Particle Flow Characteristics, Combustion and NOx Emissions of

Article pubs.acs.org/EF

Gas/Particle Flow Characteristics, Combustion and NOx Emissions of Down-Fired 600 MWe Supercritical Utility Boilers with Respect to Two Configurations of Combustion Systems Min Kuang, Zhengqi Li,* Qunyi Zhu, Lizhe Chen, Yan Zhang, and Jinping Lai School of Energy Science and Engineering, Harbin Institute of Technology, 92 West Dazhi Street, Harbin 150001, P.R. China ABSTRACT: Using a phase-Doppler anemometry (PDA) on a two-phase small-scale model for a down-fired 600-MWe supercritical utility boiler, experiments were conducted to compare the gas/particle flow characteristics between the prior and newly designed deep-air-staging combustion systems. The distributions of mean velocity, particle volume flux, and particle number concentration along several cross sections were compared between the two combustion systems, in addition to the decay and trajectory of the downward gas/particle flow. With the prior combustion system, asymmetric gas/particle flow characteristics appear in the furnace, with the gas/particle flows in the front-half of the furnace penetrating greatly further and occupying much more furnace volume than those in the rear-half of the furnace. The longitudinal gas/particle velocity components, particle volume flux, particle number concentration, decay curve, and trajectory of the downward gas/particle flow all display a severe asymmetric pattern along the furnace center. In applying the deep-air-staging combustion technology as a replacement for the prior art, the original asymmetric gas/particle flow characteristics that are seen in the furnace all develop a symmetric pattern. Industrial-sized measurements performed within the full-scale furnace uncovered that asymmetric combustion characterized by gas temperatures being much higher near the rear wall than near the front wall, developed in the boiler with the prior art. In comparison with the boiler with the prior art, the newly designed boiler applying the deep-air-staging combustion system achieved perfectly symmetric combustion, NOx emission reduction by around 40%, and large improvement in burnout.

1. INTRODUCTION Low-volatile fuels, such as anthracite and lean coal, require more time to ignite and complete combustion or come near completion because of their low hydrogen content and volatile matter.1,2 To overcome the problems associated with low ignitability and combustibility of this type of fuels, down-fired boilers, enhancing the coal burnout by maintaining high gas temperature levels and by increasing the pulverized coal residence time in these combustion systems, were designed for industrial firing of low volatile coals.3,4 However, actual operating results show that down-fired boilers suffer from various problems such as late coal ignition and poor stability,5−7 low burnout (carbon in fly ash typically in the range 8− 15%),8−10 heavy slagging,11,12 asymmetric combustion,13−16 and high NOx emissions (in the range 1100−2000 mg/m3 at 6% O2).2−4,8−10,13,17−19 Of these problems, high carbon content in fly ash, heavy slagging, and high NOx emissions are hot topics and have attracted most of the attention paid on down-fired boilers. Again, research has been reported well on various methods in dealing with them, such as inclining downward the F-layer secondary air to improve the coal burnout,6 cutting off the burners close to the side walls to alleviating the serious slagging on the side walls,11 parametric tuning of operating conditions and combustion system retrofits to reduce NOx emissions and improve the coal burnout.2−4,6,8−10,19−22 Asymmetric combustion, with large differences in volumetric heat load distribution in the furnace, creates adverse effects in down-fired boiler operations and should be eliminated or mitigated by the greatest extent. Because of the symmetric furnace configuration and air distribution in the lower furnace, © 2012 American Chemical Society

in China serious asymmetric combustion is thought to be a very strange phenomenon that has appeared in all in-service subcritical down-fired boilers made by Mitsui Babcock Energy Limited (MBEL) Corp. However, little research has been reported on this topic, with the exception of the results acquired in several subcritical down-fired boilers with a capacity grade of 300 or 600 MWe from Li et al.,13 Kuang et al.,14 and Miao and Wang.15 According to cold small-scale airflow experiments and industrial-sized measurements for down-fired 300 and 350 MWe subcritical utility boilers, Li et al.14 and Kuang et al.15 found that serious asymmetric combustion developed in the furnaces, with large differences arising in the gas temperature levels between the regions near the front and rear walls. The appearance of a deflected flow field in the lower furnace was the major reason given for this asymmetric combustion. Afterward, Kuang et al.23−25 have put forward a comprehensive low-NOx down-fired combustion technology associated with eliminating this asymmetric combustion and minimizing slagging, that is, a deep-air-staging combustion technology based on the concept of multiple injection and multiple staging (for brevity, the “MIMSC technology”). Although the flow-field deflection characteristics in these MBEL down-fired subcritical boilers has been well studied by Li et al.13 and Kuang et al.,14,24,26 the particle behavior within this deflected flow field remains unknown. Again, all of these results involving in asymmetric combustion and flow-field deflection were found in subcritical boilers. That is, the gas/particle flow Received: February 25, 2012 Revised: May 17, 2012 Published: May 22, 2012 3316

dx.doi.org/10.1021/ef300324f | Energy Fuels 2012, 26, 3316−3328

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Figure 1. Schematics of the furnace and combustion system of the MBEL down-fired 600-MWe supercritical boiler (dimensions in mm).

