DESIGN THEORY OF CIRCULATING FLUIDIZED BED.pdf

http://www.paper.edu.cn 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 100084, 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 100-150MWe 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- Normally. Appropriate understanding of the fluid mechanics inside the furnace is of fundamental importance to design a CFB boiler. especially those works linking the fundamentals with practical application is to be given.bed voidage vs. TWO PHASE FLOW IN CFB BOILER Typically.. based TH’s research and development (R&D) achievements. In the lower part. the classification of its fluidization regime has been an argument in CFB boiler research community for a long time. superficial velocity [7] However. m/s uf (m/s) Figure 1 Fluidization regimes for Al2O3 particles. the fluid mechanics inside the furnace is separately described in two parts: a lower part and upper part. the main portion and so-called free board of the bed. Theoretically. slugging bed. the main loop is a typical solid-gas two-phase flow system with chemical reaction.cn 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. 1[7]. the regimes of fluidization can be classified into stationary bed (or say fixed bed). as shown in Fig.paper. turbulent bed. size distribution is rather wide with many coarse particles and bulk density is rather high. Thus. Pneumatic Transport Al2O3 Beads dp=52µm ρp=3580kg/m3 vp: particle velocity. particulate fluidization. bed voidage and physical properties (e. depending on the gas superficial velocity uf.6]. a summary of the two-decade R&D works on CFB boilers by the TH research group. the so-called dense bed. the associated fluidization regime is not necessarily fast bed.g. the main loop of a CFB boiler is composed of a riser. For some small units. fast bed and pneumatic transport. In this paper.edu. Nevertheless.http://www.5. separators and loop seals. in the upper part. single separator and single loop seal might be applied. bubbling bed. it can be bubbling bed or turbulent bed depending mainly on the uf. size and density) of the solid particles. Since the bulk density of most coal-fired CFB boiler furnaces (tens of kg/m3 or even less) [8] is much smaller than that of fast bed reactors in chemical engineering process (in the -2- . MATERIAL BALANCE IN CFB BOILER Based on the observation on coal-fired CFB boilers. Given the upper part of a CFB boiler is a fast bed. In a bubbling bed.25 ρ f1. However. which is often called bed quality. a conceptual model was built up by TH’s CFB boiler research group [10]. the most distinguished feature of a fast bed is the formation of cluster in the riser. for certain particles.5-4)ut where.627 (2) where. even the combustion keeps going in the gas-solid flow and the furnace is surrounded by water-cooled membrane. the hydrodynamic and thus combustion and heat transfer behaviors inside the bed become CFB-alike and qualitatively different from bubbling bed. Then the onset superficial velocity of fast bed for certain size particle is defined as uc [9]: (1) uc=(3. However. Thus. m/s.164[gd p (ρp − ρf )]0. However. It can be seen from (2). 1. ut is the terminal velocity of particle. -3- . and g is the gravity. an accumulation process for size selection exists during the operation. the fluidization regime inside a bubbling bed boiler is totally different from that inside a CFB boiler. kg/m3. only small amount of particles are entrained into the free board so that combustion fraction in the dense bed is about 75-85%.edu. in a CFB boiler. It was found that for a bubbling bed boiler retrofitted with fly ash recirculation. During the CFB boiler evolution history in China. the authors suggest that the upper part of a CFB boiler still belongs to fast bed rather than pneumatic transport. Gs is also assumed to be the solid flux at the separator entrance. it was easily intended to classify the fluidization regime as pneumatic transport.627 0. The minimum solid circulating rate to enter the fast bed regime Rmin can be estimated by [7]: Rmin = uc2. shown in Fig. the average size of bed inventory. According to our observation. The temperature in the dense bed can be even too low to keep stable combustion. CFB boiler can operate at various states in fast bed regime because the bed inventory in CFB boiler is composed of different size particles. For engineering simplicity. ρf is the gas density. and no convective heat transfer surfaces are necessary to be arranged there. much more particles are entrained into the free board so that combustion fraction in the dense bed only occupies about 50-60%. resulting in strong vertical mixing. As we