DESIGN THEORY OF CIRCULATING FLUIDIZED BED BOILERS

Transcription

1 DESIGN THEORY OF CIRCULATING FLUIDIZED BED BOILERS Guangxi YUE, Junfu LU, Hai ZHANG, Hairui YANG, Jiansheng ZHANG, Qing LIU, Zheng LI, Eric JOOS*, Philippe JAUD* Department of Thermal Engineering, Tsinghua University, Beijing , China; * EDF France Paris 78401, France ABSTRACT Studies on circulating fluidized bed (CFB) boilers have being conducted at the Tsinghua University (TH) for about two decades and much of works are done to link the fundamentals with practical application. A full set of design theory was developed and some key elements of this theory are presented in this paper. First, a classification of state of the solid-gas two-phase flow in CFB boiler is given. TH s studies validated that a CFB boiler can be generally described as the superposition of a fast bed in the upper part with a bubbling bed or turbulent bed in the bottom part. A concept model of material balance for the open system of CFB boiler was developed and later improved as a more comprehensive 1-D model taking ash formation, particle attrition and segregation in bed into account. Some results of the models are discussed. Then the concept of State Specification of a CFB boiler is defined and discussed. The State Specification is regarded as the first step to design a CFB and a base to classify different style of CFB boiler technologies for various CFB boiler manufacturers. The State Specification adopted by major CFB boiler makers is summarized and associated importance issues are addressed. The heat transfer model originally developed by Leckner and his coworkers is adopted and improved. It is further calibrated with experimental data obtained on the commercial CFB boiler measurements. The principle, improvements and application of the model are introduced. Some special tools developed for heat transfer field test are also given. Also, combustion behaviors of char and volatile content are studied, and the combustion difference between a CFB boiler and a bubbling bed is analyzed. The influence of volatile content and size distribution is discussed. The concept of vertical distribution of combustion and heat in CFB boiler furnace is introduced and discussed as well. In the last, the suggested design theory of CFB boiler is summarized. Keywords: circulating fluidized bed boilers, design theory, state specification, fast bed INTRODUCTION Circulating fluidized bed (CFB) technology has gained a great progress in coal-firing boilers since the successful operation of the world s first demonstration of circulating fluidized bed (CFB) boiler in Germany [1]. The largest CFB boiler, a supercritical unit with capacity of 460MWe made by Foster Wheeler Corporation, is under construction in Lagisza, Poland [2]. In China, the number of commercial CFB boilers that have been put into operation is over 800, among which the units with capacity MWe are near 30 [3]. The first 300MWe CFB boiler (Alstrom licensed) is in construction [3]. Studies on CFB boilers have being conducted at the Tsinghua University (TH), Beijing, China since 1985, in both fundamental research and commercial development. A series of CFB - 1 -

2 boilers with capacities ranging from 20t/h to 460t/h have been put into commercial operation and some other units with larger capacities and higher steam parameters are under design or feasibility study [4,5,6], based TH s research and development (R&D) achievements. In this paper, a summary of the two-decade R&D works on CFB boilers by the TH research group, especially those works linking the fundamentals with practical application is to be given. TWO PHASE FLOW IN CFB BOILER Typically, the main loop of a CFB boiler is composed of a riser, separators and loop seals. For some small units, single separator and single loop seal might be applied. Nevertheless, the main loop is a typical solid-gas two-phase flow system with chemical reaction. Appropriate understanding of the fluid mechanics inside the furnace is of fundamental importance to design a CFB boiler. Theoretically, the regimes of fluidization can be classified into stationary bed (or say fixed bed), particulate fluidization, bubbling bed, slugging bed, turbulent bed, fast bed and pneumatic transport, depending on the gas superficial velocity u f, bed voidage and physical properties (e.g., size and density) of the solid particles, as shown in Fig. 1[7]. Normally, the fluid mechanics inside the furnace is separately described in two parts: a lower part and upper part. In the lower part, the so-called dense bed, size distribution is rather wide with many coarse particles and bulk density is rather high. Thus, the associated fluidization regime is not necessarily fast bed, it can be bubbling bed or turbulent bed depending mainly on the u f. Pneumatic Transport Al 2 O 3 Beads d p =52µm ρ p =3580kg/m 3 v p : particle velocity, m/s u f (m/s) Figure 1 Fluidization regimes for Al 2 O 3 particles- bed voidage vs. superficial velocity [7] However, in the upper part, the main portion and so-called free board of the bed, the classification of its fluidization regime has been an argument in CFB boiler research community for a long time. Since the bulk density of most coal-fired CFB boiler furnaces (tens of kg/m 3 or even less) [8] is much smaller than that of fast bed reactors in chemical engineering process (in the - 2 -

