Green Hydrogen, a Solution for Replacing Fossil Fuels to Reduce CO₂ Emissions

Abstract

The article examines the role of green hydrogen in reducing CO₂ emissions in the transition to climate neutrality, highlighting both its benefits and challenges. It starts by discussing the production of green hydrogen from renewable sources and provides a brief analysis of primary resource structures for energy production in European countries, including Romania. Despite progress, there remains a significant reliance on fossil fuels in some countries. Economic technologies for green hydrogen production are explored, with a note that its production alone does not solve all issues due to complex and costly compression and storage operations. The concept of impure green hydrogen, derived from biomass gasification, pyrolysis, fermentation, and wastewater purification, is also discussed. Economic efficiency and future trends in green hydrogen production are outlined. The article concludes with an analysis of hydrogen-methane mixture combustion technologies, offering a conceptual framework for economically utilizing green hydrogen in the transition to a green hydrogen economy.

1. Introduction

The industrial and energy utilization of green hydrogen is a component of the broader decarbonization strategy aimed at reducing CO₂ emissions. The European Union has been a leader in this effort, initiating the “Clean Energy for All Europeans” package in 2016 following the 2015 Paris Agreement. The first steps involve capturing CO₂ emissions to transform fossil fuels into less chemically hostile fuels. During the transition to a zero CO₂ emission energy system, renewable energy sources may include the gradual integration of fossil fuel energy, particularly natural gas and biomass, provided they are equipped with CO₂ capture systems. A circular economy, encompassing circular energy, entails the valorization of biomass, which includes woody biomass, agricultural residues, industrial and municipal waste, and treated wastewater. Achieving climate neutrality through biomass energy production necessitates sophisticated technologies tailored to the energy characteristics of biomass. The process involves the introduction of hydrogen from synthesis gas generated through various technological processes such as biomass gasification, pyrolysis, fermentation, and wastewater treatment. Although the origin of this hydrogen may be considered green, its association with CO and CO₂ gas components renders it environmentally harmful [1,2].

In terms of direct biomass combustion, relying solely on the Kyoto Protocol is no longer viable. However, capturing carbon dioxide (CO₂) during combustion, akin to hydrocarbon combustion, could be a potential option.

For the transition period under consideration, where the gradual injection of H₂ into the natural gas network is being contemplated, the following association can be made:

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Synthesis gas, comprising a mixture of H₂, CO, and CO₂, can be burnt similarly to the blend of H₂ with CH₄ (this gas may also be termed impure hydrogen).

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Synthesis gas can be introduced into the natural gas (CH₄) network.

In synthesis gas, hydrogen can represent proportions of 15–35%, with the power of the gaseous fuel reaching up to 40–50%.

In addition to CO₂ emissions, renewable sources include:

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Solar energy;

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Wind energy;

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Hydraulic energy;

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Geothermal energy;

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Nuclear energy;

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Energy produced from biomass.

Among these sources, only solar and wind energy exhibit random variations, linked to climatic factors.

From renewable energy in a zero CO₂ emissions society, the following are expected to be produced:

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Electrical energy;

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Thermal energy;

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Green hydrogen.

Green hydrogen becomes a complex energy vector contributing to:

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The production of electrical and thermal energy.

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The production of synthetic fuels, through reaction with captured CO₂, leading to methane (CH₄) and methanol (CH₃OH).

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The period from 2020 to 2024, characterized by the existence of up to 6 GW of power in electrolyzers, with a production of 1 million tons of green hydrogen.

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The period from 2024 to 2030, involving the expansion of electrolyzer power to 40 GW, with a production of 10 million tons of green hydrogen.

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The period from 2030 to 2050, during which the total demand for green hydrogen will be met.

Green hydrogen originates from renewable electricity sources, with electricity used in electrolyzers to carry out the process of alkaline water dissociation [3,4,5,6].

The EU has a supportive policy for a hydrogen-based economy, and for this purpose, the following bodies have been established: Hydrogen Europe Industry and the European Hydrogen Bank. This overarching policy is complemented by governmental and local policies [7,8].

Increasing the competitiveness of green hydrogen requires support measures and subsidies for production (grants and capital financing, eased tax credits, tax exemptions, preferential tariffs, and long-term contracts), as well as support for research and development and infrastructure subsidies [9].

