Although the emission concentration of trace elements such as Hg in coal-fired power plants is not high, they have become the main targets for pollutant control due to the cumulative effect and high toxicity of trace elements themselves. China has issued and implemented emission standards for air pollutants from coal-fired power plants four times, but none of the standards have set emission limits for mercury. In the new "Emission Standards for Air Pollutants from Thermal Power Plants" (GB 13223-2011), mercury emission indicators have been added. After studying the emission standards of thermal power plants in the United States, the European Union, and Germany, the concentration limit for mercury and its compounds emissions from thermal power plants in China has been determined to be 0 03mg/m3 (implemented from January 1, 2015).

With the increasingly strict environmental emission standards, mercury pollution prevention and control work has been included in the "Twelfth Five Year Plan" of power enterprises. The "Twelfth Five Year Plan" for comprehensive prevention and control of heavy metal pollution and the "Twelfth Five Year Plan" for joint prevention and control of air pollution in key areas have made arrangements for the control of atmospheric mercury emissions from coal-fired power plants. At present, research on mercury emission control strategies, the distribution of mercury forms in coal-fired power plants, emission mechanisms, and control technologies has been put on the agenda.

The hazards of mercury emissions

With the adoption of pollution control technologies such as high-efficiency electrostatic precipitators, flue gas desulfurization, flue gas denitrification, and high chimney emissions, the pollution of smoke, SO2, and NO x has been effectively controlled, and the problem of mercury pollution in coal-fired power plants is gradually emerging. As mercury begins to evaporate at 36 ℃, the higher the temperature, the faster the evaporation.

Mercury vapor can migrate to distant places with atmospheric circulation. With the emission of coal-fired flue gas, this mercury is diffused into the air and dissolved in water. Due to its high proportion, it often deposits at the bottom of rivers. In the presence of methyl vitamin B12, through the action of anaerobic bacteria, mercury ions deposited on the riverbed form methylmercury and dimethylmercury. Methylmercury can accumulate in aquatic organisms, participate in the food chain, and enrich and concentrate mercury in fish, reaching extremely high concentrations, up to 200000 times higher! Methylmercury mainly harms the nervous system, especially the central nervous system, after entering the human body.

The data shows that from January to November 2010, the amount of coal used for power generation reached 1.005 billion tons. If calculated based on the average mercury content of 0.2 mg/kg in foreign coal, the mercury content in coal reaches 201 tons. In the United States, coal-fired power plants have become the largest source of mercury emissions, delivering 48 tons of mercury to the atmosphere each year, accounting for over 40% of all mercury emissions in the United States.

In 2003, the United States proposed a presidential plan called the Clean Sky Program, which aimed to reduce mercury emissions by 69% by 2018. In March 2005, the US Environmental Protection Agency launched the Clean Air Mercury Act (CAMR), which for the first time regulated mercury emissions from thermal power plants in the country, making it the world's first regulation to regulate mercury emissions. By the implementation of the Clean Air Mercury Act, the upper limit of mercury emissions from thermal power plants will be controlled at 15 tons by early 2018.

China's "Emission Standards for Air Pollutants from Thermal Power Plants" (GB 13223-2011) stipulate emission standards for SOx, NOx, dust, mercury, and their compounds in flue gas. Table 1 shows the maximum allowable emission concentrations of mercury and its compounds in wastewater and atmosphere according to current standards in China. Table 2 shows the total Hg standards for drinking water and groundwater.

According to Tables 1 and 2, China already has a set of control indicators for mercury and its compounds.

Mercury pollution control technology for coal-fired power plants

2.1 Collaborative control effect of existing pollution control facilities on mercury

Using existing flue gas treatment equipment to remove mercury can improve equipment utilization, reduce control costs, achieve joint control of pollutants such as mercury, SO ₂, NOx, etc., and achieve integrated dust removal, desulfurization, denitrification, and mercury removal. At present, electrostatic precipitators (ESP), bag filters (FF), and electric bag composite dust collectors are commonly used in power plants both domestically and internationally to remove fly ash, and wet flue gas desulfurization (WFGD) devices are used to remove SO ₂ from flue gas. Denitration mainly uses selective catalytic reduction (SCR) and selective non catalytic reduction (SNCR) methods. Below is an analysis of the mercury removal efficiency of existing pollution control facilities:

(1) Dust removal equipment.

Electrostatic precipitators and bag filters can reduce particulate matter emissions while effectively removing mercury pollution, but their removal rates for Hg 0 and Hg 2+are relatively low. The adsorbent injection technology based on electrostatic precipitators and bag filters can improve the removal efficiency of mercury in coal-fired flue gas by adopting the method of adsorbent injection. For example, injecting activated carbon into flue gas is the simplest and most natural technology for controlling mercury pollution in coal-fired flue gas. However, this technology has high investment and operating costs, which power plants generally find difficult to bear. Therefore, many studies focus on low-cost adsorbents. According to the changes in coal types, the adsorption injection technology based on bag filter can remove 80% to 90% of mercury, while the adsorbent injection technology based on electrostatic precipitator can remove 50% to 90% of mercury. In order to integrate the advantages of electrostatic precipitators and bag filters, electric bag composite dust collectors have emerged in recent years. Their working principle is that the front stage electric field pre collects more than 80% to 90% of the particulate matter in the flue gas, and the back stage bag filter device intercepts and collects the remaining finer particles in the flue gas. Its dust removal efficiency is high, and the removal rate of mercury also integrates the characteristics of electrostatic precipitators and bag filters.