Adsorption Mechanism Of Methane Research Articles

Quantum physisorption behavior of methane in nanoporous shales: Model and new mechanism

Methane adsorption in nanoporous shales plays a significant role in the storage and flow of shale gas. Although previously numerous theoretical models have been proposed to describe the gas adsorption behavior, how the potential energy distribution in nanopores impacts gas adsorption remains unclear. The mechanism of methane adsorption in extremely complicated nanopores is still no satisfactory explanation. In this study, a total of 28 shale samples located in the Paleozoic-Mesozoic strata from different sedimentary environments were utilized to investigate the nature and process of quantum physisorption of methane in nanoporous shales. It was suggested that methane confined within nanoscale space shows a quantum effect, and the physisorption behavior of methane may be the result of molecular energy level transition. Further, the quantum physisorption model was developed by considering the quantum potential energy distribution, maximum adsorption amount, gas temperature and gas pressure, which matches the isothermal adsorption data well for a various of shales under widely varied temperature and gas pressure condition. The adsorption amounts of methane with different energy levels can also be obtained by the model. Finally, a new mechanism of the methane quantum physisorption in nanoporous shales was proposed. Uneven spatial distribution of gas molecules and elementary space of gas adsorption are the intrinsic factors of controlling the quantum physisorption behavior. The former is mainly influenced by minimum potential unit (E0) and gas temperature, while the latter is represented by the parameter of maximum adsorption amount (nL) which impacted by specific surface area and gas temperature. Gas pressure only affects the molecule concentration with different energy levels, but does not affect the spatial distribution of molecule. E0 and nL vary in shales from different sedimentary environments, due to the difference in organic matter properties (TOC content and kerogen type), which indirectly impacts the methane physisorption behavior. These results are helpful to deeply understand the storage of methane in shales and improve the efficient development of shale gas. In addition, this study is expected to open a new research direction about the quantum physisorption of gas in nanoporous media.

Investigation of methane adsorption mechanism in Low-Rank bituminous coal considering internal water Distribution: A molecular simulation and experimental study

The impact of high internal water in low-rank bituminous coal on methane adsorption properties has significant implications for the extraction and utilization of low-rank coalbed methane. Mercury-in-pressure (MIP), low-pressure N2/CO2 adsorption (LPNA/CA), low-field nuclear magnetic resonance (NMR), and isothermal adsorption of methane experiments were conducted on two types of low-rank bituminous coal from neighboring mines in the same region. Additionally, molecular dynamics simulations were performed by constructing a molecular model of the low-rank bituminous coal. Experimental and simulation results indicate that the unique internal water distribution characteristics of low-rank bituminous coal and the methane adsorption mechanism under its influence. The initial adsorption of water molecules occurs within the micropores of coal, forming a microporous filling. As relative humidity increases, water molecules migrate spontaneously to the macropores, but they do not seal the pores. This phenomenon is observed in low-rank bituminous coals. The internal water primarily affects the microporous filling of methane, yet it does not alter the absolute predominance of this form in adsorption, and the internal water spontaneously migrates to provide part of the filling site for the methane molecules with the increase in adsorption pressure. The results provide valuable insights into optimizing the extraction strategies for low-rank coalbed methane resources.

Application of metal-organic frameworks in natural gas storage

In recent years, fossil energy sources have been diminishing as global energy demand grows. The combustion of traditional petroleum fuels also produces large amounts of greenhouse gases, contributing to global warming. Natural gas has attracted widespread attention as a relatively cleaner and cheaper energy source. The main component of natural gas is methane. However, due to the poor safety of methane under standard conditions, its application in practical storage and transmission is greatly limited. To use methane fuel routinely, the development of a safe, low-cost, and efficient methane storage strategy is an urgent issue. Metal-organic skeletons (MOFs), as a new class of efficient adsorption and separation materials, have received great attention in the field of gas storage due to their high specific surface area, designability, and size tunability. In this paper, the basic characteristics of MOFs are first introduced. Then, the mechanism of methane adsorption by MOFs is explained and its main influencing factors are summarized. Finally, this paper also proposes relevant modification strategies to improve the ability of MOFs to store methane.

