Limestone forced oxidation

As it is well-known, FGD scrubber systems are used to reduce the SO2 emissions and are based on the reaction of SO2 formed during coal combustion with limestone CaCOa to form CaS04. FGD systems typically fall into two broad categories. Wet FGD systems, which are cuirently installed on about one-third of the total coal-fire generating capacity, include the commonly used limestone-forced oxidation and the magnesium-enhanced lime scrubbers. Diy FGD systems, which are found on <5% of the capacity (MW), are typically spray diyer absorbers that are usually installed in combination with an FF [38],... 

Ireland, P., and Ogden, G, 1991, FGD Economics A Comparison of Magnesium-Enhanced Lime and Limestone Forced Oxidation Systems," Proceedings Acid Rain Retrofit Seminar, The Effective Use of Lime, sponsored by the National Lime Assoc. (Arlington, VA), Pittsburgh, PA, Jan. 9-10, pp. 123-137. 

Assume a 180 MW boiler burning coal with 2.5% sulfur by weight, and a heating value of 12,767 BTU/lb. An appropriate limestone scrubber with forced oxidation would operate with a liquid/gas ratio of 130 gal. liquid per 1000 ft. of flue gas, and a pressure drop of Sin. water. Such a scrubber would consume 2.549 MW to operate, with the breakdown as shown in Table 3. This corresponds to 1.42% of the total power output of the plant. Such a scrubber would remove approximately 93% of the sulfur, while consuming approximately 13,000 Ib/hr of limestone being added at 35% solids. ... 

Table 3 Typical power requirements for limestone scrubber with forced oxidation of sludge to gypsum...

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). 

Borgwardt (JL, 2) has recently discussed forced oxidation in limestone FGD scrubbing systems. It is shown (2) that even in a single loop limestone scrubber that forced oxidation increases the SO2 removal efficiency and utilization of limestone. 

Borgwardt, R. H., "Effect of Forced Oxidation on Limestone SO Scrubber Performance", in proceedings ... 

Nine forced oxidation runs were performed with manganese, manganese plus iron, Springfield limestone, or Springfield limestone plus fly ash. The common factor involved in all these runs was the presence of manganese ions in the scrubber liquor. The presence of manganese caused a significant reduction in the adipic... 

Forced Oxidation. The mechanism by which adipic acid promotes SO2 removal is not affected by forced oxidation. Therefore, it can be used with both lime and limestone in systems with or without forced oxidation. 

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. 

Lime with forced oxidation Limestone with forced oxidation Limestone with forced oxidation, no adipic acid (Windows Creek simulation) Lime and limestone (Venturi only) Limestone factorial Limestone without forced oxidation... 

Limestone without forced oxidation, no adipiG acid (Glitsch Grid packing)... 

Forced oxidation is achieved by air sparging of the slurry in an oxidation tank, either on the bleed stream to the solids dewatering system or on the recirculated slurry within the scrubber slurry loop. For a one-scrubber-loop forced oxidation system, the slurry effluent from all scrubbers in the system (e.g., the venturi scrubber and spray tower at Shawnee constitute a two-scrubber system, and the spray tower alone or TCA, a one-scrubber system) are sent to a single effluent hold tank, which is the oxidation tank. For a two-loop forced oxidation system, there are two scrubbers in series (e.g., venturi and spray tower at Shawnee) with effluent from each scrubber going to a separate tank the effluent hold tank for the upstream scrubber (with respect to gas flow) is the oxidation tank. For either one-loop or two-loop forced oxidation systems, the oxidation tank may be followed by a second tank, in series, to provide further limestone dissolution and gypsum desupersaturation time prior to recycle to the scrubber. 

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 Tests with Two Scrubber Loops and Forced Oxidation. The venturi/spray tower system was modified for two-scrubber-loop operation with forced oxidation as shown in Figure 2. Two tanks were used in the oxidation loop (venturi loop) air was injected to the first of these tanks through a simple 3-inch diameter pipe below the agitator. Adipic acid was dry-fed to the spray tower effluent hold tank. This was accomplished by manually adding one-pound increments hourly to maintain specified concentration, usually totaling only a few pounds per hour. A small screw feeder would serve the purpose in a full-scale plant. 

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,... 

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. 

Limestone Long-Term Test with One Scrubber Loop and Forced Oxidation. A one-scrubber-loop system has an inherent advantage over a two-scrubber-loop system in its simple design and lower capital and operating costs. If a simple one-loop limestone (or lime) system is operated with adipic acid, which offers the advantage of lower operating pH, then both good SO2 removal and sulfite oxidation can be achieved with minimum cost. 

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 ... 

In any within-scrubber-loop forced oxidation system, irrespective of whether it is additive promoted or not, the possibility exists for calcium sulfite blinding of limestone because the recirculated slurry lacks the solid CaSC>3 crystal seeds. 

Limestone Tests with Bleed Stream Oxidation. A major advantage of the bleed stream oxidation is its simple flow configuration. In operation without forced oxidation, the scrubber bleed stream would be sent directly to the solids dewatering system. 

Figure 6. Effect of adipic acid concentration and slurry flow rate on SOs removal in the TCA with four grids and 15 in. of spheres. Key V, pH 5.6 0, pH 5.3 gpm/ft2 and 6, 19 gpm/ft2. Inlet SOs, 1800-2800 ppm gas velocity, 8.4-12.5 ft/s scrubber inlet pH, 5.6-6.1 (limestone stoich., 1.2) height of spheres, 15 in. with and without forced oxidation.

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. 

The economics of limestone scrubbing, with or without additive, have been projected for forced oxidation systems designed to... 

Conditions for Economic Analysis of Limestone Scrubbing with Forced Oxidation and with or without Additive... 

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