(656g) Investigation of Sorption Kinetics and Isotherm Behaviour of Coal for the Prediction of Enhanced Coalbed Methane (ECBM) Recovery Potentiality

In the last few decades, the world has witnessed an exponential intensification in climate change activities, either in the form of floods, wood fires, extremely hot summers, short winters, drought, and devastating cyclones leading to huge losses of lives, infrastructure, and the environment. Climate change caused by the Industrial Revolution has resulted in an increase in mean global surface temperature of 1.07°C since the last half of the nineteenth century, nearly doubling every 30 years, and is the greatest threat to the animal kingdom ever faced. Since the middle of the 20th century, the emission of greenhouse gases produced by human activity has been the major cause of observed climate change. The industrial revolution leads to excessive consumption of carbon-rich compounds like fossil fuels in response to ever-increasing energy demand causing an increase in the concentration of greenhouse gases.. Carbon dioxide (CO₂) accounts for 76% of the global GHG emission has been recognized as the most potent greenhouse gas as compared to other greenhouse gases such as methane (CH₄), nitrous oxide (N₂O), Halocarbons, Sulfur Hexafluoride (SF₆), etc. The concentration of atmospheric CO₂ has increased from about 280 parts per million (ppm) in the pre-industrial era in the year 1880 to 400 ppm in the year 2015 to a level of around 424 ppm in 2023. To restrict this rapidly increasing concentration of the anthropogenic CO₂, a global meeting organized by Intergovernmental Panel on Climate Change (IPCC) in Paris in the year 2015 has set a target to restrict the increase in global average temperature below 1.5°C to control global warming over pre-industrial levels, understanding that this restriction would greatly decrease climate change risks and impacts. In this situation, sustainable development is being sought, with a particular emphasis on meeting the world's rising energy needs while leaving the smallest possible carbon footprint.

Hence, in this alarming scenario, tree plantation, forest preservation, focus towards enhancement of renewable energy production, implementation of energy efficiency technologies etc., can be possible options for CO₂ mitigation. However, all these efforts are not sufficient to meet net-zero targets by 2050. Hence, to achieve the net zero emission target, permanent storage of CO₂ in geological sites can be considered as the most sustainable solution towards achieving the net zero emission target. Increased interest in CO₂ geo-sequestration has resulted in substantial investment by the public and the private sector in developing the requisite technology and determining strategies for CO₂ control to be adopted safely and effectively. Some potential carbon sequestration sites include depleted oil reservoirs, saline aquifers, unconventional gas (coal/shale) reservoirs, gas-hydrate reservoirs, basalts etc. Among all the permanent storage options of CO₂ in geological sites, CO₂ sequestration in coal seams enhanced coalbed methane (ECBM) serves triple purposes simultaneously, i.e., it reduces CO₂ emissions by storing CO₂ into the coalbeds, recovering high calorific value methane (CBM) as a fuel present in the coalbed as a potential source of energy and also by ensuring the miner’s safety by recovering methane prior to coal mining.

However, the methane (CH₄) recovery and CO₂ sequestration potential is dependent on several factors, as the gas storage and transport mechanism in the coalbeds differs significantly from the conventional reservoirs. The gas storage mechanism in coal is dominated by physical adsorption, accounting for more than 80% of the total storage capacity and the rest as free gas and solution gas. CO₂ sequestration in coalbeds occurs based on the principle of the relatively stronger affinity of CO₂ towards coal than CH_4. As a result, CO₂ gets adsorbed in the coal matrix without being released. Due to its small kinetic diameter compared to CH₄, CO₂ can enter the smallest pores, adsorb resolutely to the coal surface at a near-liquid density, and store with negligible chances of leakage. Hence, understanding the sorption behaviour of CH₄ recovery and CO₂ storage on coal is a prerequisite for estimating the ability of a coalbed to store these gases, which must be evaluated experimentally due to the complex physical and chemical structural heterogeneity.

For the present investigation, coal samples were collected from different coal mines/collieries of the Jharia coalfield, India. As a part of the characterization study proximate and ultimate analyses were performed in order to study the elemental composition of the coal. Adsorption isotherm was measured at 30 °C for all the samples. Prior to analysis, moisture-equilibrated (96 to 97% relative humidity at 30 °C) samples were prepared following ASTM D1412/D1412M standard to replicate the reservoir moisture condition. The mass balance equation, real gas law, and equation of state were then employed to calculate the adsorbed gas volume. A total of 9 steps at the incremental pressure of 1000 KPa for CH₄ and 7 steps at the incremental pressure of 1000 KPa for CO₂ were measured to calculate the isotherm. Pressure drop data was recorded at 1-second intervals in the data acquisition for the evaluation of the adsorption kinetics and fitted to Unipore and modified Unipore model. Approximately 12 hours of time interval was kept between each succeeding pressure step both for the CH₄ and CO₂ adsorption isotherms.

The results of the proximate analysis show that the coals fixed carbon content ranges from 77.2 to 79.1 % (pdf), and it is linearly increasing from upper (shallow depth) to lower seams (higher depth). The result of the ultimate analysis reveals that the samples exhibit high carbon content, varying within the range 86.5-91.7% (dmmf). The Langmuir volume (daf basis) for CH₄ isotherm varies in a comparatively narrow range as compared to CO₂ isotherm. It ranges between 19.4-23.8cc/g and 26.6-43.6cc/g (daf) for CH₄ and CO₂ respectively (Table 1). For both the gases Langmuir volume is higher for upper seam coals. The daf-wise ratio of Langmuir volume between CH₄ and CO₂ varies between 1:1.37 to 1:1.83The Langmuir pressure for CH₄ and CO₂ varies between 1608-3646 KPa and 1294-3123KPa respectively.

The depicted adsorption kinetic curve obtained from experimental adsorption resembles a Type I isotherm curve in terms of shape, which increases linearly at the early stage of adsorption, and the curve flattens a bit with time. The experimental adsorption kinetic plots [fractional uptake (V_t/V_∞) versus time (t^0.5)] of two representative samples of the studied coals are shown in Figure 1. The kinetic plots show that for all coal samples, the rate curve for CO₂ adsorption is steeper at the initial stage, i.e. after the gas is injected into sample cells. However, CO₂ takes comparatively less time to reach equilibrium than methane. The slope of the adsorption kinetic plots is determined in order to calculate the precise sorption kinetic rate. A clearly decreasing trend of the slope with the increase in pressure has been observed for both gases, which depicts that the sorption rate has a strong dependency on the pressure. It also depicts that in the initial stage, the adsorption rate is faster due to the availability of a large number of vacant sites. The driving force and the number of available sites decline with the increased contact time. Moreover, adsorbate molecules in the sorbed and bulk phases inhibit each other as the stage approaches equilibrium. The comparatively slower kinetics of CO₂ at higher pressure is also attributed to the swelling that occurs during gas injection, which inhibits the CO₂ storage and release of CH₄.