Hydrogen Electrolysis
Hydrogen electrolysis, also referred to as Water Electrolysis is the process of splitting water (H2O) into its constituent elements, hydrogen (H2) and oxygen (O2), using electric current. The process uses an electrolyser, a device that contains two electrodes and an electrolyte separating them. When an electric current is applied between the electrodes, water molecules are dissociated into hydrogen and oxygen gases.[1]
Currently H2 is produced from fossil fuels (natural gas, coal, oil), by biomass gasification, and from non-carbon sources, such as water electrolysis. The use of fossil media, such as the reforming of natural gas or methane, generates CO2 that would need to be captured and stored, at a cost. Water electrolysis today is expensive and its CO2 emissions are related to the availability of green electricity. It requires nearly 5 kWh/m3 of H2.[2] The utilisation of waste heat from coke ovens for COG reforming to produce H2 is being investigated in Japan.[3]
In the steel industry Hydrogen finds several applications, while the emissions is determined by the method of hydrogen production. Injection of H2 into a blast furnace to partly replace coal for reduction is being explored. H2 can also be used as the reductant in conventional direct reduction reactors.[4] If H2 is produced by water electrolysis using hydro or nuclear electricity, then CO2 emissions could be lowered to less than 300 kg/t HRC.[5]
Hydrogen Electrolysis methods
There are three prominent methods of hydrogen electrolysis:
Alkaline Electrolysis
Alkaline electrolysis is one of the most established methods of hydrogen production. Alkaline electrolysis is a process that involves splitting water molecules (H2O) into hydrogen (H2) and oxygen (O2) using an alkaline electrolytic cell. This method employs an alkaline electrolyte solution, typically potassium hydroxide (KOH) or sodium hydroxide (NaOH), which acts as a conducting medium to facilitate the flow of ions between electrodes.
The electrolysis cell consists of two electrodes, an anode, and a cathode, submerged in the alkaline electrolyte solution. The anode is typically made of a durable and corrosion-resistant material such as nickel, same as the cathode that can also be made of nickel, stainless steel, or other metals. When an electric current is passed through the electrolyte solution via the electrodes, several reactions take place at the anode and cathode:
At the anode, oxidation of water molecules occurs, leading to the release of oxygen gas (O2).
4OH- -> O2(g) + H2O(aq) + 4e-
At the cathode reduction of water molecules occurs, leading to the production of hydrogen gas (H2).
2H2O(l) + 2e- → H2(g) + 2OH-(aq)
Overall, the electrolysis of water using alkaline hydrogen electrolysis can be summarized by the balanced equation:
2H2O(l) → 2H2(g) + O2(g) [1]
Proton Exchange Membrane (PEM) Electrolysis
PEM electrolysis, which stands for Proton Exchange Membrane hydrogen electrolysis, is another method used for splitting water into hydrogen and oxygen. The PEM electrolysis cell consists of two electrodes, an anode, and a cathode, separated by a solid polymer electrolyte membrane, typically made of a specialized proton-conducting material like Nafion. The anode and cathode are usually made of precious metals such as platinum, but efforts are ongoing to develop more cost-effective alternatives. When an electric current passes through the PEM electrolysis cell, the following reactions occur at the anode and cathode:
At the anode, water oxidation takes place, resulting in the release of oxygen gas (O2).
2H2O(l) → O2(g) + 4H+(aq) + 4e-
Formed hydrogen ions (protons) migrate through the electrolyte to the cathode. At the cathode, reduction of protons occurs, leading to the production of hydrogen gas (H2).
4H+(aq) + 4e- → 2H2(g)
Overall, the PEM hydrogen electrolysis process can be summarized by the balanced equation:
2H2O(l) → 2H2(g) + O2(g)
The solid polymer electrolyte membrane used in PEM electrolysis plays a crucial role by allowing the passage of protons (H+ ions) while preventing the mixing of hydrogen and oxygen gases. This design ensures the selective separation of the generated gases and minimizes crossover, improving the efficiency of hydrogen production. The PEM electrolyser is compact and best suitable for small-scale applications. [1]
Solid Oxide Hydrogen Electrolysis
Solid oxide hydrogen electrolysis is a process that uses a solid oxide electrolyte to split water into hydrogen and oxygen gases at elevated temperatures. This technology is similar to solid oxide fuel cells (SOFCs) but operates in reverse to generate hydrogen instead of electricity. In solid oxide electrolysis, the cell consists of three main components: an electrolyte, an anode, and a cathode. The electrolyte is a solid oxide material, such as yttria-stabilized zirconia (YSZ) or ceria-based materials, which conducts oxygen ions (O2-) at high temperatures (typically above 700 degrees Celsius). The anode and cathode are porous electrodes that facilitate electrochemical reactions. The anode is usually made of nickel-based materials, while the cathode can be composed of perovskite-type oxides or other mixed ionic-electronic conducting materials. When an electric current is applied across the solid oxide hydrogen electrolysis cell, the oxygen ions migrate through the electrolyte to the cathode. At the cathode surface, the oxygen ions combine with electrons from the external circuit to form oxygen gas (O2).
