Liquid carbon dioxide drying and subsequent combustion behavior of high-moisture coal at high pressure

Abstract

• LCO 2 drying increases combustion rate by 37% for coal with 20% water at 500 °C. • The faster combustion of LCO 2 –coal slurry produces 35% higher particle temperature. • Both NMR and weight loss data support LCO 2 drying mechanism in high moisture coal. • The high pressure boiler with LCO 2 drying cycle increases power output by 11.3%. In this work, the combustion and radiation characteristics of high-moisture coal treated with liquid carbon dioxide (LCO 2 ) at high pressure were examined. The coal conversion and gas yield were measured to determine the combustion rate constants at four different temperatures. The particle temperature and radiative heat flux emitted from the burning coal bed were measured using a two-color pyrometer and radiometer, respectively. The changes in the moisture content of the coal microstructure after LCO 2 treatment were measured using nuclear magnetic resonance (NMR) and thermogravimetric analysis (TGA). The combustion kinetic results showed faster LCO 2 –coal slurry combustion, which frequently occurred at low temperatures. The particle temperature and radiative heat flux of the LCO 2 –coal slurry increased faster to higher steady-state values than those of raw coal. The influence of LCO 2 on the combustion of high-moisture coal was not observed on graphite devoid of water or volatile content. This indicates an effective exchange between LCO 2 and water in coal micropores when coal is exposed to LCO 2 . In addition to the TGA data, the LCO 2 drying mechanism in high-moisture coal was confirmed using NMR relaxation data. The performance of the LCO 2 drying process was compared with that of the conventional boiler with oven drying unit in terms of the water removal effect, combustion, and radiation behavior. An energy analysis was undertaken to calculate the amount of total energy produced from three power cycles occurring in a high-pressure boiler, including one combining the LCO 2 drying cycle with the steam power cycle.