Article pubs.acs.org/EF
Inner and Outer Secondary-Air Distance-Effect Study within a Cold Small-Scale Model of a New Down-Fired 600 MWe Supercritical Utility Boiler Min Kuang, Zhengqi Li,* Qunyi Zhu, Hongyu Zhang, Xingying Zhu, and Yan Zhang School of Energy Science and Engineering, Harbin Institute of Technology, 92, West Dazhi Street, Harbin 150001, People’s Republic of China ABSTRACT: To achieve significant reductions in particularly high NOx emissions and to eliminate severe asymmetric combustion in down-fired boilers, a multiple-injection and multiple-staging combustion technology was developed in our previous study. That technology was trialed in a newly designed down-fired 600 MWe supercritical utility boiler, without applying overfire air. In consequence, a reconfiguring of the secondary air for the redesigned burners on the arches, composed of directflow inner and outer secondary air, had to be performed for the newly reconfigured furnace. The present work reports our experimental investigation on the impact of the distance between inner and outer secondary air on aerodynamic characteristics within the furnace, determined by cold airflow experiments within a cold 1:20-scaled model of the furnace. Aerodynamic field measurements were conducted at four different settings, each distinguished by the ratio (denoted by Cs) of the distance between the inner and outer secondary air to the arch depth; values for Cs were fixed at 7, 9, 12, and 15%. At the two lower settings, the aerodynamic field and velocity distributions at certain cross-sections as well as airflow penetration depths were symmetric along the furnace center. At the two higher settings, a deflected flow field appeared in the lower furnace. In considering a symmetric flow field along with an appropriate airflow penetration depth, an optimal distance between the inner and outer secondary-air ports on the arch should be at Cs = 7−9% for the reconfigured furnace.
1. INTRODUCTION Low-volatile fuels, such as anthracite and lean coal, have less than one-third of the volatile matter of other fuels and require more time to ignite and complete combustion or come near completion. To overcome the problems associated with low ignitability and combustibility of these types of fuels, downfired boilers were designed for industrial firing of low-volatile coals. Aside from the higher char/volatile ratio of low-volatile coals, the characteristic high-temperature levels and long residence times of pulverized coal in these combustion systems mainly determine the higher NOx emission levels than those for bituminous coal and lignite firing. Four types of down-fired utility boilers are manufactured: the Foster Wheeler (FW), the Babcock and Wilcox (B&W), the Stein, and the Mitsui Babcock Energy Limited (MBEL) down-fired boilers. Differences between these four types referring to the burner arrangements and air distribution are available in the literature.1 Since the 1960s, down-fired boilers have operated widely in regions with extensive reserves of anthracite and lean coal, such as Western Europe and East Asia. Admittedly, most of these boilers suffer from problems, such as late coal ignition and poor stability, low burnout (carbon in fly ash typically in the range of 8−15%), and high NOx emissions (in the range of 1100−2000 mg/m3 at 6% O2). For FW boilers, reports have appeared on aerodynamic characteristics,2 combustion,2−5 slagging,6 and NOx reductions by parametric tuning of operating conditions7−9 or overfire air (OFA) application.10−12 Research on B&W and Stein boilers has been reported by Fan et al.,13 who investigated the combustion characteristics and NOx formation within a 300 MWe B&W down-fired boiler, and Burdett,14 who performed industrial tests to investigate the effects of air staging on NOx © 2011 American Chemical Society
emissions from a 500 MWe Stein down-fired boiler unit. However, investigation on MBEL boilers is lacking and needs urgent focus. This is because, besides the above operation problems, MBEL down-fired boilers also suffer from asymmetric combustion, but with different levels.15−18 Asymmetric combustion caused large differences in volumetric heat load between the zones near the front and rear walls; this created adverse effects in boiler operations. Reducing the current high NOx emissions to acceptable levels is a huge challenge, because of high gas temperatures and long residence times needed to achieve good burnout in anthracite and lean coal fed in a down-fired boiler. For MBEL down-fired boilers, asymmetric combustion and heavy slagging in the lower furnace compound the difficulties in NOx reductions. To date, only Kuang et al. have put forward a comprehensive low-NOx combustion technology associated with eliminating this asymmetric combustion and minimizing slagging, i.e., a deep-air-staging combustion technology based on the concept of multiple injection and multiple staging (for brevity, the “MIMSC technology”).19 After nearly 1 year of a collaborative effort between our group and boiler plant technicians, implementation of this new technology has been approved for the newly designed down-fired 600 MWe supercritical utility boiler, which had been set to adopt the prior MBEL art in its initial design scheme. A unique combustion system, incorporating a staged-air declination with a large increase in the staged-air mass flow rate as well Received: August 29, 2011 Revised: November 28, 2011 Published: November 30, 2011 417
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Figure 1. Schematics of the furnace and combustion system of the down-fired boiler (dimensions in millimeters).
