Influence of Staged-Air on Airflow, Combustion Characteristics and NO

Jan 5, 2010 - Cold airflow experiments were conducted to investigate the aerodynamic field in a small-scale furnace of a down-fired pulverized-coal 30...
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Environ. Sci. Technol. 2010, 44, 1130–1136

Influence of Staged-Air on Airflow, Combustion Characteristics and NOx Emissions of a Down-Fired Pulverized-Coal 300 MWe Utility Boiler with Direct Flow Split Burners ZHENGQI LI,* MIN KUANG, JIA ZHANG, YUNFENG HAN, QUNYI ZHU, LIANJIE YANG, AND WEIGUANG KONG School of Energy Science and Engineering, Harbin Institute of Technology, 92, West Dazhi Street, Harbin 150001, People’s Republic of China

Received October 10, 2009. Revised manuscript received December 7, 2009. Accepted December 17, 2009.

Cold airflow experiments were conducted to investigate the aerodynamic field in a small-scale furnace of a downfired pulverized-coal 300 MWe utility boiler arranged with direct flow split burners enriched by cyclones. By increasing the staged-air ratio, a deflected flow field appeared in the lower furnace; larger staged-air ratios produced larger deflections. Industrial-sized experiments on a full-scale boiler were also performed at different staged-air damper openings with measurements taken of gas temperatures in the burner region and near the right-side wall, wall heat fluxes, and gas components (O2, CO, and NOx) in the near-wall region. Combustion was unstable at staged-air damper openings below 30%. For openings of 30% and 40%, late ignition of the pulverized coal developed and large differences arose in gas temperatures and heat fluxes between the regions near the front and rear walls. In conjunction, carbon content in the fly ash was high and boiler efficiency was low with high NOx emission above 1200 mg/m3 (at 6% O2 dry). For fully open dampers, differences in gas temperatures and heat fluxes, carbon in fly ash and NOx emission decreased yielding an increase in boiler efficiency. The optimal setting is fully open staged-air dampers.

Introduction NOx is an extremely toxic pollutant harmful to human health and detrimental to the atmosphere. Its main source derives from primary emissions of coal-fired power plants into the air (1–3). Down-fired boilers are designed to burn anthracite and lean coal, but unfortunately, greater quantities of NOx are produced than in tangential-fired furnaces and wallarranged boilers. Four types of down-fired utility boilers are manufactured: the Foster Wheeler (FW), the Babcock & Wilcox (B&W), the Stein, and the Mitsui Babcock Energy Limited (MBEL) down-fired boilers. The Supporting Information summarizes these four types and the main differences between them. All suffer similarly from problems of poor * Corresponding author phone: +86 451 8641 8854; fax: +86 451 8641 2528; e-mail: [email protected]. 1130

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stability, low burnout and high NOx emissions. Actual operating results usually show that the carbon in fly ash is 10%-15%, the boiler efficiency is about 85%, and the NOx emissions are above 1300 mg/m3 (at 6% O2 dry). For FW boilers, reports have appeared on flow characteristics, NOx emissions, and reductions by overfire air application (4, 5). Combustion characteristics and NOx emissions of B&W boilers have also been investigated (6). Burdett (7) has carried out industrial tests on a 500 MWe Stein boiler to investigate air-staging effects on NOx emissions. However, no research reports appears on MBEL boilers, aspects of which are partially addressed in this work. Aerodynamic characteristics are of great importance in pulverized-coal combustion. Performing experiments with small-scale models is widely popular in obtaining approximate aerodynamic characteristics with boilers. Gas temperatures and components, heat fluxes, and NOx emissions of a boiler are obtainable by industrial experiments (6, 8–10). To acquire flow-field influences on combustion characteristics and NOx emissions specifically for a MBEL down-fired 300 MWe utility boiler, cold airflow experiments with a small-scale furnace were conducted to obtain aerodynamic fields at different staged-air distributions. On a fullscale boiler under industrial settings, measurements also were taken of gas temperatures in the burner region and near the right side wall, wall heat fluxes, and gas components in the near-wall region at various staged-air damper openings.

