Copyright © 2019 by ASME.This paper investigates the effectiveness of oxygen-enriched combustion process at low temperatures to avoid the unburnt carbon that remains in ash during conventional burning process. For this, thermal treatment of low-quality fuels such as olive pomace and Turkish lignite (Afsin-Elbistan) under oxygen-enriched conditions was tested in a tube furnace at temperatures between 400 and 700 °C under O2/N2 mixtures containing O2 ratios in the range of 25-50 vol %. The calorific value and the unburnt carbon content of the residues from these tests were used to investigate the combined effects of temperature and O2 concentration on unusable part of fuels. Thermal reactivity of untreated parent samples and the residues obtained from oxygen-enriched combustion was also compared based on differential thermal analysis (DTA) and derivative thermogravimetry (DTG) profiles. It was determined that oxygen-enriched conditions are able to remove the organic part of the fuels at low temperatures easily as O2 concentration increases and the oxygen-enriched conditions shifted complete burning temperature to lower values.