Predicting ash deposition from non-isothermal, turbulent parallel flows

摘要

Staged, pressurized oxy-combustion (SPOC) is a unique process that burns coal under pressure with low flue gas recycle, and is a promising technology for CO_2 mitigation in coal-based power plants. One of the challenges of this technology is to minimize ash deposition and slagging in the boiler. The SPOC boiler features a co-axial, turbulent parallel flow, which is completely different from the swirl- or recirculation-based flows in conventional coal-fired boilers. The difference in flow leads to different ash deposition behaviors. To better understand ash deposition under these conditions, the particle transport and heat transfer processes are modeled in a non-isothermal, turbulent parallel flow. The particle impact rates and particle deposition temperatures (i.e., the temperature of a particle when it hits the wall) for different sized particles are predicted. These two parameters are critical in determining the ash slagging tendency in a pressurized boiler. Simulation results show that, with the boundary layer being accurately resolved and an appropriate turbulence model being carefully chosen, the predicted particle impact rate agrees well with experimental data from the literature. For particle cooling, we defined a non-dimensional particle deposition temperature, T_d~+, and a non-dimensional characteristic time for particle cooling, τ_T~+. Simulation results showed T_d~+ is a function of only τ_T~+ and the non-dimensional particle relaxation time, τ~+. When τ~+ is less than 5, the deposition temperature is always close to the wall temperature. When τ~+ is larger than 5, T_d~+ increases with τ_T~+/τ~+.