Introduction
Coal is the main primary energy in China, most of which are anthracite and high-sulfur coal. In the process of coal production, a large number of inferior coals such as gangue and coal slime are produced [
1–
3]. The circulating fluidized bed (CFB) boiler plays a significant role in clean and efficient utilization of such coal resources and petroleum coke, oil shale [
4–
6]. As one of the matured clean coal power generation technologies, the CFB boiler has obtained an increasing application in China, whose unit capacity has been enlarged continuously [
7–
10]. Compared with the traditional pulverized coal (PC) boiler, the CFB boiler has many advantages such as high desulfurization efficiency when limestone powder is simply injected into the furnace, low NO
x emission because of low combustion temperature, wide fuel adaptability and excellent load adjustment ability [
11–
15].
By the end of 2018, a total number of 440 large-scale CFB boilers with a capacity of above 100 MW
e (410 t/h) had been put into operation in China, and the total installed capacity had exceeded 82.3 GW
e. CFB boilers are widely applied in power and heat supply, the petroleum and petrochemical industry, the chemical chlor-alkali industry, and the paper and textile industry. In the past nearly 40 years, experience in design, manufacture, installation and commissioning, operation and maintenance, equipment management of CFB boilers has been accumulated, and the reliability, economy and environmental protection level of the units are increasing year by year [
16,
17].
This paper studies the development and application of CFB boilers in China, and analyzes the technical sources, application fields, model units and unit distribution. Besides, it gives an outlook on the development of the CFB boiler technology in China from reliability, economy, emission, and energy conservation. Moreover, it introduces solutions to the problems such as water wall wear and bottom ash cooling. China has accumulated rich experiences in the design, manufacture, installation and maintenance management of the CFB boiler. The characteristics of resource environment and energy structure in some countries are similar to that in China. Therefore, related experiences revealed in this paper could be used for reference.
Development and application
Technical sources
Chinese researchers began to study bubbling fluidized bed (BFB) in the 1960s. By the end of the 1970s, more than 3000 BFB boilers (4–130 t/h) had been put into operation [
18]. Since the 1980s, China has begun to study the feasibility of CFB boiler technology and its scale-up application. The first unit of large-scale 100 MW
e CFB boiler was put into operation in late 1990s [
19,
20]. Large-scale CFB boilers with a capacity of 135 MW
e, 300 MW
e and supercritical boilers were commissioned in batches after 2003, 2008, and 2013, respectively [
21,
22]. The current 135 MW
e and 300 MW
e boilers are the main components of large-scale CFB boilers in China, accounting for 73.8% of the total installed capacity, as shown in Fig. 1.
Nowadays, major manufacturing plants in China such as Dongfang Boiler Group Co., Ltd. (DBC) in Sichuan Province, Harbin Boiler Co., Ltd. (HBC) in Heilongjiang Province, and Shanghai Boiler Works, Ltd. (SBW) in Shanghai have formed independent CFB boiler design theories and production abilities with the support of Xi’an Thermal Power Research Institute (TPRI), Institute of Engineering Thermophysics of Chinese Academy of Sciences (IET), and Tsinghua University (THU), built a complete industry chain, and issued a series of technical standards. The design and manufacturing capability of manufacturers for large-scale CFB boilers with a capacity of 100–600 MWe are listed in Table 1.
Application statistics
The large-scale CFB boilers in China are mostly distributed in the North and East region as demonstrated in Fig. 2. The top five provinces are Inner Mongolia, Shanxi, Guangdong, Shandong, and Liaoning. The detail distribution of the units in each region is tabulated in Table 2.
