Electrolysis and byproduct hydrogen, such as that from chlor-alkali processes, represent two distinct pathways for hydrogen production. Each method has unique characteristics in terms of carbon intensity and purity, which are critical factors in evaluating their suitability for different applications. A life cycle assessment (LCA) approach provides a comprehensive comparison of these methods, focusing on their environmental and operational performance.
Carbon intensity is a key metric for assessing the environmental impact of hydrogen production. Electrolysis, particularly when powered by renewable electricity, can achieve very low carbon emissions. The carbon footprint of electrolysis is directly tied to the energy source used. For example, grid-powered electrolysis in regions with a high share of fossil fuels may emit between 10 to 30 kg CO2 per kg of hydrogen produced. In contrast, renewable-powered electrolysis, such as wind or solar, can reduce emissions to nearly zero. Alkaline and proton exchange membrane (PEM) electrolyzers are the most common technologies, with PEM systems offering higher efficiency but at a higher cost. Solid oxide electrolyzer cells (SOEC) are emerging as another option, with potential for higher efficiency when integrated with heat sources.
Byproduct hydrogen from chlor-alkali processes, on the other hand, is often considered a low-carbon source because it is not the primary product of the process. The chlor-alkali industry produces hydrogen as a byproduct during the electrolysis of brine to manufacture chlorine and sodium hydroxide. Since the primary goal is not hydrogen production, the carbon intensity is allocated across all products. Depending on the energy mix used, the effective carbon intensity of byproduct hydrogen can range from 5 to 15 kg CO2 per kg of hydrogen. However, this varies significantly based on regional grid emissions and the efficiency of the chlor-alkali plant. In some cases, byproduct hydrogen may have a lower carbon footprint than grid-powered electrolysis but higher than renewable-powered electrolysis.
Purity is another critical factor when comparing these two hydrogen production methods. Electrolysis typically produces high-purity hydrogen, often exceeding 99.9% purity, especially in PEM and SOEC systems. This makes electrolytic hydrogen suitable for applications requiring stringent purity standards, such as fuel cells or electronics manufacturing. Alkaline electrolyzers may produce slightly lower purity hydrogen, around 99.5%, due to the presence of residual oxygen and moisture, but this is still sufficient for most industrial uses.
Byproduct hydrogen from chlor-alkali processes usually contains impurities such as chlorine, oxygen, and trace amounts of other gases. The purity of this hydrogen typically ranges from 98% to 99.5%, depending on the purification steps applied. While this level of purity is adequate for many industrial applications, such as refining or ammonia production, it may require additional purification for use in sensitive applications like fuel cells. The presence of chlorine, even in small quantities, can be particularly problematic due to its corrosive nature and potential to damage downstream equipment.
The life cycle assessment of these methods extends beyond direct emissions and purity to include upstream and downstream impacts. For electrolysis, the LCA must account for the manufacturing of electrolyzers, the sourcing of materials like rare earth metals for PEM systems, and the disposal or recycling of components at end-of-life. Renewable-powered electrolysis has minimal operational emissions but may have higher embodied emissions due to the production of solar panels or wind turbines. Byproduct hydrogen, while leveraging existing infrastructure, still requires energy for purification and compression, adding to its life cycle emissions.
Energy efficiency is another consideration. Electrolysis systems, particularly PEM and SOEC, can achieve efficiencies of 60% to 80%, depending on operating conditions. Byproduct hydrogen does not have a direct energy input allocated to its production, but the overall efficiency of the chlor-alkali process is lower, typically around 50% to 60% for chlorine production. The energy required to purify and compress byproduct hydrogen further reduces its net efficiency.
Scalability and infrastructure also play a role in the comparison. Electrolysis can be deployed at various scales, from small modular units to large industrial plants, making it adaptable to different energy systems. Byproduct hydrogen is limited by the scale of chlor-alkali production, which is driven by demand for chlorine and sodium hydroxide rather than hydrogen. This means that byproduct hydrogen availability is inherently constrained and may not align with growing hydrogen demand.
In summary, electrolysis and byproduct hydrogen each have distinct advantages and limitations in terms of carbon intensity and purity. Electrolysis, especially when powered by renewables, offers low-carbon, high-purity hydrogen but at a higher cost and with greater infrastructure requirements. Byproduct hydrogen provides a lower-carbon alternative to fossil-based hydrogen but is limited by availability and purity constraints. The choice between these methods depends on regional energy resources, infrastructure, and the specific requirements of end-use applications. A life cycle assessment approach ensures that all environmental and operational factors are considered, enabling informed decision-making for hydrogen production strategies.
The future of hydrogen production will likely involve a mix of methods, with electrolysis playing a central role in decarbonized energy systems and byproduct hydrogen serving as a complementary source. Advances in electrolyzer technology, renewable energy integration, and purification techniques will further enhance the viability of these pathways. Understanding the trade-offs between carbon intensity and purity is essential for optimizing hydrogen production and achieving sustainability goals.