Article pubs.acs.org/EF
Characterization of Coal Combustion and Steam Temperature with Respect to Staged-Air Angle in a 600 MWe Down-Fired Boiler Min Kuang,†,‡ Zhengqi Li,§ Zhongqian Ling,*,† Zhuofu Chen,† and Danyan Yuan∥ †
Institute of Thermal Engineering, China Jiliang University, Hangzhou, Zhejiang 310018, People’s Republic of China State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou, Zhejiang 310027, People’s Republic of China § School of Energy Science and Engineering, Harbin Institute of Technology, Harbin, Heilongjiang 150001, People’s Republic of China ∥ Zhejiang Feida Corporation, Zhuji, Zhejiang 311800, People’s Republic of China ‡
ABSTRACT: The staged-air angle was adjusted from 45° to 20° on a down-fired 600 MWe supercritical utility boiler suffering from superheat and reheat steam temperatures operating at below their designed values. This modification was expected to elevate the flame kernel position and increase the combustion share in the upper furnace, thereby raising superheat and reheat steam temperatures. To evaluate the effect of this modification on coal combustion, NOx emissions, and the aforementioned steam temperatures, normal full-load industrial-size measurements were taken within the furnace before and after the angle reduction. A comparison of the data from the two angle settings revealed that there were slight changes in superheat and reheat steam temperature levels as well as NOx emissions. Unexpectedly, the angle reduction caused several adverse effects, including a reduction of gas temperatures in the furnace, a sharp increase of carbon in fly ash, and a decrease in boiler efficiency. In conclusion, the staged-air angle reduction failed to improve the low steam temperatures and, simultaneously, resulted in poorer coal combustion within the furnace. adding bituminous coal or biomass into anthracite),18,19 exchange of the fuel-rich and fuel-lean coal/air flow nozzle locations,15 and downward inclination of the F-layer secondary air8,20,21 have been tested in real furnaces and achieved improvements in furnace performance. With respect to the angled F-layer secondary air used to improve the furnace performance, additional findings in the published work8,15,20−23 need to be reviewed here because of its close relation with the objective in this study. The F-layer secondary air, which is supplied through the front and rear walls (i.e., a jet belonging to wall air) of the low furnaces of downfired boilers applying the Foster Wheeler (FW) Corporation down-fired combustion technology, accounts for about 70% of the total secondary air supply and is horizontally fed into the primary combustion zone.8 Because of the strong interception of the F-layer secondary air on the downward coal/air flow, experimental and numerical results by Li et al.,8 Fang et al.,20 and Hui’s group22,23 all confirmed that the downward coal/air flow could not penetrate the F-layer secondary air zone, resulting in a particularly shallow airflow penetration depth and short residence times of coal particles in the lower furnace. Inclining downward the F-layer secondary air by a shallow angle of 25−30° (called “the angle F-layer secondary air” in the published work8) was thus put forward to weaken its transverse interception on the downward coal/air flame. Accordingly, the prolonged airflow penetration depth and increased furnace utilization through this solution have been confirmed by coldmodeling experimental and numerical results for 300, 600, and
1. INTRODUCTION Anthracite and lean coal, typically low-volatile and hard-to-burn fuels,1−3 are available in abundant reserves and are widely fired in thermal power generators around the world. Down-fired furnaces, which apply various carefully designed strategies to seek satisfactory firing of these fuels, are therefore believed to be better at firing these fuels than tangential-fired and wallarranged furnaces.4−6 However, the actual combustion performance deviates from the designed combustion concept. Problems have been previously reported, such as late ignition, poor combustion stability, high levels of unburnt carbon in fly ash (usually 8−15%), heavy slagging, particularly high NOx emissions (rarely below 1500 mg/m3 at 6% O2), and asymmetric combustion.1,3,7−11 Accordingly, specific investigations into down-fired furnaces have been performed, and various solutions for these problems have then been developed. For example, shutting down burners near sidewalls, reducing boiler load, and burning coal with a low slagging tendency were suggested as ways to alleviate the heavy slagging problem.9 In the asymmetric combustion aspect, the published work revealed that a deflected flow-field formation rather than a symmetric Wshaped pattern as in the designed combustion concepts accounted for the asymmetric combustion found in downfired furnaces.10,11 With respect to high NOx emissions, parametric tuning of operating conditions to regulate airstaging conditions has been confirmed to reduce NO x emissions by approximately 20−35%12−14 and a further lowcost NOx reduction up to about 50%1,15−17 must rely on comprehensive combustion modifications to establish deep airstaging circumstances. To advance coal ignition, raise combustion stability, and improve burnout, the application of fuel-preheat burner nozzles,1 burning of blended coals (i.e., © 2014 American Chemical Society
Received: March 15, 2014 Revised: May 1, 2014 Published: May 20, 2014 4199
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Figure 1. Schematics of furnace configuration, burner, and staged-air slot layout patterns and measuring port layout in industrial experiments for the down-fired 600 MWe boiler.
