Supercritical water gasification of manure: A thermodynamic equilibrium modeling approach

This study aims to investigate for the first time the equilibrium partitioning of elements on the basis of compounds and phases in the supercritical water gasification of mixed pig–cow manure for a temperature interval of 100–580 °C and at a pressure of 24 MPa. The software programs FactSage 5.4.1 and SimuSage 1.12 with user-defined databases have been used for thermodynamic equilibrium calculations. The temperature interval and the weight fraction of water have been chosen as 100 °C–580 °C and 80%, respectively. The effects of pressure and water fraction on main product gases have also been investigated. The results show that three phases are formed for the subcritical region: gas, liquid and solid. In addition to water vapor, CO2, CH4, N2 and H2 are the main gas products of the gas. The liquid phase is composed of an aqueous solution composed of stable ionic compounds such as HCO 3 − , K+, Na+ and Cl− and dissolved CH4 and N2. The solid phase compounds are mainly composed of C, Ca, Mg and P. Aqueous hydrates exist in the supercritical region but start precipitation at high temperatures. The results show that working above 340 °C would result in good carbon gasification efficiency under equilibrium conditions as the solid carbon completely disappears. In the supercritical region, backward methanation reaction takes place and CH4 decomposes and forms H2. While the water fraction has a significant effect on the amount of main product gases, pressure was found to be less significant.