Understanding the Material Circulation Process in a Circulating Fluidized Bed Boiler

In the realm of industrial heat generation, the circulating fluidized bed (CFB) boiler is a highly efficient and versatile technology. Central to its operation is the material circulation process, which ensures the optimal utilization of the fuel and enhances the heat transfer efficiency. This article delves into the two primary components of the material circulation process in a CFB boiler: internal circulation and external circulation.

What is a Circulating Fluidized Bed (CFB) Boiler?

A circulating fluidized bed boiler is a type of thermal power generating system that combines fluidization, combustion, and recirculation of the solid fuel particles. This unique design provides efficient fuel utilization and reduces emissions, making it a preferred choice in many industries.

The Material Circulation Process

The material circulation process in a CFB boiler is critical for the effectiveness and sustainability of the system. It comprises two key parts: internal circulation and external circulation. Both processes play a significant role in the overall performance of the boiler, ensuring that fuel is continuously and optimally utilized.

Internal Circulation

Internal circulation refers to the movement of particles within the reactor vessel. When the particles in the combustion zone rise in the gas-solid two-phase flow, they eventually reach a point where their terminal velocity is greater than the apparent velocity of the fluid. These particles and particle clusters continue to collide until they form larger clusters. These clusters of particles then return to the lower bed of the furnace via the walls of the combustion chamber. This return flow of particles is known as the 'internal circulation'. It is characterized by the natural behavior of particles in the fluidized state, where particles move and collide with each other, creating a continuous flow of material within the reactor. This movement ensures a more even distribution of particles and optimal fuel-to-air mixing, leading to better combustion and heat transfer efficiency.

External Circulation

In contrast, external circulation involves the movement of particles and particle clusters through the flue gas. As the combustion process occurs, the unburned particles and clusters are entrained by the flue gas and transported to the separator. The separator removes the larger particles, and the separated particles are then returned to the furnace through the return valve. This process, known as external circulation, is crucial for the continuous operation of the boiler by ensuring that only fine, combustible particles reach the combustion zone while larger particles are recirculated back to the primary bed for further combustion. This separation ensures a high level of fuel efficiency and minimizes wear and tear on the boiler components, contributing to better overall performance.

The Role of Particles and Particle Clusters

The behavior of particles and particle clusters is essential in maintaining the proper operation of a CFB boiler. Particles with a terminal velocity greater than the apparent velocity participate in internal circulation, while those with a lower terminal velocity are involved in the external circulation process. These dynamics create a continuous and efficient flow of particles, which is vital for the heat transfer and combustion processes within the system.

Conclusion

The two-part material circulation process in a circulating fluidized bed boiler, including internal and external circulation, is a key factor in the operation and efficiency of the system. Understanding these processes is critical for optimal performance and maintenance. By recognizing the role of particle behavior and circulation processes, operators can enhance the efficiency and longevity of CFB boilers, making them an indispensable component in the world of industrial heat generation and power generation.