Circulating fluidized bed boiler

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Circulating fluidized bed boilers are a high-efficiency, low-pollution clean combustion technology that has been developed over the past decade or so. It has advantages such as high combustion efficiency, wide adaptability to different coal types, low emission concentrations of harmful gases in the flue gas, a large load regulation range, and the possibility of comprehensive utilization of ash and slag. Working principle of circulating fluidized bed boilers: The process by which solid particles are transformed into a fluid-like state upon contact with gas or liquid is called fluidization. The fluidization process is used in fuel combustion, namely fluidized combustion, and the furnace used for this is called a fluidized bed boiler. 　　The circulating fluidized bed boiler is a new type of boiler developed on the basis of bubbling fluidized bed boiler technology. Its main difference from bubbling bed boilers is that the fluidization wind speed inside the furnace is higher (usually 4–8 m/s), and a gas-solid material separator is installed at the furnace outlet. The fine solid particles carried out of the furnace by the flue gas are separated by a separator and then sent back into the furnace for cyclic combustion. Circulating fluidized bed boilers can be divided into two parts: the first part consists of the furnace (rapid fluidized bed), gas-solid material separator, solid material recirculation equipment, and an external heat exchanger (some circulating fluidized bed boilers do not have this equipment); these components form a solid material circulation loop. The second section is the convective flue, which is equipped with superheaters, reheaters, economizers, and air preheaters, similar to that of other conventional boilers. 　The primary air and secondary air required for combustion in a circulating fluidized bed boiler are supplied from the bottom and side walls of the furnace respectively; the combustion of fuel takes place mainly within the furnace, and water-cooled walls are arranged around it to absorb part of the heat generated by combustion. The solid material carried out of the furnace by the gas flow is collected in a gas-solid separation device and sent back to the furnace via a return mechanism. 　　Basic technical 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, thereby enabling a repeated cycle of combustion. 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 primarily 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 rate 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. As can be seen from Figure 3, the solid materials in the circulating fluidized bed boiler (including fuel, char, ash, desulfurizing agents, and inert bed material) undergo an external circulation that involves 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 movements. 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 change 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. Advantages of circulating fluidized bed boilers. 　　(1) Wide fuel adaptability This is one of the main advantages of circulating fluidized bed boilers. In a circulating fluidized bed boiler, fuel accounts for only 1–3% of the bed material by weight; the remainder consists of non-combustible solid particles such as desulfurization agents and ash. Therefore, the fresh coal particles added to the bed are surrounded by the scorching ash particles, which function like a \"large heat storage tank\". Due to the intense mixing within the bed, these hot ash particles essentially act as an infinite \"ideal arch,\" heating the coal to its ignition temperature and causing it to burn. During this heating process, the heat absorbed accounts for only a few thousandths of the total heat capacity of the bed, so its impact on the bed temperature is minimal. Meanwhile, the combustion of coal particles releases heat, which helps to maintain a certain temperature level in the bed. This is why fluidized beds generally start burning easily and can handle a wide range of coal types. 　　(2) High combustion efficiency The combustion efficiency of circulating fluidized bed boilers is higher than that of bubbling fluidized bed boilers, typically ranging from 95% to 99%, and it can be compared to that of coal powder boilers. Circulating fluidized bed boilers have high combustion efficiency due to the following characteristics: good gas-solid mixing ； The combustion rate is high, followed by the recycled combustion of fly ash. 　　(3) Efficient desulfurization 　　Due to the cyclic combustion of fly ash, the limestone and lime that have not undergone desulfurization reactions in the bed and are blown out of the combustion chamber can be returned to the bed for reuse ； Furthermore, in the areas where desulfurization has taken place, large particles of calcium sulfate are formed; these particles collide and break apart during the cyclic combustion process, exposing the surfaces of new calcium oxide particles to the atmosphere conducive to sulfidation reactions. In this way, the desulfurization performance of circulating fluidized bed combustion is **improved** compared to bubbling fluidized bed combustion. When the calcium-sulfur ratio is 1.5–2.0, the desulfurization rate can reach 85–90%. In bubble fluidized bed boilers, the desulfurization efficiency needs to reach 85–90%, the calcium-sulfur ratio must be 3–4, and the consumption of calcium is twice as high. Compared to coal powder combustion boilers, there is no need for rear desulfurization and denitrification equipment, resulting in significantly lower investment and operating costs. 　　(4) Low nitrogen oxide (NOX) emissions Low nitrogen oxide emissions are another highly attractive feature of circulating fluidized bed boilers. Operational experience shows that the NOX emission levels for circulating fluidized bed boilers range from 50–150 ppm or 40–120 mg/MJ. The low NOX emissions of circulating fluidized bed boilers are due to two reasons: first, combustion at low temperatures, during which nitrogen in the air generally does not produce NOX ； Second is segmented combustion, which suppresses the conversion of nitrogen in the fuel into NOX and enables the reduction of some of the NOX that has already been formed. 　　(5) High combustion intensity and small furnace cross-sectional area The high heat load per unit cross-sectional area of the furnace is another major advantage of circulating fluidized bed boilers. Its sectional heat load is approximately 3.5–4.5 MW/m2, which is close to or higher than that of coal-fired boilers. Under the same heat load, the furnace cross-sectional area required for a bubbling fluidized bed boiler is 2 to 3 times larger than that of a circulating fluidized bed boiler. 　　(6) Large load regulation range and fast load adjustment. When the load changes, it is only necessary to adjust the coal feed rate, air volume, and material circulation rate; there is no need to use bed-pressure reduction techniques as in bubbling fluidized bed boilers. It also isn’t like coal-fired boilers, which require oil to assist combustion at low loads in order to maintain stable burning. Generally, the load regulation ratio of circulating fluidized bed boilers can reach (3–4):1. The load adjustment rate is also very fast, generally reaching 4% per minute. 　　(7) Easy comprehensive utilization of ash and slag The circulating fluidized bed combustion process is a type of low-temperature combustion; moreover, the favorable combustion conditions within the furnace result in low carbon content in the boiler’s ash and slag (with a carbon content of less than 1%), which facilitates their comprehensive utilization, such as as additives in cement or as building materials. At the same time, thorough heating at low temperatures also facilitates the extraction of rare metals from the ash. 　　(8) No buried tube heating surfaces are installed in the bed. Circulating fluidized bed boilers do not have buried tube heating surfaces in their beds, so there is no issue of wear associated with such heating surfaces found in bubbling fluidized bed boilers. Furthermore, since there are no tube-type heating surfaces inside the furnace, startup, shutdown, and coking treatment times are short, allowing for prolonged operation with the fire extinguished. 　　(9) Simple fuel preprocessing system The coal particle size in circulating fluidized bed boilers is generally less than 13 mm; therefore, compared to pulverized coal boilers, the system for preparing and crushing the fuel is greatly simplified. 　　(10) Fewer coal feeding points The furnace cross-sectional area of a circulating fluidized bed boiler is small, and the good mixing as well as the expanded combustion zone result in a **reduced number of coal feeding points required. It is beneficial for combustion and also simplifies the coal feeding system.