Novel designs of two types of down draft pyrolysis reactors are disclosed. One is a solid fuel reactor including a novel arrangement of down draft air inlet entrances, air distribution means, a consumable/replenishable catalytic bed, a heat exchanger for preheating inlet gas with the sensible heat of the exiting gas, and an infrared radiation trap below the reactor's screen grate. The other is an off gas pyrolysis reactor which includes a down draft reaction chamber with a fixed catalytic bed, a similar heat exchanger arrangement, an infrared radiation shield, an infrared radiation trap outside the gas outlet of the reaction chamber, and a unique relationship between the infrared radiation shield and the surface of the fixed catalytic bed.