oxygen-lean coal/air flame mixing with the angled F-layer secondary air was inevitably postponed. The present limitation in dealing with problems of downfired furnaces is that few studies have reported on comprehensive solutions to all of these problems. To address this, a multiple-injection and multiple-staging combustion technology (MIMSCT) was developed and tested in a 600 MWe pulverized-coal down-fired supercritical boiler. The corresponding real-furnace results showed a well-formed symmetric and stable combustion pattern at normal full-load operations, accompanied by relatively low levels of carbon in fly ash and NOx emissions (about 3% and 1100 mg/m3 at 6% O2, respectively). However, unfortunately, the superheat and reheat steam temperature levels were about 20 and 40 °C below their designed values, respectively. Various combustion adjustments were testified to act slightly on this problem. In light of the successful experiences in the published work8,15,20,21 (i.e., a 25− 30° downward inclination of the F-layer secondary air lowering apparently the flame kernel position to decrease gas temper-
660 MWe down-fired furnaces.8,20,22,23 In light of the apparent improvement in the airflow penetration depth and furnace utilization ratio, the angle F-layer secondary air solution was used to (i) retrofit 300 and 660 MWe large-scale down-fired furnaces8,15,20,21 and (ii) improve the combustion atmosphere within a 0.7 MWe pilot down-fired test facility.23 Real-furnace results in the published work8,15,20,21 suggested that the furnace performance in all related furnaces improved greatly in four aspects: (i) The flame kernel, which was unreasonably located in the upper furnace because of the late coal ignition and particularly shallow flame penetration depth (resulting from the horizontally fed F-layer secondary air), moved downward to a reasonable position in the lower furnace. (ii) Both residence times of coal particles in the whole furnace and gas temperature levels in the lower furnace increased apparently because of the prolonged flame penetration depth. (iii) A decrease in both carbon in fly ash (from 7−8 to 4−5%) and exhaust gas temperature resulted in a boiler efficiency increase. (iv) NOx emissions reduced to some extent because the relatively 4200
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Table 1. Proximate and Ultimate Analyses of Coals Used in Industrial-Size Measurements proximate analysis (wt %) (as received) volatile matter a
8.95 /7.58
a
b
ash a
32.61 /27.25
fixed carbon
moisture b
a
b
a
7.60 /7.52 50.84 /57.65 ultimate analysis (wt %) (as received)
b
net heating value (MJ/kg) 19.46a/21.70b
carbon
hydrogen
sulfur
nitrogen
oxygen
51.50a/57.48b
2.12a/2.08b
3.83a/3.29b
0.85a/0.79b
1.49a/1.59b
Coal corresponding to the staged-air angle of 45°. bCoal corresponding to the staged-air angle of 20°. furnace and uniformly positioned along the furnace breadth. As a schematic of staged air, Figure 1d only graphs the staged-air slot layout and major slot dimensions along one half of the front (or rear) wall. Staged air, used to organize air-staging conditions and adjust the flame penetration, is partitioned hot air from the secondary-air box. The MIMSCT concept is regulated to form a deep air-staging combustion configuration consisting of fuel-rich and fuel-lean combustion and two layers of secondary air supplying in the burner zone, staged air supplying in the primary combustion zone, and the future overfire air supplying in the burnout zone (the overfire air application is absent in the furnace at this time, and the overfire air space is reserved). With the high-speed secondary air and staged air carrying the low-speed coal/air flow to penetrate deep into the lower furnace, methods that realize the MIMSCT are listed as follows: (i) the fuel-rich and fuel-lean coal/air flows are vented through nozzles centered over the furnace and positioned near the front and rear walls to form fuel-rich and fuel-lean combustion, respectively; (ii) two layers of secondary air are supplied through the furnace arches to form a first air staging in the burner