Transition energy systems must ensure the production of green hydrogen that replaces hydrogen produced from hydrocarbons (a process with CO₂ emissions) currently used in industry and energy production. Figure 1 illustrates the usage of hydrogen as of the year 2022 [4].

The current proportion of energy consumed in applications such as steel production, glass manufacturing, and industrial chemistry is extremely low, with only green hydrogen being considered in this sector. The energy use of green hydrogen includes the following applications [5,6,7,8]:

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Transportation;

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Heating and residential household use;

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Industrial and agricultural applications;

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Electricity generation as part of energy storage operations during periods of renewable energy production variability.

Green hydrogen is produced using a technology that aligns with environmental ecology (electrolysis does not lead to the formation of environmental pollutants). Additionally, electrolysis is a sustainable technology because it does not affect the macroecological balance and ecosystems, even though there is a local increase in water consumption [9].

At present, the use of green hydrogen is mainly achieved through fuel cells, with direct combustion still in the early stages of research due to difficulties related to the very high combustion speed (approximately 8 times faster than methane) and the dynamics of hydrogen and air jets. However, it is noteworthy that the theoretical combustion temperature is very close to that obtained from methane combustion. Therefore, initially, the option of mixing green hydrogen with methane remains viable.

The use of green hydrogen for energy production necessitates compression and storage operations, both of which are challenging and incur high investment and operating costs [8,9,10].

During the energy transition period, fuel cell applications are most easily realized in transportation. Significant advancements have been made in the development of buses, trains, and automobiles. Centralized storage and the existence of refueling stations are expected to be the main future achievements in the field of transportation. The residual heat accompanying the transformation of hydrogen into electricity is an advantage during periods of low temperatures but a disadvantage during warmer periods of the year.

2. Transition to a Low CO₂ Energy System

As highlighted in various studies, the imperative to transition from electricity generation fueled by fossil fuels and eventually nuclear energy to renewable sources is becoming increasingly evident. This transition is not uniform across nations but is contingent upon the specific composition of each country’s energy production infrastructure. It is important to note that renewable resources often face challenges due to the considerable mismatch between their intermittent power capacities over time. In

Table 1. The share of energy sources for comparison.

The Country	The Energy Source
	Fossil Fuels, %	Nuclear, %	Regenerative, %
			Hydro, %	Wind, %	Solar, %	Biomass and Other Sources, %
Romania	35.91	19.98	26.04	12.75	3.27	1.06
France	7.50	67.10	13.00	7.90	2.50	2.00
Germany	51.40		-	32.20	10.00	6.40

, the share of energy sources for comparison is presented for three EU countries regarding Romania, the primary source of energy for electricity production (for 10 months of 2023, based on data from enel.ro/electripedia).

The significant proportion of renewable energy sources in Romania, at 43.06%, is noteworthy, with hydroelectric power being the dominant source, representing 60%.

For the energy transition period, renewable sources will have to replace coal production in the first stage (about 19%) and gas production (about 18%) in the second stage.

The primary energy sources structure in Germany for the year 2023 (for the first 9 months of the year).

The share of renewable energy resources in the annual average is around 42.2%. It is mentioned that for the summer months, this share varied between 57 and 59%.

The structure of primary energy sources in France, according to the “International Energy Agency”, for 2022.

The share of renewable resources is 25.4%, of which the hydraulic source is dominant with 13%. The situation in these three countries presented is typical for the European average where the share of primary renewable energy sources has average values between 25 and 50%, even if nuclear energy is not yet considered an energy of the future. An analysis of electricity production in the EU still indicates a massive reliance on fossil fuels, even for the countries ranked 2–5 in the ranking of the use of renewable sources (respectively Denmark, Latvia, Lithuania and Austria). For the countries in the middle area of the ranking presented, among which is Romania, the transition to green energy becomes possible within the time horizon of the European Union program, with the implementation of the transition to green hydrogen energy. It is mentioned that the production of green hydrogen will involve increasing the power installed in renewable sources, both as a consequence of increasing the efficiency of the technological flow and its use in other energy-consuming sectors such as transport, chemistry, etc., thus that the data presented are only an initial estimate of the problem [11,12].

3. Accessible Technologies for Producing and Storing Green Hydrogen

It should provide a concise and precise description of the experimental results and their interpretation.

