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Negative Emission Technologies Tackle Decarbonization in the US Part 2


Net zero 2050 forest

The following article is the second part of Dr. Eric Larson’s presentation, entitled “Negative Emission Technologies in US Decarbonization Pathways,” at the Twenty-Eighth International Conference on the Unity of the Sciences (ICUS XXVIII) in 2022. (See here the first part of his presentation.)  

 

In this [second] part of my talk, I will look at the potential role that negative emissions technologies might play in the United States if the US is to achieve its government-announced goal of net-zero emissions by 2050.

 

For this, I want to draw on a study that I co-led, which we published in 2021, called Net-Zero America: Potential Pathways, Infrastructure, and Impacts. It can be accessed at https://netzeroamerica.princeton.edu. We tried to paint a picture in as much detail as possible of what the US energy economy would look like if net-zero emissions were achieved by 2050.


What Might the US Energy/Industrial System Look Like as the Country Reduces Emissions to Net-Zero by 2050?

US greenhouse gas emissions
Figure 1

We started with the knowledge that today’s US net emissions are about 6 gigatons (Gt) CO2/year. We drew a straight line for the net emissions to reach zero by 2050 (Figure 1). The analysis takes into account that there is a land sink today, with trees growing and soils absorbing carbon, and that we would enhance that land sink through various measures. There were experts on our team who helped us to understand that.


There are also non-CO2 emissions that have to be considered, like methane and nitrous oxide that come from agricultural production. These tend to be more difficult to completely eliminate. Therefore, when you have non-CO2 emissions and a land sink, the difference between those is what the energy system will need to provide.


In our study, we modeled the energy and industrial system and ended up with basically slightly negative emissions for those sectors by 2050 to meet the net-zero economywide target. We did a variety of other modeling that I will not go into detail on, but Figure 2 shows the results of our study. This shows the primary energy supply in 2050 under different pathways to net-zero emissions.


5 Net-Zero Pathways Deliver the Same Energy Services, But With Different Energy Demand and Supply Mixes

renewable energy
Figure 2. RE: renewable energy

The left bar shows the 2020 mix of energy sources of over 80% fossil fuels. By 2050, our reference scenario (second-from-the-left bar), without any new policy measures, looks quite similar. Then there are five scenarios that all meet the target of net-zero emissions by 2050. They all deliver the same energy services—that is, the vehicle miles traveled, the square meters of building space heated and cooled, and so on are the same. However, they do this with different mixes of energy-demand and energy-supply technologies.


As an example, what we call the E+ scenario is a high-electrification scenario where buildings and vehicles are electrified very aggressively. The E− scenario involves less aggressive electrification. As one can see, electrification gives you some efficiency benefits. In fact, an electric vehicle, for example, has maybe three times the efficiency of a comparable internal combustion engine vehicle, so we have less of an energy requirement overall in the E+ scenario versus the E− scenario.


An electric vehicle, for example, has maybe three times the efficiency of a comparable internal combustion engine vehicle.

I want to point out the green bars in Figure 2 represent biomass. In all our scenarios, biomass is a very important contributor by 2050. Most of the biomass is used with CO2 capture and storage as well. In four of the five scenarios (scenarios E+, E-, and E+ RE-, E+ RE+, excluding scenario E- B+), we limited the amount of biomass that could be used in the energy system to that which could be delivered without changing land use from today. Taking land for bioenergy that might otherwise be used for agriculture has its potential problems. Therefore, we wanted to minimize that issue. One can see that in these four scenarios, bioenergy is at about the same level.

 

In the scenario E- B+ (the middle pathway of the five net-zero bars), we allowed more biomass, including some land use change, and you can see that much more biomass is adopted there. In all five of our pathways, biomass is a very valuable energy resource when coupled with CO2 capture and storage.

In the last two scenarios E+ RE- and E+ RE+ (right two bars), we changed the level of wind and solar generation. In the fourth bar (E+ RE-), we limited the amount of wind and solar capacity that could be added annually to about 40% more than the maximum single-year addition achieved in the recent past. In the last pathway E+ RE+ (the bar on the right), we did not place any constraint on wind or solar additions, and we required the energy system to be completely fossil-fuel free by 2050.


