Research articleTransformation mechanisms of organic S/N/O compounds during microwave pyrolysis of oil shale: A comparative research with conventional pyrolysis
Graphical abstract
Introduction
Oil shale, essentially a geologically immature form of petroleum, is predicted to be a significant oil alternative [[1], [2], [3], [4], [5]]. For many decades, conventional pyrolysis of oil shale (CON), aboveground retorting by heat conduction, has been widely applied in shale oil production [[6], [7], [8]]. Nonetheless, CON has been confronted with an inherent disadvantage, a substantial amount of organic S/N/O compounds in resulted shale oil. Besides causing fuel instability, viscosity increase, and gum formation and discoloration during shale oil transportation and storage [9], these heteroatoms lead to SOX and NOX emissions during the combustion, contributing to the photochemical smog and acid rain pollutions [10,11]. According to reported structural units from oil shale models, organic sulfur compounds mainly exist in forms of mercaptan, thiophene, thioether and sulfoxide, organic nitrogen compounds in pyrrolic N, pyridinic N, amine N and nitrogen oxide, and organic oxygen compounds in carboxyl, carbonyl, ether and hydroxyl [[12], [13], [14], [15], [16]].
Microwave heating emerged as an innovative technique in various fields [[17], [18], [19], [20], [21], [22]]. It has been applied in oil shale pyrolysis, exhibiting numerous advantages over CON. Microwave pyrolysis of oil shale (MW) dramatically shortens reaction times bringing about substantial energy efficiency boosting [23,24]. This should be understood in terms of shale oil produced per unit of electricity consumption. Compared with conventional electric heating, microwave heating has a faster heating rate [25] and leads to no need for feedstock grinding [24], thus the consumed electricity was reduced. Instantaneous starting and stopping of MW equipment also leads to enhanced ease of operation and maintenance [26]. In particular, the selective and volumetric heating improves the quality of shale oil, such as reducing the contents of S/N/O and increasing the proportion of light hydrocarbons [[27], [28], [29]].
Hence, considerable researches on MW have been performed [[30], [31], [32], [33]]. Although these researches provide useful information for MW, studies investigating transformation mechanisms of S/N/O during MW have never been reported. An essential pathway to study the S/N/O transformation mechanisms is identifying these heteroatom compounds in shale oils at a molecular level. In this regard, several studies about compositions and structures of shale oil have been conducted using gas chromatography–mass spectrometry (GC–MS) [34,35]. Nevertheless, GC–MS cannot analyze compounds of molecular weight more than 300 Da accurately [35]. Based on this, Wang et al. [35,36] proposed that shale oil was divided into two parts: <300 °C fraction characterized by GC–MS, and >300 °C fraction by Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance (NMR). However, they only provided information of functional groups and still failed to present a detailed characterization of S/N/O compounds with high molecular weight. In this regard, Electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS) analysis provides a viable route to characterization of organic S/N/O compounds. Relevant research has been conducted to investigate the evolution mechanism of bio-oil during pyrolysis using ESI FT-ICR MS [37].
In this study, a comprehensive comparison of molecular compositions of organic S/N/O compounds in shale oils derived from MW and CON was firstly conducted by ESI FT-ICR MS analysis. An unprecedented insight was gained into transformation mechanisms of organic S/N/O compounds during MW.
Section snippets
Material
Oil shale chosen for this study was obtained from the Tarfaya deposit, Morocco. It was crushed and sieved into particles (≤3 mm). The bulk properties are given in Table 1, and each analysis was conducted in triplicate to obtain mean values.
Experimental apparatus and procedure
Fig. 1a displays the newly-devised laboratory apparatus for MW. It consists of four systems for heating, reacting, temperature controlling and measuring, and condensing, respectively. The heating system converts electrical energy to microwave energy with
Pyrolysis time-temperature curves
Heating-up characteristics provide vital information for reaction mechanisms, but direct and real-time monitoring under MW has rarely been published [43]. Herein, time-temperature curves of oil shale during MW and CON are depicted in Fig. 2. Notably, due to a fixed program control, the curve under CON can act as a reference standard. To heat up oil shale to 520 °C, it took 50 min by CON whereas only about 17 min by MW. Time-temperature curves of MW-alt and MW-con almost coincided before 520 °C.
Conclusions
In this study, we have gained in-depth knowledge in transformation mechanisms of organic S/N/O compounds during MW. Results reveal MW could effectively reduce S/N/O contents in shale oil by up to 76.13, 13.22 and 27.86%, respectively. At temperatures of 300–520 °C, major differences of S/N/O compounds reactions during MW and CON are pronounced. Characteristic reactions of S/N/O compounds mainly involve cracking and polymerization which happen simultaneously. However, the dominant reaction is
CRediT authorship contribution statement
Lu He: Conceptualization, Methodology, Investigation, Software, Writing - Original Draft.
Yue Ma: Writing - Review & Editing.
Changtao Yue: Validation, Writing - Review & Editing.
Jianxun Wu: Visualization, Data Curation, Investigation.
Shuyuan Li: Resources, Supervision.
Qingqiang Wang: Validation.
Bin Wang: Investigation.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors thank the State Key Laboratory of Heavy Oil Processing, China University of Petroleum, for providing the 9.4 T ESI FT-ICR MS. The work is supported by the Research Fund of Efficient and Clean Utilization of Semi-coke Replacing Raw Coal (grant number 2015KTZDSF01-04).
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