zone, i.e., inner secondary air positioned between the fuel-rich and fuel-lean coal/air flows and outer secondary air located near the front and rear walls; and (iii) the inclined staged air is supplied at the lower part of the lower furnace to form a second combustion stage in the primary combustion zone. Additionally, the future overfire air application, aiming at regulating a third combustion stage in the burnout zone, is used to form the deep air-staging conditions. More detained information about technical principles of MIMSCT can be found elsewhere.24 2.2. Experiment Setups and Methods in Real-Furnace Data Acquiring. In an effort to improve steam temperatures, the staged-air declination angle (denoted by θ in Figure 1a) was adjusted from the originally installed 45° to a much shallower 20°. To evaluate the effects of the staged-air angle reduction on coal combustion, NOx emissions, and steam temperatures, normal full-load industrial-size measurements were taken within the furnace before and after the angle retrofit. To ensure that the results are comparable, the damper-opening settings must be kept within two runs of industrial-size measurements. This is in addition to considerable efforts to ensure minimum variation in coal characteristics and boiler operating conditions. As shown in Table 1, the experimental coal properties were similar before and after the angle retrofit. Over the 5 h duration of each experimental run, those operation parameters listed in Table 2, including fluxes and temperatures for primary air and secondary air, gas temperature at the furnace outlet, superheat and reheat steam temperatures, and exhaust gas temperature, were recorded once every 30 min in the digital dashboard to calculate mean values. Carbon in fly ash and NOx emissions were acquired by sampling fly ash and flue gas, respectively, at the air preheater exit once every 30 min. Mean values for these were also listed in Table 2. Simultaneously, additional data were acquired by performing the following measurements during each experimental run: (1) General gas temperature distribution in the furnace. Through observation ports (Figure 1) on the two wing walls connected with the right-side wall, a 3i hand-held infrared thermometer (with a measurement range from 600 to 3000 °C and accurate to within 1 °C) was used to measure gas temperatures in the front and rear-half parts of the lower furnace. These data were used to evaluate the effect of the staged-air angle reduction on the general gas temperature distribution in the lower furnace. The local combustion status was taken as the major source of
ature levels in the upper furnace), reducing sharply the large declination angle (45°) of staged air (not exceeding 20% of the total secondary air supply) in the present 600 MWe boiler may elevate the flame kernel position to raise gas temperatures in the upper furnace, thereby improving the low steam temperatures. Consequently, a measure characterized by reducing the staged-air declination angle from 45° to 20° was proposed as a simple and low-cost solution for this low stem temperature problem and recently tested in the 600 MWe boiler. A limited unburnt loss can be acceptable if the low steam temperature problem is resolved. Considering that (i) no such staged-angle retrofit has been reported to improve steam temperatures, (ii) here, staged air has large differences of air ratio and angle value from the reported F-layer secondary air, and (iii) the present furnace configuration and combustion system differ a lot from those with F-layer secondary air (detailed differences listed in the following Experimental Section), this work focuses on the effect of the staged-air angle reduction on combustion and steam gas temperature and evaluates the validity of the angle retrofit in the real furnace. Accordingly, normal full-load industrial-size measurements were performed within the furnace before and after the staged-air angle retrofit. Results from this study provide useful information on adjusting the combustion status to deal with low superheat and reheat steam temperature levels.
