| Description |
In coal gasification the char-slag transition is a process in which porous char transforms into molten slag at temperatures above ash fluid temperature. It is associated with physical changes of the char particle, such as particle density, size, porous structure and mineral-carbon association. Despite the large number of investigations on coal gasification, the physical phenomena during char-slag transition have not been well studied. In addition, little data regarding ash deposition behavior on gasifier walls during char-slag transition have been reported. This study aims to clarify the physical changes of char particles and ash deposition behavior on gasifier walls during char-slag transition in pulverized coal gasification. To achieve these objectives, two types of experiments were carried out using a laminar entrained-flow reactor: (1) char and ash formation experiments and (2) ash deposition experiments. In the first type of experiment, char and ash particles with different conversions were prepared using two bituminous coals and a subbituminous coal. The prepared char and ash samples were characterized using various techniques to obtain information on particle density, size, porous structure and mineral-carbon association. These data were used to identify the point of the char-slag transition for different coals. Results show that during the transition: (1) particle size decreases, which is caused by shrinkage in the initial stage and by fragmentation in the later stage; (2) particle density increases due to particle size reduction; and (3) particle internal surface area decreases because of ash melting induced pore blockage. In the second type of experiment, particle collection efficiency was measured for a bituminous coal at various conversions. Such information was used to derive the variation of particle stickiness during the char-slag transition. Results indicate that the particle stickiness increases dramatically during the transition. This dramatic increase is attributed to exposure of included minerals on the particle surface, which is caused by particle shrinkage and fragmentation. An empirical model is developed for the prediction of the char-slag transition by considering the ash content of the parent coal, which can be determined by proximate analysis. A hypothetical mechanism is proposed to describe the particle fates upon impaction on gasifier walls during char-slag transition. A simple correlation is established for characterizing the evolution of the particle stickiness during the transition. |
