Effect of sodium carbonate on supercritical water gasification and oxidation of sludge-lignin at continuous counter-feed of the reagents

[Display omitted] •Gasification and oxidation of sludge-lignin in SCW and SCW/O2 fluid flow were investigated.•The addition of Na2CO3 to sludge-lignin during SCW gasification increases the yield of H2, CH4, CH3SH, and CO2, but decreases the yield of CO and phenols.•The presence of Na2CO3 in sludge-lignin ensures its nearly complete oxidation in SCW/O2 fluid.•Neutralization of acids formed upon oxidation of Cl- and S-containing components of sludge-lignin with sodium carbonate reduces corrosion. The article presents the research results of supercritical water (SCW) gasification and oxidation of watered waste of pulp and paper manufacturing (sludge-lignin), aimed at solving the problem of its environmentally friendly utilization. The experiments were performed at continuous counter-feed of sludge-lignin (without and with the addition of Na2CO3) and SCW or SCW/O2 fluid under temperature gradient along the axis of a vertically arranged reactor (425 °C at the top, and 600 °C at the bottom) at a pressure of 30 MPa. To get the temperature dependencies of the yield of the product, sludge-lignin gasification was also carried out by pumping water vapor and SCW through its bed at the uniform (1.5 °C/min) heating of the reactor up to 650 °C. In SCW gasification products, along with H2, CO, CO2, and hydrocarbons, toxic S-, Cl-, N-, and O-containing substances were detected. The yield of volatile hydrocarbons and heteroatomic substances during SCW gasification in counter-current flows was greater than at pumping SCW through a sludge-lignin bed. It was revealed that the addition of Na2CO3 to the sludge-lignin at the counter-feed of SCW leads to an increase in the yield of combustible gases, as well as to a decrease in the yield of CO, phenols, and carbonized residue. The addition of Na2CO3 to the sludge-lignin intensifies the oxidation of its organic components in SCW/O2 fluid. It was established that the bulk of hydrochloric and sulfuric acids resulted from the oxidation of Cl- and S-containing organic components of sludge-lignin in SCW/O2 fluid are neutralized by sodium carbonate with the formation of NaCl and Na4(CO3)0.61(SO4)1.39, respectively.