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Influence of CFB (circulating fluidized bed) boiler bottom ash heat recovery mode on thermal economy of units
Energy 35 (2010) 3863e3869
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Influence of CFB (circulating fluidized bed) boiler bottom ash heat recovery mode on thermal economy of units Bing Zeng*, Xiaofeng Lu**, Hanzhou Liu Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Chongqing University, Ministry of Education, Chongqing 400044, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 January 2010 Received in revised form 24 May 2010 Accepted 25 May 2010 Available online 23 June 2010
CFB (circulating fluidized bed) boiler bottom ash contains large amounts of physical heat. A BAC (bottom ash cooler) is often used to treat high temperature bottom ash to reclaim heat, and to have the ash easily transported. The unit thermal economic indicators of three CFB power plants in China were derived based on heat balance calculation and analysis on the principled thermal system in turbine heat acceptance condition, taking the influence of two different bottom ash heat recovery modes into account. One of the two bottom ash heat recovery modes was the FBAC (fluidized bed ash cooler) mode, and the other was the RAC (rolling-cylinder ash cooler) mode. The results indicated that two modes both improved the thermal economy of units. Compared with the RAC mode, the FBAC mode obtained higher plant thermal efficiency, lower plant heat rate and less standard coal consumption. The standard coal consumption rate of the FBAC mode was less nearly 2 g/(kW h) than the RAC mode in the three CFB power plants, when the net calorific power of standard coal was 29.27 MJ/kg. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: CFB (circulating fluidized bed) BAC (bottom ash cooler) Heat recovery Regenerative system Thermal economy
1. Introduction CFB (Circulating fluidized bed) boiler bottom ash contains large amounts of physical heat. The physical heat loss is up to 2% without cooling the bottom ash while the boiler combusts a low-calorie fuel with more than 30% ash content. In addition, the red-hot bottom ash is not conducive to be mechanized for handling and transportation, as the upper limit temperature of ash handling machinery is mostly between 150 and 200 C. Consequently, a BAC (bottom ash cooler) is often used to treat high temperature bottom ash to reclaim heat, and to have the ash easily transported [1,2]. There are many kinds of BACs equipped for large-scale CFB boilers with the continuous development and improvement of CFB boiler, such as water-cooled ash cooling screw [2,3], RAC (rollingcylinder ash cooler) [2e5], FBAC (fluidized bed ash cooler) [6e9] and high-strength steel belt ash cooler [10]. The RAC and FBAC have large capacity, and have been commonly and reasonably applied in China. The cooling medium applied to recover the heat of the bottom ash in a BAC could be condensation water, cold air or combined cold air and condensation water. Generally, the condensation water was usually used in a RAC to cool the bottom ash while the
* Corresponding author. Tel./fax: þ86 23 65102475. ** Corresponding author. E-mail addresses: [email protected] (B. Zeng), xfl[email protected] (X. Lu). 0360-5442/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2010.05.040
condensation water and cold air were used in an FBAC to do that. The single-use of condensation water to cool the bottom ash has no guarantee on satisfactory results for some CFB boilers combusted the very high-ash coal (the ash content even greater than 70%) in China. More seriously, sometimes all of the condensation water may be not sufficient for cooling the bottom ash, which brings about a great impact on the turbine regenerative system. Thus the condensation water and cold air had to be used as the cooling medium to cool the bottom ash together. Up to now, researches on the BAC were mainly focused on the adaptability of bottom ash particle size, characteristics of gasesolid flow and ash discharging, as well as pressure distribution [11e15]. Those researches were tended to the actual operational aspects of a power plant boiler system. However, there was little investigation on the influence of bottom ash heat recovery mode on thermal economy of units. In Garrea et al.’s study [16], a new dry ash removal system was proposed. A portion of the combustion air was used to cool ash. It returned some heat loss back to the furnace. The actual data from operating units and various heat balances were provided to demonstrate the actual performance conditions about ash cooling vs. boiler efficiency effects. It was only concerned that the overall effect on the boiler efficiency vs. the required quantity of ash cooling air, but did not investigate the influence of other ash cooling modes on the plant thermal economy. Zhang et al. [17] researched a water-cooled waste heat recovery system in a 410t/h CFB boiler, and analyzed the economic benefits
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Nomenclature BAC BFPT DTR FBAC HP LP RAC SG THA Z bs d0 h h0 h q0 qcp qL qrh tfw wi
bottom ash cooler boiler feed pump turbine deaerator fluidized bed ash cooler high pressure (heater) low pressure (heater) rolling-cylinder ash cooler shaft gland steam-leakage heater turbine heat acceptance the steps of steam extraction standard coal consumption, g/(kW h) specific steam consumption, kg/(kW h) specific enthalpy, kJ/kg the specific enthalpy of main steam, kJ/kg the specific enthalpy of extraction steam, kJ/kg turbine heat rate, kJ/(kW h) plant heat rate, kJ/(kW h) the net calorific power of standard coal, MJ/kg the heat absorption of reheat steam, kJ/kg temperature of feedwater, C specific internal work, kW
Greek letter coefficient or rate
ag a h hg hcp he D Dbs q, d
gland packing-leakage rate extraction coefficient efficiency generator efficiency plant thermal efficiency turbine absolute electric efficiency the plant standard coal consumption difference between FBAC mode and RAC mode, g/(kW h) the plant standard coal consumption difference between FBAC (or RAC) mode and “No” mode, g/(kW h) the terminal temperature difference of extraction heaters, C
