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Experimental research of heat transfer uniformity for fluidized bed heat exchangers in a 300 MW CFB boiler
Applied Thermal Engineering 130 (2018) 938–950
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Research Paper
Experimental research of heat transfer uniformity for fluidized bed heat exchangers in a 300 MW CFB boiler Guoliang Song a,b,⇑, Qinggang Lyu a,b, Feng Xiao c, Yunguan Sun d a
Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China University of Chinese Academy of Sciences, Beijing 100049, China c Shanghai Boiler Works, Ltd., Shanghai 200245, China d China Guodian Corporation Xiaolongtan Power Plant, Kaiyuan 661601, China b
h i g h l i g h t s The average heat transfer coefficient of different heat exchanger chambers is linear with the boiler. The wall temperature distribution at the outlet of the different heat exchange chambers is different. There is a non-uniform heat transfer phenomena in the symmetrical arrangement of four FBHEs. The optimized layout of the heating surfaces in the FBHEs was proposed.
a r t i c l e
i n f o
Article history: Received 2 September 2017 Revised 23 October 2017 Accepted 16 November 2017 Available online 20 November 2017 Keywords: Circulating fluidized bed (CFB) Fluidized bed heat exchanger (FBHE) Solids flow Wall temperature Heat transfer uniformity
a b s t r a c t In order to investigate the heat transfer uniformity of fluidized bed heat exchangers (FBHEs), a series of experimental tests were carried out in a commercial 300 MW CFB boiler. The test results indicate that there is a good linear correspondence between the FBHEs conical valve openings and circulating ash flow rate, the average heat transfer coefficient of different heat exchangers takes on a monotonically increasing trend with the increase of the boiler loads. Moreover, there is a non-uniform heat transfer process between the different measuring points under the same load as well as the different loads for the same measuring points, the heat absorption proportion of two FBHEs (71# and 72#) on the left side is larger than that of two FBHEs (73# and 74#) on the right side of the furnace, there is a non-uniform heat transfer phenomena in the symmetrical arrangement of four FBHEs, the heating surface arrangement in the FBHEs must be adjusted from snake tube perpendicular to the ash flow direction to parallel to ash flow direction, the test results can provide a good reference for the optimized design and operation of 600 MW supercritical CFB boiler with the FBHEs in the future. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Circulating fluidized bed (CFB) boiler technology with large capacity and high steam parameters is the main development direction due to its energy saving and environment protection [1]. With the development of large-scale 300 MW CFB boilers, specially for the development of CFB boilers up to 600–1000 MWe capacities with supercritical or ultra- supercritical steam parameters [2,3], the imbalance increase between heat release and heat absorption will result in the problem of heating surface arrangement, the fluid bed heat exchangers (FBHEs) are placed in the ⇑ Corresponding author at: Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China. E-mail address: [email protected] (G. Song). https://doi.org/10.1016/j.applthermaleng.2017.11.081 1359-4311/Ó 2017 Elsevier Ltd. All rights reserved.
hot materiel circulating loop, which is a very effective approach to resolve the heating surface arrangement problem brought by the scale-up of CFB boilers [4–7], some heating surfaces are transferred from the furnace to the FBHEs, on the one hand, the combustion process and heat transfer process is partitioned by the FBHEs, which will increase the operation and control methods of furnace, bed temperature and reheat steam temperature can be adjusted flexibly by the solids control valves during the boiler operation. On the other hand, the heating surfaces in the furnace decrease, the heat expansion and seal problems of furnace will be resolved effectively by the FBHEs, some heating surfaces are placed in the FBHEs under the very low fluidization velocity within the range of 0.3 m/s to 0.5 m/s, so the attrition problem of heating surfaces will be greatly improved [8,9].
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Nomenclature Q F h K H T 4T
r
n
heat, kW flow rate, kg/s steam enthalpy, kJ/kg heat transfer coefficient, W/(m2°C) heating surface, m2 temperature, °C logarithm temperature difference, °C standard deviation, °C sample number
Subscripts and superscript t total s steam
Though CFB boiler with the FBHEs has many advantages, the pipe failure of the FBHEs is easy to occur during the operation, which is due to the nonuniformity of fluidization in the FBHEs, thus the distribution of heat absorption for heat exchanger pipes in the FBHEs along the ash flow direction is inconsistent [10,11], and resulting in the thermal deviation among the different heat transfer pipes in axial and radial direction, it is very dangerous for the FBHEs’ safe operation under high temperature. A lot of research work has been finished in the test rigs in order to investigate the heat transfer and flow characteristics in the FBHEs [12–14], which is very helpful for the design of large-scale FBHEs. However, due to limit of experiment condition, the experimental results about thermal deviation along the ash flow direction is very difficult to obtain from the small-scale test rigs. At present, the research data about thermal deviation of the FBHEs’ pipes for large-scale CFB boilers cannot still be acquired from the public published literatures, in the current study, the wall temperature of the heat exchanger pipes in four FBHEs of a commercial 300 MWe CFB boiler along the ash flow direction was measured under five different boiler loads, and obtained the heat transfer characteristics and the thermal deviation of heat exchanger pipes, the measured results will provide a very good reference for the optimal design and operation of 600 MWe level and above supercritical CFB boiler with the FBHEs.
2. Experimental 2.1. Introduction of a commercial 300 MW CFB boiler The experimental tests were carried out in a commercial 300 MWe CFB boiler with a pantleg type furnace, the general arrangement of 300 MWe CFB boiler is shown in Fig. 1, Four hot cyclone separators are arranged symmetrically at left wall and right wall of the furnace, with a loop seal and a FBHE at the end of each cyclone standpipe respectively. Circulating solids are captured by cyclones enter loop seal and FBHE respectively, and then high and low temperature solids respectively from loop seal and FBHE return to the furnace again, realizing the adjustment of bed temperature and reheat steam temperature. The cross section of the furnace is 15,050 mm (width) 14,700 mm (depth), the height of the furnace is 36,500 mm, There were 36 extended evaporation heating surfaces arranged in the upper part of the furnace, the size of the water wall tube of furnace is £57 6.5 mm in the upper and £76 8 mm in the bottom, the main design parameters of 300 MWe CFB boiler are listed in Table 1.
o i rs ms is m ls a av A ar daf
outlet inlet reheat steam main steam pumping steam at all levels at a moment adjust the valve door and shaft seal leakage steam circulating ash average value fluidization air as received basis dry ash free basis
Fig. 1. General arrangement of 300 MWe CFB boiler.
Table 1 Boiler main design parameters. Parameter
Units
BMCR
Superheat steam flow rate Superheat steam Pressure Superheat steam temperature Reheat steam flow rate Reheat steam inlet pressure Reheat steam outlet pressure Reheat steam inlet temperature Reheat steam outlet temperature Bed temperature
t/h MPa °C t/h MPa MPa °C °C °C
1025.0 17.50 540.0 846.0 3.99 3.80 327.0 540.0 850.0
2.2. Structure and layout of FBHEs The arrangement of FBHEs is shown in Fig. 2. Four FBHEs are arranged symmetrically at left wall and right wall of the furnace, each of FBHEs is divided into three chambers, the first chamber is empty, and there are the heating surfaces in the other two chambers. Each of the FBHE chambers has independent air grid and wind box, a cone valve is set at the inlet of each FBHE to control the circulation solids flow into FBHEs by adjusting the cone valves’ opening. Low temperature superheater (LTS) and high temperature reheater (HTR) are arranged respectively in 71# FBHE and 74#
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G. Song et al. / Applied Thermal Engineering 130 (2018) 938–950 Table 2 Heat transfer chamber dimension of two FBHEs.
Rear wall
Furnace
Right wall
Left wall
Component
Width (mm)
Depth (mm)
Height (mm)
73# FBHE
Empty chamber ITS2 chamber ITS1 chamber
700 3500 3500
4400 4400 4400
5300 5300 5300
74# FBHE
Empty chamber HTR chamber LTS chamber
700 4200 3500
4400 4400 4400
5300 5300 5300
The steam is leaded out from the high pressure cylinder of turbine into the low temperature reheater (LTR) arranged in the back pass, then the heated steam flows into HTR in the two FBHEs (71# and 74#), finally it is leaded into the middle pressure cylinder of the turbine.
Front wall Fig. 2. General layout of four FBHEs.
FBHE, and middle temperature superheater1 (ITS1) and middle temperature superheater2 (ITS2) are arranged respectively in 72# FBHE and 73# FBHE, due to the structural arrangement symmetry of the FBHEs, the wall temperature test experiments were carried out in two typical FBHEs (73# and 74#) at the right wall of the furnace, the dimension of two typical FBHEs is showed in Table 2. 2.3. Steam-water circulation Steam-water circulation of 300 MWe CFB boiler is shown in Fig. 3. Firstly, the boiler feed water is sent into the economizer, and then feed water after heated in the economizer is sent into the drum. The saturated steam from the drum is sent into the enclosure superheater, then superheated steam is led in turn to LTS in the two FBHEs (71# and 74#), ITS1 and ITS2 in the other two FBHEs (72# and 73#), then goes into the high temperature superheater (HTS) arranged in the back pass, finally the steam goes into the high pressure cylinder of turbine.
2.4. Arrangement of measurement points Since the thermal deviation mainly occurs in the high temperature chambers in the FBHEs, wall temperature measure points along ash flow direction are arranged in the ITS2 of 73# FBHE and HTR of 74# FBHE. In order to investigate the thermal deviation along ash flow direction for ITS2 and HTR, Twenty wall temperature measurement points are uniformly distributed in the outlet of ITS2 and HTR along ash flow direction, the arrangement of wall temperature measurement points is show in Fig. 4. All the wall temperature measurement points are assembled in the perpendicular segments before entering the steam header; it is 300 mm distance from the topmost pipe center line (see Fig. 5). The relationship between twenty wall temperature measurement points and the tube bundles of ITS2 and HTR is shown in Table 3, the installation of ten wall temperature measurement points for ITS2 and ten wall temperature measurement points for HTR is shown in Fig. 6(a) and (b), the structure of the thermocouple protection sleeve is shown in Fig. 6(c), the contact area between three sides of the heat collection block and the pipe wall is fully welded, and its interior is equivalent to an isothermal chamber, it may make the chamber temperature of heat collection block to the maximum extent close to the outer wall temperature of the heating surface
Fig. 3. Steam-water circulation diagram of 300 MWe CFB boiler.
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8ITSA
1
4
10ITSB
7
10 13
16
10ITSC
19 22 25 28
1
4
7
Ash flow direction
8HTRA Ash flow direction
HTR outlet header central line
ITS2 outlet header central line
12HTRB
10
(a) ITS2 outlet
14
18
10HTRC
21
24
27
30
(b) HTR outlet
Fig. 4. Schematic diagram of ITS2 and HTR outlet wall temperature measurement points.
