ARTICLE pubs.acs.org/EF
Combustion Characteristics of the External Circulation Loop on Baima’s 300 MWe Circulating Fluidized Bed Boiler Jiayi Lu,* Xiaofeng Lu,* Honghao He, Hu Wang, Lu Gan, Peng Zhao, and Xiuneng Tang Key Laboratory of Low-grade Energy Utilization Technologies and Systems of Ministry of Education, Chong Qing University, Chong Qing 400044, P. R. China ABSTRACT: Local measurements of concentrations of O2, CO, SO2, and NOx were carried out inside Baima’s 300 MWe circulating fluidized bed (CFB) boiler. Custom-made probes with a length of 4.8 m were used to take samples from the boiler’s external circulation loop, especially from the cyclones and fluidized bed heat exchangers (FBHEs). Eighteen points were used to introduce the probes, and the penetration depth inside the boiler was up to 3.7 m. Solid particles at the entrance and exit regions of both the cyclones and FBHEs were also sampled for analyzing the particle size distribution and carbon content. Results implied that there was significant combustion in the boiler’s external circulation loop, and most of the combustion was from the cyclones. The flue gas temperature at the cyclone outlet could be 135 °C higher than that at the cyclone inlet. The ratio of the oxygen consumption in the cyclone to that of the entire combustion process of the boiler was nearly 12%. The O2 consumption rate might mainly come from the burn-out of char particles rather than the combustible gas. With regard to the FBHEs, the inside combustion phenomenon was not as severe as that in the cyclones. Obvious differences of gas concentrations were observed at the outlet between those FBHEs with different functions. The experimental results can provide information for adjusting and designing the heat-surface arrangement and lay a foundation for modeling the flow structure, combustion, and heat transfer in similar large-scale CFB boilers.
1. INTRODUCTION As a highly efficient clean combustion technology, the circulating fluidized bed boiler, which plays a significant role in energy saving and environmental protection, has been greatly developed and widely popularized. So far, a number of subcritical 300 MWe CFB boilers have already been in operation, and the only supercritical CFB boiler in the world, developed by Foster Wheeler, has been in commercial operation since 2009. Currently, the total number of 300 MWe CFB boilers in commission or in operation in China is nearly 40, which is more than the sum of all other countries around the world.13 Among these 300 MWe CFB boilers, the boiler with imported technology has very high boiler efficiency, nearly up to 93%.4,5 As equipped with 4 hot cyclones, it is analyzed that there may be a relatively high combustion or heat release fraction in the external circulation loop of the boiler, which leads to a highly burn-out rate of fuel and hence gets higher boiler efficiency. Usually, partial combustion is found in the hot cyclone, causing an increment of flue gas temperature of about 20— 50 °C.6 However, field data of the imported 300 MWe CFB boiler shows the flue gas temperature at the cyclone outlet could be nearly 150 °C higher than that at the cyclone inlet, which is much higher than that in existing reports.6,7 Although combustion in the external circulation loop contributes to the burn-out rate of fuel, such phenomenon also deviates the heat balance from the designated values, potentially causing overheating problems for reheaters and superheaters.8,9 Meanwhile, when the cyclone is under overheat operating condition or operating near the heat-resistant limit for a long time, the inner refractory material would be damaged and the aging of the vent-pipe would be accelerated, creating a safety risk for the unit’s long-time running.10 Previous studies showed the combustion characteristic of external circulation loop (usually focused on cyclones) was strongly r 2011 American Chemical Society
impacted by fuel properties, mainly by the volatile content, and operational parameters including total air flow rate, primary/ secondary air ratio, bed inventory, and feeding coal size.6,7 However, these studies were mainly concerned with cyclones and some of them were rather qualitative. At the same time, the very limited tests were all concentrated on 150 MWe or less CFB boilers and experimental data such as gas concentrations and carbon content were insufficient.7 As a result, the combustion characteristics of the external circulation loop on larger CFB boilers are still seen as black boxes where only output data are recorded. Additionally, the imported 300 MWe CFB boiler is equipped with 4 FBHEs, the solids inventory of these FBHEs could be twice as that of the furnace, and there may be slight combustion in them. However, most of the tests on large-scale CFB boilers were focused on the furnace; the external circulation loop such as cyclones and FBHEs was scarcely involved.1119 Consequently, experimental studies of the combustion characteristics of the external circulation loop were carried out on the imported 300 MWe CFB boiler in the present work. After analyzing the measurement results, some suggestions are given in an attempt to provide reference for the explanation of the high boiler efficiency and the research and development of larger CFB boilers.
