Moving Bed Gasification of Low Rank Alaska Coal

This paper presents process simulation of moving bed gasifier using low rank, subbituminous Usibelli coal from Alaska. All the processes occurring in a moving bed gasifier, drying, devolatilization, gasification, and combustion, are included in this model. The model, developed in Aspen Plus, is used to predict the effect of various operating parameters including pressure, oxygen to coal, and steam to coal ratio on the product gas composition. The results obtained from the simulation were compared with experimental data in the literature. The predicted composition of the product gas was in general agreement with the established results. Carbon conversion increased with increasing oxygen-coal ratio and decreased with increasing steam-coal ratio. Steam to coal ratio and oxygen to coal ratios impacted produced syngas composition, while pressure did not have a large impact on the product syngas composition. A nonslagging moving bed gasifier would have to be limited to an oxygen-coal ratio of 0.26 to operate below the ash softening temperature. Slagging moving bed gasifiers, not limited by operating temperature, could achieve carbon conversion efficiency of 99.5% at oxygen-coal ratio of 0.33. The model is useful for predicting performance of the Usibelli coal in a moving bed gasifier using different operating parameters. 1. Introduction Gasification is considered to be one of the most efficient technologies to convert raw, low-cost coal into clean and highly priced chemicals, fuels, and power. Therefore, as energy prices continue to fluctuate and/or rise, gasification is often brought up as an alternate source of energy and chemicals. The massive coal reserves of Alaska [1] make it a good location for exploiting the technology for energy and chemicals. Chaney and Sheets [2] studied the feasibility of using coal gasification for power generation and chemical byproducts. The plant was to be located in the Cook Inlet region of Alaska and would use coal from the nearby Beluga coal fields (mine permit process underway). The intent of the study was to look at modifying the feedstock of a local fertilizer plant from natural gas to coal, while generating power at the same time. In a similar study, Bibber et al. [3] studied the gasification of the low-rank Usibelli coal for producing Fischer-Tropsch liquids. In both cases, only entrained flow and fluidized bed gasifiers were studied, leaving out moving bed gasifiers. Therefore, the primary objective of this research work was to evaluate performance of Usibelli coal in a moving bed gasifier. 1.1. Moving Bed Gasifier In