Table 3 Wall temperature measurement points layout of ITS2 and HTR tube outlet. ITS2
Ti Outlet steam header
Heating surface
HTR
Number
Location
Number
Location
1# 2# 3# 4# 5# 6# 7# 8# 9# 10#
1st row tube 4th row tube 7th row tube 10th row tube 13th row tube 16th row tube 19th row tube 22nd row tube 25th row tube 28th row tube
1# 2# 3# 4# 5# 6# 7# 8# 9# 10#
1st row tube 4th row tube 7th row tube 10th row tube 14th row tube 18th row tube 21st row tube 24th row tube 27th row tube 30th row tube
pressure gauge are installed on the side of each header respectively. In order to obtain the flow characteristics of each chamber, the flow meter and the thermocouple, to measure the air flow and air temperature of each chamber entrance, are equipped at the entrance of each air duct. 2.5. Method of experiments
Inlet steam header Fig. 5. Arrangement of wall temperature measurement points in the perpendicular segments.
pipes, which will improve the measurement accuracy of all the thermocouples. The thermocouple stainless protection sleeve is embedded in the heat collection block with an inner diameter equal to the outside diameter of thermocouple stainless protection sleeve. In order to obtain the heat transfer characteristics of high temperature chambers, the ash temperature measurement points are set at the outlet of each heat exchanger chamber, the arrangement of ash temperature measurement points is shown in Fig. 7, the installation of ash temperature measurement points for 73# FBHE and 74# FBHE is shown in Fig. 8, two thermocouples are set symmetrically at the left and right side wall of each chamber outlet, the average of two thermocouple temperature readings is regarded as experiment results of ash temperature. Moreover, the temperature and pressure measurement points of the steam are set at the inlet and outlet of each heating surface header. A thermocouple and a
In this research, four basic parameters, namely, ash temperature, steam temperature and pressure as well as steam flow rate, are measured separately, the enthalpy increment, that is, the steam enthalpy difference between inlet and outlet of each chamber, can be calculated by the steam temperature and pressure readings of inlet and outlet header of each chamber, the total absorbed heat (Q t ) of each chamber is defined as follows:
Q t ¼ F s ðho hi Þ
ð1Þ
where Fs is the steam flow rate, kg/s; ho and hi are the steam enthalpy at the outlet and inlet of each chamber, kJ/kg. The superheated steam flow rate (Fms) can be calculated by the feed water flow rate and three-level water spraying quantity of the desuperheaters, feed water flow rate and three-level water spraying quantity are shown in DCS system. The reheat steam flow rate can be calculated by the steam turbine mass balance, under the steady state, the reheat steam flow rate (Frs) can be calculated as follows:
F rs ¼ F ms
X F is F ls
ð2Þ
i
where Frs is the reheat steam flow rate at the inlet of the intermediate pressure cylinder, kg/s; Fms is main steam flow rate, kg/s;
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(a) ITS2
(b)HTR
(c) Thermocouple installation
Fig. 6. Installation of wall temperature measurement points in ITS2 and HTR outlet.
Fis is pumping flow rate at all levels before reheating, kg/s; Fls is adjust the valve door and shaft seal leakage steam flow rate, kg/s. Fis and Fls can be obtained by DCS data acquisition system. The heat transfer coefficient (K) of each chamber in the FBHEs is defined as follows:
K¼
Q t 1000 H DT
ð3Þ
where Q t is the total absorbed heat of each chamber in FBHEs, kW; H is the total heating surface of each chamber, m2; and 4T is logarithm temperature difference, °C, defined as:
DT ¼
ðT ia T os Þ ðT oa T is Þ
where
i
o
a
s
a T s Þ ln ðT ðT o T i Þ
T ia
and
T oa
ð4Þ
ð7Þ
Q A ¼ F A ðhAo hAi Þ
ð8Þ
where Q s is the total absorbed heat of steam in each FBHEs, kW; Q a is the released heat of circulating ash in each FBHE, kW; Fa is the circulating ash flow rate, kg/s; hai and hao are the circulating ash enthalpy in the inlet and outlet of each FBHE, kJ/kg; Q A is the absorbed heat of fluidization air in each FBHE, kW; FA is the fluidization air flow rate, m3/s; hAi and hAo are the fluidization air enthalpy in the inlet and outlet of each FBHE, kJ/m3. The fluidized air flow rate can be measured by the air flow meter before each heat transfer chamber, and the air flow meter data can be obtained by the DCS data acquisition system. 2.6. Experiment operating conditions
are the circulating ash temperatures at the inlet
and outlet of each chamber in the FBHEs, °C, T is and T os are the steam temperatures at the inlet and outlet of each chamber in the FBHEs, °C. In order to evaluate the fluctuation range of wall temperature of the FBHEs outlet along the ash flow, the standard deviation is introduced as the evaluation index of the fluctuation range of wall temperature, twenty wall temperature measurement points, along the ash flow in the ITS2 and HTR chamber outlet, are measured once one minute, two hours of continuous measurement, then the standard deviation of the wall temperature data for two hours is calculated, the larger the standard deviation is, the greater the wall temperature fluctuations is, standard deviation (r) of wall temperature within two hours is defined as follows.
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P P n T 2m ð T m Þ2 r¼ nðn 1Þ
Five typical operation conditions were chosen in this experiment, that is 90 MW, 150 MW, 225 MW, 270 MW and 300 MW respectively. For each condition, the test runs for two hour and all the measured data are automatically recorded and saved to the PC by the IMP data acquisition system. In order to get more accurate and representative results, the averaged test value within two hour, under the steady operation condition, is treated as the final test results. The experimental fuel is the low grade lignite, the proximate and ultimate analyses of the fuel are listed in Table 4. 3. Results and discussion 3.1. Ash temperature distribution of FBHEs under different loads
ð5Þ
where n is the measurement points number, Tm is the wall temperature of the measurement points at a moment, °C. Under different boiler loads, the circulating ash flow rate could be derived by the thermal balance of FBHEs, and defined as follows.
Qa ¼ Qs þ QA
Q a ¼ F a ðhai hao Þ
ð6Þ
The relation between the conical valve opening of FBHEs and the boiler loads is showed in Fig. 9, under different loads, the conical valve opening corresponding to different FBHEs is not the same, In general, with the increase of boiler loads, the conical valve opening of two FBHEs takes on a monotonically increasing trend. For the different FBHEs, the increase of the conical valve opening is not the same. Under the minimum load (90 MW), the conical
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TI2
TI1
Outlet steam header
Outlet steam header
Ash outlet
Ash flow direction
Ash inlet
T3i
T3o
Weir
T3
T4
ITS2
ITS1
(a) 73# FBHE (T3 and T4) Outlet steam header
TH1
TL1 Outlet steam header
Ash outlet
Ash inlet
Ash flow direction Weir
T4o
Weir
T4i
T1
T2
HTS
LTS
(b) 74# FBHE (T1 and T2) Fig. 7. Ash temperature measurement points schematic diagram of 73# and 74# FBHE.
(a) HTS outlet (T1)
(b) LTS outlet (T2)
(c) ITS2 outlet (T3)
Fig. 8. Ash temperature measurement points installation of 73# and 74# FBHE.
(d) ITS1 outlet (T4)
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Table 4 Ultimate and proximate analysis of test coal. Ultimate analysis (wt.%)
Proximate analysis (wt.%)
Car
Har
Oar
Nar
Sar
Aar
War
Vdaf
Qnet. (MJ/kg)
39.74
2.61
13.04
1.16
0.67
6.18
36.6
50.45
14.4
Fig. 9. Conical valve opening change of FBHEs under different loads.
(a) 73# FBHE
(b) 74# FBHE Fig. 10. Ash flow rate change of two FBHEs under different loads.
valve opening of 73# and 74# FBHEs is close to 8.7%, showing high temperature circulating ash quantity into 73# and 74# FBHEs is almost equal. With the increase of boiler load, the conical valve opening of 73# FBHE takes on a trend of rapid growth, the conical valve opening of 74# FBHE shows a slow growth trend, when attained boiler maximum continuous rating (BMCR) condition (300 MW), the conical valve opening of 73# FBHE is 54.8%, while 74# FBHE is only 28.1%. From the above analysis, under the highload conditions, high temperature circulating ash quantity entering 73# FBHE was significantly higher than that of 74# FBHE. Under the different loads, the ash flow rate change of 73# and 74# FBHEs is showed in Fig. 10, it can be seen that the ash flow rate increase with the increase of boiler load, the ash flow rate of 73# FBHE is significantly higher than that of 74# FBHE, which is consistent with the conical valve opening change trend with boiler loads. When the boiler load increased from 90 MW to 300 MW, the ash flow rate of 73# FBHE increased from 58.2 kg/s to 292.4 kg/s, while the ash flow rate of 74# FBHE increased only from 27.3 kg/s to 116.0 kg/s.
Fig. 11. Ash temperature change of 73# and 74# FBHEs under different loads.