known.cn range of hundreds of kg/m3) [3]. In order to study the material balance. ρp is the particle density. temperature distribution is rather uniform in the bed not only in the core region in radial direction but also along the furnace height. and a rather amount of immersed tube has to be arranged there. for a certain uf. if the recirculation flow rate is above a critical amount. is finer than that of bubbling bed boiler and even finer than that of feeding raw coal. Such temperature uniformity can be only maintained by the existence of strong vertical solids mixing and thus the existence of clusters. flow dynamics of the two-phase flow. or called hydrodynamic state can be defined by two parameters: superficial velocity uf (m/s) and solid circulating rate Gs (kg/m2⋅s). m/s2.paper. kg/m3. a CFB boiler was once regarded as nothing else than the traditional bubbling bed boiler with an extended free board.http://www. we have: Gout = Gout (i ) X (i ) (10) Some interesting and valuable results can be derived from the model.2 [11]. so X(i) Gin(i)=Gout(i)+F(i) (3) where. 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. It can be seen that ηm first -4- .paper. a CFB boiler is an open system for both gas flow and solid flow. The entrained flow rate of particles with size di is accounted as E(i)×X(i). inert sands for making up. The separator efficiency for size di based on the entrained flow is: ηs (i )=1 − F (i ) E (i ) ⋅ X (i ) (4) Then. the overall efficiency of the system ηm to maintain particles with size di is: η m (i )=1 − Gout (i )+F (i ) = η oi + η i − 1 E (i ) ⋅ X (i ) (7) Material balance equation can be expressed as: Gin(i)=Gout(i)+E(i)⋅X(i)⋅(1-ηi) (8) ΣX(i)=1 (9) Provided E(i) is properly given in literature and segregation in dense bed can be neglected. F(i)=E(i)⋅X(i)⋅(1-ηi) (5) If we define the bed ash drain efficiency ηo based on the entrained flow as: ηo(i)=1-Gout(i)/E(i)×X(i) (6) Then. Gout(i) is the flow rate of drained bed ash with size di. then after solving the equation group. Figure 3 depicts the variation of overall system efficiency ηm with particle size d and the size distribution of bed inventory for given separator efficiency ηs and ash drain efficiency ηo. The solid inputs are ashes formed from feeding fuel. sometimes.edu.cn CFB boiler is an open system for solid flow F(i) E(i)× X(i) Different from most chemical reactors.http://www. F(i) is the flow balance of CFB boiler rate of fly ash with size di. and E(i) is the entrainment rate of particles with size di. limestone and. Gin(i) Conceptual model of material balance in CFB boiler Solid particles of any size interval should be kept in balance during the stable operation. as shown in Fig. which is from the ash formation of Figure 2 Concept of material coal and limestone or make-up sands. Gin(i) is the flow rate of solids with size di Gout(i) entering the system. X(i) is the fraction of particles with size di in dense bed. the mean particle size decreases and fewer particles can be entrained and thus Gs decreases.0 27 160 distributions of bed inventory are 0. the particle size corresponding to the peak value of the size distribution 0 curves is smaller. namely smaller d50 and 4.3 5. and consequently the overall distribution of the particles becomes wider though the mean particle value keeps nearly the same.5 27 160 0. Moreover.0 17 110 material balance is built up. as the 5.5 27 160 the same. The size distributions of bed inventory for two separators with different cut sizes d50 and d100 are shown in Figure 4. As uf increases.1 Bed material size distribution 20 0 Frequence distributionPi %/µm is larger than the peak value. At the same time. The particle size distribution of bed inventory. more fine particles are entrained into and stored in the free board. It is clear that although the size of feeding particles into system is widely distributed. and very fine particles which are difficult to be capture by the separator are carried out the system by flue gas. It can be seen. for three different 0. for different ufs. the size 0. ηm is Separator efficiency 80 Efficiency ηi % value.5 uf m/s d50 µm 17 d99 µm 110 5. For the same different cyclone efficiencies separator. also a solution for given ηs and ηo.6 Frequence distributionPi %/µm increases with the increasing of di and after it reaches a peak value it decreases with the further increasing of di. For a separator with better 4. When balance is reached.http://www.cn dominated by ηo.4 5. the CFB boiler system behaves like size selection machine.paper.1 d100.edu. As a result.5 17 110 performance. forcing more ash particles including the particles less than d100 are