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Article

Assessment of Greenhouse Gas Emissions from Hydrogen Production Processes: Turquoise Hydrogen vs. Steam Methane Reforming

by
Gayatri Udaysinh Ingale
1,2,
Hyun-Min Kwon
2,
Soohwa Jeong
2,
Dongho Park
2,
Whidong Kim
2,
Byeingryeol Bang
2,
Young-Il Lim
3,
Sung Won Kim
4,
Youn-Bae Kang
5,
Jungsoo Mun
6,
Sunwoo Jun
7 and
Uendo Lee
1,2,*
1
Green Process and Energy System Engineering, University of Science and Technology, Daejeon 34113, Republic of Korea
2
Carbon Neutral Technology R&D Department, Korea Institute of Industrial Technology, Cheonan-si 31056, Republic of Korea
3
Center of Sustainable Process Engineering (CoSPE), Department of Chemical Engineering, Hankyoung National University, Anseong-si 17579, Republic of Korea
4
Department of Chemical and Biological Engineering, Korea National University of Transportation, Chungju-si 27469, Republic of Korea
5
Graduate Institute of Ferrous and Energy Materials Technology, Pohang University of Science and Technology, Pohang 37673, Republic of Korea
6
Institute of Technology, Lotte Engineering & Construction Co., Ltd., Seoul 06515, Republic of Korea
7
Carbon Neutral Research Institute, Samchully Co., Ltd., Osan-si 18102, Republic of Korea
*
Author to whom correspondence should be addressed.
Energies 2022, 15(22), 8679; https://doi.org/10.3390/en15228679
Submission received: 20 October 2022 / Revised: 11 November 2022 / Accepted: 16 November 2022 / Published: 18 November 2022
(This article belongs to the Special Issue Selected Papers from the 11th Asia-Pacific Forum on Renewable Energy (AFORE 2022))
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Abstract

:
Hydrogen has received substantial attention because of its diverse application in the energy sector. Steam methane reforming (SMR) dominates the current hydrogen production and is the least expensive endothermic reaction to produce grey hydrogen. This technology provides the advantages of low cost and high energy efficiency; however, it emits an enormous amount of CO2. Carbon capture storage (CCS) technology helps reduce these emissions by 47% to 53%, producing blue hydrogen. Methane pyrolysis is an alternative to SMR that produces (ideally) CO2-free turquoise hydrogen. In practice, methane pyrolysis reduces CO2 emissions by 71% compared to grey hydrogen and 46% compared to blue hydrogen. While carbon dioxide emissions decrease with CCS, fugitive methane emissions (FMEs) for blue and turquoise hydrogen are higher than those for grey hydrogen because of the increased use of natural gas to power carbon capture. We undertake FMEs of 3.6% of natural gas consumption for individual processes. In this study, we also explore the utilization of biogas as a feedstock and additional Boudouard reactions for efficient utilization of solid carbon from methane pyrolysis and carbon dioxide from biogas. The present study focuses on possible ways to reduce overall emissions from turquoise hydrogen to provide solutions for a sustainable low-CO2 energy source.

1. Introduction

The success of hydrogen technology depends on producing large-capacity hydrogen in response to growing demand while mitigating greenhouse gas (GHG) emissions during production [1]. Regarding climate impact, hydrogen (H2) production methods are not equal, and hydrogen can be classified into several colors to provide information about its production process: feedstock, energy sources, and climate neutrality [2]. The hydrogen color spectrum comprises black/grey, brown, blue, turquoise, green, pink/purple/red, yellow, and white [3]. Previous studies have reported the advantages and disadvantages of various hydrogen production methods [4,5,6,7]. According to the Royal Society report [8], 95% of global hydrogen production is from fossil fuels, including natural gas and coal. Henceforth, it is essential to focus on hydrogen production technologies based on natural gas (NG), considering its lowest carbon-to-hydrogen ratio and worldwide supply chain for industrial, domestic, and transportation applications.
Steam methane reforming (SMR) is the most widely used technique for industrial hydrogen production [9,10,11]; however, it contributes significantly to the co-production of carbon dioxide (CO2) [12]. For example, 1 kg of grey hydrogen production can generate approximately 13 kg of CO2, if one includes the primary reactions [13], such as the heat source for the process and product purification with pressure swing adsorption (PSA). Therefore, blue hydrogen, which captures and utilizes CO2 generated during SMR, is the most realistic way to reduce GHG emissions by using grey hydrogen infrastructure in the short term. However, the considering limited number of CO2 storage sites [14], the impact of blue hydrogen is limited. Moreover, the carbon capture and storage (CCS) application is affected by the GHG released during the electricity generation provided to CCS operations [15,16].
From this perspective, turquoise hydrogen with methane pyrolysis is receiving greater attention [17,18,19]. This technology is analogous to SMR in the case of feedstock utilization; that is, methane, with the difference being the solid carbon byproduct and its lower heat of reaction [20]. However, turquoise hydrogen production requires quite higher reaction temperature than that of SMR [21]. Then, how much lower GHG emissions can turquoise hydrogen have compared to grey hydrogen, and how effective is CCS for each hydrogen production technology? Is it better to limit the scope of the CCS application to products or to apply it to the emissions from process heat generation? Each hydrogen production technology requires a different amount of energy, and integrating renewable power for the process energy is crucial for low-carbon hydrogen production. The answers to these questions can be obtained through a realistic evaluation of the technology options applied to each process.
It is essential to consider fugitive methane emissions whenever natural gas is utilized for hydrogen production [22]. On the other side, biogas is a promising carbon-neutral alternative to natural gas; hence, it is necessary to evaluate the impact of switching from natural gas to biogas. [23]. For turquoise hydrogen production, the system integrated with flue-gas capture (FGC) and the Boudouard reaction enhances process efficiency with lower CO2 emissions [24]; it also boosts the utilization of solid carbon and CO2 in the form of carbon monoxide (CO). Such synergy can be maximized if about 50% of the CO2-contained biogas is used.
This study evaluated the GHG emissions from grey, blue, and turquoise hydrogen. In addition, the impact of incorporating other processes, such as CCS, the usage of biogas, and the conversion of carbon and CO2 into CO through the Boudouard reaction, on both technologies that SMR and methane pyrolysis use to reduce GHG emissions have also been investigated. The present study is a comprehensive analysis based on the emission assessment of Cornell University that includes grey and blue hydrogen [25]. Here, turquoise hydrogen is additionally included in the comparison. For calculation, gross calorific value (GCV) is used to express emissions per unit of energy generated during hydrogen combustion. Hydrogen has a GCV of 0.286 MJ/mol H2, [26] or 3.5 mol H2/MJ. Therefore, the further emission estimations are based on producing 3.5 moles of H2 from grey, blue, and turquoise hydrogen processes. Table 1 provides the properties of the compounds used for unit conversions.