Under the different loads, the ash temperature distribution characteristics along the ash-flow direction for 73# and 74# FBHEs is displayed in Fig. 11, it can be seen that the ash temperature in each of heat exchanger chambers was monotonically increased with increasing boiler load. When boiler load increases, the amount of coal and air increase accordingly, circulating ash and circulating ash temperature also gradually increased. Along the ash-flow direction of two FBHEs, the circulating ash passes through in turn the empty room, the first heat exchanger chamber (IST2 and HTR) and the second heat exchanger chamber (IST1 and LTS), and there is heat exchange between the circulating ash and the fluidized air as well as the steam in the heat transfer tubes, so that the circulating ash temperature gradually decreases from the empty chamber, the first-stage heat exchange chamber to the second-stage heat exchange chamber. Under the different loads, the ash temperature distribution characteristics of 73# FBHE is different from that of 74# FBHE,
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the flow rate of circulating ash in FBHE is, the smaller the corresponding ash temperature decrease is. It can be seen from Fig. 10 that the circulating ash flow rate of 73# FBHE is higher than that of 74# FBHE under different load conditions, so the ash temperature decrease of 74# FBHE is higher than that of 73# FBHE along the ash flow direction. The maximum ash temperature drop appear in 90 MW condition for 73# FBHE, the ash temperature drop is up to 69 °C from the empty chamber to IST2, and 132 °C from IST2 to ITS1. While for 74# FBHE, the maximum ash temperature drop occurs 150 MW condition, the ash temperature drop is up to 198 °C from the empty chamber to HTR, and 223 °C from HTR to LTS. 3.2. Heat transfer characteristics of FBHEs under different loads
(a) 73# FBHE
(b) 74# FBHE Fig. 12. Fluidization velocity change of 73# and 74# FBHEs under different loads.
the ash temperature decrease of 74# FBHE, along the ash flow direction, was significantly higher than that of 73# FBHE, In the case of constant steam flow and steam heat absorption, the greater
Under the different loads, the fluidization velocity changes of different heat exchanger chambers for 73# and 74# FBHEs are displayed in Fig. 12. Fig. 12(a) shows that the fluidization velocity of the empty chamber is above 0.5 m/s under different loads, at full load, up to 0.73 m/s, much higher than the design value of 0.5 m/s (BMCR), the fluidization velocity of ITS2 is relatively stable, and fluctuates between 0.39 m/s and 0.54 m/s, but the fluidization velocity of ITS1 is relatively high, and fluctuates between 0.53 m/s and 0.87 m/s, for ITS2 and ITS1, the fluidization velocity is higher than the design value of 0.3 m/s (BMCR), it can be seen that the fluidizing velocity in three chambers of 73# FBHE is higher than the design value. Fig. 12(b) shows that the fluidizing velocity in the empty chamber of the 74# FBHE is higher than that of 73# FBHE under different loads, and fluctuates between 0.54 m/s and 0.71 m/s, but the fluidizing velocity of HTR and LTS is lower than that of ITS2 and ITS1, at full load conditions, it is the same for three chambers of the 74# FBHE that the fluidizing velocity is higher than the design value. As the fluidization velocity is high, the wear between the heated surfaces and the high temperature circulating ash is serious, so it is necessary to control the fluidization velocity within the design speed range. The operational parameters of two tested FBHEs under five different loads is showed in Table 5, In general, with the increase of load, the operating parameters of the inlet and outlet for different heating surfaces increase correspondingly, but the operating parameters fluctuate at low load (150 MW). The average heat transfer coefficient of different heat exchanger chambers with
Table 5 Operational parameters of two FBHEs under different loads. Items
73# FBHE
Unit
ITS1
ITS2
74# FBHE
LTS
HTR
Boiler loads 90 MW
150 MW
225 MW
270 MW
300 MW
Inlet steam temperature Outlet steam temperature Inlet steam pressure Outlet steam pressure Steam flow rate Inlet steam temperature Outlet steam temperature Inlet steam pressure Outlet steam pressure Steam flow rate
°C °C MPa MPa t/h °C °C MPa MPa t/h
363.3 407.5 10.9 10.9 170.7 383.0 460.2 10.9 10.8 173.6
345.6 417.0 9.8 9.7 227.6 366.6 477.7 9.6 9.5 236.4
358.4 421.4 14.7 14.6 326.7 382.5 473.4 14.5 14.4 344.1
370.6 422.3 17.3 17.1 404.0 401.2 477.7 17.0 16.9 420.4
371.1 425.3 17.7 17.5 449.5 408.4 480.3 17.4 17.3 466.4
Inlet steam temperature Outlet steam temperature Inlet steam pressure Outlet steam pressure Steam flow rate Inlet steam temperature Outlet steam temperature Inlet steam pressure Outlet steam pressure Steam flow rate
°C °C MPa MPa t/h °C °C MPa MPa t/h
340.7 355.9 11.0 10.9 167.8 446.6 527.4 1.1 1.1 152.9
331.4 346.8 9.9 9.8 218.8 453.5 534.2 1.7 1.7 211.0
353.2 368.7 14.8 14.7 309.3 442.2 538.6 2.6 2.5 306.4
361.1 376.9 17.4 17.3 387.5 429.5 539.2 3.1 3.0 363.9
362.1 376.8 17.9 17.7 432.7 423.8 538.8 3.4 3.3 399.0
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Fig. 13. Average heat transfer coefficient of two FBHEs under different loads.
the boiler load variation is shown in Fig. 13, with the increase of the boiler load, the average heat transfer coefficient of different heat exchanger chambers takes on a monotonically increasing trend, but the increase rate of average heat transfer coefficient for different heat exchanger chamber is different, which is basically consistent with the test results of the literature [11]. In general, the average heat transfer coefficient of heating surfaces in 73# FBHEs (IST2 + ITS1) was higher than that of heating surfaces in 74# FBHEs (HTR + LTS) under different loads, the average heat transfer coefficient of ITS1 is highest among the four heat exchanger chambers. At full load, the average heat transfer coefficient of ITS1 reached 288.0 W/(m2°C). Since the conical valve opening at the entrance of the 73# FBHE is the largest (see Fig. 9), and thus the ash flow rate into ITS1 heat exchanger chamber in the 73# FBHE (see Fig. 10) is high, the high ash concentration and high fluidization velocity of ITS1 enhance the heat transfer process, resulting in a high average heat transfer coefficient. The situation is basically the opposite for the LTS, before 225 MW boiler load, the average heat transfer coefficient of LTS is lowest, after 225 MW boiler load, the average heat transfer coefficient of HTR is lowest. 3.3. Thermal deviation analysis of FBHEs under different loads In order to ensure the uniformity of steam distribution and heat transfer, there are three zones in turn along ash flow direction for the heating surface pipes in the ITS2 and HTR, the pipe dimension of three zone for the ITS2 and HTR is showed in Table 6. The pipe size of three zones for ITS2 is different, but the material of them is same, while for the HTR, the pipe size of three zones for ITS2 is same, the material of them is different. For the 73# FBHE, under the different boiler loads, the wall temperature distribution along the ash flow direction of ITS2 is shown in Fig. 14 (a). At the inlet of ITS2, the circulating ash temperature is decreased due to the heat absorption of fluidized air in the empty
chamber, so that the first third measuring point’s wall temperature of the ITS2 is low under different boiler loads. Along the flow direction, with the further increase of the fluidized air temperature, the heat transfer process between the low temperature heating surface and the high temperature circulating ash is enhanced, so that the wall temperature after the third measuring point of ITS2 is increased. At the same time, the circulating ash temperature along the ash flow direction gradually decreased with the heat transfer, so the wall temperature after ninth measuring point of ITS2 began to decline. In general, the wall temperature along the ash flow direction of ITS2 takes on a tendency to rise first and then decrease, which is consistent with the wall temperature distribution trend of the FBHEs in the 600 MW supercritical CFB boiler [15,16]. The higher the boiler load is, the higher the wall temperature is also, but when the boiler load is up to 225 MW, the wall temperature of ITS2 tends to be stable. Under five different boiler loads, the highest wall temperature point appears on the 7th column tubes in third zone (9# measuring point in Table 3), the lowest wall temperature point appears on the 10th column tubes in third zone (10# measuring point in Table 3), under 90 MW boiler load, the wall temperature fluctuations in the first nine measuring points is not large, when the boiler load increase from 150 MW to 300 MW, the wall temperature fluctuations in the front nine measuring points is very large, the trend of wall temperature fluctuation under different loads is basically the same. Based on the steam pipes design of the FBHEs, all the heating surface tubes of ITS2 are made of U-type header and adopt the partitioned design to ensure the uniform arrangement, in theory, the steam flow distribution is uniform. Therefore, the main reason for the large wall temperature deviation of ITS2 is due to the uneven heat absorption, which is caused by the localized nonuniform flow of circulating ash in ITS2. For the 74# FBHE, under the different boiler loads, the wall temperature distribution along the ash flow direction of HTR is shown in Fig. 14(b). Based on the test results, the wall temperature distribution trend along the ash flow direction of HTR is the same as that of ITS2, that is, the wall temperature along the ash flow direction of HTR takes on a tendency to rise first and then decrease, at the same time, the wall temperature distribution along the ash flow direction is relatively uniform, which is consistent with the measured wall temperature distribution at the tube panel outlets of HTR at different loads in the literature [10,17]. Compared with ITS2, the position of the highest wall temperature point moves forward from the third zone in ITS2 to the second zone in HTR. When the boiler load reaches 270 MW, the wall temperature distribution of HTR tends to be stable, when the boiler load is further increased, the wall temperature changes of HTR at different measuring points are not obvious. Under five different boiler loads, the highest wall temperature point appears on the second column tubes in second zone (4# measuring point in Table 3), the lowest wall temperature point appears on the 10th column tubes in third zone (10# measuring point in Table 3). In general, the flow and heat transfer process of the FBHE in HTR is more uniform than that of ITS2. Under different boiler load conditions, the average wall temperature of ITS2 and HTR respectively
Table 6 Pipe dimension of FBHE heating surface. Component
Number
Pipe size (mm)
Material
ITS2
8ISTA(IZone) 10ISTB(IIZone) 10ISTC(III Zone)
40 50 50
£63.5 10.5 £51.0 8.0 £51.0 7.11
SA-213T91 SA-213T91 SA-213T91
HTR
8HTRA(IZone) 12HTRB(IIZone) 10HTRC(III Zone)
40 60 50
£63.5 5.59 £63.5 5.59 £63.5 5.59
SA-213 PT321H SA-213 PT321H SA-213T91
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(a) ITS2
(a) ITS2
(b) HTR
(b) HTR
Fig. 14. Wall temperature distribution of ITS2 and HTR tubes along ash flow direction.
Fig. 16. Standard deviation of wall temperature of ITS2 and HTR tubes.
Fig. 17. Heat absorption proportion of four FBHEs under different loads.
Fig. 15. Average wall temperature of two heat transfer chambers outlet under different loads.
are shown in Fig. 15. With the increase of boiler load from 90 MW to 300 MW, the average wall temperature of ITS2 and HTR increases accordingly, ITS2 from 457 °C to 481 °C, HTR from 525 °C to 534 °C. In general, the average wall temperature of HTR
is higher than that of ITS2. When the boiler load increases, the bed temperature and the circulating ash temperature rises, the amount of circulating ash increases, the heat transfer process between the circulating ash and the steam is enhanced, so the average wall temperature of ITS2 and HTR rises. Under the different boiler loads, the wall temperature standard deviation within two hours for the ITS2 and HTR is showed in
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Fig. 14), that is, the high wall temperature of the measurement points correspond to the large wall temperature standard deviation, which indicate that when the wall temperature of heating surface is high, the heat transfer process between high temperature circulating ash and low temperature steam is enhanced, at the same time, the nonuniformity of the heat transfer process increases, and the fluctuation and the deviation of wall temperature also increase correspondingly, the above large number of test results show that there are objective facts of non-uniform heat transfer problems, between different measuring points of the same load and different loads of the same measuring point, which is mainly due to the non-uniform heat transfer caused by nonuniform flow of the high temperature circulating ash in the heat exchange chambers. 3.4. Thermal matching characteristics of FBHEs under different boiler loads Fig. 18. Heat absorption proportion of left and right sides FBHEs of the furnace.
