*3.4. Hydrides*

Another promising method for hydrogen storage is the use of metal hydrides [58]. This method enables keeping of hydrogen in a solid-state using hydride-forming metals and alloys, hydrogen-absorbing materials with a high specific surface area, metal-hydridecarbon and amide-imide composites, hydrolysable metals and hydrides, reversibly hydrogenated organic compounds [59]. The technology under consideration, according to [60], facilitates the transportation of hydrogen fuel and reduces the volume of the system by almost three times, without requiring high economic costs for the conversion and liquefaction of hydrogen, as well as the manufacture of special vessels. It is mainly used for stationary and portable devices, as well as transport systems [61].

Hydrogen production is carried out in two reactions—hydrolysis and dissociation. As noted in [62], hydrogen production doubles during hydrolysis; however, it is an irreversible process. Hydrogen accumulators are created by thermal dissociation of hydride. In this case, minor changes of temperature and pressure cause a significant change in the equilibrium of the reaction of hydride formation.

The chemical reaction of the formation of metal hydrides is the interaction of a hydrideforming metal with H2 in a gaseous phase [63]:

$$\text{M (solid)} + \text{H}\_2 \text{ (gas)} \leftrightarrow \text{MH}\_2 \text{ (solid)} + \text{Q} \tag{4}$$

where M—metal used.

A rapid increase in the pressure of hydrogen in the gaseous phase and, at the same time, a decrease in temperature, leads to a direct shift of equilibrium towards hydrides, and the reverse process causes their decomposition [64].

The whole process of hydrogen absorption consists in the mobility of H2 molecules to the surface of the material, dissociation of adsorbed hydrogen molecules, as well as the transition of its atoms into the bulk of the material with the formation of an interstitial solid solution— α-phase, then the hydride—β-phase [56].

Kinetics of absorption/release of hydrogen under mild conditions in intermetallic hydrides is suitable for creating hydrogen storage systems [65].

As a rule, in the literature, metal hydrides are divided into high- and low-temperature ones. When considering low-temperature metal hydrides, it should be noted that stable hydrogen pressure above atmospheric pressure is observed in the temperature range up to 400 K. Low-temperature metal hydrides are intermetallic compounds: AB5, AB2, AB, as well as hydrides of body-centered crystal lattice alloys based on vanadium (V) and the

Ti-Cr system. The reaction of the formation of hydrogen hydrides with these compounds takes place with a heat of formation (<45 kJ/mol H2), as well as high rates of sorption and desorption at medium temperature and pressure [66]. It should be noted that it is advisable to use such materials precisely for the formation of stable hydrogen fuel storage systems with an emphasis on simplifying operation and reducing energy costs. However, they have a low hydrogen content (1.5–3 wt.%). In addition, it should be noted that hightemperature metal hydrides include intermetallic compounds based on magnesium and alloy hydrides [56], which are characterized by elevated temperatures of hydrogen sorption and desorption in a temperature range of about 600 K and a high hydrogen capacity [67].

Metals-N-H systems, consisting of metal amides and hydrides are promising materials forstoringhydrogenontheboardofvehicles [59].

 For instance: 
$$2\text{Li}\_2\text{NH}\_2 + \text{MgH}\_2 \rightarrow \text{LiMg(NH}\_2)\_2 + 2\text{H}\_2\text{MgH}\_2 \leftrightarrow \text{Mg[NH}\_2]\_2 + 2\text{LiH} \ (5.6 \text{ mass} \% \text{ hydrogen}). \tag{5}$$

It should be noted that the methods to optimize the magnesium-hydrogen system are gaining popularity [68,69]. The kinetic properties of Mg/MgH2 are improved with the help of nanostructured materials and the addition of catalysts, e.g., transition metals, their oxides or rare earth metals. As mentioned in [25], an increase in the rate of absorption and the release of hydrogen is achieved by grinding magnesium into a powder with particles of 50–75 μm and alloying with Ni, La, Ce, Cd, Fe, Lu, Sn, Er, Ti, Mn. The addition of destabilizing agents, for example Si, helps to reduce the dehydrogenation temperature. In the reaction of Si with MgH2, stable MgSi2 is formed, rather than Mg, and the enthalpy of the process decreases by almost 40 kJ/mol [66,70].

