Annual Study Shows Decrease in Average Emission Rates for PJM Footprint


An annual study of generators in the PJM footprint shows a drop in average emission rates for carbon dioxide, nitrogen oxides and sulfur dioxide, continuing a downward trend of nearly two decades.

According to the latest Emission Rates Report (PDF), the average CO2 emission rate for electric generators in the PJM footprint decreased approximately 4% from 2021 to 2022. The average nitrogen oxides (NOx) emission rate dropped about 13%. The average emission rate of sulfur dioxide (SO2) fell by about 8%.

The report is published to support the efforts of regulators, stakeholders and other interested parties as they work toward environmental goals. It identifies both average and marginal emission rates.

The PJM system average annual value is a weighted average that accounts for higher loads during the summer and winter months. Emissions are measured in pounds per megawatt-hour.

PJM also publishes 5-minute marginal emission rates for individual load nodes, hourly marginal emission rates and hourly total emissions  on Data Miner. In addition, PJM recently updated its new Emissions page and PJM Now app to include a visualization of overall average hourly emissions information for the region PJM serves.

The report contains figures for the past five years. Since 2018:

  • The average CO2 emission rate decreased approximately 9%.
  • The average SO2 emission rate declined about 31%.
  • The average NOx emission rate fell about 33%.

The average emission rates of all three categories continued a steady 18-year decline.

From 2005 to 2022, carbon dioxide emission rates fell 37% across PJM’s footprint; emission rates for nitrogen oxides are down 87% and sulfur dioxide 95%, as competitive wholesale electricity markets continue to encourage the entry of new technologies, and lower-emitting, more efficient resources replace older, less efficient units. (This data reaching back to 2005 is not part of this report.)

The report also measures the emission rates of marginal units during peak and off-peak hours. (Peak periods are all non-holiday weekdays from 7 a.m. to 11 p.m., and off-peak periods are all other hours.)

A marginal unit is the final generation resource committed during a dispatch interval to maintain system reliability and match energy supply and demand. Variations in dispatch patterns – based on customer demand – may change the set of marginal units for a dispatch interval, which can shift marginal emission rates.

In 2022, on average:

  • Combined-cycle gas-fired generators were the marginal units 61.66% of the time, compared with 59.75% the previous year.
  • Coal units, on average, were the marginal units 10.02% of the time, down from 14.15% in 2021.
  • Wind made up 11.12% of marginal units, on average, up from 11.04% in 2021.