be because after volatilization and char cracking, the remaining nitrogen in the char is of high thermal stability and can only be released gradually with the slow burnout of char. In the third stage with fuel-rich stream flowing up from monitoring port 2 to 3 and mixing with OFA, NOx concentrations vary slightly from 596 ppm to 616 ppm. The situation can be analyzed as follows: in the period before the addition of OFA at this stage, oxidation of the remaining char-N becomes more difficult with the continued decrease in oxygen levels. The main reaction open to nitrogen is the reduction with char of NOx to N2. Thus, total NOx production begins to fall until the addition of OFA. After cold OFA has been fed into the upper furnace, temperature fell significant from 1470 to 1285 °C (see Figure 3). With lower temperatures, NOx formation is very limited above the OFA nozzle. Thus, NOx concentrations almost stop increasing in this stage. Table 1 lists the content of gas components at the furnace exit and in the flue gas for the original boiler and the boilers retrofitted by DSA and CHELNO. Carbon content in fly ash is also listed. It is evident that CO concentrations are low in all three boilers, albeit slightly higher with CHELNO. Carbon content in fly ash produced with CHELNO is larger than with DSA, but a little lower than from the original boiler. It indicates that, although OFA introduction forces the coal to burn in a reducing atmosphere for a longer period and raises levels of unburnt carbon, by applying CHELNO, declining F-tier 3930

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secondary air and introducing vent air through the oil air port improves the ignition of fuel-rich flow. Furthermore, coal residence times increase in the high-temperature area, thus compensating the loss from incomplete combustion. For the same reason, the thermal efficiency of the boiler retrofitted by CHELNO is a little higher than in the original boiler. In the table, NOx concentrations can also be observed to be very large (at over 1900 mg/m3) in the original and DSA-retrofitted boilers. The main reason is that all the air was concentrated into the lower furnace making oxygen abundant during coal combustion even in the area where peak temperatures reside. This creates a good environment for the formation of both fuel and thermal NOx. With CHELNO retrofitting, fuel NOx was restricted for longer times in the reducing area under the arches. Thermal NOx was also restricted as the peak temperature zone is always accompanied with a reducing atmosphere. The application of CHELNO lowers NOx emissions to 1057 mg/m3. The removal rate was 50% compared with the original boiler and only 45% compared with the DSA-retrofitted boiler. It can be concluded that by applying CHELNO, gas temperature distributions in the furnace were found to be more reasonable in the following sense. Peak temperatures were lowered from the upper to the lower furnace and flame stability is improved. NOx emissions can be lowered by as much as 50% without increasing levels of unburnt carbon in fly ash and decreasing boiler thermal efficiency despite the use of low-volatile coals.

Acknowledgments This work was sponsored by the Hi-Tech Research and Development Program of China (863 program) (Contract No.: 2006AA05Z321).

Literature Cited (1) Robertus, R. J.; Nielsen, K. L.; Crowe, C. T.; Pratt, D. T. Attempt to reduce nitrogen oxide (NOx) emissions from pulverized coal furnaces. Environ. Sci. Technol. 1975, 9, 859–862. (2) Arenillas, A.; Rubiera, F.; Pis, J. J. Nitric oxide reduction in coal combustion: role of char surface complexes in heterogeneous reactions. Environ. Sci. Technol. 2002, 36 (24), 5498–5503. (3) Yao, M. Y.; Che, D. F.; Liu, Y. H.; Liu, Y. H. Effect of Volatile-Char Interaction on the NO Emission from Coal Combustion. Environ. Sci. Technol. 2008, 42, 4771–4776. (4) Li, Z. Q.; Ren, F.; Zhang, J.; Zhang, X. H.; Chen, Z. C.; Chen, L. Z. Influence of vent air valve opening on combustion characteristics of a down-fired pulverized-coal 300 MWe utility boiler. Fuel 2007, 86, 2457–2462. (5) Ren, F.; Li, Z. Q.; Jing, J. P.; Zhang, X. H.; Chen, Z. C.; Zhang, J. W. Influence of the adjustable vane position on the flow and combustion characteristics of a down-fired pulverized-coal 300 MWe utility boiler. Fuel Process. Technol. 2008, 89, 1297–1305. (6) Ren, F.; Li, Z. Q.; Sun, S. Z.; Zhang, X. H.; Chen, Z. C. Influence of the secondary air-box damper opening on airflow and combustion characteristics of a down-fired 300-MWe utility boiler. Energy Fuels 2007, 21, 668–676. (7) Li, Z. Q.; Kuang, M.; Zhang, J.; Han, Y. F.; Zhu, Q. Y.; Yang, L. J.; Kong, W. G. Influence of staged-air on airflow combustion characteristics and NOx emissions of a down-fired pulverizedcoal 300 MWe utility boiler with direct flow split burners. Environ. Sci. Technol. 2010, 44, 1130–1136. (8) U.S. Environmental Protection Agency. Alternative control technologies document; EPA: Washington, DC, 1996.

(9) Costa, M.; Azevedo, J. L. T. Experimental characterization of an industrial pulverized coal-fired furnace under deep staging conditions. Combust. Sci. Technol. 2007, 179, 1923–1935. (10) Costa, M.; Azevedo, J. L. T.; Carvalho, M. G. Combustion characteristics of a front-wall-fired pulverized-coal 300 MWe utility boiler. Combust. Sci. Technol. 1997, 129, 277–293. (11) Costa, M.; Silva, P.; Azevedo, J. L. T. Measurements of gas species, temperature, and char burnout in a low NOx pulverized-coalfired utility boiler. Combust. Sci. Technol. 2003, 175, 271–289. (12) Pedersen, K. H.; Jensen, A. D.; Berg, M.; Olsen, L. H.; DamJohansen, K. The effect of combustion conditions in a full-scale low- NOx coal fired unit on fly ash properties for its application in concrete mixtures. Fuel Process. Technol. 2009, 90, 180–185. (13) Fan, W. D.; Lin, Z. C.; Li, Y. Y.; Kuang, J. G.; Zhang, M. C. Effect for air-staging on anthracite and NOx formation. Energy Fuels 2009, 23, 111–120. (14) Garcia-Mallol, J. A.; Steitz, T.; Chu, C. Y.; Jiang, P. Z. Ultra-Low NOx Advanced FW Arch Firing: Central Power Station Applications; 2nd U.S. China NOx and SO2 Control Workshop: Dalian, 2005. (15) Li, Z. Q.; Ren, F.; Chen, Z. C.; Chen, Z.; Wang, J. J. Influence of declivitous secondary air on combustion characteristics of a down-fired 300-MWe utility boiler. Fuel 2010, 89, 410–416. (16) Huang, L. K.; Li, Z. Q.; Sun, R.; Zhou, J. Numerical study on the effect of the Over-Fire-Air to the air flow and coal combustion in a 670 t/h wall-fired boiler. Fuel Process. Technol. 2006, 87, 363–371.

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