Efficiency assessment of underground gasification of ortho- and meta-lignite: High-pressure ex situ experimental simulations

Experimental simulations of underground coal gasification (UCG) using large bulk samples of two different kinds of lignite were conducted in an ex situ laboratory installation. The gasified media applied was oxygen under two distinct pressure regimes: 10 and 35 bars. The main aim of the experiments was to assess the feasibility of gas production from low-grade, high-moisture lignite through in situ gasification. The ortho-and meta-lignites subjected to gasification originated from the Oltenia (Romania) and Velenje (Slovenia) deposits. The average moisture content of the raw lignite feed was 45.64% wt and 31.62% wt, with gross calorific values of 10.64 MJ/kg and 13.62 MJ/kg for the Oltenia and Velenje samples, respectively. Changes in the composition of the process gas, its production rates, rates of oxidant supply and the distribution of temperature in the experimental coal seam were measured over the course of both experiments. The UCG trials, each lasting 72 h, constituted of three stages - Initialization, Gasification and Termination, with stage II being considered as the main part of the experiments. The process gas produced during the stage II had an average calorific value of approximately 3.9 MJ/Nm(3) and 5.0 MJ/Nm(3) for the Oltenia and the Velenje trials, respectively. The overall gross energy efficiency of the process during the stage II was estimated at 25.0% and 29.9% for the Oltenia and Velenje samples, respectively. The experiments conducted demonstrate that the physico-chemical properties of the feed lignite, as well as the pressure at operating conditions, considerably affect the in-seam gasification process. The gasification of the Oltenia lignite resulted in more stable syngas production in terms its chemical composition but lower calorific value. Gasification in a higher-pressure regime resulted in increased methane and carbon dioxide content in the produced syngas, with lower hydrogen and carbon monoxide content.