Synergistic emission reduction and distribution characteristics of heavy metals for ultra-low emission coal-fired power plants

Abstract

Heavy metals pollutants (As, Pb, Se and Cr) emitted from coal combustion has caused widespread concern. In order to study the emission and distribution characteristics of heavy metals from ultra-low emission coal-fired power plant, the Method 29 of the United States Environmental Protection Agency (EPA) and total suspended particulate (TSP) sampling system were used to conduct parallel isokinetic sampling analysis for the gaseous and particulate heavy metals at multiple points on a 350 MW coal-fired power unit equipped with selective catalytic reduction (SCR), low-temperature economizer-electrostatic precipitator (LLT-ESP), wet flue gas desulfurization (WFGD), and wet electrostatic precipitator (WESP). The enrichment, migration, distribution, and synergistic control characteristics of heavy metals in different forms in flue gas were investigated. The results show that the relative enrichment factors of four heavy metals (As, Pb, Se and Cr) in fly ash were obviously higher than that of bottom ash, in which the enrichment degree of different heavy metals in fly ash/bottom ash has positive/negative correlation with its volatility. The mass balance rates of four heavy metals across different zones were all in acceptable ranges. And four heavy metals (As, Pb, Se and Cr) were mainly distributed in fly ash and bottom ash during coal combustion with the ratios of 92.12%−95.33% and 2.31%−6.70%, respectively. However, a small amount of them existed in the WFGD and the stack, with sum in the range of 0.47%−2.98%. The ESP had high removal efficiency of 98.70%−99.52% for heavy metals. Both WFGD and WESP could further capture heavy metals, which resulted in the overall removal efficiencies of gaseous and particulate heavy metals across ESP+WFGD+WESP of 62.96%−92.99% and 99.72%−99.96%, respectively. Thus, the promotion on the enrichment of heavy metals into fly ash and the conversion of heavy metals into soluble compounds could be effective technical means for an efficient removal of heavy metals pollutants in coal-fired power plant. Finally, the concentrations of gaseous and particulate heavy metals (As, Pb, Se and Cr) emitted to atmosphere were extremely low with the range of 0.11−1.89 and 0.06−3.83 μg/m3, respectively, and the emission factors were 0.15−1.72 g/TJ, in which the emission differences of different heavy metals in different forms mainly depended on their contents in coal and volatility.

Keywords

• coal-fired power plants
• ultra-low emission
• heavy metals in different forms
• synergistic control
• emission characteristic