Methods and devices are provided for transfer of particulate material, such as biomass feedstocks, into and out of pressurized reactors. Improved sluice devices have an L-shaped sluice chamber having an upper, vertical component in communication with a horizontal loading chamber and a lower component in communication with a vertical reactor inlet or outlet. Piston valves seal the sluice inlet and outlet by axial displacement across the vertical component of the sluice chamber and across the vertical reactor inlet or outlet. Relative to other methods for reactor unloading, these devices consume less steam and significantly reduce furfural content of unloaded, pretreated biomass. An optional hybrid plug/sluice method of biomass feeding using the devices permits biomass loading at sluice pressures intermediate between atmospheric and reactor pressure, thereby reducing “pump cycle” time and increasing biomass throughput capacity.