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Review

Green Energy Revolution and Substitution of Hydrocarbons with Hydrogen: Distribution Network Infrastructure Materials

by
Reza Ghomashchi
School of Electrical and Mechanical Engineering, The University of Adelaide, Adelaide 5000, Australia
Energies 2023, 16(24), 8020; https://doi.org/10.3390/en16248020
Submission received: 11 October 2023 / Revised: 7 November 2023 / Accepted: 29 November 2023 / Published: 12 December 2023
(This article belongs to the Section A5: Hydrogen Energy)
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Abstract

:
Global warming is an accepted fact of life on Earth, posing grave consequences in the form of weather patterns with life-threatening outcomes for inhabitants and their cultures, especially those of island countries. These wild and unpredictable weather patterns have persuaded authorities, governments, and industrial leaders to adapt a range of solutions to combat the temperature rise on Earth. One such solution is to abandon fossil fuels (hydrocarbons) for energy generation and employ renewable energy sources, or at least use energy sources that do not generate greenhouse gases. One such energy carrier is hydrogen, which is expected to slowly replace natural gas and will soon be pumped into the energy distribution pipeline network. Since the current energy distribution network was designed for hydrocarbons, its use for hydrogen may pose some threat to the safety of urban society. This is the first time an overview article has examined the replacement of hydrocarbons by hydrogen from a totally different angle, by incorporating material science viewpoints. This article discusses hydrogen properties and warns about the issue of hydrogen embrittlement in the current pipeline network if hydrogen is to be pumped through the current energy distribution network, i.e., pipelines. It is recommended that sufficient study and research be planned and carried out to ensure the safety of using the current energy distribution network for hydrogen distribution and to set the necessary standards and procedures for future design and construction.

