Environ. Sci. Technol. 2010, 44, 6510–6516
Influence of the Overfire Air Ratio on the NOx Emission and Combustion Characteristics of a down-Fired 300-MWe Utility Boiler FENG REN, ZHENGQI LI,* ZHICHAO CHEN, SUBO FAN, AND GUANGKUI LIU School of Energy Science and Engineering, Harbin Institute of Technology, 92 West Dazhi Street, Harbin 150001, P.R. China
Received March 25, 2010. Revised manuscript received May 26, 2010. Accepted July 13, 2010.
Down-fired boilers used to burn low-volatile coals have high NOx emissions. To find a way of solving this problem, an overfire air (OFA) system was introduced on a 300 MWe down-fired boiler. Full-scale experiments were performed on this retrofitted boiler to explore the influence of the OFA ratio (the mass flux ratio of OFA to the total combustion air) on the combustion and NOx emission characteristics in the furnace. Measurements were taken of gas temperature distributions along the primary air and coal mixture flows, average gas temperatures along the furnace height, concentrations of gases such as O2, CO, and NOx in the near-wall region and carbon content in the fly ash. Data were compared for five different OFA ratios. The results show that as the OFA ratio increases from 12% to 35%, the NOx emission decreases from 1308 to 966 mg/ Nm3 (at 6% O2 dry) and the carbon content in the fly ash increases from 6.53% to 15.86%. Considering both the environmental and economic effect, 25% was chosen as the optimized OFA ratio.
Introduction Nitrogen oxides (NOx) have long been recognized as acid rain precursors that pose a significant threat to the environment. They are mainly derived from primary emissions of coal-fired power plants into the air (1–3). This has led to the establishment of regulatory measures and the development of technologies to reduce NOx emissions from both existing and new power plants. In general, there are two categories of NOx control technologies: primary control technologies and secondary control technologies. Primary control technologies reduce the amount of NOx produced in the coal combustion zone, whereas secondary control technologies reduce the NOx present in the flue gas away from the coal combustion zone. Much research has revealed that the former are generally more cost-effective for a certain range of NOx removal (4–6). Of the methods employing primary control technology, the use of overfire air (OFA) is the most mature and widely used. The technology creates a fuel-rich zone near the burners by introducing only a portion of the combustion air into the main-burner region and introducing the remaining combustion air (the so-called overfire air) downstream. This allows most of the coal nitrogen to be * Corresponding author tel.: +86-451-86413231, ext 806; fax: +86451-86412528; e-mail:
[email protected]. 6510
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 44, NO. 16, 2010
released in a fuel-rich region, which favors the formation of N2 over NOx. Many laboratory experiments and industrial applications have confirmed that the introduction of OFA is an effective way to reduce the formation of NOx. Major drawbacks of this modification to reduce NOx emissions include reduced coal burnout and increased carbon monoxide, which are financial disadvantages (7–11). For boilers equipped with OFA ports, finding an optimized OFA air ratio to balance the economic and environmental aspects of the retrofits is especially necessary. The ratio strongly depends on the furnace type and coal quality. To date, the majority of reports on the effects of OFA retrofitting have focused on tangential- and wall-fired boilers burning coals of relatively high volatility. There are very few applications of low-NOxemission retrofits to boilers burning low-volatile coals. In China, reserves of low-volatile coals such as anthracite and lean coal are abundant. Such coals account for 25% of the total coal consumption in electricity generation. Downfired combustion technology, which is employed in burning low-volatile coals, has developed quickly in the past 20 years. To better understand the combustion characteristics of downfired boilers, several experimental and numerical investigations have been carried out (12–16). The technology increases the coal burnout rate by increasing particulate residence times in the furnace. However, in practice, extremely high NOx emissions persist. The uncontrolled NOx emissions exceed 1700 mg/Nm3 (at 6% O2 dry), and are at times as high as 2100 mg/Nm3 (12–14), almost the highest reported for any type of boiler. The proposed Chinese emission standard for anthracite is 1100 mg/Nm3 (at 6% O2 dry). Up to now, the only reported effort to lower the NOx emission in downfired boilers is made by Foster Wheeler Power Group. They proposed a retrofit plan called “vent-to-OFA”. However, the practical effect of this retrofit has never been reported and still remains unknown. Li et al. proposed and employed a low-NOx retrofit of a 300 MWe down-fired utility boiler by equipping the boiler with OFA ports. In the present work, in situ experiments were conducted for this boiler at five different OFA ratios and measurements were taken of gas temperature distributions, ignition of the primary air/fuel mixture flow, and gas component concentrations in the furnace. Data for the five cases were compared. The data will be of benefit in the design and operation of similar boilers and provide comparison values for theoretical and numerical calculations.
