Gas Turbine Glossary 2026: 80+ Key Terms — Hydrogen, Combustion, Grid & Decarbonisation Defined

By Green Gas Turbines Editorial · Published April 1, 2026 · 22 min read

A

Additionality

In the context of clean hydrogen production (particularly IRA Section 45V), the requirement that electricity powering an electrolyser must come from new clean generation sources — not existing facilities. Prevents claiming renewable energy certificates from legacy wind or solar farms without adding new clean capacity to the grid.

Aeroderivative Gas Turbine

A gas turbine derived from an aircraft jet engine design, adapted for stationary power generation or mechanical drive applications. Aeroderivatives are characterised by fast start-up times (typically under 10 minutes to full load), high power-to-weight ratios, and modular construction. Examples include the GE LM6000, Siemens SGT-A65, and Rolls-Royce MT30. They are commonly used for peaking, fast-response grid services, and distributed generation.

Air-Cooled Combustor

A combustor design that uses compressor discharge air rather than steam to cool the combustor liner and transition pieces. Air-cooled combustors simplify combined-cycle integration (no steam piping to the combustor) and improve startup flexibility compared to steam-cooled designs.

Ammonia Cracking

The process of thermally decomposing ammonia (NH₃) into hydrogen (H₂) and nitrogen (N₂) at 400–900°C. Ammonia is being explored as a hydrogen carrier because it is easier to transport and store than pure hydrogen. Cracking recovers the hydrogen at the point of use — potentially adjacent to a gas turbine power plant.

Ancillary Services

Services provided to the grid operator to maintain system reliability and power quality, including frequency regulation, spinning reserve, voltage support, and black start capability. Gas turbines are major providers of ancillary services due to their fast ramp rates and dispatchability.

Autothermal Reforming (ATR)

A hydrogen production process that combines partial oxidation and steam reforming of natural gas in a single reactor. ATR is favoured over conventional SMR for blue hydrogen projects because it produces a more concentrated CO₂ stream, making carbon capture more efficient and less expensive.

B

Baseload

The minimum level of power demand on the grid over a given period. Baseload generation runs continuously and is optimised for efficiency and low fuel cost rather than flexibility. Combined-cycle gas turbines increasingly serve baseload duty in markets where coal plants are retiring.

BESS (Battery Energy Storage System)

An electrochemical storage system (typically lithium-ion) that stores and discharges electricity. In hybrid gas turbine installations, BESS provides bridging power during turbine start-up, absorbs rapid load transients, and enables frequency regulation services.

Black Start

The ability to restart a power plant and begin generating electricity without relying on external grid power. Gas turbines with black start capability use diesel generators, batteries, or compressed air to crank the turbine and establish initial generation, then progressively re-energise the local grid.

Blending (Hydrogen)

Mixing hydrogen with natural gas in the fuel supply to a gas turbine. Blending is typically expressed as a volumetric percentage (e.g., "30% H₂ blend"). Because hydrogen has roughly one-third the energy density of methane by volume, a 30% volumetric blend provides approximately 10–12% of total energy input from hydrogen.

Blue Hydrogen

Hydrogen produced from natural gas via steam methane reforming (SMR) or autothermal reforming (ATR) with carbon capture and storage (CCS). The "blue" designation indicates that CO₂ emissions from the reforming process are captured rather than vented, though upstream methane emissions remain a lifecycle concern.

Bond Coat

A metallic layer (typically MCrAlY — metal-chromium-aluminium-yttrium) applied between the turbine blade substrate and the ceramic thermal barrier coating (TBC). The bond coat provides oxidation resistance and mechanical adhesion for the TBC system.

Borescope Inspection

A non-destructive examination of gas turbine internal components (combustor liners, transition pieces, first-stage vanes and blades) using a flexible or rigid optical probe inserted through access ports. Borescope inspections are performed between major overhauls to assess coating condition, cracking, and erosion.

