The combustion of some types of plant biomass leads to the sintering of their mineral part with the formation of deposits on the heating surfaces. Operational experience shows that the joint combustion of several types of biomass can help to solve this problem. The aim of this work is to study the effect of co-combustion of waste from flour milling and highly mineralized peat on the sintering of the ash residue. Various analysis methods (ISO, EDXRF, TGA, and DSC), as well as a experiment in the constructed experimental combustion unit, were used in the work. Wheat bran with a low initial deformation temperature (tA = 780 °C) and highly mineralized peat from the Tomsk region (Ad = 22.8%) were taken for the investigation. These resources are unpromising when considered individually as fuel. Using a bran-peat fuel mixture with a peat fraction of 5 wt % and more makes it possible to obtain an unsintered powdery ash residue. The optimal amount of peat addition to bran during combustion is 5 wt %. A decrease in the proportion of peat leads to sintering of the ash residue, an increase in the proportion of peat increases the ash content of the mixture.