modeling criteria, cold-flow fields in small-scale furnaces can more or less describe aerodynamic characteristics in full-scale versions. Again, because of the high cost and unacceptable risks for power plant managers, such burner configuration optimization studies cannot be performed on a full-scale hot furnace. Circumspectly, we conducted cold airflow experiments within a 1:20-scaled model of the furnace to investigate the aerodynamic field at the different ratio settings associated with port separation.
as a burner redesign and reorganization, was installed in the boiler. Special direct-flow inner and outer secondary-air ports, separated by a particular, were created for the redesigned burners on arches. The burner locations on the arches, as well as this secondary-air separation, significantly affect the aerodynamic characteristics within the furnace and, thus, influence the pulverized coal burnout. This is because the air ejected through the burners (with inner and outer secondary air being the major contributors) accounts for nearly 60% of the total air flowing into the furnace (as estimated from the air mass flow rates). Therefore, uncovering the impact of the above two factors on the aerodynamic characteristics and establishing optimal burner settings for the newly reconfigured furnace are important. Below, we present our results involving the effect of inner and outer secondary-air separation. As is well-known, it is very difficult to collect aerodynamic parameters, such as airflow velocities, in full-scale furnaces. Therefore, to analyze the influence of aerodynamic behavior on pulverized coal combustion, most experiments are performed in small-scale models.1,2,5,15,20−22 The results of small-scale cold experiments show certain differences from those performed on full-scale hot furnaces. However, following certain similarities in
2. EXPERIMENTAL SECTION 2.1. Utility Boiler with the MIMSC Technology. Figure 1 presents a schematic of the furnace and combustion system of the boiler. A total of 24 louver concentrators symmetrically arranged on the arches divide the primary air/fuel mixture into fuel-rich and fuellean coal/air flows needed to regulate fuel-rich/fuel-lean combustion. A detailed description of the configuration and operational principle of the louver concentrator can be found elsewhere.23 There are 12 burners lining the front and rear arches. A total of 8 fuel-rich coal/air flow nozzles, 8 fuel-lean coal/air flow nozzles, 14 inner secondary-air ports, and 12 outer secondary-air ports feed each burner. The method that realizes the MIMSC technology here is as follows: (1) The fuelrich coal/air flow is channeled vertically down through nozzles 418
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Figure 2. Small-scale cold-experiment system and measurement sites. centered over the furnace, while the fuel-lean coal/air flow is similarly injected near the front and rear walls. (2) Two rows of short secondary-air ports are located on arches across the breadth of the furnace, i.e., one row of inner secondary-air ports positioned between the fuel-rich and fuel-lean coal/air flow nozzles and another row of 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. (3) Staged-air ports are arranged in groups on the front and near walls across the breadth of the furnace, and a high staged-air ratio (25%) is established to lower the stoichiometry to close to 0.9 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°. A detailed description of the technical principles of the MIMSC technology can be found elsewhere.19 2.2. Cold Small-Scale Airflow Experiments. The experimental system is shown in Figure 2. The similarity criteria for the small-scale airflow experiments are as follows. (1) Geometric similarity: The ratio of the small-scale apparatus to the full scale is 1:20. (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 the Reynolds number 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 were thought to be self-modeling. (3) The same momentum ratios: The momentum ratios among the airflows of the small-scale model are consistent with the full-scale furnace. All airflow rates into the model were measured by Venturi tube flowmeters, with measurement errors of less than 10%. An IFA300 constant-temperature anemometer system equipped with a 1240-type
two-dimensional probe with two hot-film sensors was used to measure the air velocity at various locations within the furnace, giving a measurement error of less than 5%. Details can be found elsewhere of methods used in quantifying the measurement errors related to these Venturi tube flowmeters, the velocity measurement principle of the anemometer system, methods in the probe calibration and measurement error determination, and uncertainties in this intrusive velocity measurement.22 Although the flow field in the lower furnace in a down-fired boiler 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 (Figure 1). 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 fullscale 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, non-axial velocities are relatively small in the zone below the burners. Here, our cold airflow experiments focus on the W-shaped flow field in the lower furnace along a vertical cross-section through the burners. Thus, a two-dimensional probe was employed. The dimensionless ratio Cs has been introduced to quantify the inner-to-outer secondary-air separation on the arch. Here, this is expressed as
Cs =
S × 100% L
(1)
