Numerical Study of the Influence of Secondary Air Uniformity on Jet Penetration and Gas-Solid Diffusion Characteristics in a Large-Scale CFB Boiler

Abstract

The uniformity of secondary air (SA) in large-scale CFB boilers has an important influence on gas-solid flow and combustion, but was seldom considered in previous studies. Numerical simulation based on the Eulerian–Eulerian and RNG k-ε turbulence models was conducted to explore the influence of SA uniformity and load variation on jet penetration, diffusion characteristics and gas-solid mixing in the first 600 MW supercritical CFB boiler. The results showed that better SA uniformity was conductive to the uniformity of SA penetration and gas-solid mixing along the furnace height, although the penetration depth and diffusion distance showed an opposite trend. In addition, the penetration depth and diffusion distance got enhanced with higher boiler load. The inner and outer SA jets could not cover the furnace width, and the uneven SA uniformity led to a huge deviation of the solid concentration within 10 m of the air distributor. Eventually, a calculation model was successfully established for predicting the penetration depth of inclined thermal SA jets during boiler operation.

MP-PIC Simulation on Solid Dispersion in a 350 MW CFB Boiler

“…33,34 It is reported that the solid dispersion in a large-scale CFB boiler is vastly different from that in a laboratory-scale bed. 13,19,21 In addition, as particle size d p and density ρ p affect fuel dispersion characteristics, the particle property changing pattern might be an important effect. From previous studies, there are generally three treatments to describe the size and density change during combustion: particle size shrinkage with constant density, density decrease with constant size, and both particle size and density decrease simultaneously.…”

“…Differing from the CFD-DEM model, the two-fluid model (TFM) is also applied. ,− Hernández-Jiménez et al used the TFM to study solid dispersion by setting two types of solid phases and evaluating the mixing completion level according to the relative volume fraction of two solid fluids. They found that it took little time to reach complete mixing with smaller particle sizes.…”

“…Recently, the MP-PIC method has been used to simulate gas–solid flow and combustion in the fluidized bed boilers. − However, there are few studies on solid dispersion using the model, and the available works are limited to a laboratory-scale fluidized bed. , It is reported that the solid dispersion in a large-scale CFB boiler is vastly different from that in a laboratory-scale bed. ,, …”