Sub-scripts b boiler c exhaust steam (or condensate) d heater drainage ej valve stem steam-leakage fw-in inlet feedwater fw-out outlet feedwater H heater m mechanical p piping rh reheat sg shaft gland steam-leakage
a
gained by transferring the waste heat of high temperature bottom ash to the turbine regenerative system. In this study, effects on the plant thermal economy resulted from the parallel installation of BAC and different low-pressure heaters were also calculated by equivalent enthalpy drop method. However, the differences in thermal economy of units caused by different heat recovery modes were not included. At present, the practical operating situation of CFB boilers in China shows that, the increase in ash content of coal resulted in a substantial increase in the amount of bottom ash. Much more cooling medium (water or air) was required to cool the bottom ash. The difference between the influences of different cooling modes on thermal economy of units also increased accordingly. Therefore, it was necessary to do some in-depth investigations on the influence of bottom ash heat recovery modes on the thermal economy of units, which was significant to improve the unit thermal economy. In this paper, we have conducted the unit thermal economy based on the heat balance calculation and analysis of three typical CFB power plants with FBAC or RAC to recover the heat of boiler bottom ash. Thermal economic indicators of the three plants with different bottom ash heat recovery modes have also been obtained. 2. Calculative bases and fundamental assumptions Fig. 1(a)illustrates the principled heat flow diagram of a 150 MWe CFB power plant (unit I) located in Panzhihua, Sichuan province, Fig. 1(b) represents that of Baima 300 MWe CFB demonstrational power plant (unit II) constructed in Baima, Sichuan province, and Fig. 1(c) displays that of a 600 MWe CFB power plant (unit III) in design stage which will be built in Baima, Sichuan province, China in 2012. Unit I used a CFB boiler (DG460/ 13.73-II4) manufactured by DongFang Boiler Group Co., Ltd, and equipped with a reheat condensing turbine (N150-13.24/535/535) made by Shanghai Turbine Co., Ltd. ALSTOM supplied the 300 MWe
CFB boiler (unit II), with most of the manufacturing being done in China and subcontracted to DongFang Boiler Group [18]. DongFang Turbine Co., Ltd provided two reheat condensing turbines N30016.7/537/537 and N600-24.2/566/566 for unit II and unit III, respectively. The supercritical CFB boiler of unit III was designed by DongFang Boiler Group Co., Ltd. The terminal temperature difference (q and d) of extraction heaters are also shown in Fig. 1, as well as the inleteoutlet position of the cooling water system of BACs. All the three CFB power plants are only used for electric power generation. General technical parameters of the three different grade CFB power plants in THA condition and the main design parameters of BACs are shown in Tables 1 and 2, respectively. The following fundamental assumptions were made for the calculation and analysis: Changes of extracted steam parameters (pressure and temperature) were neglected in calculations. The mechanical efficiency (hm) and the generator efficiency (hg) were selected as 99.5% and 99%, respectively. The 1# LP1 (low-pressure heater) of 150 MWe unit, the LP1 and LP2 of 300 MWe and 600 MWe units were all located in the condenser throat of their respective systems. The feed pump of 150 MWe was driven by an electric motor, while the feed pumps of 300 MWe and 600 MWe units were driven by BFPT (boiler feed pump turbines). The steam sealing systems of the three units were all used selfsealing systems. The boiler efficiency was calculated with 150 C of bottom ash temperature, and the value was initially assumed as a constant. It should be noted that if the FBAC was used to reclaim the heat of the bottom ash, the boiler efficiency had to be modified. The amount of the bottom ash was calculated according to the designed coal of a CFB boiler. The designed coal characteristics of the three CFB power plants were shown in Table 3.
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a
b
c Fig. 1. Principled heat flow diagrams of three CFB power plants.
The cooling water of BAC was taken from the main condensation water while the amount was determined by the heat balance of BAC. A material balance equation could be derived by taking the cooling water system of BAC as a branch of the condensation water system. Based on the above conditions, plant thermal economic indicators (e.g. plant heat rate, plant thermal efficiency and standard coal consumption) were calculated by employing the constant power algorithm [19].
d0 ¼
3600 ¼ wi hm hg
3600 h0 þ arh qrh
z X
and hfw-out represent the specific enthalpy of inlet and outlet feedwater, respectively, hdi and hd(iþ1) represent the specific enthalpy of the heater’s drainage and higher heater’s drainage, respectively. The drainages of regenerative heaters (HP heaters and LP heaters) were the sequential self-flow. Therefore, the extraction coefficients of the regenerative heaters were determined in series by employing Eq. (1). The specific steam consumption (d0) may be calculated as: where, wi is the specific internal work, arh is the reheat factor, qrh is
!
(2)
ai hi ac hc aej hej ag hg asg hsg hm hg
1
As a regenerative heater is a surface heater, the energy balance equation for the regenerative heater can be given as:
ai hi þ aH hfwin þ adðiþ1Þ hdðiþ1Þ ¼ aH hfwout þ adi hdi
(1)
where, ai represents the extraction coefficient, aH represents the flow rate of feedwater, ad(iþ1) and adi represent higher heater’s drainage coefficient and the heater’s drainage coefficient, respectively, hi represents the specific enthalpy of extraction steam, hfw-in
the heat absorption of reheat steam, Z is the step of steam extraction, h0, hc, hej, hg and hsg are the specific enthalpy of main steam, exhaust steam, valve stem steam-leakage, gland packing-leakage and shaft gland steam-leakage, respectively. The turbine heat rate (q0) is given by,
q0 ¼ d0 h0 hfw þ arh qrh
here hfw is the specific enthalpy of boiler feedwater.