3 range of hundreds of kg/m 3 ) [3], it was easily intended to classify the fluidization regime as pneumatic transport. However, the authors suggest that the upper part of a CFB boiler still belongs to fast bed rather than pneumatic transport. As we known, the most distinguished feature of a fast bed is the formation of cluster in the riser, resulting in strong vertical mixing. According to our observation, temperature distribution is rather uniform in the bed not only in the core region in radial direction but also along the furnace height, even the combustion keeps going in the gas-solid flow and the furnace is surrounded by water-cooled membrane. Such temperature uniformity can be only maintained by the existence of strong vertical solids mixing and thus the existence of clusters. During the CFB boiler evolution history in China, a CFB boiler was once regarded as nothing else than the traditional bubbling bed boiler with an extended free board. However, the fluidization regime inside a bubbling bed boiler is totally different from that inside a CFB boiler. In a bubbling bed, only small amount of particles are entrained into the free board so that combustion fraction in the dense bed is about 75-85%, and a rather amount of immersed tube has to be arranged there. However, in a CFB boiler, much more particles are entrained into the free board so that combustion fraction in the dense bed only occupies about 50-60%, and no convective heat transfer surfaces are necessary to be arranged there. It was found that for a bubbling bed boiler retrofitted with fly ash recirculation, if the recirculation flow rate is above a critical amount, the hydrodynamic and thus combustion and heat transfer behaviors inside the bed become CFB-alike and qualitatively different from bubbling bed. The temperature in the dense bed can be even too low to keep stable combustion. Given the upper part of a CFB boiler is a fast bed, shown in Fig. 1, for certain particles, flow dynamics of the two-phase flow, or called hydrodynamic state can be defined by two parameters: superficial velocity u f (m/s) and solid circulating rate G s (kg/m 2 s). For engineering simplicity, G s is also assumed to be the solid flux at the separator entrance. Then the onset superficial velocity of fast bed for certain size particle is defined as u c [9]: u c =(3.5-4)u t (1) where, u t is the terminal velocity of particle, m/s. The minimum solid circulating rate to enter the fast bed regime R min can be estimated by [7]: uc ρf Rmin = [g dp( ρp ρf)]. (2) where, ρ f is the gas density, kg/m 3 ; ρ p is the particle density, kg/m 3 ; and g is the gravity, m/s 2. It can be seen from (2), for a certain u f, CFB boiler can operate at various states in fast bed regime because the bed inventory in CFB boiler is composed of different size particles. MATERIAL BALANCE IN CFB BOILER Based on the observation on coal-fired CFB boilers, the average size of bed inventory, which is often called bed quality, is finer than that of bubbling bed boiler and even finer than that of feeding raw coal. Thus, an accumulation process for size selection exists during the operation. In order to study the material balance, a conceptual model was built up by TH s CFB boiler research group [10].

