The Search For Efficient, Low-Emission Fuels

New pollution-free fuels are being trialed to enhance and eventually replace fossil fuels. A diesel-based fuel with an added hydrocarbon compound has been shown to significantly reduce soot output from engines. Switching from diesel to this new fuel could lower atmospheric carbon dioxide levels and other emissions that are harmful to human health.

The majority of the world’s population is heavily reliant on hydrocarbon fuels for transportation and energy. These come from fossil fuels; material that nature biorefined through photosynthesis millions of years ago. This process created versatile materials that we now use in various forms, like petroleum, kerosene, and diesel, to meet our energy needs.

Fossil fuel combustion is well known to have negative effects on the environment and human health, but it is proving difficult to reduce our reliance on it. Unfortunately for the planet, there are many advantages to using fossil fuels. They have a high energy density and are much safer to transport than batteries, which also tend to degrade over time. They are a reliable, consistent energy source compared with cleaner options like solar and wind power. Furthermore, the majority of our infrastructure – both physical and “soft” infrastructure, like financial and healthcare systems – relies on the use of fossil fuels.

The impact of fossil fuel combustion is easy to see and measure – just get stuck behind an old bus while cycling to work and the side-effects of engine combustion are impossible to ignore. As well as being unpleasant in the short-term, this type of pollution has lasting effects on human health. High levels of particulate matter and other emissions, like nitrogen oxides, negatively affect 91% of the world’s population. The World Health Organization estimates that 4.2 million deaths per year are due to outdoor (ambient) air pollution, with one in nine deaths due to air pollution from ambient and indoor exposure combined.

The problem of engine emissions goes further than the immediate impact on human health from soot and other airborne particles. The burning of fossil fuels also releases carbon dioxide gas into the atmosphere, contributing to global warming. Today, carbon dioxide levels are higher than they have been in the past three million years. The impacts of this increased atmospheric carbon are well known, from rising temperatures and sea levels to unpredictable weather patterns.

To resolve these problems from an environmental perspective, an immediate and complete shift to green energy is an ideal scenario. However, due to our near-total reliance on fossil fuels, this would cause many other problems and affect livelihoods in other ways. It is imperative that reliable and sustainable alternatives are found in order to make a smooth transition from fossil fuels.

Recent collaborative research from the University of Cambridge, National University of Singapore, and Nanyang Technological University has shed light on the combustion characteristics of a new fuel blend already known to reduce soot formation in engine combustion. The combination of diesel and polyoxymethylene dimethyl ether (PODE) has the potential to mitigate some of the environmental issues caused by combustion while retaining the positive qualities of hydrocarbon fuels. It is also compatible with current diesel engines. PODE is a particularly good additive due to its high oxygen content and lack of sulfur and aromatics, which can produce harmful combustion by-products. It can also be produced in large quantities, with a production price comparable to conventional diesel.

To find out which diesel/PODE fuels combust most cleanly, researchers used a simple burner (ASTM D1322 smoke point burner) to measure the smoke points of various blends. Their results support previous research on the main causes of soot reduction from diesel/PODE. The biggest impact comes from diluting the aromatic fraction in the diesel; a physical as opposed to a chemical effect, like oxygen content. (Soot reduction through dilution can also be achieved with other additives, like alcohols, ethers and esters.) Decreasing the hydrocarbon chain length of the PODE and increasing its oxygen content also have an effect, although it is minimal.

Their research also proposes a further explanation for this soot reduction, and one that may have a big impact: that the chemical species that form during combustion affect how much soot is produced. Some of these species are better at suppressing soot formation than others, which is due to the molecular structure and decomposition pathways of the particular PODE used. Furthermore, no noticeable difference in soot reduction was observed between the various types of additives. This is significant as it allows for a wider range of additives to be considered. While there is further work to be completed in this area, this research shows great promise for identifying the PODE additives that will have the greatest impact on soot reduction.

The development of efficient, low-emission fuels is crucial for the world’s transition to clean energy. This deeper understanding of how these fuels work goes a long way towards addressing the sustainability issues around traditional fossil fuels within our existing energy infrastructure.

These findings are described in the article Sooting characteristics of polyoxymethylene dimethyl ether blends with diesel in a diffusion flame, recently published in the journal Fuel. This work was conducted by Yong Ren Tan, Maria L. Botero, and Wenming Yang from the National University of Singapore; Yuan Sheng and Rong Xu from Nanyang Technological University; Jochen A. H. Dreyer and Markus Kraft from the University of Cambridge.

This research is supported by the National Research Foundation, Prime Minister’s Office, Singapore under its CREATE programme, through the Cambridge Centre for Advanced Research and Education in Singapore Ltd.