(360f) Small-Scale Production of Platform Chemicals from Coal with Low-Temperature Microwave Plasma

The United States is endowed with extraordinary riches: it holds more than a quarter of the world's coal reserves. Regrettably, this wealth is severely undervalued. Although coal has catalyzed industrial development in the 19th and early 20th century not only as a ready source of energy, but also as the principal source of synthetic chemicals and materials – from aspirin to benzene to naphthalene to tarmac, – over the past decades coal became essentially synonymous with production of low-cost electricity.

Shale revolution has resulted in an abundance of low-cost natural gas and prompted the fall of coal’s share in electricity production from 50% to less than 30% over the course of a single decade. With significantly depressed domestic thermal coal prices, there is an opportunity to reorient the chemical industry back to reliance on this accessible and abundant feedstock. It has already happened in China, for instance, where the present capacity of coal gasification/Fischer-Tropsch conversion to methanol targeting production of olefins is 13.6Mtpa, with $55 billion worth of new capacity planned through 2026. FT plants, however, have conventionally been associated with outsized CAPEX investment (in billions USD) and a large environmental footprint, precluding their adoption in the United States.

A viable alternative method for production of coal-derived platform chemicals is direct, thermal conversion (pyrolysis) of coal. Known and industrially employed since 1800s, destructive distillation of coal yields hydrocarbon gases, liquids (monoaromatics and coal tar), and coke. In fact, until 1950s, the coke ovens were responsible for virtually 100% of the domestic benzene supply, but were quickly displaced by petroleum refiners. While the first benzene produced from petroleum appeared only in 1950, by 1961 petroleum benzene accounted for more than 75% of the total production.

Today, benzene and other monoaromatics (BTEX) are the crude oil-derived platform chemicals at the base of many important industrial supply chains: plastics, rubbers, fabrics, solvents, detergents, dyes, and pharmaceuticals. The global demand for benzene alone is in the order of 40 million tons per year. Domestic benzene deficit is expected to grow from 1.8 million tons in 2014 to 4.4 million tons by 2023, due to impact of the shale revolution on domestic BTEX sources. Refineries are forced to run reformers at reduced rates, while BTEX streams from naphtha cracking are reduced as low-cost high-ethane shale gas becomes preferred feedstock for ethylene production.

H Quest has developed and demonstrated a novel coal conversion process that avoids heat transfer and scale requirements limitations of conventional coal conversion technologies, and is well-suited for small-scale, distributed production of chemicals and material precursors from coal. In place of conventional thermal inputs, this rapid, continuous process uses focused microwave energy to create non-equilibrium plasma within inert or hydrogen-rich (CH4, H2 or syngas) process gas, localized around entrained coal particles. Optical (spectrometry) diagnostics are used for reaction characterization, species identification, and temperature determination. Observed intensity ratios or spectra band shapes yield temperature by Boltzman analysis. Pyrometry measurements of the solid particles falling through the MW reaction zone yield a temperature of ~ 1200 K. The entraining gas remains relatively cool: thermocouple measurements at the outlet of the microwave reactor typically do not exceed 300 C. Thus, rapid (< 1 sec) pyrolysis of coal releases liquid products and intermediates into the relatively cool process gas, where immediate quenching prevents secondary cracking reactions. The result are the high yields of the relatively high-value liquids (50-60wt%, d.a.f) and minimal yields of the low-value gases (<5wt%).

Process gas composed of methane or syngas serves as a hydrogen source within the single-stage reactor, eliminating external hydrogen production units and the associated CO2 production, water consumption, and capital costs. Its ionization within the localized reaction zones produces active CHx* and H* radical species, which promote further coal decomposition, prevent retrogressive radical recombination reactions of oil intermediates, and improve quality of the liquid products through partial hydrogenation and alkylation reactions. Direct activation of a hydrogen-rich gas within localized reaction zones eliminates requirements for high pressures, which conventional coal-to-liquids processes relies on to overcome mass transfer limitations.

Thus, the process enables deployment of a continuous, small-footprint, high-throughput conversion system with a thermal efficiency approaching 80% and life-cycle CO2 emissions and water consumption at least 50% lower than conventional coal conversion technologies.

Varying energy input, feed rate, and process gas composition allows adjusting the liquid product composition and properties. Bituminous coals processed in inert atmosphere (process gas includes inert, recycleable argon) yield highly aromatic liquids. Up to 25% of the liquids are benzene and other monoaromatics. Co-processing with activated hydrogen-donating gases results in methylation, hydrogenation, and ring-opening of the aromatic compounds, as shown by GC/MS traces of the resulting products and corroborated by optical spectrometry. Experimental data from testing high-sulfur (>3%) bituminous coals from Illinois and Ohio shows reduction of sulfur content relative to the parent coal in both char and liquid products of 50% and 80% respectively, effectively beneficiating both products. Production of excess hydrogen from methane decomposition may be used in downstream processes or sold as a valuable by-product.

In the past, coal conversion processes were hindered by inefficiencies, vast capital costs, high environmental impact, and, as a result, by non-competitive costs of production. New technologies relying on unconventional pathways and methods can enable cost-effective, small-scale, clean, efficient value-added production of high-value chemicals and materials directly at the coal mine sites. Most importantly, deployment of these technologies would have a direct and rapid impact on job creation and economic development in the rural communities most affected by the recent downturn in the coal industry.

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. Work supported by the Department of Energy, Office of Science under award DE-SC0015895 and by the Illinois Clean Coal Institute under subgrant 15-01 have contributed to materials presented herein.