Limestone adipic acid-enhanced

The results of this investigation show that CaCC>3 dissolution is controlled by mass transfer and not surface reaction kinetics. Buffer additives such as adipic acid enhance mass transfer by increasing acidity transport to the limestone surface. Dissolution is enhanced at low sulfite concentration but inhibited at high sulfite concentration, indicating some kind of surface adsorption or crystallization phenomenon. The rate of dissolution is a strong function of pH and temperature as predicted by mass transfer. At high CO2 partial pressure, the rate of dissolution is enhanced due to the CO2 hydrolysis reaction. 

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... 

Limestone Utilization. At a scrubber inlet pH of about 5.2, the corresponding limestone utilization is normally 80 percent or higher for an adipic acid-enhanced system, as compared to 65 to 70 percent in unenhanced limestone systems at an equivalent S0 removal. Thus the quantity of waste solids generated is reduced in an adipic acid-enhanced system. Higher limestone utilization also contributes to more reliable scrubber operation by reducing the fouling tendency. This increased reliability is a very attractive feature of adipic acid-enhanced systems, since reliability problems have historically plagued limestone FGD. 

With the lower operating pH (about 4.6 to 5.4) in an adipic acid-enhanced limestone system, compared to the higher pH (about 5.5 to 5.8) usually needed for an unenhanced limestone system, the system becomes more amenable to other process concepts and improvements. Potential advantages of low pH operation are ... 

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. 

Adipic acid-enhanced limestone scrubbing has lower projected capital and operating costs than unenhanced limestone or MgP-enhanced limestone scrubbing. This is due primarily to the reduced limestone consumption at the lower operating pH, the reduced grinding cost, and the reduced quantity of waste sludge generated. 

Run 907-1A was a month-long adipic acid-enhanced limestone run with forced oxidation, designed to demonstrate operational reliability with respect to scaling and plugging and to demonstrate the removal enhancement capability of the adipic acid additive. This run was controlled at a nominal limestone stoichiometry of 1.7 (compared to 1.4 for the base case run, Run 901-1A) and 1,500 ppm adipic acid in the spray tower. Venturi inlet pH was controlled at a minimum of 4.5 by the occasional addition of limestone to the venturi loop. 

Adipic Acid-Enhanced Limestone Tests on the Two-Loop Venturi/Spray Tower System with Forced Oxidation... 

Following Run 907-1A, a second adipic acid-enhanced limestone long-term run with forced oxidation was made during which flue gas monitoring procedures were evaluated by EPA. This run, Run 907-1B, was made under the same conditions as Run 907-1A except that the gas flow rate was varied according to a "typical" utility boiler load cycle rather than the actual Unit No. 10 boiler load. Run 907-1B began on November 13, 1978 and terminated January 29,... 

This was illustrated in a long-term adipic acid-enhanced limestone run, Run 917-1A, conducted on the Shawnee spray tower system from December 26, 1980, to March 13, 1981. Figure 4 shows the flow diagram for this long-term run with forced oxidation using two series tanks in the slurry loop. Oxidation was forced in the first tank while fresh limestone was added to the second. Use of two tanks in series in a within-scrubber-loop forced oxidation system has several advantages over a single tank ... 

Figure 4. Flow diagram for adipic acid-enhanced limestone scrubbing in the spray tower system...

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. 

Table 7 gives the results of a typical bleed stream oxidation test, Run 915-1C, which was conducted with adipic acid-enhanced limestone on the venturi/spray tower system. The effluent slurries from the venturi and the spray tower were discharged into a common effluent hold tank. The scrubber bleed stream was pumped from the effluent hold tank to an oxidation tank into which air was injected through a 3-inch diameter pipe. The final system bleed was withdrawn from the oxidation tank and sent to the solids dewatering system. 

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. 

Rickenbacker Industrial Boiler Demonstration. In February, March, and April 1981, the EPA, through its contractor, PEDCo Environmental, Inc., conducted adipic acid-enhanced limestone scrubber tests on an industrial-sized system. The testing was carried out at the Rickenbacker Air Force Base on a Research-Cottrell/Bahco system rated at 55,000 scfm, or about 27 MW equivalent. The tests, conducted with certified instrumentation, indicated an SO2 removal efficiency increase from 55 percent without adipic acid, to 90 to 95 percent with adipic acid. This improvement was achieved at a scrubber inlet pH of 5.0 and adipic acid concentrations of between 2,000 and 2,500 ppm. More complete data will be reported separately by others. 

As shown In Table 10, both the total capital investment and the first-year revenue requirement are the lowest for adipic acid-enhanced limestone scrubbing at low pH (Case 4). The total capital investment is reduced by 4.8 percent, and the first year revenue requirement reduced by 5.8 percent for the limestone/ adipic acid/low pH case (Case 4), compared with the conventional... 

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. 

In the slurry scrubbing process, limestone dissolves at pH A to 6 and 55°C in both absorber and the hold tank/crystallizer. Because of HC1 accumulation from the flue gas, typical scrubbing solution contains 0.01 to 0.2 M CaCl2 C02 partial pressure can vary from near zero with forced oxidation to one atmosphere with CO2 evolution from the hold tank and is typically 0.1 atm in the absorber. Sulfite/bisulfite buffer can be present in concentrations up to 0.1 M. CaS03 and/or CaS04 crystallization must occur simultaneously with limestone dissolution. Buffer additives such as adipic acid should enhance both SO2 removal and CaC03 dissolution at concentrations of 3 to 10 mM (5). 

Organic buffer additives such as acetic, adipic, acrylic, and sulfosuccinic acid enhance the rate of calcite dissolution by providing for mass transfer of acidity to the limestone surface. 

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). 

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. 

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