Hydrothermolysis of brown coal in cyclic pressurization–depressurization mode

The new technique of usage of supercritical water (SCW) for brown coals conversion into both liquid (condensed), and solid and gas (volatile) fuels is proposed. The method consists in cyclic filling of SCW up to the given pressure (pressurization) into the tubular reactor, filled with coal particles, and fast discharging of conversion products dissolved in SCW (depressurization). The dynamics and composition of conversion products in a mode of stepwise temperature (310–460 °C) and pressure (9–31 MPa) rise at cyclic filling and discharging of, first, the water vapor (T < 374 °C), and then of SCW have been investigated. The vapor stage of conversion has prevented agglomeration of coal particles at the stage of SCW conversion. The maximum yield of the condensed products is obtained at 390 °C, 29.0 MPa. High heating value of combustible products of conversion and solid residue appeared to be much more than that of initial coal because of removal of a great bulk (up to 85.2%) of native oxygen as CO2. H/C atomic ratio in the volatile combustible and condensed products is much higher than in the conversion residue because of redistribution of native hydrogen in coal organic matter (COM). It has been established that sulfur is mainly removed from COM as H2S (up to 77.2%) and nitrogen is concentrated (up to 67.2%) in the solid conversion residue.