Illinois Coal Research Articles

Understanding poromechanical response of a biogenic coalbed methane reservoir

Biogenic coalbed methane (BCBM) reservoirs aim to produce methane from in situ coal deposits following microbial conversion of coal. Success of BCBM reservoirs requires economic methane production within an acceptable timeframe. The work reported here quantifies the findings of previously published qualitative work, where it was found that bioconversion induces strains in the pore, matrix and bulk scales. Using imaging and dynamic strain monitoring techniques, the bioconversion induced strain is quantified here. To understand the effect of these strains from a reservoir geomechanics perspective, a corresponding poromechanical model is developed. Furthermore, findings of imaging experiments are validated using core-flooding flow experiments. Finally, expected field-scale behavior of the permeability response of a BCBM operation is modeled and analyzed. The results of the study indicated that, for Illinois coals, bioconversion induced strains result in a decrease in fracture porosity, resulting in a detrimental permeability drop in excess of 60% during bioconversion, which festers itself exponentially throughout its producing life. Results indicate that reservoirs with high initial permeability that will support higher Darcian flowrates, would be better suited for coal bioconversion, thereby providing a site-selection criteria for BCBM operations.

The Herrin Massacre of 1922: Blood and Coal in the Heart of America

The Herrin Massacre of 1922: Blood and Coal in the Heart of America

Water quality implications of the neutralization of acid mine drainage with coal fly ash from India and the United States

Subsurface coal mining often induces the formation of acid mine drainage (AMD) in active and abandoned coal mines while coal combustion generates coal combustion residuals (CCR), including fly ash (FA), with elevated levels of toxic metals. Decades of AMD and CCR production have caused major environmental and human health impacts. Given the typically elevated level of oxides in FA, previous studies have examined its potential to neutralize AMD and remove the associated metals. While the neutralization of AMD through reaction with FA has been demonstrated to successfully remove cationic metals, the fate of oxyanion forming elements are less well studied and is the focus of this study. Here we conducted 49 different experiments in which simulated AMD solutions were interacted with representative U.S. (n = 7) and Indian (n = 6) FA samples through controlled liquid to solid ratios in short-term (24 h) and long-term (up to 5 weeks) lab-scale experiments. We show that Class-F FA, originating from Gondwana and Northeastern Tertiary coals in India, has limited neutralization capacity, while Class-C FA, with high CaO and MgO contents from Powder River coals in the U.S. has the greatest AMD neutralization capacity among the studied fly ashes. The neutralization experiments show that AMD-FA reactions cause the removal of cationic elements (i.e., Fe, Mn, and Al) from solution, while at the same time, leaching oxyanion forming elements (i.e., As, Se, Mo, Cr, B, Tl, and Sb) from the FA, increasing the potential environmental risks from the treated leachates. The magnitude of mobilization of these elements depends on their concentrations in the FA and the pH conditions. We show that using FA from the Appalachian and Illinois coals efficiently neutralizes AMD, but also results in secondary contamination of the treated effluents with oxyanion forming elements to levels exceeding drinking water and ecological standards, which could contaminate the ambient environment, whereas neuralization with Powder River Basin Class-C FA induces only limited contamination.

Experimental study on the adverse effect of gel fracturing fluid on gas sorption behavior for Illinois coal

Hydraulic fracturing is an effective technology for coal reservoir stimulation. After fracturing operation and flowback, a fraction of fracturing fluid will be essentially remained in the formation which ultimately damages the flowability of the formation. In this study, we quantified the gel-based fracturing fluid induced damages on gas sorption for Illinois coal in US. We conducted the high-pressure methane and CO2 sorption experiments to investigate the sorption damage due to the gel residue. The infrared spectroscopy tests were used to analyze the evolution of the functional group of the coal during fracturing fluid treatment. The results show that there is no significant chemical reaction between the fracturing fluid and coal, and the damage of sorption is attributed to the physical blockage and interactions. As the concentration of fracturing fluid increases, the density of residues on the coal surface increases and the adhesion film becomes progressively denser. The adhesion film on coal can apparently reduce the number of adsorption sites for gas and lead to a decrease of gas sorption capacity. In addition, the gel residue can decrease the interconnectivity of pore structure of coal which can also limit the sorption capacity by isolating the gas from the potential sorption sites. For the low concentration of fracturing fluid, the Langmuir volume was reduced to less than one-half of that of raw coal. After the fracturing fluid invades, the desorption hysteresis of methane and CO2 in coal was found to be amplified. The impact on the methane desorption hysteresis is significantly higher than CO2 does. The reason for the increasing of hysteresis may be that the adsorption swelling caused by the residue adhered on the pore edge, or the pore blockage caused by the residue invasion under high gas pressure. The results of this study quantitatively confirm the fracturing fluid induced gas sorption damage on coal and provide a baseline assessment for coal fracturing fluid formulation and technology.

