Basic science ensures that burning fossil fuels in an internal combustion engine will result in carbon dioxide emissions. When that happens, the carbon dioxide will go into the atmosphere, contributing to the greenhouse effect. With enough vehicles on the road, enough time, and a lack of carbon dioxide sinks, this will cause catastrophic global warming leading to food insecurity, sea level rise, and unpredictably more intense and more frequent natural disasters. Research and data produced by Our World in Data suggest that transport emissions account for 24% of the carbon dioxide emitted by the energy sector. Of that, road transport accounts for a combined 74.5% of carbon dioxide, which means that around 18% of global carbon dioxide emissions are from road transportation of goods and people[i].
Not only does road transport account for a large portion of global carbon dioxide emission, but the modes of transportation are highly energy intense. Trucks with trailers (3.75 kWh/pas-km), trucks (3.03 kWh/pas-km), transit motor buses (0.6 kWh/pas-km), and small passenger vehicles (0.56 kWh/pas-km) are 4 of the 5 most intense modes of transportation in the United States as of 2018[ii]. The need for decarbonization is clear, but what is the path forward? There will be electrification and more efficient fuel, but replacement of vehicles is slow and far away. As a stepping stone to a fully electrified transport system, carbon dioxide must be captured at the source: the tailpipe. I will discuss innovations in tailpipe carbon capture and several uses for captured carbon.
How much carbon dioxide is released with standard unleaded fuel
A simple equation to represent the burning of gasoline is: Fuel (some form of hydrocarbon) + oxygen + spark = water + carbon dioxide + heat. According to the EIA, this results in 8.89 kilograms of carbon dioxide per gallon of gasoline[iii]. In a standard sedan with a fuel tank of 12 gallons and assuming all carbon dioxide is captured and stored on the vehicle, this would add 106.68 kilograms to the weight of the vehicle. For a standard SUV with a fuel tank of 17 gallons, an additional 151.13 kilograms to the payload. Unfortunately, this means that unloading by hand of carbon dioxide would be a difficult endeavor, and if there was any attempt to capture and store it at home, the storage tank would need to be on solid ground. Luckily, with gas station infrastructure already able to accommodate large tanker trucks, there is an opportunity to quickly scale both storage and distribution of liquid carbon dioxide from the trucking industry.
How to capture carbon dioxide in the tailpipe
Nearly all of the carbon dioxide produced from a vehicle exits through the tailpipe. In order to capture this, some extra device needs to be a barrier between the air inside the exhaust system and that outside. There are several methods that have been researched to perform this capture.
- In a 2014 paper by from Sri Krishna College of Engineering & Technology, commercial zeolite crystals are placed in a trap similar to catalytic converters. As the exhaust gas enters the trap, zeolite and carbon dioxide pass over each other, and the zeolite captures the carbon dioxide. This method is effective, but requires emptying of the zeolite which is not feasible due to the weight[iv].
- Researchers at École polytechnique fédérale de Lausanne patented a system for trucks that would capture 90% of carbon dioxide exiting the exhaust by a complicated chemical process[v]. Through this process, pure liquid carbon dioxide is created and stored in a tank on the truck, where it can be later removed during gas fill ups.
- Some other methods need more research in order to determine their applicability. One study suggests pumping the exhaust into a tank full of algae water. The algae, as a plant, uses the carbon dioxide to grow and reproduce, where the carbon is then sequestered in a form that also fixes nitrogen. Another method involves a road full of columns covered in solar panels that have a form of direct air capture, the captured carbon dioxide can then be used for clean fuel for vehicles on the road.
How to move and store the carbon dioxide
Carbon dioxide storage is complicated. Because it is such a small molecule, tanks must be leak proof. Otherwise, there would be no point in capturing carbon dioxide if it would just leak out to the atmosphere. Pumping systems are complicated, requiring high levels of precision. Additionally, safety is a concern with in-home or near home storage. Some effects of carbon dioxide leaks are dizziness, vomiting, headaches, drowsiness, nose and throat irritation, and even unconsciousness. In a liquid form, there is potential for skin and eye frostbite if leakage occurs[vi].
In the above truck system from EPFL, the researchers envisioned storage at gas stations. This concept for the moving and storage of carbon dioxide would be more standardized (and thus safer) as companies strive to make processes easier and simpler for drivers to reduce costs and the chance of accidents. The storage system would also allow greater amounts of liquid carbon dioxide to be stored, as mentioned above. Gas stations have monitoring, infrastructure, and safety standards already in place, and the addition of liquid carbon dioxide storage would be invaluable.
Uses for carbon dioxide
Carbon dioxide is a valuable additive to food and industry. In home, the most common use is in adding carbonation to water, like in a Sodastream. Currently, this carbon dioxide is either created or captured and then bottled, but it could be done sustainably by using exhaust fume carbon dioxide. Some less common home uses would include fog machines for parties and fire extinguisher fuel.
