Experimental Investigation on the Pore Structure Evolution of Coal in Underground Coal Gasification Process

Abstract

Underground coal gasification (UCG) has been shown to be a promising method for deep coal resources. A series of complicated chemical reactions can induce a considerable change in the pore structure of coal and thus promote the UCG process in turn. Currently, most studies on the effect of elevated temperature on the pore structure of coal were not involved in an air atmosphere, bringing a series of difficulties to understanding the pore structure evolution of gasified coal in the UCG process. The objective of this work was to investigate the pore structure evolution of coal heated in nitrogen and air atmospheres at elevated temperatures. Thermogravimetry tests were first conducted to gasify coal samples, and then, the method scanning electron microscopy was used to observe the microscopic morphology of the pore structure. Besides, the effect of final temperature, atmosphere, pressure, and residence time on the thermal dynamics of coal at elevated temperatures was comprehensively discussed. Results indicated that the temperature range of a heating process of coal can be classified into three stages, 25–320 °C, 320–750 °C, and 750–1000 °C. For the three temperature ranges, drying, primary pyrolysis, and secondary pyrolysis can dominate, respectively, under a nitrogen atmosphere, while the combustion and gasification process will prevail at a high temperature under an air atmosphere. Because of mass loss, the coal was becoming porous during the heating process. Compared with the intact structure of coal when the temperature is lower than 300 °C, the pore space can interconnect under moderate-temperature conditions (500 °C) like a honeycomb, and then, only an ash framework remained under a higher-temperature condition (700 °C) under an air atmosphere. In comparison, the coal heated in a nitrogen atmosphere can gradually turn into a porous char. This investigation can provide some new insights into the pore structure evolution of gasified coal and contribute to the mechanisms of a UCG process.

Study on the Effect of Pore Structure on Desorption Hysteresis of Deep Coking Coal under High-Temperature and High-Pressure Conditions

“…Under the same equilibrium pressure, as the temperature increases, the total specific surface area of the coal sample shows a decreasing trend. Due to the reforming effect of temperature on the pore structure of the coal, 41 the thermal effect leads to the cracking of the coal matrix, an increase in the number of large pores, and a corresponding decrease in the number of small pores as the temperature increases. The adsorption capacity of the coal to gas decreases, thereby weakening the desorption lag effect of gas in the coal.…”

Critical review of methods for intensifying the gas generation process in the reaction channel during underground coal gasification (UCG)

“…In addition to the rock mass pressure, the caving intensity is affected by the high temperature accompanying the underground gasification process [139]. As a result of the penetration of heated gases into fissures, the roof rocks are heated, expanded and destroyed [140]- [143]. Collapsed rocks change the gasification channel geometry along the combustion face, and also have a negative impact on heterogeneous processes in the reaction channel.…”

Increasing the underground coal gasification efficiency using preliminary electromagnetic coal mass heating

“…Thermodynamic analyses indicate that multigeneration system integrated with a coal gasification subsystem has a high potential for efficiency [46]. UCG is a complex thermalchemical-physical process with a slow heating rate, controlled by a combination of heat and mass transfer [47], [48]. The main methods of electrophysical influence on coal mass in the process of underground gasification may include the following:…”