Utility Boiler Figure 1 depicts a cross-sectional schematic view of the Foster-Wheeler designed 300 MWe furnace. Arches divide the furnace into two parts: a lower furnace and upper furnace. Figure 2 depicts the combustion system of the boiler. Cyclone burners are set on the arches to form a W-shaped flame. At each side of the furnace, there are 18 cyclone burners. The cyclone burner centrifugally separates the primary air/coal stream into two flows: a fuel-rich flow and fuel-lean flow. For each cyclone burner, a single group of secondary airboxes A-F is provided to assist the combustion. The three tiers of secondary air slots D, E, F are under the arch, between the vertical waterwall tubes in the front and rear walls feeding the majority of the combustion air. Further details of this boiler including the dimensions are described elsewhere (12–14). As a design concept, the separation of the fuel lean flow from the primary air/fuel by the cyclone burner on the arches and the staged secondary air admission under the arch can 10.1021/es100956d
2010 American Chemical Society
Published on Web 07/28/2010
FIGURE 1. Furnace and combustion system of the 300 MWe down-fired boiler. lower NOx emissions. However, in practical operation, the situation is quite the contrary in that the effects of these two combustion modifications are very limited. The cause for it is that to ensure flame stability and burnout of the lowvolatile fuel, the lower furnace of a down-fired boiler is partially or completely insulated by refractory lining, whereas it is water-cooled in the other tangential- or wall-fired boilers. As a result, combustion temperature is higher, leading to increased NOx production. In Li et al.’s low-NOx retrofit of the combustion system, several OFA ports were set above the windboxes on the arches (see Figure 2). From the view of coal combustion, the position of the OFA ports above the arches seems a little higher than in conventional OFA configurations of other types of boilers, which is disadvantageous to the latter gas-solid mixing and coal burnout. The structure of a down-fired boiler restricts the design of the OFA port positions. As the pulverized coal rises from the F-tier secondary flow zone to the upper furnace, combustion air can be supplied to support the combustion from nowhere, which makes the pulverized coal burn in a reducing atmosphere for too long a period and is not advantageous for coal burnout. To compensate, the OFA ports are designed to angle down so that the OFA can mix with the coal as early as possible. The specific structure of OFA is shown in Figure 2. The overall flow duct is divided into two parts: the inner and the outer OFA ducts. The outer duct is equipped with axial vanes to swirl the peripheral OFA so that it easily spreads to the surrounding area and mixes with the upflowing gas near the wall. The inner OFA has direct flow so that it has sufficient momentum to reach the central part of the upper furnace and mix with the upflowing gas there.