Fig. 19. Total heat absorption proportion of four FBHEs under different loads.
Fig. 16. With the increase of boiler loads, the standard deviation of wall temperature tends to decrease gradually. For the ITS2, when the boiler load is less than 225 MW (75% BMCR), the wall temperature standard deviation of the same measuring point was large, especially at 150 MW (50% BMCR) load, the fluctuates of wall temperature standard deviation between the different measuring points is the biggest, fluctuating from 2.9 °C to 5.2 °C, which indicates that the heat transfer process between the high temperature circulating ash and the low temperature steam is unstable at low load conditions (less than 225 MW). When the boiler load increases to 225 MW (75% BMCR), the wall temperature standard deviation is small, fluctuating only from 1.2 °C to 1.9 °C, which indicates that the heat transfer process between the high temperature circulating ash and the low temperature steam is stable. For the HTR, with the increase of boiler loads, the change of wall temperature standard deviation shows a similar phenomenon. That is, under high boiler load conditions (225 MW and above), the standard deviation and the fluctuation of wall temperature is relatively low, under low boiler load conditions (less than 225 MW), the situation is just the opposite. In addition, the high standard deviation of wall temperature for ITS2 appears on the 7th column tubes in third zone (especially 150 MW load), and the high standard deviation of wall temperature for HTR occurs in the 4–7 columns of the first zone and 2 columns of the second zone, under low boiler load conditions (less than 225 MW), the distribution of wall temperature standard deviations for the different measuring points is basically the same as the distribution of wall temperature corresponding measuring points (see
Under the different load conditions, the ratios of the heat absorption of four FBHEs to the boiler effective heat are shown in Fig. 17. It can be seen that the heat absorption proportion of four FBHEs is different with the increase of the boiler load. For the 71# and 74# FBHEs, with the increase of the boiler load, the proportion of heat absorption took on a monotonically increasing trend, As the boiler load increased from 90 MW to 300 MW, the proportion of heat absorption for 71# FBHE increased from 5.11% to 8.57%, and the heat absorption proportion of 74# FBHE increased from 3.83% to 6.79%. However, for the 72# FBHE, under the 270 MW load conditions, the heat absorption proportion reached the maximum of 11.39%, for 73# FBHE, under the 225 MW load conditions, the heat absorption proportion reached a maximum of 10.85%. For five different load conditions, the heat absorption proportion of 72# FBHE is the largest, the heat absorption proportion of 74# FBHE is the smallest. The heat absorption proportion variation of two FBHEs on the left and right sides of the furnace is shown in Fig. 18. Under different load conditions, the heat absorption proportion of two FBHEs (71# and 72#) on the left side of the furnace is larger than that of two FBHEs (73# and 74#) on the right side of the furnace, when boiler loads increased from 90 MW to 300 MW, the total heat absorption proportion of the left two FBHEs (71# and 72#) increased correspondingly from 14.3% to 19.7%, and that of the right two FBHEs (73# and 74#) increased correspondingly from 10.5% to 17.5%, which proves that there are asymmetric heat transfer phenomena in the symmetrical arrangement of four FBHEs, the phenomenon is due to uneven heat transfer caused by uneven flow of high temperature circulating ash in four FBHEs, this problem needs to be given enough attention in the design of the FBHEs in the future. The variation of the total heat absorption proportion in four FBHEs under different load conditions is shown in Fig. 19, the total heat absorption proportion of four FBHEs increases linearly with the increase of boiler loads, when the boiler load was increased from 90 MW (30% BMCR) to 300 MW (100% BMCR), The total heat absorption proportion of four FBHEs increased from 25.20% to 38.20% monotonically. At the full load (300 MW), the total heat absorption proportion (38.20%) of four FBHEs is very close to that of other 300 MW CFB boilers at the same level, which was shown in Table 7 [18]. Under different load conditions, the heat absorption proportion variation of four heat transfer chambers (ITS2, ITS1, HTR and LTS) in four FBHEs is shown in Fig. 20, In general, the heat absorption proportion of the four heat transfer chambers increases accordingly with the increase of boiler loads. Under five different loads, the heat absorption proportion of ITS2 in the four heat exchange chambers is the largest, and the heat absorption proportion ranges
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G. Song et al. / Applied Thermal Engineering 130 (2018) 938–950 Table 7 Comparison of heat absorption proportion of FBHEs with 300 MW boiler. Objects
Poland Lagiza (460 MW)
Poland Turow4-6 (262 MW)
USA JEA (300 MW)
China Baima (300 MW)
This project (300 MW)
Number of FBHE FBHE heat absorption proportion /%
8 30.4
8 33.6
6 40.0
4 38.7
4 38.2
absorption proportion of 72# FBHE (ITS1 + ITS2) and 73# FBHE (ITS1 + ITS2) is relatively high, which is consistent with the previous analysis results shown in Fig. 17. Based on the above test results, in order to improve the heat transfer nonuniformity of four FBHEs, it is necessary to improve the arrangement of the heating surfaces in four FBHEs, The heating surface of the serpentine tubes is changed from the direction perpendicular to the ash flow direction to parallel to the ash flow direction (see Fig. 21), and set aside some maintenance space, at the same time, it is suggested to optimize the fluidized air distributor structure of four FBHEs, the above test results can provide some references for the optimal design and operation of 600 MW grade supercritical CFB boiler with FBHEs.
Fig. 20. Heat absorption proportion of four heat transfer chambers under different loads.
from 10.66% to 12.03%, the heat absorption proportion of LTS is the smallest, and the heat absorption proportion range from 2.86% to 6.35%, which is consistent with the previous analysis about heat absorption proportion test results of four FBHEs with the boiler load changes (see Figs. 10 and 11). Under different boiler load conditions, the heat absorption proportion of each heat transfer chamber is different, when the boiler load increases from 90 MW to 300 MW, the increase of heat absorption proportion in ITS1 is the largest in the four heat transfer chambers, reaching 5.56%, and the increase of heat absorption proportion in ITS2 is the smallest, only 1.37%. When the boiler load increases to 150 MW and 225 MW, the heat absorption proportion of ITS2 is about 12.0%, which is not affected basically by the boiler load. Under different load conditions, the heat absorption proportion of ITS2 is the largest, and the heat absorption increase of ITS1 is the biggest, compared with the heat absorption proportion of 71# FBHE (HTR + LTS) and 74# FBHE (HTR + LTS), the heat
(a)Old arrangement of heating surface
4. Conclusion The tests of the wall temperature distribution in two fluidized bed heat transfer chambers outlet were carried out in 300 MW subcritical CFB boiler under different boiler loads, some important key parameters are obtained, and the main conclusions are summarized as follows. (1) The measurement results indicate that four FBHEs in the 300 MW subcritical CFB boiler are of a very good heat transfer performance, it is one of very effective methods to solve the inadequate heating surface for the large-scale circulating fluidized bed boilers. (2) For four FBHEs, there is a good linear correspondence among boiler loads and the conical valve openings as well as circulating ash flow rate. With the increase of the boiler load, the average heat transfer coefficient of different heat exchanger chambers takes on a monotonically increasing trend, under five different loads, the average heat transfer coefficient of ITS1 is highest, and that of LTS is the lowest.
(b) New arrangement of heating surface
Fig. 21. Optimized layout of heating surface in FBHEs.
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(3) Under the different boiler load conditions, the wall temperature distribution at the outlet of the different heat exchange chambers is different, there is an non-uniform heat transfer process between the different measuring points of the same load as well as the different loads of the same measuring point, the wall temperature test results show that the heating surface arrangement of HTR is more reasonable. (4) Under five different boiler load conditions, the heat absorption proportion of 72# FBHE is the largest, and that of 74# FBHE is the smallest. The heat absorption proportion of two FBHEs on the left side is larger than that of two FBHEs on the right side of the furnace, there is a non-uniform heat transfer phenomena in the symmetrical arrangement of four FBHEs. (5) In order to improve the non-uniform heat transfer between high temperature circulating ash and low temperature steam in four FBHEs, it is recommended the fluidizing air velocity of the heat exchange chambers is reduced to the design value. Simultaneously, the heating surface arrangement direction in four FBHEs was adjusted from snake tube perpendicular to the ash flow direction to parallel to ash flow direction, the above test results can provide a good reference for the optimized design of 600 MW supercritical or ultra supercritical CFB boiler with the FBHEs in the future.
Acknowledgments This work is financially supported by the National Key Research & Development Program of China, Grant No. 2016YFB0600202. References [1] G. Yue, R. Cai, J. Lu, H. Zhang, From a CFB reactor to a CFB boiler – the review of R&D progress of CFB coal combustion technology in China, Powder Technol. 316 (2017) 18–28.
