Abstract
Low-rank coal contains more inherent moisture, high alkali metals (Na, K, Ca), high oxygen content, and low sulfur than high-rank coal. Low-rank coal gasification usually has lower efficiency than high-rank coal, since more energy has been used to drive out the moisture and volatile matters and vaporize them. Nevertheless, Low-rank coal comprises about half of both the current utilization and the reserves in the United States and is the largest energy resource in the United States, so it is worthwhile and important to investigate the low-rank coal gasification process. In this study, the two-stage fuel feeding scheme is investigated in a downdraft, entrained-flow, and refractory-lined reactor. Both a high-rank coal (Illinois No.6 bituminous) and a low-rank coal (South Hallsville Texas Lignite) are used for comparison under the following operating conditions: 1) low-rank coal vs. high-rank coal, 2) one-stage injection vs. two-stage injection, 3) low-rank coal with pre-drying vs. without pre-drying, and 4) dry coal feeding without steam injection vs. with steam injection at the second stage. The results show that 1) With predrying to 12% moisture, syngas produced from lignite has 538 K lower exit temperature and 18% greater Higher Heating Value (HHV) than syngas produced from Illinois #6. 2) The two-stage fuel feeding scheme results in a lower wall temperature (around 100 K) in the lower half of the gasifier than the single-stage injection scheme. 3) Without pre-drying, the high inherent moisture content in the lignite causes the syngas HHV to decrease by 27% and the mole fractions of both H2 and CO to decrease by 33%, while the water vapor content increases by 121% (by volume). The low-rank coal, without pre-drying, will take longer to finish the demoisturization and devolatilization processes, resulting in delayed combustion and gasification processes.
Highlights
Gasification is the process of converting various carbonbased feedstocks to clean synthetic gas, which is primarily a mixture of hydrogen (H2) and carbon-monoxide (CO) with minor methane (CH4) and inert nitrogen gas, through an incomplete combustion
This paper focuses on a low-rank coal gasification study
SHT Lignite was used as the low-rank coal and Illinois No.6 bituminous coal was used as the high-rank coal in this study
Summary
Gasification is the process of converting various carbonbased feedstocks to clean synthetic gas (syngas), which is primarily a mixture of hydrogen (H2) and carbon-monoxide (CO) with minor methane (CH4) and inert nitrogen gas, through an incomplete combustion. To help narrow down the number of experimental variables and to guide preliminary design development, the objective of this study is to employ a Computational Fluid Dynamics (CFD) scheme to investigate the low-rank coal gasification process. Both a high-rank coal (Illinois No. bituminous coal) and a low-rank coal (South Hallsville Texas (SHT) Lignite) are used for comparison. A third condition of using low-rank coals without a predrying process is studied This condition is not common in real applications, its result could provide a reference for demonstrating the influence of high inherent moisture in coal on the thermal-flow behavior during the gasification process
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