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In this paper, the design principle of Fluidization State Specification (FSS) is reviewed and its applications in CFB boiler are introduced. In the early 2000s, FSS design principle was proposed to determine the fluidization state in the upper furnace as the basis of engineering design. The principle was initially adopted by engineers to select a proper superficial gas velocity to prevent the severe erosion on water walls. It was used in many CFB furnace designs including the world's largest 600 MW supercritical one in China. Later, FSS design principle was extended to reduce the pressure head of the draft fan to save energy consumption. Industrial application showed that by changing the amount of bed inventory, CFB boilers could operate at a re-constructed fast bed state, not only saving ~30% power of the draft fans, but also further reducing the erosion on water walls and improving combustion efficiency. Recently, FSS design principle was further improved and applied to achieve ultra-low NOx and SO 2 emission. A third coordinate with improved bed quality, i.e., size distribution of bed material was proposed to add on the original two-dimensional fluidization state diagram defined by the superficial gas velocity and the solid circulation rate. Industrial practices showed that under the guideline of the improved FSS design principle, it was possible for CFB boilers to reach over 99% desulfurization efficiency with limestone injected into furnace, and less than 50 mg/Nm 3 (at 6% O 2 ) NOx emission at the furnace exit. FSS design principle promotes the booming of ultra-low emission CFB boilers in China. [ABSTRACT FROM AUTHOR]

In this paper, experiments were carried out to obtain the actual operation characteristics of external heat exchangers (EHEs) in the first 600 MW supercritical CFB boiler. The furnace temperatures and some operational parameters in EHEs were measured and the heat transfer coefficients of heating surfaces in EHEs were calculated. The results showed that by controlling the solid flow rates to the EHEs, the furnace temperature were adjusted for good performance at various boiler loads and the bed temperature were maintained stable at full load. The heat transfer coefficients in EHEs were mostly over 200 W/(m 2 ∙ K), which contributed to the flexible temperature regulation. The tube panel wall temperatures were found to be a saddle-shaped distribution, which was high in the bed center but low near the wall. Packet renewal theory was used to explain such uneven temperature distribution. These results confirm the importance of EHEs to the furnace temperature control of the supercritical CFB boilers, but the uneven tube wall temperatures need to be well solved in the future. [ABSTRACT FROM AUTHOR]

Circulating fluidized bed (CFB) technology was applied for coal combustion after CFB chemical reactors have been widely used and studied in chemical engineering. However, there were still many special phenomena found in the CFB boiler. In this paper, the main differences between a conventional CFB reactor and a CFB boiler are summarized. A CFB boiler is a special CFB reactor. It is an open circulating system with a wide size distribution feedstock and a low circulating solid flow rate. The CFB boiler behaves as particle selection machine, retaining particles within a certain size range in the furnace. Its fluidization regime is distinguished from a conventional CFB reactor, composing of a bubble or turbulent dense bed in the lower furnace and a fast dilute bed in the upper furnace. A large portion of heat is released in the dilute bed, and there is an oxygen-deficient combustion zone in the center of the upper furnace of a CFB boiler. Based on the theoretical studies and engineering practice, the principles and a guidance map of fluidization state specification for CFB boiler design are presented. In addition, the paper reviews the research and development (R&D) history and important achievements of the Chinese CFB combustion technology. [ABSTRACT FROM AUTHOR]

The world first 550-MWe ultra-supercritical circulating fluidized bed (CFB) boiler is designed with integrated recycle heat exchangers (INTREX) and internal solid circulation. In this research, in order to investigate the heat transfer behavior and the significance of the internal circulation, the operation characteristics of the four INTREXes as final superheaters including heat transfer coefficients and ash flow rates under different load conditions are experimentally studied. The calculation of ash circulating rate shows that as the boiler load decreases, the furnace bed temperature and the circulating ash temperature in the loop seal decrease, while the flow rate of the internal circulation and its proportion to the total solid flux increase as the boiler load decreases. In particular, under 60% load condition, the internal flow rate is found to be higher than that of external flow rate for the two INTREXes located in front of the middle sections of the furnace. This indicates that the internal circulation is of more significance in the low-load condition than in the high-load condition. The key factor determining the internal-circulation flow rate is found to be the bed pressure drop in the furnace. The phenomenon of non-uniform distribution of solid flux among INTREXes is also discussed. • Experimental tests were performed on a 550-MWe USC CFB boiler designed with INTREX. • Internal circulation is found to be significant under a low-load condition. • Heat transfer in the INTREX is found to be enhanced by the internal circulation. • The flow rate of internal circulation is found to be increased as load decreases. [ABSTRACT FROM AUTHOR]

