Numerical modeling of axial bed-to-wall heat transfer in a circulating fluidized bed combustor

The water-wall surfaces located above the secondary air inlet within the circulating fluidized bed (CFB) combustor are exposed to the axial bed-to-wall heat transfer process. In the current work, the axial bed-to-wall heat transfer coefficients are estimated for three different axial voidage profiles (covering three widely occurring average particle concentrations) in order to investigate the effect of voidage, time, initial and fixed temperature of the bed and annulus, and gas gap between wall and solid particles; on the axial heat transfer process. A 2D thermal energy balance model is developed to estimate the axial heat transfer values for the gas–solid suspension along the height of the riser column with horizontally changing mass distribution. The gas–solid mass distribution is fixed with time thus providing a spectrum of changes in axial bed-to-wall heat transfer profile with time. The current work provides an opportunity to understand the axial heat transfer relationship with particle concentration and instantaneous behaviour. The results from the work show that: (i) first few seconds of the suspension temperature near the wall has maximum energy thus providing a small time frame to transfer more heat to the surface (CFB wall); (ii) both axial and horizontal particle concentrations (influenced by the operating conditions) affect the axial heat transfer locally; (iii) initial temperature of the bed between average and maximum values provide end limits for the axial heat transfer; (iv) annulus region has higher thermal energy than the core due to increased particle presence; and (v) a particle-free zone near the wall (gas gap) having a maximum thickness of 1 mm, tends to reduce up to 25% of axial heat transfer value. The model trends have close agreement with experimental trends from published literature; but the model values differ when correlating with real values due to inconsistencies in riser diameter and nature of variation in parameters.