A Brighter, Sustainable Future for Coal: Coal Products Have a Myriad of Industrial Uses

By B. J. Arnold, J. H. Adair, A. Dahi Taleghani, A. Gharpure, J. P. Mathews, S. V. Pisupati and R. L. Vander Wal, Pennsylvania State University

Pennsylvania State University has a long history of developing advanced carbon products from coal. This legacy continues with a focus on producing carbon-reinforced cement for wellbores, graphene and graphite, a critical mineral, from coal through various pathways. Some of these products have promise in battery applications. Coal properties and coal quality are important factors in this transformation and coal beneficiation will play a role, especially when addressing recovery of coal/carbon from waste coals.

Given the importance of coal to Pennsylvania (the leading cumulative coal-producing state with ~23 percent by 2009) (Hook and Alekett 2009), it is not surprising that Pennsylvania State University is a center for coal research (Mathews et al., 2013). Penn State has a +100-year history of coal research. The mining program (with a significant coal focus, including coal preparation) celebrated its 125th year in 2015, having its first mining graduate in 1894 (Miller 1992). Historically, Penn State coal research has followed the trends of combustion, ash chemistry, mineral processing, gasification, liquefaction (Mathews et al. 2013), coal-to-materials, atomistic simulations and more recently, CO2 sequestration or storage along with coalbed methane extraction. A re-emerging field is coal to carbon materials where the (Pennsylvanian) bituminous and anthracite ranks have extensive potential.

The Consortium for Premium
Carbon Products from Coal (CPCPC) was Penn State-housed and active between 1998 and 2010 with 58 member companies and universities. The CPCPC funding by the U.S. DOE National Energy Technology Laboratory resulted in academic/industrial collaborations with numerous coal production and carbon manufacturing companies. One hundred research projects with budgets totaling USD$13 million covered activated carbons, anode cokes, graphite electrodes, foams, fibers and nanotubes, graphite, nanofiber sheets, needle coke and pitches. Penn State researchers maintain several ongoing initiatives, including mesophase pitch and needle coke production from coal, coal-based adsorbents for pollution control and liquids purification and nanocarbons. Several projects are highlighted here.

Natural Low-cost Coal-based Additive to Improve Well Cement Integrity The access to underground U.S. energy reserves is mainly provided by wellbores. It is estimated that more than 90 percent of the U.S. total energy supply depends on some sort of wellbore systems, with the annual number of wells drilled continuing to grow.

Ensuring long-term integrity and zonal isolation in these wells is key for preventing methane emissions as well as successful injections, such as with carbon dioxide sequestration/storage. Cement integrity plays a vital role in safe and long-term sealing of the wells meant for CO2 flooding or geological carbon storage. Cement was reinforced at the micro level using cost-effective coal-based particles to change the hydrophobic surface properties of anthracite particles to hydrophilic. These surface-modified anthracite microparticles at 0.2 vol. percent improved the compressive strength and the flexural strength by 309 percent and 60 percent, respectively, as compared to the neat cement.

Graphene Extraction from Coal Penn State researchers are evaluating the extraction of graphene from anthracite coal. For decades, mixed media milling (aka attrition milling) in the Adair labs has been used to comminute ceramic materials to primary particle sizes. Our initial work has demonstrated the ability to extract 12 percent of the coal mass in the hydrophobic liquid with nanoparticle structures (~2 nm nanocrystals by Raman recently discussed by Schuepfer et al. (2020), ~2 nm particles by dynamic laser light scattering and 2 nm-thick platelet particles via atomic force microscopy measurements). Suspensions of the extracted graphene also have spectra similar to graphene quantum dots.

Coal-derived Graphite
Coal tar pitch (CTP) is used in formulations to produce graphite products, amorphous smelter electrodes and refractories and, in some cases, carbon fibers. It is recovered from coal tar, a byproduct of coke-making in the steel industry.

Shifts away from the blast furnace iron-making route (by the steel industry in the U.S., with its attendant elimination of furnace coke as an iron-making requirement) have reduced coke-making capacity, causing a significant shift to pitch production offshore. The Synpitch process (Dadyburjor et al. 2004; Kennel et al. 2009, 2012) being examined by Penn State researchers does not produce a char product as do some alternative processes or pyrene derivative compounds (undesirable for health concerns).

Laboratory Comparison
Commercial petroleum pitch (PP), coal-tar pitch (CTP) and a low-quality aliphatic pitch (LQP) were obtained from leading local vendors. Another coal-derived pitch (CDP) from the Synpitch process was obtained by solvent extraction of bituminous coal. The CDP showed a markedly better nanostructure compared to PP and better nanostructure compared to the PP after graphitization. The graphitic quality of coal-derived pitches can be further improved by synergistically blending different pitches providing complementary chemical compositions.

Therefore, coal-derived precursors can provide higher quality graphite than commercial petroleum precursors. Coal-derived precursors can not only accommodate lower quality aliphatic precursors but also interact synergistically to provide a higher degree of graphitization and stacking height in the produced graphite.

Production and Optimization of Synpitch Graphite
Isotropic and anisotropic pitches can be formed from coal-derived Synpitch to obtain multiple carbon products (Fig. 1). Penn State researchers are teaming with coal companies and project developers to move the technical readiness level of the Synpitch process from laboratory scale to a level where process flow diagrams, piping and instrumentation diagrams and energy and material balances can be developed for a 1 TPD demonstration plant. Bituminous coal and anthracite will be evaluated in this work, along with beneficiated waste coal. The researchers will control the Synpitch processing conditions to generate isotropic and anisotropic pitches, needle coke and various green cokes suitable for graphitization for application in the nuclear graphite industry and separately as electrodes in Li-ion battery applications. These different cokes will then be graphitized to generate the industrially relevant graphite and tested in comparison to commercial products.

Coal has long been recognized as a vital source of carbon for steelmaking, for chemicals, for water treatment, etc. As its use for electricity generation is being curtailed in some countries, its more valuable use as a carbon source will remain. Carbon fibers, carbon foams, nanotubes, graphene, quantum dots and graphite can and will come from coal, as superior products will be developed from this global resource. Adding strength to materials, supplying battery and nuclear materials and continued coal production as well as the recovery of coal from waste will supply the technology needs of the future.

The authors are researchers at the Pennsylvania State University.