Hydrogen production technology

Published: August 24, 2008, 4:40 pm

Updated: October 9, 2011, 9:14 pm

Source: DOE

Topics:

Energy (main) Physics & Chemistry (main) Chemical Engineering (main)

Introduction Hydrogen (Hydrogen production technology) can be produced using diverse feedstocks including fossil fuels, such as natural gas and coal (with carbon sequestration); nuclear power; and biomass and other renewable energy technologies, such as wind, solar, geothermal, and hydro-electric power. The overall challenge to hydrogen production is cost reduction. For transportation, hydrogen must be cost-competitive with conventional fuels and technologies on a per-mile basis in order to succeed in the commercial marketplace.

Thermal Processes

Some thermal processes use the energy in various resources, such as natural gas, coal, or biomass, to release hydrogen, which is part of their molecular structure. In other processes, heat, in combination with closed chemical cycles, produces hydrogen from feedstocks such as water—these are known as "thermochemical" processes.

Reforming of Natural Gas

Most hydrogen currently is produced from fossil materials, such as from natural gas at this oil refinery. Photo: DOE

Distributed natural gas reforming is an important pathway for near-term hydrogen production during the transition to a hydrogen economy. Natural gas contains methane (CH₄) that can be used to produce hydrogen via thermal processes, such as steam methane reformation and partial oxidation.

Steam Methane Reforming

About 95% of the hydrogen produced today in the United States is made via steam methane reforming, a process in which high-temperature steam (700 - 1000°C) is used to produce hydrogen from a methane source, such as natural gas. In steam methane reforming, methane reacts with steam under 3-25 bar pressure (1 bar = 14.5psi) in the presence of a catalyst to produce hydrogen, carbon monoxide, and a relatively small amount of carbon dioxide. Steam reforming is endothermic—that is, heat must be supplied to the process for the reaction to proceed.

Subsequently, in what is called the "water-gas shift reaction," the carbon monoxide and steam are reacted using a catalyst to produce carbon dioxide and more hydrogen. In a final process step called "pressure-swing adsorption," carbon dioxide and other impurities are removed from the gas stream, leaving essentially pure hydrogen. Steam reforming can also be used to produce hydrogen from other fuels, such as ethanol, propane, or even gasoline.

Steam Reforming Reactions
Methane:
CH₄ + H₂O (+heat) → CO + 3H₂

Propane:
C₃H₈ + 3H₂O (+heat) → 3CO + 7H₂

Ethanol:
C₂H₅OH + H₂O (+heat) → 2CO + 4H₂

Gasoline (using iso-octane and toluene as example compounds from the hundred or more compounds present in gasoline):
C₈H₁₈ + 8H₂O (+heat) → 8CO + 17H₂
C₇H₈ + 7H₂O (+heat) → 7CO + 11H₂

Water-Gas Shift Reaction
CO + H₂O → CO₂ + H₂ (+small amount of heat)

Partial Oxidation

In partial oxidation, the methane and other hydrocarbons in natural gas are reacted with a limited amount of oxygen (typically, from air) that is not enough to completely oxidize the hydrocarbons to carbon dioxide and water. With less than the stoichiometric amount of oxygen available for the reaction, the reaction products contain primarily hydrogen and carbon monoxide (and nitrogen, if the reaction is carried out with air rather than pure oxygen), and a relatively small amount of carbon dioxide and other compounds. Subsequently, in a water-gas shift reaction, the carbon monoxide reacts with water to form carbon dioxide and more hydrogen.

Partial oxidation is an exothermic process—it gives off heat. It is, typically, a much faster process than steam reforming and requires a smaller reactor vessel. As can be seen from the chemical reactions of partial oxidation (below), this process initially produces less hydrogen per unit of the input fuel than is obtained by steam reforming of the same fuel.