It can be admitted that an energy based on renewable sources inevitably leads to the implementation of the use of green hydrogen, in the previously presented areas. Such a neutral society from the point of view of CO₂ emissions will be decisively influenced in the next stage, both by the investment factor, and also by the energy efficiency and reliability factor [4].

Green hydrogen is obtained through the electrolysis of alkaline water, its price being a decisive factor for making the transition towards hydrogen energy. In the new energy stage, green hydrogen will have to replace that produced from hydrocarbons in the chemical industry (see Figure 1) in addition to ensuring consumption for ecological transport, heating, industrial and agricultural applications, and last but not least, for the production of electric power. The technological chain for the use of green hydrogen, regardless of the field of application, includes an affordable price for green hydrogen being achieved by minimizing both investment and operation for all components of the technological chain shown in Figure 2.

A first technological stage will be reached when the price of green hydrogen obtained by the electrolysis of alkaline water will be comparable to that for hydrogen obtained from hydrocarbons [4,5].

Figure 3 shows the current economic parameters for the most efficient electrolyzers.

Green hydrogen production systems are shown schematically in Figure 4. The cost of green hydrogen depends mainly on the cost of the electrolysis apparatus and the cost of the catalysts used (reliability and maintenance will also be sensitive) [8].

The economic aspects include supporting the following actions: subsidies not only for production and infrastructure, advantages of trading CO₂ emission certificates, subsidies for end-users, and incentives for transportation, storage, and distribution stations. Among the EU co-funding projects, the NATURALHY project is mentioned [9].

Hydrogen is a consequence of using plant resources, waste, and water treatment for energy purposes. This issue does not entail a massive shift to these resources, as one might think.

The use of installations for the production of biohydrogen include those for the production of biogas from vegetable matter or from protein through anaerobic reactions [13].

Wastewater treatment serves as a significant source for the production of biogas, a renewable energy source primarily composed of methane and carbon dioxide, alongside trace amounts of hydrogen and other gases. The process of treating wastewater involves the breakdown of organic matter by bacteria through anaerobic digestion, which naturally produces biogas as a byproduct.

Hydrogen is also generated in varying proportions, typically ranging from 10% to 45%, depending on the specific conditions of the digestion process. However, this hydrogen is often considered impure as it is accompanied by other gases such as carbon monoxide, nitrogen, and traces of methane.

Furthermore, gasification and biomass pyrolysis, which involve heating organic materials such as wood or agricultural waste in the absence of oxygen, also yield hydrogen along with a mixture of other gases.

Despite the impurities, the hydrogen produced from these biological processes still holds potential value, especially in the context of transitioning towards a hydrogen-based economy. Technologies such as chemical hydrogenation can be employed to enhance the purity of the hydrogen produced, increasing its proportion within the gas mixture.

The transition towards carbon-neutral energy, also known as green hydrogen energy, must involve not only the industrial and residential sectors but also the significant sectors represented by agriculture and forestry. Agriculture should not only be seen as an energy consumer but also as a potential producer of green energy, including hydrogen. This integration leads to the concept of a global circular economy, where resources are reused and recycled, minimizing waste and environmental impact.

New technologies for biomass conversion into synthesis gas, which contains a significant proportion of hydrogen, provide opportunities for aligning with carbon-neutral energy goals. These technologies include fermentation, pyrolysis, and gasification, each offering distinct advantages in terms of efficiency and output. Among these, pyrolysis and gasification have shown particularly promising results for plant biomass, offering high yields of hydrogen-rich gas suitable for various energy applications. The participation of agriculture in energy and green hydrogen production needs to align with the vast potential of raw materials in this sector, which often remain underutilized even after intensive agricultural activities.

To be utilized in the synthesis gas production process, it’s crucial for biomass to possess specific characteristics. Therefore, it is recommended to have an optimal moisture content ranging between 10 and 20%, and the ash content should not exceed 7% (A < 7%). The volatile components are essential in this process, representing between 60 and 65% of the total biomass mass [14].

Pyrolysis technology is a vital process in converting biomass into synthesis gas. This process involves heating biomass in the absence of an oxidizing agent, within a temperature range of 400 to 750 °C. Sometimes, temperatures can reach up to 900 °C. During pyrolysis, volatile components from biomass are released as a combustible gas, while the remaining solid fraction is transformed into a product known as “char”.

