This Bureau of Mines report presents the design for a commercial- sized flue gas desulfurization (fgd) demonstration plant that uses the citrate process. The goal of the Bureau's citrate process is to minimize the undesirable environmental impacts of industrial plants emitting so2-bearing gas. The fgd plant is located at the George F. Weaton Powerplant, Monaca, Pennsylvania. Construction was completed in April 1979 and will be followed by preliminary testing and a 1-year testing and evaluation program. Design capacity of the fgd plant is 156.000 Scfm of 0.2-Volume-percent-so2 flue gas yielding about 16 tons of sulfur per day. The plant is intended to (1) clean fly ash, so3, and cl-7 from the gas while cooling the gas in a Venturi scrubber, (2) absorb so2 from the gas using 1,200 gpm of a countercurrent-flowing citric acid-sodium citrate-sodium thiosulfate solution, (3) react the absorbed so2 in two 13,000-gal stirred closed vessels with added h2s, thus precipitating elemental sulfur and regenerating the citrate solution for recycle, and (4) recover the sulfur from the slurry by air flotation, followed by melting in a heat exchanger and separation from the occluded citrate solution in a sulfur decanter at 35 psi and 135 deg c. The work upon which this report is based was done under an agreement between the Bureau of Mines, U.S. Department of the Interior, U.S. Environmental Protection Agency, and St. Joe Minerals Corp.