Swelling characteristics of three kinds of single coal particles heated with a well-characterized CO2 laser were directly observed and quantitatively evaluated by combined application of a high-speed video camera with an image analysis system. The results are as follows:  (1) The interval time between swelling and shrinking of bubbles of coking coal particles (Blue Creek, Goonyella) could be used to express the ease of reforming bubbles after bursting and the growth of the bubbles before bursting. The distribution of the interval time of coking coals depends on laser intensity, particle size, and coal properties. (2) For the high-volatile weak coking coal (Witbank), there is no formation of bubbles during heating and the maximum swelling ratio is much smaller than that of coking coals. (3) The maximum swelling ratio and final swelling ratio evaluated with the relative projection area decrease with the increasing of laser intensity (or temperature and heating rate) and particle size. (4) There is a monotonously increasing relationship between the maximum relative projection area and the maximum Gieseler fluidity for all three kinds of coal, although the maximum relative projection area increases with increasing laser intensity. This result suggests that the relative projection area of coal particles on heating can be used for evaluating both the swelling and fluidity properties of coals. (5) The present results suggest a possibility that the “surface tension” of bubble film of the coking coal on heating seems to be attainable from the calculation of the rupture pressure of bubbles.

No abstract

An unsteady state boundary layer model was developed in order to analyse the combustion of single char particles in radiation fields. The model is based on the following assumptions, a spherical char particle with small diameter is combustedat atmospheric pressure, the temperature difference between the surface and the inside of the particle is negligible, particle diameter shrinks due to the reaction on the char surface, the surrounding gas is stagnant, but Stefan flow is taken into consideration and bulk gas flow is neglected. The model predicts the particle temperature increase and the distributions of gas temperature and gas components in the boundary layer. The computed distribution of gas componentproves clearly that the reaction rate is mainly controlled by oxygen diffusion in the boundary layer except during the initial heating and reaction, Using the simulation model, the effects of particle size, oxygen content in bulk gas and input laser power is discussed. The results predict that smaller particles heat up faster in initial heating and higher laser input has the same effect. Further, it is shown that a higher oxygen content raises the sustained combustion temperature and shortens the combustion period.