The synthesis gas resulting from pyrolysis has a hydrogen content of up to 10–15%. This combustible gas can be used in various applications due to its versatile energy and chemical properties.

Gasification involves converting biomass into combustible gas by reducing CO₂ to CO and converting H₂O into H₂ and CH₄ through reactions in a carbon bed at temperatures above 650 °C.

The synthesis gas comprises CO, H₂, CH₄, N₂, and traces of CO₂. Gasification can be directed toward obtaining a larger quantity of H₂, which can represent 0.3–0.55 of the combustible components. Experiments on cereal straw gasification conducted at the Polytechnic University of Bucharest have shown a high percentage of H₂ up to 40% in the synthesis gas, with a calorific value ranging between 5600 and 9000 kJ/m³. In the initial period of implementing green hydrogen energy, the synthesis gas can be considered similar to that obtained from CH₄ with H₂. For periods aiming for CO₂ neutrality, synthesis gas must be used only in energy facilities equipped with CO₂ capture.

Electrolysers with polymer electrolyte membranes (PEM) achieve medium power levels and can also be used for residential applications. For higher power levels, anion exchange membrane (AEM) electrolyzers are used [4,8]. A schematic diagram of green hydrogen production and utilization is presented in Figure 5.

Among all the methods for obtaining green hydrogen, the pathway followed is that of water electrolysis.

The concept of impure hydrogen produced from biomass (including household and industrial waste and wastewater) has been linked in the literature with green hydrogen, as its production from a renewable resource does not result in CO₂ emissions. This hydrogen has the advantage of being able to be burned at the point of production, making the energy production chain direct and much simpler.

For green hydrogen, regarding storage, technologies are determined by the field of application, which can include mobile applications (for transportation) as well as stationary applications. Storage includes the compression phase, which involves various procedures, including those with mechanical compression. Metal hydride storage systems have proven to be efficient for low to medium-power installations. The use of fuel cells has an advantage over lithium-ion battery applications due to the delivery of both heat and electricity, enabling combined heat and power generation, especially for residential or industrial use.

Recent research indicates advantages for magnesium hydride and for compounds of green hydrogen with metals. Adsorption installations for hydrogen on porous nanocomposite materials are also mentioned, especially with applications in the transportation sector.

4. The Effectiveness of Producing Green Hydrogen

The transition period to green hydrogen energy is under the general requirement of lowering the cost of hydrogen production and use. Electrolyzers have an efficiency in the range of 70–85% (η = 70–85%). Their performances according to the technology used are presented in the data in Figure 6.

This energy consumption for the production of green hydrogen leads to the current technological level of the following prices [4,8,12]:

(20 ÷ 50) [EU/(kgH₂)], for centralized production sources;

(30 ÷ 55) [EU/(kgH₂)], for point sources of production.

However, the financial information is quite distorted by the crises in the country in recent years (the health crisis and the energy crisis).

Thus, for Romania, for the current price of electricity, the following prices result for green hydrogen:

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Alkaline electrolysis: 2.21 – 2.30 EU/(kgH₂);

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PEM electrolysis: 2.34 – 2.73 EU/(kgH₂).

Depending on the field and mode of use of green hydrogen, the efficiency varies as shown in Figure 7 [12,13,14].

An economic analysis demonstrates higher costs for using green hydrogen compared to hydrocarbons or other renewable energy sources (including hydroelectric power), [15,16].

The direct use of green hydrogen is primarily achieved in industry, including the chemical industry. It should be noted that the initial efficiency from electrolysis will be adjusted by the industrial process chain, the same aspect intervening in electricity production as well. [17,18]. Data regarding the efficiency of green hydrogen utilization reveal an essential aspect, summarized by the fact that electricity production from hydrogen is still too costly. [18,19]. However, for some industries, such as refining, chemical synthesis, and the glass industry, the use of hydrogen is mandatory. Regarding impure green hydrogen presented as a source of generation in Figure 5, it is mentioned that it is in a proportion of 15–35% in a gas mixture, predominantly nitrogen (N₂ = 45–55%) if the oxidizing agent for gasification is air, with methane in the range of 8–15%, the rest being represented by CO and CO₂. The use of these combustible biogases somewhat conceptually corresponds to the situation of using a mixture of H₂ in CH₄, with the combustion of this mixture ultimately leading to a reduction in CO₂ emissions [16].

