Power generation from biomass is a fairly new area, and boilers that utilize various types of biomass have in many cases experienced serious problems with slagging, fouling and corrosion of boiler tubes. Mineral matter in these fuels can deposit on the heat-exchanger surfaces in the boiler and generate an insulating layer, which will significantly reduce the degree of heat-transfer from flue gas to water and steam. Our investigations were focused on the slag characteristics of different kinds of herbaceous biomass fuels. Since there is usually a reducing atmosphere present in the direct combustion zone of modern low-NO, firing systems, it is important to study mineral matter transformation of burned fuel residues in a reducing atmosphere. An excellent device for this type of study is the electric-resistance heated Bunte-Baum softening temperature testing instrument, which was used in this work. Ash chemical composition was analyzed via flame atomic absorption spectrometry and the microstructure of ash was determined using a scanning electron microscope. Crystalline compounds of the ashes were identified by using X-ray powder diffraction. This paper provides an overview of results on the combustion and slag characteristics of herbaceous biomass fuels. The results include chemical compositions, morphology and softening properties of these fuels, with special attention to switch grass and sunflower seed shell. (C) 2009 Elsevier B.V. All rights reserved.