Systematic Literature Review on Pipeline Transport Losses of Hydrogen, Methane, and Their Mixture, Hythane

Abstract

The transition to cleaner energy sources necessitates an in-depth understanding of the transport characteristics, losses, and opportunities associated with various gaseous fuels, including hydrogen, methane, and their mixtures, such as hythane. Hydrogen (H₂), the most abundant element in the universe, is increasingly recognized as a viable alternative to fossil fuels, primarily due to its potential to reduce carbon footprints as a cleaner energy source. Gradually gaining prominence in the energy market, it is displacing other fuels such as methane. In some transport systems, hydrogen is mixed with methane (CH₄) in order to reduce the carbon footprint while using the same existing production equipment. As more and more large methane consumers are implementing this mixture, we would like to see how the research has followed the market trend. An up-to-date research, development, and implementation status review is critical. This study aims to identify the main indicators of H₂ and CH₄ transport losses in pipes, providing a review of the state of the art in the specific literature. To deliver this, a systematic literature review (SLR) was carried out using preferred reporting items for systematic reviews and meta-analyses (PRISMA) methodology, pinpointing the research trends and results in peer review-published articles over a period of twelve years (2012–2024). Findings: this review identifies and points out, in numbers, the boundaries of the 2012–2024 timeline research.

1. Introduction

As global energy policies shift towards sustainability, the role of hydrogen and other gaseous fuels like methane in pipeline transportation becomes increasingly relevant. This review identifies the challenges and opportunities associated with the transport of hydrogen, methane, hythane (a blend of hydrogen and methane), and their mixtures. The aim is to synthesize existing knowledge and identify gaps for future research.

Transitioning from an energy market dominated by fossil fuels to one incorporating hydrogen offers numerous benefits, including environmental, social, economic, and energy-related advantages. Key stakeholders across political, environmental, and economic sectors are united in their support for the rapid implementation of this transition. The primary focus has been on hydrogen production and usage, but, similar to all energy forms and carriers, a big challenge lays in the transport and deployment from the point of production to the point of usage.

The development of new transport facilities for hydrogen (H₂) is at odds with the need for infrastructure that can support the increasing demands of H₂ production and consumption. A key strategic approach is to substitute methane (CH₄) with H₂, leveraging the substantial existing capital investments (CAPEX) that can be readily adapted to facilitate a blend of CH₄ and H₂, or even entirely pure H₂. It is anticipated that the current CH₄ transport infrastructure will play a crucial role in all strategic planning. Using the existing CH₄ pipeline to transport hydrogen gas is a highly cost-effective solution for delivering significant quantities of H₂.

In 2008, the only H₂ pipe network was a limited one (750 km in USA and 1500 in EU [1]). In 2021, the USA had 2600 km [2], and the EU, in 2040, will have 40,000 km of hydrogen transport dedicated pipeline [3].

Utilizing CH₄ infrastructure or incorporating H₂ with CH₄ up to a 30% volume of H₂—commonly referred to as hythane—within the same transport system introduces potential unknown risks due to the differing chemical properties of H₂ and CH₄. These risks are not entirely understood or clearly defined and mitigated. It is essential to guarantee the same safety standards as those applied to conventional hydrocarbon technologies before implementing the transport network [4].

Blending hydrogen with natural gas can greatly cut greenhouse gas emissions, especially if the hydrogen comes from low-carbon sources like biomass, solar, wind, nuclear, or fossil fuels with carbon capture. The environmental benefits of sustainable hydrogen can be attributed to the natural gas blend, proportional to the hydrogen content. This is similar to the introduction of biogas into natural gas pipelines, offering a renewable option [5]. A credit trading system, akin to the renewable energy credit system in electricity, could incentivize the conversion of renewable energy into hydrogen, boosting the sustainability of natural gas [6,7].

In recent years, hydrogen and natural gas have gained recognition for their potential as clean energy fuels in the transportation sector, especially in ecological vehicles such as city buses. Hydrogen, in particular, offers the advantage of producing zero tailpipe emissions when used in fuel cells, making it a promising alternative to traditional fossil fuels. Similarly, natural gas, though a fossil fuel, produces fewer greenhouse gas emissions compared with gasoline or diesel. When used in a blended form, such as hythane (a mixture of hydrogen and natural gas), both fuels can contribute to reducing the overall carbon footprint of vehicles, while utilizing the existing fuel infrastructure. Several studies [8] have explored the use of hydrogen and natural gas in public transportation systems, highlighting their role in reducing emissions and improving air quality, particularly in urban areas. These alternative fuels represent a crucial step toward the decarbonization of the transportation sector, aligning with global sustainability goals.

When we are talking of H₂ generation, we all see green H₂, but the reality is that, as of 2021, 96% of world H₂ generation is still made from fossil fuels and just 4% accounts for generation that could use only green energy [9,10]. Figure 1 shows the current potential share of renewable sources for hydrogen generation, along with the efficiency and costs associated with each generation method [10].

The generally recognized round-trip conversion efficiency of power to hydrogen and back to power is approximately 46% [10], so we must try to optimize all aspects of generation, storage, transport, and usage.

