Reducing NOx Emissions for a 600 MWe Down ... - ACS Publications

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Reducing NOx Emissions for a 600 MWe Down-Fired Pulverized-Coal Utility Boiler by Applying a Novel Combustion System Lun Ma,† Qingyan Fang,*,† Dangzhen Lv,*,‡ Cheng Zhang,† Yiping Chen,‡ Gang Chen,† Xuenong Duan,‡ and Xihuan Wang§ †

State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan, Hubei 430074, People’s Republic of China ‡ Electric Power Research Institute, Hunan Power Company, Changsha, Hunan 410000, People’s Republic of China § Jinzhushan Thermal Power Company, Datang Huayin Electric Power Company, Limited, Loudi, Hunan 417500, People’s Republic of China S Supporting Information *

ABSTRACT: A novel combustion system was applied to a 600 MWe Foster Wheeler (FW) down-fired pulverized-coal utility boiler to solve high NOx emissions, without causing an obvious increase in the carbon content of fly ash. The unit included moving fuellean nozzles from the arches to the front/rear walls and rearranging staged air as well as introducing separated overfire air (SOFA). Numerical simulations were carried out under the original and novel combustion systems to evaluate the performance of combustion and NOx emissions in the furnace. The simulated results were found to be in good agreement with the in situ measurements. The novel combustion system enlarged the recirculation zones below the arches, thereby strengthening the combustion stability considerably. The coal/air downward penetration depth was markedly extended, and the pulverized-coal travel path in the lower furnace significantly increased, which contributed to the burnout degree. The introduction of SOFA resulted in a low-oxygen and strong-reducing atmosphere in the lower furnace region to reduce NOx emissions evidently. The industrial measurements showed that NOx emissions at full load decreased significantly by 50%, from 1501 mg/m3 (O2 at 6%) to 751 mg/m3 (O2 at 6%). The carbon content in the fly ash increased only slightly, from 4.13 to 4.30%.

1. INTRODUCTION The control and reduction of NOx emissions from coal combustion have attracted extensive attention around the world. This is because NOx is a precursor for photochemical smog, contributes to acid rain, causes ozone depletion, and threatens human health.1,2 Typically, its control and reduction in pulverized-coal boilers can be achieved via the following methods: low-NOx combustion technology,1,3 treatment of post-combustion flue gas, fuel additive technology,4,5 and oxygen-enriched coal combustion.6,7 Low-NOx combustion technology is preferred as a low-cost and highly effective approach to reduce NOx emissions. Its main principle is to reduce combustion temperature and delay the mixing between fuel and oxygen in the combustion air, thereby creating a fuelrich lean-oxygen atmosphere, where most of the generated NOx can be reduced to N2.8−12 In the real-time application, low-NOx combustion technology and treatment of post-combustion flue gas are generally combined to reduce NOx emissions as much as possible. In China, reserves of low-volatility solid fuels (mainly including lean coal and anthracite) are widely used in coalfired power plants and supply a considerable proportion of current electricity demands, approximately 30%.13,14 Downfired boilers have some merits, such as the high-temperature © XXXX American Chemical Society

level in the lower furnace and the long travel path of the pulverized-coal particles, and these characteristics allow for the use of coal with low volatility and poor reactivity.11,15 However, the high-temperature level in the lower furnace has a negative influence and causes NOx emissions at the furnace outlet to increase considerably (approximately 1100−1800 mg/m3, with O2 at 6%, before the flue gas denitrification system),16,17 despite the utilization of conventional air staging that tends to inhibit NOx production. To significantly reduce NOx emissions, some researchers have concentrated on optimizing operating parameters and modifying the combustion system, i.e., reforming of the burner design, staging the air/fuel ratio, and introducing overfire air (OFA). In particular, the combustion system modification is a relatively high-efficiency technique, and various technologies for different types of down-fired boilers have been put forward. The “multiple-injection and multiple-staging concept” (MIMSC) was proposed on Mitsui Babcock Energy Limited (MBEL) boilers.18 Industrial-sized measurements of a 600 MWe supercritical furnace with this Received: June 10, 2015 Revised: September 8, 2015 Accepted: October 9, 2015