2. Emission Assessment of Grey Hydrogen (SMR Process)

SMR is an endothermic process that produces hydrogen and CO2 according to the overall reaction indicated in Equation (1). The process flow diagram (PFD) for grey hydrogen via SMR is explained in Figure 1.
CH4 + 2H2O → 4H2 + CO2, ΔH0 = 41.25 kJ/mol H2
Here, the production of 3.5 mol H2/MJ coproduces 38.47 g/MJ of CO2 indicated in Table 2 which is calculated by Equation (2) in Table 3. Since one mole of methane produces four moles of hydrogen in SMR (Equation (1)), the estimated methane consumption for SMR is 14.02 g/MJ (Equation (3)) (Table 2). Note that the calculation results are summarized in Table 2, and the equations for calculations are listed in Table 3.

2.1. Heat Source of SMR

The thermal efficiency of SMR in Equation (1) is considered to be 75% [27,28]; hence, this process requires a significant amount of input energy that is 0.60 kWh/m3 H2 or 0.052 MJ/mol H2 (Equation (4)). Natural gas (CH4) combustion is utilized to fulfill this energy demand; hence, the amount of methane combusted for the heat source is 3.36 g/MJ (Equation (5)) (Table 2), or 0.21 mol/MJ. One mole of methane combustion produces one mole of CO2; therefore, the CO2 emissions from heat sources are 9.23 g/MJ (Equation (6)) (Table 2).

2.2. Indirect Upstream Emissions by Grey Hydrogen

Indirect upstream emissions are caused during the production, processing, and transportation of NG, which are 7.5% of the CO2 produced during the overall process [25,29]. The total CO2 produced during the overall process is 47.70 g/MJ i.e., the sum of CO2 from the SMR (38.47 g CO2/MJ) and the heat source (9.23 g CO2/MJ) (sum of Equations (2) and (6)). Therefore, indirect upstream emissions can be estimated at 7.5% of 47.70 g CO2/MJ, which are 3.58 g CO2/MJ (Equation (7)) (Table 2). Further, adding these indirect upstream emissions (3.58 g CO2/MJ) into the total direct CO2 emission (47.70 g CO2/MJ), the total CO2 emitted from grey hydrogen is 51.28 g/MJ (sum of Equations (2), (6) and (7)) (Table 2).

2.3. Fugitive Methane Emissions (FME) by Grey Hydrogen

The escape or loss of some amount of methane to the environment during the production, processing, and transportation of NG is known as fugitive methane emissions (FMEs) [30]. Routine venting and flaring also contribute to FME [31]. According to research conducted on ten natural gas fields in the USA, the FMEs account for an average of 2.6% of total NG production [32]. Moreover, methane emissions during storage and transportation account for an additional 0.8% of total NG production [32,33,34]. Then, collectively, 3.4% of total NG production represents fugitive methane emissions.
The global NG production and consumption in 2019 were 159.60 EJ and 154.58 EJ (converting terajoules to exajoules), respectively [35]. By this data, the FMEs are 3.5% of NG consumption, as determined in Equation (8). Now, the CH4 consumed by SMR and the heat source are 14.02 g/MJ (Equation (3)) and 3.36 g/MJ (Equation (5)), respectively; hence, the FMEs are 0.49 g CH4/MJ (Equation (9)) and 0.12 g CH4/MJ (Equation (10)), respectively (Table 2). Methane’s global warming potential (GWP) has been estimated by the Intergovernmental Panel on Climate Change (IPCC) to be between 84 and 87 when using a 20-year timeframe (GWP20) and between 28 and 36 when using a 100-year timeframe (GWP100) [36]. For further calculations, the GWP20 of 86 has been considered [25]. Thereby, the conversion of FME in Equations (9) and (10) is 42.33 g CO2eq/MJ and 10.15 g CO2eq/MJ (Equations (11) and (12)), respectively. Altogether, from Equations (11) and (12), 52.49 g CO2eq/MJ are emitted as FMEs from SMR. Lastly, adding these FMEs to the total CO2 emitted (51.28 g/MJ) (sum of Equations (2), (6) and (7)), the overall emissions from grey hydrogen production are 103.76 g CO2eq/MJ (Table 2).

3. Emission Assessment of Blue Hydrogen (SMR Process)

Blue hydrogen is principally the same as grey hydrogen; however, the released CO2 is captured and stored (CCS), refraining from entering the atmosphere [37]. Since blue hydrogen mitigates environmental CO2 impact more than grey hydrogen and therefore is gaining attention as an eco-friendly alternative [38]. However, the CCS facility is also responsible for excessive emissions.

3.1. CO2 Capture of Blue Hydrogen and Flue-Gas Capture (FGC)

According to statistics of the Shell plant in Alberta, the average capture efficiency of CO2 emitted from SMR is 78.8%, with daily rates of 53% to 90% [25,39]. Here, 85% efficiency has been considered, which is between 78.8% and 90% [25]; as a result, 5.77 g/MJ of CO2 (Equation (13)) (Table 2) is directly released into the atmosphere. The heat source is also responsible for flue gases or CO2 emissions due to fossil fuel combustion. Nevertheless, in blue hydrogen, carbon capture has been solely concerned with the CO2 emissions from SMR and not those from the heat source. The difference between blue hydrogen and blue hydrogen with flue-gas capture (FGC) is indicated in Figure 2. No plant data is available for the flue-gas or CO2 capture from an NG-fired heat source. Howarth et al. [25] stated that the CO2 capture rate of the coal power plant’s exhaust ranges from 55% to 72% [40,41,42]. Therefore, 65% flue-gas capture efficiency is employed here [25], and consequently, 3.23 g/MJ of flue-gas CO2 (Equation (14)) (Table 2) is directly released into the atmosphere.

3.2. Heat Source of CO2 and Flue-Gas Capture Facility of Blue Hydrogen

The energy demand of the CO2 capture facility is fulfilled by NG combustion, which results in emissions. These CO2 emissions are 25% of the CO2 captured from SMR and 39% of the flue-gas CO2 captured from the heat source [25]. Therefore, the energy consumption by the CO2 capture facility of SMR contributes to additional emissions of 8.17 g CO2/MJ (Equation (15)) consuming 2.98 g/MJ (Equation (17)) of methane for blue hydrogen and 10.51 g CO2/MJ emissions (sum of Equations (15) and (16)) consuming 3.83 g/MJ (Equation (18)) of methane for blue hydrogen (FGC) (Table 2). Here, the amount of methane is calculated based on the methane combustion reaction.

