Removal of sulfur dioxide from an air stream by selected coal fly ashes in a bench-scale fixed-bed reactor

In a series of exploratory tests the capacity for five United States coal fly ashes to absorb sulfur dioxide was measured under various temperature and moisture conditions. Tests were run at temperatures ranging from 75° F to 350° F. A simulated flue gas was passed through a fixed-bed of the test material. The circular bed was 1.5 inches in diameter and approximately 1/16 inch deep. The gas flow rate, SO₂ concentration, sample size, pressure, and length of the test period were all held constant at conditions similar to those found in a typical fabric filter collecter used to control fly ash emissions from a coal-fired boiler. The degree of water vapor in the test gas was varied from dry to complete saturation. The total alkali oxide in each of the fly ashes tested ranged from 6.4 to 39.4 weight percent.

Moderate SO₂ retention was seen under certain conditions for the highly alkaline Leland Olds fly ash from North Dakota. The 39.47o alkali oxide corresponds to 0.0062 mole alkali oxide per gram of fly ash. The greatest sulfur dioxide retention was noted at the lowest temperature tested, 75° F; and at complete vapor saturation. Under these conditions, a one gram sample of Leland Olds fly ash could retain a significant 0.0812 gram of SO₂. This amount corresponds to 18.3% conversion of the available alkali oxide in the fly ash to the reaction product. At the same temperature, when the test gas contained no water vapor, the Leland Olds fly ash was unable to retain any sulfur dioxide. At the highest temperatures in this study (250-350° F) the Leland Olds fly ash also had negligible capacity to retain SO₂, regardless of the water vapor content.

None of the four low alkaline fly ashes showed ability to retain SO₂ in this study. However, it must be pointed out that due to limited time and availability of these fly ashes, not every condition was tested. Therefore, direct comparison can not be made between the two types of fly ashes in this study.