One Bureau of Mines goal is to minimize the undesirable environmental impacts associated with industrial plants emitting waste gases containing so2. To help meet this goal, a regenerable flue gas desulfurization process was developed. This process, known as the citrate process, uses a buffered weak acid solution to absorb so2 from the waste gas. The absorbed so2 is reacted with h2s to precipitate elemental sulfur and regenerate the solution for recycle. The h2s feedstock for the process, if not otherwise available, may be produced by reacting two-thirds of the recovered elemental sulfur with natural gas and steam. Laboratory investigations and pilot plant operations were conducted by the Bureau to determine if h2s from the natural gas, sulfur, and steam reaction was suitable for the citrate process. The laboratory investigations, in which an h2s generator was integrated with other citrate process operations, provided a basis for design and operation of the pilot plant. The objective of the pilot plant was primarily to provide h2s for the citrate process pilot plant at the Bunker Hill Company lead smelter in Kellogg, Idaho. The design capacity of the pilot plant was 0.4 to 1.25 Tons of h2s per day. The operation demonstrated that (1) a 77- to 79-dry-volume-percent h2s product could be produced from pilot-plant product sulfur, natural gas, and steam; (2) the product h2s regenerated the so2-loaded citrate solution; and (3) under certain conditions over 95 percent of the natural gas could be converted to h2s.