Grate furnaces (represented by mechanical grate furnaces) and traditional fluidized bed (FB) boilers exhibit significant differences in technical characteristics, which can be clearly distinguished in terms of combustion method, system configuration, and operational adjustments:
- Core Combustion Method:
- Grate Furnace: Employs grate combustion (stoker firing). Waste is conveyed and combusted via a mechanical grate, sequentially undergoing drying, combustion, and burnout stages on the grate surface. Mechanical movement of the grate ensures adequate contact between the waste and combustion air.
- Fluidized Bed Boiler: Based on the principle of fluidized combustion. Using inert material as bed material, air distributed from the bottom fluidizes the bed, creating a “boiling” state. Fuel particles undergo intense mixing and combustion with hot bed material and air within the fluidized layer.
- System Configuration:
- Grate Furnace: Features a dedicated air preheater system. A separately installed air preheater preheats combustion air, allowing for more precise temperature matching to the specific combustion stage requirements along the grate length.
- Fluidized Bed Boiler: The air preheater is typically integrated directly into the flue gas ductwork at the boiler rear, utilizing waste heat from the flue gas for air preheating. This design is closely tied to the FB boiler’s need for large volumes of high-temperature air to maintain fluidization and its overall flue gas flow design – the rear location simplifies the system while accommodating the FB’s high air volume and pressure requirements.
- Key Auxiliary Systems:
- Grate Furnace: Requires a unique hydraulic system. The reciprocating motion of the grate, pushing action of the feeder, and operation of the ash discharger all rely on a hydraulic oil station for power. Hydraulic transmission ensures stable and precise coordination of mechanical components.
- Fluidized Bed Boiler: Its critical system is the solids return system (loop seal/return leg). This system recirculates high-temperature solid particles (including unburned fuel and bed material) captured by the cyclone separator back into the furnace for further combustion. This is essential to maintain the material balance and combustion efficiency inherent in fluidized bed combustion.
- Operational Adjustments:
- Grate Furnace: Adjustment focuses on matching combustion air supply with feed rate. If waste moisture content is high, air supply must be increased to enhance drying and combustion support, while the feed rate must be reduced to ensure sufficient burnout time on the grate.
- Fluidized Bed Boiler: Adjustments are more complex. Beyond controlling air volume and feed rate, strict management of bed material particle size and fluidization velocity is crucial:
- Bed material size must be uniform (typically ≤10mm); otherwise, localized defluidization can occur.
- Fluidization velocity must be maintained above the critical value; too low causes bed material accumulation (“slumping”), while too high leads to excessive particle entrainment (“elutriation”). Both conditions can cause combustion instability and potentially furnace slagging.
Industry Context: While some power plants utilize FB technology for waste combustion, the mainstream technology in the waste incineration sector is the grate furnace. This is primarily because FB boilers burning waste often require coal blending to maintain combustion stability and bed temperature, unable to rely solely on the inherent calorific value of the waste. Conversely, grate furnaces can directly adapt to the complex composition of municipal solid waste (MSW), achieving efficient combustion without auxiliary fuel. This allows for more complete utilization of the waste’s energy value, better aligning with the waste treatment goals of waste reduction and resource recovery.
