- Email: [email protected]
Development of a novel fluidized bed ash cooler for circulating fluidized bed boilers: Experimental study and application
Powder Technology 212 (2011) 151–160
Contents lists available at ScienceDirect
Powder Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p ow t e c
Development of a novel fluidized bed ash cooler for circulating fluidized bed boilers: Experimental study and application Bing Zeng ⁎, Xiaofeng Lu ⁎⁎, Lu Gan, Maolong Shu Key Laboratory of Low-grade Energy Utilization Technologies and Systems (Chongqing University), Ministry of Education, PR China
a r t i c l e
i n f o
Article history: Received 7 April 2011 Accepted 5 May 2011 Available online 13 May 2011 Keywords: Circulating fluidized bed Bottom ash cooler Gas–solid flow characteristic Particle separation Industry application
a b s t r a c t A novel bottom ash cooler (BAC) called compound fluidized bed ash cooler (CFBAC) was developed in this paper. The CFBAC combined the major technical features of spouted bed and bubbling bed, and could achieve the selective discharge on the bottom ash. Experiments about the gas–solid flow characteristics of the CFBAC were conducted in a visible cold test rig. The experimental results indicated that the separation chamber working in the spouted bed state had a good particle separation effect on the boiler bottom ash. A small quantity of fluidizing air was needed for the cooling chambers to work in the bubbling bed state. The particle separation effect could be controlled by the fluidizing air flow and physical dimensions such as the height of separation partition. The CFBAC had also been industrially applied in a 300 MW circulating fluidized bed unit. The application showed that the CFBAC had a well separation effect, an excellent adaptability on the bottom ash, a good cooling effect and a large ash discharge capacity over 30 t/h. Compared with the water-cooled BAC, the CFBAC had a better energy conservation performance. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Circulating fluidized bed (CFB) boiler bottom ash contains large amounts of physical heat. While the boiler combusts the low-calorie fuel, the ash content is normally more than 40% and the physical heat loss is approximately 3% if the bottom ash is discharged without cooling. .In addition, the red-hot bottom ash is bad for mechanized handling and transportation, as the upper limit temperature of the ash handling machinery is 200 °C. Therefore, a bottom ash cooler (BAC) is often used to treat the high temperature bottom ash to reclaim heat, and to have the ash easily handled and transported [1,2]. As a key auxiliary device of CFB boilers, the BAC has a direct influence on the secure and economic operation of the boiler. There are many kinds of BACs equipped for large-scale CFB boilers with the continuous development and improvement of the CFB boiler, such as watercooled ash cooling screw [2], rolling-cylinder ash cooler (RAC) [2–4], fluidized bed ash cooler (FBAC) [5–8] and high-strength steel belt ash cooler [9]. The RAC and FBAC have a large capacity, and have been commonly and reasonably applied in China. As the coal resources are in short supply, the China government has strongly supported using CFB boilers to combust the low-calorie fuel such as coal gangue, stone coal and petroliferous shale, whose ash content are even greater than 70%. The as fired coal of CFB boilers in China is poor and changeable, and the actual operations of the boilers ⁎ Corresponding author. Tel./fax: + 86 23 65102475. ⁎⁎ Corresponding author. E-mail addresses: [email protected] (B. Zeng), xfl[email protected] (X. Lu). 0032-5910/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2011.05.005
deviate greatly from the original design state. If the CFB boiler burns a large quantity of the low-calorie fuel, the combustion condition of the furnace would be changed greatly and the discharge capacity of the bottom ash would be substantially increased with the coarsening particle size. More seriously, sometimes all of the condensation water may be not sufficient for cooling the bottom ash while the watercooled BAC such as RAC is used, which brings about a great impact on the turbine regenerative system. Because of the insufficient output capacity of existing BACs, the slag discharge temperature would be increased accordingly and then, the conveying equipment would be broke down frequently. According to statistics, the forced shutdown rate caused by the active defect of BACs exceeded 60%. With the development of CFB boilers towards large-scale and highparameter, the single unit capacity is enlarging. In order to meet the needs of the furnace load and the particle concentration in the splash zone, it requires more fine ash in bed material to maintain a higher circulating ratio. Consequently, it is particularly important that BACs have the function of discharging coarse slag and returning fine ash to the furnace. As we all know, the FBAC has many advantages, such as a higher heat-transfer coefficient, a greater cooling capacity, no mechanical equipments, a low maintenance cost and a better thermal economy on reclaiming heat of the bottom ash. Compared with the RAC, the FBAC can also be used to cool the circulating ash discharged from the loop seal or the external heat exchanger. The FBAC used for the future supercritical CFB boiler could even be considered adopting both feed-water and condensate water as cooling medium [5,10]. The most important advantage of the FBAC is that partial fine particles of the bottom ash could be returned to the furnace to improve the bed material size
152
B. Zeng et al. / Powder Technology 212 (2011) 151–160
distribution. Moreover, the previous research revealed that using FBACs to reclaim heat of the bottom ash obtained higher plant thermal efficiency, lower plant heat rate and less standard coal consumption than using the RAC to do [11]. But the further development of existing FBACs is restricted by some disadvantages such as poor flow ability of the bottom ash, bad particle size control performance, large air-cooled proportion and frequent slag bridging and building [5]. Therefore, it is necessary to develop a novel FBAC which can meet the needs of largescale CFB boilers, can adapt to the situation of the fuel as fired and have a wide adaptability to the bottom ash granularity. The objective of this paper was to introduce a compound fluidized bed ash cooler (CFBAC). The CFBAC has obtained an invention patent in China [12]. Experiments about the gas–solid flow characteristics of the CFBAC were investigated in a visible cold test rig. Furthermore, the experimental results were used in the CFBAC of a 300 MWe CFB boiler. The industrial application was studied and the operation aspect was found to be satisfactory.
2. Compound fluidized bed ash cooler Fig. 1 illustrates the basic structure of the CFBAC, which combines the advantages of various FBAC technologies and is based on the actual coal quality and the discharging ash particle size of CFB boilers in China. The operational principle and working process of the CFBAC is as follows: Firstly, the bottom ash discharged from the CFB boiler passes from the ash feed chute to the separation chamber of the CFBAC. The separation chamber was designed as a rectangular spouted bed. The high-temperature bottom ash is separated by the spouting air in the separation chamber. On the one hand, the high-temperature bottom ash exchanges heat with the cold air. On the other hand, the fine ash (e.g. particle size smaller than 4 mm) having small mass and good fluidity fluidizes into the cooling chamber I over the separation partition under the effects of overflowing and spouting. The coarse slag is discharged by a screw-type slag extractor from the separation chamber. Secondly, after cooling by the fluidizing air and the water-cooled tube in the cooling chamber I, the fine ash fluidizes and flows through the bottom hole of the intermediate partition into the cooling
Separation chamber
Air vent
chamber II for further cooling. After cooled to 100 ~ 150 °C, the fine ash is overflowed into the fine ash discharge chute and is discharged by a rotary discharge valve. Besides, the flue gas containing the extremely fine ash and the heat-absorbing fluidizing air returns back to the furnace through the air vent. The cooling chamber I and II need a small amount of fluidizing air to work in a way of bubbling fluidized bed. The fluidizing air could be the cold primary air. The cooling water could be the condensate water or feedwater. Compared with existing FBACs, the CFBAC has the following technical characteristics: • The spouting separation technology was adopted to carry out the particle separation of coarse slag and fine ash in the separation chamber. After cooling to 100 ~ 150 °C in the separation chamber, most of the coarse slag was discharged through the slag discharge chute and the screw-type slag extractor. Only a very small part of the coarse slag was overflowed into the cooling chamber I together with the fine ash. Moreover, the high speed air jet of the separation chamber enhanced the flow ability and adaptability of the bottom ash, and effectively avoided such phenomenon as bad fluidization, slag bridging and building caused by the deposition of coarse slag. • The initial bubbling fluidization technology was deployed in the cooling chamber to reduce the fluidizing airflow and the heating surface wear, and to improve heat transfer efficiency. The particles consisting of fine ash and a little coarse slag ensured that the cooling chamber was able to work in the bubbling bed state. • The bottom ash flowed in a mode of “overflow–underflow–overflow” in the CFBAC, due to the bottom hole and partitions in different heights. The especial flow mode efficiently controlled the directional flow of particles and lengthened the retention time of the bottom ash. • The selective ash discharge could be achieved in the CFBAC. It meant discharging coarse slag from the furnace and returning partial fine particles of the bottom ash to the furnace. The selective ash discharge could be of benefit to adjust the bed material size distribution and to improve the fluidization quality of the furnace. • It could accommodate to the development of large-scale CFB boilers. The membrane wall and the feedwater could be used as parts of the heat transfer surface and cooling medium of the CFBAC, respectively, to enhance the unit economy.
Separation partition
Intermediate partition
Cooling chamber I
Bottom ash cooler
Cooling chamber II Cooling tube
Ash inlet duct
Small ash discharge chute
Rotary ash discharge valve
Wind box
Screw-type slag extractor
Bottom hole
Fig. 1. Structural diagram of the CFBAC.
Emergency discharge chute
B. Zeng et al. / Powder Technology 212 (2011) 151–160
3. Experiments
153
Table 1 Physical properties of bed material.
