Overall Evaluation of Combustion and NOx Emissions for a Down

Article pubs.acs.org/est

Overall Evaluation of Combustion and NOx Emissions for a DownFired 600 MWe Supercritical Boiler with Multiple Injection and Multiple Staging Min Kuang, Zhengqi Li,* Chunlong Liu, and Qunyi Zhu School of Energy Science and Engineering, Harbin Institute of Technology, 92, West Dazhi Street, Harbin 150001, P.R. China S Supporting Information *

ABSTRACT: To achieve significant reductions in NOx emissions and to eliminate strongly asymmetric combustion found in down-fired boilers, a deep-air-staging combustion technology was trialed in a down-fired 600 MWe supercritical utility boiler. By performing industrial-sized measurements taken of gas temperatures and species concentrations in the near wing-wall region, carbon in fly ash and NOx emissions at various settings, effects of overfire air (OFA) and staged-air damper openings on combustion characteristics, and NOx emissions within the furnace were experimentally determined. With increasing the OFA damper opening, both fluctuations in NOx emissions and carbon in fly ash were initially slightly over OFA damper openings of 0−40% but then lengthened dramatically in openings of 40−70% (i.e., NOx emissions reduced sharply accompanied by an apparent increase in carbon in fly ash). Decreasing the staged-air declination angle clearly increased the combustible loss but slightly influenced NOx emissions. In comparison with OFA, the stagedair influence on combustion and NOx emissions was clearly weaker. Only at a high OFA damper opening of 50%, the staged-air effect was relatively clear, i.e., enlarging the staged-air damper opening decreased carbon in fly ash and slightly raised NOx emissions. By sharply opening the OFA damper to deepen the air-staging conditions, although NOx emissions could finally reduce to 503 mg/m3 at 6% O2 (i.e., an ultralow NOx level for down-fired furnaces), carbon in fly ash jumped sharply to 15.10%. For economical and environment-friendly boiler operations, an optimal damper opening combination (i.e., 60%, 50%, and 50% for secondary air, staged-air, and OFA damper openings, respectively) was recommended for the furnace, at which carbon in fly ash and NOx emissions attained levels of about 10% and 850 mg/m3 at 6% O2, respectively.

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INTRODUCTION NOx is an extremely toxic pollutant that is harmful to human health and detrimental to the atmosphere. Its main source derives from primary emissions of coal-fired power plants into the air.1,2 Currently, low-volatile fuels such as anthracite and lean coal are widely used in power generators in the world but, unfortunately, large quantities of NOx are produced by burning these fuels.3−7 Down-fired boilers, designed especially for industry to fire anthracite and lean coal, are thought to be better than tangential-fired and wall-arranged furnaces for burning these fuels.3,6 However, problems such as late ignition and poor combustion stability,8 heavy slagging,9 poor burnout,8,10,11 particularly high NOx emissions (typically in the range 1400−2100 mg/m 3 at 6% O 2 ),3,8,12,13 and asymmetric combustion10,11 are widely present in practical operations of down-fired boilers. Accordingly, much research dealing with these problems has reported various solutions, such as burning blended coals to advance coal ignition and raise combustion stability,6,8 shutting down burners close to the side walls to alleviate heavy slagging,9 inclining downward the Flayer secondary air to improve burnout,14 and parametric tuning of operating conditions or combustion system retrofits to reduce NOx emissions.3,12,13,15,16 In view of the high gas temperatures and long residence times needed to achieve good burnout in down-fired boilers, © 2013 American Chemical Society

reducing the high NOx emissions to acceptable levels by forming deep-air-staging conditions (such as those in refs 17−21) but without apparently increasing combustible loss should be a huge challenge. Again, poor combustion stability, heavy slagging, and asymmetric combustion in these furnaces compound difficulties in NOx reduction. To deal with these problems, Kuang et al. put forward a comprehensive deep-airstaging combustion technology based on the concept of multiple injection and multiple staging (MIMSC).22 The proposed MIMSC technology was first trialed in a newly designed, down-fired 600 MWe supercritical utility boiler. Because the boiler was of large capacity and with a high steam parameter, the first industrial application of the MIMSC technology followed a low risk course in not installing an OFA system, which was essentially a part of MIMSC technology. In the present study, this was called the first step of the technology application. However, industrial operation results after this step uncovered that, aside from the reheated steam temperature being lower than the designed value by about 40 °C (because of the unreasonable design in the heating surface of the reheater), Received: Revised: Accepted: Published: 4850