(3)
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Table 1 Technical parameters of three CFB power plants in THA condition. Properties
Steam turbine parameters (rated power/pressure of main steam/temperature of main steam/ temperature of reheat steam) Exhaust steam parameters (Pressure/Specific enthalpy) Extraction Parameters HP3 (to 3# hp heater) (Pressure/Temperature) HP2 (to 2# hp heater) HP1 (to 1# hp heater) De (to deaerator) BFPT (to BFPT) LP4 (to 4# lp heater) LP3 (to 3# lp heater) LP2 (to 2# lp heater) LP1 (to 1# lp heater) Feedwater temperature, tfw Shaft gland steam-leakage rate, asg Gland packing-leakage rate, ag Valve stem steam-leakage rate, aej Pressure loss coefficient of extraction line Boiler parameters (maximum continue rate e pressure of superheated steam/temperature of superheated steam/temperature of reheat steam) Make-up water rate The amount of bottom ash Boiler efficiency, hb
Values I
II
III
150 MWe/13.24 MPa /535 C/535 C
300 MWe/16.7 MPa/537 C/537 C
600 MWe/24.2 MPa/566 C/566 C
0.0058 MPa/2404.82 kJ/kg
0.0064 MPa/2448.98 kJ/kg
0.0055 MPa/2311.8 kJ/kg
0.01 4% 460t/he13.73 MPa/540 C/540 C
3% 1900t/he25.4 MPa/571 C/569 C
0% 37.5t/h 0.89
0% 60t/h 0.91
0% 120t/h 0.92
0.5552 MPa/335 C 0.2304 MPa/231.6 C 0.0579 MPa/101.9 C 243.85 C 0. 0017
Then the turbine absolute electric efficiency (he) can be written as:
he ¼ 3600=q0
(4)
The plant heat rate (qcp) and the plant thermal efficiency (hcp) may be expressed as follows:
qcp ¼
3600
(5)
hb hp he
hcp ¼ hb hp he
(6)
where, hb is the boiler efficiency and hp is the piping thermal efficiency. When the net calorific power of standard coal (qL) was equal to 29.27 MJ/kg, the standard coal consumption of a power plant (bs) could be calculated by the equation given as follows:
bs ¼
3600 3600 ¼ z0:123=hcp kg=ðkW: hÞ qL hcp 29 270hcp
(7)
3. Results and discussion Based on the constant power algorithm, the thermal economic indicators of a condensing power plant mainly include specific
steam consumption (d0), turbine heat rate (q0), plant heat rate (qcp), plant thermal efficiency (hcp) and standard coal consumption (bs). The calculated results of the thermal economic indicators in THA condition were shown in Table 4, while different BACs were equipped in CFB power plants. The “No” condition in Table 4 could be considered as the bottom ash were cooled by external working substance, which had no influence on the turbine regenerative system. The results shown in Table 4 reveal that, compared with the FBAC heat recovery mode, the RAC mode has less specific steam consumption (d0), lower turbine heat rate (q0), better turbine-side thermal economy besides higher exhaust coefficient (condensate rate) (ac). The reason was that the RAC mode had larger consumption of condensation water. The larger consumption of condensation water induced less condensation water flowing through the last stage low-pressure (LP) heater(s) and reduced the steam extraction flow (shown in Fig. 2), namely the steam extraction of last stage LP heater(s) was pushed out. The last stage LP heater(s) referred to LP1 of the 150 MWe power plant, LP1 and LP2 of the 300 MWe and 600 MWe power plants. Simultaneously, more steam did expansion work in the turbine resulted in larger condensing steam flow and greater condensation loss. On the other hand, as the steam turbine power remained unchanged, the work
Table 3 The designed coal analysis on as-received basis of the three CFB power plants. Values
Table 2 The main design parameters of BACs. Properties
Inlet temperature of cooling water ( C) Outlet temperature of cooling water ( C) Water cooling ratio Inlet temperature of bottom ash ( C) Outlet temperature of bottom ash ( C)
6.687 MPa/370.9 C 4.418 MPa/315.3 C 2.164 MPa/471.5 C 1.086 MPa/370.1 C 1.086 MPa/370.1 C 0.367 MPa/236.9 C 0.197MPa/170.1 C 0.102MPa/107.4 C 0.046 MPa/79.2 C 282.4 C 0.0008 0.0066
5.78 MPa/383.1 C 3.527 MPa/318.1 C 1.652 MPa/438.9 C 0.796 MPa/336.6 C 0.796 MPa/336.6 C 0.467 MPa/272 C 0.259 MPa/205.2 C 0.136 MPa/140.2 C 0.063 MPa/87.4 C 273 C 0.001 0.00535 0.0029 3% 1025t/he17.4 MPa/540 C/540 C
3.7133 MPa/358.5 C 2.6032 MPa/313.4 C 1.0233MPa/416.4 C
Values FBAC
RAC
40 80 0.6 900 150
40 80 1 900 150
Carbon, Car Hydrogen, Har Oxygen, Oar Nitrogen, Nar Sulfur, Sar Moisture, Mar Ash, Aar The net calorific power, Qnet.ar
I
II and III
33.27% 1.97% 3.42% 0.59% 0.24% 8.00% 52.51% 12.12 MJ/kg
49.20% 2.09% 1.65% 0.56% 3.54% 7.69% 35.27% 18.50 MJ/kg
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Table 4 The calculated results of thermal economic indicators in THA condition for three CFB power plants equipped different BACs. Indicators
I (150 MWe)
Cooling water flow rate of BAC, Dbac, t/h Exhaust coefficient, ac Specific steam consumption, d0, kg/(kW. h) Turbine heat rate, q0, kJ/(kW. h) Turbine absolute electric efficiency, he Boiler efficiency, hb Plant thermal efficiency, hcp Plant heat rate, qcp, kJ/(kW. h) Plant standard coal consumptiona, bs, g/(kW. h) a
II (300 MWe)
III (600 MWe)
No
FBAC
RAC
No
FBAC
RAC
No
FBAC
RAC
0 0.6906 2872.6 8068.0 0.4462 0.89 0.3938 9141.5 312.3
100.9 0.7079 2862.0 8038.1 0.4479 0.8992 0.3994 9014.2 308.0
168.1 0.7194 2854.9 8018.2 0.4490 0.89 0.3963 9085.2 310.4
0 0.6059 3012.2 7918.1 0.4547 0.91 0.4124 8730.3 298.3
161.4 0.6188 3005.7 7901.0 0.4556 0.9176 0.4167 8639.7 295.2
269.0 0.6277 3001.3 7889.4 0.4563 0.91 0.4139 8698.7 297.2
0 0.5789 2814.5 7497.8 0.4801 0.92 0.4401 8179.8 279.5
322.8 0.5930 2807.6 7479.4 0.4813 0.9280 0.4450 8089.5 276.4
538.1 0.6026 2802.9 7466.9 0.4821 0.92 0.4419 8146.1 278.3
The net calorific power of standard coal, qL is equal to 29.27 MJ/kg.
