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| Fig. 1: Intrinsic energy and mass flows for the production of 1 ton of hydrogen through SMR/WGS and MP. (Image Source: H. Moise) |
Over the course of human history, few discoveries have had as profound an impact on our society and the global economy as the utilization of fossil fuels. Much of the prosperity seen today has been driven by our access to these ancient deposits. While there are enough geological reserves of coal, oil, and natural gas (NG) to likely fuel our society for at least several more decades, the environmental consequences associated with the continued emission of CO2 into our atmosphere has sparked conversations of an earlier transition away from them. [1] As of 2022, most forms of renewable energy are not able to scale in their current technological states to meet the energy demands of the primary and secondary economic sectors. This means our society must pursue advanced fuels as bridging technologies until renewables are able to scale.
An advanced fuel is termed as any energy source that reduces greenhouse gas emissions (GHG) by at least 50%. Hydrogen has been touted as a possible candidate for such a fuel due to its zero direct emissions when combusted or used in a fuel cell. Indirectly, however, the current process for hydrogen production is a heavy GHG emitter which means fuel-switching with it simply shifts the emissions elsewhere.
Natural gas is converted to hydrogen through steam-reforming of methane (SMR) followed by water-gas shift (WGS), which emits 8-12 kg CO2 per kg H2. [2] Of these emissions, 5.5 kg CO2 per kg H2 is produced directly from the stoichiometric combination of SMR (Table 1, Eq. 1) with WGS (Table 1, Eq. 2), with the remainder emitted indirectly through process units and heat input.
SMR is highly endothermic and requires high operating temperatures of 850-950°C in its primary reactor to achieve sufficient CO and H2 production. This can increase to 1050°C in secondary SMR reactors if low CH4 concentrations (<1%) are required in the effluent. Incorporation of small amounts of oxidants (0.2-0.5:1 O2:CH2) in the feed allows for autothermal reforming (ATR) (Table 1, Eq. 3) or partial oxidation (POX) (Table 1, Eq. 4), which can also aid in mitigating this carbon formation, introducing heat into the reactor, and tuning the produced syngas ratio. WGS is thermodynamically opposed to SMR and as a slightly exothermic reaction operates at lower operating temperatures in two separate reactors high temperature (HT) WGS at 300- 400°C and low temperature (LT) WGS at 200-300°C. Industrial HT WGS reactors experience CO feed concentrations of 10-15% and industrial LT WGS reactors experience CO feed concentrations of 2-3%. [3] This process is highly energy intensive and consumes substantial amounts of NG to provide the required heat.
While 1 kg of methane has an energy content of roughly 56 MJ, the intrinsic energy content of the hydrogen produced through SMR/WGS is 71 MJ per kg CH4 that is consumed directly in the SMR/WGS reaction. The first law of thermodynamics is maintained by the extrinsic energy costs of hydrogen production in SMR/WGS and as well as half the hydrogen being derived from the steam co-feed. This also highlights the importance of normalizing energy contents with their emissions. Directly combusting methane would stoichiometrically produce the same quantity of CO2 that is emitted through the SMR/WGS reaction. Since burning methane is a downhill reaction, the additional costs associated with the SMR/WGS process outlined previously make fuel-switching with this hydrogen for power generation more carbon intensive.
