Effect of hydrothermal treatment on Na and Ca migration behavior during pyrolysis of Baishihu coal

Vitrinite-rich Baishihu coal (BSR) was hydrothermally treated in an autoclave. FT-IR spectra peak fitting was used to investigate change of oxygen functional groups. Fixed bed was applied to pyrolysis of hydrothermal treated samples with and without separation of liquid waste, then contents of Na and Ca in these pyrolysis products were determined by atomic absorption spectrophotometer (AAS).The results show that moisture and chlorine content in Baishihu coal and Na₂O content in ash decrease significantly after hydrothermal treatment. The hydrolysis of aryl ethers of Baishihu coal and the ion exchange reaction of carboxylate lead to increase of hydrogen content and H/C atomic ratio, promoting increase of tar yield during pyrolysis. The organic Ca can be removed by hydrothermal treatment at 300℃ and is separated with the hydrothermal liquid waste. Because of catalysis of inorganic elements such as sodium and calcium in hydrothermal liquid on coal pyrolysis, the hydrothermal treated samples without separation of liquid waste has higher gas yield and lower tar yield than those with separation of liquid waste. Na content and distribution in pyrolysis products of BSR and treated samples decease as:char > water > tar > gas, while the order of Ca is:char > tar > water > gas. As the hydrothermally treated temperature increases, the released content of sodium and calcium during pyrolysis process decreases. The released Na during pyrolysis mainly distributes in water, followed by tar, while Ca is just opposite.