Experimental Section Utility Boiler. Vertical and horizontal cross sections of the furnace showing the layout of the combustion system are presented in Figure 1. The arches separate the furnace into two regions, the rectangular upper furnace and the octagonal lower furnace with four wing walls. Sixteen cyclones symmetrically arranged on the arches divide the primary air/ fuel mixture into fuel-rich and fuel-lean flows needed in regulating fuel-bias combustion. The fuel-rich flow is channeled through nozzles centered over the furnace, while the fuel-lean flow is injected through nozzles near the front and rear walls. There are eight burners lining the front and rear arches. Four fuel-rich flow nozzles, four fuel-lean flow nozzles, and eight secondary-air ports fed each burner. The secondary air box is partitioned in two, one on the arches and the other below. Each part has eight small boxes, each box on the arches corresponding to one of the burners while each box below the arches feeds a group of staged-air ports. The distribution of secondary air and staged-air is adjustable by damper openings associated with each box. Small-Scale Cold Aerodynamic Experiments. The experimental apparatus is a 1:15 scaled version of the original and presented in Figure S1 of the Supporting Information. Venturi tube flowmeters measured all airflow rates into the small-scale furnace. Measurement errors were less than 10%. An IFA300 constant-temperature anemometer system measured the air velocity at various locations within the furnace. Errors for these velocity measurements were less than 5%. An important concept in understanding small-scale experiments is the staged-air ratio defined as the ratio of the stagedair mass flux to the total air mass flux into the small-scale furnace. To investigate the influence of staged-air ratios on the flow field four different values, viz., 0%, 4%, 10%, and 15%, were chosen. The velocities of the fuel-rich and fuellean flows are 12.80 and 20.58 m/s, corresponding to mass flow rates of 0.096 and 0.116 kg/s, respectively. The second air velocities are 33.04, 31.44, 28.99, and 27.00 m/s and those of staged-air are 0, 5.62, 14.04, and 21.07 m/s corresponding 10.1021/es903085v

 2010 American Chemical Society

Published on Web 01/05/2010

FIGURE 1. Vertical and horizontal cross sections through the furnace of the down-fired boiler showing combustion system and monitoring port layout in industrial experiments (dimensions in mm). to the four chosen values, respectively. Corresponding mass flow rates of secondary air are 0.967, 0.920, 0.848, and 0.790 kg/s and those of staged-air are 0, 0.055, 0.137, and 0.205 kg/s. Cold aerodynamic fields were measured along the longitudinal cross-section intersecting the vertical centerline of one of the fuel-rich nozzles. Industrial Experiments. At a load of 300 MWe, industrial experiments were conducted at various staged-air damper openings. With openings below 30%, the negative gas pressure in the furnace fluctuated significantly and combustion was unstable. In consequence, we performed three experiments with staged-air damper openings of 30%, 40%, and 100%. The staged-air mass flow rates are about 22, 32, and 38 kg/s, corresponding to staged-air ratios of 7%, 10% and 12%, respectively. It should be emphasized that, during experimental procedures, 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. Throughout the experiments, the coal used was anthracite from the Nayong region in China. Because coal characteristics vary from one day to the next, we performed three successive experiments to obtain comparable data using similar coal. Table 1 lists coal characteristics and design parameters as well as averaged operating parameters and heat fluxes measured through the viewing ports over the duration of each experimental run lasting for five hours. During the experiments, the following measurements were made: (1) Gas temperature in burner region. Along the sight port pipe, a thermocouple device with a 0.3-mm-diameter and 10-m-length nickel-chromium/nickel-silicon wire located in a 6-mm-diameter sheath placed inside a 8.5-m stainless steel tube was inserted into the furnace through the secondary-air port next to the flow-rich nozzle. The end of the sheath was exposed in the furnace while taking a temperature measurement. The specific locations at which the thermocouple was inserted are shown in Figure 1; (2) VOL. 44, NO. 3, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Aerodynamic fields for the cases with different staged-air ratios.

