(322b) Wave Liquefaction™ Processing of Carbon Materials for the Production of Value-Added Chemicals and Feedstocks: Spectroscopic Diagnostics and Material Characterization

This paper presents optical spectroscopic diagnostics as used in an engineering pilot implementation of H Quest Vanguard’s microwave plasma process applied to coal, and material characterization results from application of the process to H₂ generation from natural gas. Plasma diagnostics for process control will be illustrated. Chemical analyses are presented of coal conversion products. Recent results yielding nanographene as a premium carbon by microwave-plasma processing of methane will be shown.

Microwave-driven plasma-mediated reaction engineering represents process intensification for energy efficient and economical hydrocarbon conversion processes. Examples include coal-to-liquids and natural gas conversion to hydrogen and premium carbon products. Optical emission spectroscopy and analysis provides species identification and determination of temperatures in different energy modes in the non-equilibrium plasma.

Microwave energy, variable between 1 - 4 kW is coupled into a continuous flow reactor. Wave Liquefactionâ„¢ process applied to coal implements in-situ product upgrading via hydrogenation and/or methylation reactions enabled by activating hydrogen or methane present in the process gas. Pulverized coal is fed by a particle seeder. Gas ratios are optimized for the particular hydrocarbon feedstock conversion. An alternative application of the microwave plasma processing is methane decomposition to H₂ and premium carbon products. Reactor residence times are sub-second and performed at atmospheric pressure. Filters, traps and cyclone stages downstream of the reactor perform gas/liquid/solid separations.

2.0 Optical Emission Spectroscopy

Optical diagnostics are central to reaction characterization and hold particular value for species identification and temperature determination [1]. Observed intensity ratios or spectra band shapes can yield temperature by Boltzman analysis using spectral constants. Moreover, optical emission serves to identify reactive species and intermediates, albeit indirectly inferred by the observation of their electronically excited counterparts, e.g. CH* and C₂* radicals. For example, the presence of key atomic or diatomic radicals can support postulates of electron impact dissociation, and radical mediated bond insertion or radical capping reactions and provide mechanistic insights from the temperatures associated with the different degrees of freedom – electronic, vibrational, such as from the C₂* (d³_g – a³_u) Swan band emission. In this study spectroscopic data collection was performed using a fiber-optically coupled Ocean Optics HR2000 spectrometer fitted with a UV-NIR grating for a spectral range of 900 nm. All optical emission spectra were captured with equipment spectrally calibrated using a Hg/Ar lamp and corrected for instrument response function using a NIST-traceable tungsten lamp as spectral calibration standard. Temperatures of C₂* are determined using custom spectral band-fitting algorithms, taking into account the electronic baseline and underlying blackbody radiation. Spectral fits are performed at 1 nm spectral resolution. Carbon particle temperatures are determined by fitting Planck’s radiation law with black body conditions. Ar* emission lines are identified by reference to the NIST spectroscopic database.

3.0 Gas/Liquid/Carbon Analyses

Microwave (MW) activated natural gas decomposition for H₂ generation produces no CO₂ and requires zero process water, in contrast to the present industrial process – steam reforming. Notably it opens a path for renewable energy storage by coupling electrical energy into chemical bond energy – with H₂ being particularly versatile as fuel or chemical precursor. Moreover, MW activated natural gas decomposition does not require additional carbon as catalyst but rather produces carbon [2]. Therein MW driven decomposition of methane achieves green hydrogen production and production of value-added carbon materials [3]. Here a suite of characterization tools have been applied to the light gases, liquid and solid carbon products. Gas and liquid analyses were performed using gas chromatography-mass spectrometry. For the solid carbon products SEM has been applied to coal chars for cenosphere analysis, and for the carbon products from methane decomposition, scanning electron microsocpy (SEM) for morphology, transmission electron microscopy (TEM) for (nano)structural assessment and identification of different carbon forms (i.e. sp² phases), X-ray diffraction (XRD) for evaluation of graphitic structure, thermo-gravimetric analysis (TGA) for bulk determination of oxidative reactivity of the carbons as a means by which to assess graphitic content and gas (N₂) adsorption analyses for texture – i.e. surface area and porosity. Elemental analysis was also performed for HCNS and O by difference. Complementary thermo-gravimetric analyses assess the oxidative reactivity of the carbon products. Optical diagnostics such as multi-wavelength pyrometry relate these material characteristics to reactor conditions and can be applied for process control.