2. EXPERIMENTAL STUDY 2.1. Experimental Object. The measurements were carried out on Baima’s 300 MWe CFB boiler of the demonstration power plant in Sichuan province. This boiler, designed by ALSTOM, a 1025 t/h CFB steam generator, has been in commercial operation since April 2006. At Received: April 18, 2011 Revised: July 2, 2011 Published: July 05, 2011 3456
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Figure 3. Design of the gas suction probe.
Figure 1. Schematic diagram of the boiler.
Figure 4. Particle sampling setup.
Figure 2. Position of measuring points on the boiler. that time, it was the first 300 MWe CFB boiler in China and one of the largest CFB boilers in the world.20 The external circulation loop of the boiler mainly consists of four hot cyclones and four FBHE; the overall structure of the boiler is shown in Figure 1. To extract gas and solid particles from the external circulation loop, 18 measuring points were installed specifically, which are at the cyclone entrance region both on the front wall and rear wall, cyclone entrance region on the ceiling membrane wall, cyclone exit region on the vent-pipe (for gas sampling), cyclone exit region on the stand pipe (for particles
sampling), and the exit region of FBHEs. Each point is constituted by a steel sleeve and a sealing ball valve. Considering there may be some influences on the combustion characteristic of each external circulation loop due to the difference functions between the front-wall side FBHEs and rear-wall side FBHEs, the measuring points were located on two cyclones and FBHEs at diagonal position, as shown in Figure 2. 2.2. Experimental Methodology. To measure local gas concentrations inside the combustion chamber, water-cooled gas suction probes were usually used. This was done to avoid accidents due to the deformation of the probe caused by the long tube’s weight or particles hitting, and meanwhile reactions within the probe could be quenched. However, since the width of fins between the neighboring membrane tubes is only 25 mm, it is impossible to place universal water-cooled probes. Although a rectangular cross-section water-cooled probe was designed by Hartge et al,19 it is not realistic to use in our measurements as there are several measuring points located on the ceiling membrane wall where is difficult to conduct measurements with such a complicated sampling system. Therefore, custom-made probes were used, the design of which is shown in Figure 3. The sampling probe is made of stainless steel tubing, equipped with corundum as inside lining. The design was put out under the following considerations: first, all the measuring points on the furnace are located at the furnace exit region, where the probe would not be easily bent by fine particles hitting. Second, the probe would not be bent by the weight of itself at the vertically installed ceiling water wall points. Last, after being sucked through a filter at the tip of the probe, flue gas would be completely drawn into the corundum tube which prevents the gas from contacting and reacting with the steel tube’s inner side. During the test, probes with different lengths were used depending on the accessibility of the measurement location. The standard probe had a total length of 4.8 m which allowed a penetration into the combustion chamber of 3.7 m, covering most of the cyclone inlet region. To prevent condensation inside the probe, the gas temperature was kept at 120 °C 3457
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Table 1. Typical Experimental Conditions during the Measurements item
unit
value
item
unit
value
main steam flow rate
t/h
913
main steam temperature
°C
539
main steam pressure
MPa
16.57
reheat steam temperature
°C
540
reheat steam pressure
MPa
3.11
total bed pressure drop
KPa
17.1
coal feeding rate
t/h
210
feed water flow rate
t/h
780
total air flow rate
Nm3/h
874183
cyclones inlet temperature
°C
821
cyclone outlet temperature (front wall side)
°C
960
cyclone outlet temperature (rear wall side)
°C
952
FBHE intlet temperature (front wall side)
°C
927
FBHE intlet temperature (rear wall side)
°C
930
FBHE outlet temperature (front wall side)
°C
431
FBHE outlet temperature (rear wall side)
°C
579
