A study of chemical structure changes of Chinese lignite during fluidized-bed drying in nitrogen and air

A Chinese lignite coal was dried in a fluidized-bed reactor in the presence of air and nitrogen to examine the effect of temperature, particle size, gas flow rate, and sample mass. The changes of chemical structure during drying were investigated using FTIR technique. The drying rate is shown to increase with increasing drying temperature and gas flow rate, and decreasing particle size. It has been found that the aliphatic hydrogen absorbance decreased with increasing drying temperature. These changes in aliphatic absorption were more significant after drying in air compared to in nitrogen. In air drying the absorption of oxygen-containing functional groups increased gradually with increasing temperature up to 200 °C and declined thereafter due to the decomposition of these groups to release CO and CO2 or react with nearby hydroxyl groups to produce esters. Carbonyl groups decreased progressively with temperature in nitrogen while carboxylate and aromatic carbon absorption increased up to 250 °C and decreased thereafter. During air drying, the aromatic carbon absorption remained relatively unchanged suggesting the reaction of aliphatic groups with oxygen may be a primary oxidation mechanism.