1. Introduction

The issue of global warming threatening the world population is principally associated with human activities and their dependence on energy for a vibrant and sustainable society. A quick glance through the last three centuries verifies that the issue of global warming has been exacerbated by the increased industrialization of countries and societies [1,2]. It is now well accepted by the majority of climate scientists, and is well publicized by active political green movements and societies, that the prime source of global warming is the application of fossil fuels for energy production. This is probably due to cheap hydrocarbon fuels, such as coal, oil, and gas, sustained through the political manipulation of fossil-fuel-producing countries’ government officials and the world oil market by the governments of the more industrialized countries. This cheap energy enabled the governments of industrialized countries to sustain development through most of the twentieth century but with massive emission of greenhouse gases, the prime source of global warming. One of the consequences of global warming is the unpredictable weather patterns we are witnessing around the world. This requires the global energy system to undergo a significant transformation over the next few decades to reduce greenhouse gas emissions and work towards limiting global warming to less than 2 °C, which implies a reduction in emissions to close to net zero by around 2050 [3]. This translates into reducing the use of hydrocarbon-based energy sources through the introduction of more efficient use of fossil fuels, as more than 67% of generated energy is lost as waste heat [4], and incorporating renewable energy sources, like solar, wind, geothermal, and/or wave energy. The most intensive fossil fuel consumption is in electricity generation, where the amount of energy lost as waste heat is the highest as well; for instance, 24.3% of energy used for electricity generation in the United States is lost as waste heat [4].
Amongst more industrialized countries’ dependency on coal, Australia has a prominent position in spite of recent surges in wind and solar energy. Australia utilizes coal as the main energy source for electricity generation, with more than 50% of the electricity generated in 2021 powered by coal [5]. Australia is the world’s worst coal power emitter when accounting for population size [5]. Australia’s CO2 emission per capita from coal power is five times higher than the world average, far ahead of China and India (Figure 1) [5]. However, in recent years, some of the countries around the Persian Gulf, like Qatar, Kuwait, and Saudi Arabia, generated quite high CO2 emissions, as shown in Table 1 [6]. Coal-based electricity generation must be completely phased out in Australia by around 2030 in order to keep up with the Paris Accord, in which governments pledged to keep the global temperature rise below 2 °C above pre-industrial levels. However, the set goal is to keep the temperature rise to around 1.5 °C, rather than 2 °C, as approved by the world’s governments in 2018 [7]. This is not compatible with the announced Australian coal retirement schedule, since there will remain a share of around 7 GW of coal capacity in the electricity generation mix by 2039–40 [5,8]. The good news, however, is that announcements of the retirement of coal-fired power stations have accelerated in recent years. In early 2022, Origin Energy, one of the main energy provider companies in Australia, announced the closure of the Eraring power station—Australia’s largest coal-fired power plant—in 2025, seven years earlier than initially planned [9]. AGL, another major energy provider in Australia, with the largest share of greenhouse gas emissions, has also accelerated the closure of the coal-fired power stations of Bayswater (2030–2033) and Loy Yang A (2040–2045), confirming that the coal power phase-out may proceed faster than the announced schedule [8].
Australia progressed well in improving electricity generation through renewable sources and increased its share in electricity generation via solar photovoltaic (PV) sources to 13%, in contrast to world electricity generation of 4.4% by solar in 2022 [5]. It is interesting to note that the trend in the world’s use of solar energy for electricity generation improved in 2022, compared to 1.1% in 2015 [5].
A new line of thinking is finding greater voice these days regarding reducing greenhouse gas (GHG) emissions by using energy sources that do not generate CO2, the main constituent of GHGs responsible for global warming. One such energy resource is hydrogen gas, which helps the environment by generating water when combusted, or even generating oxygen if produced through water electrolysis. It is important to emphasize that hydrogen is not an energy source, but is instead an energy-storing medium or energy carrier; there is a need to spend energy to produce hydrogen, as hydrogen does not exist as H2 on Earth, and once hydrogen is burnt, the stored energy is released. There is a range of hydrogen gas classification based on the generation procedure and the energy sources employed [10], i.e., whether the production of hydrogen results in the generation of GHGs:
  • Black/brown H2: produced from coal/lignite (brown coal) by gasification to generate hydrogen and other gases like CO, CO2, CH4, and H2O.
  • Gray H2: produced from natural gas, and the resultant CO2 is released into the atmosphere.
  • Blue H2: produced from natural gas, but the resultant CO2 is stored and not released into the atmosphere.
  • Green H2: produced via electrolysis of water and is environment-friendly, since the power used for electrolysis is also provided by non-fossil fuel sources, like solar.
There are further nomenclatures for hydrogen, like purple/pink or turquoise hydrogen, based on the energy source used or the product, as shown in Figure 2 [10]. In spite of the classification of hydrogen based on carbon dioxide generation, there are many reports that consider this classification to be less realistic, for instance, with regard to blue hydrogen generation [11], or even green hydrogen, when using electrolysis and solar energy (photo voltaic) for water electrolysis [12].
There are two approaches proposed or adapted by energy producers when it comes to using hydrogen as a replacement for hydrocarbon-based fossil fuels:
  • Partial mixing of hydrogen in hydrocarbon gas pipelines with gradual increase to eventually achieving full replacement of hydrocarbon-based gases;
  • Total replacement of hydrocarbons with hydrogen.
In either case, there are two important characteristics of hydrogen that need to be examined carefully as hydrogen is extremely flammable and highly diffusible. This means the leakage of hydrogen into buildings and factory sites could be critical due to its flammability, as history remembers the hydrogen-filled Hindenburg airship disaster on 6 May 1937. The more recent hydrogen-related accidents include the Muskingum River power plant vapor cloud explosion in 2007 [13], the Silver Eagle Refinery vapor cloud explosion in 2009 [14], the explosion of a hydrogen refueling station in Norway in 2018 [15], and the explosion of a hydrogen tank in Gangneung, Korea, in May 2019 [16].
In addition to flammability and high diffusivity, hydrogen may dissolve within the microstructure of the container vessel material and alter its mechanical properties. These are the main concerns with the safety of hydrogen transport and storage especially within highly populated urban areas. This is the main theme of this report: To view the usage of hydrogen as an energy source from another angle, a materials science viewpoint, and highlight the risks associated with storage and transportation of hydrogen as an energy carrier/generator. The discussion on energy consumption for hydrogen production or the energy generation on combustion of hydrogen gas, energy efficiency, and most importantly hydrogen usage effects on global warming are not the theme of this report and is left to experts in the field, e.g., [17,18,19,20,21,22]. This report is about the material of pipeline networks and pressure vessels used for hydrogen distribution and attempts to highlight the possible danger and risks associated with using the current hydrocarbon distribution infrastructure. Pumping or storing hydrogen through and within current pipeline networks and pressure vessels runs the risk of altering the fracture characteristics of their material, steels (usually high-strength low-alloy HSLA steels, specified as X60, 70, 80, …), making the pipeline. They are not designed and constructed for highly diffusible hydrogen gas.