by an electrical heating system. During the measurement, high-temperature resistance fiber paper, aluminosilicate fiber cotton, and tape were used for sealing the whole sampling system. A “MGA5” infrared gas analyzer was employed to monitor the online gas concentrations. With respect to the ash sampling, three corundum tubes (inside diameter 6 mm) were used as suction probes. To prevent the slender sampling tubes from being broken by the weight or the hitting of particles, a stainless steel tube was placed outside each corundum tube as casing pipe. The standard probe had a total length of 3 m which allowed penetration into the combustion chamber as far as 2 m. The suction circuit consisted of a small cyclone (inside diameter 8 cm), a particle collector, an air filter apparatus, and a vacuum pump. The installation of the ash sampling system is illustrated in Figure 4. Additionally, considering there may be carbonate in the sampled particles, the carbon content was obtained by the universal loss-on-ignition method after an acid cleaning and drying preparation.21,22 Other operating parameters, such as the overall air flow rate, the cyclone inlet/outlet temperatures, and furnace temperature were obtained from the Distributed Control System (DCS) database in the control center of the plant. 2.3. Experimental Procedure. At each gas measuring point a vertical (horizontal) profile was measured. For each position the gas was sampled for about 10 min. Furthermore, during the measurement of each profile, at least one of the measurements was repeated, to get an indication of the reproducibility of the measurements and the stability of the combustor operation. For ash sampling, it was determined that at the same horizontal level, particle size distribution was nearly the same at different penetration depth.11 Moreover, the closer the sampling location to the furnace exit, the more obvious of such effect was found.13 Meanwhile, at the same horizontal level, the carbon content was nearly the same for particles sampled at different radial locations.11 Iherefore, only two radial locations were set to sample solid particles at the same horizontal level. During the experimental procedure, the boiler load was at 280 ( 3 MWe. Other typical experimental conditions are shown in Table 1, and the characteristics of feeding coal are summarized in Table 2.
3. COMBUSTION CHARACTERISTIC IN CYCLONE 3.1. Gas Concentration Analysis. During the measurements, the gas concentrations were fairly steady especially at the outlet of the cyclones and FBHEs. The deviations of the gas concentrations between the repeated test and its previous test were always below 5%. However, since the gassolid disturbance was more severe at the cyclone inlet region, the deviations were more obvious. For O2 and CO, the deviations were about 7%, while those of NOx and SO2 were much higher, normally about 15%, sometimes even up to 20%. Figures 5 and 6 give the gas concentrations at the two cyclones inlet sampled from the ceiling water wall measuring points. It is
Table 2. Proximate and Ultimate Analysis of Fuel proximate analysis
ultimate analysis
Mar
2.59%
Car
48.51%
Aar
47.31%
Har
2.01%
Vadf Fc,d
15.76% 43.24%
Nar Oar
0.24% 1.65%
Qnet
14.98 MJ/kg
Sar
2.83%
found that O2 concentration at this region could be as high as 5%, which provides a prerequisite for further combustion in the cyclones. Meanwhile, the CO concentration at this region was very high, implying the existence of inadequate gas lateral mixing hence leading to incomplete combustion of the combustible gas. Moreover, at the middle and upper part of the exit duct (penetration depth less than 2 m), both O2 and CO concentrations were relatively low, while those at the lower part of the exit duct (penetration depth over 2 m) were higher. Lackermeier et al.23 conducted a comprehensive study on the flow phenomena in the exit zone of a circulating fluidized bed and their results illustrated that the horizontal solids velocity reached its maximum at the middle and upper part of the exit duct. Therefore, it can be concluded that the gassolid lateral mixing would be more severe at this region, which contributes to the combustion and hence leads to the lower O2 and CO concentrations. Additionally, the low emission of NOx at the