Hydrogen is the most abundant chemical element in the universe. However, it is rarely found on its own and must be separated from compound forms to be used in fuel. There are many methods to produce hydrogen fuel. This article examines some of the most common hydrogen fuel technologies using fossil fuels and renewable resources.
Fossil fuels vs. renewable sources
The majority of hydrogen produced today comes from fossil fuels. However, using low-carbon, renewable alternatives can make the process more eco-friendly. The main issue with this is the lower cost for production and storage, but cleaner options that minimize environmental impact are available at a higher cost. Learn more about the advantages and disadvantages of different sources and technologies for hydrogen fuel production below.
Fossil fuels hydrogen production
Fossil fuels are non-renewable resources made from hydrocarbons. Examples include coal, natural gas, petroleum, crude oil, etc. Methods for producing hydrogen with fossil fuels include:
- Steam-methane reforming
- Methane pyrolysis
- Coal gasification
- Partial oxidation
Steam methane reforming
Steam methane reforming is one of the most popular hydrogen production methods, accounting for approximately 80 per cent of hydrogen production in the United States (according to the 2021 Global Hydrogen Review).
Steam methane reforming produces hydrogen using natural gas as the primary fuel source but can be done with other fuels such as gasoline, propane, or ethanol.
The process occurs in three stages. The first stage involves high-temperature steam (1292 to 1832 F or 700 to 1,000 C) reacting with methane in the presence of a catalyst. This produces carbon monoxide, hydrogen, and carbon dioxide. In the next stage, there is a “water-gas shift reaction,” meaning that the carbon monoxide reacts with the steam to produce more hydrogen and carbon dioxide. In the final stage, called the “pressure-swing adsorption,” carbon dioxide and other impurities are removed from the gas stream, leaving behind pure hydrogen.
Advantages of steam methane reforming:
Here are the advantages to steam methane reforming:
- High hydrogen yield
- Creates hydrogen with a high level of purity
- Cheapest source of industrial hydrogen
- Natural gas is widely available in the U.S and Canada
- Reduces CO2 emissions (when compared to the burning of conventional fuels. This can be further reduced by carbon capture and storage methods).
- The efficiency of the process is approximately 65 to 75 per cent (source)
Disadvantages of steam methane reforming
Here are the disadvantages of steam methane reforming:
- Uses fossil fuels
- Emits CO2 (more than twice the amount of hydrogen produced)
- High energy consumption
- High production cost (despite being cheaper than other methods)
- Can contribute to increased demand for natural gas
- Sulfur in the fuel sources may poison certain catalysts used in the reaction
- Coking may also impact catalysts used in the reaction
Methane pyrolysis
Methane pyrolysis is a newer process using thermal decomposition to split methane (CH4) directly into its two components – hydrogen and solid carbon. It is a one-step endothermic process that breaks down methane by exposing it to high temperatures. Methane enters a chamber and is heated to temperatures around 1065 °C (1949 °F), splitting the methane molecule into gaseous hydrogen and solid carbon. Further processes may be needed to purify or upgrade the hydrogen produced.
Advantages of methane pyrolysis
Here are the advantages of methane pyrolysis
- Does not produce CO2 (when using molten metal catalyst)
- Eliminates the need for removal or storage of CO2
- The solid carbon byproduct can be sold
- Can reduce overall greenhouse gas emissions caused by hydrogen production
- – Produces a high volume of hydrogen (albeit less than steam reforming)
Disadvantages of methane pyrolysis
Here are the disadvantages of methane pyrolysis:
- Uses fossil fuels
- Newer process and more research is needed to address current issues
- High process temperatures
- Higher cost compared to other methods
- Less efficient than other methods (e.g., approximately 58 per cent efficiency for methane pyrolysis vs. 65 to 75 per cent for steam methane reformation)
- There is not a large market for solid carbon currently, so storage or disposal may be needed
- Additional separation and purification processes are needed (which contribute to a higher cost)
Coal gasification
Coal consists of carbon, nitrogen, oxygen, hydrogen, and sulphur. This process separates hydrogen from the carbon-based matter in coal.
Coal gasification begins with coal being partially burned in the presence of a catalyst. This creates reactions needed to produce carbon dioxide, which reacts with the coal to form carbon monoxide. Next, the carbon monoxide goes through a “water-gas shift.” This is where it reacts with steam to produce hydrogen. The hydrogen is then purified to eliminate carbon monoxide and other impurities. Gasification can also produce hydrogen from carbon sources, such as petroleum coke.
