Singlet-diradical character in large PAHs triggers spontaneous-ignition of coal

doi:10.1016/j.combustflame.2019.10.035

Combustion and Flame

Volume 212, February 2020, Pages 279-281

https://doi.org/10.1016/j.combustflame.2019.10.035 Get rights and content

Abstract

Chemical phenomena dictating the spontaneous-ignition of coal remain highly speculative with some of the most intriguing questions appearing unaddressed. In this communication, we deploy a synergistic experimental-theoretical approach to elucidate the definite role of singlet-biradical PAHs in driving the low-temperature oxidation of coal. We establish that, five linearly-fused benzene rings (i.e., pentacene, as a model compound for the PAH content in coal, and an example of short-chain graphene nanoribbons) ignites 200 °C lower than the corresponding temperature for naphthalene.

Introduction

Despite decades of fundamental and applied research, the chemical phenomena underpinning the occurrence of spontaneous-ignition and accidental fires of coal (and other solid hydrocarbon fuels) remain indefinable. The most widely discussed hypotheses for spontaneous fires of coal encompass activation of ground-state triplet oxygen into its highly reactive singlet state by minute loads of transition metal oxides in coal [1,2], oxidation of pyrite (FeS₂) in low-ranked coals [3], and formation of reactive oxygen species (peroxides or hydroperoxide) from the inherent water content [4]. Overall, the low-temperature oxidation of coal is dictated by a complex array of chemical reactions, mass transport phenomena, morphological and geological factors, and some external physical conditions [5]. In addition to the factors above, the frequent occurrence of spontaneous ignition in hydrocarbons may entail a propensity for atmospheric oxygen molecules to attack highly reactive sites in certain structural constituents in these fuels. Testing this hypothesis via a synergistic theoretical-experimental approach constitutes the core of this contribution.

Coal is chiefly composed of networks of polycyclic aromatic hydrocarbons (PAHs) stacked into three-dimensional matrices. The content of conjugated fused rings correlates with the degree of coal maturity and thus follows the order of anthracite > bituminous > sub-bituminous > lignite [6]. Herein, we argue that, spontaneous-ignition of coal and its low-temperature oxidation partly stems from the π-conjugated systems of its PAHs structural entities. Owing to localised π electrons, spin-singlet PAHs exhibit diradical characters with long linear acenic PAHs [X, 1] (X ≫ 7) and large rectangular PAHs, PAH[X, Y] (X, Y ≫ 7, where X and Y signify the number of fused rings along the zigzag and armchair edges, respectively) displaying multiradical centres [7]. PAHs, with a closed-shell electronic structure, incur diradical character (i.e., a y_o value) of 0.0 (i.e., benzene PAH[1,1]) while pure diradical PAHs, such as PAH[7,7] nanoflakes, entail a y_o of 1.0 [7].

We study herein the initial combustion stages of pentacene, PAH[5,1] as a model species prototypical of the PAH constituents in coal. The onset of the combustion process and the pertinent energy profile are contrasted with the analogous values of naphthalene PAH[2,1]. The latter signifies a PAH with a near-free diradical character at y_o of 0.05 (i.e., a nearly closed-shell singlet species) while pentacene holds a modest y_o value of 0.42 [7]. Being a long acene, the radical character in pentacene mainly appears at the centre of its zigzag chain, away from the armchair edges [7,8].

Section snippets

Methodology

We investigated this notion in a vertically entrained reactor (Fig. 1). The tubular flow reactor comprises an ultra-pure quartz tube (ultra-pure quartz, >99.99%) with i.d. of 10 mm, mounted vertically along the centreline of the three-zone furnace. We ground the samples (i.e., pentacene and naphthalene) into particle size < 250 µm, before feeding approximately 3 mg into the reactor, operating at constant temperatures under synthetic air atmosphere (200 mL/min, STP). The use of small sample

Results and discussions

The experimental result shows that the five linearly-fused benzene rings (pentacene) ignite at a relatively low temperature (ca. 300 °C) as compared to binary fused benzene rings (naphthalene) that ignites around 500 °C (reported autoignition temperature is 526 °C [12]). As discussed earlier, the existence of diradical character on larger PAH enables their higher reactivity with O₂. Such a phenomenon does not transpire under pyrolytic condition [12], in the absence of oxygen in our system, the

Conclusions

We demonstrated herein that radical characters in PHAs facilitate addition of oxygen molecules. We argue that the π-conjugated systems of the abundant PAHs structural entities in coal can contribute substantially to the common occurrence of accidental coal fires.

