Characterization of Macromolecular Structure Elements from a Green River Oil Shale, II. Characterization of Pyrolysis Products by ¹³C NMR, GC/MS, and FTIR

Abstract

This paper is Part II of a study of the chemical structural components of the organic matter of oil shale in the Green River formation. Three sections of a well-characterized oil shale core from the Utah Green River formation were demineralized, and the resulting kerogen was pyrolyzed at 10°C/min in nitrogen at atmospheric pressure at temperatures up to 525°C. The pyrolysis products (light gas, tar, and char) were analyzed using 13 C NMR, GC/MS, and (FTIR). Pyrolysis yields of 80% (daf basis) were achieved at these conditions, with 60% daf tar yield at the highest temperature. The solid-state NMR results indicate that the aromaticity of the kerogen char increased from 20% (at RT) to 80% during pyrolysis, with a corresponding decrease in the average aliphatic carbon chain length from 12 to less than 1. The average number of aromatic carbons per cluster increased from 12 to 20 in a narrow temperature window between 425 and 525°C, with an increase in the number of attachments per cluster from 6 to 8 in that same temperature window. Liquid-state NMR results of the condensed tars showed predominance of n-alkyl chains, with similar carbon aromaticities at each temperature. The alkyl chains were also observed in the GC/MS data. The light gases determined by FTIR were primarily CH 4 , CO, and CO 2 . The combination of gas, tar, and char yields and chemical structure analyses are valuable for modeling of oil shale processes based on chemical structure rather than based on empiricism.

Modeling Light Gas and Tar Yields from Pyrolysis of Green River Oil Shale Demineralized Kerogen Using the Chemical Percolation Devolatilization Model

“…These predictions did not compare well to the thermogravimetric analysis (TGA) data 3 or tar and total yield data obtained in a laboratory kerogen retort by Hillier et al 25 and Fletcher et al 18 The chemical structure of the kerogen in oil shale is much different from the chemical structure in coal; therefore, this result was not surprising. Rate coefficients were determined for a first-order distributed activation energy model (DAEM) of mass release using TGA pyrolysis data on the parent oil shales of the demineralized samples referenced in Table 1.…”

“…The molecular weight of the light gas (MW lg ) was set to an average of 20 amu, and the molecular weight of the heavy gas (MW hg ) was then calculated from the combined gas molecular weight (calculated from the NMR parameters by the CPD model 10 (1) where y lg = g 2 /(g 1 + g 2 ) and y hg = g 1 /(g 1 + g 2 ), and y lg and y hg were set to 0.2 and 0.8, respectively. The bridge reaction rate coefficients were then adjusted slightly to obtain best fit with the kerogen retort data of Fletcher et al 18 The new rate coefficients are shown in Table 3.…”

“…Miknis et al then postulated that the oil yield could be correlated with the aliphatic content of the kerogen. More detailed chemical structure characterization with advanced NMR techniques were obtained by Solum et al 16 and Fletcher et al, 18 which have been used to develop the model reported here for three samples of a Utah Green River oil shale. This model is only applicable to the Green River kerogen samples referenced herein.…”

“…Lattice structure parameters reported by Solum et al 16 from the solid-state analysis of kerogen from three demineralized Green River oil shale samples are shown in Table 1. The nomenclature of GR1.9, etc., is the same as used by Solum et al 16 and Fletcher et al, 18 meaning the sample from the ninth demineralization step of the parent GR1 sample. As noted in Table 1, the value of p 0 was arbitrarily set to 0.5 in the absence of a true measurement.…”

Dynamic imaging of oil shale pyrolysis using synchrotron X‐ray microtomography

“…The largest known deposit is the Eocene Green River Formation in Western Colorado, Eastern Utah, and Southern Wyoming [Dyni, 2003;Johnson et al, 2009]. Kerogen, which constitutes most of oil shale's organic matter, is a highly cross-linked, macromolecular material distributed in a heterogeneous inorganic matrix [Fletcher et al, 2014;Solum et al, 2013]. Pyrolysis of oil shale breaks down the complex kerogen network structure to produce shale oil and natural gas [Hillier et al, 2013;Sun et al, 2014].…”