November 19, 2012 March 25, 2013 March 26, 2013 March 26, 2013 dx.doi.org/10.1021/es304492j | Environ. Sci. Technol. 2013, 47, 4850−4858

Environmental Science & Technology

Article

Figure 1. Schematics of furnace and combustion system of the down-fired 600 MWe supercritical boiler as well as the monitoring port layout in industrial experiments.

NOx emissions remained still relatively high. According to the recently announced emission standards for power plants in China, the permissible NOx emissions for down-fired boilers are 200 mg/m3 at 6% O2 as of July 1, 2014. To improve the reheated steam temperature (by elevating the flame kernel position and increasing the combustion share in the upper furnace) and comply with the new emission standards, recently adding OFA was performed in the furnace, in addition to adjusting the staged-air declination angle from 45° to 20°. In this study, the OFA addition was called the second step of the technology application. To understand well the coal combustion and NOx formation characteristics with the full technology and to evaluate effects of the staged-air angle adjustment and OFA addition on the furnace performance,

detailed industrial-sized data on pulverized-coal combustion and NOx emissions within the furnace must be acquired after each step application. This study focuses on reporting the findings at various boiler’s operating settings with the full technology. Additionally, the comparison of results before and after the second step was made in this study. The information in this study is useful to establish deep-air-staging conditions for modifications of down-fired furnaces already in service and for new designs. Considering that we have published a series of investigations10,11,22,23 into the flow, combustion, and NOx emission characteristics within down-fired furnaces, differences between this work and our previous studies10,11,22,23 were summarized in the Supporting Information so as to highlight 4851

dx.doi.org/10.1021/es304492j | Environ. Sci. Technol. 2013, 47, 4850−4858

4852

53.72 2.44 2.77 0.84 2.99

51.50 2.12 1.49 0.85 3.83

7.60 8.95 32.61 50.84 19.46

45° 600 2.54 17 1292 91.52

0° 600 8.5 34 1467 87.83

7.40 7.94 30.04 54.62 20.40

2 60% 30%

1 60% 30%

Section (i)a Section (ii)

57.48 2.08 1.59 0.79 3.29

7.52 7.58 27.25 57.65 21.70

3 60% 30% 0% 20° 600 7.84 14 1289 89.04

4 60% 30% 15% 20° 600 5.94 13 1244 90.00

6 7 8 60% 60% 60% 30% 30% 30% 40% 50% 70% 20° 20° 20° 600 600 600 7.74 11.47 15.10 72 338 572 1047 823 503 88.57 86.61 83.97 Analysis, wt % (as received)

Ultimate Analysis, wt % (as received)

5 60% 30% 30% 20° 600 7.78 42 1077 89.03 Proximate

OFA panel

56.38 2.02 1.01 0.78 3.34

7.76 7.26 28.71 56.27 21.25

9 60% 10% 50% 20° 600 13.05 176 696 85.30

10 60% 20% 50% 20° 600 11.85 283 739 86.12

11 60% 30% 50% 20° 600 11.47 338 823 86.61

high OFA opening

Section (iii)

12 60% 50% 50% 20° 600 9.81 65 878 86.70

54.86 2.02 1.13 0.76 3.30

7.83 7.36 30.10 54.71 20.63

13 60% 10% 15% 20° 550 4.87 29 1365 89.97

staged-air panel 14 60% 20% 15% 20° 550 5.22 28 1331 89.85

15 60% 30% 15% 20° 550 5.24 28 1234 89.72

low OFA opening 16 60% 50% 15% 20° 550 4.47 32 1360 90.22

a Data are from the literature.23 Cases 7 and 11 are indeed one case, which is used twice with different assigned case number; case 4 shares the same damper openings with those of case 15 except for the boiler load.

