Fluidized bed furnace technology has gained increasing attention due to its energy-saving potential, particularly in the utilization of overflow ash heat. Wang Wei from Harbin University of Architecture, Department of Building Thermal Energy, has conducted a preliminary study on this topic. Fluidized bed furnaces, developed in the early 1960s, use a unique combustion method that lies between traditional layer combustion and fire chamber combustion. This technique is especially suitable for low-quality fuels such as coal gangue, oil shale, and inferior anthracite, and it offers the advantage of in-furnace desulfurization with reduced emissions. Additionally, fluidized bed furnaces have good heat transfer performance, simple structure, and lower costs, making them ideal for China’s abundant low-grade coal resources.
However, despite these benefits, fluidized bed furnaces suffer from certain disadvantages, including relatively low thermal efficiency and severe wear on heat exchange surfaces. These issues limit their widespread application. The author proposes exploring ways to improve thermal efficiency, focusing on the recovery of heat from overflow ash, which typically exits at around 900°C. This high-temperature ash represents a significant energy resource that can be effectively utilized.
One approach is to install an economizer on the ash side, similar to how flue gas economizers are used. By passing the hot ash over a series of water tubes, the heat can be transferred to the water, reducing physical heat loss and improving boiler efficiency. A heat transfer calculation model was established, taking into account conduction, radiation, and convection. The heat release coefficient of the ash on the tube surface was calculated using specific formulas, considering factors like ash temperature, contact time, and thermal resistance.
Through detailed calculations, the author determined the optimal structure for the heat recovery device, ensuring maximum heat utilization while accounting for real-world conditions such as partial washing of the tube bundle. The results showed that recycling the heat from overflow ash could significantly enhance overall system efficiency.
In addition to this, the study suggests that the hot ash could be directly mixed with water or used in a water tank via a coil system, further increasing the economic value of this energy recovery process. For a 4,1 MW fluidized bed furnace operating continuously throughout the year, the potential energy savings from such a system are substantial.
Overall, the thermal utilization of overflow ash is technically feasible and offers great potential for energy conservation and cost reduction in industrial applications. This research provides a foundation for future improvements in fluidized bed furnace design and operation.
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