combustion system reconfigurations of new designs, research on the flow and combustion characteristics are clearly necessary to be devoted to these four newly designed down-fired supercritical boilers. Aside from the flow field, pulverized-coal particle behaviors are of great importance in pulverized-coal combustion in a furnace. In practice situations, process parameters such as flow field and pulverized-coal particle behaviors in a full-scale furnace are almost impossible to obtain. Although cold modeling methods are commonly believed to be unable to model accurately enough actual hot coal/air flows because of differing heat expansions within the flows in the fullscale furnace, cold gas/particle flow characteristics acquired in

and combustion characteristics within MBEL down-fired supercritical boilers have not been uncovered. One of the first two 600 MWe MBEL supercritical down-fired boilers has been in service since January, 2012, in China. Meanwhile, another two newly designed down-fired 600 MWe supercritical utility boilers (with the same furnace as those for the previous two MBEL supercritical down-fired boilers), which had been set to adopt the prior MBEL art in its initial design scheme, finally implemented the proposed MIMSC technology, but without applying over-fire air. To date, one of the two newly designed boilers is in a trial run before its 168-h test. For good operations of down-fired supercritical boilers in service and for 3317


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Figure 2. Schematics of the combustion system of the down-fired 600-MWe supercritical boiler applying the MIMSC technology (dimensions in mm). upper furnace and the octagonal lower furnace with four wing walls constitute the whole furnace. A total of 24 cyclones symmetrically arranged on the arches divide the primary air/fuel mixture into fuelrich and fuel-lean coal/air flows needed to regulate fuel rich/lean combustion. The fuel-rich coal/air flow is vertically supplied into the regions near the front and rear walls, while the fuel-lean coal/air flow is injected with a 15° angle inclined toward the furnace center. There are 24 burners evenly lining the front and rear arches, aiming at establishing a relatively uniform heat load distribution in the furnace. Two fuel-rich coal/air flow nozzles, two fuel-lean coal/air flow nozzles, and four secondary-air ports feed each burner. Staged-air is horizontally fed into the lower furnace through 12 groups of stagedair slots (47 slots for each group, each slot with an outlet area of 16 mm × 750 mm). The present air distribution pattern is similar to those of the subcritical boilers in the literature.13,14,24,26 Figure 2 depicts a schematic diagram of the deep-air-staging downfired combustion system configured with the proposed MIMSC technology on the same furnace. Here, the 24 cyclones are replaced by 24 louver concentrators with a much lower operation resistance. A detailed description of the configuration and operational principle of the louver concentrator can be found elsewhere.8 Staged air, with a large increase in each slot outlet area (15 slots for each group, each slot with an outlet area of 38 mm × 905 mm), is fed at a set declination of 45°, instead of the previously horizontal direction. The detailed

small-scale models (following certain similarities in modeling criteria) can more or less describe those in full-scale furnaces. In this way, the pulverized-coal combustion dynamics can be predicted to a certain extent. Therefore, the gas/solid flow behavior of full-scale furnaces is investigated widely by performing gas/solid two-phase flow experiments in smallscale models.27−30 We present our experimental comparison of the gas/particle flow characteristics involving the two combustion systems (i.e., applying the prior MBEL art and MIMSC technology, respectively), acquired within a 1:40-scaled model of the fullscale furnace for the four mentioned supercritical down-fired boilers. In addition, in situ measurements of the gas temperature distribution in the regions near the front and rear walls, carbon content in fly ash, and NOx emissions in flue gas for the two mentioned boilers with the prior MBEL art and MIMSC technology, respectively, are used to uncover their furnace performance.

2. EXPERIMENTAL SECTION 2.1. Utility Boiler. Figure 1 presents the schematic of the furnace and combustion system with the prior MBEL art. The rectangular 3318


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Figure 3. Cold small-scale gas/solid two-phase flow experimental system incorporating the prior MBEL art.

Table 1. PDA Parameters for the Measurable Range and Measurement Error quantity

velocity

particle diam.

particle concn.

measurable range measurement error

0−500 m/s 1%

0.5−1000 μm 4%

0−106 particles/cm3 15%

method that realizes the MIMSC technology and description of technical principles of the proposed technology can also be viewed in the literature.23−25 2.2. Small-Scale Gas/Particle Flow Experiments. Figure 3 shows the experimental system incorporating the prior MBEL art. Within the 1:40-scaled model of the real furnace, the origin of the coordinate system is set at the centerline. For each measurement point, Vy denotes the longitudinal-velocity component in the downward direction. V0 signifies the outlet airflow velocity of the fuel-rich coal/flow nozzle for each technology. X0 is the horizontal distance between the front and rear walls in the lower furnace. X signifies the distance from the furnace centerline to a measurement point along the x-direction. H0 is the vertical distance between the center of the fuel-rich coal/air flow nozzle outlet and the upper edge of the bottom hopper, whereas H is the vertical distance from a measurement point to the center of the fuel-rich coal/air flow nozzle outlet. The experimental system with the MIMSC technology applies the same model, but with the model burner arrangement depicted in Figure 2. There is no change in the location of the fuel-rich flow nozzle centerlines for either technology; H0 and H remain pertinent for both. A three-dimensional phase-Doppler anemometry (PDA) produced by DANTEC Company was used as the test apparatus for the smallscale gas/particle flow experiments in the present work. The PDA uses proven phase Doppler principles for simultaneous nonintrusive and