2. EXPERIMENTAL SECTION 2.1. Utility Boiler. The aforementioned 600 MWe down-fired boiler based on a MIMSCT16 was used in this work. Figure 1 shows schematics of the furnace configuration and combustion system. As illustrated in Figure 1a, the furnace arches divide the furnace into two zones: the octagonal lower furnace with four wing walls (see the horizontal cross-section in Figure 1b) and the rectangular upper furnace, which act as the fuel-burning zone and the fuel-burnout zone, respectively. Obviously, here, the huge lower furnace space and short upper furnace create a pyknic furnace pattern, which allows for major combustion to be completed in the lower furnace if a good burnout rate is achieved. On the contrary, those down-fired furnaces with the F-layer secondary possess a thin, tall furnace pattern to allow for more combustion share in their tall upper furnaces.8,15,20,21 A total of 24 louver concentrators, symmetrically arranged on the two furnace arches, divide the primary air/fuel mixture into fuel-rich and fuel-lean coal/air flows to regulate fuel rich/lean combustion. Besides giving the major dimensions and horizontal cross-section of the lower furnace with two arches and four wing walls, Figure 1b shows the correspondence between these louver concentrators and coal/air ducts (connected with six millers labeled as A−F) on furnace arches. Again, the burner layout pattern is clearly illustrated in Figure 1b. That is, 12 burner groups symmetrically line the front and rear arches and are uniformly positioned along the furnace breadth, with 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 feeding each burner group (Figure 1c). Corresponding to burner groups on furnace arches, there are 12 groups of staged-air slots symmetrically located at the lower parts of the front and rear walls in the lower 4201
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(3) Local gas temperature and species concentration distributions in the near-wall region. To perform these measurements, the previously described thermocouple device and a 3 m long water-cooled stainless-steel probe, comprising a centrally located 10 mm inner diameter sampling pipe (with a gas sampling flow rate of 1 L/min) surrounded by a 60 mm inner diameter stainless-steel tube with a high-pressure cooling water flow rate of 60 L/min for probe cooling, were inserted in turn into the furnace through observation ports 1 and 2 (Figure 1). Gas samples were obtained with a vacuum pump and the sampling pipe, between which there was a cyclone separator, gas collector, flow meter, and other devices. With the help of the large-flux and high-pressure cooling water, the chemical reactions in the captured gas samples were quickly quenched and the gas samples were cooled in a short time. Subsequently, the cooled gas samples were analyzed online by a Testo 350M gas analyzer with measurement errors of 1% for O2, 5% for CO, and 50 ppm for NOx. Sources of uncertainty in the fine-wire thermocouple temperature measurements originated from (i) radiation loss occurring from the gas to the thermocouple and from the thermocouple to the surrounding wall and (ii) ash deposition on the thermocouple. Those uncertainties in the gas species concentration measurements were associated with the quenching of chemical reactions and aerodynamic disturbances of the flow. According to calculations in the published work,25,26 the uncertainty in the fine-wire thermocouple temperature measurements should be below 8%. It should be noted that the thermocouple used is easy to burnout when placed in a high-temperature atmosphere exceeding 1300 °C. To avoid this burnout, the thermocouple was removed from the furnace when readings were close to 1300 °C during the measurement of gas temperatures. Ports 1 and 2 are not far from the front wall and can be directly affected by the downward coal/air flow and staged air, respectively (Figure 1). Apparently, the burner region and ports 1 and 2 are all relatively low-temperature zones. These circumstances usually allow for the thermocouple to acquire the local maximum value of the flame temperature at limited distances from the wall. Before the above industrial-size measurements, all data-acquiring apparatuses, including the thermometer, thermocouple device, and gas analyzer, were calibrated to ensure the stated accuracy.
