Aerodynamic influences on the outer ash deposition rates during oxy-coal combustion

Abstract

Second generation oxy-combustion and load following operations are accompanied by a significant (60–70%) reduction in combustor flue gas flow rates when compared to baseload operation. Understanding the aerodynamic influences on the fly-ash particle deposition characteristics during these novel operational scenarios are critical towards anticipating operational challenges. To fill this void, a novel Computational Fluid Dynamic (CFD) methodology in conjunction with an ash deposition module was used to model the outer ash deposition process coal during combustion of coal in AIR and O₂/CO₂ (70/30 vol%, OXY70) oxidizer compositions and the predictions were compared with measurements. The measured fly-ash particle size distributions (PSD) were employed as inputs to represent the particle Stokes numbers near the deposition surface accurately and the capture criterion was based on a recently proposed particle viscosity and kinetic energy (PKE) based formulation. Prediction sensitivities to the particle viscosity model and fly-ash PSD were also assessed. Deposition rate predictions in AIR were in excellent agreement with the measurements. The deposition rate enhancement (OXY70/AIR) across all scenarios were in the range 2.8–3.3 which was in reasonable agreement with the measured values of 2.1–2.4. Predicted capture efficiencies were nearly 100% across all scenarios. The calculations were repeated by employing the corresponding fly-ash composition information rather than the bulk-ash compositions and did not alter the predictions significantly. The study therefore supports the hypothesis that ash deposition rates in these novel operational scenarios are likely to be dominated by PKE.