3.3. Indirect Upstream Emissions and FME by Blue Hydrogen

The estimation of indirect upstream emissions is the same as in Section 2.2. For blue hydrogen, the total CO2 produced during the overall process is 55.87 g/MJ (sum of Equations (2), (6) and (15)). Therefore, indirect emissions are 4.19 g CO2/MJ (Equation (19)). Similarly, for blue hydrogen (FGC), the total CO2 produced is 58.21 g/MJ (sum of Equations (2), (6), (15) and (16)). Therefore, indirect emissions are 4.37 g CO2/MJ (Equation (20)). Further, these indirect upstream emissions are added to the respective total direct CO2 emissions to obtain the total CO2 emitted.
The FME estimation is the same as in Equations (9) and (11) in Section 2.3. The CH4 consumption by the CO2 capture facility of blue hydrogen is 2.98 g/MJ and by blue hydrogen (FGC) is 3.83 g/MJ; hence, the FMEs are 9.00 g CO2eq/MJ (Equation (21)) and 11.57 g CO2eq/MJ (Equation (22)), respectively (Table 2).

4. Emission Assessment of Turquoise Hydrogen (NG-NG)

This analysis is based on methane pyrolysis, which uses NG as feedstock and a heat source. In this reaction, methane is thermally decomposed into hydrogen and solid carbon (C) as illustrated in Equation (23).
In pyrolysis, the production of 3.5 mol H2/MJ coproduces 1.75 mol/MJ of solid carbon (Equation (24)) or 21.00 g/MJ. Since one mole of methane produces two moles of hydrogen in pyrolysis, the estimated methane consumption for this process is 28.05 g/MJ (Equation (25)) (Table 2).

4.1. Heat Source of Turquoise Hydrogen (NG-NG)

Considering 75% thermal efficiency of Equation (23), pyrolysis requires 0.54 kWh/m3 H2 or 0.047 MJ/mol H2 (Equation (26)) energy. Therefore, the methane combusted for the heat source to produce 3.5 moles of H2 is 3.05 g CH4/MJ (Equation (27)). Correspondingly, the flue-gas CO2 emissions from the heat source are 8.37 g CO2/MJ (Equation (28)). This flue-gas CO2 needed to be captured. Considering the same flue-gas capture efficiency as in Section 3.1, 2.93 g/MJ of CO2 (Equation (29)) (Table 2) from the heat source is directly discharged into the atmosphere. Figure 3 shows the turquoise hydrogen production (NG-NG) with and without FGC.

4.2. Heat Source of Flue-Gas Capture Facility of Turquoise Hydrogen (NG-NG)

NG combustion is a heat source for the flue-gas capture facility, which results in emissions, as described in Section 3.2. These emissions are 39% of the flue-gas CO2 captured from the heat source [25], which is 2.12 g CO2/MJ (Equation (30)). Accordingly, based on the methane combustion reaction, the CH4 consumption is 0.77 g/MJ (Equation (31)) (Table 2).

4.3. Indirect Upstream Emissions and FME by Turquoise Hydrogen (NG-NG)

This calculation process follows the same steps outlined in Section 2.2. For turquoise hydrogen, the total CO2 produced is a sum of the amounts produced by pyrolysis and the heat source; however, pyrolysis eliminates CO2 production. Therefore, the CO2 amount is calculated considering the combustion of the same amount 28.05 g CH4/MJ of methane that is 76.94 g CO2/MJ (Equation (32)). Hence, for turquoise hydrogen (NG-NG), the total CO2 produced during the overall process is 85.31 g/MJ (sum of Equations (28) and (32)). Therefore, indirect upstream emissions are 7.5% of 85.31 g CO2/MJ which is 6.40 g CO2/MJ (Equation (33)). For turquoise hydrogen (NG-NG) with FGC, the total CO2 produced during the overall process is 87.43 g/MJ (sum of Equations (28), (30) and (32)). Resultantly, these emissions are 6.56 g CO2/MJ (Equation (34)) (Table 2).
FME estimation follows the same procedure as in Equations (9) and (11) in Section 2.3, based on methane consumption. Hence, the FME estimation is provided in Equations (35), (36) and (37).

5. Emission Assessment of Turquoise Hydrogen (NG-RE)

This analysis is based on methane pyrolysis, which uses NG feedstock and renewable energy (RE) as a heat source. The fundamental concepts of hydrogen production, amount, methane consumption for pyrolysis, and energy requirement for the heat source are the same as in Section 4 and Section 4.1; however, the heat source mechanism is replaced by solar PV. Since fossil fuel combustion is prevented, the CO2 capture facility is not needed. However, solar PV contributes to GHG emissions, which are considered in this process. Figure 4 illustrates the turquoise hydrogen production with RE.

5.1. Heat Source of Turquoise Hydrogen (NG-RE)

As stated in Section 4.1, pyrolysis requires the energy of 0.54 kWh/m3 H2 or 0.013 kWh/mol H2 (Equation (38)). Solar PV fulfills this energy need to produce hydrogen 3.5 mol H2, while emitting GHG. The life-cycle GHG emissions of solar PV range from 12.5 to 126 g CO2eq/kWh [43], hence the average of 69.25 g CO2eq/kWh has been considered. Thus, the GHG emissions by the heat source are 3.14 g CO2eq/MJ (Equation (39)) (Table 2).

5.2. Indirect Upstream Emissions from Turquoise Hydrogen (NG-RE)

According to Section 4.3, the calculated CO2 amount is 76.94 g/MJ (Equation (32)). Therefore, indirect upstream emissions are 7.5% of 76.94 g CO2/MJ [25], which are 5.77 g CO2/MJ (Table 2).

6. Emission Assessment of Turquoise Hydrogen (BG-RE)

Here, methane pyrolysis uses biogas (BG) feedstock with solar PV (RE) as a heat source. The fundamentals of hydrogen production remain the same as in Section 4. Since fossil fuel combustion is avoided, the CO2 capture facility is not needed. The details of BG are provided in Table 4.
The production of 3.5 mol H2/MJ or 0.084 m3 H2/MJ (converted using the molecular weight and density of H2 from Table 1) consumes the same amount of bio-CH4 that is in Equation (25), which is 28.05 g CH4/MJ or 0.043 m3 CH4/MJ (converted using density). Hence, from Equation (40), 0.075 m3/MJ or 88.89 g/MJ (converted using density) (Table 2) of BG is required to produce 0.043 m3/MJ of bio-CH4 via pressure swing adsorption (PSA). The CO2 separated from bio-CH4 is 0.032 m3/MJ (Equation (41)) (or 60.22 g/MJ or 1.37 mol/MJ, converted using density).