A developed dual-site Langmuir model to represent the high-pressure methane adsorption and thermodynamic parameters in shale

Comprehending the mechanism of methane adsorption in shales is a crucial step towards optimizing the development of deep-buried shale gas. This is because the methane adsorbed in shale represents a significant proportion of the subsurface shale gas resource. To properly characterize the methane adsorption on shale, which exhibits diverse mineral compositions and multi-scale pore sizes, it is crucial to capture the energy heterogeneity of the adsorption sites. In this paper, a dual-site Langmuir model is proposed, which accounts for the temperature and pressure dependence of the density of the adsorbed phase. The model is applied to the isothermals of methane adsorption on shale, at pressures of up to 30 MPa and temperatures ranging from 40 to 100 °C. The results show that the proposed model can describe the adsorption behavior of methane on shale more accurately than conventional models, which assume a constant value for the density of adsorbed phase. Furthermore, the proposed model can be extrapolated to higher temperatures and pressures. Thermodynamic parameters were analyzed using correctly derived equations. The results indicate that the widely used, but incorrect, equation would underestimate the isosteric heat of adsorption. Neglecting the real gas behavior, volume of the adsorbed phase, and energy heterogeneity of the adsorption sites can lead to overestimation of the isosteric heat of adsorption. Furthermore, the isosteric heat evaluated from excess adsorption data can only be used to make a rough estimate of the real isosteric heat at very low pressure.

Methane adsorption of nanocomposite shale in the presence of water: Insights from molecular simulations

Accurately predicting gas adsorption in shale gas is essential for estimating production capacity and optimizing extraction processes. However, the complex nature of shale reservoirs (heterogeneous structure, organic–inorganic nanopores, moisture content, etc.) presents significant challenges in understanding the mechanism of methane adsorption. To address these challenges, a nanocomposite shale model incorporating kerogen and illite with slit pore widths of 1 nm, 2 nm, and 4 nm is created. The hybrid grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations are used to investigate methane adsorption in rigid and flexible shale models (kerogen matrix and clay mineral) at three different temperatures, several pressures, and various moisture contents. The simulation results show that the methane density distributions revealed asymmetrical behavior due to the heterogeneous surfaces. Factors such as kerogen flexibility, pore width, moisture content, temperature, and pressure were found to significantly influence fluid behavior. A comparative analysis between the rigid and flexible shale models, considering varying pore widths under both dry and moist conditions, revealed noteworthy differences. Kerogen's flexibility led to increased surface roughness and decreased adsorbed density. Conversely, the presence of water reduced methane adsorption sites in illite, resulting in diverse and complex behaviors dependent on pore widths in the shale model. Finally, the results suggest that using adsorbed volume instead of classical adsorption models is recommended for converting excess adsorption into absolute adsorption. This alternative approach can provide more accurate predictions and enhance the understanding of adsorption in shale gas systems.

Simulation Study on Molecular Adsorption of Coal in Chicheng Coal Mine.

To study the importance of the adsorption mechanism of methane (CH4) and carbon dioxide (CO2) in coal for coalbed methane development, we aimed to reveal the influence mechanism of adsorption pressure, temperature, gas properties, water content, and other factors on gas molecular adsorption behavior from the molecular level. In this study, we selected the nonsticky coal in Chicheng Coal Mine as the research object. Based on the coal macromolecular model, we used the molecular dynamics (MD) and Monte Carlo (GCMC) methods to simulate and analyze the conditions of different pressure, temperature, and water content. The change rule and microscopic mechanism of the adsorption amount, equal adsorption heat, and interaction energy of CO2 and CH4 gas molecules in the coal macromolecular structure model establish a theoretical foundation for revealing the adsorption characteristics of coalbed methane in coal and provide technical support for further improving coalbed methane extraction.

Control mechanism of small organic molecules on methane adsorption capacity of coal

Small organic molecules have a significant influence on methane adsorption of different rank coals. To explore the control mechanism of small organic molecules on methane adsorption capacity of coal, in this paper, based on the relative content of small organic molecules in different rank coals obtained by extraction experiment, the complex models of raw and residual coal with different ranks were constructed. The influence of small organic molecules on the methane adsorption of coal was analyzed by molecular simulation calculation. The control mechanism of small organic molecules on the methane adsorption capacity of coal was further discussed. The results showed that for the low-rank coal with high content of small organic molecules, small organic molecules mainly play the role of blocking and separating so that separate the larger coal pores into more micropores, thereby the methane adsorption capacity of coal pores is increased. For the middle-rank coal with the same high content of small organic molecules, micropores are closed by small organic molecules, so that its methane adsorption capacity is reduced. For the high-rank coal, due to its low content of small organic molecules, few pores are blocked or closed, that the existence of small organic molecules has no significant effect on its methane adsorption capacity. The research results help to reveal the microscopic mechanism of methane adsorption of coal, which has great significance to the development of coalbed seam gas and mine gas disaster prevention.

