ARTICLE pubs.acs.org/EF
Impact of the Overfire Air Location on Combustion Improvement and NOx Abatement of a Down-Fired 350 MWe Utility Boiler with Multiple Injection and Multiple Staging Min Kuang, Zhengqi Li,* Shantian Xu, Xingying Zhu, Yan Zhang, and Qunyi Zhu School of Energy Science and Engineering, Harbin Institute of Technology, 92, West Dazhi Street, Harbin 150001, People’s Republic of China ABSTRACT: A new technology based on multiple injection and multiple staging has been developed specifically for severe asymmetric combustion and particularly high NOx emissions within a down-fired 350 MWe utility boiler. The aim of the present work is to evaluate the performance of the furnace with the new technology with respect to different overfire air (OFA) locations and then to obtain an optimal setup to recommend as a retrofit for the furnace in the future. Numerical simulations of combustion characteristics and NOx emissions were conducted within the furnace at three different OFA location settings, i.e., OFA on arches but close to the furnace center, OFA on the furnace throat, and OFA on the upper furnace. Good agreement was found between simulations and in situ measurements with the original technology. Asymmetric combustion removals and significant NOx reductions were validated for all three OFA location settings. NOx emissions, carbon in fly ash, and gas temperatures at the furnace exit varied largely over different OFA settings. Both carbon in fly ash and gas temperatures in the furnace outlet for OFA on the furnace throat and OFA on the upper furnace are higher than for OFA on arches. The setting with OFA on the upper furnace achieved the highest NOx emissions, whereas that with OFA on arches enabled the lowest. To establish efficient furnace-operating conditions with the new technology, OFA on arches but close to the furnace center was recommended as the optimal OFA location according to simulation results.
1. 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. Reserves of anthracite and lean coal are abundant and globally distributed. With their low volatile content, anthracite and lean coal present difficulties in ignition and burnout. Down-fired boilers are specifically designed to burn anthracite and low-volatile coal, but unfortunately, a larger quantity of NOx is produced in these boilers than in tangential-fired furnaces and wall-arranged 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) downfired boilers. Differences between these four types referring to the burner arrangements and air distribution are described in detail in previously published literature.1 For FW boilers, reports have appeared on aerodynamic characteristics,2 combustion,3 5 slagging,6 and NOx reductions by overfire air (OFA) application.7 9 Research on B&W and Stein boilers has been reported by Fan et al.,10 who investigated the combustion characteristics and NOx formation within a 300 MWe B&W down-fired boiler, and by Burdett,11 who performed industrial tests to investigate the effects of air staging on NOx emissions from a 500 MWe Stein down-fired boiler unit. However, investigation on MBEL boilers is lacking and needs urgent focus. According to published literature,12 14 MBEL down-fired boilers suffer similarly from problems of severe asymmetric combustion, high NOx emissions typically in the range of 1100 1500 mg/m3 but sometimes as high as 1700 mg/m3 at 6% O2, and serious slagging in the lower furnace. r 2011 American Chemical Society
For primary removal of NOx, modifications of the combustion system are generally less costly; of these, air staging or OFA are the more mature and widely used. However, the resulting low NOx combustion environment applied deep air staging or OFA usually leads to high carbon levels in fly ash that are financially disadvantageous for power plant management.15,16 For downfired boilers, research to date has reported only on low NOx combustion retrofits in FW boilers, i.e., “vent-to-OFA” technology,7 “combined high efficiency and low NOx technology”,8 and “swirl stage (DS) burner and OFA application”.9 Especially with the latter two technologies, NOx emissions could be loweredh by as much as 50%, without increasing clearly levels of unburnt carbon in fly ash within the respective furnace after the retrofits. For MBEL down-fired boilers, no technology has been reported on significant reductions in NOx emissions or anything comprehensive enough that would also eliminate asymmetric combustion and minimize slagging in the lower furnace. In this paper, a new deep-air-staging technology was developed specifically for a MBEL down-fired pulverized-coal 350 MWe utility boiler. An exclusive combustion system, comprising a staged-air declination combined with a large increase in staged-air mass flow rate and overfire air application, as well as a burner redesign and reorganization, was installed in the boiler. To evaluate the performance of the furnace with the new technology and then to obtain an optimal OFA location to recommend as a retrofit for the furnace in the future, numerical simulations of the pulverized-coal Received: June 25, 2011 Revised: September 1, 2011 Published: September 06, 2011 4322
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Figure 1. Schematics of the furnace and combustion system of the down-fired boiler (dimensions in millimeters).
combustion process and NOx emissions within the furnace were conducted with respect to different OFA locations.
