An evaluation of the waste product from a calcium based dry flue-gas desulfurization system

Integrated processes for removal of particulate and gaseous pollutants from flue-gas streams are receiving considerable interest from a number of electric utilities. One such process involves the injection of a calcium hydroxide slurry into a coal fired boiler exhaust gas stream in order to collect S0₂. Slurry moisture is evaporated in the gas duct and a dry waste product, consisting of calcium hydroxide-sulfur dioxide reaction products and fly ash, is collected by a fabric filter.

A review of the literature indicated that little information concerning the waste products of calcium based flue-gas desulfurization (FGD) was available. Most authorities seem to agree that waste disposal problems associated with these systems should be small in comparison to those of wet scrubber FGD systems, A literature review was initiated involving the disposal and reuse characteristics of several materials similar in composition to dry calcium FGD wastes. This review indicated several physical and engineering parameters commonly associated with utility wastes. The effects of the Resource Conservation and Recovery Act of 1976 (RCRA) upon utility wastes in general was investigated.

Laboratory investigations conducted with the subject wastes involved the determination of physical and engineering properties, and their hazardous waste potential. Particle size distribution and specific gravity determinations indicated that FGD waste solids were somewhat larger and more dense than lignite fly ash. Laboratory compaction tests indicated that the compactive behavior of these wastes is dependent upon the rela tive amount of free calcium hydroxide present. Waste materials compacted near the optimum .moisture content did undergo significant strength gain with increased curing time, The magnitude of strength gain, as determined by unconfined compression tests, was a function of calcium hydroxide in the waste materials.

Hazardous waste potential was measured via unbuffered unbuffered water leaching and the Toxicant Extraction Procedure (TEP). Unbuffed water leaching indicated that heavy metals release from this waste from landfills constructed of dry FGD wastes. The TEP indicated that heavy metals release was not significant from cured, compacted samples typical of anticipated disposal practice.

This study concluded that dry calcium FGD wastes may be disposed The hazardous waste in a landfill type manner with little problem. potential of this material is dependent upon the coal used and disposal waste technique. There is opportunity for reuse or utilization of this if certain additional engineering properties are adequate.