The following article is the first part of Prof. Eric Larson’s presentation, entitled “Negative Emission Technologies in U.S. Decarbonization Pathways,” at the Twenty-Eighth International Conference on the Unity of the Sciences (ICUS XXVIII) in 2022.
I would like to speak today about negative emissions technologies, and in particular, the role that these might play in the decarbonization of the United States economy.
I will begin by explaining why negative emissions are needed and then describe different negative emissions technologies (or NETs). Finally, I will discuss possible roles of NETs in technological pathways for the United States to reach net-zero emissions by 2050.
Cumulative CO2 Emissions Determine Warming
Remaining Emissions “Budget” for 1.5-2 °C Is Shrinking
I will start by reviewing the science that we understand about the relationship between global warming and the emissions of greenhouse gases, especially CO2. Figure 1 shows a graph from the Intergovernmental Panel on Climate Change’s Special Report on Global Warming of 1.5 °C. (I will not go into all the details here.)
This graph tells us that the warming that we can anticipate for the world is a function directly related to the cumulative emissions of carbon dioxide that we have put into the atmosphere since the preindustrial period, starting with the mid-1800s. With this understanding of the relationship, we can estimate the remaining amount of carbon budget that we can emit before we hit certain thresholds of temperature increase. As of December 2018, when this study came out, there was a 50% probability that we could stay below 2 °C warming globally if we emit no more than 1500 Gt of CO2 cumulatively from that point forward. To stay within a 1.5 °C carbon budget, it would be, of course, much lower—closer to 600 Gt.
If we want to stay within 1.5 °C of warming, we have about ten years left of emissions at the current global emissions rate before we hit that threshold.
Since that estimate was made, we have, as a world, already emitted an additional 150 Gt [as of 2022]. We have spent some of our budget already, which means you can estimate that if we want to stay within 1.5 °C of warming, we have about ten years left of emissions at the current global emissions rate before we hit that threshold, and we have a bit more time if we are satisfied with staying below 2 °C.
When we look at a graph like in Figure 2, it shows the trajectory of global CO2 emissions to stay below the 2 °C threshold. This graph was made a few years back, and scientists at the time started their modeled emissions trajectory with the year 2005. As we know now, emissions from 2005 to 2015 continued to increase beyond the modeled level.
Global Emissions Trajectory for a Carbon Budget
Corresponding to a Warming of 20C
The pathway is not precise here, but it is reflective of the kind of change that the world needs to see to stay below 2 °C of warming: the world would need to reach zero emissions by about 2070; in other words, the world would have used up its carbon emissions budget by that date.
This budget can essentially be extended if we allow for the possibility of negative emissions. In this case, we are on the pathway shown in Figure 3. Again, these are modeling results, and this pathway results in higher emissions than following the budget to 2 °C, but we compensate for that by negative emissions, beginning as early as 2030 and growing considerably beyond that. This then allows us to stay on a net-zero emissions trajectory for 2 °C.
Cumulative Emissions Can Be Reduced Using
Negative Emissions Technology (NETs)
Essentially, negative emissions allow us to increase our budget of positive emissions and to still stay below our temperature targets. There are a variety of negative emissions technologies, and we understand many of these quite well, such as those in Figure 4.
Essentially, negative emissions allow us to increase our budget of positive emissions and to still stay below our temperature targets.
For example, through restoration and management of terrestrial and aquatic ecosystems, we can absorb CO2 out of the atmosphere. We can do the same by changing agricultural practices—so-called carbon farming. We can increase the carbon content in soils, which takes CO2 out of the atmosphere. These are largely biological measures (left side of Figure 4). As we move to the right in Figure 4, we move toward more engineered measures, starting with bioenergy with CO2 capture and storage. This is based on plant matter that has absorbed CO2 from the atmosphere as it has grown. The plant matter is then converted into a convenient form of energy, for example, electricity, and the by-product CO2 of the conversion process is captured and stored underground.
Negative Emissions Technology (NETs)
More fully engineered negative emissions systems include direct air capture (DAC), where we are taking the CO2 directly out of the air by a chemical process and then storing the CO2 below ground. There is also enhanced mineral weathering, which is basically creating carbonate rocks using CO2 and natural rocks that combine to make carbonate rocks and thereby store CO2.
Biological processes tend to be less costly per ton of CO2 that is removed. They are closer to deployment in part because they are less costly and because we know how to do these quite well.