augmentation of the turbine accordingly caused the reduction of the specific steam consumption (d0). In addition, the positive effect of “specific steam consumption (d0) reduction” was above the negative effect of “condensation loss increment”. As a result, the turbine heat rate (q0) would be decreased and the turbine absolute electric efficiency (he) would be increased in the condition of assuming the main steam and feedwater parameters as constants (except the main steam flow). It could be known that the FBAC mode has a better thermal economy on boiler-side than RAC mode from Table 4. It could be attributed to that the fluidizing air of the FBAC was directly returned to the furnace after absorbing partial physical heat of the bottom ash, which really helped heat recovery from the bottom ash and distinctly improved the boiler efficiency (hb). Plant thermal efficiency (hcp), plant heat rate (qcp) and plant standard coal consumption (bs) are three common thermal economic indicators of condensing power plant. From Figs. 3 and 4, it could be seen that the hcp increased linearly with the increase of rated power whatever the heat recovery mode was. On the contrary, the qcp decreased linearly with the increase of rated power. It was consistent with the general knowledge: the higher the parameters, the higher the energy conversion efficiency. It could also be concluded from Table 4, Figs. 3 and 4 that, compared with utilizing external working substance to cool the bottom ash (i.e. the “No” condition), the FBAC mode and the RAC mode both improved the hcp, and the FBAC mode had the higher hcp. The FBAC mode ameliorated not only the turbine absolute electric efficiency (he) but also the boiler efficiency (hb), while the RAC mode only meliorated the he. As the steam extraction of last stages
0.06
Dbs ¼ bsFBAC bsNo
(8)
or
Dbs ¼ bsRAC bsNo
(9)
Fig. 5 shows the calculated results about variation of Dbs with rated power for each unit. The legend “D” in Fig. 5 refers to the plant standard coal consumption difference between FBAC and RAC mode, thus,
D ¼ bsFBAC bsRAC
(10)
As shown in Fig. 5, using the FBAC to recover physical heat of the bottom ash saved more standard coal consumption than using the RAC. And Dbs decreased with the increase of rated power from 150 MWe to 300 MWe, then increased slightly from 300 MWe to 600 MWe. In addition, the FBAC mode had higher Dbs than the RAC
No FBAC RAC
0.05
0.45 0.44
0.04
0.43
ηcp
Extraction coefficient ( i)
heater(s) reduced, the total amount of the turbine steam admission (i.e. the specific steam consumption) decreased accordingly in the condition of the turbine power remained constant and hence the turbine efficiency decreased. Furthermore, the increment of hb was much higher than the increment of he (from Table 4). Thereby combined the two increments, the FBAC mode had the better hcp. The plant standard coal consumption bs is the most comprehensive indicator for evaluating technological perfection level of energy conversion process in a condensing power plant. We had defined the saving of the plant standard coal consumption Dbs as the plant standard coal consumption difference between FBAC (or RAC) mode and “No” mode. The equations were given as:
0.03
0.42
0.02
0.41
0.01
0.40
No FBAC RAC
0.39
0.00 LP3 LP2 LP1
LP4 LP3 LP2 LP1
LP4 LP3 LP2 LP1
150MWe
300MWe
600MWe
Fig. 2. Extraction coefficient (ai) of low-pressure heaters for each unit in different heat recovery modes.
150
300
450
600
Rated power (MWe) Fig. 3. Plant thermal efficiency (hcp) with rated power for each unit in different heat recovery modes.
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9200 9000
qcp (kJ/(kW. h))
mode, approximately 2 g/(kW h), when the net calorific power of standard coal was 29.27 MJ/kg. Moreover, the 150 MWe power plant had the highest Dbs with value of 4.35 g/(kW h) and brought considerable economic benefits. It was because that the design coal of 150 MWe power plant had higher ash content than the other two plants (see Table 3). It could be seen that the higher the ash content of coal as fired, the greater the Dbs. The 300 MWe CFB power plant was taken for an example to compare the economic benefits. The FBAC mode could save the cost of coal as fired about 2.34 million yuan RMB per year than the RAC mode, on the assumption that the plant operated 200 days annually at full load and the net calorific power of coal cost 500 yuan/t (including tax) was 18 MJ/kg.
No FBAC RAC
8800 8600 8400 8200 8000 150
300
450
600
4. Suggestions
Rated power (MWe) Fig. 4. Plant heat rate (qcp) with rated power for each unit in different heat recovery modes.
Fig. 5. Dbs with rated power for each unit in FBAC and RAC modes.
(1) The waste heat recovered in a power plant system was mostly made an entry into the turbine regenerative system and hence pushed partial steam extraction aside, which engendered additional condensation loss and reduced the turbine absolute internal efficiency. Accordingly, the regenerative steam extraction should not be affected, no matter which kind of BAC was used to reclaim heat. However, as long as the BAC used condensation water as cooling medium, the turbine regenerative system was bound to be adversely affected. The singly use of cold air for cooling could bring such problems as low heat transfer coefficient and large air consumption. This part of air returned to the furnace would have a great impact on combustion conditions. Therefore, in order to reach the optimal plant thermal economy for reclaiming heat of the bottom ash, it had to take the boiler, steam turbine and BAC as a whole system to make an integrated design. (2) In China, the high-ash content of coal as fired and the large amount of stones mixed in the coal resulted in a large percentage (the largest being 15%) of coarse slag in the bottom ash, while the biggest particle size of the bottom ash was even up to 200 mm. The coarse slag was referred to the bottom ash whose particle size was larger than the required biggest particle size (about 8 mm) of the coal as fired. As a result, the problems of slag bridging and building occurred frequently
Bottom ash inlet
Coarse slag discharging outlet
Seperation chamber
Return air outlet
Water-cooled chamber I
Water-cooled chamber II
Accident ash discharging outlet Fig. 6. The cutaway view of a mixed-flow BAC.
Main ash discharging outlet
B. Zeng et al. / Energy 35 (2010) 3863e3869
when the traditional FBAC was used to treat such bottom ash. However, using the patent technology of mixed-flow BAC (as shown in Fig. 6) [20] could solve the problems encountered by the coarse bottom ash in an FBAC, through the separation and respective cooling of the coarse and fine bottom ash. The coarse bottom ash was cooled by air while the fine bottom ash was cooled by both air and water. Such a patent mixed-flow BAC had been put into operation in a 300 MWe CFB boiler. 5. Conclusions In this paper, the influence of two different bottom ash heat recovery modes on thermal economy of three CFB power plants in THA condition had been calculated and analyzed based on heat balance and constant power algorithm. The heat recovery of waste heat in power plant had also been discussed. Some primary conclusions were summarized as follows: In the three CFB power plants, both FBAC mode and RAC mode improved the thermal economy of units. Besides, the FBAC mode had higher plant thermal efficiency (hcp), lower plant heat rate (qcp) and less standard coal consumption (bs) than the RAC mode. The FBAC mode could save more bs, approximately 2 g/(kW h). No matter which kind of BAC was used to reclaim heat, it should not affect the power plant systems except boiler, especially the turbine regenerative system. Therefore, in order to achieve the optimal plant thermal economy for reclaiming heat of bottom ash, it had to take the boiler, steam turbine and BAC as a whole system to make an integrated design. Acknowledgements Financial support of this work by the Key Project of the National Eleventh-Five Year Research Program of China (2006BAA03B02-06) is gratefully acknowledged. References [1] Cen KF, Ni MJ, Luo ZY, Yan JH, Chi Y, Fang MX, et al. Theory design and operation of circulating fluidized bed boiler. Beijing: China Electric Power Press; 1997 [in Chinese]. [2] Lu XF. Large-scale circulating fluidized bed boiler equipment and operation. Beijing: China Electric Power Press; 2006 [in Chinese].
