SCR NOx Reduction Unit

Overview

A selective catalytic reduction unit removes nitrogen oxides (NO and NO2, together called NOx) from combustion exhaust by reacting them with ammonia over a solid catalyst. It sits downstream of a boiler, gas turbine, engine, or process heater and ahead of the stack. NOx forms when nitrogen and oxygen combine at flame temperatures, and it contributes to smog, acid rain, and respiratory harm, so emission permits cap it. The unit injects a metered ammonia stream into the flue gas, mixes the two, and passes the mixture through stacked catalyst layers held in the Catalyst Reactor Housing. Over the catalyst the ammonia reduces NOx to nitrogen and water. Removal of 80–95% is routine, taking inlet concentrations of 200–800 mg per Nm3 down to 20–100 mg per Nm3.

The reaction is selective because the ammonia reacts with NOx in preference to being oxidized by the surrounding oxygen, provided the gas stays inside the catalyst temperature window. That window is the central design constraint and shapes where the reactor is placed in the gas path.

Reaction chemistry

The main reactions consume ammonia and oxygen to convert NOx into harmless products. The dominant path with nitric oxide is:

4 NO + 4 NH3 + O2 -> 4 N2 + 6 H2O

A second path handles nitrogen dioxide, which makes up a smaller fraction of the inlet NOx:

2 NO2 + 4 NH3 + O2 -> 3 N2 + 6 H2O

A fast reaction runs when NO and NO2 arrive in equal amounts:

NO + NO2 + 2 NH3 -> 2 N2 + 3 H2O

The unit doses ammonia at a molar ratio of 0.8 to 1.0 against the measured NOx. Dosing below the ratio leaves NOx untreated; dosing above it pushes unreacted ammonia out of the reactor as ammonia slip. The Dosing Control System holds outlet NOx at the permit value while keeping slip inside the 2–5 ppm guarantee, since excess ammonia forms ammonium bisulfate that fouls downstream air heaters and adds to particulate.

Catalyst and reactor

The active material is usually vanadium pentoxide on a titania carrier, with tungsten or molybdenum oxides added to widen the temperature window and resist sulfur. For high-temperature service above 450 °C, zeolite catalyst is used instead. The catalyst comes as extruded Honeycomb Catalyst Element blocks with parallel channels, or as coated Plate Catalyst Element sheets that shed ash better in dirty gas. Elements pack into the Catalyst Modules, which crane into the reactor and rest on beam grids. Most reactors carry 2 to 4 active layers plus one empty layer, so operators can add catalyst as activity decays instead of replacing it all at once.

Performance depends on temperature. Below about 300 °C the reaction rate drops and ammonium salts condense on the catalyst; above about 400 °C the ammonia starts to oxidize and the catalyst can sinter. Gas residence is set by space velocity of 3,000 to 6,000 per hour, the ratio of volumetric gas flow to catalyst volume. Pressure drop across the layers runs 5 to 12 mbar, which the upstream fan must overcome. Catalyst lasts 24,000 to 40,000 hours before a layer is reloaded, with life cut short by arsenic, alkali metals, and fine ash that blind the pores.

Reagent supply

Ammonia reaches the reactor from the Reagent Storage and Dosing. Plants store either anhydrous ammonia, 19% aqueous ammonia, or 40% urea solution, with urea chosen where handling pure ammonia onsite is a hazard. Urea installations add a Urea Decomposition Chamber that hydrolyzes urea into ammonia, carbon dioxide, and water before injection. The reagent leaves a heated tank, passes through metering Dosing Pump units running on shared ball-bearing shafts, and joins a dilution air stream from a blower. Trace heating with a heating-element on the lines keeps urea above its 0 °C crystallization point. A pressure-sensor on the discharge confirms supply before the controller opens the injection valves.

Injection and mixing

The diluted ammonia enters the duct through the Ammonia Injection Grid, a pipe array spanning the cross section. Each lateral carries injection nozzles, and manual balancing valves trim flow to each zone so the ammonia profile matches the measured NOx profile. Even distribution matters because the catalyst cannot correct a maldistributed feed; a rich pocket slips ammonia while a lean pocket passes NOx. Downstream of the grid the Flow Straightening and Mixing uses a static mixer and turning vanes to blend the streams and flatten the velocity profile entering the reactor. Designers target an ammonia-to-NOx variation under 5% root mean square at the catalyst face.

Ash handling and cleaning

Coal and biomass exhaust carries fly ash that settles on the catalyst and plugs channels. The Sootblower System sweeps each layer with steam or compressed air on a timed cycle, and acoustic horns shake loose the fine fraction between sweeps. Ash that drops off the catalyst collects in the Ash Collection Hopper, where heated walls stop bridging and a rotary airlock meters the ash out while holding duct pressure. Gas-fired and engine applications carry far less ash and may omit sootblowers entirely.

Instrumentation and control

NOx analyzers at the Process Instrumentation inlet and outlet, an ammonia slip analyzer, an oxygen analyzer, and thermocouples across the layers feed the Dosing Control System. The controller runs a feedforward estimate from inlet NOx and gas flow, then trims it with feedback from the outlet analyzer. It locks out dosing when gas temperature falls below the catalyst window to prevent ammonium salt deposition, and it raises an alarm when slip climbs, which signals catalyst aging or a maldistributed grid. Relay outputs drive the pumps, valves, and blower, and the operator works from an HMI panel showing setpoints, trends, and alarms.