Biological processes tend to be less costly per ton of CO2 that is removed. They are closer to deployment in part because they are less costly and because we know how to do these quite well. On the other hand, they are more vulnerable to reversal—that is, soil carbon can be rereleased to the atmosphere if the methods are not properly managed. On the other hand, there are environmental co-benefits with carbon and soil that often increase the productivity of the soil, which is a positive result of that system.
As we move toward the more engineered systems, we see that they are generally more costly and often need more research and development and certainly more complicated deployment and demonstration of commercial capability. On a positive note, they are less vulnerable to reversal. Potential co-benefits would be technology leadership for countries or companies that are at the forefront in developing these potentially new employment opportunities.
Among the various negative emissions technologies, two are generally considered to be the most prospective in terms of the role that they can play in net-negative emissions overall: bioenergy with carbon capture and storage (BECCS) and DAC with CO2 storage.
Figure 5 shows the carbon flows for a BECCS system. The widths of the arrows in this picture are roughly equivalent to the magnitude of the carbon flows. There are emissions at various points along this process, from tractors that might be used in the cultivation and harvesting of biomass, from unavoidable emissions at the conversion plant, and if a hydrocarbon fuel is being made, some carbon will return to the atmosphere when the fuel is used.
However, a large amount of the by-product CO2 in the conversion process is captured and put underground for storage. This carbon had been removed from the atmosphere via photosynthesis as the biomass grew. Looking at the net balance across all arrows, there is a net flow of carbon from the atmosphere to the subsurface on an annual basis.
Carbon Flows for Bioenergy with CO2 Capture and Storage (BECCS)
Technologies for the biomass conversion process are rather well understood. My group has analyzed many of these. There are other researchers around the world who also have been looking at these technologies. The challenge has been primarily over their cost because most of these processes are not economical under today’s conditions. Therefore, although we have a quite good understanding of how these technologies work from an engineering perspective, they are not widely deployed commercially today.
Direct air capture (DAC) concepts are also well understood, but the technologies themselves are at a relatively early stage of development.
Direct air capture (DAC) concepts are also well understood, but the technologies themselves are at a relatively early stage of development. Two of the leading concepts are shown in Figure 6 and Figure 7. One involves passing air over a dry sorbent that then selectively pulls the CO2 molecules out of the air. The sorbent is then regenerated through some means, typically by heat addition to drive off the CO2. That CO2 is collected and compressed for transportation through a pipeline to an underground storage site. The scheme in Figure 6 uses such a dry sorbent. One company has now built a 4000 t CO2/year capture facility in Iceland. That is a relatively small facility by comparison to the levels of CO2 capture that we want in order to address the 2 °C or even the 1.5 °C challenge, but it is a start.
Process flow diagram for dry-sorbent DAC
Figure 6 Green lines represent gaseous flows, and blue lines represent liquid flows. The dashed green line from the contactor to the vacuum pump represents the initial phase of desorption where residual air is removed from the contactor to prevent dilution of the produced CO2 after evolution from the sorbent. (McQueen, et. al. “A review of direct air capture (DAC),” Progress in Energy, 3, 2021. https://doi.org/10.1088/2516-1083/1bf1ce)
Process flow diagram for the liquid-solvent DAC process
Figure 7 Green lines represent gaseous flows, blue lines liquid flows, and brown lines solid flows. The H20 streams undergo temperature changes not represented in this diagram. (McQueen, et. al. “A review of direct air capture (DAC),” Progress in Energy, 3, 2021. https://doi.org/10.1088/2516-1083/1bf1ce)
The other concept (Figure 7) is centered around a liquid solvent—potassium hydroxide—that captures the CO2 and then goes through a process to separate the CO2 from the solvent so that the solvent can be recycled and used again. The captured CO2 is then compressed and stored. A different company is developing this concept and has plans to have a 1 million t CO2/year facility starting up in 2024. This begins to get to the commercial scale that will be needed in the longer term.
With both BECCS and DAC, plus storage, there is a requirement for underground storage resources. Fortunately, around the world, there are many geological formations that have the capacity to store CO2.
However, the distribution of these resources varies from country to country. The United States is particularly well endowed with CO2 storage geology. On the order of 40 million t CO2/year is currently being captured and stored across the world, not just in the US, via a number of demonstration projects. We understand the CO2 storage process rather well. The challenge is characterizing the subsurface sufficiently so that one can have confidence that the storage is secure.
*Eric Larson has a Ph.D. in Mechanical Engineering and is the Senior Research Engineer at the Andlinger Center for Energy and the Environment, Princeton University, USA.