4.0 Summary

H Quest Vanguard is developing broad-spectrum microwave plasma processes targeting conversion of hydrocarbon feedstocks such as coal and natural gas to value-added materials, chemicals and fuels [4,5].

A microwave pyrolysis approach was originally proposed in response to the DARPA initiative for green-house gas (GHG)-emission-free and cost-effective production of US Air Force jet fuel from the domestic coal resources. In H Quest Vanguard’s process, natural gas can be used in single-stage reactor as a hydrogen source, eliminating external hydrogen production units and the associated CO₂ production, water consumption, and capital costs, and providing excess hydrogen sufficient for downstream hydro-treating. The MW process produces fast heating resulting in flash devolatilization and pyrolysis followed by fast quenching, preserving the primary pyrolysis products and volatiles’ molecular structure.

Since company formation in 2014, H Quest Vanguard, Inc. has developed ample material base and expertise in development of chemical and catalytic processes enhanced by microwave plasma. In particular, H Quest has developed a conversion process that applies microwave energy to rapidly co-pyrolyze solid hydrocarbons (e.g. coal) and natural gas to produce liquid hydrocarbons. A wide range of carbon materials including graphene and ordered carbon blacks have been observed across a wide range of experiments. These forms have potential for high value applications: electrical conductivity additives for plastics, and as electrode material in supercapacitors and Li-batteries. Subsequent and ongoing work has demonstrated feasibility of a microwave driven plasma mediated methane decompostion for H₂ production and nanographene formation as a premium carbon.

5.0 Acknowledgements

H Quest Vanguard, Inc. is a privately held technology company, based in Pittsburgh, Pennsylvania, focused on the development and commercialization of novel hydrocarbon conversion technologies. This material is based on work supported by the Department of Energy, Office of Science through subaward agreement no. 186949 with H Quest Vanguard, Inc. under the Prime Award DE-SC0015895 Phase I SBIR.

6.0 References

[1] Vander Wal, R. L., Gaddam, C. K., and Kulis, M. J. (2014). An Investigation of Micro-Hollow Cathode Glow Discharge Generated Optical Emission Spectroscopy for Hydrocarbon Detection and Differentiation. Applied spectroscopy, 68(6), 649-656.

[2] Dagle, R., Dagle, V., Bearden, M., Holladay, J., Krause, T., and Ahmed, S., (2017). An Overview of Natural Gas Conversion Technologies for Co-Production of Hydrogen and Value-Added Solid Carbon Products (No. PNNL-26726; ANL-17/11). Pacific Northwest National Lab. (PNNL), Richland, WA (United States); Argonne National Lab. (ANL), Argonne, IL (United States).

[3] Abbas, H. F., and Daud, W. W. (2010). Hydrogen production by methane decomposition: A review. International Journal of Hydrogen Energy, 35(3), 1160-1190.

[4] Skoptsov, George L., and Alan A. Johnson. Method for processing hydrocarbon fuels using microwave energy. U.S. Patent 9,682,359, issued June 20, 2017.

[5] Strohm, J.J., Linehan, J.C., Roberts, B.Q., McMakin, D.L., Sheen, D.M., Griffin, J.W. and Franz, J.A., Battelle Memorial Institute, 2012. Heavy Fossil Hydrocarbon Conversion and Upgrading Using Radio-Frequency or Microwave Energy. U.S. Patent Application 13/401,216.