furnace exit region indicates that under the low-temperature (about 850 °C) operation condition with staged-combustion, the formation of NOx could be efficiently suppressed.24 Meanwhile, measurements from both the industrial-scale and pilot-scale circulating fluidized bed showed that SO2 mainly generated in the furnace (especially at the bottom part).19,25,26 Therefore, according to the high sulfur content of the fuel and the low SO2 concentration at the furnace exit region, it can be concluded that the effect of desulfurization was obvious. Previous studies showed that the desulfurization efficiency of this boiler could be close to 94% when the Ca/S mol ratio was about 1.7.4,27 Maybe it is due to the gassolid disturbance, these pollutant concentrations were found almost unchanged along the penetration depth. To verify the credibility of these results, auxiliary measurements were carried and the results are shown in Figures 7 and 8. From the variation of gas concentrations along the penetration depth in the furnace, it is found that a core/annulus structure still existed at the furnace exit region and the wall layer thickness was less than 1 m, which is consistent with Werhter’s findings.19 Because the area of the wall layer occupied just about 5% of the furnace cross-sectional area28 and the vertical net (time-averaged) 3458
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Figure 5. Gas concentrations of cyclone inlet at front wall side ceiling measuring points. Figure 7. Gas concentrations of cyclone inlet at front wall furnace exit.
Figure 6. Gas concentrations of cyclone inlet at rear wall side ceiling measuring points.
gas velocity within the wall layer could be seen as zero or near zero,29 only the gas concentrations in the core region can approximately reflect those at the cyclone inlet region. It is seen that after passing through the wall region, results coincide with those sampled from the ceiling water wall measuring points. Figure 9 shows the gas concentrations at the cyclones outlet sampled from the vent-pipe measuring points. It is found that O2 and CO concentrations were obviously lower than those at the cyclone inlet, proving combustion did occur in the hot cyclones. Additionally, compared to the cyclone inlet, SO2 concentration at the cyclone outlet was a little higher. Generally, the generation of SO2 by coal combustion under the circulating fluidized bed condition showing an evident character of phases and the formation of SO2 from pyritic sulfur can last for several minutes. Furthermore, the production rate increases with the increment of bed temperature.30 In this way, SO2 would be generated in the cyclones. On the other hand, the best desulfuration temperature using limestone is 850890 °C,31,32 while the temperature in the cyclones during the test could be as high as 960 °C. It has been studied that if the temperature is too high
Figure 8. Gas concentrations of cyclone inlet at rear wall furnace exit.
(above 920 °C), the pores in CaO would be blocked quickly to prevent its deeper desulfuration action, which results in lower desulfuration efficiency.32,33 From both aspects above, the SO2 concentration increased slightly at the cyclone outlet. At the same time, the concentration of NOx at the cyclone outlet was a little lower than that at the cyclone inlet, and results from the gas analyzer showed most of the NOx was NO. Because CO concentration was relatively high at the cyclone inlet region and the gassolid disturbance was more severe in cyclone than in furnace, the reduction of NO by CO would occur in the cyclones. Furthermore, since most of the volatiles could be seen as released in the furnace,28 the NOx generated in the cyclone may be mainly due to the char-N oxidation.34 Considering the transient residence time of particles staying in the cyclone, only a small amount of NOx would be generated, some of which may be reduced simultaneously by both the homogeneous and heterogeneous reduction reactions.35,36 As a result, less NOx emission was found at the cyclone outlet. 3459
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Figure 10. Particle size distribution of particles at cyclone inlet.
Figure 9. Gas concentrations of cyclone outlet on vent-pipe points.