[2] A. Hotta, K. Kauppinen, A. Kettunen, Towards New Milestones in CFB Boiler Technology – CFB 800 MWe/new 460MWe Supercritical Plant with CFB Boiler in Lagisza – First Experience Update. Power-Gen Europe 2010, Amsterdam, Netherlands, June 8–10, 2010, pp. 1–17. [3] H. Hack, A. Hotta, A. Kettunen, et al., Ultra-supercritical CFB technology to meet the challenge of climate change, Presented at Power-Gen International 2008, Florida, USA, December 2–4, 2008 [4] B. Xiong, Gas-solid flow in an integrated external heat exchanger for CFB boiler, Powder Technol. 202 (2010) 55–61. [5] Q. Wang, Z. Luo, M. Fang, M. Ni, K. Cen, Development of a new external heat exchanger for a circulating fluidized bed boiler, Chem. Eng. Process. 42 (4) (2003) 327–335. [6] H. Wang, Y. Li, G. Qiu, G. Song, W. Yang, Measurement of gas–solids flow in loop seal and external heat exchanger in a circulating fluidized bed, Powder Technol. 266 (2014) 249–261. [7] M. Gao, Q. Lu, J. He, An experiment study on pneumatic control valve in circulating fluidized bed, Proc. CSEE 1 (2006) 51–57. [8] A. Blaszczuk, W. Nowak, Heat transfer behavior inside a furnace chamber of large-scale supercritical CFB reactor, Int. J. Heat Mass Transf. 87 (2015) 464– 480. [9] Y. Zhang, Z. Li, Design and operation experience of domestic first 300 MW CFB boiler, Boiler Manuf. 4 (2006) 10–13. [10] H. Wang, X. Lu, W. Zhang, Q. Wang, J. Chen, Y. Kang, W. Kuang, S. Xin, C. Liu, X. Xie, Study on heat transfer characteristics of the high temperature reheater tube panel in a 300MW CFB boiler with fluidized bed heat exchanger, Appl. Therm. Eng. 81 (2015) 262–270. [11] M. Zhang, H. Wu, Q. Lu, Y. Sun, G. Song, Heat transfer characteristics of fluidized bed heat exchanger in a 300 MW CFB boiler, Powder Technol. 222 (2012) 1–7. [12] I. Bolea, L. Romeo, D. Pallarès, Heat transfer in the external heat exchanger of oxy-fuel fluidized bed boilers, Appl. Therm. Eng. 66 (1–2) (2014) 75–83. [13] P. Ludowski, D. Taler, J. Taler, Identification of thermal boundary conditions in heat exchangers of fluidized bed boilers, Appl. Therm. Eng. 58 (1–2) (2013) 194–204. [14] I. Bolea, L. Romeo, D. Pallarés, The role of external heat exchangers in oxy-fuel circulating fluidized bed, Appl. Energy 94 (2012) 215–223. [15] X. Zheng, P. Xu, X. Hu, X. Yang, Q. Zhou, Experimental research on adjustment of operation of high temperature external heat exchanger for the 600MW supercritical CFB boiler, Dongfang Electr. Rev. 30 (118) (2016) 23–26. [16] X. Zhou, Q. Guo, X. Zheng, Q. Zhou, L. Nie, H. Su, H. Yang, G. Li, W. Cheng, Operation characteristics of external heat exchanger in 600MW supercritical CFB boiler, Dongfang Electr. Rev. 31 (121) (2017) 26–29. [17] X. Sun, C. Hu, X. Li, X. Lei, S. Tao, W. Kuang, Tube wall temperature characteristic of external heat exchanger in 600 MW supercritical CFB boiler, Power Constr. 35 (4) (2014) 6–9. [18] V. llkka, R. Psik, 460 MWe supercritieal CFB boiler design for Lagisza power plant, Power-Gen Europe, May 25–27, 2004, Barcelona, Spain.
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research Paper
Experimental research of heat transfer uniformity for fluidized bed heat exchangers in a 300 MW CFB boiler Guoliang Song a,b,⇑, Qinggang Lyu a,b, Feng Xiao c, Yunguan Sun d a
Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China University of Chinese Academy of Sciences, Beijing 100049, China c Shanghai Boiler Works, Ltd., Shanghai 200245, China d China Guodian Corporation Xiaolongtan Power Plant, Kaiyuan 661601, China b
h i g h l i g h t s The average heat transfer coefficient of different heat exchanger chambers is linear with the boiler. The wall temperature distribution at the outlet of the different heat exchange chambers is different. There is a non-uniform heat transfer phenomena in the symmetrical arrangement of four FBHEs. The optimized layout of the heating surfaces in the FBHEs was proposed.
a r t i c l e
i n f o
Article history: Received 2 September 2017 Revised 23 October 2017 Accepted 16 November 2017 Available online 20 November 2017 Keywords: Circulating fluidized bed (CFB) Fluidized bed heat exchanger (FBHE) Solids flow Wall temperature Heat transfer uniformity
a b s t r a c t In order to investigate the heat transfer uniformity of fluidized bed heat exchangers (FBHEs), a series of experimental tests were carried out in a commercial 300 MW CFB boiler. The test results indicate that there is a good linear correspondence between the FBHEs conical valve openings and circulating ash flow rate, the average heat transfer coefficient of different heat exchangers takes on a monotonically increasing trend with the increase of the boiler loads. Moreover, there is a non-uniform heat transfer process between the different measuring points under the same load as well as the different loads for the same measuring points, the heat absorption proportion of two FBHEs (71# and 72#) on the left side is larger than that of two FBHEs (73# and 74#) on the right side of the furnace, there is a non-uniform heat transfer phenomena in the symmetrical arrangement of four FBHEs, the heating surface arrangement in the FBHEs must be adjusted from snake tube perpendicular to the ash flow direction to parallel to ash flow direction, the test results can provide a good reference for the optimized design and operation of 600 MW supercritical CFB boiler with the FBHEs in the future. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Circulating fluidized bed (CFB) boiler technology with large capacity and high steam parameters is the main development direction due to its energy saving and environment protection [1]. With the development of large-scale 300 MW CFB boilers, specially for the development of CFB boilers up to 600–1000 MWe capacities with supercritical or ultra- supercritical steam parameters [2,3], the imbalance increase between heat release and heat absorption will result in the problem of heating surface arrangement, the fluid bed heat exchangers (FBHEs) are placed in the ⇑ Corresponding author at: Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China. E-mail address: [email protected] (G. Song). https://doi.org/10.1016/j.applthermaleng.2017.11.081 1359-4311/Ó 2017 Elsevier Ltd. All rights reserved.
hot materiel circulating loop, which is a very effective approach to resolve the heating surface arrangement problem brought by the scale-up of CFB boilers [4–7], some heating surfaces are transferred from the furnace to the FBHEs, on the one hand, the combustion process and heat transfer process is partitioned by the FBHEs, which will increase the operation and control methods of furnace, bed temperature and reheat steam temperature can be adjusted flexibly by the solids control valves during the boiler operation. On the other hand, the heating surfaces in the furnace decrease, the heat expansion and seal problems of furnace will be resolved effectively by the FBHEs, some heating surfaces are placed in the FBHEs under the very low fluidization velocity within the range of 0.3 m/s to 0.5 m/s, so the attrition problem of heating surfaces will be greatly improved [8,9].
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Nomenclature Q F h K H T 4T
r
n
heat, kW flow rate, kg/s steam enthalpy, kJ/kg heat transfer coefficient, W/(m2°C) heating surface, m2 temperature, °C logarithm temperature difference, °C standard deviation, °C sample number
Subscripts and superscript t total s steam
Though CFB boiler with the FBHEs has many advantages, the pipe failure of the FBHEs is easy to occur during the operation, which is due to the nonuniformity of fluidization in the FBHEs, thus the distribution of heat absorption for heat exchanger pipes in the FBHEs along the ash flow direction is inconsistent [10,11], and resulting in the thermal deviation among the different heat transfer pipes in axial and radial direction, it is very dangerous for the FBHEs’ safe operation under high temperature. A lot of research work has been finished in the test rigs in order to investigate the heat transfer and flow characteristics in the FBHEs [12–14], which is very helpful for the design of large-scale FBHEs. However, due to limit of experiment condition, the experimental results about thermal deviation along the ash flow direction is very difficult to obtain from the small-scale test rigs. At present, the research data about thermal deviation of the FBHEs’ pipes for large-scale CFB boilers cannot still be acquired from the public published literatures, in the current study, the wall temperature of the heat exchanger pipes in four FBHEs of a commercial 300 MWe CFB boiler along the ash flow direction was measured under five different boiler loads, and obtained the heat transfer characteristics and the thermal deviation of heat exchanger pipes, the measured results will provide a very good reference for the optimal design and operation of 600 MWe level and above supercritical CFB boiler with the FBHEs.
2. Experimental 2.1. Introduction of a commercial 300 MW CFB boiler The experimental tests were carried out in a commercial 300 MWe CFB boiler with a pantleg type furnace, the general arrangement of 300 MWe CFB boiler is shown in Fig. 1, Four hot cyclone separators are arranged symmetrically at left wall and right wall of the furnace, with a loop seal and a FBHE at the end of each cyclone standpipe respectively. Circulating solids are captured by cyclones enter loop seal and FBHE respectively, and then high and low temperature solids respectively from loop seal and FBHE return to the furnace again, realizing the adjustment of bed temperature and reheat steam temperature. The cross section of the furnace is 15,050 mm (width) 14,700 mm (depth), the height of the furnace is 36,500 mm, There were 36 extended evaporation heating surfaces arranged in the upper part of the furnace, the size of the water wall tube of furnace is £57 6.5 mm in the upper and £76 8 mm in the bottom, the main design parameters of 300 MWe CFB boiler are listed in Table 1.
o i rs ms is m ls a av A ar daf
outlet inlet reheat steam main steam pumping steam at all levels at a moment adjust the valve door and shaft seal leakage steam circulating ash average value fluidization air as received basis dry ash free basis
Fig. 1. General arrangement of 300 MWe CFB boiler.
Table 1 Boiler main design parameters. Parameter
Units
BMCR
Superheat steam flow rate Superheat steam Pressure Superheat steam temperature Reheat steam flow rate Reheat steam inlet pressure Reheat steam outlet pressure Reheat steam inlet temperature Reheat steam outlet temperature Bed temperature
t/h MPa °C t/h MPa MPa °C °C °C
1025.0 17.50 540.0 846.0 3.99 3.80 327.0 540.0 850.0
2.2. Structure and layout of FBHEs The arrangement of FBHEs is shown in Fig. 2. Four FBHEs are arranged symmetrically at left wall and right wall of the furnace, each of FBHEs is divided into three chambers, the first chamber is empty, and there are the heating surfaces in the other two chambers. Each of the FBHE chambers has independent air grid and wind box, a cone valve is set at the inlet of each FBHE to control the circulation solids flow into FBHEs by adjusting the cone valves’ opening. Low temperature superheater (LTS) and high temperature reheater (HTR) are arranged respectively in 71# FBHE and 74#
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G. Song et al. / Applied Thermal Engineering 130 (2018) 938–950 Table 2 Heat transfer chamber dimension of two FBHEs.
Rear wall
Furnace
Right wall
Left wall
Component
Width (mm)
Depth (mm)
Height (mm)
73# FBHE
Empty chamber ITS2 chamber ITS1 chamber
700 3500 3500
4400 4400 4400
5300 5300 5300
74# FBHE
Empty chamber HTR chamber LTS chamber
700 4200 3500
4400 4400 4400
5300 5300 5300
The steam is leaded out from the high pressure cylinder of turbine into the low temperature reheater (LTR) arranged in the back pass, then the heated steam flows into HTR in the two FBHEs (71# and 74#), finally it is leaded into the middle pressure cylinder of the turbine.
Front wall Fig. 2. General layout of four FBHEs.
FBHE, and middle temperature superheater1 (ITS1) and middle temperature superheater2 (ITS2) are arranged respectively in 72# FBHE and 73# FBHE, due to the structural arrangement symmetry of the FBHEs, the wall temperature test experiments were carried out in two typical FBHEs (73# and 74#) at the right wall of the furnace, the dimension of two typical FBHEs is showed in Table 2. 2.3. Steam-water circulation Steam-water circulation of 300 MWe CFB boiler is shown in Fig. 3. Firstly, the boiler feed water is sent into the economizer, and then feed water after heated in the economizer is sent into the drum. The saturated steam from the drum is sent into the enclosure superheater, then superheated steam is led in turn to LTS in the two FBHEs (71# and 74#), ITS1 and ITS2 in the other two FBHEs (72# and 73#), then goes into the high temperature superheater (HTS) arranged in the back pass, finally the steam goes into the high pressure cylinder of turbine.
