Hydrogen purification is any technology used to purify hydrogen. The impurities in hydrogen gas depend on the source of the H2, e.g., petroleum, coal, electrolysis, etc. The required purity is determined by the application of the hydrogen gas. For example, ultra-high purified hydrogen is needed for applications like proton exchange membrane fuel cells. [1]
The default large-scale purification of H2 produced in oil refineries exploits its very low boiling point of −253 °C. Most impurities have boiling points well above this temperature. Low temperature methods can be complemented by scrubbing to remove particular impurities. [1]
Hydrogen can be purified by passing through a membrane composed of palladium and silver. This alloy with a ca. 3:1 ratio for Pd:Ag is more structural robust than pure Pd, which is the active component that allows the selective diffusion of H2 through it. Diffusion is faster near 300 °C. This method has been used for purification of hydrogen on a laboratory scale, but not in industry. Silver-palladium membranes are unstable toward alkenes and sulfur-containing compounds. [1]
Dense thin-metal membrane purifiers are compact, relatively inexpensive and simple to use. [2] [3] [4]
Pressure swing adsorption is used for the removal of carbon dioxide (CO2) as the final step in the large-scale commercial synthesis of hydrogen. It can also remove methane, carbon monoxide, nitrogen, moisture and in some cases, argon, from hydrogen.
Hydrogen purifiers are used in metalorganic vapour phase epitaxy reactors for LED production. [5]
Fuel cell electric vehicles commonly use polymer electrolyte membrane fuel cells (PEMFC) that are susceptible to a range of impurities. Impurities impact PEMFC using a range of mechanisms, these may include poisoning the anode hydrogen oxidation reaction catalysts, reducing the ionic conductivity of the ionomer and membrane, altering wetting behaviour of components or blocking porosity in diffusion media. The impact of some impurities like carbon monoxide, formic acid, or formaldehyde is reversible with PEMFC performance recovering once the supply of impurity is removed. Other impurities, for example sulphurous compounds, may cause irreversible degradation. [6] The permissible limits of hydrogen impurities are shown below.
Maximum Permissible Concentration / μmol mol−1 | |
---|---|
Total non-hydrogen gasses | 300 |
Water | 5 |
Total Hydrocarbons Except Methane [Carbon atom basis] | 2 |
Methane | 100 |
Oxygen | 5 |
Helium | 300 |
Nitrogen | 300 |
Argon | 300 |
Carbon Dioxide | 2 |
Carbon Monoxide | 0.2 |
Total Sulphur Compounds [Sulphur atom basis] | 0.004 |
Formaldehyde | 0.2 |
Formic Acid | 0.2 |
Ammonia | 0.1 |
Halogenated Compounds [Halogen ion basis] | 0.05 |
Maximum Particulate Concentration | 1 mg kg−1 |
Efforts to assess the compliance of hydrogen supplied by hydrogen refuelling stations against the ISO-14687 standard have been performed. [8] [9] [10] While the hydrogen was generally found to be 'good' [8] violations of the standard have been reported, most frequently for nitrogen, water and oxygen.
Combustion applications are generally more tolerant of hydrogen impurities than PEFMC, as such the ISO-14687 standard for permissible impurities is less strict. [11] This standard has itself been criticised with revisions proposed to make it more lenient and therefore suitable for hydrogen distributed through a repurposed gas network. [12]
Impurity | Maximum Permissible Concentration / μmol mol−1 |
---|---|
Total non-hydrogen gasses | 20 000 |
Water | Non-condensing |
Total Hydrocarbons [Carbon atom basis] | 100 |
Carbon Monoxide | 1 |
Sulphur [Sulphur atom basis] | 2 |
Combined water, oxygen, nitrogen, argon | 19 000 |
Permanent Particulates | Shall not contain an amount sufficient to cause damage. |
The presence of impurities in hydrogen depends on the feedstock and the production process. Hydrogen produced by electrolysis of water may routinely include trace oxygen and water. Hydrogen produced by reforming of hydrocarbons contains carbon dioxide and carbon monoxide as well as sulphur compounds. [12] Some impurities may be added deliberately, for example odorants to aid detection of gas leaks. [14]
As the permissible concentrations for many impurities are very low this sets stringent demands on the sensitivity of the analytical methods. Moreover, the high reactivity of some impurities requires use of a properly passivated sampling and analytical systems. [15] Sampling of hydrogen of is challenging and care must be taken to ensure that impurities are not introduced to the sample and that impurities do not absorb on or react within the sampling equipment, there are currently different methods for sampling but rely on filling a gas cylinder from the refuelling nozzle of a refuelling station. [16] Efforts are underway to standardise and compare sampling strategies. [17] [18] A combination of different instruments is needed to assess hydrogen samples for all of the components listed in ISO 14687-2. [19] Techniques suitable for individual impurities are indicated in the table below.