3.1. Experimental apparatus The experimental apparatus of the CFBAC is illustrated in Fig. 2. It consists of hopper, test rig body, wind boxes, etc. The cold test rig was manufactured by plexiglass, angel steel and steel plate, convenient for observing the gas–solid flow characteristic. The air distribution plate was made by steel plate for strengthening the pressure resistance. A layer of canvas was set on the distribution plate to prevent the bed material leaking into wind boxes. The air from an air compressor was blown into each chamber through respective wind boxes and measured by the glass rotameter. The pressures were measured by differential pressure transmitter to accomplish the real time data acquisition. The internal dimensions of the cold test rig were 900 mm×200 mm× 600 mm. The cold test rig was divided equally to four chambers by three partitions: separation chamber, cooling chamber I, cooling chamber II and discharge chamber from feed side to discharge side. The separation chamber was designed as a rectangle spouted bed, and its bottom vertical section liked a “V” in shape. The other three chambers were designed as rectangle bubbling beds. The partitions were set as modular structure in order to change the partition heights. Except the separation partition, the other two partitions had a bottom hole (60 mm×60 mm). The air vent was located on the top of the cooling chamber II.
Properties
Values
Real density, kg/m3 Bulk density, kg/m3 Mean particle size, μm Minimum spouted velocity, m/s Minimum fluidizing velocity, m/s
2421 1530 564 0.4 0.28
The minimum spouted velocity and minimum fluidized velocity were gotten by Figs. 4 and 5, respectively. To investigate the solid flow characteristics, separation characteristics of the separation chamber, elutriation and ash discharge characteristics in the cold test rig, the solid samples from each chamber, discharge chute and air vent were collected during each experiment. The size analysis by sieving of the samples was done to determine the mean particle size and PSD. The separation mass flow rate (Gs) and the separation particle size (ds) were used to represent the separation characteristics of the separation chamber. Gs was defined as the whole particle weights of the solids separated from the separation chamber into the cooling chamber per unit time. ds was defined as the mean particle size of the solids separated into the cooling chamber. Pressure drop was considered to be a driving force of the solid flow. The height of partitions was a key factor to determine the solid flow, the bed pressure and the disposition of the heating surface. Especially, the separation partition between the separation chamber and the cooling chamber I played a very important role in the separation process of the bottom ash. Several experiments with
3.2. Experiments The bottom ash from a CFB boiler was used as the bed material after screening in all tests. Its properties were listed in Table 1, and the accumulative particle size distribution (PSD) was showed in Fig. 3.
A
A Air vent
Hopper
Cold Test Bed
Separation chamber
Cooling chamber II
Cooling chamber I
600
Discharge chute
Feed chute Discharge chamber
B
B
Wind box
B Bottom hole
Partition
A
A
900 Fig. 2. The visible cold test rig of the CFBAC.
200
B Separation partition
154
B. Zeng et al. / Powder Technology 212 (2011) 151–160
Fig. 6. Screening granularity of samples. Fig. 3. Accumulative particle size distribution of bed material.
Fig. 4. Bed pressure difference of separation chamber with superficial velocity.
various partition heights had been completed to study the characteristics of controlling the particle separation and the ash discharge. 4. Experimental results and discussion 4.1. Flow characteristics of bed material v, u1, u2, and u3 expressed the fluidizing velocities of separation chamber, cooling chamber I, cooling chamber II and discharge chamber, respectively.
Fig. 5. Bed pressure difference of cooling chamber with superficial velocity (descent method).
The mean particle size of each sample sampled from every chamber as well as discharge ash was showed in Fig. 6 after 2 h continuous run of the cold test rig, when ν, u1, u2, and u3 were 1.26 m/s, 0.52 m/s, 0.40 m/s and 0.28 m/s, respectively, and the height of separation partition (H) was 350 mm. The bottom pressures of each chamber were presented in Fig. 7. As shown in Fig. 6, the mean particle size decreased from feed side to discharge side (i.e. along the flow direction of particles). It revealed that the proportions of small particles in each chamber increased along the flow direction of particles. The reason was that the small particles had a better flowability than the coarse particles. The small particles were able to flow quickly from a chamber into the next chamber, whereas the coarse particles lagged far behind. As can be seen in Fig. 7, the bottom pressure of each chamber also decreased along the particle flow direction. The small pressure differences between the last three chambers provided a driving force for the particle flow. The flow behavior of bed material particles can be directly observed through the plexiglass of the test rig. Most of the coarse particles bobbed up and down in the bottom area of the separation chamber; only a very few coarse particles could be entrained by the small particles into the upper space or fluidized over the separation partition into the cooling chamber I. Meanwhile, the bubbling phenomenon in dense bed and the particles throwing in bed surface were clearly seen in the last three chambers (i.e. cooling chamber I, cooling chamber II and discharge chamber). It could also be found that the particles were fluidized and mainly passed through the bottom holes of partitions into the next chamber in the last three chambers. Then the particles were overflowed into the discharge chute in the discharge chamber.
Fig. 7. The bottom pressure of each chamber.
B. Zeng et al. / Powder Technology 212 (2011) 151–160
Fig. 8. Separation particle size with static bed material heights in different fluidizing velocities.
4.2. Separation characteristics in separation chamber The separating effect in the separation chamber had a direct impact on the fluidization quality of the next three chambers, and was a determinant of the operation quality of the CFBAC. The properties of particles, fluidizing velocity (v), static bed height (h) and separation partition height (H) were considered as the influential factors. The properties of particles could be considered as remaining practically unchanged in each experiment. The separation characteristics were obtained in the case of a single run of the separation chamber. The variations of the separation particle size (ds) and the separation mass flow rate (Gs) with static bed height (h) in different fluidizing velocities were showed in Figs. 8 and 9, respectively, when the separation partition height (H) was 350 mm. As shown in Fig. 8, ds increased with the increase of v. It was because the larger the v, the more the coarse particles were separated into the cooling chamber. Meanwhile, the ds roughly increased with an increase in h. The reason was that the lower the h, the greater the height difference between H and h and the higher the difficulty of particles overflowing the separation partition. The entrainment height of coarse particles was lower than that of small particles in a same v. It could be known from Fig. 9 that Gs increased with the increase of h in any v, and increased with increasing v in any h. When h was 150 mm, the Gs was always very small no matter how the change in v. It was because the fluidizing velocities could not provide the sufficient drag force for particles to overcome the height difference. When the v
Fig. 9. Separation mass flow rate with static bed material heights in different fluidizing velocities.
155
raised, the amount of particles whose terminal velocity were lower than the v increased, the interactions between particles and fluidizing air or particles and particles became increasingly intense, thus the entrainment effect strengthened. To sum up, it could be concluded that the higher the v and h, the greater the Gs and ds from the experimental results of the separation chamber. However, it can be known from the flow characteristics of spouted bed that h cannot be increased unlimited. If the h was higher than the maximum spouting height, the spouted bed state cannot be formed. The v should also be determined by combining the limitation of fluidizing airflow in actual operation. Furthermore, Gs and ds were a pair of mutual restraint factors. A larger ds means a bigger particle size in cooling chambers, therefore more fluidizing airflow is needed to maintain a good fluidized state. Fig. 10 illustrated the effects of H on Gs and ds when v was 1.58 m/s, as the height difference between H and h remaining constant. As shown in Fig. 10, both Gs and ds first increased and then decreased with the increase of H, and simultaneously reached the maximum point as H reaching 250 mm. The resistance for lifting particles from the bed surface to the top of the separation partition remained unchanged when the height difference maintained as constant. When H and h were low, the separation chamber operated in the shallow bed state and had an obvious particles delamination; hence the effect of the pneumatic separating on small particles became stronger. However, there was only a little of bed material in total, the amount of particles that could be thrown over the separation partition per unit time was small (i.e. Gs was small). With gradually increasing of H and h, the spouting phenomenon became more severe as well as a stronger entrainment, which resulted in the increments of Gs and ds. Then, when H and h further increased to certain heights, the bed resistance raised more, and the original v was insufficient to maintain a good spouted bed state in separation chamber. As a result, Gs and ds decreased by the weaken separation effect. At this moment, if v was increased, Gs and ds would increase accordingly. 4.3. Elutriation The terms elutriation, entrainment and carryover are used to describe a variety of phenomena ranging from the selective removal of fine particles from the surface of large granules in a fluidized conveyer or drier. As bubbles break at the surface of the bed, particles in the leading bulge of the bubble and the bubble wake are thrown up above the bed surface and are entrained by the upward flowing gas stream. Several researchers have made important contributions to understand the mechanism of elutriation [13–17]. The recapitulative conclusions were that particles elutriated from the surface of a fluidized bed into the freeboard depended on the mechanism of
Fig. 10. Separation mass flow rate and separation particle size with partition heights.
156
B. Zeng et al. / Powder Technology 212 (2011) 151–160
bubble eruption. Particles might be ejected into the freeboard from the roofs or wakes of bursting bubbles, depending on bed geometry, fluidizing velocity and particle type. The relational expression of the elutriation rate constant (k) could be written as:k ∝ u 1 ~ 7dp− 1 ~ − 2, where u was the fluidizing velocity and dp was the particles size. It indicated that the higher the fluidizing velocity (u) and the finer the particles size (dp), the greater the elutriation rate constant (k) would be. For the CFBAC, the elutriation directly determined the particle residence time and the amount of particles returning to the furnace, and then influenced the PSD of the furnace. It was difficult to measure the amount of particles from the CFBAC to the furnace during hot operation and collect the elutriation rate (Er) during experiments. Therefore, the suspension space pressure (ps) was used to qualitatively describe Er. The relationships of the suspension space pressure and the elutriation rate with the mean fluidized velocity (um) of the last three chambers of the test rig were plotted in Fig. 11. As shown in Fig. 11, both ps and Er increased linearly with increasing the um of the last three chambers of the test rig. It was because there were more particles could be ejected into the suspension space with the increase of the um, and the particle concentration of the suspension space increased, hence the ps accordingly increased.