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Influence of CFB (circulating fluidized bed) boiler bottom ash heat recovery mode on thermal economy of units Bing Zeng*, Xiaofeng Lu**, Hanzhou Liu Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Chongqing University, Ministry of Education, Chongqing 400044, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 January 2010 Received in revised form 24 May 2010 Accepted 25 May 2010 Available online 23 June 2010
CFB (circulating fluidized bed) boiler bottom ash contains large amounts of physical heat. A BAC (bottom ash cooler) is often used to treat high temperature bottom ash to reclaim heat, and to have the ash easily transported. The unit thermal economic indicators of three CFB power plants in China were derived based on heat balance calculation and analysis on the principled thermal system in turbine heat acceptance condition, taking the influence of two different bottom ash heat recovery modes into account. One of the two bottom ash heat recovery modes was the FBAC (fluidized bed ash cooler) mode, and the other was the RAC (rolling-cylinder ash cooler) mode. The results indicated that two modes both improved the thermal economy of units. Compared with the RAC mode, the FBAC mode obtained higher plant thermal efficiency, lower plant heat rate and less standard coal consumption. The standard coal consumption rate of the FBAC mode was less nearly 2 g/(kW h) than the RAC mode in the three CFB power plants, when the net calorific power of standard coal was 29.27 MJ/kg. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: CFB (circulating fluidized bed) BAC (bottom ash cooler) Heat recovery Regenerative system Thermal economy
1. Introduction CFB (Circulating fluidized bed) boiler bottom ash contains large amounts of physical heat. The physical heat loss is up to 2% without cooling the bottom ash while the boiler combusts a low-calorie fuel with more than 30% ash content. In addition, the red-hot bottom ash is not conducive to be mechanized for handling and transportation, as the upper limit temperature of ash handling machinery is mostly between 150 and 200 C. Consequently, a BAC (bottom ash cooler) is often used to treat high temperature bottom ash to reclaim heat, and to have the ash easily transported [1,2]. There are many kinds of BACs equipped for large-scale CFB boilers with the continuous development and improvement of CFB boiler, such as water-cooled ash cooling screw [2,3], RAC (rollingcylinder ash cooler) [2e5], FBAC (fluidized bed ash cooler) [6e9] and high-strength steel belt ash cooler [10]. The RAC and FBAC have large capacity, and have been commonly and reasonably applied in China. The cooling medium applied to recover the heat of the bottom ash in a BAC could be condensation water, cold air or combined cold air and condensation water. Generally, the condensation water was usually used in a RAC to cool the bottom ash while the
* Corresponding author. Tel./fax: þ86 23 65102475. ** Corresponding author. E-mail addresses: [email protected] (B. Zeng), xfl[email protected] (X. Lu). 0360-5442/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2010.05.040
condensation water and cold air were used in an FBAC to do that. The single-use of condensation water to cool the bottom ash has no guarantee on satisfactory results for some CFB boilers combusted the very high-ash coal (the ash content even greater than 70%) in China. More seriously, sometimes all of the condensation water may be not sufficient for cooling the bottom ash, which brings about a great impact on the turbine regenerative system. Thus the condensation water and cold air had to be used as the cooling medium to cool the bottom ash together. Up to now, researches on the BAC were mainly focused on the adaptability of bottom ash particle size, characteristics of gasesolid flow and ash discharging, as well as pressure distribution [11e15]. Those researches were tended to the actual operational aspects of a power plant boiler system. However, there was little investigation on the influence of bottom ash heat recovery mode on thermal economy of units. In Garrea et al.’s study [16], a new dry ash removal system was proposed. A portion of the combustion air was used to cool ash. It returned some heat loss back to the furnace. The actual data from operating units and various heat balances were provided to demonstrate the actual performance conditions about ash cooling vs. boiler efficiency effects. It was only concerned that the overall effect on the boiler efficiency vs. the required quantity of ash cooling air, but did not investigate the influence of other ash cooling modes on the plant thermal economy. Zhang et al. [17] researched a water-cooled waste heat recovery system in a 410t/h CFB boiler, and analyzed the economic benefits
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Nomenclature BAC BFPT DTR FBAC HP LP RAC SG THA Z bs d0 h h0 h q0 qcp qL qrh tfw wi
bottom ash cooler boiler feed pump turbine deaerator fluidized bed ash cooler high pressure (heater) low pressure (heater) rolling-cylinder ash cooler shaft gland steam-leakage heater turbine heat acceptance the steps of steam extraction standard coal consumption, g/(kW h) specific steam consumption, kg/(kW h) specific enthalpy, kJ/kg the specific enthalpy of main steam, kJ/kg the specific enthalpy of extraction steam, kJ/kg turbine heat rate, kJ/(kW h) plant heat rate, kJ/(kW h) the net calorific power of standard coal, MJ/kg the heat absorption of reheat steam, kJ/kg temperature of feedwater, C specific internal work, kW
Greek letter coefficient or rate
ag a h hg hcp he D Dbs q, d
gland packing-leakage rate extraction coefficient efficiency generator efficiency plant thermal efficiency turbine absolute electric efficiency the plant standard coal consumption difference between FBAC mode and RAC mode, g/(kW h) the plant standard coal consumption difference between FBAC (or RAC) mode and “No” mode, g/(kW h) the terminal temperature difference of extraction heaters, C