Partial Oxidation Reactions
Methane:
CH₄ + ½O₂ → CO + 2H₂ (+heat)

Propane:
C₃H₈ + 1.5O₂ → 3CO + 4H₂ (+heat)

Ethanol:
C₂H₅OH + ½O₂ → 2CO + 3H₂ (+heat)

Gasoline (using iso-octane and toluene as example compounds from the hundred or more compounds present in gasoline):
C₈H₁₈ + 4O₂ → 8CO + 9H₂ (+heat)
C₇H₈ + 3.5O₂ → 7CO + 4H₂ (+heat)

Water-Gas Shift Reaction
CO + H₂O → CO₂ + H₂ (+small amount of heat)

Gasification of Coal

Coal is converted into a gaseous mixture of hydrogen, carbon monoxide, carbon dioxide, and other compounds by applying heat under pressure in the presence of steam and a controlled amount of oxygen (in a unit called a gasifier). The coal is chemically broken apart by the gasifier's heat, steam, and oxygen, setting into motion chemical reactions that produce a synthesis gas, or "syngas"—a mixture of primarily hydrogen, carbon monoxide, and carbon dioxide. The carbon monoxide is reacted (in a separate unit) with water to form carbon dioxide and more hydrogen. Adsorbers or special membranes can separate the hydrogen from this gas stream.

Chemically, coal is a complex and highly variable substance. The carbon and hydrogen in coal may be represented in approximate manner as 0.8 atoms of hydrogen per atom of carbon in bituminous coal. Its gasification reaction may be represented by the (unbalanced) reaction equation:

CH_0.8 + O₂ + H₂O → CO + CO₂ + H₂ + other species

An advantage of this technology is that carbon dioxide can be separated more easily from the syngas and captured, instead of being released into the atmosphere. If carbon dioxide can be successfully sequestered, hydrogen can be produced from coal gasification with near-zero greenhouse gas emissions.

Coal gasification can also be used to produce electricity by routing the syngas to a turbine to generate electricity. Coal gasification technology could be used to generate both electricity and hydrogen in one integrated plant operation. The U.S. Department of Energy's (DOE's) FutureGen is a $1 billion, 10-year initiative to demonstrate the world's first coal-based, zero-emissions power plant.

Coal gasification technology is most appropriate for large-scale, centralized hydrogen production. This is due to the nature of handling large amounts of coal and the carbon capture and sequestration technologies that must accompany the process.

Gasification of Biomass

Hydrogen can be manufactured from biomass, such as this switchgrass, via pyrolysis and gasification.

Biomass, a renewable organic resource, includes: agriculture crop residues, such as corn stover or wheat straw; forest residues; special crops grown specifically for energy use, such as switchgrass or willow trees; organic municipal solid waste; and animal wastes.

Biomass is converted into a gaseous mixture of hydrogen, carbon monoxide, carbon dioxide, and other compounds by applying heat under pressure in the presence of steam and a controlled amount of oxygen (in a unit called a gasifier). The biomass is chemically broken apart by the gasifier's heat, steam, and oxygen, setting into motion chemical reactions that produce a synthesis gas, or "syngas"—a mixture of primarily hydrogen, carbon monoxide, and carbon dioxide. The carbon monoxide is then reacted with water to form carbon dioxide and more hydrogen (water-gas shift reaction). Adsorbers or special membranes can separate the hydrogen from this gas stream.

Simplified Example Reaction
C₆H₁₂O₆ + O₂ + H₂O → CO + CO₂ + H₂ + other species
Note: The above reaction uses glucose as a surrogate for cellulose. Actual biomass has highly variable composition and complexity, with cellulose as one major component.

Water-Gas Shift Reaction
CO + H₂O → CO₂ + H₂ (+small amount of heat)

Pyrolysis is the gasification of biomass in the absence of oxygen. In general, biomass does not gasify as easily as coal, and it produces other hydrocarbon compounds in the gas mixture exiting the gasifier; this is especially true when no oxygen is used. As a result, typically an extra step must be taken to reform these hydrocarbons with a catalyst to yield a clean syngas mixture of hydrogen, carbon monoxide, and carbon dioxide. Then, just as in the gasification process for hydrogen production, a shift reaction step (with steam) converts the carbon monoxide to carbon dioxide. The hydrogen produced is then separated and purified.

Biomass gasification technology is most appropriate for large-scale, centralized hydrogen production, due to the nature of handling large amounts of biomass and the required economy of scale for this type of process.

Reforming of Renewable Liquid Fuels

Biomass resources can be converted to ethanol, bio-oils, biodiesel or other liquid fuels that can be transported at relatively low cost to a refueling station or other point-of-use and reformed to produce hydrogen.

Reforming renewable liquids to hydrogen is very similar to reforming natural gas.

