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The significance of fluidized bed boilers in industrial gypsum applications: Industrial gypsum mainly includes flue gas desulfurization gypsum and chemical phosphogypsum. Desulfurized gypsum is a by-product of wet flue gas desulfurization using limestone-gypsum processes in thermal power plants, steel mills, smelters, and other facilities. Phosphogypsum is a by-product of the wet-process production of phosphoric acid in phosphate fertilizer plants. Compared to natural gypsum, industrial gypsum has many similarities with it. However, there are many differences as well; for example, phosphogypsum contains higher levels of phosphorus, fluorine, and acidic impurities, and it must be purified and specially dried before use. Treated industrial gypsum can generally be used to produce cement retarders, building gypsum, plastering gypsum, and gypsum board. As industrial gypsum derived from flue gas desulfurization projects and waste from the phosphating industry, it can replace some natural gypsum after treatment, and is used in the construction industry as an environmentally friendly material. At present, in the developed countries of Europe and America abroad, industrial gypsum is widely used, and a similar trend is also emerging in China. The hot air furnace is a key device for drying and heating industrial gypsum, and it determines the drying output and quality of industrial gypsum. There are mainly three categories of hot air stoves in use today: gas-fired hot air stoves, oil-fired hot air stoves, and coal-fired hot air stoves. However, given our country’s conditions, the vast majority of regions and enterprises do not have access to gas, and the cost of fuel is too high. Therefore, domestic enterprises mostly choose coal-fired hot air stoves. At the current stage, coal-fired hot air stoves mainly come in several types: inclined reciprocating grate stoves, horizontal reciprocating chain grate stoves, and fluidized bed stoves. Both the inclined reciprocating grate stoves and the horizontal reciprocating chain grate stoves use a layer combustion method, and they share several common disadvantages and limitations when used for drying industrial gypsum: 1. High mechanical wear, short lifespan of the grates, and high equipment failure rates. 2. Low combustion efficiency and high energy consumption. 3. There are selective application requirements for raw coal. 4. The coal feeding amount is not controlled accurately, resulting in unstable heating and large fluctuations in hot air temperature. 5. The combustion environment is poor, resulting in high labor intensity for the stoker. 6. The adjustment of operating conditions relies heavily on human factors. A fluidized-bed coal-fired hot air furnace (boiling furnace) can overcome the aforementioned shortcomings of layer-fired furnaces. It is the most energy-efficient and environmentally friendly type of furnace in coal-fired hot air furnaces, offering stable heat supply, good thermal control performance, simple operation, and low maintenance requirements. Its use is very widespread in the metallurgy, building materials, and power industries. With the technological advancements in fluidized bed boilers, especially as the problems of dust content in the exhaust fly ash and burnout rate were gradually resolved, these boilers have made significant progress in applications within the fertilizer industry and for drying and heating industrial gypsum. The environmental and social benefits they offer are quite evident. Fluidized bed hot blast stoves have become the trend in the clean and energy-efficient combustion of coal. Introduction to Boiling Combustion Technology 1. The boiling furnace combustion method is the common name for a coal-fired hot air furnace with a bubbling fluidized bed. Its combustion mechanism involves coal being crushed into particles smaller than 10 mm, which are then fed evenly into the furnace’s air distribution plate by coal feeding machinery. The air distribution plate is integrated with a constant-pressure air blower; the high-pressure air from this blower passes through numerous small holes in the air distribution plate and enters the furnace. The air spreads in all directions, forming a \"gas cushion\" that lifts the coal particles and other furnace materials, enabling them to burn in a fluidized state. The coal particles move up and down, rubbing against each other, colliding, and breaking down, gradually shrinking in size until they are completely burned out. Since the solid particles are in a fluidized state, there are specific characteristics related to gas-solid flow, heat and mass transfer, and chemical reactions. As a result, this type of boiler, which lies between layer-fired boilers and pulverized coal boilers, possesses some features that differ from those of conventional coal burning methods. 2. Advantages of boiling furnaces: 1. Wide fuel adaptability. It can burn high-quality coal as well as various low-quality fuels—coal with high ash content, coal with high sulfur content, coal gangue, oil shale, petroleum residue, tailings, sludge, slag, bark, rice husks, wood shavings, garbage, and so on. With the variability in fuel sources, types, and quality, fluidized bed combustion technology is more suitable. 2. It has a relatively low combustion temperature (850–950°C), which enables effective control of the generation and emission of Nox and Sox during combustion; it is a \"clean\" combustion technology that eliminates the need for large and expensive flue gas desulfurization and denitrification systems required in coal-fired boilers. 3. High combustion efficiency, ranging from 90 to 99%. 4. The cross-sectional thermal strength is high, resulting in excellent heat transfer capacity of the bed layer, which allows for a reduction in the furnace volume and steel consumption. 5. It offers a wide range of load regulation with excellent control performance. 6. The ash is not prone to softening or sticking; it has good reactivity, making it suitable for comprehensive utilization. 