TABLE 1. Coal Characteristics, Boiler Design and Operation Parameters and Wall Heat Fluxes proximate analysis, wt.% (as received) volatile matter 7.71

ash 35.02

moisture 8.1

case flow rate of main steam (ton/h) temperature of main steam (°C) total flux of primary air (Nm3/s) temperature of primary air (°C) total flux of secondary air (Nm3/s) temperature of the secondary air (°C) coal feed rate (ton/h) NOx in flue gas (mg/m3 at 6% O2 dry) O2 at the furnace exit (dry volume %) carbon in fly ash (%) carbon in slag (%) boiler efficiency (%) port 1 port 2 heat flux (kW/m2) port 3 port 4

ultimate analysis, wt.% (as received)

fixed carbon 49.17 design

net heating value (kJ/kg) C 18640 50.45 operation parameters in experiments

parameters

30%

40%

100%

909.6 540 35.1 115 196.2 339 113 650 3.45 9.0 14.6 91.63

846.4 539.9 42.4 96 209.6 358 153 1252 2.90 15.03 13.71 83.42 238 312 116 105

843.5 542.3 42.8 97 211.3 354 146 1230 2.79 13.87 11.76 84.24 245 311 121 109

847.5 537.9 41.1 99 212.5 362 144 1089 2.80 10.51 12.52 86.41 251 271 212 189

Local gas temperatures and components. The same thermocouple and a 5-m-long water-cooled stainless steel probe, comprising a centrally located 10-mm-i.d. tube surrounded by a tube for probe cooling, were inserted in turn into the furnace through monitoring ports 1-4 perpendicular to the wing wall. These measured local gas temperatures and captured gas samples to be analyzed online by a Testo 350 M instrument (Figure 1). On inserting the thermocouple or water-cooled probe into furnace through the port, gaps in the port entrance were plugged with a piece of 50-mm-thick asbestos. The probe was cleaned frequently by blowing highpressure air through it to maintain a constant suction rate. We performed zero and span calibrations with standard mixtures before and after each measurement session. The measurement error associated with the Testo 350 M was 1% for O2, 5% for CO and 50 ppm for NOx. Radiation losses originating from radiation from the gas to the thermocouple and from the thermocouple to the surrounding wall represent the major source of uncertainty in the temperature measurements. Calculations indicate that in regions of highest temperatures, the “true” temperature does not exceed the measured values by more than 8% (8, 11). An additional source of uncertainty relates to ash deposition on the thermocouple. This is avoided by retracting the thermocouple from the furnace within 60 s. After retraction, we examined the probe for any deposition, which was then removed. The 1132

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H 2.10

S 1.67

N 0.81

O 1.85

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. Quantifying probe flow disturbances was not attempted; (3) Wall heat fluxes. Heat flux measurements were recorded through monitoring ports 1-4 using a heat flux meter with a measuring range of 0-1200 kW/m2 and an error of 2%.

Results and Discussion Aerodynamic Field. As shown in Figure 2, for staged-air ratios of 0% and 4%, the aerodynamic field is essentially symmetric in the zone near the front and rear walls; the airflow velocity is highest in the dry bottom hopper. After penetrating the dry bottom hopper region, airflow near the rear wall deflects upward toward the region near the front wall suppressing the airflow near the front wall and forcing it closer to the dry bottom hopper to penetrate further. A large recirculation zone appears in the lower furnace below the rear arch. As the ratio increases to 10% and 15%, the downward airflow near the rear wall deflects upward sooner and the deflection angle evidently steepens, leaving a low-pressure region near the right side of the bottom hopper to allow the airflow near the left side of the bottom hopper partly flowing into after