A

DOI: 10.1021/acs.est.5b02827 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology

Figure 1. Schematics of the original and novel combustion system of the down-fired boiler.

by the combustion environment, these solutions may be useful, i.e., positing OFA near the main combustion zone to extend the reaction time between un-burnout char and OFA, decreasing the OFA ratio to make coal particles burn more completely in the lower furnace, or enhancing the coal residence time in the lower furnace. However, these will inevitably cut down the reducing region or weaken the reducing atmosphere below the OFA, which does not contribute to the NOx reduction. Thus, it is necessary to take measures to further lower NOx emissions without bringing an increase in the carbon content of fly ash. A novel and comprehensive low-NOx combustion technology was applied to a 600 MWe FW down-fired pulverized-coal utility boiler to reduce NOx emissions without producing an obvious increase of unburnt carbon in fly ash. It mainly included moving fuel-lean nozzles from the arches to the front/ rear walls and rearranging staged air as well as introducing SOFA. Numerical simulations were carried out to reveal more details of combustion and NOx emissions in the furnace, before and after adopting the novel combustion system. Meanwhile, industrial measurements were also performed to validate combustion improvements and NOx reduction.

technology confirmed the development of coal ignition, symmetric and stable combustion, and low NOx emissions. Leisse et al. employed a comprehensive combustion modification for a 350 MWe B&W furnace in Spain.19 It mainly consisted of applying OFA and adopting the so-called DS swirl burner as a replacement of the original swirl burner. The actual performance of the B&W boiler demonstrated that not only was sharp NOx reduction attained but also low levels of the carbon content in fly ash were accompanied. Some modifications on Foster Wheeler (FW) down-fired boilers also appeared to reduce NOx emissions. The FW Company introduced two technologies: “vent-to-OFA”20 and “fuel preheat nozzle” technologies.11 The “vent-to-OFA” supplied lean-fuel/air mixtures through OFA nozzles into the furnace, which could promote coal ignition as a result of the vent air carrying much of the moisture from the coal. The “fuel preheat nozzle” involved shortening the fuel nozzle and displacing the rod with a hollow cylinder, which was in favor of (1) enhancing the fuel-rich concentration, (2) strengthening the rigidity of fuel-rich coal/air flow, (3) making the hot B-layer secondary air and the fuel-rich coal/air streams mix more intensive near the burner outlet, and (4) producing more char that was favorable to lower NOx. The performance of the boiler improved in aspects of enhancing the flame stability, reducing NOx emissions by 50%, but bringing the obvious increase of carbon in fly ash. Li et al. proposed the “combined high-efficiency and low-NOx” (CHELNO) technique,21 which consisted of inclining F-layer secondary air, moving fuel-lean nozzles from the face-fire side to the back-fire side, and introducing OFA. According to the reported results of a modified furnace with the “CHELNO” technique, NOx emissions were reduced by 50% and the flame stability was enhanced but, unfortunately, the level of carbon in fly ash was relatively high (about 7.84%).22 Of these NOx-controlling technical schemes, introducing OFA is undoubtedly the simplest, lowest cost, and highest efficiency technique. However, the introduction of OFA also changes the dominant combustion condition and may elicit the problem of high burnout loss, which is economically disadvantageous for power plants.23−25 To deal with the high burnout loss caused

2. METHODOLOGY 2.1. Utility Boiler. The unit is a 600 MWe FW down-fired pulverized-coal utility boiler. The total height is 50.150 m. The furnace is divided into two zones by the arches: the upper furnace and the lower furnace. The dimensions are 34.480 × 9.906 m for the upper furnace and 34.480 × 16.062 m for the lower furnace. More details about the furnace can be found in the Supporting Information. 2.2. Novel Combustion System. As illustrated in Figure 1, the following steps are included: (1) The original 72 fuel-lean coal/air nozzles are merged into 36 nozzles, and their locations are moved from the arches to the position below the arches. The fuel-lean/air stream is fed to the furnace at a declination angle of approximately 30°. (2) At the original fuel-lean location (centered over the furnace on the arches), A-layer secondary air is injected through circular nozzles, instead of through the original annular nozzles. (3) D- and E-layer B