The generation of H₂ being covered in another review [10] and storage being relatively simple, with little or no loss and with no higher CAPEX, we will try to focus on the transport side and the link between generation and usage.

While this review focuses on the transport systems for hydrogen, methane, and hythane, detailed topographical data for specific pipeline areas are unavailable. As a result, this study does not include topographical analysis. Future research may incorporate geographical variations to assess their impact on pipeline efficiency and pressure loss.

While the generation of hydrogen is addressed in other reviews [9] and storage is relatively straightforward with minimal loss and no significant increase in capital expenditure, our emphasis will be on the transportation aspect, which serves as the connection between generation and consumption.

In our paper, we provide a comprehensive analysis of the existing literature on the transport losses associated with hydrogen, methane, and hythane. Our contributions include the following:

• Synthesis of current research. We systematically compile and synthesize research findings related to the losses experienced during the transportation of hydrogen, methane, and hythane, highlighting key factors that influence these losses.
• Identification of knowledge gaps. Our review identifies critical gaps in the current body of knowledge, pointing out areas that require further investigation to optimize transport systems for these gases.
• Comparative analysis. We offer a comparative assessment of transportation losses for hydrogen, methane, and hythane, providing insights into how the transport characteristics differ between these gases and the implications for efficiency and safety.
• Recommendations for future research. Based on our findings, we propose recommendations for future research directions, focusing on enhancing transport efficiency and reducing losses, which can contribute to the development of more sustainable energy systems.

While significant advancements have been made in understanding the individual transport characteristics of methane and hydrogen, limited studies have systematically compared the transport losses of hydrogen, methane, and their mixture, hythane. This paper addresses this gap by providing a comprehensive analysis of transport losses across these gases, highlighting the implications for efficiency and safety in pipeline transport. By synthesizing research from various studies and providing a comparative framework, this review offers new insights that are critical for the development of future hydrogen–methane blended transport systems.

By addressing these aspects, our review not only consolidates existing knowledge but also serves as a foundation for future studies aimed at improving the transport systems for hydrogen, methane, and hythane.

2. Materials and Methods

Given the complexity and interdisciplinary nature of the research area that this study is focused on, a review is necessary to assess the field’s current readiness.

Identifying key features in peer-reviewed research papers—such as research focus, findings, economic factors, performance metrics, and technologies utilized—can serve as a foundational resource for manufacturers, researchers, managers, and other decision makers involved in H₂ transport.

We use the systematic literature review (SLR) method as per preferred reporting items for systematic reviews and meta-analyses (PRISMA) [11] in order to deliver a review of the H₂ transport topic accessible to researchers worldwide. This study follows a systematic literature review (SLR) methodology, which is designed to systematically collect, review, and analyze relevant research on the transport of hydrogen, methane, and their mixture, hythane. This method ensures that all research is comprehensively identified and evaluated. Furthermore, the review adheres to the preferred reporting items for systematic reviews and meta-analyses (PRISMA) guidelines, which provide a transparent and reproducible framework for conducting systematic reviews.

The objectives of this review are to identify the status of H₂ transport research and market readiness.

The research questions raised during this review are as follows:

RQ1:

can a natural gas pipeline network be used for H₂ transport?

RQ2:

what are the main pressure loss indicators in H₂ transport?

RQ3:

are the risks of transporting H₂ vs. CH₄ higher?

In order to comply with the Kyoto Protocol [12] and the Paris Agreement [13], it is not enough to merely invest in hydrogen generation technologies and efficient usage, we also need to establish a transport network that can connect production and consumption at a finer resolution than what is currently available for methane (CH₄).

This process follows three large steps [14,15]: (i) review the planning, (ii) conduct the review, and (iii) disseminate the results.

(i) During this stage, we define the review target and identify the limits of our research.

• Pipes for H₂ transport;
• H₂ and CH₄ blended transport;
• Replacement of CH₄ with H₂ in the actual pipe transport networks.

Based on our above targeted research area, we defined our procedure, like a guide that unites all the steps that are performed in this SLR. First, we defined our data pool, the EBSCO Discovery Service. This was our option because it contains an exhaustive list of research papers and because it has a no bias/high-quality policy.

Because a single search did not cover the whole spectrum of our research, we used the six strategies [5] defined in

Table 1. Search criteria used to populate the review research papers’ database.

	Criterion 1	Criterion 2	Criterion 3	Criterion 4	Criterion 5	Criterion 6
Key words	H2 losses in pipes	Losses in pipes	Fluid transmission losses	Hythane losses	CH4 losses	NA 1
Disciplines	Appendix A					All
Expanders	Also search within the full text of the articles/apply equivalent subjects					NA
Search modes	Appendix A					Appendix A
Results’ limits	Full text peer-reviewed
Published date	2012 ÷ 2024					No time limit
Language	English
Result [papers]	13	200	40	34	88	5

¹ The sixth criterion refers to the papers that were recommended by online page recommendation during the full text review of the research papers defined in criteria 1 to 5.

.