4.4. Ash discharge characteristics Researches on the bed material flow and ash discharge characteristics of other types FBAC were mainly focused on such influencing factors as the pressure distribution [18], the pressure difference between FBAC and the furnace [19], the structure of fluidizing air distributor [20] and the equal or unequal air distribution [21]. The air distribution modes including equal distribution and unequal distribution of the last three chambers were investigated in this study. The equal distribution refers that the cooling chamber I, cooling chamber II and discharge chamber have a same fluidizing air flow. The unequal distribution means that the fluidizing air flow of each chamber is different, but the total fluidizing air flow remains constant. The effects of fluidizing air flow on the ash discharge rate (Ds) under equal distribution and unequal distribution were presented in Figs. 12 and 13, respectively. It could be known from Fig. 12 that, Ds increased first linearly and then slightly with the increase of the fluidizing air flow in each chamber. The reason was that there were more fine particles entrained or elutriated out of the test rig through the air vent, when the fluidizing air flow was larger than 32 N m 3/h. It counteracted a part of the increment of Ds caused by increasing the
Fig. 12. Ash discharge rate with fluidizing air flow of the last three chambers in equal distribution.
fluidizing air flow. Meanwhile, the equal distribution mode made the three chambers had the very similar particles flow state and bubble behavior, as well as the same bottom pressure, which weakened the lateral moving ability of particles. That's the reason why the Ds increased slightly with the increase of the fluidizing air flow after 32 Nm 3/h. As shown in Fig. 13, Ds in the fluidizing air flow ratio of 60:48:36 was greater than that in other ratios. Compared with the equal distribution mode, the unequal distribution mode had a higher Ds. The reason was that the bubble behaviors were different in the last three chambers under the unequal distribution. The greater the difference in fluidizing air flow in each chamber, the greater the difference in moving intensity of particles and the greater the difference in the bottom pressure would be. In the chamber which had the larger fluidizing air flow, on one hand, the solids and bubbles movement were more intense and the bubbles entrainment was stronger; on the other hand, the bed voidage was larger and the motion resistance was smaller. These resulted in the stronger lateral moving ability of particles and the larger Ds. However, when the air distribution was 72:48:24, the Ds decreased a lot and was smaller than that in the condition of 48:48:48. It was because the small fluidizing air flow leaded to a low quality of fluidization in the discharge chamber, although the pressure difference became larger. 5. Industrial application 5.1. Application introduction On the basis of the experimental results, the earliest CFBAC was designed and used in a 150 MWe CFB unit for trial operation in
Fig. 11. Suspension space pressure and elutriation rate with mean fluidizing velocity.
Fig. 13. Ash discharge rate with fluidizing air flow ratio of the last three chambers in unequal distribution.
B. Zeng et al. / Powder Technology 212 (2011) 151–160
Access door
Ash inlet duct
Separation chamber
Cooling chamber
157
Slag discharge vent
Separation chamber
Separation chamber
Coolong chamber
Cooling chamber
Access door Ash discharge vent Fluidizing nozzle
Cooling tube Collecting box Fig. 14. Internal structure diagram of the bottom ash cooler after rebuilt to a CFBAC.
Panzhihua, Sichuan province, China in 2009[10???]. The trial operation results indicated that the CFBAC had met the design output requirement (15 t/h). A detailed description of the cold test and hot test could be found in [10]. According to the early application and the experimental results, the optimized CFBAC had been industrially applied to a BAC reconstruction project of a 300 MWe CFB unit in Huaibei, Anhui province, China in 2010. The design coal of the 300 MWe CFB boiler was blended coals, including gangue, schlamm and middlings of washing coal. The boiler bottom ash had a coarse particle size, and the boiler was often required to discharge circulating ash. The internal structure, water-cooled tubes and air distributors of the original FBAC were reformed to a CFBAC in the case of remaining the main framework unchanged, as shown in Fig. 14. Its internal length, width and height were 7450 mm, 2100 mm and 2900 mm, respectively. The separation chamber was divided into three chambers by two partitions, and three separation chambers communicated with each other in the bottom. A long partition with a certain height was set up between separation chambers and cooling chambers. The cooling chamber was also divided into three chambers by two partitions. The heights of the two partitions were lower than that of the long partition. The partition between the cooling chamber I and the cooling chamber II had a through hole in the bottom, while the partition between the cooling chamber II and the cooling chamber III did not have. The separation chamber used a grid plate as air distributor while the cooling chamber used capped nozzles. The main design parameters of the CFBAC were shown in Table 2.
III were about 100 °C, which indicated that the discharge temperatures of the coarse slag and the fine ash were about 100 °C. Consequently, the CFBAC had a good cooling effect on the bottom ash. In order to investigate the output capacity of the CFBAC, the heat balance was calculated, as listed in Table 3. The heat balance calculation of the cooling water side and fluidizing air side showed that the CFBAC had a 35.55 t/h of ash cooling capacity while the discharge temperature of the coarse slag and the fine ash were 138.6 °C and 128.4 °C, respectively.
5.2.2. Particles separation effect The screening results of samples were shown in Fig. 16. The samples were sampled from the bottom ash inlet duct, cooling chamber I, slag discharge chute and ash discharge chute during hot operation. The proportion of particles larger than 4 mm in the coarse slag discharged from the separation chamber was over 70%, while the percentage of particles larger than 4 mm in the cooling chamber was about 5%. And the ratio of particles larger than 1 mm in the fine ash discharged from the cooling chamber was just under 3%. Moreover, the calculated mean particle size of the bottom ash, the coarse slag and the fine ash according to the screening results were 1.37 mm, 4.01 mm and 0.35 mm, respectively. It indicated that the separation chamber of the CFBAC had strong granularity adaptability and good separation effect on the bottom ash.
5.2. Application results 5.2.1. Temperature distribution and heat balance calculation Fig. 15 represented the bed temperature distribution of the CFBAC during a continuous operation. It revealed that the CFBAC operated stably and had a good and reasonable temperature gradient. The bed temperatures of the separation chamber III and the cooling chamber Table 2 Main design parameters of CFBAC. Properties
Values
Ash cooling capacity, t/h Ash inlet temperature, °C Ash outlet temperature, °C Fluidizing air volume, N m3/h Temperature of return flue gas, °C Cooling water requirement, t/h Water inlet temperature, °C Water outlet temperature, °C
30 850–920 ~ 150 22,000 300–450 100 ~ 35 ~ 85
Fig. 15. Operation data of the CFBAC (bed temperature distribution).
158
B. Zeng et al. / Powder Technology 212 (2011) 151–160
Table 3 Heat balance calculation of CFBAC. Properties
Values
Bottom ash inlet temperature, °C Coarse slag outlet temperature, °C Fine ash outlet temperature, °C Cooling water inlet temperature, °C Cooling water inlet pressure, MPa Cooling water outlet pressure, MPa Cooling water outlet temperature of tube bank 1, °C Cooling water requirement of tube bank 1, t/h Cooling water outlet temperature of tube bank 2, °C Cooling water requirement of tube bank 2, t/h Fluidizing air volume, N m3/h Fluidizing air inlet temperature, °C Return flue gas temperature, °C Calculated ash cooling capacity, t/h
885 138.6 128.4 25.6 0.66 0.29 70.5 74.63 38.1 69.69 22,445 35 333.1 35.55
5.2.3. Average heat transfer coefficient of tubes For the design and layout of the tube in the cooling chamber of the CFBAC, it is important to investigate the average heat transfer coefficient between water-cooled tubes and the mixture of the fluidizing air and particles. The relationship between the average heat transfer coefficient of tubes and the bed height could be calculated and analyzed based on the operation data and the assumption that the fluidizing velocity, the structure of water-cooled tubes and the PSD in the cooling chamber remained unchanged. The calculated results of many operating points were plotted in Fig. 17, and a curve was fitted according to the results. As shown in Fig. 17, the average heat transfer coefficient increased approximately linearly with the increase of the bed height. It was because that the immersed area of tubes and the particle concentration of the suspension space increased with the increasing of the bed height. The heat transfer coefficient of the tubes submerged in the bed material was far greater than that of the tubes in the suspension space. 5.2.4. Technical–economic comparison It was concluded by the previous study [11] that, the standard coal consumption rate of the FBAC mode was less nearly 2 g/(kW h) than the RAC mode in three CFB power plants (150 MWe, 300 MWe and 600 MWe, respectively), when the net calorific power of the standard coal was 29.27 MJ/kg. The FBAC and RAC were two different bottom ash heat recovery modes. The FBAC combined cold air and condensation water as the cooling medium, while the RAC only used the condensation water. However, the own energy consumption of the FBAC and RAC was not included in the previous study, such as
Fig. 16. Accumulative particle size distribution of samples during hot operation of the CFBAC.
the driving motor power consumption of the RAC and the increased fan power for supplying the fluidizing air of the FBAC. In general, the fluidizing air of the FBAC was taken from the cold primary air. The return air of the FBAC was returned back to the furnace as the secondary air, which resulted in a corresponding reduction in the proportion of the secondary air. Therefore, the ratio between the primary air and the secondary air of a CFB boiler with the FBAC or the RAC were different. The saving of electric power (ΔP0) in the turbine heat acceptance (THA) condition may be calculated as:
ΔP0 =
Δbs P bs 0
ð1Þ
where, P0 is the rated power, b s is the standard coal consumption in the THA condition and Δb s is the standard coal consumption difference between the FBAC mode and the RAC mode. The output power of the driving motor (Pg) of a fan can be expressed as follows:
Pg =
qv p ηηtm
ð2Þ
where, qv is the fan outlet flow rate, p is the fan outlet total pressure, η is the fan total pressure efficiency and ηtm is the mechanical transmission efficiency (taken to be 1). The fan outlet total pressure was assumed to remain constant, as the fan outlet flow rate changed relatively little. Hence the fan power consumption difference of the primary fan (ΔPg1) and the secondary fan (ΔPg2) caused by the change of the outlet flow rate can be written as follows:
ΔPg1 =
Δqv1 ·p1 η1 ηtm
ð3Þ
ΔPg2 =
Δqv2 ·p2 η2 ηtm
ð4Þ
where Δqv represents the fan outlet flow rate difference (namely the total fluidizing air flow of FBACs), p1 and p2 represent the fan outlet total pressure of the primary fan and secondary fan, respectively, and η1 and η2 represent the fan total pressure efficiency of the primary fan and secondary fan, respectively.