Experimental study of the influence of water-based fracturing fluids on the pore structure of coal

To investigate the interaction between coal and fracturing fluid and its influence on the pore structures of the coal, three commonly used water-based fracturing fluids were selected and used to treat Illinois coal. A series of low-temperature nitrogen ad-/desorption experiments and nuclear magnetic resonance measurements were conducted to comparatively analyse the evolution of the coal pore structure after treatment. The results show that the intrusion of the water-based fracturing fluid reduces the pore volume and pore area of the coal. In addition, the residue of the VES fracturing fluid may change the shape of the ink-bottled pores of coal. The evolution of the fractal dimension of the coal pore system after fracturing-fluid treatment illustrated that the residue of the fracturing fluid may adhere primarily to the relatively rough region of the pore wall. There is no significant chemical reaction between the water-based fracturing fluids and the Illinois coal.

Evaluating the changes of sorption and diffusion behaviors of Illinois coal with various water-based fracturing fluid treatments

The water-based fracturing stimulation is commonly used for low permeability coalbed methane (CBM) reservoirs. The influence of various fracturing fluids on the gas sorption and diffusion behaviors were experimentally studied in the work. The Illinois basin coal was selected for the experimental measurements. We used slickwater, guar gel and viscoelastic surfactant (VES) fracturing fluids to treat the coal. The sorption and diffusion behaviors were measured and quantified by the volumetric sorption experiments. The microscope was employed for the coal sample surface observation to qualitatively characterize the coating and residue behavior of fracturing fluids. It was found that all types of fracturing fluids can significantly influence the gas sorption capacity of coal. Apart from the low concentration of polyacrylamide (0.1% and 0.3%) slickwater, all other fracking fluids damage the gas sorption capacity. For most fracturing fluids, the sorption capacity can be recovered closely back to its virgin coal condition with extensive water washing. For the coal samples treated by the VES fracturing fluid, both the Langmuir volume and Langmuir pressure were dramatically increased. The adverse effect of fracturing fluids on coal is apparent and all diffusion coefficients decrease with the treatment. Even with extensive washing, the diffusion coefficient cannot recover for slickwater and VES fluids. But guar gel fluid has the best diffusion recovery because of the easy removal of the guar gel. This study provides a preliminary data set for fracturing fluid selection for Illinois coal in terms of sorption and diffusion.

Laboratory study of cryogenic treatment induced pore-scale structural alterations of Illinois coal and their implications on gas sorption and diffusion behaviors

Cryogenic fracturing using liquid nitrogen is a choice for CBM reservoir stimulation. However, the development of cryogenic fracturing is still at its infancy. This work focused on induced pore structural alterations due to cryogenic treatment and their effects on gas sorption and diffusion behaviors. The changes in the pore structure of coal induced by cyclic nitrogen injections were studied by physical adsorption at low temperatures. A micromechanical model is proposed to simulate the microscopic process and predict the degree of deterioration due to low temperature treatments. As a common characteristic of modeled results and experimental results, the total volume of mesopore and macropore increased with cryogenic treatment, but the growth rate of pore volume became much smaller as freezing-thawing were repeated. Pores of different sizes experienced different degrees of deterioration. In the range of micropores, no significant alteration of pore volume occurred with the repetition of freezing and thawing. In the range of mesopores, pore volume increased with the repetition of freezing and thawing. In the range of macropores, pore volume increased after the first cycle of freezing and thawing but decreased after three cycles of freezing and thawing. Because of pore structural alteration, cryogenic treatment enhanced gas transport process as the diffusion coefficients of the freeze-thawed coal samples were increased by 18.76% and 30.18% in the adsorption and desorption process. The effect of cyclic cryogenic treatment on the pore structure of tested coal varied depending on coal properties. For the studied Illinois coal sample, repetitive applications of cryogenic treatment reduced macropore volume and increase mesopore volume. For the tested sample, the diffusion coefficient of the coal sample that underwent three cycles of freezing-thawing was lower than that of the coal sample that underwent a single cycle of freezing and thawing. The outcome of this study provides justification for a post-cryogenic fracturing effect on diffusion improvement and gas production enhancement, especially for high “sorption time” CBM reservoirs.