6.1. Heat Source of Turquoise Hydrogen (BG-RE)

The PSA needs 0.35 kWh/m3 BG [45,46] energy to process 0.075 m3 BG/MJ (Equation (40)), and pyrolysis requires 0.54 kWh/m3 H2 (Section 4.1) energy to produce 0.084 m3 H2/MJ (Section 6). Solar PV meets these requirements while emitting GHG, as described in Section 5.1. Therefore, GHG emissions from the heat sources of PSA and pyrolysis are 1.82 g CO2eq/MJ and 3.14 g CO2eq/MJ, respectively (Equations (42) and (43)) (Table 2).
The quantity of carbon produced through pyrolysis is three times greater than that of hydrogen. For example, 1 ton of turquoise hydrogen production co-produces 3 tons of solid carbon. If the hydrogen is commercially produced via pyrolysis, a substantial amount of carbon will be co-produced, which may surpass the market demand. Therefore, it is essential to develop a way to utilize the excess carbon effectively. Furthermore, the CO2 extracted from biomethane is readily available in concentrated form. In this case, the addition of the Boudouard reaction is proposed for the effective utilization of excess carbon produced by pyrolysis and concentrated CO2 extracted from BG, as indicated in Equation (44).
The carbon monoxide (CO) produced in this process has a market value in the chemical and biomedical industries [47]. The turquoise hydrogen production with and without the Boudouard reaction is shown in Figure 5. The CO2 extracted from BG is 0.032 m3/MJ (Equation (41)) or 1.37 mol/MJ (calculated using the molecular weight and density of CO2 from Table 1), whereas the carbon produced by pyrolysis is 1.75 mol/MJ (Equation (24)). Since one mole of CO2 produces two moles of CO (Equation (44)), the total CO produced is 2.74 mol/ MJ. Considering the 75% thermal efficiency of Equation (44), 2.54 kWh/m3 CO2 of energy needs to be provided by solar PV. Therefore, the GHG emissions by the heat source of Boudouard reaction are 5.68 g CO2eq/MJ (Equation (45)) (Table 2).

6.2. FME from Turquoise Hydrogen (BG-RE)

The FMEs from the BG plant typically range from 1% to 7% of the biomethane production [48,49]; however, it can be decreased to 2% [48]. The global BG production and consumption in 2019 were 1.434 EJ and 1.429 EJ (converting terajoule to exajoule) [35], respectively; thus, the losses were 0.32%. The global biomethane production in 2019 was 3.96 bcm [50]. Due to limitations in data availability, the predicted quantity of consumption by applying 0.32% losses is 3.95 bcm. Thus, the FMEs are 2% of biomethane consumption (Equation (46)). Now, the biomethane consumed for the pyrolysis process is 28.05 g/MJ; therefore, the FMEs are 0.56 g CH4/MJ (Equation (47)) (Table 2).

7. Emission Assessment of Turquoise Hydrogen (BG-BG)

In this case, methane pyrolysis uses BG as both a feedstock and a heat source. The overall process remains the same as in Section 6.

7.1. Heat Source of Turquoise Hydrogen (BG-BG)

The PSA requires an energy input of 0.35 kWh/m3 BG [45] energy to process 0.075 m3 BG/MJ (Equation (40)) also, pyrolysis requires an energy input of 0.047 MJ/mol H2 (Equation (26)) to produce 3.5 mol H2/MJ. BG combustion meets these requirements. Thus, for PSA, 5.8 g/MJ (Equation (48)) of BG is combusted, which contains 1.83 g/MJ (Equation (49)) of methane. For pyrolysis, 10.05 g/MJ (Equation (50)) (Table 2) of BG is combusted, which contains 3.17 g/MJ of methane (calculation based on Equation (49)).
As in Section 6.1 an additional Boudouard reaction is recommended here, which consumes the CO2 (1.37 mol/MJ) extracted from BG for CO production. This reaction requires the input energy of 0.22 MJ/mol CO2 (Equation (51)), which is supplied by 18.14 g/MJ of BG combustion (Equation (52)) (Table 2), which comprises 5.72 g/MJ of methane (calculation based on Equation (49)). The turquoise hydrogen production with and without the Boudouard reaction is shown in Figure 6.

7.2. FME from Turquoise Hydrogen (BG-BG)

FMEs are estimated in the same way described in Section 6.2 and are based on bio-CH4 consumption and methane content in BG provided in the respective sections.

8. Energy Requirements of Hydrogen Production Processes

The energy required for each process is provided in Table 2. It is calculated based on the feedstock consumption for the process and fuel consumption for the heat source. For example, in the case of blue H2 FGC, the feedstock consumption for SMR is 14.02 g CH4/MJ (Table 2), the fuel consumption for the heat source of SMR is 3.36 g CH4/MJ (Table 2) and the fuel consumption for the heat source of CO2 capture facility is 3.83 g CH4/MJ (Table 2). Therefore, the summation of all these amounts is the total methane consumption for the process, which is 21.22 g CH4/MJ (Table 2). Further, multiplying it with the calorific value of methane provided in Table 1 gives the total energy required for the process, which is 1.14 MJ (Equation (53)). Now, this energy is required to produce 3.5 moles of H2 hence, the energy required per mol of H2 is given in Equation (54), which is 0.33 MJ/mol H2 (Table 2). Similarly, it is calculated for all the remaining routes where methane is used as both a feedstock and a heat source.
In the case of turquoise H2 (NG-RE), the feedstock is methane, whereas the heat source is renewable energy. Here, the feedstock consumption for pyrolysis is 28.05 g CH4/MJ (Table 2). As mentioned in Section 5.1, the energy required for pyrolysis is 0.013 kWh/mol H2 (Equation (38)). Solar PV is used to complete this energy demand and produce 3.5 moles of H2. Therefore, the total energy required for the process is 1.67 MJ (Equation (55)). Now, this energy is required to produce 3.5 moles of H2 hence, the energy required per mol of H2 is given in Equation (56) and is 0.48 MJ/mol H2 (Table 2).
In turquoise H2 (BG-RE Boud), the feedstock is biogas, whereas the heat source is renewable energy. Here, the feedstock consumption for pyrolysis is 88.89 g BG/MJ (Table 2); thus, the energy content of the biogas consumed is calculated by multiplying the calorific value provided in Table 4. Further, as mentioned in Section 6.1, the PSA needs 0.35 kWh/m3 BG [45,46] of energy to process 0.075 m3 BG/MJ (Equation (40)), and then the multiplication of these gives the energy consumed for the PSA. Now, pyrolysis requires the energy of 0.54 kWh/m3 H2 to produce 0.084 m3 H2/MJ as per Section 6.1, so the multiplication of these gives the total energy consumed for pyrolysis. Additionally, as per Section 6.1, the energy required for the conversion of 0.032 m3 CO2/MJ (Equation (41)) of CO2 via Boudouard reaction is 2.54 kWh/m3 CO2, so the multiplication of these gives the total energy consumed by the Boudouard reaction. Therefore, the total energy consumed for the process of turquoise H2 (BG-RE Boud) is 2 MJ (Equation (57)). Now, this energy is required to produce 3.5 moles of H2; hence, the energy required per mol of H2 is given in Equation (58), which is 0.57 MJ/mol H2 (Table 2). Similar calculations were performed for the remaining pathways where biogas serves as the feedstock and renewable energy serves as a heat source.
Biogas is utilized in the production of turquoise H2 (BG-BG Boud) as both a feedstock and a heat source. In this case, Table 2 shows that the feedstock consumption for SMR is 88.89 g BG/MJ, whereas the fuel consumption for the heat sources of PSA, pyrolysis, and the Boudouard reaction is 5.80 g BG/MJ, 10.05 g BG/MJ, and 18.13 g BG/MJ, respectively. As a result, the total quantity of biogas used in the process is 122.87 g BG/MJ (Table 2). Further, multiplying it by the calorific value of the biogas listed in Table 4 provides a total energy requirement for the process of 2 MJ (Equation (59)). Now, this energy is required to produce 3.5 moles of H2 hence, the energy required per mol of H2 is given in Equation (60); that is, 0.57 MJ/mol H2 (Table 2). It is computed similarly for all the other methods where biogas served as both the feedstock and the heat source.