Fig. 17. Average heat transfer coefficient of tubes with bed height in cooling chamber.
B. Zeng et al. / Powder Technology 212 (2011) 151–160 Table 4 Technical–economic comparison calculation. Properties
Values
Rated power of the unit, P0, MWe Standard coal consumption, bs, g/(kW·h) Standard coal consumption difference, Δbs, g/(kW·h) Saving of electric power, ΔP0, kW Total fluidizing air flow of 4 CFBACs, Δqv, Nm3/h Total pressure of primary air, p1, kPa Total pressure of secondary air, p2, kPa Primary fan total pressure efficiency, η1 Secondary fan total pressure efficiency, η2 Mechanical transmission efficiency, ηtm Fan power consumption difference, ΔPg, kW Driver motor power consumption of a RAC, PRAC, kW Total Net power saving, ΔP, kW Net power saving of a CFBAC, ΔPCFBAC, kW
300 305 2.11 2075 88,000 30.9 15.4 0.9 0.9 1 421 20 1574 393.5
Then the power consumption difference between the primary fan and the secondary fan (ΔPg) is given by, ΔPg = ΔPg1 −ΔPg2 =
Δqv ·p1 Δqv ·p2 − η1 ηtm η2 ηtm
ð5Þ
Finally, compared with the RAC mode, the total net power saving (ΔP) in the FBAC mode can be calculated by the equation given as follows: ΔP = ΔP0 −ΔPg −nPRAC
ð6Þ
where, PRAC is the driving motor power consumption of a RAC and n is the number of RACs. The 300 MWe CFB unit in Huaibei, Anhui province, China, was taken as an example to compare the technical economy, when four BACs (CFBACs or RACs) were used to cool the bottom ash and to reclaim heat. The primary data and calculated results were shown in Table 4. It was found that the CFBAC had a better technical economy, and the net power saving was 1574 kW/h. The CFBAC could save the cost of about 2.8 million yuan RMB per year than using the RAC, on the assumption that the plant operated 200 days annually at rated load and the pool purchase price was 0.37 yuan/(kW h).
159
Nomenclature BAC bottom ash cooler CFB circulating fluidized bed CFBAC compound fluidized bed ash cooler FBAC fluidized bed ash cooler PSD particle size distribution RAC rolling-cylinder ash cooler THA turbine heat acceptance Ds ash discharge rate, g/s Er elutriation rate, g/s Gs separation mass flow rate, g/s H height of separation partition, mm PRAC power consumption of the driving motor of a RAC, kW P0 rated power, kW Pg output power of driving motor, kW bs standard coal consumption, g/(kW·h) mean particle size, mm dp ds separation particle size, mm h static bed height, mm k elutriation rate constant n numbers p outlet total pressure of the fan, Pa ps suspension space pressure, Pa qv outlet flow rate of the fan, Nm 3/h um mean fluidizing velocity, m/s v, u1, u2, u3 fluidizing velocities of the separation chamber, cooling chamber I, cooling chamber II and discharge chamber, respectively, m/s Greek letters ΔP total net power saving, kW ΔPCFBAC net power saving of a CFBAC, kW ΔP0 saving of electric power, kW ΔPg power consumption difference, kW Δb s standard coal consumption difference, g/(kW·h) Δqv outlet flow rate difference of the fan, total fluidizing air flow of FBACs, Nm 3/h η total pressure efficiency of the fan ηtm mechanical transmission efficiency
6. Conclusions A new type of FBAC called CFBAC was developed in this paper. Experiments had been conducted in a visible cold test rig, and the 30 t/h CFBAC had been successfully applied in a 300 MWe CFB boiler. Some primary conclusions were summarized as follows: (1) The pneumatic separation technology of the spouted bed was used to achieve the separation of the coarse slag and fine ash in the CFBAC. It improved the flowability and size adaptability of the bottom ash, and ensured that the cooling chamber could work in the bubbling bed state to reduce the fluidizing air flow. (2) The experimental results indicated that the CFBAC had a good particle flow characteristic. Fluidizing velocity and the height of the separation partition were two key parameters, which influenced the separation effect of the separation chamber. There was an optimum combination of the fluidizing velocity and the height of the separation partition. The elutriation rate increased linearly with increasing the mean fluidizing velocity of the last three chambers of the test rig. And the ash discharge rate in a certain unequal fluidizing air distribution (60:48:36) was greater than that in the equal distribution. (3) The industrial application revealed that the operation of the CFBAC was found to be satisfactory. The CFBAC had a well separation effect and a good cooling effect. And the actual ash cooling capacity exceeded the design capacity of 30 t/h. The CFBAC had a better energy conservation than the water-cooled BAC (e.g. RAC).
References [1] K.F. Cen, M.J. Ni, Z.Y. Luo, J.H. Yan, Y. Chi, M.X. Fang, et al., Theoretical Design and Operation of Circulating Fluidized Bed Boiler, China Electric Power Press, Beijing, 1998, pp. 647–663. [2] X.F. Lu, Equipments and Operation of Large-scale Circulating Fluidized Bed Boiler, China Electric Power Press, Beijing, 2006, pp. 125–133. [3] Y.C. Liu, H.C. Yin, J.P. Liu, Cold and hot test study on the rolling-cylinder type slagcooler of CFB boiler, Power Syst. Eng. 22 (5) (2006) 35–38. [4] W. Wang, X.D. Si, H.R. Yang, H. Zhang, J.F. Lu, Heat-transfer model of the rotary ash cooler used in circulating fluidized-bed boilers, Energy Fuels 24 (2010) 2570–2575. [5] M.L. Shu, J.H. Chen, X.F. Lu, H.Z. Liu, Investigation on fluidized bed bottom ash coolers in large-scale CFB boilers, Power Syst. Eng. 23 (2) (2007) 29–31. [6] J.J. Butler, N.C. Mohn, J.C. Semedard, CFB Technology: Can the Original Clean Coal Technology Continue to compete, Power Gen International Conference, Nevada, USA, 2005. [7] W. Nowak, Design and operation experience of 230MWe CFB boilers at Turow Power Plant in Poland, Proceedings of the 15th International Conference on Fluidized Bed Combustion, ASME, New York, 1999. [8] P. Xiao, T. Guo, Z.Q. Xu, J.Z. Jiang, H.S. Lu, Q. Li, et al., Large capacity fluidized bottom ash cooler and its operation performance, Proceedings of the CSEE, 29, 2009, pp. 113–117, (S). [9] Z.H. Cheng, Y.C. Liu, G. Li, Application of negative pressure type super-strength steel strip slag cooler, Electr. Equip. 6 (2) (2005) 43–47. [10] B. Zeng, X.F. Lu, H.Z. Liu, Industrial application study on new-type mixed-flow fluidized bed bottom ash cooler, Proceedings of the 20th International Conference on Fluidized Bed Combustion, 2009, Xi'an, China. [11] B. Zeng, X.F. Lu, H.Z. Liu, Influence of CFB boiler bottom ash heat recovery mode on thermal economy of units, Energy 35 (9) (2010) 3863–3869. [12] X.F. Lu, B. Zeng, M.L. Shu, H.Z. Liu, Y.L. Chen, W.L. Wang, et al, Compound bottom ash cooler, Chinese Patent ZL200710092413.7, 2010. [13] M. Leva, Elutriation of fines from fluidized system, Chem. Eng. Prog. 47 (1951) 39.
160
B. Zeng et al. / Powder Technology 212 (2011) 151–160
[14] D. Geldart, Elutriation, in: J.F. Davidson, R. Clift, D. Harrison (Eds.), Fluidization, Academic Press Inc., London, 1985, pp. 383–409. [15] C.Y. Wen, L.H. Chen, Fluidized bed freeboard phenomena: entrainment and elutriation, Am. Inst. Chem. Eng. J. 28 (1982) 117–128. [16] S.T. Pemberton, J.F. Davidson, Elutriation from fluidized beds—I. Particle ejection from the dense phase into the freeboard, Chem. Eng. Sci. 41 (2) (1986) 243–251. [17] D. Santana, J.M. Rodríguez, A. Macías-Machín, Modelling fluidized bed elutriation of fine particles, Powder Technol. 106 (1999) 110–118. [18] T. Guo, X.F. Lu, H.Z. Liu, et al., Cold model experiments and numerical simulation on a selective fluidized bed bottom ash cooler in a 410t/h CFB boiler, Proceedings
of 19th International Conference on Fluidized Bed Combustion, 2006, Vienna, Austria. [19] X.F. Lu, Y.L. Li, A cold model experimental study on the flow characteristics of bed material in a fluidized bed bottom ash cooler in a CFB boiler, J. Therm. Sci. 9 (4) (2000) 381–384. [20] M. Zhang, R.S. Bie, Q.G. Xue, Fluidized bed ash cooler used in a circulating fluidized bed boiler: an experimental study and application, Powder Technol. 201 (2010) 114–122. [21] X.M. Ye, C.X. Li, X. Fan, et al., Cold-state slag discharge characteristics of an air– water cooler selective slag cooler under a non-uniform air distribution, J. Eng. Therm. Energy and Power (China) 20 (1) (2005) 73–76.
