Analysis of syngas production and reaction zones in hydrogen oriented underground coal gasification

Abstract

Underground Coal Gasification (UCG) is an emerging technology that utilizes unmined coals. In UCG process, coal is burned and gasified underground. Produced syngas is collected through drilled wells to be processed or used directly. Syngas comprises mainly CO2, CO, H2 and CH4 and its hydrogen content majorly depends on gasification agents and properties of coal gasified. It is assumed that the combustible components of syngas, such as CO and H2, are products of heterogeneous gasification reactions and oxygen is almost solely consumed by homogeneous combustion reactions of these combustible components. These assumptions are widely used to simplify dynamic UCG models. Here, a lab-scale hydrogen oriented underground coal gasification experiment was performed for Turkish lignite. Using the experimental data, a two-dimensional underground coal gasification computational fluid dynamics (CFD) model was built to investigate cavity formation, reaction zones, reaction rates and the syngas properties. By developing the CFD model, valuable insights regarding which reactions are dominant in which region are gained and UCG process is better understood. Two-stage gasification method was employed in the experiment. Experiments were performed using lignite having high volatile, high ash and moderate moisture contents in a prism-shaped ex-situ reactor. Oxygen and steam were successfully supplied as gasification agents. In the steam-gasification stage with a supply rate of 5 m3/h, hydrogen content and calorific value of syngas were measured up to 40 (vol.) % and 8 MJ/Nm3, respectively. Experimental and CFD model results were compared to validate the mathematical model and assumptions used in UCG syngas production process.