Environmental behavior of power plants fly ash containing FGD solids utilized in deep coal mines

The environmental implications of underground use of fly ash (FA) containing end products from dry flue gas desulfurization (D-FGDS) and semi-dry flue gas desulfurization (FA + SD-FGDS) were exemplified in the evaluation of the respective wastes from 3 power plants (Rybnik, Opole, Laziska) utilized in deep coal mines in the Upper Silesia coal basin (USCB, poland). Following load-based criteria, environmental behavior of these end products and their mixtures with water used in the deep mines was evaluated and compared with that of "pure" FA without FGDS that might originate from the electric utilities either not using desulfurization process or applying wet flue gas desulfurization technology. The end product properties were shown to highly depend upon the FA rate, which moderated the effect of FGDS. Solids from dry and semi-dry flue-gas desulfurization process exerted a considerable effect on the end product properties and release of constituents from this material due to influence on solidification process, hydraulic conductivity, pH, contents and forms of sulfur and calcium compounds, as well as on concentrations of trace elements. With respect to trace element contents, flue gas desulfurization products were found to cause positive "diluting" effect on the FA. The phase and chemical composition of FGDS in dry (CaSO4, residual CaO), and semi-dry process (CaSO3, CaSO4, Ca(OII)(2), CaCO3), besides quantity and quality of mine water, determined properties and the environmental effect of the end product utilization in mine working in the form of mine water: FA + FGDS mixture. The most environmentally beneficial way of utilization either of pure FA, or FA + FGDS was found to be its use in the deep mines as a sealing material in the form of low-ratio mixture with saline mine water placed in dry workings, ilnsulated from recoverable usable water resources. Using an optimum saline mine water: end product ratio, which assures transportability of mixture to the place of deposition at minimum leachate, would cause considerable reduction of aquatic environment contamination in the period of mining and after mine closure. Application of fresh water or low TDS-mine water for mixture preparation, as well as placing FA + SD-FGDS in the feeding zone of water recoverable resources should be avoided. Use of FA + SD-FGDS in dry mine workings appeared to be the most rational approach, taking into consideration long solidification time and certain thixotropic properties of this material.