SNCR and dioxins: how urea and additives can reduce PCDD/F

What dioxins are

“Dioxins” refers to two families of chlorinated organic compounds: polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF), together PCDD/F. They form in traces in many combustion processes and are persistent and toxic even at very low concentrations.

That is why they are measured in nanograms (billionths of a gram) of toxic equivalent: ng I-TEQ/Nm³. For incineration plants, EU Directive 2010/75/EU sets a stack limit of 0.1 ng I-TEQ/Nm³, referred to 11% O₂.

Where they form

Any dioxins in the fuel are destroyed in the combustion chamber at high temperature. The problem comes later: as the flue gas cools in the boiler and ducts, between about 250 and 450 °C, dioxins can re-form on the surface of the fly ash.

There are two main mechanisms:

• “de novo” synthesis: unburnt carbon in the ash reacts with chlorine and oxygen, with copper and other metals in the ash acting as catalysts;
• from precursors: already chlorinated compounds, such as chlorophenols and chlorobenzenes, combine and become dioxins.

Both need three ingredients: available chlorine, a metal catalyst and the right temperature window. Inhibition techniques act on the first two.

Why SNCR urea can slow them down

In an SNCR system urea is injected at around 850–950 °C. It decomposes and releases ammonia, which reduces NOx to nitrogen and water. Part of the ammonia, however, travels with the flue gas to the cooler zone where dioxins form. There, according to the studies, it can act in three ways:

• it removes chlorine: it reacts with hydrochloric acid to form ammonium chloride, leaving less chlorine to chlorinate the carbon;
• it “poisons” the catalyst: it binds to the copper in the ash and makes it less active;
• it interrupts chlorination: it blocks some of the reactions leading from organic compounds to dioxins.

In practice, besides cutting NOx, the SNCR system introduces an inhibitor of dioxin formation into the flue gas.

Nitrogen and sulfur additives

The same principle applies to other compounds that release ammonia and sulfur dioxide (SO₂) at high temperature. Sulfur is a particularly effective inhibitor: SO₂ consumes chlorine and, like ammonia, deactivates copper. Compounds studied as SNCR reagents or additives include:

• ammonium sulfate: releases both ammonia and SO₂;
• ammonium thiosulfate and persulfate: nitrogen and sulfur compounds;
• thiourea: a “urea with sulfur”.

The idea is to use the same SNCR injection system to dose a solution that reduces NOx and at the same time hinders dioxin re-formation. With urea alone the effect comes from ammonia; sulfur additives add the effect of sulfur.

What the studies say

The reference closest to practice is the study by Wielgosiński and colleagues (Sustainability, 2020), carried out on a real medical waste incinerator with a rotary kiln of about 400 kg/h. Reagents were injected between the post-combustion chamber and the recovery boiler, at around 950 °C, the normal SNCR position. Compared with injecting water only:

In the same tests NOx fell by up to about 90% with urea. The authors nevertheless call the results preliminary: a single plant, few measurements and no cost analysis.

Other studies confirm the mechanism, mostly in the laboratory:

• the study on the effect of ammonium sulfate and urea on PCDD/F formation from activated carbon describes the inhibiting effect and a possible mechanism;
• Wang and colleagues (Scientific Reports, 2016) show that some ammonium compounds block copper and stop it from transferring chlorine to carbon;
• studies on the effects of gaseous NH₃ and SO₂ on dioxin concentrations analyse the role of the two gases;
• the review by Conesa (Sustainability, 2021) on sewage sludge, rich in nitrogen and sulfur, reports inhibition of up to about 90% in real plants and identifies sulfur as the more effective inhibitor.

Limits and caveats

• It does not replace dedicated systems: activated carbon and bag filters remain the reference techniques for meeting the dioxin limit. Inhibition with SNCR is a supporting measure.
• Variable results: the effect depends on fuel, chlorine, ash, temperatures and residence times. It must be verified with measurements on the plant.
• Residual ammonia: increasing dosing to curb dioxins raises ammonia slip at the stack.
• Salts and fouling: ammonium chloride and bisulfate can deposit on cold surfaces of boiler and heat exchangers.
• Sulfur: sulfur additives increase SO₂ in the flue gas, with effects on corrosion and emissions to be assessed.

In short

• Dioxins re-form in the cooler part of the boiler, between about 250 and 450 °C, helped by chlorine and copper in the ash.
• Ammonia from SNCR urea can remove chlorine and deactivate copper, slowing their formation.
• Nitrogen and sulfur additives, such as ammonium sulfate, thiosulfate and persulfate, showed PCDD/F reductions of around 60–75% in a study on a real plant.
• These are preliminary results: useful as a supporting measure, to be verified case by case, without replacing activated carbon and filters.

For SNCR basics and the comparison with SCR see our SNCR and SCR guide; for an example of SNCR on a waste-to-energy plant, the 18.7 MW waste-to-energy reference.

Frequently asked questions

Sources

1. Wielgosiński G. et al., «Simultaneous NOx and Dioxin Removal in the SNCR Process», Sustainability 12(14), 5766, 2020
2. Conesa J. A., «Sewage Sludge as Inhibitor of the Formation of Persistent Organic Pollutants during Incineration», Sustainability 13(19), 10935, 2021
3. Wang S.-J. et al., «Amino Compounds as Inhibitors of De Novo Synthesis of Chlorobenzenes», Scientific Reports, 2016
4. «Effect of ammonium sulfate and urea on PCDD/F formation from active carbon and possible mechanism of inhibition» (AGRIS, FAO)
5. Study on the effects of gaseous NH₃ and SO₂ on PCDD/F concentrations (Aston University)
6. Directive 2010/75/EU on industrial emissions (EUR-Lex)