Preparing solid biomass particles for catalytic conversion includes agitating solid biomass particles and providing a biomass-catalyst mixture to a conventional petroleum refinery process unit. The biomass-catalyst mixture includes the solid biomass particles and a catalyst. Agitating solid biomass particles includes flowing a gas to provide a velocity to at least a portion of the solid biomass particles sufficient to reduce their sizes. Co-processing a biomass feedstock and a conventional petroleum feedstock includes liquefying at least a portion of a biomass-catalyst mixture and co-processing at least a portion of the liquefied biomass feedstock and a conventional petroleum feedstock in a conventional petroleum refinery process unit. The biomass feedstock includes a plurality of solid biomass particles and a catalyst, which is liquefied to produce a liquefied biomass feedstock.