Abstract
Mercury (Hg) emissions from coal-fired power plants are of increasing concern around the world. In this study, field tests were carried out to understand the Hg emission characteristics and its migration behaviors in a commercial CFB boiler unit with the electricity generation capacity of 25 MW. This boiler is equipped with one electrostatic precipitator (ESP) and two fabric filters (FFs) in series for removing particulates from the flue gas. The EPA 30B method was used for simultaneous flue gas Hg sampling at the inlet of the ESP and the outlet of the second FF. The Hg mass balance in the range of 104.07% to 112.87% was obtained throughout the CFB unit by measuring the Hg contents in the feed fuel, the fly ash and the bottom ash, as well as in the flue gas at the outlet of the particulate control device (PCD) system. More than 99% of Hg contained in the feed fuel was captured by the fly ash, whilst less than 1% of Hg was remained in the bottom ash or the flue gas after passing the PCD system. The gaseous Hg obviously migrated from the flue gas to the fly ash in the air pre-heater, where the flue gas temperature decreased from 250 °C at the inlet to 120 °C at the outlet. Other gaseous Hg migrated from the flue gas to the fly ash in the PCD system, as the Hg concentrations in the flue gas ranged from 3.14 to 4.14 μg/m3 at the inlet of the ESP and ranged from 0.30 to 0.36 μg/m3 at the outlet of the second FF. The average Hg contents in the fly ash samples collected from the ESP, the first FF and the second FF were 912.3, 1313.6 and 1464.9 ng/g, respectively, while the mean particle diameters of these fly ash samples tend to decrease along the flow pass in the PCD system. Compared to large fly ash particles, smaller fly ash particles exhibit higher Hg capture performance due to their high unburned carbon (UBC) content and large specific surface area. The migration of gaseous Hg from the flue gas to the fly ash downstream of the CFB boiler unit was easier than that downstream of the PC boiler unit due to high UBC content and specific surface area.
Highlights
Mercury (Hg) is a serious toxic heavy metal for human health due to its volatility, persistence and bioaccumulation [1]
Our previous work investigated the effect of the temperature on the of Hg on the fly ash collected from another circulating fluidized bed (CFB) boiler unit [35]
The results suggested that the adsorption of Hg0 on the fly ash collected from another CFB boiler unit [35]
Summary
Mercury (Hg) is a serious toxic heavy metal for human health due to its volatility, persistence and bioaccumulation [1]. Coal combustion for electricity and heat generation is the dominant source of anthropogenic mercury emissions [2,3,4]. It is estimated that coal-fired power plants account for Energies 2020, 13, 1040; doi:10.3390/en13051040 www.mdpi.com/journal/energies. 26% of the global anthropogenic Hg emissions to the atmosphere [5]. The United States proposed the Mercury and Air Toxics Standards (MATS) rule to be enforced by April 15, 2015 [7]; China issued the latest coal-fired power plants air pollutant emission standard (GB13223-2011) in 2011 [8], which limited. Hg emissions from coal-fired power plants should be considered to be a high-priority regulatory environment concern
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