Is the just transition socially accepted? Energy history, place, and support for coal and solar in Illinois, Texas, and Vermont

Given a myriad of challenges and technological advancement, the energy system has been constantly evolving and transforming, which requires compatible adjustments in energy policies. For regions with highly diverse energy sources, such as the United States, it's important to not only analyze public perceptions of various energy sources, but also examine relative levels of support for energy transition policies. We examine public perceptions of two energy sources, coal and solar energy, as well as the public's support for energy transition policies that assist the transition from coal to renewable energy, specifically solar. To better understand how place influences attitudes and policies of coal and solar, we sampled respondents from three areas with varying ties to coal and solar: Saline County, IL, Houston, TX, and Burlington, VT. Our results suggest that one’s place has an important effect in differentiating the public perceptions of renewable and nonrenewable energy and for support in an energy transition from fossil fuels to renewable energy. While residents of places with historical attachment to coal mining have positive attitudes toward coal, our data show that they have even more positive attitudes toward sustainable energy sources. These attitudes hold for future development and government assistance. If federal or state governments seek to reduce greenhouse gas emissions (and consequently energy from coal, a major source of greenhouse gases), solar and other sustainable energy policies need to be targeted at places with historical ties to coal and other fossil fuels to ensure a procedurally just energy transition.

Sustaining biogenic methane release from Illinois coal in a fermentor for one year

To evaluate coal biogasification in a larger reactor over a longer duration as compared to studies reported so far, a 3-liter fermentor was established. During the one-year study, a nutrient recipe was added three times to sustain methane release from Illinois bituminous coal. The cumulated methane production was 5171 ft3/ton with a methane content of 75.4% on day 365. After the fermentation was terminated, the residual coal and fermentation broth were characterized in detail. Compared to the untreated coal, the treated coal residue appeared to be finer and highly degraded with less carbon but more ash. Based on mass balance, volatile and fixed carbon decreased 15.9% and 29.6%, respectively, using the untreated coal as the baseline. According to GC/MS analysis, the fermentation broth contained mainly three groups of compounds: fatty acids and their derivatives, aromatics, and hydrocarbons. In addition, the fermentation broth was found to have effect on flocculation and contained compounds that possessed surface-active properties. Further investigations are needed to identify these chemicals responsible for these activities and develop ways to further enhance coal biogasification based upon results obtained then.

Development of a nutrient recipe for enhancing methane release from coal in the Illinois basin

Recent literature has shown that in order to enhance methane release from coal through biogasification, suitable nutrient solutions are needed to stimulate microbial activities toward coal depolymerization and conversion to biogas. Specific for bituminous coal in the Illinois basin, a nutrient recipe that can be used to enhance coal biogasification in situ is not available yet. To develop such a recipe, the formation water and the indigenous microbial community were first characterized in detail. Based on these characteristics and our previous experience working with Illinois coal, four parameters, Fe powder, methanol, ethanol, and trace minerals were optimized through using a Box-Behnken design. The optimal condition predicted by the models was: Fe-powder at 74mM; methanol at 97.9mM, ethanol at 100mM, and trace minerals at 100%. Under these conditions, the predicted methane yield and content was 1417.35ft3/ton and 80.7%, respectively. These results were then validated by experimental studies. In addition, each added component was evaluated in terms of its contribution to methane generated. Specifically, the role of coal in the biogasification process was studied against two other solid materials. Overall, this study demonstrated that coal can be converted to methane and nutrient solution can definitely enhance methane release from coal. The real effect of this recipe in improving methane release from coal in situ needs to be further evaluated in a field scale.

