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Article

Methane Adsorption Properties in Biomaterials: A Possible Route to Gas Storage and Transportation

by
Sanya Du
1,2,
Yixin Qu
3,
Hui Li
1,* and
Xiaohui Yu
2,*
1
Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China
2
National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
3
College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China
*
Authors to whom correspondence should be addressed.
Energies 2022, 15(12), 4261; https://doi.org/10.3390/en15124261
Submission received: 3 May 2022 / Revised: 1 June 2022 / Accepted: 4 June 2022 / Published: 9 June 2022
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Abstract

:
Methane can be stored in biomaterials rapidly in hydrate form with low energy consumption. Considering the high cost of biomaterials (vegetables or fruits), agricultural wastes may be more practical. In this work, the characteristics of methane storage in two low-cost agricultural wastes, eggplant, and static water, are studied and compared. The methane adsorption rates and capacities were greatly enhanced in three biomaterials compared with that in the static water, while only corncob pith maintained relatively high gas adsorption capacity (72 v/v) and adsorption rate (~0.0300 MPa/min) in repeatable gas adsorption-desorption processes. Further investigations on the gas adsorption behavior in the corncob pith revealed that the porous structure of corncob pith generates larger specific surface areas, providing more nucleation sites for hydrate nucleation. In addition, the hydrophobic and hydrophilic performance of corncob pith components also affect the hydrate formation. The porous structure of corncob pith reduces its water activity, which decreases the stability of methane hydrate (~0.6 MPa higher at 273.15 K for equilibrium pressure than bulk phase). These results demonstrate the great gas adsorption performance and mild storage-transportation conditions of low-cost agricultural wastes and provide significant information in promoting their application in gas storage and transportation.

1. Introduction

As the growing concentration of carbon dioxide in the atmosphere causes serious environmental problems, it is of great significance to seek cleaner energy. Hydrogen may be a good solution to solve this problem, and can be produced by a hydrolysis reaction [1], but the transition to a hydrogen economy faces huge challenges [2,3]. Methane, the main component of natural gas (NG), can also serve as a cleaner fuel due to its high H to C ratio and low sulfur and nitrogen content [4]. NG reduces the CO2 emissions by about 50% compared to coal when employed for power generation, and results in a more than 20% reduction in CO2 emissions compared to gasoline when used as a transportation fuel [5,6]. In addition, methane production and storage-transportation technologies are relatively mature. Therefore, the demand for NG as an important energy source continues to grow, and the development of economical and safe technologies to store and transport NG is imperative.
Several methods have been applied to store and transport NG worldwide. Compressed natural gas (CNG) can store and transport NG in a small volume, but it suffers from safety issues and relatively poor volumetric storage capabilities [7,8]. Liquefied natural gas (LNG) has a high volumetric and energy density (600 v/v), which is based on a high cooling energy requirement [8]. Adsorbed natural gas (ANG) adsorbs NG in porous materials (such as carbon), which have relatively large surface areas and high porosity making their storage capacity higher. However, this slightly increased storage capacity and high cost of the materials impedes their application in gas transportation [8,9,10].
Clathrate hydrates are nonstoichiometric crystalline compounds composed of water and gases alone. One volume of clathrate hydrate can contain 150~180 v/v (standard temperature and pressure) of gas [11,12,13], fixing NG in a solid form in water cages at a relative high gas density. Meanwhile, its transportation cost is expected to be 18%~33% lower than LNG [13,14,15]. Therefore, clathrate hydrate provides an environmentally friendly, safe, and economical way for gas storage and transportation. The natural gas hydrate (NGH) formation reaction occurs between a two-phase (gas-liquid or gas-solid) interface, and the rate of hydrate formation decreases with increased thickness of the hydrate layer and the area covered by the hydrate particles. The addition of porous materials [16,17,18,19,20,21], such as activated carbon (AC) [18,19], dry water [17,20] and carbon nanotubes [21], can accelerate hydrate growth. Zhou et al. [19] reported that wet activated carbon with appropriate water content can adsorb methane gas over 1.6 times that of dry activated carbon due to the hydrate formation. Wang et al. [20] reported that the addition of dry water produced a much shorter induction time (usually 5~10 min) under conditions of 273.2 K and an initial pressure of 8.6 MPa. For hydrophobic multiwalled carbon nanotubes (MWCNT) with a load of 5 or 10 ppm, the methane hydrate formation rates increased by~6%; for hydrophilic MWCNT with both 0.1 ppm and 10 ppm loadings, and the methane hydrate formation rates increased by~16% [21]. Although the presence of porous materials can facilitate methane uptake and improve hydrate formation kinetics, their extra materials costs, the greatly reduced storage capacity and reproducibility issues during hydrate formation-decomposition cycles to some extent hinder the large-scale application of these materials for NG storage and transportation.
It has been found that nature can tackle gas transport through biological structures such as alveoli in lungs and stomata in the leaves of plants. Natural, prestructured materials can be used as potential media for gas storage and transportation. Gases such as carbon dioxide and methane can be stored in the form of hydrate using plants and fungi as supports (referred as bioclathrate), with accelerating gas clathrate formation kinetics and better gas storage. For example, at 273.2 K, methane adsorption capacity can reach 123 v/v for mushroom and 100 v/v for eggplant within 500 min [22]. Polyphenols and saponins in tea greatly improved methane clathrate formation kinetics, which can achieve 90% of saturated adsorption capacity in 20 min [23]. Therefore, the combination of hydrate synthesis and biological materials has great development prospects.
To date, the studied biological materials are mainly vegetables [22], which have poor reusability and high-cost. The influence of the microstructure, internal surface characteristics and the properties of internal water in biomaterials on methane gas adsorption behavior and adsorption stability has also not been clearly studied. Here, agricultural wastes are used as supporting materials to solve the above problems, which have advantages of their environmental friendliness, wide sources, short renewable cycle, low costs, and porous structures. The performance of two common agricultural wastes (corncob pith and sorghum stalk) as NG transportation and storage materials in terms of gas adsorption properties and reusability were investigated, and were compared with eggplant. The methane gas adsorption behaviors and storage and transportation conditions of these biomaterials are discussed in depth. Our work further reduces the material cost on the basis of ensuring the rapid adsorption and storage of methane and provides better insight into the positive role of biomaterials in NG storage and transportation.

