| Abstrakt: |
A mathematical model of a one-dimensional char flame is used to predict the temperature profiles, char burn-out and oxygen depletion profiles, ignition time, and flame speed for different initial char concentrations, particle sizes, and size distributions without and with preheat. The model is based on the conservation equations using the radiative Equation of Transfer as the basis for the energy conservation. The relevant predictions are shown to be in good agreement with available char and coal flame data obtained by Cogoli (1) and Howard (2). Three reactivity models are examined: Case 1for surface reaction at constant density (shrinking particle); Case 2for internal reaction at constant radius (non-shrinking particle); and Case 3, a combined model. the low reactivity chars fit the Case 1 model which predicts, in agreement with experiment, ignition times of about 1 sec. The high reactivity chars and bituminous coal fit Case 3, the combined model, with about equal reaction externally and internally, which predicts, in a greement with experiment, ignition times of about 0.1 sec. Both monodisperse and polydisperse distributions are examined, with good agreement with experiment. The peak flame temperatures and final temperatures are shown to be substantially insensitive to particle diameter, excess air, and preheat; however, these factors strongly affect rate of heating and ignition distance or time. Ignition time is found to be most sensitive to the reactivity mode; this affects the variation of particle diameter along the flame so that there is coupling between the reaction mode and flame speed through the influence of the cloud absorptivity on the radiative flux through the flame. This is regarded as one of the most significant results of the work. |