To understand the combustion characteristic in a quantitative way, a dimensionless coefficient β is introduced. β equals the ratio of the oxygen consumption in the cyclone to that of the whole combustion process of the boiler: β¼
O2cyc, in O2cyc, out O2f ur, in O2cyc, out
ð1Þ
where O2cyc, in, O2cyc, out and O2fur, in represent the arithmetic weighted mean O2 concentrations at the inlet/outlet of the cyclone and the entrance region of furnace. Based on the experiment results, β was about 12%. It is implied that there was significant combustion in the cyclone, which explains why the flue gas temperature at the cyclone outlet could be much higher (about 135 °C) than that at the cyclone inlet. It was analyzed that during the test, the fuel of Baima’s 300 MWe CFB boiler was anthracite, the volatile content of which is low and the pores size are too small to be conducive to the spread of mass for burning, causing difficulties in burning out the combustibles in furnace.37 Meanwhile, the above measurement results showed that there was a lot of combustible gas at the furnace exit region, indicating an inadequate gas lateral mixing. While in the cyclone, the disturbance is much stronger so it is easier to burn out the combustible gas. Additionally, some unburned particles would be exposed in the oxygen environment again under the attrition process,38 which contributes to the further combustion of char particles in the cyclones. Finally, our previous studies showed the separation efficiency of the cyclone was greatly improved by the use of some unique technologies,39,40 which hampers the escape of some high reactivity fine particles. The burning of this part of fine particles also increased the combustion fraction of the cyclone. 3.2. Particle Size Distribution and Carbon Content Analysis. Figure 10 shows particle size distribution curves of solid particles sampled from the cyclone inlet. It can be seen that particles were mainly concentrated in the range of 70—200 μm. Mass content of 70—90 μm particles was more and that of either d 200 μm particles were less. Figure 11 presents carbon content distribution of the particles sampled from the cyclone inlet and outlet (stand pipe). During the process of determining the carbon content, a series of standard sieves (sieve pore size (μm): 63, 75, 90, 150, 210,
Figure 11. Carbon content of particles at cyclone inlet and outlet (stand pipe).
300) was used to divide the sampled ashes into several particle size groups. To facilitate the presentation, each group was characterized by the average pore size of the two adjacent sieves. From Figure 11, it is found that carbon content of 70—180 μm particles was very low while that of either d < 50 μm or d > 200 μm particles was high. This is because 70—180 μm particles were the major component of recycled ashes, and they could be separated effectively from gas to take part in circulating combustion by cyclones, which would lengthen the residence time of particles being in the furnace. As a result, the carbon content of 70—180 μm particles was lower. For particles smaller than 50 μm, the separation efficiency of cyclone was low, and only a small part of the particles could be separated away from hot gas to involve in circulating combustion. So carbon content of d < 50 μm particles was higher. For particles larger than 200 μm, the burn-out time of these particles is usually longer. Under the same combustion condition and residence time, the burn-out rate of large particles is lower. Meanwhile, after a long time of burning in the furnace, the particle-size of large particles would be reduced due to attrition. Additionally, it is calculated that the arithmetic weighted-average carbon content of particles at the stand-pipe was nearly 0.3% lower than that at the cyclone inlet, which shows combustion of particles continued in the cyclones. From these test results, the arithmetic weighted mean O2 and CO consumption in the cyclones could be obtained, which were 2.12.4% and 0.40.7%, respectively. In this case, if all CO were 3460