FIGURE 3. Decay curve comparison of the longitudinal velocity component of the fuel-rich flow with respect to different staged-air ratios. it turning upward. For this reason, an oppositely directed velocity factor appears to that for 0% and 4% staged-air ratios. The phenomenon is more clearly observed at a staged-air ratio of 15%. The recirculation zone gets larger and draws near the rear arch, while decreasing both the airflow velocity in the dry bottom hopper and the depth that the airflow reaches near the rear wall. Repetition of measurements for one setting demonstrated that the flow field deflection is steady. Figure 3 presents a comparison of decay curves of the fuel-rich flow with respect to various staged-air ratios. Here (Vy)max is the largest longitudinal velocity component of the fuel-rich flow at fixed measurement point depths below the front and rear arches. V0 signifies the outlet velocity of the fuel-rich nozzle along the y-direction. H0 is the vertical distance between the outlet of the fuel-rich nozzle and the upper edge of the furnace hopper, while H is the vertical distance from the measurement point to the outlet of the fuel-rich nozzle (Figure S1 of the Supporting Information). After the fuel-rich flow leaves the outlet of nozzles, the values of (Vy)max/V0 in the zone below the front and rear arches are greater than unity. This is to say flow velocities are greater than their outlet velocities. Subsequently, velocities increase at a high rate initially, but then decrease fast after attaining maxima at H/H0 ) 0.1-0.15. This is because secondary-air velocities are far greater than are those of the fuel-rich flow, although the latter rises with secondary air mixing. After slight increments at H/H0 ) 0.38-0.45, flow velocities near the front wall remains essentially the same while that near the rear wall keeps decaying. Increasing the staged-air ratio decreases the velocity of the fuel-rich flow. Moreover, its decay quickens because of the decrease in secondary air velocity and the total momentum ratios of secondary air and the fuel-rich flow. The flow field deflection clearly results in different decays near the front and rear walls. The deflection reasons are as follows: First, before the two upward airflows near the front and rear walls flow through the entrance of the upper furnace, flow suppression between the two upward airflows is strong as the ratio between the width at the entrance of the upper furnace (W2) and the width of the lower furnace (W1) is small (Figure 1). In association with their co-operating flow suppression, a deflected flow field forms easily as some difference exists between the two airflows. For 300 MWe down-fired boilers, W2/W1 values are 0.54, 0.54, and 0.51 for FW, B&W, and Stein boilers, respectively (4, 6, 12), their large values corresponding to weak suppression and flow fields were all symmetric within the furnaces. Here the W2/W1 is smaller at 0.47, yielding stronger suppression between the two airflows, which counts as one factor favoring a ready formation of a deflected flow

field within this boiler. Second, boiler noise may be the second factor that imposes some effect on the upward flow near the rear wall causing some difference from the upward flow near the front wall. Third, depending on angle and ratio of the staged-air when injected into furnace, i.e., either horizontally or at a small declination angle (the angle between the stagedair direction and the horizontal), staged-air has a large transverse momentum component providing a strong impetus on the two downward airflows. When injecting stagedair horizontally, staged-air ratios of 10% and 15% evidently enhance this impetus. This may be contributing toward the ready formation of a deflected flow field. In addition, the large span of the secondary air port, located on the center of an arch and covering almost half of the arch along the W1 direction (Figure 1), and the high ratio of secondary air (about 70% of total air mass flux) may be factors enhancing the suppression between the two upward airflows leading to the formation of a deflected flow field. The above analysis represents a preliminary study of this phenomenon and further investigation will be forthcoming. Industrial Experiments. Figure 4 presents various gas temperature profiles in the burner region and the zone near the wing walls. Profiles in this burner region are given in Figure 4(a),(b). Here, distances are measured from the outlets of the secondary-air ports to where the thermocouple was inserted into the furnace (Figure 1). These profiles show the process of coal ignition. For burner 1 on the rear arch, temperatures along the sight port pipe are higher than 650 °C at measurement points beyond 0.8 m from the outlet. Temperatures near the outlet and in burner region all rise as staged-air dampers are opened. For burner 2 on the front arch, temperatures within 1.6 m from the outlet along the sight port pipe are all below 500 °C then rise quickly with increasing distance. Temperatures also rise on opening the staged-air dampers with the exception of 30% opening for which the temperature within 1.6 m from the outlet is higher than that for the wider openings due to combustion fluctuations. Temperatures near the outlet and in the burner region of burner 2 are lower than those of burner 1. Results from the cold aerodynamic experiments explain this observation based on the similarity of modeling criteria. Temperature measurement points near the burner outlet are near the furnace center. In the large recirculation zone that exists in the lower region below the rear arch, high temperature recirculation gas mixes with the downward flow of pulverized coal-air to enhance combustion. A large heat release greatly increases temperatures in the burner region below the rear arch. After leaving the burner outlet, secondary air very easily mixes with the high-temperature recirculation gas near the furnace center. Thus, the gas temperature measured near VOL. 44, NO. 3, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Gas temperatures in burner region and the zone near the wing walls with different staged-air damper openings. the burner outlet is actually the gas temperature of the mixture of the high-temperature recirculation gas near the furnace center and secondary air near the burner outlet. In the absence of a recirculation zone such as that below the rear arch, temperatures are relatively lower in the lower part of the zone below the front arch. Gas temperatures measured near the burner outlet are the actual temperature of secondary air. Thus, temperatures near the outlet of burner 2 are lower than those near burner 1 and change slightly. With increasing staged-air ratios, the recirculation zone enlarges and approaches the rear arch, allowing high temperature recirculation gas to reach the outlet of the fuel-rich flow. Decreasing secondary air velocities then result in a decline in velocities of the downward flow of pulverized coal-air in the burner region. Therefore, temperatures near the outlet and in the burner region rise below the rear arch on opening the stagedair dampers. Concurrent with increasing temperatures in this region, temperatures rise in the upward flowing gas deflected toward the region near the front wall. Thus, temperatures in the burner region below the front arch also rise with an increased staged-air damper opening. Gas temperature profiles in the zone near the wing walls are presented in Figures 4(c)-(f). Monitoring ports 1 and 2, and monitoring ports 3 and 4 are close to the rear and front walls, respectively (Figure 1). Gas temperatures measured through ports 1 and 3 indicate temperatures of the flow of pulverized coal-air at an early stage of ignition, while those measured through ports 2 and 4 are gas temperatures near 1134

