Investigating Fuels Produced from Waste Tires

Pyrolysis of recycled rubber tires produces a liquid called tire pyrolysis oil (TPO) which has potential as an alternative to replace petroleum-based fuels. However, scientists need to know more about TPO’s properties before it can be used as an energy source.

old tires

Image Credit: Yeongsik Im/

TPO is a complex mixture of multiple hydrocarbon families and contains sulfur, nitrogen, and oxygen as well. Its physical, chemical, and combustion properties are challenging to measure experimentally, so researchers are looking to predict them based on structural information. Molecular distribution and constituent functional groups can affect combustion properties, in particular, as can the degree and position of branching in molecules. Scientists hope that identifying and quantifying functional groups could help them predict TPO’s combustion characteristics.

In this analysis, the characteristics of two TPO samples are studied. One is a sulfur-containing TPO, and the other contains calcium oxide added during pyrolysis (TPO[CaO]). The researchers used Fourier transform ion cyclotron resonance mass spectrometry, specifically Bruker’s 9.4 T SolariX FT-ICR MS system equipped with an APPI source, to identify ions at the molecular level and detect extremely low mass differences at the order of one electron. They also employed 1H and 13C nuclear magnetic resonance spectrometry (NMR) to quantify the type of hydrogen and carbon atoms present. Combining data from both techniques allowed the researchers to predict the overall molecular structure.

The APPI FT-ICR MS results showed that pure hydrocarbons appeared in larger quantities in TPO[CaO] (78.6%) than in TPO (74.9%). Molecules containing one sulfur atom (S1) were found slightly more in TPO (14.3%) than TPO[CaO] (13.9%), suggesting that the core skeletal structures of the molecules might be thiophenic or thiolic. Only TPO contained any molecules containing two sulfur atoms (0.43%). FT-ICR MS analysis also showed that condensed aromatic structures appeared in significant amounts in both samples.

The NMR studies revealed that around 80% of the hydrogen atoms in both fuels are present in methylene, methyl, naphthenic, and aromatic groups. Carbon atoms in paraffinic groups, including both methylene and methyl groups, and protonated carbons in aromatic structures, accounted for more than half of all carbon atoms in TPO and TPO[CaO].

The paper provides fresh insight into the composition and structural characteristics of TPO, highlighting its promise as a fuel and proposing various ways to enhance its potential. In particular, these results will help researchers improve their understanding of the combustion properties of TPO.

This study indicated that TPO could be used as a fuel without substantial modifications. Distillation could be used to separate it into discrete fractions and increase its potential for specific applications. What is more, distillation would concentrate high molecular weight sulfur-containing compounds into the leftover fraction, which enhances the characteristics of the other fractions. The results also suggest that oxidative desulfurization would work better than other sulfur-removing techniques, such as hydrodesulfurization, as it is highly selective for aromatic sulfur compounds.

NMR is the single most important key technology for investigating molecular properties in real time. With Bruker’s high-resolution NMR spectrometer portfolio, researchers around the world study polymer branching, cross-linking positions and functional end groups. These analyses provide the necessary insight to transform waste into valuable products as shown here. Furthermore, it is unique to Bruker’s solution offering not to only focus on comprehensive research instrumentation. The Minispec Time-Domain (TD) NMR benchtop system provides hydrogen content analysis of various fuels by the push of a button. This key performance indicator drives combustion properties of fuels and so directly influences the exhaust profiles.


This information has been sourced, reviewed and adapted from materials provided by Bruker BioSpin - NMR, EPR and Imaging.

For more information on this source, please visit Bruker BioSpin - NMR, EPR and Imaging.


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