- Grey hydrogen is the most commonly produced form of hydrogen today and is generated primarily through the process of steam methane reforming (SMR) using natural gas as the feedstock. In this process, methane (CH₄) reacts with steam (H₂O) at high temperatures (700–1,000°C) to produce hydrogen gas (H₂) and carbon dioxide (CO₂):
- CH₄ + H₂O → CO + 3H₂
- CO + H₂O → CO₂ + H₂
- While grey hydrogen provides a reliable and cost-effective supply of hydrogen, it comes at a significant environmental cost. The carbon dioxide produced during the process is released directly into the atmosphere, making grey hydrogen a major contributor to greenhouse gas emissions. For every tonne of hydrogen produced, approximately 9–10 tonnes of CO₂ can be emitted, depending on the efficiency of the process and the carbon content of the fuel used.
- Grey hydrogen currently accounts for the vast majority of hydrogen production globally, primarily because SMR is a well-established, relatively inexpensive technology. It is widely used in industries such as refining (to remove sulfur from fuels), ammonia production for fertilizers, and various chemical manufacturing processes. Its affordability and established infrastructure make it the dominant form of hydrogen, especially in countries where natural gas is abundant and cheap.
- However, the environmental impact of grey hydrogen has come under increased scrutiny in the context of global efforts to reduce carbon emissions and transition to clean energy systems. As hydrogen gains attention as a potential clean fuel for transportation, power generation, and industry, the source of hydrogen production becomes a critical factor. Grey hydrogen is considered unsustainable in the long term because it reinforces reliance on fossil fuels and contributes significantly to climate change.
- Efforts to address the environmental downsides of grey hydrogen have led to the development of blue hydrogen, where carbon dioxide emissions from SMR are captured and stored using carbon capture and storage (CCS) technologies. While blue hydrogen mitigates some of the emissions, it still depends on fossil fuels and infrastructure with inherent risks, such as methane leaks, which are potent contributors to global warming.
- Transitioning away from grey hydrogen toward low-carbon alternatives—such as green hydrogen, produced from renewable electricity via electrolysis—is a major goal of current energy policy and research initiatives. Nonetheless, grey hydrogen is expected to remain a part of the hydrogen supply chain in the short term due to its cost advantage and the time needed to scale up cleaner production methods.