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| Table 1: Reactions of interest with their associated endotherms. [6] | ||||||||||||||||||||||||
Methane pyrolysis (MP) (Table 1, Eq. 5) has been proposed in recent decades as a potential chemical route to decarbonize America's low-cost and abundant NG as well as utilize some of the existing SMR/WGS infrastructure, which includes 2.6 million miles of NG pipelines that stretch across the United States. [4] MP decomposes methane in a non-oxidative environment at a moderate endotherm of 74 kJ/mol CH4, producing hydrogen gas and a solid carbon that can either find value in the market or be sequestered in a much more stable and manageable form as compared to CO2. The management and removal of this solid carbon within the reactor remains to be a major barrier preventing the deployment of this technology at commercial scales. Sustaining high rates of methane decomposition is rapidly impeded by the generation of coke on catalyst surfaces, which cannot be removed through traditional means (i.e. oxidative burning) due to the associated CO2 emissions. The inability to separate this carbon from the reactor internals and convey it out of the reactor will also inevitably leads to clogging. This endothermic reaction is thermodynamically limited and high operating temperatures are required to overcome the activation barrier, generally requiring temperatures above 800°C if catalytically driven and above 1100°C if driven thermally. [5]
Since MP has not been commercialized yet, any price point discussed in literature is theoretical and relies heavily on assumptions. Thus, I will mainly focus on the intrinsic thermodynamics of this process compared to SMR/WGS to fairly compare the two. Fig. 1 depicts the energy and mass flows for the production of 1 ton of hydrogen produced through SMR/WGS and MP. The intrinsic energy content of the hydrogen produced through MP is 36 MJ per kg CH4 consumed, meaning that 36% of methane's intrinsic energy, or 20 MJ, is lost as heat when converting it to hydrogen through this process. [6] This means customers will pay at least 1.56 times more for their energy if it comes from this hydrogen as opposed to directly combusting the methane. This energy penalty, in the form of wasted heat and the extrinsic energy costs of a MP process, indirectly becomes a pre-combustion sequestration cost that users will compare to any existing carbon tax or carbon price point available. Of course, this value must also eventually include additional costs sure to be found further downstream, such as transportation, distribution, storage, and retail.Whether the produced hydrogen is used as a fuel or a chemical feed stock, its cost will be compared to existing technologies that must utilize post-combustion sequestration techniques such as flue gas amine scrubbers.
Some believe that the carbon produced in MP is capable of offsetting some of the hydrogen production costs, but the laws of economics do not align with this viewpoint. During MP, carbon is stoichiometrically produced at three times the rate of hydrogen by mass. For this reason, carbon cleanliness becomes a high priority for any process that implements expensive media or catalysts that may leave the reactor with the solid carbon during a gas-solid separation step - resulting in high media recovery and makeup costs. The volume at which this carbon would be produced if the global demand for hydrogen were met using MP would drive its value down to levels that might make it challenging to recover much of that process cost. The current market size for H2 in 2022 is 95 million metric tons (MT), which would result in 283 MT of carbon if this H2 was produced through MP. This value is 14 times larger than the 2021 market for all forms of carbon (<20 MT) and is only worsened if H2 begins to play a greater role in energy. [7,8] The market size for coal is certainly large enough to absorb this volume of carbon, but cannot be used since this would inadvertently produce CO2. Materials that have been shown to benefit from the addition of carbon materials, and whose markets are large enough to absorb this volume of material, are polymers, cements, ceramics, and textiles all of which require low cost process feeds. [9] For these reasons, the carbon produced from MP is not likely to offset any costs of production in the immediate future until the carbon market becomes more elastic for this flux of material.
In closing, there is no free lunch. If our society wants to continue using fossil fuels, but also wants to prevent the further emission of carbon dioxide into our atmosphere, we must pay a thermodynamic cost for that. Whether hydrogen is to be used as a fuel or as its traditional role as a chemical feedstock, its current production is a heavy GHG emitter and we must choose whether we want to capture and sequester that carbon as a solid with MP or as carbon dioxide with SMR/WGS.
© Henry Moise. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] "BP Statistical Review of World Energy 2022," British Petroleum, June 2022
[2] "Global Hydrogen Review 2022," International Energy Agency, September 2022.
[3] C. H. Bartholomew and R. J. Farrauto, Fundamentals of Industrial Catalytic Processes, 2nd Ed. (Wiley-AlChE, 2006).
[4] U.S. Department of Transportation, "Pipeline Safety: Gas Pipeline Leak Detection and Repair," Federal Register 88 FR 31890, 18 May 23.
[5] N. Sáchez-Bastardo, R. Schlögl and H. Ruland, "Methane Pyrolysis for Zero-Emission Hydrogen Production: A Potential Bridge Technology From Fossil Fuels to a Renewable and Sustainable Hydrogen Economy," Ind. Eng. Chem. Res. 60, 11855 (2021)
[6] CRC Handbook of Chemistry and Physics, (CRC Press, 2010).
[7] "ECGA Annual Report," European Carbon and Graphite Association, 2016.
[8] "Carbon Black User Guide," International Carbon Black Association, 2016.
[9] E. T. Thostenson, C. Li and T.-W. Chou, "Nanocomposites in Context," Compos. Sci. Technol. 65, 491 (2005).