3. The main issue with conventional boiling furnaces in the industrial drying of gypsum is related to their performance. The most successful application of boiling hot-air furnaces is the \"two-stage U-shaped combustion, low-pollution high-temperature flue gas boiling furnace\" designed by the Research Group on Coal Combustion at Huazhong University of Science and Technology. Since 1989, thousands of units have been deployed nationwide, and they have also been exported to countries in Southeast Asia. Boiling furnaces are primarily used in industries such as building materials and metallurgy for drying materials. The main issue that still needs to be addressed in the industrial gypsum applications of fluidized bed furnaces is the high amount of fly ash in the hot flue gases generated by these furnaces. This is especially a problem when burning coal with a high ash content, as it places high demands on dust removal equipment. If conventional dust collectors are used, the dust emission concentration in the flue gases remains high, which limits direct heat supply and increases the system resistance. 4. Introduction to the ZDFR type two-stage separation clean fluidized-bed coal-fired hot air furnace. This furnace (patent number: ZL03 2 54894.X) is equipped with a two-stage inertial gas-solid separation device as well as a flue tube secondary air system; these features improve the combustion efficiency and reduce the amount of fly ash carried in the exhaust gases, thereby providing cleaner hot air for heating purposes and meeting the heating requirements of various industries. The dome shape is relatively flexible, and a designed mixed flue allows for air supply, enabling the temperature of the hot exhaust gas at the outlet to be adjusted evenly. Based on the fluidized bed hot blast stove (boiling furnace), a two-stage gas-solid separation device is designed inside the furnace: 1. Structural features of the U-shaped groove refractory brick impact separator: Above the fire barrier between the fluidized bed furnace chamber and the flue chamber, two or three rows of U-shaped groove refractory brick walls are constructed; each row of such bricks is laid overlapping vertically, with the flue gas channels offset from side to side ； The ventilation ducts in the front and rear rows are offset from each other. As the flue gas passes by, the airflow strikes the U-shaped groove bricks and then bends toward the ventilation channels on both sides. The solid particles in the flue gas are deflected and collide multiple times with the brick walls before being separated from the airflow. Under the effect of gravity, they settle along the U-shaped grooves and fall back into the furnace for further combustion, thereby reducing the amount of ash residue and the concentration of ash in the downstream flue chamber. Given that an excessive concentration of ash in the U-shaped channel can lead to entrainment, after every few layers are built, the refractory bricks in the U-shaped channel are replaced with those for a U-shaped inclined channel, so that the ash accumulates to a certain amount within the channel before settling in segments. Thus, a U-shaped groove refractory brick impact separator is formed. Refractory bricks are chosen due to their high-temperature resistance, oxidation resistance, ease of use in constructing furnaces, and relatively low manufacturing cost. 2. The structural design of the furnace chamber for the U-type combustion sedimentation chamber features two separate U-type combustion chambers, which is different from previous designs. Structural features: The airflow moves downward and then upward, undergoes direction changes, accelerates and then decelerates, causing the solid particles in the flue gas to separate from the airflow due to inertia and gravity. These particles settle at the bottom of the chamber, where they continue to burn, thereby increasing the burning time of the carbon particles and improving thermal efficiency. The bottom of this chamber does not have an ash collection tray; instead, a slope is provided, allowing the accumulated ash to flow down along this slope to the bottom of the mixing chamber, from where it is discharged through the ash tray located at the bottom of the mixing chamber. Depending on the size of the furnace, several ventilation holes are designed at the bottom of the settling chamber and the mixing flue. Below these ventilation holes, flue tubes with holes are installed; these tubes are connected to the furnace’s main bellows. The flue tubes have two rows of holes, one row facing upward, in order to supply oxygen for secondary combustion inside the furnace ； A row of horizontal openings is provided in the slag discharge hopper, aimed at loosening and blowing away the dust accumulated there to facilitate slag discharge. Manual valves are installed between the main and secondary air ducts to control the amount of secondary air distributed. The improved design featuring a sloped structure and secondary bell-shaped ducts makes the furnace more compact, simplifies its structure, reduces costs; it also results in lower resistance in the separator, fewer slag discharge ports, and facilitates ash accumulation for combustion, all of which contribute to improving combustion efficiency. It also contributes to the structural strength of the fire wall, thereby extending the lifespan of the furnace. The furnace design covered by this patent, in addition to possessing the common advantages of boiling furnaces, also has the following benefits: 1. A two-stage separation system is employed, which **reduces the amount of dust carried by the hot flue gases. The dust content in the flue gas is less than 10%. 2. The carbon particle content in the flue gas decreases, increasing the thermal efficiency of the furnace. 3. The temperature of the exhaust flue gas can be adjusted evenly, and the outlet position is flexible. 4. The furnace has a compact structure, is simple to install, and is economical and practical. 5. It is widely applicable to heating industry production lines with special requirements regarding fly ash content and residual carbon content. Such as fertilizer granulation and drying, etc. This post was last edited by mingjiazhao on 2007-12-20 20:23.]