In this article, combustion properties of biomass fuels in boiler power systems are briefly reviewed. Brief summaries of the basic concepts involved in the combustion of biomass fuels are presented. Biomass is an attractive renewable fuel in utility boilers. The compositions of biomass among fuel types are variable. Ash composition for the biomass is fundamentally different from ash composition for the coal. Especially inorganic constituents cause critical problems of toxic emissions, fouling, and slagging. Metals in ash, in combination with other fuel elements, such as silica and sulfur, and facilitated by the presence of chlorine, are responsible for many undesirable reactions in combustion furnaces and power boilers. Elements including Si, K, Na, S, Cl, P, Ca, Mg, Fe are involved in reactions leading to ash fouling and slagging in biomass combustors. Chlorine in the biomass may affect operation by corrosion. Ash deposits reduce heat transfer and may also result in severe corrosion at high temperatures. Other influences of biomass composition are observed for the rates of combustion and pollutant emissions.