Data Acquisition Methods and Experimental Conditions Measurements were performed in the 300 MWe down-fired pulverized-coal boiler equipped with OFA ports to investigate
changes in the combustion process and NOx formation in the furnace with different OFA ratios. Here, the OFA ratio is defined as the ratio of the OFA mass flux to the total air mass flux. Before the boiler began operation, cold flow experiments were carried out in the full-scale boiler. The scope of the cold experiment is to determine the relationships between the damper openings and several air ratios (including the ratios of primary air, D, E, F secondary air, and OFA). The cold flow experiments were finished in 24 h with a large measurement campaign of 10 persons. In the experiment, the flow rates at all the ports were measured by physical instruments. Venturi pipes were used to measure the flow rates of the primary air and OFA. For secondary air, hot-wire anemometers are put at several positions of the port outlets, thus obtaining the approximate flow rate through those ports. The dampers were regulated to make the air even along the width of the furnace and between the front and rear walls. After the regulation, the velocities varied by less than 10%. The results show that the relation between the damper opening and flow rate is parabolic. The increase of the flow rate becomes slower with the increase of damper opening. It should be emphasized that during hot experimental procedures, the boiler operators made considerable efforts to ensure minimum variation in boiler operating conditions and chemical and particle-size characteristics of the coal. The use of soot blowers in the furnace was avoided during measurements. Coal used in all experiments was Yangquan anthracite. Sample characteristics, listed in Table 1, indicate that all coal samples had a high heating value and a low volatile content. Altogether, five OFA ratios were considered: 12%, 20%, 25%, 31%, and 35%. The reason for not including the case with OFA 0% is to avoid the burnout of the nozzle. All the five cases were completed in one day, about four hours for each, ensuring the comparability of the data among the five cases. Hardly any time was left to repeat the cases, thus the measurement results may not be statistically representative. To compensate for it, we took each case very carefully. For each case, the boiler condition took more than 2 h enough time to stabilize. While waiting for the stabilization of the boiler conditions, we took the oxygen content at the furnace outlet as the key signal to judge whether the boiler condition was stable or not. Only when it fluctuated under (0.1 did we start the measurement. At each measurement point, we put the probe in the field for about 60 s, and recorded the average readings. Table 1 lists the average operating parameters over the duration of each experimental run. The stoichimetry of the lower furnace was determined by the measured furnace exit oxygen and the experiential air leakage rates of various equipment and the OFA ratio which determines the air flux into the upper furnace. Coal feeding rate can be calculated by the method of heat balance closure. The total flux of primary air was measured by Venturi pipe with the error of (2.1 × 104 N m3/h. The total flux of combustion air could be calculated from the total coal feeding rate and the stoichiometry of the furnace, thereafter the secondary air flux was also obtained. The absolute errors for all the air ratios are within (2% according to the measurement in the cold flow experiment. During the experiments, the following parameters were measured simultaneously. (a) The furnace gas temperature was measured with a hand-held pyrometer (Raytek 3i pyrometer, U.S.) inserted through monitoring ports in the front, rear, and side walls. Measurement information including specific measuring locations and procedures has been discussed in previous reports (16, 20). It is essential to state that narrow-band, infrared radiation pyrometry measurements are not absolute measurements of gas temperature at specific points but along a line of sight from the detector, and the value is not linear, but approximately to the fourth VOL. 44, NO. 16, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
6511
FIGURE 2. Combustion system for the down-fired boiler.
