by Jordan Wilkerson
figures by Daniel Utter
Ever since the Industrial Revolution around 150 years ago, most of the electricity used in the United States has come from burning fossil fuels. These fuels, such as coal and oil, are all made of carbon. They’re such an important form of the element that the name “carbon” comes from the Latin word for coal. However, burning all of this carbon for energy has increased the amount of carbon dioxide (CO2) entering our atmosphere at an unprecedented rate. CO2 acts as a greenhouse gas by retaining heat emitted by the Earth that would otherwise escape to space, which is the dominant cause of recent global climate change. The Earth’s average temperature has increased by 1.4 °F since 1880. This may not seem like a lot, but we’re already seeing many consequences of this recent temperature hike, such as sea level rise and increasingly intense heat waves. These effects translate to sizeable infrastructural damage and increased mortality rates – especially in southern coastal states. The fact is that climate change is going to cost money and lives. Scientists and policymakers are considering a portfolio of strategies to reduce future warming. One solution being proposed is, oddly enough, to continue burning carbon for energy like we have been doing – but with one difference: instead of fossil fuels, burn carbon that comes from plants.
Burning a Different Kind of Carbon
Electricity that comes from burning plant-based fuel is referred to as bioenergy. You are likely familiar with the use of corn-based ethanol to supplement gasoline. Another version of this solution focuses on coal power plants. Instead of burning coal at these facilities, we can burn compressed pellets of plant matter. Power plants that have been refurbished to use biomass instead of, or in addition to, coal already exist to a limited extent across the United States.
There’s another step we could take in refurbishing these coal power plants. If we capture the CO2 that flows out of the facility’s smokestacks and bury it deep underground, then we’d prevent the greenhouse gas from escaping to the atmosphere entirely. This process is known as carbon capture, and when coupled with an underground storage capability, carbon capture and storage (CCS). CCS technology is commonly tied to clean coal, but it would work just as well for bioenergy.
At this point, you might be wondering: why is burning carbon from plants better for the climate than burning carbon from coal? This is because it’s not simply burning carbon that’s causing our climate to rapidly change; it matters where that carbon comes from and where it eventually goes. So where does the carbon from fossil fuels and biomass come from and go?
The Flow of Carbon
The element carbon is constantly flowing through different facets of the Earth and atmosphere through what is called the carbon cycle. However, carbon deep in the ground stays there for much longer than carbon found in plants. This is because, as shown in Figure 1, the plants come from a significantly faster part of the carbon cycle, known as the terrestrial carbon cycle, which flows through all life on land. Plants grow by taking CO2 from the air and using photosynthesis to convert it to new leaves and stems. When plants die, they’re naturally eaten up by fungus and microbes in the soil, which release the CO2 back into the air.
Let’s say we instead harvest the plants for fuel. This doesn’t really exacerbate climate change or alter the carbon cycle because the plants can regrow. By fully regrowing, the plants pull the same amount of CO2 that we emitted by burning them in the first place. Therefore, we only increase the amount of CO2 in the atmosphere for a short time because new plants can grow and remove that CO2 again.
Let’s compare this to the time it takes for fossil fuels to form. The red arrows in Figure 1 indicate a very slow part of this cycle known as the geologic carbon cycle. Fossil fuels, being deep underground, are part of this cycle. When ancient life died and was naturally buried en masse over time, the immense pressure slowly transformed them into the coal and oil we use today. Carbon also naturally enters into deep sediment when aquatic organisms in the ocean die and are buried over millennia at the ocean floor. These processes are very slow and fossil fuels taken millions of years to form. In contrast, the exchange of carbon between plants and the atmosphere is over 1000 times faster than the exchange of carbon between the sediment deep beneath us and the skies above.
Since the Industrial Revolution, humans have been extracting fossil fuels and burning them for energy, creating a route for deeply buried carbon to enter the atmosphere. Humans are currently releasing 100 times more carbon to the atmosphere per year than from volcanic activity, which is the geologic carbon cycle’s natural method for releasing carbon to the atmosphere (Figure 2).
How long does this carbon stay in our atmosphere? It depends on the marine life’s ability to make carbonate shells, which are made from CO2 that dissolved into the oceans primarily from the atmosphere (Figure 1). The extremely slow burial of these shells is the only natural way that atmospheric CO2 is ultimately placed back deep into the Earth crust. This process is much longer than the time it takes plants to regrow. Therefore, the CO2 we emit from fossil fuels represents a much more long-lasting increase in greenhouse gas concentrations than when we emit CO2 from burning plant matter.
Removing Carbon from the Air
We can potentially do more than just change what kind of carbon we burn. By retrofitting a power plant with carbon capture and storage (CCS) technology, the CO2 we produce during electricity generation can be pumped into an underground geologic formation. CCS technology is still in its incipient stages with only one coal power plant in the US currently using the technology. Technological and economic hurdles aside, the allure of CCS is clear: it reduces the amount of carbon we transfer from deep rocks to the atmosphere (Figure 3).
CCS simply means we don’t emit more carbon to the air. We get the carbon (fossil fuels) from the ground, and we inject the carbon dioxide back into the ground in a closed loop. However, let’s take that same idea and apply it to a biomass power plant. The carbon we use from biomass comes from a tree or a bundle of grass, both of which got that carbon from the air relatively recently. By burning these plants and burying the CO2 deep underground, we are effectively taking CO2 that was in the air and moving it back into deep sediment – the opposite of what we have been doing for the past 150 years by burning fossil fuels.
Dedicating land and resources to growing crops for bioenergy certainly makes the environmental benefits less clear-cut. However, this isn’t necessary to consider for the initial stages of increasing biomass power. Many existing biomass power plants are conveniently located near sawmills where the mill waste is transported to the power plant. The crops are grown regardless of whether the biomass power plant exists nearby, so the fuel cost – and environmental burden – is close to zero. There are other sources of biomass waste, too, such as forest and agricultural plant residues, which are often being burned, anyway, as part of land management practices. However, if we do start growing crops just for bioenergy, we could use cheap, fast-growing grasses. Grasses require fewer resources than traditional crops and obviate the need to worry about competition with the food industry.
Despite complications associated with drastically increasing bioenergy, it is still considered a crucial component of our future energy portfolio if we are to address climate change as effectively as possible. Successfully meeting the climate-change mitigation goal outlined by the United Nations’ Paris Climate Agreement assumes heavy reliance on contributions from bioenergy with carbon capture in addition to other rigorous strategies. Biomass power plants are starting to be proposed by policymakers as an important component of renewable energy portfolios. To be clear, these proposals focus on using plant waste, not on growing crops specifically for bioenergy. Separately, several coal power plants have been refurbished to have the capability of burning biomass. However, there are many abandoned coal power plants still scattered across the United States that could be revived and refurbished to burn plant waste. These abandoned buildings could be put to use again, the jobs could be regained, and waste management could improve. All the while, we’d be helping address the growing problem of our Earth’s drastically changing climate.
Jordan Wilkerson is a fifth-year Ph.D. student in the Department of Chemistry and Chemical Biology at Harvard University.
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For more information on the carbon cycle, check out this report by the Intergovernmental Panel on Climate Change (IPCC) a United Nations’ panel.