SNCR vs. SCR: What Is the Difference?
From Korea's emission charges and cap system to principles, limits and how to choose
01 · REGULATION
Why is NOx reduction required in Korea?
In Korea, DeNOx is needed because NOx emissions are controlled by three systems at once, covering concentration, total load and charges, and costs rise with emissions. Nitrogen oxides (NOx) is the collective term for nitric oxide (NO), nitrogen dioxide (NO₂) and related gases formed during combustion. Most of the NOx in flue gas is NO, and because NOx is a precursor of fine particulate matter and ozone, its emissions are regulated.
| System | What it limits | Impact on facilities |
|---|---|---|
| Emission standards (concentration) | An upper limit on pollutant concentration at each stack. Limits vary by facility type, size and installation date. | Exceeding the limit triggers an excess emission charge.[1] |
| Regional total-load (cap) management | Under Korea's Special Act on Air Quality Control Areas, each cap-managed facility is allocated an annual emission cap. | Exceeding the cap incurs a cap-exceedance penalty and a smaller allocation the following year. Pollutants under cap management are exempt from the basic emission charge.[2][3] |
| Emission charges | Calculated from a unit rate of KRW 2,130 per kg of NOx (excess emission charge criteria), multiplied by exceedance-rate and regional coefficients.[1] | Costs rise with emissions. At the unit rate, 1 metric ton of NOx comes to about KRW 2.13 million. |
Cap management began in the Seoul metropolitan area and, from 2020, was extended to three more regions, the central, southern and southeastern regions, where 799 facilities at the time were allocated annual emission caps.[2] Meeting instantaneous concentration limits is therefore not enough; annual emissions themselves have to come down. The amounts actually charged and the coefficients depend on each facility's conditions, so treat the unit rate above only as an indication of scale.
Concentration limits are set by facility type and size in Annex 8 of Korea's Clean Air Conservation Act Enforcement Rule. For example, under the current Annex 8, the NOx limit for wastewater, waste and waste gas incineration facilities is 70 ppm for an incineration capacity of 2 tons per hour or more and 90 ppm below 2 tons per hour, expressed as NO₂ and corrected to a standard oxygen concentration of 12%.[4] Where NOx comes from, and why it is expressed as NO₂ and corrected for oxygen, is covered in Where Does Nitrogen Oxide (NOx) Come From?.
02 · REACTION
What reaction does DeNOx rely on?
DeNOx is treatment that converts NOx into nitrogen (N₂) and water (H₂O) with a reagent such as ammonia or urea. The key reaction is 4NO + 4NH₃ + O₂ → 4N₂ + 6H₂O.
Figure 1. NOx in flue gas (typically NO and NO₂) meets the reagent ammonia (NH₃) and is converted into nitrogen (N₂) and water (H₂O). Red is nitrogen, dark blue is oxygen and light blue is hydrogen. The equation is a concept based on NO, showing the coefficients and the oxygen supply.
Source: Based on Micro-one in-house technical data · concept illustration
A catalyst is a substance that lowers the activation energy barrier of a reaction without being consumed itself. Because it lets the same reaction proceed at a lower temperature, SCR can reach high efficiency at far lower temperatures than SNCR.
Nitrous oxide (N₂O) is managed separately from NOx as a greenhouse gas. EPA data also note that in SNCR with urea as the reagent, part of the NOx can be converted to N₂O.[5]
03 · COMPARISON
How do SNCR and SCR differ in principle, temperature and efficiency?
SNCR works in a high-temperature zone of about 870–1,100°C (1,600–2,000°F) with a typical field efficiency of 30–50%,[5] while SCR operates at about 250–427°C (480–800°F) and is expected to achieve 70–90%.[6] The figures below are based on U.S. EPA data and vary with equipment, fuel and load.
Figure 2. Top: in SNCR, the reagent reacts with NOx directly in the high-temperature zone. Bottom: in SCR, flue gas mixed with the reagent reacts as it passes through the catalyst bed. The NOx left over in the SNCR panel illustrates the efficiency gap between the two methods; for actual efficiencies, see the table below.