(ii) During the conduct the review stage, the database extracted from the EBSCO Discovery Service, namely 258 research papers, in July 2024 was considered.

(iii) The reporting and dissemination stage is covered under the results topic.

Due to the large number of research papers processed in this review, a flow of exclusion was identified and is presented in Figure 2. This flow chart follows the PRISMA methodology for reporting systematic reviews [11].

3. Results

3.1. RQ1: Can a Natural Gas Pipeline Network Be Used for H₂ Transport?

Firstly, we need to examine the properties of both gases and determine how a functional structure designed to safely transport CH₄ can meet the same functional and safety requirements for transporting H₂.

3.1.1. Parametric Characteristics of the Two Gases

The physical proprieties and the chemical proprieties of CH₄ are different from those of H₂ (Appendix B). Also, the blend of the two gasses that are the scope of multiple investments WW is detailed in the extract hereunder (

Table 2. Physical proprieties of H₂, CH_4, and mixtures (volume %).

Properties	Hydrogen (H2)	Methane (CH4)	Hythane 10% H2-90% CH4	Hythane 25% H2-75% CH4	Hythane 30% H2-70% CH4	Mixture 40% H2-60% CH4	Mixture 60% H2-40% CH4	Unit
Lower calorific value by volume at 1 atm	11.00	35.00	32.60	27.20	33.32	25.40	20.60	MJ/m3
Higher calorific value by volume at 1 atm	13.00	39.00	36.00	30.00	27.00	28.00	20.00	MJ/m3
Maximum flame temperature	1800.00	1495.00	1525.50	1594.13	1516.35	1617.00	1678.00	K
Auto-ignition temperature	560.00	600.00	590.00	590.00	580.00	580.00	570.00	C
Laminar burning velocity	3.10	0.40	0.68	1.31	0.60	1.52	2.08	m/s

, fully reproduced in Appendix B [4,15,16,17,18,19,20,21,22]).

Hythane types are used both in automotive transport and in industry (30% H₂), where the presence of H₂ contributes to the engine combustion yield and to the CO₂ emission reduction. Also, the mixture can be a competitive alternative fuel for existing combustion plants [18]. On the other hand, in discussions about the natural gas transportation network, there is typically an established upper limit of 10% hydrogen volume [23].

The properties of hydrogen (H₂) and methane (CH₄) (presented in Table 2) highlight significant differences that impact their transport through pipelines, both in existing infrastructure and future networks. The transition from methane (CH₄) to hydrogen (H₂) as an energy carrier presents several challenges, primarily due to the physical properties of hydrogen. Hydrogen’s lower molar mass and higher diffusivity allow it to permeate materials more easily than methane CH₄, resulting in higher permeability in pipeline materials and a greater risk of embrittlement, particularly in steel pipes commonly used for methane transport. This compatibility issue is significant, as steel may become brittle and prone to cracking when exposed to hydrogen over time.

Hydrogen’s lower auto-ignition temperature and broader flammability limits compared with methane require stricter safety protocols to manage the associated risks in its transport and use. While methane is known for its higher calorific value by volume, which makes it more efficient for energy transport, blending hydrogen with methane—known as hythane—presents a potential solution that could strike a balance between lowering carbon emissions and leveraging the existing gas infrastructure [24].

However, adapting current pipeline systems to accommodate this H₂/CH₄ mixture poses challenges. Maintaining pipeline integrity and safety will necessitate significant modifications, especially for older networks that were not originally designed to accommodate the unique properties of hydrogen. These modifications may involve material upgrades, enhanced sealing techniques, and rigorous safety evaluations to ensure that both the efficiency of the energy transport and the safety of the infrastructure can be maintained [6].

3.1.2. H₂ Transportation

We can see three main methods of H₂ transportation.

• In a gaseous state in specialized cylinders or tube trailers and pipeline transport (compressed hydrogen to high pressures—typically around 35–70 MPa—is one of the most common methods; this approach is suitable for short to medium distances and for smaller volumes);
• In a liquid state (H₂ can be transported in liquid form, which is advantageous for long distances and large-scale transport due to its higher energy density compared with gaseous hydrogen; however, the liquefaction process is energy-intensive and requires cryogenic temperatures around −253 °C);
• In a bound form using solid or liquid carriers, including by land transport in cylindrical containers (H₂ can also be transported using solid or liquid carriers, such as metal hydrides or chemical hydrogen storage systems [25]; these carriers allow for safer and more efficient storage and transportation, adding additional complexity to the hydrogen release process);

The safest and most practical method for transporting significant quantities of hydrogen (H₂) over long distances is through pipelines in a compressed form. In order to achieve a technology readiness level of 9 and establish a cost-effective production method, hydrogen’s share in the energy market must account for at least 10% [26].

We will direct our focus on the existing natural gas transport piping network. Here, we can see that the vast majority are built out of steel [2,26]. Due to the size of H₂ atoms, one proton and one electron, all metals are permeable to this gas [27], steel being one of them.

The H₂ steel penetration from gas phase can be divided into the following stages [28].