4 CFB boiler is an open system for solid flow Different from most chemical reactors, a CFB boiler is an open system for both gas flow and solid flow. The solid inputs are ashes formed from feeding fuel, limestone and, sometimes, inert sands for making up. There are two outlets for solids to exit: one is on the furnace bottom for draining bed ashes and the other is on the separator top for blowing fly ash, as shown in Fig.2 [11]. Conceptual model of material balance in CFB boiler Solid particles of any size interval should be kept in balance during the stable operation, so G in (i)=g out (i)+f(i) (3) where, G in (i) is the flow rate of solids with size d i entering the system, which is from the ash formation of coal and limestone or make-up sands; F(i) is the flow rate of fly ash with size d i ; G out (i) is the flow rate of drained bed ash with size d i ; X(i) is the fraction of particles with size d i in dense bed; and E(i) is the entrainment rate of particles with size d i. The entrained flow rate of particles with size di is accounted as E(i) X(i). The separator efficiency for size d i based on the entrained flow is: Fi () ηs()=1 i (4) Ei () Xi () Then, F(i)=E(i) X(i) (1-η i ) (5) If we define the bed ash drain efficiency η o based on the entrained flow as: η o (i)=1-g out (i)/e(i) X(i) (6) Then, the overall efficiency of the system η m to maintain particles with size d i is: η Gout ()+ i F() i m()=1 i = oi i 1 Ei () Xi () η + η (7) Material balance equation can be expressed as: G in (i)=g out (i)+e(i) X(i) (1-η i ) (8) ΣX(i)=1 (9) Provided E(i) is properly given in literature and segregation in dense bed can be neglected, then after solving the equation group, we have: Gout () i Gout = (10) X () i Some interesting and valuable results can be derived from the model. Figure 3 depicts the variation of overall system efficiency η m with particle size d and the size distribution of bed inventory for given separator efficiency η s and ash drain efficiency η o. It can be seen that η m first G in (i) E(i) X(i) X(i) F(i) G out (i) Figure 2 Concept of material balance of CFB boiler

5 increases with the increasing of d i and after it reaches a peak value it decreases with the further increasing of d i. As d is smaller than the peak value, η m is dominated by η s and as d is larger than the peak value, η m is dominated by η o. The particle size distribution of bed inventory, also a solution for given η s and η o, exhibits a cap-like curve. Moreover, the particle size corresponding to the peak value on the size distribution curve is consistent with that corresponding to the highest η m. The size distributions of bed inventory for two separators with different cut sizes d 50 and d 100 are shown in Figure 4, for three different u f s while the ash drain efficiency η o is the same. It can be seen, as the material balance is built up, the size distributions of bed inventory are remained. For a separator with better performance, namely smaller d 50 and d 100, the particle size corresponding to the peak value of the size distribution curves is smaller. This result is straight forward since more fine particles are captured if separation efficiency increases. For the same separator, for different u f s, the particle Efficiency ηi % 100 Frequence distribution Pi %/µm Overall efficiency Ash drain efficiency Bed material size distribution Separator efficiency Particle size d i µm Figure 3 Overall efficiency of the system Particle size d i µm size corresponding to the peak values on the frequency distribution curves are nearly constant. As u f increases, more fine particles are entrained into and stored in the free board. At the same time, G s increases and the amount of returning particles increase, forcing more ash particles including the particles less than d 100 are drained from the bottom. As a result, the mean particle size decreases and fewer particles can be entrained and thus G s decreases. When balance is reached, more particles around the mean value are drained, and consequently the overall distribution of the particles becomes wider though the mean particle value keeps nearly the same. It is clear that although the size of feeding particles into system is widely distributed, the CFB boiler system behaves like size selection machine. Coarse particles which can not be entrained are drained out from bottom of bed, and very fine particles which are difficult to be capture by the separator are carried out the system by flue gas. Only those particles that can be entrained by the u f m/s d 50 µm d 99 µm Figure 4 Size distribution of bed inventory for different cyclone efficiencies Frequence distribution Pi %/µm

