Modeling of smoldering process in a porous biomass fuel rod

Abstract

Modeling of the smoldering process in a porous fuel bed consists of two parts: kinetic modeling of pyrolysis and combustion reactions and CFD modeling of the flow and energy equations. A transient axisymmetric two-dimensional (2D) model based on the first principles was developed for the forward and natural smoldering of a porous biomass fuel rod. Heat transfer is represented by two energy equations whereby the solid and gas phases are considered to have separate and distinct temperatures interacting through an interface heat exchange. The starting material first undergoes pyrolysis prior to oxidation of the remaining carbonaceous residue. The pyrolysis is assumed to consist of a set of first-order reactions. Oxidation of the carbonaceous residue is accompanied by the formation of gas-phase combustion products, the concentration of which is also computed. Calculations under unsteady conditions are done for both natural and forward smoldering for a variety of operating and boundary conditions as well as physical properties of the fuel and the packing density. The computation captures the development of a steady combustion region in which it moves at a constant rate. The results for the gas and solid temperature profiles, oxygen concentration profile and pressure distribution are also presented. The effects of surface heat loss, heating value of fuel, airflow rate, porosity of the fuel rod on temperature and smolder velocity are discussed.