Various processes have been developed in the past few decades to decrease the environmental impact of coal burning.
Firstly the coal can be “washed” prior to burning to reduce the levels of some of the combustion products (such as ash and sulphur). For example, sulphur-consuming bacteria can be used to reduce the sulphur level.
Secondly, the process of combustion can be improved so that lower levels of byproducts are released. In fluidized bed combustion air is blown through a bed of sand heated to incandescent heat. The coal is introduced and it burns at a lower temperature than normal, the efficiency of combustion is higher, and lower levels of sulphur-dioxide and nitrous-oxide are released. Some of the sulphur can also be captured if limestone is included in the fluidized bed.
Thirdly, various technologies (such as “scrubbers”) can be used at the post-combustion end of the process, primarily to reduce the amount of sulphur emitted. New technology is now being developed which is hoped to allow extraction of energy from coal while producing almost no emissions. The process involves generation of hydrogen from coal, and then use of that hydrogen in a fuel cell.