Electrochemical membrane process for flue gas desulfurization

Abstract

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.

Electrochemical membrane flue‐gas desulfurization: K2SO4/V2O5 electrolyte

“…Generation was calculated as the difference between outlet SO, and inlet SO,. At 480"C, the rate of generation was reduced to approximately 30% of that previously (McHenry and Winnick, 1994) observed with a pyrosulfate mechanism. SO, generation was observed to virtually disappear at 520"C, with observed SO, levels attributed solely to the unoxidized portion passing through the preoxidation reactor at the highest flow rate.…”

“…Other contaminant gases such as HCI were not tested; CO,, H,O, and NO, (N,O, NO, and NO,) have been found to pass through unchanged (Scott et al, 1988;Townley and Winnick, 1981;McHenry and Winnick, 1994). SO, was therefore the gas of interest for this study.…”

“…A small ($86/ton) credit was taken for sulfuric acid production. A preoxidation reactor ($10.6 million) and sulfuric acid production plant ($9 million) were included in operating and capital costs (McHenry and Winnick, 1994). The low -300 ppm SO, outlet projected from this process should not condense in the process, as preheater temperatures of 230°C are well above the dew point of 165°C.…”

“…Their porous nature allowed intimate contact, high interfacial surface area, between the electrode, gas, and electrolyte, necessary for electrochemical removal, placing mass-transfer limitations in bulk film diffusion to the electrode surface as previously discussed. Both the cathode and anode porous gas-diffusion electrodes in these experiments were lithiated nickel oxide (Li,Ni,O) (McHenry and Winnick, 1994). A 0.001-m-thick 86% porous nickel Fibrex sheet (National Standard) was cut to a cylinder of 0.051 m diameter, providing an area of 20X m2, and soaked in 1 M LiOH solution for 20 min.…”

Electrochemical polishing of hydrogen sulfide from coal synthesis gas

“…The primary gaseous pollutant in each of these cases has been H2S in a fuel gas stream, but removal of SO, has also been achieved from flue gas streams [5]. The membrane is exposed to the same pressure on both sides, so there is no theoretical limit to the pressure at which the process can operate.…”