Background: Combusting or gasifying biomass with high ash can create operational issues, such as slagging and corrosion, as well as environmental issues, such as hazardous air emission, thus limiting the types of biomass that can be used in these conversions. Methodology: Exploration of a reduced severity liquid hot water treatment was performed on poplar wood, corn stover, switchgrass and Miscanthus to investigate the potential of this treatment to upgrade the biomass quality for combustion. Results: The results indicate that a significant reduction of ash occurs regardless of the treatment severity. Additionally, the treatment was able to increase the energy density up to 25% through increased heating values and pellet density. Conclusion: This treatment shows promise to improve the solid fuel quality of pelleted biomass.