Digestion of cellulosic biomass to produce a hydrolysate may be accompanied by the formation of cellulosic fines which may be damaging to system components. Biomass conversion systems that may address the issue of cellulosic fines may comprise a fluid circulation loop comprising: a hydrothermal digestion unit; a solids separation unit that is in fluid communication with an outlet of the hydrothermal digestion unit; where the solids separation unit comprises a plurality of filters and the filters are in fluid communication with the fluid circulation loop in both a forward and a reverse flow direction; and a catalytic reduction reactor unit that is in fluid communication with an outlet of the solids separation unit and an inlet of the hydrothermal digestion unit; where at least one of the plurality of filters is in fluid communication with an inlet of the catalytic reduction reactor unit.