real-time measurements of each velocity component and turbulence characteristics and utilizes new triangulation methods using phase differences between Doppler signals received by three detectors located in different positions. A more detailed description of the PDA operation principle can be found elsewhere.27,28 Using the PDA, lots of data such as the mean velocity, particle volume flux, and particle number concentration of a two-phase flow can be measured. Table 1 lists the PDA parameters for the measurable range and measurement error. Although the flow field in a down-fired furnace is very complex and three-dimensional, velocities along the furnace breadth are relatively small in the lower furnace, except for the zone below large gaps between the burners (Figures 1 and 2). This is because, in the direct-flow split burner layout, either a nozzle or port is separated by a very small gap (i.e., about 100 mm in the real furnace along the furnace breadth direction). The split-shaped jet emanating from either a nozzle or port outlet is narrow, because of the inhibiting nearby jet. Therefore, nonaxial velocities are relatively small in the zone below the burners. Here, our cold experiments focus on the W-shaped flow field in the lower furnace along a vertical cross-section through one burner. Thus, the present PDA measurements only focused on twodimensional results (i.e., results along the x- and y-directions). The gas/particle flow characteristics in the furnace were acquired along the longitudinal cross section intersecting the center line of one pair of the fuel-rich coal/airflow nozzles. 3319


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whose Reynolds numbers range 20−40 in the main flow zone of the lab-scale model, were chosen in the present experiments, leading the lab-scale flows in the same transition region as those in the full-scale ones. According to Ren et al.,28 the inertia force of the particles with such a large diameter makes the experimental flows deviate from the original full-scale ones more or less, whereas the calculation indicates that both relaxation times for the particles with diameters of 10 and 42 μm are very small (i.e., the order of magnitude is from 10−4 to 10−2 s). From the comments on the accuracy,28 the present labscale experimental results can still indicate the particle motion in the full-scale furnace, though not very accurately. Of the used glass beads, fine particles with diameters from 2 to 8 μm are hollow glass particles, whose mean diameter is around 5 μm, density is 600−650 kg/m3, and proportion in total glass beads is 2.5 wt %. The other larger glass particles, whose density is 2500 kg/m3, are full solid particles. Reynolds numbers of these fine particles are less than 1; thus, these fine particle flows are in the Stokes regime, where particles can trace the gas motions tightly. Therefore, particles with diameters from 2 to 8 μm were used to trace the airflow in the present experiments, and particles with diameters from 10−100 μm were used to represent particle phase flow. Particles with diameters between 2 and 100 μm were used for analysis of the particle-volume flux. (5) The same ratios of momentum flux rate. The ratios of momentum flux rate among the airflows of the small-scale model are consistent with the full-scale furnace. For the two experiment settings corresponding to the two combustion technologies, the airflow velocity and air mass flow rate for each of airflows into the cold small-scale model are modeled from design parameters of the real furnace at normal full load. Of design parameters for the prior art, the velocities of the fuel-rich and fuel-lean coal/air flows, near-wall air, secondary air, and staged air are 11.00, 22.57, 26.78, 36.20, and 21.60 m/s, corresponding to mass flow rates of 49.23, 73.84, 35.30, 393.38, and 75.65 kg/s, respectively. Corresponding air velocities in the modeling experiments are 11.04, 19.20, 17.43, 23.01, and 14.19 m/s, respectively. For the design parameters with the MIMSC technology, the velocities of the fuel-rich and fuel-lean coal/air flows, secondary air (the same velocity for inner and outer secondary air), and staged air are 15.00, 22.57, 55.00, and 55.00 m/s, corresponding to mass flow rates of 49.23, 73.84, 312.97, and 156.85 kg/s, respectively. Corresponding air velocities in the cold airflow experiments are 16.00, 19.59, 36.80, and 37.19 m/s, respectively. The pulverized-coal loads within the fuel-rich and fuellean coal/air flows of the real furnace are 1.38 and 0.16 kg (coal)/kg (air), respectively. Accordingly, glass particle loads within gas/particle flows of the small-scale model are the same as those for the full-scale version. 2.3. In Situ Measurements. As mentioned previously, the boiler with the prior MBEL art has been in service since January, 2012, whereas the newly designed one with the MIMSC technology is still in a trial run before its 168-h test. In consequence, in situ experiments with measurements taken of gas temperatures in the near right-wall regions, carbon content in fly ash, and NOx emissions in flue gas, were performed at normal full load for the real furnace with the prior MBEL art and at a 550 MWe load during the trial run for the newly designed one with the MIMSC technology. A 3i hand-held pyrometer (a type of noncontact infrared thermometer made by Raytek, Santa Cruz, CA),8 with a measurement range from 600 to 3000 °C, accurate to within 1 °C and with an error of ±30 °C, was inserted through several monitoring ports on the wing walls adjacent to the right-side wall to measure the highest gas temperatures in the near-wall regions adjacent to the front and rear walls. The captured gas samples at the air preheater exit were analyzed online by a Testo 350 M instrument. The measurement error associated with the Testo 350 M was 1% for O2 and 50 ppm for NOx. Although industrial-sized data at full load are absent within the newly designed furnace, it is thought the present in situ measurements at 550 MWe would be close to those at full load because only one twelfth burner nozzles were decommissioned at 550

All airflow rates into the model were measured by Venturi tube flowmeters with measurement errors of less than 10%. In considering that the burners on the arches are independent from each other and no difference exists between them, we thus selected two symmetric burners to supply glass beads into the zone below the front and rear arches by the four powder feeders. Glass beads were fed into the fuelrich and fuel-lean coal/air flow ducts and then carried by the primary airflow into the small-scale model. During the experiments, some of the smaller particles were lost due to the low efficiency of the cyclone separator. Thus, fine particles needed to be introduced at a certain interval so as to make sure that the particle size distribution did not change much. The similarity criteria for particle laden flows refer to Reynolds (Re), Froude (Fr), and Stokes (St) numbers, whose definitions are provided as follows:

Re =

Fr =

St =

ug l v

(1)

ug2 gl

(2)

ρp ug nd n + 1 cμg n lρg1 − n

(3)

where ug is the gas velocity, l is the characterized length, v is the kinematic viscosity of gas, g is the acceleration of gravity, ρp is the particle density, ρg is the gas density, d is the particle diameter, and μg is the dynamic viscosity of gas. For burner nozzles and the lower furnace cross section, the characterized lengths are their equivalent diameters in the model. In the main flow zone of the real furnace, the most particle Reynolds numbers (Rep) are in 2−5; thus, the flows are in the transition region (also called Allen region where 1 < Rep < 750), which is between the Stokes region and Newton region. Accordingly, values of parameters n and c are 23.4 and 0.725, respectively. The similarity criteria for particle laden flows are quite complex and difficult to satisfy. Usually, we could only ignore some of the criteria of less importance and make approximate modeling. The main criteria are discussed as follows:

(1) Geometric similarity. The ratio of the small-scale apparatus to the full scale is 1:40.

(2) Euler rule for self-similar flow. The limited Reynolds numbers of the primary and secondary airflows through direct-flow split pulverized-coal burner nozzles are 1.48 × 105 and 0.75 × 105, respectively. However, the flow behavior of the jets after leaving their respective nozzle outlets and the mixture characteristics between airflows do not change when the Reynolds number is higher than 1 × 104. The limited Reynolds number for the flows through the small-scale model of the utility boiler is 3 × 104. In the present small-scale model, the Reynolds number for the airflows through the burner nozzles is set as 1.5 × 104 and that for the airflows through the small-scale model is higher than 5 × 104. Thus, given these comparative values, the airflows through the model burner nozzle and in the model furnace are thought to be self-modeling. (3) Froude criterion. For most of the forced flows with small size particles, the Froude number shows much less importance than other criteria, such as Reynolds number. For the modeling of inertia equipment, with the diameters of the particles less than 200 μm, the influence of the Froude number can be ignored.28 Because all particles were 2−100 μm in diameter and some recirculating air exists in the model, the Froude criterion is ignored in the present gas/particle flow experiments. (4) Stokes criterion. To meet the requirement of the Stokes criterion, the lab-scale particle should be less than 10 μm in diameter. Such fine particles are impractical in small-scale experiments because of high cost and non-recoverability. Therefore, glass beads with a mean diameter of 42 μm, 3320


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MWe. Table 2 lists coal characteristics and the two boilers’ mean operation parameters during the measurements. The relatively small

secondary air ejection (for gas velocity peaks) and the fuelrich and fuel-lean coal/air flows supplying (for particle velocity peaks) (Figure 2). At cross sections H/H0 = 0.035 and 0.075, which is close to the burner outlets, only a few very fine particles from the fuel-rich and fuel-lean coal/air flows can diffuse into the zone just below inner and outer secondary-air port outlets. Therefore, at the first two cross sections H/H0 = 0.035 and 0.075, the ejection of the fuel-rich and fuel-lean coal/ air flows produces these particle velocity peaks and their peak locations do not coincide with those of gas velocity peaks. Again, because of the MIMSC technology based on the highspeed inner and outer secondary air carrying the fuel-rich and fuel-lean coal/air flows to penetrate in the lower furnace,23−25 inner secondary air (with its port location being close to the fuel-rich coal/air flow nozzle) mixes more easily with the lowspeed fuel-rich coal/air flow and thus decays much faster than outer secondary (with its port location being adjacent to the front and rear wall), resulting in only two velocity peaks existing after H/H0 = 0.114. This means that the mixing point between the two parallel jets of the fuel-rich coal/air flow and inner secondary air should be in H/H0 = 0.075−0.114. The distribution patterns along the six cross sections reveal that, with the prior art, the longitudinal-velocity components of the gas/particle flows are severely asymmetric along the furnace center. The peak value decays more quickly and the peak position moves toward the furnace center more sharply in the rear-half furnace-half than in the front furnace, in addition to the gas/particle flows beginning to reverse upward much later near the rear wall (at X/X0 = 0.10−0.20) than near the front wall (at X/X0 = −0.35). Negative values of Vy/V0 for both air and particles appear in the region close to the rear wall after H/ H0 = 0.161. Furthermore, at H/H0 = 0.468 equaling to the upper part of the hopper region (Figure 3), Vy/V0 values of air and particles are all negative in the right-half zone, whereas large downward velocities occupy most of the left-half hopper region. This means the downward gas/particle flows near the rear wall can only penetrate the staged-air zone (near H/H0 = 0.341), instead of the upper part of the hopper region such as the depth level H/H0 = 0.468, whereas those near the front wall penetrate much further. According to similar asymmetric airflow field patterns in the literature,13,14,26 the present circumstances would allow the zones below the shorter downward gas/particle flows and near the rear wall being filled with the upward gas/particle flows, thereby resulting in the above phenomenon with negative Vy/V0 values appearing in the rear-half furnace. For the left cross section Y = 0 in the lower part of the upper furnace, most of the gas/particle velocities are negative and the upward gas/particle flows mainly deflect toward the zone near the front wall. These observations reveal that a similar flow-field deflection as reported previously in a MBEL subcritical boiler13,14,26 occurs in the present furnace, which can also be viewed in the downward gas/particle flow trajectories in the front-half and rear-half of the furnace in the next three subsections. In contrast, for the MIMSC technology, well-formed symmetric patterns appear for the longitudinalvelocity component distributions of the gas/particle flows at all eight cross sections, with both the gas/particle flows near the front and rear walls penetrating the hopper region (i.e., symmetric positive Vy/V0 at H/H0 = 0.468). The longitudinal-velocity differences between gas and particles with the prior art, depicted in Figure 5, is used to verify the validation of the selected fine glass particles (with the diameter range 2−8 μm) in tracing gas. It can be seen that,