Declaration of Competing Interest

Authors have no competing interests to declare.

Acknowledgment

This study has been supported by grants of computing time from the National Computational Infrastructure (NCI) and from the Pawsey Supercomputing Centre, Australia as well as funds from the Australian Research Council (ARC).

References (12)

J. Al-Nu'airat et al.
Effect of feonanoparticles on combustion of coal surrogate (Anisole): enhanced ignition and formation of persistent free radicals
Proc. Combust. Inst.
(2019)
WangH. et al.
Coal oxidation at low temperatures: oxygen consumption, oxidation products, reaction mechanism and kinetic modelling
Prog. Energy Combust. Sci
(2003)
C. Achten et al.
Native polycyclic aromatic hydrocarbons (PAH) in coals – a hardly recognized source of environmental contamination
Sci. Total. Environ.
(2009)
H. Nagai et al.
Signature of multiradical character in second hyperpolarizabilities of rectangular graphene nanoflakes
Chem. Phys. Lett.
(2010)
J. Al-Nu'airat et al.
Role of singlet oxygen in combustion initiation of aromatic fuels
Energy. Fuels
(2018)
DengJ. et al.
Effects of pyrite on the spontaneous combustion of coal
Int. J. Coal. Sci. Technol.
(2015)

There are more references available in the full text version of this article.

Cited by (10)