drained from the bottom.2 60 40 0. Coarse particles which can not be entrained are drained out from bottom of bed.3 Overall efficiency Ash drain efficiency 0. exhibits a cap-like curve.0 0 200 400 600 Particle size d i µm 800 1000 Figure 3 Overall efficiency of the system 0. ηm is dominated by ηs and as d 100 0. the particle size corresponding to the peak value on the size distribution curve is consistent with that corresponding to the highest ηm. more particles around the mean value are drained. Only those particles that can be entrained by the -5- . the particle size corresponding to the peak values on the frequency distribution curves are nearly constant.5 ufs while the ash drain efficiency ηo is 0. Gs increases and the amount of returning particles increase.2 remained. This result is 0 200 400 600 800 straight forward since more fine Particle size d i µm particles are captured if separation Figure 4 Size distribution of bed inventory for efficiency increases. As d is smaller than the peak 0. are needed to keep fine circulating ash in bed. STATE SPECIFICATION FOR CFB BOILER DESIGN 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.cn flue gas and also be captured by the separators are retained in system for circulating. This result is not only important for the design of separator but also important for determination of bed ash drain characteristics. Frequence (%/micron) distributionPi %/µm 质量频度 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].edu.010 ash between the data measured in the . The uf is a design and operating parameter. the state of a CFB boiler can be represented by the superficial gas velocity uf and solid recirculation rate Gs. ash drain facilities with specific size classification. Our studies on the commercial CFB boilers showed that Gs is typically in the order of 103 larger than the feeding rate of such size particles. Thus the overall system efficiency.008 field of this boiler and those predicted by the 1-D model. In engineering practice. 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. especially the efficiency for circulating ash (near the d99 of separator) is very important and sensitive for the circulating rate. so the efficiency near this size should be over 99. combined with ash cooler.http://www.004 attrition on ash size formation. remarkable discrepancy would be induced. 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.006 there is an important impact of .002 Without taking the attrition of solid 0.7%. It can be seen that .000 particles into account. besides the superficial velocity and ash formation characteristics of coal and limestone. The model was calibrated by the field test data from three boilers in China and successfully applied to predict the .016 Measurement 床测试数据 Province循环 material balance in the Gardanne’s Intrinsic . Figure 5 . . The particle segregation in dense bed was taken into account in the model to characterize the bed ash drain.014 Gardanne煤种本 征 成灰分 布 Model prediction 布 Gardanne计算成灰分 250MWe CFB boiler. The prediction on resident time of different size particles and its impact on attrition is a novel feature of the model.paper.012 compares the size distributions of fly . 10 100 10000 Particle size d i µm1000 Figure 6 is the comparison of the 颗粒直 径 (micron) Figure 5 Comparison of model prediction on the bulk density along the height of ash formation w/o attrition with the data measured in furnace by field test and model the field for a 250MWe CFB prediction. From previous discussions. separation -6- . sometimes. while the Gs is a dependent variable on the uf. and the fractional fuel heat releasing along the furnace height could not be kept stable during 1 operation. the heat transfer coefficients between water-wall membrane and solid-gas flow in furnace.Case 2 0.cn efficiency.0 Dot: measurements Line: Model Prediction Dot: measurements Line: Model Prediction 0 .2 0. limestone or make-up sands are varying.paper. 2.4 increasing of bulk density in furnace. the state of a CFB boiler might keep changing as well if Gs can not be controlled. as we Dimensionless height discussed in the material balance section.8 controlled by adjusting the bed 1.2 State Specification plays a fundamental role in CFB boiler 0 design. 12 3 and thus the increasing of Gs at the furnace outlet.8 1 before conducting the detailed design.http://www.5 1 . Fortunately.6 3. 0 0.2 0.Case 3 Shown in Fig. For an industrial combustion process. In engineering practice. the increasing of the bed inventory leads the 0.0 0. Gs can be manually 0. 1.0 0.0 Dimensionless Pressure 0 .0 0 . 