Date of Award
Doctor of Philosophy (PhD)
The North Dakota portion of the Williston basin holds huge, but economically unmineable lignite resources in the Fort Union formation. A technology coupling the underground coal gasification with carbon capture and storage (UCG-CCS) is proposed in this study to recover these lignite resources in North Dakota. The UCG-CCS system provides a cost-effective and environment-friendly approach to convert the lignite to electricity and beneficially utilize the by-product of CO2 at the same time. The target coal seam is the Harmon lignite in the Fort Union formation in western North Dakota. The main objectives of this study are to set up the technology roadmap, conduct the preliminary feasibility study, and identify necessary future research works.
Based on literature review, three UCG candidate sites were screened out, located in Dunn, Golden Valley, and Slope Counties, respectively. The selected site in Dunn County has the best potential to host the UCG-CCS project because of its suitable geological conditions and proximity to oil fields. A three-dimensional geological model, a facies model and an aquifers distribution model were built. It is also estimated that the nearby oil fields have a CO2 storage capacity of 18 million tones. So there exists a big market for beneficial utilization of CO2 in the study area.
Environmental risks associated with UCG are always worth noting. The environmental risks usually result from the change of formation properties and the in situ stress field during the gasification process. Good understanding to the geomechanical, petrophysical and hydrogeological characteristics of the coal-bearing formation is important. A laboratory geomechanical study was conducted by using rock samples of the Harmon bed. The results show that the low strength of the adjoining rock would be considered as a disadvantage for structural stability. On the other hand, the low-permeable adjoining rocks function as a hydraulic seal to prevent the escape of contaminants during gasification process. An analytical study and numerical modeling of a conceptual commercial scale UCG plant were also carried out to analyze the stability of the cavities and the mining recovery factor of the coal seam. The allowable size of the UCG cavities and reasonable spacing between the cavities were estimated based on the stress profile and safety consideration. The results indicate the mining recovery factor is significantly affected by the presence of discontinuity in the formation. The methodologies and results provide a convenient and fast approach to estimate the economics of a UCG plant, once the fundamental properties of the coal-bearing formation are known.
In the last part, the plant performance and cost of the UCG-CCS system were analyzed by analogue to an integrated gasification combined cycle (IGCC) plant with CO2 capture. The results indicate that, as there is no surface gasifier and fuel handling system, the capital cost of a UCG-CCS system is significantly reduced by 50%, and the UCG-CCS system presents advantages over the IGCC plant.
Pei, Peng, "Study on underground coal gasification combined cycle coupled with on-site carbon capture and storage" (2012). Theses and Dissertations. 223.