Table 2. Coal Characteristics and Operational Parameters proximate analysis, wt % (as received) volatile matter 7.94a/ 10.6b

ash

moisture

fixed carbon

net heating value (MJ/kg)

30.04a/ 7.4a/8.0b 54.62a/ 34.2b 47.2b ultimate analysis, wt % (as received)

carbon

hydrogen

sulfur

53.72a/48.53b

2.44a /2.64b

2.99a/3.43b

20.4a/18.7b

nitrogen 0.84a/ 0.81b

oxygen 2.77a/2.39b

operational parameters case

prior MBEL art

boiler load (MWe) flow rate of main steam (t/h) total flux of primary air (kg/s) temp. of primary air (°C) total flux of secondary air (kg/s) temp. of secondary air (°C) coal feed rate (ton/h) exhaust gas temp. (°C) O2 in flue gases (dry vol. %) carbon content in fly ash (%) NOx in flue gas (mg/m3 at 6% O2 dry)

605 1615 100.2 100 508.7 334 236.1 125 3.40 8.5 1467

MIMSC technology 550 1503 111.3 115 435.3 326 230.4 132 3.65 5.1 822

a

Coal for the furnace with the prior MBEL art. bCoal for the furnace with the MIMSC technology. differences in the coal characteristics and boiler load between the two boilers indicate that the measuring results are comparable with respect to the two configurations of combustion systems.

3. RESULTS AND DISCUSSION As shown in Figure 3, eight cross sections of H/H0 = 0.035, 0.075, 0.114, 0.161, 0.224, 0.341, 0.468, and Y = 0 were selected to present the main distribution patterns of gas/particle velocities, particle volume flux, and particle number concentration in the model. In addition, the velocity decay and trajectories of the downward gas/particle flows in the front-half and rear-half of the furnace were compared so as to examine the gas/particle flow symmetries. 3.1. Gas/Particle Velocities. Figure 4 presents the longitudinal-velocity component distribution profiles for both gas and particle phases. Although the burner configurations are different between the two down-fired combustion technologies, their longitudinal-velocity component distributions at the first six cross sections (i.e., H/H0 = 0.035−0.341, covering the region from the burner zone to the staged-air zone, Figure 3) are generally similar in two aspects: (i) From the front and rear walls to the furnace center, both Vy/V0 values of air and particle increase initially but then decrease after reaching maxima at X/ X0 = −0.45 to −0.4 in the front-half of the furnace and X/X0 = 0.35−0.45 in the rear-half of the furnace, because of the gas/ particle flows ejected through burners on the front and rear arches. (ii) In the furnace central part zone, values of Vy/V0 are all negative and vary little after the gas/particle flows turning upward. Consequently, a dual-peak distribution pattern appears, with the exception of four-peak distribution pattern at cross sections H/H0 = 0.035 and 0.075 for the MIMSC technology because of the high-speed inner and outer 3321


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Figure 4. Gas/particle longitudinal-velocity component profiles along different cross sections.

section Y = 0), because of the gas/particle flow diffusing sufficiently after reaching these regions. 3.2. Particle Volume Flux and Particle Number Concentration. Figure 6 depicts the distributions of the particle volume flux along the y-direction and particle number concentration. For both of the two technologies, the particle volume flux distribution along the all eight cross sections (Figure 6a) and particle number concentration along the first three cross sections (i.e., H/H0 = 0.035, 0.075, and 0.114 in Figure 6b) show similar symmetric patterns as for the gas/ particle longitudinal-velocity component distributions. With the

along the first three cross sections and at the right side of cross sections H/H0 = 0.224, 0.341, and 0.468, large longitudinalvelocity differences between gas and particles appear in the zone near the previous velocity peak locations (Figure 4). The downshot gas/particle flows ejected through the two arches, large differences in the gas and particles inertia, and asymmetric gas/particle velocity distribution (Figure 4) account for the observations in the velocity differences. Much smaller longitudinal-velocity differences develop in the whole furnace central part and the lower part of the upper furnace (cross 3322