9. Emission Index

Each of the routes listed in Table 2 produces the hydrogen amount of 3.5 mol H2/MJ or 7.05 g H2/MJ (converted using the molecular weight of H2 from Table 1), and this production results in certain direct CO2 emissions, including upstream indirect emissions. The emission index estimates the amount of CO2 emitted to the atmosphere per kilogram of hydrogen produced. As an illustration, for grey H2, the total CO2 emitted is 51.28 g CO2/MJ (Table 2); therefore, the emission index is 51.28 g CO2/MJ divided by 7.05 g H2/MJ, which is 7.27 kg CO2/ kg H2. Similar calculations are conducted for the other routes, which are presented in Table 5.

10. Discussion

Figure 7 compares the carbon footprints of grey, blue, and turquoise hydrogen, whereas Figure 8 shows the energy demand. Compared to grey H2, blue H2 emits 47% less CO2 while using 17% more energy. Further, blue H2 FGC reduces emissions by 53%, consuming 22% more energy than grey H2. The CO2 emissions are significantly decreased in turquoise H2 (NG-NG) compared to grey and blue H2 by 71% and 46%, respectively; meanwhile, energy consumption is raised by 79% and 53%, respectively. The introduction of FGC to turquoise H2 appears to be appropriate because, in the case of pyrolysis (NG-NG FGC), the CO2 emissions are reduced by 21% compared to pyrolysis (NG-NG), whereas it requires only 2% additional energy. The replacement of the heat source of pyrolysis (NG-NG) from NG to RE cuts CO2 emissions by 61%. In turquoise H2, when the feedstock is switched from NG to BG and the heat source from NG to RE, the CO2 emissions are zero, with a 2% extra energy demand. This extra energy is required for PSA to obtain bio-methane for pyrolysis. The introduction of the Boudouard reaction consumes 17% more energy compared to turquoise H2 (BG-BG).
Table 5 displays the emission index, or the amount of CO2 emitted per kg of hydrogen production. The turquoise H2 (BG-RE Boud) and (BG-BG Boud) show a negative emission index because BG combustion is regarded as carbon neutral; moreover, the CO2 separated from bio-methane is converted into carbon monoxide via the Boudouard reaction. Subsequently, the amount of CO2 used for this conversion is indicated as a negative emission index.
The fugitive methane emissions, along with feedstock that is methane or BG consumption, are indicated in Figure 9. These emissions are directly proportional to the amount of methane or BG consumed. In the case of pyrolysis, methane consumption is considerably high in comparison with SMR. The FMEs from turquoise hydrogen (NG-NG) are significantly higher than those from grey hydrogen because it consumes 79% more methane; however, according to Figure 7, the CO2 emissions are 71% lower. The highest amount of methane is consumed by turquoise hydrogen (NG-NG FGC) because of flue-gas capture; therefore, the FMEs are correspondingly significant. On the other hand, the FMEs are relatively low when BG is consumed for the process. In comparison to NG, the biogas-producing unit is comparatively small. Furthermore, biogas requires less handling, storage, and transportation than natural gas, potentially making it possible to reduce FME at the time of BG production.
In Figure 10, the overall GHG emissions are quite high for some scenarios of turquoise H2, such as NG-NG and NG-NG FGC, compared to SMR, which is predominantly attributed to FMEs. These two cases demonstrate that pyrolysis consumes significantly higher amounts of NG than SMR. Although GHG emissions are high in these cases, pyrolysis produces less CO2 than SMR. The World Resources Institute highlighted several federal and state regulations as well as industry practices for reducing fugitive methane emissions at a reasonable cost [51]. Considering the implementation of these policies and industry standards, a 50% reduction in FME has been taken into account for sensitivity analysis. This leads to a substantial reduction in overall GHG emissions, as indicated in Figure 11.
This study evaluated the effects of several technological alternatives for SMR and methane pyrolysis on the CO2 emission index and process energy requirements. In the case of SMR, CO2 emissions could be decreased in the sequence of grey > blue > blue with FGC, but the energy requirement increased in the same order. More diverse options, such as switching the feedstock (NG vs. BG), CCS or FGC application, RE utilization, and additional Boudouard reactions, were examined to make methane pyrolysis more economical and carbon neutral. The energy consumption of turquoise hydrogen always surpassed that of SMR because of the nature of the pyrolysis process, which requires more methane than SMR to produce the same quantity of hydrogen. Furthermore, considerable impacts were observed due to the employment of different CO2 reduction strategies, including flue-gas capture and RE utilization. The CO2 emission index was reduced to zero by switching from NG to BG, and a negative CO2 emission index was achieved using the Boudouard reaction to convert CO2 extracted from biogas into CO.
Meanwhile, the total GHG emissions for each process were dominated by fugitive emissions, which must be considered when using methane or biogas. When using the average FMEs estimated in the current NG global supply chain of 3.6%, SMR and pyrolysis processes with NG exhibited total GHG emissions of more than 87 g CO2eq/MJ; hence, technical approaches to minimize CO2 emissions were meaningless in this context. Due to the nature of BG, which operates on a small scale and is consumed on-site, its FMEs are smaller than those of NG, and thus its overall GHG emissions were about half those of NG cases. (Note that actual FMEs are considered carbon neutral when using BG). The appropriate possible way to make the overall GHG emissions net zero is the additional employment of CO production using carbon produced by the pyrolysis process and CO2 in biogas.
In other words, the strict management of FMEs is critical for the technology to extract hydrogen from methane. On this account, the efforts to minimize FMEs in the global supply chain of NG should be carried out with technological development to reduce CO2 generation in the SMR or methane pyrolysis. At the same time, the effect of BG on reducing overall GHG emissions is certain, and the use of biogas as a decentralized CO2-free hydrogen production base is essential.

