Hydrogen is being produced by manufacturers throughout the world through electrolysis, steam methane reforming, and gasification. After these production techniques, hydrogen is utilized in what is now called the “hydrogen economy” as a tool to help lower carbon emissions since it releases no greenhouse gas emissions at usage. The produced hydrogen can be used for electricity and heating, fuel cells, energy storage, etc. The “green” factor of hydrogen, or the level of carbon emissions associated with hydrogen, depends on the production method utilized to produce the hydrogen. As illustrated in figure 1, green and blue hydrogen produced via electrolysis, steam methane reforming, and gasification produce the lowest carbon emission intensive hydrogen. For blue hydrogen, the hydrogen is decarbonized through carbon capture and storage which does not release carbon emissions.
Figure 1: The different methods of hydrogen production and color-coding system (Hydrogen Power Partners)
Using water and electricity, electrolyzers produce hydrogen through splitting the water molecule into hydrogen and oxygen by utilizing the power current of electricity. Currently, there are three different types of electrolyzer technologies: Polymer Electrolyte Membrane (PEM), Alkaline, and Solid Oxide. The most used technology is the PEM electrolyzer shown in figure 2 below. During the PEM process, there is an anode and cathode reaction that takes place. The splitting of water occurs at the anode where it reacts to form oxygen and protons, these produced protons (or hydrogen atoms) flow towards the cathode to combine with electrons, resulting in hydrogen gas. The description is illustrated in figure 3, showing the flow of atoms and the chemical reactions taking place during electrolysis.
In addition to the chemical reactions taking place, the electricity source utilized during electrolysis plays a key role in the hydrogen’s classification. The electricity used in electrolysis processes can be produced from renewable energy sources like solar or wind, nuclear power, or fossil fuels like oil and gas. When the electricity used is from renewable energy or nuclear, the resulting hydrogen is “green” and zero-carbon. However, when the electricity used is carbon intensive, the hydrogen produced is not considered zero-carbon, due to the higher carbon emissions associated with the process.
Figure 2: Green hydrogen production method of Polymer Electrolyte Membrane Electrolyzer (Mancera et al)
Figure 3: The anode and cathode reaction that takes place in a PEM electroylzer (U.S. Department of Energy)
2. Steam Methane Reforming
The most common method of hydrogen production today, steam methane reforming, uses natural gas as an input with the process outlined in figure 4. Next, the methane from the natural gas is heated to chemically react with steam and produce hydrogen, carbon monoxide, and carbon dioxide (DoE). Once the methane and steam react, a “water gas shift” reaction occurs during which a catalyst creates an additional reaction with the carbon monoxide and steam producing more hydrogen and carbon dioxide. Lastly, “pressure swing absorption” is performed to purify the hydrogen by removing impurities like carbon dioxide (Student Energy). This process has the potential to be categorized as producing decarbonized or “blue hydrogen” if carbon capture exists as shown in process flow diagram in figure 4.
Figure 4: Process flow diagram of steam methane reforming (Lyndon Energy)
The gasification process uses coal as a feedstock to react with steam in a gasifier to create synthesis gas containing carbon monoxide, carbon dioxide and hydrogen. Once this synthesis gas is created, it is purified and a water gas shift reaction, similar to the steam methane reforming process, occurs to enable hydrogen separation. The gasification process is shown in figure 5. In addition to coal, the gasification process can use biomass as an input and is called pyrolysis. To decarbonize the gasification process, carbon capture can be used since the carbon emissions from gasification are two to three times higher in comparison to steam methane reforming (Matzen et al).
Figure 5: Process flow diagram of gasification (Matzen et al)
Once hydrogen is produced, it further progresses into the hydrogen economy (pictured in figure 6) to be consumed in various forms like fuel cells, grid injection, and industrial applications. All the listed uses of hydrogen have a commonality of reducing the carbon emissions of a current process by using green hydrogen which is zero-carbon. For example, when fuel cells are utilized in vehicles, the vehicle no longer releases the emissions associated with burning gasoline. Hydrogen fuel cells are more effective than battery powered vehicles due to the longer mileage range so there is more of an opportunity for long-haul transport. Additionally, when hydrogen is injected into the natural gas grid, it is used for electricity, heating, and cooking. Lastly, when using hydrogen for industry applications it can be used for “refining petroleum, treating metals, producing fertilizer and other chemicals, and processing foods” (EIA). While hydrogen can be used immediately following production, it can also be stored in “salt caverns, lined hard rock caverns, depleted oil and natural gas fields, and aquifers” for later use (EIA).
As hydrogen research continues, new technologies may emerge for additional hydrogen usage to lower carbon emissions and meet the net-zero emissions goal by 2050. Today, steam methane reforming produces 95% of hydrogen with a growing emphasis on electrolysis for zero-carbon hydrogen (DoE). To then use the hydrogen, governments in Europe are beginning to inject hydrogen into the natural gas grid and many fuel cell technologies are emerging for vehicles and power plants. The figure below shows how Sensus gas products fit into the hydrogen economy.
Figure 7: Hydrogen economy using Sensus industrial turbo, residential and commercial meters