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the staged-air region. In the region near ports 1 and 2, gas temperatures fall within a range of 1100-1200 °C within 0.4 m from the wing wall in the lower furnace. Gas temperatures in the region near port 2 are slightly higher than are those near port 1. As the staged-air damper opening increases, the temperatures near port 1 rise while those near port 2 decrease. This is because ports 1 and 2 are near the burner and stagedair port, respectively. As discussed earlier (Figure 4(a)), temperatures in the burner region rise and staged-air mixing with the gas near port 2 increases as the staged-air dampers open. In contrast, gas temperatures in the area near ports 3 and 4 increase as staged-air dampers open. The reason for this runs along similar lines to that discussed earlier concerning Figure 4(b). Temperatures rise in the upward flowing gas deflected toward the region near the front wall that raises temperatures in the region near the front wall. Additional staged-air near port 4 slightly lowers gas temperatures compared with those near port 3. Gas temperatures near port 3 rise with distance from the wing wall in the range of 0-1.2 m due to high-temperature gas accumulating in this region. At distances beyond 1.2 m from the wing wall, gas temperatures decrease with distance because measurement points encroach on the burner near the wing wall (Figure 1) and measurements are affected by the unburnt flow in burner region below the front arch (Figure 5 (b)). Gas temperatures though are stable in the area near port 4. It can be seen that gas temperatures in the region near the rear wall are significantly higher than those near the front wall,

FIGURE 5. Local mean gas species concentrations in the zone near the wing walls with different staged-air damper openings. and temperature differences between the two regions with fully opened dampers is significantly lower than those at 30% and 40% openings. Heat flux measurements are given in Table 1. Heat fluxes in the region near the rear wall (measured through ports 1 and 2) are significantly higher than are those near the front wall (measured through ports 3 and 4). With increasing staged-air damper opening, only the heat fluxes measured through port 2 decrease; in contrast, those measured through the other three ports increase. At full opening, the difference in heat flux between the front and rear walls regions is obviously lower than that at 30% and 40% openings. Thus, the flux distribution is similar to gas temperature distributions shown in Figure 4(c)-(f). Figure 5 presents gas component profiles near the wing walls. In the region near port 1, O2 and NOx content decrease while CO content increases on increasing the staged-air damper opening. This is because secondary air diminishes which produces a reductive atmosphere in the burner zone, inhibiting then the formation of NOx. In the region near port 2, opening the staged-air dampers increases O2 content and