2.4. Arrangement of measurement points Since the thermal deviation mainly occurs in the high temperature chambers in the FBHEs, wall temperature measure points along ash flow direction are arranged in the ITS2 of 73# FBHE and HTR of 74# FBHE. In order to investigate the thermal deviation along ash flow direction for ITS2 and HTR, Twenty wall temperature measurement points are uniformly distributed in the outlet of ITS2 and HTR along ash flow direction, the arrangement of wall temperature measurement points is show in Fig. 4. All the wall temperature measurement points are assembled in the perpendicular segments before entering the steam header; it is 300 mm distance from the topmost pipe center line (see Fig. 5). The relationship between twenty wall temperature measurement points and the tube bundles of ITS2 and HTR is shown in Table 3, the installation of ten wall temperature measurement points for ITS2 and ten wall temperature measurement points for HTR is shown in Fig. 6(a) and (b), the structure of the thermocouple protection sleeve is shown in Fig. 6(c), the contact area between three sides of the heat collection block and the pipe wall is fully welded, and its interior is equivalent to an isothermal chamber, it may make the chamber temperature of heat collection block to the maximum extent close to the outer wall temperature of the heating surface
Fig. 3. Steam-water circulation diagram of 300 MWe CFB boiler.
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8ITSA
1
4
10ITSB
7
10 13
16
10ITSC
19 22 25 28
1
4
7
Ash flow direction
8HTRA Ash flow direction
HTR outlet header central line
ITS2 outlet header central line
12HTRB
10
(a) ITS2 outlet
14
18
10HTRC
21
24
27
30
(b) HTR outlet
Fig. 4. Schematic diagram of ITS2 and HTR outlet wall temperature measurement points.
Table 3 Wall temperature measurement points layout of ITS2 and HTR tube outlet. ITS2
Ti Outlet steam header
Heating surface
HTR
Number
Location
Number
Location
1# 2# 3# 4# 5# 6# 7# 8# 9# 10#
1st row tube 4th row tube 7th row tube 10th row tube 13th row tube 16th row tube 19th row tube 22nd row tube 25th row tube 28th row tube
1# 2# 3# 4# 5# 6# 7# 8# 9# 10#
1st row tube 4th row tube 7th row tube 10th row tube 14th row tube 18th row tube 21st row tube 24th row tube 27th row tube 30th row tube
pressure gauge are installed on the side of each header respectively. In order to obtain the flow characteristics of each chamber, the flow meter and the thermocouple, to measure the air flow and air temperature of each chamber entrance, are equipped at the entrance of each air duct. 2.5. Method of experiments
Inlet steam header Fig. 5. Arrangement of wall temperature measurement points in the perpendicular segments.
pipes, which will improve the measurement accuracy of all the thermocouples. The thermocouple stainless protection sleeve is embedded in the heat collection block with an inner diameter equal to the outside diameter of thermocouple stainless protection sleeve. In order to obtain the heat transfer characteristics of high temperature chambers, the ash temperature measurement points are set at the outlet of each heat exchanger chamber, the arrangement of ash temperature measurement points is shown in Fig. 7, the installation of ash temperature measurement points for 73# FBHE and 74# FBHE is shown in Fig. 8, two thermocouples are set symmetrically at the left and right side wall of each chamber outlet, the average of two thermocouple temperature readings is regarded as experiment results of ash temperature. Moreover, the temperature and pressure measurement points of the steam are set at the inlet and outlet of each heating surface header. A thermocouple and a
In this research, four basic parameters, namely, ash temperature, steam temperature and pressure as well as steam flow rate, are measured separately, the enthalpy increment, that is, the steam enthalpy difference between inlet and outlet of each chamber, can be calculated by the steam temperature and pressure readings of inlet and outlet header of each chamber, the total absorbed heat (Q t ) of each chamber is defined as follows:
Q t ¼ F s ðho hi Þ
ð1Þ
where Fs is the steam flow rate, kg/s; ho and hi are the steam enthalpy at the outlet and inlet of each chamber, kJ/kg. The superheated steam flow rate (Fms) can be calculated by the feed water flow rate and three-level water spraying quantity of the desuperheaters, feed water flow rate and three-level water spraying quantity are shown in DCS system. The reheat steam flow rate can be calculated by the steam turbine mass balance, under the steady state, the reheat steam flow rate (Frs) can be calculated as follows:
F rs ¼ F ms
X F is F ls
ð2Þ
i
where Frs is the reheat steam flow rate at the inlet of the intermediate pressure cylinder, kg/s; Fms is main steam flow rate, kg/s;
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(a) ITS2
(b)HTR
(c) Thermocouple installation
Fig. 6. Installation of wall temperature measurement points in ITS2 and HTR outlet.
Fis is pumping flow rate at all levels before reheating, kg/s; Fls is adjust the valve door and shaft seal leakage steam flow rate, kg/s. Fis and Fls can be obtained by DCS data acquisition system. The heat transfer coefficient (K) of each chamber in the FBHEs is defined as follows:
K¼
Q t 1000 H DT
ð3Þ
where Q t is the total absorbed heat of each chamber in FBHEs, kW; H is the total heating surface of each chamber, m2; and 4T is logarithm temperature difference, °C, defined as:
DT ¼
ðT ia T os Þ ðT oa T is Þ
where
i
o
a
s
a T s Þ ln ðT ðT o T i Þ
T ia
and
T oa
ð4Þ
ð7Þ
Q A ¼ F A ðhAo hAi Þ
ð8Þ
where Q s is the total absorbed heat of steam in each FBHEs, kW; Q a is the released heat of circulating ash in each FBHE, kW; Fa is the circulating ash flow rate, kg/s; hai and hao are the circulating ash enthalpy in the inlet and outlet of each FBHE, kJ/kg; Q A is the absorbed heat of fluidization air in each FBHE, kW; FA is the fluidization air flow rate, m3/s; hAi and hAo are the fluidization air enthalpy in the inlet and outlet of each FBHE, kJ/m3. The fluidized air flow rate can be measured by the air flow meter before each heat transfer chamber, and the air flow meter data can be obtained by the DCS data acquisition system. 2.6. Experiment operating conditions
are the circulating ash temperatures at the inlet
and outlet of each chamber in the FBHEs, °C, T is and T os are the steam temperatures at the inlet and outlet of each chamber in the FBHEs, °C. In order to evaluate the fluctuation range of wall temperature of the FBHEs outlet along the ash flow, the standard deviation is introduced as the evaluation index of the fluctuation range of wall temperature, twenty wall temperature measurement points, along the ash flow in the ITS2 and HTR chamber outlet, are measured once one minute, two hours of continuous measurement, then the standard deviation of the wall temperature data for two hours is calculated, the larger the standard deviation is, the greater the wall temperature fluctuations is, standard deviation (r) of wall temperature within two hours is defined as follows.
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P P n T 2m ð T m Þ2 r¼ nðn 1Þ
Five typical operation conditions were chosen in this experiment, that is 90 MW, 150 MW, 225 MW, 270 MW and 300 MW respectively. For each condition, the test runs for two hour and all the measured data are automatically recorded and saved to the PC by the IMP data acquisition system. In order to get more accurate and representative results, the averaged test value within two hour, under the steady operation condition, is treated as the final test results. The experimental fuel is the low grade lignite, the proximate and ultimate analyses of the fuel are listed in Table 4. 3. Results and discussion 3.1. Ash temperature distribution of FBHEs under different loads
ð5Þ
where n is the measurement points number, Tm is the wall temperature of the measurement points at a moment, °C. Under different boiler loads, the circulating ash flow rate could be derived by the thermal balance of FBHEs, and defined as follows.
Qa ¼ Qs þ QA
Q a ¼ F a ðhai hao Þ
ð6Þ
The relation between the conical valve opening of FBHEs and the boiler loads is showed in Fig. 9, under different loads, the conical valve opening corresponding to different FBHEs is not the same, In general, with the increase of boiler loads, the conical valve opening of two FBHEs takes on a monotonically increasing trend. For the different FBHEs, the increase of the conical valve opening is not the same. Under the minimum load (90 MW), the conical
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TI2
TI1
Outlet steam header
Outlet steam header
Ash outlet
Ash flow direction
Ash inlet
T3i
T3o
Weir
T3
T4
ITS2
ITS1
(a) 73# FBHE (T3 and T4) Outlet steam header
TH1
TL1 Outlet steam header
Ash outlet
Ash inlet
Ash flow direction Weir
T4o
Weir
T4i
T1
T2
HTS
LTS
(b) 74# FBHE (T1 and T2) Fig. 7. Ash temperature measurement points schematic diagram of 73# and 74# FBHE.
(a) HTS outlet (T1)
(b) LTS outlet (T2)
(c) ITS2 outlet (T3)
Fig. 8. Ash temperature measurement points installation of 73# and 74# FBHE.
(d) ITS1 outlet (T4)
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Table 4 Ultimate and proximate analysis of test coal. Ultimate analysis (wt.%)
Proximate analysis (wt.%)
Car
Har
Oar
Nar
Sar
Aar
War
Vdaf
Qnet. (MJ/kg)
39.74
2.61
13.04
1.16
0.67
6.18
36.6
50.45
14.4
Fig. 9. Conical valve opening change of FBHEs under different loads.
(a) 73# FBHE
(b) 74# FBHE Fig. 10. Ash flow rate change of two FBHEs under different loads.
valve opening of 73# and 74# FBHEs is close to 8.7%, showing high temperature circulating ash quantity into 73# and 74# FBHEs is almost equal. With the increase of boiler load, the conical valve opening of 73# FBHE takes on a trend of rapid growth, the conical valve opening of 74# FBHE shows a slow growth trend, when attained boiler maximum continuous rating (BMCR) condition (300 MW), the conical valve opening of 73# FBHE is 54.8%, while 74# FBHE is only 28.1%. From the above analysis, under the highload conditions, high temperature circulating ash quantity entering 73# FBHE was significantly higher than that of 74# FBHE. Under the different loads, the ash flow rate change of 73# and 74# FBHEs is showed in Fig. 10, it can be seen that the ash flow rate increase with the increase of boiler load, the ash flow rate of 73# FBHE is significantly higher than that of 74# FBHE, which is consistent with the conical valve opening change trend with boiler loads. When the boiler load increased from 90 MW to 300 MW, the ash flow rate of 73# FBHE increased from 58.2 kg/s to 292.4 kg/s, while the ash flow rate of 74# FBHE increased only from 27.3 kg/s to 116.0 kg/s.
