NOVELTY - A pyrolytic hydrogen generator comprises an array of hydrogen generating elements (1-18) arranged side-by-side and separated from one another into cells by partitioning provided with directional venting designed to permit gases to exit laterally in outward directions only with respect to the centre of the array.    USE - As pyrolytic hydrogen generator, particularly portable hydrogen generator (claimed), used in larger fuel cells, hydrogen engines or gas chromatographs.    ADVANTAGE - The housing is lightweight and inexpensive to produce, allows hydrogen diffusion through its walls, prevents cross-ignition, can be perforated to direct the flow of evolved hydrogen and does not produce contaminants. The housing can be directly attached to a circuit board and can be readily adapted to receive different ignitor devices. The arrangement is lightweight, quick to assemble, and is compact, permits higher energy densities to be achieved, and avoids the need for external wires to be attached directly to each tray, or attached along the stack. The baffles are substantially porous so that most of their surface area allows gases to vent freely.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) modular pellet tray assembly for use in a pyrolytic hydrogen generator comprising several pellet holder trays mounted on top of each other by several support rods to form a stack, where each tray comprises several pellet holders and associated ignitors for igniting any pellets loaded, in use, into the respective pellet holders, and where the support rods also provide electrical connectivity within the stack; and    (2) pellet tray assembly for use in a pyrolytic hydrogen generator and comprising at least one pellet holder tray (19) comprising several pellet holders (1-18), where at least some of the more outwardly disposed pellet holders contain only outwardly facing vents.    DESCRIPTION OF DRAWING(S) - The figure shows cross-sectional view of heat baffle system for pellet holder tray.    Hydrogen generating elements (1-18)    Pellet holder tray (80) Extending baffle element (19)    Solid walled baffle elements (82)    Radii (84)    Dead Spaces (86)