SNCR system: how it works and what it is made of

What is an SNCR system

SNCR stands for Selective Non-Catalytic Reduction. It is one of the two technologies used to remove NOx, the nitrogen oxides formed in every combustion process; the other is SCR, which works with a catalyst.

In SNCR the reagent, usually a urea solution in water, is atomised into the upper part of the combustion chamber. At the right temperature urea releases ammonia, which reacts with NOx and turns it into nitrogen and water vapour, the same components as the air we breathe.

No catalyst or separate reactor is needed: the boiler itself is the reactor. This is why SNCR takes up little space and is well suited to existing plants.

How it works, step by step

1. Storage. The aqueous urea solution is delivered by tanker or in IBC containers and kept in a dedicated tank, sized for several days or weeks of operation.
2. Dosing. The metering pumps draw the amount of urea needed at that moment and send it to the lances. Where required, the solution is diluted with water before injection.
3. Atomisation. In each lance urea meets compressed air, which breaks it into very fine droplets.
4. Injection. The lances spray the urea mist into the zone of the boiler where the flue gas is between about 850 and 950 °C.
5. Reaction. The water evaporates, urea decomposes and releases ammonia, which reduces NOx to nitrogen and water vapour as the flue gas rises.
6. Measurement. The analyser at the stack continuously measures the NOx leaving the plant and sends the signal to the control panel.
7. Regulation. The PLC panel increases or reduces pump dosing to stay below the limit with the lowest urea consumption.

What it is made of: the components

An SNCR system is made of a few blocks, each with a precise task. In our systems the dosing components are mounted on skids, pre-assembled and tested in the workshop, so only the connections are left for the site.

• Storage tank: in a material compatible with urea, with level indicator or load cells. In cold climates a heater is added: the 32.5% solution starts to crystallise at around −11 °C.
• Dosing skid: metering pumps, normally in a 1+1 arrangement so the system keeps running if one fails, with filters, safety valves, pressure gauges and flow meters.
• Dilution line, where needed: brings urea to the right concentration for the boiler and for the distance the jet has to cover.
• Atomising air: compressed air from the plant network or from a dedicated compressor.
• Injection lances: enter the boiler through nozzles in the wall. They are cooled to withstand the chamber temperature.
• Piping: stainless steel where in contact with urea, from the skid to the lances.
• PLC control panel: manages pumps and alarms, allows manual or automatic operation with the 4–20 mA signal from the NOx analyser.

The parameters that decide the result

Two SNCR systems with the same components can give very different results. Four parameters make the difference.

Temperature

The reaction works well between about 850 and 950 °C. Below, the reaction is slow and part of the ammonia leaves through the stack unreacted. Above 1,000 °C ammonia burns and produces more NOx instead of removing it.

Residence time

After injection, the flue gas must stay in the hot zone long enough for the reaction to complete: at least 0.3 seconds, preferably more than one second.

Coverage of the cross-section

Urea must reach the whole cross-section crossed by the flue gas. If part of the gas passes without meeting the reagent, that NOx leaves through the stack. Coverage depends on the number and position of the lances, on how fine the atomisation is and on the direction of the jets.

Amount of reagent

More urea is dosed than theoretically needed, but without overdoing it: the ratio between reagent and NOx to be reduced is adjusted continuously based on the stack measurement. Too little urea will not meet the limit; too much raises costs and the residual ammonia (ammonia slip), which permits usually limit to 5–10 mg/Nm³.

Where the lances go

Lance positions are decided case by case, starting from the boiler geometry and the temperature map. This is why every system we build starts with a site visit and the plant’s operating data.

Two aspects matter more than the others:

• Boiler load. We position the lances based on how the boiler actually runs, from minimum to full load, so the reaction stays effective when output changes.
• Jet direction. In new boilers the ports are made already angled; in retrofits, with existing nozzles, the jet can be angled instead of the port. We cover this in our insight on offset spray in SNCR lances.

Efficiency and limits

A well-sized SNCR system typically reduces NOx by 30–60%, up to 70% in favourable conditions: stable temperature, good residence time, good coverage of the cross-section.

It is the best fit when emissions must go below limits such as 200–300 mg/Nm³ starting from values that are not too far off, as is often the case with biomass boilers and small waste-to-energy plants.

When deeper reductions are needed, above 80–90%, SCR is used: it works with a catalyst at lower temperatures. The two technologies can also be combined: the SNCR in the boiler lightens the load on the catalyst, as in our reference on an 18.7 MW waste-to-energy plant. The full comparison is in our DeNOx SNCR and SCR guide.

Where it is used

• Biomass boilers burning wood chips, bark or pellets, for district heating and industrial processes. Some examples: our references in France, in Switzerland and in Cosio Valtellino.
• Waste-to-energy plants and incinerators, on their own or upstream of an SCR.
• Industrial furnaces and boilers with a flue gas zone at the right temperature and accessible for the lances.

Retrofit on existing plants

Most SNCR systems are installed on boilers already in operation, when emission limits or the fuel change. Retrofit is possible because the system needs no major structural work:

• the skid arrives pre-assembled and is placed near the boiler;
• the lances enter through existing nozzles or new ones made during a planned shutdown;
• plant downtime is limited to the time needed to insert and connect the lances.

An existing SNCR system that underperforms can also be improved: in the revamping of a 3.5 MW biomass boiler we reworked the lances and the jet direction to spread urea better across the chamber.

Operation and maintenance

An SNCR system needs little maintenance, but on a regular basis:

• Lances: periodic check of the nozzles, which can foul or wear over time and worsen atomisation.
• Pumps and filters: filter cleaning and pump checks according to the manufacturer’s instructions.
• NOx analyser: periodic calibration, because automatic dosing is only as reliable as the measurement.
• Tank: level check and, in winter, heating check.

Why urea lines need flushing, and how, is explained in our insight on flushing SNCR urea lines and lances.

The main operating cost is the urea consumed, which depends on how much NOx has to be removed and on how well the reagent is distributed in the flue gas.

What the cost depends on

Every SNCR system is sized on the boiler, so there is no standard price. The cost depends mainly on:

• flue gas flow and amount of NOx to be reduced, which set urea consumption and pump size;
• number and position of the lances;
• tank capacity, i.e. how many days of autonomy are needed;
• whether a dilution line and tank heating are required;
• level of automation and integration with the boiler control system;
• installation complexity, especially in retrofits.

With the boiler data and the limits to be met, we can prepare an estimate from the first enquiry.

Frequently asked questions

Sources

1. U.S. EPA, Air Pollution Control Cost Manual, chapter “Selective Noncatalytic Reduction” (7th edition)