Hydrogen is one of the highest-volume gases consumed by the heat-treat industry and is used for a broad range of process applications because of its unique chemical and physical properties (Table 1). Examples include annealing, brazing, metal injection molding, sintering and welding operations as both a protective and reducing atmosphere for steel and stainless steels. In general, processing of stainless steels requires a higher purity level than carbon steel, in which the atmosphere is often a blend (e.g., nitrogen/hydrogen) or gas dilution (e.g., endothermic/nitrogen).
A number of production methods exist for generating hydrogen. These include steam reforming (i.e., collection and extraction of hydrogen from carbon-based fuels), electrolysis and thermolysis (both of which extract hydrogen from water molecules). On-site hydrogen-generation systems are a popular choice for the heat-treatment industry and include methods such as water electrolysis, which matches typical industry-required flowrate ranges and usage patterns.
Hydrogen can be as pure as 99.9995+% (or better). Traditionally, hydrogen has been purchased through large-scale reformers at or near the site of use or from industrial gas suppliers via truck delivery or dedicated pipeline to the site. Both approaches have advantages and drawbacks. Large-scale on-site generation provides a reliable supply of hydrogen. However, few heat-treating applications require hydrogen in such a large volume to justify the high capital expense of a conventional reforming generator, which is the technology most widely used in on-site applications. For most heat treaters, the only practical option has been to rely on delivered hydrogen, avoiding capital investment. The downside of this approach is typically higher total cost of hydrogen, delivery surcharges and the potential for supply disruptions.
With respect to electrolysis, three cell technologies are in use: solid-oxide electrolysis cells (SOECs), alkaline electrolytic cells (AECs) operating at high electrolyte (KOH or K2CO3) concentrations and polymer electrolyte membrane cells (PEMs). SOECs operate at high temperature (typically at or near 800°C/1472°F), while AECs operate in the range of 200°C. By contrast, polymer electrolyte membrane cells (PEM) typically operate below 100°C.
On-Site Hydrogen Generation
For many heat treaters, systems employing PEMs are being used on-site to produce hydrogen from water by means of electrolysis (Fig. 1). The technology typically employs a platinum catalyst in combination with a membrane separator to split deionized water into its constituent parts (hydrogen and oxygen), and the hydrogen is sent off as a process gas. When a DC voltage is applied to the electolyzer, water molecules at the anode are oxidized to oxygen and protons (H+ ions) while electrons are released. The protons pass through the membrane to the cathode, where they combine with electrons from the other side of the circuit reducing to hydrogen gas (Fig. 2). Once generated, the hydrogen is dried and supplied to the desired process application. It is produced at the same rate that it is used, requiring no storage of hydrogen.
The need for hydrogen in the heat-treatment industry continues to grow, and on-site hydrogen production capability is an intriguing alternative that should be explored by heat treaters.
- Herring, Daniel H., Vacuum Heat Treatment, BNP Media Group, 2014.
- Wikipedia (www.wikipedia.org)
- Wolff, David, “Advances in Water Electrolysis Creating New Hydrogen Capabilities,” Technical Presentation, Proton On-Site
- Harness, John, “On-Site Hydrogen Generation Can Benefit Heat Treaters,” Heat Treating Progress, September/October 2006
- Wolff, David, “Hydrogen Supply Methods for Heat Treating Operations,” Heat Treating Process, July/August 2006
- David Wolff, regional manager, Proton OnSite, technical contributions and private correspondence