In the search to develop alternatives to petroleum-derived fuels, scientists are increasingly looking at ways to develop new fuels from waste, such as plastics and waste tires. Waste tires are a good potential source because they are discarded in large numbers but also because they contain a renewable portion in the form of natural rubber .
The pyrolysis of waste tires produces, among other compounds, a liquid called tire pyrolysis oil (TPO). With its high energy density and natural rubber component, it shows promise as a component in alternative fuels and complies with global guidelines on promoting renewable energy, such as the 2009/28/EC European directive. However, a lack of knowledge of TPO’s basic fuel characteristics and combustion properties is hampering its development as a fuel.
The Structure and Properties of TPO
TPO is a complex mixture of several hydrocarbon families ranging from a carbon number of five to 50. Other components include sulfur and nitrogen, and oxygen to a lesser extent. Some of the main compounds in TPO include light aromatics (benzene, toluene, xylene, and ethylbenzene), polyaromatics (naphthalene), aliphatics (dodecane and tridecane), and monoterpenes (limonene) .
The physical, chemical, and combustion properties of TPO are difficult to measure experimentally, but they are known to be governed by a range of factors such as tire composition and conditions during pyrolysis. One of the factors affecting combustion, in particular, is molecular distribution and constituent functional groups . Functional groups and structural characteristics, such as the degree and position of branching, all contribute to combustion reactivity. That’s why identifying and quantifying functional groups could help scientists predict TPO’s properties and combustion characteristics, which would help its development as an alternative fuel.
For example, one of the main disadvantages of TPO as a fuel is that it contains different sulfur-containing compounds. These react differently when treated with various desulfurization processes. Knowing the molecular characteristics of these sulfur-containing compounds would help scientists design the most appropriate desulfurization steps. It would also be vital in understanding emissions and impacts on the environment and human health.
Analyzing Structural Features using FT-ICR MS and NMR
However, the complexity of TPO has made determining its structural features challenging. In this study, the team from Colombia and Saudi Arabia analyzed the characteristics of two samples, a sulfur-containing TPO and a second TPO containing calcium oxide added during pyrolysis (TPO[CaO]). This meant they could compare and contrast structural characteristics .
The researchers used Bruker’s 9.4 T SolariX FT-ICR MS system equipped with an APPI source. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) allows identification of ions at the molecular level and also detects extremely low mass differences at the order of one electron . Both samples were diluted in pure toluene and directly injected into the APPI source. The FT-ICR mass spectra of the samples were acquired using the positive APPI ionization mode with a mass range of 154−1200 m/z.
The team also employed 1 H and 13 C nuclear magnetic resonance spectrometry (NMR) to quantify the type of hydrogen and carbon atoms present. This information combined with data from FT-ICR MS allowed the researchers to estimate the overall molecular structure of these complex mixtures.
Analysis with APPI FT-ICR MS showed that the main molecular classes in TPO and TPO[CaO] are pure hydrocarbons and hydrocarbons containing one sulfur atom (S1). Pure hydrocarbons appeared in larger quantities in TPO[CaO] (78.6%) than in TPO (74.9%), while the S1 class was found more in TPO (14.3%) than TPO[CaO] (13.9%). The presence of S1 compounds in these quantities suggests that the core skeletal structures of the molecules could be thiophenic or thiolic. Molecules containing two sulfur atoms (S2) were detected only in TPO (0.43%). FT-ICR MS results also indicated condensed aromatic structures were present in significant amounts in both samples.
According to 1 H NMR, around 80% of the hydrogen atoms in both fuels are present in methylene, methyl, naphthenic, and aromatic groups. 13C NMR revealed that carbon atoms in paraffinic groups, including both methylene and methyl groups, and protonated carbons in aromatic structures, together form more than half of the carbon atoms in TPO and TPO[CaO].
In summary, these results shed new light on the composition and structural characteristics of TPO. This knowledge should aid scientists to increase their understanding of the combustion properties of TPO and demonstrates TPO’s potential as a fuel as well as possible approaches to upgrading.
The team concludes that both samples could be used as fuels without having to be modified significantly. They suggest that separating TPO into distinct fractions by distillation would increase its potential for specific applications. Furthermore, distillation could be used to concentrate high molecular weight sulfur-containing compounds into the leftover heavy fraction, improving the characteristic of the other ones. In addition, the researchers suggest that oxidative desulfurization might be the best method for removing sulfur from TPO compared to other techniques, such as hydrodesulfurization, as it is better at selecting 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.
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