Impurity | Possible Analytical Methods | Detection Limits |
---|---|---|
Total non-hydrogen gasses | ||
Water | Quartz crystal microbalance or CRDS | 1.3 or 0.030 |
Total Hydrocarbons Except Methane [Carbon atom basis] | GC-Methaniser-FID | 0.1 |
Methane | GC-Methaniser-FID, GC-EPD | 0.1 |
Oxygen | GC-PDHID, GC-EPD | 0.3 |
Helium | GC-TCD | 10 |
Nitrogen | GC-PDHID, GC-EPD | 1 |
Argon | GC-PDHID, GC-EPD | 0.3 |
Carbon Dioxide | GC-Methaniser-FID, GC-EPD | 0.02 |
Carbon Monoxide | GC-Methaniser-FID, GC-EPD | 0.02 |
Total Sulphur Compounds [Sulphur atom basis] | GC-SCD, GC-EPD | 0.001 |
Formaldehyde | GC-Methaniser-FID | 0.1 |
Formic Acid | FTIR | 0.2 |
Ammonia | GC-MS or UV-visible spectroscopy or FTIR | 1 or 0.03 or 0.1 |
Halogenated Compounds (Halogen Ion Equivalent) | TD-GC-MS | 0.016 |
Techniques such as electrochemical sensors [22] [23] and mass spectrometry. [24]
A fuel cell is an electrochemical cell that converts the chemical energy of a fuel and an oxidizing agent into electricity through a pair of redox reactions. Fuel cells are different from most batteries in requiring a continuous source of fuel and oxygen to sustain the chemical reaction, whereas in a battery the chemical energy usually comes from substances that are already present in the battery. Fuel cells can produce electricity continuously for as long as fuel and oxygen are supplied.
A regenerative fuel cell or reverse fuel cell (RFC) is a fuel cell run in reverse mode, which consumes electricity and chemical B to produce chemical A. By definition, the process of any fuel cell could be reversed. However, a given device is usually optimized for operating in one mode and may not be built in such a way that it can be operated backwards. Standard fuel cells operated backwards generally do not make very efficient systems unless they are purpose-built to do so as with high-pressure electrolysers, regenerative fuel cells, solid-oxide electrolyser cells and unitized regenerative fuel cells.
Proton-exchange membrane fuel cells (PEMFC), also known as polymer electrolyte membrane (PEM) fuel cells, are a type of fuel cell being developed mainly for transport applications, as well as for stationary fuel-cell applications and portable fuel-cell applications. Their distinguishing features include lower temperature/pressure ranges and a special proton-conducting polymer electrolyte membrane. PEMFCs generate electricity and operate on the opposite principle to PEM electrolysis, which consumes electricity. They are a leading candidate to replace the aging alkaline fuel-cell technology, which was used in the Space Shuttle.
The alkaline fuel cell (AFC), also known as the Bacon fuel cell after its British inventor, Francis Thomas Bacon, is one of the most developed fuel cell technologies. Alkaline fuel cells consume hydrogen and pure oxygen, to produce potable water, heat, and electricity. They are among the most efficient fuel cells, having the potential to reach 70%.
Steam reforming or steam methane reforming (SMR) is a method for producing syngas (hydrogen and carbon monoxide) by reaction of hydrocarbons with water. Commonly natural gas is the feedstock. The main purpose of this technology is hydrogen production. The reaction is represented by this equilibrium:
A proton-exchange membrane, or polymer-electrolyte membrane (PEM), is a semipermeable membrane generally made from ionomers and designed to conduct protons while acting as an electronic insulator and reactant barrier, e.g. to oxygen and hydrogen gas. This is their essential function when incorporated into a membrane electrode assembly (MEA) of a proton-exchange membrane fuel cell or of a proton-exchange membrane electrolyser: separation of reactants and transport of protons while blocking a direct electronic pathway through the membrane.
Electrolysis of water is using electricity to split water into oxygen and hydrogen gas by electrolysis. Hydrogen gas released in this way can be used as hydrogen fuel, but must be kept apart from the oxygen as the mixture would be extremely explosive. Separately pressurised into convenient 'tanks' or 'gas bottles', hydrogen can be used for oxyhydrogen welding and other applications, as the hydrogen / oxygen flame can reach approximately 2,800°C.
Formic acid fuel cells (direct formic acid fuel cells or DFAFCs) are a subcategory of direct liquid-feed fuel cells (DLFCs), in which the liquid fuel is directly oxidized (electrochemically) at the anode instead of reforming to produce hydrogen. Formic acid-based fuel cells represent a promising energy supply system in terms of high volumetric energy density, theoretical energy efficiency, and theoretical open-circuit voltage. They are also able to overcome certain problems inherent to traditional hydrogen (H2) feed fuel cells such as safe handling, storage, and H2 transportation.