2. METHODOLOGY 2.1. Utility Boiler. Figure 1 presents the schematic of the furnace and combustion system with the prior MBEL art. The arches separate the furnace into two regions, the rectangular upper furnace and the octagonal lower furnace with four wing walls. A total of 16 cyclones symmetrically arranged on the arches divide the primary air/fuel mixture into fuel-rich and fuel-lean coal/air mixtures needed to regulate the fuelrich and fuel-lean combustion. The fuel-rich coal/air mixture is vertically
channeled through nozzles near the front and rear walls, while the fuellean coal/air mixture is injected with a 15° angle inclined toward the furnace center through nozzles centered over the furnace. The fuel-rich coal/air mixture nozzles and secondary-air ports are positioned in an alternating manner and close to each other. There are eight burners lining the front and rear arches. Four fuel-rich coal/air mixture nozzles, four fuel-lean coal/air mixture nozzles, and eight secondary-air ports feed each burner. The air box is partitioned into two parts, one on and the other below the arches. Each part has eight small boxes, with each box on the arches corresponding to one of the burners and each box below the arches feeding a group of staged-air slots. Near-wall air is partitioned air fed to the air box on the arches and designed to mitigate 4323
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slagging on the front and rear walls. The distribution of the vertically ejected secondary air and the horizontally fed staged air is adjustable by damper openings associated with each box. 2.2. In Situ Measurements. To verify the simulation validity, in situ experiments on the full-scale furnace with the original technology were performed at full load, with measurements of gas temperatures taken in the right-wall region and gas concentrations (O2 and NO) taken in the near-wall region. A 3i hand-held pyrometer (a type of noncontact infrared thermometer made by Raytek, Santa Cruz, CA),8 with a measurement range from 500 to 2000 °C, accurate to within 1 °C and with an error of (30 °C, was inserted through monitoring ports 1 4
Table 1. Characteristics of the Coal Used in Industrial Experiments and Simulations proximate analysis (wt %, as received) volatile matter
ash
9.66
31.50
moisture fixed carbon net heating value (kJ/kg) 6.7
52.14
20500
ultimate analysis (wt %, as received) carbon
hydrogen
sulfur
nitrogen
oxygen
54.25
2.51
1.35
0.68
3.01
(Figure 1) to measure the highest gas temperatures in regions from the wing walls to just below the burners adjacent to wing walls near the rightside wall. A 3 m long water-cooled stainless-steel probe, comprising a centrally located 10 mm inner diameter tube surrounded by a tube for probe cooling, was inserted into the furnace through monitoring port 3 perpendicular to the wing wall (Figure 1). The captured gas samples were analyzed online by a Testo350 M instrument. Upon inserting the water-cooled probe through the port, gaps in the port entrance were plugged with asbestos to avoid air leak. The probe was cleaned frequently by blowing high-pressure 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 350M was 1% for O2 and 50 ppm for NOx. The major sources of uncertainty in concentration measurements were associated with the quenching of chemical reactions and aerodynamic disturbances of the flow. Because of 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/s. Coal characteristics and operating conditions during experiments are listed in Table 1. 2.3. New Technology. As illustrated in Figure 2, the method that realizes this new technology is as follows: (1) The fuel-rich coal/air mixture is channeled vertically down through nozzles centered over the furnace, while the fuel-lean coal/air mixture is similarly injected near the front and rear walls. The fuel-rich coal/air mixture nozzles are short and rectangular in shape; these are arranged side-by-side in pairs to postpone
Figure 2. Schematics of the combustion system of the down-fired boiler applying MIMSC technology. 4324
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Table 2. Operational Parameters, Experimental Data, and Simulation Results operation parameters and results simulation (MIMSC) upper case coal feed rate (kg/s)
fuel-rich coal/air mixture
fuel-lean coal/air mixture
experiment
simulation (original)
arch OFA
throat OFA
furnace OFA
42.62
42.62
42.62
42.62
42.62
temperature (°C)
138
138
138
138
138
flow rate (kg/s)
25.50
25.50
25.50
25.50
25.50
velocity (m/s)
11.02
11.02
15.00
15.00
15.00
coal concentration (kg/kg)
1.50
1.50
1.50
1.50
1.50
temperature (°C)
138
138
138
138
138
flow rate (kg/s) velocity (m/s)
25.50 23.39
25.50 23.39
25.50 23.39
25.50 23.39
25.50 23.39
coal concentration (kg/kg)
0.17
0.17
0.17
0.17
0.17
temperature (°C)
323
323
323
323
323
flow rate (kg/s)
276.94
276.94
162.30
162.30
162.30
velocity (m/s)
50.90
50.90
50.90
50.90
50.90
temperature (°C)
323
323
323
323
323
flow rate (kg/s)
53.46
53.46
96.96
96.96
96.96
velocity (m/s) temperature (°C)
25.45 323
25.45 323
50.90 N/A
50.90 N/A
50.90 N/A
flow rate (kg/s)
6.44
6.44
N/A
N/A
N/A
velocity (m/s)
10
10
N/A
N/A
N/A
temperature (°C)
N/A
N/A
323
323
323
flow rate (kg/s)
N/A
N/A
77.56
77.56
77.56
velocity (m/s)
N/A
N/A
50.90
50.90
50.90
flow rate of main
1080
N/A
N/A
N/A
N/A
steam (ton/h) gas temperature at the
990
877
911
974
999
1682
1840
923
784
1012
3.07
5.56
5.51
7.18
8.94
secondary air
staged air
near-wall air
OFA
furnace outlet (°C) NOx in flue gas (mg/m3 at 6% O2 dry) carbon in fly ash (%)