O2- (from electrolyte) + 2e- → O2(g)
At the anode, water vapor (steam) or a mixture of steam and carbon dioxide is supplied. At high temperatures, steam is reduced by the electrons flowing through the external circuit, leading to the production of hydrogen gas (H2).
H2O (steam) + 2e- → H2(g) + O2-
Overall, the solid oxide hydrogen electrolysis process can be summarized by the balanced equation:
2H2O (steam) → 2H2(g) + O2(g)[1]
Electrolytic H2 blending
In 2020, the European Commission initiated a strategy focused on hydrogen to advance Europe towards climate neutrality. This strategy outlined steps to incorporate clean hydrogen into mainstream energy usage, including goals for installing renewable hydrogen electrolysers by 2024 and 2030. The concept of blending hydrogen with existing gases in the gas grid emerged as an initial tactic for reducing natural gas emissions. Initially, hydrogen integration into the gas grid, at levels of approximately 5-10% by volume, can occur without significant modifications to transmission infrastructure or consumer installations. Towards the latter part of the decade, this percentage could rise to 15-20% after necessary infrastructure adjustments. This could allow for the integration of up to 18.4 GW of electrolyser capacity across the EU, tripling the 2024 target. With a 20% blending threshold, this capacity could increase to 40-70.8 GW, aligning with the EU's 2030 target of 40 GW and resembling capacities outlined in various national strategies (ES, NL, FR, IT, DE, PT). The volume of hydrogen introduced into the gas grid through electrolysis, capped at 20%, heavily relies on how electrolysers are integrated into the power market and, under market-driven setups, on the extent of price support.[6]
Hydrogen Plasma reduction of iron ore
Given the emission potential of existing iron and steel manufacturing technology, alternative reduction technologies are being explored for extracting iron from its ores. Use of hydrogen for reducing iron ores yields water vapour instead of CO2 with a carbon-based reductant. The water in the offgas can be easily separated by condensation. The overall decrease in CO2 emissions is depends largely on the CO2 emissions associated with the H2 generation process.[4] The hydrogen-based direct reduction has been explored as a sustainable route to mitigate CO2 emissions, where the reduction kinetics of the intermediate oxide product FexO into iron is the rate-limiting step of the process. The total reaction has an endothermic net energy balance.[7]
Reduction based on a hydrogen plasma may offer an attractive alternative. Molecular H2 cannot reduce liquid iron oxide: atomic or ionised H2 is required. These states, however, can only be achieved at very high temperatures such as in the vicinity of an electric or plasma arc. In a procedure proposed by a research team investigating the reduction of iron ore fines using an H2 plasma at Montanuniversität Leoben in Austria, iron ore fines and lime additives (to achieve the required slag viscosity) are introduced into the smelter. A gas mixture of H2/argon, forming the plasma, is introduced through hollow graphite electrodes, with H2 serving as both a heat carrier and reducing agent. The resulting hot metal (with a reduction degree exceeding 97%) is tapped, degassed to eliminate dissolved H2 and O2, and then alloyed to reach the desired grade. To minimize energy consumption, the sensible heat from the smelter offgas is utilized to preheat and pre-reduce the iron ore fines (achieving a reduction degree of approximately 33%) within a fluidized bed system. The sensible heat from the preheater offgas is recovered using a boiler, and the steam generated is directed to the steam reformer for H2 production. Following passage through the boiler, the offgas undergoes cleaning before being recycled back into the process.[4]
References
- ↑ 1.0 1.1 1.2 1.3 "The Basics of Hydrogen Electrolysis". stargatehydrogen.com. Retrieved 2024-05-03.
- ↑ Birat, JP; Borlée, J (2008). "ULCOS: the European steel industry's effort to find breakthrough technologies to cut its CO2 emissions significantly". Carbon dioxide reduction metallurgy: 59–69.
- ↑ Kojima, A (October 2009). "Steel industry's global warming measures and sectoral approaches". Quarterly Review. 33: 55–68.
- ↑ 4.0 4.1 4.2 Carpenter, Anne (January 2012). "CO2 abatement in the iron and steel industry" (PDF). USEA. Retrieved 03 May 2024.
{{cite web}}: Check date values in:|access-date=(help)CS1 maint: url-status (link) - ↑ Ranzani da Costa, A; Wagner, D; Patisson, F; Ablitzer, D (2008). "Modeling of DR shaft operated with pure hydrogen using a physical-chemical and CFD approach". 4th ULCOS seminar,: 6.
{{cite journal}}: CS1 maint: extra punctuation (link) - ↑ Kanellopoulos, Konstantinos; Busch, Sebastian; De, FELICE Matteo; Giaccaria, Sergio; Costescu, Anca (2022-01-14). "Blending hydrogen from electrolysis into the European gas grid". JRC Publications Repository. Retrieved 2024-05-04.
- ↑ Souza Filho, I. R.; Ma, Y.; Kulse, M.; Ponge, D.; Gault, B.; Springer, H.; Raabe, D. (2021-07-01). "Sustainable steel through hydrogen plasma reduction of iron ore: Process, kinetics, microstructure, chemistry". Acta Materialia. 213: 116971. doi:10.1016/j.actamat.2021.116971. ISSN 1359-6454.
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