gray hydrogen

What is Gray Hydrogen? 

Hydrogen can be produced in many ways. It is often categorized into a color-coded system, describing the processes and sources used and outlining the technology’s environmental impact. This article focuses on gray hydrogen. Learn about how it is produced and used below. 

What is gray hydrogen? 

Gray hydrogen is similar to blue hydrogen. It uses fossil fuels to produce hydrogen through the process of steam reforming. However, unlike blue hydrogen, carbon dioxide (CO2) made during the process is not captured and stored. Instead, CO2 is released into the atmosphere. Therefore, gray hydrogen technology emits greenhouse gases, causes more significant environmental impacts, and is less renewable than other technologies. 

How is gray hydrogen produced? 

Steam methane reforming produces hydrogen using methane or natural gas as the primary fuel source. Still, it can be done with other non-renewable fuels such as gasoline, propane, or ethanol. 

Gray hydrogen is produced through steam methane reforming. The process is also called “natural gas reforming” or “steam reforming”. It is one of the most popular hydrogen technologies, accounting for approximately 80 percent of hydrogen production in the United States, according to the 2021 Global Hydrogen Review.   

The gray hydrogen process is an endothermic (absorbs heat) reaction in three stages.

  • The first stage involves heating liquids to high temperatures (around 1292 to 1832 F or 700 to 1,000 C) to produce steam. Next, methane (CH4) reacts with the steam to produce hydrogen, carbon monoxide, and carbon dioxide. A nickel catalyst can make the process react quickly and help retain more hydrogen gas. 
  • In the second stage, there is a “water-gas shift reaction.” The reactor is filled with water and a catalyst (e.g., iron oxide). The steam breaks down oxygen and hydrogen in the water molecules. The hydrogen gas is captured, and the oxygen joins the carbon monoxide, turning it into carbon dioxide. Although CO2 is a greenhouse gas, it is less harmful to the environment than carbon monoxide produced in the first step. And in blue hydrogen technologies, this CO2 can be captured and stored for industrial use. 
  • The final stage is called “pressure-swing adsorption.” This step removes carbon dioxide and other impurities from the gas stream. This includes carbon dioxide, methane, carbon monoxide, and water. Pressure swing absorption involves absorption materials trapping contaminants at high pressure. Next, pure hydrogen is pumped out, and the reforming vessel is de-pressurized to release trapped pollutants. 


Benefits and drawbacks of gray hydrogen technologies 

One of the significant benefits of steam methane reforming is the cost. This technology is currently the cheapest source of industrial and commercial hydrogen, accounting for almost 50 percent of global hydrogen production (source). Some of the reasons for this include efficiency and availability. 

Steam methane reforming produces a high hydrogen yield, with an efficiency of approximately 65 to 75 per cent (source). In contrast, methane pyrolysis, used to produce turquoise hydrogen, is only around 58 per cent efficient. The hydrogen produced also has a high purity level, meaning fewer steps are needed to remove impurities. Natural gas is also widely available in many countries, such as the U.S. and Canada. 

The low cost associated with this technology can mean that more significant amounts of hydrogen can be produced and used in several applications. And these applications can reduce other environmental impacts. For example, gas cars emit carbon dioxide, methane, nitrous oxide, etc. Cheaper and more accessible hydrogen fuel sources can help speed up the production of hydrogen cars, and a switch to hydrogen electric cars can reduce overall greenhouse gas emissions. 

Steam methane reforming is more mature than other methods. For example, green hydrogen technologies are carbon neutral and use renewable energy, making them more sustainable. However, electrolysis is newer and less widely used and tested. It can also use high-cost materials or electricity sources. Therefore, wide-scale production of green hydrogen is currently more expensive and takes longer to produce than gray hydrogen.

One of the major drawbacks of gray hydrogen technology is that it uses fossil fuels and releases CO2 into the atmosphere. 

Fossil fuels are a finite resource, and using them in hydrogen production can increase the demand for natural gas. One of the ways to combat this is by using biomass. Biomass is a renewable organic resource, such as wood, crops, manure, etc. It is readily available in many parts of the world and can help offset the emissions used to produce hydrogen because biomass recycles CO2 (e.g., plants consume CO2 from the atmosphere, and plants are a type of biomass). This does not eliminate greenhouse gas emissions that occur with steam methane reforming but offers a more sustainable fuel source. 

One of the only ways to reduce emissions is to capture and store CO2, like with blue hydrogen technologies. Naturally, this would add costs to production, but the captured CO2 could be used for other industrial processes. 

Other issues with gray hydrogen technologies include high energy consumption, high production cost (despite being cheaper than other methods), and problems with catalysts. For example, the sulfur in specific fuel sources may poison catalysts used in the reaction. 

General hydrogen challenges also impact gray hydrogen production. This includes difficulties with storage. Hydrogen has a high energy content by weight but low energy content by volume, and it must be compressed and stored at low temperatures. Hydrogen is also highly combustible, so if a gas leak comes in contact with an ignition source, it can cause dangerous explosions. 

How is gray hydrogen used? 

There are vast applications for hydrogen, such as oil refining, methanol production, treating metals, hydrogen fuel cell cars (e.g., BMW’s hydrogen SUVs), rocket fuel, producing electricity, producing fertilizers, etc.  

Hydrogen produced by steam methane reforming can help further hydrogen applications by offering a cost-effective and well-developed alternative. Although gray hydrogen has environmental concerns, it has the potential to reduce overall CO2 emissions when compared to the burning of conventional fuels (e.g., gasoline-fueled cars). However, this can be reduced further by carbon capture and storage methods. That is why countries like South Korea favor moving away from gray hydrogen to focus on more sustainable hydrogen production. This was one of the issues touched on when Prime Minister Han Duck-soo announced new hydrogen policies designed to “kick the industry’s growth into high gear” at a meeting of the Hydrogen Economy commission (read more).