Model unit
The technology resources of large-scale CFB boilers in China mainly come from equipment import, technology introduction, and independent development. The most widely-used CFB boilers before 2005 were manufactured by technology introduction. Three giant boiler manufacturers of China, DBC, HBC and SBH, introduced 100–135 MW
e CFB boiler technologies from Foster Wheeler (FW), Energie und Verfahrenstechnik (EVT) and Asea Brown Boveri-Combustion Engineering (ABB-CE), respectively in the first stage. Then, Alstom’s 300 MW
e CFB boiler technology was jointly introduced by main electric power design institutes of China. The first demonstration of the 300 MW
e CFB with a pant-leg structure produced by using the Alstom technology was operated in Baima in Sichuan Province in 2006. After adsorption and simplifying the process, the first independent developed 300 MW
e CFB boiler with a single furnace without an external heat exchanger (EHE) was put into operation in Baolihua in June 2008, and the first self-developed 330 MW
e CFB boiler with a single furnace and a compact heat exchanger (CHE) was applied in Fenyi in Jiangxi Province in January 2009. The world’s first 600 MW
e supercritical CFB boiler was operated in Baima in April 2013 [
23]. By expanding the 600 MW
e supercritical CFB boiler technology, China has successfully developed the 350 MW
e supercritical CFB boiler, and the world’s first 350 MW
e CFB boiler was put into operation in Guojin in Shanxi Province in September 2015. The basic information for all types of large-scale CFB boilers in China is presented in Table 3.
300 MWe subcritical CFB boiler
The 300 MWe CFB boiler in Baima is designed by Alstom and manufactured by DBC, as illustrated in Fig. 3. The boiler adopts a double-furnace and pant-leg structure. Four cyclones with a diameter of 8.7 m are arranged on both sides of the furnace. EHE is equipped with superheater I, superheater II, and reheater III, respectively. The coal is fed from 8 coal inlets on both sides of the furnace (located on the return leg) and 4 coal inlets on both sides of the wall. The back pass is arranged with superheater III, reheater I, economizer, and rotary air heater, respectively. The boiler is equipped with 4 fluidized bottom ash coolers, starting with 4 in-duct burners and 12 over-bed start-up burners. The main guaranteed performance parameters and the corresponding test results of the Baima 300 MWe CFB boiler is given in Table 4.
The 300 MW
e CFB boiler in Baolihua, whose schematic diagram is depicted in Fig. 4, is designed and manufactured by DBC. Three steam-cooled cyclones, each with an inner diameter of 8.5 m, are arranged between furnace and back pass, and a dual-loop seal is arranged under the separator [
25–
27]. To simplify the system, the boiler does not adopt EHE, while a part of the superheater and the reheater are arranged in the form of panel on the upper part of the furnace. The back pass is a double-flue structure, and a-low temperature reheater is arranged on the upper part of the front flue. In addition, a high-temperature superheater and a low-temperature superheater are arranged on the upper part of the rear flue, while an economizer and a tubular air heater are combined and arranged after the lower part of the flue. Coal is fed from 8 coal inlets from the front wall of the furnace. The boiler is equipped with 6 roller type ash coolers, starting with 2 in-duct burners and 8 start-up burners.
The Baolihua 300 MWe CFB boiler is designed without EHE. Therefore, this kind of design is called simple design. There exist great differences between the two kinds of design in structure, controlling, etc. Taking a 300 MWe CFB boiler for example, boilers designed with EHE and without EHE are exhibited in Fig. 5 while the main structures and performance parameters are shown in Table 5.
The boiler with a simple design has ① a simpler structure, more safety boiler circulation and no limitation of steam parameter [
28]; ② a single furnace to avoid bed flop [
29,
30]; and ③ a lower system energy consumption without EHE [
31].
Besides, the cost of the boiler with an EHE design is 15% higher than that of a simple design (Fig. 6). The 300 MWe CFB boilers with an EHE design are only used in 17 units in China. However, boilers with a simple design are used in more than 70 units in China, of which, 53 units are put into operation by DBC. In conclusion, simple design is mainly used in China.
The 330 MW
e CFB boiler in Fenyi adopts a single furnace structure as 6 pieces of water-cooling panels are arranged in the furnace (Fig. 7), and 4 insulated cyclones, each with a diameter of 7.7 m are arranged on both sides of the furnace, respectively [
32]. Each cyclone corresponds to one CHE, which is arranged with superheater I and reheater II. The back pass is arranged with superheater III, reheater I, economizer, and rotary air heater, respectively. The boiler is equipped with 5 rotary ash coolers, each with a capacity of 26 t/h, starting with 4 in-duct burners and 8 start-up burners.