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flow nozzles. As the gas/particle flows penetrate and spread in the furnace, the peak value decays quickly, whereas in the furnace central part the particle volume flux and number concentration vary little, regardless of the type of technology used. Similar to the longitudinal-velocity component, the distributions of the particle volume flux and number concentration for the prior art also present an asymmetric pattern along the furnace center: (i) The peak value of the particle volume flux decays more quickly, and the peak position moves toward the furnace center much more sharply in the rear-half of the furnace than in the front-half of the furnace after H/H0 = 0.114. Consequently, the particle volume flux fully attains negative values in the rear-half of the furnace at H/H0 = 0.468. Moreover, the particle volume flux is negative in the region close to the rear wall after H/H0 = 0.114. These observations can be explained by the gas/particle longitudinal-velocity component distribution characteristics in the rear-half of the furnace (Figure 4). (ii) The particle number concentration is clearly higher near the rear wall than near the front wall after H/H0 = 0.114. This is because the mentioned upward flow, accompanied by a great number of fine particles, mixes with the downward gas/particle flows, resulting in the particle number concentration raising sharply near the rear-wall zone. However, with the MIMSC technology, well-formed symmetric patterns appear in the particle volume flux and number concentration distributions along the furnace center. In conjunction with the velocity distributions in Figure 4, additional discussion in Figure 6 is as follows: For the prior art, the peak locations of the particle volume flux and number concentration fully coincide with those of gas/particle velocities, which means that a high-particle-concentration zone corresponds to high gas velocities. This is because the respective fuel-rich coal/air flow nozzles and secondary air ports are arranged in alternating positions and are closely spaced (Figure 1). After leaving the port outlet, secondary air with velocities much higher than that of the fuel-rich coal/air flow mixes rapidly with the slower gas/particle flows. With these circumstances appearing, late coal ignition and high NOx emissions are very likely to occur, such as the results reported in the literature.13,14 Meanwhile, for the MIMSC technology, in the burner zone (i.e., the zone from burner outlets to the depth H/H0 = 0.224), the peak locations of the particle volume flux and number concentration approach the high-temperature furnace center more apparently than those of gas/particle velocities. Only after the gas/particle flows reach the depth H/ H0 = 0.341, these peak locations can coincide with those of gas/ particle velocities. In conjunction with the fact that no large differences appear in the particle volume flux and number concentration in the zone below arches at H/H0 = 0.341, it can be concluded that the mixing point between the two parallel jets of the fuel-rich coal/air flow and outer secondary air should be close to H/H0 = 0.341. This is because the MIMSC technology is developed on the basis that a static pressure difference exists between airflows with different velocities.23−25 In the fuel-rich coal/air flows, gas, which has inertia much smaller than that of particles, deviates from the fuel-rich coal/ air flows much more easily and quickly than particles, with the carrying of high-speed inner and outer secondary air, leaving a relatively high-particle-concentration and low-velocity zone forming below the arches. These circumstances potentially facilitate a timely ignition with the heating from the high-

Figure 5. Longitudinal-velocity differences between gas and particle with the prior art along different cross sections.

exception of a four-peak pattern at cross sections H/H0 = 0.035 and 0.075 for the MIMSC technology (due to the fuel-rich and fuel-lean coal/air flows supplying) and the apparently high particle number concentrations in the near rear-wall zone (due to a great number of fine particles accompanied by the upward flow in this zone (Figure 4b)), the distributions of the particle volume flux and number concentration for both technologies present a dual-peak pattern at the first six cross sections (i.e., H/H0 = 0.035−0.341). With the MIMSC technology at cross sections H/H 0 = 0.035 and 0.075, the peak values corresponding to the fuel-rich coal/air flow are far higher than those for the fuel-lean coal/air flow because of about 80% of particles into the furnace fed through the fuel-rich coal/air 3323


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Figure 6. Particle volume flux and number concentration profiles along different cross sections.

the respective fuel-rich coal/air flow nozzles and secondary air ports are arranged in alternating positions and are closely spaced (Figure 1). Additionally, the velocity decay of the gas/ particle flows in the front-half and rear-half of the furnace displays a severe asymmetric pattern along the furnace center, with the decay rates being far higher in the rear-half of the furnace than in the front-half of the furnace. The gas/particle flows in the rear-half of the furnace stop going down and reverse upward before the depth H/H0= 0.5, whereas those in the front-half of the furnace still keep slowing down after the depth H/H0= 0.67. These observations on the whole (Vy)max/ V0 change trends, determined by the present gas/solid twophase flow experiments, are similar to those acquired within a single airflow model for a 300 MWe subcritical boiler13,14,26 because of the essentially similar combustion system application.

temperature gas near the furnace center and may restrain the fuel-NOx formation in the burner zone. 3.3. Decay of the Downward Gas/Particle Flows. Figure 7 presents the decay curves for the longitudinal-velocity of the downward gas/particle flows. Here, (Vy)max is the largest longitudinal-velocity component of the downward gas/particle flows at fixed measurement depths below the front and rear arches. The change trends of (Vy)max/V0 with respect to the dimensionless depth (i.e., H/H0) is used to reveal the downward gas/particle flows decay. With the prior art, both (Vy)max/V0 values of gas and particle below the front and rear arches are greater than unity after the gas/particle flows leave the burner nozzle outlet. Moreover, (Vy)max/V0 increases initially for a short distance but then decreases sharply after reaching maxima in the depths near H/H0 = 0.1. As mentioned previously, these change trends are attributed to the fact that 3324


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Figure 7. Decay curves for longitudinal-velocity components of the down-ward gas/particle flows.

