Scrubbers magnesium oxide

Fig. 6 Magnesium oxide regeneration and sulfur dioxide recovery section for a magnesium oxide scrubber. (From Ref.. )...

A furnace similar to the Tomlinson kraft recovery furnace is used for the combustion of magnesium-based sulfite spent liquors. In this case, however, no smelt is obtained instead the base is completely recovered as magnesium oxide in dust collectors the sulfur escapes as sulfur dioxide and is absorbed from the combustion gases in scrubber towers. However, because magnesium hydroxide has a very low solubility in water, a complete recovery of sulfur dioxide meets difficulties. 

The magnesium oxide formed in the regeneration process is recycled back to the scrubber system and the sulfur dioxide is sent to the sulfuric acid plant. 

No magnesium sulfate was added to the system for run MG-3. The objective of this run was to evaluate the system performance with decreasing Mg2+ concentration. The mass balance indicated that the total Mg2 concentration should drift down to below 500 ppm. During the run, the total Mg2+ concentration decreased from 1000 ppm to about 625 ppm toward its end. A leak was discovered at the scrubber bleed/quench recirculation pump inlet which introduced air into the process stream and therefore caused high oxidation. The high oxidation, as confirmed by solids analysis results in Table 3, was reflected by increases of the sulfate-to-sulfite ratio to above 2.5. After the air leak problem was corrected, the sulfate-to-sulfite ratio decreased, but the test average was 2.4. 

Fig. 2 Basic schematic of a wet scrubber column. Absorbent slurry percolates down through the packing, while the flue gases flow upward. The most common absorbents for sulfur oxides are limestone (calcium carbonate), lime (calcium hydroxide), and magnesium-enhanced lime made from dolomite. The sulfur-bearing sludge for some scrubbers is market-grade gypsum, but for other scrubbers it is a waste product that must be landfilled.

Case 2 — A limestone case with MgO addition. Oxidation of the scrubber bleed stream was chosen because in-loop oxidation is incompatible with magnesium-enhanced scrubbing. As in Case 1, long-term reliability has not been demonstrated at Shawnee for this mode of operation. 

Quicklime, with about 7 % MgO, is slaked to produce a milk of lime. This is fed to the scrubber and removes oxides of sulfur (29.10, 29.11). The liquor that collects in the sump of the scrubber is a suspension of calcium sulfite and sulfate and a solution of magnesium hydrogen sulfite/sulfate. 

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],... 

The other point deals with scrubbers. I agree with your number. I would even say that calcium oxide is not an adequate scrubber for the simple reason that calcium oxide started life as calcium carbonate. You could use magnesium silicate instead. Again, the transportation issue would indeed be overwhelming. 

---