A brief discussion on the design and key usage points of fluidized bed boilers

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Industrial raw materials, fuel materials, semi-finished products, and finished products are mostly dried in order to facilitate better preparation and storage for subsequent processing steps; large-scale industrial production necessarily relies on drying equipment. The effectiveness of a drying system is not only reflected in the thorough and comprehensive design of its components, such as low heat loss and efficient gas exchange ； Many factors within the system also need to be taken into account to achieve the optimal combination. The selection, design, and use of hot blast stoves are very important aspects. I. Selection of hot air stoves The hot air stove is the source of heat for the drying system. The thermal efficiency of a hot air stove depends on the amount of hot flue gas supplied and the temperature of the medium; in practical applications, there are various types of hot air stoves. Manual furnace: Coal is fed manually, and it can burn granular coal with a diameter of 50 mm or less. Continuous feeding of coal and removal of slag are required, which results in high labor intensity for the workers. A large amount of cold air enters the furnace, leading to an unstable combustion process, low temperatures of the flue gases inside the furnace, and significant losses due to incomplete combustion. As a result, coal consumption is high, thermal efficiency is low, and the amount of heat generated is limited. Coal-fired furnace: Strict control is required over the depth of the flame; if the flame is too deep, it can easily damage the internal cylinders and material lifting plates of the dryer, and even alter the physical properties of the material ； If it is too short, the temperature of the flue gas entering the dryer is insufficient, resulting in a reduced drying capacity. In addition, strict requirements are placed on coal quality and fineness; combustion is unstable, and operation is difficult. Fuel (gas) furnace: Uses oil or gas as fuel; its advantages are high burnout efficiency and ease of operation. The downside is the high requirements for operation, especially regarding safety; leaks are strictly prohibited to prevent explosions. Boiling furnace: It lies between layer combustion and suspended combustion; the combustion process takes place in a boiling state. It offers advantages such as enhanced combustion, good heat transfer performance, simple structure, and the ability to burn low-quality fuels. However, due to unreasonable local structural design, the presence of many right-angled sections, traditional boiling furnaces have a short service life; slag tends to form inside them, eddy currents are severe, coal consumption is high, and the combustion temperature is low. Coal-saving high-temperature boiling furnace: It is a new KF furnace type that represents a comprehensive redesign and optimized design based on the traditional boiling furnaces I work with. It features a small furnace bed with an integral frame structure; the volume of the furnace bed is reduced compared to conventional designs, and the structure of the furnace is more stable, **which increases the furnace’s service life and the heat intensity per unit volume ； It reduces sharp right angles and lowers the frequency of slag formation, enabling coal savings of 40–60% compared to conventional boiling furnaces. The furnace temperature can be increased significantly and controlled freely, further improving the ability to use lower-quality coal. A comparison of the technical and economic indicators for several types of furnaces is shown in Table 1, while a comparison of the heat intensity per unit volume is shown in Table 2. Table 1 Comparison of Technical and Economic Indicators for Different Combustion Methods
Furnace Type | Lower Heating Value of Coal / kcal/kg | Combustion Temperature °C | Carbon Content in Ash % | Coal Consumption kg/t | Investment (10,000 yuan)
--- | --- | --- | --- | --- | ---
Layer-type manual furnace | 5000 | 600 | 18 | 40 | 8
Spray-type coal powder furnace | 6000 | 900 | 12 | 30 | 12
Conventional bubbling furnace | 4500 | 700 | 8 | 28 | 16
Coal-saving bubbling furnace | 3000 | 1100 | 4 | 18 | 16

Table 2 Comparison of Heat Intensity per Volume of Furnace Chambers for Various Furnace Types
Furnace Type | qv (kW/m³)
--- | ---
Coal powder furnace | 175–233
Coal-throwing furnace | 233–291
Conventional bubbling furnace | 930–1170
Coal-saving bubbling furnace | 1350–1861
Fuel oil furnace | 291–349
Blast furnace gas furnace | 233–349
The coal-saving high-temperature bubbling furnace is favored by many enterprises due to its strong adaptability to coal, ability to burn low-quality coal, simple operation, and significant coal savings. II. Selection of parameters: The determination of the combustion chamber size, that is, the area of the furnace grates and the volume of the furnace chamber. 1. Heat consumption and coal consumption: Heat consumption and coal consumption are calculated using the following formulas: q = lc1t1/η, g = lc1t1/ηQDW, gc = (lc1t1(W1 – W2))/ηQDW × 100 – W1. GC = Wg, or GC = 1000Ggc. In these formulas, l represents the amount of hot gas required to evaporate 1 kg of water, in Nm3/kg of water; c1 represents the average specific heat of the hot gas entering the dryer, in kJ/Nm3. ℃） ； t1 – Temperature of the hot gas entering the dryer,℃ ； W1 – Initial moisture content of the material entering the dryer, %
W2 – Final moisture content of the material exiting the dryer, %
q – Heat consumption of the dryer, kJ/kg of water ； g – Coal consumption of the dryer, kg of coal/kg of water ； GC – Coal consumption of the dryer, in kg of coal per kg of dried material.
GC – Coal consumption in the combustion chamber, in kg of coal per hour.
QDW – Lower calorific value of coal, in kJ/kg of coal.
η – Thermal efficiency of the combustion chamber, as shown in Table 3 below.

