Process evaluation of underground coal gasification: an exergy analysis

A dissertation submitted to the Faculty of Engineering and the Built Environment, University of Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science in Engineering Johannesburg, 29 August 2016This study discusses underground coal gasification (UCG) and the analysis thereof. Two main methods were used. The first is the Bond Equivalent Diagram, which gives an ideal of where operations should take place in relation to their coal and product gas compositions. This method was used to analyze several real life sites for their idealized and actual operations. The second consisted of a comparative exergy simulation study. This was done for an air-blown UCG plant with a downstream Fischer-Tropsch reactor and an oxygen-blown UCG plant with upstream air separation. The plants were analyzed by their overall exergy efficiency as well as their exergy outputs with respect to coal inputs (fuel). It was discovered that the air-blown simulation with downstream Fischer-Tropsch was the better choice from an exergy point of view due to it having higher efficiencies (1.5 for overall, 1.38 for fuel) as opposed to the oxygen-blown simulation (0.77 overall, 0.8 for fuel). This coupled with other design and safety factors led to the conclusion that the air-blown simulation was better.MT201