Limestone utilization

Spray-dry scrubbers are an alternative to conventional wet scrubbers. In this type of scrubber, an alkaline slurry or solution is sprayed in fine droplets into a reaction vessel, along with the flue gas. The droplefs rapidly react with the sulfur dioxide while drying to a fine powder of sulfite salts. This powder is entrained in the gas stream, and is carried to a dust precipitator where it is collected, as shown in Fig. 7. Most of the sulfur dioxide is collected in liquid-phase reactions while the droplets are drying, but 10-15 /o additional sulfur dioxide can be absorbed in gas/solid reactions, as the absorbent powder is swept through the ductwork and particulate collector. These are cocurrent devices, and so the limestone utilization and sulfur removal efficiency are inherently lower than those of countercurrent devices such as wet scrubbers. Partial recycle of the sorbent is often used to improve the sorbent utilization. 

The bulk of the limestone dissolution in most SO2 scrubbers occurs in the 5-6 pH range. Thus, both solution mass transfer properties and the nature of the limestone must be considered at typical operating conditions. Therefore, the pH, solution composition, solution buffer capacity, and the nature of the limestone are important considerations when designing for maximum limestone utilization. This paper deals primarily with the measurement of the influence of limestone properties on the overall dissolution rate. 

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). 

The testing at Shawnee has shown that adipic acid is effective at concentration levels of 3 to 20 mM. SO2 removal at typical operating conditions was increased from 80-85% without adipic acid to 95-99% with 10 to 20 mM adipic acid. Simultaneously limestone utilization was increased from 70-80% to 90-95%. 

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. 

Potential for essentially complete limestone utilization with improved scrubber operating reliability... 

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... 

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. 

Overall limestone utilization during this run was 92 percent. Sulfite oxidation averaged 98 percent and the waste sludge filter cake quality was excellent, having a solids content of 85 percent. 

In summary, the objectives of this long-term test were met. High removal was consistently achieved at a good limestone utilization, and no fouling, scaling, or plugging occurred. 

Limestone utilization is improved with two tanks in series... 

Limestone utilization averaged 92.6 percent. Sulfite oxidation was excellent at 99.8 percent and the filter cake solids content was high, averaging 86 percent. Gypsum saturation in the scrubber inlet liquor was only 93 percent. 

Onstream hours Fly ash loading Adipic acid concentration, ppm (controlled) Scrubber gas velocity, ft/sec Liquid-to-gas ratio, gal/Mcf Slurry solids concentration, wt % (controlled) Scrubber inlet pH (controlled) Oxidation tank level, ft Oxidation tank residence time, min Effluent hold tank residence time, min Average percent SO2 removal Average inlet SO2 concentration, ppm SO2 make-per-pass, m-moles/liter Percent oxidation of sulfite to sulfate Air stoichiometry, atoms oxygen/mole SO2 absorbed Oxidation tank pH Percent limestone utilization Scrubber inlet liquor gypsum saturation, % Filter cake solids content, wt % 1,688 High 1,300 1,700 5.4- 9.4 85 150 15 5.0- 5.1 18 2.8 8.3 93.4 2,660 4.0- 8.9 99.8 1.4- 2.4 4.9 92.6 93 86... 

Test Gas Flow 1()3 dscfm Slurry Flow gpm Inlet S02 ppm dry Scrubber Inlet pH Adipic Acid Cone. ppm Av. S02 Removal % Limestone Util. % Test Period hrs. 

Table IV also shows that the adipic acid makeup rate must be held below 15.4 lb/ton of SO2 absorbed in order to recover the cost of the additive when operating at 90 percent limestone utilization. This limit is increased only slightly, to 16.5 lb/ton of SO2, at a 97 percent level of limestone utilization. The makeup rate used in Table IV was obtained from two independent Shawnee measurements. One measurement is the average of eight runs (934-2A to -2H) made without fly ash and with 78 percent solids in the discharged sludge these runs required an average makeup rate of 9.95 lb/ton SO2. The second measurement was obtained from...