Contents lists available at ScienceDirect
Powder Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p ow t e c
Development of a novel fluidized bed ash cooler for circulating fluidized bed boilers: Experimental study and application Bing Zeng ⁎, Xiaofeng Lu ⁎⁎, Lu Gan, Maolong Shu Key Laboratory of Low-grade Energy Utilization Technologies and Systems (Chongqing University), Ministry of Education, PR China
a r t i c l e
i n f o
Article history: Received 7 April 2011 Accepted 5 May 2011 Available online 13 May 2011 Keywords: Circulating fluidized bed Bottom ash cooler Gas–solid flow characteristic Particle separation Industry application
a b s t r a c t A novel bottom ash cooler (BAC) called compound fluidized bed ash cooler (CFBAC) was developed in this paper. The CFBAC combined the major technical features of spouted bed and bubbling bed, and could achieve the selective discharge on the bottom ash. Experiments about the gas–solid flow characteristics of the CFBAC were conducted in a visible cold test rig. The experimental results indicated that the separation chamber working in the spouted bed state had a good particle separation effect on the boiler bottom ash. A small quantity of fluidizing air was needed for the cooling chambers to work in the bubbling bed state. The particle separation effect could be controlled by the fluidizing air flow and physical dimensions such as the height of separation partition. The CFBAC had also been industrially applied in a 300 MW circulating fluidized bed unit. The application showed that the CFBAC had a well separation effect, an excellent adaptability on the bottom ash, a good cooling effect and a large ash discharge capacity over 30 t/h. Compared with the water-cooled BAC, the CFBAC had a better energy conservation performance. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Circulating fluidized bed (CFB) boiler bottom ash contains large amounts of physical heat. While the boiler combusts the low-calorie fuel, the ash content is normally more than 40% and the physical heat loss is approximately 3% if the bottom ash is discharged without cooling. .In addition, the red-hot bottom ash is bad for mechanized handling and transportation, as the upper limit temperature of the ash handling machinery is 200 °C. Therefore, a bottom ash cooler (BAC) is often used to treat the high temperature bottom ash to reclaim heat, and to have the ash easily handled and transported [1,2]. As a key auxiliary device of CFB boilers, the BAC has a direct influence on the secure and economic operation of the boiler. There are many kinds of BACs equipped for large-scale CFB boilers with the continuous development and improvement of the CFB boiler, such as watercooled ash cooling screw [2], rolling-cylinder ash cooler (RAC) [2–4], fluidized bed ash cooler (FBAC) [5–8] and high-strength steel belt ash cooler [9]. The RAC and FBAC have a large capacity, and have been commonly and reasonably applied in China. As the coal resources are in short supply, the China government has strongly supported using CFB boilers to combust the low-calorie fuel such as coal gangue, stone coal and petroliferous shale, whose ash content are even greater than 70%. The as fired coal of CFB boilers in China is poor and changeable, and the actual operations of the boilers ⁎ Corresponding author. Tel./fax: + 86 23 65102475. ⁎⁎ Corresponding author. E-mail addresses: [email protected] (B. Zeng), xfl[email protected] (X. Lu). 0032-5910/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2011.05.005
deviate greatly from the original design state. If the CFB boiler burns a large quantity of the low-calorie fuel, the combustion condition of the furnace would be changed greatly and the discharge capacity of the bottom ash would be substantially increased with the coarsening particle size. More seriously, sometimes all of the condensation water may be not sufficient for cooling the bottom ash while the watercooled BAC such as RAC is used, which brings about a great impact on the turbine regenerative system. Because of the insufficient output capacity of existing BACs, the slag discharge temperature would be increased accordingly and then, the conveying equipment would be broke down frequently. According to statistics, the forced shutdown rate caused by the active defect of BACs exceeded 60%. With the development of CFB boilers towards large-scale and highparameter, the single unit capacity is enlarging. In order to meet the needs of the furnace load and the particle concentration in the splash zone, it requires more fine ash in bed material to maintain a higher circulating ratio. Consequently, it is particularly important that BACs have the function of discharging coarse slag and returning fine ash to the furnace. As we all know, the FBAC has many advantages, such as a higher heat-transfer coefficient, a greater cooling capacity, no mechanical equipments, a low maintenance cost and a better thermal economy on reclaiming heat of the bottom ash. Compared with the RAC, the FBAC can also be used to cool the circulating ash discharged from the loop seal or the external heat exchanger. The FBAC used for the future supercritical CFB boiler could even be considered adopting both feed-water and condensate water as cooling medium [5,10]. The most important advantage of the FBAC is that partial fine particles of the bottom ash could be returned to the furnace to improve the bed material size
152
B. Zeng et al. / Powder Technology 212 (2011) 151–160
distribution. Moreover, the previous research revealed that using FBACs to reclaim heat of the bottom ash obtained higher plant thermal efficiency, lower plant heat rate and less standard coal consumption than using the RAC to do [11]. But the further development of existing FBACs is restricted by some disadvantages such as poor flow ability of the bottom ash, bad particle size control performance, large air-cooled proportion and frequent slag bridging and building [5]. Therefore, it is necessary to develop a novel FBAC which can meet the needs of largescale CFB boilers, can adapt to the situation of the fuel as fired and have a wide adaptability to the bottom ash granularity. The objective of this paper was to introduce a compound fluidized bed ash cooler (CFBAC). The CFBAC has obtained an invention patent in China [12]. Experiments about the gas–solid flow characteristics of the CFBAC were investigated in a visible cold test rig. Furthermore, the experimental results were used in the CFBAC of a 300 MWe CFB boiler. The industrial application was studied and the operation aspect was found to be satisfactory.
2. Compound fluidized bed ash cooler Fig. 1 illustrates the basic structure of the CFBAC, which combines the advantages of various FBAC technologies and is based on the actual coal quality and the discharging ash particle size of CFB boilers in China. The operational principle and working process of the CFBAC is as follows: Firstly, the bottom ash discharged from the CFB boiler passes from the ash feed chute to the separation chamber of the CFBAC. The separation chamber was designed as a rectangular spouted bed. The high-temperature bottom ash is separated by the spouting air in the separation chamber. On the one hand, the high-temperature bottom ash exchanges heat with the cold air. On the other hand, the fine ash (e.g. particle size smaller than 4 mm) having small mass and good fluidity fluidizes into the cooling chamber I over the separation partition under the effects of overflowing and spouting. The coarse slag is discharged by a screw-type slag extractor from the separation chamber. Secondly, after cooling by the fluidizing air and the water-cooled tube in the cooling chamber I, the fine ash fluidizes and flows through the bottom hole of the intermediate partition into the cooling
Separation chamber
Air vent
chamber II for further cooling. After cooled to 100 ~ 150 °C, the fine ash is overflowed into the fine ash discharge chute and is discharged by a rotary discharge valve. Besides, the flue gas containing the extremely fine ash and the heat-absorbing fluidizing air returns back to the furnace through the air vent. The cooling chamber I and II need a small amount of fluidizing air to work in a way of bubbling fluidized bed. The fluidizing air could be the cold primary air. The cooling water could be the condensate water or feedwater. Compared with existing FBACs, the CFBAC has the following technical characteristics: • The spouting separation technology was adopted to carry out the particle separation of coarse slag and fine ash in the separation chamber. After cooling to 100 ~ 150 °C in the separation chamber, most of the coarse slag was discharged through the slag discharge chute and the screw-type slag extractor. Only a very small part of the coarse slag was overflowed into the cooling chamber I together with the fine ash. Moreover, the high speed air jet of the separation chamber enhanced the flow ability and adaptability of the bottom ash, and effectively avoided such phenomenon as bad fluidization, slag bridging and building caused by the deposition of coarse slag. • The initial bubbling fluidization technology was deployed in the cooling chamber to reduce the fluidizing airflow and the heating surface wear, and to improve heat transfer efficiency. The particles consisting of fine ash and a little coarse slag ensured that the cooling chamber was able to work in the bubbling bed state. • The bottom ash flowed in a mode of “overflow–underflow–overflow” in the CFBAC, due to the bottom hole and partitions in different heights. The especial flow mode efficiently controlled the directional flow of particles and lengthened the retention time of the bottom ash. • The selective ash discharge could be achieved in the CFBAC. It meant discharging coarse slag from the furnace and returning partial fine particles of the bottom ash to the furnace. The selective ash discharge could be of benefit to adjust the bed material size distribution and to improve the fluidization quality of the furnace. • It could accommodate to the development of large-scale CFB boilers. The membrane wall and the feedwater could be used as parts of the heat transfer surface and cooling medium of the CFBAC, respectively, to enhance the unit economy.
Separation partition
Intermediate partition
Cooling chamber I
Bottom ash cooler
Cooling chamber II Cooling tube
Ash inlet duct
Small ash discharge chute
Rotary ash discharge valve
Wind box
Screw-type slag extractor
Bottom hole
Fig. 1. Structural diagram of the CFBAC.
Emergency discharge chute
B. Zeng et al. / Powder Technology 212 (2011) 151–160
3. Experiments
153
Table 1 Physical properties of bed material.
