Modelling Outer Ash Deposition Rates in Second Generation Atmospheric Pressure Oxy-Fuel Combustion Systems

ABSTRACT

The particle size distribution (PSD) measurements of coarse fly-ash particles (diameter, d_p >10 µm) near the depositing surface are often not available, creating challenges in formulating mechanistic models for ash deposition. This void may be partially alleviated if measurements of deposition rate and PSD of the ash deposits are available. This is demonstrated using well-resolved Computational Fluid Dynamic simulations in conjunction with a particle viscosity and particle kinetic energy based capture criterion to model the outer ash deposition processes for three different fuels (two types of coal and Rice Husk) during their combustion in AIR and O₂/CO₂ (70/30 vol %, OXY70; 50/50 vol %, OXY50) oxidizer compositions. First, deposition rate predictions for monodisperse fly-ash particles (10 µm < d_p < 150 µm) encompassing the aforementioned scenarios were carried out. The PSD of the fly-ash particles were then inferred by mass-weighted averaging the monodisperse fly-ash diameters to match the measured deposition rates as well as the deposit ash PSD. While the methodology proved to be promising in the coal-firing scenarios, the deposit ash PSD predictions for Rice Husk were smaller than their corresponding measurements and may be attributed to irregular particle sizes or necessary modifications to surface capture characteristics.

KEYWORDS:

• Ash deposition
• oxy-combustion
• CFD
• coal
• Rice Husk

Nomenclature

CD	=	Particle drag coefficient
d	=	diameter, m
PKE	=	Particle kinetic energy, J
Re	=	Particle Reynolds Number
Stk	=	Stokes Number
T	=	Temperature, K
V	=	velocity, m/s
$x$	=	mass fraction in fly-ash
$y$	=	mass fraction in ash deposit
Greek Symbols	=
$\rho$	=	density, kg/m3
$\mu$	=	Viscosity, kg/m-s
$\eta$	=	Efficiency (impaction or capture)
Subscripts	=
p	=	denotes particle

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This research was partially funded through the University Coal Research Program being administered by DOE-NETL (Award Number: DE-FE0031741).