R&D and Demonstration of Large Domestic CFB Boilers

In order to develop large CFB boilers with independent intellectual property, Xi'an Thermal Power Research Institute (TPRI) established a laboratory with complete functions for the technical development of CFB boilers. This laboratory consists of a 1-MW and a 4-MW CFB combustion test facilities and a laboratory for limestone desulphurization performance evaluation. It carried out tests on CFB combustion and desulphurization for Chinese typical coals and limestone and research on heat-transfer characteristics and key parts, and developed the first home-made 100-MW CFB boiler. Based on the experience of R&D, the laboratory further researched key techniques for enlarging capacity systematically, and cooperating with Harbin Boiler Co. (HBC), developed the first domestic 210-MW CFB boiler with independent intellectual property and put it into engineering demonstration, laying a solid foundation for the development of CFB boilers of even larger capacity.

Fault Analysis and Innovation in Slag Discharge System of 440-t/h CFB Boiler

CFB boilers have been widely used in China in recent years with their perfect performances in coal adaptability, load variation capability and lower pollutant emission. The No.3 135-MW CFB unit in Lianzhou Power Plant is the f irst 440-t/h series CFB unit in Guangdong Province. It f inished 72-hour trial operation in Feb. 2004 and was transferred to trial operation. During the trial operation and the next commercial operation, there were some problems happened in the boiler slag discharging system, seriously affecting the safe and reliable operation and the loading capability. After innovation, these problems have been completely solved. Hopefully the solutions may be used for reference to the units with similar problems.

Comprehensive Solution to Flue Gas Desulfurization and Denitration of Circulating Fluidized Bed Boiler

With the advantages of high combustion efficiency, wide fuel flexibility and low concentrations of discharged pollutants, circulating fluid- ized bed （CFB） boiler has been widely used in recent years. However, in order to meet the requirement of new emission standard, it＇s necessary to add flue gas desulfurization and denitration devices. In this paper, the choice of flue gas purification processes for CFB boiler has been discussed firstly, and then the economy and rationality of the SNCR ＋ CFB-FGD ＋ COA comprehensive solution to flue gas desulfurization and denitration have been analyzed.

CPFD modeling of hydrodynamics,combustion and NOx emissions in an industrial CFB boiler

The ultra-low NOx emission requirement(50 mg/m^(3))brings great challenge to CFB boilers in China.To further tap the NOx abatement potential,full understanding the fundamentals behind CFB boilers is needed.To achieve this,a comprehensive CPFD model is established and verified;gas-solid flow,combustion,and NOx emission behavior in an industrial CFB boiler are elaborated;influences of primary air volume and coal particle size on furnace performance are evaluated.Simulation results indicate that there exists a typical core-annular flow structure in the boiler furnace.Furnace temperature is highest in the bottom dense-phase zone(about 950℃)and decreases gradually along the furnace height.Oxygen-deficient combustion results in high CO concentration and strong reducing atmosphere in the lower furnace.NOx concentration gradually increases in the bottom furnace,reaches maximum at the elevation of secondary air inlet,and then decreases slightly in the upper furnace.Appropriate decreasing the primary air volume and coal particle size would increase the CO concentration and intensify the in-furnace reducing atmosphere,which favors for NOx reduction and low NOx emission from CFB boilers.

Influence of Limestone Addition on Combustion and Emission Characteristics of Coal Slime in the 75 t/h CFB Boiler with Post-Combustion Chamber

Coal slime can be disposed in quantity and fully utilized in a well-designed circulating fluidized bed(CFB)boiler,but the nitrogen oxides(NO_(x))and sulphur dioxide(SO_(2))emissions generated in the combustion of coal slime have contributed to serious atmospheric pollution.High Temperature&Post-combustion Technology,a novel and high-efficient way to reduce the NO_(x)emission in the process of combustion,is applied to a 75 t/h CFB boiler burning exclusively coal slime,which will succeed to meet the ultra-low NO_(x)emission standard.To further explore an appropriate method to reduce the SO_(2)emission under the condition of new technology,the experiments were conducted on a 75 t/h CFB boiler with post-combustion chamber to study the influence of limestone addition on the combustion and emission characteristics of coal slime.The experimental results showed that High Temperature&Post-combustion Technology combined with the sorbent injection in the furnace is a very promising technology to control the NO_(x)and SO_(2)emissions simultaneously.Limestone addition can cause the slight decrease in combustion temperature.Limestone addition will lead to the increase in NO_(x)emission in the combustion of coal slime.In 75 t/h coal slime CFB boiler,the desulfurization efficiency of limestone injection in furnace is close to 98%,achieving the ultra-low SO_(2)emission.To meet the standard of ultra-low NO_(x)and SO_(2)emission,the two technologies for simultaneous removal of NO_(x)and SO_(2)emissions are economical and feasible currently:Removal of SO_(2)under ultra-low NO_(x)emission and Removal of NO_(x)under ultra-low SO_(2)emission.