2. Materials and Methods

2.1. Materials

Materials used in our work were fresh waxy corncob pith from Yunnan province, eggplant (aubergine), sorghum stalk and deionized water. Fresh waxy corncob pith and sorghum stalk were chosen because they are common agricultural wastes. Pure methane gas (from Hua Yuan Gas Chemical Co., Ltd. with a gas purity of 99.9%, Beijing, China) was applied in experiments.

2.2. Gas Adsorption and Desorption Measurements

The gas adsorption method measures the methane adsorption kinetics of samples such as static deionized water, fresh waxy corncob pith, sorghum stalk and eggplant. Experimental steps were as follows. First, the sample was placed and sealed into identical high-pressure reactor, which was equipped with two probes for the temperature sensing and a pressure sensor (from BRIGHTY INSTRUMENT CO. LTD, Beijing, China) with an accuracy of~0.05 MPa in the range of 0~20 MPa; then the reactor was flushed with methane gas 3~4 times to vent the air in the reactor before pumping to the required initial pressure (~8 MPa). The adsorption process was conducted at 273.15 K until the pressure remained constant for 8 h, and the gas adsorption was considered to be saturated. The experimental setup is shown in Figure 1. After the gas adsorption experiments finish, gas desorption experiments were conducted. At first, the pressure was decreased to ~1.5 MPa manually, and the hydrates began to decompose until the pressure no longer changes. Then the pressure was decreased to ordinary pressure and the pressure changes observed. The pressure changes were recorded throughout the process. All samples were measured with the same mass except deionized water.