Table 2. Major Operational Parameters of the Boiler in FullLoad Experimental Settingsa staged-air declination angle quantity
45°
20°
total rate of primary air (kg/s) temperature of primary air (°C) primary air ratio (%) total rate of secondary air (kg/s) temperature of secondary air (°C) staged-air share in the total secondary air (%) secondary air ratio (%) staged-air ratio (%) pulverized-coal fineness (R90, %) gas temperature at the furnace exit (°C) superheat steam temperature (°C) reheat steam temperature (°C) exhaust gas temperature (°C) carbon content in fly ash (%) NOx in flue gas (mg/m3 at 6% O2) boiler efficiency (%)
118.0 95 17.6 554.2 360 12.7 71.9 10.5 8.6 1055 553 531 149.7 2.54 1292 91.52
115.1 98 17.0 563.1 352 13.3 72.5 11.0 8.0 1042 548 529 147.2 7.84 1289 89.04
a
The designed superheat and reheat steam temperatures at full load are 571 and 569 °C, respectively. Here, the gas temperature at the furnace exit is the mean value of the multiple groups of data recorded frequently in the digital dashboard, because of the temperature inevitably fluctuating within a small range during each experimental run. The angle reduction allows for a slight staged-air ratio increase because of an airflow resistance decrease through staged-air slots. uncertainty in this type of temperature measurement because it clearly affected the temperature reading obtained by the handheld infrared thermometer. Consequently, the measurement at each port was taken 10 times within 5 min, and then these values were averaged. Repetition of the multiple measurements demonstrated that, with considerable efforts to ensure minimum variation in the local combustion status, the temperature reading variation was within 20 °C. Because of coal combustion mainly occurring in the lower furnace of down-fired boilers and no flame observation port located on walls of the upper furnace, gas temperature measurements were only performed within the lower furnace and those of the upper furnace were absent. As the complementary material, the acquisition of gas temperatures at the furnace exit is necessary to uncover more clearly the angle reduction on the overall gas temperature levels in the furnace. Fortunately, there are several online gas temperature monitoring points at the furnace exit, with the automatically recorded temperature data appearing in the digital dashboard of the boiler. In light of the limited data fluctuation appearing in the digital dashboard because of considerable efforts to ensure minimum operational parameter variation, the obtained gas temperatures at the furnace exit were actually the mean values of the multiple groups of data recorded frequently in the digital dashboard during each experimental run. (2) Gas temperatures in the burner region. A K-type thermocouple with a 0.3 mm diameter and 8 m length nickel−chromium/ nickel−silicon fine wire, located in a 4 mm diameter twin-bore stainless-steel sheath, was placed inside a 6 m water-cooled stainless-steel probe to form a thermocouple device. As shown in panels a−c of Figure 1, along the sight port pipe of the F2 concentrator, the thermocouple device was inserted into the furnace through the inner secondary-air port next to the flowrich coal/air flow nozzle to obtain gas temperatures in the burner region. The end of the sheath was exposed in the furnace while taking a temperature measurement.
3. RESULTS AND DISCUSSION Figure 2 presents gas temperature and species concentration distributions before and after the staged-air angle reduction. The comparison of the overall gas temperature distribution in the lower furnace before and after the retrofit, as shown in Figure 2a, reveals that the staged-air angle reduction, which is essentially put forward to improve steam temperatures by elevating the flame kernel position without dropping gas temperatures, actually decreases the overall gas temperature levels in the lower furnace. This occurs because that the angle reduction strengthens the staged-air block on the downward coal/air flow, thereby shorting the flame penetration. Consequently, shorter pulverized-coal residence times occur, and less heat is released during coal combustion in the lower furnace. Gas temperatures in the burner region and the near-wall zone adjacent to ports 1 and 2 are presented in panels b and c of Figure 2, respectively. As shown in Figure 2b, the angle reduction lowers gas temperatures in the burner region yet no change occurs in the temperature pattern. The observation is attributed to (i) the two angle settings sharing the same burner configuration and air distribution and (ii) the angle reduction decreasing the overall gas temperature levels in the lower furnace (Figure 2a). In Figure 2c, gas temperature profiles correspond to ports 1 and 2 at an early stage of coal ignition 4202
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Figure 2. Gas temperature and species concentration distributions at staged-air angles of 45° and 20°.