3.1. Experimental apparatus The experimental apparatus of the CFBAC is illustrated in Fig. 2. It consists of hopper, test rig body, wind boxes, etc. The cold test rig was manufactured by plexiglass, angel steel and steel plate, convenient for observing the gas–solid flow characteristic. The air distribution plate was made by steel plate for strengthening the pressure resistance. A layer of canvas was set on the distribution plate to prevent the bed material leaking into wind boxes. The air from an air compressor was blown into each chamber through respective wind boxes and measured by the glass rotameter. The pressures were measured by differential pressure transmitter to accomplish the real time data acquisition. The internal dimensions of the cold test rig were 900 mm×200 mm× 600 mm. The cold test rig was divided equally to four chambers by three partitions: separation chamber, cooling chamber I, cooling chamber II and discharge chamber from feed side to discharge side. The separation chamber was designed as a rectangle spouted bed, and its bottom vertical section liked a “V” in shape. The other three chambers were designed as rectangle bubbling beds. The partitions were set as modular structure in order to change the partition heights. Except the separation partition, the other two partitions had a bottom hole (60 mm×60 mm). The air vent was located on the top of the cooling chamber II.
Properties
Values
Real density, kg/m3 Bulk density, kg/m3 Mean particle size, μm Minimum spouted velocity, m/s Minimum fluidizing velocity, m/s
2421 1530 564 0.4 0.28
The minimum spouted velocity and minimum fluidized velocity were gotten by Figs. 4 and 5, respectively. To investigate the solid flow characteristics, separation characteristics of the separation chamber, elutriation and ash discharge characteristics in the cold test rig, the solid samples from each chamber, discharge chute and air vent were collected during each experiment. The size analysis by sieving of the samples was done to determine the mean particle size and PSD. The separation mass flow rate (Gs) and the separation particle size (ds) were used to represent the separation characteristics of the separation chamber. Gs was defined as the whole particle weights of the solids separated from the separation chamber into the cooling chamber per unit time. ds was defined as the mean particle size of the solids separated into the cooling chamber. Pressure drop was considered to be a driving force of the solid flow. The height of partitions was a key factor to determine the solid flow, the bed pressure and the disposition of the heating surface. Especially, the separation partition between the separation chamber and the cooling chamber I played a very important role in the separation process of the bottom ash. Several experiments with
3.2. Experiments The bottom ash from a CFB boiler was used as the bed material after screening in all tests. Its properties were listed in Table 1, and the accumulative particle size distribution (PSD) was showed in Fig. 3.
A
A Air vent
Hopper
Cold Test Bed
Separation chamber
Cooling chamber II
Cooling chamber I
600
Discharge chute
Feed chute Discharge chamber
B
B
Wind box
B Bottom hole
Partition
A
A
900 Fig. 2. The visible cold test rig of the CFBAC.
200
B Separation partition
154
B. Zeng et al. / Powder Technology 212 (2011) 151–160
Fig. 6. Screening granularity of samples. Fig. 3. Accumulative particle size distribution of bed material.
Fig. 4. Bed pressure difference of separation chamber with superficial velocity.
various partition heights had been completed to study the characteristics of controlling the particle separation and the ash discharge. 4. Experimental results and discussion 4.1. Flow characteristics of bed material v, u1, u2, and u3 expressed the fluidizing velocities of separation chamber, cooling chamber I, cooling chamber II and discharge chamber, respectively.
Fig. 5. Bed pressure difference of cooling chamber with superficial velocity (descent method).
The mean particle size of each sample sampled from every chamber as well as discharge ash was showed in Fig. 6 after 2 h continuous run of the cold test rig, when ν, u1, u2, and u3 were 1.26 m/s, 0.52 m/s, 0.40 m/s and 0.28 m/s, respectively, and the height of separation partition (H) was 350 mm. The bottom pressures of each chamber were presented in Fig. 7. As shown in Fig. 6, the mean particle size decreased from feed side to discharge side (i.e. along the flow direction of particles). It revealed that the proportions of small particles in each chamber increased along the flow direction of particles. The reason was that the small particles had a better flowability than the coarse particles. The small particles were able to flow quickly from a chamber into the next chamber, whereas the coarse particles lagged far behind. As can be seen in Fig. 7, the bottom pressure of each chamber also decreased along the particle flow direction. The small pressure differences between the last three chambers provided a driving force for the particle flow. The flow behavior of bed material particles can be directly observed through the plexiglass of the test rig. Most of the coarse particles bobbed up and down in the bottom area of the separation chamber; only a very few coarse particles could be entrained by the small particles into the upper space or fluidized over the separation partition into the cooling chamber I. Meanwhile, the bubbling phenomenon in dense bed and the particles throwing in bed surface were clearly seen in the last three chambers (i.e. cooling chamber I, cooling chamber II and discharge chamber). It could also be found that the particles were fluidized and mainly passed through the bottom holes of partitions into the next chamber in the last three chambers. Then the particles were overflowed into the discharge chute in the discharge chamber.
Fig. 7. The bottom pressure of each chamber.
B. Zeng et al. / Powder Technology 212 (2011) 151–160
Fig. 8. Separation particle size with static bed material heights in different fluidizing velocities.
4.2. Separation characteristics in separation chamber The separating effect in the separation chamber had a direct impact on the fluidization quality of the next three chambers, and was a determinant of the operation quality of the CFBAC. The properties of particles, fluidizing velocity (v), static bed height (h) and separation partition height (H) were considered as the influential factors. The properties of particles could be considered as remaining practically unchanged in each experiment. The separation characteristics were obtained in the case of a single run of the separation chamber. The variations of the separation particle size (ds) and the separation mass flow rate (Gs) with static bed height (h) in different fluidizing velocities were showed in Figs. 8 and 9, respectively, when the separation partition height (H) was 350 mm. As shown in Fig. 8, ds increased with the increase of v. It was because the larger the v, the more the coarse particles were separated into the cooling chamber. Meanwhile, the ds roughly increased with an increase in h. The reason was that the lower the h, the greater the height difference between H and h and the higher the difficulty of particles overflowing the separation partition. The entrainment height of coarse particles was lower than that of small particles in a same v. It could be known from Fig. 9 that Gs increased with the increase of h in any v, and increased with increasing v in any h. When h was 150 mm, the Gs was always very small no matter how the change in v. It was because the fluidizing velocities could not provide the sufficient drag force for particles to overcome the height difference. When the v
Fig. 9. Separation mass flow rate with static bed material heights in different fluidizing velocities.
155
raised, the amount of particles whose terminal velocity were lower than the v increased, the interactions between particles and fluidizing air or particles and particles became increasingly intense, thus the entrainment effect strengthened. To sum up, it could be concluded that the higher the v and h, the greater the Gs and ds from the experimental results of the separation chamber. However, it can be known from the flow characteristics of spouted bed that h cannot be increased unlimited. If the h was higher than the maximum spouting height, the spouted bed state cannot be formed. The v should also be determined by combining the limitation of fluidizing airflow in actual operation. Furthermore, Gs and ds were a pair of mutual restraint factors. A larger ds means a bigger particle size in cooling chambers, therefore more fluidizing airflow is needed to maintain a good fluidized state. Fig. 10 illustrated the effects of H on Gs and ds when v was 1.58 m/s, as the height difference between H and h remaining constant. As shown in Fig. 10, both Gs and ds first increased and then decreased with the increase of H, and simultaneously reached the maximum point as H reaching 250 mm. The resistance for lifting particles from the bed surface to the top of the separation partition remained unchanged when the height difference maintained as constant. When H and h were low, the separation chamber operated in the shallow bed state and had an obvious particles delamination; hence the effect of the pneumatic separating on small particles became stronger. However, there was only a little of bed material in total, the amount of particles that could be thrown over the separation partition per unit time was small (i.e. Gs was small). With gradually increasing of H and h, the spouting phenomenon became more severe as well as a stronger entrainment, which resulted in the increments of Gs and ds. Then, when H and h further increased to certain heights, the bed resistance raised more, and the original v was insufficient to maintain a good spouted bed state in separation chamber. As a result, Gs and ds decreased by the weaken separation effect. At this moment, if v was increased, Gs and ds would increase accordingly. 4.3. Elutriation The terms elutriation, entrainment and carryover are used to describe a variety of phenomena ranging from the selective removal of fine particles from the surface of large granules in a fluidized conveyer or drier. As bubbles break at the surface of the bed, particles in the leading bulge of the bubble and the bubble wake are thrown up above the bed surface and are entrained by the upward flowing gas stream. Several researchers have made important contributions to understand the mechanism of elutriation [13–17]. The recapitulative conclusions were that particles elutriated from the surface of a fluidized bed into the freeboard depended on the mechanism of
Fig. 10. Separation mass flow rate and separation particle size with partition heights.
156
B. Zeng et al. / Powder Technology 212 (2011) 151–160
bubble eruption. Particles might be ejected into the freeboard from the roofs or wakes of bursting bubbles, depending on bed geometry, fluidizing velocity and particle type. The relational expression of the elutriation rate constant (k) could be written as:k ∝ u 1 ~ 7dp− 1 ~ − 2, where u was the fluidizing velocity and dp was the particles size. It indicated that the higher the fluidizing velocity (u) and the finer the particles size (dp), the greater the elutriation rate constant (k) would be. For the CFBAC, the elutriation directly determined the particle residence time and the amount of particles returning to the furnace, and then influenced the PSD of the furnace. It was difficult to measure the amount of particles from the CFBAC to the furnace during hot operation and collect the elutriation rate (Er) during experiments. Therefore, the suspension space pressure (ps) was used to qualitatively describe Er. The relationships of the suspension space pressure and the elutriation rate with the mean fluidized velocity (um) of the last three chambers of the test rig were plotted in Fig. 11. As shown in Fig. 11, both ps and Er increased linearly with increasing the um of the last three chambers of the test rig. It was because there were more particles could be ejected into the suspension space with the increase of the um, and the particle concentration of the suspension space increased, hence the ps accordingly increased.