2050 Energy Mix in the Five Net-Zero US Pathways

net-zero US pathways
Figure 2 excerpt

In the first four pathways (scenarios E+, E-, E- B+, and E+ RE-) (see Figure 2 excerpt), we still have fossil fuel use in 2050. Part of the reason we can continue using fossil fuels there is because we have CO2 capture and storage involved and, in fact, in these first four scenarios, we have between 1 billion and 2 billion tons per year of CO2 capture and storage. In the fifth scenario, E+ RE+, we did not allow carbon storage, but there is still capture of CO2, with the carbon being recycled back into fuels that are needed in the energy system.


Part of the reason we can continue using fossil fuels … is because we have CO2 capture and storage involved.

 

All five of our pathways rely on six decarbonization pillars (Figure 3) deployed at unprecedented rates. “Unprecedented” means we have not seen such rates of change historically in the US; it does not necessarily mean they are impossible rates of change.


Our study delved into each of these important pillars. Here, I will show a snapshot of some results for the BECCS (bioenergy with carbon capture and storage) and DAC (direct air capture) technologies—in other words, the engineered negative-emissions technologies included in our model.


Unprecedented Rates of Physical Change Across Six Essential Pillars of Decarbonization

Decarbonization
Figure 3

We did detail mapping and prospective siting of bioenergy facilities. Figure 4 displays our map for 2050. We did this mapping in five-year time steps. I am just showing the 2050 map. The green point sources represent bioenergy conversion with CCS (carbon capture and storage). The sites are widespread, particularly around the Midwest, but also in the Southeast as well as along the western part of the US.


For the Five Net-Zero Pathways, Annual Capture at BECCS Facilities Ranges from 0.4–1.5 Gt CO2 in 2050

net-zero pathways
Figure 4

We designed a pipeline network for CO2 collection and transportation to underground storage locations. The gray-shaded regions in Figure 4 represent the most prospective regions for CO2 storage in the country. The volume of CO2 capture and storage that is going on in the E+ scenario, which is not our most aggressive one, is comparable to total current US oil production. This gives you an indication that CO2 capture, transport, and storage is a very significant new industry in our net-zero pathways.


CO2 capture, transport, and storage is a very significant new industry in net-zero pathways.

Annual Direct Air Capture (DAC) in 2050 Reaches 0.7 Gt CO2 in the Most DAC-Intensive Pathway (E-)

CO2 sources in 2050
Figure 5
source

In the upper panel of Figure 5, we see all the sources of CO2 capture in 2050 in each of our five net-zero scenarios. The lower panel in Figure 5 shows where captured CO2 (from all sources) goes. The gray in the lower panel represents CO2 stored. Most of the CO2 that is captured in the first four scenarios is stored underground. You can see that bioenergy (BE) with carbon capture and storage (CCS—BE+CCS = BECCS) plays a big role in all five of these scenarios.


Direct air capture (DAC) really comes in only in the E− scenario. That is the case where we did not electrify vehicles and buildings as aggressively as we did in the E+ pathway. That leads to additional fossil fuel use in 2050, so, that then the emissions from those need to be offset. Since we have consumed the full biomass potential, here we need to adopt DAC, which in our modeling is a more expensive CO2 removal option than BECCS, and that is why it comes in later and mainly in that one scenario. Thus, both the BECCS and DAC technologies play very important roles in the US in potentially getting to net-zero emissions—and potentially in other countries as well.


Just to recap, why are we interested in negative emissions? Cumulative emissions of CO2 determine future global warming. To stay below 1.5–2 °C of warming, the carbon budget that we have left to spend is shrinking quite rapidly. Negative emissions can essentially help us stay within our budget and meet those emissions thresholds.


What are the various net-zero emissions or negative emissions technologies? I reviewed a number of them, including restoring and managing terrestrial and aquatic ecosystems, mineralizing carbon, BECCS, and DAC with CO2 storage. All of these have a role to play in meeting the carbon challenge. But BECCS and DAC prospectively have the largest roles. I showed some results from our US study, including quite detailed modeling results, that really highlight the critical roles for those two future industries in reaching net-zero targets.

 

*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.

 

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