the decay in the fuel-rich coal/air mixture. (2) Two rows of short secondary-air ports are located on arches across the breadth of the furnace, i.e., one row of so-called inner secondary-air ports positioned between the fuel-rich and fuel-lean coal/air mixture nozzles and another row of so-called outer secondary-air ports located near the front and rear walls. Accordingly, secondary air is fed through arches in a two-stage manner, and a first combustion stage forms in the burner region. The near-wall air is then canceled by the outer secondary air adjacent to the front and rear wall. (3) Staged-air slots are arranged uniformly on the front and near walls across the breadth of the furnace, and a high stagedair ratio (25%) is established to lower the stoichiometry to close to 0.65 in the zone above the staged air. Next, a second combustion stage is formed. Again, staged air is fed at a set declination of 45°, and (4) OFA is introduced to achieve deep staging conditions. Direct-flow OFA ports are arranged across the breadth of the furnace with an experimentally optimized declination of 40°, to lower the stoichiometry to nearly 1.0 in the zone below the OFA. With this technique, a third combustion stage forms in the furnace throat region. There exist three different schemes in the location for OFA arrangement along the furnace height, i.e., on the front and near arches but close to the furnace center (denoted by “arch OFA” in the following text), on the furnace throat (the conjunction of the lower and upper furnaces, denoted by “throat OFA” in the following text), and on the front and rear walls in the upper furnace (denoted by
“upper furnace OFA” in the following text) (Figures 1 and 2). The present work focuses on the performance of the furnace with the three OFA locations above and then obtaining an optimal OFA location recommended to retrofit the furnace in the future. It is necessary to point out that the proposed technology is based on the injection mechanism derived from Bernoulli’s principle and defined as follows: The injection mechanism is such that the higher the air flow velocity, the lower the air flow static pressure. Thus, for the two parallel air flows injected into the furnace, a static-pressure difference arises that depends upon the difference in air flow velocities. Owing to the static-pressure difference, the lower velocity air flow is deflected toward and gradually mixes with the higher velocity air flow. Carried by the higher velocity air flow, the lower velocity air flow can now penetrate further into the furnace. Here, the technology solving the above-mentioned problems is based on three levels of air injection and comprehensive deep-staged combustion. Because of the presence of multiple air flow injections under deep air staging, we call the new technology “multi-injection multi-stage combustion” (MIMSC). To understand more clearly the underlying principle behind MIMSC technology, Figure 2 presents a schematic diagram of the multiinjection air flow trajectories. For a combustion system with this technology, the design parameters, such as the air flow rate and velocity, are listed in Table 2. 2.4. Numerical Simulation Method. A commercial computational fluid dynamics code (Fluent, version 6.3.26),17 was used in 4325
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Figure 3. Grid divisions and comparison of in situ measurements and calculated results with prior MBEL technology. conducting the simulations. In this work, gas turbulence was specifically taken into account by the so-called realizable k ε model. The Lagrangian stochastic tracking model was applied to analyze the gas/particle flow field, while calculations of gas/particle two-phase coupling employed the particle-source-in-cell method. Radiation was described using the P-1 model, and devolatilization was modeled with the two-competing-rate Kobayashi model.18 The combustion of volatiles was modeled by employing a probability density function theory, and char combustion was modeled by employing a diffusion/kinetics model. More detailed descriptions of these models can be found in the literature.19 The formation of NOx includes thermal NOx and fuel NOx but little prompt NOx. Here, only the production of NO was taken into account, because NOx emitted into the atmosphere from combusting fuels consists mostly of NO, with there being much lower concentrations of NO2 and N2O. The concentration of thermal NOx was calculated using the extended Zeldovich mechanism (specifically, N2 + O f NO + N, N + O2 f NO + O, and N + OH f NO + H).20 The fuel NOx concentration was calculated using De Soete’s model.21 The formation of prompt NOx was neglected in calculations. Reflection symmetry in the furnace design allows for simulations to be performed over a computation domain corresponding to the right half of the furnace. A partition meshing method was adopted to achieve high-quality grid, and the mesh system of
the calculation domain consists of 1 046 100 cells. Mesh is refined in the neighborhood of the burners where the combustion processes actively take place. Grid-dependent tests were performed with the current grid system as well as two additional grid systems of approximately 900 000 and 1 200 000 cells. The results have shown that the current grid system sufficiently provides grid-independent solutions. The SIMPLE algorithm of pressure correction was applied to consider the coupling of velocity and pressure fields. Using the first-order finite difference method, the conservation equations of the gas phase were solved with successive under-relaxation iterations until the solution satisfied a pre-specified tolerance. The wall function method and temperature wall were employed for the velocity and thermal boundary conditions, respectively. The size distribution of the pulverized-coal particles obeys the Rosin Rammler algorithm, with an average diameter of 50 μm according to the test result. The operational parameters were set to simulate those at the full load, with coal characteristics that are the same as those in the in situ experiments (Table 1).