With the MIMSC technology, a well-formed symmetric pattern appears in the velocity decay of the gas/particle flows in the front-half and rear-half of the furnace. Values of (Vy)max/V0 for gas in the zone not far away from the burner outlets are also greater than unity. This occurs because, in the downward gas/ particle flows, primary air is carried to penetrate by inner and outer secondary air,23−25 whose velocities are much higher than that of the fuel-rich coal/air flow. After the gas/particle flows leaving the port and nozzle outlets, values of (Vy)max/V0 for gas decrease initially at a high rate, up to large depths of H/H0 = 0.035−0.34. Undergoing a rise in the zone near H/H0= 0.40 due to the ejection of staged air with a high velocity and large mass flow rate, flow velocities continually slow down. Velocity decay curves for particles are similar as those for gas, with the exception of (Vy)max/V0 values being lower than unity just after the gas/particle flows leaving the burner nozzle outlet and the occurrence of a velocity increase stage in H/H0 = 0.035−0.11. The causes for the phenomena, presented previously in the explanation for particle velocity peaks at cross section H/H0 = 0.035 (Figure 4b), are located in the following facts: (i) Particles, which have inertia much larger than that of gas, are fed with primary air, which has velocities much lower than those of inner and outer secondary air. (ii) In the zone close to the burner outlets, only a few very fine particles can diffuse into inner and outer secondary air. 3.4. Trajectories of the Downward Gas/Particle Flows. Apparently, the measurement points with a maximum particle volume flux at a fixed measurement depth below the front and rear arches, are located on the trajectories of the downward gas/particle flows in the front-half and rear-half of the furnace. Accordingly, it is acceptable to graph the downward gas/ particle flow trajectories by using the location of each measurement point with a maximum particle volume flux at a fixed measurement depth. As shown in Figure 8, with the prior art severe asymmetric gas/particle flow trajectories characterized by the downward gas/particle flows near the front wall penetrating much further than those near the rear wall, appear in the lower furnace. The gas/particle flows near the front wall penetrate deep into the lower part of the hopper region (i.e., particularly deep penetration depth of H/H0 = 0.76), from which they deflect upward toward the right side of the hopper region and then mix with the upward gas/particle flows emanating from the zone near the rear wall, before being

Figure 8. Downward gas/particle flow trajectories in the lower furnace.

redirected upward toward the left side of the furnace throat zone. Meanwhile, it is also found the particles accompanied by the upward deflected gas/particle flows wash over the right hopper wall; this may cause serious slagging at the right hopper wall in the real furnace. Lifted by the upward gas/particle flows emanating from the zone near the front wall, the downward gas/particle flows near the rear wall incline toward the furnace center as flowing downward, and finally reverse upward only after reaching the staged-air zone (i.e., particularly shallow penetration depth of H/H0= 0.37). Here, the fact of the severe asymmetric gas/particle flow trajectories reveals that seriously deflected gas/particle flow field develops in the furnace, which accounts for the asymmetric distributions of velocities, particle volume flux, and particle number concentration, and decay in 3325


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flow goes down in the lower furnace, its volume expands quickly as the continual increase of the temperature. The density of the fuel-rich coal/air flow also falls together with the flow momentum. All these changes mean that, when extrapolating the results to the combustion environment, the specific data from the cold gas/solid two-phase flow experiments should be corrected. However, upon in situ measurements of gas temperatures, carbon content in fly ash, and NOx emissions shown in Figures 1 and 9, as well as in Table 2, the qualitative conclusions in the present cold modeling gas/ particle experiment are still valid. Detailed discussion is as follows.

the down-ward gas/particle flows (Figures 4, 6, and 7). In addition, a large recirculation zone (schematically graphed by the dashed line) was found below the rear arch. Although the deflected characteristics of the present gas/particle flow field are similar to those acquired within a single airflow model for a 300 MWe subcritical boiler,13,14 the present asymmetries are more serious. Maybe the causes are mainly attributed to the much shorter upper and larger boiler nose in the present supercritical boiler than in the previous subcritical one (see the detailed discussion in the next subsection). While with the MIMSC technology, a well-formed symmetric pattern appears in the gas/particle flow trajectories in the fronthalf and rear-half of the furnace. Both the downward gas/ particle flows near the front and rear walls reverse upward toward the furnace center after penetrating the middle part of hopper region, so as to form a well-formed symmetric Wshaped gas/particle flow field. 3.5. Explanation of the Formation for the Asymmetric and Symmetric Gas/Particle Flow Characteristics. According to previously published work13,14,23,26 on MBEL downfired subcritical boilers, the deficiency in the furnace configuration and combustion system design accounts for the asymmetric combustion and flow-field deflection that appear at normal operation models. The direct cause for the formation of the asymmetric flow field can be attributed to their asymmetric and short upper furnace, with their boiler nose and upper furnace outlet located on the rear wall (see respective Figure 1 in the literature13,14,23,26); this is despite a symmetric configuration in the lower furnace. Additional factors include the sharp shoulder joining the lower and upper parts of the furnace, the large span of the secondary-air port and high secondary-air ratio (about 70% of the total air mass flow rate), and the horizontally fed direction of staged air, all of which favor the formation of an asymmetric flow field. A more detailed explanation of the asymmetric flow field formation can be found elsewhere.13,14,23,26 The deficiency in the furnace configuration is enlarged in the present supercritical boiler to favor the formation of a more asymmetric gas/particle flow field. That is, in the present upper furnace, the ratio between the straight section height and the upper furnace depth (i.e., 10812 and 12512 mm in Figure 1, respectively) is small at 0.86, far smaller than those (i.e., ratios of 1.22 and 1.53) for the two subcritical boilers in the literature.13,14,23,26 The application of the present shorter upper furnace and larger boiler nose strengthens the passive effect of the asymmetric upper furnace configuration on the gas/particle flow characteristics in the lower furnace. Nevertheless, applying the MIMSC technology, a symmetric gas/particle flow field develops in the furnace replacing the asymmetric one with the MBEL prior art. That change is attributed to a reconfigured combustion system and air distribution with the MIMSC technology. Two factors25 weaken the interactional extrusion existing between the two upward coal/airflows that circumvents the formation of the asymmetric flow field;13,14,23,26 one is the significant reduction in the secondary-air mass flow rate and the rearrangement of secondary air and the fuel-lean coal/air flow toward the front and rear walls, and the other is a large increase in the mass flow rate of staged-air accompanied by a 45° declination. 3.6. Evaluation on the Modeling Experiment Validity and In Situ Measurement Results. Actually, it is quite certain that cold modeling with isothermal condition cannot accurately describe the complex physical and chemical progress of fuel combustion in the furnace. When the fuel-rich coal/air

Figure 9. Gas temperature distribution at a 550 MWe load in the 600MWe supercritical boiler, applying the MIMSC technology.