Numerical Investigation on the Heat Transfer Characteristics of Supercritical Water in a Non-Uniformly Heated Tube for a Supercritical CFB Boiler

In this paper,a numerical model was built by ANSYS FLUENT to investigate the heat transfer performances of supercritical water in a circumferential non-uniformly heated vertical tube.The Shear Stress Transport(SST)k-ωmodel was adopted for describing turbulence.The operating parameters are chosen according to a 660 MW ultra-supercritical CFB boiler.The heat transfer performances under different operating parameters,such as boiler load,flow direction and heat flux distribution are analyzed.The temperature and heat flux on inner wall varies along the circumference and show symmetric distributions.The overall heat transfer performances at each cross section are better than the local heat transfer performance of midpoint of heating side.Flow direction has a great influence on heat transfer performance;it changes the radial distribution of axial velocity and then affects the turbulence distribution.Therefore,upward flow condition shows a better heat transfer performance.Smaller heat flux improves both the overall and local heat transfer performances.Reducing the heat flux area is not conducive to the overall heat transfer,but does not affect the local heat transfer at the midpoint of heating side.Finally,a new correlation is fitted based on the simulated results of supercritical water heat transfer with circumferential non-uniform heat flux distributions.

Model prediction of the operating behavior of a circulating fluidized bed boiler

An improved mathematical model for a circulating fluidized bed (CFB) boiler based on the model developed earlier by the authors was applied to simulate the operation of a 12 MW CFB boiler. The influences of the excess air ratio, primary air ratio, coal particle size distribution, coal properties (ash content and volatile content) and Ca/S ratio on the boiler operation were analyzed. The results showed that the model simulation may be applied to the optimum design and economic operation of the CFB boiler.

The Heat Transfer Coefficients of the Heating Surface of 300 MWe CFB Boiler

A study of the heat transfer about the heating surface of three commercial 300 MWe CFB boilers was conducted in this work. The heat transfer coefficients of the platen heating surface, the external heat exchanger (EHE) and cyclone separator were calculated according to the relative operation data at different boiler loads. Moreover, the heat transfer coefficient of the waterwall was calculated by heat balance of the hot circuit of the CFB boiler. With the boiler capacity increasing, the heat transfer coefficients of these heating surface increases, and the heat transfer coefficient of the water wall is higher than that of the platen heating surface. The heat transfer coefficient of the EHE is the highest in high boiler load, the heat transfer coefficient of the cyclone separator is the lowest. Because the fired coal is different from the design coal in No.1 boiler, the ash content of the fired coal is much lower than that of the design coal. The heat transfer coefficients which calculated with the operation data are lower than the previous design value and that is the reason why the bed temperature is rather high during the boiler operation in No.1 boiler.

COAL COMBUSTION EFFICIENCY IN CFB BOILER

The carbon content in the fly ash from most Chinese circulating fluidized bed (CFB) boilers is much higher than expected, thus directly influencing the combustion efficiency. In the present paper, carbon burnout was investigated both in field tests and laboratory experiments. The effect of coal property, operation condition, gas-solid mixing, char deactivation, residence time and cyclone performance are analyzed seriatim based on large amount of experimental results. A coal index is proposed to describe the coal rank, defined by the ratio of the volatile content to the coal heat value, is a useful parameter to analyze the char burnout. The carbon content in the fly ash depends on the coal rank strongly. CFB boilers burning anthracite, which has low coal index, usually have high carbon content in the fly ash. On the contrary, the CFB boilers burning brown coal, which has high coal index, normally have low carbon content. Poor gas-solid mixing in the furnace is another important reason of the higher carbon content in the fly ash. Increasing the velocity and rigidity of the secondary air could extend the penetration depth and induce more oxygen into the furnace center. Better gas solid mixing will decrease the lean oxygen core area and increase char combustion efficiency. The fine char particles could be divided into two groups according to their reactivity. One group is 'fresh' char particles with high reactivity and certain amount of volatile content. The other group of char particles has experienced sufficient combustion time both in the furnace and in the cyclone, with nearly no volatile. These 'old' chars in the fly ash will be deactivated during combustion of large coal particles and have very low carbon reactivity. The generated fine inert char particles by attrition of large coal particles could not easily burn out even with the fly ash recirculation. The fraction of large coal particles in coal feed should be reduced during fuel preparation process. The cyclone efficiency controls the particle residence time in CFB loop, especially for that of the fine particles. So the cyclone efficiency, especially the cut size, will greatly influence the carbon content in the fly ash.

