Observation of the Coal Thermoplastic Layer Using μ-focus X-ray CT and Sole-heated Oven

The pore structure of coke is one of the factors governing coke strength. As a step toward technology development to control the coke pore structure, we observed the behavior of coal thermoplastic layers in which coke pores are formed using our newly developed technique with sole-heated oven and μ-focus X-ray CT.Coal packed bed was heated in the sole-heated oven, and then cooled rapidly after the sample temperature reached 350°C at the top and 550°C at the bottom. Next, many cross-section images of the coal and coke layers were observed nondestructively and continuously by the μ-focus X-ray CT. As a result, the following knowledge was obtained.The thermoplastic layer of coal goes through the following four processes: 1) initial process of pore formation, 2) initial softening process whereby the growth of pores, expansion of coal, filling of voids between coal particles, and decrease in porosity occur, 3) intermediate softening process whereby the porosity becomes maximum, and 4) final softening process (resolidification process) whereby the pore size and porosity decrease.With a given type of coal, firstly pores are formed inside the particles over 1 mm in diameter, while the pore formation inside the particles less than 1 mm occurs at approximately 10°C temperature higher than the temperature at which pore formation inside large particles.