**Items:** Combustion chamber type, Manual operation, Rotary grate, Inclined push grate, Vibrating grate, Pulverized coal combustion chamber, Boiling furnace combustion chamber.
**Thermal efficiency η:** 0.80, 0.85–0.9, 0.85, 0.85–0.9, 0.9, 0.9–0.95.
**Adaptability to coal types:** Wide adaptability; not suitable for burning anthracite or low-quality coals with high caking tendency and high ash content; not suitable for burning coals with low volatile matter content. Efficiency decreases when burning anthracite or low-quality coals. Wide adaptability; can use low-quality coals and gangue with particle sizes of 0–10 mm.
**Blowing air pressure (Pa):** 800–1000, 900–1500, 900–1500, 900–1500, 2000–3000, 6000–9000.

2. **Area of the combustion chamber grate:**
F = GC × QDW / qF
Where F is the area of the combustion chamber grate, including the area of the grates and ventilation openings, calculated based on its projection on the horizontal plane, in m². qF is the heat intensity per unit area of the grate, in kJ/m²·h or kW/m², as shown in Table 4.

**Ventilation method and coal type:**
**Combustion chamber type:** Manual operation, Rotary grate, Inclined push grate, Vibrating grate, Pulverized coal combustion chamber.
**Ventilation method:** Manual ventilation.
**Coal type:** Bituminous coal: 292×104 – 335×104, 335×104 – 378×104, 292×104 – 335×104, 335×104 – 418×104.
**Heat intensity of the furnace chamber:** 684×104 – 828×104.
**Coal type:** Anthracite: 335×104 – 378×104, 209×104 – 292×104.
**Ventilation method:** Natural ventilation.
**Coal type:** Bituminous coal: 126×104 – 209×104, 187×104 – 252×104.
**Coal type:** Anthracite: 169×104 – 252×104.

3. **Volume of the combustion chamber furnace:**
V = GC × QDW / qV
Where V is the volume of the combustion chamber furnace, in m³ ； qv – Heat intensity of the furnace volume, kJ/m3·h or Kw/m3 ； See Table 5 for the heat intensity of the furnace volume in kJ/m3·h. Notes: For layer combustion chambers, the value ranges from 104×104 to 126×104; the lower value is used for bituminous coal, while the higher value is used for anthracite. For pulverized coal combustion chambers, the range is from 50×104 to 83×104. 4. General technical parameters of fluidized bed boilers: The cold-state fluidization wind speed is approximately 1 m/s ； The opening rate of the air distribution plate is 2.2%–3% ； The wind speed at the small holes of the wind cap is approximately 30~45 m/s ； The wind speed at the interface of the gaps between the caps is approximately 2~2.5 m/s ； The hot air velocity in the suspension section is approximately 0.8~1.2 m/s ； The cross-sectional wind speed under the air distribution plate is less than or equal to 1.5 m/s ； The wind speed at the blower inlet is less than 5 m/s. 5. A blower pressure of 6000–8000 Pa is appropriate for a boiling furnace. III. Points to note in structural design 1. For ease of operation, the depth of the grate should not exceed 2 mm ； A furnace door is provided along the width of the grate, at 1-1.5m intervals. 2. The furnace height is determined based on the principle of ease of operation. 3. The length-to-width ratio of the furnace chamber should be 3:1. 4. The furnace chamber should have an enlarged cross-section, at 22° to the vertical plane. 5. The distance between the air cap on the farthest wall inside the furnace chamber and the inner wall of the furnace is 20–30 mm. 6. When laying the inner and outer layers of fire-resistant bricks and red bricks, the brick joints should be staggered, with overlapping bricks between them. 7. After the outer layer of bricks is laid, a steel frame should be used to support the outer perimeter in order to enhance its stiffness and overall stability. 8. When the coal consumption is greater than 180 kg/h, mechanical coal feeding and slag discharge should be used. 9. A reasonable and appropriate height of the fire wall helps to control the temperature of the hot gases entering the dryer, thereby preventing damage to the front part of the dryer cylinder. 10. The feed pipe passing through the boiling furnace should be lined with heat-resistant concrete or made of heat-resistant stainless steel capable of withstanding high temperatures, and proper sealing between them must be ensured. 11. A temperature-sensing thermocouple should be installed near the exhaust outlet of the boiling furnace to facilitate control of the temperature of the exhaust gases. IV. Issues to Note During Use 1. Control of coal particle size fed into the furnace: To meet the requirements for boiling in a fluidized bed furnace, the particle size of the coal fed into the furnace should be controlled at around 3 mm, with the maximum size not exceeding 8 mm. 2. Production organization and the operation of hot blast stoves have a significant impact on coal consumption. The level of coal consumption is influenced not only by factors such as the temperature of the exhaust gases, the insulation of the system to reduce heat loss, and the degree of combustion of the coal, but also by production organization and the operation of the hot blast stoves. The latter is easily overlooked; some manufacturers have limited storage capacity, resulting in intermittent production – they stop operating for several days after each baking cycle, and frequent shutdowns and restarts of the furnaces lead to high coal consumption ； Some manufacturers do not master the proper operation of hot air stoves; different methods need to be employed for various types of stoves (boiling stoves, coal-fired stoves, fuel oil-fired stoves, etc.). It is important to focus on the key aspects and operate the stove in a way that minimizes air leakage, thereby improving the combustion efficiency of coal. 3. Use air reasonably and operate frequently to prevent coking. Operators must receive thorough training in identifying the color of the flame in order to determine the temperature inside the furnace and make appropriate adjustments to ensure proper calcination within it.