TABLE 1. Coal Characteristics and Boiler Operation Parameters coal characteristics proximate analysis, wt.% (as received) volatile
ash
moisture
fixed carbon
8.24
27.39
7.0
57.37
ultimate analysis, wt.% (air-dried wt %)
net heating value (kJ/kg)
C
21740 62.72 coal fineness: R90 ) 5%; R74 ) 7%
H
S
N
O
1.84
1.47
0.96
2.23
boiler operation parameters operation parameters in experiments quantity total flux for the primary air (104 N m3/h) temperature of the primary air (°C) total flux for the secondary air (104 N m3/h) ratio of D and E secondary air ratio of F secondary air stoichiometry of the lower furnace temperature of the secondary air (°C) fuel rich flow fuel lean flow secondary air D average velocity (m/s) secondary air E secondary air F OFA O2 at the furnace exit (dry volume %, exclude in-leakage in air preheaters) O2 in flue gases (dry volume %, include in-leakage in air preheaters)
root of the radiative energy. Spectral effects due to gas radiation bands can affect the measurement via gas composition and combustion conditions, which is a rather difficult effect to ascertain. Such measurements have value because with its wide range of measurement, we can get the approximate temperature variation for the whole furnace and indicate the boiler operation. (b) Temperatures of primary air/fuel flow, as shown in Figure 2, were measured with a fine-wire thermocouple with a 0.3-mm-diameter nickel-chromium/nickel-silicon thermistor inserted along the line parallel to the axis of the cyclone. The farthest insertion of the thermocouple was 2400 mm. Bare thermocouples were used here because radiation error would be low due to the wall refractory lining in the lower furnace. 6512
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 44, NO. 16, 2010
12% 15.6 105 60.5 14% 53% 0.90 320 18.3 3.8 4.3 5.8 11.2 11.7 2.74 4.12
20% 15.7 105 60.3 13% 47% 0.81 320 18.4 3.8 4.0 5.4 9.9 19.39 2.69 4.32
25% 15.9 105 59.3 11% 42% 0.76 320 18.6 3.8 3.4 4.5 8.7 24.0 2.54 3.995
31% 15.6 105 60.0 10% 38% 0.70 320 18.3 3.8 3.1 4.1 7.9 29.9 2.72 4.46
35% 15.8 105 59.5 9% 35% 0.66 320 18.5 3.8 2.7 3.7 7.3 33.6 2.64 4.38
When measuring the temperature, the thermocouples were not kept in the furnace more than 60 s to avoid heavy deposition of soot or ash on the thermocouples. The condition of the thermocouple was frequently examined after it was pulled out of the furnace and any depositions were removed once found. More detailed descriptions and error estimations of the measurement have been given in 20. The burner at which we measured is at the middle along the furnace width. Before the formal cases began, we also measured the temperature at three other burners to make a comparison with the formally measured one. We found that among these burners, the measured values were not significantly different. Thus, in the formal cases, to save time, we choose only one. (c) Gas species compositions were determined by sampling
FIGURE 4. Gas temperature distributions in the burner zone. FIGURE 3. Gas temperature variations along the furnace height. gases using a 2.5-m-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 to cool the probe through which quenched samples were evacuated. The water-cooled probe was inserted into the furnace through monitoring ports 1-3 (see Figure 2; port 1 is in the D and E airflow zones, port 2 is in the F airflow zone, and port 3 is over the OFA nozzles). Gases were withdrawn and analyzed online using a Testo 350 M system. The flue gas after the air heater was also analyzed online. Calibrations using standard mixtures including those with zero concentrations were performed before each measurement session. The major sources of uncertainty in concentration measurements were associated with the quenching of chemical reactions and aerodynamic disturbances of the flow. Due to the high water-cooling rate, quenching of the chemical reactions was rapidly achieved upon samples being drawn into the probe. Estimated quenching rates were approximately 106 K/sec. (d) Unburnt carbon in fly ash was determined by collecting fly ash using a particle-sampling device with constant suction speed. With the confidence of 95% and normal distribution, the statistic random error for the temperature measured by thermocouple is below 70 °C, while that for O2 is (1%, for CO (50 ppm, and for NOx is (20 ppm.
