EPA Rules and Sulfur Dioxide

Flue Gas Desulfurization by URS

A number of regulations are set to target SO₂ emissions, including the Cross State Air Pollution Rule (CSAPR) and the SO₂ National Ambient Air Quality Standards (NAAQS). In addition, SO₂ is important to consider when preparing for the Mercury and Air Toxics Standard (MATS), as it is a surrogate for acid gas hazardous air pollutants (HAPs).

With so many regulations in the mix, it seems as though SO₂ is an all-encompassing concern in the scope of emissions control. If a utility can get a solid control on SO₂, other air toxins will be controlled.

According to the EPA, power plants are responsible for 66 percent of worldwide SO₂ emissions, with the majority — more than 98 percent — coming from coal-fired power plants. Wet flue-gas desulfurization (FGD), dry FGD, dry sorbent injection (DSI) and other technologies have been responsible for cutting SO₂ emissions by 57 percent between 1980 and 2008, according to the Edison Electric Institute.

Wet Scrubber

Inter-state SO2

CSAPR requires a total of 27 states to reduce annual SO₂ and nitrogen oxide (NOX) emissions from power plants. CSAPR was stayed by a federal court in December but may be re-released this summer. More than three dozen lawsuits have been filed against CSAPR, accusing the rule of being one of the "most costly, burdensome and arbitrary" rules ever issued under the Clean Air Act.
Since the stay, EPA has released minor changes to the rule. The Final Revisions Rule revises state SO₂ and NOx budgets.

SO2 NAAQS: The One-hour Challenge

The SO₂ NAAQS are another concern for utilities. The Clean Air Act requires EPA to set national air quality standards for SO₂ and five other emissions types. On June 2, 2010, the primary SO₂ NAAQS was revised to set the SO₂ standard at 75 parts per billion (ppb), which is attained when the three-year average of the 99th percentile of one-hour daily maximum concentrations does not exceed 75 ppb.
The one-hour measurement of SO₂ raises a new set of challenges for utilities, especially compared to the previous standard, which allowed no more than 140 ppb over 24 hours. In contrast, many other EPA standards have concentration measurements taken over a span of eight hours, one day or even one year.

MATS: SO2 as Surrogate

One of the biggest overarching concerns for power generators is the recently finalized Mercury and Air Toxics Standard, EPA's first mandate in U.S. history to limit mercury emissions from coal-fired units. The rule will also curtail a number of hazardous air pollutants (HAPs), including lead, arsenic, hydrogen chloride, hydrogen fluoride and dioxins/furans. MATS control will require the power industry to reach compliance by 2015. Since SO₂ is a surrogate for HAPs, MATS could also introduce or reinforce a push toward SO₂ control installation.

MATS is EPA's replacement for the Clean Air Mercury Rule (CAMR), an interstate cap-and-trade program issued in 2005. EPA estimates that MATS will reduce mercury emissions from covered power plants by 91 percent, acid gas emissions by 91 percent and SO₂ emissions by 55 percent.

BART

Best Available Retrofit Technology (BART) guidelines under the Clean Air Visibility Rule will also take a toll on SO₂ emissions (as well as NOx, ammonia and certain volatile organic compounds), affecting facilities built or reconstructed between Aug. 7, 1962, and Aug. 7, 1977, that have the potential to emit more than 250 tons a year of emissions and fall into one of 26 different categories, including power plants. The EPA's presumptive BART emission limit for SO₂ is 0.15 lb/mmBtu, or 95 percent removal.

Flue Gas Desulfurization Scrubber

(Pollution Engineering, by Lindsay Morris, June 2012)