2.3. Pressure-Temperature and Raman Measurements

To assess the stability of methane hydrate in biomaterials, phase equilibrium diagrams of pure methane hydrate and methane hydrate in biomaterials were measured. All methane hydrate samples were synthesized at an initial pressure of ~8 MPa and a circulating temperature of −5~5 °C in the high-pressure reactor. It was considered that hydrate synthesis was completed when no further pressure drop was observed. After hydrate synthesis, the pressure in the reactor was decreased (~0.1 MPa) at constant temperature and then stabilized for 2 h to observe whether the pressure increased significantly. If there was no significant increase, further pressure reduction (~0.1 MPa at a time) was required until a significant pressure increase occurred. The pressure value at this time was below the equilibrium pressure required at the temperature point, and the hydrate would continue to decompose until the pressure barely changed (ΔP < 0.02 MPa) over ~8 h, indicating that a new equilibrium was reached. The equilibrium pressure value was recorded and the corresponding equilibrium point could be obtained. Subsequently, the temperature was decreased and the previous steps were repeated to obtain a series of equilibrium points. The equipment used here is the same as that used in gas adsorption and desorption measurements, as shown in Figure 1.
Raman experiments were further conducted at low temperatures (liquid nitrogen environment) to investigate the formation of methane hydrate in biomaterials, which also compared fresh biomaterials and frozen biomaterials. Raman spectra of the sample were recorded using a LabRAM HR Evolution (Horiba Scientific, Kyoto, Japan) instrument, which employed a 532 nm diode-pumped solid-state laser as the excitation light source and could focus a beam diameter of ~2.5 μm on the sample.

2.4. Microstructure Measurement

An Environmental Scanning Electron Microscope (ESEM) can observe samples with a certain water content, or non-conductive samples under conditions very close to the atmospheric environment, and was used to obtain microstructure information of plant tissues. The related microstructural characterization of porous structures was carried out on an ESEM (Thermo Fisher Quattro, Waltham, MA, USA). In order to avoid sample damage during the measurement process, the measurement had to be completed as soon as possible. As a supplement, the pore size distribution was also measured using specific surface and porosity analyzer (Micromeritics ASAP 2460) by nitrogen adsorption at 77 K. The pre-vacuum dried samples were outgassed at 80 °C for 1 h prior to the pore size distribution measurements.

2.5. Relaxation Time Measurements

1H T1 and 1H T2 of pure water and water in corncob pith under different temperatures were performed on a solid state nuclear magnetic spectrometer (Ascend 600WB BRUKER) with a proton resonance frequency of 600 MHz at different temperatures (278 K, 288 K and 298 K). T2 transverse relaxation measurements of corncob pith with different moisture content were conducted on a Low Field Nuclear Magnetic Resonance (LF-NMR) analyzer (MesoMR23-060V-I).

3. Results and Discussions

3.1. Structures of Corncob Pith and Hydrate within It

The cross-sectional morphology of corn cob is shown in Figure 2a. As can be seen, the corn cob consists of the glume, woody ring, and pith. Corncob pith has a spongy-like structure with extremely low density and a large surface area [24]. Its microstructure and internal pore distributions were investigated through ESEM and BET (Brunner−Emmett−Teller) methods, respectively. As shown in Figure 2b, the corncob pith is composed of a number of bundles with micron channels. There are many dark oval−like areas on the walls of these bundles and many small pores exist in each oval−like area. As the magnification increases (Figure 2c–e), the presence of nanoscale aperture can be clearly observed, such as pores with diameters of 25.47 nm and 63.96 nm. Due to the high electron beam intensity of ESEM, the measurement process damages the sample and the internal pores of the sample expand. Therefore, it is difficult to observe smaller nanopores. This can be supplemented through the measurement of the pore size distributions. The results shown in Figure 3 and Table 1 indicate corncob pith has a wide pore size distribution, ranging from a few nanometers to several hundred nanometers (2~74 nm and 110~220 nm). Both ESEM and BET results show that corncob pith has a multi-stage pore structure, which increases the gas-liquid contact area and is beneficial to mass transfer. Therefore, corncob pith can be used as a potential material to store and transport NG in hydrate form.
In order to determine the existence form of methane in corncob pith, a methane adsorption experiment in fresh corncob pith was conducted and the results are shown in Figure 4. It can be seen that the macroscopic morphology of the corncob pith does not significantly change (Figure 4a). When the corncob pith is placed in the water at room temperature, many bubbles escape from its surface. This implies that methane can be stored in the corncob pith and the gas may be stored in the hydrate form.
Bubbles seen on the surface of the corncob pith could be methane gas produced by the decomposition of hydrates formed in the corncob pith. To verify this, Raman experiments were conducted on the samples after the methane adsorption experiment, and the results are shown in Figure 4b. The fresh and frozen corncob pith had no distinct Raman peaks, while two identified Raman peaks of hydrate characteristics appeared in the hydrates synthesized in corncob pith, which indicates the formation of methane hydrates. The two peaks with an area ratio of 3 appear at 2904 cm−1 and 2914 cm−1, indicate that structure-I (sI) [25] hydrate was formed in the corncob pith. Therefore, the methane stored in corncob pith was in the sI hydrate form.

