A vapor generating system in which a furnace section is provided that is formed by four upright walls, the lower portion of two opposed walls being slanted inwardly to form a hopper portion. A plurality of openings are formed in each of the opposed walls immediately above its slanted portion. Two gasifiers extend adjacent said opposed wall portions, respectively, and surround the respective slanted wall portions and openings, so that the respective interiors of the gasifiers communicate with the openings. A bed of adsorbent material is supported in each gasifier for adsorbing the sulfur generated as a result of the gasification of fuel introduced into the gasifier and air is passed through the bed of adsorbent material to fluidize said material so that, upon combustion of said fuel, a substantially sulfur-free product gas is produced which passes from the gasifier, through the openings and into the furnace section.