furnace when readings were close to 1300 °C, resulting in the profile terminating at a short distance. Comparisons of gas species concentrations in the near-wall region before and after the angle reduction are presented in Figure 2d. The change trends of gas species concentrations generally present the following patterns: (i) The O2 content initially varies slightly but then increases with distance; the CO content was low the entire time, except for the sharp increase after 1.0 m in the zone near port 2 at 45°; and the NOx content generally increased with distance, except for some fluctuations. (ii) Both O2 and NOx levels at the two settings are higher near port 1 than near port 2, except for the NOx content beyond 1.2 m at 20°. (iii) Reducing the angle increases the O2 content while decreasing both CO and NOx contents, except for the NOx content within distances of 0.4−1.2 m in port 2. The explanation of these observations is based on two factors. First, in the zone near port 1 where coal combustion is in the preceding combustion stage, the relatively poor coal combustion and secondary air diffusion enable the O2 content to remain at relatively high levels and to increase moderately with distance. In the zone near port 2, where coal combustion is in the primary combustion stage and has already consumed large amounts of O2 to develop an oxygen-lean atmosphere,
and in the primary combustion stage, respectively. The dataacquiring locations account for the phenomenon that, in the near wing-wall region, gas temperatures are higher near port 2 than near port 1. As the distance increases, gas temperatures initially increase and then decrease in port 1 at both angles and in port 2 at 20°, whereas temperatures in port 2 at 45° increase sharply before measurements terminate at a short distance of 0.9 m. In accordance with the decrease in the overall gas temperature distribution (Figure 2a), the angle reduction decreases both gas temperatures near ports 1 and 2. This occurs for several reasons: (i) The high-temperature gas accumulates in the near-wall region, resulting in the initial increase stages. (ii) Port 1 is not far from the burner nozzle outlets, and the zone near port 2 at 20° is directly affected by cold staged air (see Figures 1 and 2a). As the thermocouple is inserted further, measuring points gradually approach the zones affected by the low-temperature secondary air and staged air, thereby entering those temperature decrease stages. (iii) At the 45° setting, the much weaker influence of the sharply declivous staged air on the zone near port 2 and the relatively higher gas temperature levels at port 2 (Figure 1) supports the sharp temperature increase pattern. The thermocouple was removed from the 4203
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angle modifications solely for a possibly existing optimal angle, which necessitate lots of industrial-size measurements in the furnace, instead of the simple, convenient, and low-cost benchscale test circumstances, cannot be allowed by boiler managers because of the potential operation risk induced by the frequent angle modifications and high cost. Therefore, only two angle settings before and after the angle retrofit were tested in the real furnace. It should be noted that the equipped angle 20° was determined by previous cold-modeling airflow experiments by evaluating the flow field fullness and downward airflow penetration depth. Because of the relatively low staged-air ratio (15−25%) and the originally installed large angle 45° developing a weak block of staged air on the downward airflows, the furnace obtained a high flow field fullness and large downward airflow penetration depth, thereby achieving particularly high burnout performance. With only reducing the angle to a much shallower angle not exceeding 20°, the airflow penetration depth was found to be shortened greatly. To elevate apparently the flame kernel position and simultaneously achieve a relatively deeper airflow penetration depth for burnout, 20° was finally equipped in the staged-air angle retrofit. Solely considering the combustion characteristic and NOx emissions without coal particles washing over the hopper wall, 45° should be the optimal staged-air angle setting because of the deep flame penetration, high gas temperatures, and late mixing of the downward flame with staged air. Any angle reduction may more or less induce adverse effects on burnout and NOx emissions based on the gained results from 45° to 20° in this study. Nevertheless, the limited data acquisition and angle settings presented in this work fail to exhibit more detail on the effect of staged-air angle reduction on the overall gas temperature and NOx distributions in the furnace, and it is also very difficult to collect the data on furnace flow and gas temperature fields using industrial-size measurements. Therefore, the corresponding numerical simulations into the flow, coal combustion, and NOx formation within the furnace need to be performed in the near future in an attempt to uncover the comprehensive staged-air effect by evaluating a series of angle settings. In any case, here, the failed retrofit suggests that a similar method characterized by adjusting wall− air angles (such as the F-layer secondary air angle,8,20,21,23 tertiary air in the literature,27 and staged air in this study) should not be recommended as the preferred solution to deal with problems of low superheat and reheat steam temperatures in down-fired boilers, because this method may cause uncertain effects on coal combustion.