Brayton Cycle

The thermodynamic cycle that describes gas turbine operation: air is compressed (compressor), heated at constant pressure (combustor), expanded through a turbine (extracting work), and exhausted. Combined-cycle plants pair the Brayton cycle (gas turbine) with a Rankine cycle (steam turbine) to recover waste heat.

C

Capacity Factor

The ratio of actual energy output over a period to the maximum possible output if the plant ran at full rated capacity for the entire period. A gas turbine peaker might have a 10–20% capacity factor; a baseload CCGT might achieve 60–85%.

Carbon Capture and Storage (CCS)

A suite of technologies that capture CO₂ from industrial or power generation processes, transport it, and store it permanently in geological formations. In gas turbine applications, CCS can be applied post-combustion (capturing CO₂ from flue gas) or integrated with hydrogen production (blue hydrogen).

CBAM (Carbon Border Adjustment Mechanism)

The EU's tariff on carbon-intensive imports, designed to prevent carbon leakage by equalising carbon costs between domestic producers (subject to EU ETS) and importers. Relevant to gas turbine operators whose products or electricity may be subject to cross-border carbon accounting.

CCGT (Combined-Cycle Gas Turbine)

A power plant configuration that pairs a gas turbine (Brayton cycle) with a heat recovery steam generator (HRSG) and steam turbine (Rankine cycle). The steam turbine generates additional electricity from the gas turbine's exhaust heat, boosting overall efficiency from 35–42% (simple cycle) to 58–64% (combined cycle).

CEMs (Continuous Emissions Monitoring System)

An array of instruments that continuously measure pollutant concentrations (NOx, CO, CO₂, O₂, particulates) in the exhaust stack. CEMs data is used for regulatory compliance reporting and real-time combustion tuning.

CHP (Combined Heat and Power)

Also called cogeneration. A system that simultaneously generates electricity and useful thermal energy (steam, hot water, or chilled water) from a single fuel source. Gas turbine CHP systems can achieve 80%+ overall energy utilisation efficiency.

CMAS (Calcium-Magnesium-Alumino-Silicate)

Airborne dust and volcanic ash particles that melt at high temperature and infiltrate the porous structure of thermal barrier coatings (TBCs), causing stiffening, delamination, and spallation. CMAS protection is a major research area for advanced TBC development.

Combustion Dynamics

Pressure oscillations within the combustor caused by the interaction between heat release, acoustic modes, and fuel-air mixing. High-amplitude combustion dynamics can damage hardware and limit operating range. Hydrogen's faster flame speed and different acoustic characteristics make combustion dynamics management a central challenge for H₂ gas turbines.

Compressor

The front section of a gas turbine that draws in ambient air and compresses it (typically to 15:1–23:1 pressure ratio) before delivering it to the combustor. Axial compressors use multiple stages of rotating and stationary airfoils; centrifugal compressors use impellers.

D

Demand Response

The adjustment of electricity consumption in response to grid signals (price or reliability). Gas turbines participate on the supply side of demand response by ramping up generation during high-demand periods.

Digital Twin

A virtual model of a physical gas turbine that mirrors its real-time operating state using sensor data, physics-based models, and machine learning. Digital twins are used for predictive maintenance, performance optimisation, and remaining-useful-life estimation of hot-section components.

Dispatch

The process of selecting which power generation units to activate and at what output level to meet real-time electricity demand. Gas turbines are valued for their dispatchability — the ability to start, stop, and change output quickly on command.

DLE (Dry Low Emissions)

A combustion technology (Siemens/European terminology, equivalent to DLN) that achieves low NOx emissions by premixing fuel and air before combustion, avoiding high-temperature stoichiometric flames. "Dry" indicates no water or steam injection is used for NOx control.

DLN (Dry Low NOx)

A combustion technology (GE/US terminology, equivalent to DLE) that uses lean premixed combustion to reduce thermal NOx formation. DLN combustors are the standard for modern gas turbines and the primary technology being adapted for hydrogen operation.

E

EBC (Environmental Barrier Coating)

A coating system applied to ceramic matrix composite (CMC) components to protect against water-vapor-driven surface recession, oxidation, and chemical attack. Distinct from TBCs (which primarily reduce heat flow), EBCs primarily protect the substrate from environmental degradation — especially critical in hydrogen combustion environments with high steam content.