Sub-scripts b boiler c exhaust steam (or condensate) d heater drainage ej valve stem steam-leakage fw-in inlet feedwater fw-out outlet feedwater H heater m mechanical p piping rh reheat sg shaft gland steam-leakage
a
gained by transferring the waste heat of high temperature bottom ash to the turbine regenerative system. In this study, effects on the plant thermal economy resulted from the parallel installation of BAC and different low-pressure heaters were also calculated by equivalent enthalpy drop method. However, the differences in thermal economy of units caused by different heat recovery modes were not included. At present, the practical operating situation of CFB boilers in China shows that, the increase in ash content of coal resulted in a substantial increase in the amount of bottom ash. Much more cooling medium (water or air) was required to cool the bottom ash. The difference between the influences of different cooling modes on thermal economy of units also increased accordingly. Therefore, it was necessary to do some in-depth investigations on the influence of bottom ash heat recovery modes on the thermal economy of units, which was significant to improve the unit thermal economy. In this paper, we have conducted the unit thermal economy based on the heat balance calculation and analysis of three typical CFB power plants with FBAC or RAC to recover the heat of boiler bottom ash. Thermal economic indicators of the three plants with different bottom ash heat recovery modes have also been obtained. 2. Calculative bases and fundamental assumptions Fig. 1(a)illustrates the principled heat flow diagram of a 150 MWe CFB power plant (unit I) located in Panzhihua, Sichuan province, Fig. 1(b) represents that of Baima 300 MWe CFB demonstrational power plant (unit II) constructed in Baima, Sichuan province, and Fig. 1(c) displays that of a 600 MWe CFB power plant (unit III) in design stage which will be built in Baima, Sichuan province, China in 2012. Unit I used a CFB boiler (DG460/ 13.73-II4) manufactured by DongFang Boiler Group Co., Ltd, and equipped with a reheat condensing turbine (N150-13.24/535/535) made by Shanghai Turbine Co., Ltd. ALSTOM supplied the 300 MWe
CFB boiler (unit II), with most of the manufacturing being done in China and subcontracted to DongFang Boiler Group [18]. DongFang Turbine Co., Ltd provided two reheat condensing turbines N30016.7/537/537 and N600-24.2/566/566 for unit II and unit III, respectively. The supercritical CFB boiler of unit III was designed by DongFang Boiler Group Co., Ltd. The terminal temperature difference (q and d) of extraction heaters are also shown in Fig. 1, as well as the inleteoutlet position of the cooling water system of BACs. All the three CFB power plants are only used for electric power generation. General technical parameters of the three different grade CFB power plants in THA condition and the main design parameters of BACs are shown in Tables 1 and 2, respectively. The following fundamental assumptions were made for the calculation and analysis: Changes of extracted steam parameters (pressure and temperature) were neglected in calculations. The mechanical efficiency (hm) and the generator efficiency (hg) were selected as 99.5% and 99%, respectively. The 1# LP1 (low-pressure heater) of 150 MWe unit, the LP1 and LP2 of 300 MWe and 600 MWe units were all located in the condenser throat of their respective systems. The feed pump of 150 MWe was driven by an electric motor, while the feed pumps of 300 MWe and 600 MWe units were driven by BFPT (boiler feed pump turbines). The steam sealing systems of the three units were all used selfsealing systems. The boiler efficiency was calculated with 150 C of bottom ash temperature, and the value was initially assumed as a constant. It should be noted that if the FBAC was used to reclaim the heat of the bottom ash, the boiler efficiency had to be modified. The amount of the bottom ash was calculated according to the designed coal of a CFB boiler. The designed coal characteristics of the three CFB power plants were shown in Table 3.
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a
b
c Fig. 1. Principled heat flow diagrams of three CFB power plants.
The cooling water of BAC was taken from the main condensation water while the amount was determined by the heat balance of BAC. A material balance equation could be derived by taking the cooling water system of BAC as a branch of the condensation water system. Based on the above conditions, plant thermal economic indicators (e.g. plant heat rate, plant thermal efficiency and standard coal consumption) were calculated by employing the constant power algorithm [19].
d0 ¼
3600 ¼ wi hm hg
3600 h0 þ arh qrh
z X
and hfw-out represent the specific enthalpy of inlet and outlet feedwater, respectively, hdi and hd(iþ1) represent the specific enthalpy of the heater’s drainage and higher heater’s drainage, respectively. The drainages of regenerative heaters (HP heaters and LP heaters) were the sequential self-flow. Therefore, the extraction coefficients of the regenerative heaters were determined in series by employing Eq. (1). The specific steam consumption (d0) may be calculated as: where, wi is the specific internal work, arh is the reheat factor, qrh is
!
(2)
ai hi ac hc aej hej ag hg asg hsg hm hg
1
As a regenerative heater is a surface heater, the energy balance equation for the regenerative heater can be given as:
ai hi þ aH hfwin þ adðiþ1Þ hdðiþ1Þ ¼ aH hfwout þ adi hdi
(1)
where, ai represents the extraction coefficient, aH represents the flow rate of feedwater, ad(iþ1) and adi represent higher heater’s drainage coefficient and the heater’s drainage coefficient, respectively, hi represents the specific enthalpy of extraction steam, hfw-in
the heat absorption of reheat steam, Z is the step of steam extraction, h0, hc, hej, hg and hsg are the specific enthalpy of main steam, exhaust steam, valve stem steam-leakage, gland packing-leakage and shaft gland steam-leakage, respectively. The turbine heat rate (q0) is given by,
q0 ¼ d0 h0 hfw þ arh qrh
here hfw is the specific enthalpy of boiler feedwater.