Mechanism of Methane Adsorption and Diffusion and its Influencing Factors Based on the Fractal Structure of Coal-based Porous Media

Mechanism of Methane Adsorption and Diffusion and its Influencing Factors Based on the Fractal Structure of Coal-based Porous Media

Insights into shale gas adsorption and an improved method for characterizing adsorption isotherm from molecular perspectives

• The specific multiscale inter-connected pore models of shale are proposed. • The dependences of diverse media properties and pore size on CH 4 adsorption behaviors are clearly examined by GCMC simulations. • An improved method for calibrating isothermal adsorption experimental result is proposed. Gas adsorption behavior in shale complex porous media is of great importance to gas in place (GIP) evaluation and exploration. At present, the mechanism of methane adsorption has not been adequately investigated in multi-size nanoscale space via experimental and simulation methods. In this work, the specific multi-size inter-connected pore models are proposed to study the adsorption isotherm and density profiles based on the veritable pore structure characterizations of shale samples. The grand canonical Monte Carlo (GCMC) simulations are utilized to give insight into methane adsorption behavior in organic matter and inorganic clay minerals with multi-size pores at reservoir conditions. Based on the proposed models, the effects on adsorption behaviors of methane molecules under the simultaneous action of diverse media surfaces and variable pore sizes are carefully examined. In addition, the adsorbent-adsorbate interactions, which involve the cation exchange and the isosteric heat variations with media surfaces, are carefully investigated to reveal complex adsorption behaviors in shale nanopores. Considering the identified factors influencing methane adsorption and the shortcomings of experimental test, an improved method for calibrating the gas adsorption isotherms in shale is proposed from a molecular perspective, which takes into account several factors, including i) the accessible volume of methane molecules, ii) the pore size distribution in shale, and iii) the composition of organic matter and clay minerals. The GCMC simulation results are integrated into the process of calibrating experimental results in a case study with a shale sample from the Longtan Formation in the Sichuan Basin, China. The findings of this study advance the in-depth understanding of gas adsorption mechanisms in complex nanoporous structure and provide a reliable approach to evaluate the GIP in shale.

Nanopore Structure and Origin of Lower Permian Transitional Shale, Eastern Ordos Basin, China

Nanopores in the shale play a vital role in methane adsorption, and their structural characteristics and origins are of great significance for revealing the mechanism of methane adsorption, desorption, and diffusion. In this paper, through low-temperature ashing and low-pressure gas adsorption experiments, the nanopore structure of original shales and ashed shales was quantitatively characterized, and the nanopore origins in the transitional shale of lower Permian in eastern Ordos Basin were analyzed. The results show that the pore volume (PV) and specific surface area (SSA) of nanopores in transitional shale reservoirs are 0.0217–0.0449 cm3/g and 13.91–51.20 m2/g, respectively. The average contribution rates of micropores (<2 nm), mesopores (2–50 nm), and macropores (50–100 nm) to PV are 18.78, 72.26, and 8.96%, respectively, and the average contribution rates to SSA are 66.19, 33.10, and 0.71%, respectively. In addition, it is found that the average contribution rates of inorganic minerals and organic matter to the SSA of micropores are 55.9 and 44.1%, respectively, and the average contribution rates to the SSA of mesopores are 92.3 and 7.7%, respectively. Combining the adsorption properties of the main clay minerals and kerogen in shale, it is concluded that organic pores control the adsorption of methane with an absolute advantage in transitional shales. It is of great significance to understand the mechanism of methane occurrence, desorption, and diffusion in shales by clarifying the origins of multiscale pores.