CHE has an excellent adjustable performance and the bed temperature changes gently during the fluctuation of boiler load. The problem of heating surface abrasion in EHE is greatly avoided in CHE (Fig. 8). A bed temperature of above 870°C could be maintained in the 330 MWe CFB boiler in Fenyi, which has a higher combustion efficiency even in 30% of BMCR operation when using inferior coal, because of CHE adjustment (Fig. 9).
600 MWe supercritical CFB boiler
After more than ten years of research, the huge theoretical and engineering challenges in the technical leap over from 300 MW
e subcritical natural circulation to 600 MW
e supercritical forced circulation have been systematically solved, and a complete technical system including boiler, auxiliary equipment, auxiliary system, installation, debugging, dynamic simulation and control strategy has been developed for demonstration project construction [
33]. Designed and manufactured by DBC, the 600 MW
e CFB boiler in Baima is the world’s largest capacity CFB boiler, as displayed in Fig. 10. The furnace has a double-furnace and pant-leg structure, and 6 steam-cooled cyclones, each with an inner diameter of 9 m, are arranged on both sides of the furnace, respectively [
34–
36]. In addition, one CHE is connected to the lower part of each separator. Of the 6 CHEs, two ones are arranged with the high-temperature reheater, and four ones are arranged with the secondary intermediate temperature superheater. The furnace is arranged with 14 platen superheaters (superheater III), and the back pass is arranged with the low-temperature superheater, low-temperature reheater, the economizer, and the rotary air heater, respectively [
37]. The boiler is equipped with 6 roller type bottom ash coolers, starting with 2 in-duct burners and 14 start-up burners. The boiler has an excellent performance, the sulfur capture efficiency of limestone in the furnace exceeds 97%, and the original NO
x emission concentration is only 112 mg/m
3 (Table 6).
The performance has fully satisfied the expectation. The Baima project was regarded as a milestone in the development of international CFB technology, indicating that the CFB technology in China has taken the lead in the world.
350 MWe supercritical CFB boiler
The 350 MW
e CFB boiler in Guojin is also designed and manufactured by DBC, as presented in Fig. 11. Three steam-cooled cyclones with an internal diameter of 8.6 m are arranged between the furnace and the back pass. A dual-loop seal is arranged under the separator instead of an EHE. The furnace is arranged with 14 platen superheaters and 6 platen reheaters. The back pass is a double-flue structure. A low-temperature reheater is arranged on the upper part of the front flue, while an intermediate temperature superheater and a low-temperature superheater are arranged on the upper part of the rear flue. An economizer and a rotary air heater are arranged after the lower part of flue is combined. The coal is fed from the 10 coal inlets of the front wall. The boiler is equipped with 6 roller type ash coolers, starting with 4 in-duct burners and 6 start-up burners. The 350 MW
e CFB boiler in Guojin adopts selective non-catalytic reduction (SNCR) denitrification and rear CFB semi-dry gas desulfurization in controlling gas emission. So the emission of SO
2 and NO
x could stably satisfy the ultra-low emission standard (Fig. 12) [
38].
660 MWe ultra-supercritical CFB boiler
Further improvement in steam parameters of CFB boilers is a requirement for technological development [
39–
41]. The design of the ultra-supercritical CFB boiler at 29.4 MPa, 605°C and 623°C (Fig. 13) have been completed by DBC, HBC and SBW. Two 660 MW
e projects are about to be construction by DBC and HBC [
42,
43], respectively. Taking the design scheme of the 660 MW
e ultra-supercritical CFB boiler completed by HBC as an example, it is expected that the thermal efficiency of this boiler is higher than 93.5%, the coal consumption for power supply is lower than 290 g/(kW·h), and the emission concentrations of SO
2, NO
x and dust are less than 35, 50 and 5 mg/m
3, respectively. It will become the CFB boiler in the world with the lowest emission, energy consumption, and the highest capacity and efficiency.