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Table 1. Proximate and Ultimate Analyses of Coal, Parameter of Boiler Operation, and Measured and Calculated Results under the Original and Novel Combustion Systems (Simulated Boiler) proximate analysis (wt %, air dried) V

M

A

8.27

2.00

39.80

ultimate analysis (wt %, air dried) FC

C

H

49.93 51.56 2.26 original combustion system

O

S

2.46

1.24

Qnet (kJ/kg, as received)

N

0.68 17370 novel combustion system

item

air rate (%)

velocity (m/s)

temperature (K)

air rate (%)

velocity (m/s)

temperature (K)

fuel-rich flow fuel-lean flow A-layer secondary air B-layer secondary air C-layer secondary air D-layer secondary air E-layer secondary air F1-layer secondary air F2-layer secondary air separated overfire air leaking air

11.11 11.11 10.00 10.00 15.00 4.00 5.00 5.00 23.78

17.98 15.75 31.05 30.07 29.89 5.41 5.26 5.92 8.34

393 393 594 594 594 594 594 594 594

11.11 11.11 10.00 10.00 15.00 7.78

18.44 22.81 33.35 32.50 32.30 32.10

403 403 642 642 642 642

5.00 item

carbon content in the fly ash (%) O2 content (vol %) exhaust gas temperature (K) NOx (mg/m3, 6% O2, before the flue gas denitrification system)

300 600 MW (original) measured calculated measured calculated measured measured calculated

4.13 3.96 2.25 2.22 400.2 1501 1351 5.62 45 1875 818 816 91.05

carbon content in the bottom slag (%) CO (ppm) flow rate of main steam (ton/h) main steam temperature (K) reheat steam temperature (K) boiler efficiency (%)

10.00 42.10 20.00 42.84 5.00 450 MW (original) 600 MW (novel) 3.98 3.77 2.67 2.46 390 1127 1058 3.51 23 1174 821 819 90.52

4.30 4.05 2.31 2.30 395.4 751 729 4.73 76 1905 817 815 91.31

642 642 300 450 MW (novel) 4.54 4.36 2.45 2.43 390.7 648 615 3.70 10 1353 815 814 90.16

2.3. Numerical Simulation. Pulverized coal combustion is a series of complex physical and chemical reactions consisting of turbulent flow, combustion, heat and mass transfer, etc. In this work, a computational fluid dynamics (CFD) program, Fluent (version 6.3.26), was used to carry out the numerical simulation. The realizable k−ε method was applied to the gasphase turbulent fluid flow simulation. The coal particle motion in the furnace was calculated by the stochastic particle method. The radiation heat transfer and the heat absorption coefficient of combustion gases were taken into account by the P-1 model and the cell-based weighted-sum-of-gray-gas (WSGG) method, respectively. Particle emissivity and particle scattering factor in pulverized coal combustion were set at average constants of 0.9 and 0.6, respectively.26−28 The pulverized-coal combustion process generally consists of two stages: devolatilization and char combustion. Coal devolatilization was simulated by the two-competing-rate model, and char combustion was calculated with the diffusion/kinetics model. The parameters of the twocompeting-rate model and the diffusion/kinetics model can be found in the Supporting Information. The gas-phase turbulent combustion was employed by the mixture fraction/probability density function (PDF) method. Detailed descriptions about these above-mentioned models can be found in the literature.29−31 The NOx formation is a complex process because it includes fuel NOx, thermal NOx, and prompt NOx. In this work, prompt NOx formation was not taken into account because of its low proportion in the pulverized furnace; thus, only fuel NOx and thermal NOx were taken into account.