3. RESULTS AND DISCUSSION 3.1. Validation for the Simulated Results. Figure 3 presents grid divisions along the longitudinal cross-section, intersecting 4326
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Figure 4. Calculated flow field, temperature field, and gas components with prior MBEL technology.
the vertical centerline of one of the fuel-rich coal/air mixture nozzles, as well as the comparison of gas temperatures and component concentrations acquired from in situ experiments and simulations with prior MBEL technology. Figure 3b clearly shows that the combustion characteristics in the lower furnace are asymmetric, with gas near the front wall (ports 1 and 2) being clearly hotter than those near the rear wall (ports 3 and 4). An explanation of this phenomenon can be found in the following discussion on the asymmetric flow seen in Figure 4a. As shown in Figure 3d, the NO concentration is at high levels, with an average value of 850 ppm. As listed in Table 1, NOx emissions are as high as 1682 mg/m3 at 6% and carbon in fly ash is at a low level of about 3%, although asymmetric combustion occurs. The high NOx emissions and asymmetric combustion are in accordance with the common problems already mentioned in the Introduction. Figure 3 also shows that calculated gas temperatures near the right wall, as well as O2 and NO concentrations, are all consistent with measured values, with the exception of some apparent deviations in gas component concentrations at several measuring points near the 1.0 m location. This is because of the occurrence of the asymmetric combustion in the furnace. Any variation in boiler operating parameters will strongly affect the extent of the asymmetric combustion. Therefore, it is very difficult to achieve full consistency between the simulation result and the experimental data under the present asymmetric combustion condition, although many attempts were performed. The temperatures near the right wall show similar asymmetric combustion characteristics to those measured in industrial experiments. The calculated gas temperature at the furnace outlet and NOx emissions listed in Table 2 also show good agreement with those measured, with a calculation error of 11.4% in the gas temperature at the furnace outlet and a 9.4% discrepancy in NOx concentrations in flue gas. As the dominant component of NOx, NO in flue gas was 1578 mg/m3 (6% O2 dry) in the in situ measurement and 1722 mg/m3 (6% O2 dry) in simulations, with a calculation error of 9.1%. However, the result for carbon in fly ash is not
satisfactory, with an absolute difference of 2.49% between measured and calculated values of 3.07 and 5.56%, respectively. These results indicate that the models adopted in the present study are suitable for investigating the effects of MIMSC technology on pulverized-coal combustion processes and NOx formation within the furnace. 3.2. Details on the Calculated Results with Prior MBEL Technology. For more details on the current flow and combustion characteristics as well as NOx formation within the furnace, the calculated flow field, gas temperature field, and O2 and NO concentration fields with the original technology are extracted and presented in panels a d of Figure 4, respectively. These are taken along the longitudinal cross-section, intersecting the vertical centerline of one of the fuel-rich coal/air mixture nozzles. Also, calculated results with the new technology were extracted along the same cross-section; these are presented in the next subsection. As shown in Figure 4a, there is a deflected flow field in the lower furnace for the original technology, with the downward coal/air flow near the rear wall clearly not penetrating as deep as that near the front wall. The downward coal/air flow near the front wall penetrates deep into the middle and lower parts of the dry bottom hopper region, from where it deflects upward along the right side of the hopper, filling most of the area in the hopper. Blocked by the staged air and lifted by the upward gas near the staged-air zone, the downward coal/air flow near the rear wall deflects upward toward the furnace center after reaching the upper part of the staged-air zone. Causes for the formation of the asymmetric flow field are largely the same as those listed in the published literature.1,12,22 Figure 4b shows that, for the original technology, asymmetric combustion appears with the hightemperature zone near the front wall penetrating the upper part of the hopper region but that near the rear wall being truncated above the staged-air slot. In general, calculated gas temperatures are higher in the front half of the furnace than in the rear, in agreement with measured results (Figure 3b). Cashew-shaped hot regions with gas temperatures reaching as high as 1900 K 4327
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Figure 5. Calculated flow fields associated with different OFA locations for MIMSC technology.