According to [70], in the near future, the combined hydrogen storage composed of several systems of storage and processing of hydrogen fuel, will be a serious competitor to the current technologies. The new systems can be composed of small to medium sized metal hydride hydrogen storage units as well as thermo-sorption compressors. It is worth noting that the buffer installations for storage, purification and controlled supply of hydrogen to the consumer may also be in demand.

In addition to the reaction of formation of metal hydrides, it is important to determine how the reverse process of obtaining hydrogen from them is carried out. The hydrogen from metal hydride is produced either by its heating to above 400 ◦C or by its reaction with water. In heating process, the metal hydrides are extruded as rods, and are decomposed by heating using electrical heaters or flue gas [56].

It can be concluded that the application of the metal hydrides is promising, as it will allow for compact, environmentally friendly and cost-effective storage and transportation of hydrogen fuel in a chemically bound state.

#### **4. Concepts of Development of Oil and Gas Fields in the Arctic Shelf**

The development of gas condensate fields is composed of many operations during which a wide range of gaseous and liquid products is obtained, e.g., methane, ethane, propane-butane mixture, unstable gas condensate, etc. As a result, in order to transport those products, the large investments are needed like gas or condensate pipelines. In the case of sea routes, it is additionally needed to build plants for the liquefaction of natural gas [71].

When arranging an Arctic oil field, similar problems arise related to logistics and transportation of products. For the export of oil, pipeline or tanker transport can be used, but the situation is complicated by the presence of significant volumes of associated petroleum gas. For its transport, it is not economically feasible to build LNG production units and long gas pipelines [10]. In this case, the possibility of its utilization on site for the production of heat and electricity should be considered. In the case when heat and electric energy is not required in volumes comparable to the amount of assisted associated petroleum gas (APG), surplus flaring is used. However, there are fields where there is so much APG that the field should be considered as oil and gas, and APG as a valuable raw material that should be converted into final products with high added value [72].

In these cases, it would be recommended to apply a Gas-To-Chem approach of creating installations, a concept developed in Saint-Petersburg Mining University. The proposed concepts for the development of remote Arctic oil and gas and gas condensate fields with the possibility of producing and transporting low-carbon products by tankers are presented in Figure 2a,b, respectively.

**Figure 2.** Concept for the development of a remote Arctic oil and gas field (**a**), gas condensate (**b**) with the possibility of producing and transporting blue hydrogen bound to methanol, ammonia and cyclohexane.

In this article there is introduced the concept of development of oil and gas and gas condensate fields, taking into account the current trends to reduce the carbon footprint of products, creation of chemical commodities, as well as hydrogen-based power generation.

#### *4.1. Ammonia, Methanol and Cyclohexane as Hydrogen Carriers*

As a promising technology for the preparation of fat gases, it is proposed to use the process of aromatization of the produced gas, which will allow converting the fat gas with C1–C5 components and unstable gas condensate into the lean C1–C2 gas (methane and ethane), liquid aromatic hydrocarbons benzene-toluene-xylene, as well as hydrogen. In this case, the lean gas can be used as a gas-chemical feedstock for the production of hydrogen, ammonia and methanol, or it will be pumped into the pipeline mixed with hydrogen.

Benzene-toluene-xylene is a valuable chemical raw material. It can be hydrogenated to cycloalkanes and next used as an intermediate product for transport and storage of hydrogen as an energy source. Additional hydrogen for the hydrogeneration of the benzenetoluene-xylene can be obtained by steam reforming and partial oxidation of natural gas, and the greenhouse gases formed in the process, together with exhaust gases from turbine power generators. It could be injected into the underground reservoir carbon capture and storage or processed into methanol for subsequent implementation carbon capture and utilization.

The main elements of the proposed schemes are the unit for reforming C1–C2 hydrocarbon gases, a power generation unit and waste gas utilization. Electricity generation block will include a steam turbine with a binary cycle to utilize excess heat from other gas chemical processes [73].

A schematic diagram of a methane steam reforming unit with water gas reforming reactors and amino purification is shown in Figure 3. Natural gas, over 90% methane, is mixed with the required amount of water vapor. The mixture is heated in heat exchangers H-1-5 and fed to the mixer M-1, where the prepared O2 mixture with an equal volume of water vapor is supplied. The conversion of methane with steam takes place in the R-1 reactor. Next, the products of conversion are sent to equilibrium reactors R-2-3 for the conversion of CO with water vapor and obtaining a mixture of carbon dioxide with hydrogen. In the absorber A-1, the mixture is separated into components by an aqueous

solution of MDEA. After amine purification, the hydrogen-containing mixture is sent to a pressure swing adsorption unit (PSA) to obtain hydrogen with a purity of up to 99.999%.