Technical performance
Reliability
The combustion mode of the CFB boiler is different from that of the PC boiler in that the inferior coal with a low calorific value and high ash content can be burned. Consequently, the heating surface of the furnace always experiences serious wear due to limited operation and maintenance experience in the early stage, which leads to high unplanned shut-downs. Statistics on 30 units of 100–135 MWe CFB boilers showed that the number of unplanned shut-down was 5.61 times per unit and annually in 2004. The average-level boiler can continuously operate for 74 days, while the longest continuous operation was 139 days, and the shortest continuous operation was only 33 days.
With the accumulation of operating experience and the promotion of anti-wear technology, the reliability of most CFB boilers were improved and the boilers can operate for a long period of time. The statistics of 81 units of 100–300 MW
e CFB boilers in 2017 showed that the average number of unplanned shut-down was only 0.37 times per year, of which the 135 MW
e was 0.26 times per year, and 300 MW
e was 0.46 times per year, which was basically the same as the PC boiler [
44]. The average available hour of the CFB boiler was 7880 h, which composing 7884 h for 135 MW
e and 7875 h for 300 MW
e. The continuous operation time of the 300 MW
e in Baolihua (#5 unit) was 434 days, that of the 200 MW
e in Yili (#1 unit) was 341 days, that of the 150 MW
e in Shangwan (#1 unit) was 384 days, and that of the 135 MW
e in Pansan (#1 unit) was 311 days [
45].
Economics
The coal consumption for power supply and auxiliary power ratio are technical indicators for the energy efficiency of a boiler. Affected by fuel characteristics, the thermal efficiency of CFB boilers is relatively low. During the combustion process, a large amount of materials is in suspension, and it is necessary to overcome the resistance of the air distributer and separator. Therefore, the auxiliary power ratio of a CFB boiler is higher than that of a PC boiler. In particular, the selected scale of the fans of some CFB boilers was large and the operating efficiency was low, which increased the power consumption rate of the plant. However, the auxiliary power ratio of some 300 MW
e CFB boilers based on flow-pattern reconstruction can be reduced to about 4%, which is close to that of PC boilers [
46], as given in Fig. 14.
Emission
Since the application of large-scale CFB boilers, the main implementation has been the 2003 and 2011 editions of the Emission Standard of Air Pollutants for Thermal Power Plants (GB 13223) [
47,
48]. The main binding indicators are dust, sulfur dioxide, and nitrogen oxide [
49,
50]. In terms of dust control, there is no significant difference between the CFB boiler and the PC boiler. The early CFB boiler mainly adopted the electrostatic precipitator (ESP), but the addition of limestone desulfurization in the furnace affected the resistivity and dielectric property of fly ash. Therefore, some CFB boilers adopted the electrostatic and fabric composite filter. In the later period, most CFB boilers used bag filters as a result of further improvement in environmental standards.
As for SO2 control, the CFB boiler mainly uses limestone desulfurization and the calcium-sulfur molar ratio is generally 1–2 in the furnace when the emission concentration standard is above 400 mg/m3, so the environmental protection cost is relatively low. The calcium-sulfur molar ratio needs to be increased to 2–4 when the emission concentration standard is 200–400 mg/m3. The combination of desulfurization in the furnace and external desulfurization is generally adopted when the emission concentration standard is lower than 200 mg/m3, or only the external desulfurization device is used. The semi-dry desulfurization process is mostly used for external desulfurization, and the wet desulfurization processes is also adopted.
As for NOx control, CFB boilers can meet the NOx emission concentration standards in GB 13223-2003 by burning different types of coal in different periods. Most in-service CFB boilers can also be used in accordance with NOx emission concentration standards (200 mg/m3) required in GB 13223-2011. For new boilers (100 mg/m3) and key areas (50 mg/m3), the SNCR technology can be used, and the denitrification efficiency can generally reach 60%–85%. Only a small part of the CFB boiler adopts the selective catalytic reduction (SCR) technology or the SNCR/SCR joint technology.
