The thermodynamic analysis carried out focuses on biomass mixing to reduce the formation of corrosive (mainly alkali) chlorides during straw combustion. The calculations confirm the reduction abilities of sewage sludge and peat and provide information on the addition levels at which no corrosive compounds are expected to form. The calculations provide insight into the mechanisms responsible for the disappearance of alkali chlorides. The mechanisms that can potentially take place are known (reaction with sulfur and reaction with or adsorption on aluminosilicates or other ash compounds). However, many aspects remain unclear, and calculations cast light on several of them. The main result obtained in this study is that, in a given binary mixture, the chemical elements involved in the decomposition of corrosive alkali chlorides (or preventing them from forming) change with the mixing proportions, an important fact never mentioned to our knowledge. The practical implications are significant: in a real system, local elemental concentrations will vary; this means that several mechanisms will simultaneously fight the formation of corrosive alkali compounds. This new result may explain why the experimental results from the literature are often confusing or even contradictory even for a given mixture; the overall chemical picture is not static. The chemical elements reacting with alkalis during co-combustion of straw with sewage sludge or peat are predicted to be S, Ca-S, and aluminosilicates.