The temperature effect on the methane and CO2 adsorption capacities of Illinois coal

The sorption capacity of coal is influenced by temperature, coal rank, mineral composition and moisture content. In the study, we experimentally measure the methane and carbon dioxide sorption capacity at various temperature for Illinois coal. A volumetric adsorption apparatus was employed to determine the excess gas adsorption capacity, and then the absolute adsorption capacity was estimated through the adsorbed gas phase density correction at each tested pressure. The adsorption isotherms were estimated for various temperatures from 283.15K to 343.15K. The experimental results indicated that both the methane and carbon dioxide adsorption capacities decrease linearly with an increase in temperature from 283.15K up to 323.15K, and the adsorption capacity reduction for CO2 is more significant than that for methane. When the temperature is equal to or greater than 323.15K, the adsorption capacities are independent of the temperature and remain constant for both gases. This means that there is a “critical temperature point” at which the influence of the temperature on the adsorption capacity vanishes, and the adsorption isotherms are very similar when the temperature is beyond the critical temperature point. The ratios of the CO2/CH4 absolute adsorption capacities vary from 2.62 and 3.80 with the tested pressures and temperatures.

What did Unions do? The Case of Illinois Coal Mining in the 1880s

This study asks what unions did for Illinois coal miners in the 1880s. It measures the outcomes provided by a traditional union and the Knights of Labor, primarily via difference-in-differences applied to a panel of coal mining counties in Illinois. Neither the traditional union nor the Knights of Labor was able to raise wages, provide benefits, or reduce hours per workday. The traditional union reduced days of work per year. Circumstantial evidence suggests the unions were able to ensure prompt payment of wages due.

Amphibian habitat creation on postindustrial landscapes: a case study in a reclaimed coal strip-mine area

Although habitat loss is a major driver of amphibian and reptile declines globally, a subset of postindustrial landscapes, reclaimed and restored, are creating habitat for these animals in the United States. In southwestern Indiana, along the southeastern edge of the Illinois Coal Basin, post-SMCRA (Surface Mining Control and Reclamation Act of 1977) grassland restorations are recreating prairies in a region where they occurred naturally as openings (pockets) in deciduous forest, but were destroyed by agricultural activities following Euro-American settlement. Furthermore, it is likely, given the speed of recolonization by 34 species of amphibians and reptiles (9 frog, 5 salamander, 13 snake, 5 turtle, and 2 lizard species), that the grasslands associated with railroad spur-line right-of-ways act as corridors to facilitate movement of these species into these relatively large sites. We suggest that reclaimed, restored, and properly managed landscapes can support reproducing populations of amphibians and reptiles, including species in decline across other portions of their range.

Roof Rockmass Characterization in an Illinois Underground Coal Mine

Among all United States underground coal fields, those in Illinois have the highest rate of roof fall events due to their weak and severely moisture sensitive roof rock units. Rockmass characterization is the key initial step in designing safe and economical roof control measures in underground coal mines. In this study, a performance-based roof rockmass characterization is investigated. The geologic conditions as well as underground mine geographic specifications, roof fall analysis, mining method, utilized supplemental roof control measures, and geotechnical properties of roof rock units were considered to link the roof performance to rockmass characterization. The coal mine roof rating (CMRR) rockmass characterization method was used to evaluate the roof conditions and roof support design for an underground coal mine located in the Illinois Coal Basin. The results of several mine visit mappings, laboratory test results, and geotechnical issues and concerns are presented and discussed. The roof support designs are analyzed based on the rockmass characterization and are compared with the observed performance. This study shows that (1) CMRR index is a reasonable method for characterizing roof rockmass; (2) moisture sensitivity and bedding strengths in the horizontal direction are essential parameters for roof support design in mines with weak roof conditions; and (3) the applicability of the analysis of roof bolt system for roof support design of the studied mine is questionable.

Acid Mine Drainage (AMD) Treatment with Sludge from Wastewater Treatment Plant in a Two-Chamber Microbial Fuel Cell

