Scales and Sediments Formed during Liquefaction of Victorian Brown Coal

with a 50 ton (dry coal) /day Pilot Plant

Abstract

The wall scales and reactor sediments formed during the liquefaction of Victorian brown coal with a 50 ton (dry coal) / day pilot plant were analyzed to clarify the mechanisms of their formation and the relationship between their amount and the operation time of the pilot plant, because they are very troublesome for long-term operation of a coal liquefaction plant.
The scales on the preheater and the connecting pipe between the preheater and the first reactor consisted of NaCl (50-60 wt%) and Fe_1-xS (30-40 wt%) originating from the used catalyst (Fe₂O₃ and S) and coal. The wall scales on the reactors mainly consisted of carbonates of Ca, Mg and Na which originated from ion exchangeable cations in the coal.Their main components were CaCO₃, CaMg (CO₃) ₂ and Na₆Mg₂ Cl₂ (CO₃) ₄ and changed in order from the first reactor to the fourth reactor. All scales were confirmed to be hard inorganic materials which had a layered structure.
The total amount of the scale formed during the operation of the pilot plant was correlated with IEC / Ash (CLB) ×COT ((kg/kg) ·Eh), where ICE and Ash (CLB) were the amount of the ion exchangeable cations (Ca, Mg and Na) in the coal and catalyst, and that of the ash in the heavy liquid product (coal liquid bottom, CLB) used as a part of the solvent, respectively. This result suggested that unreactive solid particles (ash) in the feed slurry suppressed the scale formation during the liquefaction. In addition, an increase in pressure drop observed during the operation was also found to be mainly caused by the preheater scale.
The sediments of the particles found in the reactor bottom were spheres of 0.5-1.0 mm and consisted of similar components to those of the wall scales. Their accumulation was prevented by withdrawing a small amount of the slurry from the first reactor bottom.