The steam-gasification of biochar with concentrated solar radiation is experimentally investigated using a 3 kW solar reactor prototype consisting of a cylindrical cavity-receiver containing an opaque tubular absorber. Particles of beech charcoal are used as the biomass feedstock in a continuous steam-particle flow through the tubular absorber. A reactor model that couples radiative, convective, and conductive heat transfer to the chemical kinetics is formulated and validated by comparing numerically computed and experimentally measured temperatures and carbon conversions. The simulation model is further applied to examine the thermal performance of 100 kW and 1 MW scaled-up solar reactor containing multiple tubular absorbers, yielding a theoretical maximum solar-to-chemical energy conversion efficiency of 39% and 50%, respectively. Major sources of irreversibility are associated with re-radiation losses through the cavity's aperture. (C) 2009 Elsevier B.V. All rights reserved.