Mercury Migration Behavior from Flue Gas to Fly Ashes in a Commercial Coal-Fired CFB Power Plant

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.

Numerical Study on the Homogeneous Reactions of Mercury in a 600 MW Coal-Fired Utility Boiler

“…Mercury is released from coal during the combustion process, with only about 2% of mercury remaining in the bottom slag. Mercury in flue gas mainly exists in three forms: gaseous elemental mercury (Hg 0 ), gaseous oxidized mercury (Hg 2+ ) and particulate mercury (Hg p ) [3][4][5]. Among them, gaseous oxidized mercury has high solubility and can be removed in wet flue gas desulfurization devices.…”

Mercury Speciation in Various Coals Based on Sequential Chemical Extraction and Thermal Analysis Methods

“…Mercury (Hg) is regarded as a serious pollutant worldwide to public health due to its volatility, toxicity, persistence, and bioaccumulation in the human body and natural environment [1]. Although the average content of Hg in coal is only around 0.1 mg/kg [2,3], coal utilization through pyrolysis, gasification, and combustion is the main source of global anthropogenic mercury emissions because of the huge consumption in the coal processing industry with a limited Hg control technology [4].…”

A Preliminary Study on Dependence of Mercury Distribution on the Degree of Coalification in Ningwu Coalfield, Shanxi, China

“…With a huge amount of coal consumed, coal utilization remains one of the largest anthropogenic sources of Hg in the environment. Due to the increasing environmental pollution and the enhancement of analytical technique, the geochemical study on Hg in coal has caused a rise in public concern [1][2][3]. The Hg content in coal varies considerably, from 0.025 mg/kg to 1000 mg/kg, with a worldwide average of 0.10 ± 0.01 mg/kg regardless of location and coal rank [4][5][6].…”