A flue gas desulfurization process being developed for coal-fired power plants removes and concentrates sulfur oxides (SO,) IntroductionA large number of flue gas desulfurization processes are presently under development throughout the world to combat the acid rain problem with most using wet or dry sorption to preferentially remove SOz from the flue gas. Most of these processes need expensive reagents, a flue gas reheating step and a landfill for disposal of the captured sulfur. This leads to high operating and capital costs. With the recent 1991 Clean Air Act, these processes will see continued interest as utilities attempt to reduce SO2 emissions by 10 million tons from 1980 levels.Here we present the development of an electrochemical membrane flue gas desulfurization process, which overcomes the problems of most other flue gas desulfurization processes. As a membrane separation process, it has similarities to other membrane processes, with one major difference: the driving force for the separation is an applied electric field instead of a conventional pressure or concentration gradient. This technique has been applied to other separation schemes, such as the production of oxygen from air (Langer and Haldeman, 1964), the removal of carbon dioxide from dilute streams (Kang Walke et al., 1988) and hydrogen sulfide from coal gas (Weaver and Winnick, 1991) or natural gas (Alexander and Winnick, 1990). The key is that the components to be removed must be the strongest electron acceptors (Lewis acids) in the gas mixture. Sulfur oxides are the most acidic of all species present in flue gas and are therefore theoretically separable by this technique.When development is complete, this technique offers several advantages over standard limestone scrubbing. The operation is continuous and totally in situ; no liquid is pumped and no regeneration is required. No reagents are necessary, other than about 1-2070 of the plant electric power production. Furthermore, a salable byproduct, oleum, (anhydrous sulfuric acid) is produced; no waste is emitted. Finally, operation at fluegas temperature eliminates the need for stack-gas-reheat.While the goal of process development is to produce a process for the removal of SO, and NO,, the focus of this article is on the SO, removal capabilities.

Lithiated nickel oxide (LiNi 0) electrodes have been successfully used with molten K2S207/V205 at 400 t and K2S04/V205 at 480 and 520 CC in tice electrochemical membrane removal of sulfur oxides from simulated flue gases. The porous gas-diffusion electrodes manufactured in the laboratory from commercial Ni electrode material remained stable and conductive in this environment for periods up to 40 days. Polarizations are reported in these systems for current densities up to 50 mA/cm2. The results show exchange current densities from 0.113 mA/cm2 at 400 CC to 0.8 mA/cm2 at 520 CC.

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