Proposed System for Hydrogen Production from Methane Hydrate with Sequestering of Carbon Dioxide Hydrate

Kinetic mechanisms of methane hydrate replacement and carbon dioxide hydrate reorganization

Gas hydrates, which contain the largest amount of methane on our planet, are a promising source of natural gas after the depletion of traditional gas fields, the reserves of which are estimated to last about 50 years. Therefore, it is necessary to study the methods for extracting gas from gas hydrates in order to select the best of them and make reasoned technological and engineering decisions in the future. One of these methods is the replacement of methane in its hydrate with carbon dioxide. This work studies the construction of a mathematical model to observe this method. The following process is considered in this article: on one side of a porous reservoir, initially saturated with methane and its hydrate, carbon dioxide is injected; on the opposite side of this reservoir, methane and/or carbon dioxide are extracted. In this case, both the decomposition of methane hydrate and the formation of carbon dioxide hydrate can occur. This problem is stated in a one-dimensional linear formulation for the case of negative temperatures and gaseous carbon dioxide, which means that methane, carbon dioxide, ice, methane, and carbon dioxide hydrates may be present in the reservoir. A mathematical model is built based on the following: the laws of conservation of masses of methane, carbon dioxide, and ice; Darcy’s law for the gas phase motion; equation of state of real gas; energy equation taking into account thermal conductivity, convection, adiabatic cooling, the Joule — Thomson effect, and the release or absorption of latent heat of hydrate formation. The modelling assumes that phase transitions occur in an equilibrium mode and that methane can be completely replaced by carbon dioxide. The results of numerical experiments are presented.

During the formation of gas hydrate crystals, stable isotope fractionation of the guest molecules occurs. For methane and ethane, hydrogen stable isotope fractionation has been reported in the range of a few per mil, but for carbon stable isotope fractionation, there is no difference in 13C, or the difference is below the detection limit. As for carbon dioxide in natural gas hydrates, little is known about stable isotope fractionation of carbon dioxide. In this study, we report the stable isotope fractionation during formation of synthetic carbon dioxide hydrates under various temperature conditions. Fine ice powder and pure carbon dioxide gas were introduced into a pressure cell and adjusted to be above the equilibrium pressure of the hydrate and below the liquefaction pressure of the carbon dioxide at each temperature. After the formation of carbon dioxide hydrate, the residual gas was collected and the hydrate was recovered under the liquid nitrogen temperature to obtain the hydrate-bound gas. The carbon stable isotope ratios of carbon dioxide in the hydrate and residual gases were determined using a stable isotope mass spectrometer. Over a wide range from 243 K to 278 K, the 13C of residual gas was always larger than the hydrate phase, indicating that the carbon dioxide hydrate preferentially enclathrated lighter molecules. This trend is consistent with the results of a previous study. These results suggest that the equilibrium pressure of 13CO2 hydrate is slightly higher than that of 12CO2 hydrate.

Nowadays, methane hydrates are viewed as a potential energy source but their exploitation is challenging for many technical and environmental reasons. The thermodynamic conditions that govern the formation, stability, and dissociation of gas hydrates, mostly methane and carbon dioxide hydrates, their structure, mechanical, thermal, and flow properties are addressed in a fluid-saturated porous media context and brought in a computational format. Standard and prospective production methods are reviewed. Gas hydrates are solid crystalline compounds, looking like ice, formed by natural gas components, e.g. methane, ethane, hydrogen sulfide, carbon dioxide, which occupy lattice positions in the water structure. The gas is not chemically bound to water. Hydrates contribute to a large part to the mechanical properties of the hydrate-bearing sediments. Therefore, their dissociation by depressurization, thermal or chemical methods, leaves a weakened mechanical resistance that may trigger underwater slope slides over large areas. Attempts to replace methane by carbon dioxide briefly reviewed, a process that at once would enhance methane production, maintain the mechanical properties of the hydrates, and sequestrate a greenhouse gas.

