Greenhouse Gas Emissions from Generation

As regional and state policies shift towards a desire to reduce emissions and deploy clean energy resources, it is important to understand historical emissions, changes in the generation stack and dispatch, and trends (both past and present). The electricity sector is one of the primary sources of greenhouse gas emissions. An analysis from the Energy Information Agency (EIA) shows that amongst energy-related sectors at the national level, the electricity sector was the greatest emitter of carbon dioxide (CO₂) until 2016 when the transportation sector surpassed it. As electricity sector emissions have declined (see United States Generation and Carbon Dioxide Emissions), transportation sector emissions have increased, while emissions from the industrial, residential, and commercial sectors have remained fairly constant.

EIA: U.S. energy-related CO₂ emissions by sector

Greenhouse gas emissions attributed to electricity generation are emitted at the source, along the supply chain, to the point-of-combustion – often termed lifecycle emissions. Upstream emissions typically describe the emissions released when fuel is sourced, extracted, and transported, while point-of-combustion emissions typically refer to the on-site (power plant) burning of fossil fuel for electricity. The following analysis includes only fossil fuel emissions from point-of-combustion. For more information on upstream emissions, and in particular methane emissions, (see the upstream methane emissions analysis).

The primary greenhouse gas emissions related to electricity generation are carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O). Greenhouse gases affect the climate by trapping heat within the atmosphere, absorbing energy, and then slowing the rate at which the energy escapes. While each of these gases are naturally present in the atmosphere and release a level of emissions from natural sources (oceans, soil, etc.), anthropogenic (human caused) emissions, for example emissions from fossil fuel combustion at electric power plants, add more emissions to the atmosphere faster than the Earth’s natural ability to cycle or sink them. These additional trapped emissions can have the effect of raising the Earth’s temperature and is synonymous with climate change.

Carbon dioxide (CO₂) 

Carbon dioxide is a naturally occurring chemical compound that produces a colorless, odorless gas. The Earth has a natural carbon cycle that circulates and exchanges carbon atoms through phases of photosynthesis, decomposition, respiration, and combustion. When additional CO₂ is produced and emitted into the atmosphere through the combustion of fossil fuels (as an example of anthropogenic causes), it results in more carbon getting added to the atmosphere faster than the natural carbon cycle can remove it.

Carbon dioxide is a direct byproduct of fossil fuel production. The burning of different fossil fuels – coal, natural gas, petroleum/oil – will produce varying rates of emissions.

CO₂ Emissions by Fuel Type

In the Pacific Northwest, where subbituminous coal is most commonly burned, coal emits CO₂ at a rate of about 1.8 times that of natural gas.

Methane (CH₄)

Methane is highly flammable, colorless, odorless gas. Natural sources of methane include wetlands, oceans, and termites (to name a few). Methane is the primary component of natural gas. Methane emissions from fossil-fuel combustion are fairly insignificant, particularly when compared to CO₂ emissions. However, upstream methane emissions from natural gas sourcing and extraction, coal mining, and transport are more substantial.

Other anthropogenic methane emissions occur in agriculture and landfill waste processes – see opportunities to turn this methane into power through biomass technologies – and manmade reservoirs. A 2017 study by Washington State University suggested that methane emissions from reservoirs have been historically miscalculated and underreported. Manmade reservoirs at dams (electrical or non-powered) tend to flood, trap, and collect large amounts of organic matter such as plants, branches, and algae (more so than natural bodies of water) that then emit greenhouse gases as they decompose. A 2017 Council analysis found that while there wasn’t enough data to make detailed emissions estimates specific to the region’s reservoir system, the Columbia River system in general doesn't produce the kind of nutrient-rich environment that supports excessive plant growth, and therefore it's unlikely to emit large levels of methane gas. The Environmental Protection Agency (EPA) kicked off a 3-yr measurement program at 120 reservoirs across the United States. The results of this analysis may prove helpful in future PNW reservoir methane emissions estimates.

Nitrous oxide (N₂O)

Nitrous oxide (N₂O) is a colorless, non-flammable gas that naturally occurs in soils and oceans and is part of the natural nitrogen cycle. Nitrous oxide is sometimes referred to as nitrogen oxide (NO_X), a shorthand reference that includes both N₂O and nitric oxide (NO).

Nitrous oxide is emitted as a byproduct of fossil fuel combustion, with the magnitude of emissions dependent on the type of fuel and equipment used. In addition to atmospheric impacts, nitrous oxide is linked with a number of adverse effects on the respiratory system. Since the implementation of regulations under the Clean Air Act Amendments of 1990, N₂O emissions have declined by 86% (according to the EPA’s Clean Air Market power plant emission trends) through the installation of combustion control technologies such as selective catalytic reduction (SCR).

By far the biggest anthropogenic source of N₂O is agricultural soil management. The use of harmful nitrogen-based fertilizers in agricultural soils is inherently tied to our reliance on current food production methods and represent a significant portion of N₂O emissions.

Global Warming Potential

A common metric used to compare the atmospheric impacts of greenhouse gases over time is the Global Warming Potential (GWP), which measures the radiative efficiency of the GHG – it’s ability to absorb energy – with how long that energy is trapped in the atmosphere - it’s lifetime. Using CO₂ as the reference gas, the GWP allows us to compare the potency (the energy absorbed by a gas over X number of years) of GHGs in the near-term (20 years) and the long-term (100 years). In addition, it normalizes all GHGs to units of carbon dioxide equivalents (CO₂e), enabling us to inventory them in a common metric.

Global Warming Potential (IPCC AR5)

Gases with shorter lifespans, such as methane, will have a higher GWP at 20 years than at 100 years. Even though methane has a relatively short lifetime in the Earth’s atmosphere compared to CO₂, it is more efficient at trapping radiation. Therefore, when comparing the impact of the two greenhouse gases, methane is 86 times more potent than CO₂ over a 20-year period and 34 times more potent over a 100-year period.

When looking at point-of-combustion emissions, carbon dioxide is by far the most significant greenhouse gas emitter. The U.S. Environmental Protection Agency (EPA) produces an annual Inventory of U.S. Greenhouse Gas Emissions and Sinks that quantifies anthropogenic greenhouse gas emissions and trends. While this data is derived from a different methodology than the EIA data used in the historical emissions analysis below, it is very similar. In 2018, CO₂ emissions accounted for over 98.5% of total GHG emissions from the combustion of fossil fuels for electricity production. In a far distant second, N₂O emissions accounted for about 1.4%, and methane emissions were negligible. These ratios have been consistent over time, emphasizing the magnitude and weight of CO₂ in terms of GHG emissions for electricity production. 

2018 U.S. Greenhouse Gas Emissions from Combustion of Fossil Fuels for Electric Power

(data from the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2018, Table 3-7, page 3-12)