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oxidized to CO2, just about 0.3% of O2 would be consumed, indicating that only a small part of the O2 consumption was due to the burn-out of combustible gas. Consequently, it could be inferred that most of the combustion in the cyclones might come from the burn-out of char particles. 3.3. Simple Method to Estimate the Circulating Mass Flow. A feature of the measurement results is that it allows estimating the circulating mass flow. Currently, the circulating mass flow cannot be measured directly and is usually calculated on the basis of fluid mechanical modeling. In the present work, the volatile fraction of the fuel was low and if we suppose the volatiles were all released and combusted in the furnace,28 CO would be the main combustible gas at the cyclone inlet. Meanwhile, from the test results, it can be seen that the CO concentration was rather low at the cyclone outlet. So it can be assumed that both the CO and char particles were oxidized to CO2 as final product. In this way, the quantity of reacted carbon could be obtained, which also equals the difference of carbon content of particles between the cyclone inlet and outlet (both at the stand pipe and the vent pipe): Mcar, rea ¼ Mcir, cyc, in 3 Acar, cyc, in ðMcir, cyc, out 3 Acar, cyc, out þ Mf lyash 3 Af lyash Þ Mcir, cyc, in ¼ Mcir, cyc, out þ Mflyash
ð2Þ ð3Þ
where Mcar, rea is the quantity of reacted carbon in the cyclone, Mfly ash, Mcir, cyc, in, and Mcir, cyc, out represent the fly ash flow and ash flow both at cyclone inlet and outlet (stand pipe). Afly ash, Acar, cyc, in and Acar, cyc, out are the fly ash carbon content and carbon content of solid particles at cyclone inlet and outlet (stand pipe). At the same time, the circulating mass flow can be seen as unchanged from the cyclone inlet to the stand pipe since the fly ash mass flow (always less than 50 t/h for 300 MWe CFB boilers) only accounts for a small part of the circulating mass flow (up to thousands of tons per hour). So eq 2 can be simplified as Mcar, rea ¼ Mcir 3 ðAcar, cyc, in Acar, cyc, out Þ Mf lyash 3 Af lyash ð4Þ According to the proximate analysis of fuel, the total ash flow from the feeding fuels can be determined, and the ratio of fly ash to bottom ash could be obtained from the procedure explained in our previous study,4 thus the fly ash flow can be known. The fly ash mass flow can also be determined on large time intervals by counting the number and loading of trucks which transport the ashes to a disposal site.41 Meanwhile, the fly ash carbon content in the test was about 4%, corresponding to the previous studies, about 3.6% (with similar fuel and under the full work load).4,42 In this way, it can be calculated that the circulating mass flow was about 5700 t/h, which is on the same order of magnitude as our previous modeling result of about 6300 t/h.43 It should be indicated that 5700 t/h is just an estimated result which greatly relies on the measurement accuracy. Considering the boiler’s size, it is impossible to take samples at all the theoretical representative positions in the test.44 So there are certain differences between the simple measurement-based result and the modeling result. However, since the circulating mass flow cannot be measured directly and the modeling is usually complicated and some of the fluid mechanisms such as the influence of secondary air have not been solved completely,29 the method
Figure 12. Gas concentrations at the outlet of FBHEs.
proposed in the present work did provide an approximate result in a very simple way, and the result will be more accurate with the progress of the measurement technology. In this part, it is illustrated that there was significant combustion in the hot cyclones of the imported 300 MWe CFB boiler, and the combustion might mainly come from the burn-out of char particles. Additionally, a simple method was proposed to estimate the circulating mass flow.