3.2. Gas Adsorption Characteristics

After the storage form of methane gas in biomaterials was confirmed, methane uptake kinetics at 273.15 K in samples of waxy corncob pith, sorghum stalk, and eggplants were further investigated and compared with the kinetics for bulk, unstirred water. The results are shown in Figure 5a. It can be seen that the methane adsorption kinetics of these samples were much faster than that of bulk water, which adsorbed less quantities of gas at the same time under the same conditions. This indicates that gas adsorption in biomaterials is faster. Among these three biomaterials, the pressure drop of eggplant was highest after methane gas adsorption for 600 min, followed by the corncob pith; sorghum stalk has the lowest pressure drop. However, the highest pressure drop of eggplant only appeared in the first methane adsorption cycle. In the second and third methane adsorption cycles, as shown in Figure 5b, the methane adsorption capacity of eggplants significantly dropped, while the methane adsorption capacities of corncob pith and sorghum stalk changed little (Figure 5c,d). This indicates that although eggplant has the best ability to adsorb methane for the first time, its reusability is poor.
To gain a more intuitive comparison, methane molar consumption was calculated using the following equation [26,27]:
Δ n = n 0 − n t = P 0 V Z 0 R T − P t V Z t R T
where Δn represents the moles of methane gas consumed during hydrate formation, n0 and nt are the moles of methane gas at the initial stage (0 min) and t mins, respectively, P0 and Pt represent the gas pressure at the initial stage and t mins, respectively, which can be continuously recorded by a computer-controlled acquisition system. Based on the Redlich–Kwong contrast equation of state [28], the compressibility factors (Z0 and Zt) are obtained.
The gas storage capacity (GSC) of hydrate refers to the volume of methane gas that can be stored in per unit volume of biomaterial. The calculation process is as follows:
GSC = V t V m
where Vt and Vm represent the volume of the methane gas consumed at t mins and related biomaterial, which are calculated as follows:
V t = V − V m − V c
V m = M ρ
where V, Vm, and Vc are the volumes of stainless-steel reactor, related biomaterial, and biomaterial container, respectively. The volume of the biomaterial (Vm) is calculated by dividing the biomaterial mass by its true density.
Through the above formula, the amount of gas adsorbed each time can be obtained in the three-cycle methane adsorption process. The results Table 2 show that although eggplant has a high gas adsorption capacity, its reusability is poor, while sorghum stalk has a good reusability, but its GSC is low. Only corncob pith has relatively good GSC and good reusability.
The initial adsorption rates of the three kinds of biomaterials were also calculated from Figure 5b–d. For eggplant, the initial adsorption rates of the three cycles were 0.01824, 0.00742, and 0.00433 MPa/min, respectively. For corncob pith, the initial adsorption rates of the three cycles were 0.02426, 0.04002, and 0.03408 MPa/min, respectively. As for sorghum stalk, the initial adsorption rates of the three cycles were 0.05473, 0.05023, and 0.01956 MPa/min, respectively. Due to the relatively low gas uptake of sorghum stalk, its adsorption rate was not as accurate as that of eggplant and corncob pith. It can be seen that the initial adsorption rate of eggplant was relatively low and it greatly decreased with increase in the number of cycles. Although the initial adsorption rate of sorghum stalk was the highest, it also significantly decreased as the number of cycles increased. Corncob pith not only has a good adsorption rate, but does not decrease as the cycle number increases. Combined with the previous analysis of gas adsorption capacity, corncob pith has relatively high adsorption capacity and adsorption rate, and has good stability as well, which is essential for material reuse.