• Condensation of gaseous H₂ on a steel surface (dispersive forces or Van der Waals forces) leads to surface coverage. The coverage of hydrogen atoms on the steel surface can be up to a monolayer under typical conditions. At room temperature and moderate pressures (0.1–1 MPa), surface coverage might range from 10⁻³ to 10⁻² molecules per square nanometer (molecules/nm²) [26,29].
• Dissociation of molecules into atoms—chemisorption. The energy of chemisorption is the energy required for hydrogen molecules to dissociate and chemisorb onto steel surfaces and typically ranges between 20 and 50 kJ/mol. The degree of dissociation depends on temperature and surface properties but could result in a significant proportion of surface hydrogen in an atomic form at temperatures above 200 °C [26,29].
• Transition of atoms through a steel surface—gas dissolution in steel. The surface permeation flux representing the flux of hydrogen atoms penetrating the steel surface (depending on pressure and temperature) could range between 10⁻⁶ and 10⁻⁴ mol/m²/s under typical industrial conditions (e.g., 200–400 °C and 1–10 MPa pressure). The actual flux depends significantly on the steel type and environmental conditions [26,29].
• Diffusion of H₂ atoms from the surface into the interior of the steel wall, characterized by:

  ➢

  Diffusion coefficient. The diffusion coefficient of hydrogen in steel varies widely with temperature, typically ranging from 10⁻⁷ to 10⁻⁴ cm²/s at temperatures between 100 °C and 400 °C. At lower temperatures (e.g., 25 °C), the diffusion coefficient may be around 10⁻⁹ to 10⁻⁷ cm²/s [26,29].

  ➢

  Penetration depth. The penetration depth of hydrogen into steel over extended periods (e.g., 1000 h) at room temperature might be in the range of 10 to 100 micrometers (µm), increasing significantly at higher temperatures due to the exponential increase in the diffusion coefficient [26,29].

The hydrogenation intensity is determined by the gaseous H₂ pressure, temperature, and type of steel. The solubility of hydrogen in iron, which indicates the maximum amount of hydrogen that can exist in a solid solution form in Fe at a temperature of 23 °C and a pressure of 0.1 MPa, is ~0.0001 m³/0.1 kg Fe [26]. This process is directly proportional to the working pressure and the strength of the steel. The hydrogenation of steel pipes changes the mechanical properties of steel pipes and increases the risk of failure in connecting surfaces (welding, T junctions, etc.) [2,26,30].

Some issues related to the injection of H₂ into the CH₄ pipes are related to the pressure difference between the two gasses. This pressure difference leads to a Joule–Thomson effect, causing the temperature of the H₂ to increase during its expansion (pressure decrease) [4].

Having such a high number of increased risks for transportation in existing natural gas steel pipes, we would assume that a steel piping network would not be a viable solution for H₂ transport. However, mitigating the risks (as performed in early CH₄ pipe systems) the economical and sustainability criteria are the ones that drive the transport industry [31].

Based on the status outlined, it appears that the strategy involves repurposing the existing steel network instead of implementing significant changes. This approach leverages the current infrastructure while adapting to a new risk matrix that addresses any potential challenges. By doing so, the system can maintain the same safety standards that are currently in place for methane usage. This strategy not only ensures continuity of operations but also optimizes the existing resources in a way that aligns with safety and risk management objectives [27].

3.2. RQ2: What Are the Main Pressure Loss Indicators in H₂ Transport?

The main research indicators identified in this review are as follows:

• Mixture H₂/CH₄ [%];
• Gas or liquid state;
• Pipeline diameter [mm];
• Wall thickness [mm];
• Length of pipe [m];
• Pipe material;
• Strength class;
• Outside pipe pressure [Pa];
• Inside pipe pressure [Pa];
• Velocity [m/s];
• Temperature [°C];
• Diffusion coefficient of H₂ in metal [m²/s];
• Solubility of H₂ in Fe [m³/m³ solid*MPa0.5];
• H₂ leaks [m³];
• Power usage for transport [MW];
• Load loss [MPa];
• Inner surface roughness of pipe [μm];
• Loss due to diffusion [%]
• Enthalpic jump [kJ/kg];
• Mass flow [kg/s];
• Pipe production technology.

In order to visualize the focus of the research on hythane transport, we present hereunder some charts to summarize the KPIs and results. Figure 3 presents the specified working pressure of the hythane (hydrogen–methane) transported [25,26,27,28,29,30,31,32,33,34].

Most research (94.12%) is aimed at the gas transport of H₂ and only 5.88% is directed to the liquid phase of H₂ [29,35]. This preference for the gas phase of hythane and H₂ is determined by the economics of reusing or sharing the existing infrastructure of CH₄ transport. Figure 4 displays a box-and-whisker plot illustrating the pipe parameters of the modeled or physically studied pipeline network. In Figure 5, we add the materials used in the research regarding the hythane transport in real or modeled networks [35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54].

The coating of pipelines [55] or new methods (e.g., pulsed laser ablation [44]) of delivering pipes for hythane or H₂ transport could reduce the associated risks. Building pipelines for H₂ transport only drastically differs in costs. Estimated costs of operating the existing CH₄ pipeline infrastructure for H₂ or hythane increases the cost by 30% to 50% more than CH₄ [24].