Fig. 11. Ash temperature change of 73# and 74# FBHEs under different loads.
Under the different loads, the ash temperature distribution characteristics along the ash-flow direction for 73# and 74# FBHEs is displayed in Fig. 11, it can be seen that the ash temperature in each of heat exchanger chambers was monotonically increased with increasing boiler load. When boiler load increases, the amount of coal and air increase accordingly, circulating ash and circulating ash temperature also gradually increased. Along the ash-flow direction of two FBHEs, the circulating ash passes through in turn the empty room, the first heat exchanger chamber (IST2 and HTR) and the second heat exchanger chamber (IST1 and LTS), and there is heat exchange between the circulating ash and the fluidized air as well as the steam in the heat transfer tubes, so that the circulating ash temperature gradually decreases from the empty chamber, the first-stage heat exchange chamber to the second-stage heat exchange chamber. Under the different loads, the ash temperature distribution characteristics of 73# FBHE is different from that of 74# FBHE,
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the flow rate of circulating ash in FBHE is, the smaller the corresponding ash temperature decrease is. It can be seen from Fig. 10 that the circulating ash flow rate of 73# FBHE is higher than that of 74# FBHE under different load conditions, so the ash temperature decrease of 74# FBHE is higher than that of 73# FBHE along the ash flow direction. The maximum ash temperature drop appear in 90 MW condition for 73# FBHE, the ash temperature drop is up to 69 °C from the empty chamber to IST2, and 132 °C from IST2 to ITS1. While for 74# FBHE, the maximum ash temperature drop occurs 150 MW condition, the ash temperature drop is up to 198 °C from the empty chamber to HTR, and 223 °C from HTR to LTS. 3.2. Heat transfer characteristics of FBHEs under different loads
(a) 73# FBHE
(b) 74# FBHE Fig. 12. Fluidization velocity change of 73# and 74# FBHEs under different loads.
the ash temperature decrease of 74# FBHE, along the ash flow direction, was significantly higher than that of 73# FBHE, In the case of constant steam flow and steam heat absorption, the greater
Under the different loads, the fluidization velocity changes of different heat exchanger chambers for 73# and 74# FBHEs are displayed in Fig. 12. Fig. 12(a) shows that the fluidization velocity of the empty chamber is above 0.5 m/s under different loads, at full load, up to 0.73 m/s, much higher than the design value of 0.5 m/s (BMCR), the fluidization velocity of ITS2 is relatively stable, and fluctuates between 0.39 m/s and 0.54 m/s, but the fluidization velocity of ITS1 is relatively high, and fluctuates between 0.53 m/s and 0.87 m/s, for ITS2 and ITS1, the fluidization velocity is higher than the design value of 0.3 m/s (BMCR), it can be seen that the fluidizing velocity in three chambers of 73# FBHE is higher than the design value. Fig. 12(b) shows that the fluidizing velocity in the empty chamber of the 74# FBHE is higher than that of 73# FBHE under different loads, and fluctuates between 0.54 m/s and 0.71 m/s, but the fluidizing velocity of HTR and LTS is lower than that of ITS2 and ITS1, at full load conditions, it is the same for three chambers of the 74# FBHE that the fluidizing velocity is higher than the design value. As the fluidization velocity is high, the wear between the heated surfaces and the high temperature circulating ash is serious, so it is necessary to control the fluidization velocity within the design speed range. The operational parameters of two tested FBHEs under five different loads is showed in Table 5, In general, with the increase of load, the operating parameters of the inlet and outlet for different heating surfaces increase correspondingly, but the operating parameters fluctuate at low load (150 MW). The average heat transfer coefficient of different heat exchanger chambers with
Table 5 Operational parameters of two FBHEs under different loads. Items
73# FBHE
Unit
ITS1
ITS2
74# FBHE
LTS
HTR
Boiler loads 90 MW
150 MW
225 MW
270 MW
300 MW
Inlet steam temperature Outlet steam temperature Inlet steam pressure Outlet steam pressure Steam flow rate Inlet steam temperature Outlet steam temperature Inlet steam pressure Outlet steam pressure Steam flow rate
°C °C MPa MPa t/h °C °C MPa MPa t/h
363.3 407.5 10.9 10.9 170.7 383.0 460.2 10.9 10.8 173.6
345.6 417.0 9.8 9.7 227.6 366.6 477.7 9.6 9.5 236.4
358.4 421.4 14.7 14.6 326.7 382.5 473.4 14.5 14.4 344.1
370.6 422.3 17.3 17.1 404.0 401.2 477.7 17.0 16.9 420.4
371.1 425.3 17.7 17.5 449.5 408.4 480.3 17.4 17.3 466.4
Inlet steam temperature Outlet steam temperature Inlet steam pressure Outlet steam pressure Steam flow rate Inlet steam temperature Outlet steam temperature Inlet steam pressure Outlet steam pressure Steam flow rate
°C °C MPa MPa t/h °C °C MPa MPa t/h
340.7 355.9 11.0 10.9 167.8 446.6 527.4 1.1 1.1 152.9
331.4 346.8 9.9 9.8 218.8 453.5 534.2 1.7 1.7 211.0
353.2 368.7 14.8 14.7 309.3 442.2 538.6 2.6 2.5 306.4
361.1 376.9 17.4 17.3 387.5 429.5 539.2 3.1 3.0 363.9
362.1 376.8 17.9 17.7 432.7 423.8 538.8 3.4 3.3 399.0
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Fig. 13. Average heat transfer coefficient of two FBHEs under different loads.
the boiler load variation is shown in Fig. 13, with the increase of the boiler load, the average heat transfer coefficient of different heat exchanger chambers takes on a monotonically increasing trend, but the increase rate of average heat transfer coefficient for different heat exchanger chamber is different, which is basically consistent with the test results of the literature [11]. In general, the average heat transfer coefficient of heating surfaces in 73# FBHEs (IST2 + ITS1) was higher than that of heating surfaces in 74# FBHEs (HTR + LTS) under different loads, the average heat transfer coefficient of ITS1 is highest among the four heat exchanger chambers. At full load, the average heat transfer coefficient of ITS1 reached 288.0 W/(m2°C). Since the conical valve opening at the entrance of the 73# FBHE is the largest (see Fig. 9), and thus the ash flow rate into ITS1 heat exchanger chamber in the 73# FBHE (see Fig. 10) is high, the high ash concentration and high fluidization velocity of ITS1 enhance the heat transfer process, resulting in a high average heat transfer coefficient. The situation is basically the opposite for the LTS, before 225 MW boiler load, the average heat transfer coefficient of LTS is lowest, after 225 MW boiler load, the average heat transfer coefficient of HTR is lowest. 3.3. Thermal deviation analysis of FBHEs under different loads In order to ensure the uniformity of steam distribution and heat transfer, there are three zones in turn along ash flow direction for the heating surface pipes in the ITS2 and HTR, the pipe dimension of three zone for the ITS2 and HTR is showed in Table 6. The pipe size of three zones for ITS2 is different, but the material of them is same, while for the HTR, the pipe size of three zones for ITS2 is same, the material of them is different. For the 73# FBHE, under the different boiler loads, the wall temperature distribution along the ash flow direction of ITS2 is shown in Fig. 14 (a). At the inlet of ITS2, the circulating ash temperature is decreased due to the heat absorption of fluidized air in the empty
chamber, so that the first third measuring point’s wall temperature of the ITS2 is low under different boiler loads. Along the flow direction, with the further increase of the fluidized air temperature, the heat transfer process between the low temperature heating surface and the high temperature circulating ash is enhanced, so that the wall temperature after the third measuring point of ITS2 is increased. At the same time, the circulating ash temperature along the ash flow direction gradually decreased with the heat transfer, so the wall temperature after ninth measuring point of ITS2 began to decline. In general, the wall temperature along the ash flow direction of ITS2 takes on a tendency to rise first and then decrease, which is consistent with the wall temperature distribution trend of the FBHEs in the 600 MW supercritical CFB boiler [15,16]. The higher the boiler load is, the higher the wall temperature is also, but when the boiler load is up to 225 MW, the wall temperature of ITS2 tends to be stable. Under five different boiler loads, the highest wall temperature point appears on the 7th column tubes in third zone (9# measuring point in Table 3), the lowest wall temperature point appears on the 10th column tubes in third zone (10# measuring point in Table 3), under 90 MW boiler load, the wall temperature fluctuations in the first nine measuring points is not large, when the boiler load increase from 150 MW to 300 MW, the wall temperature fluctuations in the front nine measuring points is very large, the trend of wall temperature fluctuation under different loads is basically the same. Based on the steam pipes design of the FBHEs, all the heating surface tubes of ITS2 are made of U-type header and adopt the partitioned design to ensure the uniform arrangement, in theory, the steam flow distribution is uniform. Therefore, the main reason for the large wall temperature deviation of ITS2 is due to the uneven heat absorption, which is caused by the localized nonuniform flow of circulating ash in ITS2. For the 74# FBHE, under the different boiler loads, the wall temperature distribution along the ash flow direction of HTR is shown in Fig. 14(b). Based on the test results, the wall temperature distribution trend along the ash flow direction of HTR is the same as that of ITS2, that is, the wall temperature along the ash flow direction of HTR takes on a tendency to rise first and then decrease, at the same time, the wall temperature distribution along the ash flow direction is relatively uniform, which is consistent with the measured wall temperature distribution at the tube panel outlets of HTR at different loads in the literature [10,17]. Compared with ITS2, the position of the highest wall temperature point moves forward from the third zone in ITS2 to the second zone in HTR. When the boiler load reaches 270 MW, the wall temperature distribution of HTR tends to be stable, when the boiler load is further increased, the wall temperature changes of HTR at different measuring points are not obvious. Under five different boiler loads, the highest wall temperature point appears on the second column tubes in second zone (4# measuring point in Table 3), the lowest wall temperature point appears on the 10th column tubes in third zone (10# measuring point in Table 3). In general, the flow and heat transfer process of the FBHE in HTR is more uniform than that of ITS2. Under different boiler load conditions, the average wall temperature of ITS2 and HTR respectively
Table 6 Pipe dimension of FBHE heating surface. Component
Number
Pipe size (mm)
Material
ITS2
8ISTA(IZone) 10ISTB(IIZone) 10ISTC(III Zone)
40 50 50
£63.5 10.5 £51.0 8.0 £51.0 7.11
SA-213T91 SA-213T91 SA-213T91
HTR
8HTRA(IZone) 12HTRB(IIZone) 10HTRC(III Zone)
40 60 50
£63.5 5.59 £63.5 5.59 £63.5 5.59
SA-213 PT321H SA-213 PT321H SA-213T91
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(a) ITS2
(a) ITS2
(b) HTR
(b) HTR
Fig. 14. Wall temperature distribution of ITS2 and HTR tubes along ash flow direction.