Food preservation is a huge category of use for carbon dioxide. It can be used as a refrigerant, creating a more sustainable cooling system. In impermeable space, the gas can replace natural atmosphere. This technique, called modified atmosphere processing, reduces spoilage by preventing natural oxygen reactions from occurring while also increasing food safety by not allowing dangerous microbes to reproduce[vii].
Another option developed by researchers at Argonne National Lab is using electrolysis to turn carbon dioxide into a source of fuel for backup generators, stabilizing the grid and providing resiliency[viii]. This method would be beneficial for both at home emergency use and for distributed energy resource management by providing power when other renewable resources like solar and wind are offline. This could also be used for utilities looking to supplement their normal fossil fuel generation with cleaner, recycled fuel. Economies of scale would prove useful if the electrolysis process is net positive for fuel. Utilities and trucking companies or delivery fleets could partner to provide a source for carbon dioxide from the vehicles and a sink in the generation.
Going even further from traditional use of carbon dioxide, there is a growing sector of aquaculture using fast growing algae to sequester carbon and then multiply the effects by turning the algae into fertilizer using its high nitrogen content. One could picture a utopian circular economy where vehicles that run on conventional fuel then convert the exhaust into food for algae, which in turn becomes food for plants, which then becomes food for people who drive those vehicles. It is a little farfetched, but certainly conceivable.
Industrially, carbon dioxide can be pumped into the ground to perform enhanced oil recovery. This is a form of oil extraction in which carbon dioxide is mixed with oil, enabling it to be more viscous and flow to the well to allow more oil in place to be extracted. According to the NRDC, 80% of carbon dioxide used in enhanced oil recovery in 2017 comes from naturally occurring underground accumulations[ix]. If tailpipe carbon capture improves, then this number could drop dramatically.
Another use for liquid carbon dioxide is that it can be further cooled into dry ice pellets for use in sandblasting, which is used to remove rust and clean surfaces for treatment. This would reduce cleanup (as the dry ice would sublimate), but one downside is that it would not permanently sequester the carbon dioxide. Dry ice is also less destructive than other solid blasting grit, which could reduce the need for more expensive (in both cost and material) repairs. Another downside of this conversion is the excess energy needed to make the dry ice usable.
Lastly, carbon dioxide has many uses in concrete. One of the best involves injecting gaseous carbon dioxide into the mixer as cement is mixed with water and other ingredients. The production process of concrete is a sneaky 7% of global greenhouse gas emissions and replacing some of the ingredients with carbon dioxide allows for both reducing the impact of the raw cement production and creating a market for captured carbon dioxide.
Tailpipe carbon capture is a messy, infrastructure heavy operation that would require cooperation from stakeholders ranging from gas stations to utilities to heavy industry to be worth the effort. While decarbonization of the transport sector is a necessary step to achieving global zero-carbon, there is a long path ahead that will involve reducing carbon emissions while internal combustion engine vehicles remain on the road. Fortunately, there are new innovators in this space and the capture, storage, and movement of carbon dioxide is becoming more defined and plausible. Additionally, there exist a myriad of uses for captured carbon dioxide, but at this point market supply of naturally occurring carbon dioxide has kept pace. As more uses come to market, the market for capture technology will expand as well.
[i]Cars, planes, trains: where do CO2 emissions from transport come from? Our World in Data https://ourworldindata.org/co2-emissions-from-transport.
[ii] Energy intensity of transport per passenger-kilometer. Our World in Data https://ourworldindata.org/grapher/energy-intensity-transport.
[iii] Environment — U.S. Energy Information Administration (EIA) — U.S. Energy Information Administration (EIA). https://www.eia.gov/environment/emissions/co2_vol_mass.php.
[iv] Carbon capture and storage from automobile exhaust to reduce co2 emission: 7.522. http://www.ijirset.com/upload/2014/special/tapsa/7_TAPSAAUTO001.pdf (2014).
[v] staff, E. editorial. CO2 could be captured directly from truck exhausts. https://eandt.theiet.org/content/articles/2019/12/co2-could-be-captured-directly-from-truck-exhausts/ (2019).
[vi] Government of Canada, C. C. for O. H. and S. Carbon Dioxide : OSH Answers. https://www.ccohs.ca/oshanswers/chemicals/chem_profiles/carbon_dioxide.html (2021).
[vii] Djenane, D. & Roncalés, P. Carbon Monoxide in Meat and Fish Packaging: Advantages and Limits. Foods 7, (2018).
[viii] Turning carbon dioxide into liquid fuel | Argonne National Laboratory. https://www.anl.gov/article/turning-carbon-dioxide-into-liquid-fuel.
[ix] Strengthening the Regulation of Enhanced Oil Recovery to Align It with the Objectives of Geologic Carbon Dioxide Sequestration. NRDC https://www.nrdc.org/resources/strengthening-regulation-enhanced-oil-recovery-align-it-objectives-geologic-carbon-dioxide.