Process for Removal of Sulfur Oxides From Waste Gases

This invention relates to a process for absorbing sulfur oxides from industrial waste gases with a sorbent containing scrubbing liquor and regenerating the sorbent for reuse.

In the combustion of fossil fuels, and in many industrial processes, a serious problem is presented by the combustion of the sulfur-containing components therein. The noxious sulfur oxides produced are an environmental pollutant and in recent years considerable effort has been made to remove the sulfur oxides from the combustion gasses exhausted to the atmosphere. Several methods for removing such oxides are known. One of the processes presently used is wet lime scrubbing. This can either be a slurry process or the so-called dual alkali process. Both of these processes were described at the Environmental Protection Agency sponsored Symposium on Flue Gas Desulfurization, Oct. 28-31, 1980, Houston, Tex.

In some of the existing processes, e.g., the dual alkali process, the sulfite ion is an alkalinity carrier. While oxidation of the sulfite ion to sulfate improves the waste product characteristics, it is detrimental to the SO.sub.2 removal capability. The process of the invention overcomes this difficulty, as well as making several other improvements.

Briefly, the process of the invention comprises treating the waste gas containing sulfur oxides (which is principally and hereinafter for convenience referred to as sulfur dioxide) with an alkaline aqueous scrubbing liquor consisting essentially of a sorbent salt in which the cation is sodium or potassium and the anions include borate in an amount sufficient to react with substantially all of the sulfur dioxide present. Other than the borate ion, the particular anion employed is of little significance as this is an acid base reaction and the anion does not take place in the reaction. Where the anion is a weak base such as carbonate, bicarbonate or sulfite, the excess of borate ion renders the contribution of the other anions to SO.sub.2 removal insignificant. Typical anions other than borate include hyroxide, sulfate, chloride, carbonate, bicarbonate, sulfite, bisulfite and mixtures. Generally, an amount between about 0.05 and about 2 molal borate ion (moles of solute ion per 1000 grams of solvent) will be sufficient, and preferably between about 0.1 and 0.6 molal.

The borate ion [(B(OH).sub.4.sup.- ], an alkaline species, reacts with dissolved sulfur dioxide to form unionized boric acid.

The flue gas, substantially free of sulfur dioxide, is then vented to the atmosphere and the spent sorbent liquor is regenerated with lime. The lime converts the unionized boric acid to borate ion and precipitates the sulfur dioxide as insoluble calcium salts:

Some of the sulfur in the plus four oxidation state is oxidized to the plus six oxidation state.

A calcium sulfate waste is produced as a result of this. After separation of the waste solids the sorbent liquor is recycled for treating waste gas.

From the above reaction it may be seen that the sodium cation, Na.sup.+, does not enter into the reaction. Potassium would suffice equally as well. The sodium ion is required for electrical neutrality of the solution.

In previous lime based processes, the alkalinity for SO.sub.2 removal was provided by the sulfite ion, SO.sub.3.sup.- 2. The presence of borate ion, however, in accordance with the invention, permits the liquid-to-gas ratio to be decreased because it provides additional alkalinity for SO.sub.2 removal. Further, a high degree of oxidation of sulfite to sulfate can be tolerated which produces by-product sulfate solids. These are more easily dewatered so that smaller thickeners can be used and the waste makes a more acceptable landfill. In addition, less makeup chemicals, e.g., soda ash, are required because high sulfite levels are not required for alkalinity and if forced oxidation is used, the water content of the waste solids will be lower and less sodium is lost with the sludge.

The drawing is a schematic flow diagram of the process of the invention.

Referring to the drawing, a scrubbing liquor or slurry at a pH of about 12 is passed via conduit 1 to absorber 3, which is a gas-liquid contacting device. It may contain trays or packing to provide an increased mass transfer area. A flue gas containing sulfur dioxide is passed to absorber 3 via conduit 2. After treatment, the flue gas, substantially free of sulfur dioxide, is vented to the atmosphere via conduit 4.

The spent liquor at a pH of about 6 is passed via conduit 5 to a regeneration reactor 6 where lime is added via conduit 7. In a preferred embodiment, recycle tank 9 is employed to recycle the spent liquor via conduit 13 to increase the total mass transfer area in the absorber 3. Lime reacts with the dissolved sulfite and sulfate to precipitate calcium sulfite and/or calcium sulfate solids.

