Biomass fuels differ in many ways from the conventional fossil fuels used in combustion processes, such as coal. They often have high moisture contents, lower heating values, and a variety of minor constituents, such as chlorine, sulfur, phosphorus, nitrogen, and a variety of ash-forming metals. These special properties of biomass fuels cause several challenges, but in many cases also provide advantages, to their use in combustion processes. Design of the combustion devices and choice of their operating parameters are very dependent on the detailed properties of the biomass fuel or fuels to be used. Often these challenges are connected to the fate and chemistry of the many minor constituents or impurities of the fuels. This paper reviews some of such chemical details related to biomass combustion that are important to take into consideration in the use of biomass combustion processes. The focus of the paper is in large, industrial scale combustion technologies for biomass and biomass derived waste fuels, either in district heating or also power production. Areas discussed are biomass particle conversion and biomass char oxidation reactivity, nitrogen and sulfur reactions in furnaces, superheater fouling and corrosion due to biomass ashes, low-temperature corrosion, and bed sintering in fluidized bed furnaces. The advances in understanding chemical details of biomass combustion have strongly contributed to the development of more reliable and efficient boiler technologies. Unresolved challenges are still connected to simultaneous combustion of several different biomasses and interaction of fuel ashes in such applications.