staged air mixing with gas in this zone is much weaker at 45° than at 20° (Figure 2a). Second, reducing the angle to 20° weakens combustion intensity and decreases gas temperatures in the furnace. Changes in major operation parameters are listed in Table 2. At the original 45° setting, the furnace attains good burnout and still high NOx emissions (i.e., carbon in fly ash and NOx emissions of 2.54% and 1292 mg/m3 at 6% O2, respectively). However, both the superheat and reheat steam temperatures are lower than the designed values of 571 and 569 °C, respectively. As mentioned previously, the staged-air angle reduction was designed to deal with the low steam temperatures by elevating the W-shaped flame without decreasing gas temperature levels in the furnace. On the contrary, the angle reduction retrofit actually decreases the gas temperature at the furnace exit, with the change trend remaining in accordance with the overall gas temperature distribution in the lower furnace (Figure 2a). Evidently, various temperature decreases (i.e., those of gas temperatures in the lower furnace and at the furnace exit, superheat and reheat steam temperatures, and exhaust gas temperature; see Figure 1a and Table 2) confirm that the staged-air angle reduction fails to deal with this problem. Additionally, carbon in fly ash increases to 7.84%. These adverse events are all attributed to the fact that the angle retrofit can shorten the coal/air flame penetration depth in the lower furnace, thereby decreasing the overall gas temperatures (Figure 2a). According to the published work,8,20,21,23 moderately inclining the F-layer secondary air downward from 0° to 25° or 30° (the jet location generally equal to that of staged air in this work) deepened the coal/air flame penetration and postponed the mixing of the F-layer secondary air with the downward coal/air flame, thereby improving burnout, raising boiler efficiency, and reducing NOx emissions. Reversing the angle retrofit in this study from 20° to 45° resulted in the change trends in burnout and boiler efficiency remaining in accordance with those in the aforementioned published work. However, that in NOx emissions was different, where no apparent change in NOx emissions occurred following the angle reduction. The explanation behind this phenomenon is the combined effects of (i) the drop in the overall gas temperatures and (ii) the enhancement in the mixing of staged air with the ignited coal/air mixture in the primary combustion zone after the angle reduction. To summarize, aside from causing adverse effects in coal combustion (such as lowering gas temperatures and increasing combustible loss to drop boiler efficiency), the staged-air angle reduction is essentially unable to deal with the low steam temperature problem. Realistically, regulating a series of industrial-size staged-air angle settings within a large angle range is expected, to experimentally uncover the comprehensive effect of staged-air angle on coal combustion and steam temperatures. However, unfortunately, the real-furnace circumstances only allow the two angle settings to be tested before and after the retrofit, instead of a series of angles following an assumed understanding. Reasons are mainly in two aspects: (i) An adjustable angle setup cannot be established for the large quantities of grouped staged-air slots (Figure 1d) if staged-air jets necessitate the air-channeling prevention to maintain a strong rigidity after leaving the slot outlets. A strong staged-air rigidity favors good mixing of staged air and the downward coal/air flame, thereby assisting an intense coal combustion behavior in the primary combustion zone and adjusting effectively the flame penetration. (ii) A series of staged-air
4. CONCLUSION The work presented here is an experimental investigation into the effect of a staged-air angle reduction (from 45° to 20°) on coal combustion and steam temperature within a down-fired 600 MWe supercritical boiler. Normal full-load industrial-size measurements were performed within the furnace before and after the angle reduction. At the originally installed 45° angle setting, the furnace attained high gas temperature levels and particularly good burnout but, unfortunately, suffered from low superheat and reheat steam temperatures and still high NOx emissions. Reducing the angle to 20° affected slightly these steam temperatures while simultaneously causing several adverse effects on coal combustion, such as lowering gas temperatures and sharply increasing carbon in fly ash, despite a lack of apparent change in NOx emissions. The experience from this failed retrofit suggests that a similar method, such as 4204
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(20) Fang, Q. Y.; Wang, H. J.; Zhou, H. C.; Lei, L.; Duan, X. L. Energy Fuels 2010, 24, 4857−4865. (21) Li, Z. Q.; Ren, F.; Chen, Z. C.; Chen, Z.; Wang, J. J. Fuel 2010, 89, 410−416. (22) Liang, L.; Hui, S. E.; Zhao, S.; Zhou, Q. L.; Xu, T. M.; Zhao, Q. X. Exp. Therm. Fluid Sci. 2012, 42, 240−247. (23) Zhao, S.; Hui, S. E.; Liang, L.; Zhou, Q. L.; Zhao, Q. X.; Li, N.; Tan, H. Z.; Xu, T. M. Exp. Therm. Fluid Sci. 2013, 45, 180−186. (24) Li, Z. Q.; Chen, Z. C.; Kuang, M.; Sun, R.; Zhu, Q. Y.; Zeng, L. Y. W-staged flame boiler for multi-stage combustion with multiejection and its method thereof. U.S. Patent Application Publication US 20110253066 A1, 2011. (25) De, D. S. J. Inst. Energy 1981, 54, 113−116. (26) Costa, M.; Azevedo, J. L. T.; Carvalho, M. G. Combust. Sci. Technol. 1997, 129, 277−293. (27) Fan, J. R.; Zha, X. D.; Cen, K. F. Energy Fuels 2001, 15, 776− 782.