To produce a more desirable, predominantly sulfate waste product, an oxygen-containing material such as air can be added to reactor 6. It has been found that between one and two molecules of oxygen are typically required to provide the driving force required to oxidize one molecule of sulfite to sulfate.

All or a portion of the resultant regenerated slurry is then passed from reactor 6 to a solid-liquid separation section, e.g., thickener 8, via conduit 20 and the solid waste product, now in a more concentrated form, is passed via conduit 21 to filter 12. Wash water is added to filter 12 via conduit 22 to remove sodium and borate additives. The washed solid waste is passed to solid waste disposal via conduit 23. The combined wash water and filtrate are passed from filter 12 to the thickener via conduit 24 and the clarified liquid from thickener 8 is passed to mix tank 10 vial conduit 25.

Makeup sodium species, e.g., sodium carbonate, is added to mix tank 10 via conduit 26. If the makeup sodium is sodium sulfate it may be added to reactor 6. Makeup borate is added to mix tank 10 via conduit 27.

In the event a slurry, rather than a predominantly solids-free liquor is used for scrubbing, a substantial portion of the slurry from reactor 6 is passed to absorber 3 via conduit 11.

If HCl is present in the flue gas, the sodium addition must be increased to neutralize the chloride ion which will be absorbed by the scrubbing solution.

The borate ion or "alkalinity carrier" aids by converting the solid phase alkalinity of the lime to liquid phase alkalinity of the borate ion. Solid phase alkalinity is not readily available for SO.sub.2 removal while liquid phase alkalinity is.

The borate ion in the circulating solution may be supplied by addition of boric acid, borax, sodium borate, sodium tetraborate, and the like. With the use of the latter compound, sufficient Na.sub.2 CO.sub.3 should be added at a rate which will neutralize the HCl absorbed from the flue gas and the excess borate ions which will result since Na.sub.2 B.sub.4 O.sub.7 produces four boric acid molecules and only two sodium ions. Thus, one mole of Na.sub.2 CO.sub.3 per mole of Na.sub.2 B.sub.4 O.sub.7 is required.

The following examples will serve to illustrate the preferred embodiments of the invention.

Flue gas (1,056,600 acfm) at 1 atm pressure and 300.degree. F., containing 420 lb-mole/hr SO.sub.2 and 20.3 lb-mole/hr HCl is treated in a packed tower gas-liquid absorber with 2,440 gal/min (liquid-to-gas ratio of 3.0 gal/1000 acf (actual cubic feet) of gas saturated with water at 125.degree. F.) of clear liquor containing 0.4 molal borate ion and additional other dissolved species including sodium, chloride, sulfate, sulfite, carbonate, etc. to remove 95 percent of the SO.sub.2 and essentially 100 percent of the HCl from the gas.

The spent liquor is then treated with 430 lb.-mole/hr (100% CaO basis) lime to form insoluble calcium-sulfur compounds. The solids formed are separated and washed to recover dissolved species and sent to disposal. The reclaimed liquor from the separation and washing steps is recycled to the absorber with makeup sodium (e.g., sodium carbonate) and makeup borate (e.g., boric acid) added. The amount of makeup sodium and borate required depends on the effectiveness of the solid-liquid separation and washing steps. With no washing, and a waste sludge suspended solids content of 63 wt. %, the borate makeup rate is 3.3 lb-moles/hr (as Na.sub.2 B.sub.4 O.sub.7), and the sodium makeup rate is 18.5 lb-mole/hr (as Na.sub.2 CO.sub.3).

Comparison of this example to the additional examples in the following table show the relative effects of the presence of borates and that forced oxidation has no significant impact on the required liquid-to-gas ratio with borates present.

The general procedure of Example I is repeated with the exception that the borate concentration is adjusted from 0.4 to 0 molal as indicated in the following Table I. In Example IV forced oxidation is employed by bubbling air in the reactor to provide two molecules of oxygen for every mole of sulfite desired to be oxidized. From the data reported, it can be seen that even a minor amount of 0.1 molal borate ion substantially reduces the liquid-to-gas ratio and that forced oxidation has no significant effect.