Understanding Low Temperature Coking in CFB Boilers
Introduction to Low Temperature Coking
The term low temperature coking in the context of circulating fluidized bed (CFB) boilers refers to a phenomenon where the coking process is limited to certain localized areas within the boiler. This differs from high-temperature coking which affects the entire bed material, leading to more severe blockages and damages.
Characteristics of Low Temperature Coking
Low temperature coking, unlike its high-temperature counterpart, primarily manifests in the material layer separator, feeder, and slag cooler. The visual appearance of the coked material is very similar to the normal bed material; the original particle shape can be distinctly observed, and the particles are loosely held together. This distinguishing feature is one of the key indicators of low temperature coking.
The texture of the low-temperature coking blocks is notably loose. These blocks are easily broken under external forces, which makes it more manageable compared to high-temperature coking. Another telling characteristic is that the density and hardness of these blocks are lower, reflecting their less consolidated nature. This loose texture also enhances the possibility of partial disintegration and facilitates the removal of coked materials.
Causes of Low Temperature Coking
The primary cause of low temperature coking in CFB boilers is the inadequate fluidization of certain localized areas. When the overall temperature of the material within the boiler is lower than the coal deformation temperature and lower than the melting point of the fully melted ash, the localized fluidization becomes weak. This results in a delayed heat release and inadequate local cooling, leading to the formation of coking blocks. However, it is crucial to note that other materials within the boiler exhibit normal physical and chemical indicators.
Therefore, while low temperature coking affects specific areas, the overall performance and health of the boiler remain unaffected by the presence of these coked formations. This localized issue can be addressed without necessitating a complete shutdown or major overhaul, making it a more manageable and less disruptive phenomenon compared to high-temperature coking.
Conclusion
In summary, low temperature coking in CFB boilers is a localized issue that affects only specific areas such as the material layer separator, feeder, and slag cooler. The characteristics of loose, easily breakable, and lower density blocks help in identifying and managing this issue. Understanding the causes, such as weak localized fluidization, is essential for preventive maintenance and effective operational practices. Proper monitoring and maintenance can help mitigate the impact of low temperature coking, ensuring efficient and safe operation of CFB boilers.