A model for enhanced coal bed methane recovery aimed at carbon dioxide storage: The role of sorption, swelling and composition of injected gas

Numerical simulations on the performance of CO2 storage and enhanced coal bed methane (ECBM) recovery in coal beds are presented. For the calculations, aone-dimensional mathematical model is used consisting of mass balances describing gas flow and sorption, and a geomechanical relationship to account for porosity and permeability changes during injection. Important insights are obtained regarding the gas flow dynamics during displacement and the effects of sorption and swelling on the ECBM operation. In particular, initial faster CH4 recovery is obtained when N2 is added to the injected mixture, whereas pure CO2 allows for a more effective displacement in terms of total CH4 recovery. Moreover, it is shown that coal swelling dramatically affects the gas injectivity, as the closing of the fractures associated with it strongly reduces coal's permeability. As a matter of fact, injection of flue gas might represent a useful option to limit this proble

Prediction of competitive adsorption on coal by a lattice DFTmodel

Adsorption is one of the main mechanisms involved in the ECBM process, a technology where CO2(or flue gas, i.e. a CO2/N2mixture) is injected into a deep coal bed, with the aim of storing CO2by simultaneously recovering CH4. A detailed understanding of the microscopic adsorption process is therefore needed, as the latter controls the displacement process. A lattice DFT model, previously extended to mixtures, has been applied to predict the competitive adsorption behavior of CO2, CH4and N2and of their mixtures in slit-shaped pores of 1.2 and 8nm width. In particular, the effect of temperature, bulk composition and density on the resulting lattice pore profiles and on the lattice excess adsorption isotherms has been investigated. Important insights could be obtained; when approaching near critical conditions in the mesopores, a characteristic peak in the excess adsorption isotherm of CO2appears. The same effect could be observed neither for the other gases nor in the micropores. Moreover, in the case of mixtures, a depletion of the less adsorbed species close to the adsorbent surface is observed, which eventually results in negative lattice excess adsorption at high bulk densitie

Pure and binary adsorption of CO2, H2, and N2 on activated carbon

A new developing field of application for pressure swing adsorption (PSA) processes is the capture of CO2 to mitigate climate change, especially the separation of CO2 and H2 in a pre-combustion context. In this process scheme the conditions of the feed to the separation step, namely a pressure of 3.5 to 4.5MPa and a CO2 fraction of around 40% are favorable for an adsorption based separation process and make PSA a promising technology. Among the commercial adsorbent materials, activated carbon is most suitable for this application. To evaluate the potential, to benchmark new materials, and for process development a sound basis of the activated carbon thermodynamic data is required, namely equilibrium adsorption isotherms of the relevant pure components and mixtures, Henry's constants and isosteric heats. In this work pure adsorption equilibria of CO2, H2 and N2 on commercial activated carbon (AP3-60 from Chemviron, Germany) are measured using a Rubotherm Magnetic Suspension Balance (MSB) (Bochum, Germany) in a wide temperature and pressure range. The data is used to fit the temperature dependent parameters of Langmuir and Sips (Langmuir-Freundlich) isotherms and to determine the Henry's constants as well as isosteric heats. Based on this evaluation different methods to evaluate the data are compared and discussed. With the pure isotherm parameters of the Sips isotherm binary adsorption is predicted using an empirical binary Sips equation and ideal adsorbed solution theory (IAST). The results are compared to binary measurements in the same MSB applying a gravimetric-chromatographic metho

Fixed bed adsorption of CO2/H2 mixtures on activated carbon: experiments and modeling

We present breakthrough experiments in a fixed bed adsorber packed with commercial activated carbon involving feed mixtures of carbon dioxide and hydrogen of different compositions. The experiments are carried out at four different temperatures (25°C, 45°C, 65°C and 100°C) and seven different pressures (1bar, 5bar, 10bar, 15bar, 20bar, 25bar and 35bar). The interpretation of the experimental data is done by describing the adsorption process with a detailed one-dimensional model consisting of mass and heat balances and several constitutive equations, such as an adsorption isotherm and an equation of state. The dynamic model parameters, i.e. mass and heat transfer, are fitted to one single experiment (reference experiment) and the model is then further validated by predicting the remaining experiments. Furthermore, the choice of the isotherm model is discussed. The assessment of the model accuracy is carried out by comparing simulation results and experimental data, and by discussing key features and critical aspects of the model. This study is valuable per se and a necessary step toward the design, development and optimization of a pressure swing adsorption process for the separation of CO2 and H2 for example in the context of a pre-combustion CO2 capture process, such as the integrated gasification combined cycle technolog

