Module 5

1. Module 5 Sulfur Oxide Control

2. MCEN 4131/5131 Preliminaries

3. MCEN 4131/5131 SO2 formation • S + O2 --> SO2 and SO3 • SO2 criteria pollutant, What is the NAAQS annual average concentration? • SO2 toxic gas causes respiratory irritation and bronchial constriction

4. MCEN 4131/5131 Sulfuric acid formation • In presence of high humidity, the following reactions form H2SO4 • 2SO2 + 2H2O + O2 --> 2H2SO4 • Occurs rapidly in presence of metal salts, such as the sulfates and chlorides, or Fe and Mn • SO3 + H2O --> H2SO4 • Sulfur acid is very strong, and very hygroscopic. Water collects around the H2SO4 to form very acidic small droplets • Sulfur emissions promote formulation of very acidic aerosols • Can be retained for long periods of time (days). Eventually they are removed by rain out

5. MCEN 4131/5131 SO2 control schemes • Two main ways of controlling SO2 emissions • Get rid of sulfur before the fuel is burned • Remove SO2 from post-combustion emissions • Easiest thing to do is choose a low-S fuel of course

6. MCEN 4131/5131 FUEL DE-sulfurization • Liquid and gaseous fuels can be desulfurized by reacting the organic sulfur with hydrogen: • H2 + R-S --> H2S + R (R is generic hydrocarbon) • Then need to recover the H2s, 1st oxidize to SO2, then reduce it to elemental sulfur • H2S + 1.5 O2 --> H2O + SO2 • 2H2S + SO2 --> 2H2O + 3S • Difficult to remove organic sulfur from a solid fuel like coal • FeS is where most of S comes from in coal, so usually crush the coal, remove by density differences

7. MCEN 4131/5131 Post combustion control • Several ways to get rid of SO2 • Sometimes products of cleaning the FLUE GAS can be recovered, but often the waste must be disposed of • Most common way to remove SO2 is to exploit the acidic nature of the gas • Called FLUE GAS DE-sulfurization

8. MCEN 4131/5131 Fuel Desulfurization • Coal currently mined in US is 2-2.5% sulfur