• The liquid fuel is reacted with steam at high temperatures in the presence of a catalyst to produce a reformate gas composed mostly of hydrogen and carbon monoxide.
• Additional hydrogen and carbon dioxide are produced by reacting the carbon monoxide (created in the first step) with high temperature steam in the "water-gas shift reaction."
• Finally, the hydrogen is separated out and purified.
• Steam Reforming Reaction (Ethanol): C₂H₅OH + H₂O (+heat) → 2CO + 4H₂
• Water-Gas Shift Reaction: CO + H₂O → CO₂ + H₂ (+small amount of heat)

Biomass-derived liquids, such as ethanol, biodiesel and bio-oils, can be produced at large, central facilities located near the biomass source to take advantage of economies of scale and reduce the cost of transporting the solid biomass feedstock. The liquids have a high energy density and can be transported with minimal new delivery infrastructure and at relatively low cost to distributed refueling stations or stationary power sites for reforming to hydrogen.

High-temperature Water Splitting

High-temperature water splitting (a "thermochemical" process) is a long-term technology in the early stages of development. High-temperature heat (500 - 2000°C) drives a series of chemical reactions that produce hydrogen. The chemicals used in the process are reused within each cycle, creating a closed loop that consumes only water and produces hydrogen and oxygen. The high-temperature heat needed can be supplied by next-generation nuclear reactors under development (up to about 1000°C) or by using sunlight with solar concentrators (up to about 2000°C).

Researchers have identified cycles appropriate to specific temperature ranges and are examining these systems in the laboratory. The more than 200 possible cycles identified have been screened and down-selected to about twelve for initial research. High temperature water splitting is most suitable for large-scale, centralized production of hydrogen, although semi-central production from solar driven cycles might be possible.

High-temperature Water Splitting Using Solar Concentrators: A solar concentrator uses mirrors and a reflective or refractive lens to capture and focus sunlight to produce temperatures up to 2,000°C. This high temperature heat can be used to drive chemical reactions that produce hydrogen.

Chemical cycle example: zinc/zinc oxide cycle: Zinc oxide powder is passed through a reactor heated by a solar concentrator operating at about 1,900°C. At this temperature, the zinc oxide dissociates to zinc and oxygen gases. The zinc is cooled, separated, and reacted with water to form hydrogen gas and solid zinc oxide. The net result is hydrogen and oxygen, produced from water. The hydrogen can be separated and purified. The zinc oxide can be recycled and reused to create more hydrogen through this process.

2ZnO + heat → 2Zn + O₂
2Zn + 2H₂O → 2ZnO + 2H₂

High-temperature Water Splitting Using Nuclear Energy: Similar to a solar concentrator, a nuclear reactor produces energy as high-temperature heat that can be used to drive high-temperature thermochemical water splitting cycles. The next-generation nuclear reactors under development could generate temperatures of 800 to 1,000°C — these temperatures are much lower than those produced by a solar concentrator. A thermochemical process based on nuclear heat would use a different set of chemical reactions to produce hydrogen.

Chemical cycle example: sulfur-iodine cycle: Sulfuric acid, when heated to about 850°C, decomposes to water, oxygen, and sulfur dioxide. The oxygen is removed, the sulfur dioxide and water are cooled, and the sulfur dioxide reacts with water and iodine to form sulfuric acid and hydrogen iodide. The sulfuric acid is separated and removed, and the hydrogen iodide is heated to 300°C, where it breaks down into hydrogen and iodine. The net result is hydrogen and oxygen, produced from water. The hydrogen can be separated and purified. The sulfuric acid and iodine are recycled and used to repeat the process.

2H₂SO₄ +heat at 850°C → 2H₂O + 2SO₂ + O₂
4H₂O + 2SO₂ + 2I₂ → 2H₂SO₄ + 4HI
4HI + heat at 300°C → 2I₂ + 2H₂

Electrolytic Processes

Electricity is used to disassociate water into hydrogen and oxygen. Photo courtesy of the Schatz Energy Research Center, Humboldt State University.

Electrolysis is the process of using electricity to split water into hydrogen and oxygen. This reaction takes place in a unit called an electrolyzer. Electrolyzers can be small, appliance-size equipment and well-suited for small-scale distributed hydrogen production. Research is also underway to examine larger-scale electrolysis that could be tied directly to renewable or other non-greenhouse gas-emitting electricity production. Hydrogen production at a wind farm generating electricity is an example of this.

