화학공학소재연구정보센터
Fuel, Vol.251, 636-643, 2019
Coal drying study for a direct syngas fired supercritical CO2 cycle integrated with a coal gasification process
The Allam Cycle is a semi-closed, oxy-fuel, supercritical carbon dioxide (sCO(2)) Brayton power cycle fueled with natural gas or syngas derived from coal, biomass or other carbonaceous fuels, offering advantages over simple cycle and combined cycle arrangements. For the coal based Allam Cycle, a gasification system is required to convert coal to clean syngas for downstream power generation. These systems require heat input to dry as-delivered coal to certain specifications. Produced steam is typically used for this purpose, but since the Allam Cycle is based on use of sCO2, and not steam, it is preferable to utilize alternative methods for coal drying. An experimental study was carried out at Energy & Environmental Research Center (EERC) to determine the heat energy required for drying lignite coal down to moisture levels acceptable for the operation of different gasifiers (range of moisture levels inserted here), and to investigate the effect of the heat transfer medium (steam and/or N-2) and heat transfer type (direct or indirect) on the lignite drying performance. The results show that optimal operating temperature of coal drying is 90 degrees C. For direct heating system, the result shows a relatively linear trend of drying energy versus the coal's moisture content, and the drying data suggests that individual parameters of velocity, residence time, and bed temperature are of secondary importance compared to how they combine to determine energy transfer to the coal. When the results of the direct heating tests using different heat transfer medium were compared, they were in good agreement, in that they indicate an approximate heat of evaporation around 2791.2 kJ/kg moisture removed. The energy consumption of the indirect heating steam coil tests is about 4652-5815 kJ/kg moisture removed, which is much higher than direct heating test data since not all the steam can condense at the exit of the steam coil. All the test results are important for designing the lignite drying process for the Allam Cycle. In the coal based Allam Cycle power generation system, low grade energy at the temperature of 70-100 degrees C from the turbine exhaust stream is sufficient for the coal drying process. Nitrogen from air separation unit can be used via direct heating and coal fluidization medium. For an indirect heating coal dryer system, a closed loop high pressure water system can be designed to transfer low grade heat from turbine exhaust stream to the dryer to remove moisture from coal.