Transportation is one of the biggest sources of greenhouse gas (GHG) emissions in the United States, as this sector accounts for 28.5% of this country’s total emissions. These emissions primarily originate from the burning of fossil fuels, 90% of which are petroleum-based, that is used by cars, trucks, ships, trains and planes.
In an effort to reduce GHG emissions, several Northeast and Mid-Atlantic states have adopted the Regional Greenhouse Gas Initiative carbon-trading plan. Furthermore, liberal environmental activists, like U.S. Representative Alexandria Ocasio-Cortez, have publicly stated their support for The Green New Deal. The Green New Deal, which is a radical approach to tackle some of the most concerning environmental issues plaguing the globe, plans to have all vehicles achieve 100% zero-emission rates by 2030.
While these initiatives continue to arise on the legislative level, a significant amount of research has been made on the fuel production side to address growing environmental concerns. Fuel formulations and additive packs are now being designed to improve fuel consumption and reduce harmful emissions.
In particular, soot emissions, which arise as a result of incomplete combustion, contribute significantly to the greenhouse effect. While this may be true, a reduction in soot emission can be achieved much more easily than carbon dioxide (CO2) emissions through the use of additives that help to promote a more complete combustion. In addition to reducing soot emission, cleaner combustion also produces less carbon monoxide and nitric oxide (NOx) emissions, both of which have detrimental effects on air quality, especially in urban areas that commonly have heavy amounts of road traffic.
The expanded set of tools that has been made available to researchers through advances in nanotechnology is currently being used to create novel fuel additives that aim to improve the combustion cleanliness in engines. This article will explore a few examples of these fuel nanoadditives and their potential benefits.
Figure 1. Transport fuels are one of the largest contributors to global greenhouse gas emissions. Fuel producers are working to improve their environmental footprint by enhancing fuel economy and emissions using tailored additive packages. Image Credits: EPA.
Cerium Oxide Nanoparticles
Cerium oxide has the ability to catalyze combustion reactions by donating oxygen atoms from its lattice structure. This catalytic activity is dependent on surface area; therefore, using nanoscale cerium oxide particles can offer distinct advantages over bulk material or larger particles.
Adding cerium oxide nanoparticles to fuel can help assist in the decomposition of unburnt hydrocarbons and soot, thereby reducing the emission of these pollutants from the exhaust while simultaneously reducing the amount of fuel used. It has also been shown that cerium oxide decreases the pressure in the combustion chamber, which reduces the production of NOx and makes combustion reactions more efficient.
Cerium oxide nanoparticles can also be used as a short-term treatment for particulate filters in diesel engines. These nanoparticles help to clear away soot, which clogs up the filters, in an effort to drastically improve the performance of the filters and the overall cleanliness of the exhaust emissions. For example, the use of Karanja oil biodiesel that have been mixed with cerium oxide nanoparticles additives improved engine performance by 5%, while reducing NOx emissions by as much as 26%. Furthermore, this specific study found that this cerium oxide nanoadditive also significantly reduce particulate size number distribution, lowered particle size surface area distribution and reduce total particulate number concentration as compared to diesel fuel emission characteristics.
Nanoparticles and microparticles of aluminum have also been extensively investigated as potential fuel additives. The main reason or this is because of aluminum’s ability to increase the power output of engines as a result of its high combustion energy. Recent advances in the fabrication and characterization of nanoparticles have provided researchers with a more detailed understanding into the relationship that exists between particle size and structure with performance benefit, thereby supporting research into aluminum nanoparticle fuel additives.
A study carried out by researchers at Purdue University, Indiana, found that nano-sized particles of aluminum provided a greater performance benefit as compared to aluminum microparticles. The characteristics of nanoaluminum in suspension were found to be more conducive to the formation of micro explosions during combustion, which assists the air-fuel mixing and leads to cleaner, more efficient combustion.
In addition, the aluminum nanoparticle suspensions in ethanol-based fuels were much better than those in model hydrocarbons, suggesting that nanoaluminum could be effective in additive packs for bio-ethanol fuels. In fact, a 2018 study found that incorporating aluminum oxide (Al2O3) nanoparticles into Jojoba biodiesel-diesel fuel significantly improved engine performance while simultaneously reducing the NOx emissions by 70%, carbon monoxide (CO) emissions by 80% and smoke opacity by 35%.
Figure 2. More and more public transport vehicles are being run on biofuels, to improve their environmental profile. Although the carbon may be from a renewable source, inefficient combustion of biodiesel could lead to an overall increase in the output of pollutants and greenhouse gases. Additives are needed to ensure the fuel burns cleanly and does not produce large amounts of soot. Image Credits: senate.iowa.gov.