carbon hydrogen oxygen nitrogen sulfur

moisture volatile matter ash fixed carbon net heating value (MJ/kg)

case number secondary-air damper opening staged-air damper opening OFA damper opening staged-air declination angle boiler load (MWe) carbon in fly ash (%) CO in flue gas (ppm) NOx emissions (mg/m3, 6% O2) boiler efficiency (%)

Quantity

Table 1. Detailed Damper Opening Settings, Major Operational Results, and Coal Characteristics for Sections (i)−(iii)

Environmental Science & Technology Article



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Figure 2. Gas temperature distributions in the furnace under the circumstances of sections (i)−(iii) (temperature data are acquired using a handheld pyrometer; data in panel a are from the literature23).

the main points of the present contribution as compared to our previous work.

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technical principles of the MIMSC technology can be found in a numerical investigation.22 After the second step application, the present combustion configuration differs from that described in the literature22 in two aspects: (i) applying a much shallower staged-air declination angle of 20°, instead of the originally designed 45°; (ii) supplying OFA at a set declination of 20°, instead of 40° (i.e., the OFA angle designed for a 350 MWe down-fired furnace in the literature22). The OFA angle adjustment lies on the fact that the present upper furnace depth (i.e., the distance 12 512 mm in Figure 1) is much larger than that (i.e., 7176 mm) of the 350 MWe downfired furnace. OFA is thus supplied at a much shallower

EXPERIMENTAL SECTION

Utility Boiler. As shown in Figure 1, the arches divide the furnace into two: the octagonal lower furnace with four wing walls and the rectangular upper furnace. 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 fuel rich/lean combustion. Introductory material about the combustion system, air distribution, and 4853



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declination angle of 20° (optimized previously by cold modeling experiments), thereby maintaining a relatively high transverse momentum of OFA to reach the furnace central part. During the second step application, each of the original secondary-air box fixed symmetrically on furnace arches is divided into two layers; the upper layer supplies OFA whose flow rate is controlled by OFA damper, and the lower layer allows secondary air flowing. By changing secondary-air, stagedair, and OFA damper openings, boiler managers can adjust the furnace air distribution to address various combustion conditions. Industrial-Sized Measurements. To achieve the three objectives, that is, confirming the validation of the MIMSC technology in eliminating asymmetric combustion and reducing NOx emissions, evaluating effects of the staged-air angle adjustment, and understanding well coal combustion and NOx emissions characteristics in the furnace with the full MIMSC technology, at full load (or a 550 MWe load) three sections of industrial-sized data needed to be presented in this study. These are: (i) at a conventional operation setting (60% and 30% for secondary-air and staged-air damper openings, respectively) within another 600 MWe supercritical furnace (whose furnace dimensions are the same as those of the present furnace) with a prior MBEL art reported in the literature;23 (ii) at a conventional operation setting (the same damper openings as those in section (i)) within the present furnace after the first step application; (iii) at various operation settings (i.e., changing OFA and staged-air damper openings) within the present furnace after the second-step application. This means that various damper opening settings needed to be established for section (iii): First, to evaluate the effect of OFA on coal combustion and NOx emissions, the OFA damper opening was adjusted in turn to 0%, 15%, 30%, 40%, 50%, and 70% settings, with the secondary-air and staged-air damper openings fixed at 60% and 30%, respectively; second, to determine the staged-air effect, the staged-air damper opening was adjusted in turn to settings of 10%, 20%, 30%, and 50%, whereas a 60% opening for the secondary-air damper remained unchanged and the OFA damper opening was fixed at 50% and 15%, respectively. To highlight their differences, detailed damper opening settings for sections (i)−(iii) are listed in Table 1 accompanied by the assigned number for each case. In this study, the data of section (i) are from the literature,23 whereas those of sections (ii) and (iii) were acquired by parts of or the whole industrial-sized measurements described as follows: (1) General gas temperature distribution in the 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 through each observation port (Figure 1) on the wing walls to measure the highest gas temperatures in the near-wall regions. These temperature data were used to verify this new technology in establishing symmetric combustion and to evaluate mainly the effect of the OFA damper opening on the general gas temperature level in the furnace. (2) Local gas temperatures and species concentrations in the near-wall region. Assured that symmetric combustion had developed for all experimental settings after the first and second steps (Figure 2), we thus selected only the front-half part of the furnace to perform these gas

