How to Reduce NOx Emissions: NOx Control Methods Explained

NOx is made in the flame, not in the stack

Nitrogen oxides are a gas, not a dust you can trap in a filter. They are built in the flame, in a few thousandths of a second. Before you compare cleanup equipment, you need to know which formation route your plant is running. That decides what can work at all.

NOx is shorthand for nitric oxide (NO) and nitrogen dioxide (NO2). At the burner, 90 to 95 percent of it is normally NO. It becomes NO2 later, in the duct and in the air. That split matters: some cleanup methods work well on NO2 and hardly touch NO.

Thermal, fuel and prompt NOx

Thermal NOx comes from the nitrogen in the combustion air. At high temperature, oxygen radicals break the strong triple bond of the N2 molecule. Below roughly 1,300 °C it is slow; above that it climbs steeply, so a flame at 1,800 °C makes far more NOx than the same fuel burnt at 1,500 °C. Peak flame temperature is the strongest lever on this route.

Fuel NOx comes from nitrogen bound chemically inside the fuel. Coal, heavy oil, biomass, sludge and waste all carry it. It leaves the fuel early in the flame and becomes NO if oxygen is there at that moment. In coal firing it is often the bigger half of the total. Natural gas carries almost no bound nitrogen, so a gas flame is close to pure thermal NOx.

Prompt NOx is the third route: hydrocarbon fragments at the front edge of a rich flame attack N2 directly. It is usually a few percent, and it sets the floor that burner tuning cannot get under. The three routes do not answer to the same levers. Thermal NOx follows peak temperature, oxygen and residence time in the hot zone. Fuel NOx follows the nitrogen bound in the fuel and the oxygen present where it is released, so cooling the flame does little for it. Prompt NOx needs a fuel-rich, oxygen-poor flame front, so it moves the other way again.

Primary measures that reshape the flame

Primary measures change the combustion itself, so less NOx is ever formed, with no reagent and no extra vessel in the gas path. Low-NOx burners mix fuel and air in stages and pull cooled furnace gas into the flame root, spreading the heat release over a longer, cooler flame. A burner swap alone typically cuts NOx by 30 to 50 percent.

Air staging goes further. The main burner zone runs with less air than it needs, so fuel nitrogen is released where there is little oxygen to turn it into NO. The rest of the air comes in higher up as over-fire air, to burn out the fuel. Fuel staging, or reburning, works the other way round: 10 to 20 percent of the fuel goes in above the main zone and makes a reducing zone that turns NO already formed back into N2. Together these often reach a third to a half of the untreated level, but rarely a modern permit limit on their own.

Cooling the flame, and what it costs

Flue-gas recirculation feeds 10 to 30 percent of the cooled flue gas back into the combustion air. The extra inert mass absorbs heat and dilutes the oxygen, so peak temperature drops. On gas this can remove half the NOx. On dusty solid fuels it is harder, because the returned gas brings ash and moisture back into the furnace.

Water or steam injection does the same job with a different coolant and is common on gas turbines. It cuts NOx strongly and follows load well. It also needs demineralised water in large amounts, a plant of its own.

None of this is free, and each measure is paid for differently. Water or steam injection carries heat up the stack as steam and typically costs one to a few percent of turbine efficiency. Flue-gas recirculation costs fan power and duct work and can worsen burnout. Running the burner zone short of air raises carbon monoxide and unburnt carbon in the ash, and a reducing atmosphere near the walls can corrode them.

Secondary measures: what SNCR and SCR do

Secondary measures remove NOx after it has formed. The two established ones work by chemical reduction: a reagent, ammonia or urea, turns NO into harmless N2 and water.

SNCR sprays the reagent into the furnace, into a temperature window of roughly 850 to 1,050 °C. It is cheap to install, little more than lances and a tank, but limited. Real plants get 30 to 50 percent removal, and the window moves with load. Too cold, and unreacted ammonia leaves as slip. Too hot, and the reagent itself burns to NO.

SCR runs the same chemistry on a catalyst, usually between 300 and 400 °C, and reaches 80 to 95 percent. The catalyst is the whole story: dust blinds it, alkali and arsenic poison it, and it turns part of the SO2 into SO3, which joins ammonia slip to form sticky ammonium bisulphate downstream. It needs space, a reagent supply chain and catalyst replacement. The two are compared side by side, with figures, in the separate SCR versus SNCR article on this site.

