SO2 Emission Calculator

Sulfur dioxide from combustion is calculated by mass balance rather than by an empirical emission factor, because essentially all sulfur in the fuel oxidizes regardless of burner design. Fuel rate times sulfur mass fraction gives sulfur throughput, and multiplying by 2.0 converts it to SO2: the molar masses are 32.06 and 64.06 grams per mole, a ratio of almost exactly two. In practice 95 to 99 percent of fuel sulfur leaves the stack as SO2, with a small share forming SO3 or retained in ash, the retained fraction being larger for high-alkali fuels. Sulfur content varies enormously by fuel: pipeline natural gas is essentially sulfur-free, western US subbituminous coals run about 0.3 to 0.6 percent, eastern bituminous coals 1 to 4 percent, and heavy fuel oil 1 to 3.5 percent. The control efficiency term represents flue gas desulfurization, with wet limestone scrubbers achieving 90 to 98 percent removal and dry sorbent or spray dryer systems roughly 70 to 90 percent. The method assumes constant sulfur content and continuous scrubber availability, so it understates real annual emissions when fuel lots vary or when a scrubber is bypassed during startup, shutdown, or malfunction.

The link between burning sulfurous fuel and damaged air was documented long before the arithmetic was formalized. Robert Angus Smith, a Manchester chemist, described acidified rain around industrial towns and used the term acid rain in work published in 1852 and expanded in his 1872 book Air and Rain. The London smog of December 1952 forced the issue politically and produced the British Clean Air Act of 1956, while Svante Oden's work in Sweden during the 1960s showed sulfur deposition crossing national borders and acidifying Scandinavian lakes. Flue gas desulfurization was demonstrated early, with a lime-water washing plant fitted to Battersea Power Station in London in the 1930s, but it spread widely only after the 1970 US Clean Air Act made SO2 a criteria pollutant. Title IV of the 1990 Clean Air Act Amendments created the Acid Rain Program, the first large-scale cap-and-trade market for an air pollutant, and US SO2 emissions fell by roughly 90 percent over the following three decades. The EPA replaced the older 24-hour and annual limits with a 1-hour standard of 75 parts per billion in 2010, and the International Maritime Organization capped marine fuel sulfur at 0.50 percent worldwide in January 2020.