6 flue gas and also be captured by the separators are retained in system for circulating. The results indicate that the average size of bed inventory (bed quality) and the circulating rate of ash are depending on the performance of separator and bed ash drain characteristics, besides the superficial velocity and ash formation characteristics of coal and limestone. Thus the overall system efficiency, especially the efficiency for circulating ash (near the d 99 of separator) is very important and sensitive for the circulating rate. Our studies on the commercial CFB boilers showed that G s is typically in the order of 10 3 larger than the feeding rate of such size particles, so the efficiency near this size should be over 99.7%. This result is not only important for the design of separator but also important for determination of bed ash drain characteristics. In engineering practice, sometimes, ash drain facilities with specific size classification, combined with ash cooler, are needed to keep fine circulating ash in bed. 1-D model for CFB material balance A 1-D material balance model was developed by the co-research work between TU and EDF [12]. Standard bench-scale facility and test procedure were implemented to measure the coal ash formation and attrition characteristics [13] that are used as input data for the model. The particle segregation in dense bed was taken into account in the model to characterize the bed ash drain. The prediction on resident time of different size particles and its impact on attrition is a novel feature of the model. The model was calibrated by the field test data from three boilers in China and successfully applied to predict the material balance in the Gardanne s 250MWe CFB boiler. Figure 5 compares the size distributions of fly ash between the data measured in the field of this boiler and those predicted by the 1-D model. It can be seen that there is an important impact of attrition on ash size formation. Without taking the attrition of solid particles into account, remarkable discrepancy would be induced. Figure 6 is the comparison of the bulk density along the height of furnace by field test and model prediction Particle size d i µm Figure 5 Comparison of model (micron) prediction on the STATE SPECIFICATION FOR CFB BOILER DESIGN Frequence (%/micron) distribution Pi %/µm Province Measurement Gardanne Intrinsic Gardanne Model prediction ash formation w/o attrition with the data measured in the field for a 250MWe CFB State Specification and its importance The State Specification for a CFB boiler means to keep the CFB boiler in a specific state such that it can operate stably and continuously. From previous discussions, the state of a CFB boiler can be represented by the superficial gas velocity u f and solid recirculation rate G s. The u f is a design and operating parameter, while the G s is a dependent variable on the u f, separation - 6 -

7 efficiency, ash drain efficiency and solid inputs in the open system. For an industrial combustion process, the operating state has to be controlled to a stable state Case1 Case Dot: measurements Line: Model Prediction 0.8 Dot: measurements Line: Model Prediction Dimensionless Height Dimensionless Height Dimensionless Pressure Dimensionless Pressure Figure 6 Comparison of model prediction on the pressure drop profiles along the furnace height with the data measured in the field for a 250MWe CFB boiler In case that the feedings of particles such as coal, limestone or make-up sands are varying, the state of a CFB boiler might keep changing as well if G s can not be controlled. Consequently, the heat transfer coefficients between water-wall membrane and solid-gas flow in furnace, which strongly depends on the bulk density [14], and the fractional fuel heat releasing along the furnace height could not be kept stable during operation. Fortunately, as we discussed in the material balance section, G s can be manually controlled by adjusting the bed inventory. Shown in Fig. 7, the increasing of the bed inventory leads the increasing of bulk density in furnace, and thus the increasing of G s at the furnace outlet. State Specification plays a fundamental role in CFB boiler design. In engineering practice, before conducting the detailed design, CFB boiler designers usually selected a specific state in fast bed regime for Dimensionless height Demensionless bulk density Figure 7 Bed inventory vs. the bulk density in furnace and circulating rate G s Case 1 2- Case 2 3- Case 3

8 the CFB boiler, namely to perform State Specification for a CFB boiler. After State Specification (with fixed u f and G s ), designers started to collect much referential data such as heat transfer coefficients and fractional heat releasing along height of furnace, mainly from the field test on demonstration boilers collaborated with laboratory researches. This accumulation is actually a long-term R&D work. Based on State Specification and the following data accumulation, a program, so-called Design Code, would be developed to design the layout and components of the CFB boiler. Once CFB boilers designed by the Design Code are put into commercial operation, more data are provided to improve and mature the Design Code. As a result, on one hand, each CFB boiler manufacturer owns a specific Design Code as a commercial secret and makes CFB boilers in different styles; on the other hand, it is also very difficult and challenging to change the Design Code once it becomes a design standard because all design data based on a specific state of a CFB boiler need to be re-accumulated. Consequently, special cautious should be paid in State Specification. Major Consideration in State Specification The determination on superficial velocity u f The u f in a CFB boiler should be higher than the onset velocity of fast bed corresponding to particle size as mentioned before. Some designers favor higher u f s in order to obtain higher specific cross section load. However, u f is limited by the erosion on the vertical water wall, besides the resident time for fine coal particles burnout and de-no X [15]. The determination of solid circulating rate G s The G s should be more than the minimum solid circulating rate of fast bed regime -R min as discussed before; otherwise the bed is a bubbling bed. The upper limitation for G s depends on several considerations. For example, since G s is related to the total bed inventory (Fig. 7), it is related to the power consumption of draft fan. In addition, the total bed inventory can be divided into circulating ash inventory that is important for keeping an enough amount of G s, and the coarse particle inventory that is important for keeping sufficient resident time for the burnout of coarse coal particles. Recent research in China shows that the solid suspension in furnace influences gas diffusion, thereby the burnout efficiency of coal char. Another factor limiting G s is the erosion in furnace. State Specification Practices The G s -u f diagram shown in Fig. 8 summarizes the State Specification done by several major CFB boiler manufacturers in the world. Because few data on circulating rate for commercial CFB boilers were published, much of the data were by our estimation or by our field measurements (most CFB boiler makers have demonstration boilers in China). In above state diagram, the dot-dash curve close to the u f axis is the onset circulating rate of fast bed which is based the calculation assuming the particle size is around 200µm (according our observation, the cut size of circulating material for most CFB boilers is around µm [4,16,17]. Below the line, fast bed state can not be realized. Above this curve, there are two curves (one in dot-dash, and the other in dash) representing the maximum circulating rates for CFB boiler with one stage cyclone and two stage cyclones in serial respectively, both predicted by TH- EDF material balance model assuming no limestone or inert additives are added. Two dot curves - 8 -

