8.8. Sulfur Oxides (SOx) Emissions From Coal

Fossil fuels such as coal contain sulfur in two inorganic forms, pyritic sulfur (FeS₂) and sulfates (Na₂SO₄, CaSO₄, FeSO₄), and organic forms such as sulfides, mercaptans, bisulfides, etc. Typically coal contains anywhere from 0.2 to 5 percent sulfur by dry weight. During high-temperature gasification of coal (or other applicable solid fuels), most of the sulfur constituent is released and converted to hydrogen sulfide (H₂S), as well as a small amount of carbonyl sulfide (COS), due to the reduced oxygen environment. H₂S and COS (which along with carbon dioxide [CO₂] are called acid gases) are mostly removed from the syngas in the acid gas removal equipment prior to combustion in gas turbines or other forms of fuel conversion (e.g., fuel cell) for electricity production or conversion to other products such as liquid fuels or chemicals.

Acid gas removal equipment extracts from 95% to greater than 99% of the H₂S and COS, once hydrolyzed, from the fuel gas and converts it to a salable sulfur or sulfuric acid byproduct. The small amount of residual sulfur that remains in the syngas is converted to SO₂ in the combustion turbine and released to the atmosphere in the HRSG stack gas. Other secondary sources of SO₂ emissions in an IGCC plant will typically include the sulfur recovery system’s tail gas incinerator stack, auxiliary boilers (if applicable), and the syngas flare during gasifier startup and system upset conditions. These secondary SO₂ sources are typically significantly smaller than the HRSG stack emissions.

SO₂ Effects
SO₂ causes a wide variety of health and environmental problems due to its reactivity with other airborne substances. Among these are:

Respiratory Problems - During peak levels of airborne gaseous SO₂ those with asthma and who are active outdoors can experience difficulty breathing and long-term exposure can result in respiratory illness and can aggravate existing heart disease. In addition, airborne SO₂ can interact with other chemicals to form sulfate particles which when inhaled are associated with respiratory disease, breathing difficulty, and premature death.
Visibility Reduction - Airborne sulfate particles also scatter and absorb light to cause haze and reduce visibility.
Acid Rain - Along with nitrogen oxides, SO₂ reacts with airborne substances to form acid which damages forests, crops, soil, lakes, and streams when the acid falls to earth in the form of rain, fog, snow, or dry particles.
Corrosion - SO₂ causes premature decay of paint and building materials, monuments, statues, and sculptures.

SO₂ Emissions
U.S. commercial IGCC plants Polk and Wabash River both achieve total SO₂ emissions below 0.15 lb SO₂/10⁶ Btu heat input (< 1.3 lb/MWh for 8,600 Btu/kWh heat rate) or greater than 97% sulfur reduction when operating with coal. Edwardsport IGCC has very low SO₂ emissions from the combustion turbines/HRSG at only about 5% of the 0.15 lb SO₂/10⁶ Btu heat input limit specified in its Class V Air Permit under the New Source Performance Standards (NSPS).

In general, current IGCC technologies reliably provide nearly an order of magnitude reduction in SO₂emissions compared with their conventional pulverized coal combustion counterparts. The graph below compares typical gasifier types in use for commercial power production to PC based generation, showing the superior environmental performance of gasification in terms of SOx emissions.

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