Direct-fired supercritical CO2 cycles are one of the promising approaches to eliminate CO2 emissions in the power energy sector, while maintaining high efficiency and the ability to burn fossil fuels. One of the key elements of such cycles is the combustor, in which natural gas is burned in an O2/CO2 environment at supercritical pressure. Oxy-fuel CO2-diluted combustion differs significantly from traditional air-fuel combustion, which creates the need to adapt existing numerical modeling techniques. Experimental data are required to verify numerical methods, but present experimental studies are fragmented and non-formalized. This paper presents the results of an experimental test of the 15 kW oxy-fuel burner device in a tunnel gas furnace at atmospheric pressure. Experimental tests were carried out with oxygen-fuel ratios (α) of 1, 1.45, and 1.75 in the range of CO2 mass fractions in the oxidizer (γ) from 0 to 0.9. An experimental temperature profile is obtained, the adiabatic combustion temperature is calculated, and the boundaries of stable combustion are determined. Based on the experimental results, the main similarity criteria of combustion were calculated, and it was shown that by changing the composition of the model mixture, it is possible to reduce the discrepancy of the similarity criteria between the model and full-scale oxy-fuel combustors.