Geologic accumulations of natural gas hydrates hold vast organic carbon reserves, which have the potential of meeting global energy needs for decades. Estimates of vast amounts of global natural gas hydrate deposits make them an attractive unconventional energy resource. As with other unconventional energy resources, the challenge is to economically produce the natural gas fuel. The gas hydrate challenge is principally technical. Meeting that challenge will require innovation, but more importantly, scientific research to understand the resource and its characteristics in porous media. Producing natural gas from gas hydrate deposits requires releasing methane from solid gas hydrate. The conventional way to release methane is to dissociate the hydrate by changing the pressure and temperature conditions to those where the hydrate is unstable. The guest-molecule exchange technology releases methane by replacing it with a more thermodynamically stable molecule (e.g., CO2, N2). This technology has three advantageous:it sequesters greenhouse gas,it releases energy via an exothermic reaction, andit retains the hydraulic and mechanical stability of the hydrate reservoir. Field testing of the guest-molecule exchange technology is currently in the planning stages for site within the Prudhoe Bay area, Alaska North Slope, where the average hydrate-bearing layer conditions are 6.9 MPa and 4.4°C. Previously, a numerical simulator was developed with capabilities for modeling gas hydrate production from geologic reservoirs using four technologies:depressurization,thermal stimulation,inhibitor injection andCO2-CH4 guest molecule exchange. The original form of the simulator assumed equilibrium conditions between the mobile and hydrate concentrations of CO2 and CH4; where, the mobile components are those in the aqueous, gas, and liquid-CO2 phases. Additionally the simulator ignored dissolution of CH4 into the liquid-CO2 phase. Simulation results from this simulator predicted rapid pore plugging by the injected CO2, regardless of the form of injected CO2 (i.e., subcritical gas, liquid, supercritical gas, aqueous dissolved). In support of the technical planning for the arctic field demonstration, this paper re-examines the injectivity of liquid CO2 into hydrate bearing formations, using a new kinetic exchange model between the mobile and hydrate concentrations of CO2 and CH4. Simulation results using the kinetic implementation of the simulator demonstrate the importance of guest molecule exchange kinetics in maintaining formation injectivity. Introduction G as hydrates are clathrate compounds in which water molecules encapsulate a guest molecule within a lattice structure. The lattice structure of gas hydrates form under low-temperature, high-pressure conditions via hydrogen bonding between water molecules. Gas hydrates with methane (CH4) guest molecules occur as accumulations in sedimentary formations offshore and permafrost environments where sufficiently low temperatures and high pressures exist. From an energy resource perspective, these geologic accumulations of natural gas hydrates represent a significant component of the world's organic carbon. Assessments by the United States Geological Survey (USGS) have estimated that reserves of methane in hydrate form exceed the all other fossil fuel forms of organic carbon (Booth et al., 1996). Under geologic environmental conditions, the lattice structure of a gas hydrate depends primarily on the guest molecule (Englezos, 1993; and Sloan, 1998). Interestingly, the two most prevalent emitted greenhouse gases (U.S. EPA, 2006) carbon dioxide (CO2) and methane (CH4) both form sI hydrate structures under geologic temperature and pressure conditions. Whereas their clathrate structures are similar, CO2 hydrates form at higher temperatures and have a higher enthalpy of formation compared with CH4 hydrates (Sloan, 1998).

Today the world is faced with two major energy challenges, i.e., shortage of primary energy sources and climate change. The latter is widely believed to be related to CO2 released from combustion of fossil fuels. Many options are being considered for reducing the emission of CO2 to the atmosphere, including using low carbon fuels (e.g., natural gas) and CO2 capture and storage. There are vast quantities of methane in the form of gas hydrates in marine sediments and permafrost regions. However, there are many technological challenges in recovering this low carbon fuel. As for CO2 storage, several techniques have been suggested, including their storage in the form of hydrates in sediments. It might be possible to integrate CO2 storage with methane gas production. In this work, we present the results of a series of experiments on the thermodynamic conditions and kinetics of integrated methane recovery and CO2 sequestration. The preliminary experiments were conducted at different temperature and pressure conditions, in the presence or absence of excess water, in the presence of gaseous or liquid CO2. Silica glass beads and a kaolinite-sand mixture were used to simulate marine sediments. Results of the experimental tests show a higher methane recovery rate in the system inside the methane hydrate stability zone (HSZ) and outside the CO2 HSZ. The presence of excess water noticeably slowed down the CO2 displacement reaction. It was observed that in the kaolinite-sand mixture methane recovery rate was significantly lower than those observed in the tests with silica glass beads. The experimental results infer that mass transfer plays a crucial role in methane recovery through CO2 replacement. The study suggests that the thermodynamic conditions inside methane HSZ and outside CO2 HSZ could be the optimum conditions for integration of methane recovery and CO2 storage in marine sediments. Introduction It has been identified by seismic survey there are enormous sedimentary deposits of methane hydrates worldwide (Kvenvolden, 1988 and 1993; Milkov, 2004). The methane trapped in the hydrates has been considered as potential energy source in the near future, while the known geological reserves of conventional natural gas and oil are rapidly declining parallel to expanding demands of fossil fuels. In marine sediments under seafloor naturally-occurring gas hydrates may form from either bacterial or thermogenic methane. Bacterial methane, generated by bacteria via either reduction of CO2 or acetate fermentation, usually forms structure I hydrates, while thermogenic methane is generated from organic matters buried underground (for example, in marine sediments) under high pressure and temperature conditions (Kvenvolden, 1993; Coleman, et al., 1995; Sassen, et al., 1999). Thermogetic methane is always accompanied with non-negligible concentrations of ethane, propane, and butane, and therefore, usually forms structure II hydrates.