3.3. Gas Recovery and Thermodynamic Characteristics

In addition to considering the methane adsorption behaviors and reusability, the recovered amount of methane gas and the conditions for methane storage and transportation in the form of hydrates in biomaterials also need to be considered, which are closely related to the actual utilization efficiency and economic effectiveness. Therefore, gas recovery experiments were carried out. Besides, the thermodynamic stabilities of methane hydrate in three biomaterials were also measured and compared with bulk methane hydrate.
To avoid the rapid decomposition of hydrate caused by a sharp drop in pressure, the methane recovery from biomaterials was attempted in two steps at 273.15 K. The pressure was first decreased to ~1.5 MPa. At this point, methane hydrate began to decompose and the pressure in the reactor began to rise. After the pressure in the reactor remained stable for a period of time, the pressure was further reduced to normal pressure and its change recorded. As shown in Figure 6a–c, there was almost no hydrate decomposition behavior in the second stage, indicating that the methane hydrate was completely decomposed in the first pressure drop stage. It can also be seen that for all biomaterials, the adsorbed methane was almost completely released during the recovery process.
Thermodynamic stabilities of methane hydrates in biomaterials are shown in Figure 6d. It can be seen that at the same temperatures, the equilibrium pressure of bulk methane hydrate was about 0.6 MPa lower than those in corncob pith and eggplants, and was about 0.8 MPa lower than that in sorghum stalk. When methane was stored and transported in corncob pith, eggplant or sorghum stalk at 273.15 K, the required stable pressure only had a slight difference compared to bulk hydrate, whereas the adsorption capacity and adsorption rate of methane significantly increased in the same time. In addition, the methane stored in the three biomaterials could also be obtained easily.
From the results, it can be seen that corncob pith and other agricultural wastes with low potential value are expected to accelerate hydrate formation rates to store and transport methane in the absence of any extra power technology. Some of them can be reused many times while maintaining the gas storage abilities. Methane storage in corncob pith was reversible, and almost all the methane could be released at 273.15 K in several hours (Figure 6a–c). Besides, the required transportation conditions are only slightly higher than that of bulk methane hydrates. Therefore, the use of agricultural wastes for the storage and transportation of methane has very important economic and practical significance.

3.4. Hydrate Formation Kinetics Investigation

As corncob pith is a common agricultural waste with the relatively great performance, corncob pith was chosen as an example to explore the deep reasons for rapid storage of methane in biomaterials. In the gas adsorption process, methane gas is adsorbed in the form of hydrate. When methane is adsorbed in bulk water, a methane hydrate film is formed and thickened at the gas-water/ice interface along with the reaction process. The gas-liquid contact area gradually decreases. Therefore, molecular diffusion is hindered, and the gas adsorption rate slows down, thereby further reducing the reaction rate. By contrast, the kinetics of hydrate formation in biomaterials is greatly accelerated with a significantly increased adsorption capacity. One of the reasons for the good GSC in corncob pith may be its intrinsically large surface-to-volume ratio and micro-channel structure. Corncob pith contains a multi-level pore structure, with smaller pores in each micrometer-scale pore wall. These pores vary widely in size, ranging from a few nanometers to hundreds of nanometers. Similar to porous materials, the porous structure in biomaterials enables them to have larger specific surface areas and can serve as nucleation sites, playing a significant role in promoting hydrate nuclei. In addition, water in biomaterials is dispersed in various parts of the interior of the plant, increasing the gas-water contact area and enhancing the mass transfer. Therefore, biomaterial morphology plays a significant role in determining methane uptake kinetics in bioclathrates.
In addition to the structure, high water content plays an important role. If gas is stored in a dry porous material by means of gas adsorption under moderate conditions, GSC will be greatly reduced. For example, Zhou et al. [19] reported that wet AC with proper water content could greatly enhance methane adsorption compared with dry AC, which shows the importance of water content. As a consequence, material morphology and water content are both important in determining gas uptake kinetics in clathrates formed in biomaterials. Fresh corncob pith, which contains enough water and has a micro–nano pore structure, can greatly promote the formation of methane hydrate.
Moreover, the characteristics of the corncob pith itself have a certain influence on the adsorption of methane. The pith is mainly composed of cellulose, hemicellulose, and lignin [24,29,30]. Among the three chemical components, cellulose and lignin are hydrophobic [31,32,33], while hemicellulose is hydrophilic. The hydrophobic effect makes water molecules locally aggregate near cellulose and lignin, and does not compete with methane molecules for water molecules, thereby promoting the formation of hydrates and the adsorption of methane [34,35]. The hydrophilic effect has an opposite effect. Therefore, the hydrophobic surface can form nucleation sites of hydrate formation. Whether the internal surface as a whole promotes or inhibits the formation of hydrate and methane adsorption is the result of competition between the two effects. Due to fast methane uptake in the three cycles, it is believed that the chemical components of corncob pith promote hydrate formation and methane adsorption.

