(Basic knowledge) What are the basic characteristics of circulating fluidized bed boilers?

What are the basic characteristics of circulating fluidized bed boilers? The basic characteristics of circulating fluidized bed boilers are as follows: (1) Low-temperature power-controlled combustion. Its combustion rate mainly depends on the rate of chemical reaction, which is determined by the temperature level. Physical factors are no longer the dominant factor controlling combustion. (2) A high-speed, high-concentration, high-throughput fluidization and circulation process for solid materials. All combustion in a circulating fluidized bed boiler is gradually completed within these two forms of circulation. (3) High-intensity processes of heat, mass, and momentum transfer. The heat in a circulating fluidized bed boiler is primarily generated by the back-and-forth circulation of solid materials at high speed, high concentration, and high flow rate. The transfer and exchange of heat, mass, and momentum within the furnace occur very rapidly, resulting in a very uniform temperature distribution throughout the entire furnace. (4) As the load changes, the fluidization state changes, with the lowest value being 0.

(1) Low-temperature combustion: Fluidized bed combustion is very different from layer combustion and pulverized coal combustion. At all times, there must be a large amount of high-temperature inert material (such as ash, limestone, or sand) available in the furnace; these inert materials account for 97–98% of all the solid materials present in the furnace. In other words, the proportion of solid combustibles in the furnace never exceeds 2–3% of the total bed material. Therefore, even at combustion temperatures of only 850–900°C, any solid fuel can be completely burned as long as there is sufficient oxygen. Additionally, the long residence time of the fuel in the furnace along with strong turbulent mixing within the bed ensure that fluidized bed boilers can burn any type of fuel steadily and efficiently at low temperatures of 850–900°C. (II) Excellent fuel adaptability: Due to the low-temperature combustion characteristic mentioned above, fluidized bed boilers have excellent fuel adaptability; for almost any type of fuel, it is possible to design a fluidized bed boiler capable of burning that fuel, while ensuring stable combustion and high efficiency ； For existing fluidized bed boilers, even when the fuel properties vary over a wide range, the boilers can still maintain stable combustion. To date, the fuels that have been successfully burned in fluidized bed boilers include all types of coal, such as lignite with high ash and moisture content and anthracite with low volatility; coal gangue, washed gangue, and coal washing slurry from various coals; peat; various types of petroleum coke; oil shale; turfs; municipal waste; oily sludge; used tires; and agricultural and forestry biomass waste such as bark, wood chips, rice husks, and sugarcane bagasse. It can also be used to burn various liquid and gaseous fuels; these fuels can be burned either individually or in combination, a feature that is unmatched by any other combustion method. (III) Low pollutant emissions: The characteristic of low-temperature combustion in fluidized bed boilers effectively suppresses the formation of thermally induced NOx. By employing staged combustion, the emission of fuel-induced NOx can be controlled. As a result, the amount of NOx generated in fluidized bed boilers is only 1/3 to 1/4 that of coal powder boilers. The NOx emission levels in bubbling fluidized bed boilers can be kept below 0.03–0.04%, while those in circulating fluidized bed boilers can be controlled at 0.01–0.02%. Furthermore, if limestone or dolomite is added directly into the furnace during combustion, and considering that the combustion temperature of 850–900°C is the optimal temperature for the reaction between lime (CaO) and sulfur dioxide (SO2) to achieve desulfurization, then by adding an appropriate amount of limestone to the furnace of a fluidized-bed boiler, a desulfurization efficiency of around 90% can be attained, depending on the sulfur content in the coal. Therefore, the fluidized bed is an economical and effective low-pollution coal combustion technology, which is the fundamental reason for its rapid acceptance and development around the world. (IV) High combustion intensity: Due to the intense turbulent mixing that occurs during fluidized bed combustion, and since the combustion in a circulating fluidized bed boiler takes place throughout the entire furnace volume, this **significantly increases its combustion intensity, boosts the output per unit of furnace volume, and reduces the cross-sectional area and volume of the furnace. The volume of a circulating fluidized bed boiler can be made smaller than that of a conventional boiler. (5) Strong heat transfer within the bed: The high-intensity heat transfer characteristics in the fluidized bed enable reduced consumption of metal for the heating surfaces inside the furnace. For bubbling fluidized bed boilers, the heat transfer coefficient of the gas/solid mixture within the bed to the tubes embedded in it can range from 233 to 326 W/(m2·K). In circulating fluidized bed boilers, the heat transfer coefficient of the gas/solid mixture in the furnace to the water-cooled walls lies within the range of 450 to 100 W/(m2·K). (VI) Good load regulation performance: Thanks to the large reserve of hot bed material in the furnace, fluidized bed boilers exhibit excellent load regulation capabilities, with a wide range of load adjustments; they can maintain stable combustion even at as low as 20% of the rated load. (7) Easy to operate and maintain: Due to the low combustion temperature, the ash does not soften or stick together, so there is no issue of slag formation inside the furnace, and it is not necessary to install soot blowers in the furnace chamber. A lower furnace temperature results in a lower heat flux on the heated surfaces inside the furnace, reducing the likelihood of tube failure due to heat transfer crises. The corrosive effect of combustion is also lower in fluidized bed boilers and pulverized coal boilers. All these make fluidized bed boilers easy to operate and maintain. As for the wear problems that often occur in fluidized bed boilers, these have been resolved by implementing a series of anti-wear measures such as anti-wear refractory coatings on the parts prone to wear. (8) Easy comprehensive utilization of ash: The ash produced by low-temperature combustion possesses good reactivity, and its fly ash and bottom ash have a low carbon content, usually below 4–5%. It can be used as an admixture in cement production or as a raw material for other construction materials, facilitating the comprehensive utilization of this ash.

