Solar-thermal conversion of biomass through pyrolysis process is an alternative option to storing energy in the form of liquid fuel, gas, and biochar. Fast pyrolysis is a highly endothermic process and essentially requires high heating rate and temperature > 400 °C. This study presents a theoretical study on biomass fast pyrolysis in a solar-thermal reactor heated by a parabolic trough concentrator. The reactor is part of a novel closed loop pyrolysis-gasification process. A Eulerian–Eulerian flow model, with constitutive closure equation derived from the kinetic theory of granular flow and incorporating heat transfer, drying, and pyrolysis reaction equations, was solved using ANSYS Fluent computational fluid dynamics (CFD) software. The highly endothermic pyrolysis was assumed to be satisfied by a constant solar heat flux concentrated on the reactor external wall. At the operating conditions considered, the reactor overall energy efficiency was found equal to 67.8% with the product consisting of 51.5% bio-oil, 43.7% char, and 4.8% noncondensable gases. Performance analysis is presented to show the competitiveness of the proposed reactor in terms of thermal conversion efficiency and environmental impact. It is hoped that this study will contribute to the global effort on securing diverse and sustainable energy generation technologies.