Clean & Secure Energy from Domestic Oil Shale & Oil Sands Resources
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Multiscale Thermal Processes

Investigators

Milind Deo, Tom Fletcher, Eric Eddings, Pankaj Tiwari

Project Purpose/Goals

  • Perform thermal gravimetric analysis (TGA) and bench scale experiments on cores of different sizes to create mechanistic pathways for the conversion of kerogen to oil.
  • Conduct pyrolysis of oil shale at high temperature and pressure, as would exist under in situ conditions, for a range of heating rates
  • Collect an analyze condensable pyrolysis products from demineralized kerogen
  • Develop kerogen pyrolysis models that integrate observations at various scales. One model combines heat and mass transport mechanisms along with reaction kinetics. The second model is based on the Chemical Percolation Devolatilization model (CPD).

Project Sponsor

Department of Energy, National Energy Technology Laboratory

Project Description

When oil shale is heated in an oxygen-free environment either on the surface or in situ (e.g. pyrolysis), oil is produced. Product composition and rate(s) of production depend on raw material composition, temperature, heating rate, pressure, and a host of other factors. Model accuracy in predicting product amounts and compositions depends on accurate kinetic data. Intrinsic kinetic data is measured in a thermal gravimetric analyzer (TGA) using oil shale that is finely ground.

The decomposition kinetics of complex materials such as oil shale are not easily described. It is also difficult to establish the proportions and compositions of the primary products of pyrolysis, e.g. oil, gas and coke, because the industrial processes are occurring at different scales. One must consider how the material is heated (heat transfer) and how the products come into production pathways on their way to production manifolds or wells (mass transfer). A concept called distribution of activation energies with conversion can be used to unify what is observed in the laboratory with what transpires on the geologic time-scale. In this project, researchers have determined the distribution of kinetic energies of the kerogen decomposition process using advanced isoconversional methods (Figure 1).

Figure 1: Distribution of activation energy on overall oil shale pyrolysis with TGA.

TGA combined with online mass spectrometry (TGA-MS) affords the opportunity to obtain compositional information while the decomposition is measured quantitatively. In a recently completed TGA-MS analysis of Green River oil shale from Utah, compounds of about 300 atomic mass units were targeted. Alkanes such as hexane and decane were detected at slightly lower temperatures than their equivalent carbon number aromatic compounds, but the differences were not significant. Higher heating rates generated more alkenes compared to the respective alkanes, and as the carbon number increased, this ratio decreased. Kinetics of the formation of naphtha group of compounds (C5-C12) were derived using the advanced isoconversion method. The activation energies, in the range of 41-206 kJ/mol, were lower than for the entire decomposition process. However, because the compound evolution signals as detected by mass spectrometry are noisier than the overall weight loss data, the uncertainties in these measurements were much greater in certain conversion ranges.

Multiscale pyrolysis of oil shale cores has also been performed. Results of experiments at two different scales at 500°C and 500 psi pressure are shown in Table 1. The significant increase in gas yield in the 2.5" core sample is likely due to secondary reactions that occur before the product is withdrawn. The gas chromatograms and single carbon number distribution show that the composition of oil produced at these two scales is not much different. The peak differences reveal that the 3/4" sample has relatively more C10-C14 compounds while the 2.5" sample exhibits a shift in distribution to C24-C26 compounds.

Table 1: Overall mass balance of pyrolysis products.
Results 2.5" core 3/4" core
Wt loss % 24.52% 18.69%
Oil yield % 7.96% 10.63%
Coke % 4.14% 1.03%
Gas% 16.56% 8.06%
Unreacted organic% 0.05% 0.43%