We identified only one paper that mentions two indicators related to the velocity increase and pressure drop after injecting/blending H₂ into CH₄ [56]. For 17% H₂ in hythane, we see a velocity increase of 0.16 [m/s] and a pressure drop of 0.019 [MPa].

In Figure 6, we highlight the outline of the transport pressure, velocity, and temperature (only for the gas phase of H₂; for the liquid phase, we have only one research at −252 °C [41]).

As most of our research focus includes steel pipes, we mention some characteristics that define the transport and material specifications.

• Solubility of H₂ in iron 0.0001 cm/0.1 kg Fe [26].
• Solubility of H₂ in steel 5.9 × 10⁻⁵ [m³ gas/(m³ solid*MPa0.5)] [26].
• Diffusion coefficient of H₂ in steel [m²/s] varies from

  ➢

  iron—5.8 × 10⁻¹⁰ [28];

  ➢

  K42 steel—4.32 × 10⁻¹¹ [26];

  ➢

  100Cr6 steel—4.9 × 10⁻¹² [42];

  ➢

  304 steel—7.37 × 10⁻¹⁶ [28].

• H₂ permeability of the piping coating:

  -

  Diglycidyl ether (DGEBA)/polyetheramine (D-400) epoxy coating, 0.35 Barrer [55];

  -

  Crosslinked poly (vinyl alcohol) coating, 0.084 Barrer [55].

3.3. RQ3: Are Risks of Transporting H₂ vs. CH₄ Higher?

In order to identify and assess the risk associated with each process and segment of the transportation area, a standard risk matrix is provided in

Table 3. Risk matrix for H₂ utilization in existing methane pipelines.

Consequences						Occurrence Probability
						0–20%	21–40%	41–60%	61–80%	81–100%
						A	B	C	D	E
Severity	Health/Safety	Environment	PR	Corporate Impact	Almost Certain	Rare	Unlikely	Possible	Likely	Certain
5 Severe	Potential for multiple fatalities	Potential catastrophic damage	Governmental level	Lack of functionality >7 days	>3 days	5A	5B	5C	5D	5E
4 Major	Potential for single fatality	Potential long-term effects	Public disruption	Lack of functionality >1 day	<3 days	4A	4B	4C	4D	4E
3 Moderate	Potential for a single major injury	Potential medium-term effects	Small public disruption	Lack of functionality <1 day	>2 days	3A	3B	3C	3D	3E
2 Minor	Potential for lost time	Potential long-term effects	Local media coverage	Corrective maintenance	>2 days	2A	2B	2C	2D	2E
1 Marginal	Potential for first aid injury	Potential long-term effects	No media coverage	Malfunction	No imp.	1A	1B	1C	1D	1E

. The matrix is suitable for CH₄ and for H₂ [57,58].

In

Table 4. Maximum allowable H₂ mixture in CH₄ transport networks.

CH4 Infrastructure	Pressure Regulation	Meters	CHG Storage Tanks	House Install	Seals /Valves	Transmission Pipelines	Co-Gen Plants	Home Gas Burners/Stove	Compression Station	Gas Turbines
H2 [%]	67	30	30	30	30	30	20	10	2	1
Transport network	Steel pipes	PVC pipeline	Sealants	Connectors	Fittings	Flow valves	Domestic pipe
H2 [%]	30	70	30	30	30	15	30
Transport equipment	Pipeline	Turbine	Compressor
	30	1	5

, we try to summarize the upper limit of hythane H₂% blending so that the risk remains a green one (as per above matrix) [59].

In Figure 7, we present the maximum acceptable H₂ mixture in CH₄ transport within piping networks across different countries. Any higher mixture of H₂ into CH₄ would increase the risk associated with yellow labeling [56,57,60].

Due to the proprieties of H₂ versus CH₄ mentioned in Table 2, the risk assessment for the same event could be higher for H₂ on a severity scale and on occurrence probability even though the associated hazards are similar [61,62]. All the assessment vaguely presented in Table 3 should take into consideration the following:

• Correctly assessing the vulnerabilities of metal piping;
• Understanding the behavior of H₂ released into the atmosphere, including its rapid dispersal and tendency not to accumulate near the ground, where ignition sources are more likely to be present;
• Accidental release of H₂ means a much higher flammable mixture, necessitating a review of applicable safety measures;
• Compared with methane (CH₄), hydrogen possesses a significantly higher flame propagation velocity, which can increase the risk of fire hazards;
• The flammability and radiation hazard should be reconsidered;
• Equipment built for use in gas group IIC (ATEX) is to be used [58];
• Gas measurement and leak detection equipment must be specifically suitable for hydrogen to effectively identify potential leaks and mitigate risks associated with its release.

4. Expanded Comparative Analysis and Safety Considerations in Hydrogen and Methane Transport Systems

Although hydrogen and methane share many similarities, there are notable differences that lead to a knowledge gap, as presented in

Table 5. Comparative properties of methane and hydrogen relevant to transport and safety.