3.5. Thermodynamic Stability Investigation

The microporous structure of corncob pith also provides a special environment, which may affect the properties of the water in it, and affects the properties of the hydrate. Due to the micro–nano pore structure of the corncob pith, the existing capillary effect may influence the characteristics of water [36,37], such as water molecular motion, which further influences the gas transmission kinetics and the formation of hydrates. Moreover, the dynamics of water in biological tissue may also be related to water–biosurface interaction. Therefore, the nature of water in the corncob pith was studied with the help of 1H-NMR (Nuclear Magnetic Resonance) and LF-NMR.
First, the spin-lattice relaxation rates (1H-1/T1) and 1H spin-spin relaxation rates (1H-1/T2) of pure water and fresh corncob pith were measured at different temperatures (278 K, 288 K, and 298 K). As can be seen from Figure 7a, the 1/T1 values for water molecules in corncob pith always relaxed faster than molecules in the bulk liquid under experimental conditions. The 1/T1 for water molecules in corncob pith were 1.8 s−1 (278 K), 1.6 s−1 (288 K), and 1.2 s−1 (298 K), respectively, while the 1/T1 for bulk water molecules were 0.77 s−1 (278 K), 0.56 s−1 (288 K), and 0.35 s−1 (298 K), respectively. Regardless of temperatures, 1/T1 for confined water molecules in corncob pith was larger than bulk water, which is consistent with previous work [38,39]. This phenomenon indicates the molecular motion of water was inhibited in biomaterials compared to that of bulk water.
The 1H-1/T1 value of water provides an insight into the faster component of motions and involves an intramolecular component that is associated with rotational diffusion, and an intermolecular component that is associated with proton diffusion as well as translational diffusion. Tsukahara et al. [40] studied water molecules confined in an expanded nano-space and found that only intermolecular interaction varied with pore size, due to the inhibition of molecular translational diffusion (hydrodynamic mobility) and/or enhancement of proton diffusion (protonic mobility including proton hopping between water molecules) associated with the Grotthus proton-transfer mechanism [41]. Compared with 1H-1/T1, 1H-1/T2 can provide insight into the slower component of motions, such as water adsorbed on the internal surface. It can be seen that the decreasing temperatures increased the value of 1/T2 (Figure 7b), the same way that 1/T1 changed with temperature. In addition, both 1/T1 and 1/T2 were larger than that of pure water, indicating a faster relaxation time in biomaterials. There is a relationship between relaxation time and water activity [38]. With a slower relaxation time, the water activity is larger. As a result, the water in biomaterials (bio-water) has a smaller water activity. This phenomenon, caused by porous structure, is similar to thermodynamic inhibitors [42,43], which reduces water activity, changes the thermodynamic equilibrium conditions between water molecules and hydrocarbon molecules, and destroys the structural relationship between water molecules having cage structures. The partial pressure of interface steam partial pressure of interface vapor is reduced, thereby reducing the crystallization point of hydrates and inhibiting the formation of hydrates.
The distributions of relaxation time in corncob pith with different water contents were also measured by LF-NMR experiments, as shown in Figure 8. For fresh corncob pith “as it was”, the relaxation time showed a much wide range from 0.1 ms to more than 1 s, and two main peaks appeared at about 1 ms and 1000 ms. The distribution of relaxation time reflects some heterogeneity in the internal pore space of corncob pith [44,45,46]. Similar with the result obtained from ESEM characterization, the corncob pith had a wide pore size distribution, in which water was slowly exchanged on the NMR relaxation timescale. As the moisture decreased, longer relaxation time (peak at ~1000 ms) moved to the left and faded away, which was to be expected for the larger pores that are easy to empty. In addition, the peak at around 150 ms migrated to a faster relaxation time, while the peak at 1 ms had no obvious reduction, which may be due to the 1H nuclei of water and macromolecules with restricted mobility (cellulose, hemicelluloses, and lignin). This demonstrates that water distributes widely in biomaterials with reduced activity, consistent with 1H-NMR results.
In conclusion, the porous structure, high water content and chemical composition of corncob pith promote hydrate formation and enhance methane adsorption. Moreover, due to the limited environment provided by the porous structure of corncob pith and the interaction between its surface and water, molecular motion and activity of water molecules in corncob pith are inhibited. The reduced water activity suppressed hydrate formation conditions, explaining its slightly reduced stability.