4.4. Ash discharge characteristics Researches on the bed material flow and ash discharge characteristics of other types FBAC were mainly focused on such influencing factors as the pressure distribution [18], the pressure difference between FBAC and the furnace [19], the structure of fluidizing air distributor [20] and the equal or unequal air distribution [21]. The air distribution modes including equal distribution and unequal distribution of the last three chambers were investigated in this study. The equal distribution refers that the cooling chamber I, cooling chamber II and discharge chamber have a same fluidizing air flow. The unequal distribution means that the fluidizing air flow of each chamber is different, but the total fluidizing air flow remains constant. The effects of fluidizing air flow on the ash discharge rate (Ds) under equal distribution and unequal distribution were presented in Figs. 12 and 13, respectively. It could be known from Fig. 12 that, Ds increased first linearly and then slightly with the increase of the fluidizing air flow in each chamber. The reason was that there were more fine particles entrained or elutriated out of the test rig through the air vent, when the fluidizing air flow was larger than 32 N m 3/h. It counteracted a part of the increment of Ds caused by increasing the
Fig. 12. Ash discharge rate with fluidizing air flow of the last three chambers in equal distribution.
fluidizing air flow. Meanwhile, the equal distribution mode made the three chambers had the very similar particles flow state and bubble behavior, as well as the same bottom pressure, which weakened the lateral moving ability of particles. That's the reason why the Ds increased slightly with the increase of the fluidizing air flow after 32 Nm 3/h. As shown in Fig. 13, Ds in the fluidizing air flow ratio of 60:48:36 was greater than that in other ratios. Compared with the equal distribution mode, the unequal distribution mode had a higher Ds. The reason was that the bubble behaviors were different in the last three chambers under the unequal distribution. The greater the difference in fluidizing air flow in each chamber, the greater the difference in moving intensity of particles and the greater the difference in the bottom pressure would be. In the chamber which had the larger fluidizing air flow, on one hand, the solids and bubbles movement were more intense and the bubbles entrainment was stronger; on the other hand, the bed voidage was larger and the motion resistance was smaller. These resulted in the stronger lateral moving ability of particles and the larger Ds. However, when the air distribution was 72:48:24, the Ds decreased a lot and was smaller than that in the condition of 48:48:48. It was because the small fluidizing air flow leaded to a low quality of fluidization in the discharge chamber, although the pressure difference became larger. 5. Industrial application 5.1. Application introduction On the basis of the experimental results, the earliest CFBAC was designed and used in a 150 MWe CFB unit for trial operation in
Fig. 11. Suspension space pressure and elutriation rate with mean fluidizing velocity.
Fig. 13. Ash discharge rate with fluidizing air flow ratio of the last three chambers in unequal distribution.
B. Zeng et al. / Powder Technology 212 (2011) 151–160
Access door
Ash inlet duct
Separation chamber
Cooling chamber
157
Slag discharge vent
Separation chamber
Separation chamber
Coolong chamber
Cooling chamber
Access door Ash discharge vent Fluidizing nozzle
Cooling tube Collecting box Fig. 14. Internal structure diagram of the bottom ash cooler after rebuilt to a CFBAC.
Panzhihua, Sichuan province, China in 2009[10???]. The trial operation results indicated that the CFBAC had met the design output requirement (15 t/h). A detailed description of the cold test and hot test could be found in [10]. According to the early application and the experimental results, the optimized CFBAC had been industrially applied to a BAC reconstruction project of a 300 MWe CFB unit in Huaibei, Anhui province, China in 2010. The design coal of the 300 MWe CFB boiler was blended coals, including gangue, schlamm and middlings of washing coal. The boiler bottom ash had a coarse particle size, and the boiler was often required to discharge circulating ash. The internal structure, water-cooled tubes and air distributors of the original FBAC were reformed to a CFBAC in the case of remaining the main framework unchanged, as shown in Fig. 14. Its internal length, width and height were 7450 mm, 2100 mm and 2900 mm, respectively. The separation chamber was divided into three chambers by two partitions, and three separation chambers communicated with each other in the bottom. A long partition with a certain height was set up between separation chambers and cooling chambers. The cooling chamber was also divided into three chambers by two partitions. The heights of the two partitions were lower than that of the long partition. The partition between the cooling chamber I and the cooling chamber II had a through hole in the bottom, while the partition between the cooling chamber II and the cooling chamber III did not have. The separation chamber used a grid plate as air distributor while the cooling chamber used capped nozzles. The main design parameters of the CFBAC were shown in Table 2.
III were about 100 °C, which indicated that the discharge temperatures of the coarse slag and the fine ash were about 100 °C. Consequently, the CFBAC had a good cooling effect on the bottom ash. In order to investigate the output capacity of the CFBAC, the heat balance was calculated, as listed in Table 3. The heat balance calculation of the cooling water side and fluidizing air side showed that the CFBAC had a 35.55 t/h of ash cooling capacity while the discharge temperature of the coarse slag and the fine ash were 138.6 °C and 128.4 °C, respectively.
5.2.2. Particles separation effect The screening results of samples were shown in Fig. 16. The samples were sampled from the bottom ash inlet duct, cooling chamber I, slag discharge chute and ash discharge chute during hot operation. The proportion of particles larger than 4 mm in the coarse slag discharged from the separation chamber was over 70%, while the percentage of particles larger than 4 mm in the cooling chamber was about 5%. And the ratio of particles larger than 1 mm in the fine ash discharged from the cooling chamber was just under 3%. Moreover, the calculated mean particle size of the bottom ash, the coarse slag and the fine ash according to the screening results were 1.37 mm, 4.01 mm and 0.35 mm, respectively. It indicated that the separation chamber of the CFBAC had strong granularity adaptability and good separation effect on the bottom ash.
5.2. Application results 5.2.1. Temperature distribution and heat balance calculation Fig. 15 represented the bed temperature distribution of the CFBAC during a continuous operation. It revealed that the CFBAC operated stably and had a good and reasonable temperature gradient. The bed temperatures of the separation chamber III and the cooling chamber Table 2 Main design parameters of CFBAC. Properties
Values
Ash cooling capacity, t/h Ash inlet temperature, °C Ash outlet temperature, °C Fluidizing air volume, N m3/h Temperature of return flue gas, °C Cooling water requirement, t/h Water inlet temperature, °C Water outlet temperature, °C
30 850–920 ~ 150 22,000 300–450 100 ~ 35 ~ 85
Fig. 15. Operation data of the CFBAC (bed temperature distribution).
158
B. Zeng et al. / Powder Technology 212 (2011) 151–160
Table 3 Heat balance calculation of CFBAC. Properties
Values
Bottom ash inlet temperature, °C Coarse slag outlet temperature, °C Fine ash outlet temperature, °C Cooling water inlet temperature, °C Cooling water inlet pressure, MPa Cooling water outlet pressure, MPa Cooling water outlet temperature of tube bank 1, °C Cooling water requirement of tube bank 1, t/h Cooling water outlet temperature of tube bank 2, °C Cooling water requirement of tube bank 2, t/h Fluidizing air volume, N m3/h Fluidizing air inlet temperature, °C Return flue gas temperature, °C Calculated ash cooling capacity, t/h
885 138.6 128.4 25.6 0.66 0.29 70.5 74.63 38.1 69.69 22,445 35 333.1 35.55
5.2.3. Average heat transfer coefficient of tubes For the design and layout of the tube in the cooling chamber of the CFBAC, it is important to investigate the average heat transfer coefficient between water-cooled tubes and the mixture of the fluidizing air and particles. The relationship between the average heat transfer coefficient of tubes and the bed height could be calculated and analyzed based on the operation data and the assumption that the fluidizing velocity, the structure of water-cooled tubes and the PSD in the cooling chamber remained unchanged. The calculated results of many operating points were plotted in Fig. 17, and a curve was fitted according to the results. As shown in Fig. 17, the average heat transfer coefficient increased approximately linearly with the increase of the bed height. It was because that the immersed area of tubes and the particle concentration of the suspension space increased with the increasing of the bed height. The heat transfer coefficient of the tubes submerged in the bed material was far greater than that of the tubes in the suspension space. 5.2.4. Technical–economic comparison It was concluded by the previous study [11] that, the standard coal consumption rate of the FBAC mode was less nearly 2 g/(kW h) than the RAC mode in three CFB power plants (150 MWe, 300 MWe and 600 MWe, respectively), when the net calorific power of the standard coal was 29.27 MJ/kg. The FBAC and RAC were two different bottom ash heat recovery modes. The FBAC combined cold air and condensation water as the cooling medium, while the RAC only used the condensation water. However, the own energy consumption of the FBAC and RAC was not included in the previous study, such as
Fig. 16. Accumulative particle size distribution of samples during hot operation of the CFBAC.
the driving motor power consumption of the RAC and the increased fan power for supplying the fluidizing air of the FBAC. In general, the fluidizing air of the FBAC was taken from the cold primary air. The return air of the FBAC was returned back to the furnace as the secondary air, which resulted in a corresponding reduction in the proportion of the secondary air. Therefore, the ratio between the primary air and the secondary air of a CFB boiler with the FBAC or the RAC were different. The saving of electric power (ΔP0) in the turbine heat acceptance (THA) condition may be calculated as:
ΔP0 =
Δbs P bs 0
ð1Þ
where, P0 is the rated power, b s is the standard coal consumption in the THA condition and Δb s is the standard coal consumption difference between the FBAC mode and the RAC mode. The output power of the driving motor (Pg) of a fan can be expressed as follows:
Pg =
qv p ηηtm
ð2Þ
where, qv is the fan outlet flow rate, p is the fan outlet total pressure, η is the fan total pressure efficiency and ηtm is the mechanical transmission efficiency (taken to be 1). The fan outlet total pressure was assumed to remain constant, as the fan outlet flow rate changed relatively little. Hence the fan power consumption difference of the primary fan (ΔPg1) and the secondary fan (ΔPg2) caused by the change of the outlet flow rate can be written as follows:
ΔPg1 =
Δqv1 ·p1 η1 ηtm
ð3Þ
ΔPg2 =
Δqv2 ·p2 η2 ηtm
ð4Þ
where Δqv represents the fan outlet flow rate difference (namely the total fluidizing air flow of FBACs), p1 and p2 represent the fan outlet total pressure of the primary fan and secondary fan, respectively, and η1 and η2 represent the fan total pressure efficiency of the primary fan and secondary fan, respectively.
