Terminology
The technical glossary
Definitions for the terms used throughout the publication. Each entry links to an explanation with context and sources.
- Anode
- The electrode at which oxidation occurs. In a hydrogen fuel cell generating power, hydrogen is oxidized at the anode. Read the explanation.
- Autothermal reforming
- Reforming combined with oxidation reactions that release heat inside the reacting system. The oxygen feed and heat balance affect hydrogen output. Read the explanation.
- Balance of plant (BoP)
- Equipment supporting the main process component, such as pumps, blowers, controls, cooling and gas handling. Its energy use affects net system output. Read the explanation.
- Bioethanol
- Ethanol made from biological feedstocks. Its chemistry is ethanol chemistry; its origin and processing route matter for lifecycle emissions. Read the explanation.
- Catalyst
- A material that increases a reaction rate without being consumed by the overall reaction. It does not change the equilibrium constant at a given temperature. Read the explanation.
- Cathode
- The electrode at which reduction occurs. Oxygen is reduced at the cathode of a hydrogen PEM fuel cell. Read the explanation.
- Combined heat and power (CHP)
- Production of electrical power and useful recovered heat in one system. Combined efficiency must count heat that can actually be used. Read the explanation.
- Concentration polarization
- A composition difference between bulk gas and the membrane surface caused by transport limitations. Local hydrogen depletion can reduce permeation. Read the explanation.
- Conversion
- The fraction of a specified reactant consumed relative to its feed amount. Complete ethanol conversion does not establish hydrogen yield or recovery. Read the explanation.
- Distributed production
- Production close to the consuming site. It changes delivery requirements, scale, utilization and maintenance duties. Read the explanation.
- Electrolysis
- A process driven by electrical energy. Water electrolysis produces hydrogen and oxygen; it is distinct from a fuel cell generating electricity. Read the explanation.
- Fluidized bed
- A bed of solid particles suspended and moving in an upward fluid flow. It changes mixing and transfer conditions and can expose membranes to erosion. Read the explanation.
- Gas diffusion layer (GDL)
- A porous fuel-cell layer that supports gas transport, electrical conduction and water management between the flow field and catalyst region. Read the explanation.
- Heating value: HHV and LHV
- HHV includes the heat recovered by condensing product water; LHV excludes it. Efficiency percentages require the same heating-value basis to be compared. Read the explanation.
- Hydrogen recovery
- The fraction of hydrogen in a specified feed or generated stream captured in the desired product. The denominator must be stated. Read the explanation.
- Membrane electrode assembly (MEA)
- The fuel-cell electrolyte and adjacent catalyst layers. Some supplier definitions also include gas diffusion layers, so component scope should be checked. Read the explanation.
- Membrane reactor
- A device combining chemical reaction and selective membrane transport, commonly product removal or reactant delivery. Read the explanation.
- Partial pressure
- The pressure contribution of a gas in a mixture. Under an ideal-gas approximation, it is the mole fraction multiplied by total pressure. Read the explanation.
- PEM fuel cell
- A proton exchange membrane, or polymer electrolyte membrane, fuel cell. It conducts protons through a polymer electrolyte and electrons through an external circuit. Read the explanation.
- Permeance
- Flux divided by a defined transport driving force for a membrane. The pressure exponent and units must be stated when comparing values. Read the explanation.
- Permeate
- The stream that has crossed a membrane. For a hydrogen-selective membrane, it is generally enriched in hydrogen. Read the explanation.
- Pressure swing adsorption (PSA)
- Cyclic separation using preferential adsorption at elevated pressure and regeneration at lower pressure. Multiple beds can supply a continuous hydrogen product. Read the explanation.
- Purity
- The fraction of the desired substance in a product on a stated basis. A bulk percentage does not identify trace contaminants or demonstrate fuel-quality compliance. Read the explanation.
- Reformate
- The gas mixture produced by reforming. It contains hydrogen and other constituents that depend on feedstock, reactions and operating conditions. Read the explanation.
- Retentate
- The stream remaining on the feed or reaction side of a membrane after selective permeation. Read the explanation.
- Selectivity
- A measure of relative preference for one species over another. Intrinsic material selectivity and an installed module’s product quality are distinct. Read the explanation.
- Steam reforming
- Catalytic conversion of a fuel with steam into a hydrogen-containing gas mixture. It requires a heat supply and downstream conditioning suited to the end use. Read the explanation.
- Sweep gas
- A stream supplied on the permeate side to lower the partial pressure of the permeating species. It changes product composition and downstream separation duties. Read the explanation.
- Water–gas shift
- The reaction CO + H₂O ⇌ CO₂ + H₂. It converts carbon monoxide and steam into carbon dioxide and additional hydrogen. Read the explanation.