**Figure 3.** Steam methane reforming unit with water gas reforming reactors and amine purification. R-1—conversion reactor; R-2-3—equilibrium reactor for additional oxidation of CO and CO2; M-1—mixer; H-1-5—heat exchanger; C-1-2— refrigerator-condenser; S-1—separator for separating liquid products; A-1—absorber; SP—adsorber.

In the reforming unit, depending on the selected configuration of the equipment, it is proposed to use the technology of steam reforming or partial oxidation of C1–C2 components. It is important to note that in the production of hydrogen, it is possible to implement one of several methods of utilization of CO2 released from synthesis gas and exhaust gases of a power plant (except for injection into wells to maintain intra-reservoir pressure and increase oil recovery). The method consists in mixing in the presence of a catalyst hydrogen and carbon dioxide with the formation of products—water and carbon monoxide, which, when hydrogen is added, forms synthesis gas suitable for processing to produce methanol and motor fuels [74,75]. The produced synthesis gas will be next processed into methanol, Figure 4.

**Figure 4.** Preparation of synthesis gas with its subsequent processing into methanol. R-1—methanol synthesis reactor; M-1—mixer; H-1—heat exchanger; C-1—refrigerator-condenser; S—separator for separating liquid products; D—flow divider; CC—centrifugal compressor.

A mixture of carbon monoxide and hydrogen is purified from impurities, compressed in a multistage compressor CC, then mixed with unreacted gases in a mixer M-1. Next the gas enters the tubular heat exchanger H-1, heating up to 320 ◦C with hot reaction products. The heated gas enters the contact synthesis reactor R-1, where methanol is formed in the catalyst bed. The main stream of the gas mixture, heated in the heat exchanger H-1, is introduced into the upper part of the column R-1 and enters the catalyst bed. The reaction products are cooled in the tubes of the heat exchanger and removed through the bottom of the column. The synthesis products leaving the column are cooled to 100 ◦C in the heat exchanger H-1, and then in the cooler-condenser C-1 to 25–30 ◦C. The resulting liquid methanol is separated from the unreacted substances in the separator S and collected in the collector.

In the absence of the possibility of building a gas pipeline, hydrogen could be directed to the hydrogenation of benzene-toluene-xylene with the production of cyclohexane, methanol and ammonia from a nitrogen-hydrogen mixture associated petroleum gas, Figure 5.

**Figure 5.** Scheme of ammonia production from a nitrogen-hydrogen mixture. R-1—ammonia synthesis column; R-2-3— condensation column; M-1-3—mixer; H-1—remote heat exchanger; C-1-2—refrigerator-condenser; S-1-2—separator for separating liquid products; D-1-2 filter—flow divider; CC-1—turbocharger; V-1-3—valves.

Raw gas and unreacted circulating gases enter M-1, then flow through the tubes of the heat exchanger H-1 to the synthesis column R-1. Next, liquid ammonia, passing through M-2, mixes with the circulation gas, entering the top of the condensation column R-3, where, due to the evaporation of liquid ammonia in the annular space, the gas mixture is further cooled down to −10–15 ◦C. A mixture of gas and condensed ammonia enters the separation section of the R-2 column to separate liquid ammonia from gases. The mixture from the separation column R-3 is discharged into the external heat exchanger H-1 and, the flow enters the coolers C-1-2 The condensed ammonia is separated in the separator S1, and the gas mixture enters the suction of the circulation stage of the CC-1 compressor, where it is compressed to pressure not higher than 24 MPa, compensating for pressure losses in the system.

From the circulation stage of the CC-1 compressor, the circulation gas is fed to the secondary condensation system, which consists of a condensing column R-1 and liquid ammonia evaporators R-2-3.

In this case, associated petroleum gas for the synthesis of ammonia is obtained from the waste gases of the methane reforming unit. There is used the method of partial oxidation and conversion of steam with subsequent removal of CO2 by amine purification. In addition, according to the presented scheme, the other gases, e.g., exhaust gases from turbine electric generators operating on methane, can be purified by the removal of CO2.