The file Upgrade and Reconstruction Action Plan for Thermal Power Conservation and Emission Reduction (2014–2020) in China requires that new coal-fired units should meet the emission limit, which means that dust, SO2, and NOx emission concentrations should be lower than 10, 35, and 50 mg/m3, respectively. CFB boilers have implemented many technical routes to meet environmental protection requirements. Comparing with PC boilers, there are more economical and reasonable technical routes for CFB boilers with different coal types.
From the current technical practice and taking 2 × 300 MW
e CFB boilers as an example, the combination of boiler combustion optimization, furnace limestone desulfurization, SNCR denitrification, and rear CFB semi-dry gas desulfurization can meet the technical requirements of ultra-low emission. The initial investment of that is about 24 million dollars, saving about 6 million dollars for investment and 2 million dollars for operation compared to the technical route of the PC boiler. The technical route is adaptable, and the environmental protection cost and operating cost are relatively low [
51].
Problems and solutions
Typical problem
As the development of CFB boilers is relatively fast, the operating conditions for burning inferior coal are insufficiently considered due to the lack of design experience in the early stage. The following problems often occur after the boilers are built.
1) The water wall of the furnace, especially the junction area of dense phase and dilute phase, the four corners of furnace, the top and the outlet of furnace, the wall-through zone of platen heat surface and other parts, is abraded. Consequently, the continuous operation period of the unit is short [
52,
53];
2) The nozzle wear causes ununiformed local fluidization, coking of slag, and slag leakage from windbox, which needs a lot of replacement during maintenance [
54];
3) The refractory material on the furnace, the separator, and the loop seal wear and fall off [
55];
4) The coal supply system is frequently blocked and interrupted, and the coal feeder belt is burned, causing boiler fuel interruption;
5) The bottom ash cooler and the slag conveying system are frequently blocked [
56–
58];
6) The electricity consumption rate and coal consumption for power plant is high, resulting in poor economics of the unit;
7) The ignition oil consumption is large and the start-up time is long [
59].
Fortunately, the above problems have been solved by technicians. Section 4.2 which follows is a description about heating surface wear and bottom ash cooling that significantly affect the operation of CFB boilers.
Water wall wear
The CFB boiler has a high adherent flow velocity and high concentration, which is the main reason for the wear of furnace wall. The flue gas velocity in the furnace is generally controlled to 5.5 m/s or even below 5 m/s for the newly-designed CFB boiler to reduce wear, while the tube designed in the transition zone is used to avoid local vortex in the meantime. Technologies like anti-wear beam, anti-wear clapboard, laser cladding and cladding are applied in many CFB boilers to reduce wear.
The technical principle of the anti-wear beam is shown in Fig. 15, which blocks the descending adherent flow by applying anti-wear and refractory material on the water wall. The test results show that the speed of adherent flow is reduced from about 8 m/s to less than 3 m/s after using this technique, thereby reducing the wear of heating surface in the furnace. At present, the technology has been adopted by more than 200 CFB boilers with a capacity of 50–350 MWe in China.
The anti-wear clapboard is more commonly applied in 50–100 MWe CFB boilers. The effect of protection is related to the quality of welding process. Even if only few anti-wear clapboards are fallen off, the water wall will be worn seriously in a shot time.
Bottom ash cooling
The in-service CFB boilers in China in the early stage mainly matched fluidized bottom ash coolers. Fluidized bed bottom ash coolers have a high heat transfer coefficient and a large output. However, large particles of stones and debris are often mixed into the coal of CFB boilers to make the bottom ash flow deteriorate because of the poor coal quality [
60,
61]. Sometimes the blockage caused by coke slag results in the fact that the ash does not enter the bottom ash cooler, or a large amount of ash cannot enter in a short-term. Therefore, the bottom ash cooler cannot be operated stably, and the ash discharge temperature fluctuates frequently, or the ash is extremely over-temperature. To solve the problem of bottom ash cooling, the CFB boilers in China have generally been switched to roller type ash coolers (Fig. 17).