2. Hydrogen Facts

Hydrogen is odorless, colorless, and tasteless and is as flammable and combustible as natural gas and gasoline. It is much lighter than air, 14 times lighter, which enables hydrogen to rise up in open space at a speed of 20 m/s, (six times faster than natural gas) [23]. It is usually accumulated near the ceiling in enclosed space. However, this does not mean that hydrogen cannot be near ground level, as cryogenic hydrogen at around ~30 K (~ −245 °C) is heavier than air. Unlike natural gas, which is mixed with sulfur to enable users to smell (detect) the gas, there is not any known odorant that is light enough to move with hydrogen and thus enable its detection via smell. Hydrogen is more readily combustible as its combustion energy is significantly lower than that of natural gas and gasoline. The ignition energy of hydrogen is 0.02 mJ (Millie Joule) and its lower explosion limit (LEL) is as low as 4 vol%; Figure 3 provides useful information on hydrogen and other fuels’ combustion levels [24]. Hydrogen requires an oxidizer like oxygen to burn and the flame has low radiant heat, much lower than that of natural gas. Although the flame is as hot as natural gas, its low radiative power reduces the risk of secondary fire. Therefore, hydrogen is far less effective in heating buildings via radiation. The dangerous point about hydrogen fire is its invisibility, which means individuals may walk into a hydrogen fire zone without realizing, i.e., direct contact with flame. Unlike other gases (except oxygen), hydrogen does not cause asphyxiation due to its rapid rise and dispersion. Hydrogen is neither toxic, nor poisonous, and is not an atmospheric pollutant [23]. The heat generated by hydrogen is compared with some other fossil fuel sources in Table 2 [25]. The heat values in Table 2 considers the latent heat of vaporization of water vapor generated during combustion, i.e., measuring the released heat at 25 °C when water is liquid.

3. Materials Failure

The materials used in engineering applications usually fail in the two modes of brittle and ductile fractures. The brittle or ductile nature of fracture is dependent on many parameters including the magnitude of applied stress and its direction with respect to the crystallographic orientation of grains; the nature of applied stress including cyclic, impact, shear, etc.; factors of the environment like temperature, oxidizing, etc.; and the chemical composition of alloys, including the gases dissolved or absorbed or the microstructure, containing certain phases segregated at certain regions and locations. In most engineering applications of materials, it is recommended to select materials that are able to behave in ductile manner when in service. Ductility is a material property where plastic deformation is expected to occur before failure. In other words, some energy should be spent, or work be performed to create fracture (initiation of cracks and their propagation), i.e., energy is employed to induce plastic deformation before failure. This means that failure occurs over a time span. This is in contrast to brittle fracture, which occurs suddenly in very short time. The energy spent or work performed for ductile fracture is orders of magnitude greater than the work needed for brittle fracture. This is interpreted as ductile failure occurring over a time span where cracks propagate slowly, giving maintenance engineers enough time to detect the deterioration of the engineering structure and prevent catastrophic break down and fracture. This is not the case for brittle fracture, as the propagation of cracks may take place at speeds equal or greater than the speed of sound with catastrophic failures, and with possible loss of life and property, Figure 4 [26].

