This page explains where NOx comes from in a gas turbine and the three main ways to control it.
Where NOx comes from
Most NOx in a gas turbine is thermal NOx, formed when nitrogen and oxygen in the air combine at very high flame temperatures. The hotter the flame and the longer gas stays at that temperature, the more NOx forms. Control methods either lower the peak flame temperature or remove NOx from the exhaust afterwards.
Method 1: dry low NOx combustion
Dry low NOx combustors, known as DLN or DLE depending on the maker, mix fuel and air thoroughly before burning, so the flame burns lean and cooler. They are now standard on many new heavy-duty and aeroderivative turbines burning natural gas. Their limits: they are designed for natural gas within a specified composition range, they need careful tuning across the load range, and low emissions are usually only guaranteed above a minimum load. Fuel quality matters; see the fuel gas section of our new gas turbine buying guide.
Method 2: water or steam injection
Injecting demineralised water or steam into the combustor cools the flame and cuts NOx on conventional diffusion combustors. It is common on dual-fuel units when running on liquid fuel. The trade-offs are a supply of high-purity water, a water treatment plant, and slightly higher fuel use and maintenance. On sites where water is scarce, this can be a deciding factor.
Method 3: selective catalytic reduction
Selective catalytic reduction (SCR) removes NOx from the exhaust by injecting ammonia or urea solution upstream of a catalyst, which converts NOx to nitrogen and water. It can reach very low outlet levels and is often required in strict air quality areas, usually combined with DLN or water injection. The catalyst must sit in a specific exhaust temperature window, which on simple cycle units often means a tempering air system, and it adds reagent storage, handling and catalyst replacement to the operating plan.
Comparing the options
| Method | How it works | Main trade-off |
|---|---|---|
| Dry low NOx | Lean premixed flame | Fuel quality limits and tuning; minimum load for compliance |
| Water or steam injection | Cools the flame | Water supply and treatment; small efficiency loss |
| Selective catalytic reduction | Converts NOx in the exhaust | Reagent handling, catalyst life, exhaust temperature control |
Monitoring and permits
Many permits require continuous emissions monitoring of NOx and often carbon monoxide, with records kept for the regulator. Low NOx tuning can raise carbon monoxide at part load, so permits and operating ranges must be agreed together. In the USA, limits depend on the location, the size of the plant and federal and state rules, so the permit strategy should be settled before the turbine is ordered.
Questions to ask before ordering
What NOx and CO levels does the maker guarantee, at what loads and ambient conditions? What fuel composition range applies? Is water or a catalyst system needed to meet the permit? What are the tuning and inspection intervals for the combustor? These answers belong in the specification. Our page on commissioning gates covers how emissions are proven during start-up, and power station design shows where emissions equipment sits in the layout.
For US hyperscale campuses, the same questions apply to the New USP&E FX-45 45MW Modular Gas Turbine for the Hyperscale Data Center sector in the USA.
Permit-ready gas turbine plants
USP&E designs and builds gas turbine power stations for data centers, utilities and industry, including the emissions systems and permit support they need. With 150+ power projects in 45+ countries and 25,000 MW contracted since 2002, we plan emissions from the start. See how gas turbines serve data centre power projects.
Offices: Colorado Springs (1755 Telstar Dr, Suite 300), Knoxville (800 S Gay St, Suite 700), London (162-168 Regent Street), Johannesburg (22 Kildoon Rd, Bryanston), Cape Town (31 Brickfield Road, Woodstock) and China (No. 1 Ningzhou Road, Wuxi). Regions: USA | AFRICA | UK | UAE | CHINA. ISO 9001 and ISO 45001 certified.
Gas turbine NOx questions
How do gas turbines reduce NOx?
By lowering flame temperature with dry low NOx combustors or water and steam injection, and by removing NOx from the exhaust with selective catalytic reduction.
What is a dry low NOx combustor?
A combustor that premixes fuel and air so the flame burns lean and cooler, which reduces thermal NOx without water.
When is selective catalytic reduction needed on a gas turbine?
When the air permit limit is lower than the combustor alone can achieve, which is common in strict air quality areas.
Does NOx control affect efficiency?
Water injection and catalyst systems add small losses and operating costs. Dry low NOx combustion has little efficiency penalty but limits fuel range and minimum compliant load.
Working on a gas turbine permit?
Tell us the site, the fuel and the permit limits, and we will outline the combustor and emissions options that fit.
For a first conversation, use the gas turbine engineers, ring +27 10 822 2324 or send the details to info@uspeglobal.com. Available new and remanufactured units are in the gas turbine inventory.