temperature and species concentration measurements. A thermocouple device (with a 0.3 mm diameter and 8 m length nickel−chromium/nickel−silicon wire) and a 3 m long water-cooled stainless steel probe comprising a centrally located 10 mm i.d. sampling pipe (with a gas sampling flow rate of 1 L/min) surrounded by a 60 mm i.d. stainless steel tube with a high-pressure water flow rate of 60 L/min for probe cooling were inserted in turn into the furnace through observation ports 1 and 2 (Figure 1) parallel to the front wall. On inserting the thermocouple or water-cooled probe through ports, asbestos was used to plug gaps in each port entrance to avoid air leaks. The captured gas samples were analyzed online by a Testo 350 M instrument; measurement errors were estimated to be 1% for O2, 5% for CO, and 50 ppm for NOx. The major sources of uncertainty in gas concentration measurements were associated with the quenching of chemical reactions and aerodynamic disturbances of the flow. Because of the high gas velocity and low gas flow rate in the thin sampling pipe and the high flow rate of the cooling water in the external tube, the water-cooling rate was particularly high and quenching of the chemical reactions was rapidly achieved upon samples being drawn into the probe. Calculated from the inlet and outlet temperatures of the captured gas samples and the required residence times for cooling gas samples in the sampling pipe, the estimated quenching rates were approximately 106 K/sec. Quantifying probe flow disturbances was not attempted. With respect to the gas temperature measurements performed by thermocouple device, 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%.24,25 Accordingly, bare thermocouples were used here because radiation error would be low due to the wall refractory lining in the lower furnace. An additional source of uncertainty in the gas temperature measurement relates to soot or ash deposition on the thermocouple. To avoid heavy deposition while taking measurements, the thermocouple was frequently examined and any depositions found were removed. The relatively small differences in coal characteristics (listed in Table 1) and values of the main operating parameters (given in the Supporting Information) indicate that the measuring results are comparable among sections (i)−(iii). This article presents in detail various industrial-sized data in section (ii) and at three typical OFA damper openings (i.e., 0%, 30%, and 50%) in section (iii) to evaluate mainly effects of the staged-air angle adjustment and OFA flux on combustion and NOx emissions. For those cases in Table 1, only important data such as gas temperature distribution, carbon in fly ash, NOx emissions, and boiler efficiency are provided.

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RESULTS AND DISCUSSION Before providing various industrial-sized results before and after OFA addition and at various damper openings, the partitioning of the combustion zones in the furnace is given as follows: (1) In the region below the furnace arch but not far from the burner outlets, the pulverized-coal is ignited and the two-stage secondary air (i.e., inner and outer secondary air) guides the 4854



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Figure 3. Gas temperatures in the near wing-wall region (temperature data are acquired using a thermocouple device).