The cylinder of membrane wall bottom ash cooler is composed of steel pipes distributed along the circumference, the steel pipes are welded together by fins, and the inside of steel pipe is provided with cooling water. The cylinder which is composed of a steel pipe and fins functions as both the inner cylinder and the outer cylinder, and the inner cylinder wall is welded with louver blades or other heat transfer elements. To increase the output of the bottom ash cooler, the cylinder may be divided into 3 to 5 cooling chambers to increase the cooling area. The advantage of this bottom ash cooler is that it has a strong heat exchange effect, a great output regulation performance, a strong pressure bearing capacity, and the bottom ash is not easy to be blocked. Because its actual output can reach more than 35t/h. However, the disadvantage of this bottom ash cooler is that its structure is complicated, the requirements for manufacturing and installation are high, and it is difficult to overhaul.
Energy conservation and other new technologies
The selection of a new technology is an effective way to reduce energy consumption of the CFB boiler. The 300 MW
e CFB boiler in Longyan in Guangdong Province uses the flow-pattern reconstruction technology that dramatically optimizes the bed quality and reduces the auxiliary power ratio. The ratio could be dropped closed to that of the PC boiler [
62]. The 350 MW
e CFB boiler in Hepo in Shanxi Province uses 100% capacity turbine driven feed water pump which reduces power consumption rate of plant by 0.3%–0.5% compared with the traditional design of 100% capacity electrically driven feed pump. The elimination of the dry ash handling system and fly ash silo in power plant design not only saves construction cost, but also reduces the number of air compressors from 10 to 4. The resistance of the air and flue system is reduced by optimizing the piping layout, setting air, and gas guiding device, increasing the height of the nozzle of secondary air, increasing the elevation of lower secondary air outlet from 11 to 12.5 m and the elevation of upper secondary air outlet from 14.95 to 15.6 m. During operation, the pressure of windbox is only 8–9 kPa. The primary fan, the secondary fan, and the induced draft fan are adjusted by the variable frequency drive (VFD). Therefore, the power consumption of the fan is significantly reduced [
38,
63].
Coal washing and processing will produce large amount of coal slime, coal gangue, middlings and other coal by-products with a low calorific value. The operation practice of the 300 MW
e CFB boiler in Pingshuo in Shanxi, the 300 MW
e CFB boiler in Panbei in Guizhou, and the 330 MW
e CFB boiler in Jinghai in Inner Mongolia suggests that the CFB power generation technology is the best of its kind for clean and efficient utilization of low calorific value coal in large scale [
45,
64]. In addition, the 300 MW
e CFB boiler in Yongan in Fujian Province also proves that biomass and urban solid waste can be used in a clean and efficient way through the CFB boiler [
65].
By improving the design of the boiler and optimizing the flue gas cleaning system of the 300 MWe CFB boiler in Guofeng in Shanxi Province, a new technical route is proposed for CFB boilers burning low-rank coals. The results indicate that an ultra-low emission of NOx, SO2, and dust can be realized. Fly ash and slag are used to replace part of cement and coarse aggregate to make bricks. Moreover, this technical route has obvious advantages in economy over the conventional technologies.
Conclusions
The technical advantages of CFB boilers determine its wide application in China, which contributes to the rational use of coal resources and the reduction in pollutant emissions. With the future application of 350 MWe supercritical CFB boilers and 660 MWe ultra-supercritical CFB boilers, the CFB boiler technology will continue to develop and occupy an important position in China’s thermal power generation market.
China has solved the problems occurred in the early application of CFB boiler technology, and significantly improved the reliability, economics, and environmental protection of CFB boilers. There are many developing countries around the world, which are suitable for the application of efficient, energy-saving, and environmentally friendly CFB boilers. China has accumulated abundant experience in CFB design, manufacturing, installation, maintenance, and management, which can provide experience for these countries.
There are 40 units CFB boiler of supercritical and ultra-supercritical under construction and some have already been put into operation in recent years. Their operation performances are still being tested through actual operation experience at this time. However, it is believed that the large scale CFB boiler will have a bright application prospect in China.
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