Colors of Hydrogen
The Colorful World of Hydrogen Production
As hydrogen gains momentum as a clean energy carrier, attention is turning to how this versatile fuel is produced. Not all hydrogen is created equal when it comes to environmental impact. There are a range of production methods, each with pros and cons. Understanding the different “colors” of hydrogen will be key for scaling up sustainable solutions.
What Exactly is Hydrogen Production?
Hydrogen itself is abundant in compounds like water (H2O) but very rarely exists as pure diatomic hydrogen gas (H2) naturally. To get usable hydrogen fuel, production processes are needed to isolate the hydrogen molecules.
The most common industrial method today is called steam methane reforming. This takes natural gas as the feedstock and uses high-temperature steam to extract hydrogen, producing carbon monoxide and carbon dioxide as byproducts.
Other production methods also exist, like the gasification of coal or biomass, extracting hydrogen from oil fractions, or the increasingly promising electrolysis of water. Each process results in hydrogen of varying purity, cost, and environmental impacts. Let’s explore the main options.
Steam Methane Reforming of Natural Gas
As mentioned above, steam methane reforming (SMR) is currently the cheapest way to produce large volumes of hydrogen, accounting for about 95% of global dedicated hydrogen generation. This process reacts methane from natural gas with high-temperature steam to produce hydrogen along with carbon monoxide and carbon dioxide.
The reaction between methane and steam is catalyzed in a reformer unit. Then a secondary water-gas shift reaction converts the carbon monoxide to additional hydrogen and carbon dioxide. The resulting gas mixture goes through further processing to remove impurities and isolate the hydrogen.
SMR benefits from advanced technology and the widespread availability of natural gas. But the carbon dioxide emissions range from 9–12 kilograms per kilogram of hydrogen. Capturing some of this CO2 does reduce the environmental impact, which brings us to our next category.
Natural Gas Reforming With Carbon Capture (Blue Hydrogen)
The SMR process can be coupled with carbon capture technology to create so-called “blue hydrogen”. Here, up to 90% of the CO2 generated from the SMR plant is captured before being stored permanently underground or used in industrial applications. This reduces the lifecycle carbon emissions of the resulting hydrogen.
However, questions remain about the long-term viability and permanence of carbon sequestration. Small amounts of methane and CO2 can still escape in the process as well. While considered “low-carbon”, blue hydrogen is not completely emissions-free over its life cycle. But it can serve as a lower environmental impact transition stepping stone.
Coal Gasification (Brown Hydrogen)
Next up is hydrogen derived from coal, which accounts for a very small but notable fraction of current production. Here, coal is reacted at high temperatures with oxygen and/or steam to create synthesis gas, a mixture of hydrogen and carbon monoxide. After further processing to increase the hydrogen concentration, we are left with so-called “brown hydrogen”.
The emissions from coal gasification are even higher than natural gas SMR, with over 20 kilograms of CO2 per kilogram of hydrogen produced. While this coal-sourced hydrogen may make sense in regions with abundant coal reserves, the high carbon intensity limits its sustainability potential.
Biomass Gasification
An emerging method for lower-impact hydrogen production is to gasify biomass resources like agricultural crop residues, forestry waste, and organic municipal waste. The resulting “biogenic hydrogen” has a carbon intensity as low as green hydrogen under optimal conditions.
The interconnected process here is that biomass resources absorb CO2 as they grow. When waste biomass is gasified into hydrogen, this releases the same captured CO2 back out. With sustainable harvesting and plant regrowth, a cyclical capture-emission balance is achieved.
The drawback is that the truly large-scale production of hydrogen from biomass gasification would require tremendous land resources. And there are still some lifecycle emissions from harvesting, transportation, and gasification. But biohydrogen does show promise as a lower carbon option.
Electrolysis of Water (Green Hydrogen)
The ideal from a sustainability standpoint is electrolytic hydrogen produced using renewable electricity such as solar, wind, or hydropower. Water molecules are split into hydrogen and oxygen gas in specialized electrolyzer devices.
Because the electricity input is emissions-free, the resulting “green hydrogen” is completely carbon neutral. Green hydrogen currently makes up less than 1% of total production, but rapid growth is anticipated thanks to falling electrolyzer costs and increasing renewable energy penetration.
The main limitation currently is the relatively high cost of dedicating large amounts of zero-carbon electricity to hydrogen production. But transitions already underway in the power sector make green hydrogen viable at scale in the not-so-distant future.
Research continues into other methods for better hydrogen production such as thermal cracking of methane, photoelectrochemical routes, and even nuclear-powered electrolysis. While not currently ready for wide deployment, innovations in hydrogen generation will be crucial for the growth of the hydrogen economy.
Weighing the Options
It becomes clear that hydrogen production methods span a wide range in terms of environmental sustainability. From brown coal through blue natural gas to green renewables, not every pathway is created equal.
As hydrogen steps into a more prominent global energy role, it will be vital to critically assess the resources and processes used for generation. While low or zero-carbon hydrogen may carry a price premium today, climate policies that account for emissions and changing cost structures in renewables will shift the economic equation over time.
The Future is Green
Green electrolytic hydrogen holds the greatest promise for affordable, sustainable growth. Renewable electricity costs continue to fall, making electrolysis more competitive. In optimal regions, green hydrogen could achieve costs below $2 per kilogram by 2030.
Coupled with dedicated renewable energy generation, green hydrogen’s potential is immense. Strong policy support for production and usage will be key, as will public-private collaboration and scaling up projects. As nations target aggressive decarbonization of energy systems, hydrogen produced using emissions-free power has an essential role to play.
The array of colors today will give way to green in the future hydrogen economy. Though the road ahead still requires work, the destination is a carbon-free energy carrier enabling a sustainable world.
References
Energy Information Administration (2020). Hydrogen explained — Production of hydrogen. https://www.eia.gov/energyexplained/hydrogen/production-of-hydrogen.php
International Energy Agency (2019). The Future of Hydrogen — Seizing today’s opportunities. https://www.iea.org/reports/the-future-of-hydrogen
Hydrogen Council (2020). Path to hydrogen competitiveness — A cost perspective. https://hydrogencouncil.com/en/path-to-hydrogen-competitiveness-a-cost-perspective/
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