This article presents how the combustion of coal char was studied optically in a specially designed drop tube reactor at 1123 K under varying oxygen concentrations and residence times. The char particles were produced in a drop tube reactor (at 1123 K) with nitrogen flow from pulverized coal that was sieved to a size fraction of 100–125 µm. The oxygen concentrations were set to 3, 12, and 30 vol-% in N2, and 30 vol-% in CO2. The drop tube reactor was equipped with movable feeding and collecting probes, and the sample particles were quenched in nitrogen flow. A two-color pyrometer was used to measure the temperature, size, and velocity of the particles, and a charge-coupled device camera was used to measure particle size and velocity. The results of the experiments show that an increase in the oxygen concentration causes an increase in the char surface temperature and a decrease in the reaction time. Carbon dioxide in turn reduces the surface temperature of the particles significantly. By replacing N2 with CO2 at the same O2 concentration from the atmosphere inside the reactor, the average particle surface temperature shows a decrease of approximately 300 K. This result is notable for boiler design in the future because it shows that the combustion temperature inside the boiler can be moderated.