decrease in the general gas temperature field is initially slight over the OFA damper opening 0−30%, but then lengthens apparently in openings of 30−50%, because of the oxygen-lean atmosphere continuously deepening in the lower furnace. With the OFA damper fixed at a high opening of 50%, opening the staged-air damper, in general, also results in a continuous decrease in levels of gas temperature. These observations are attributed to: (1) Opening the staged-air damper at this high OFA flux setting further deteriorates the oxygen-lean atmosphere in the preceding stage combustion zone, thereby inhibiting coal combustion and dropping gas temperature levels in this zone; (2) with this shallow staged-air angle of 20°, an increase in the cold staged-air flux acts negatively on gas temperatures in the staged-air zone. Gas temperature profiles in the near wing-wall region are depicted in Figure 3. Those temperatures measured through ports 1 and 2 indicate temperatures of the coal/air mixture at an early stage of ignition and in the primary combustion stage, respectively. This explains the increasing sequence port 1 < port 2 in gas temperature levels in the near wing-wall region beyond 0.6 m. The profile of section (ii) in part b of Figure 3 shows temperature measurements terminating at a short distance from the wing wall; this is to protect the thermocouple, which was removed from the furnace when readings were close to 1300 °C. As seen in part a of Figure 3, in the zone near port 1 gas temperatures present a similar twophase pattern with distance for all four settings regardless of the OFA addition and OFA damper opening settings used. That is, gas temperatures initially rise rapidly with distance due to hightemperature gas accumulating in the near-wall region (called the first phase). Subsequently, temperatures decrease with distance because measurement points gradually approach the zone just below the burner near the wing wall (Figure 1) (called the second phase). Affected by low-temperature secondary air, temperatures thus present a decreasing trend with distance. Compared with the case before applying OFA (i.e., case 2 in section (ii)), gas temperature levels for those cases with OFA are lower in the first phase but higher in the second phase. Again, with opening OFA, gas temperatures decrease in the first phase but increase in the second phase. These observations occur because: (1) Applying OFA acts negatively in dropping the general furnace gas temperatures and opening the OFA damper deepens this negative effect because of the enhancement in the oxygen-lean atmosphere in the lower furnace (parts b and c of Figure 2); (2) apparently, the negative effect of the low-temperature secondary air on gas temperatures in the second phase weakens with increasing the OFA flux.

relatively low-temperature, fuel-rich, and oxygen-deficient chemical atmosphere downstream into the lower furnace. This combustion zone is hereafter referred to as the preceding stage combustion zone; (2) after the downward coal/air flow penetrates the middle part of the lower furnace, the ignited coal/air mixture from the preceding stage combustion zone is then combusted intensely in the staged-air zone where the air stoichiometric ratio is nearly 1. This area is hereafter referred to as the primary combustion zone; (3) after entering the furnace throat zone, the unburned coal particles mix with OFA and complete combustion (or come near to completion) in the upper furnace. The area for this process is hereafter referred to as the burnout zone. Figure 2 presents gas temperature patterns within the furnace with respect to various settings for sections (i)−(iii). As seen from parts b−d of Figure 2, these temperature data demonstrate that, with the MIMSC technology (including the circumstances before and after OFA application), the boiler accomplished well-formed symmetric combustion within the furnace regardless of the damper opening settings used. In parts b−d of Figure 2, gas temperature differences between nearfront and near-rear walls are within 100 °C. This contrasts with those reaching 300−600 °C in down-fired boilers (with the prior MBEL down-fired combustion technology10,22,23) operating under severe asymmetric combustion conditions, such as the case shown in part a of Figure 2. These circumstances occur because a strongly deflected airflow field was found in those down-fired boilers.10,22,23 For the present supercritical boiler, the burner configuration and air distribution develops essentially a symmetric gas/particle flow field aided by the new technology.23 No matter what damper opening setting is used, applying the OFA system strengthens the oxygen-lean atmosphere in the lower furnace and then restrains coal combustion, thereby resulting in the phenomena that levels of gas temperatures are clearly lower in section (iii) than in section (ii) (parts b−d of Figure 2). Additional explanation needs to be provided for this comparison between section (ii) and case 3 in section (iii). Because the OFA damper is fully closed (i.e., a 0% opening), case 3 in section (iii) indeed shares the same air distribution with section (iii), with the exception of the reduction in the staged-air declination angle (i.e., from 45° to 20°). This angle adjustment can apparently strengthen the staged-air block on the downward coal/air mixture and thus shrinks the flame penetration in the lower part of the furnace. Consequently, in comparison with section (iii), less heat release from coal combustion develops in the lower furnace at case 3. Again, part c of Figure 2 reveals that, as the OFA damper opens, the 4855



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Figure 4. Local mean gas species concentrations in the near wing-wall region.