Advantages of coal gasification
Here are the advantages of coal gasification
- Filters out impurities (e.g., sulfur) from coal, which can be sold commercially
- Allow for easier capture and storage of CO2 produced
- Efficient process
Disadvantages of coal gasification
Here are the disadvantages of coal gasification
- Emits CO2 and other greenhouse gases
- Increased use of coal reserves may lead to the return of coal mining (which is a relatively dangerous process)
- Higher cost than coal combustion plants
Partial oxidation
Partial oxidation is an exothermic reaction that uses natural gas. The hydrocarbon in natural gas reacts with oxygen (often from the air), primarily producing hydrogen and carbon monoxide. The carbon monoxide goes through a water-gas shift reaction where it reacts with water to create carbon dioxide and hydrogen.
Advantages of partial oxidation
Here are the advantages of partial oxidation
- Typically faster than steam reforming
- Requires a smaller reactor vessel
- High efficiency (around 70 to 80 per cent – source)
Disadvantages of partial oxidation
Here are the disadvantages of partial oxidation
- Uses fossil fuels
- Lower hydrogen yield
- Produces CO and CO2
- Typically less efficient than steam reforming
Renewable process for hydrogen production
Most hydrogen is produced using fossil fuels resulting in the emission of carbon dioxide and other greenhouse gases. However, hydrogen can also be created using renewable sources such as solar or wind. This can make the process more eco-friendly and help push towards more sustainable fuels. Common renewable hydrogen methods include:
- Electrolysis
- Solar water splitting practices
- Biological/microbial practices
Electrolysis
Electrolysis involves the separation separating hydrogen and oxygen from water molecules. The process takes place in an electrolyzer that operates on electricity.
An electrolyzer has an anode (where electricity moves into), a cathode (where electricity flows out of), and an electrolyte (substance that separates into ions when dissolved in water). An electric current passes through the water, separating hydrogen and water molecules.
Advantages of electrolysis
Advantages of electrolysis include:
- Does not typically use fossil fuels
- Does not emit greenhouse gases
- Low carbon process if renewable electricity sources are used
- Water is easily accessible in many places
Disadvantages of electrolysis
Disadvantages of electrolysis include:
- Often, a high-energy process (especially in methods like SOFC, which require high temperatures)
- Not as widely used and tested
- High carbon process if electricity is sourced from fossil fuels
- May be costly depending on technologies, materials, electricity source
There are three main types of electrolysis technologies.
- Polymer electrolyte or proton exchange membrane (PEM) electrolysis – Uses a solid plastic material as the electrolyte.
- Alkaline electrolysis– Uses a liquid alkaline solution (e.g., sodium hydroxide) as the electrolyte.
- Solid oxide fuel cells (SOFC)/Solid oxide electrolyte cells (SOEC) electrolysis – Uses a ceramic material as the electrolyte.
Another electrolysis technology is anion exchange membrane water (AEM) electrolysis. Electrolytes use low-cost catalyst materials used in alkaline electrolysis and polymer materials used in PEM. This process is cost-effective and has a low interaction with CO2, which improves performance. However, it is still in the early stages of development.
PEM
Water reacts with the anode and splits into oxygen, protons (hydrogen ions), and electrons. The electrons move through the circuit, and the protons move through the membrane towards the cathode. Finally, the protons and electrons combine at the cathode to produce hydrogen.
This technology produces high-purity hydrogen and has high efficiency. However, it uses high-cost catalysts and expensive materials. It is also not as established as other methods.
Alkaline electrolysis
A current is applied to the anode, cathode, and membrane (“cell stack”). The hydrogen ions protons (hydrogen ions) move through the electrolyte from the cathode to the anode. The reaction generates hydrogen gas bubbles on the cathode side and oxygen gas on the anode side.
This technology is well established, cost-effective (in terms of materials), and operates at lower temperatures. However, some drawbacks include the low current density, low purity, and corrosive properties of the electrolyte (which can minimize the lifespan of the electrolyzer).
SOFC/SOEC
Steam is applied to the cathode. It combines with electrons from the external circuit to create hydrogen gas and negatively charged oxygen ions. The oxygen passes through the electrolyzer and reacts with the anode to produce oxygen gas and electrons for the external circuit.
One of the main drawbacks of the process is that it must operate at high temperatures (over 500 °C/932 °F) for the electrolyzer to operate correctly, making it energy intensive, which may be more expensive. It is also newer and less established. However, the process is very efficient and creates high hydrogen purity.
Solar water splitting processes
These processes use solar energy to split water into hydrogen and oxygen. Examples include:
- Photoelectrochemical water splitting– Uses light energy and semiconductors to separate water into hydrogen and oxygen.
