Experimental study on liquid hydrocarbon formation during pressurized underground coal gasification conditions

Abstract

UCG is a gasification process used to produce synthesis gas from coal in situ (underground in the coal seam) by injecting air or oxygen, with or without steam, into coal seams and extracting the product gas via surface wells. The resulting synthetic gas (“syngas”) can be used to produce electricity, as well as chemicals, liquid fuels, hydrogen and synthetic natural gas. Together with the resulting syngas, some of the coal components, especially condensable water, oils, tars, inorganic trace elements and a small fraction fly ash as particulate matter make its way to the surface via the production well and can cause negative impacts on downstream processes. Tar and oil formation during pressurised Underground Coal Gasification condition will be discussed in this paper. The project aims at generating and characterizing tar/oil (condensable hydrocarbon fraction) from a South African Highveld coal sample under conditions that imitate the pyrolysis stage of Underground Coal Gasification (UCG). For the generation of the tar/oil fraction, a pressurized pyrolysis plant of the Institute of Energy Process Engineering and Chemical Engineering at TU Bergakademie Freiberg is used. The experimental equipment is a counter-current fixed bed reactor, in which the preheated purge gas Argon and if necessary the previously generated steam is introduced at the bottom of the reactor. The fixed bed is gradually building up during the experiment as the feedstock is introduced from a storage tank via a screw feeding system into the reactor. The system is electrically heated and can be operated at pressures up to 30 bars. The tar/oil fraction is collected within a condensation system, whereby a solvent is used to remove the tar/oil from the condensers. Permanent gases are analysed after pressure reduction. To generated tar/oil under UCG conditions, the following conditions will be used: Pressure: 30 bars Superficial flow velocity: 0.05 m/s Particle size: 2.0 - 6.3 mm Temperature(s): 650 °C; 800 °C Steam addition: 0 - 0. 15 kg/kg coal The results from this study, both on quality and quantity, on the liquid hydrocarbon formation during a UCG process will be discussed