Adding ozone reduces the heat needed to burn biofuels


Low-temperature chemistry (LTC) is a special set of reactions that take place at what chemists consider to be relatively low temperatures: around 400 to 700 Kelvin (260 to 800 degrees F). The study of LTC is important for the basic science of chemistry. It is also useful for understanding how internal combustion engines burn fuel. In these engines, fuel ignition is surprisingly complex, taking place in two distinct stages. This two-step process affects how engines produce harmful emissions. One way to speed up an LTC reaction is to inject ozone into a system. This study investigated the use of ozone injection with methyl hexanoate (MHX). Researchers use this substance to study the chemistry of the main components of biodiesel. The study found new ways in which combustion can operate at very low temperatures in the LTC range.

The impact

This research discovered a new combustion process for MHX. It occurs at relatively low temperatures (around 440 Kelvin or 330 degrees F) when small amounts of ozone are added to a mixture of MHX and oxygen. This discovery may help reduce soot and nitrogen oxide emissions from internal combustion engines. The results are relevant for both biodiesel and conventional fuel engines. The results also offer new insights into the chemistry of oxidation. They will also contribute to research on atmospheric chemistry.


Researchers investigated the reactivity of methyl hexanoate/oxygen mixtures over a range of temperatures (460-940 Kelvin) in the presence of small amounts of ozone in an externally heated, atmospheric pressure jet stirred reactor. The researchers identified a previously undetected oxidation regime at temperatures below the typical low-temperature regime (Extreme Low Temperature Combustion, or ELTC). Researchers identified key intermediate species using a mass spectrometer and ultraviolet radiation generated by the chemical dynamics beam line of the Advanced light source, a Department of Energy (DOE) user facility. Experimental data indicate that the chemistry in the ELTC regime is initiated by the thermal decomposition of ozone, followed by reactions of methyl hexanoate by the oxygen atoms of ozone.

There has been much research on the oxidation of unsaturated hydrocarbons by ozone. However, research on interactions with molecules without a C=C double bond has been sparse. The observation in this research of a new ELTC regime responds to the societal need for clean combustion, as the addition of ozone allows for sustained oxidation close to 500 Kelvin, thus eliminating the emission of toxic by-products such as nitrogen oxide and soot. In addition, this work provides researchers in atmospheric chemistry with information on the formation pathways of highly oxygenated species considered to form secondary organic aerosols, another pollutant. Since ozone is a long-lived air-plasma intermediate, this research also improves the understanding of plasma-initiated oxidation processes.


This work was supported by the DOE Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences and Biosciences Division, under the Gas-Phase Chemical Physics Program.


Kevin A. Perras