secondary air are supplied through annular ports around the fuel-lean nozzles to cool the burner nozzles (named as D-layer secondary air). (4) The F1- and F2-layer secondary air are merged into a new secondary air scheme called the F-layer secondary air, and it is supplied at a declination angle of 20° through the novel nozzles instead of the original slit ports. (5) A part of the secondary air (accounting of approximately 20% in total air mass) is separated from the secondary air box to be directed as SOFA to form a low-oxygen and reducing atmosphere in the lower furnace. The direct-flow SOFA nozzles are arranged symmetrically on the front and rear walls in the upper furnace zone at a declination angle of approximately 30°. The introduction of vent air below the arches and novel-staged air at a declination angle is in favor of (i) further increasing the residence time of pulverized-coal particles to obtain more reaction times and promoting fuel-rich ignition because of the vent air carrying much of the moisture from the coal and (ii) extending the flame penetration depth and postponing the mixing of the coal/air stream with secondary air to inhibit NOx formation. Positing SOFA in the upper furnace is to strengthen air-staging conditions and enlarge the reducing zone in the lower furnace region as far as possible. With this technology, the most important merit is to reduce NOx emissions without causing an obvious increase in the carbon content of fly ash after introducing SOFA. The detailed schematic of the novel combustion system can be found in Figure S3 of the Supporting Information to understand clearly the flow and combustion principle. C


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Figure 2. Comparison of experimental and calculated gas temperatures.

Figure 3. Comparison of calculated flow field and the trajectories of coal particles.

The model by De Soete was used to simulate the formation of fuel NOx.32 It was assumed that fuel N was distributed between the char and volatiles. The char N directly converted to NO, and the volatile N converted to intermediates, such as CHN

and NH3 (CHN/NH3 = 9:1). The chemical percolation devolatilization (CPD) model was adopted to calculate the mass fraction of volatile N and char N,33 and they were 0.2309 and 0.7691 in this paper, respectively. The conversion fraction D


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For the original combustion system (Figure 3a), it is noted that the arch recirculation zones below the arches are relatively small and not obvious. This is disadvantageous to the initial mixing of the high-temperature flue gas with the coal/air mixture to weaken the convection and radiation heat transfer and postpone coal ignition. The relatively small recirculation zones may be explained as follows: (1) The weak rigidity of the arch airflow causes the airflow to decay more quickly. As a result, the arch airflow redirects downward at a short travel path and then flows upward. (2) The secondary air is blocked by the horizontal D, E, F1, and F2 layers (especially the F1- and F2layer secondary air), which also shallows the penetration depth of the arch coal/air mixture flow. Because of the arrangement with positing the vent air ports over the furnace center on the arches, it rapidly redirects its travel path and then flows toward the upper furnace. In addition, suppressed by the downward coal/airflow, the F2-layer secondary air maintains its initial horizontal direction for a long distance from the slit outlets and then redirects upward. As a consequence, it is also observed that a considerable volume is not shared in the lower part of the lower furnace. After adopting a novel combustion system, a symmetric flow field appears obvious. It is observed in planes b and c of Figure 3 that the recirculation zones below the arches become considerably larger and the travel path of the arch airflow is prolonged. These are favorable for (i) entraining more hightemperature flue gases to promote the convection and radiation heat transfer and to enhance flame stability and (ii) increasing the airflow fullness level and the particle residence time to enhance pulverized-coal burnout degree. These can be explained by the following: (1) In contrast with the original, A-layer secondary air with high speed and strong rigidity is jetting into the furnace over the furnace center zone to sharply prolong its travel path in the lower furnace. (2) Secondary air from the D and F layers is fed at a set declination angle, thereby creating a forceful ejecting effect on the arch airflow to further prolong the airflow travel path in the lower furnace. (3) It is also observed that vent airflow is fed to the furnace with a declination angle below the arches, which also results in a longer penetration and travel path for the fuel-lean coal/air mixture. In addition, A-layer secondary air forms a wind shadow near the furnace throat region to prevent flame short circuit, which further improves the flame stability in the initial combustion stage. The F-layer secondary air lifts the arch airflow to prevent the arch airflow from rushing into the ash hopper wall. Meanwhile, SOFA with high speed and strong rigidity initially flows downward and toward the furnace center with an initial momentum. When airflow begins to collide with upward flue gas, the collisions are redirected into flowing vertically upward. Planes d−g of Figure 3 show the trajectories of the coal particles. The majority of them follow the airflow travel path, i.e., flowing downward into the lower furnace and then redirecting upward. For the coal particles from the fuelrich nozzles, the trajectories of the novel (shown in Figure 3f) penetrate deeper than that of the original (shown in Figure 3d) in the lower furnace, which helps increase the residence time of the primary coal particles. For the coal particles from the fuellean (i.e., vent) nozzles, the travel path significantly increases by comparing planes e and g of Figure 3, which contributes to further increase the residence and reactive time of fuel-lean particles in the lower furnace. Thus, the longer pulverized-coal travel path of the novel is beneficial to the coal burnout.