4. Hydrogen in Steels

The dissolution and high diffusivity of hydrogen in metals, especially in steels, Figure 5 [27], as the main conduit infrastructure material for gas supply (pipeline) or for storage (pressure vessels), imposes stringent quality requirements on the selection of the steel and the welding procedure, as the main fabrication route for energy pipelines and pressure vessel construction [28,29,30,31,32]. This is due to the reduction in steel ductility that results from the dissolution and diffusion of hydrogen in steel microstructure, which makes the steel behave like a brittle material. This means that the dissolution of hydrogen in steels may increase the probability of premature failure with a considerable element of unpredictability, Figure 4 [26]. The dissolution of hydrogen in steels varies with temperature as shown in Figure 5 [27] and Figure 6 [33]. It is well established that the solubility of hydrogen in steels is also dependent on the crystal structure as the FCC γ-iron has higher capacity for hydrogen dissolution than the BCC crystal structures (δ and α), see Figure 6 [33]. That is why the construction of storage tanks and gas pipelines calls for careful selection of steel grade and mechanical properties along with appropriate welding procedures if hydrogen is to replace hydrocarbon-based energy sources, partially or fully.
The subject of hydrogen embrittlement in steels is an important consideration in the oil and gas industry and is a major chapter in many metallurgical and materials textbooks on steels, e.g., [34]. The need for replacing hydrocarbons with hydrogen to mitigate global warming has created the impetus for “The American Society of Mechanical Engineers”, (ASME), to develop the ASME B31.12 [35] code to provide guidelines for the design, construction, and operation of hydrogen piping and pipeline systems. A similar attempt has been made by the European regulating agencies to re-examine their current pipeline networks for the partial or full substitution of hydrogen for natural gas. Europeans have started to carry out materials testing to verify the current pipeline infrastructures and propose guidelines for the construction of future hydrogen distribution pipeline networks. This is evident in the recent report by Germans [Deutscher Verein des Gas- und Wasserfaches e.V.] to investigate the suitability of the current natural gas steel pipeline for hydrogen [36]. These comprehensive codes and standards are to ensure the integrity and safety of hydrogen infrastructure, but due diligent care and further research are still necessary and extremely important to achieve safe and efficient transmission, storage, and use of hydrogen in densely populated urban regions. Fortunately, such studies have also started in Australia as in a recent report by APA, the owner of 15,000 km of natural gas pipelines in Australia, which is considering the conversion of the most southern section of the Parmelia Gas Pipeline in Western Australia to a pure hydrogen service. This would be the first conversion of a natural gas transmission pipeline to one for pure hydrogen in Australia [37].
The dissolution of hydrogen in steels may take place in two different stages:
Dissolution of hydrogen in molten steels
Hydrogen dissolution in steels may occur during steel production or during welding of steel where molten steel comes in contact with hydrogen. The dissolved hydrogen may have a wide range of sources including the humidity of the environment, the raw materials, and consumables, like welding electrodes used during steel structure production and fabrication, the distribution pipeline, or transportation/storage media.
Dissolution of hydrogen in solid steels
This is the case where steel structures come to direct contact with pressurized hydrogen gas. There are two ways that hydrogen can be introduced into steels:
  • Electrochemical discharge, where the steel is immersed in a basic (e.g., NaOH) or acidic (e.g., H2SO4) solution as the cathode;
  • The steel is placed in a hydrogen-gas-rich environment at specific pressure. The latter is the case encountered during the storage/transportation of hydrogen as the inner wall of the vessel/pipeline is exposed to hydrogen at pressure.
The dissolution of hydrogen in molten steels is now under strict control by steel producers and engineering firms designing and manufacturing gas transportation infrastructures from pipelines to pressure vessels. There are a range of treatments during steel making including vacuum degassing during steel making or covering the steel billets and slabs in hot boxes (around 600 °C) to slowly cool them down to near room temperature to allow hydrogen effusion out of the steel. Also, some steel producers bury the blooms or billets in a sand pit in the sun when the steels have a temperature of about 400–500 °C to slowly cool down to room temperature. The slow cooling enables effusion of the dissolved and diffusing hydrogen out of steel.
As for welding, low-hydrogen consumables (electrodes) are used with preheating the joint to reduce the cooling rate and allow hydrogen to effuse out of the weld joint. The cooling rate of steel weld between the 800 and 500 °C temperature interval (cooling time of the weld to cool from 800 °C to 500 °C, t8/5) as well as the time for weld to reach 100 °C, (t100), from 800 °C are critical parameters that can control the weld susceptibility to hydrogen cracking [38]. The formation of hydrogen-assisted cold cracking (HACC) is the consequence of hydrogen dissolving during welding. The dissolved hydrogen in the weld joint diffuses to stressed regions of the weld, like crack tips that lead to fracture [31,39]. However, the cooling times (reflecting cooling rate) t8/5 and t100 along with the preheating of the joint before welding play an important role in mitigating weld-joint cracking.
For the dissolution of hydrogen in solid steel, which is the main concern for the hydrogen distribution pipeline network, there is an increase in hydrogen dissolution in steel with increasing pressure, Figure 7 [40]. It is also worth mentioning that there are a range of detrimental effects encountered when hydrogen comes in contact with metals through an electrochemical reaction. These include hydrogen-induced cracking (HIC), where cracks develop along the rolling direction and stepwise through thickness [41] due to steel exposure to environments containing H2S, and stress corrosion cracking (SCC), where diffusion of hydrogen to the crack tip is generated due to stress and the exposure of steel to corrosive environment. Both HIC and SCC lead to pipeline failure if not detected early [29].
Therefore, it is important to verify the rate of hydrogen dissolution in the energy-distributing steel pipeline network and study the changes in ductility due to dissolution before using the existing pipelines for hydrogen distribution.
In the next section, hydrogen embrittlement in steel is discussed briefly along with the proposed theoretical principles associated with induced steel brittleness. There is no doubt that stringent quality checks must be implemented on the current energy distributing network pipeline if they are “to be converted into hydrogen transporting media”. It is necessary to ensure that they meet the requirements for being leak-proof especially through weld joints, valves and accessory parts [42], as well as that the steel grade has sufficient resistance to hydrogen embrittlement.