- Photobiological water splitting – Uses the photosynthesis of bacteria and green algae to split water into hydrogen and oxygen.
- Solar thermochemical water splitting – Uses high heat concentrated from solar power systems and chemical reactions to split water into hydrogen and oxygen.
- Photocatalytic water splitting – This artificial photosynthesis process uses light energy to split water into hydrogen and oxygen.
Advantages of solar water splitting processes
Advantages of solar water splitting processes include:
- Uses renewable energy
- Minimal to no greenhouse gas emissions
- Fairly simple process and set-up
Disadvantages of solar water splitting processes
Disadvantages of solar water splitting processes include:
- Low hydrogen yield
- Still in the early stages of development
Biological/microbial processes
These technologies use biological or microbial processes or reactions to produce hydrogen. Examples include:
- Biomass gasification – Similar to coal gasification, but uses biomass as the fuel source. Biomass is a renewable organic resource, such as wood, crops, manure, etc., readily available in the United States and many parts of the world. Using biomass can also cause less greenhouse gas emissions because biomass recycles carbon dioxide (e.g., plants consume CO2 from the atmosphere, and plants are a type of biomass), which can offset the emissions used to produce hydrogen.
- Microbial biomass conversion/fermentation – Microbes break down organic matter to produce hydrogen or byproducts that can be combined with enzymes to produce hydrogen.
- Photobiological water splitting (discussed above).
Advantages of biological/microbial processes
Advantages of biological/microbial processes include:
- Uses organic sources
- Biomass is an abundant resource
- Minimal to no carbon emissions
- Algae and microbes can be grown in water not suitable for personal or agricultural uses
Disadvantages of biological/microbial processes
Disadvantages of biological/microbial processes include:
- Technologies are very new and in early stages of development
- Low hydrogen yield
More renewable hydrogen production technologies
Other types of renewable hydrogen production technologies include:
- Chemically assisted electrolysis – Make use of the oxygen produced by introducing fuel into the oxygen side of the reactor. This can reduce the required energy for the system and the overall cost of hydrogen.
- Radiolysis – Separating water molecules by exposing them to ionizing nuclear radiation. This produces a low but not insignificant yield of hydrogen. However, a 1976 study deems a facility of this nature would not be feasible due to safety concerns (source).
- Thermolysis – Separating water by heating it to the temperatures required to dissociate (around 2500 °C/4532 °F. This is not the most feasible option since energy would be needed to reach such high temperatures. Similarly, most hydrogen-producing equipment is not made to withstand these temperatures.
- Acid-alkaline amphoteric water electrolysis – Hydrogen is produced in an acidic solution, and oxygen in an alkaline solution. It also has an electrolyzer that stops neutralization. This uses less energy and produces four times more hydrogen than alkaline electrolysis. It is a promising method, but more research and development are needed.
Different types of hydrogen by color code
Hydrogen is a colorless element. However, it is classified into different color groups depending on the source and process in which hydrogen is produced. This system is discussed below.
What is Green Hydrogen?
Green hydrogen uses renewable energy to create hydrogen through electrolysis. No CO2 is emitted throughout the production process. This is considered carbon neutral since no CO2 is produced or emitted.
What is Blue Hydrogen?
Blue hydrogen is produced using fossil fuels (e.g., steam methane reforming). It results in the splitting of hydrogen and CO2. The CO2 is not released into the atmosphere but is stored underground for industrial use. Therefore, blue hydrogen is low-carbon since there are no CO2 emissions.
What is Gray Hydrogen?
Gray hydrogen is similar to blue hydrogen, except the CO2 created during the process is released into the atmosphere. As a result, these methods emit greenhouse gases and cause more significant environmental impacts.
What is Yellow Hydrogen?
Yellow hydrogen refers to electrolysis methods that use solar energy to produce hydrogen (e.g., photoelectrochemical water splitting). It may also refer to techniques that use a mix of renewable and fossil fuels. These methods would be considered low carbon.
What is Purple/Pink Hydrogen?
This refers to electrolysis methods that use nuclear energy/electricity to produce hydrogen. These methods are also low-carbon.
What is Brown/Black Hydrogen?
Black/brown hydrogen is produced from coal (e.g., coal gasification). This is characterized as being one of the most damaging hydrogen production methods.
What is Turquoise Hydrogen?
Turquoise hydrogen is created using methane pyrolysis. No CO2 is produced during this process.
What is White Hydrogen
White hydrogen is derived form natural sources in both the earth and from the air. Read more about white hydrogen and its uses and methods used to extract it.