where S denotes the separation between the two rows of inner and outer secondary-air ports and L denotes the arch depth (5577 mm; Figure 1). At the lowest Cs value of 7%, there is no gap left between the inner and outer secondary-air ports, except the fuel-lean coal/ airflow nozzle. Various cold small-scale airflow experiments were conducted, whereby fuel-rich coal/air flow nozzles and inner secondary-air ports were moved toward the furnace center at predetermined distances that yielded Cs values of 7, 9, 12, and 15%; fuel-lean coal/air flow nozzles and outer secondary-air ports remained unchanged. The airflow velocity and air mass flow rate for each of the airflows into the cold small-scale model, modeled from design 419
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parameters of the full-scale furnace at normal full load, were kept constant throughout all four settings. Of these design parameters, the velocities of the fuel-rich and fuel-lean coal/air flows, secondary air (the same velocity in inner and outer secondary air), and staged air are 15.00, 22.57, 55.00, and 55.0 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 20.50, 22.58, 41.28, and 42.27 m/s, respectively. Cold aerodynamic field velocities were measured along the longitudinal cross-section intersecting the vertical center line of one of the fuel-rich coal/airflow nozzles. In a down-fired boiler, the pulverized-coal combustion process mainly depends upon the flow field in the lower furnace because pulverized-coal combustion is almost complete in the lower furnace. Consequently, only the cold aerodynamic flow in the lower furnace and the lower part of the upper furnace is acquired.
hopper wall; this protective layer prevents fast particulates washing over the hopper walls, thus avoiding slagging in the hopper. For the two higher Cs settings, an asymmetric flow-field deflection appears, with the downward airflow near the front wall clearly penetrating further than that near the rear wall; in fact, it penetrates deep into the middle and lower parts of the hopper region. From there, it deflects upward and then mixes with the upward airflow emanating from the zone near the rear wall, before being redirected upward toward the furnace center and finally reaching the zone below the front arch. Meanwhile, the downward airflow near the rear wall deflects upward only after reaching the upper part of the hopper region. Repetition of the measurements for these higher settings demonstrated that the flow-field deflection is steady. As Cs increases (i.e., the fuel-rich coal/air flow and inner secondary air moves toward the furnace center), the flow-field deflection becomes turbulent and the downward airflow reach near the rear wall decreases. In addition, the recirculation zone below the rear arch shrinks, and airflow velocities in the recirculation zone decrease. Admittedly, the situations with an asymmetric flow-field deflection formation would appear at extreme parameter setups, regardless of the type of down-fired boiler used. From the literature,4,26 flow-field deflections were reported as appearing at excessively inclined angles of the F-layer secondary air for a 300 MWe FW down-fired boiler and at unreasonable air distribution models in the burners for a 300 MWe B&W down-fired boiler. Evidently, here, our object (i.e., the separation between the inner and outer secondary air) shares no overlap with those in the literature4,26 (i.e., the F-layer secondary air inclined angles and the air distributions in the burners). 3.2. Explanation of the Symmetric and Asymmetric Flow Field Formation. As mentioned above, avoiding the occurrence of an asymmetric flow-field deflection is difficult when extreme parameter settings are in place, regardless of the type of down-fired boiler used.1,4,8,10,16,24,26 MBEL down-fired boilers, with asymmetric combustion appearing at normal operation models, are unfortunately much weaker in overcoming the flow-field deflection than the other three types, because of deficiencies in the furnace configuration and combustion system design.1,15,22 Nevertheless, applying MIMSC technology, a symmetric flow field develops in the lower furnace replacing the original asymmetric field of the MBEL prior art reported in previously published literature.1,15,22 That change is attributed to a reconfigured combustion system and air distribution with the MIMSC technology. Two factors weaken the interactional extrusion existing between the two upward coal/airflows that circumvents the formation of the asymmetric flow field;1,15,22 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 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 inhibits the formation of this deflected flow field in the lower furnace. Here, additional changes in the secondary-air mass flow rate and burner arrangement strengthen suppression of an asymmetric flow field. In contrast, drastically increasing the separation between the inner and outer secondary-air ports advances the fuel-rich coal/air flow and inner secondary air toward the furnace center and, thus, induces adverse effects. This occurs because moving
3. RESULTS AND DISCUSSION 3.1. Aerodynamic Field. As seen in Figure 3, a symmetric W-shaped flow field appears in the lower furnace for the two
Figure 3. Aerodynamic fields associated with different distance settings between the inner and outer secondary air (the solid red directed line is presented as a visual aid to the airflow trajectory near the front and rear walls).