occur in the front- and rear-wall zones but not in the central part of the furnace. In this high-temperature zone, slagging could occur because of fast flowing particulates washing over the walls (Figure 4a). It is this deflected flow field that leads to the asymmetric combustion. As shown in panels c and d of Figure 4, both the calculated O2 and NO concentrations exhibit similar asymmetric patterns as the gas temperature distribution. Low O2 and high NO concentration regions extend further in the front half than rear half of the lower furnace, because of the asymmetric combustion. In the high-temperature zones near the front and rear walls, both O2 and NO concentrations decrease with the flame penetration depth, with the highest O2 and NO concentrations appearing in the zone below the arches not far from the nozzle outlet. This is a consequence of premature mixing of secondary air with the fuelrich coal/air mixture because of an especially narrow gap between a fuel-rich coal/air mixture nozzle and the secondaryair port nearby. Because secondary air carries the coal/air flow that washes over the walls (Figure 4a), relatively low O2 concentrations (6 8%) appear in the front and rear wall zones. Thus, a high-temperature low-oxygen region forms that facilitates slagging there. Levels of NO are particularly high in the whole furnace, and reasons for this are 3-fold: (1) the premature mixing of secondary air with the fuel-rich coal/air mixture at the early stage of ignition (the respective nozzles and ports are arranged in alternating positions and are closely spaced; after leaving the port outlet, secondary air with velocities much higher than that of the fuel-rich coal/air mixture mixes rapidly with the slower flow, resulting in early ignition of pulverized coal in this oxygen-rich atmosphere but produces a large amount of fuel NOx), (2) a shallow staging condition [to ensure high velocities of secondary air and, thus, guarantee a long flame travel by propelling the fuel-rich coal/air mixture into the hopper region, the staged-air ratio (i.e., the ratio of the staged-air mass flux to the
total air mass flux in the furnace) is set to about 13% (calculated in Table 1) and shallow staging conditions are established], and (3) high gas temperatures in the lower furnace, especially in the zone below arches (Figure 4b), needed for good burnout, facilitating thermal NOx formation. 3.3. Calculated Results with MIMSC Technology Associated with Different OFA Locations. In Figure 5, no matter what kind of OFA location is set up, a perfectly symmetric flow field appears, with coal/air flows penetrating downward into the lower furnace adjacent to the front and rear walls. Meanwhile, staged-air flowing downward at high speed along the hopper wall forms a distinctive air flow layer covering the wall. This layer serves to prevent fast particulates washing over the hopper walls, thus avoiding slagging in the hopper. With the same declination angle of 40°, fast OFA jets for all three OFA location settings can reach the furnace center and then mix well with the upward gas after leaving the port outlet. In comparison to the “upper furnace OFA”, OFA jets of “arch OFA” and “throat OFA” begin to mix with the upward gas much earlier, allowing for a longer reaction time for OFA and unburnt particles. No evident difference appears among the three flow fields in the lower furnace because of the same combustion system, except for the OFA location. Explanation of the formation of the symmetric flow field with the MIMSC technology: For a combustion system and air distribution with MIMSC technology, two factors weaken the interactional extrusion1,12,22 existing between the two upward coal/air flows that circumvents the formation of the asymmetric flow field; one is the significant reduction in the secondary air mass flow rate and the rearrangement of secondary air and the fuel-lean coal/air mixture toward the front and rear walls, and the other is a large increase in the mass flow rate of staged air accompanied with a 45° declination. As reported previously,1 only an adjustment of the feed direction of staged air to a declination of 45°, instead of the original horizontally fed direction, 4328
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Figure 6. Calculated gas temperatures associated with different OFA locations for MIMSC technology.