Results and Discussion Figure 3 shows the furnace temperatures measured with the pyrometer inserted through monitoring ports of the furnace. As mentioned above, narrow-band, infrared radiation pyrometer cannot warrant accurate measurement of the gas temperature since the value is affected by the gas species in the flame. Although we have not ruled out this effect, this measurement seems still indicative combining with the gas species measurement and primary air/fuel temperature measurement with thermocouple. This measurement indicates that in all five cases the measured gas temperatures in the lower furnace exceeded 1400 °C, which is much higher than gas temperatures in the upper furnace and different from previous results recorded for no OFA in refs 14–16 and 20. If this phenomenon is repeatable, it seems different from the former experience from the boilers burning with highvolatile coals which are of high reactivity. For these coals, volatiles release easily and combustion generally occurs most intensely in that part of the furnace where the stoichiometry is close to 1.0. Under air staging conditions, 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 would be lower
than 0.9. Here, for this low-volatile-coal, we try to explain the phenomenon as follows. 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 this reason, with a certain amount of secondary air removed from the lower furnace to the upper, the temperature rises. This can be validated in the three cases with OFA ratios of 12%, 20%, and 25%. When the OFA ratio rises in this range, less secondary air is fed into the lower furnace and the gas temperature in the lower furnace continues to increase. Figure 3 also indicates that the flame kernel in the furnace lowers as the OFA ratio increases, which lengthens the coal combustion time at high temperature and is advantageous for coal burnout. However, when the OFA ratio exceeds 30%, the gas temperatures begin to fall because the air supplied to the lower furnace is not sufficient for the primary combustion of the pulverized coal. From this point of view, the optimized OFA ratio seems to be 25%. Figure 4 shows gas temperature distributions in the burner zone along the cyclone axes of the burners. All distances were measured from the fuel-rich throat in the furnace. The five distributions show that at 1300 mm temperatures cannot reach 600 °C. This measurement result suggests that the ignition positions of primary air and pulverized coal mixtures are far from the fuel-rich nozzle irrespective of the OFA ratio. This is due to the low reactivity of low-volatile coal. Comparison of the average fuel-rich flow temperature gradients between measurement points at 0 and 1400 mm follow the decreasing sequence 12% > 25% > 20% > 31% > 35%. This temperature sequence is similar to that determined using the pyrometer at the furnace height of 21 m (Figure 3) except for the case of an OFA ratio of 12%. The temperature gradient of the fuel-rich flow with an OFA ratio of 12% is the greatest, although there are obviously higher temperatures in the lower furnace for other OFA ratios. If we accept this trend, then it could be explained as follows: in these situations, after a significant fraction of the air is channeled to the upper furnace, it is possible that the total gas flux in the lower furnace decreases. Thus, the quantity of recirculating gas that flows back to the arch (shown in Figure 2) decreases. Even at higher temperatures, the total heat supplied to the fuel-rich flow by the recirculating gas is less than that in the 12% case. Comparing the cases with OFA ratios of 20%, 25%, 31%, and 35%, the temperature gradient sequence of the fuel-rich flow is mainly governed by the recirculating gas temperature sequence. Because of the mixing of larger quantities of secondary air, temperature gradients of the fuel-rich flow in the 12% case VOL. 44, NO. 16, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
6513
FIGURE 5. Local mean gas species concentrations in the zone near the water-cooled wall.