Oxidation, scrubbing and adsorption

The third route is oxidation followed by washing. NO barely dissolves in water, so it is first oxidised to NO2 or higher oxides with ozone, hydrogen peroxide or chlorine dioxide, then absorbed in an alkaline scrubber. It works cold and suits plants that already have a wet scrubber. The price is a steady oxidant supply and nitrate-loaded wastewater.

The fourth route is adsorption. The gas passes through a solid bed that holds NOx on its surface. Nothing is burned and nothing is dosed. When the bed fills, it is regenerated and used again, and the NOx comes off as a small concentrated stream instead of being destroyed. That removes reagent logistics from the plant and turns captured nitrogen into a product stream. The bed sets the limits: it must be kept clear of dust and of anything competing for the same sites, and moisture and inlet load decide how often it is regenerated.

The six questions that decide the choice

Start with the gas temperature where you would take the gas off. That one number removes most options. SNCR needs a hot furnace section. SCR needs a few hundred degrees, and a colder take-off means a reheat stage that burns fuel all year. Scrubbing and adsorption prefer cold gas, so they suit a spot after the existing dust and acid gas cleaning.

Then look at dust and sulphur. They kill catalysts and foul beds, and are why many SCR units sit at the tail end of the plant. Next, how steady is the load? A plant that cycles daily, or changes fuel, moves the temperature window under an SNCR system and makes dosing hard to control.

The fourth question is the outlet your permit demands. Many European limits now sit at 50 mg/Nm3, about 24 ppm; U.S. rules use other units, ppm corrected to 15 percent oxygen for turbines or pounds per million Btu for boilers. Primary measures alone rarely reach that level, and single-digit ppm takes most of the field out. The fifth is practical: can you store and handle reagent? Ammonia brings safety distances and its own permit; urea is safer but costs more per mole removed.

The sixth question is usually asked last, and it should not be. What happens to the captured nitrogen? SCR and SNCR destroy it, turning it back into N2. That is clean and final, and it throws away nitrogen that was expensive to fix. Scrubbing and adsorption keep it, as nitrate in water or as a concentrated gas stream, where it can become a feedstock instead of a waste.

Measure the real gas before you choose

Most NOx projects start from design data, a datasheet or an annual average. Real gas rarely matches them. The NO to NO2 ratio, water content, SO2, HCl and dust loading, trace metals, and how all of these move over a production cycle decide whether a technology works on your stack or only on paper.

Measuring first changes the answer often enough to be worth the delay. A high NO2 share makes scrubbing more attractive. A trace poison nobody looked for rules out a catalyst. Two cold hours every morning explain a missed SNCR guarantee.

Krajete runs industrial gas sampling and analysis for exactly this step: EUR 4,900 fixed, a decision-grade report within four weeks, standard NDA. If the analysis finds no actionable pathway, the fee is refunded. Enquiries get a reply within one business day.

Where a dry, regenerative adsorber fits

Krajete is an Austrian gas-purification specialist with two services: the gas analysis above, and regenerative NOx removal. The removal step is a zeolite adsorber that works at ambient temperature, with no ammonia, no catalyst, no process water and no reagents at all. The adsorbent is regenerated with gentle heat and used again, cycle after cycle.

In industrial service, outlets reach the 1 ppm class. In CO2 matrices, 99.9 percent NOx capture has been demonstrated, and CO2 polishing has reached below 10 ppb. A published cement-kiln pilot took a slipstream of about 100 Nm3/h of hot, dusty kiln gas and reached roughly 10 ppm at the outlet, with SOx captured alongside. Day one was conditioning, shown openly; on day two uptake stabilised above 85 percent.

Nothing is destroyed, so the NOx released in regeneration is concentrated and recoverable as nitric acid or fertiliser feedstock. Pilot units cover 50 to 18,000 m3/h, are containerised and slipstream-ready. A pilot draws a small side stream from the duct and runs beside the existing abatement, which keeps operating.

Krajete delivers to pilot scale. Full-scale construction is carried out by the client's engineering partner of choice, with Krajete supplying the design basis, adsorbent media and licence. The owner picks the constructor, and the pilot runs on the plant's own gas before any full-scale commitment.