Introduction: Sulfur Oxides - Formation Mechanisms
Sulfur dioxide and sulfuric acid are formed during the combustion of fuel or waste that
contains sulfur compounds. Sulfur oxides can also be released from chemical reactors
and sulfuric acid plants.
The sulfur in the fuel or waste being fired enters the combustion process in a variety of
chemical forms including but not limited to inorganic sulfates, organic sulfur compounds,
and pyrites.
A small fraction of the fuel or waste sulfur (usually less than five percent) remains in the
bottom ash leaving the combustion processes. The remaining 95+ percent is converted to
sulfur dioxide, which remains in the gaseous form throughout the combustion system.
A small fraction of the sulfur dioxide generated in the combustion zone is oxidized
further to form sulfur trioxide. The reaction mechanisms that could contribute to the
formation of this pollutant are not entirely known; however, they probably include the
following: