(699c) Upgrading Coal for the Production of Value-Added Chemicals and Feedstocks By Wave Liquefaction™: Spectroscopic Diagnostics and Material Characterization

Coal liquids and tars are valuable chemicals with applications ranging from metallurgy, energy storage, wood preservatives and carbon fibers [1]. Coal pyrolysis, a thermochemical conversion of coal in oxygen deficient environment at high temperature produces liquid (coal-tar), solid (coal-char) and gases [2]. The main chemicals present in the coal-tar are benzene, toluene, xylene and phenolics, which serve as valuable feedstocks for the petrochemical industry [3]. In conventional pyrolysis the solid particles are heated to desired pyrolysis conditions by the external heating, e.g. electrical heating, hot sand or process steam [2]. These conventional heating strategies suffer from heat transfer limitations, which reduces the overall process energy efficiency and product yield [4]. Liquid, tar yields and their product distributions depend upon heating rate, generally increasing with temperature (up to a limit) [5]. Microwave (MW) driven pyrolysis is a promising approach to resolve these limitations. Energy efficiency is a dramatic advantage of microwave heating: the typical energy efficiency of a kitchen microwave oven magnetron at 1kW output power is around 65-70%, while industrial scale magnetrons demonstrate about 80% energy efficiency at output power on the order of 100 kW [6]. A key innovation is the ability to process coal with little to no CO₂ emissions or water consumption. In the literature to-date, microwave pyrolysis of virgin coal has been largely applied to other coal processing purposes, rather than devolatilization.

In reactive atmospheres (H₂, CH₄ or both) microwave radiation forms a plasma with energetic electrons creating H-atoms and methyl radicals [7]. These reactive species can cap radicals, add to -bonds and initiate bond scission [8]. Radical capping through hydrogenation or methylation (in H₂/CH₄ reactive atmosphere), suppresses both retrogressive and secondary cracking reactions [9] that are typical of the conventional pyrolysis processes, decreasing liquid and tar yields [10]. In fact radical termination is essential to liberating liquids and tars as labile groups depart and matrix linkages scission, preventing charring [11]. Such capping reactions by H-atoms and methyl radicals also upgrade unsaturated and aromatic compounds by increasing the H/C ratio. Moreover these reactive species can potentially promote coal matrix deconstruction by radical abstraction/addition reactions, initiating bond scission.

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.

2.0 Measurements and Analyses

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.

Correspondingly optical diagnostics are central to reaction characterization and hold particular value for species identification and temperature determination [12]. 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.

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.

3.0 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 [13,14]. 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.

4.0 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 American Chemical Society, Petroleum Research Foundation, Award No. PRF# 58973-ND4 and 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.

5.0 References

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