inhibits formation of this deflected flow field in the lower furnace. Here, additional changes in the secondary air mass flow rate and burner arrangement strengthen the inhibition on the formation of an asymmetric flow field. As shown in Figure 6, for the MIMSC technology with any OFA location mentioned previously, a perfectly symmetric gas temperature field appears throughout the furnace. Each temperature field has three high-temperature zones in the lower furnace; two smaller zones are located in the region near the front and rear walls below arches but not far away from the nozzle outlet, and a larger zone is located in the central part of the furnace extending from the throat to the lower part of the hopper region. The two smaller zones are similar, whereas the larger one is clearly different from the others, with the larger one for the “upper furnace OFA” being much bigger than those for the two other settings. Generally speaking, gas temperatures would be higher after adoption of deep staging conditions. In comparison to the original configuration, here, the highest gas temperature is a little lower (1800 versus 1900 K) but the general gas temperature levels in the lower furnace are higher. The different coal combustion intensities in the burner region explain this temperature difference between the original and MIMSC technologies. With the original technology, the rapid mixing of secondary air with the fuel-rich coal/air mixture results in the combustion of pulverized coal in an oxygen-rich atmosphere. Thus, coal combustion is intense in the burner region. However, under the gradual mixing of secondary air with the fuel-rich coal/air mixture in MIMSC technology, a particularly low O2 concentration zone (2%; Figure 4c), through which pulverized coal passes, is just below the fuel-rich flow nozzle outlet and extends for a long distance from the nozzle outlet. Pulverized coal burns in a particularly oxygen-lean atmosphere for quite some time. Therefore, a restrained combustion develops, and the highest gas temperatures appear further away than those in the original. Secondary air ejected into the lower furnace adjacent to the front and rear walls and staged-air flowing downward along the hopper wall, screened by the air flow layer, cause gas temperatures in the near-wall region to be lower.
In Figure 7, each O2 concentration field shows a similar symmetric pattern as for the corresponding gas temperature field. Also, these O2 concentration fields in the lower furnace share lots of common characteristics. High-temperature zones, depicted in Figure 6, all maintain the lowest O2 concentrations, as a consequence of the intensity of combustion. Owing to the later mixing of OFA with the upward gas, the wedge-shaped lowoxygen zone in the central part of the furnace for the “upper furnace OFA” setting extends further in the upper furnace than those for the other two settings. Because of the injection of outer secondary air, a high O2 concentration (14 20%) appears not only in both front and rear wall zones. Again, particularly high O2 concentrations appear along the left and right walls of the hopper, because of the air flow layer formed by the downward moving staged air (Figure 5). This is to say, low-temperature high-oxygen regions form in these zones that mitigate slagging in the lower furnace. In the region near the furnace throat (for the “arch OFA” and “throat OFA”) and the lower part of the upper furnace region (for the “upper furnace OFA”), OFA flows from the front and rear walls toward the furnace center and mixes with the upward flue gas; each O2 concentration distribution features a low-level center and two high-level ends. Near the front and rear walls, O2 concentrations decrease rapidly, with gases rising into the upper furnace and, finally, because of the reaction with the unburnt particles, falling to the same levels as found at the furnace outlet in the original configuration. The occurrence of intense combustion in the upper furnace, thus, results in levels of gas temperature in the upper furnace rising to some extent compared to the original (Figures 4d and 6). Also in this region, the O2 concentration distribution along the furnace height varies greatly among the three settings because of the injection of OFA at different locations. In Figure 8, each NO concentration field with MIMSC technology shows a strong asymmetric pattern, with concentrations throughout the furnace being much lower than those in the original configuration, i.e., values almost one-half of those of the original. This is because in MIMSC, mixing of secondary air with the fuel-rich coal/air mixture is clearly postponed and a deep air 4329
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Figure 7. Calculated O2 concentrations associated with different OFA locations for MIMSC technology.
Figure 8. Calculated NO concentrations associated with different OFA locations for MIMSC technology.
staging condition is formed with a large increase in the staged-air ratio (25% compared to the original 13%, calculated in Table 2) and the introduction of OFA. Therefore, pulverized coal ignites in an oxygen-lean environment for a long combustion time, thereby reducing NOx production dramatically. With a similar pattern to gas temperature fields above, each NO concentration field has three high NO concentration zones in the lower furnace, two below arches but not far away from the nozzle outlet and one left in the furnace central part, all of which are just positioned in
the high-temperature zones depicted in Figure 6. The high NO concentration zone in the furnace central part extends much larger, and values are clearly higher for the “upper furnace OFA” than for the two other settings, because of a much larger hightemperature zone in the central part of the lower furnace. The fact that levels of the NO concentration for the “arch OFA” are a little higher than those for the “throat OFA” can be attributed to a relatively smaller distance between the fuel-rich coal/air mixture nozzle and the OFA port compared to that of the “throat OFA” 4330
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Figure 9. Mean gas temperature and component concentration distributions along the furnace height for different OFA locations.