Author Contributions

Writing—original draft preparation, visualization, and investigation, G.U.I.; Conceptualization, H.-M.K. and S.J. (Soohwa Jeong); methodology, D.P. and W.K.; validation, B.B. and Y.-I.L.; investigation, S.W.K. and Y.-B.K.; resources, J.M. and S.J. (Sunwoo Jun); data curation, G.U.I.; supervision and project administration, U.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Ministry of Science and ICT (MSIT) grant funded by the Korean Government’s National Research Foundation of Korea (NRF), grant number 2021M3I3A1084772.

Conflicts of Interest

The authors declare no conflict of interest.

Abbreviations

BGBiogas
BG-REBiogas as a feedstock and renewable energy as a heat source
BG-RE BoudBiogas as a feedstock and renewable energy as a heat source with a Boudouard reaction
BG-BGBiogas as both a feedstock and a heat source
BG-BG BoudBiogas as both a feedstock and a heat source with a Boudouard reaction
Bio-CH4Bio-methane
Blue FGCBlue hydrogen with flue-gas capture
BoudBoudouard reaction
CCSCarbon capture and storage
FGCFlue-gas capture
FMEFugitive methane emission
GHGGreenhouse gas
GCVGross calorific value
NGNatural gas
NG-NGNatural gas as both a feedstock and a heat source
NG-NG FGCNatural gas as both a feedstock and a heat source with flue-gas capture
NG-RENatural gas as a feedstock and renewable energy as a heat source
PFDProcess flow diagram
PSAPressure swing adsorption
RERenewable energy
SMRSteam methane reforming
Greek letters
ΔH0Standard heat of reaction (kJ/mol)