of char N directly converting to NO was 0.5. The formation of thermal NOx was expressed by the extended Zeldovich mechanism.34 Grid divisions and the pulverized-coal size distributions in the calculation can be found in the Supporting Information. 2.4. Full-Scale Industrial Measurements. Industrial measurements on the boiler performance before and after the combustion modification were performed at 600 and 450 MWe loads, respectively. During the experiments, pulverized coal was sampled from the exit of various coal hoppers and was mixed before testing for properties. The used coal of pre- and postmodification is similar. The total mass rate of coal is 93.4 kg/s (as received, at full load). The coal characteristics and the respective operating parameters are listed in Table 1. The mean gas temperatures located near the right-side wing wall and the burner region were performed with a fine-wire thermocouple device. A Testo-925 was used to measure the exhaust gas temperature at the air preheater exit by adopting a grid-based method, and the final exhaust gas temperature was acquired by averaging the data of measured points. A MSI EURO-type flue gas analyzer was employed with the grid-based method to measure the O2, CO, and NOx concentrations in the flue gas at the entrance of the air preheater. Fly ash and bottom ash samples were collected and tested by using a proximate analyzer. More details about the measurements can be found in the Supporting Information.

3. RESULTS AND DISCUSSION 3.1. Validation of the Numerical Results. Figure 2 shows the trend of temperature changes at the axis of one burner and the measured and calculated gas average temperatures near the right-side wall (about 1.5 m). The overall trend of gas temperatures achieves well consistency between measured values and calculated results, except for some apparent deviations. This may be because any variation of in-furnace operating parameters will strongly influence the level and extent of the combustion, or some ash may deposit on the thermocouple, despite trying to keep the device clean. Therefore, it is very difficult to attain full consistency between the measured values and calculated data under the present complicated combustion conditions. The comparison between the calculated and measured carbon contents in fly ash, O2, and NOx concentrations of flue gas at the entrance of the air preheater is summarized in Table 1. These predicted values are also consistent with those measured. Therefore, the mesh and models adopted in this work are reasonable to investigate the characteristics of flow, combustion, and NOx emissions for the studied boiler. 3.2. Comparison of Simulated Results to the Original and Novel Combustion Systems. The calculated flow field, trajectories of coal particles, flame stability curves, gas temperature distribution, and concentration distributions of O2 and NOx between the original and novel combustion systems are presented in Figures 3−6, respectively. The fuelrich and fuel-lean (i.e., vent) cross-sections are taken out along the longitudinal vertical section of the fuel-rich and fuel-lean coal/air nozzles, respectively. For the phenomenon regarding the penetration depth of the flow field in the taken-out fuel-rich cross-section, temperature and concentration distributions are deeper in the front-wall region than in the rear-wall region. This may be caused by the asymmetric arrangement of the C-layer secondary air ports on the arches (as seen from the Supporting Information). E


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Figure 4. Comparison of calculated temperature distributions.

