Group combustion of char/carbon particles

Abstract

Extensive literature exists for the experimental data on coal/char ignition and combustion. While most of the experiments are performed with a cloud or stream of particles, the theoretical modeling used to compare and interpret the experimental data is based on the individual particle combustion (IPC) model. As opposed to individual particle modeling, a group combustion (GC) theory is proposed for the combustion modeling of char/carbon particles. For a cloud of liquid drops, the group behavior implies the formation of a flame (group flame) around a large number of drops rather than a flame around each drop. More generally, the group behavior for a cloud of particles represents the change in the burning characteristics due to collective behavior of particles with or without a group flame. To gain a basic understanding of the group behavior, a model such as the analysis of a spherically symmetric cloud of particles burning in quiescent air is presented here. Each particle within the cloud produces CO, due to both the oxidation of C to CO and the reduction of CO 2 to CO which subsequently oxidizes to CO 2 in the homogeneous gas phase. Generalized results for the burning rate and the flame structure are given as a function of group combustion number (G). Predicted results show unexpected results including the independence of the burning rate of CO kinetics. Quantitative results for both the cases of frozen and fast CO kinetics are given. There is a group flame for the case of fast CO kinetics. It is shown that the group flame occurs at G > 5 while for a cloud of liquid drops, the group flame occurs at G > 0.1. The higher critical group combustion number is attributed to the lower burning rate of particle inside the cloud compared to the burning rate of liquid drops inside the cloud. The results show that there exists mainly three modes of combustion: (i) Individual Particle Combustion (IPC, low G), (ii) Group Combustion (GC, intermediate G) and (iii) Sheath Combustion (SC, high G). Criteria are given for identifying the mode of combustion from the experimental conditions. The criteria and the establishment of modes of combustion are independent of the extent of CO kinetics. It is found that the experimental data, obtained with a stream of particles and commonly interpreted with the IPC model, indicate the combustion modes to vary from IPC to SC modes. These data are now reinterpreted with the group theory.