lowest Cs settings. Two large symmetric zones comprising recirculating air flowing toward the nozzle outlet of the fuel-rich coal/air flow but with some differences in the recirculating air reversal develop below the front and rear arches; this will be beneficial in pulverized-coal ignition. The recirculating air below the arches is an exclusive characteristic of down-fired boilers,24,25 which can also be found widely in relevant literature referring to experimental and numerical investigations on other types of down-fired boilers.2,4,6,13,26 The declined staged air, flowing downward at high speed along the bottom hopper wall, forms a distinctive airflow layer covering the 420
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the fuel-rich coal/air flow and inner secondary air toward the furnace center shrinks the space between the two downward airflows, which restrains the free expansion of rapid downward jets below the arches and the upward airflow in the central part of the lower furnace and strengthens the interactional extrusion existing between the two upward airflows. The strengthening in the interactional extrusion and the cooperation of the asymmetric configuration in the short upper furnace (i.e., the boiler nose and furnace exit located on the rear wall; Figure 1) favor the formation of the asymmetric flow field again.1,15,22 3.3. Velocity Distribution Patterns along the Selected Cross-Sections. In the following, a comprehensive discussion of airflow velocity distributions along several horizontal and vertical cross-sections is provided. In Figure 2, X0 is the horizontal distance between the front and rear walls in the lower furnace. X is the distance from the furnace centerline to the measurement point along the x direction. The location of the fuel-rich coal/air flow nozzle varies a little among these five settings. For a better comparison in airflow velocities among various settings, we thus select the location for the fuel-rich coal/air flow nozzle corresponding to a ratio Cs = 9% as the reference location of the fuel-rich coal/air flow nozzle. Accordingly, for ratio Cs = 9%, H0 in Figure 2 is the vertical distance between the outlet of the fuel-rich coal/air flow nozzle and the upper edge of the hopper, whereas H is the vertical distance from the measurement point to the outlet of the fuelrich coal/air flow nozzle. The origin of the coordinate system is set at the furnace centerline. Given the 16 successive depths between the fuel-rich coal/air flow nozzle outlet and each row of measurement points below the arches, the dimensionless distances (H/H0) are 0.035, 0.071, 0.107, 0.142, 0.178, 0.213, 0.262, 0.309, 0.341, 0.369, 0.397, 0.445, 0.492, 0.563, 0.657, and 0.752 for Cs = 9%. For each measurement point, y and Vy denote the vertical position and longitudinal-velocity component in the downward direction, whereas x and Vx are the horizontal position and transverse-velocity component toward the right. V0 signifies the outlet velocity of the fuel-rich flow nozzle along the y direction. From the viewpoint of combustion, the recirculating air existing below the arches can direct hot up-flowing gas toward the fuel-rich coal/air flow and ensure sufficient heat for the coal/air flow to produce a timely ignition. Analyses on the recirculating air behavior below the arches can be found widely in various investigations on the velocity field in down-fired boilers.4,6,10,13,24,25 Accordingly, with regard to the MIMSC technology in the MBEL boiler, the recirculating air behavior below the arches needs to be presented here. Figure 4 shows the transverse-velocity component of the upward airflow toward the fuel-rich nozzle outlet (i.e., the recirculating air below the front and rear arches) along the vertical crosssections X/X0 = ±0.322, where the recirculating air flows. Because of the positive Vx from the left toward the right, here, Vx along X/X0 = −0.322 is negative, whereas that along X/X0 = 0.322 is positive. For a better comparison to different settings, the dimensionless transverse-velocity component (i.e., Vx/V0) along X/X0 = −0.322 is therefore cast as (−Vx)/V0 in the figure. V̅ x denotes the average value of Vx at all measurement points along each vertical cross-section. A parameter (−V̅ x)f/ (V̅ x)r is used to quantify the difference in the recirculating air velocity in the zone below the front and rear arches. Here, (V̅ x)f and (V̅ x)r signify the values of V̅ x in the zone near the front and rear walls, respectively. The figure shows that, despite a symmetric flow field developing for all four settings, except for
Figure 4. Transverse-velocity components along vertical cross-sections X/X0 = ±0.322, as well as a comparison between these in the recirculation zones below the arches.
