Investigation of co-firing of carbon-containing waste with a high content of ash and biomass was carried out using methods of thermogravimetric analysis. As raw materials coal sludge, characterized by high ash content, and straw, which has a high content of volatile products, were chosen. Combustion characteristics, namely, characteristic temperatures and weight loss rates for raw materials and for their blends of various compositions were measured. It is shown that despite the fact that intense combustion of the selected raw materials occurs in different temperature ranges, the additive principle cannot be used to evaluate the combustion characteristics of their mixtures. The presence of coal sludge in blend leads to a decrease in weight loss rate both in the temperature range typical for the emission and oxidation of volatile products of straw pyrolysis and in the temperature range at which its solid residue is oxidized. The cause of an increase in combustion reactivity of coal sludge, when straw is added, is the catalytic effect of straw ash.