Characterization and reactivity studies for chemical loop gasification of high sulfur lignites

Kanca, Arzu
The objective of this study was to characterize and to determine the gasification reactivity of Tuncbilek lignites. The ultimate analysis of Tuncbilek lignite revealed that the elemental composition is 37.7% C, 3.6% H, 1.6% N, and 5.4% S, while the proximate analysis indicated 4.7 ± 0.9% moisture 27.9 ± 0.1% volatiles and 37.9 ± 0.2% ash. In this context, four different reactions during gasification namely, pyrolysis, oxidation, hydrogenation, and wet air oxidation were investigated separately. Carbon residues of all these processes were analyzed by XRD, DRIFTS, and 1H and 13C (CP) NMR spectroscopy in order to associate between chemical structure and reactivity. A semi-batch reactor system was used for pyrolysis, oxidation, and hydrogenation experiments, while a high-pressure batch reactor was used for wet air oxidation experiments. Pyrolysis and oxidation experiments revealed that carbon conversion of Tuncbilek lignite is quite high in the presence of oxygen. In addition, hydrogenation experiments displayed that the sulfur removal is the most efficient in the presence of gas phase hydrogen. On the other hand, desulfurization yield of wet air oxidation reaction at 5 bar and 150ºC, was lower than hydrodesulfurization yields. The results of the experiments indicated that high pressure and temperature are necessary to enhance the yield. Co and Pb based pure and mixed metal oxides were investigated as oxygen source and sulfur trapping agents for chemical looping systems. The oxygen transfer potential of Co-Pb metal oxides was monitored by TGA and the maximum weight loss was recorded when coal to metal oxide ratio is higher than 1. Additionally, XRD revealed sulfur capturing ability of these oxides during both pyrolysis and oxidation processes. A process flow diagram is proposed to utilize the mixed metal oxides as chemical looping agents for oxygen and sulfur transfer