In many combustion applications a switch from fossil to renewable fuels, e.g. from fueloil to wood powder, may result in a reduction of production capacity in the boiler,furnace or kiln. Oxygen enrichment of the combustion air can be used to improve thethermal efficiency of practical combustors, i.e. reduce heat losses and promote fuelsavings. In addition, oxygen enrichment can reduce NOx emissions and also facilitateCO2 scrubbing and capture processes in such systems. In this work, flame characteristicsand furnace temperature profiles during oxygen enriched combustion were studied whenoxygen was added to the combustor at different enrichment levels by the use of a lance.The experiments were carried out in a pilot-scale furnace fired with (i) wood powder and(ii) heavy fuel oil (no.5). The results show that for the wood flame, the average furnacetemperature becomes higher and the furnace temperature profile becomes more flat.Thus, compared to conventional air combustion, there are smaller differences betweennear-burner and back-end temperatures as oxygen is added to the process. For the oilflame, as oxygen was added to the process, a higher average furnace temperature wasobserved along with a distinct shift in furnace peak temperature towards the central partsof the furnace, creating a relatively strong temperature gradient towards the back-end ofthe furnace. Comparing the two flames, the furnace temperature profile of the oxygenenriched wood flame becomes more flat compared to the oxygen enriched oil flame. Thisis interpreted as an effect of differences in overall fuel reactivity, in which the oil, being aliquid fuel, ignites and burns faster than the solid fuel wood powder. The results found inthis work shows that the burner that was used, being designed for conventional aircombustion by feeding of air through the primary, secondary and tertiary air vanes, couldhandle the changes in aerodynamics caused by the reduced air flows. The general resultsfrom this work are useful for furnace and kiln applications in which a more controllableflame and process temperature is required, e.g. in a lime kiln where a fuel switch fromfossil fuels to biomass is considered.