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Solidification/Stabilization of Power Plant Wastes: Water Pollution Solutions

Discover the potential water pollutants from coal power plants and innovative solidification/stabilization techniques to prevent contamination. Learn about fly ash, FGD gypsum, and the methods to mitigate environmental impact.

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Solidification/Stabilization of Power Plant Wastes: Water Pollution Solutions

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  1. Solidification/Stabilization of Power Plants Wastes- Potential Water PollutantsSlobodanka Marinković, Aleksandra Kostić-Pulek,Svetlana PopovFaculty of Mining and Geology, the University of Belgrade,Belgrade, Serbia and Montenegro

  2. Introduction • Disposal of solid wastes from coal power plants (bottom ash, fly ash and flue gas desulphurization gypsum) is becoming a major issue because of their potential to contaminate surface and groundwater with arsenic, boron, heavy metals, sulphate anions, etc. • What is fly ash? • Fly ash is composed of oxides of iron, silicon, aluminium, magnesium, calcium, sodium and potassium. Along with oxides, fly ash contains trace elements (As, Sb, Cd, Cu, Cr, Ni, Zn, Mn, Hg, Pb, etc.). Laboratory studies reveal the possibility that trace elements can be easily mobilized and pollute the surrounding waters.

  3. What is FGD gypsum? • Flue gas desulphurization (FGD) gypsum is a waste from power plants, obtained in desulphurization process based on SO2 absorption and reaction with Ca(OH)2. FGD-gypsum disposal is a source of contamination of the surrounding waters by sulphate ions. • The lignite power plant“Nikola Tesla” is the biggest power plant in Serbia. It produces about 5 – 6 million tones of coal ashes per annum. The disposal sites of these wastes are located in an area rich in ground and surface waters. Hence contamination of the surrounding waters and the water of the river Sava (which is the final recipient of waters from the power plant disposal sites) is predictable. • Moreover, the concentration of SO2 from the “Nikola Tesla” power plant exceeds the recommended air limit of the Europe Union. Hence, a flue gas desulphurisation system will have to be built at the “Nikola Tesla” power plant. There, an additional solid waste – FGD gypsum will be produced.

  4. Lets find a solution • Solidification/stabilization (S/S) is a technique presently widely practiced • for remediation of coal wastes containing harmful constituents. This treatment inhibits the migration of hazardous constituent into the surrounding environment. Solidification refers to changes in the physical properties of wastes. They include an increase in the compressive strength, a decrease in permeability, and the encapsulation of hazardous constituents. Stabilization refers to chemical changes of the hazardous constituents in a waste, including converting the constituents into a less soluble, mobile, or toxic form. • Many lignite coals produce a low calcium fly ash. This fly ash reacts with lime forming calcium silicate and calcium aluminate hyrates (C-S-H and C-A-H). When sulphates are present (from FGD-gypsum, for example), they may combine with lime, alumina from the fly ash and water to form calcium-aluminate-sulphate hydrat ettringite(Ca6Al(OH)6224H2O(SO4)32H2O).

  5. The structure of ettringite consists of columns of Ca6Al(OH)6224H2O6+ with the intercolumn space (channels) occupied by anions (SO4)32H2O6. • Structural Ca2+ and Al3+ can be replaced by other metal cations (Zn2+, Cd2+, Cu2+, Ni2+, Pb2+, Cr3+, Fe3+, Si4+, Ti4+, etc.). Furthermore, SO42 and H2O can be replaced by anions (CrO42, AsO43, ZnO22, CO32, B(OH)4, etc.). • The aim of the present study was to test the possibility of solidification/stabilization of fly ash from the Serbian lignite power plant “Nikola Tesla” and FGD gypsum from the Bohemian lignite power plant “Hvaletice”, in the presence of lime.

  6. Experimental • Fly ash from the Serbian lignite power plant “Nikola Tesla”, FGD gypsum from the Bohemian lignite power plant “Hvaletice” (no Serbian plant has a FGD system installed yet) and lime from a mineral source (Serbia) were used in this study. • Calcined gypsum (CaSO40.5H2O), used in this work, was prepared by heating FGD gypsum (CaSO42H2O) in dryer at 135 oC. • Two mixtures were prepared at room temperature): • 1. fly ash-FGD gypsum-lime-water (the mass ratio of components was 16:3:1:16, respectively), and • 2. fly ash-calcined FGD gypsum-lime-water (the mass ratio of components was 7:2:1:5, respectively). • The samples prepared in this way were placed in cylindrical moulds and cured in ambient air for 30 and 180 days. After these periods the specimens were examined by means of DT, TG and XRD analysis. In addition, all the specimens were tested for their compressive strength.

  7. Results and discussion • The DTA curves of the specimens formed in the two mentioned mixture • showed two endothermic peaks which corresponded to ettringite and gypsum. The exothermic peaks in the DTA curves resulted from ignition of residual particles of coal, or carbon particles from the fly ash. • The results of thermogravimetric analysis showed that the total content of formed hydrates was greater in the system fly ash-calcined FGD gypsum-lime–water, than in the system fly ash-FGD gypsum-lime-water. • It must be to emphasized that gypsum is a reactant in the system fly ash-FGD gypsum-lime-water, but it is a product of the hydration reaction in the system fly ash-calcined FGD gypsum-lime-water. • The values of the compressive strength of the specimens from the system fly ash-calcined FGD gypsum-lime-water were greater than those of specimens from the system fly ash-FGD gypsum-lime-water. This is in accordance with literature data that the compressive strength of hardened specimens from the system fly ash-CaO-CaSO4-H2O is directly related to the number of new phases and their content (the number and the total content of formed new phases are greater in the system fly ash-calcined FGD gypsum-lime-water than in the system fly ash-FGD gypsum-lime-water).

  8. Conclusion • The present study showed that in the system: • 1. fly ash-FGD gypsum-lime-water (with a mass ratio of the components 16:3:1:16, respectively) and • 2. fly ash-calcined FGD gypsum-lime-water (with a mass ratio of the components 7:2:1:5, respectively) solidification/stabilization processes can be performed. • These systems satisfied the compressive strength (0.34 MPa) stipulated for solidification/stabilization processes, especially the system involving calcined FGD gypsum. • The formation of ettringite, in both systems, is very important because it can result in the incorporation of trace constituents (cations and oxyanions) into the ettringite structure, with the result of loss leaching of them in ground and surface water. • Acknowledgements • This work was financially supported by the Serbian Ministry of Science and Environmental Protection and the Serbien power plant “NikolaTesla”.

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