Magnesium-Aluminium and Cobalt Oxide Nanoparticles
In a 2011 study, researchers from Anna University in India investigated the potential of cobalt oxide (Co3O4) and magnesium-aluminum (magnalium) nanoparticles as additives for biodiesel fuels.
The oxygen atoms in Co3O4 particles can moderate the combustion reactions in a similar mechanism as to how cerium oxide nanoadditives function. As a result, when Co3O4 nanoparticles were applied to the fuel, the researchers found that the combustion was cleaner and that both CO unburnt hydrocarbon emission were significantly reduced.
The cobalt nanoadditives were also shown to reduce NOx production. This is especially significant with biodiesel combustion, since biodiesel fuels are often prone to high NOx emissions as compared to regular petrochemical diesel.
Magnalium nanoparticles serve a similar function as fuel additives as compared to aluminium nanoparticles, in that these nanoparticles exhibit a high energy combustion that produces micro explosions. These micro explosions ultimately improve combustion efficiency to help to improve fuel efficiency or increase power output.
In this study, it was also noted that the magnalium particles acted as a heat sink within the combustion chamber, which allowed for a reduction in the overall temperature of the engine to help avoid hotspots and reduce NOx production.
Toxicity and Environmental Impact of Nanoadditives
As with many nanomaterials, the main challenge that prevents full commercial deployment of these nanoadditives is their potential environmental impact. Whilst nanoadditives have demonstrated the potential to improve fuel efficiency and the quality of exhaust emissions, they may also cause environmental issues if the nanoparticles themselves are carried into the exhaust gases during combustion.
Several studies have shown that addition of cerium oxide nanoparticles to fuel results in emission of small amounts of the particles. These particles have the potential to accumulate in the environment, particularly in roadside areas and cause adverse health effects. For example, diesel fuels that have been enhanced with cerium oxide nanoparticles have been found to cause adverse pulmonary effects in the form of increased bronchial alveolar lavage fluid and lung inflammation in exposed rats.
Although the potential effects associated with this level of exposure to nanoadditives is not yet fully understood, it is likely that the overall toxicity of the emissions will be reduced as a result of the reduction of particulate emissions. Combustion-derived nanoparticles, which are generally particles of soot and other residues from incomplete combustion, have been shown to be a major contributor to the toxicity of diesel fumes. While this may be true, it is crucial for environmental agencies to determine whether the potential adverse side effects of these fuel additives outweigh the known health and environmental implications of GHG emissions like CO and NOx.
The ability to explore the effect of fuel nanoadditives on engine performance has opened up a wide range of potential ways in which engineers can reduce fuel consumptions, as well as counteract some of the performance compromises associated with the use of internal combustion engines and biofuels.
The main challenge moving forward will be to fully assess the potential environmental and health impacts associated with the release of these nanoadditives, while also investigating the extent to which these nanoadditives can improve current GHG emission rates.
References and Further Reading
- “Sources of Greenhouse Gas Emissions” – The United States Environmental Protection Agency
- "Toxicological Review of Nano Cerium Oxide" - NIA Prospect
- Pandey, A., Nandgaonkar, M., Pandey, U., Suresh, S., et al., (2018). The effect of cerium oxide nanoparticles fuel additive on performance and emission of Karanja biodiesel fueled compression ignition military 585kW heavy duty diesel engine. SAE Technical Paper. DOI: 10.4271/2018-01-1818.
- "Combustion characteristics of fuel droplets with addition of nano and micron-sized aluminium particles", Y. Gan & L. Qiao, Combustion and Flame 2011. DOI: 10.1016/j.combustflame.2010.09.005
- El-Seesy, Ahmed & Attia, Ali. (2018). The effect of Aluminum oxide nanoparticles addition with Jojoba methyl ester-diesel fuel blend on a diesel engine performance, combustion and emission characteristics. Fuel. 224. 147-166. DOI: 10.1016/j.fuel.2018.03.076.
- "Effect of nano-fuel additive on emission reduction in a biodiesel fueled CI engine", D. Ganesh & G. Gowrishankar, 2011 International Conference on Electrical and Control Engineering. DOI: 10.1109/ICECENG.2011.6058240
- "Exploring Nano-sized Fuel Additives" - EPA Science Matters newsletter
- Snow, S., McGee, J., Miller, D., Bass, V., Schladweiler, M., et al. (2014). Inhaled Diesel Emissions Generated With Cerium Oxide Nanoparticle Fuel Additive Induce Adverse Pulmonary and Systemic Effects.. Toxicological sciences : an official journal of the Society of Toxicology. 142. DOI: 10.1093/toxsci/kfu187.
This article was updated on 25th February, 2019.