Basic characteristics of circulating fluidized bed combustion boilers: (1) Low-temperature, power-controlled combustion. Circulating fluidized bed combustion is a type of fluidized combustion process in which rapidly moving flue gas comes into close contact with solid particles carrying strong turbulent disturbances, and there is significant backmixing of these particles within the furnace； At the same time, the vast majority of the high-temperature solid particles are captured outside the furnace and sent back into it to participate in the combustion process again, thus enabling a cyclic combustion process. Obviously, the time that the fuel burns in the furnace has increased. Under this combustion mode, the temperature level inside the furnace is generally around 850°C, limited by the optimal temperature for desulfurization. Such a temperature is far lower than the temperature levels in conventional coal-fired boilers, and it is also below the ash melting point of ordinary coal, which eliminates the various problems associated with ash melting. This \"low-temperature combustion\" method has many advantages: slag formation and the precipitation of alkali metals inside the furnace are significantly reduced compared to coal-fired boilers; the sensitivity to the properties of ash is decreased; no large space is required to cool the hot ash; nitrogen oxide emissions are low; and it is possible to implement inexpensive and efficient desulfurization processes within the furnace, among others. From the perspective of combustion reaction kinetics, the combustion reaction in a circulating fluidized bed boiler is controlled within the power combustion zone (or transition zone). Since the temperature in a circulating fluidized bed boiler is relatively low, and there is intense mixing of a large amount of solid particles, the combustion rate under such conditions depends mainly on the rate of chemical reactions, that is, on the temperature level; physical factors are no longer the dominant factors controlling the combustion rate. The burnout degree of fuel in circulating fluidized bed boilers is very high; typically, well-performing circulating fluidized bed boilers can achieve a combustion efficiency of 95–99% or higher. (2) High-speed, high-concentration, high-throughput fluidization cycle process for solid materials The solid materials in a circulating fluidized bed boiler (including fuel, char, ash, desulfurizing agents, and inert bed material) undergo an external circulation consisting of the furnace, separator, and return device. At the same time, an internal circulation also exists inside the furnace due to wall effects; therefore, the material in a circulating fluidized bed boiler participates in both external and internal circulation processes. The entire combustion process as well as the desulfurization process are gradually completed within the dynamic cycle of these two forms. (3) Intense processes of heat, mass, and momentum transfer In circulating fluidized bed boilers, large amounts of solid material pass through the furnace under intense turbulence. By manipulating operational parameters, it is possible to adjust the amount of material in circulation as well as the distribution of this material within the furnace, thereby adapting it to different combustion conditions. Under this organizational arrangement, the heat, mass, and momentum transfer processes within the furnace are very intense, which results in a uniform temperature distribution throughout the entire furnace height.