4.1. Hydrogen Embrittlement

The detrimental effect of hydrogen dissolution in steels on their mechanical properties was first reported in 1875 [43]. Johnson [43] reported that there is significant reduction in fracture toughness and fracture strain when steel is exposed to a hydrogen-rich environment. The effect of hydrogen content on the ductility of steel is shown in Figure 8 [44] where the time (minutes) for hydrogen charging (hydrogen content) is plotted against the area reduction, which is a measure of ductility. Firstly, the ductility of all steels reduced with the dissolution of hydrogen in steels. Secondly, the steels that had almost the same ductility (~45–48% area reduction) behaved differently when exposed to the same level of hydrogen dissolution. Steels with higher strength (e.g., 1862 MPa) were affected more intensely than those of the softer (lower strength) ones (e.g., 1379 MPa). Since a steel’s strength is dependent on its microstructure, this indirectly reveals that steels with different microstructures react differently to hydrogen dissolution and hydrogen embrittlement (HE) accordingly, as pointed out in the later paragraphs in this section.
The introduction of hydrogen into steels and most metals, even as low as 1 ppm (0.0001 wt%), can cause cracking in structures [45]. The susceptibility of metals to HE is mainly observed in BCC and HCP crystal structures metals and alloys, while FCC metals like aluminum alloys, although susceptible to hydrogen embrittlement [46], are not as severely susceptible as BCC and HCP metals. This is due to the ability of FCC metals to dissolve higher amounts of hydrogen with traps (irreversible) that are capable of holding the hydrogen atoms and preventing them from diffusing. A range of defects in the aluminum FCC crystal structure such as lattice vacancies, dislocations, and grain boundaries are potential trapping sites for hydrogen [46], thus preventing the dissolved hydrogen from diffusing due to stress fields within the metal crystal lattice, e.g., crack tips, and also encouraging crack growth. For steels, the most effective sites are the microvoids and the interface of steel and non-metallic inclusions like Fe2O3, Fe3O4, MnS, Al2O3, TiC, Ce2O3 [38] and the percentage of the austenitic (FCC, γ-iron) phase.
In other words, it is the diffusible hydrogen that is the main problem for inducing embrittleness, and if hydrogen atom is pinned down (trapped) within the crystal lattice, the problem is removed.
One of the characteristics of steels is the formation of a range of phases that form upon the solidification of molten steel as well as thermal treatments, i.e., temperature and cooling rates, employed through the austenitic region, (723–1493 °C). The resultant microstructure has a profound effect on the mechanical properties of steel as well as its susceptibility to HE. This is because of the dissolution and diffusion of hydrogen, which is different for different phases. For interested readers (to review the phases formed in steels), references should be made to [47,48].
The main characteristics of hydrogen embrittlement in metals are their sensitivity to strain rate, temperature, and the delayed nature of failure as detected in weld joints [28,29,30,31,32]. Steel structures loaded at lower strain rates (below 10−3 to 10−5 s−1) [49] and intermediate temperatures (−70 to 25 °C) [50] are more susceptible to HE. The effect of hydrogen on steel ductility is evident in Figure 9 [39], where laboratory charged micro-cantilevers prepared from X70 pipeline steel structure resulted in reduction in plastic deformation. In addition, the area under the stress–strain, (load–displacement), which is an indication of toughness, is also much less than that of the uncharged steel.
When steel comes in contact with hydrogen, molecular hydrogen, (H2), is first absorbed on the surface. The molecular hydrogen is not detrimental and only becomes a concern when it transforms into atomic hydrogen. At this point, the hydrogen atom dissolves in the metal structure and diffuses to stressed regions such as dislocations, grain boundaries, or micro-cracks within the metal microstructure. Therefore, the main task to prevent HE in steels is to prevent the hydrogen ingress into steels. However, if the hydrogen has already dissolved in steel, the solution would be “how to keep the hydrogen atoms stationary”, i.e., not letting them diffuse within the crystal lattice structure.
In order to prevent hydrogen from ingress into steel, it is necessary to create a barrier in hydrogen’s path into the crystal lattice by employing diffusion barrier coatings. This is a well-researched area, and the findings are useful for implementation in pipelines to prevent hydrogen dissolution in steels, [51,52,53,54,55,56,57,58,59,60].
In the case of dissolved hydrogen, it is required to make it immobile or non-diffusible. To achieve this, it is required to design alloys and implement specific thermal treatments that encourage the formation of phases or defects that are capable of trapping the dissolved hydrogen permanently. The effect of retained austenite as a phase capable of dissolving a higher concentration of hydrogen with permanent trapping is shown in Figure 10 [61]. The reduction in hydrogen diffusivity with increasing austenite content confirms the H-trapping capability of austenite.

