Limestone scrubbers

Figure 2 is a plot of the correction parameter C as a function of magnesium concentration and pH. Note that for the typical range of limestone scrubber inlet liquor pH, 5.2 to... 

For a typical limestone scrubber inlet liquor pH range of 5.2-6.0, and for liquors having a chloride-to-magnesium ratio of 0.2 mole/mole or less, the following simplified equation can be used to determine gypsum saturation from calcium measurements ... 

The settling characteristics of the solid sulfur compounds produced in the lime/limestone scrubber systems vary considerably. Although the clarifier-thickeners are being designed by the vendors, C-E is developing a computer program that could be used to design the clarifier-thickeners for lime/limestone wet scrubber systems. 

Fig. 4 Circuit for a wet limestone scrubber, with oxidation of the solids to gypsum. The absorbent tank simplifies control of the process, while the hydrocyclone and filter remove coarse gypsum particles.

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...

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. 

The theoretical basis for the effect of adipic acid on the performance of lime and limestone scrubbers was first developed in detail by G. Rochelle in 1977 ( 3). In October 1977, EPA began an investigation of adipic acid with the 0.1 MW IERL-RTP pilot plant to determine its effectiveness as an additive to limestone scrubbers for improving S02 removal efficiency (4). Initial results demonstrated, as predicted by Rochelle, that adipic acid was indeed an attractive and powerful additive. 

Springfield Full-Scale Demonstration. In August and September 1980, the EPA, through its contractor, Radian Corporation, conducted the first demonstration of the commercial feasibility of adipic acid addition to a full-scale limestone scrubber (8). 

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. 

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. 

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. 

Figure 5. Gross electric power demand of limestone scrubbers using hot (300°F)...

The requirements for water pollution control have been forcing a shift to concentrating spent sulfite pulping liquor by evaporation, followed by incineration with heat recovery (102). When calcium-base liquor is burned, the sulfur emerges as calcium sulfate and is not available for recycle to the pulping process. The flue gas from such furnaces in Sweden is reported to contain 0.2—0.3% sulfur dioxide, and in one Swedish mill a Bahco wet limestone scrubber is used to treat the gases (115). 

Absorption of sulfur oxides from coal-fired power plant flue gases by limestone slurries is the current method of choice of the EPA. List some of the more important problems you would anticipate in the design of operable equipment, and draw a sketch and describe the internal column flow arrangement of a limestone scrubber. 

If coal or oil is the fuel source, the FGD control technologies result in the generation of solid wastes. Wet lime/limestone scrubbers produce a slurry of ash, unreacted lime, calcium sulfate, and calcium sulfite. Dry scrubber systems produce a mixture of unreacted sorbent (e.g., lime, limestone, sodium carbonates, and calcium carbonates), sulfur salts, and fly ash. 

Current practice with lime and limestone scrubber systems often injects slurry into the scrubber loop beyond the boilers. These scrubbers can remove well over 90% of both SO2 and fly ash when operating properly. In addition to corrosion and scaling problems, disposal of lime sludge poses formidable obstacles. The quantity of this sludge may be appreciated by considering that approximately 1 ton of limestone is required for each 5 tons of coal. 

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