At the same time, benzene-toluene-xylene is a valuable chemical raw material; in addition, aromatic hydrocarbons can be used as an intermediate product for the transport and

storage of hydrogen by hydrogenation. Additional hydrogen for benzene-toluene-xylene hydrogenation can be obtained by reforming natural gas.

This arrangemen<sup>t</sup> will ensure a low carbon footprint of the products, and the produced oil, condensate and produced hydrogen, ammonia, cyclohexane and methanol will be classified as blue. In addition, hydrogen can be released in close proximity to the consumer in the process of dehydrogenation of ammonia and cyclohexane, and reduced nitrogen and aromatic hydrocarbons could be sold on local markets as an independent product.

It is important to note that it is possible to implement one of the several methods of utilizing CO2 released from synthesis gas and exhaust gases of a power plant, e.g., CO2 injection into the well to increase the oil recovery factor by 5–40% [76].

The proposed schemes for the development of remote gas condensate and oil and gas fields will ensure a low carbon footprint of the products. In addition, hydrogen can be released in close proximity to the consumer in the process of dehydrogenation of ammonia and cyclohexane, and the reduced nitrogen and aromatic hydrocarbons could be sold at the local markets. The produced oil, condensate and hydrogen, ammonia, cyclohexane and methanol will have a low carbon footprint.

#### *4.2. Low Carbon Hydrogen Bound in Hydrates*

Another option for converting hydrocarbon gas into pure hydrogen can be based on combining pyrolysis of hydrocarbon gases and formation of metal hydrides. In this case, in the process of pyrolysis, solid carbon is formed, which could be collected in still bags or briquetted for transportation. The produced hydrogen could be converted into the solid hydrides. At the same time, the products could be transported from a remote field by dry cargo ship, Figure 6. In this case, solid carbon could be used locally and the hydride would be dissociated with the release of hydrogen in the immediate vicinity of the consumer. The hydride-forming materials are returned to the remote Arctic port for reuse. The produced hydrogen can be classified as turquoise when it uses a renewable source [77].

**Figure 6.** Concept of arranging a remote Arctic gas field with the possibility of producing and transporting solid carbon and turquoise hydrogen as a hydride using a dry cargo ship.

In the absence of a source of hydrocarbon gases, it is suggested to consider the process of electrolysis of seawater for hydrogen production. The disadvantage of this option is its high energy consumption. Therefore, there is an argumen<sup>t</sup> to implement the process of electrolysis from seawater and the binding of hydrogen into hydrides, to use low-carbon high-potential green energy sources, e.g., hydro energetics or geothermal systems as well as nuclear power plants.

#### **5. Extraction of Hydrogen from Ammonia, Methanol and Cyclohexane**

To facilitate the use of hydrogen by consumers, it is important to consider ways of its recovery from substances in which hydrogen has been bound to facilitate its storage and transport.

#### *5.1. Extraction of Hydrogen from Ammonia*

A nitrogen-hydrogen mixture is created in the decomposition reaction of ammonia (6) at a temperature of 900 ◦C:

$$2\text{NH}\_3 \rightarrow \text{N}\_3 + 2\text{H}\_2, \ \Delta\text{H} = +46.22\text{ kJ/mol.} \tag{6}$$

Ammonia in liquid form enters a high-pressure liquid evaporator heated by electric heaters, where it is transformed into a gaseous state. From the evaporator, gaseous ammonia, NH3, enters into a cracker (dissociator), in which it is decomposed into nitrogen and hydrogen. The use of a catalyst (Fe2O3, NiO) lowers the dissociation temperature by 100 ◦C or more. Next, the mixture of hydrogen and nitrogen is returned to the evaporator, where heat is recovered, and the liquid ammonia evaporates. Furthermore, the dissociated ammonia is dried to a dew point of −45 to −60 ◦C. The obtained mixture contains up to 75% hydrogen and 25% nitrogen [78].

Next, the reaction mixture is sent to a membrane nitrogen recovery unit. After membrane separation, the hydrogen-containing gas is fed to the adsorbers, where the remaining nitrogen is removed. The commercial hydrogen will be sent to the consumer.