Electrolyser

A device that splits water (H₂O) into hydrogen (H₂) and oxygen (O₂) using electricity. The two main types are PEM (proton exchange membrane) and alkaline. Electrolysers powered by renewable electricity produce green hydrogen.

Embrittlement (Hydrogen)

The degradation of metal mechanical properties (ductility, toughness, fatigue life) caused by hydrogen diffusion into the crystal lattice. Hydrogen embrittlement affects fuel system piping, valves, and manifolds, requiring the use of hydrogen-compatible alloys (austenitic stainless steels, nickel alloys) in hydrogen gas turbine fuel systems.

Emissions Index (EI)

A measure of pollutant mass emitted per unit of fuel consumed, typically expressed as grams of pollutant per kilogram of fuel. Used alongside concentration-based measurements (ppm) for comparing emissions across different fuels and operating conditions.

F

Film Cooling

A technique where cool air is ejected through small holes in turbine blade and vane surfaces to create a thin protective layer (film) between the hot combustion gas and the metal surface. Film cooling effectiveness is affected by hole geometry, blowing ratio, and mainstream gas composition — all of which change with hydrogen fuel.

Firing Temperature

The temperature of the gas entering the first stage of the turbine section, after combustion. Higher firing temperatures improve thermodynamic efficiency but increase demands on materials, coatings, and cooling systems. Modern large-frame turbines operate at firing temperatures exceeding 1,500°C.

Flashback

The upstream propagation of a flame into the premixing zone of a DLN/DLE combustor. Flashback is a primary safety and operability concern for hydrogen gas turbines because hydrogen's flame speed is 6–8x faster than methane's, making the flame more likely to propagate against the fuel-air flow.

Fuel Flexibility (Fuel Flex)

The ability of a gas turbine to operate on multiple fuels — natural gas, hydrogen, biogas, syngas, liquid fuels — either individually or in blends. Fuel-flexible turbines are designed with combustion systems and controls that accommodate varying fuel properties (heating value, flame speed, Wobbe index).

G

Green Hydrogen

Hydrogen produced by electrolysis of water using electricity from renewable sources (wind, solar, hydropower). Green hydrogen has near-zero lifecycle carbon emissions and is the target fuel for fully decarbonised gas turbine operation.

Grey Hydrogen

Hydrogen produced from natural gas via steam methane reforming (SMR) without carbon capture. Grey hydrogen is the cheapest production method but has significant CO₂ emissions (~9–12 kg CO₂ per kg H₂). It represents approximately 95% of current global hydrogen production.

Grid Forming

The capability of a generator or inverter to establish voltage and frequency on a de-energised grid segment, rather than synchronising to an existing grid signal. Grid-forming controls are increasingly important as synchronous generation (including gas turbines) is displaced by inverter-based renewables that traditionally require an existing grid signal to operate.

GREET Model

Greenhouse gases, Regulated Emissions, and Energy use in Technologies — a lifecycle analysis model developed by Argonne National Laboratory. The 45VH2-GREET variant is the mandated tool for calculating lifecycle emissions under the IRA Section 45V hydrogen tax credit.

H

Heat Rate

The amount of fuel energy (in BTU or kJ) required to generate one unit of electrical energy (kWh). Lower heat rate = higher efficiency. A modern CCGT has a heat rate of approximately 5,500–6,000 BTU/kWh (58–62% efficiency); a simple-cycle turbine is typically 8,500–10,000 BTU/kWh (34–40% efficiency).

Heat Recovery Steam Generator (HRSG)

A heat exchanger that captures waste heat from gas turbine exhaust to produce steam, which drives a steam turbine in a combined-cycle configuration. Modern HRSGs are multi-pressure (typically triple-pressure with reheat) to maximise energy recovery.

