There are two types of CO2 emission sources: natural and anthropogenic (manmade). Natural sources include respiration from animals and plants, volcanic eruptions, forest and grass natural fires, decomposition of biomass material (plants and trees), and naturally occurring sources in geologic formations. Anthropogenic sources result from human activity, including the burning of fossil fuels for electricity generation, cement production and other industrial processes, deforestation, agriculture, and changes in natural land usage. Although CO2 emissions from natural sources are estimated to be greater than the anthropogenic sources, natural sources are believed to maintain equilibrium through a process known as the global carbon cycle, in which carbon is exchanged between the land, ocean, and atmosphere. This natural system keeps CO2 levels in the atmosphere stable over time. Increases in anthropogenic emissions over the last 200 years have led to an overall increase in the concentration of CO2 and other greenhouse gases in the atmosphere. While nature’s carbon cycle keeps CO2 levels in balance, human activity, mostly resulting from the burning of fossil fuels, produces more CO2 than nature can absorb.1Department of Energy. (n.d.). CO2 stationary sources. U.S. Department of Energy. Retrieved June 24, 2021, from https://www.netl.doe.gov/node/6096
One straightforward way of reducing CO2 emissions into the atmosphere is to capture CO2 at the point it is produced; this is called a point source. Point sources can be large or small, and mobile or stationary. Capturing the emissions from the source is the first step in carbon, capture, utilization and storage.
Mobile Point Sources
Examples of mobile point sources include cars, ships, airplanes, construction vehicles—essentially, any vehicle that uses a carbon-emitting fuel to move. Due to the large number of mobile sources of air pollution, and their ability to move from one location to another, mobile sources are regulated differently from stationary sources, such as power plants. Instead of monitoring individual emitters, such as an individual vehicle, mobile sources are often regulated more broadly through design and fuel standards.2Mobile Source Air Pollution. (2021, April 24). In Wikipedia. https://en.wikipedia.org/wiki/Mobile_source_air_pollution
Stationary Point Sources
Stationary point sources include power plants, cement plants, gas refineries, and other types of industrial facilities that emit CO2 as a direct or indirect byproduct of production.3Kelemen, P., Benson, S. M., Pilorgé, H., Psarras, P., & Wilcox, J. (2019). An overview of the status and challenges of CO2 storage in minerals and geological formations. Frontiers in Climate, 1, 9.
Although in theory it is possible to capture CO2 at every single point source, there are millions of vehicles in motion all around the world, and millions more stationary sources, all emitting varying amounts of CO2 into the atmosphere. Carbon capture makes the most sense logistically and economically when it is applied to large, stationary point sources such as power plants and refineries. Additionally, an International Energy Agency survey from 2000 found that the top 13% of emission sources account for 85% of emissions. So, if we want to efficiently and effectively capture CO2 at the source, large stationary point sources should be a main focus for carbon capture projects.
Image Credits: “Traffic jam in the rush hour” by bibiphoto via Shutterstock.com; courtesy Jeffrey Phillips; U.S. Department of Energy