Fig. 17. Average heat transfer coefficient of tubes with bed height in cooling chamber.
B. Zeng et al. / Powder Technology 212 (2011) 151–160 Table 4 Technical–economic comparison calculation. Properties
Values
Rated power of the unit, P0, MWe Standard coal consumption, bs, g/(kW·h) Standard coal consumption difference, Δbs, g/(kW·h) Saving of electric power, ΔP0, kW Total fluidizing air flow of 4 CFBACs, Δqv, Nm3/h Total pressure of primary air, p1, kPa Total pressure of secondary air, p2, kPa Primary fan total pressure efficiency, η1 Secondary fan total pressure efficiency, η2 Mechanical transmission efficiency, ηtm Fan power consumption difference, ΔPg, kW Driver motor power consumption of a RAC, PRAC, kW Total Net power saving, ΔP, kW Net power saving of a CFBAC, ΔPCFBAC, kW
300 305 2.11 2075 88,000 30.9 15.4 0.9 0.9 1 421 20 1574 393.5
Then the power consumption difference between the primary fan and the secondary fan (ΔPg) is given by, ΔPg = ΔPg1 −ΔPg2 =
Δqv ·p1 Δqv ·p2 − η1 ηtm η2 ηtm
ð5Þ
Finally, compared with the RAC mode, the total net power saving (ΔP) in the FBAC mode can be calculated by the equation given as follows: ΔP = ΔP0 −ΔPg −nPRAC
ð6Þ
where, PRAC is the driving motor power consumption of a RAC and n is the number of RACs. The 300 MWe CFB unit in Huaibei, Anhui province, China, was taken as an example to compare the technical economy, when four BACs (CFBACs or RACs) were used to cool the bottom ash and to reclaim heat. The primary data and calculated results were shown in Table 4. It was found that the CFBAC had a better technical economy, and the net power saving was 1574 kW/h. The CFBAC could save the cost of about 2.8 million yuan RMB per year than using the RAC, on the assumption that the plant operated 200 days annually at rated load and the pool purchase price was 0.37 yuan/(kW h).
159
Nomenclature BAC bottom ash cooler CFB circulating fluidized bed CFBAC compound fluidized bed ash cooler FBAC fluidized bed ash cooler PSD particle size distribution RAC rolling-cylinder ash cooler THA turbine heat acceptance Ds ash discharge rate, g/s Er elutriation rate, g/s Gs separation mass flow rate, g/s H height of separation partition, mm PRAC power consumption of the driving motor of a RAC, kW P0 rated power, kW Pg output power of driving motor, kW bs standard coal consumption, g/(kW·h) mean particle size, mm dp ds separation particle size, mm h static bed height, mm k elutriation rate constant n numbers p outlet total pressure of the fan, Pa ps suspension space pressure, Pa qv outlet flow rate of the fan, Nm 3/h um mean fluidizing velocity, m/s v, u1, u2, u3 fluidizing velocities of the separation chamber, cooling chamber I, cooling chamber II and discharge chamber, respectively, m/s Greek letters ΔP total net power saving, kW ΔPCFBAC net power saving of a CFBAC, kW ΔP0 saving of electric power, kW ΔPg power consumption difference, kW Δb s standard coal consumption difference, g/(kW·h) Δqv outlet flow rate difference of the fan, total fluidizing air flow of FBACs, Nm 3/h η total pressure efficiency of the fan ηtm mechanical transmission efficiency
6. Conclusions A new type of FBAC called CFBAC was developed in this paper. Experiments had been conducted in a visible cold test rig, and the 30 t/h CFBAC had been successfully applied in a 300 MWe CFB boiler. Some primary conclusions were summarized as follows: (1) The pneumatic separation technology of the spouted bed was used to achieve the separation of the coarse slag and fine ash in the CFBAC. It improved the flowability and size adaptability of the bottom ash, and ensured that the cooling chamber could work in the bubbling bed state to reduce the fluidizing air flow. (2) The experimental results indicated that the CFBAC had a good particle flow characteristic. Fluidizing velocity and the height of the separation partition were two key parameters, which influenced the separation effect of the separation chamber. There was an optimum combination of the fluidizing velocity and the height of the separation partition. The elutriation rate increased linearly with increasing the mean fluidizing velocity of the last three chambers of the test rig. And the ash discharge rate in a certain unequal fluidizing air distribution (60:48:36) was greater than that in the equal distribution. (3) The industrial application revealed that the operation of the CFBAC was found to be satisfactory. The CFBAC had a well separation effect and a good cooling effect. And the actual ash cooling capacity exceeded the design capacity of 30 t/h. The CFBAC had a better energy conservation than the water-cooled BAC (e.g. RAC).
References [1] K.F. Cen, M.J. Ni, Z.Y. Luo, J.H. Yan, Y. Chi, M.X. Fang, et al., Theoretical Design and Operation of Circulating Fluidized Bed Boiler, China Electric Power Press, Beijing, 1998, pp. 647–663. [2] X.F. Lu, Equipments and Operation of Large-scale Circulating Fluidized Bed Boiler, China Electric Power Press, Beijing, 2006, pp. 125–133. [3] Y.C. Liu, H.C. Yin, J.P. Liu, Cold and hot test study on the rolling-cylinder type slagcooler of CFB boiler, Power Syst. Eng. 22 (5) (2006) 35–38. [4] W. Wang, X.D. Si, H.R. Yang, H. Zhang, J.F. Lu, Heat-transfer model of the rotary ash cooler used in circulating fluidized-bed boilers, Energy Fuels 24 (2010) 2570–2575. [5] M.L. Shu, J.H. Chen, X.F. Lu, H.Z. Liu, Investigation on fluidized bed bottom ash coolers in large-scale CFB boilers, Power Syst. Eng. 23 (2) (2007) 29–31. [6] J.J. Butler, N.C. Mohn, J.C. Semedard, CFB Technology: Can the Original Clean Coal Technology Continue to compete, Power Gen International Conference, Nevada, USA, 2005. [7] W. Nowak, Design and operation experience of 230MWe CFB boilers at Turow Power Plant in Poland, Proceedings of the 15th International Conference on Fluidized Bed Combustion, ASME, New York, 1999. [8] P. Xiao, T. Guo, Z.Q. Xu, J.Z. Jiang, H.S. Lu, Q. Li, et al., Large capacity fluidized bottom ash cooler and its operation performance, Proceedings of the CSEE, 29, 2009, pp. 113–117, (S). [9] Z.H. Cheng, Y.C. Liu, G. Li, Application of negative pressure type super-strength steel strip slag cooler, Electr. Equip. 6 (2) (2005) 43–47. [10] B. Zeng, X.F. Lu, H.Z. Liu, Industrial application study on new-type mixed-flow fluidized bed bottom ash cooler, Proceedings of the 20th International Conference on Fluidized Bed Combustion, 2009, Xi'an, China. [11] B. Zeng, X.F. Lu, H.Z. Liu, Influence of CFB boiler bottom ash heat recovery mode on thermal economy of units, Energy 35 (9) (2010) 3863–3869. [12] X.F. Lu, B. Zeng, M.L. Shu, H.Z. Liu, Y.L. Chen, W.L. Wang, et al, Compound bottom ash cooler, Chinese Patent ZL200710092413.7, 2010. [13] M. Leva, Elutriation of fines from fluidized system, Chem. Eng. Prog. 47 (1951) 39.
160
B. Zeng et al. / Powder Technology 212 (2011) 151–160
[14] D. Geldart, Elutriation, in: J.F. Davidson, R. Clift, D. Harrison (Eds.), Fluidization, Academic Press Inc., London, 1985, pp. 383–409. [15] C.Y. Wen, L.H. Chen, Fluidized bed freeboard phenomena: entrainment and elutriation, Am. Inst. Chem. Eng. J. 28 (1982) 117–128. [16] S.T. Pemberton, J.F. Davidson, Elutriation from fluidized beds—I. Particle ejection from the dense phase into the freeboard, Chem. Eng. Sci. 41 (2) (1986) 243–251. [17] D. Santana, J.M. Rodríguez, A. Macías-Machín, Modelling fluidized bed elutriation of fine particles, Powder Technol. 106 (1999) 110–118. [18] T. Guo, X.F. Lu, H.Z. Liu, et al., Cold model experiments and numerical simulation on a selective fluidized bed bottom ash cooler in a 410t/h CFB boiler, Proceedings
of 19th International Conference on Fluidized Bed Combustion, 2006, Vienna, Austria. [19] X.F. Lu, Y.L. Li, A cold model experimental study on the flow characteristics of bed material in a fluidized bed bottom ash cooler in a CFB boiler, J. Therm. Sci. 9 (4) (2000) 381–384. [20] M. Zhang, R.S. Bie, Q.G. Xue, Fluidized bed ash cooler used in a circulating fluidized bed boiler: an experimental study and application, Powder Technol. 201 (2010) 114–122. [21] X.M. Ye, C.X. Li, X. Fan, et al., Cold-state slag discharge characteristics of an air– water cooler selective slag cooler under a non-uniform air distribution, J. Eng. Therm. Energy and Power (China) 20 (1) (2005) 73–76.