Methane	Hydrogen
greater density	higher heat capacity
greater viscosity	higher diffusivity
higher calorific value by volume	higher calorific value by mass
higher solubility in water	higher flame temperature
	higher auto-ignition temperature
	wider explosive and fire danger

, primarily resulting from the insufficient experimental data, which have largely focused on decades of methane transport. In comparing methane and hydrogen for transport and safety considerations, it is essential to examine their respective physical and chemical properties. These properties directly influence their behavior during pipeline transport and the associated risks, such as energy losses, safety hazards, and transport efficiency. Table 5 highlights the key differences between methane and hydrogen, focusing on properties relevant to their transport and safety performance.

RQ1: can the natural gas pipeline network be used for H₂ transport?

The existing natural gas (CH₄) pipeline infrastructure has the potential to be repurposed for hydrogen (H₂) transport, but several technical challenges and limitations need to be addressed. Due to its status as the smallest molecule, hydrogen exhibits greater diffusivity and permeability than methane. These properties increase the likelihood of leakage and raise the risk of embrittlement in metals, particularly in the high-strength steel typically utilized in natural gas pipelines [58,59].

Hydrogen, methane, and their mixture, hythane, exhibit distinct transport losses due to their differing physical and chemical properties. Hydrogen, with its lower molecular weight and higher diffusivity, tends to leak more easily from pipelines, leading to greater pressure losses compared with methane. On the other hand, methane, being denser, experiences lower leakage rates but higher energy losses over long transport distances due to friction and viscosity. Hythane, as a blend of hydrogen and methane, balances these properties, but the exact losses depend on the ratio of hydrogen to methane in the mixture. This review highlights that the efficiency of transport systems for hydrogen will require significant adaptations in pipeline materials and sealing technologies to minimize losses, whereas methane can be transported more reliably using the current infrastructure.

Studies have shown that blending up to 20% H₂ by volume with natural gas can be managed within the existing infrastructure with minimal modifications [24]. However, as the concentration of hydrogen increases, so do the risks of pipeline embrittlement, leakage, and reduced integrity of the pipeline materials [33]. In recent years, several real-world projects have tested the feasibility of hydrogen transport through existing natural gas pipelines. For instance, the “HyDeploy” project in the United Kingdom demonstrated that up to 20% hydrogen could be blended into the existing natural gas network without significant safety or efficiency issues [63]. However, this study also noted challenges related to pipeline materials, particularly the need for coatings and seals that are resistant to hydrogen embrittlement [63]. Similarly, in the United States, the “H₂@Scale” initiative has identified key technical barriers, including pressure regulation and monitoring for hydrogen leakage [64]. These examples underline the importance of tailored solutions for hydrogen transport, which must account for both material compatibility and operational safety. Research indicates that additional measures, such as the application of internal coatings, the use of low-carbon steel, and ongoing monitoring of pipeline integrity, would be necessary to mitigate these risks [29,34,35].

Moreover, the operational conditions for H₂ transport differ from those for CH₄. Hydrogen’s lower energy density requires higher pressures to achieve the same energy throughput as methane [65]. This difference could necessitate modifications in compressors and other pipeline network components.

Several studies have conducted comparative analyses on the transport efficiency and safety of hydrogen versus methane. For example, a recent study [66] found that the energy loss per kilometer of pipeline is approximately 15% higher for hydrogen than for methane under similar conditions, primarily due to hydrogen’s higher diffusivity. Another comparative study [67] demonstrated that, while hydrogen offers a cleaner alternative, the cost of retrofitting pipelines to safely handle hydrogen adds significant operational expenses. In contrast, methane, while less prone to leakage, poses greater environmental risks due to its higher greenhouse gas potential. These findings highlight the trade-offs involved in transitioning from methane to hydrogen in existing pipeline networks. Pipelines designed for natural gas or hythane do not face the same degree of embrittlement or permeation challenges. However, for the safe transport of hydrogen, solutions such as composite materials, lined pipelines, or advanced coatings are essential. These materials not only provide the necessary mechanical strength but also prevent hydrogen from penetrating the pipeline structure, reducing the risk of leakage and maintaining pipeline integrity over time [68]. Our review emphasizes the need for more robust material standards and suggests future research on developing cost-effective materials that can accommodate both hydrogen and methane in blended systems [69]. Recent advancements in materials science have enabled the development of high-strength polymers and composites that can withstand the higher pressures required for hydrogen transport [70], while also preventing hydrogen diffusion through the pipeline walls. Compared with traditional steel, these flexible systems not only reduce the risk of pipeline failure but also allow for easier installation and maintenance. Additionally, the use of flexible pipelines in hydrogen distribution networks provides a cost-effective alternative to retrofitting existing natural gas pipelines

RQ2: what are the main pressure loss indicators of H₂ transport?

