The experimental characterisation of solid fuel flames lacks systematic analysis regarding the influence of inherent boundary conditions. Heat losses to the environment often require continuous gas co-firing, complicating the separate investigation of this effect in existing literature. Additionally, studies on scaling effects from laboratory to semi-industrial scales are limited. This chapter addresses the transition from methane co-fired flames to self-sustained flames and examines the transition from thermal loads typically investigated in laboratory experiments (up to \({50}~{\text {k}\text {W}_{\text {th}}}\) ) to semi-industrial scales ( \({100}~{\text {k}\text {W}_{\text {th}}}\) ). Further on, flame stabilisation in oxy-fuel combustion is explored by comparing different oxygen contents in the oxidiser. Experiments are conducted in a brick-lined combustion chamber with constant wall heating, enabling self-sustained solid fuel combustion and gas co-firing across various thermal loads and fuel compositions. Rhenish lignite and walnut shell flames are studied with and without methane co-firing. Measurements include solid fuel particle velocities and narrow-band imaging to assess how particle flow field and flame shape are affected. The findings reveal that flame stabilisation is sensitive to variations in parameters such as added gas content, volatile content of the fuel, local flame temperature, and local gas composition.