Cs = 12 and 15%, the transverse-velocity component of the recirculating air displays a poor symmetric pattern at the two cross-sections along the furnace center for all four settings, with the transverse-velocity component being much higher along X/ X0 = −0.322 in the left-half zone than along X/X0 = 0.322 in the right-half zone at Cs = 12 and 15%. A quantitative result, i.e., values of (−V̅ x)f/(V̅ x)r along the pair of cross-sections diverging from unity to different extents, is consistent with this asymmetric pattern. As the value of Cs increases, the transverse-velocity component decreases initially but then increases along X/X0 = −0.322 and decreases continually along X/X0 = 0.322. Again, increasing Cs initially has little affect on the symmetry of the transverse-velocity component along the furnace center in Cs = 7−9% but then decreases it dramatically in Cs = 9−15%, which is also revealed in the change trend of the value of (V̅ x)r/(−V̅ x)f. The above observation is attributed to the occurrence of a symmetric flow field at Cs = 7 and 9% but with a little asymmetry in the recirculating air below the front and rear arches and the presence of the flow-field deflection at Cs = 12 and 15%. In the flow-field deflection, on one hand, the downward airflow is directed upward much earlier near the rear wall than near the front wall, leaving a relatively smaller region to allow for the upward air flow to partly flow; on the other hand, the deflected airflow reaching the zone below the front arch strengthens the recirculating air toward the burner outlet below the front arch (panels c and d of Figure 3). Increasing the distance between the inner and outer secondary air raises the flow-field deflection first and then deteriorates it (Figure 3), resulting in the values of (V̅ x)r/(−V̅ x)f at settings of Cs = 12 and 15% being not only 421
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clearly higher than unity but also far larger than those at Cs = 7 and 9% (close to unity). Figure 5 presents the longitudinal-velocity component profiles along the horizontal cross-section H/H0 = 0.369 that
and mixing processes with staged air near the front wall are affected slightly, because of deflections suppressing the airflow near the front wall to flow downward in the near-wall zone (panels c and d of Figure 3). 3.4. Decay and Penetration Depth of Downward Airflows. Figure 6 presents a comparison of the decay curves of the downward airflow at different settings. Here, (Vy)max is the largest longitudinal-velocity component of the downward airflow at fixed measurement point depths below the front and rear arches. After the airflows leave the nozzle outlet, both of the values of (Vy)max/V0 in the zone below the front and rear arches decrease at a high rate initially for a large distance. After a rise in the zone near H/H0 = 0.37 because of the staged-air ejection, flow velocities continually slow down for all four settings, with the exception of a continual sharp decay before the airflow reversal at H/H0 = 0.61 and 0.70 near the rear wall at the respective setting of Cs = 12 and 15% because of the flow-field deflection. Increasing the value of Cs generally decreases both (Vy)max/V0 near the front and rear walls but with much higher decrease rates near the rear wall than near the front wall. This is because, in the lower furnace, the space between the inner and outer secondary air enlarges, allowing for the downward airflow to expand easily and decay rapidly in the zone below the arch. A consequence of deflections suppressing the downward airflow near the front wall to flow downward in the near-wall zone, accounts for the fact that the smaller decrease rates in (Vy)max/V0 with Cs appear near the front wall than near the rear wall. Of all four settings, only the decay in (Vy)max/V0 at two smaller distance settings of Cs = 7 and 9% shows an acceptable symmetric pattern in the zone near the front and rear walls. As one of the more important aerodynamic characteristics, the penetration depth of the downward airflow is critical to the formation of a proper aerodynamic field in a down-fired furnace and determines the flame penetration depth during combustion that influences the position of the flame center and the degree of mixing of combustion air and coal.4,6,10,13,19,24,25 In the present work, we bring in a criterion fixing the effective depth that the downward airflow reaches into the lower furnace with the MIMSC technology, i.e., the distance from the fuel-rich coal/air flow nozzle outlet to the position at which (Vy)max/V0 has decayed to 0.35. The ratio between this effective depth and H0 is the dimensionless penetration depth of the downward airflow.
