Hydrogen can be produced from a wide range of feedstocks, and any hydrogen-rich material can serve as a possible fuel source for fuel cells.
Hydrocarbon fuels, novel feedstocks such as landfill gas, anaerobic digester gas, and biomass can also produce hydrogen, as can compounds containing no carbon, such as ammonia or borohydride.
The vast majority of today’s hydrogen is produced via steam reformation of natural gas (roughly 48% globally), but alternative sources such as biogas are growing in popularity.
Hydrogen can be produced from a variety of sources:
- Traditional: natural gas, gasoline, diesel, propane
- Renewable/Alternative Fuels: methanol, ethanol, landfill gas, biogas, methane
- Water: using electrolysis, solar or wind power
- Innovative: sodium borohydride, algae, peanut shells
Gas Reforming
Hydrocarbon fuels – methanol, ethanol, natural gas, petroleum distillates, liquid propane, and gasified coal – can yield hydrogen in a process called reforming.
Natural gas, the feedstock of choice for most of today’s mass-produced hydrogen, contains methane (CH4) that can be used to produce hydrogen via a thermal process known as steam-methane reformation.
In steam-methane reforming, methane reacts with steam in the presence of a catalyst to produce hydrogen, carbon monoxide, and a relatively small amount of carbon dioxide. Steam reforming is endothermic, meaning heat must be supplied to the process for the reaction to proceed.
The process is approximately 72 percent efficient. This type of reforming works similarly for other hydrocarbon fuels, combining the fuel with steam by vaporizing them together at high temperatures.
Hydrogen is then separated out using membranes. Another type of reformer is the partial oxidation (POX) reformer. Some CO2 is emitted in the reforming process, but the emissions of NOX, SOX, particulates, and other smog producing agents are cut to zero.
Water Electrolysis
When an electric current is introduced to water (H2O), hydrogen and oxygen are separated, with hydrogen forming at the cathode and oxygen forming at the anode.
Electricity can be provided from any source, however, to minimize greenhouse gas emissions, electricity generation using renewable energy technologies, such as wind, solar, geothermal, and hydroelectric power, nuclear energy, or coal and natural gas with carbon sequestration are preferred.
Electricity can be provided from any source, but using solar and wind energy to electrolyze water provides the cleanest pathway to produce hydrogen.
This model is being used in some hydrogen refueling stations and in renewable energy storage systems that utilize hydrogen.
Gasification
Gasification is a process in which coal or biomass is converted into gaseous components by applying heat under pressure and in the presence of air/oxygen and steam.
A subsequent series of chemical reactions produces a synthesis gas, which is then reacted with steam to produce a gas stream with an increased hydrogen concentration that then can be separated and purified.
With carbon capture and storage, hydrogen can be produced directly from coal with near-zero greenhouse gas emissions.
Since growing biomass consumes CO2 from the atmosphere, producing hydrogen through biomass gasification releases near-zero net greenhouse gases.
Renewable Liquid Reforming
Biomass can also be processed to make renewable liquid fuels, such as ethanol or bio-oil, which are relatively convenient to transport and can be reacted with high-temperature steam to produce hydrogen at or near the point of use.
Researchers are also exploring a variation of this technology known as aqueous-phase reforming.
Nuclear High-Temperature Electrolysis
Heat from a nuclear reactor can be used to improve the efficiency of water electrolysis to produce hydrogen.
By increasing the temperature of the water, less electricity is required to split it into hydrogen and oxygen, which reduces the total energy required.
High-Temperature Thermochemical Water-Splitting
Another water-splitting method uses high temperatures generated by solar concentrators (mirrors that focus and intensify sunlight) or nuclear reactors to drive a series of chemical reactions to split water into hydrogen and oxygen through a series of chemical reactions.
All of the intermediate process chemicals are recycled within the process.
Enzymes
Another method to generate hydrogen is with bacteria and algae. Cyanobacteria, an abundant single-celled organism, produce hydrogen through its normal metabolic function.
Cyanobacteria can grow in the air or water, and contain enzymes that absorb sunlight for energy and split the molecules of water, thus producing hydrogen. Since cyanobacteria take water and synthesize it to hydrogen, the waste emitted is more water, which becomes food for the next metabolism. Sodium borohydride (NaBH4) is an inorganic compound that can dissolve in water in the absence of a base.
Hydrogen can be generated through catalytic decomposition.
Photoelectrochemical
Hydrogen can be produced directly from water using sunlight and a special class of semiconductor materials.
These highly specialized semiconductors absorb sunlight and use the light energy to completely separate water molecules into hydrogen and oxygen.
Doesn’t producing hydrogen require more energy than the energy subsequently released by the hydrogen?
When judging the efficiency of hydrogen sourced from natural gas, the entire production and consumption chain has to be factored into the equation.
A hydrogen fuel cell is highly efficient. This means that hydrogen matches or exceeds the energy efficiency of conventional transport fuels.
However, crude oil reserves are dwindling and will no longer be available in sufficient amounts to power vehicles for the next generation. And even though today’s combustion engines are becoming cleaner and more efficient, they continue to emit CO2 and other harmful substances such as nitrogen oxides and particulate matter. Hydrogen offers us a route to oil-free mobility, for generations to come. It will improve the quality of air in cities and play an important role in reducing and – ultimately – eliminating CO2 emissions.
Source: hfcarchive.org, eere.energy.gov