There are almost 46,000 abandoned mines all over the United States. These mines produce heavy-metal-rich sulfuric acid solution called acid mine drainage (AMD), which has brought long-term water and soil pollution impact, according to the US department of the interior [1]. Different technologies have been used to remediate the surface water, groundwater and soil contaminated by the AMD. These methods include physical, biological, and chemical methods which are generally chemical intensive or the treatment period is relatively long. A better approach for AMD treatment is using microbial fuel cell (MFC), a type of bioelectrochemical systems (BES). MFC uses microorganisms to convert the chemical energy in the biodegradable materials to electricity and at the same time removes the heavy metals in AMD by precipitation and enhances the pH of the drainage. Dry sludge from wastewater treatment plant was used as electron donors for the first time for AMD remediation in the MFC. This treatment technology offers a cheap and environmentally benign approach for integrated waste utilization, energy production and wastewater treatment. Using BES with different mechanisms, different heavy metals have been removed, such as nickel, copper, lead, cadmium and zinc. [2&3&4]. In this study, AMD was treated in the cathode chamber of the MFC with continuous air injection into the chamber, and dry sludge was placed in the anode chamber of the MFC. AMD samples were taken from an abandoned coal-mine in Illinois Coal Basin around Carbondale IL, and the dry sludge was from Carbondale Southeast Wastewater Treatment Plant in Carbondale, IL. The plant used activated sludge tank as secondary treatment process. The dry sludge was formed after activated sludge digestion and drying. Anion exchange membrane (Membrane International Inc) was used to separate the two chambers. Carbon fiber brush was used as the anode electrode, and platinum coated carbon sheet was used as the cathode electrode. With catalyst used, the removal speed of heavy metals can be enhanced. The volume of each chamber was 150 mL. The dry sludge in the anode chamber of the MFC was saturated with D.I. water and a mixture of bacteria from different sources was inoculated into the chamber. No external nutrients were added into the chamber as the dry sludge contains reach nutrition, including organic compounds and different salts. Air was injected into the cathode chamber filled with the AMD. A multimeter was used to record the whole cell potential with an external resistor of 1 kΩ resistance, and the potential of each half cell was also recorded by a data logger (DataLoggerInc) with an Ag/AgCl reference electrode placed in the cathode chamber. The results showed that the closed circuit voltage of the whole cell was kept increasing during the 5 days of operation with the maximum reached voltage of 0.1 V. With the density peak method, the internal resistance of the MFC was measured to be 650 Ω. The pH and the ORP of the AMD changed from 2.57 and 619.9 mV to 4.49 and 263.2 mV respectively after 5 days. White-gray colored precipitation was observed to be formed on the bottom of the cathode chamber. The color of the AMD was changed from yellow-brown before the test to transparent after 5 days. Scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS) was used to test the composition of the precipitate and to observe its morphology. The EDS test showed the existence of Al, Cu, and Fe in the precipitation. Precipitates with different sizes were formed with the maximum less than 5 µm. The preliminary results from this study indicated the possible treatment pathway of AMD by MFC with electricity generation during the treatment process and by using dry sludge from the wastewater treatment plant; no additional chemicals are needed for the treatment process.

Combining a Spatial Model and Demand Forecasts to Map Future Surface Coal Mining in Appalachia

Predicting the locations of future surface coal mining in Appalachia is challenging for a number of reasons. Economic and regulatory factors impact the coal mining industry and forecasts of future coal production do not specifically predict changes in location of future coal production. With the potential environmental impacts from surface coal mining, prediction of the location of future activity would be valuable to decision makers. The goal of this study was to provide a method for predicting future surface coal mining extents under changing economic and regulatory forecasts through the year 2035. This was accomplished by integrating a spatial model with production demand forecasts to predict (1 km2) gridded cell size land cover change. Combining these two inputs was possible with a ratio which linked coal extraction quantities to a unit area extent. The result was a spatial distribution of probabilities allocated over forecasted demand for the Appalachian region including northern, central, southern, and eastern Illinois coal regions. The results can be used to better plan for land use alterations and potential cumulative impacts.

Studies on the effect of coal particle size on biodepyritization of high sulfur coal in batch bioreactor

Abstract The moderate thermophilic mix culture bacteria were used to depyritize the Illinois coal of varying particle sizes (-100 μm, 100-200 μm, +200 μm). Mineral libration analysis showed the presence of pyrite along with other minerals in coal. Microbial depyritization of coal was carried out in stirred tank batch reactors in presence of an iron-free 9K medium. The results indicate that microbial depyritization of coal using moderate thermophiles is an efficient process. Moreover, particle size of coal is an important parameter which affects the efficiency of microbial depyritization process. At the end of the experiment, a maximum of 75% pyrite and 66% of pyritic sulphur were removed from the median particle size. The XRD analysis showed the absence of pyrite mineral in the treated coal sample. A good mass balance was also obtained with net loss of mass ranging from 5-9% showing the feasibility of the process for large scale applications.