distance in the lower furnace, which leads to the burning of more pulverized coal in the upper region. After adoption of the novel combustion system (planes d and e of Figure 4), symmetric temperature distributions also appear and the highest temperature zones are positioned in two locations where pulverized coal passed through: the first region is not far from the fuel-rich nozzle outlet, and the second region is not far from the fuel-lean nozzle outlet. The highest temperature zones are present further away and larger than that of the original combustion system. The flame penetration depth significantly increases in the lower furnace to bring a positive effect on the pulverized-coal burnout. Figure 4b shows the flame images of the original and novel combustion systems through the observation port 2 with a camera and also demonstrates the improvement of the flame penetration depth. Meanwhile, a restrained combustion and releasing heat in a larger region develops, which contributes to attenuate the centralized combustion. This is because of the following reasons: (1) The mixing between the coal/air and secondary air stream is gradual; thereby, the reaction between pulverized coal and air needs more time. (2) The combusting pulverized-coal penetration depth significantly increases. (3) The fuel-lean/air stream is transported through nozzles from the rear and front walls, therefore making the fuel-lean coal combust in a lower location zone. (4) The introduction of SOFA causes the lower

Figure 4 displays the comparison to temperature distributions and fields of the original and novel combustion systems. As shown in Figure 4a, the ignition position is defined as the point at which the temperature at the center of the burner is 1000 K.35 In comparison to the original, the ignition position of the novel combustion system advances by approximately 1.0 m, which indicates the improvement of flame stability markedly. This is caused by (i) the larger recirculation zones below the arches and (ii) moving vent air from the arches to the front/ rear walls (the vent air carrying much the moisture from the coal). For temperature distributions at the cross-section, the high-temperature region is located below the arches but not far from the burner outlet in the original (Figure 4c). An explanation for this is that the rapid mixing of secondary air with the coal/air mixture results in the intense and centralized combustion of pulverized coal in an oxygen-rich atmosphere below the arches. It can also be seen that the flame penetration depth is relatively shallow because of the horizontal staged air (D-, E-, F1-, and F2-layer secondary air) hindering the air/coal mixture penetration. The fullness of the flame is also relatively poor in the lower furnace, especially in the zones below the F2 layer. In addition, the flame center is relatively high, which corresponds to a large de-superheating water rate during actual operation. This can be explained by the coal short travel F


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Figure 5. Comparison of calculated O2 concentration distributions.

Figure 6. Comparison of calculated NOx concentration distributions.

region. The average temperature curve of the novel shows a drop in the SOFA port regions as a result of SOFA jetting but then increases for the continuous combustion of char. Figure 5 presents the O2 concentrations for the two settings. In the original (Figure 5a), the low-O2-concentration region is mainly located above the E-layer secondary air. The O2 concentration near the F1- and F2-layer secondary air is relatively high. This confirms that the fuel/air stream cannot penetrate the F-tier airflow zone and that no combustion occurs in this area. After adoption of the novel combustion system (planes b and c of Figure 5), a perfectly symmetric O2 concentration field appears and corresponds to the symmetric flow and temperature fields. The high-temperature and low-O2concentration region moves downward as a consequence of the deeper flame penetration depth. This process is helpful for increasing the reactive time of the pulverized-coal particles in the low-O2-concentration reducing atmosphere. In the upper region, SOFA is directed toward the furnace center and mixes with the upward high-temperature flue gas to supply sufficient oxygen needed during the pulverized-coal burnout process. As a

furnace to be in an oxygen-lean atmosphere, which further results in the release of heat in the larger region. The fullness of the flame significantly increases in the lower furnace to decrease the volume heat load. The F-layer secondary air below the fuellean coal/air nozzles forms an air screen to lift the downward flame, which is beneficial to prevent the hopper walls from slagging. In the upper region, the char continues to combust and releases heat as a result of the injection of SOFA, which further improves coal burnout. In addition, the flame center moves downward, and the temperature in the upper furnace region is lower than that of the original combustion system to some extent. This phenomenon corresponds to a small desuperheating water rate and, during actual operation, attributes to the safe operation. The mass-weighted average temperature versus the furnace height is presented in Figure 4f, and some differences exist compared to the original combustion system. The average gas temperature of the novel is sharply higher than that of the original in the lower part region of the lower furnace. This is because the flame penetration depth increases and the restrained combustion releases heat in the lower (and larger) G


