Despite its high practical value, the process of coal from production to consumption may cause serious pollution to the environment. The impact on the environment can be divided into: ① emissions from fuel consumption systems; ② land disturbance and mining area settlement; ③ water, dust and noise pollution; ④ methane emissions (World Coal Association-Energy Security 2005).
Coal production requires surface mining or underground mining. During surface mining, the removal of coal will produce pits. In order to prevent soil erosion and other environmental damage, the land needs to be refilled with soil to restore it to its original state. In addition, during surface mining, mining dust is a major problem affecting the environment near the mining area. When mining underground, acidic mineral water can cause environmental problems. The water seeps into the mine, and the oxygen in the atmosphere reacts with the pyrite in the coal mine (iron sulfide) to form acid mine water. The acidic mine water is pumped into nearby rivers, streams or lakes, which will further dissolve the heavy metals such as copper, lead and mercury discharged to the ground, causing serious pollution. Another result of underground mining is the release of methane, a type of greenhouse gas (US Environmental Protection Agency 2006). Frondel et al. (2007) and Steenblik and Coronyannakis (1995) pointed out that approximately 15 tons of methane are emitted for every ton of coal produced.
The main pollutants of coal are produced during its use, especially the combustion of coal. When coal is used in power plants or factories, flue gas is generated. The flue gas mainly includes CO2, SO2, NOx, particulate matter and mercury. SO2 and NOx can cause acid rain and photochemical smog, which is very harmful to the human body and the environment. Since the Industrial Revolution, acid rain and photochemical smog have occurred in many large cities. For example, in the 19th and 20th centuries, London was notorious for its industrial smog. The most serious smog occurred in 1952. The poisonous smog lasted for 5 days and caused more than 4,000 deaths. The combustion of coal in the atmosphere caused a large amount of CO2 to accumulate, which led to the greenhouse effect. Many extreme weather phenomena in the 20th century were related to the increase in the concentration of CO2 in the atmosphere (IPCC2007). Coal has been continuously used as a major fuel source in developed countries for two and a half centuries, which has increased the amount of greenhouse gases in the atmosphere. The impact of climate change on public health and physical ecosystems is obvious, such as the rise of sea level, the melting of glaciers, the melting of frozen soil, the migration of species to high latitudes, the frequent occurrence of winds and floods, heat waves and droughts, The spread of disease, etc. Climatologists predict that if greenhouse gas emissions are not urgently and efficiently controlled, it will cause heavy casualties and huge economic losses (IPCC2007; Jayarama Reddy 2010). Thermal power plants are one of the major sources of environmental pollution and the largest emission source of toxic mercury. Burning 1 kg of coal releases approximately 2.93 kg of CO2. A 500MW thermal power plant releases approximately 3×106 CO2 a year. The power generation sector contributes 60% of anthropogenic mercury emissions and 35% of sulfides that cause acid rain (Maruyama and Eekelman 2009). However, by 2030, carbon emissions from energy use are expected to continue to increase by 26%, with an annual growth rate of 1.2% from 2011 to 2030. By 2030, 4.5×1010tCO2 will be emitted every year (BP2012). The large accumulation of burnt waste in power plants is another potential source of pollution. The hazard of waste lies in the potential hazards of the lead, mercury and other toxic chemicals contained in it that dissolve in water and diffuse (NRC2006).
