SCR Ammonia Injection Calculator

Calculator Inputs

Enter values, then press Calculate to update results above.

Exhaust flow

Use normalized flow when available.

Inlet NOx

Treat NOx as NO2-equivalent unless specified.

NOx molecular weight

g/mol

Commonly 46 for NO2-equivalent reporting.

Target mode

Choose how you define performance.

Reduction target

%

Used when Target mode is percent reduction.

Outlet NOx target

ppmv

Used when Target mode is outlet NOx.

Stoichiometric coefficient

mol NH3/mol NOx

Use 1.0 for NO-dominant; increase if NO2 fraction is high.

NH3/NOx molar ratio

-

Often 0.95-1.05 depending on strategy.

Utilization factor

0-1

Accounts for mixing, distribution, and effectiveness.

Safety factor

%

Adds margin for variability and control tolerance.

Reagent selection

Choose based on site logistics and safety requirements.

Aqueous NH3 concentration

wt%

Typical range 19-30 wt%.

Aqueous solution density

kg/L

Enter supplier density for better accuracy.

Urea concentration

wt%

Use 32.5 wt% for standard DEF.

Urea solution density

kg/L

Typical DEF density is about 1.09 kg/L.

Notes (optional)

Notes are included in CSV/PDF exports.

Reset

Example data table

A realistic scenario to verify your setup.

Case	Exhaust flow (Nm3/h)	Inlet NOx (ppmv)	Target	Reagent	NH3 required (kg/h)	Solution flow (L/h)
Example	2,500	450	85% reduction	32.5% DEF	0.63	5.3
High-load	4,000	700	80% reduction	32.5% DEF	1.52	12.9
Tight outlet	2,200	520	Outlet 70 ppmv	Aqueous NH3	0.71	3.1

Example outputs are illustrative. Use site-specific densities and NOx basis for design work.

Formula used

Core relationships used in the calculator.

Exhaust molar flow at normal conditions
Exhaust_kmol/h = Flow_Nm3/h ÷ 22.414
22.414 Nm3/kmol is the normal molar volume.
Convert inlet NOx to molar flow
NOx_kmol/h = Exhaust_kmol/h × (NOx_ppmv ÷ 1,000,000)
If mg/Nm3 is used: ppmv = (mg/Nm3) × 22.414 ÷ MW.
NOx removed
Removed_kmol/h = NOx_in_kmol/h × Reduction
Or computed from an outlet target: inlet minus outlet.
NH3 requirement with utilization and safety
NH3_kmol/h = (Removed × Stoich × Ratio ÷ Utilization) × (1 + Safety%)
NH3 mass flow: NH3_kg/h = NH3_kmol/h × 17.031.
Reagent conversion
Urea: 1 mol urea produces 2 mol NH3. Solution volume uses your density and concentration.

How to use this calculator

A practical workflow for design and checks.

Measure or estimate normalized exhaust flow at the operating point.
Enter inlet NOx as ppmv or mg/Nm3 using a consistent basis.
Select either a reduction goal or an outlet concentration target.
Set stoichiometric coefficient and molar ratio for your strategy.
Use utilization and safety factors to reflect field variability.
Choose reagent type, then input concentration and density if needed.
Press Calculate, review results, then export CSV or PDF.

Technical note

Professional guidance for applying SCR dosing results.

1) Purpose of ammonia injection in SCR

Selective catalytic reduction reduces NOx by reacting it with NH3 over a catalyst to form nitrogen and water. Dosing must track exhaust flow and inlet NOx, while preventing excess NH3 slip. This calculator converts your operating point into a required NH3 mass rate and an optional reagent solution flow for design work.

2) Typical input data ranges used in field checks

For many stationary and mobile duty cycles, normalized exhaust flow can range from 500 to 10,000 Nm3/h. Inlet NOx often falls between 100 and 1,200 ppmv (or equivalent mg/Nm3). Targets commonly specify 70% to 95% reduction or an outlet limit such as 50 to 150 ppmv, depending on permits and catalyst condition.

3) Factors that influence required NH3

The stoichiometric coefficient reflects NO/NO2 chemistry and how NOx is reported (frequently NO2‑equivalent). The NH3/NOx ratio captures control strategy, while utilization accounts for mixing, distribution, and catalyst effectiveness. Many commissioning teams begin with utilization of 0.85 to 0.95 and apply a 2% to 10% safety margin to cover sensor drift and transient operation.

4) Using outputs for pump, line, and nozzle sizing

The solution flow in L/h and injection rate in mL/min help select a metering pump and confirm turndown. For mid-load operation, DEF injection often falls near 3 to 8 L/h. Verify the minimum stable pump flow against low‑load operation, and confirm line velocities avoid crystallization when using urea solutions. For aqueous ammonia, check material compatibility and ventilation.

5) Engineering checks before issuing a dosing setpoint

Confirm the same reference conditions are used for flow and concentration, and validate NOx basis and molecular weight. Compare predicted outlet NOx with recent test data and catalyst age. Finally, document assumptions, densities, and safety factors using the CSV or PDF export so calculations remain traceable.

FAQs

Quick answers for common setup and interpretation questions.

1) Should I use NO or NO2 molecular weight?

Use the basis your analyzer reports. Many systems report NOx as NO2‑equivalent, so 46 g/mol is common. If you have speciated NO and NO2, adjust the coefficient and basis accordingly.

2) What does utilization factor represent?

It represents how much injected reagent effectively reacts with NOx after mixing and catalyst effects. Poor distribution, short residence time, or deactivation lowers utilization, increasing the required dosing rate.

3) Why add a safety factor?

Safety margin covers instrument uncertainty, load swings, and control lag. A small margin can prevent non‑compliance, but excessive margin can increase NH3 slip, deposits, and operating cost.

4) How is mg/Nm3 converted to ppmv here?

The calculator uses ppmv = (mg/Nm3) × 22.414 ÷ MW. This assumes normalized conditions consistent with the stated molar volume. Always match the reference conditions used by your data source.

5) Does urea solution dosing equal NH3 dosing?

Not directly. Urea thermally decomposes to produce NH3. The calculator converts required NH3 to an equivalent urea amount using 1 mol urea producing 2 mol NH3, then applies solution concentration and density.

6) What reagent should I select for construction sites?

Many sites prefer 32.5% urea solution due to handling and logistics. Aqueous or anhydrous ammonia may be used for stationary systems with trained personnel, safety controls, and clear regulatory requirements.

7) Can I use these results as a final control setpoint?

Use results for sizing and initial estimates. Final setpoints should be tuned with emissions testing, slip monitoring, and catalyst performance data. Record the operating point and assumptions with the exports for traceability.