Differentiated emission control strategy based on comprehensive evaluation of multi-media pollution: Case of mercury emission control.

In order to comprehensively evaluate the environmental impact of multi-media mercury pollution under differentiated emission control strategies in China, a literature review and case studies were carried out. Increased human exposure to methylmercury was assessed through the dietary intake of residents in areas surrounding a typical coal-fired power plant and a zinc (Zn) smelter, located either on acid soil with paddy growth in southern China, or on alkaline soil with wheat growth in northern China. Combined with knowledge on speciated mercury in flue gas and the fate of mercury in the wastewater or solid waste of the typical emitters applying different air pollution control devices, a simplified model was developed by estimating the incremental daily intake of methylmercury from both local and global pollution. Results indicated that air pollution control for coal-fired power plants and Zn smelters can greatly reduce health risks from mercury pollution, mainly through a reduction in global methylmercury exposure, but could unfortunately induce local methylmercury exposure by transferring more mercury from flue gas to wastewater or solid waste, then contaminating surrounding soil, and thus increasing dietary intake via crops. Therefore, tightening air emission control is conducive to reducing the comprehensive health risk, while the environmental equity between local and global pollution control should be fully considered. Rice in the south tends to have higher bioconcentration factors than wheat in the north, implying the great importance of strengthening local pollution control in the south, especially for Zn smelters with higher contribution to local pollution.

Effect of the Coal Preparation Process on Mercury Flows and Emissions in Coal Combustion Systems.

Coal preparation is effective in controlling primary mercury emissions in coal combustion systems; however, the combustion of coal preparation byproducts may cause secondary emissions. The inconsistent coal preparation statistics, unclear mercury distribution characteristics during coal preparation, and limited information regarding the byproduct utilization pathways lead to great uncertainty in the evaluation of the effect of coal preparation in China. This study elucidated the mercury distribution in coal preparation based on the activity levels of 2886 coal preparation plants, coal mercury content database, tested mercury distribution factors of typical plants, and then traced the mercury flows and emissions in the downstream sectors using a cross-industry mercury flow model. We found that coal preparation altered the mercury flows by reducing 68 tonnes of mercury to sectors such as coking and increasing the flows to byproduct utilization sectors. Combusting cleaned coal rather than raw coal reduced the mercury emissions by 47 tonnes; however, this was offset by secondary mercury emissions. Coal gangue spontaneous combustion and the cement kiln coprocessing process were dominant secondary emitters. Our results highlight the necessity of whole-process emission control of atmospheric mercury based on flow maps. Future comprehensive utilization of wastes in China should fully evaluate the potential secondary mercury emissions.

Highly Resolved Inventory of Mercury Release to Water from Anthropogenic Sources in China.

This study developed an up-to-date and point-source-based inventory of mercury (Hg) releases to water in China by applying probabilistic release factors that combined industry removal efficiencies, reuse of reclaimed water, and receiving water types. In 2017, the national mercury release to water was estimated to be 50 (35-66) tons, in which 47%, 8%, 7%, and 25% were from nonferrous metal smelting, vinyl chloride monomer (VCM) production, coal-fired boilers, and domestic sewage, respectively. Approximately 95% of mercury was released to inland rivers, and the rest was discharged to lakes or coastal water. The significant sources were identified based on their mercury releases to water. The control of mercury release to water in China shall focus on zinc smelting plants, municipal sewage treatment plants, and the VCM production process. For zinc smelting plants, China can tighten the limit of mercury concentration in discharged wastewater and combine Hg-catcher device in traditional integrated treatment. For municipal sewage treatment plants and the VCM production process, promoting processes of Hg-free production can reduce mercury inputs at the source. Our study provides insights for other parties to identify the relevant sources of mercury release to water and to conduct control measures, so as to promote the global convention implementation.

Mercury emission characteristics and mechanism in the raw mill system of cement clinker production.

Mercury pollution has attracted worldwide attention due to its toxicity, bioaccumulation and persistence. Cement clinker production is the top emitter of atmospheric mercury in China and the emissions from raw mill systems account for about 85% of all emissions. However, the mercury emission characteristics and mechanisms as a function of time during an operation cycle are still unclear. This study aims to reveal the mercury emission characteristics and mechanisms in cement plants by comprehensively using offline and online field measurements, control experiments and heat transfer analysis. Research results indicated that an intermediate temperature (300-500 °C) desorption and the heterogeneous oxidation of mercury in the precalciner, the selective adsorption of oxidized gaseous mercury (Hg2+) to raw meal, and Hg2+ re-vaporization in the conditioning tower jointly caused an increase in the Hg2+ ratio (15.3%-83.6%) during the mill-off mode. In addition, mercury concentrations remained at approximately 6.5 µg/Nm3 during the mill-on mode while the values reached a peak of 1835.4 µg/Nm3 during the mill-off mode. Thus, atmospheric mercury emissions during the mill-off mode accounted for 35.0%- 71.7% of the emissions during the entire cycle, although the mill-off period only lasted for 5%- 17% of the whole cycle. Our results therefore suggest that supervisory monitoring of mercury in cement clinker production should specify the operating status of raw mills. Mercury control technologies targeting a relatively short period for the mill-off mode can substantially reduce mercury emissions from cement clinker production, and thus, the related impacts on ecosystems and human health.

Impact of emission reductions and meteorology changes on atmospheric mercury concentrations during the COVID-19 lockdown.

Controlling anthropogenic mercury emissions is an ongoing effort and the effect of atmospheric mercury mitigation is expected to be impacted by accelerating climate change. The lockdown measures to restrict the spread of Coronavirus Disease 2019 (COVID-19) and the following unfavorable meteorology in Beijing provided a natural experiment to examine how air mercury responds to strict control measures when the climate becomes humid and warm. Based on a high-time resolution emission inventory and generalized additive model, we found that air mercury concentration responded almost linearly to the changes in mercury emissions when excluding the impact of other factors. Existing pollution control and additional lockdown measures reduced mercury emissions by 16.7 and 12.5 kg/d during lockdown, respectively, which correspondingly reduced the concentrations of atmospheric mercury by 0.10 and 0.07 ng/m3. Emission reductions from cement clinker production contributed to the largest decrease in atmospheric mercury, implying potential mitigation effects in this sector since it is currently the number one emitter in China. However, changes in meteorology raised atmospheric mercury by 0.41 ng/m3. The increases in relative humidity (9.5%) and temperature (1.2 °C) significantly offset the effect of emission reduction by 0.17 and 0.09 ng/m3, respectively, which highlights the challenge of air mercury control in humid and warm weather and the significance of understanding mercury behavior in the atmosphere and at atmospheric interfaces, especially the impact from relative humidity.