0.0 D im e n s io n le s s P re s s u re 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. the bulk density in furnace a specific state in fast bed regime for and circulating rate Gs -7- .4 0. which strongly depends on the bulk density [14]. 7.5 1. Demensionless bulk density CFB boiler designers usually selected Figure 7 Bed inventory vs. the operating state has to be controlled to a stable state.edu.6 0 .Case 1 inventory. ash drain efficiency and solid inputs in the open system.6 0. Consequently.6 0.8 1 .2 0 .4 0.8 Dimensionless Height Dimensionless Height 0.4 0 .0 Case 2 Case1 0.0 0 . would be developed to design the layout and components of the CFB boiler. both predicted by TH. the dot-dash curve close to the uf axis is the onset circulating rate of fast bed which is based the calculation assuming the particle size is around 200µm (according our observation. besides the resident time for fine coal particles burnout and de-NOX [15].EDF material balance model assuming no limestone or inert additives are added. Above this curve. Major Consideration in State Specification The determination on superficial velocity uf The uf 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 ufs in order to obtain higher specific cross section load. namely to perform State Specification for a CFB boiler. In addition. Consequently. Below the line. Two dot curves -8- . As a result. much of the data were by our estimation or by our field measurements (most CFB boiler makers have demonstration boilers in China). However. there are two curves (one in dot-dash. since Gs is related to the total bed inventory (Fig. 7). Another factor limiting Gs is the erosion in furnace. more data are provided to improve and mature the Design Code. Recent research in China shows that the solid suspension in furnace influences gas diffusion. designers started to collect much referential data such as heat transfer coefficients and fractional heat releasing along height of furnace. In above state diagram.http://www. the total bed inventory can be divided into circulating ash inventory that is important for keeping an enough amount of Gs. uf is limited by the erosion on the vertical water wall. and the other in dash) representing the maximum circulating rates for CFB boiler with one stage cyclone and two stage cyclones in serial respectively. otherwise the bed is a bubbling bed. a program.16. State Specification Practices The Gs-uf diagram shown in Fig. each CFB boiler manufacturer owns a specific Design Code as a commercial secret and makes CFB boilers in different styles. The upper limitation for Gs depends on several considerations. special cautious should be paid in State Specification. mainly from the field test on demonstration boilers collaborated with laboratory researches. Because few data on circulating rate for commercial CFB boilers were published. fast bed state can not be realized. so-called Design Code. Once CFB boilers designed by the Design Code are put into commercial operation. 8 summarizes the State Specification done by several major CFB boiler manufacturers in the world. The determination of solid circulating rate Gs The Gs should be more than the minimum solid circulating rate of fast bed regime -Rmin as discussed before. on one hand. and the coarse particle inventory that is important for keeping sufficient resident time for the burnout of coarse coal particles. Based on State Specification and the following data accumulation. 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. thereby the burnout efficiency of coal char. on the other hand. it is related to the power consumption of draft fan.edu.cn the CFB boiler. This accumulation is actually a long-term R&D work.paper. the cut size of circulating material for most CFB boilers is around 150-250µm [4.17]. For example. After State Specification (with fixed uf and Gs). no erosion problem on vertical water wall has been reported for the boilers using same technology while lignite is burning. for some CFB boiler technologies of which uf is near or over 6m/s. bituminous or anthracite coal. The bench-scale tests on ash formation and attrition characteristics for coal and limestone are strongly suggested to be done first. it is safer for the designers to select uf to be lower than 5. a reliable material balance model is needed for designers to validate the material balance for design coal. which is safe for most coal types and the Gs is also far away from the material balance limit.paper. With those experimental data. Clearly. the selection of acceptable fast bed state is limited within a small area in the state diagram.5m/s in case fuel quality can not be guaranteed. the hardness of ash and the superficial velocity have more significant