Hydrogen, a colorless gas, is widely recognized as a potential energy carrier with significant promise for decarbonizing industrial and transportation sectors. While its combustion produces only water, its environmental profile depends heavily on how it is manufactured. The industry classifies hydrogen by color according to the production method, which reflects the resulting carbon footprint. Gray hydrogen is the most common and historically established classification, defined by its source material and the subsequent release of greenhouse gases during manufacture.
The Core Production Process
Gray hydrogen is created through a mature industrial process known as Steam Methane Reforming (SMR), which utilizes natural gas as its primary raw material. This method is favored for its technical maturity, reliability, and relatively low production cost. The SMR process is highly energy-intensive and involves two distinct chemical reactions, typically conducted at high temperatures in specialized reformer tubes.
The initial step of SMR involves combining methane, the main component of natural gas, with superheated steam at temperatures ranging between \(700^\circ\text{C}\) and \(1,000^\circ\text{C}\). This mixture is passed over a metal catalyst, usually nickel-based, to drive the primary reaction. This endothermic reaction produces synthesis gas, or syngas, composed primarily of carbon monoxide and hydrogen.
The carbon monoxide is then channeled into a second reactor, where it undergoes the Water-Gas Shift Reaction (WGSR). In this phase, it reacts with additional steam to generate even more hydrogen, maximizing the final yield. This secondary reaction also produces carbon dioxide as a byproduct, which must be separated from the purified hydrogen stream.
The Defining Characteristic Carbon Emissions
The defining feature that assigns the “gray” classification is the fate of the carbon dioxide byproduct generated during the SMR process. In gray hydrogen production facilities, this large volume of carbon dioxide is vented directly into the atmosphere without any attempt at capture or storage. This direct release makes gray hydrogen a high-carbon-intensity product.
The chemical reactions inherent to SMR result in a significant emission ratio. For every kilogram of hydrogen gas produced, the process typically releases between \(7\) and \(10\) kilograms of carbon dioxide into the air. This substantial carbon footprint links gray hydrogen production to the use of fossil fuels and the resulting contribution to atmospheric greenhouse gas concentrations.
This environmental outcome is the distinction between gray hydrogen and classifications like “blue” hydrogen, which utilizes the same SMR process but incorporates Carbon Capture and Storage (CCS) technology to mitigate emissions. Since the carbon byproduct is treated as a waste product and released, the process is categorized as gray.
Role in Today’s Global Hydrogen Supply
Gray hydrogen currently accounts for the vast majority of the world’s commercially produced hydrogen, often cited as over \(90\%\) of the total supply. This dominance is primarily due to the low cost of production, estimated to be around \(\\)1$ to \(\\)2$ per kilogram, making it the most economically viable option. The SMR technology is mature, and the global infrastructure necessary for extracting and transporting the natural gas feedstock is already well-established.
The reliable and scalable supply of gray hydrogen is indispensable for several large industrial sectors today. Its largest application is in petroleum refining, where it is used in hydrocracking and hydrotreating processes to remove impurities, such as sulfur, from crude oil. Another major use is as a feedstock in the production of ammonia through the Haber-Bosch process.
Ammonia is a foundational chemical for manufacturing nitrogen-based fertilizers, a process that underpins global food production. Gray hydrogen’s role in fertilizer and chemical production, alongside its use in methanol manufacturing, ensures that it remains the default source for industries requiring large, steady volumes of hydrogen gas.