Study on gasification kinetics of hydrogen production from lignite in supercritical water

Abstract

Supercritical water gasification provides a clean and efficient way to produce hydrogen from high-moisture lignite. The development of kinetic model is a demanding task for the understanding of the reaction pathway and the reactor optimization. A novel gasification kinetics model mainly concentrating on the gas products (H2, CO, CH4 and CO2) was established to omit the unimportant reactions and intermediates owing to the complexity of the gasification process. Seven reactions were selected as the main routes of lignite gasification in supercritical water based on the present gasification mechanisms. The kinetics model was used to fit the experimental data obtained from the tubular reactor for continuous lignite gasification in supercritical water (operating in 560 °C, 25 MPa, lignite slurry concentration 5%, residence time 4.66 s–12.41 s). Rate constants were determined through minimizing the sum of the square of prediction errors. The gas product concentration as a function of time can be predicted by the model and it indicates that the concentrations of CO and CH4 increased first and then decreased to be negligible after 30 min. The concentrations of H2 and CO2 increased and remained unchanged and the fractions of H2 and CO2 were 65.62% and 34.29% respectively. The predictions agreed well with the thermodynamic results by minimizing Gibbs free energy. Gas formation and consumption pathways can also be predicted. Most hydrogen was produced by steam reforming reaction and consumed by methanation reaction.