Abstract Chars of Sugar cane bagasse (1 & 2), Cotton stalk and low rank Pakistani coal have been studied by TGA under low oxidative environments with O2 concentrations of 1% and 3%. The maximum reactivity of the chars was found to be greater by a factor of 2 under 3% oxygen compared to 1% O2 conditions. Overall conversion levels at 3% O2 for Sugar cane bagasse-2 increased from 63% to 100%, Sugar cane bagasse-1; 54% to 97%, Cotton stalk; 45% to 100% and Pakistani coal; 63% to 90% in comparison to 1% O2. The maximum average rate of weight loss was found in Region III compared to Region I and II supported by CO/CO2 FTIR Chemigram analysis. On the other hand, % conversion was maximum in Region II under 1% and 3% O2 concentration. Overall average rates of weight losses were dependant on O2 concentration and temperature ranges, however for all the regions % conversion and average weight loss were twice in 3% compared to 1% O2 concentration. Biomass chars were found to be more reactive than the coal studied here during each region of the oxidation process. Evaluated apparent energy of activations for biomass chars was found within range of 41.2–105.8 kJ mole−1 under 1%, 46.9–125.6 kJ mole−1 under 3% compared to coal; 70.3–183.9 kJ mole−1 under 1% and 83.1–167.4 kJ mole−1 in 3% O2 concentration for order of reaction (n) varying between 0.5 ≤ n ≤ 2. From the tests carried under O2 levels of 1% and 3%, it is possible to give the following sequence to the apparent activation energies under any of the fixed value of n, obtained for the biomasses and coal; Pakistani coal > Cotton stalk > Sugar cane bagasse-2 > Sugar cane bagasse-1.