Contribution of atmospheric deposition to halogenated polycyclic aromatic hydrocarbons in surface sediments: A validation study
2022, Science of the Total Environment
Citation Excerpt :
It was also suspected that atmospheric deposition is another possible source (Sun et al., 2011). Cl/Br-PAHs have similar physical and chemical properties with PCDDs, PCDFs, PCBs and PAHs (Riaz et al., 2019; Zhao et al., 2019; Altarawneh et al., 2019; Altarawneh et al., 2020). It has been proven that atmospheric deposition is a definite source of polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), polychlorinated biphenyls (PCBs), and PAHs (Bidleman, 1988; Tsapakis et al., 2006; Moon et al., 2006).
Surface sediments are both sinks and sources of chlorinated and brominated polycyclic aromatic hydrocarbons (Cl/Br-PAHs) in the environment. It is important to study the source of Cl/Br-PAHs in the surface sediment for controlling the ecological risk of Cl/Br-PAHs. Clues from the previous research suggested that atmospheric deposition may be one of the main sources of Cl/Br-PAHs in sediment. However, due to the lack of matched sediment and atmospheric Cl/Br-PAHs data, the contribution of atmospheric deposition to Cl/Br-PAHs in sediment has not been confirmed. This study investigated the characteristics of 37 Cl/Br-PAHs and validated the contribution of atmospheric sedimentation to Cl/Br-PAHs in sediment by a case study in the surface sediments of the Chaobai River, China. To the best of our knowledge, four Cl-PAHs and eleven Br-PAHs were found in the sediments for the first time. The total concentrations of 18 Cl-PAH species were 76–2301 pg/g, while those of Br-PAHs were 6–238 pg/g. The toxic equivalent quantities (TEQ) of the Cl-PAHs in surface sediments in the water conservation area and in the urban comparison area were 0.73 pg TEQ/g and 2.21 pg TEQ/g, respectively. The TEQ of the Br-PAHs in surface sediments in the water conservation area and in the urban comparison area were 2.85 × 10⁻² pg TEQ/g and 6.6 × 10⁻² pg TEQ/g, respectively. Based on the characteristics comparison and correlation analysis of Cl/Br-PAHs in both sediment and ambient air, it was initially confirmed the contribution of atmospheric deposition to Cl-PAHs in sediments. However, there was no conclusion of Br-PAHs in sediment similar to Cl-PAHs in sediment. It was inferred that the sources of Br-PAHs in sediment were different from Cl-PAHs in sediment.
On the formation chemistry of brominated polycyclic aromatic hydrocarbons (BrPAHs)
2022, Chemosphere
Citation Excerpt :
Mounting experimental and DFT evidences have confirmed the potent role of transition metal oxides in mediating surface-based transformation of substituted phenols/benzenes and PAHs into their respective EPFRs (Jia et al., 2018; Ahmed et al., 2020; Liu et al., 2020). Very recently, we have established via a combined experimental-DFT framework, that the π-conjugated system in large PAHs facilitates oxygen addition leading to surprisingly low onset combustion temperature (Altarawneh et al., 2020). The delocalized electronic system in large PAHs render these entities to display diradical character (Nagai et al., 2010).
Brominated polycyclic aromatic hydrocarbons (BrPAHs) have been consistently detected in various environmental matrices, and measured at alarming rates in stack emissions. However, formation mechanisms and bromination patterns of BrPAHs remain unclear. This contribution constructs detailed mechanistic pathways for the synthesis of selected BrPAHs (namely bromine-bearing naphthalene, acenaphthylene, anthracene, and phenanthrene). Mapped-out pathways follow the Bittner-Howard's route in the hydrogen abstraction acetylene addition (HACA) mechanism, in which a second C₂HBr molecule is added to the first one. Constructed kinetic model portrays temperature-dependent profiles of major and minor species. Direct loss of an H atom from the acetylenic fragment appears to be more important at elevated temperatures, when compared with further addition of C₂HBr cuts or ring-cyclization reactions. The occurrence of closed-shell Diels-Alder pathway should be inhibited owing to sizable enthalpic barriers. Fukui Indices for electrophilic substitutions (f⁻¹) establish bromination’ s pattern of selected BrPAHs. The diradical character of BrPAHs coupled with electron-deficient C(Br) sites, render BrPAHs as potent precursors for the formation of environmentally persistent free radicals (EPFRs). Findings reported herein shall be useful in comprehending the formation chemistry of BrPAHs, a less-investigated category of toxicants in thermal systems.
Soot inception: Carbonaceous nanoparticle formation in flames: Soot inception
2022, Progress in Energy and Combustion Science
The route by which gas-phase molecules in hydrocarbon flames form condensed-phase carbonaceous nanoparticles (incipient soot) is reviewed. These products of incomplete combustion are introduced as particulates and materials revealing both their useful applications and unwanted impacts as pollutants. Significant advances in experimental techniques in the last decade have allowed the gas phase precursors and the transformation from molecules to nanoparticles to be directly observed. These measurements combined with computational techniques allow for various mechanisms known to date to be compared and explored. Questions remain surrounding the various mechanisms that lead to nanoparticle formation. Mechanisms combining physical and chemical routes, so-called physically stabilised soot inception, are highlighted as a possible “middle way”.
Dissect the capacity of low-temperature oxidation of coal with different metamorphic degrees
2021, Fuel
Citation Excerpt :
Both vacuum drying and pyrolysis of coal will increase the concentration of radicals, and it will also increase after exposure to oxygen [30–32]. It has been shown that the radical properties of polycyclic aromatic hydrocarbon are related to the addition reaction of oxygen molecules, which may trigger coal spontaneous combustion [33]. Although the effect of radicals on the coal low-temperature oxidation has been demonstrated [12,34,35], it has long been thought that the degree of oxidation of coal depends on the absolute concentration of radicals in coal.
Radical chemistry is a major factor affecting the low-temperature oxidation of coal, which plays a critical role in the coal oxygen complex reaction. However, the characteristics of radical for judging its ability to oxidize are still poorly understood. The electron paramagnetic resonance (EPR) spectroscopy has emerged as a powerful approach in studying the behavior of radicals. So the EPR spectra of coal low-temperature oxidation were investigated by in-situ EPR spectroscopy. Analysis of the initial temperature of CO evolution and its increment and crossing-point temperatures (CPTs) of coal reveal that the oxidizability of low-rank coal is more prominent than others. The EPR linear profile of low-rank coal has a well temperature dependence and a large g factor, an indicator of oxidizability. However, it is not consistent with the increasing trend of the absolute content of radical concentration. This study provides a novel approach to quantify the oxidizability of coal using the relative growth rate R of radials, and the results are consistent with the coal spontaneous combustion tendency. The main objective of the current study is to obtain a comprehensive understanding of low-temperature oxidation of coal with different ranks.
Analysis of the Current Status and Hot Technologies of Coal Spontaneous Combustion Warning
2023, Processes
Raman Spectroscopy of Nascent Soot Oxidation: Structural Analysis During Heating
2022, Frontiers in Energy Research

View all citing articles on Scopus

View full text

Brief CommunicationsSinglet-diradical character in large PAHs triggers spontaneous-ignition of coal

Abstract

Introduction

Section snippets

Methodology

Results and discussions

Conclusions

Declaration of Competing Interest

Acknowledgment

Proc. Combust. Inst.

Prog. Energy Combust. Sci

Sci. Total. Environ.

Chem. Phys. Lett.

Role of singlet oxygen in combustion initiation of aromatic fuels

Energy. Fuels

Effects of pyrite on the spontaneous combustion of coal

Int. J. Coal. Sci. Technol.

Brief Communications
Singlet-diradical character in large PAHs triggers spontaneous-ignition of coal