(Figure 2). A small part of OFA could reverse to mix with the fuel-rich coal/air mixture under the taking of the recirculating gas below arches (Figure 5a). This mixing process facilitates the formation of NO. Figure 9 presents a comparison of calculated data (mean gas temperature and component concentration distributions along the furnace height) with respect to different OFA locations. Along the furnace height, for all three settings, gas temperature curves present a pattern of an initial increase but then a continual decrease, except for one or two valleys caused by the injection of OFA. O2 concentrations decrease at a high rate initially but then increase fast before continually slowing down. NO concentrations also increase initially but then decrease fast before varying little for a long distance. CO concentrations initially increase rapidly but then decrease fast. Although the highest CO concentrations reach vast values above 57 000 ppm near y = 15 m because of the occurrence of three high-temperature and low-O2 zones at this height level (Figures 6 and 7), levels of CO concentrations near the furnace outlet are close to zero because of the needed air supply from OFA for all three settings. Relatively low levels of gas temperatures and NO and CO concentrations, as well as high O2 concentrations, appear in the zone near the start of these curves. This is because this zone is located in the middle part of the hopper region where coal combustion is relatively weak and most of the hopper region is filled with the low-temperature staged air. After a comparison between curves of gas temperatures and component
concentrations, it can be found that gas temperatures and NO concentrations reach maxima, while O2 concentrations drop to minima at y = 16 m for the “arch OFA” and “throat OFA” settings and at y = 21 m for the “upper furnace OFA” setting (except for the NO peak at y = 16 m). This is because hightemperature and low-O2 zones associated with high NO concentrations cover most of the area at the mentioned furnace height levels (Figures 6 8). The OFA injection results in these gas temperature and NO concentration valleys and O2 concentration peaks (at y = 20 m for the “arch OFA” and “throat OFA” settings and at y = 24 m for the “upper furnace OFA” setting). Here, the comparison of calculated results among different OFA settings reveals that the higher the location where OFA is positioned, the higher the levels of gas temperatures. This is because a higher OFA location setting maintains a general oxygen-lean atmosphere in the lower furnace for a longer time before OFA and delays the reaction of unburnt coal particles and OFA with lots of heat releasing in the upper furnace. For the “upper furnace OFA”, the higher levels of gas temperatures account for the fact that NO concentrations are clearly higher than for the other two settings and the delay in the reaction above explains why O2 and CO concentrations in the upper furnace are higher than for the other two settings. The fact that NO concentrations for the “arch OFA” are higher than those for the “throat OFA” might be attributed to the mixing of the fuel-rich coal/air mixture and some OFA mentioned above. These circumstances with the highest levels of gas temperatures 4331
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Energy & Fuels and NO and CO concentrations in the upper furnace for the “upper furnace OFA” should not be acceptable. Table 2 lists the calculated gas temperature at the furnace outlet, carbon in fly ash, and NOx emissions. With the prior MBEL technology being replaced by the deep-air-staging MIMSC technology, calculated NOx emissions decreased by 50, 57, and 45% for the “arch OFA”, “throat OFA”, and “upper furnace OFA”, respectively, but unfortunately, both the gas temperature at the furnace outlet and carbon content in fly ash increase with different extents for all three settings, with the exception of almost no change in carbon in fly ash for the “arch OFA”. The setting with OFA on the upper furnace achieved the highest gas temperature at the furnace outlet and carbon content in fly ash, whereas that with OFA on arches enabled the lowest. This is because the reaction of unburnt coal particles and OFA releases lots of heat in the upper furnace, and hence, these temperatures are higher than the original temperature. A higher location for OFA not only postpones this heat-releasing process along the furnace height to elevate the gas temperature at the furnace outlet but also shortens the reaction time for unburnt particles to raise levels of unburned carbon. The equitable secondary air and staged-air configurations enabling adequate residence times of pulverized-coal particles in the lower furnace, in combination with adequate reaction times between OFA and unburnt particles, achieve this slight decrease in the calculated carbon content in fly ash for the “arch OFA”, instead of an apparent increase for the other two settings. Highest levels for the gas temperature throughout almost the entire furnace and for O2 concentrations in the upper furnace result in the highest NOx emissions for the “upper furnace OFA” setting, whereas the lowest levels for O2 concentrations throughout the furnace account for the main cause favoring the lowest NOx emissions for the “throat OFA” setting. To establish efficient furnace-operating conditions with the new technology, NOx emissions must be reduced significantly but with levels for the gas temperature at the furnace outlet and carbon content in fly ash that approach the original as closely as possible, despite some loss in NOx reduction. In consequence, the “arch OFA” is the optimal OFA location for MIMSC technology that should be recommended for low NOx retrofitting for furnaces. The applicability of the new technology will be verified in future work with the modification of in-service MBEL down-fired boilers and new designs.
4. CONCLUSION In applying MIMSC technology as a replacement for the MBEL prior art, the original asymmetric flow field, gas temperature, and gas component distributions that were seen in the furnace all developed a symmetric pattern, regardless of the OFA location. Also, relatively low gas temperatures and high O2 concentrations were found in the front- and rear-wall zones, as well as in the hopper wall zone, thereby mitigating slagging in the lower furnace. NOx emissions can be lowered by as much as 45 57% for different OFA position locations, but unfortunately, both the gas temperature at the furnace outlet and carbon content in fly ash rose to different extents for all three settings, with the exception of almost no change in carbon in fly ash for the “arch OFA”. The “throat OFA” achieved the largest reduction in NOx emissions, whereas the “upper furnace OFA” achieved the lowest, both enabling a considerable increase in the gas temperature at the furnace outlet and carbon content in fly ash. After comprehensive consideration of both NOx reduction and
ARTICLE
efficient furnace-operating conditions, the “arch OFA” was found to give the optimal OFA location for MIMSC technology for the recommendation with low NOx retrofitting for furnaces.
