Article pubs.acs.org/EF
Molecular Characterization and Comparison of Shale Oils Generated by Different Pyrolysis Methods Jang Mi Jin,† Sunghwan Kim,*,†,‡ and Justin E. Birdwell*,§ †
Department of Chemistry, Kyungpook National University, Daegu, 702-701 Republic of Korea Green-Nano Materials Research Center, Daegu, 702-701 Republic of Korea § U.S. Geological Survey, Denver Federal Center, Box 25046 MS 977, Denver, Colorado 80225, United States ‡
S Supporting Information *
ABSTRACT: Shale oils generated using different laboratory pyrolysis methods have been studied using standard oil characterization methods as well as Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) with electrospray ionization (ESI) and atmospheric photoionization (APPI) to assess differences in molecular composition. The pyrolysis oils were generated from samples of the Mahogany zone oil shale of the Eocene Green River Formation collected from outcrops in the Piceance Basin, Colorado, using three pyrolysis systems under conditions relevant to surface and in situ retorting approaches. Significant variations were observed in the shale oils, particularly the degree of conjugation of the constituent molecules and the distribution of nitrogen-containing compound classes. Comparison of FT-ICR MS results to other oil characteristics, such as specific gravity; saturate, aromatic, resin, asphaltene (SARA) distribution; and carbon number distribution determined by gas chromatography, indicated correspondence between higher average double bond equivalence (DBE) values and increasing asphaltene content. The results show that, based on the shale oil DBE distributions, highly conjugated species are enriched in samples produced under low pressure, high temperature conditions, and under high pressure, moderate temperature conditions in the presence of water. We also report, for the first time in any petroleum-like substance, the presence of N4 class compounds based on FT-ICR MS data. Using double bond equivalence and carbon number distributions, structures for the N4 class and other nitrogen-containing compounds are proposed.
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temperatures (350−400 °C), and under the influence of high lithostatic pressures from overburden and hydrostatic pressures from generated gas/oil vapor.6−8 The liquid oil yield from in situ retorting is lower, but the oil product is typically of higher quality than that of surface retorts.9,10 Other advantages of in situ retorting are reduced costs associated with mining and less surface disturbance. Although shale oils have been extensively characterized using a wide range of analytical techniques, including Fourier transform ion cyclotron mass spectrometry (FT-ICR MS),11 there has not been a study applying ultrahigh resolution MS to examine how the molecular composition of shale oil differs when generated under different pyrolysis conditions from the same source rock. In this study, we have examined a series of shale oils using FT-ICR MS coupled with electrospray ionization (ESI) and atmospheric photoionization (APPI) to assess how different retorting conditions affect shale oil at the molecular level. The results are compared to more commonly used oil characterization techniques in order to emphasize the utility of FT-ICR MS for improved understanding of highly complex mixtures, like shale oil.
INTRODUCTION Despite the growing interest in using energy sources other than liquid hydrocarbons (e.g., electricity, natural gas, hydrogen) in the transportation sector, it will be decades before widespread, substantive change to our energy and transportation infrastructure can be implemented. In the meantime, the increasing demand for liquid transportation fuels has led to significant interest in the development of alternative hydrocarbon sources, like oil shale, in several countries. Oil shale, generally defined as sedimentary rock rich in kerogen, is already an important energy source in a few countries and could be used to supplement petroleum supplies in many places throughout the world.1 In the western United States, large oil shale deposits exist in Colorado, Utah, and Wyoming2−4 and have been considered for utilization during periods of high crude oil prices. Generation of liquid hydrocarbons from oil shale requires the application of heat to convert solid kerogen present in the rock matrix into a petroleum-like liquid product. A great deal of research has been conducted on oil shale pyrolysis (or retorting) over the years and much of this work has been focused on how different conditions affect the yield and quality of shale oil. The most dramatic differences are typically seen when comparing shale oil generated by surface and in situ retorting approaches. Surface retorting of mined and crushed oil shale generally involves rapid heating to high temperatures (≥500 °C) at near atmospheric pressure. This approach produces a high shale oil yield, but the product generally requires significant upgrading before it can be refined into transportation fuels.5 In situ pyrolysis methods involve heating the rock in place more slowly, to lower maximum © 2011 American Chemical Society
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EXPERIMENTAL SECTION
Source Rock. Oil shale samples were collected from outcrop at the Anvil Points Mine (APM) near Rifle, Colorado and are representative of lacustrine oil shales from the Eocene Green River Formation Received: October 6, 2011 Revised: December 16, 2011 Published: December 26, 2011 1054
dx.doi.org/10.1021/ef201517a | Energy Fuels 2012, 26, 1054−1062
Energy & Fuels
Article
Mahogany zone in the Piceance Basin of northwestern Colorado. The total organic carbon (TOC) content of the APM oil shale is 19.3 wt %, as determined by the Leco method (Weatherford International, Shenandoah, Texas). On the basis of Rock Eval and Leco TOC analyses of the whole rock and elemental analysis of the isolated insoluble organic matter, the sample contains predominantly type I kerogen.12 Prior to pyrolysis, oil shale was crushed and sieved to obtain a uniform material with a particle size range (0.5−2.38 mm) consistent with the American Society for Testing and Materials (ASTM) Fischer Assay procedure.13 Pyrolysis Methods. Oil samples were generated using three pyrolysis systems. Modified Fischer Assay13 was used to obtain oils similar to those produced by surface retorts and is the standard approach for determining the oil yield from oil shale.1 Briefly, the Fischer Assay procedure involves heating a 100 g source rock sample to 500 °C at a rate of 12 °C/min (∼40 min heat-up time) and holding the sample at that temperature for 40 min or until oil generation is complete. Hydrous pyrolysis is an approach used to generate pyrolysates similar to natural petroleum, and a typical approach involves heating a 200 g source rock sample with ∼400 mL of deionized water in a 1-L batch reactor to 360 °C at a rate of ∼6 °C/min and holding for 72 h, although other combinations of time and temperature can be used depending on the nature and goals of the particular experiment.14,15 Batch experiments to mimic in situ retorting were conducted using a system described by Birdwell et al.12 The in situ simulator (ISS) consists of a reactor and gas/oil vapor collection vessel connected by stainless steel tubing or flexhose. Experiments involve heating 100 g samples of oil shale to 360 °C at a heating rate of ∼3 °C/min and holding at that temperature for a predetermined time. In this study, oil shale samples were held for 6, 24, 72, 120, and 288 h. Before the end of the heating period, the collection vessel is prepared by evacuating to N1 > N2, whereas the oil generated after 288 h at 360 °C showed a different trend, with N1 > N3 > N2. Both the standard and low temperature Fischer Assay oils had roughly similar (+) ESI nitrogen class distributions that were dominated by the N1 species, although the sample generated at the lower heating rate (1 °C/min) showed a higher relative abundance of N2 and N4 class compounds than the other Fischer Assay retort oils. The hydrous pyrolysis oil was unique in that the N2 class was dominant (∼30% relative abundance). The APPI distributions for the shale oil nitrogen classes (Figure 3 b) showed N1 class compounds to be most abundant (35−45%) in all shale oil samples examined, accompanied by significant amounts of N2 (10−20%) and small relative abundances (