9 approximately parallel to the G s axis stand for the erosion limitation for lignite combustion and hard ash content coal combustion respectively. These limitations are from our observation for a group of CFB boilers with different design statuses and for burning different coals in China. According to our observation, the hardness of ash and the superficial velocity have more significant impact on erosion than the circulating rate. We have to point out here, for some CFB boiler technologies of which u f is near or over 6m/s, serious erosion has been found on the vertical water wall in furnace within limited operating period burning lean coal, bituminous or anthracite coal. As shown in Fig. 8, those CFB boilers are operating at the states near to the erosion line. Although, no erosion problem on vertical water wall has been reported for the boilers using same technology while lignite is burning, it is safer for the designers to select u f to be lower than 5.5m/s in case fuel quality can not be guaranteed. Ash circulating rate GS kg/(m 2 s) A B C D E Commended F G H I Limits for erosion protection (Two stage cyclone) One stage cyclone Fast bed limit 5 Soft coal Hard coal Fluidizing velocity u f m/s Figure 8 State Specification by several major CFB boiler manufacturers In fact, the selection of acceptable fast bed state is limited within a small area in the state diagram. TH also suggest its own state (marked in asteroid *), which is safe for most coal types and the G s is also far away from the material balance limit. Clearly, after stated of a CFB boiler is specified, a reliable material balance model is needed for designers to validate the material balance for design coal. The bench-scale tests on ash formation and attrition characteristics for coal and limestone are strongly suggested to be done first. With those experimental data, designers can use the model to check if the maximum ash circulating rate has enough margins for the specified state. If it does not, the model can estimate the quantity and quality of make-up sands

10 HEAT TRANSFER IN CFB FURNACE There are enormous literatures on heat transfer research in CFB boiler s [18~21]. They are valuable for understanding the mechanisms of heat transfer in bed, but difficult to be directly used into application. For engineering purpose, TU has conducted a series of experimental studies on the commercial CFB boilers. A Heat Flux Probe and a Solid Suspension Density Probe were developed to measure the heat transfer coefficients and solid density respectively and successfully applied in the field tests. The schematics of heat flux probe and local bulk density probe are shown in Fig. 10 and Fig. 11 [22]. At the same, a semi-empirical water inlet water outlet Slide Guide Upper Cover Slide Control Bar Bottom Cover Figure 10 Sampling probe for bulk density Thermocouples in center Thermal insulation layer Protecting shell Thermocouples in probe surface Figure 11 Schematic of Heat Flux Probe model was developed based on the suggestions from Bo Andersson and Leckner [23] and further correlated with the field data. The overall heat transfer coefficient between two phase flow and the water wall, α b, is mainly composed of two components particle suspension convective heat transfer coefficient α c and particle suspension radiative heat transfer coefficient α r. α b =α c +α r (11) The α c is expressed as the function of local bulk density of solid suspension ρ as: α b =aρ b (12) where, a and b are correlation parameters with data from the field test. The α r is calculated by following equation: α r = 1/( + 1) σ( Tb + Tw )*( Tb + Tw ) (13) ε ε w b where, T and ε denote for temperature emissivity respectively, and the subscripts of b and w denote respectively the suspension and water wall. Later on, the heat transfer model was improved by taking the geometric factor of water membrane into account. TH s heat transfer mode has been proved to be simple and with satisfied accuracy for engineering purpose, and it has been practiced in the design of more than one hundred units of CFB boilers with different capacities. More detailed information about the model can be found in other publications [24]. Probe COMBUSTION IN CFB