4. Conclusions

In this work, gas adsorption-desorption experiments were conducted to investigate gas adsorption property, gas recovery and reusability of two agricultural wastes (corncob pith and sorghum stalk), eggplant, and static water. The pressure-temperature equilibrium curves of methane hydrates in these biomaterials were also measured and compared with those of bulk methane hydrates to obtain the storage and transportation conditions of natural gas. Then, taking corncob pith as an example, the effects of its porous structure, surface hydrophilicity and hydrophobicity on gas adsorption behavior and adsorption stability were investigated. The conclusions are as follows:
(1)
Both two agricultural wastes (corncob pith and sorghum stalk) and eggplant exhibited much faster gas adsorption rates and higher adsorption capacities than a static water system. Regardless of high gas recovery, only corncob pith maintained high gas adsorption rate and adsorption capacity in multiple gas adsorption-desorption cycle measurements, showing excellent gas adsorption characteristics and high gas recovery.
(2)
The thermodynamic stability of methane hydrate in the three kinds of biomass (bioclathrate) did no change significantly compared with that of bulk hydrates. At 273.15 K, the equilibrium pressures of methane hydrate in biomass were only 0.6~0.8 MPa higher than that in bulk methane hydrate.
(3)
The rapid adsorption behavior in corncob pith can be attributed to its high-water content, porous structure, and chemical composition. The porous structure produces larger specific surface area, providing more nucleation sites for hydrate formation. The overall hydrophobic properties of corncob pith also contribute to hydrate formation. Moreover, the porous structure reduces the activity of water and, therefore, slightly decreases the thermodynamic stability of bioclathrate.
The results show that low-cost agricultural waste has many advantages, such as good gas adsorption performance, high gas recovery, good reusability and moderate transportation conditions, providing a new resource for natural gas storage and transportation.

Author Contributions

Conceptualization, X.Y.; investigation, S.D.; data curation, H.L.; writing—original draft preparation, S.D.; writing—review and editing, Y.Q. and H.L.; funding acquisition, X.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Strategic Priority Research Program and Key Research Program of Frontier Sciences of the Chinese Academy of Sciences, grant numbers XDB33000000, XDB25000000, and QYZDBSSW-SLH013; the National Key R&D Program of China, grant Numbers 2021YFA1400300 and 2018YFA0305700; the Youth Innovation Promotion Association of Chinese Academy of Sciences, grant number Y202003.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