Heavy-Duty Gas Turbine

A gas turbine designed specifically for stationary power generation (as opposed to aeroderivative designs adapted from aircraft engines). Heavy-duty turbines are optimised for long continuous operation, high output, and long maintenance intervals. Examples include GE 7HA, Siemens SGT5-9000HL, and Mitsubishi M701JAC.

Hot Gas Path (HGP)

The components exposed to high-temperature combustion gases: combustor liners, transition pieces, first-stage nozzles (vanes), and first-stage buckets (blades). HGP inspections and part replacements are the primary driver of gas turbine maintenance cost and outage scheduling.

Hydrogen-Ready

A designation indicating a gas turbine can be converted to operate on hydrogen (partially or fully) through defined hardware and software upgrades. There is no universal standard for "hydrogen-ready" — it can mean anything from "fuel system is H₂-compatible" to "combustor can be swapped for an H₂ variant during a planned outage." Always ask for the specific H₂ percentage, combustion mode, and required modifications.

I–K

Inertia (Grid/Rotational)

The kinetic energy stored in the rotating mass of synchronous generators (including gas turbines) that resists changes in grid frequency. Higher system inertia means the grid is more resilient to sudden generation or load changes. As synchronous machines are displaced by inverter-based renewables, declining grid inertia is a growing reliability concern.

Inlet Guide Vanes (IGVs)

Adjustable vanes at the compressor inlet that control airflow and, consequently, turbine output. IGVs allow part-load operation with improved efficiency and are used in load-following strategies.

Interconnection Queue

The waiting list for new generation or large-load projects to connect to the transmission grid. Queue times of 4–7 years in congested US markets are a primary driver of behind-the-meter gas turbine deployment for data centres and industrial facilities.

ISO Conditions

Standard ambient conditions used for rating gas turbine performance: 15°C (59°F), 101.325 kPa (sea level), 60% relative humidity. Actual performance varies with site conditions — hot, high-altitude, or humid sites derate turbine output relative to ISO ratings.

L

LCOE (Levelised Cost of Energy)

The total lifecycle cost of building and operating a power plant, divided by total energy output over its lifetime. Expressed in $/MWh. Allows comparison across generation technologies with different capital costs, fuel costs, and capacity factors.

LCOH (Levelised Cost of Hydrogen)

The total lifecycle cost of producing hydrogen, expressed in $/kg. Includes capital cost of the production facility, energy input costs, operating and maintenance costs, and any applicable incentives (e.g., Section 45V credits). Green hydrogen LCOH in 2026 ranges from $3–6/kg without incentives, $1–3/kg with 45V Tier 1 credits.

Lean Premix

A combustion strategy where fuel and air are thoroughly mixed at a fuel-lean ratio before entering the flame zone. Lean premix reduces peak flame temperatures, which in turn reduces thermal NOx formation. The basis for DLN/DLE combustion systems.

LTSA (Long-Term Service Agreement)

A contractual arrangement between a gas turbine operator and OEM (or third-party maintainer) covering scheduled maintenance, parts supply, and sometimes performance guarantees over a multi-year period (typically 10–20 years). LTSAs are being renegotiated across the industry to address hydrogen fuel switching and its impact on hot-section part life.

M–N

Micro-Mix Combustion

A combustion technology using a very large number of miniature fuel injection points to create rapid, distributed fuel-air mixing at extremely short length scales. Developed primarily by Kawasaki for hydrogen DLE applications, micro-mix combustion provides inherent flashback resistance because the mixing length is too short for flame propagation upstream.

MNQC (Multi-Nozzle Quiet Combustor)

A GE diffusion-flame combustor design that uses multiple fuel nozzles to reduce combustion noise and dynamics. MNQC systems can operate on 100% hydrogen today but produce higher NOx than premix (DLN) systems, typically requiring SCR for emissions compliance.

NOx (Nitrogen Oxides)

A collective term for NO and NO₂, the primary regulated air pollutants from gas turbine combustion. Thermal NOx forms at high flame temperatures (above ~1,500°C). Hydrogen combustion can produce more thermal NOx than methane due to higher flame temperature, making DLN/DLE premix or SCR essential for compliance.

