Limestone spray tower

Spray Tower. Figure 2 shows the SO2 removals reported by Head ( ) and by Burbank and Wang(10) for a limestone spray tower, plotted as a function of the net work input. The feed liquor pH was again 5.8 (stoichiometric ratio = 1.4 to 1.5). Gas velocities and slurry recirculation rates were systematically varied from 5.4 to 9.4 ft/sec, and 15 to 30 gpm/sq ft, respectively. The data include scrubber configurations with and without a venturi preceding the spray tower. All data are for single-loop mode of operation i.e., the venturi and spray tower were fed from a single EHT. 

Of the removal processes that have attained commercial status, the current favorite employs a shiny of lime or limestone. The activity of the reagent is promoted by the addition of small amounts of carboxylic acids such as adipic acid. The gas and the shiny are contacted in a spray tower. The calcium salt is discarded. A process that employs aqueous sodium citrate, however, is suited for the recoveiy of elemental sulfur. The citrate solution is regenerated and recycled. (Kohl and Riesenfeld, Gas Purification, Gulf, 1985, p. 356.)... 

Bischoff A flue-gas desulfurization process. A slurried mixture of lime and limestone is sprayed into the gas in a spray tower. The calcium sulfite in the product is oxidized by air to calcium sulfate. Used in Europe in the 1980 s. Lurgi Bishoff is a part of the Lurgi group. The process is offered by Lentjes, Germany, a subsidiary of Lurgi. 

S02 emissions from sulfuric acid plants are controlled in spray towers. Effluent gases contain less than 0.5 percent S02. The S02 emissions have to be controlled (or recovered as elemental sulfur by, for example, the Claus process). An approach is to absorb the S02 in a lime (or limestone) slurry (promoted by small amounts of carboxylic acids, such as adipic acid). Flow is in parallel downward. The product calcium salt is sent to a landfill or sold as a by-product. Limestone is pulverized to 80 to 90 percent through 200 mesh. Slurry concentrations of 5 to 40 percent have been used in pilot plants. 

Manufacture Synthetic hydrated calcium chloride is produced from the calcium chloride containing-residual brines of the Solvay process by evaporation initially in vacuum, then at atmospheric pressure. Calcium chloride is also produced from waste acid by reaction with limestone. Anhydrous calcium chloride is obtained e.g. by evaporation in fluidized bed dryers or spray towers. 

Originally, bubble cap plates had been used for absorption of pollutant gases such as sulfur dioxide. However, the solids in the slurries used as absorbents can more readily plug bubble caps. Typical absorbents used in current processes include, for example, conventional lime slurry lime-limestone slurries mixed sodium sulfite/ lime slurries and magnesium sulfite/bisulfite mixed with lime slurries. Conventional lime slurry towers may consist of a multilevel spray tower combined with a venturi scrubber. Venturi scrubbers will be discussed briefly below. Mixed sodium sulfite/lrme slurries may be contacted in a plate tower. Sieve plates might be used with larger than normal holes to help prevent plugging due to the solids in the slurries. 

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. 

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... 

Spray tower limestone stoich. ratio (controlled) 1.36 1.77 1.70... 

Venturi and spray tower inlet pH averaged 4.65 and 5.45, respectively. Overall limestone utilization was 88 percent and the spray tower limestone utilization was 56 percent, demonstrating the advantage of good limestone utilization in a two-scrubber-loop operation. 

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...

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. 

Factorial Test Results. Full or partial factorial tests have been conducted at Shawnee, primarily to investigate the effects of adipic acid concentration and pH on SO2 removal. These tests usually lasted 12 hours or longer, including at least 5 to 7 hours of steady-state operation. Scrubber configurations used were venturi alone, spray tower alone, combined venturi and spray tower, and TCA. Limestone was used in all scrubber configurations. Lime was used only with the venturi alone. Only the typical results from the TCA and spray tower tests are presented below to show the degree of effect of pH and adipic acid concentration on SO2 removal. 

Figures 8 and 9 show the results of partial factorial limestone runs made on the spray tower. Common operating conditions for these runs were ...

The gross electric energy demands of the three types of limestone scrubbers are compared in Table III as percentages of the total plant power production. The comparison is based on 2420 cfm per MW (saturated flue gas at 125°F, 1 atm) and 70 percent fan efficiency. In no case does the electric demand for 90 percent S02 removal exceed 0.83 percent of production. It is interesting to note that the differential between the highest demand (spray tower) and the lowest demand (TCA) amounts to only 0.16 percent of the total plant power production. From this viewpoint, the simplicity and increased reliability of the spray tower are not expensive attributes in terms of additional energy drain. 

Figure 7-5. Photograph of a scale model of the limestone process FGD plant on Unit 2 of Alabama Electric Cooperative s Tombigbee Station. Components from left to right are inlet ductwork I.D. fans common recycle tank, spray tower absorbers, one behind the other slurry preparation tank limestone silo and limestone ball mill (in enclosure). Courtesy of Peabody Process Systems, tnc.

Comparative Operating Conditions for SO2 Absonitlon by Limestone and Ume Slunles In a Spray Tower... 

Characteristics of the dissolution of limestone in acid solution were investigated to clarify the limestone dissolution in the spray tower and the oxidation tank limestone reacts with sulfurous acid in the spray tower and it reacts with sulfuric acid in the oxidation tank. 

Limestone slurry CaCOs CaSOj solids (gypsum) CaC03 + SO2=CaSOs+CO2 2HjO+CaSOs + 1/2O2=CaS04 2H2O 1000-4500 -95 Open spray tower... 

Spray 10 Tower System Limestone High Yes 1-Loop 2... 

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