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1、OPERATIONAL STATUS OF 135MWe CFB BOILERS IN CHINA Abstract: CFB boiler technology has been rapidly developed in China and China has the largestinstalled number and capacity of CFB boiler in the world. The number of 135MWe CFB boilers isover 150. The operational performance of 135MWe CFB boilers was
2、summarized in the article.And the reliability, economy, environmental protection index are also discussed in the article. Itnot only provides information of the development status of CFB industry, but also provides someexperience and guidance to improve the operation and to further develop CFB combu
3、stiontechnology.Keywords: 135MWe, CFB boilers, status, performance, summaryINTRODUCTIONOver the past two decades, because of its three outstanding advantages: broad fuel flexibility, excellent environmental performance, large range of load adjustment, CFB boiler technology has being under rapid deve
4、lopment and becomes an international acceptable commercial clean coal combustion technology (Lu et al.2006). CFB boiler has an irreplaceable position in Chinese boiler market. Up to now, the total number of CFB boiler operation is over 3,000, most of them are of small and medium capacity. The number
5、 of 410480t/h(100150MWe) CFB boilers is over 150 (Yue et al., 2006). CFB boilers are important for the improvement of the electric power supply structure. However, as the CFB boiler has only been adopted into coal combustion for less than 30 years and only is used power generation in recent years, m
6、any aspects are expected to be improved.The current status of the 135MWe is introduced in this paper. The aims of this paper are not only to provide information of the development status of CFB industry, but also to provide some experience and guidance to improve the operation and to further develop
7、 CFB combustion technology.RELIABILITY STATUSReliability is the fundamental of economy. Low reliability has been one of the main depute topics in CFB boiler development for years. The reliability of CFB boiler must be considered at first.In this article, two indices are used to measure the boilers r
8、eliability. One is unplanned outage times caused by various accidents and the other is continuous running time, namely the longest running time of the boiler.Unplanned outage timesFigure1 shows the variation of unplanned outage times for 135MWe CFB boilers in recent years. As the improving design an
9、d operations, the unplanned outage times decreases significantly. However, the times of unplanned outage for some boilers is still over 9.Continuous running timeAs CFB boilers were first appeared in China, they were frequently forced to shutdown by variousaccidents. Heating surface wear, slag discha
10、rger and crusher malfunction were the most common ones. Fortunately, with the improvement in design, manufacture operation and management, the continuous running time was prolonged gradually. As shown in Figure3, the continuous running time of 135MWe CFB boilers becomes longer and longer. To some CF
11、B power stations, the longest running time is more than 350 days, comparable to pulverized coal fired ones (Mao, 2006).ECONOMIC STATUSEonomic index includes carbon content in fly and bottom ash, loading rate, flue gas temperature, ignition and support oil, power consumption rate and supply coal cons
12、umption etc.Carbon content in fly and bottom ashFigure 4 shows that the carbon content in fly ash (LOI) for 135MWe boilers decreases in the year and the present average LOI is 7%.In the initial stage of CFB boiler commissioning and operation, the high LOI is often a severe problem, decreasing the ef
13、ficiency of CFB boiler. In some early CFB boilers, the LOI was as high as 40%. With better understanding CFB combustion theory and technology, LOI was decreased by optimizing the boiler design and operation (Xiao et al., 2005). Nowadays, LOI of some boilers is decreased by 2%, or even smaller (Hou e
14、t al., 2007). For most CFB units, the fuel is low rank coal or even coal stone, so LOI is relatively higher. However, for the same kind of coal, the combustion efficiency of CFB boiler is near to that of pulverized coal boiler.Average load rateFigure 5 shows the average load rate of CFB boilers in t
15、hese years. It can be seen that the load rate of 135MWe CFB boilers is only 64.4% in the year of 2007.That is because some 135MWe units used for mere power generation were regarded as smaller units which were allowed for limited operation time under the newly promulgated energy-saving policy. Howeve
16、r the load rate of some 135MWe CFB boilers used for self-supply power supply was more than 95%. Inevitably, the load rate variation affected the economic performance of units.Flue gas temperatureFlue gas temperature is an important factor for the boiler efficiency. Generally speaking, in CFB boilers
17、, sulfur content in flue gas is relatively low and the low temperature corrosion is not serious, so the exhausted flue gas temperature can be set to low to reduce heat loss.As Figure 6 shown, however the flue gas temperature is higher than the designate value at high loads. For example, in the year
18、of 2005, the average flue gas temperature is almost 20 higher than the designate value. The first reason is ash deposition on heating surface. The second reason is high air temperature at primary fan outlet. The phenomenon is unique for CFB boilers. As calculation, air temperature at primary fan out
19、let is about 20oC higher than ambient temperature, certainly decreasing the temperature difference between the air and the heat transfer surfaces in the tail duct of the boiler. This phenomenon must be considered in CFB boiler design. The convection heating surfaces in the units after were enlarged
20、and the economizers in commissioning units were modified, so that the flue gas temperature was decreased to some extent. In the same time, soot blowing was improved and excessive air ratio was controlled during operation which decreased the flue gas temperature as well.Remarkably, because of lower S
