Urea Dosing Pump for SCR and SNCR: What It Does and How to Specify One

A urea dosing pump has one job in a DeNOx plant: deliver the right amount of urea solution, steadily, to the place where it becomes a reducing agent. In SNCR, that means the furnace injectors. In SCR systems that use urea as the reagent source, it means the decomposition chamber or hydrolyzer, where urea is turned into ammonia before it reaches the catalyst. Feed too much and you waste reagent and raise ammonia slip; feed too little and NOx goes over the limit. This article is about stationary plants such as power and industrial boilers, cement kilns and waste-to-energy lines, not vehicle DEF systems, and is written for process and maintenance engineers.

What is a urea dosing pump?

In most plants it is a positive displacement metering pump, usually a diaphragm type. Each stroke moves a fixed volume, and flow is changed by stroke rate or stroke length. Unlike a centrifugal transfer pump, its output barely changes when discharge pressure moves, which is exactly what you want when flow must follow a control signal.

On DeNOx drawings the term is sometimes used loosely for a whole metering module: pumps, flow meters, control valves, gauges and filters on one skid. Read the P&ID carefully. Some designs meter directly with the urea dosing pump, others use a circulation pump with control valves and flow meters, and the pump requirements differ between the two.

What a urea dosing pump does in SNCR

SNCR uses no catalyst. Diluted urea solution is sprayed into the furnace in the appropriate temperature zone, where it decomposes and reduces NOx. It is common on circulating fluidized bed boilers, cement kilns and waste incinerators. Here the urea dosing pump directly sets how much reagent enters the furnace, and with several injector levels the flow often has to be split by level.

What a urea dosing pump does in SCR

In SCR the reduction happens on the catalyst. When urea is the reagent, it is first converted to ammonia in a decomposition chamber or hydrolyzer. The pump feeds that conversion unit, so its accuracy carries straight through to the ammonia injection grid.

Both processes punish unstable metering. In SNCR, excess reagent leaves the stack as ammonia slip. In SCR, slipped ammonia can react with SO₃ in the flue gas to form ammonium bisulfate, which deposits on the air preheater and raises its pressure drop.

Signs your urea metering is off

• NOx readings swing in ways that do not follow load;
• urea consumption runs well above design and slip readings climb;
• air preheater differential pressure creeps up over weeks;
• some injectors spray heavily while others barely spray.

Before retuning the control loop, check whether the urea dosing pump is actually delivering what it is set to deliver. Urea crystallizes easily, and sticking valves are the most common cause of low output. Our guide to a urea metering pump not dosing in SCR systems gives the troubleshooting order.

How to specify a urea dosing pump

1. Materials. Urea solution is hard on copper alloys and plain carbon steel, so pump heads, valves and piping are commonly stainless steel, with seals compatible with urea and flush water.
2. Flow and turndown. Boiler load varies widely, so the gap between minimum and maximum urea demand is large. The pump must stay in a sensible stroke range at low load as well. Our article on mechanical diaphragm pump sizing and stroke margin explains how to leave margin.
3. Discharge pressure. Take the injector or conversion unit requirement, add line losses, and fit a back pressure valve and relief valve.
4. Control. Most units take a 4-20 mA signal from the DCS, with motor-driven pumps on a VFD. See 4-20 mA vs pulse control for metering pumps.
5. Redundancy. Continuously running units need duty and standby pumps, rotated on a schedule.
6. Heat tracing and flushing. In cold climates the pump and lines need tracing and insulation, and every urea dosing pump head should have a flush connection so it can be cleaned with demineralized water before shutdown.

For larger flows with variable speed, a mechanical diaphragm metering pump is the usual choice, and the complete metering and distribution module can be built as a chemical dosing skid.

Two things projects often miss

The first is low-load operation. Many projects size the urea dosing pump for full load, then the boiler spends most of its life at part load. The pump ends up at very low stroke or speed, accuracy drops and crystals stick valves more easily. Calculate urea demand at minimum load too, and consider a small and large pump combination where the range is wide.

The second is distribution across injectors. When one pump feeds several lances, differences in pipe length and resistance give uneven spray. Fitting a flow meter or regulating valve ahead of each lance and checking each one during commissioning beats adding offsets in the DCS later.

Safety note: stop the pump, close valves and release pressure before opening a pump head or line. Isolate power before working on heat tracing, and watch for hot solution when breaking connections.

Conclusion

A urea dosing pump meters urea solution to SNCR injectors or to an SCR decomposition or hydrolysis unit in step with load and NOx, and poor metering shows up as NOx excursions or ammonia slip. To specify one, focus on materials, turndown, pressure, control signal, redundancy, heat tracing and flushing. Send us the load range, urea concentration, number of injectors and required pressure, and contact our engineers to check the pump and configuration.

FAQ

Is a urea dosing pump different from a normal chemical dosing pump?

The principle is the same positive displacement metering. The differences are materials, heat tracing and flushing provisions, and tighter turndown requirements.

Can a solenoid metering pump be used for DeNOx?

For very small SNCR systems, possibly. Most boiler and kiln projects need more flow, so motor-driven diaphragm pumps on a VFD are typical.

Does the pump need recalibrating if urea concentration changes?

Yes. The same volume now carries a different amount of urea, so the conversion factor in the control system must change too.

We have both a pump setting and a flow meter. Which one do we trust?

Trust the flow meter reading and use the pump setting as a reference. A growing gap between them points to valve or diaphragm problems.

Can one urea dosing pump serve both an SCR and an SNCR system?

Not recommended. The two systems need different pressures and flows, so give each its own pumps for clearer control and maintenance.