This chapter addresses the gap in the current understanding and validation of mathematical models for pulverised solid fuel flames, particularly under oxy-fuel conditions. Traditional invasive and optical measurement techniques have been insufficient for capturing the coupled processes of fluid mechanics, particle dynamics, and chemical reactions. Therefore, this study employs advanced, non-invasive optical methods in a combustion chamber with large optical access in gas-assisted solid fuel flames, enabling detailed measurements and well-defined boundary conditions. Experiments gradually increased complexity from single-phase, non-reacting flows to two-phase, reacting flows. Parametric studies examined the influence of the fuel (lignite, bituminous coal and biomass) and oxidiser composition on the flame. Inert particles were also used for fundamental studies of particle-flow interaction. Key findings indicate that flame stabilisation of biomass and coal in oxy-fuel combustion is dominated by the combustion of volatiles and that the transition of boilers from air to oxy-fuel operation requires an increase in the oxygen content in the oxidiser.