1f: Quantifying mercury emissions from energy generation

Mercury (Hg) emitted from coal-fired power plants (CFPPs) is one of the most important anthropogenic sources. China is the largest coal consumer in the world, and CFPPs burned one half the coal used in China. During the past 10 years, Chinese CFPPs have sequentially installed the flue gas desulfurization (most limestone-gypsum) and DeNOx system (most SCR). Since FGD and DeNOx system have obviously synergetic effect on Hg removal, more work are needed to update the Hg emission factors. Hg isotope signatures from CFPPs are also important for source apportionment. Guizhou province in southwest China is rich in coal, and ranked No.1 in southern China. Coal in Guizhou feathered with high sulfur and ash content. In this research, we studied 14 CFPPs in Guizhou during 2014 to 2016, including 2 CFB (circulating fluidized bed) boilers and 12 PC (pulverized coal) boilers. Hg in the flue gas were sampled with Ontario Hydro Method (OHM) and detected with CVAAS (F732). Total Hg in solid samples such as coal, bottom ash, fly ash, flue gas filters, limestone and gypsums were determined with Lumex CVAAS. Hg isotope signatures in the flue gas and solid samples were detected with MC-ICP-MS (Nu Instruments, Nu plasma model II). Total Hg in the stack flue gas ranged from 0.14 to 3.92 μg/m³, with CFB boiler emitted (average 0.21 μg/m³, N=2) much lesser than PC boilers (average 1.17 μg/m³, N=12). The total Hg removal efficiency for CFB boilers ranged from 98-99%, and 87-99% for PC boilers. Hg emission levels reduced dramatically for PC boiler than several years ago. Most Hg emitted from the CFPPs is in Hg⁰ form. Hg concentration and speciation in the flue gas were determined by the coal property, boiler type and air pollution control devices. Mass-dependent fraction (MDF) was found between different coal combustion products, with lightest for the fly ash, and heaviest for the flue gas, with up to 6‰ difference in δ²⁰²Hg. MDF were also found between different Hg species in the flue gas, with Hgp the lightest, then Hg²⁺, and Hg⁰ the heaviest. Mass-independent fraction (MIF) was found in flue gas of some CFPPs, which may be caused by the catalytically oxidation of Hg⁰ in SCR and H₂SO₄ production process during the organic amine desulfurization. Hence, Hg isotopic emissions from CFPPs can’t be predicted by the feed coal.

The Illinois Basin is the third largest producer of U.S. coal, after the Powder River Basin and the Appalachian Basin, respectively. Due to its proximity to U.S. coal-fired power generation and resulting low transportation costs, interest in high sulfur Illinois Basin coals has increased despite an overall decline in U.S coal production resulting from replacement by gas-fired power generation. Greater adoption of flue gas desulfurization in the U.S. fleet has made the high sulfur content of these coals less limiting. Nonetheless, Illinois Basin coals are commonly prepared prior to delivery to reduce mineral matter and improve heating value, with corresponding sulfur reduction. Mercury reduction occurs as a co-benefit of coal optimization for these other parameters.

As part of a larger investigation of the controls on mercury distribution in Illinois Basin coals, in the present study, mine mouth coal preparation circuits in in southern Illinois and southwestern Indiana were sampled to determine reduction of mercury and pyritic sulfur in these operations. In southern Illinois, the cleaning circuit processes Herrin (#6) coal, the most widely used coal in Illinois. Feed coal to this circuit contains 5.4% total sulfur, 4.5% pyritic sulfur, and 0.24 ppm Hg (as-received basis). The cleaned product, a combination of three cleaned size fractions, has a total sulfur content of 2.4%, pyritic sulfur of 0.9% and a Hg concentration of 0.09 ppm, which is below U.S. (0.11-0.12 ppm) and world (0.10 ppm) averages for mercury in delivered coal. Regression of pyritic sulfur vs. mercury for all eight samples from this circuit yields a positive correlation with an r2 of 0.89, supporting an association of Hg with pyrite as indicated in previous studies.

For Illinois Basin coal in Indiana, preparation circuit feed coals include the Springfield (#5), Bucktown (#5a), and Hymera (#6), coals, producing a composite product, and a separate preparation circuit for the Danville (#7) coal. For the Danville feed coal, having a total sulfur content of 1.7%, and a pyritic sulfur content of 0.8%, these values are reduced to 1.6% and 0.7%, respectively (as-received basis). In the composite circuit feed, pyritic sulfur ranges from 0.5% (Hymera coal) to 7.9% (Springfield coal), with a cleaned product pyritic sulfur of 1.9%. Determination of mercury in each Indiana circuit is in progress. In the absence of nationwide limits for mercury emissions from U.S coal-fired power generation, coal preparation remains an important means of reducing mercury emissions from coal use.

The Mercury and Air Toxics Standard (MATS) came into effect for almost all coal-fired power plants in the US on April 16, 2016. Power plants must now report their hourly mercury emissions to the US Environmental Protection Agency (EPA) on a quarterly basis. Plants in the US employ a variety of strategies to reduce mercury air emissions. The most common mercury control strategies are powdered activated carbon injection (ACI) and flue gas desulfurization (FGD) systems. The addition of halogen additives to the coal is sometimes used with both these technologies. In addition, ACI is sometimes used on boilers with FGD systems. The result is a myriad of different technology combinations in use for mercury control. Using information on the control technologies at specific plants as well as the reported Hg emissions, we assess the efficacy of different control strategies. This information will prove useful to countries that are considering wide-spread implementation of mercury control on coal-fired boilers.

As one of the primary atmospheric point sources in the Minamata Convention on Mercury, coal-fired power plant bears the brunt of mitigation stress, especially for the relevant source which covers at least 75 percent of the emissions. In order to identify relevant source and obtain a deep insight of temporal and spatial characteristics of atmospheric mercury emissions from coal-fired power plants in China, a unit-based method is developed based on detailed information of unit capacity, electricity generation, coal consumption, air pollution control devices and geographical locations. The national total Hg emission in 2015 is estimated at 75 t, of which more than 60% is released from units with size lager than 300MW. The top three provincial emitters are Inner Mongolia, Jiangsu and Shandong, where power plants are densely concentrated. The average Hg-intensity is about 18.9 g/kWh in 2015. Based on the current mercury emission inventory, three scenarios of coal consumption and application of particulate matter/ sulfur dioxide/ nitrogen oxides/mercury (PM/SO2/NOx/Hg) control devices are projected to discuss in the time horizon from 2015 to 2030. Under the current coal consumption pattern and air pollution control policies without additional activated carbon injection (ACI), the mercury emissions would increase to 88 t in 2030. With the widespread application of fabric filter (FF) and application of ACI on 50% of Relevant Source, the mercury emission could reduce by 40% in 2030. Through the application of FF on 100% and ACI on 80% of relevant source, the mercury emission in 2030 can be decreased by 64% from the level of 2015. Considering these three scenarios, GEOS-Chem model is used for the evaluation of future mercury pollution levels.