O–P

OEM (Original Equipment Manufacturer)

The company that designed and manufactured the gas turbine. The four major gas turbine OEMs are GE Vernova, Siemens Energy, Mitsubishi Power (MHPS), and Kawasaki Heavy Industries.

Open Cycle

See Simple Cycle. A gas turbine operating without heat recovery or a bottoming steam cycle.

Pattern Factor

A measure of temperature non-uniformity in the combustor exit gas delivered to the first-stage turbine nozzles. A high pattern factor means some areas receive much hotter gas than the average, reducing component life. Hydrogen's different combustion characteristics can alter pattern factors, requiring combustor redesign.

Peaker (Peaking Plant)

A power plant that operates primarily during periods of high electricity demand (peaks). Gas turbine peakers are valued for fast start-up, low capital cost, and the ability to provide grid reliability without running baseload hours. Peakers typically have low capacity factors (5–20%) but earn revenue from capacity payments and energy margin during high-price hours.

PEM Electrolyser

A proton exchange membrane electrolyser that uses a solid polymer electrolyte to split water into hydrogen and oxygen. PEM electrolysers offer fast response times, compact footprint, and good compatibility with variable renewable power inputs. They are the preferred technology for co-location with gas turbine plants.

Pink Hydrogen

Hydrogen produced by electrolysis using nuclear electricity. Pink hydrogen has very low lifecycle carbon emissions and benefits from the high capacity factor of nuclear plants, enabling electrolyser utilisation rates of 85–95%.

PPA (Power Purchase Agreement)

A long-term contract between a power generator and an electricity buyer (utility, corporation, or grid operator) specifying the price, volume, and terms for electricity delivery. PPAs provide revenue certainty for gas turbine projects and are the primary financing instrument for both renewable and gas-fired generation.

Pressure Ratio

The ratio of compressor discharge pressure to inlet pressure. Higher pressure ratios generally improve thermal efficiency but increase mechanical and thermal loads on compressor and turbine components. Modern large-frame gas turbines operate at pressure ratios of 20:1–24:1.

R

Ramp Rate

The speed at which a gas turbine can increase or decrease its power output, typically expressed in MW per minute or as a percentage of rated output per minute. Fast ramp rates (10–30 MW/min for large frames; even faster for aeroderivatives) are essential for grid balancing and renewable integration.

Rankine Cycle

The thermodynamic cycle used by the steam turbine in a combined-cycle plant. Water is heated to steam (using gas turbine exhaust heat in the HRSG), expanded through a steam turbine to extract work, condensed, and pumped back to the HRSG.

RNG (Renewable Natural Gas)

Biogas that has been upgraded to pipeline quality (predominantly methane) through removal of CO₂, H₂S, moisture, and other contaminants. Derived from anaerobic digestion of organic waste (landfills, wastewater, agricultural waste) or thermal gasification of biomass. RNG is considered carbon-neutral or carbon-negative and can be used in existing gas turbines without modification.

S

SCR (Selective Catalytic Reduction)

A post-combustion emission control technology that injects ammonia or urea into the exhaust gas stream upstream of a catalyst bed, converting NOx to harmless nitrogen (N₂) and water. SCR is required when DLN/DLE combustion alone cannot meet permit limits — particularly relevant for gas turbines operating on hydrogen in diffusion mode.

Simple Cycle

A gas turbine operating alone without a heat recovery system or steam turbine. Simple-cycle efficiency is typically 35–42%. Used for peaking, backup, mechanical drive, and applications where fast start and low capital cost are more important than fuel efficiency.

SMR (Steam Methane Reforming)

The dominant industrial process for hydrogen production. Natural gas reacts with steam at 700–1,000°C over a nickel catalyst to produce hydrogen and CO₂. Without CCS, SMR produces grey hydrogen; with CCS, it produces blue hydrogen.

Spallation

The failure mode where a thermal barrier coating (TBC) delaminates and separates from the underlying bond coat, exposing the metal substrate to hot gas. Spallation is driven by thermal cycling, TGO growth, sintering, and moisture-assisted degradation — all of which can be accelerated by hydrogen combustion environments.