(3)
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Table 1 Technical parameters of three CFB power plants in THA condition. Properties
Steam turbine parameters (rated power/pressure of main steam/temperature of main steam/ temperature of reheat steam) Exhaust steam parameters (Pressure/Specific enthalpy) Extraction Parameters HP3 (to 3# hp heater) (Pressure/Temperature) HP2 (to 2# hp heater) HP1 (to 1# hp heater) De (to deaerator) BFPT (to BFPT) LP4 (to 4# lp heater) LP3 (to 3# lp heater) LP2 (to 2# lp heater) LP1 (to 1# lp heater) Feedwater temperature, tfw Shaft gland steam-leakage rate, asg Gland packing-leakage rate, ag Valve stem steam-leakage rate, aej Pressure loss coefficient of extraction line Boiler parameters (maximum continue rate e pressure of superheated steam/temperature of superheated steam/temperature of reheat steam) Make-up water rate The amount of bottom ash Boiler efficiency, hb
Values I
II
III
150 MWe/13.24 MPa /535 C/535 C
300 MWe/16.7 MPa/537 C/537 C
600 MWe/24.2 MPa/566 C/566 C
0.0058 MPa/2404.82 kJ/kg
0.0064 MPa/2448.98 kJ/kg
0.0055 MPa/2311.8 kJ/kg
0.01 4% 460t/he13.73 MPa/540 C/540 C
3% 1900t/he25.4 MPa/571 C/569 C
0% 37.5t/h 0.89
0% 60t/h 0.91
0% 120t/h 0.92
0.5552 MPa/335 C 0.2304 MPa/231.6 C 0.0579 MPa/101.9 C 243.85 C 0. 0017
Then the turbine absolute electric efficiency (he) can be written as:
he ¼ 3600=q0
(4)
The plant heat rate (qcp) and the plant thermal efficiency (hcp) may be expressed as follows:
qcp ¼
3600
(5)
hb hp he
hcp ¼ hb hp he
(6)
where, hb is the boiler efficiency and hp is the piping thermal efficiency. When the net calorific power of standard coal (qL) was equal to 29.27 MJ/kg, the standard coal consumption of a power plant (bs) could be calculated by the equation given as follows:
bs ¼
3600 3600 ¼ z0:123=hcp kg=ðkW: hÞ qL hcp 29 270hcp
(7)
3. Results and discussion Based on the constant power algorithm, the thermal economic indicators of a condensing power plant mainly include specific
steam consumption (d0), turbine heat rate (q0), plant heat rate (qcp), plant thermal efficiency (hcp) and standard coal consumption (bs). The calculated results of the thermal economic indicators in THA condition were shown in Table 4, while different BACs were equipped in CFB power plants. The “No” condition in Table 4 could be considered as the bottom ash were cooled by external working substance, which had no influence on the turbine regenerative system. The results shown in Table 4 reveal that, compared with the FBAC heat recovery mode, the RAC mode has less specific steam consumption (d0), lower turbine heat rate (q0), better turbine-side thermal economy besides higher exhaust coefficient (condensate rate) (ac). The reason was that the RAC mode had larger consumption of condensation water. The larger consumption of condensation water induced less condensation water flowing through the last stage low-pressure (LP) heater(s) and reduced the steam extraction flow (shown in Fig. 2), namely the steam extraction of last stage LP heater(s) was pushed out. The last stage LP heater(s) referred to LP1 of the 150 MWe power plant, LP1 and LP2 of the 300 MWe and 600 MWe power plants. Simultaneously, more steam did expansion work in the turbine resulted in larger condensing steam flow and greater condensation loss. On the other hand, as the steam turbine power remained unchanged, the work
Table 3 The designed coal analysis on as-received basis of the three CFB power plants. Values
Table 2 The main design parameters of BACs. Properties
Inlet temperature of cooling water ( C) Outlet temperature of cooling water ( C) Water cooling ratio Inlet temperature of bottom ash ( C) Outlet temperature of bottom ash ( C)
6.687 MPa/370.9 C 4.418 MPa/315.3 C 2.164 MPa/471.5 C 1.086 MPa/370.1 C 1.086 MPa/370.1 C 0.367 MPa/236.9 C 0.197MPa/170.1 C 0.102MPa/107.4 C 0.046 MPa/79.2 C 282.4 C 0.0008 0.0066
5.78 MPa/383.1 C 3.527 MPa/318.1 C 1.652 MPa/438.9 C 0.796 MPa/336.6 C 0.796 MPa/336.6 C 0.467 MPa/272 C 0.259 MPa/205.2 C 0.136 MPa/140.2 C 0.063 MPa/87.4 C 273 C 0.001 0.00535 0.0029 3% 1025t/he17.4 MPa/540 C/540 C
3.7133 MPa/358.5 C 2.6032 MPa/313.4 C 1.0233MPa/416.4 C
Values FBAC
RAC
40 80 0.6 900 150
40 80 1 900 150
Carbon, Car Hydrogen, Har Oxygen, Oar Nitrogen, Nar Sulfur, Sar Moisture, Mar Ash, Aar The net calorific power, Qnet.ar
I
II and III
33.27% 1.97% 3.42% 0.59% 0.24% 8.00% 52.51% 12.12 MJ/kg
49.20% 2.09% 1.65% 0.56% 3.54% 7.69% 35.27% 18.50 MJ/kg
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Table 4 The calculated results of thermal economic indicators in THA condition for three CFB power plants equipped different BACs. Indicators
I (150 MWe)
Cooling water flow rate of BAC, Dbac, t/h Exhaust coefficient, ac Specific steam consumption, d0, kg/(kW. h) Turbine heat rate, q0, kJ/(kW. h) Turbine absolute electric efficiency, he Boiler efficiency, hb Plant thermal efficiency, hcp Plant heat rate, qcp, kJ/(kW. h) Plant standard coal consumptiona, bs, g/(kW. h) a
II (300 MWe)
III (600 MWe)
No
FBAC
RAC
No
FBAC
RAC
No
FBAC
RAC
0 0.6906 2872.6 8068.0 0.4462 0.89 0.3938 9141.5 312.3
100.9 0.7079 2862.0 8038.1 0.4479 0.8992 0.3994 9014.2 308.0
168.1 0.7194 2854.9 8018.2 0.4490 0.89 0.3963 9085.2 310.4
0 0.6059 3012.2 7918.1 0.4547 0.91 0.4124 8730.3 298.3
161.4 0.6188 3005.7 7901.0 0.4556 0.9176 0.4167 8639.7 295.2
269.0 0.6277 3001.3 7889.4 0.4563 0.91 0.4139 8698.7 297.2
0 0.5789 2814.5 7497.8 0.4801 0.92 0.4401 8179.8 279.5
322.8 0.5930 2807.6 7479.4 0.4813 0.9280 0.4450 8089.5 276.4
538.1 0.6026 2802.9 7466.9 0.4821 0.92 0.4419 8146.1 278.3
The net calorific power of standard coal, qL is equal to 29.27 MJ/kg.
