Spatial Air Quality Impacts of Increased Natural Gas Development and Use in Texas

Compared to coal-fired power plants on a per MWh basis, natural-gas electricity generators in the grid of the Electricity Reliability Council of Texas (ERCOT) emit substantially less nitrogen oxides (NOx) and sulfur dioxide (SO2), which are precursors for the formation of ozone (O3) and fine particulate matter (PM2.5). In addition, several life-cycle assessments have concluded that the development and use of shale gas resources will likely lead to air quality benefits, despite emissions associated with natural gas production, due to changes in fuel utilization in the electricity generation sector. The formation of ozone and PM2.5 is non-linear, however, and depends on spatial and temporal patterns associated with the precursor emissions. This study used Texas as a case-study for the changes in regional ozone and PM2.5 concentrations associated with natural gas production and use in electricity generation in the state. Texas makes a compelling case study since it was among the first states with large-scale shale gas production with horizontal drilling and hydraulic fracturing technologies, since it has a self-contained electric grid (ERCOT), and since it includes several regions which do not currently meet Federal standards for ozone. This study utilized an optimal power flow model for electricity generation in ERCOT, coupled with a regional photochemical model to estimate the ozone and PM2.5 impacts of changes to natural gas production and use in the state. The utilization of natural gas is highly dependent on the relative price of natural gas compared to coal. Thus, the amount of natural gas consumed in power generation in ERCOT was estimated for a range of prices from 1.89-7.74, which have occurred in Texas since 2006. Sensitivity scenarios in which natural gas production emissions in the Barnett Shale were raised or lowered depending on demand for the fuel in the electricity generation sector were also examined. Overall results indicate that regional ozone and fine PM2.5 concentrations are reduced as the price of natural gas decreased in Texas. The air quality impacts were predominantly driven by changes in the electricity generation sector rather than in the fuel-supply chain. The areas in which the largest changes in ozone and fine PM were modeled were regions with several coal-fired power plants, which were dispatched less frequently in our model as the price of natural gas decreased. Ozone decreases were largest in magnitude in the afternoon hours during times which were relevant for the daily maximum 8-hour ozone concentration, on which the Federal ozone standard is based. Despite localized increases in NOx and volatile organic compound (VOC) emissions associated with the natural gas production in the Barnett Shale, ozone concentrations were modeled to decrease in the region with decreasing natural gas prices.