Reliability Run 932-2A, which was made with fly ash, discharged a sludge containing 61 percent solids, and required a makeup of 17.5 lb/ton S02 When these data are normalized to a fly-ash-free basis, at equal limestone utilization, and equal sludge moisture, they yield the same makeup rate of 10.7 lb/ton SO2 for a concentration of 1500 ppm adipic acid in the scrubbing liquor. 

The limitation on adipic acid makeup rate likewise implies a minimum acceptable filtration efficiency for break-even operation on a fly-ash-free basis, the sludge must contain more than 46 percent solids when operating at 90 percent limestone utilization and 1500 ppm adipic acid. For both economic and environmental reasons, filter washing should be employed whenever additives are used in limestone scrubbers. 

Adipic acid will be most effective as a means of improving limestone utilization potential reductions in sludge disposal and limestone makeup costs are greater than the value of electric power savings. 

Month S02 Removal Efficiency Limestone Feed Stoichiometry (mols CaC03/ mol AS02) Soda Ash Feed Stoichiometry (mols Na2C03/ mol AS02) Limestone Utilization (% of CaC03 Reacted) Waste Solids Range Cake % (wt. %) Average... 

Limestone Utilization. Two types of limestone were used for regenerating the spent scrubber liquor Fredonia limestone from Kentucky, ground to 96.A wt. % through 325 mesh, with a Ca content of 93.6 wt. % as CaCO and Mg content of A.2 wt. % as MgCO and Sylacauga limestone from Alabama, ground to 97.7 wt. % through 325 mesh, with a Ca content of 95.7 wt. %, and Mg of 1.3 wt. %. 

Analyses were made to determine the progress of the regeneration reactions, and thus limestone utilization, through the reactor train. The results of these analyses are shown in Figure 3. 

The effect that the solids carried over in the thickener overflow liquor (and thus fed to the reactor train after passing through the absorber and scrubber) had on the limestone utilization in the first reactor is of particular interest. The utilization in this reactor ranged from 23% at solids carryover of 104 ppm to 63% at 1910 ppm. Since the solids carried over in the thickener overflow were essentially fully reacted, they undoubtedly contributed to an "apparent" high limestone utilization in the first reactor. In addition, it is also possible that solids entering the first reactor facilitated the precipitation of calcium sulfur salts. This, in turn, would have promoted an increase in the rate at which calcium from the limestone dissolved in the liquor and reacted with the sodium bisulfite. The high utilizations that accompany the solids carryover have been observed before (1) however, further studies would be needed to verify the exact mechanism involved. 

As reacting slurry passed through the reactors, the holdup time in the first reactor becomes small ( 5 minutes) in relation to the overall holdup time in the reactor train ( 100 minutes), and the spread reduced to limestone utilizations of 82 to 99% in the last reactor. Additional reaction in the thickener further reduced the differences in limestone utilizations. 

Thus, it seems that regardless of the extent of reaction achieved in the first reactor, the limestone will be efficiently consumed in the remainder of the reactor train and dewatering system. The solids carryover in the thickener overflow, therefore, do not appear to have a substantial effect on the overall limestone utilization by the system they do, however, impact on the settling properties of the solids generated as will be discussed later when addressing the properties of the waste solids. 

As the concentration of sulfate increases relative to sulfite, the amount of sulfate precipitation increases. Thus, as the rate of oxidation increases, the ratio of sulfate to sulfite in solution will increase until the rate of calcium sulfate precipitation is sufficient to keep up with the rate of sulfate formation by oxidation. This self-adjustment by the system may, however, be limited by the need to maintain a high active sodium concentration which will limit sulfate concentrations (and consequently the sulfate/ sulfite ratio) simply by solution saturation considerations. Furthermore, the sulfate/sulfite ratio may also be limited by the need to ensure high limestone utilizations and good solids properties. 

Although the generation of poor settling solids does not appear to affect the SO2 removal efficiency or the limestone utilization, it does however impact on the quality of the final waste product. These solids are more difficult to dewater and can result in a waste cake that is too moist, with attendant sodium losses. Furthermore, the advantages of using a clear liquor rather than a slurry in the absorber loop would be lost. Thus, there is still a need to refine the process in order to better control and understand the settling behavior of the waste solids. 

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