A Discussion on CO2 Sequestration in the UCG Space Based upon the Review of the UCG Residue Physicochemical Properties

Abstract

The strategic goal of “carbon peaking and carbon neutrality” has promoted further reform of the global energy system. Additionally, coal is still the dominate form of energy consumption. Underground coal gasification, which changes the method of coal utilization, is an important way to execute clean coal development and clean utilization, and is also an effective method for the development of deep coal resources, belonging to the technical category of clean energy resources. Compared with the traditional method of coal mining surface gasification, it has obvious advantages of being economical, safe and environmentally friendly. After high-temperature gasification, the physicochemical properties of the residues in the three reaction zones of UCG are significantly different from those of the raw coal. Therefore, this paper summarizes the transformation characteristics and the evolution of organic matter and minerals during UCG. Moreover, the paper analyzes the transformation of pore structure caused by UCG and its influencing mechanism, and discusses the possible utilization of UCG residue based on its physicochemical properties. The results show that: (1) after the UCG, the gasification center residue was mainly composed of acidic and alkaline oxides, accompanied by glassy silica that wrapped the carbon residue, which was located far from the gasification center. Due to the weakening of the oxygen supply, the chemical reaction changed from oxidation to reduction, and the influence of baking on the pyrolysis of coal and minerals gradually weakened. Furthermore, the organic carbon content in the residue gradually increased, whereas the inorganic mineral content decreased. Additionally, the thermal decomposition declined. In the boundary between the dry distillation zone and the raw coal, the organic skeleton of coal and inorganic minerals remained basically unchanged. (2) It is suitable for carbonation to sequestrate CO2 for the oxidation residue after the oxidation process because the residue was composed of slag, whereas the baked roof rock fell off. It is suitable for high-pressure adsorption to sequestrate CO2 in the reduction coal due to the porous and high specific surface of the pyrolysis coal. Some pyrolytic minerals were conducive to mineralization to sequestrate CO2. The dry distillated coal had higher specific surface area and volume of pores in the dry distillation zone than those of the raw coal, whereas the values were lower than those of the reduction zone. The pyrolysis of minerals was not obvious, and the carbonation of CO2 was relatively low. The study points out that CO2 sequestration in the UCG space is an important way to reduce greenhouse gas emissions under the dual carbon peak and neutralization targets. The use of UCG cavities for CO2 sequestration is important for achieving the strategic goal of carbon neutrality. However, the current research on CO2 sequestration using UCG cavities is at the theoretical stage, further field tests are lacking and the commercialization process of CO2 sequestration has not yet been realized.