Abstract It is difficult to maintain enough high circulating ash rate in circulating fluidized bed (CFB) boilers if they burn the Indonesian coals due to their low ash content. Hence, the ash formation data for these coals is of significance to assessing mass balance in CFB boilers. Two kinds of Indonesian coal that were burned in the 550 MW supercritical CFB boilers in Korea, Kideco and KCH, were tested with static combustion and cold sieving method to study their ash formation characteristics. Results showed that the average primary ash particle size of these two coals is quite fine overall and nearly no gangue exists. Besides, their attrition rate constant K af is much higher than that of some other bituminous or lean coals, and with the increase of particle size, the K af decays exponentially for Kideco, while it initially increases and then decreases for KCH. In addition, the physical and chemical properties of these two Indonesian coals' ash obviously change in particle size, including density, pore structure, chemical composition, etc. The larger ash particles are composed of some hard materials, such as quartz (SiO 2) and mullite (Al 6 Si 2 O 13). While some other materials with lower hardness value, like hematite (Fe 2 O 3) and gehlenite (Ca 2 Al 2 SiO 7), become major mineral components of smaller and softer ash particles. These results indicate that the difference of chemical compositions results in the discrepancy of attrition capability for different size ash particles. Graphical abstract The ash attrition rate of low-rank coals including these two Indonesian coals is much higher than that of some other bituminous or lean coals, and larger ash particles are more difficult to be worn. Unlabelled Image Highlights • Two kinds of Indonesian coal were tested to obtain their ash formation data. • The ash properties obviously change in particle size. • The difference of chemical compositions may result in the discrepancy of attrition rate for different size ash particles. [ABSTRACT FROM AUTHOR]

A 1-dimensional CFB boiler model has been developed to simulate the mass balance and SO 2 emission of the world first 550 MW e ultra-supercritical circulating fluidized bed boiler. • A 1-dimensional CFB boiler model has been developed. • Mass balance and SO 2 emission of the world first USC CFB boiler were simulated. • The modelling results agree well with the field test data. • Improving cyclone efficiency can improve in-situ desulphurization performance. • Properly reducing limestone size can obtain higher desulphurization efficiency. In the present work, the in-situ desulphurization performance of the world first 550 MWe ultra-supercritical (USC) circulating fluidized bed (CFB) boiler was investigated mainly through simulation method. A pseudo steady-state 1-dimensional CFB model has been developed that integrates and simultaneously predicts the particle balance, combustion, lime sulphation and some other aspects. The specially designed integrated recycle heat exchanger (INTREX) and bottom ash self-circulation system of this boiler are also considered. The predicted median diameters of fly ash, bottom ash and circulating ash are 16 μm, 238 μm and 142 μm, respectively, which agree well with the field test data. Both test and modelling results reveal that the when the fine limestone with a median diameter of 50 μm is applied, the in-situ desulphurization efficiency can reach up to 95% and the final SO 2 emission is only around 20 ppm, while no any back-end FGD system is applied. A sensitivity analysis is carried out using the model to examine the effects of cyclone efficiency and limestone particle size on the in-situ desulphurization performance. It indicates that by improving the cyclone efficiency, the SO 2 emission in the outlet of furnace significantly decreases. Besides, appropriately reducing the limestone size can obtain higher desulphurization efficiency. The model presented in this paper can be used to further study the operation characteristics of USC CFB boilers, also including in-situ desulphurization, which is important for accurate prediction and cost-effective control of SO 2 emission. [ABSTRACT FROM AUTHOR]