21、O2 content in flue gas and higher flue gas temperature, waste heat in flue gas was attemptted to utilize by lithium bromide absorption refrigeration units in Shuangshui power plant. This attempt, which has achieved a splendid result in supplying heat cold and power in CFB power plant, is worth popul
22、arizing.Ignition and support oilAs there are massive inert bed materials in the furnace, the performance at low load rate of CFB boilers is superior. Generally speaking, CFB boilers can be still stable burning at 35% load rate. However, the ignition oil consumption for a CFB boiler is more than that
23、 used in the same capacity PC boiler. Figure 7 shows the ignition and assist oil consumption of 135MWe CFB boiler. The amount decreased as more operational experiences were obtained (Zhang et al, 2005; Meng et al, 2007).The average amount of oil consumption for cold start-up is about 30 tons each ti
24、me. For hot start-up, the amount is only 13 tons each time. In addition, with the decreasing of unplanned outage times, the ignition oil consumption decreased largely.Power consumption rateIn a CFB boiler, the distributor, bed material and cyclone cause large resistances to gas-solid flow, leading t
25、o the increasing of primary fanspressure head and power consumption. If air-water combined slag coolers are used, the plant power consumption will further increases.In the aspect of boiler and system design, the designate pressure head of primary fan is usually larger than the required value to keep
26、 the bed material in normal fluidization state. This design rule increases the plant power consumption.As shown in Figure 8, the power consumption of a 135MWe CFB boiler unit is about 9%, larger than that of PC boiler unit. Between 2004 and 2005, the power consumption was greatly decreased. The reas
27、ons can be attributed to 3 aspects. Firstly, the designate pressure head of primary fan was reduced reasonably. Secondly, some improvements such as adjustment of primary and secondary air ratio was applied in practical operation.Thirdly air-water combined slag coolers was replaced by drum type coole
28、r in some boilers.From Fig. 8, the power consumption of heat supply units is larger than that of condensed steam generating units. It is because of the reduction of generated energy and energy loss due to heat supply.Net coal consumptionAs shown in Fig. 9, the net coal consumptionof CFB boilers is h
29、igh. As the coal consumption due to heat supply, the net coal consumption of heat supply units is smaller than that of condensed steam generating units.From Fig. 9, it can be seen that the trend of net coal consumption variation is the same as that of boiler load rate. In 2007, net coal consumption
30、decreased because of the increase of boiler load rate. Moreover, all the condensed steam generation units, the smallest net coal consumption one is Shenhuo power plant. Its net coal consumption is 369g/kWh. And the heat supply power plant with smallest net coal consumption is Taishan power plantin D
31、a Lian. Its net coal consumption,which is similar to that of PC boilers, is only 346g/kW.h.ENVIRONMENTAL STATUSAs ESP or bag filter is used in lots of power plants, the dust emission is not considered.SO2 emission control statusOne of the outstanding advantages of a CFB boiler is in-furnace desulfur
32、ization. As shown in Fig. 10, the desulfurization efficiency of 135MWe CFB boilers in China is higher than 85%, and many of them are even higher than 95%, at same level of advanced international unit.From Fig. 11, it can be seen that SO2 emission of CFB boilers meets the national emission standard.
33、It was reported the lowest SO2 emission for a 135MWe CFB boiler is 45.3mg/Nm3.待添加的隐藏文字内容3NOx emission control statusAs low temperature combustion and air staging combustion are used in a CFB boiler, and rather high on uncompleted burned carbon in the furnace, NOx emission is far below the Chinese na
34、tional standard (GB13223-2003). It was reported that lowest NOx emission is 13mg/Nm3, in Minjiang power plant,Ash and slag utilizationIt can be seen from Figure12 that ash and slag utilization of CFB boilers is improved year by year. The ash and slag from CFB boilers are generally used as raw materi
35、al of building materials. Nowadays, the ash and slag deep processing is investigated by many researchers.CONCLUSIONSChina has the largest amount of 135MWe CFB boilers. The current operational status and performance were summarized in this paper. Each unit of the 135MWe CFB boiler is in normal operat
36、ion, demonstrating the great advantages of the CFB combustion technology. The boilers in this capacity are still superior in burning low rank fuels and should not be all shut down.The power consumption rate and net coal consumption of a CFB boiler is larger than those of PC boiler. To enlarge the bo
37、iler capacity, and increase the steam parameters in supercritical state is suggested. The great achievements on design and operation of 135MWe CFB boilers are helpful for development of 300 and 600MWe CFB boiler.REFERENCESGB13223-2003. Emission standard of air pollution for thermal power plants. Bei
38、jing: State Environmental ProtectionAdministration of China. 2003.12.30.Hou X.S, Chang D.W, Zhang H, et al. (2007). CSPE8.4, Chengdu,: 27-34.Lu Q.G, Song G.L, Sun Y.K, et al.(2006) In: Yue G, Hao J eds. Proceedings of the 1st CCCCFB & the 6th annual meeting ofCFBCEC,: 56-67.Mao J.X. (2006) Boiler Te
39、chnology, , 37(3): 24-28.Meng L.W; Xu Z.Q. (2007) Electric Power, , 40(9): 65-68.Xiao X.B, Yang H.R, Zhang H, Lu J.F, Yue G.X. (2005). Energy and Fuels.,19 (4):1520-1525.Yue G.X. Hao J eds. (2006) Proceedings of the 1st CCCCFB & the 6th annual meeting of CFBCEC: 2-30.Zhang Q.S, Tian W.D, Wang B.J, et al. (2005) Electrical Equipment, 6: 77-79.