Figure 5. Longitudinal-velocity component profiles along the horizontal cross-section H/H0 = 0.369.
lies in the airflow zone just below the staged-air slots. As shown in Figure 5, from the front and rear walls to the furnace center, longitudinal velocity components decrease rapidly for all four settings, before finally attaining negative values (i.e., airflows begin to deflect upward) in the zone X/X0 = from −0.30 to −0.25 near the front wall and X/X0 = 0.20−0.25 near the rear wall, from which these change little. The longitudinal-velocity component distribution displays a well-formed symmetric pattern along the furnace center for all four settings, with the exception of Vy/V0 values being higher and the airflow deflecting upward earlier near the front wall than near the rear wall at settings of Cs = 12 and 15%. This is because of the occurrence of the flow-field deflection. Increasing the distance between the inner and outer secondary air has little effect on the airflow velocity in the zone not far away from the staged-air slot outlet near the rear wall (i.e., X/X0 = from −0.5 to −0.35) but decreases it continually in the symmetric zone near the rear wall (i.e., X/X0 = 0.35−0.5). This is because increasing the distance above not only accelerates the downward airflow decaying but also postpones the mixing process between two components in the downward airflow (i.e., the fuel-rich coal/air flow and the inner secondary air) and the high-speed staged air near the rear wall. However, the corresponding airflow decaying
Figure 6. Decay curves for longitudinal-velocity components of the downward airflow. 422
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achieve good burnout in the furnace without slagging in the hopper. 3.5. Evaluation of the Results. Actually, cold modeling under isothermal conditions cannot accurately describe the complex physical and chemical progresses of fuel combustion in the furnace. Thus, if the flow field is predicted in a full-scale furnace by cold modeling, the influence of temperature variation should also be taken into consideration. When the fuel-rich coal/air flow descends into the lower furnace, its volume expands quickly as the temperature continuously increases. Thus, the density of the fuel-rich coal/air flow also falls together with the flow momentum. Then, it can be concluded that the actual penetration depth for the fuel-rich coal/air flow in the full-scale furnace is shallower than the labscale results. Thus, when the results are extrapolated to a combustive environment, the specific data from the cold experiment should be modified, whereas the qualitative conclusions that the flow-field deflection formation and the particularly shallow penetration depths near the rear wall at the two higher settings (i.e., Cs = 12 and 15%) are still valid, although choosing between the lower values Cs = 7 and 9% is not possible. As mentioned above, Cs cannot go below 7% because of physical limitations. Again, recently published numerical work19 on MIMSC technology with Cs = 9% for a 350 MWe MBEL down-fired boiler gave credence to the new technology in three aspects: (i) In applying the MIMSC technology as a replacement for the MBEL prior art, the original asymmetric combustion that was seen in the furnace developed a symmetric pattern. (ii) 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. (iii) NOx emissions could be lowered by as much as 50%, without increasing levels of unburnt carbon in fly ash. When the above numerical simulation results are combined from a similar boiler using the same MIMSC technology in the published literature,19 we believe that these results from cold small-scale airflow experiments in the present work are valid and the optimal setting for Cs should be around 7−9%, thereby uniquely fixing in the newly reconfigured furnace the burner locations on the arches. Further investigation on the validity of the optimized distance between the inner and outer secondaryair ports will be reported by in situ measurements after the boiler starts commercial operations in the near future.
