Impact of Wintertime SCR/SNCR Optimization on Visibility Impairing Nitrate Precursor Emissions

Title: Impact of Wintertime SCR/SNCR Optimization on Visibility Impairing Nitrate Precursor Emissions
Orgranization: MANE-VU Technical Support Committee
Publication Date: November 20, 2017
Role: Primary Analyst and Author
Executive Summary:
During the first planning phase for regional haze, programs that were put in place focused on reducing sulfur dioxide (SO2) emissions. The reductions achieved led to improvements in visibility at the MANE-VU Federal Class I Areas due to reduced sulfates formed from SO2 emissions. The reduction in visibility impairment from sulfates resulted in nitrates driving the visibility impairment rather than sulfates in some MANE-VU Class I Areas on the 20% most impaired days, in particular, during the winter months. Nitrogen oxide (NOX) emissions are an important precursor to the formation of nitrates.

Often Electric Generating Units (EGUs) only run NOX emission controls to comply with ozone season trading programs; consequently, emissions of NOX are uncontrolled during the winter. Controlling winter-time NOX emissions at EGUs using existing controls is generally more cost-effective compared to other sectors that would have to install and bear the capital costs of control equipment solely for improving visibility.

We looked at the visibility data and observed emission rates from EGUs with installed selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) controls, and compared those rates to projected emissions using the ERTAC EGU tool, to show the potential NOX emissions reductions from running existing SCR and SNCR during the winter months.

We found that the number of most impaired days occurring during the winter is increasing at all Class I areas, in particular Brigantine, and that Brigantine and Lye Brook are seeing nitrate impairment in high levels during those times. We also found that back trajectories from those sites during the winter often traverse MANE-VU and LADCO states with power plant emissions. We found that running existing installed controls is one of the most cost-effective ways to control NOX emissions from EGUs and that running existing SCRs and SNCRs on EGUs could substantially reduce the NOX emissions in many of the states upwind of Class I areas in MANE-VU that lead to visibility impairment during the winter from nitrates.

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