Scientific and technological innovations are needed to realize effective production of natural gas hydrates. Whereas global estimates of natural gas hydrate reservoirs are vast, accumulations vary greatly in nature and form. Suboceanic deposits vary from disperse concentrations residing at low saturations in the pore space of unconsolidated sediments with sand-sized particles to higher concentrations residing in the fractures of sediments with clay-sized particles. Conventional methods for gas hydrate production include depressurization, thermal stimulation, and inhibitor injection. For suboceanic accumulations in sandy sediments, depressurization has been shown, through numerical simulation, to be the most feasible production technology. However, recovery efficiencies are too low to justify pursuing these energy reservoirs. Under high pressure, low temperature suboceanic conditions the hydrate structure can accommodate small molecules other than methane (CH4), such as carbon dioxide (CO2) and nitrogen (N2) in both the small and large cages. Although CO2 and N2 clathrates generally are not naturally as abundant as those of CH4, their occurrence forms the foundation of an unconventional approach for producing natural gas hydrates that involves the exchange of CO2 with CH4 in the hydrate structure. This unconventional concept has several distinct benefits over the conventional methods:the heat of formation of CO2 hydrate is greater than the heat of dissociation of CH4 hydrate, providing a low-grade heat source to support additional methane hydrate dissociation,exchanging CO2 with CH4 will maintain the mechanical stability of the geologic formation, andthe process is environmentally friendly, providing a sequestration mechanism for the injected CO2. An operational mode of the STOMP simulator has been developed at the Pacific Northwest National Laboratory that solves the coupled flow and transport equations for the mixed CH4-CO2 hydrate system under nonisothermal conditions, with the option for considering NaCl as an inhibitor in the pore water. This paper describes the numerical simulator, its formulation, assumptions, and solution approach and demonstrates, via numerical simulation, the production of gas hydrates from permafrost accumulations in sandstone formations with high gas hydrate saturations and suboceanic accumulations in sandy sediments with low hydrate saturations using the CO2-CH4 exchange technology. Introduction Gas hydrates are clathrate compounds in which water molecules encapsulate a guest molecule within a lattice structure. The lattice structure of gas hydrates form under low temperature, high pressure conditions via hydrogen bonding between water molecules. Gas hydrates with methane (CH4) guest molecules are abundant as geologic accumulations in offshore and permafrost environments where sufficiently low temperature and high pressure conditions exist. From an energy resource perspective, these geologic accumulations of natural gas hydrates represent a significant component of the world's organic carbon sources. Recent surveys by the United States Geological Survey (USGS) have estimated that reserves of methane in hydrate form exceed the all other fossil fuel forms of organic carbon (Booth et al., 1996). Under geologic environmental conditions, the lattice structure of a gas hydrate depends primarily on the guest molecule (Englezos, 1993; and Sloan, 1998). Interestingly, the two most prevalent emitted greenhouse gases (U.S. EPA, 2006) carbon dioxide (CO2) and methane (CH4) both form sI hydrate structures under geologic temperature and pressure conditions. Whereas their clathrate structures are similar, CO2 hydrates form at higher temperatures and have a higher enthalpy of formation compared with CH4 hydrates (Sloan, 1998).

Phase equilibrium for clathrate hydrates formed in the (methane, carbon dioxide or ethane) + water + ammonium chloride system

Analytical study of CO2–CH4 exchange in hydrate at high rates of carbon dioxide injection into a reservoir saturated with methane hydrate and gaseous methane

Most of the natural gas hydrates are found in deep marine sediments and permafrost regions, where the presence of salts and porous media are quite evident. With this, we develop a computationally efficient mathematical model that can expound the clathrate hydrate dynamics in a reservoir-mimicking environment. Along with proposing the chemical potential difference as the driving force to take care of the thermodynamic aspect, a nonstoichiometric reaction with nth-order kinetics is for the first time introduced in the line of adsorption kinetics. To make this thermokinetic model more rigorous, the diffusion part is further formulated with the kinetic factor, along with incorporating various practical aspects, including reaction surface renewal and hydrate formation in nanometer-sized pores of irregular and distributed particles. Finally, to examine the validity of this rigorous model, experimental case studies of methane (CH4) and carbon dioxide (CO2) hydrate formation in various porous media with pure and saline water are used. In addition, we compare the developed model with the existing formulations of gas hydrate dynamics, and it is perceived that the proposed model outperforms the existing models with reference to the experimental data of methane and carbon dioxide hydrate formation at diverse geological conditions.