reducing the wall−air declination angles, should not be recommended as the preferred solution to deal with problems of low superheat and reheat steam temperatures in down-fired boilers because of uncertain effects on coal combustion caused by this adjustment. To avoid inducing adverse effects on coal combustion, reasonably adding the heating surface areas in superheaters and reheaters would be more applicable to deal with such a problem, unlike the simple and economical but high-risk angle reduction.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +86-571-86914542. Fax: +86-571-86835763. Email:
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[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Contract 51306167), the Project funded by the China Postdoctoral Science Foundation (Contract 2014M551733), and the Key Science and Technology Innovation Team of Zhejiang Province in China (Contract 2011R50017).
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REFERENCES
(1) Garcia-Mallol, J. A.; Steitz, T.; Chu, C. Y.; Jiang P. Z. Ultra-low NOx advanced FW arch firing: Central power station applications. Proceedings of the 2nd U.S. China NOx and SO2 Control Workshop; Dalian, China, Aug 2−5, 2005. (2) Lee, J. M.; Kim, D. W.; Kim, J. S. Energy 2011, 36, 5703−5709. (3) Cahill, P.; Beeley, T. J.; O’Connor, M.; Spence, J.; Gibbins, J. R.; Man, C. K.; Seitz, M. H. The assessment of low volatile fuels for downshot fired boilers. Proceedings of the 10th International Conference on Coal Science; Taiyuan, China, Sept 12−17, 1999. (4) Fan, W. D.; Lin, Z. C.; Li, Y. Y.; Kuang, J. G.; Zhang, M. C. Energy Fuels 2009, 23, 111−120. (5) Zhou, H.; Mo, G. Y.; Si, D. B.; Cen, K. F. Asia-Pac. J. Chem. Eng. 2012, 7, 624−632. (6) You, C. F.; Xu, X. C. Energy 2010, 35, 4467−4472. (7) Luo, Z. X.; Wang, F.; Zhou, H. C.; Liu, R. T.; Li, W. C. Korean J. Chem. Eng. 2011, 28, 2336−2343. (8) Li, Z. Q.; Ren, F.; Liu, G. K.; Shen, S. P.; Chen, Z. C. Combust. Sci. Technol. 2011, 183, 238−251. (9) Fang, Q. Y.; Wang, H. J.; Wei, Y.; Lei, L.; Duan, X. L.; Zhou, H. C. Fuel Process. Technol. 2010, 91, 88−96. (10) Kuang, M.; Li, Z. Q.; Zhang, Y.; Chen, X. C.; Jia, J. Z.; Zhu, Q. Y. Energy 2011, 37, 580−590. (11) Gao, Z. Y.; Sun, X. Z.; Song, W.; Fang, L. J. Proc. CSEE 2009, 29, 13−18 in Chinese. (12) Fueyo, N.; Gambón, V.; Dopazo, C.; González, J. F. J. Eng. Gas Turbines Power 1999, 121, 735−740. (13) Burdett, N. A. J. Inst. Energy 1987, 60, 103−107. (14) Cañadas, L.; Cortés, V.; Rodríguez, F.; Otero, P.; González, J. F. NOx reduction in arch-fired boilers by parametric tuning of operating conditions. Proceedings of the Electric Power Research Institute (EPRI)/ Environmental Protection Agency (EPA) Megasymposium; Washington, D.C., Aug 25−29, 1997. (15) Li, Z. Q.; Ren, F.; Chen, Z. C.; Liu, G. K.; Xu, Z. X. Environ. Sci. Technol. 2010, 44, 3926−3931. (16) Kuang, M.; Li, Z. Q.; Xu, S. T.; Zhu, Q. Y. Environ. Sci. Technol. 2011, 45, 3803−3811. (17) Leisse, A.; Lasthaus, D. VGB Power Technol. 2008, 11, 1−7. (18) Blas, J. G. Combustion 1970, 42, 6−13. (19) Steer, J.; Marsh, M.; Griffiths, A.; Malmgren, A.; Riley, G. Energy Convers. Manage. 2013, 66, 285−294. 4205
dx.doi.org/10.1021/ef500587z | Energy Fuels 2014, 28, 4199−4205