A protonic ceramic fuel cell or PCFC is a fuel cell based around a ceramic, solid, electrolyte material as the proton conductor from anode to cathode. These fuel cells produce electricity by removing an electron from a hydrogen atom, pushing the charged hydrogen atom through the ceramic membrane, and returning the electron to the hydrogen on the other side of the ceramic membrane during a reaction with oxygen. The reaction of many proposed fuels in PCFCs produce electricity and heat, the latter keeping the device at a suitable temperature. Efficient proton conductivity through most discovered ceramic electrolyte materials require elevated operational temperatures around 600-700 degrees Celsius, however intermediate temperature (200-400 degrees Celsius) ceramic fuel cells and lower temperature alternative are an active area of research. In addition to hydrogen gas, the ability to operate at intermediate and high temperatures enables the use of a variety of liquid hydrogen carrier fuels, including: ammonia, and methane. The technology shares the thermal and kinetic advantages of high temperature molten carbonate and solid oxide fuel cells, while exhibiting all of the intrinsic benefits of proton conduction in proton-exchange membrane fuel cells (PEMFC) and phosphoric acid fuel cells (PAFC). PCFCs exhaust water at the cathode and unused fuel, fuel reactant products and fuel impurities at the anode. Common chemical compositions of the ceramic membranes are barium zirconate (BaZrO3), barium cerate (BaCeO3), caesium dihydrogen phosphate (CsH2PO4), and complex solid solutions of those materials with other ceramic oxides. The acidic oxide ceramics are sometimes broken into their own class of protonic ceramic fuel cells termed "solid acid fuel cells".
Hydrogen gas is produced by several industrial methods. Nearly all of the world's current supply of hydrogen is created from fossil fuels. Most hydrogen is gray hydrogen made through steam methane reforming. In this process, hydrogen is produced from a chemical reaction between steam and methane, the main component of natural gas. Producing one tonne of hydrogen through this process emits 6.6–9.3 tonnes of carbon dioxide. When carbon capture and storage is used to remove a large fraction of these emissions, the product is known as blue hydrogen.
Several methods exist for storing hydrogen. These include mechanical approaches such as using high pressures and low temperatures, or employing chemical compounds that release H2 upon demand. While large amounts of hydrogen are produced by various industries, it is mostly consumed at the site of production, notably for the synthesis of ammonia. For many years hydrogen has been stored as compressed gas or cryogenic liquid, and transported as such in cylinders, tubes, and cryogenic tanks for use in industry or as propellant in space programs. The overarching challenge is the very low boiling point of H2: it boils around 20.268 K (−252.882 °C or −423.188 °F). Achieving such low temperatures requires expending significant energy.
Hydrogen technologies are technologies that relate to the production and use of hydrogen as a part hydrogen economy. Hydrogen technologies are applicable for many uses.
Reformed Methanol Fuel Cell (RMFC) or Indirect Methanol Fuel Cell (IMFC) systems are a subcategory of proton-exchange fuel cells where, the fuel, methanol (CH3OH), is reformed, before being fed into the fuel cell.
A hydrogen infrastructure is the infrastructure of hydrogen pipeline transport, points of hydrogen production and hydrogen stations for distribution as well as the sale of hydrogen fuel, and thus a crucial prerequisite before a successful commercialization of automotive fuel cell technology.
Electromethanogenesis is a form of electrofuel production where methane is produced by direct biological conversion of electrical current and carbon dioxide.
Proton exchange membrane(PEM) electrolysis is the electrolysis of water in a cell equipped with a solid polymer electrolyte (SPE) that is responsible for the conduction of protons, separation of product gases, and electrical insulation of the electrodes. The PEM electrolyzer was introduced to overcome the issues of partial load, low current density, and low pressure operation currently plaguing the alkaline electrolyzer. It involves a proton-exchange membrane.
Liquid organic hydrogen carriers (LOHC) are organic compounds that can absorb and release hydrogen through chemical reactions. LOHCs can therefore be used as storage media for hydrogen. In principle, every unsaturated compound can take up hydrogen during hydrogenation. The sequence of endothermal dehydrogenation followed by hydrogen purification is considered as the main drawback which limits the overall efficiency of the storage cycle. LOHC shipping without heat recycling has an energy efficiency of 60-70%, depending on the dehydrogenation rate, which is equivalent to liquid hydrogen shipping. With heat recycling, the energy efficiency increase to 80-90%.
Sorption enhanced water gas shift (SEWGS) is a technology that combines a pre-combustion carbon capture process with the water gas shift reaction (WGS) in order to produce a hydrogen rich stream from the syngas fed to the SEWGS reactor.
High Temperature Proton Exchange Membrane fuel cells (HT-PEMFC), also known as High Temperature Polymer Electrolyte Membrane fuel cells, are a type of PEM fuel cells which can be operated at temperatures between 120 and 200°C. HT-PEM fuel cells are used for both stationary and portable applications. The HT-PEM fuel cell is usually supplied with hydrogen-rich gas like reformate gas formed by reforming of methanol, ethanol, natural gas or LPG.
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