Analyzing The Mechanism Of The Process And Experimental Investigation On Smoldering Of Char

Smoldering is the fast oxidation of solid fuel without flame.To be more specific,it is the slow, low-temperature(523-1173K),flameless form of combustion,sustained by the heat evolved when oxygen directly attacks the surface of a condensed-phase fuel.Because the flame is impossible inside the powder piles or inside the particle(when the diameter of the particle is more than 5mm), smoldering exists not only in fire from solid materials,either fuel or industrial materials,but also in many thermo conversion applications of solid materials,such as biomass,urban solid waste. Mechanism of smoldering is very complex and many chemical factors and physical factors are involved in the process.Compared to the large amount of experimental investigations on chemical mechanism(kinetics of pyrolysis,oxidative polymer degradation,char oxidation)of the process in the last two decades,experiments on physical mechanism are much less.However physical mechanism of the process is of great importance in danger prevention and in utilization and treatment of biomass.As a typical solid fuel,char is chosen as the experimental material because it is much easier to detect the gas component in the production of the process than that of high molecular materials.And thus the mechanism of the process is much easier to be evaluated or testified by experiments.Firstly,a simplified 1-D model has been developed describing natural downward smoldering of char.Factors which affect the process turns out to be the following:ambient temperature,insulation of the fuel bed,ignition temperature and position,bulk density of fuel,shrinkage of fuel bed during the process,heat conductivity,void fraction of the fuel bed,heat emission of the fuel,specific heat of solid.Using the program,effects of fuel bed properties on smoldering of char at typical condition have been predicted.Results showed that propagation velocity of the process,temperature inside the whole bed and time smoldering sustained were affected by shrinkage of bed,bulk density of carbon, void size in fuel bed,oxygen diffusivity in ash bed significantly.While the effects of initial condition, environmental condition or fuel bed properties such as specific heat and heat conductivity were ignorable.Based on the aforementioned work,an experimental setup for char smoldering has been designed and manufactured.It consists of smoldering stove,electrical control system,and data acquisition system.In order to investigate the effect of carbon content and particle diameter of char on smoldering process,different size of char from fast pyrolysis and carbonization were chosen for experiments.Result showed that:1)There is no rapid change from smoldering to flame or to extinguish during smoldering of the above char.Temperature inside the bed showed that the process can be divided into heating,reaction and high temperature remaining.The reproducibility of the process is better than smoldering of agro-stalks.2)Maximum temperature of the bed and smoldering time decreases with the increase of particle diameter and increases with the increase carbon content in the char.3)The mole ratio of CO to CO₂ in char smoldering is significantly higher than that of agro-stalks smoldering.It keeps more than 1.0 for a long time at the former part of smoldering.This can be used as reference for the deep investigation of the process.