Lime/limestone adipic acid-enhanced

Burbank, D.A., and Wang, S.C., "Test Results on Adipic Acid-Enhanced Lime/Limestone Scrubbing at the EPA Shawnee Test Facility", presented at the Industry Briefing on EPA Lime/ Limestone Wet Scrubbing Test Program, Raleigh, N.C., Dec. 5, 1979. 

Adipic Acid-Enhanced Lime/Limestone Test Results at the EPA Alkali Scrubbing Test Facility... 

This report describes the results of adipic acid-enhanced lime and limestone testing at the Shawnee Test Facility from July 1978 through March 1981. It also summarizes earlier adipic acid additive test results from the IERL-RTP 0.1 MW pilot plant, which led to the testing at Shawnee. Also reported are preliminary results from the 100 MW full-scale demonstration being... 

Economics. Since limestone dissolution is not a ratecontrolling step in SO2 absorption for an adipic acid-enhanced limestone system, adipic acid should promote use of less expensive and less energy-intensive limestone rather than lime. 

Bleed stream oxidation of unenhanced lime or limestone slurry is usually not feasible because the pH rise caused by the residual alkali in the oxidation tank makes it difficult to redissolve the solid calcium sulfite. With adipic acid-enhanced limestone scrubbing, however, this constraint is removed because of the low operating pH and low residual alkali in the bleed slurry. Thus, the oxidation tank can be maintained at a low pH for good sulfite oxidation, while achieving high SO2 removal efficiency with a sufficiently high concentration of adipic acid in the scrubber liquor. 

Therefore, it would be advantageous to operate a low pH, adipic acid-enhanced limestone or lime system with within-scrubber-loop forced oxidation which, in addition to improved SO2 removal, requires low adipic acid makeup, minimizes gypsum scaling potential, and produces a sludge with good disposal properties. Based on Figure 9, 90 percent SO2 removal can be achieved at 5.0 inlet pH and only 1,100 ppm adipic acid, or at 4.6 inlet pH with 1,400 ppm adipic acid. 

Total capital investment and operating costs for adipic acid-enhanced limestone at high pH (Case 3) are higher than those for limestone/adipic acid at low pH (Case 4), but are still lower than those for the conventional limestone (Case 1) or the lime-stone/MgO case (Case 2). Total capital investment is lower by 3.9 percent, and the first-year revenue requirement is lower by 4.0 percent for Case 3, compared with Case 1. 

A primary objective of the EPA alkali wet scrubbing test program during the last several years has been to enhance SO2 removal and improve the reliability and economics of lime and limestone wet scrubbing systems by use of adipic acid as a chemical additive. 

Since forced oxidation converts sulfite to sulfate, it has an adverse effect on SO2 removal in an unenhanced lime system in which sulfite is the major SO2 scrubbing species. This is also true in MgO-enhanced lime and limestone systems in which the promotion of SO2 removal relies on an increased sulfite-bisulfite buffer. When adipic acid is used with lime, calcium adipate becomes a major buffer species therefore, both good SO2 removal and sulfite oxidation can be achieved using within-scrubber-loop forced oxidation. 

---