For the combustion of biogas containing hydrogen, one can also rely on prior energy experience from using blast furnace gas, which has a composition very similar to that of biogas [16].

For Romania, studies show that green hydrogen production will become profitable when the power of electrolyzers exceeds a minimum of 1500 MW. [13].

It is estimated that the price could decrease to 1.38 EU/(kgH₂) for alkaline electrolysis and 1.59 EU/(kgH₂) for PEM electrolysis, for an electricity price of 25 EU/(MWh). Even in this case, the predominant use of green hydrogen will be towards industry and transportation, possibly towards the residential sector as well through fuel cells in cogeneration mode, but also through the mixture of hydrogen with methane, with electricity production, especially through direct burning, remaining economically deficient (even for the role of energy storage).

5. Elements of Direct Hydrogen Combustion

As previously mentioned, direct combustion of green hydrogen represents an economically unviable energy option at present due to the extremely high upstream costs (the solution is not economically viable even for combined cycles with gas turbines, an area where some technical progress has been made). As mentioned earlier, hydrogen is indispensable to certain industries where the economic aspect is already established.

There remains the possibility of directly burning a mixture of hydrogen with methane, considering today a volumetric proportion of hydrogen of 5–15%, (x = 0.05 – 0.15) [16,17,18].

Research regarding the distribution of the CH₄–H₂ mixture has demonstrated good performance for both steel and plastic pipes according to data from the European Industrial Gases Association. The conversion of pipelines from CH₄ to H₂ reduces investments for an H₂/CH₄ proportion of up to 20%, and the EN16726/2015 standard does not impose a limit on mixing with H₂, [17].

The calorific power of the gaseous fuel formed from the mixture of methane with hydrogen decreases with the increase in the volumetric proportion x of hydrogen, denoted by x = volume H₂/volume CH₄:

$${\mathrm{H}}_{\mathrm{i}}=\mathrm{35,700}-\mathrm{24,800}\mathrm{x}\left[\mathrm{k}\mathrm{J}/{(\mathrm{m}}_{\mathrm{N}}^3)\right]$$

(1)

The carbon dioxide emission can be determined by the following relationship, the calculation formula:

$${\mathrm{C}\mathrm{O}}_2={\displaystyle \frac{\mathrm{35,700}}{\mathrm{35,700}-\mathrm{24,800}\mathrm{x}}}\left(1-\mathrm{x}\right)\left[{\mathrm{m}}_{\mathrm{N}}^3{\mathrm{C}\mathrm{O}}_2/{(\mathrm{m}}_{\mathrm{N}}^3\mathrm{c}\mathrm{o}\mathrm{m}\mathrm{b})\right]$$

(2)

The issues related to the combustion of the methane and hydrogen mixture that are currently being addressed include [16,20,21]:

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Safe and economical operation of compression stations.

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Distribution through existing natural gas pipelines.

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Reliability of control and measurement elements in distribution networks.

Data regarding calorific power and density for a certain percentage of H₂ in CH₄ allow for the calculation of the Wobbe criterion. The replacement of one gaseous fuel with another having different physico-energetic characteristics requires maintaining the Wobbe criterion. Since the values defining the Wobbe criterion are in opposition (calorific power in the numerator and density in the denominator), and since they have similar variability values, it follows that to maintain the Wobbe criterion at current burners, a very small increase in pressure in the gas supply network is required, with the value of the increase presented in the data from Figure 8. Other aspects regarding compliance with the Wobbe criterion are further detailed.

For the first mixing stage of H₂ and CH₄ at a maximum of 10%, it results in possible maintenance of the current pressure in the supply network. In Romania, a successful experiment took place in 2023 involving the use of this mixture within two communities comprising 110 households.

Figure 9 shows the variation in the calorific value of the mixture of methane and hydrogen.

