This paper reports the results of investigation on sulfur dioxide reaction in a pressurized fluidized bed. In case of fluidized bed combustion, sulfur retention depends upon excess air ratio and fluidized bed height (gas residence time) respectively. Study of SO₂ injection into various points conducted that sulfur dioxide was removed rapidly and completely in the fluidized bed and sprash zone. Bed material, which consisted in Ca compounds and coal ash, performed high sulfurization ability against the high CO₂ partial pressure. Therefore, these results suggested that it had high sulfurization performance zone and its mechanism was not able to explain only direct sulfation of CaCO₃. The coal character dependence of sulfur retention was explained the influence of coal combustion style in the fluidized bed and inorganic sulfur content in the coal. According to combustion style observation study, when coal particles burn separately and rapidly in the fluidized bed, SO₂emission of these coals was high. Addition to that inorganic sulfur in the coal tend to burn over fluidized bed zone. On the other hand, when coal particle is cohesive, its primary particle burned slowly in the bed and SO₂emission of this kind of coal was low.

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The operation of coal liquefaction pilot plant has been carried out for 269 days, including 3 kinds of coal and the stable operation being continued 1920 h, since December 1996 to September 1998, in Kashima, Japan. Through the coal-in-operation, the high oil yield and long-term consecutive operability have been confirmed.
Nevertheless after a long operation, a variety of solid deposits have been found in several locations of the plant such as liquefaction reactors and hot high pressure separator. These were characterized using proximate, ultimate, ash component, optical microscopic, so on, and EDX analyses.
The deposit was found to consist of coal minerals, catalyst, inert and unliquified active macerals which were dispersed in carbonized pitch matrix.
The mineral particles containing catalyst and coal minerals are found to be agglomerated in several layers as found as the reactors. Such a feature of deposit suggests that the viscous slurry of high density was trapped on the bottom surface of the separator.
The pitch matrix is carbonized slowly around 400°C, to form composite solid of high densities, while the light fraction is washed out by the coal liquid during the operation.
Low temperature and solid particles dispersed in the pitch prohibit development of large anisotropy.
From these results, some ideas to prevent such depositing are proposed.

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