Spinning Reserve

Generation capacity that is online, synchronised to the grid, and available to increase output within seconds to minutes in response to a sudden loss of another generator or unexpected load increase. Gas turbines provide spinning reserve by operating below maximum output with headroom to ramp up.

Syngas (Synthesis Gas)

A fuel gas mixture of hydrogen and carbon monoxide, typically produced by gasification of coal, biomass, or waste. Some gas turbines in integrated gasification combined-cycle (IGCC) plants are designed to burn syngas, giving them inherent experience with hydrogen-rich fuel blends.

T

TBC (Thermal Barrier Coating)

A ceramic coating (typically yttria-stabilised zirconia / YSZ) applied to hot-section components to insulate the metal substrate from combustion gas temperatures. TBCs can reduce metal temperature by 100–300°C, dramatically extending component life. Advanced systems for hydrogen turbines use multilayer designs with rare-earth zirconates.

TGO (Thermally Grown Oxide)

A thin alumina (Al₂O₃) layer that forms between the bond coat and TBC during operation. Controlled TGO growth is normal and necessary for TBC adhesion, but excessive TGO thickness (driven by high temperature and steam-rich environments in hydrogen combustion) generates stress that leads to spallation.

Thermal NOx

Nitrogen oxides formed by the high-temperature reaction of atmospheric N₂ and O₂ in the flame zone. Thermal NOx formation increases exponentially above ~1,500°C. It is the dominant NOx mechanism in gas turbines and the primary target of DLN/DLE lean premix combustion strategies.

Tier IV (Data Centre)

The highest classification in the Uptime Institute's data centre rating system, requiring 99.995% availability (less than 26 minutes of unplanned downtime per year), fully redundant infrastructure, and concurrent maintainability. Gas turbines are increasingly used as primary or backup power for Tier IV facilities.

Turndown

The ability of a gas turbine to operate at reduced output while maintaining stable combustion and acceptable emissions. Turndown ratio (minimum stable load as a percentage of full load) is important for load following and grid flexibility. Hydrogen operation can affect turndown limits due to different lean blowout and flashback boundaries.

Turquoise Hydrogen

Hydrogen produced by methane pyrolysis — the thermal decomposition of methane into hydrogen gas and solid carbon (rather than CO₂). If powered by clean energy, turquoise hydrogen can achieve very low lifecycle emissions because the carbon is sequestered in solid form rather than emitted as a gas.

U–W

Wobbe Index

A measure of fuel interchangeability defined as the higher heating value divided by the square root of the specific gravity. Fuels with similar Wobbe indices will deliver similar heat input through the same fuel nozzle at the same supply pressure. Hydrogen has a very different Wobbe index than natural gas (~48 vs ~50 MJ/m³), which is why hydrogen fuel systems require different nozzle sizing and pressure regulation.

Wet Compression

The injection of atomised water or fog into the compressor inlet to cool the intake air, increasing air density and turbine output (power augmentation). Commonly used on hot days to recover output lost to high ambient temperatures.

Y–Z

YSZ (Yttria-Stabilised Zirconia)

The industry-standard ceramic material for thermal barrier coatings. 6–8 wt% yttria stabilises the zirconia in its tetragonal phase, providing a combination of low thermal conductivity, adequate toughness, and manufacturability. YSZ remains the baseline TBC material but is being supplemented by multilayer systems with rare-earth zirconates for hydrogen turbine applications.

Zero-Carbon Dispatch

Operating a gas turbine on 100% green hydrogen or other zero-carbon fuel, producing electricity with zero CO₂ emissions at the stack. Zero-carbon dispatch is the end-state goal for gas turbines in a net-zero grid, enabled by hydrogen-ready turbine designs and scaled green hydrogen production.

References

1. GE Gas Power – Gas Turbine Education Resources
2. U.S. DOE – Hydrogen Production Overview
3. IEA – Hydrogen Tracking Report
4. Argonne National Laboratory – GREET Lifecycle Analysis Model