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Figure 1. The PFD of grey hydrogen production.
Figure 1. The PFD of grey hydrogen production.
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Figure 2. PFD of (a) blue hydrogen and (b) blue hydrogen with FGC.
Figure 2. PFD of (a) blue hydrogen and (b) blue hydrogen with FGC.
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Figure 3. PFD of (a) turquoise hydrogen (NG-NG) and (b) turquoise hydrogen (NG-NG) with FGC.
Figure 3. PFD of (a) turquoise hydrogen (NG-NG) and (b) turquoise hydrogen (NG-NG) with FGC.
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Figure 4. PFD of turquoise hydrogen with renewable energy (NG-RE).
Figure 4. PFD of turquoise hydrogen with renewable energy (NG-RE).
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Figure 5. PFD of (a) turquoise hydrogen (BG-RE) and (b) turquoise hydrogen (BG-RE) with Boudouard reaction.
Figure 5. PFD of (a) turquoise hydrogen (BG-RE) and (b) turquoise hydrogen (BG-RE) with Boudouard reaction.
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Figure 6. PFD of (a) turquoise hydrogen (BG-BG) and (b) turquoise hydrogen (BG-BG) with Boudouard reaction.
Figure 6. PFD of (a) turquoise hydrogen (BG-BG) and (b) turquoise hydrogen (BG-BG) with Boudouard reaction.
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Figure 7. The CO2 emission from grey, blue, and turquoise hydrogen production.
Figure 7. The CO2 emission from grey, blue, and turquoise hydrogen production.
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Figure 8. The energy requirement for grey, blue, and turquoise hydrogen production.
Figure 8. The energy requirement for grey, blue, and turquoise hydrogen production.
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Figure 9. The fugitive methane emissions from grey, blue, and turquoise hydrogen production.
Figure 9. The fugitive methane emissions from grey, blue, and turquoise hydrogen production.
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Figure 10. The overall GHG emissions for grey, blue, and turquoise hydrogen production.
Figure 10. The overall GHG emissions for grey, blue, and turquoise hydrogen production.
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Figure 11. The overall GHG emissions for grey, blue, and turquoise hydrogen production after 50% reduction in FMEs.
Figure 11. The overall GHG emissions for grey, blue, and turquoise hydrogen production after 50% reduction in FMEs.
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Table
Table 1. The molecular weight, density, and calorific values of the compounds.
Table 1. The molecular weight, density, and calorific values of the compounds.
CompoundsMolecular Weight (g/mol)Density
(kg/m3)
at Standard State		Gross Calorific Value (MJ/kg)
CH416.040.6653.6
C12.01226732.8
H22.020.085141.87
CO244.011.87-
CO28.011.1410.1
Table
Table 2. Comparison of feedstock consumption and emissions from grey, blue, and turquoise hydrogen.
Table 2. Comparison of feedstock consumption and emissions from grey, blue, and turquoise hydrogen.
Grey H2Blue H2Turquoise H2
NG-NGNG-REBG-REBG-BG
FGC 1 FGC 1 Boud 2 Boud 2
1. SMR/Pyrolysis processBG consumed (g BG/MJ) 88.8988.8988.8988.89
CH4 consumed (g CH4/MJ) 14.0214.0214.0228.0528.0528.05
CO2 produced (g CO2/MJ) 38.4738.4738.47
Fugitive CH4 emissions (g CH4/MJ) 0.490.490.490.980.980.980.560.560.560.56
Fugitive CH4 emissions (g CO2eq/MJ) 42.3342.3342.3384.6784.6784.6748.3948.3948.3948.39
Direct CO2 emissions (g CO2/MJ) 38.475.775.77
CO2 capture rate 85%85%
Carbon produced (g C/MJ) 21.0021.0021.0021.0021.0021.0021.00
2. Heat source of PSABG consumed (g BG/MJ) 5.805.80
CO2 produced (g CO2/MJ)
Fugitive CH4 emissions (g CH4/MJ) 0.040.04
Fugitive CH4 emissions (g CO2eq/MJ) 3.163.16
GHG emissions from solar PV (g CO2eq/MJ) 1.821.82
3. Heat source of SMR/Pyrolysis BG consumed (g BG/MJ) 10.0510.05
CH4 consumed (g CH4/MJ)3.363.363.363.053.05
CO2 produced (g CO2/MJ) 9.239.239.238.378.37
Fugitive CH4 emissions (g CH4/MJ) 0.120.120.120.110.11 0.060.06
Fugitive CH4 emissions (g CO2eq/MJ) 10.1510.1510.159.219.21 5.475.47
GHG emissions from solar PV (g CO2eq/MJ) 3.143.143.14
Direct CO2 emissions (g CO2/MJ) 9.239.233.238.372.930.00
Flue-gas capture rate 65% 65%
4. Boudouard reactionCO2 consumed (g CO2/MJ) 60.22 60.22
CO produced (g CO/MJ) 76.65 76.65
5. Heat source of Boudouard reactionBG consumed (g BG/MJ) 18.13
Fugitive CH4 emissions (g CH4/MJ) 0.11
Fugitive CH4 emissions (g CO2eq/MJ) 9.87
GHG emissions from solar PV (g CO2eq/MJ) 5.68
6. Heat source of CO2/flue-gas capture facilityCH4 consumed (g CH4/MJ) 2.983.83 0.77
CO2 produced (g CO2/MJ) 8.1710.51 2.12
Fugitive CH4 emissions (g CH4/MJ) 0.100.13 0.027
Fugitive CH4 emissions (g CO2eq/MJ) 9.0011.57 2.33
Direct CO2 emissions (g CO2/MJ) 8.1710.51 2.12
Summary of the processIndirect upstream CO2 emissions (g CO2/MJ)3.584.194.376.406.565.77
Total BG consumed (g BG/MJ) 88.8988.89104.74122.87
Total CH4 consumed (g CH4/MJ)17.3920.3721.2231.1031.8728.05
Total CO2 emitted (g CO2/MJ)51.2827.3623.8814.7611.615.77 −60.22 −60.22
Total fugitive CH4 emissions (g CO2eq/MJ)52.4961.4864.0693.8796.2184.6748.3948.3957.0266.89
Total GHG emissions from solar PV (g CO2eq/MJ) 3.144.9610.63
Overall emissions (g CO2eq/MJ)103.7688.8587.94108.64107.8193.5853.35−1.1957.026.67
Energy required for the process (MJ/mol H2)0.270.310.330.480.490.480.490.570.490.57
1 With flue gas capture, 2 With additional Boudouard reactions.
Table
Table 3. Table of equations.
Table 3. Table of equations.
EquationsNo.
(3.5 mol H2/MJ) × (1 mol CO2/4 mol H2) × (44.01 g/mol CO2) = 38.47 g CO2/MJ(2)
(3.5 mol H2/MJ) × (1 mol CH4/4 mol H2) × (16.04 g/mol CH4) = 14.02 g CH4/MJ(3)
(0.60 kWh/m3 H2) × (3.6 MJ/kWh) × (2.02 g/mol H2)/(0.085 × 1000 g/m3 H2) = 0.052 MJ/mol H2(4)
(0.052 MJ/mol H2) × (3.5 mol H2/MJ)/((53.6/1000) MJ/g CH4) = 3.36 g CH4/MJ(5)
