Sulfur is an undesirable part of coal because it can cause air pollution when the coal is burned. When coal is burned, the sulfur in the coal reacts with oxygen to form sulfur dioxide gas. Sulfur dioxide gas is a major contributor to acid rain, which can damage forests, lakes, and buildings. It can also cause respiratory problems in humans.

In addition, sulfur can also cause problems during the combustion of coal. Sulfur can react with other elements in the coal to form slag and ash, which can build up on the walls of boilers and other equipment. This can reduce the efficiency of the coal-fired power plant and lead to increased maintenance costs.

For these reasons, sulfur is considered an undesirable part of coal and efforts are made to remove it from coal before it is burned. This can be done by washing the coal with water or by using chemical processes to remove the sulfur.