4.2. Hydrogen Embrittlement Mechanisms

There are three more acceptable hypotheses for hydrogen embrittlement in steels, plus a few others which are no longer hold water to justify HE in steels [62]. The less accepted ones are those that are based on hydrogen pressure (e.g., [63,64,65,66]) and the formation of hydride (e.g., [67,68,69]), which were later shown to be less reliable (e.g., [70,71,72,73]). The more acceptable hypotheses are given in Table 3 and briefly explained below [62].
  • Hydrogen-Enhanced Decohesion (HEDE) mechanism
The HEDE mechanism introduced by Johnson and Troiano [74] and elaborated on by Oriani and co-workers [75,82] is based on the dissolution of hydrogen atoms in steels and their diffusion to regions of high triaxial tensile stresses. The presence of atomic hydrogen within the interstices of these highly stressed regions weakens the cohesive bond strength of the atoms in steel. As a result, the critical stress required for crack initiation and propagation is reduced [83]. The HEDE mechanism originally relied on hydrogen concentration build-up encouraged by opening and stretching of the crystal lattice due to elastic hydrostatic stresses [84], while later work clarified that trapping is also a possible mechanism for hydrogen segregation [85].
  • Adsorption-Induced Dislocation Emission (AIDE) Mechanism
Lynch initially proposed the AIDE mechanism in 1979 [78] and then elaborated further in 1989 [79]. In his updated version of the AIDE model, he proposed that the adsorption of hydrogen on the crack surface and one or two atomic layers beneath it encourages the formation of dislocations by weakening the interatomic bonds at the crack tip. The nucleation of dislocations and their activities around the crack tip aids the propagation of cracking. The dislocation movement results in the formation of voids ahead of the crack tip and the coalescence of these voids results in crack propagation and brittle fracture.
  • Hydrogen Enhanced Localized Plasticity (HELP) Mechanism
The HELP concept, which is based on localized plastic deformation despite the macroscopic brittle appearance of the hydrogen crack, was introduced by Beachem [86]. He pointed out that there are highly localized micro-plastic deformation segments that are detectable in fracture topography. Such findings helped Birnbaum and Sofronis and a few others develop the HELP hypothesis [80,81,87]. The main point in HELP theory is the assistance given to dislocation movement by the hydrogen atmosphere attached to dislocations. The H-atmosphere reduces the barrier towards dislocation movement through reducing localized flow stress.
The hydrogen segregation and reduction of elastic strain fields within crystal lattice improves dislocation mobility [80]. If the strain rate is low enough and the temperature is sufficiently high to allow hydrogen to move with kinetically faster dislocations, the reduction in elastic strain field decreases the likelihood of dislocation interactions with one another. This means that dislocations are able to move at lower stresses promoting localized plasticity at the microscale, i.e., the loss of ductility associated with hydrogen embrittlement at the macroscale does not interfere with plasticity at the microscale [80,81].