augmentation of the turbine accordingly caused the reduction of the specific steam consumption (d0). In addition, the positive effect of “specific steam consumption (d0) reduction” was above the negative effect of “condensation loss increment”. As a result, the turbine heat rate (q0) would be decreased and the turbine absolute electric efficiency (he) would be increased in the condition of assuming the main steam and feedwater parameters as constants (except the main steam flow). It could be known that the FBAC mode has a better thermal economy on boiler-side than RAC mode from Table 4. It could be attributed to that the fluidizing air of the FBAC was directly returned to the furnace after absorbing partial physical heat of the bottom ash, which really helped heat recovery from the bottom ash and distinctly improved the boiler efficiency (hb). Plant thermal efficiency (hcp), plant heat rate (qcp) and plant standard coal consumption (bs) are three common thermal economic indicators of condensing power plant. From Figs. 3 and 4, it could be seen that the hcp increased linearly with the increase of rated power whatever the heat recovery mode was. On the contrary, the qcp decreased linearly with the increase of rated power. It was consistent with the general knowledge: the higher the parameters, the higher the energy conversion efficiency. It could also be concluded from Table 4, Figs. 3 and 4 that, compared with utilizing external working substance to cool the bottom ash (i.e. the “No” condition), the FBAC mode and the RAC mode both improved the hcp, and the FBAC mode had the higher hcp. The FBAC mode ameliorated not only the turbine absolute electric efficiency (he) but also the boiler efficiency (hb), while the RAC mode only meliorated the he. As the steam extraction of last stages
0.06
Dbs ¼ bsFBAC bsNo
(8)
or
Dbs ¼ bsRAC bsNo
(9)
Fig. 5 shows the calculated results about variation of Dbs with rated power for each unit. The legend “D” in Fig. 5 refers to the plant standard coal consumption difference between FBAC and RAC mode, thus,
D ¼ bsFBAC bsRAC
(10)
As shown in Fig. 5, using the FBAC to recover physical heat of the bottom ash saved more standard coal consumption than using the RAC. And Dbs decreased with the increase of rated power from 150 MWe to 300 MWe, then increased slightly from 300 MWe to 600 MWe. In addition, the FBAC mode had higher Dbs than the RAC
No FBAC RAC
0.05
0.45 0.44
0.04
0.43
ηcp
Extraction coefficient ( i)
heater(s) reduced, the total amount of the turbine steam admission (i.e. the specific steam consumption) decreased accordingly in the condition of the turbine power remained constant and hence the turbine efficiency decreased. Furthermore, the increment of hb was much higher than the increment of he (from Table 4). Thereby combined the two increments, the FBAC mode had the better hcp. The plant standard coal consumption bs is the most comprehensive indicator for evaluating technological perfection level of energy conversion process in a condensing power plant. We had defined the saving of the plant standard coal consumption Dbs as the plant standard coal consumption difference between FBAC (or RAC) mode and “No” mode. The equations were given as:
0.03
0.42
0.02
0.41
0.01
0.40
No FBAC RAC
0.39
0.00 LP3 LP2 LP1
LP4 LP3 LP2 LP1
LP4 LP3 LP2 LP1
150MWe
300MWe
600MWe
Fig. 2. Extraction coefficient (ai) of low-pressure heaters for each unit in different heat recovery modes.
150
300
450
600
Rated power (MWe) Fig. 3. Plant thermal efficiency (hcp) with rated power for each unit in different heat recovery modes.
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9200 9000
qcp (kJ/(kW. h))
mode, approximately 2 g/(kW h), when the net calorific power of standard coal was 29.27 MJ/kg. Moreover, the 150 MWe power plant had the highest Dbs with value of 4.35 g/(kW h) and brought considerable economic benefits. It was because that the design coal of 150 MWe power plant had higher ash content than the other two plants (see Table 3). It could be seen that the higher the ash content of coal as fired, the greater the Dbs. The 300 MWe CFB power plant was taken for an example to compare the economic benefits. The FBAC mode could save the cost of coal as fired about 2.34 million yuan RMB per year than the RAC mode, on the assumption that the plant operated 200 days annually at full load and the net calorific power of coal cost 500 yuan/t (including tax) was 18 MJ/kg.
No FBAC RAC
8800 8600 8400 8200 8000 150
300
450
600
4. Suggestions
Rated power (MWe) Fig. 4. Plant heat rate (qcp) with rated power for each unit in different heat recovery modes.
Fig. 5. Dbs with rated power for each unit in FBAC and RAC modes.
(1) The waste heat recovered in a power plant system was mostly made an entry into the turbine regenerative system and hence pushed partial steam extraction aside, which engendered additional condensation loss and reduced the turbine absolute internal efficiency. Accordingly, the regenerative steam extraction should not be affected, no matter which kind of BAC was used to reclaim heat. However, as long as the BAC used condensation water as cooling medium, the turbine regenerative system was bound to be adversely affected. The singly use of cold air for cooling could bring such problems as low heat transfer coefficient and large air consumption. This part of air returned to the furnace would have a great impact on combustion conditions. Therefore, in order to reach the optimal plant thermal economy for reclaiming heat of the bottom ash, it had to take the boiler, steam turbine and BAC as a whole system to make an integrated design. (2) In China, the high-ash content of coal as fired and the large amount of stones mixed in the coal resulted in a large percentage (the largest being 15%) of coarse slag in the bottom ash, while the biggest particle size of the bottom ash was even up to 200 mm. The coarse slag was referred to the bottom ash whose particle size was larger than the required biggest particle size (about 8 mm) of the coal as fired. As a result, the problems of slag bridging and building occurred frequently
Bottom ash inlet
Coarse slag discharging outlet
Seperation chamber
Return air outlet
Water-cooled chamber I
Water-cooled chamber II
Accident ash discharging outlet Fig. 6. The cutaway view of a mixed-flow BAC.
Main ash discharging outlet
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when the traditional FBAC was used to treat such bottom ash. However, using the patent technology of mixed-flow BAC (as shown in Fig. 6) [20] could solve the problems encountered by the coarse bottom ash in an FBAC, through the separation and respective cooling of the coarse and fine bottom ash. The coarse bottom ash was cooled by air while the fine bottom ash was cooled by both air and water. Such a patent mixed-flow BAC had been put into operation in a 300 MWe CFB boiler. 5. Conclusions In this paper, the influence of two different bottom ash heat recovery modes on thermal economy of three CFB power plants in THA condition had been calculated and analyzed based on heat balance and constant power algorithm. The heat recovery of waste heat in power plant had also been discussed. Some primary conclusions were summarized as follows: In the three CFB power plants, both FBAC mode and RAC mode improved the thermal economy of units. Besides, the FBAC mode had higher plant thermal efficiency (hcp), lower plant heat rate (qcp) and less standard coal consumption (bs) than the RAC mode. The FBAC mode could save more bs, approximately 2 g/(kW h). No matter which kind of BAC was used to reclaim heat, it should not affect the power plant systems except boiler, especially the turbine regenerative system. Therefore, in order to achieve the optimal plant thermal economy for reclaiming heat of bottom ash, it had to take the boiler, steam turbine and BAC as a whole system to make an integrated design. Acknowledgements Financial support of this work by the Key Project of the National Eleventh-Five Year Research Program of China (2006BAA03B02-06) is gratefully acknowledged. References [1] Cen KF, Ni MJ, Luo ZY, Yan JH, Chi Y, Fang MX, et al. Theory design and operation of circulating fluidized bed boiler. Beijing: China Electric Power Press; 1997 [in Chinese]. [2] Lu XF. Large-scale circulating fluidized bed boiler equipment and operation. Beijing: China Electric Power Press; 2006 [in Chinese].
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