Final answer:
Coal cleaning is the process of washing crushed coal to remove heavy impurities. It differs from processes like using scrubbers to remove SO2 emissions during combustion, which prevails at coal-burning power plants to minimize the environmental effects of burning coal.
Step-by-step explanation:
Coal cleaning refers to washing crushed coal to remove heavy impurities. It is a process designed to improve the quality of coal by reducing the amounts of extraneous materials, such as soil and rock, which are termed as impurities. These impurities could be physical, like stones, or chemical impurities like sulfur compounds that can lead to environmental issues like acid rain when the coal is burned. However, while coal cleaning focuses on removing impurities through a mechanical process, coal combustion emissions are also a significant concern.For instance, during coal combustion in power plants, sulfur dioxide (SO2) can be a problematic emission because it can lead to acid rain. Thus, power plants use scrubbers to remove SO2 from the exhaust gases by reacting it with a lime (CaO) and water spray to produce calcium sulfite dihydrate (CaSO3.2H2O), a process not to be confused with coal cleaning. Additionally, clean coal technologies have been developed to reduce emissions of SO2, NOx and other harmful byproducts before and after the coal combustion process. Options such as the use of low sulfur coal, modification of burning techniques, and technologies to convert coal to gaseous or liquid fuels are paving the way for more environmentally friendly use of coal.