(0.21 mol CH4/MJ) × (1 mol CO2/1 mol CH4) × (44.01 g/mol CO2) = 9.23 g CO2/MJ(6)
7.5% × (47.70 g CO2/MJ) = 3.58 g CO2/MJ(7)
(3.4% of NG production) × (159.60 EJ/154.58 EJ) = 3.5% of NG consumption(8)
(3.5% of NG consumption) × (14.02 g CH4/MJ) = 0.49 g CH4/MJ(9)
(3.5% of NG consumption) × (3.36 g CH4/MJ) = 0.12 g CH4/MJ(10)
(0.49 g CH4/MJ) × (86 g CO2eq/g CH4) = 42.33 g CO2eq/MJ(11)
(0.12 g CH4/MJ) × (86 g CO2eq/g CH4) = 10.15 g CO2eq/MJ(12)
(100–85%) × (38.47 g CO2/MJ) = 5.77 g CO2/MJ(13)
(100–65%) × (9.23 g CO2/MJ) = 3.23 g CO2/MJ(14)
(25%) × [(38.47 g CO2/MJ) − (5.77 g CO2/MJ)] = 8.17 g CO2/MJ(15)
(39%) × [(9.23 g CO2/MJ) − (3.23 g CO2/MJ)] = 2.34 g CO2/MJ(16)
(8.17 g CO2/MJ) × (16.04 g CH4/44.01g CO2) × (1 mol CH4/1 mol CO2) = 2.98 g CH4/MJ(17)
(10.51 g CO2/MJ) × (16.04 g CH4/44.01g CO2) × (1 mol CH4/1 mol CO2) = 3.83 g CH4/MJ(18)
7.5% × (55.87 g CO2/MJ) = 4.19 g CO2/MJ(19)
7.5% × (58.21 g CO2/MJ) = 4.37 g CO2/MJ(20)
(3.5% of NG consumption) × (2.98 g CH4/MJ) × (86 g CO2eq/g CH4) = 9.00 g CO2eq/MJ(21)
(3.5% of NG consumption) × (3.83 g CH4/MJ) × (86 g CO2eq/g CH4) = 11.57 g CO2eq/MJ(22)
(CH4 → 2H2 + C,→ ΔH0 = 37.4 kJ/mol H2(23)
(3.5 mol H2/MJ) × (1 mol C/2 mol H2) = 1.75 mol C/MJ(24)
(3.5 mol H2/MJ) × (1 mol CH4/2 mol H2) × (16.04 g/mol CH4) = 28.05 g CH4/MJ(25)
(0.54 kWh/m3 H2) × (3.6 MJ/kWh) × (2.02 g/mol H2) / (0.085 × 1000 g/m3 H2) = 0.047 MJ/mol H2(26)
(0.047 MJ/mol H2) × (3.5 mol H2/MJ)/((53.6 /1000) MJ/g CH4) = 3.05 g CH4/MJ(27)
((3.05 g CH4/MJ)/(16.04 g/mol CH4)) × (1 mol CO2/1 mol CH4) × (44.01 g/mol CO2) = 8.37 g CO2/MJ(28)
(100–65%) × (8.37 g CO2/MJ) = 2.93 g CO2/MJ(29)
(39%) × [(8.37 g CO2/MJ) − (2.93 g CO2/MJ)] = 2.12 g CO2/MJ(30)
(2.12 g CO2/MJ) × (16.04 g CH4/44.01g CO2) × (1 mol CH4/1 mol CO2) = 0.77 g CH4/MJ(31)
(28.05 g CH4/MJ) × (44.01 g CO2/16.04 g CH4) = 76.94 g CO2/MJ(32)
7.5% × (85.31 g CO2/MJ) = 6.40 g CO2/MJ(33)
7.5% × (87.43 g CO2/MJ) = 6.56 g CO2/MJ(34)
(3.5% of NG consumption) × (28.05 g CH4/MJ) × (86 g CO2eq/g CH4) = 84.67 g CO2eq/MJ(35)
(3.5% of NG consumption) × (3.05 g CH4/MJ) × (86 g CO2eq/g CH4) = 9.21 g CO2eq/MJ(36)
(3.5% of NG consumption) × (0.77 g CH4/MJ) × (86 g CO2eq/g CH4) = 2.33 g CO2eq/MJ(37)
(0.54 kWh/m3 H2) × (2.02 g/mol H2)/(0.085 × 1000 g/m3 H2) = 0.013 kWh/mol H2(38)
(3.5 mol H2/MJ) × (0.013 kWh/mol H2) × (69.25 g CO2eq/kWh) = 3.14 g CO2eq/MJ(39)
(0.043 m3 CH4/MJ)/(57% vol) = 0.075 m3 BG/MJ(40)
(0.075 m3 BG/MJ) × (43% vol) = 0.032 m3 CO2/MJ(41)
(69.25 g CO2eq/kWh) × (0.35 kWh/m3 BG) × (0.075 m3 BG/MJ) = 1.82 g CO2eq/MJ (42)
(69.25 g CO2eq/kWh) × (0.54 kWh/m3 H2) × (0.084 m3 H2/MJ) = 3.14 g CO2eq/MJ (43)
C + CO2 → 2CO,→ ΔH0 = 172.5 kJ/mol CO2(44)
(69.25 g CO2eq/kWh) × (2.54 kWh/m3 CO2) × (0.032 m3 CO2/MJ) = 5.68 g CO2eq/MJ (45)
(2% of bio-methane production) × (3.96 bcm/3.95 bcm) = 2% of bio-methane consumption(46)
(2% of NG consumption) × (28.05 g CH4/MJ) = 0.56 g CH4/MJ(47)
(0.35 kWh/m3 BG) × (3.6 MJ/kWh) × (0.075 m3 BG/MJ)/((16.27/1000) MJ/g BG) = 5.8 g BG/MJ(48)
((5.8 g BG/MJ)/(1.187 × 1000 g/m3 BG)) × (57% vol) × ((0.66 × 1000 g/m3)) = 1.83 g CH4/MJ(49)
(0.047 MJ/mol H2) × (3.5 mol H2/MJ)/((16.27/1000) MJ/g BG) = 10.05 g BG/MJ(50)
(2.54 kWh/m3 CO2) × (3.6 MJ/kWh) × (44.01 g/mol CO2)/(1.87 × 1000 g/m3 CO2) = 0.22 MJ/mol CO2(51)
(0.22 MJ/mol CO2) × (1.37 mol CO2/MJ)/((16.27 /1000) MJ/g BG) = 18.14 g BG/MJ(52)
((14.02 g CH4/MJ) + (3.36 g CH4/MJ) + (3.83 g CH4/MJ)) × ((53.6/1000) MJ/g CH4)) = 1.14 MJ(53)
(1.14 MJ)/(3.5 mol H2) = 0.33 MJ/mol H2(54)
((28.05 g CH4/MJ) × ((53.6/1000) MJ/g CH4)) + ((0.013 kWh/mol H2) × (3.6 MJ/kWh) × (3.5 mol H2/MJ)) = 1.67 MJ(55)
(1.67 MJ)/(3.5 mol H2) = 0.48 MJ/mol H2(56)
((88.89 g BG/MJ) × ((16.27/1000) MJ/g BG)) + ((0.35 kWh/m3 BG) × (3.6 MJ/kWh) × (0.075 m3 BG /MJ)) + ((0.54 kWh/m3 H2) × (3.6 MJ/kWh) × (0.084 m3 H2 /MJ)) + ((2.54 kWh/m3 CO2) × (3.6 MJ/kWh) × (0.032 m3 CO2/MJ)) = 2.00 MJ(57)
(2.00 MJ)/(3.5 mol H2) = 0.57 MJ/mol H2(58)
((88.89 g BG/MJ) + (5.80 g BG/MJ) + (10.05 g BG/MJ) + (18.13 g BG/MJ)) × ((16.27/1000) MJ/g BG)) = 2.00 MJ(59)
(2.00 MJ)/(3.5 mol H2) = 0.57 MJ/mol H2(60)
Table
Table 4. The composition and thermodynamic parameters of biogas [44].
Table 4. The composition and thermodynamic parameters of biogas [44].
CH4 (% vol)57%
CO2 (% vol)43%
Molar mass (kg/mol)28.067
Density (kg/m3)1.187
Calorific value (MJ/kg)16.27
Table
Table 5. The emission index for grey, blue, and turquoise hydrogen (Excluding hydrogen purifying processes such as PSA).
Table 5. The emission index for grey, blue, and turquoise hydrogen (Excluding hydrogen purifying processes such as PSA).
Emission Index (kg CO2/kg H2)
SMRPyrolysis (Turquoise H2)
Grey H27.27NG-NG2.09BG-RE0.00
Blue H23.88NG-NG FGC1.65BG-RE Boud−8.54
Blue H2 FGC3.39NG-RE0.82BG-BG0.00
BG-BG Boud−8.54
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Ingale, G.U.; Kwon, H.-M.; Jeong, S.; Park, D.; Kim, W.; Bang, B.; Lim, Y.-I.; Kim, S.W.; Kang, Y.-B.; Mun, J.; et al. Assessment of Greenhouse Gas Emissions from Hydrogen Production Processes: Turquoise Hydrogen vs. Steam Methane Reforming. Energies 2022, 15, 8679. https://doi.org/10.3390/en15228679

AMA Style

Ingale GU, Kwon H-M, Jeong S, Park D, Kim W, Bang B, Lim Y-I, Kim SW, Kang Y-B, Mun J, et al. Assessment of Greenhouse Gas Emissions from Hydrogen Production Processes: Turquoise Hydrogen vs. Steam Methane Reforming. Energies. 2022; 15(22):8679. https://doi.org/10.3390/en15228679

Chicago/Turabian Style

Ingale, Gayatri Udaysinh, Hyun-Min Kwon, Soohwa Jeong, Dongho Park, Whidong Kim, Byeingryeol Bang, Young-Il Lim, Sung Won Kim, Youn-Bae Kang, Jungsoo Mun, and et al. 2022. "Assessment of Greenhouse Gas Emissions from Hydrogen Production Processes: Turquoise Hydrogen vs. Steam Methane Reforming" Energies 15, no. 22: 8679. https://doi.org/10.3390/en15228679

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