drop at distances further than 1600 mm and are exceeded by those for the other four OFA ratios. At distances farther than 1600 mm, the average temperature gradient sequence for OFA ratios is 25% > 20% > 31% > 35% > 12%, which is the same as the sequence determined using the pyrometer at a furnace height of 21 m. Figure 5 shows distributions of O2, CO, and NOx concentrations in zones near monitoring ports 1, 2, and 3. The measured NOx concentration is the ratio of the local NOx volume to the local gas volume. For the same volume of NOx production, a different gas volume leads to a different NOx concentration. Thus, here with different OFA ratios, the NOx concentrations are largely influenced by the different quantities of local secondary air that are supplied to the furnace and mix with the fuel-rich flow, which makes it impossible to reflect the real NOx production. To remove this influence from the mixing of secondary air, in constructing the curves in Figure 5, the NOx concentrations near port 1 are multiplied by the factor (1 - OFA ratio - F-tier secondary air ratio) and those near port 2 are multiplied by the factor (1 - OFA ratio). The F-tier secondary air ratios are listed in Table 1. If we trust in the reliability of the data in Figure 5, some explanation could be made as follows. For all OFA ratios, the O2 concentration sequence is port 1 > port 2 > port 3, which describes well the flow of coal as illustrated in each panel of Figure 2. As coal passes through the areas near monitoring ports 1, 2, and 3 in this sequence, O2 is consumed gradually. In this process, although some air is added from the F-tier secondary air ports and the OFA ports, the consumption of O2 seems faster. The figure also indicates that near monitoring ports 1 and 2, the O2 concentration approximately follows the decreasing sequence of OFA ratios of 12% > 20% > 25% 6514
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 44, NO. 16, 2010
> 31% > 35%. This shows that in the lower furnace, the O2 concentration sequence depends mainly on the secondary air flux fed from the secondary air ports under the arches and less on the combustion status. In the zone near port 3 above the arches, O2 concentrations are in the OFA ratio sequence 35% > (31%, 25%) > 20% > 12%. Since the supply of air to the furnace is complete in the zone near port 3, the O2 concentration there more or less reflects the combustion of the coal. The sequence indicates that with a lower OFA ratio, the combustion is more complete. In other words, the stoichiometry in the furnace determines the whole combustion process. In all five cases, the CO concentrations continue to rise as the coal passes through the zones near ports 1, 2, and 3. In the zone near port 3, the average CO concentrations are higher than 1500 ppm. This is due to the low stoichiometry in the lower furnace that forces combustion to occur in a lean-oxygen atmosphere in the lower furnace. In the zone near ports 2 and 3, the CO concentration sequence is OFA ratios of 35% > 31% > 25% > 20% > 12% owing to the stoichiometry sequence. Since the CO concentration in the 25% case is the highest in the zone near port 1, the coal combustion is the most intense in this case. The NOx concentration increases slowly from port 1 to port 3, which differs from the situation presented in the literature (12–14, 20). This indicates that the introduction of OFA lowers NOx emission. In the zone near monitoring port 1, the NOx production sequence is OFA ratios of 25% > 20% > 31% > 12% > 35%, which is the same as the sequence of the gas temperature at 21 m height shown in Figure 3 except for the 12% and 35% cases. In this zone, therefore, the coal combustion process controls NOx production. In the 35% case, the reducing atmosphere is so strong that NOx production is a minimum although combustion is
becomes large as the OFA ratio increases. When the OFA ratio increases from 12% to 25%, the amount of unburnt carbon increases only 1.5% from 6.53% to 8.04%, whereas when the OFA ratio increases from 25% to 35%, the unburnt carbon content increases 7.8% from 8.04% to 15.86%. The change of solid unburnt loss has the same trend as that of the unburnt carbon in fly ash. Figure 3 indicates that the reason for the difference might be that in the former range, the increase in the OFA ratio advances the coal combustion process, which more or less compensates the incomplete combustion. In the latter range, the increase in the OFA ratio not only provides a leaner-oxygen atmosphere but also delays the combustion. From this point of view, the OFA ratio should not exceed 25%. At this ratio, the thermal efficiency of the boiler is 90.72%, which is relatively acceptable. Considering both the unburnt carbon in fly ash and the NOx emission standards, the optimal OFA ratio for the studied down-fired boiler is 25%.
Acknowledgments This work was sponsored by the Hi-Tech Research and Development Program of China (863 program) (Contract 2006AA05Z321).