OFA damper opening of 50%, O 2 content decreases continuously with distance and in the zone below the burner region (i.e., beyond 1.2 m) CO content increases sharply to levels above 1000 ppm because of the formation of an extreme oxygen-lean atmosphere. In the area near port 2, with both the intense coal combustion in the primary combustion stage and the staged air mixing gradually with gas, the change trends in O2 and CO content are generally similar to those in the area near port 1, with the exception of the high levels of CO content in the zone beyond 1.2 m. Here, similar causes as those in port 1 account for these observations in O2 and CO content. Applying OFA and adjusting staged-air angle apparently influence the NOx content pattern in the area near port 2: NOx content at case 2 generally increases with distance, whereas those at the later three settings initially increase with distance but then decrease after distances beyond 1.2 m. These differences are attributed to the degree of staged air mixing with gas in this area at different staged-air declination angles (Figures 1 and 2). Because of the rapid O2 consumption and CO production, as well as staged air mixing with gas in the primary combustion stage, comparisons of gas species concentrations in the area near ports 1 and 2 generally present the following pattern: both O2 and NOx content are lower, whereas CO content is higher in the region near port 2 than in the region near port 1. In the regions near both two ports, O2 content is higher at the OFA 0% setting than at case 2 and increasing the OFA damper opening continuously decreases O2 content. In the region near port 2, NOx content at the three settings with OFA are generally higher than that at case 2 and NOx content initially increases but then decreases with opening the OFA damper. Multiple factors account for these observations, which are as follows: (1) By reducing the staged-air angle from the original 45° to the present 20°, coal combustion in the region near port 1 weakens (Figure 2) and the process of staged air mixing with gas in the region near port 2 strengthens, thereby attaining relatively higher O2 content in the regions near ports 1 and 2 at the OFA 0% setting (compared with case 2); (2) opening the OFA damper reduces the fluxes of secondary air and staged air

For the region near port 2, because secondary air is mixing well in the primary air/flame and staged air is fed at a deep declination angle of 45°, relatively intense coal combustion occurs and the negative effect of the low-temperature staged air on gas temperatures in the region is relatively weak, thereby producing a clearly faster rise in the temperature profile for case 2 in section (ii), as shown in part b of Figure 3. Moreover, a temperature decrease (as shown in part a of Figure 3) is absent because at this case the temperatures increase sharply to levels approaching 1300 °C. However, part b of Figure 3 uncovers that circumstances in the other three cases (i.e., applying OFA, accompanied by an adjustment of the staged-air angle from 45° to 20°) are obviously different. That is, aside from the relatively lower levels (compared with case 2), gas temperatures at the three cases present a two-phase pattern similar to those in part a of Figure 3. Similarly to the change trend presented in part a of Figure 3, gas temperatures in the region near port 2 also decrease in the first phase but increase in the second phase with opening OFA damper. Explanation of these observations is as follows: (1) The deepened oxygen-lean atmosphere in the lower furnace decreases the general furnace gas temperatures (parts b and c of Figure 2); (2) with this shallow declination angle of 20° (Figure 1 and part c of Figure 3), low-temperature staged air can easily affect gas temperatures in this region; (3) opening the OFA damper also decreases the staged-air flux and thereby weakening the negative effect of the low-temperature staged air. Figure 4 presents gas species concentration profiles in the near wing-wall region. In the region near each port, gas species concentrations present a similar pattern for all four settings except for the OFA 50% setting (i.e., case 7). In the area near port 1 where the pulverized-coal is ignited and coal combustion is in the preceding stage, because of coal combustion proceeding and secondary air diffusing continuously, at the first three settings (i.e., cases 2, 3, and 5) O2 content initially maintains a relatively flat stage within distances of 0.4−1.4 m but then increase with distance, CO content varies slightly all the while, and NOx content generally presents an increasing pattern with distance except for some fluctuations. At the high 4856



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operation setting (i.e., 60%, 50%, and 50% for secondary air, staged-air, and OFA damper openings in case 12, respectively) with carbon in fly ash below 10% and NOx emissions about 850 mg/m3 at 6% O2 is determined for the furnace. This means that, in comparison with the circumstances with the prior MBEL art (i.e., case 1), the optimal operation setting attains NOx reduction by 40%. Although room exists for further NOx reduction to levels of 500 mg/m3 at 6% O2 (Table 1), the present carbon in fly ash remains at a relatively high level. This is because the sharp reduction in the staged-air declination angle, aiming at improving the reheated steam temperature by elevating the flame kernel position, causes a large negative effect on burnout (carbon in fly ash at cases 2 and 3 in Table 1). Within the further application of the MIMSC technology in other down-fired furnaces already in service and new designs, the staged-air declination will be set at the originally designed 45°. In this way, carbon in fly ash is expected to reduce in comparison with the present level of about 10%. Again, to comply with the mentioned NOx emission standard of 200 mg/ m3 at 6% O2, an accompanied flue-gas denitration process is needed so as to further reduce NOx emissions.

