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Module 6: Air Pollutants and Control Techniques - Sulfur Oxides - Formation Mechanisms
Lesson Material
Practice
Problems
Objective
- Describe the basic mechanisms contributing to the formation of sulfur dioxide and sulfuric acid.
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:
- Free
radical reaction of sulfur dioxide with atomic oxygen in the high
temperature zones
- Catalytic oxidation of sulfur dioxide on the surfaces of particles
entrained in the gas stream
- Thermal reactions between sulfur dioxide and other inorganic gases
generated during combustion
The concentration of sulfur trioxide generated during combustion varies widely from unit to unit for reasons that have not been determined entirely; however, sulfur trioxide concentrations are generally related directly to the concentration of sulfur in the fuel and the concentration of oxygen in the combustion zone. The sulfur trioxide concentrations are usually 0.5 to 2 percent of the sulfur dioxide concentration. Sulfur trioxide quickly converts to sulfuric acid upon cooling in the gas stream or atmosphere.
Practice Problems
Sulfur Oxides - Formation Mechanisms
- Instructions:
- Complete the Practice Problems before proceeding to the next section. Click on the button below.