Three coals were oxidized in a fluidized bed, using air at temperatures from 175°C to 280°C and total pressures from 126 kPa. to 274 kPa., for periods of up to 24 hours. An infra-red analyzer was used to measure concentrations of carbon oxide gases in the fluidizing stream. Both CO and CO2 rates declined with time and increased with total pressure. The CO2/CO ratio increased with oxidation temperature.
Oxidation at 200°C for 12 hours caused about a 10% loss in coal heating value; with most of that loss occurring in the first two hours. The loss of heating value correlated well, according to Dulong's formula, with the decrease in carbon and hydrogen content. The total and carboxylic ion-exchange capacities of the coal increased significantly with oxidation. The enhanced cation exchange capacity could be used to introduce calcium or other cations, which could help reduce emissions of sulfur oxides during coal combustion by foaming sulfates in the ash.
FTIR spectra of coal samples were obtained using diffuse reflectance spectroscopy and peak overlap was reduced significantly using the fourier self-deconvolution technique, The spectra of oxidized coal samples showed the progressive reduction in the intensities of aromatic, methyl, and methylene C-H peaks and the appearance or enhancement of a number of carbonyl peaks.
Carbon-13 NMR spectra of solid coal samples were obtained using the cross-polarization technique together with magic-angle spinning, and carbon aromaticities were estimated which, when combined with analyses for carbon and ash content, allowed the calculation of overall consumption of aromatic and aliphatic carbon. Both aromatic carbon and aliphatic carbon were consumed, even at 175°C. This finding disproved a previous theory that aromatic rings were oxidized only above 225°C.
