Jet geometry
An SNCR system reduces NOx by injecting urea where the flue gas is at about 850–950 °C. For the reaction to work, the reagent must reach the whole flue gas section while the temperature is right.
That is why it is not only how much urea you inject that matters, but where the jet goes. As the US EPA manual on SNCR systems points out, inadequate mixing results in insufficient NOx reduction; and it is the spray angle and velocity that decide the reagent’s trajectory.
With the same amount of urea, a well-oriented jet reduces more NOx and leaves less ammonia at the stack.
New builds: ports already angled
When a new boiler is built, injection points are designed together with the SNCR system. The wall ports are usually made already angled, at an inclination studied for the chamber geometry: the lance enters at an angle and the jet, in line with the lance, goes where it is needed.
Retrofit: working with existing nozzles
On boilers and incinerators already in operation things are different: you often have to use the existing nozzles (wall ports), provided by the boiler maker for other purposes or for a previous system, and almost always perpendicular to the wall. Re-drilling means stopping the plant and working on the boiler.
With straight-spray lances in perpendicular ports, in a cylindrical chamber or a narrow duct, all jets point to the centre and collide:
- too much reagent in the centre: part of the urea finds no NOx to react with and becomes residual ammonia (slip);
- local cooling: the water in the solution evaporating in one spot lowers the flue gas temperature, and the reaction slows down;
- uncovered zones: near the walls and between lances the flue gas passes without meeting the reagent.
Angle the jet, not the port
The solution is to move the angle from the port to the jet. The lance is fitted in the existing nozzle, perpendicular to the wall; it is the nozzle tip, with its air cap, that makes the spray leave at an angle to the lance axis: the so-called offset spray.
- Mounting flange on the existing nozzle
- Urea, atomising air and cooling air inlets
- Nozzle with an air cap that turns the jet
Simplified animation, not to scale: the jet changes from on-axis to offset.
If all lances turn their jets in the same direction, the sprays follow the circumference of the chamber instead of colliding in the centre: turbulence improves, the reagent covers the whole section better and evaporation is spread out.
Together with our Italian partners we have made nozzles with different jet angles, chosen according to the chamber geometry and the position of the existing nozzles. This way the jet orientation adapts to the plant, instead of adapting the plant to the lances.
What to assess in design
- Temperatures in the injection zone at different boiler loads: the jet must work inside the useful window.
- Number and position of existing nozzles, and space available around the boiler.
- Jet angle and droplet size: larger droplets penetrate deeper into the flue gas, finer droplets evaporate sooner.
- Atomising and cooling air, which protect the lance and shape the spray.
- Check at start-up: water test of the lances, then NOx and ammonia measurements at the stack to tune dosing.
A real example
In the revamp of an SNCR system on a 3.5 MW biomass boiler the new boiler had a cylindrical chamber with three nozzles at 120°, set by the boiler maker. The offset-spray lances were fitted in the existing nozzles, with no changes to the boiler, and at start-up both the NOx limit and the residual ammonia limit were met.
For the basics of the technology see our SNCR and SCR guide.
Frequently asked questions
Why is jet geometry so important in SNCR?
Because the reagent must reach the whole flue gas section within the right temperature window. If jets overlap or leave zones uncovered, NOx reduction drops and residual ammonia rises, even with more urea.
Angled ports or offset spray?
In new builds the ports are made already angled. In retrofits you often work with existing nozzles perpendicular to the wall: in that case you angle the jet, with an offset nozzle, instead of the port.
Can an existing SNCR system be adapted without modifying the boiler?
Often yes. With an offset spray the lance fits the existing nozzle and the nozzle tip turns the spray in the right direction.
Does an offset spray increase urea consumption?
No. By distributing the reagent better, for the same NOx reduction it tends to lower the urea needed and the residual ammonia.