The development of coal liquefaction technology using a 150t/d pilot plant of NEDOL process has been carried out in Kashima city, Japan. Since December 1996 to September 1998, the pilot plant has been successfully operated without any seri-ous trouble throughout the operations. However a variety of massive solid matters appeared inside the heating coil of vacuum tower heating furnace, especially upper side of the inner wall of the coil.
In this paper, these solid matters were analyzed chemically and morphologically to investigate their formation mechanism.
The solid products were found to consist of coal minerals and catalyst which were dispersed in the semi-coke block on the upper side of the coil showing zigzagging bound-ary.
Coke forming mechanism speculated from our study was as follows.
(1) Vaporization of light fraction in coal liquid forms slug flow pattern of coal liquid inside the heating coil.
(2) Upper side of the heating coil is heated up above 450°C with high heat flux.
(3) Splashes of coal liquid containing coal minerals and catalyst were trapped first on the upper surface of the heating coil.
(4) The coal liquid trapped on the upper surface is carbonized quickly, to form semi-coke of high densities.
(5) In the next stage, light fraction in the liquid is vaporized to be further trapped among the semi-coke grains produced first on the wall, giving semi-coke of well-developed anisotropy and low mineral content.
Steam injection was useful to inhibit development of the coke. Elimination of light fraction in the coal liquid before vacuum distillation and reduction of heat flux of the fur-nace are effective to inhibit development of the coke.
