A pyrolytic hydrogen generator comprising a pressure vessel containing a plurality of cardboard receptacles for the thermally decomposable hydrogen generating material and an associated ignition system. Also, a modular pellet tray assembly for use in the generator comprises a plurality of trays having pellet holders and associated igniters and held in a stack by support rods that also provide electrical connectivity to the trays. Also, a pellet tray assembly comprises a plurality of pellet holders, wherein some of more outwardly disposed pellet holders contain only outwardly facing vents and are fired first. Also, the generator has an array of hydrogen generating elements arranged side by side and separated from one another into cells by partitioning provided with directional venting that only permits laterally exiting gases to vent outwardly. Alternatively, the elements can be separated into cells by a baffle system comprising gas confining and gas venting elements, which may be heat conductive.