Pressure losses in hydrogen transport are significantly influenced by the unique physical properties of hydrogen, which differ from those of methane. These properties include lower density and higher diffusivity, both of which contribute to greater pressure loss over long pipeline distances. In particular, hydrogen’s high flow velocity at the same pressure gradient as methane leads to greater frictional resistance in pipelines. Studies have shown that maintaining optimal flow velocity is crucial to minimizing these losses. Furthermore, the inner surface roughness of pipelines, which affects friction, plays a larger role in hydrogen transport. The use of smoother internal surfaces in hydrogen pipelines may reduce frictional losses and help maintain pressure over long distances.

Pressure loss in hydrogen pipelines is influenced by several factors, which differ from those in natural gas pipelines due to hydrogen’s unique properties. The primary indicators of pressure loss in hydrogen transport include the following:

• Pipe diameter and wall thickness. Smaller pipe diameters and thicker walls generally result in greater pressure losses due to increased friction and reduced flow area. For hydrogen, which is less dense and more prone to turbulent flow, optimizing pipe dimensions is crucial to minimize losses.

Fundamental relationship is the relationship between pipe diameter D, flow rate Q, and pressure drop ΔP in a pipeline carrying hydrogen, described by the Darcy–Weisbach Equation (1):

$$∆P= f\ast {\displaystyle \frac{L}{D}}\ast {\displaystyle \frac{\rho \ast v^2}{2}}$$

(1)

where f is the friction factor (which depends on the flow regime and pipe roughness), L is the length of the pipe, D is the pipe diameter, $\rho$ is the density of the hydrogen, and v is the average flow velocity of the hydrogen.

Flow rate relationship. The flow rate Q is related to the velocity v and the cross-sectional area A of the pipe by:

$$Q=A\ast v={\displaystyle \frac{\pi D^2}{4}}\ast v$$

(2)

Substituting into the Darcy–Weisbach equation illustrates that for, a given flow rate Q, increasing the pipe diameter D reduces the velocity v and, therefore, the pressure drop ΔP.

• Average flow velocity. Hydrogen has a higher flow velocity at the same pressure gradient compared with methane. This increased velocity can lead to higher frictional losses, especially in long-distance transport pipelines. Maintaining optimal flow velocity is essential to reducing pressure drop.
• Surface roughness. The internal surface roughness of the pipeline has a significant impact on pressure loss. Hydrogen’s low viscosity exacerbates the effects of surface roughness, increasing frictional resistance. Pipelines designed for hydrogen transport might require smoother internal surfaces to mitigate these losses [71].
• Operating pressure and temperature. Higher operating pressures tend to reduce pressure losses due to a higher density of hydrogen, which can compensate for its low molecular weight. However, operating at elevated temperatures can decrease hydrogen’s density, increasing pressure loss.

Pressure losses in hydrogen transport are significantly influenced by the unique physical properties of hydrogen, which differ from those of methane [72]. These properties include lower density and higher diffusivity, both of which contribute to greater pressure loss over long pipeline distances. In particular, hydrogen’s high flow velocity at the same pressure gradient as methane leads to greater frictional resistance in pipelines. Studies [72,73] have shown that maintaining optimal flow velocity is crucial to minimizing these losses. Furthermore, the inner surface roughness of pipelines, which affects friction, plays a larger role in hydrogen transport. The use of smoother internal surfaces in hydrogen pipelines may reduce frictional losses and help maintain pressure over long distances.

RQ3: are the risks of transporting H₂ higher than CH₄?

The transportation of hydrogen poses several distinct risks compared with methane. One of the primary concerns is hydrogen embrittlement, a phenomenon where hydrogen atoms penetrate steel pipelines, causing the material to become brittle and more prone to cracking. This issue is exacerbated in high-strength steel pipelines traditionally used for methane transport. Hydrogen leakage is also a more significant risk due to its smaller molecular size, which allows it to escape through pipeline joints and even through the steel itself. Furthermore, hydrogen’s broader flammability limits and lower ignition energy increase the risk of accidental ignition in the event of a leak. These factors necessitate stricter safety protocols, including the use of advanced coatings and materials that can withstand hydrogen’s unique properties.

The primary risks include the following:

• Hydrogen embrittlement. Hydrogen can penetrate steel pipelines, leading to embrittlement—a phenomenon whereby the metal becomes brittle and more susceptible to cracking. This risk is particularly pronounced in high-strength steels and requires the use of special alloys or coatings to mitigate [28,29,42].
• Leakage. Due to its small molecular size, hydrogen is more likely to leak through pipeline joints, fittings, and even through the steel itself. This increases the risk of explosions, especially since hydrogen has a wide flammability range (4–75% in air) and a low ignition energy compared with methane [43,57,74].
• Explosion hazard. Hydrogen’s high diffusivity and wide flammability limits make it more prone to accidental ignition. When leaks occur, hydrogen can form explosive mixtures more readily than methane, especially in confined spaces. The energy released in hydrogen explosions is also typically higher, posing a significant safety risk [7].
• Joule–Thomson effect. When hydrogen is released from high pressure (as might happen during a leak), its temperature increases, unlike methane, which cools upon expansion. This temperature rise can lead to ignition risks if there are sparks or other ignition sources nearby [36].
• Accidental damage to a pipe in an excavation activity. Here, the failure frequencies are similar to those for natural gas “from 2016 to 2022 it resulted in 11 fatalities, 36 injuries, and over USD 144 M in damages” [74] in the US only.

