Trends In Power Plant Emissions

Aspen’s multidisciplinary engineering and scientific staff and our Energy Resource Economics practice has a history of providing state energy agencies and local jurisdictions with advice and analysis on energy and the environment. While California’s natural gas-fired power plants are seeing their use decline fairly dramatically, they continue to play an important role in providing local capacity and stability to ensure a reliable grid. However many existing power plants remain situated in areas with disadvantaged communities.

Local governments and planners are interested in understanding how infrastructure is used under real-world conditions. For example, the use of gas-fired power plants may need to be considered in an Environmental Justice Element or environmental justice policies in response to Senate Bill 1000 (Leyva, 2016) or by planners looking to reduce localized air pollution through community-focused programs under Assembly Bill 617 (Garcia, 2017).

Using public data, we did some research to see how some of California’s natural gas-fired generators have been running in recent years. With California’s ongoing progress on developing and integrating ever-greater levels of renewable energy, we expect the gas fleet to produce less electricity, burn less natural gas, and emit lower levels of air pollutants like smog-forming nitrogen oxides, or NOx. On balance, this is true across California, but there are local areas where existing fossil-fueled generators are still needed.

One way to evaluate trends in power plant usage is to look at recent capacity factors, which tell us the actual amount of electricity produced as a fraction of the plant’s potential output or nameplate capacity. We are also interested in how emissions rates have been changing over time.

We studied over 130 of California’s gas-fired power plants from the U.S. EPA’s eGrid database and facility-specific information with ARB’s emissions data. We were looking to see:

• Which power plants showed capacity factor increases between 2012 to 2016 to see where power plants were used more; and

• Which power plants produced less electricity but had higher emissions to see where the fleet might be running less efficiently than before.

We found that:

• Across roughly 40,000 MW of operational gas-fired plants, only 1,100 MW had a capacity factor increase of more than 5 percent from 2012 to 2016. This reflects how most of California’s fleet is running less than it did in the first part of the decade. Only a few plants were running more frequently in 2016 than in 2012.

• A handful of sites emitted more NOx in 2016, even while running less often and producing less electricity. For this, we looked at air pollutant emissions data in the eGrid database and also the ARB’s pollution mapping tool.

• While the ARB data does not indicate electrical output, some portions of the fleet created fewer GHG emissions while emitting greater amounts of NOx. Power plants that are emitting greater amounts of NOx while producing less electricity can be indicators of locations where the gas fleet may be running less-optimally than before. Responding to the intermittent renewable energy supply requires more cycling and ramping, or more startups, shutdowns, and partial load operations. Each of these demands on California’s gas-fired generators can result locally-greater levels of NOx emissions from the power plants.

If you would like more information on our qualifications in analysis of energy issues as they relate to air quality or socioeconomic impact assessment, please contact us.