5. Conclusions

The changes in weather patterns and unexpected hot and cold temperatures along with floods and droughts around the world are attributed to the emission of greenhouse gases and the resulting global warming as explained by environmental scientists. This has forced the public and governments to find solutions for reducing greenhouse gas emissions and thus stop the unexpected climate changes on Earth. The solution is to reduce hydrocarbon-based fossil fuels consumption for energy production, improve the usage efficiency of fossil fuels, and, if possible, replace GHG-producing energy sources with less environmentally detrimental sources of energy like hydrogen and renewable energy sources including solar, wind, wave, and geothermal. One such energy carrier is hydrogen, which is abundant on Earth as a compound, water. The production of hydrogen requires the consumption of energy, which may be a source of pollution. However, the intention is to employ renewable green energy to produce hydrogen and replace fossil fuels with “green hydrogen” for energy generation. Green hydrogen is therefore expected to be pumped out into urban areas through the existing energy distribution infrastructure, the so-called pipeline networks, and pressure vessels currently in use for fossil fuels. The distribution of hydrogen as a highly diffusible gas through the existing pipeline network and knowing that hydrogen has the ability to dissolve in metals and alter their mechanical behavior is something that triggered the presentation of this article. It is believed that this report is the first time that the safety of using the current energy distribution network for hydrogen distribution has been raised.
This report tries to give some caution about such a fuel change-over, as hydrogen gas is very inflammable and has the ability to diffuse rapidly through the joints and cracks and crevices in distribution pipes, storage tanks, valves, and pressure vessels. Furthermore, hydrogen diffuses within the microstructure of steel pipeline materials and alters their mechanical properties, i.e., ductile steel pipe materials turn into brittle steels and therefore create an uncertain and unpredictable condition for distribution pipeline safety and its useful life. This article attempts to raise awareness and encourage caution and detailed research about the effect of hydrogen on our current energy distribution infrastructure. The great ability of hydrogen to escape out of its container and its highly combustible nature along with the invisibility of hydrogen flame creates a great risk for public safety.
It has been reported that some countries have already started to consider this issue and planned and even carried out some limited research to verify the safety of current energy distribution infrastructure for hydrogen distribution. This report is suggesting more caution and calling for detailed investigation of hydrogen distribution media and the introduction of new standards and specifications for future pipeline construction.
In addition, it is recommended to improve fossil fuel efficiency through conversion of the waste heat generated during energy production from fossil fuels into electricity using thermoelectric materials. (A few examples from author’s group are [88,89,90,91,92,93,94].) A green approach to energy generation requires support from governments and energy-providing authorities for researchers in this field.

Funding

This research received no external funding.

Acknowledgments

The information on hydrogen embrittlement and cracking of weld joints due to hydrogen dissolution and diffusion during welding are due to the work of several of my students whose hard work and dedication enriched my knowledge on this topic.

Conflicts of Interest

The author declares no conflict of interest.