Figure 8, hereunder, provides a comparative timeline of major accidents associated with methane (CH₄) and hydrogen (H₂) transport systems. The upper chart illustrates significant incidents involving CH₄, highlighting the frequency and severity of accidents over the years. The lower chart focuses on hydrogen-related incidents, underscoring the unique risks posed by hydrogen transport due to its higher diffusivity, lower ignition energy, and broader flammability limits compared with methane. This timeline emphasizes the importance of understanding the distinct safety challenges associated with hydrogen transport as the energy sector transitions towards cleaner alternatives.

5. Conclusions

The transition from methane (CH₄) to hydrogen (H₂) as an energy carrier presents both opportunities and challenges in the context of existing transport infrastructure. This analysis outlines key strategies and considerations for achieving this shift effectively while ensuring safety and efficiency.

• Incremental integration of hydrogen. The initial phase of integrating hydrogen into existing natural gas systems involves blending H₂ with methane in concentrations ranging from 5% to 30%. This blending is contingent upon local regulations and, as such, the adjustment of risk assessment matrices and maintenance practices for metal piping and associated infrastructure becomes imperative. Effective management of this transition requires a thorough understanding of the materials properties and behavior of piping systems under mixed gas conditions.
• Adaptation of combustion systems. Converting large-scale CH₄ combustion systems to utilize hythane (a mixture of hydrogen and natural gas) up to 30% H₂ allows for a competitive alternative that aids in maintaining the necessary flow within transport pipelines. This conversion not only facilitates the transition to hydrogen but also leverages existing infrastructure and technologies for a more gradual shift.
• Innovative liquid hydrogen transport. Exploring the feasibility of transporting liquid hydrogen through cryogenic pipelines presents a revolutionary approach to reducing transport costs, with theoretical reductions in energy costs of up to 99.5% compared with traditional methane transport [41]. This method could potentially open new avenues for hydrogen distribution, although it requires innovative material solutions to handle the extreme conditions.
• Repurposing natural gas pipelines. While repurposing existing natural gas pipelines for hydrogen transport is achievable, it necessitates extensive upgrades to ensure safety and efficiency. This includes material adjustments to withstand hydrogen’s unique properties, the implementation of enhanced monitoring systems, and limitations on hydrogen concentration to mitigate mechanical stress risks. Additionally, integrating an inner lining designed to reduce friction could further optimize energy loss during transport [55].
• Advanced pipeline design. The introduction of pipes with a 3D-structured surface—such as triangular or knife-blade geometries—aimed at minimizing drag, represents a cutting-edge solution for enhancing hydrogen flow efficiency. These advanced designs can significantly reduce turbulence and pressure losses, thus optimizing the overall performance of hydrogen transport systems.
• Optimization of transport parameters. To ensure efficient hydrogen transportation, a comprehensive analysis of pipeline dimensions, flow velocity, surface roughness, and operating conditions is critical. Effective design and optimization of these factors are essential to minimize pressure losses and maximize throughput.
• Risk management. The inherent risks associated with hydrogen transport—such as embrittlement of materials, leakage, and explosion potential—are notably higher than those for methane. Addressing these risks demands substantial advancements in technology and materials science, along with rigorous safety protocols that may include continuous monitoring and rapid response mechanisms.

In conclusion, the transition to a hydrogen economy is fraught with both challenges and potential. By strategically adapting infrastructure, investing in innovative transport methods, and rigorously managing safety risks, it is possible to pave the way for a more sustainable and efficient hydrogen transport ecosystem. The steps outlined provide a foundational framework for stakeholders in the energy sector as they navigate this critical evolution.

Research on hydrogen and hythane (a blend of hydrogen and natural gas) transport losses is critical for improving the efficiency and sustainability of these energy carriers. A few ideas for future research focused on the following topics:

• Leak detection technologies. Development of advanced sensors and monitoring systems for real-time detection of hydrogen and hythane leaks in pipelines. This can include research on nanomaterials or IoT-enabled devices for enhanced sensitivity.
• Material innovation. Investigating new materials for piping and storage that reduce permeability to hydrogen and minimize transport losses. This could include coatings or composite materials that are more resistant to hydrogen embrittlement.
• Integration with renewable energy sources. Analyzing how integrating hydrogen transport with renewable energy systems (like solar or wind) can enhance energy efficiency and minimize loss during transport.

By pursuing these research avenues, experts can help optimize hydrogen and hythane transport systems, greatly improving their feasibility and efficiency as future energy sources.

This paper provides a novel contribution to the existing body of research by systematically reviewing and comparing the transport losses of hydrogen, methane, and hythane. Our findings highlight the differences in transport characteristics between these gases and identify the technical and safety challenges associated with hydrogen transport. By filling the current gaps in the literature, this review serves as a foundation for future studies aimed at optimizing transport efficiency and enhancing the safety of pipeline systems. The comparative analysis presented here will help industry stakeholders make informed decisions regarding the integration of hydrogen into existing methane transport infrastructures