and produces a reductive atmosphere in the lower furnace, thereby reducing O2 content and weakening the staged air mixing with gas in the region near port 2; (3) sharply reducing the staged-air angle can greatly strengthen the process of staged air mixing with gas in the region near port 2, which can supply O2 timely to aid the intense coal combustion in this region and thus facilitates the NOx formation. In consequence, the NOx content in the region near port 2 is higher at cases 3, 5, and 7 than that at case 2. As shown in Table 1, in comparison with the boiler with the prior MBEL art,23 the newly designed boiler with the MIMSC technology but without OFA (i.e., under the air-staging conditions at the section (ii) step) attains NOx emission reduction by around 12% and significant improvement in burnout at a conventional operation setting (i.e., carbon in fly ash and NOx emissions from 8.5% and 1467 mg/m3 at 6% O2 to 2.54% and 1292 mg/m3 at 6% O2). This means that further NOx emission reduction needs to be achieved accompanied by no large increase in combustible loss. After applying OFA and adjusting the staged-air angle, opening the OFA damper produces a two-phase change pattern in NOx emissions, combustible loss, and boiler efficiency, that is, these data initially fluctuate slightly over OFA damper openings of 0−40% but then vary dramatically in openings of 40−70% (i.e., both CO in flue gas and carbon in fly ash increase sharply, whereas both NOx emissions and boiler efficiency decrease greatly). These changing trends are attributed to: (1) a continuous increase in the air-staging conditions in the furnace; (2) a relatively low OFA flux in openings of 0−40%; and (3) a continuous decrease in the staged-air block on the downward coal/air mixture, thereby acting positively on coal combustion in the primary combustion stage over openings 0−40%). Comparisons between cases 2 and 3 uncovered that, with the same air distribution setting, sharply reducing the staged-air angle from 45° to 20° apparently increases carbon in fly ash (because of shortening the residence times of the pulverizedcoal in the lower furnace) but slightly influences NOx emissions. With the OFA damper opening set at 15%, the high levels of NOx emissions and boiler efficiency and low combustible loss vary slightly with the staged-air damper opening and no clear change trend can be found. However, with the OFA damper opening fixed at 50%, opening the staged-air damper continuously decreases carbon in fly ash and slightly raises levels of NOx emissions. This is because a high OFA damper opening greatly reduces secondary air and staged air to form an oxygen-lean atmosphere in the lower furnace. Increasing staged-air flux facilitates coal combustion in the primary combustion stage but, unfortunately, produces more NOx. However, these circumstances can not be established at a low OFA damper opening such as 15% because an oxygen-sufficient atmosphere formed in the lower furnace at this low OFA setting, produces high NOx emissions and low carbon in fly ash (Table 1). These above observations uncover that (1) in comparison with OFA, the staged-air influence on combustion and NOx emissions is weaker; (2) the present staged air indeed acts as combustion air in the primary combustion zone (parts b−d of Figure 2), no longer as the jet for establishing staging conditions to reduce NOx (as described in the prior MBEL art10,22,23); (3) deep-air-staging conditions rely on OFA instead of staged air. To significantly reduce NOx emissions and control the combustible loss within an acceptable level, an optimal

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ASSOCIATED CONTENT

S Supporting Information *

Differences between the present study and our previous investigations into down-fired furnaces and major operation parameters in industrial-sized measurements for sections (i)− (iii), respectively. Note introducing briefly information regarding the NOx formation. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 451 86418854; fax: +86 451 86412528; e-mail: [email protected] (Z.Q. Li). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant No. 51121004).

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REFERENCES

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