Figure 7 presents the dimensionless airflow penetration depth versus the distance between the inner and outer
Figure 7. Penetration depth for downward airflow reaching the lower furnace.
secondary air. The figure shows that the penetration depth is higher near the front wall than near the rear wall for all four settings. As Cs increases, the penetration depth near the front wall decreases initially but then increases, whereas that near the rear wall decreases continually. Moreover, the difference in penetration depths near the front and rear walls initially varies little in the lower range of Cs = 7−9% but then lengthens dramatically in the higher range of Cs = 9−15%. As mentioned previously, these circumstances occur because of the presence of symmetric flow fields at settings of Cs = 7 and 9% and the occurrence of the deflected flow field at the left two higher. Moving the inner secondary air toward the furnace center not only accelerates generally the decay in downward airflows but also brings out the flow-field deflection, which is characterized by the airflow deflecting upward earlier near the rear wall than near the front wall. To exploit the relatively large hopper region in a MBEL down-fired boiler, good burnout without slagging in the hopper is enhanced when the downward coal/air flow turns upward after penetrating the middle part of the hopper region (i.e., the penetration depth should be about 0.6).1,19,24 As shown in Figure 7, both penetration depths near the front and rear walls for the lowest setting of Cs = 7% are higher than 0.65. With these circumstances appearing, an excessive penetration depth is established for the downward coal/airflow, which lowers the flame center position and achieves adequate burnout in the lower furnace. Consequently, the coal combustion share and the heat release proportion in the upper furnace are below the normal levels, and then steam temperatures may drop below the design parameters. Also, the excessive airflow reach may result in slagging in the hopper. For the higher two settings of Cs = 12 and 15%, penetration depths near the rear wall are all below 0.53, an especially shallow depth reversing the downward airflow in the uppermost part of the hopper region. According to much boiler operation experience7,9,11,24 and results from experimental and numerical investigations,2,4,5,26 unburnt carbon content would be high with such a shallow airflow penetration depth, because residence times are short for reactions to take place in the furnace. For the moderate Cs = 9% setting, penetration depths near the front and rear walls are 0.64 and 0.61, respectively, close to the optimal depth range mentioned above (i.e., ∼0.6). The symmetric flow field and an appropriate airflow reach in the lower furnace are very likely to
4. CONCLUSION The present work reported on our experimental investigation into the effect of separation between the inner and outer secondary air on aerodynamic characteristics, as determined by cold small-scale airflow experiments within a 1:20-scaled model of the furnace. Aerodynamic field measurements were conducted for different settings Cs = 7, 9, 2, and 15%, i.e., different ratios of separation/arch depth. At the two lower settings, aerodynamic field and velocity distributions along certain cross-sections and downward airflow penetration depths featured well-formed symmetric patterns along the furnace center, with the exception of slight asymmetry in the recirculating air below the front and rear arches. At the two higher settings, a deflected flow field appeared in the lower furnace and the downward airflow near the rear wall was directed upward earlier than that near the front wall. In general, increasing Cs (i.e., moving the inner secondary air toward the furnace center) decreases both penetration depths near the 423
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front and rear walls and increases their difference. To establish a symmetric flow field along with an appropriate airflow penetration depth, we found that, for the newly reconfigured furnace, an optimal separation between the inner and outer secondary-air ports on the arch should correspond to Cs = 7− 9%.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +86-451-8641-8854. Fax: +86-451-8641-2528. Email:
[email protected].
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NOTE ADDED AFTER ASAP PUBLICATION There was an error in the data presented in the fourth paragraph of the Experimental section of the version of this paper published December 14, 2011. The correct version published January 11, 2012.
ACKNOWLEDGMENTS This work was sponsored by the Hi-Tech Research and Development Program of China (863 Program) (Contract 2006AA05Z321) and supported by the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant 51121004).
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dx.doi.org/10.1021/ef201294r | Energy Fuels 2012, 26, 417−424