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Environmental Science & Technology Table 2. Proximate and Ultimate Analyses of Experimental Coal and Measured Results under the Original and Novel Combustion Systems (Another Similar 600 MW FW Boiler) proximate analysis (wt %, air dried)

ultimate analysis (wt %, air dried)

V

M

A

FC

C

9.98

1.96

38.09 item

49.97

51.86

H

O

S

2.35 4.31 0.78 600 MW (original) 450 MW (original)

carbon content in the fly ash (%) O2 content (vol %) exhaust gas temperature (K) NOx (mg/m3, 6% O2, before the flue gas denitrification system) carbon content in the bottom slag (%) CO (ppm) flow rate of main steam (ton/h) main steam temperature (K) reheat steam temperature (K) boiler efficiency (%)

3.45 2.29 425 1245 3.45 30 1834 817 813 90.72

3.36 2.93 410 961 2.25 17 1197 815 813 90.75

Qnet (kJ/kg, as received)

N

0.65 600 MW (novel)

17,476 450 MW (novel)

3.93 2.40 395.4 592 3.67 31 1901 815 814 91.46

3.40 3.10 401.3 562 3.54 16 1346 816 814 91.94

concentration atmosphere. (3) The secondary-air-staging level is considerably enhanced. The mixing of fuel with secondary air is postponed, and the reducing atmosphere regions, with a low O2 concentration, become larger in the lower furnace. (4) Applying the SOFA system restrains combustion and strengthens the reducing and oxygen-lean environment in the lower furnace. These cause the reduction of NOx in the gas. A greater amount of generated NOx is restored into N2 by coke and also into HCN, NH3, and other intermediate products in the flue gas. The mass-weighted average of the NO x concentration along the furnace height is presented in Figure 6d. Generally, the pattern shows an initial increase and then a continual decrease. Initially, the NOx concentration curve rapidly increases with the increase in the furnace height and reaches a maximum at the height of approximately 16.0 m (in this area, the combustion is very intense). Then, it gradually decreases through the furnace height. In the hopper area, with the furnace height changing from 0 to 11.0 m, the NOx concentration is higher under the novel combustion system than under the original combustion system. This can be explained by the occurrence of combustion in this area under the novel combustion system, and a greater amount of NOx is produced. In contrast, the combustion of the original combustion system is weak in this zone. In comparison to the original, the NOx concentration has a considerable decrease in the dominated combustion region. It is suggested that this is caused by the strong oxygen-lean reducing atmosphere that significantly restrains the initial formation of NOx and simultaneously reduces the formation of NOx to N2. In the upper furnace region, under the novel combustion system, the concentration of NOx is continually maintained at a low level. 3.3. Boiler Performance under the Pre- and Postmodification of the Combustion System. After modification, there are smaller fluctuations of the in-furnace negative pressures under different cases for a period of operation, which reveals more stable combustion. The de-superheating water rate greatly decreases, even though there is no de-superheating water at some cases, from 100 ton/h (the original) to 20−30 ton/h (the novel). The steam parameter values also achieve the designed values. Table 1 summarizes the measured performance parameters under 600 and 450 MWe loads with the original combustion system compared to the novel combustion system for the studied boiler in this paper. It can be seen that the NOx emissions at 600 MWe decrease by 50%, from 1501 to 751 mg/m3 (O2 at 6%, before the flue gas denitrification