impact on erosion than the circulating rate. -9- . TH also suggest its own state (marked in asteroid *). If it does not. Although. Ash circulating rate GS kg/(m2⋅s) 30 25 20 A F B G C H D I (Two stage cyclone) Limits for erosion protection E 15 One stage cyclone Commended 10 Fast bed limit Soft coal 5 Hard coal 0 1 2 3 4 5 6 7 8 9 10 Fluidizing velocity uf m/s Figure 8 State Specification by several major CFB boiler manufacturers In fact.http://www. We have to point out here. the model can estimate the quantity and quality of make-up sands. after stated of a CFB boiler is specified. serious erosion has been found on the vertical water wall in furnace within limited operating period burning lean coal.edu. designers can use the model to check if the maximum ash circulating rate has enough margins for the specified state. 8. As shown in Fig.cn approximately parallel to the Gs axis stand for the erosion limitation for lignite combustion and hard ash content coal combustion respectively. those CFB boilers are operating at the states near to the erosion line. According to our observation. These limitations are from our observation for a group of CFB boilers with different design statuses and for burning different coals in China. TU has Control Bar conducted a series of experimental Figure 10 Sampling probe for bulk density studies on the commercial CFB Thermocouples in center boilers. αb. At the same. The schematics of heat flux Thermal insulation layer Protecting shell water outlet probe and local bulk density probe Thermocouples in probe surface are shown in Fig. a semi-empirical model was developed based on the suggestions from Bo Andersson and Leckner [23] and further correlated with the field data. T and ε denote for temperature emissivity respectively. 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.paper. More detailed information about the model can be found in other publications [24].edu.cn HEAT TRANSFER IN CFB FURNACE There are enormous literatures Upper on heat transfer research in CFB Cover boiler s [18~21]. a and b are correlation parameters with data from the field test. Slide For engineering purpose.http://www. 11 Figure 11 Schematic of Heat Flux Probe [22]. A Heat Flux Probe and a Probe Solid Suspension Density Probe water inlet were developed to measure the heat transfer coefficients and solid density respectively and successfully applied in the field tests. and it has been practiced in the design of more than one hundred units of CFB boilers with different capacities. Later on. They are valuable Bottom Slide for understanding the mechanisms Cover Guide of heat transfer in bed. is mainly composed of two components – particle suspension convective heat transfer coefficient αc and particle suspension radiative heat transfer coefficient αr.10 - . 10 and Fig. The overall heat transfer coefficient between two phase flow and the water wall. The αr is calculated by following equation: 1 αr =1/( + εb ε 1 w −1) ⋅ σ(Tb2 + Tw2 ) * (Tb + Tw ) (13) where. αb=αc+αr (11) The αc is expressed as the function of local bulk density of solid suspension ρ as: αb=aρb (12) where. but difficult to be directly used into application. and the subscripts of b and w denote respectively the suspension and water wall. COMBUSTION IN CFB . Since oxygen can not be compensated from bubbling phase. The 0. the combustion theory was applied to the commercial CFB boiler design. Therefore.paper.8 Both modeling and measurement showed that 0.0 height of a bench scale CFB apparatus.0 upper part of the furnace and Dimensionless height so does fine char particles.11 - .http://www. who reported the vigorous fluctuation of oxygen in bed. proper size Figure 12 Distribution of the accumulative heat released along the height of a bench-scale CFB boiler distribution of specific feeding fuel is required to satisfy a uniform temperature distribution in CFB boiler furnace. Our studies also found that the combustion occurring in the dense bed of a CFB boiler is in fuel lean condition.8 1. and then a secondary fragmentation by combustion of char [25]. in CFB boilers. Our later research proved such phenomena is contributed to the average particle size in CFB Accumulative combustion heat fraction boilers (around 200µm) is much smaller than that in bubbling bed (around 1mm) [28]. 