Potential Evaluation of the Hydrogen Production in Underground Coal Gasification Process

Abstract

Abstract Under the new requirement of low-carbon energy transition, underground coal gasification (UCG) process has re-obtained much attention in recent years. It is a process of producing syngas by the underground in-situ conversion of solid coal resources. The main gaseous products include CO, CH4, and H2, and by-products such as CO2. Therefore, it also provides a new possibility and potential of hydrogen production from syngas. In this work, an experimental and numerical study is performed to evaluate the potential of hydrogen production in UCG process. First, a series of heat-treatment tests are conducted to gasify the coal samples. By using the methods of scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD), the microscopic pore structure and mineral transformation behavior of gasified coal samples can be evaluated. Simultaneously, from the SEM images, the element composition of gasified coal samplesis also discussed. Then, based on the laboratory observation, a field scale UCG simulation model is developed, which fully considered 10 complicated chemical reactions during UCG process, including pyrolysis, combustion and gasification processes. And it is also validated by comparing against the field test results of Swan Hills UCG test. In order to accurately evaluate the hydrogen production capacity, through adjusting the parameters of each chemical reaction, the fractions of gaseous products are also verified. Then, based on the simulation model, the factors that control the hydrogen production capacity are discussed, and the optimal operation procedure is also studied. Results show that with the temperature of heat treatment rises, the porosity and permeability of the gasified coal samples are obviously increased, and the mass loss rate has reached about 47%.The number and width of fractures in post heat-treated coal samples are also extremely increased. An exponential function relationship between porosity and permeability of gasified coal is obtained, and the obtained multiplier between porosity and permeability is about 14.93. The developed UCG simulation model can well simulate the coal gasification process. The fraction of hydrogen in syngas is about 20%, which indicates that the UCG process can be a potential source of hydrogen energy. Then, based on this model, the effect of some sensitive factors is discussed. A strategy of pure oxygen injection followed by oxygen/water co-injection is proposed to enhance the hydrogen production during UCG process. And through an optimization on the operation parameters, the maximum fraction of hydrogen in syngas can reach about 25%. The suggested injection time of oxygen/water co-injection is at the end of the first stage of combustion cavity expansion (30d), and the optimized mass ratio of oxygen/water is 1.5:1. As a typical clean coal technology, UCG process can be considered as an important source of hydrogen production in such a time of energy transition.