The work was partially carried out at high-pressure synergetic measurement station of Synergic Extreme Condition User Facility.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Schematic of experimental setup for the synthesis of methane hydrate and methane adsorption-desorption measurements.
Figure 1. Schematic of experimental setup for the synthesis of methane hydrate and methane adsorption-desorption measurements.
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Figure 2. (a) Macrostructure of corn cob; (b) Microscopic bundle structure of the corncob pith sample; (c) Dark oval−like areas on the walls of bundles; (d,e) Nanopores in the dark oval-like areas.
Figure 2. (a) Macrostructure of corn cob; (b) Microscopic bundle structure of the corncob pith sample; (c) Dark oval−like areas on the walls of bundles; (d,e) Nanopores in the dark oval-like areas.
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Figure 3. Pore size distribution in corncob pith (the illustration shows no pore size is less than 2 nm).
Figure 3. Pore size distribution in corncob pith (the illustration shows no pore size is less than 2 nm).
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Figure 4. (a) Photographs of corncob pith after synthesis of methane hydrate within it (the illustration shows the release of methane gas when the hydrate in the corncob decomposes). (b) Raman spectrum of the methane gas stored in the corncob pith in the form of methane hydrate. Fresh: fresh corncob pith. Frozen: corncob pith frozen in liquid nitrogen. Hydrate: corncob pith with methane hydrate formed within it.
Figure 4. (a) Photographs of corncob pith after synthesis of methane hydrate within it (the illustration shows the release of methane gas when the hydrate in the corncob decomposes). (b) Raman spectrum of the methane gas stored in the corncob pith in the form of methane hydrate. Fresh: fresh corncob pith. Frozen: corncob pith frozen in liquid nitrogen. Hydrate: corncob pith with methane hydrate formed within it.
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Figure 5. (a) Methane sorption kinetics for bulk water, waxy corncob pith, sorghum stalk, and eggplant; (b–d) are the curves of methane gas adsorption from eggplant, corncob pith, and sorghum stalk in three cycles, respectively.
Figure 5. (a) Methane sorption kinetics for bulk water, waxy corncob pith, sorghum stalk, and eggplant; (b–d) are the curves of methane gas adsorption from eggplant, corncob pith, and sorghum stalk in three cycles, respectively.
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Figure 6. (a–c) Curves of methane gas release from eggplant, corncob pith, and sorghum stalk; (d) equilibrium curves of methane hydrate in eggplant, corncob pith, and sorghum stalk.
Figure 6. (a–c) Curves of methane gas release from eggplant, corncob pith, and sorghum stalk; (d) equilibrium curves of methane hydrate in eggplant, corncob pith, and sorghum stalk.
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Figure 7. (a) 1H-1/T1 values and (b) 1H-1/T2 of confined water in corncob pith at different temperatures by using 600 MHz NMR measurements.
Figure 7. (a) 1H-1/T1 values and (b) 1H-1/T2 of confined water in corncob pith at different temperatures by using 600 MHz NMR measurements.
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Figure 8. Relaxation time distributions for corncob pith in different conditions: (fresh-0) “as it was”; (fresh-25) after being dried at 40 °C for 25 min; (fresh-50) after being dried at 40 °C for 50 min; (fresh-75) after being dried at 40 °C for 75 min; (dry) after vacuum drying.
Figure 8. Relaxation time distributions for corncob pith in different conditions: (fresh-0) “as it was”; (fresh-25) after being dried at 40 °C for 25 min; (fresh-50) after being dried at 40 °C for 50 min; (fresh-75) after being dried at 40 °C for 75 min; (dry) after vacuum drying.
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Table
Table 1. Summary of pore distribution of corncob pith.
Table 1. Summary of pore distribution of corncob pith.
Pore Width (nm)Cumulative Volume (cm3/g)Incremental Volume (cm3/g)Cumulative Area (m2/g)Incremental Area (m2/g)
0~20000
2~500.021960.021967.2727.272
50~2200.026900.004947.3470.075
Table
Table 2. Changeable methane adsorption capacity (v/v) of different biomaterials in three cycles.
Table 2. Changeable methane adsorption capacity (v/v) of different biomaterials in three cycles.
EggplantCorncob PithSorghum Stalk
1st cycle152.25100%89.78100%54.28100%
2nd cycle72.3247.50%60.6667.57%36.1966.67%
3rd cycle30.1319.19%65.7373.22%36.1966.67%
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Du, S.; Qu, Y.; Li, H.; Yu, X. Methane Adsorption Properties in Biomaterials: A Possible Route to Gas Storage and Transportation. Energies 2022, 15, 4261. https://doi.org/10.3390/en15124261

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Du S, Qu Y, Li H, Yu X. Methane Adsorption Properties in Biomaterials: A Possible Route to Gas Storage and Transportation. Energies. 2022; 15(12):4261. https://doi.org/10.3390/en15124261

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Du, Sanya, Yixin Qu, Hui Li, and Xiaohui Yu. 2022. "Methane Adsorption Properties in Biomaterials: A Possible Route to Gas Storage and Transportation" Energies 15, no. 12: 4261. https://doi.org/10.3390/en15124261

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