0. Compared with bubbling bed boilers. The concept so-called “vertical distribution of combustion and heat in furnace” was introduced by TH [28].13 indicate that volatile matter 0.cn Coal combustion modeling and bench-scale experiments also have been extensively conducted at TH. It was found that the coal particles. The volatile combustion occurs mainly in bubbles in the dense bed and in dilute phase in the freeboard. The result matched the experimental observation by Leckner [27]. The char combustion occurs in emulsion phase in the dense bed and also in dilute phase in the freeboard.6 the vertical distribution of combustion and heat in CFB boilers are strongly impacted 0.0 prefers to be burnt in the 0. The field test data of vertical distribution of combustion and heat were also used to correlate the 1-D combustion model developed by TH.2 results shown in Fig.4 0. as soon as fed into CFB boiler furnace. The combustion rate of char is controlled by both reaction kinetics and gas diffusion.edu. 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. which is on opposite of a bubbling bed boiler [26].2 0. 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. Figure 12 shows the experimental results of accumulative heat released along the 1.0 0.4 by the volatile content and size distribution of fuel. consuming most of oxygen over there.6 0. Char combustion mostly occurs in emulsion phase in the dense bed. experience a primary fragmentation by devolatilization or by thermal stress. the CO concentration on the boundary of dense bed of CFB boiler is very high [29] Again. An interesting result should be mentioned is that the accumulation of heat releasing in dense . 4.paper. The state of a CFB boiler is defined by superficial velocity uf and circulating rate Gs. 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. CONCLUSIONS 0.8 0.edu.4 researchers at Tsinghua University. The coal combustion also is different between a CFB boiler and a bubbling bed.cn 1 Accumulative combustion heat fraction bed of CFB boiler is much less than that of in bubbling bed. and has volatile content and coal size been applied in designing more than 100 commercial CFB boilers. Combustion of char and volatile content shows different behaviors in CFB boilers. Modeling studies shows the bed quality strongly depends on the overall system efficiency and ash size formation and attrition of coal on. A CFB boiler can operate at different states in fast bed regime with a given uf and dependent Gss by adjusting the bed inventory during operation.4 Vdaf 34. and integrated in the Design Code for commercial CFB boiler design. including local heat transfer coefficients and combustion heat releasing profiles.6 boiler has been developed by the 0.8 A set of design theory for CFB 0. 1.12 - . certain amount make-up inert sands instead of selecting a new state is recommended.6mm 0. and also tells us why we have to put certain amount of immersed heating surface in bubbling bed to keep heat balance. CFB boiler designers specified a firm state of fast bed for the CFB boiler burning design coal. 3.5~0. A simple model on heat transfer suggested by Bo Leckner and his coworkers can be adopted and improved.6mm 0. 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.4% 0.http://www. The concept of vertical distribution of combustion and heat in CFB boiler furnace was introduced. because almost all design data are based on the specified state.2 based on twenty-year research and 0 development experience on CFB 0 1 2 3 4 5 boiler. The model has satisfied accuracy in engineering practices.2 Char 0 0 1 2 Height h m 3 4 5 Accumulative combustion heat fraction 1 0. 1.0~1. 7. The State Specification is mainly performed on engineering experience. This was explained before. As first step of process design. This step is called State Specification and is the base of CFB boiler design.6 0. Followings are a few main points of the design theory. Modeling and . 6. 2. After State Specification. The theory couples the Height h m fundamental studies in the laboratory Figure 13 Vertical distributions of combustion and with the experiments on the heat inside CFB furnace burning coals with different commercial CFB boilers. 5. but it is not needed for CFB dense bed. CFB boiler is an open system for solid-gas flow. Donald L Bonk eds. Boiler Technology. Yue G. ASME. 1984. In: Xuchang Xu ed. Lu J. Nevada. In: Kunii D. Engineering Chemistry & Metallurgy. Xing X. 4: 20-45 [10] Yang H. et al. et al. Matsen J M eds. 2004. Proceeding of 4th International Symposium of Multiphase Flow and Heat Transfer. 2003. et al. 1980: 537-544. Boiler Technology. 35(3): 12~16 [7] Li Y. et al. The Experimental Investigation on the Coal Ash Formation in CFB Combustion. Wang F. et al. 9(5): 477~480 [14] Jin X. A Simple Method to Investigate the Ash Size Distribution and Its Attrition. In: Chen X. Zhang J. The First Supercritical Circulating Fluidized Bed Boiler in the World. Lu J. Yu L. 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