4. COMBUSTION CHARACTERISTIC IN FBHE 4.1. Gas Concentration Analysis. As mentioned above, the boiler is equipped with four FBHEs: two located at the front wall side and two located at the rear wall side. The low temperature superheaters and high temperature reheaters are arranged in the front wall side FBHEs, while the intermediate temperature superheaters are arranged in the rear wall side FBHEs. The front wall sides FBHEs can be used to adjust the temperature of reheat steam, and the rear wall side FBHEs play a furnace temperature adjusting role. Because the entering wind of FBHEs is compressed air from fluidization fan, only the gas concentrations at the outlet were measured and results are shown in Figure 12. In this figure, O2 concentration at the outlet was much lower than that at the inlet. It is analyzed that on the one hand, although the particles had been combusted in the furnace and cyclones, there would be still a small amount of fuel without burning out. Meanwhile, the solid inventory of these FBHEs could be twice that of the furnace, hence despite the amount of unburned particles accounting for only a small proportion of the recycled ashes, the absolute amount was still considerable. On the other hand, the inlet air quantity of FBHE was rather small and accounted for only 1% of the boiler’s total air. As a result, O2 concentration at the FBHE outlet was much lower than that at the inlet. Meanwhile, the CO concentration was very high, indicating there was combustion occurring in the FBHEs but the combustion was not complete. Moreover, since the inlet air quantity of FBHE accounted for only 1% of the boiler’s total inlet air, the O2 consumption in the FBHEs could be ignored, which means most of the combustion in the external circulation loop of this boiler was from the cyclones. 3461
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Energy & Fuels Additionally, since the heat-surfaces absorb heat from the hot ashes, the inside average temperature of the two FBHEs was just about 700 °C, which results in a low activity of the desulfurization reaction.35 While according to the low SO2 concentration at the FBHEs outlet, one can indicate that the SO2 production was fairly limited in the FBHEs, showing that generation and removal of SO2 were mainly occurring in the furnace. However, the presence of FBHE may provide some preparations which would be conducive to the sulfur retention. On the one hand, the existence of FBHE not only lengthens the residence time of particles but also contributes to the shedding of dust closely attached outside the combustible particles by attrition for further desulfurization reaction.45 On the other hand, when the temperature condition is appropriate, the FBHE may serve as a desulfurization reactor in order to facilitate the desulfurization efficiency. It can also be seen from Figure 12 that there are great differences of combustion characteristic between the two FBHEs. During the test, the openings of the incoming ash flow control valves of the FBHEs were 52% (rear wall side) and 18% (front wall side), which means the combustible mass flow that went into the rear wall side FBHE would be nearly thrice that of the front wall side FBHE. Meanwhile, the inlet air flow of the two FBHEs was nearly the same, so it is easy to understand that there were obvious gas concentration differences at the outlets of the two FBHEs. On the other hand, because of the different functions of the heat surfaces arranged in the two kinds of FBHEs, the amount of heat absorbed by the heat surfaces would be different and thus may lead to certain influences on the inside temperature distribution of the FBHEs. During the measurements, the inlet flue gas temperature of both FBHEs was about 930 °C and the outlet flue gas temperatures were 431 °C (front wall side) and 579 °C (rear wall side), respectively. Therefore, it is clear that there was a great difference of the inside temperature distribution between the FBHEs, which maybe another reason for the significant gas concentration differences at the FBHEs outlet. In Figure 12, O2 consumption of the rear wall side FBHE could be twice that of the front wall side FBHE. Note that compared to the front wall side FBHE, the concentration of CO was higher while that of NOx was also higher at the outlet of the rear wall side FBHE. As mentioned above, NOx in FBHEs would be generated from the char-N oxidation and was mainly NO as shown on the gas analyzer. Ren46 studied the generation mechanism of char-NOx under a fluidized bed condition and found NOx (NO and N2O) and CO could reach the peak value synchronously. In our measurements, there were more char flowed into the rear wall side FBHE, thus more NOx and CO would be generated. Meanwhile, it was found that the reduction of NO by CO would not take place to any great extent due to the low temperature in the FBHEs.34 As a result, high concentrations of both CO and NOx were found at the outlet of rear wall side FBHE. 4.2. Particle Size Distribution and Carbon Content Analysis. Figure 13 shows particle size distribution curves of ashes sampled from the outlet of the FBHEs. It can be seen that particles were mainly concentrated in the range of 80—150 μm, which demonstrates the separation efficiency of cyclone was high so these fine particles could be involved in ash circulation. Figure 14 presents carbon content distribution of the ashes sampled from both the inlet (stand pipe) and outlet of the FBHEs. It can be seen that the carbon content of particles at the FBHEs inlet was very close to that at the FBHEs outlet, which
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Figure 13. Particle size distribution of particles at FBHEs outlet.
