Char conversion is a highly complex process. Primarily composed of solid carbon, char has a very low vapour pressure, which prevents it from evaporating and undergoing gas-phase oxidation. Instead, char reacts directly at its surface throughout oxygen, carbon dioxide, hydrogen, and water vapour. These surface reactions generate carbon monoxide (CO), which then escapes the solid phase and oxidises into \(\text {CO}_2\) in the surrounding gas. Key factors that affect char conversion include the temperature of the surrounding gas, the composition of the atmosphere near the surface, ambient pressure, and characteristics like fuel composition, particle size, and shape. To capture the entire temperature range relevant in practical applications and to study the full span of char burnout—from the initial phase to the late stage over extended residence times—two experimental setups are employed: a fluidised bed reactor and a laminar plug flow reactor. These two test rigs also allow to study all char conversion regimes: regime I (kinetically controlled), regime II (pore-diffusion controlled) and regime III (film-diffusion controlled). Therefore char conversion is studied across a variety of fossil and biogenic solid fuels, with a broad range of temperatures and oxy-fuel gas atmospheres applied.