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Energies 16 08020 g001
Figure 1. The distribution of carbon dioxide, CO2, (tonnes per year), generation per capita for G20 countries (courtesy EMBER, [5]) (The gray colour is the world average).
Figure 1. The distribution of carbon dioxide, CO2, (tonnes per year), generation per capita for G20 countries (courtesy EMBER, [5]) (The gray colour is the world average).
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Energies 16 08020 g002
Figure 2. Classification of hydrogen produced from a range of energy sources (courtesy AEC, [10]).
Figure 2. Classification of hydrogen produced from a range of energy sources (courtesy AEC, [10]).
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Energies 16 08020 g003
Figure 3. Flammability and ignition comparison of hydrogen and other fuels (courtesy MSA, [24]).
Figure 3. Flammability and ignition comparison of hydrogen and other fuels (courtesy MSA, [24]).
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Energies 16 08020 g004
Figure 4. Catastrophic failure of a steel pressure vessel due to brittle fracture (wall thickness = 149 mm, fragments thrown 46 m away [26]).
Figure 4. Catastrophic failure of a steel pressure vessel due to brittle fracture (wall thickness = 149 mm, fragments thrown 46 m away [26]).
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Energies 16 08020 g005
Figure 5. Arrhenius plot to show the effect of temperature on hydrogen diffusivity (Deff) in different steels compared with hydrogen diffusivity (DH)in pure iron [27].
Figure 5. Arrhenius plot to show the effect of temperature on hydrogen diffusivity (Deff) in different steels compared with hydrogen diffusivity (DH)in pure iron [27].
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Energies 16 08020 g006
Figure 6. Solubility of hydrogen in pure iron in equilibrium with pure H2 (P = 1 bar) as a function of temperature, (adapted from [33]).
Figure 6. Solubility of hydrogen in pure iron in equilibrium with pure H2 (P = 1 bar) as a function of temperature, (adapted from [33]).
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Energies 16 08020 g007
Figure 7. Total hydrogen concentration of the 3.5NiCrMoV quenched-and-tempered, martensitic steel charged electrochemically and in hydrogen atmosphere under pressure [40].
Figure 7. Total hydrogen concentration of the 3.5NiCrMoV quenched-and-tempered, martensitic steel charged electrochemically and in hydrogen atmosphere under pressure [40].
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Energies 16 08020 g008
Figure 8. Effect of hydrogen content (charging time) on ductility of steel (reduction in area). The values on each graph are the fracture strength of steel in tension, (data extracted from original experiments [44].
Figure 8. Effect of hydrogen content (charging time) on ductility of steel (reduction in area). The values on each graph are the fracture strength of steel in tension, (data extracted from original experiments [44].
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Figure 9. SEM micrographs of the as-fabricated (a) and tested (b) micro-cantilever beams. The load (P)–displacement (δ) graphs (c) display the results for uncharged and H-charged micro-cantilevers [39].
Figure 9. SEM micrographs of the as-fabricated (a) and tested (b) micro-cantilever beams. The load (P)–displacement (δ) graphs (c) display the results for uncharged and H-charged micro-cantilevers [39].
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Energies 16 08020 g010
Figure 10. The diffusivity of hydrogen in nanostructured bainite (circles) and duplex stainless steels (square). α-bainite or ferrite, γ-austenite [61].
Figure 10. The diffusivity of hydrogen in nanostructured bainite (circles) and duplex stainless steels (square). α-bainite or ferrite, γ-austenite [61].
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Table 1. CO2 emission of some oil-producing countries around Persian Gulf for year 2021 [6].
Table 1. CO2 emission of some oil-producing countries around Persian Gulf for year 2021 [6].
Countries of Persian GulfCO2—Tonnes per Capita per Year (2021)
Bahrain26.7
Iran8.5
Iraq4.3
Kingdom of Saudi Arabia18.7
Kuwait25.0
Oman17.9
Qatar35.6
United Arab Emirates21.8
Table 2. The released heating values of some common fuels at 25 °C [25].
Table 2. The released heating values of some common fuels at 25 °C [25].
FuelThermal Energy of CombustionAuto Ignition Temperature
MJ/kgBTU/lb(°C)
Hydrogen141.8061,000560
Methane55.5023,900580
Ethane51.9022,400515
Propane50.3521,700455
Butane49.5020,900405
Pentane48.6021,876260
Paraffin wax46.0019,900200–240
Kerosene46.2019,862210
Diesel44.8019,300210
Coal (anthracite)32.5014,000930
Gasoline (Petrol)46.0019,800246–280
Wood fuel21.209142180–230
Table 3. Summary of more accepted hypotheses for hydrogen-enhanced embrittlement [62].
Table 3. Summary of more accepted hypotheses for hydrogen-enhanced embrittlement [62].
MechanismsDescriptionReference
Hydrogen-enhanced decohesion
(HEDE)		Reduction in interatomic cohesive bond strength due to interstitial segregation of dissolve hydrogen resulting in reduced local plasticity.[74,75,76,77]
Adsorption-induced dislocation emission
(AIDE)		Ease of dislocation generation and increased activity at the crack tip due to weakened interatomic bond strength initiated by hydrogen adsorption.[78,79]
Hydrogen-enhanced localized plasticity
(HELP)		Hydrogen dissolution and formation of hydrogen atmosphere at dislocation core reduces the barrier towards dislocation movement, promoting localized plasticity.[80,81]
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Ghomashchi, R. Green Energy Revolution and Substitution of Hydrocarbons with Hydrogen: Distribution Network Infrastructure Materials. Energies 2023, 16, 8020. https://doi.org/10.3390/en16248020

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