Figure 14. Carbon content of particles at FBHEs inlet and outlet.
also indicates the combustion was very slight in FBHEs. Meanwhile, the high carbon content of the fine and course particles still existed. In this part, it is shown that there was inside combustion phenomenon in the FBHEs, but such phenomenon was not as severe as that in the cyclones. Due to the differences of the combustible mass flow and the inside temperature distribution, obvious gas concentration differences were observed at the outlet of the two FBHEs. From the two sections above, it is shown that there was significant combustion in the external circulation loop of this imported 300 MWe CFB boiler, and most of the combustion was from the hot cyclones. This phenomenon contributes to the burn-out rate of fuel, consequently improving the boiler efficiency. At the same time, the combustion or heat release fraction distribution in the boiler must be deviated from the designated values. So, for new boilers, designers should take this phenomenon into account for designing the suitable heat-surface arrangement.
5. CONCLUSION Sampling of gas and solid particles with the custom-made probes were carried out on the external circulation loop of Baima’s 300 MWe CFB boiler. Although some of the measuring points were blocked by ash and thus turned out to be inaccessible, the measurements provide a comprehensive picture of the combustion characteristics of the external circulation loop in a 3462
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Energy & Fuels large-scale CFB boiler. Some significant conclusions are summarized as follows: (1) There was significant combustion in the boiler’s external circulation loop, and most of the combustion was from the cyclones. The flue gas temperature at the cyclone outlet could be 135 °C higher than that at the cyclone inlet. The ratio of the oxygen consumption in the cyclone to that of the whole combustion process of the boiler could be as high as 12%. The carbon content of particles at the entrance of the cyclones was about 0.3% higher than that at the exit of the cyclones. The O2 consumption in the cyclones might be mainly from the combustion of char particles. Based on the measurement results, a simple method is proposed to estimate the circulating mass flow. (2) There was inside combustion phenomenon in the FBHEs, but the combustion was not as severe as that in the cyclones. Such inside burning phenomenon maybe provide some inspiration for the desulfidation in CFB boilers. (3) Besides providing reference to large-scale CFB boilers, the combustion characteristics of the external circulation loop also gives some enlightenment to the small-scale industrial CFB boilers. Since the height of an industrial CFB boiler is usually too short and therefore combustibles cannot stay long enough to be combusted completely, FBHE can be equipped which would lengthen the residence time of the combustibles (especially combustible particles) and meanwhile could serve as an auxiliary combustor for improving the combustion efficiency.47
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’ ACKNOWLEDGMENT Financial support of this work by the Key Project of the National Eleventh-Five Year Research Program of China (2006BAA03B0206) is gratefully acknowledged. We are thankful to Xiujian Lei, Wenqing Zhang, Jianbin Chen, Liqiang Zou, Shengwei Xin, Changxv Liu, Lin Li, and Xiaoling Huang from Sichuan Baima CFB Demonstration Power Plant Co. Ltd. for valuable help. ’ NOMENCLATURE β = ratio of O2 consumption in the cyclones to that of the whole combustion process of the boiler O2cyc, in = arithmetic weighted mean O2 volume concentration at the cyclone inlet (%) O2cyc, out = arithmetic weighted mean O2 volume concentration at the cyclone outlet (%) O2fur, in = arithmetic weighted mean O2 volume concentration at the entrance of furnace (%) Mcar, rea = quantity of reacted carbon in the cyclone (t/h) Mcir, cyc, in = ash flow at cyclone inlet (t/h) Mcir, cyc, out = ash flow at stand pipe (t/h) Mcir = circulating mass flow (t/h) Mfly ash = fly ash flow (t/h) Acar, cyc, in = carbon content of solid particles at cyclone inlet (%) Acar, cyc, out = carbon content of solid particles at stand pipe (%) Afly ash = carbon content of fly ash (%)
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