Dissociation of methane and carbon dioxide hydrates: Synergistic effects

Simplified diffusion model of gas hydrate formation from ice

낮은 온도와 높은 압력에서 저분자량의 가스가 물분자들에 의해 만들어지는 격자 속으로 포집되면서 형성되는 가스하이드레이트에 대한 존재가 알려진 것은 비교적 오래 되었으나, 물과 가스에 의해 형성되어 진다는 점에서 최근 관심이 증가되고 있다. 포집되는 가스의 종류에 따라 독특한 특성을 가지고 각각의 구조 결정을 형성하는 하이드레이트는 최근 지구 온난화가스인 이산화탄소 문제와 다양한 에너지원, 특히 천연가스와 수소 에너지에 대한 연구로 크게 주목받고 있다. 따라서 본 고에서는 가스 하이드레이트 활용 분야 중에서 활발히 진행되고 있는 분야, 즉 대표적 지구온난화 가스인 이산화탄소의 심해저장과 동시에 메탄 하이드레이트 층으로부터의 천연 가스의 포집연구와 수소 저장량을 극대화시킨 수소하이드레이트에 관한 전반적인 연구동향을 소개하도록 한다. Gas hydrates are known to form by physical interactions between host water and guest gas molecules and thus can be treated as a special type of icy materials. The gas hydrates are recently highlighted because of their use to future energy source even though they were discovered naturally in the deep-sea marine sediments a long time ago. However, the present and future urgent task is to develop the efficient and safe production technology for recovering methane from gas hydrates. Here, we propose one of potential recovery processes using swapping phenomenon occurring between gaseous carbon dioxide and methane hydrate deposits. Such a swapping process provide several technological and economical advantages over conventional processes. The carbon dioxide can be directly sequestered into methane hydrate layer and simultaneously methane can be produced with a high recovery rate more than 90%. In addition, the icy powders can be effectively used as a new medium for storing hydrogen. To increase hydrogen storage capacity the icy hydrate networks need to be redesigned to create the more empty cages in which hydrogen gas can be enclathrated. Functionalized icy materials might be used in a variety of energy and environmental fields.

Molecular dynamics simulations are used to characterize the hydrates of Xe, methane, and CO2, allowing for a systematic comparison of the structural and dynamical properties for these three hydrates. Although the host−guest interaction energy for the T = 0 K structures is most attractive in the case of Xe, other structural and dynamical properties from the simulations indicate that, in fact, host−guest coupling is most important for the CO2 hydrate. Specifically, the host lattice of CO2 hydrate expands more with increasing temperature than do the lattices of the xenon and methane hydrates, and the translational and rotational dynamics of the water molecules are predicted to be most perturbed in the CO2 hydrate. The simulations predict that the CO2 and xenon hydrates have lower speed of sound values and lower themal conductivities than methane hydrate or the empty lattice.

Gas hydrates are crystalline structures formed by water molecules and compounds of low molecular weights, being formed under suitable conditions of pressure and temperature. Although initially considered as inconveniences to the natural gas industries, they are currently considered as promising alternatives for solving some important global issues, such as contributing to the reduction of effects caused by the greenhouse gases. This concern related to the control of emissions of polluting gases has mobilized hundreds of countries that, at the United Nations Climate Change Conference (COP), agreed to reduce emissions of carbon dioxide and other gases by 2100. However, despite several strategies in the reduction of carbon dioxide emissions have been proposed, many rely on political incentives and substantial investments to convert pre-existing technologies to clean technologies, making such applicability and adaptability problematic. Thus, innovative Carbon Capture and Storage (CCS) techniques are being studied, which considers the use of gas hydrates formation to trap these gases, presents perspectives of lower costs and low environmental damages, promising to overcome the above mentioned problem, besides capturing and storing adequately the carbon dioxide and methane emitted. The need for robust evaluation of the thermodynamic equilibrium of hydrate-containing systems arises in order to make the proposal feasible and used on a large scale. The present work extensively solidifies this assessment of hydrate phase equilibria by proposing the isofugacity and Gibbs energy minimization criteria coupled to the nonlinear programming for the calculation of phase equilibria in the formation of methane and carbon dioxide hydrates. The Soave-Redlich-Kong cubic equation (SRK) was used to calculate the liquid and gaseous phases, and the Van der Waals and Platteeuw models were used to describe the solid phase of the hydrate. The procedure was implemented in the General Algebraic Modeling System (GAMS) software and in the CONOPT3 solver, with some numerical procedures performed in Microsoft Office Excel. The comparison between the results obtained from the present study by the isofugacity criterion and experimental data has been carried out, allowing concluding the satisfactory prediction of the phase equilibria behavior of systems containing hydrates.