decreases NOx content, while CO content fluctuates slightly. This is because of an increase in staged-air mixing with gas and a decrease in thermal-NOx due to a decrease in gas temperatures (Figure 4(d)). Compared with that near port 1, the O2 content near port 2 is lower at a 30% opening but higher at openings of 40% and 100% because of an increase in the staged-air distribution. Meanwhile, NOx content near port 2 is continuously lower due to an increase in staged-air mixing with gas. In the areas near port 3 and port 4, an increasing staged-air damper opening decreases O2 content and increases NOx content, while CO content changes slightly. This is because, due to rising gas temperatures, combustion in these regions proceeds well consuming more O2 and producing more NOx (Figure 4(e)-(f)). In addition, the clearly higher NOx content at 100% opening over than at 30-40% opening is partly attributed to the higher temperatures that promotes thermal-NOx formation in the regions. Because of the effect of the unburned flow in the burner region below the front arch, O2 content increases with distance, while NOx content decreases in the area near port 3 at distances beyond 1.2 m from the wing wall. The O2, CO, and NOx content are VOL. 44, NO. 3, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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similar near the two ports at openings of 30% and 40%, but fully opened, the O2 content is higher and that for NOx lower in the area near port 4 compared with near port 3. This is due to increased staged-air mixing. In the area near port 1, the changing trend in O2 and NOx content with respect to staged-air damper opening is the same as that near port 3. For openings of 30% and 40%, as well as full opening but only at distances from the wing wall beyond 1.2 m, the O2 content is lower and the NOx content is higher in the area near port 1 compared with near port 3, but the CO content is almost the same near the two ports. This is because combustion is more complete and gas temperatures are higher in the burner region near the rear wall than that near the front wall due to the deflected flow field. Thus, more O2 is consumed and more NOx is produced in the area near port 1 than port 3. As seen from Table 1, at the furnace exit O2 content changes slightly. Carbon in fly ash and NOx emissions are clearly higher but boiler efficiencies are greatly lower than the designed parameters. This is because the deflected flow field results in lean combustion conditions in the region near front and rear wall. Low temperatures and poor burnout in the region near the front wall leads to high carbon content in the fly ash throughout the whole boiler. High carbon in fly ash and lower-grade coal different from designed coal (with a net heating value of 23 354 kJ/kg as received) produce during operations the notoriously low boiler efficiencies. Rapid mixing of secondary air with the fuel-rich flow after leaving the port outlets (Figure 3) accounts for a major proportion of high NOx emissions. Compared with openings of 30% and 40%, the boiler has lower carbon in fly ash, lower NOx in flue gas and higher boiler efficiency when the staged-air dampers are fully open. These circumstances are due to higher gas temperatures and burnout rates in the region near the front wall at full opening. Just by increasing the opening from 30% to 100%, carbon in fly ash drops by 4.52% points, boiler efficiency rises by 2.99% points. In addition, NOx emissions decrease by 13% due to thinning secondary air, and an enhanced reductive atmosphere in the burner region that evidently inhibits fuel-NOx formations (9, 13). Nevertheless, because of a definite rise in gas temperatures in the furnace, a slight increase results in thermal-NOx formation. It can be concluded that results of fully open dampers show earlier ignition and smaller differences in gas temperatures and components distribution between the regions near the front and rear walls. Combustion improved sufficiently in the region near the front wall to decrease carbon in fly ash, yielding an increase in boiler efficiency. In addition, NOx emission is reduced by 13%. Thus, the optimal setting is fully open staged-air dampers, which is now the setting the boiler operates at following the experimental investigation to take advantage of this optimization.

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

Supporting Information Available A summary is provided of the four types of down-fired boilers and the main differences between them. A schematic of the cold airflow experiment system is presented in Figure S1. This material is available free of charge via the Internet at http://pubs.acs.org.

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