result, the O2 concentration falls gradually in the burnout region and is due to the reaction with the unburned char in the flue gas. The mass-weighted average of the O2 concentration along the furnace height is presented in Figure 5d. In comparison to the original combustion system, the average O2 concentration in the lower furnace zone decreases dramatically for the novel combustion system, thereby resulting in an oxygen-lean atmosphere in this area. The position of the lowest level average O2 concentration is approximately z = 14.0−16.0 m (while the height is approximately z = 16.0 m for the original), which indicates that the flame center moved downward at a certain degree and that combustion occurred in a larger region. In the upper furnace region, with SOFA jetting, the O2 concentration rises slightly to approximately the z = 24.0 m level. Although the oxygen-lean atmosphere in the lower furnace may be disadvantageous for complete burnout, the longer travel path and residence time offset this negative effect. Therefore, as shown in the burnout region in Figure 5d, the O2 concentration is at a level similar to the original combustion system. Thereby, the burnout degree of pulverized coal cannot be impacted. Planes a−c of Figure 6 present the NOx concentration distributions. In the original combustion system, the pulverizedcoal combustion produces considerable amounts of NOx, especially at the stage of initial combustion. A significant amount of NOx is generated in the burner zones with the oxygen-rich and high-temperature atmosphere. One of the main reasons for the highly centralized combustion is the quick formation of initial NOx. In the lower furnace, poor air staging and a weak reducing atmosphere do not effectively restore the presence of NOx in the flue gas. After adoption of the novel combustion system, the high NOx concentration zones and maximum NOx concentration decrease. The novel combustion system not only inhibits the formation of NOx at the initial coal combustion stage but also largely reduces any formation of NOx to N2. This can be explained by the following: (1) The vent air moves from the arches to the front/rear walls. It is beneficial to postpone the mix of the fuel-rich and fuel-lean streams to reduce the fuel NOx formation near the burner region. Meanwhile, less heat released from coal combustion develops below the arches not far from the burner outlet in the oxygen-lean atmosphere, which restrains the formation of thermal NOx in the initial pulverized-coal combustion stage. (2) The penetration of the flame is deeper to increase the residence time of pulverized-coal particles in the low-O2H



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system), and at 450 MWe decrease by 42%, from 1127 to 648 mg/m3 (O2 at 6%, before the flue gas denitrification system). The carbon content in fly ash increases only slightly from 4.13 to 4.30% at 600 MW load and from 3.98 to 4.54% at 450 MWe load. The overall evaluation of the combustion and NOx emissions show that the performance of the studied boiler is successfully improved with the novel combustion system. Another similar boiler has been modified with the same novel combustion system; the steam parameter values achieve the designed values; the combustion stability is also enhanced; and the de-superheating water rate decreases. The measured performance parameters can be found in Table 2. It also indicates that the performance of the similar boiler is successfully improved with the novel combustion system. The industrial measurements show that NOx emissions decrease by 52% (at 600 MWe) and decrease by 42% (at 450 MWe). The carbon content in fly ash increases only slightly from 3.45 to 3.93% at 600 MWe load and from 3.36 to 3.40% at 450 MWe load. The performance of this boiler also improves successfully with the novel combustion system. In conclusion, for the FW down-fired boiler, the novel combustion system forms larger recirculation zones below the arches to strengthen the combustion stability considerably and markedly extends the coal/air downward penetration depth to contribute to the burnout degree. The introduction of SOFA results in a low oxygen concentration and a strong reducing atmosphere in the lower furnace region to reduce NOx emissions. The performance of the modified FW boilers indicates that NOx emissions decreased significantly, while the carbon content in the fly ash increased only slightly. The novel combustion system can achieve the effects of high-efficiency and low-NOx combustion for FW down-fired boilers.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.5b02827. Figures S1−S4, Table S1, brief introduction of the utility boiler, details of the CFD model, grid division, numerical simulation and other important settings in the simulations, and industrial measurements (PDF)

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Article

AUTHOR INFORMATION

Corresponding Authors

*Telephone/Fax: +86-27-87540249. E-mail: [email protected]. cn. *Telephone/Fax: +86-731-85605384. E-mail: dangzhenlv@ 163.com. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support for this research from the Project on the Foundation of State Key Laboratory of Coal Combustion, Youth Foundation of Huazhong University of Science and Technology (2014QN185) and the National Natural Science Foundation of China (51390494). I


Article

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