Complex sentence: The combustion of coal contributes the most to acid rain and air pollution, and has been connected with global warming, due to the chemical composition of coal and the difficulties of removing the impurities from this solid fuel prior to its combustion.
Split into simpler sentences: The combustion of coal contributes the most to acid rain and air pollution, and has been connected with global warming. Due to the chemical composition of coal there are difficulties in removing impurities from the solid fuel prior to its combustion.