With the prior MBEL art, asymmetric combustion (Figure 1) characterized by gas temperatures near the rear wall (ports 1 and 2) being much higher than those near the front wall (ports 3 and 4), develops in the lower furnace, in addition to high levels of carbon content in fly ash and NOx emissions (Table 2, 8.5% and 1467 mg/m3 at 6% O2 dry, respectively); this asymmetric combustion phenomenon tallies well with the corresponding asymmetric gas/particle flow field acquired in the above cold modeling experiments. Based on kinematic similarity in modeling criteria, results from cold model experiments can explain these phenomena of asymmetric combustion, low burnout, and high NOx emissions. The large recirculation (Figure 8) below the rear arch zone directs hightemperature recirculating gas to mix with the downward coal/ air flow; this behavior enhances coal ignition and combustion in the zone near the rear wall. Consequently, a large heat release greatly increases gas temperature levels in the rear-half of the furnace. In the absence of a recirculation zone, such as that appearing below the rear arch, temperatures are relatively lower in the zone near the front wall. That is, the severe asymmetric gas/particle flow field results in lean combustion conditions in regions near the front and rear walls. Low gas temperatures and poor burnout in the region near the front wall lead to high carbon content in fly ash throughout the whole furnace. 3326


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velocity zone in the burner zone developed in the lower furnace. A gradually mixing process appeared among the lowspeed fuel-rich coal/air flow and high-speed inner and outer secondary air. The mixing point between the fuel-rich coal/air flow and inner secondary air should be in H/H0 = 0.075−0.114, whereas that between the fuel-rich coal/air flow and outer secondary air should be close to H/H0 = 0.341. Industrial-sized experimental results uncovered that, in comparison with the boiler with the prior MBEL art, the newly designed boiler applying the MIMSC technology achieved perfectly symmetric combustion, NOx emission reduction by around 40%, and large improvement in burnout.

Because the present asymmetric combustion characteristics are similar to those within a 300 MWe subcritical boiler,13,14 more detailed explanation of the asymmetric combustion can be viewed in the literature.13,14 The high NOx emissions are consistent with the previous discussion in Figure 6, viz., a highparticle-concentration zone accompanied by high gas velocities in the burner zone facilitates the formation of a large amount of NOx. In comparison with the prior MBEL art, circumstances shown in Figure 9 and Table 2 improve greatly with the proposed MIMSC technology. Gas temperature differences near the front and rear walls are within 50 °C, with the exception of a 100−120 °C gap at two pairs of monitoring ports (Figure 9), instead of those up to 300−590 °C with the prior MBEL art (Figure 1). NOx emissions are reduced by approximately 44%, in addition to an apparent decrease in levels of unburnt carbon in fly ash. As mentioned previously, the gas/particle flow and combustion characteristics at the present 550 MWe should be close to those at full load of 600 MWe because of their slight differences in the boiler load and burner’s service model. Therefore, the occurrence of a symmetric gas/particle flow field and the formation of a relatively high-particle-concentration and low-velocity zone in the burner zone at full load, as well as the deep-air-staging combustion configuration, account for the well-formed symmetric combustion accompanied by much lower levels of carbon content in fly ash and NOx emissions. Generally, in comparison with the 550 MWe load, O2 content in the furnace should be a little lower, whereas gas temperature levels should be a little higher at full load. Moreover, the present gas temperatures (Figure 9) are not larger than 1410 °C, at which fuel-NOx still hold a dominant position in the total NOx production. Consequently, it is thought that levels of carbon content in fly ash and NOx emissions at full load will differ slightly from those at 550 MWe. Namely, with the MIMSC technology, the well-formed symmetric combustion pattern, an approximately 40% decrease in NOx emissions, and the large improvement in pulverized-coal burnout are still valid at full load, despite industrial-sized data at full load being absent up to now.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 451 86418854. Fax: +86 451 86412528. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was sponsored by the Hi-Tech Research and Development Program of China (863 program) (Contract No. 2006AA05Z321) and 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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4. CONCLUSION Using PDA measurements in gas/solid two-phase flow experiments within a cold 1:40-scaled model of the furnace, the present work reported an experimental investigation on the gas/particle flow characteristics for a down-fired 600-MWe supercritical boiler with respect to the prior MBEL art and MIMSC technology. With the prior MBEL art, a deflected gas/ particle flow field characterized by the gas/particle flows in the front-half of the furnace penetrating greatly further and occupying much more furnace volume than those in the rearhalf of the furnace, developed in the lower furnace. The longitudinal gas/particle velocity components, particle volume flux, particle number concentration, decay curves, and trajectories of the downward gas/particle flows all displayed a severe asymmetric pattern along the furnace center. A highparticle-concentration zone accompanied by high gas velocities developed in the burner zone, which is adverse to timely coal ignition and NOx reduction. In situ measurements revealed that severe asymmetric combustion and high NOx production occurred within the furnace with the prior MBEL art. With the MIMSC technology, a well-formed symmetric gas/particle flow field with a relatively high-particle-concentration and low3327


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