#### *5.2. Extraction of Hydrogen from Methanol*

Dehydrogenation of methanol can be carried out with the formation of formaldehyde in a heterogeneous process that occurs in the gas phase on a solid catalyst according to reaction (7):

$$\text{CH}\_3\text{OH} \rightarrow \text{HCOH} + \text{H}\_2, \Delta\text{H} = +84.0 \text{ kJ/mol.} \tag{7}$$

The process takes place in a reactor with a fixed bed of catalyst. The degree of conversion increase is controlled by a gas flow distributor. Reactor for the oxidative dehydrogenation of methanol is proposed in [79]. Formaldehyde formed during the reaction is an intermediate product of the organic industry. It is widely used in the production of synthetic resins and plastics, and in the synthesis of drugs and dyes. Thus, in the process of methanol dehydrogenation, hydrogen will be obtained, and the carboncontaining component, formaldehyde, will be bound in the target product.

#### *5.3. Extraction of Hydrogen from Cyclohexane*

Cyclohexane is a product for obtaining a wide range of chemical raw materials, such as caprolactam, adipic acid and cyclohexanone. It is also used as a solvent. The release of hydrogen from cyclohexane occurs in the dehydrogenation reaction (8), with the formation of an additional intermediate benzene, which is widely used in industry and is a feedstock for the production of drugs, synthetic rubber and dyes. Based on the kinetics of the process, a simplified reaction mechanism is given as:

$$\rm C\_6H\_{12} \rightarrow C\_6H\_6 + 3H\_{22} \text{ } \Delta H = +239.13 \text{ kJ/mol.} \tag{8}$$

The substances considered are raw materials used in the production of many compounds used in the production of fertilizers, plastic, resins, etc. If needed, they could be decomposed with the release of hydrogen as an energy resource. Carbonaceous substances can be used for obtaining the products binding CO2 for a long time and preventing it from entering the atmosphere.

## **6. Discussion and Conclusions**

The role of hydrogen as an energy source is constantly growing amid trends to reduce greenhouse gas emissions. In this context, the exploitation of oil and gas deposits in Arctic regions starts to be confronted with new tasks. They are related to development of new methods of hydrogen production and transport under sever climatic conditions, strict environmental rules and lack of appropriate infrastructure. The described above factors were considered in the development of the Gas-To-Chem concept for the development of oil and gas fields in the Arctic region with the production and transport of bound hydrogen. Preparation of natural or associated petroleum gas, reduction of carbon footprint of produced products, as well as production and transport of bound hydrogen, would require implementation and the improvement of several key processes:


Some of the processes described above are well known. However, they are practically not used in oil and gas fields, despite the fact that the development of gas chemical plants directly at the place of hydrocarbon production will reduce the cost of production. Such an oil and gas facility may be more efficient and more environmentally friendly than existing ones since it will have high-tech equipment and systems for the utilization of industrial wastewater and gases into absorption wells.

This article introduces conceptual Gas-To-Chem solutions for oil, gas and gas condensate fields taking into account the current trends to reduce carbon footprint of energy and chemical raw materials, as well as efforts towards the creation of low-carbon economics based on hydrogen. A low-carbon economy creates several opportunities for the use of products originating in the discussed regions: in the energy sector (hydrogen and ammonia burnt without CO2 emissions), chemical industry (cyclohexane and methanol binding carbon in chemical compounds), production of food and biomass (nitrogen from ammonia as the basis for fertilizers). The concept is based on modern gas chemical technologies with an emphasis on the production of substances with high added value and low carbon footprint. Ammonia, methanol and cyclohexane are considered as sources of a clean energy resource of hydrogen. In the article, the methods to increase production efficiency due to the complementary nature of each other gas chemical process and to ensure selling flexibility through diversification of production are presented. Such diversification will reduce investment risks and maintain profitability amid the transformation of international markets.

The concepts presented in the article could be the starting point for the transformation of the oil and gas sector in Arctic regions, considering the actual global environmental and climate agenda.

**Author Contributions:** Conceptualization, M.D. and G.B.; methodology, A.K. (Andrzej Kraslawski); validation, M.D.; formal analysis, A.K. (Andrzej Kraslawski); investigation, A.K. (Andrey Kunshin), D.S. and M.B.; resources, M.D.; data curation, G.B.; writing—original draft preparation, A.K. (Andrey Kunshin); writing—review and editing, G.B.; visualization, D.S.; supervision, M.D.; project administration, G.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author. The data are not publicly available due to Federal Law of the Russian Federation "On Export Control".

**Conflicts of Interest:** The authors declare no conflict of interest.