CO2 sorbents for a sorption-enhanced water–gas-shift process in IGCC plants: A thermodynamic analysis and process simulation study

A sorption-enhanced water–gas-shift reaction process (SEWGS) combines the WGS reaction and CO2 removal into a single process step in an integrated gasification combined cycle (IGCC) power plant. In the SEWGS, a solid sorbent is employed to capture the CO2 generated by the WGS reaction. A key technical issue for the SEWGS is to identify and develop sorbents that can capture CO2 under high pressure and high temperature syngas conditions and minimize or even eliminate the need for WGS catalysts. We present the results from a thermodynamic analysis and process simulation study that aimed to identify sorbents and optimal temperature windows to maximize the thermal efficiency of an IGCC + SEWGS plant. The results from the thermodynamic analysis identified two metal oxides, one silicate, three zirconates, and one titanate sorbents that are suitable for the SEWGS application. The results from the process simulation study revealed that the overall thermal efficiency of a 1187 MWth (thermal input) IGCC + SEWGS plant gasifying an Illinois coal, and using the seven sorbents selected from the thermodynamic analysis, was between 0.5 and 2.4 percentage points greater than that of a reference IGCC + Selexol plant.

Laboratory investigation of drying process of Illinois coals

High moisture coal leads to low power plant efficiency, increased stack emissions of pollutants and maintenance and operational problems when it is used in coal fired power plants. In this study, laboratory experiments and theoretical calculations of the coal drying process were carried out and compared in order to determine proper drying conditions.This research describes several experiments to present the effects of parameters, such as drying temperature, on drying performance. The tests were carried out with three different types of coal — Buckheart, Crown mine and Viper mine, which have initial moisture contents of 23%, 18% and 20%, respectively. The drying tests were performed at 1.25m/s air velocity and from 43°C to 60°C drying temperature. In this paper, the effect of drying temperature on drying rate of different types of coal was studied to obtain information relating to optimal operating conditions. The drying performances of each type of coal can be determined by analyzing the test data and operation conditions.A theoretical model was also utilized for the drying process based on mass balances and conservation of energy. Comparisons were made between experimental and theoretical results. Good agreement with laboratory test results was obtained especially at lower drying temperature. It is shown that this model can be reasonably used to predict the drying performance.

A molecular model for Illinois No. 6 Argonne Premium coal: Moving toward capturing the continuum structure

A large-scale molecular model for Illinois No. 6 Argonne Premium coal is generated based on an automated construction approach in an effort to move toward capturing the continuum structure. The model contains 50,789atoms within 728 molecules and is the largest, most complex coal representation constructed to-date. The aromatic ring size distribution was based on multiple high-resolution transmission electron microscope (HRTEM) lattice fringe micrographs and was duplicated with automated construction protocols (Fringe3D) in molecular modeling space. Additional structural data was obtained from the abundant literature assessing this Argonne Premium coal. Organic oxygen, nitrogen, and sulfur functionalities were incorporated primarily into the polyaromatic structures according to X-ray photoelectron spectroscopy and X-ray adsorption near-edge structure spectroscopy data. Aliphatic carbons were in the form of cross-links (bridges and loops) and pendant alkyl groups based on the combination of laser desorption ionization mass spectrometry (LDIMS), ruthenium ion catalyzed oxidation, elemental analysis, and NMR data in good agreement with the literature. Bound and bulk water was also included. Construction of the coal molecules was performed by use of Perl scripts developed in Materials Studio to eliminate personal bias and improve the accuracy and the scale of the structure generated. The large-scale model captured a broad and continuous molecular weight distribution in accordance with LDIMS data here ranging from 100 to 2850Da, enabling inclusion of structural diversity to capture a portion of the continuum structure. A theoretical pyridine extraction yield, determined by a group contribution approach, was in agreement with the experimental value. The extract and residue representations were generated from the large-scale Illinois coal model and showed consistency with NMR, elemental analysis and LDIMS trends. The distribution of heteroatomic classes and double bond equivalents was also well-defined experimentally based on electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. These data further constrain the molecular weight of extractable material and was consistent with limited pyridine extractability and model heteroatom classes. Future work will be well served by staying within the limits established by the approach and increasing the structural diversity (sampling frequency through increased scale) to better capture the complex nature of coal structural diversity, i.e., the continuum.