The operation of a gaseous fuel burner when changing gas quality or fuel delivery parameters such as pressure is characterized by the Wobbe criterion. Ideally, a thermally equivalent operation requires the constant maintenance of this criterion [13,15]:

$${\mathrm{W}}_{\mathrm{o}}=\mathrm{c}\mathrm{t}$$

(3)

where ${\mathrm{W}}_{\mathrm{o}}$ is the Wobbe criterion

$${\mathrm{W}}_{\mathrm{o}}=\sqrt{∆{\mathrm{p}}_{\mathrm{g}}}·{\displaystyle \frac{{\mathrm{H}}_{\mathrm{i}}^{\mathrm{i}}}{\sqrt{{\mathsf{\rho }}_{\mathrm{g}}}}}$$

(4)

where $∆{\mathrm{p}}_{\mathrm{g}}$ is the gas inlet pressure at the burner in Pa, ${\mathsf{\rho }}_{\mathrm{g}}$ is the gas density in $\mathrm{k}\mathrm{g}/{\mathrm{m}}_{\mathrm{N}}^3$, and ${\mathrm{H}}_{\mathrm{i}}^{\mathrm{i}}$– is the lower calorific value in $\mathrm{k}\mathrm{J}/{\mathrm{m}}_{\mathrm{N}}^3$.

The variation in density ${\mathsf{\rho }}_{\mathrm{g}}$ for a methane hydrogen mixture is shown in Figure 10.

The variation limits of the calorific value of the mixture CH₄ with H₂, for proportions of hydrogen up to 10%, following the application in the Wobbe criterion demonstrates a need to maintain the thermal load of pressure variation in the distribution network as stated in Figure 8 [16].

The validation of these models regarding CH₄–H₂ combustion was conducted through experiments carried out at the University of Construction on the operation of wall-mounted boilers, and in the ROMGAZ experiment of using this mixture in 2023 in 110 residential households. Combusting the CH₄–H₂ mixture premixed with air leads to the possibility of flame flashback. Burners used in residential settings must comply with the European standard EN437 [22].

Therefore, for these medium and high fuel flow rates, the authors recommend diffusion combustion technology with separate, concentric jets of H₂ and air for powers exceeding 300 kW regulated by EN676, as forced-draft burners. Research conducted highlighted the importance in this case of the ratio of air velocity to fuel velocity.

Ignition presents no problems since hydrogen has a minimum spark ignition energy of (0.017 ÷ 0.1) mJ; the ignition energy increases with the pressure in the combustion chamber, remaining below that of methane which has an average value of 0.2 mJ [23,24].

An analysis of the behavior of the current burners for se CH₄ when operating with a CH₄/H₂ mixture for the first stage of energy conversion to H₂ does not reach the conclusion that they do not create special problems for the combustion process as a whole as will still look. However, the issues related to metering and detection as well as anti-explosion protection remain open [25,26].

6. Conclusions

The paper presents a series of problems raised in the transition to green hydrogen energy. For a certain period, the continued use of fossil fuels with carbon capture technology has been considered. With all the present optimality, hydrogen does not represent a general universal solution, an objective that was the basis of the approaches in the paper.

The studies show that it is necessary to develop strategies that identify the determining issues, both from a thermal and economic point of view. It is clearly identified that not only the production of green hydrogen is the objective of the transition to energy without CO₂ emissions, but also the entire chain necessary for its energy recovery, which includes compression, transport (distribution), storage and energy loss.

The solution of producing hydrogen from renewable sources is only the basis of the problem because its subsequent exploitation still presents technical aspects that are not fully resolved, as well as high costs.

The issue of hydrogen produced from biomass has also been addressed, considering its technical and economic limitations. This impure hydrogen will be in smaller quantities compared to that obtained through hydrogenation.

The most effective first applications of green hydrogen will be in industry and transportation. Storing electricity in hydrogen involves very high costs, but also the advantage of a much longer storage time compared to the battery solution. The direct combustion of hydrogen is still in its early stages, being tested in transportation for internal combustion engines and gas turbines. For Romania, through the developed energy program, it is expected to achieve by 2030 an electrolysis capacity of about 3.7% of that proposed for the European Union. The paper presents technical and economic data outlining the aspects of all these issues. It is admitted that over time, the price of renewable energy can reach a sufficiently low value so that together with the ever-reduced investment costs, they also allow the realization of a final price of green hydrogen that is cost-effective compared to alternatives to fossil fuels. It is estimated that this target will be reached at the earliest for the period of 2028–2032.

Regarding the requirement of referendum 2, we would like to mention that creating a comparative table between our works on this subject and other works cannot be used as an objective. The paper presents numerous comparisons with other results in this field, highlighting the novelty of the research.