Effects of Pore Structure of Different Rank Coals on Methane Adsorption Heat

Adsorption thermodynamic characteristics are an important part of the methane adsorption mechanism, and are useful for understanding the energy transmission mechanism of coalbed methane (CBM) migration in coal reservoirs. To study the effect of coal pore characteristics on methane adsorption heat, five different types of rank coals were used for low-pressure nitrogen, low-pressure carbon dioxide, and methane adsorption experiments. Pore structure and adsorption parameters, including maximum adsorption capacity and adsorption heat, were obtained for five coal samples, and their relationships were investigated. The results show that the low-pressure nitrogen adsorption method can measure pores within 1.7–300 nm, while the low-pressure carbon dioxide adsorption method can measure micropores within 0.38–1.14 nm. For the five coal samples, comprehensive pore structure parameters were obtained by combining the results of the low-pressure nitrogen and carbon dioxide adsorption experiments. The comprehensive results show that micropores contribute the most to the specific surface area of anthracite, lean coal, fat coal, and lignite, while mesopores contribute the most to the specific surface area of coking coal. Mesopores contribute the most to the pore volume of the five coal samples. The maximum adsorption capacity has a significant positive correlation with the specific surface area and pore volume of micropores less than 2 nm, indicating that methane is mainly adsorbed on the surface of micropores, and can also fill the micropores. The adsorption heat has a significant positive correlation with the specific surface area and pore volume of micropores within 0.38–0.76 nm, indicating that micropores in this range play a major role in determining the methane adsorption heat.

Influence of the Pore Structure on the Methane Adsorption Mechanism in the Upper Triassic Lacustrine Shales from the Western Sichuan Basin, China

Influence of the Pore Structure on the Methane Adsorption Mechanism in the Upper Triassic Lacustrine Shales from the Western Sichuan Basin, China

Study on heat adsorption based on the experiment of coal adsorption of methane by using the weight method

The study of heat adsorption plays an important role in understanding the mechanism of methane adsorption from coal. The gravimetric method was used to conduct 303 K, 308 K, and 313 K isotherm adsorption experiments on two sets of coal samples; the equivalent heat adsorption of methane adsorption by coal was calculated, and the thermodynamic properties of coal adsorption of methane were analyzed. The research results showed that the calculated isometric heat of adsorption is 30.51 kJ/mol and 23.14 kJ/mol in the experimental temperature, pressure, and corresponding adsorption capacity, respectively, indicating that the adsorption of methane by coal belongs to physical adsorption. The increase in adsorption capacity increases monotonously and slowly, which is the result of the dominant interaction force between the adsorbed methane molecules. The 318 K isotherm adsorption characteristics are predicted using the 303 K and 313 K isothermic adsorption heat data, which are compared with the laboratory measured isotherm adsorption data. The two groups have the same trend. The relative errors of the two groups of coal samples are between 2.26%-5.72% and 0.29%-2.19%, respectively. A new method is proposed for using isosteric heat adsorption to predict isothermal adsorption characteristics under different temperature and pressure conditions.

Mechanisms of shale gas adsorption: Insights from a comparative study on a thermodynamic investigation of microfossil-rich shale and non-microfossil shale

Understanding the methane adsorption mechanism in shale, especially for shale with abundant microfossils, is significant for guiding the exploration and development of shale gas. In this study, two groups of organic-rich shale samples derived from the Wufeng-Lungmachi formations were selected for methane adsorption experiments: one core sample contains abundant radiolarian fossils, while the other core sample does not contain any radiolarian fossils. Through a series of calculations and comparisons of thermodynamic studies between radiolarian-rich shale and non-radiolarian shale, the results show that the absolute adsorption of methane in radiolarian-rich shale is greater than the absolute adsorption of methane in non-radiolarian shale at the same pressure and temperature. In addition, the difference in the thermodynamic parameters such as the isosteric heat of adsorption (Qst), enthalpy change (ΔH), standard entropy change (ΔS0) and Gibbs free energy change (ΔG) between radiolarian-rich shale and non-radiolarian shale indicates that the interaction between radiolarian-rich shale and methane molecules is strong, and that radiolarian-rich shale is likely to have more adsorbed methane than non-radiolarian shale under similar geological conditions. Two plausible explanations are proposed to explain the methane adsorption mechanisms in radiolarian-rich shale. On the one hand, the biological pores developed in radiolarian fossils provide some adsorption sites for methane molecules. On the other hand, the amorphous silica in the radiolarian fossils can combine with organic molecules to form a new functional group called trimethylsilane, which possesses a hydrophobic character. The organic-inorganic pore surface provides a new perspective for interpreting methane adsorption mechanisms in shale.