Competitive adsorption equilibria of CO2 and CH4 on a dry coal

Gases like CO2 and CH4 are able to adsorb on the coal surface, but also to dissolve into its structure causing the coal to swell. In this work, the binary adsorption of CO2 and CH4 on a dry coal (Sulcis Coal Province, Italy) and its swelling behavior are investigated. The competitive adsorption measurements are performed at 45 °C and up to 190bar for pure CO2, CH4 and four mixtures of molar feed compositions of 20.0, 40.0, 60.0 and 80.0% CO2 using a gravimetric-chromatographic technique. The results show that carbon dioxide adsorbs more favorably than methane leading to an enrichment of the fluid phase in CH4. Coal swelling is determined using a high-pressure view cell, by exposing a coal disc to CO2, CH4 and He at 45 and 60 °C and up to 140bar. For CO2 and CH4 a maximum swelling of about 4 and 2% is found, whereas He shows negligible swelling. The presented adsorption and swelling data are then discussed in terms of fundamental, thermodynamic aspects of adsorption and properties which are crucial for an ECBM operation, i.e. the CO2 storage capacity and the dynamics of the replacement of CH4 by CO

Capillary Heterogeneity in Sandstone Rocks During CO2/Water Core-flooding Experiments

AbstractWe have successfully applied a novel experimental technique to measure drainage capillary pressure curves in reservoir rocks with representative reservoir fluids at high temperatures and pressures. The method consists of carrying out 100% CO2 flooding experiments at increasingly higher flow rates on a core that is initially saturated with water and requires that the wetting-phase pressure is continuous across the outlet face of the sample. Experiments have been carried out on a Berea Sandstone core at 25 and 50°C and at 9MPa pore pressure, while keeping the confining pressure at 12MPa. Measurements are in good agreement with data from mercury intrusion porosimetry. The technique possesses a great potential of applicability due to the following reasons: (a) it can be applied in conjunction with steady-state relative permeability measurements, as it shares a very similar experimental configuration; (b) it is faster than traditional (porous-plate) techniques used for measuring capillary pressure on rock cores with reservoir fluids; (c) by comparison with results from mercury porosimetry, it allows for the estimation of the interfacial and wetting properties of the CO2/water system, the latter being unknown for most rocks; (d) by combination with X-ray CT scanning, the method allows for the observation of capillary pressure–saturation relationships on mm-scale subsets of the rock core. The latter are of high relevance as they directly and non- destructively measure capillary pressure curve heterogeneity in sandstone rocks

Operability-economics trade-offs in adsorption-based CO2_2 capture process

Low-carbon dispatchable power underpins a sustainable energy system, providing load balancing complementing wide-scale deployment of intermittent renewable power. In this new context, fossil fuel-fired power plants must be coupled with a post-combustion carbon capture (PCC) process capable of highly transient operation. To tackle design and operational challenges simultaneously, we have developed a computational framework that integrates process design with techno-economic assessment. The backbone of this is a high-fidelity PCC mathematical model of a pressure-vacuum swing adsorption process. We demonstrate that the cost-optimal design has limited process flexibility, challenging reactiveness to disturbances, such as those in the flue gas feed conditions. The results illustrate that flexibility can be introduced by relaxing the CO$_2$ recovery constraint on the operation, albeit at the expense of the capture efficiency of the process. We discover that adsorption-based processes can accommodate for significant flexibility and improved performance with respect to the operational constraints on CO$_2$ recovery and purity. The results herein demonstrate a trade-off between process economics and process operability, which must be effectively rationalised to integrate CO$_2$ capture units in the design of low-carbon energy systems.Comment: Pre-print paper currently under review. 32 pages, 6 figures. The first two authors contributed equally to this wor

Reliable measurement of near-critical adsorption by gravimetric method

A gravimetric apparatus is used to measure the excess adsorption at high pressure. The equipment consists of a Rubotherm magnetic suspension balance, which allows to measure also the density of the fluid. In order to obtain the excess adsorbed amount, the measured weight has to be corrected with a buoyancy term, for which the density of the adsorbing fluid has to be known at each experimental conditions. Therefore the homogeneity of density in the high-pressure cell plays a fundamental role in determining the accuracy of the measured excess adsorbed amounts. This paper is intended to show the impact of the actual approach to thermostating the unit on the density distribution of the adsorbing fluid inside the high-pressure cell. Namely, by changing the inlet position of the heating fluid, large differences in the measured excess adsorption are produced. The closer to the critical point of the fluid, the stronger are these differences. An optimum configuration for our measuring device has been found and it has been used to study the adsorption of carbon dioxide (CO2) on Filtrasorb 400 activated carbon at supercritical and near-critical condition