Hydrogen produced via electrolysis can result in zero greenhouse gas (GHG) emissions, depending on the source of the electricity used. The source of the required electricity—including its cost and efficiency, as well as emissions resulting from electricity generation—must be considered when evaluating the benefits of hydrogen production via electrolysis. In many regions of the country, today's power grid is not ideal for providing the electricity required for electrolysis because of the greenhouse gases released and the amount of energy required to generate electricity. Hydrogen production via electrolysis is being pursued for renewable (wind) and nuclear options. These pathways result in virtually zero GHG emissions and criteria pollutants.

Like fuel cells, electrolyzers consist of an anode and a cathode separated by an electrolyte. Different electrolyzers function in slightly different ways.

PEM Electrolyzer

In a polymer electrolyte membrane (PEM) electrolyzer, the electrolyte is a solid specialty plastic material.

• Water reacts at the anode to form oxygen and positively charged hydrogen ions (protons).
• The electrons flow through an external circuit and the hydrogen ions selectively move across the PEM to the cathode.
• At the cathode, hydrogen ions combine with electrons from the external circuit to form hydrogen gas.

Anode Reaction: 2H₂O → O₂ + 4H⁺ + 4e^-
Cathode Reaction: 4H⁺ + 4e^- → 2H₂

Alkaline Electrolyzers

Alkaline electrolyzers are similar to PEM electrolyzers but use an alkaline solution (of sodium or potassium hydroxide) that acts as the electrolyte. These electrolyzers have been commercially available for many years.

Solid Oxide Electrolyzers

Solid oxide electrolyzers, which use a solid ceramic material as the electrolyte that selectively transmits negatively charged oxygen ions at elevated temperatures, generate hydrogen in a slightly different way:

• Water at the cathode combines with electrons from the external circuit to form hydrogen gas and negatively charged oxygen ions;
• The oxygen ions pass through the membrane and react at the anode to form oxygen gas and give up the electrons to the external circuit.

Solid oxide electrolyzers must operate at temperatures high enough for the solid oxide membranes to function properly (about 500 - 800°C; compared to PEM electrolyzers, which operate at 80 - 100°C, and alkaline electrolyzers, which operate at 100-150°C). The solid oxide electrolyzers can effectively use heat available at these elevated temperatures (from various sources, including nuclear energy) to decrease the amount of electrical energy needed to produce hydrogen from water.

Photolytic Processes

Photobiological Water Splitting

In this process, hydrogen is produced from water using sunlight and specialized microorganisms, such as green algae and cyanobacteria. Just as plants produce oxygen during photosynthesis, these microorganisms consume water and produce hydrogen as a byproduct of their natural metabolic processes. Photobiological water splitting is a long-term technology. Currently, the microbes split water much too slowly to be used for efficient, commercial hydrogen production. But scientists are researching ways to modify the microorganisms and to identify other naturally occurring microbes that can produce hydrogen at higher rates. Photobiological water splitting is in the very early stages of research, but offers long-term potential for sustainable hydrogen production with low environmental impact.

Photoelectrochemical Water Splitting

In this process, hydrogen is produced from water using sunlight and specialized semiconductors called photoelectrochemical materials. In the photoelectrochemical system, the semiconductor uses light energy to directly dissociate water molecules into hydrogen and oxygen. Different semiconductor materials work at particular wavelengths of light and energies. Research focuses on finding semiconductors with the correct energies to split water that are also stable when in contact with water. Photobiological water splitting is in the very early stages of research, but offers long-term potential for sustainable hydrogen production with low environmental impact.

Further reading

Hydrogen Production and Delivery, The National Renewable Energy Laboratory (NREL).

Disclaimer: This article contains information that was originally published by the Department of Energy. Topic editors and authors for the Encyclopedia of Earth have edited its content and added new information. The use of information from the Department of Energy should not be construed as support for or endorsement by that organization for any new information added by EoE personnel, or for any editing of the original content.

Citation

(2011). Hydrogen production technology. Retrieved from http://editors.eol.org/eoearth/wiki/Hydrogen_production_technology