Fig. 16. Standard deviation of wall temperature of ITS2 and HTR tubes.
Fig. 17. Heat absorption proportion of four FBHEs under different loads.
Fig. 15. Average wall temperature of two heat transfer chambers outlet under different loads.
are shown in Fig. 15. With the increase of boiler load from 90 MW to 300 MW, the average wall temperature of ITS2 and HTR increases accordingly, ITS2 from 457 °C to 481 °C, HTR from 525 °C to 534 °C. In general, the average wall temperature of HTR
is higher than that of ITS2. When the boiler load increases, the bed temperature and the circulating ash temperature rises, the amount of circulating ash increases, the heat transfer process between the circulating ash and the steam is enhanced, so the average wall temperature of ITS2 and HTR rises. Under the different boiler loads, the wall temperature standard deviation within two hours for the ITS2 and HTR is showed in
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Fig. 14), that is, the high wall temperature of the measurement points correspond to the large wall temperature standard deviation, which indicate that when the wall temperature of heating surface is high, the heat transfer process between high temperature circulating ash and low temperature steam is enhanced, at the same time, the nonuniformity of the heat transfer process increases, and the fluctuation and the deviation of wall temperature also increase correspondingly, the above large number of test results show that there are objective facts of non-uniform heat transfer problems, between different measuring points of the same load and different loads of the same measuring point, which is mainly due to the non-uniform heat transfer caused by nonuniform flow of the high temperature circulating ash in the heat exchange chambers. 3.4. Thermal matching characteristics of FBHEs under different boiler loads Fig. 18. Heat absorption proportion of left and right sides FBHEs of the furnace.
Fig. 19. Total heat absorption proportion of four FBHEs under different loads.
Fig. 16. With the increase of boiler loads, the standard deviation of wall temperature tends to decrease gradually. For the ITS2, when the boiler load is less than 225 MW (75% BMCR), the wall temperature standard deviation of the same measuring point was large, especially at 150 MW (50% BMCR) load, the fluctuates of wall temperature standard deviation between the different measuring points is the biggest, fluctuating from 2.9 °C to 5.2 °C, which indicates that the heat transfer process between the high temperature circulating ash and the low temperature steam is unstable at low load conditions (less than 225 MW). When the boiler load increases to 225 MW (75% BMCR), the wall temperature standard deviation is small, fluctuating only from 1.2 °C to 1.9 °C, which indicates that the heat transfer process between the high temperature circulating ash and the low temperature steam is stable. For the HTR, with the increase of boiler loads, the change of wall temperature standard deviation shows a similar phenomenon. That is, under high boiler load conditions (225 MW and above), the standard deviation and the fluctuation of wall temperature is relatively low, under low boiler load conditions (less than 225 MW), the situation is just the opposite. In addition, the high standard deviation of wall temperature for ITS2 appears on the 7th column tubes in third zone (especially 150 MW load), and the high standard deviation of wall temperature for HTR occurs in the 4–7 columns of the first zone and 2 columns of the second zone, under low boiler load conditions (less than 225 MW), the distribution of wall temperature standard deviations for the different measuring points is basically the same as the distribution of wall temperature corresponding measuring points (see
Under the different load conditions, the ratios of the heat absorption of four FBHEs to the boiler effective heat are shown in Fig. 17. It can be seen that the heat absorption proportion of four FBHEs is different with the increase of the boiler load. For the 71# and 74# FBHEs, with the increase of the boiler load, the proportion of heat absorption took on a monotonically increasing trend, As the boiler load increased from 90 MW to 300 MW, the proportion of heat absorption for 71# FBHE increased from 5.11% to 8.57%, and the heat absorption proportion of 74# FBHE increased from 3.83% to 6.79%. However, for the 72# FBHE, under the 270 MW load conditions, the heat absorption proportion reached the maximum of 11.39%, for 73# FBHE, under the 225 MW load conditions, the heat absorption proportion reached a maximum of 10.85%. For five different load conditions, the heat absorption proportion of 72# FBHE is the largest, the heat absorption proportion of 74# FBHE is the smallest. The heat absorption proportion variation of two FBHEs on the left and right sides of the furnace is shown in Fig. 18. Under different load conditions, the heat absorption proportion of two FBHEs (71# and 72#) on the left side of the furnace is larger than that of two FBHEs (73# and 74#) on the right side of the furnace, when boiler loads increased from 90 MW to 300 MW, the total heat absorption proportion of the left two FBHEs (71# and 72#) increased correspondingly from 14.3% to 19.7%, and that of the right two FBHEs (73# and 74#) increased correspondingly from 10.5% to 17.5%, which proves that there are asymmetric heat transfer phenomena in the symmetrical arrangement of four FBHEs, the phenomenon is due to uneven heat transfer caused by uneven flow of high temperature circulating ash in four FBHEs, this problem needs to be given enough attention in the design of the FBHEs in the future. The variation of the total heat absorption proportion in four FBHEs under different load conditions is shown in Fig. 19, the total heat absorption proportion of four FBHEs increases linearly with the increase of boiler loads, when the boiler load was increased from 90 MW (30% BMCR) to 300 MW (100% BMCR), The total heat absorption proportion of four FBHEs increased from 25.20% to 38.20% monotonically. At the full load (300 MW), the total heat absorption proportion (38.20%) of four FBHEs is very close to that of other 300 MW CFB boilers at the same level, which was shown in Table 7 [18]. Under different load conditions, the heat absorption proportion variation of four heat transfer chambers (ITS2, ITS1, HTR and LTS) in four FBHEs is shown in Fig. 20, In general, the heat absorption proportion of the four heat transfer chambers increases accordingly with the increase of boiler loads. Under five different loads, the heat absorption proportion of ITS2 in the four heat exchange chambers is the largest, and the heat absorption proportion ranges
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G. Song et al. / Applied Thermal Engineering 130 (2018) 938–950 Table 7 Comparison of heat absorption proportion of FBHEs with 300 MW boiler. Objects
Poland Lagiza (460 MW)
Poland Turow4-6 (262 MW)
USA JEA (300 MW)
China Baima (300 MW)
This project (300 MW)
Number of FBHE FBHE heat absorption proportion /%
8 30.4
8 33.6
6 40.0
4 38.7
4 38.2
absorption proportion of 72# FBHE (ITS1 + ITS2) and 73# FBHE (ITS1 + ITS2) is relatively high, which is consistent with the previous analysis results shown in Fig. 17. Based on the above test results, in order to improve the heat transfer nonuniformity of four FBHEs, it is necessary to improve the arrangement of the heating surfaces in four FBHEs, The heating surface of the serpentine tubes is changed from the direction perpendicular to the ash flow direction to parallel to the ash flow direction (see Fig. 21), and set aside some maintenance space, at the same time, it is suggested to optimize the fluidized air distributor structure of four FBHEs, the above test results can provide some references for the optimal design and operation of 600 MW grade supercritical CFB boiler with FBHEs.
Fig. 20. Heat absorption proportion of four heat transfer chambers under different loads.
from 10.66% to 12.03%, the heat absorption proportion of LTS is the smallest, and the heat absorption proportion range from 2.86% to 6.35%, which is consistent with the previous analysis about heat absorption proportion test results of four FBHEs with the boiler load changes (see Figs. 10 and 11). Under different boiler load conditions, the heat absorption proportion of each heat transfer chamber is different, when the boiler load increases from 90 MW to 300 MW, the increase of heat absorption proportion in ITS1 is the largest in the four heat transfer chambers, reaching 5.56%, and the increase of heat absorption proportion in ITS2 is the smallest, only 1.37%. When the boiler load increases to 150 MW and 225 MW, the heat absorption proportion of ITS2 is about 12.0%, which is not affected basically by the boiler load. Under different load conditions, the heat absorption proportion of ITS2 is the largest, and the heat absorption increase of ITS1 is the biggest, compared with the heat absorption proportion of 71# FBHE (HTR + LTS) and 74# FBHE (HTR + LTS), the heat
(a)Old arrangement of heating surface
4. Conclusion The tests of the wall temperature distribution in two fluidized bed heat transfer chambers outlet were carried out in 300 MW subcritical CFB boiler under different boiler loads, some important key parameters are obtained, and the main conclusions are summarized as follows. (1) The measurement results indicate that four FBHEs in the 300 MW subcritical CFB boiler are of a very good heat transfer performance, it is one of very effective methods to solve the inadequate heating surface for the large-scale circulating fluidized bed boilers. (2) For four FBHEs, there is a good linear correspondence among boiler loads and the conical valve openings as well as circulating ash flow rate. With the increase of the boiler load, the average heat transfer coefficient of different heat exchanger chambers takes on a monotonically increasing trend, under five different loads, the average heat transfer coefficient of ITS1 is highest, and that of LTS is the lowest.
(b) New arrangement of heating surface
Fig. 21. Optimized layout of heating surface in FBHEs.
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(3) Under the different boiler load conditions, the wall temperature distribution at the outlet of the different heat exchange chambers is different, there is an non-uniform heat transfer process between the different measuring points of the same load as well as the different loads of the same measuring point, the wall temperature test results show that the heating surface arrangement of HTR is more reasonable. (4) Under five different boiler load conditions, the heat absorption proportion of 72# FBHE is the largest, and that of 74# FBHE is the smallest. The heat absorption proportion of two FBHEs on the left side is larger than that of two FBHEs on the right side of the furnace, there is a non-uniform heat transfer phenomena in the symmetrical arrangement of four FBHEs. (5) In order to improve the non-uniform heat transfer between high temperature circulating ash and low temperature steam in four FBHEs, it is recommended the fluidizing air velocity of the heat exchange chambers is reduced to the design value. Simultaneously, the heating surface arrangement direction in four FBHEs was adjusted from snake tube perpendicular to the ash flow direction to parallel to ash flow direction, the above test results can provide a good reference for the optimized design of 600 MW supercritical or ultra supercritical CFB boiler with the FBHEs in the future.
Acknowledgments This work is financially supported by the National Key Research & Development Program of China, Grant No. 2016YFB0600202. References [1] G. Yue, R. Cai, J. Lu, H. Zhang, From a CFB reactor to a CFB boiler – the review of R&D progress of CFB coal combustion technology in China, Powder Technol. 316 (2017) 18–28.
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