’ AUTHOR INFORMATION Corresponding Author
*Telephone: +86-451-8641-8854. Fax: +86-451-8641-2528. E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was sponsored by the Hi-Tech Research and Development Program of China (863 Program) (Contract 2006AA05Z321) and the Changjiang Scholars and Innovative Research Team in University (Contract IRT0914). ’ REFERENCES (1) Kuang, M.; Li, Z. Q.; Han, Y. F.; Yang, L. J.; Zhu, Q. Y.; Zhang, J.; Shen, S. P. Energy Fuels 2010, 24, 1603–1610. (2) Liu, R. W.; Hui, S. E.; Yu, Z. Y.; Zhou, Q. L.; Xu, T. M.; Zhao, Q. X.; Tan, H. Z. Energy Fuels 2010, 24, 5514–5523. (3) Ca~ nadas, L.; Cortes,V.; Rodríguez, F.; Otero, P.; Gonzalez, J. F. NOx reduction in arch-fired boilers by parametric tuning of operating conditions. Proceedings of the Electric Power Research Institute (EPRI)/Environmental Protection Agency (EPA) Megasymposium; Washington, D.C., 1997. (4) Fueyo, N.; Gamb on, V.; Dopazo, C.; Gonzalez, J. F. J. Eng. Gas Turbines Power 1999, 121, 735–740. (5) Fang, Q. Y.; Wang, H. J.; Zhou, H. C.; Lei, L.; Duan, X. L. Energy Fuels 2010, 24, 4857–4865. (6) Fang, Q. Y.; Wang, H. J.; Wei, Y.; Lei, L.; Duan, X. L.; Zhou, H. C. Fuel Process. Technol. 2010, 91, 88–96. (7) Garcia-Mallol, J. A.; Steitz, T.; Chu, C. Y.; Jiang P. Z. Ultra-low NOx advanced FW arch firing: Central power station applications. Proceedings of the 2nd U.S. China NOx and SO2 Control Workshop; Dalian, China, Aug 25 28, 2005. (8) Li, Z. Q.; Ren, F.; Chen, Z. C.; Liu, G. K.; Xu, Z. X. Environ. Sci. Technol. 2010, 44, 3926–3931. (9) Leisse, A.; Lasthaus, D. VGB Power Tech. 2008, 11, 1–7. (10) Fan, J. R.; Zha, X. D.; Cen, K. F. Energy Fuels 2001, 15, 776–782. (11) Burdett, N. A. J. Inst. Energy 1987, 60, 103–107. (12) Li, Z. Q.; Kuang, M.; Zhang, J.; Han, Y. F.; Zhu, Q. Y.; Yang, L. J; Kong, W. G. Environ. Sci. Technol. 2010, 44, 1130–1136. (13) Miao, C. X.; Wang, J. W. Therm. Power Gener. 2005, 34, 48–51 (in Chinese). (14) Han, K. H.; Gao, H.; Zhai, L.; Liu, J.; Wang, D. F. Boiler Technol. 2005, 36, 47–50 (in Chinese). (15) Costa, M.; Silva, P.; Azevedo, J. L. T. Combust. Sci. Technol. 2003, 175, 271–289. (16) Costa, M.; Azevedo, J. L. T. Combust. Sci. Technol. 2007, 179, 1923–1935. (17) Fluent, Inc. FLUENT 6.3 User’s Guide; Fluent, Inc.: Lebanon, NH, 2006. (18) Kobayashi, H.; Howard, J. B.; Sarofim, A. F. Symp. (Int.) Combust., [Proc.] 1977, 16, 411–425. (19) Smoot, L. D.; Smith, P. J. Coal Combustion and Gasification; Plenum Press: New York, 1989. (20) Hill, S. C.; Smoot, L. D. Prog. Energy Combust. Sci. 2000, 26, 417–458. (21) De Soete, G. G. Overall reaction rates of NO and N2 formation from fuel nitrogen. Proceedings of the 15th International Symposium on Combustion; Pittsburgh, PA, 1975. (22) Li, Z. Q.; Kuang, M.; Zhu, Q. Y.; Yang, P. F.; Wu, Y. G.; Xu, S. T. Energy Fuels 2010, 24, 4883–4892. 4332
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