Restart-up Performance of a CFB Boiler after a Sudden Power Failure Accident

Due to the massive bed materials in its typical main loop,a circulating fluidized bed(CFB)boiler may face greater trouble in operation and restart-up if an unplanned sudden power failure happens in the power plant.This study aims to explore the restart-up performance of a 170 t/h natural circulation CFB boiler after a sudden power failure and evaluate whether the hot restart-up can be realized.The heat transfer process after the restoration of the primary air is modeled.Validating the isothermal property of the bed after the restoration of the power and air supply,the correlation among the accident duration time,bed temperature after the power restores,inlet air temperature,air supply time,and air velocity is proposed.The predicted results indicate that the major influencing factor of the bed temperature during this process is primary air velocity.To provide guidelines for judgment on whether the hot restart-up can be realized,the maximum values of the air supply time and accident duration time for hot restart-up for different types of coals are given.The results show both of them have a rapid decrease as the coal ignition temperature in a CFB boiler increases from 450°C to 650°C,which also means that the coal ignition temperature plays a very important role in the restart-up process.Based on the simulation results of bed temperature during the accident,it is also estimated that the drum level drops by 77.7 mm after 8 hours but still stays in the permissible range.

Combustion and NO_(x)Emission Characteristics of Coal Slime Solid Waste at Different Feeding Positions

The NO_(x)emission of coal slime burned in circulating fluidized bed(CFB)boilers could hardly meet the increasingly strict standards in China.Feeding coal slime from the top of furnace led to uneven combustion in furnace and cyclones,short residence time and overheated tail heating surface.The effects of feeding positions on the combustion uniformity and pollutant emission characteristics of coal slime were studied.The experimental results showed that the coal slime combustion was more uniform when feeding from the front wall and longer residence time was conducive to the control of NO_(x)emission.When the boiler temperature and excess air ratio were almost identical,the initial NO_(x)emissions were 45.0 mg·m^(−3)and 70.7 mg·m^(−3)when feeding from the front wall and the top of furnace,respectively;the NO_(x)emission was cut down 36.35%when feeding from the front wall,successfully meeting the ultra-low NO_(x)emission standard of China.The adoption of feeding from the front wall greatly reduced the original emission of NO_(x);the operation costs in the practical applications were saved to a large extent.

Thermal Inertia of 330 MW Circulating Fluidized Bed Boiler during Load Change

The operating principles of Circulating Fluidized Bed(CFB)boilers involve a significant amount of heat accumulation,which forms the thermal inertia of the boiler and hinders the improvement of its variable load response rate.This study aims to characterize the thermal inertia of CFB boilers by evaluating the change in the boiler's heat accumulation corresponding to the change in unit power generation.The thermal inertia of a 330MW CFB boiler was determined through the collection of operating data under four different operating conditions of 30%,50%,75%,and 100%load.The study proposes to substitute the existing refractory material with a metal grille to reduce the thermal inertia of the boiler.The effect of the metal grille on heat transfer was confirmed through verification on a 440 t/h CFB boiler,and its performance change and thermal inertia reduction were further predicted.The results indicate that over 50%of the total thermal inertia of CFB boilers originates from the refractory material.The use of metal grille in place of refractory material improved heat transfer in the furnace,resulting in a decrease of the furnace chamber temperature by 13℃in the 330 MW CFB boiler.This reduction of thermal inertia by 30%-35%will facilitate faster load lifting and lowering of the boiler,fulfilling the requirement for flexible peaking.

Development and technical progress in large-scale circulating fluidized bed boiler in China

Circulating fluidized bed(CFB)boilers has realized the clean and efficient utilization of inferior coal like gangue and coal slime,high sulfur coal,anthracite,petroleum coke,oil shale and other resources.As a country with the largest amount of CFB boilers and the largest installed capacity in the world,China has 440100-600 MWe CFB boilers with a total capacity of 82.29 GWe,including 227 units of 135 MWe,95 units of300 MWe,and 24 supercritical units.The statistics of typical 100-300 MWe CFB boilers showed that the average number of unplanned shut-down was only 0.37 times per year,among which the 135 MWe was 0.26 times per year and 300 MWe was 0.46 times per ycai*.The auxiliary power ratio of some 300 MWe CFB boilers based on flow-pattern reconstruction can be reduced to about 4%,which is closed to the same level of pulverized coal(PC)boilers.This paper summarizes the development process and application status of China's large-scale CFB boilers,analyzes the characteristics and technical performance of the iconic units,and introduces solutions to the problems such as water wall wear and bottom ash cooling.