Urea Dosing Rate for SCR and SNCR: How to Size the Pump

The urea dosing rate for an SCR or SNCR system comes from how much NOx you need to remove, converted to ammonia through the molar ratio and then to urea solution volume. Once you have that volume at maximum and minimum load, sizing the metering pump is a straightforward step.

Buyers often ask us for a typical urea dosing rate so they can pick a pump off a catalogue page. There is no typical figure. Boiler size, fuel, inlet NOx and the emission limit all move the answer. What we can give you is the calculation route, written so a process engineer can plug in project data and a purchasing engineer can check the pump that comes out of it.

The four inputs you need before any math

Ask the boiler supplier or design institute for these four values. If one is missing, any urea dosing rate you calculate is a guess.

1. Flue gas flow Q in Nm³/h, dry basis, corrected to the reference oxygen content in the permit. Using a wet, uncorrected flow pushes the result too high.
2. Inlet NOx concentration C₁ in mg/Nm³, usually reported as NO₂. Confirm whether the figure is "as NO₂" or "as NO", because the molar mass is different.
3. Outlet limit C₂, or the target removal efficiency η = (C₁ − C₂) ÷ C₁.
4. Normalized stoichiometric ratio (NSR), the moles of ammonia injected per mole of NOx reduced. SNCR typically runs a noticeably higher NSR than SCR, since the furnace has no catalyst helping the reaction.

Flue gas flow is where most errors start. Some data sheets give rated load, others maximum continuous rating. For the top end of the urea dosing rate, take the larger one.

The urea dosing rate formula, step by step

Step 1: NOx to be removed

m₁ = Q × C₁ × η × 10⁻⁶, in kg/h. The 10⁻⁶ turns mg into kg.

Step 2: convert to pure urea

With NOx counted as NO₂, the molar mass is 46. Urea, CO(NH₂)₂, has a molar mass of 60, and each molecule releases two ammonia molecules when it decomposes. So pure urea demand is m₂ = m₁ ÷ 46 × NSR ÷ 2 × 60, in kg/h. If the inlet value was given as NO, replace 46 with 30.

Step 3: convert to solution volume

For a solution with mass fraction w and density ρ in kg/L, the volumetric urea dosing rate is V = m₂ ÷ (w × ρ), in L/h. This V is what the metering pump actually has to deliver.

Use the concentration the pump really sees. Some skids pump concentrated solution from the storage tank and meter dilution water separately downstream. Others dilute first and then pump. The two layouts can differ by several times in pump flow, so confirm the flow sheet before you size anything.

From urea dosing rate to pump capacity

One operating point is not enough. Calculate at least two:

• Maximum point: highest flue gas flow, highest inlet NOx and the strictest limit together, giving V_max.
• Minimum point: lowest stable load at normal inlet NOx, giving V_min.

Rated pump capacity is usually set somewhat above V_max. A margin of around 20 to 30 percent is common, covering nozzle wear, concentration drift and tighter limits later. Then check V_min against the pump's turndown. If the minimum urea dosing rate is only a small slice of rated flow, the stroke or motor speed ends up very low, and accuracy suffers. In that case a large and a small pump working together beats one oversized unit. How stroke and VFD share the job is covered in our urea pump adjustment guide.

Mistakes that skew the result

• Mixing ppm and mg/Nm³. Converting ppm to mass concentration needs the molar mass and molar volume.
• Skipping oxygen correction. The same raw reading looks very different once corrected.
• Treating NSR as fixed. At low load or outside the temperature window, the real NSR drifts, so do not blindly reuse the design value at the minimum point.
• Ignoring winter. Cold urea solution can crystallize. With weak heat tracing, the pump delivers less than its setting, and the urea dosing rate on the screen is not what reaches the nozzles.

If you need background on where urea goes in SCR and SNCR, read urea pumps in denitrification and the urea pump working principle.

Turning the number into hardware

For flows from tens to a few hundred L/h with a 4-20 mA signal from the DCS for load tracking, most projects use a mechanical diaphragm metering pump with a VFD motor and a stainless steel liquid end. Continuous units are usually duty and standby, with metering, dilution and distribution built into a dosing skid so site piping and commissioning stay short.

Safety note: stop the pump, close the valves, relieve pressure and isolate heat tracing power before calibration or teardown.

Conclusion

There is no universal urea dosing rate, but the route never changes. Multiply flue gas flow by inlet NOx and efficiency to get NOx removed, convert to urea using NSR and molar masses, convert to solution volume using concentration and density, then size pump capacity from the maximum point and confirm turndown at the minimum point. Send us your flue gas flow, inlet NOx, limit, NSR and urea concentration through our contact page and we will check the pump flow and margin with you.

FAQ

What is a typical urea dosing rate?

There is no typical urea dosing rate you can copy. Flue gas flow and inlet NOx change it for every project, so plug your own design values into the formula.

What NSR should I use?

Use the value from the denitrification process package. SCR and SNCR differ a lot, and the vendor's performance guarantee depends on that number.

Should I add margin on top of V_max?

Usually yes, around 20 to 30 percent. Much more than that pushes the stroke too low at minimum load.

How much does the urea dosing rate change between 40 percent and diluted solution?

It depends on the dilution ratio. Pumping concentrated solution needs far less flow, so recalculate the volume for whichever solution enters the pump.

How do I verify the dosing rate after start-up?

Measure actual pump output with a calibration column or by weighing, then compare it with outlet NOx and ammonia slip readings.