11 Coal combustion modeling and bench-scale experiments also have been extensively conducted at TH. It was found that the coal particles, as soon as fed into CFB boiler furnace, experience a primary fragmentation by devolatilization or by thermal stress, and then a secondary fragmentation by combustion of char [25]. The volatile combustion occurs mainly in bubbles in the dense bed and in dilute phase in the freeboard. The char combustion occurs in emulsion phase in the dense bed and also in dilute phase in the freeboard. The combustion rate of char is controlled by both reaction kinetics and gas diffusion. Our studies also found that the combustion occurring in the dense bed of a CFB boiler is in fuel lean condition, which is on opposite of a bubbling bed boiler [26]. The result matched the experimental observation by Leckner [27], who reported the vigorous fluctuation of oxygen in bed. Our later research proved such phenomena is contributed to the average particle size in CFB boilers (around 200µm) is much smaller than that in bubbling bed (around 1mm) [28]. Compared with bubbling bed boilers, in CFB boilers, the fraction of fluidization air into emulsion phase is smaller and the resistance of gas exchange between bubble phase and emulsion phase bed is stronger. Char combustion mostly occurs in emulsion phase in the dense bed, consuming most of oxygen over there. Since oxygen can not be compensated from bubbling phase, the CO concentration on the boundary of dense bed of CFB boiler is very high [29] Again, the combustion theory was applied to the commercial CFB boiler design. The concept so-called vertical distribution of combustion and heat in furnace was introduced by TH [28]. This concept is useful for boiler designers to arrange heating surfaces in furnace and it was also validated by gas sampling along the furnace height of some commercial CFB boilers. The field test data of vertical distribution of combustion and heat were also used to correlate the 1-D combustion model developed by TH. Figure 12 shows the experimental results of accumulative heat released along the height of a bench scale CFB apparatus. Both modeling and measurement showed that the vertical distribution of combustion and heat in CFB boilers are strongly impacted by the volatile content and size distribution of fuel. The results shown in Fig.13 indicate that volatile matter prefers to be burnt in the upper part of the furnace and so does fine char particles. Therefore, proper size distribution of specific Accumulative combustion heat fraction Dimensionless height Figure 12 Distribution of the accumulative heat released along the height of a bench-scale CFB boiler feeding fuel is required to satisfy a uniform temperature distribution in CFB boiler furnace. An interesting result should be mentioned is that the accumulation of heat releasing in dense

12 bed of CFB boiler is much less than that of in bubbling bed. This was explained before, and also tells us why we have to put certain amount of immersed heating surface in bubbling bed to keep heat balance, but it is not needed for CFB dense bed. CONCLUSIONS Accumulative combustion heat fraction V daf 34.4% Char Height h m A set of design theory for CFB boiler has been developed by the researchers at Tsinghua University, based on twenty-year research and development experience on CFB boiler. The theory couples the fundamental studies in the laboratory with the experiments on the commercial CFB boilers, and has been applied in designing more than Accumulative combustion heat fraction commercial CFB boilers. Followings are a few main points of the design theory. 1. The flow pattern inside CFB boiler furnace is classified as the superposition of a fine particle fast bed in the upper part and a bubbling bed or turbulent bed in the bottom part with bed coarse particle segregation. 2. CFB boiler is an open system for solid-gas flow. Modeling studies shows the bed quality strongly depends on the overall system efficiency and ash size formation and attrition of coal on. 3. The state of a CFB boiler is defined by superficial velocity u f and circulating rate G s. A CFB boiler can operate at different states in fast bed regime with a given u f and dependent G s s by adjusting the bed inventory during operation. 4. As first step of process design, CFB boiler designers specified a firm state of fast bed for the CFB boiler burning design coal. This step is called State Specification and is the base of CFB boiler design. The State Specification is mainly performed on engineering experience. 5. After State Specification, double check the material balance for design fuel by material balance model and corresponding ash formation and attrition experiments is suggested. If the material balance does not satisfy, certain amount make-up inert sands instead of selecting a new state is recommended, because almost all design data are based on the specified state, including local heat transfer coefficients and combustion heat releasing profiles. 6. A simple model on heat transfer suggested by Bo Leckner and his coworkers can be adopted and improved, and integrated in the Design Code for commercial CFB boiler design. The model has satisfied accuracy in engineering practices. 7. Combustion of char and volatile content shows different behaviors in CFB boilers. The coal combustion also is different between a CFB boiler and a bubbling bed. The concept of vertical distribution of combustion and heat in CFB boiler furnace was introduced. Modeling and ~0.6mm 1.0~1.6mm Height h m Figure 13 Vertical distributions of combustion and heat inside CFB furnace burning coals with different volatile content and coal size

