Alkali in the ash of annual crop biomass fuels creates serious fouling and slagging in conventional boilers. Even with the use of sorbents and other additives, power plants can fire only limited amounts of these fuels in combination with wood. The National Renewable Energy Laboratory (NREL), U.S. Department of Energy (DOE), and the biomass power industry conducted eight full-scale firing tests and several laboratory experiments to study the nature and occurrence of deposits. The goal was to increase the quantities of these biofuels which can be used. This paper describes the results of the laboratory and power plant tests which included: tracking and analyzing fuels and deposits by various methods; recording operating conditions; and extensive laboratory testing. These analyses have advanced the understanding of the role of minerals in the combustion of biomass, and their occurrence in biofuels. Deposits occur as a result of the boiler design, fuel properties and boiler operation. The limited furnace volume and high flue gas exit temperatures of most biomass boilers promote slag or deposits from biofuels which contain significant amounts of alkali, sulfur or chlorine and silica. All annual growth, whether from urban tree trimmings, annual crops and residues or energy crops contains sufficient volatile alkali, 0.34 kg GJ(-1) (0.8 lb MMBtu(-1)) or more, to melt in combustion or vaporize and condense on boiler tubes and refractory. Special boiler designs are required for annual crops, including grasses and straws. Addition of magnesium oxide and other additives may be necessary to inhibit alkali volatilization while burning these biofuels. Copyright (C) 1996 Elsevier Science Ltd.