New Adsorption Models for Entirely Describing the Adsorption Isotherm and Heat of Methane in Heterogeneous Nanopore Structures of Coal.

To understand the adsorption mechanism of methane in heterogeneous nanopore structures of coal, integral adsorption models based on linear, exponential, hyperbolic and quadratic energy distribution functions are established. The adsorption energy domain of the new models is assumed to be a finite interval. These new adsorption models can describe both the adsorption isotherm and the adsorption heat. A volumetric method of adsorption with a microcalorimetry system is used to measure the adsorption isotherms and integral heat, and then the parameters of the new models are obtained by fitting the experimental data. Since the adsorption heat can be different for different adsorption models, it is necessary to fit the adsorption isotherms and heat simultaneously. The fitting results of the adsorption isotherms and heat show that the new models are able to describe the experimental data better than the Langmuir model. By comparing the fitting results and the effective range of adsorption energy of the different adsorption models, it is shown that the exponential energy distribution function is the most reasonable model for methane adsorption in coals, which can be used to evaluate the energetic heterogeneity of nanopores in coal samples. The decreasing exponential energy distributions of three coal samples indicate that a larger adsorption energy corresponds to fewer adsorption sites in the coal samples. The proportion of high adsorption energy is related to the micro-nanopore volume in the coal samples.

Effects of nano-pore and macromolecule structure of coal samples on energy parameters variation during methane adsorption under different temperature and pressure

Research on methane adsorption in coal reservoirs by combining adsorption energy variation with molecular structure helps to explain the micro adsorption mechanism of methane. According to experimental data obtained from methane isothermal adsorption at different temperatures (20, 30 and 40 degrees centigrade) and pressure (0 ~ 10 MPa), effects of temperature and pressure on the energy parameters (adsorption potential, adsorption space, surface free energy and isosteric heat of adsorption) of middle and high rank coal samples were discussed from the angles of adsorption kinetics and thermodynamics. Moreover, pore and macromolecular structure of all the samples were elucidated by employing low temperature liquid nitrogen, carbon dioxide adsorption and infrared spectroscopy. Then, relationship between micro-pore structure, chemical structure parameters and adsorption energy variation were discussed. The following results has been achieved. 1) As adsorption pressure increases, reduction rate of surface free energy grows rapidly at the early period of lower pressure stage (0 ~ 8 MPa), then it becomes less than that of higher pressure stage (8 ~ 10 MPa). At the same adsorption pressure, reduction value rate of surface free energy decreases as coal rank increase. 2) Increasing temperature causes the decrease of adsorption capacity (VL and PL), and the sensitivity of temperature to adsorption capacity of high rank samples is stronger than that of middle rank samples. As Ro, max increases, initial isosteric heat of adsorption gradually increases. 3) In contrast to meso-macropore, micro-pore is critical affecting adsorption capacity. Larger specific surface area and adsorption potential of micro-pore results in a decrease in the surface free energy. 4) When Ro, max is less than 2.5%, the graphitization degree of aromatic increase linearly, and the length of the aliphatic chain reaches the minimum value. After that, macromolecule parameters varies smoothly. Then, functional groups variation affects the micro-pore structure evolution, whereas has a weak effect on the meso-macropore structure. Moreover, as apparent aromaticity (fa) increases, maximum surface free energy of coal sample increases linearly and the initial isosteric heat of adsorption increases exponentially, respectively. Above all, gas adsorption energy variation affects methane adsorption capacity, then it is also influenced by the micro-pore and internal chemical structure.

Methane Adsorption Interpreting with Adsorption Potential and Its Controlling Factors in Various Rank Coals