13 experimental studies were conducted on the coal combustion in CFB boilers indicated that volatile matter prefers to be burnt in the upper part of the furnace and so does fine char particles. Therefore, proper size distribution of specific feeding fuel is required to satisfy a uniform temperature distribution in CFB boiler furnace. ACKNOWLEDGMENTS Financial supports of the present investigation by EDF and Chinese National Key Projects of Tenth-Five Plan are gratefully acknowledged. REFERENCES [1] Kuhle K. Zement-Kalk-Gips. 1984, 34: 219~225 [2] Shi Y. The First Supercritical Circulating Fluidized Bed Boiler in the World. Power Station Reconnaissance and Design, 2004, 1: [3] Yu L, Lu J, Wang Z. et al. The Future Investigation of Circulating Fluidized Bed Combustion Technology. Journal of Engineering for Thermal Energy and Power, 2004, 19(4): [4] Lu J, Ling X, Yu L, et al. Concept Design of a 200MW Circulating Fluidized Bed Boiler Based on the Domestic Technology. Boiler Manufacturing, 2002, 3: 1~5 [5] Liu Q, Lu J, Xin J, et al. Steam and water wall temperature of a 600MWe supercritical pressure circulating fluidized bed boiler. Boiler Technology, 2003, 34(3): 34~38 [6] Wu Y, Lu J, Zhang J, et al. Conceptual Design of an 800MWe Supercritical Pressure Circulating Fluidized Bed Boiler. Boiler Technology, 2004, 35(3): 12~16 [7] Li Y, Kwauk, M. The Dynamics of Fast Fluidization. In: Kunii D, Matsen J M eds. Fluidization IV, Plenum Press, New York and London, 1980: , [8] Lu J, Zhang J, Xing X, et al. Solid Suspension Density Distribution in the Furnace of 75t/h Circulating Fluidized Bed Boiler with Water-cooled Square Separator. In: Chen X, Chen T, Chen Z, eds. Proceeding of 4th International Symposium of Multiphase Flow and Heat Transfer, Xi an, 1999: [9] Li Y, Chen B, Wang F, et al. Study on fast Fluidized Bed. Engineering Chemistry & Metallurgy. 1980, 4: [10] Yang H, Xiao X, Wang X, et al. Model Research on Material Balance in a Circulating Fluidized Bed Boiler. In: Xuchang Xu ed. Proceeding of the 5th International Symposium on Coal Combustion, Nanjing China, 2003: [11] Yang H, Xiao X, Lu J, et al. Modeling Research of Residence Time of Materials in a Circulating Fluidized Bed Boiler. Journal of Engineering for Thermal Energy and Power, 2003, 18(2): [12] Tang Z, Yue G, Qian M, et al. The Experimental Investigation on the Coal Ash Formation in CFB Combustion. In: Donald W Geiling, Donald L Bonk eds. Proceeding of 16th International Conference on FBC, Nevada, ASME, 2001: No.60 [13] Lu J, Yang H, Zhang J, et al. A Simple Method to Investigate the Ash Size Distribution and Its Attrition. Journal of Combustion Science and Technology, 2003, 9(5): 477~480 [14] Jin X, Lu J, Li Y, et al. Experimental Investigation on Heat Transfer in Industrial-scale

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15 Note: Will be published in the 18th International Conference on Fluidized Bed Combustion, Toronto Canada,