Literature Cited FIGURE 6. Effect of the OFA ratio on the NOx concentration, CO emission, unburnt carbon content, and boiler efficiency. more intense than in the 12% case. In the zone near port 2, the NOx concentrations for OFA ratios of 12% and 20% exceed the concentrations in the 25% case. The reason is that the relatively lean-oxygen atmosphere restricts NOx production in the F-tier airflow zone for an OFA ratio of 25%, although combustion is more intense than in the other two cases. For OFA ratios of 31% and 35%, the NOx concentration is low owing to the weak combustion and the lean-oxygen atmosphere. In this zone, therefore, the atmosphere is the main factor controlling NOx emission. In the zones near monitoring port 3, the NOx concentrations sequence is OFA ratios of 12% > 20% > 25% >31% > 35%; it is observed that the larger the OFA ratio is, the less NOx is produced. Figure 6 shows the change in CO, unburnt gas loss, and NOx concentrations in the flue gas as the OFA ratio varies. It is evident that the CO concentrations and unburnt gas loss are low in all five setups, albeit slightly higher for OFA ratios of 31% and 35%. NOx production decreases from 1308 to 966 mg/Nm3 (at 6% O2 dry) as the OFA ratio increases from 12% to 35%. With a larger OFA ratio, the quantity of gas in the lower furnace decreases, leading to a low upflow velocity there. Therefore, fuel NOx is restricted for longer time and in a stronger reducing atmosphere under the arches. Thermal NOx is also restricted as the peak temperature zone always corresponds to a reducing atmosphere. Thus, the total NOx emission decreases. According to Chinese NOx emission standards, NOx emission for low-volatile coal should not exceed 1100 mg/Nm3. Therefore, the OFA ratio should be set larger than 25%. Figure 6 also shows the changes in the amount of unburnt carbon, solid unburnt loss, and boiler thermal efficiency as the OFA ratio varies. It must be declared here that the Chinese standards of boiler thermal efficiency are based on net heating value, which may differ from those in the other countries. The carbon content in fly ash rises from 6.53% to 15.86% as the OFA ratio increases from 12% to 35%. The increase in the amount of unburnt carbon also
(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 VolatileChar Interaction on the NO Emission from Coal Combustion. Environ. Sci. Technol. 2008, 42, 4771–4776. (4) EPA. Alternative Control Technologies Document; EPA-453/R94-023; U.S. Environmental Protection Agency: Washington, DC, 1996; p 26. (5) Hill, S. C.; Smoot, L. D. Modeling of nitrogen oxides formation and destruction in combustion systems. Prog. Energy Combust. Sci. 2000, 26, 417–458. (6) McCahey, S.; McMullan, J. T.; Williams, B. C. Techno-economic analysis of NOx reduction technologies in p.f. boilers. Fuel 1999, 78, 1771–1778. (7) 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. (8) 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. (9) Li, S.; Xu, T. M.; Hui, S. E.; Wei, X. L. NOx emission and thermal efficiency of a 300 MWe utility boiler retrofitted by air staging. Appl. Energy 2009, 86, 1797–1803. (10) Dı´ez, L. I.; Corte´s, C.; Pallare´s, J. Numerical investigation of NOx emissions from a tangentially-fired utility boiler under conventional and overfire air operation. Fuel 2008, 87, 1259– 1269. (11) Pedersen, K. H.; Jensen, A. D.; Berg, M.; Olsen, L. H.; DamJohansen, K. The effect of combustion conditions in a fullscale low-NOx coal fired unit on fly ash properties for its application in concrete mixtures. Fuel Process. Technol. 2009, 90, 180–185. (12) Fan, W. D.; Lin, Z. C.; Kuang, J. G.; Li, Y. Y. Impact of air staging along furnace height on NOx emissions from pulverized combustion. Fuel Process. Technol. 2010; doi:10.1016/j.fuproc.2010.01.009. (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) 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. (15) 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 pulverizedVOL. 44, NO. 16, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
6515
coal 300 MWe utility boiler. Fuel Process. Technol. 2008, 89, 1297–1305. (16) 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. (17) 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. (18) Fang, Q. Y.; Wang, H. J.; Wei, Y.; Lei, L.; Duan, X. L.; Zhou, H. C. Numerical simulations of the slagging characteristics in a down-
6516
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 44, NO. 16, 2010
fired, pulverized-coal boiler furnace. Fuel Process. Technol. 2010, 91, 88–96. (19) Wang, H. J.; Huang, Z. F.; Wang, D. D.; Luo, Z. X.; Sun, Y. P.; Fang, Q. Y.; Lou, C.; Zhou, H. C. Measurements on flame temperature and its 3D distribution in a 660MWe arch-fired coal combustion furnace by visible image processing and verification by using infrared pyrometer. Meas. Sci. Technol. 2009, 20, 114006. (20) 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 (2), 410–416.
ES100956D
