Scrubber additive, adipic acid testing

The addition of adipic acid to limestone-based FGD wet scrubbers results in improved limestone utilization and enhanced S02 sorption kinetics. The use of adipic acid was first proposed by Rochelle (1) and has been tested by the EPA in pilot systems at the Industrial Environmental Research Laboratory, Research Triangle Park, North Carolina and at the TVA Shawnee Test Facility at Paducah, Kentucky. Adipic acid in the concentration range of 1,000-2,000 mg/1 has been found effective as a scrubber additive. During scrubber operation, however, adipic acid is lost from the system in the liquid and solid phase purge streams and by chemical degradation (2,3). 

Tests conducted at the Shawnee Test Facility indicated that adipic acid added to their limestone FGD scrubber did not degrade at pH s below 5. Since these unexpected but favorable results were important to the future application of adipic acid as an FGD additive, independent verification was desired. Radian was contracted by the EPA to carry out a systematic study of the effects of scrubber operating conditions on adipic acid degradation. 

This paper summarizes the results of tests conducted from July 1978 through March 1981 at the EPA, 10-MW equivalent, lime/limestone wet-scrubbing FGD test facility, during which adipic acid as an additive was tested and shown to be a powerful scrubber additive for improving SO2 removal. The optimum concentration of adipic acid is only 700 to 1500 ppm at a scrubber inlet pH of 5.2 or higher. SO2 removal efficiencies in excess of 90 percent and reliable operation were demonstrated in four long term, limestone/adipic acid runs. Factorial tests were also conducted to characterize SO2 removal as a function of gas and slurry flow rates, pH, and adipic acid concentration. Intermediate duration optimization runs and favorable economics are also reported. 

The results of the tests showed adipic acid to be very effective in improving SO2 removal efficiency, even when operating at chloride levels as high as 17,000 ppm. A TCA scrubber, which removed 82 percent of the inlet SO2 without the additive, yielded 89 percent S02 removal with 700 ppm adipic acid, 91 percent removal with 1,000 ppm, and 93 percent removal with 2,000 ppm adipic acid. The limestone utilization was concurrently increased from 77 percent without the additive to 91 percent with 1,600 ppm adipic acid. The observed effects thus confirmed the theoretical expectations in all respects. In addition, the tests showed no serious inteference by adipic acid on the performance of the oxidizer, operating at pH 6.1. 

Tests without forced oxidation also demonstrated the efficacy of adipic acid. Operating a TCA scrubber with 2,000 ppm adipic acid and 6 inches H2O pressure drop, 92 percent SO2 removal was obtained at a limestone utilization level of 88 percent. By comparison, only 75 percent SO2 removal would be expected in the pilot plant at these test conditions without the additive. At this adipic acid level, the unoxidized sludge filtered to 49 percent solids at lower adipic acid levels (1,500 ppm or less), the... 

Limestone Long-Term Test with One Scrubber Loop and Without Forced Oxidation. Perhaps the most straightforward illustration of the effectiveness of adipic acid is demonstrated by a long-term limestone test conducted on the Shawnee TCA system, in which the additive was introduced without any system modifications. 

Lime Test with One Scrubber Loop and Forced Oxidation. With-in-scrubber-loop forced oxidation in a single-loop scrubbing system would not be expected to give good SO2 removal for a lime scrubber because of the oxidation of the major scrubbing species, sulfite ion, into nonreactive sulfate ion. With adipic acid addition, however, satisfactory SO2 removal should be possible because calcium adipate becomes the major scrubbing species. In addition, the lower pH at which a lime/adipic acid system operates should facilitate sulfite oxidation. 

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