Water content, metamorphism (coal rank) particle size, and especially pore structure, strongly influence the adsorption capacity of coal to methane. To understand the mechanism of methane adsorption in different rank coals, and its controlling factors, isothermal adsorption experiments with different coal ranks, moisture contents and particle sizes at the temperature of 303.15 K were conducted. In addition, the pore structures of coals were investigated through N2 adsorption/desorption experiments at the low-temperature of 77 K for selected coals from the Junggar Basin of NW China, Qinshui Basin and Ordos Basin of north China. Moreover, the adsorption potential of methane on the surface of the coal matrix was calculated, the controlling factors of which were discussed. The obtained methane isothermal adsorption result shows that the Langmuir volume (VL) of coal is independent of the particle size, and decreases with the increase of moisture content, which decreases first and then increases when the coal rank increases. Combined with the pore structure by the N2 adsorption at 77 K, VL increases with the increase of pore surface area and pore volume of coal pores. Besides, the adsorption potential of all selected coals decreased with the increase of the methane adsorption volume, showing a negative relationship. The interesting phenomena was found that the surface adsorption potential of the coal matrix decreases with the increase of moisture content, and increases with the decrease of particle size at the same pressure. With the same adsorption amount, the adsorption potential on the surface of coal matrix decreases first, and then increases with the increase of coal rank, reaching a minimum at Ro,m of 1.38%, and enlarging with the increase of pore surface area and the pore volume of coal pores. These findings may have significant implications for discovering CBM accumulation areas and enhancing CBM recovery.

Microscopic mechanism of methane adsorption in shale: Experimental data analysis and interaction potential simulation

The adsorbed methane in shale determines the geological reserves of shale gas and production of the well. Therefore, it is of great importance to clarify the microscopic mechanism of methane adsorption in shale for evaluating the geological condition and formulating exploitation plan. In this study, the experimental data and the simulation result were combined to research the microscopic mechanism of methane adsorption in shale. The experimental data for 99 groups of low-temperature nitrogen adsorption and methane isothermal adsorption were collected, to calculate the number of adsorption layers of methane in shale at the micro-level. The surface, microfracture and micropore structures of the organic matter in shale were constructed by graphite structure with the hydroxyl group. Thereafter, they were used to simulate the interaction potential of the organic matter on methane molecules and the adsorption layer's number by using the theory of intermolecular interaction potential field. Furthermore, the effects of different fracture widths and pore diameters on methane adsorption were analyzed and verified. The results show that the adsorption capacity of methane in shale is related to shale's structure, and the order of the adsorption capacity is surface < microfracture < micropore. Meanwhile, the multilayer adsorption can occur in microfractures with a width of 1.44–1.59 nm and micropores with a diameter of 1.49–1.83 nm, whereas the surface structure just can be the monolayer adsorption. These quantitatively results establish a foundation for the development of adsorption theory for methane in the shale.

A Metal–Organic Framework Based Methane Nano‐trap for the Capture of Coal‐Mine Methane

AbstractAs a major greenhouse gas, methane, which is directly vented from the coal‐mine to the atmosphere, has not yet drawn sufficient attention. To address this problem, we report a methane nano‐trap that features oppositely adjacent open metal sites and dense alkyl groups in a metal–organic framework (MOF). The alkyl MOF‐based methane nano‐trap exhibits a record‐high methane uptake and CH4/N2 selectivity at 298 K and 1 bar. The methane molecules trapped within the alkyl MOF were crystalographically identified by single‐crystal X‐ray diffraction experiments, which in combination with molecular simulation studies unveiled the methane adsorption mechanism within the MOF‐based nano‐trap. The IAST calculations and the breakthrough experiments revealed that the alkyl MOF‐based methane nano‐trap is a new benchmark for CH4/N2 separation, thereby providing a new perspective for capturing methane from coal‐mine methane to recover fuel and reduce greenhouse gas emissions.

A Metal-Organic Framework Based Methane Nano-trap for the Capture of Coal-Mine Methane.

As a major greenhouse gas, methane, which is directly vented from the coal-mine to the atmosphere, has not yet drawn sufficient attention. To address this problem, we report a methane nano-trap that features oppositely adjacent open metal sites and dense alkyl groups in a metal-organic framework (MOF). The alkyl MOF-based methane nano-trap exhibits a record-high methane uptake and CH4 /N2 selectivity at 298 K and 1 bar. The methane molecules trapped within the alkyl MOF were crystalographically identified by single-crystal X-ray diffraction experiments, which in combination with molecular simulation studies unveiled the methane adsorption mechanism within the MOF-based nano-trap. The IAST calculations and the breakthrough experiments revealed that the alkyl MOF-based methane nano-trap is a new benchmark for CH4 /N2 separation, thereby providing a new perspective for capturing methane from coal-mine methane to recover fuel and reduce greenhouse gas emissions.