Source: Based on Micro-one in-house technical data · concept illustration
| Item | SNCR (selective non-catalytic reduction) | SCR (selective catalytic reduction) |
|---|---|---|
| Layout (schematic) | ||
| Catalyst | None. | Uses a catalyst bed. |
| Reaction location | Reagent is injected directly into a high-temperature zone, such as an incinerator or boiler furnace. | The reaction takes place in the catalyst bed of a separate reactor. |
| Operating temperature | Ammonia about 870–1,100°C (1,600–2,000°F), urea about 900–1,150°C (1,650–2,100°F)[5] | About 250–427°C (480–800°F), depending on catalyst and flue gas composition[6] |
| NOx removal efficiency | 30–50% in typical field applications; up to 75% reported in demonstrations[5] | 70–90%. Some designs target 90% or more.[6][7] |
| Ammonia slip | Controlled at about 5–10 ppm in reported cases[5] | U.S. permitted levels are usually 2–10 ppm, and well-designed systems keep slip at around 2 ppm or lower.[6][7] |
| Installation | Injection nozzles are added to the existing combustion unit, with no separate reactor. | Needs space for a reactor and, depending on location, reheating.[7] |
| Investment and operation | Capital cost is generally lower; managing reagent consumption is the key. | Investment is high, and catalyst replacement and maintenance costs recur. |
| Weak points | The narrow temperature window makes it sensitive to load changes.[5] | Dust and poisons reduce catalyst activity.[6] |
Source: Based on Micro-one in-house technical data · Layout drawings are schematics; actual structures and dimensions differ.
How do urea solution and aqueous ammonia differ?
Reagent choice depends less on price than on handling, spray conditions and by-product control. EPA data note that ammonia is generally cheaper than urea, but urea is more widely used in large boilers because it is safer to handle and disperses better when sprayed.[5] Aqueous ammonia is usually shipped and stored as a solution of around 29%, and urea is typically used as a 50% solution.[5][7]
In SNCR with urea, part of the NOx can be converted to N₂O,[5] and in SCR, urea is converted to ammonia before injection, with urea systems rated as having the highest capital cost.[7] The right reagent is chosen by weighing reaction temperature, storage and safety conditions, and the permitted slip together.
04 · LIMITS
Where does each method hit its limits in the field?
SNCR hits its limit where pushing efficiency raises ammonia slip; SCR hits its limits on space and dust conditions.
SNCR: adding reagent to raise efficiency increases slip
SNCR has a narrow temperature window. EPA data explain that gas temperature varies widely by location inside a boiler and falls at low load.[5] If the injection point moves outside the optimum temperature range, efficiency drops.
Over-injecting reagent to make up the shortfall then creates a vicious circle of higher chemical costs and unreacted ammonia leaving the stack. EPA data state that ammonia causes odor at 5 ppm or more and raises health concerns at 25 ppm or more.[5] The SCR data also note that salts formed from ammonia and sulfur compounds can plug or foul equipment.[6]
No. Beyond a certain point, the extra NOx reduction becomes small and only slip may increase. First check the injection location, nozzle angle and dosing control linked to load and oxygen concentration, and if needed, confirm the temperature distribution with computational fluid dynamics (CFD).
SCR: space, investment and catalyst deactivation
In SCR, providing enough residence time in the catalyst bed makes the reactor larger, and more catalyst also means higher pressure drop and maintenance. Where space is short, elevated installation and longer ductwork complicate the layout and raise construction difficulty and cost. Total investment varies widely with the site, ductwork, reheating and auxiliary equipment, so it should be compared using site-specific quotes.
Flue gas inReagent injectionCatalyst layersTreated gas out
1NOx · NH₃ in2Reaction on catalyst3N₂ · H₂O outFigure 3. Left: an SCR reactor in which reagent-dosed flue gas passes downward through the catalyst layers. Right: a close-up of the channels in one catalyst block. Concept illustration; the number of layers, size and cell arrangement vary by site.
Source: Based on Micro-one in-house technical data · AI-assisted concept illustration
Flue gas enters the top of the reactor, mixes with the reagent and passes downward through the catalyst layers. The honeycomb catalyst is densely packed with long, narrow channels, so NOx and reagent react and turn into nitrogen and water as the flue gas flows along the channel walls. The design provides a large reactive surface within the same reactor size.
Catalysts deactivate in service. EPA data list alkali metals such as potassium and sodium, as well as arsenic, chlorine, fluorine and lead, as catalyst poisons,[7] and explain that dust can cover or plug the catalyst surface while particle impact and high gas velocity erode it.[7] As activity falls, NOx removal drops and ammonia slip rises.[6]
Reactor location reverses the trade-offs
EPA data divide SCR arrangements into high-dust, low-dust and tail-end.[7] A high-dust arrangement places the reactor upstream of the dust collector and is considered the most economical and simplest design when space allows, but dust and poisons reach the catalyst directly. A tail-end arrangement after all other control equipment gives the catalyst clean gas, but the flue gas must be reheated, which raises operating cost significantly.[7] In short, protecting the catalyst costs reheating, and avoiding reheating costs catalyst life.
05 · COMBINATION
How are the two methods used together under stricter limits?
Under stricter limits, the more efficient SCR is used, typically run in series with dust collection as separate equipment. Dust is removed from the flue gas first, the gas is brought to reaction temperature, and it then passes through the SCR reactor. The full sequence of an incinerator flue gas treatment system is covered in How Is an Incinerator Flue Gas Treatment System Arranged?.
Dust collection is handled by a baghouse or an electrostatic precipitator; the difference between the two is covered in Electrostatic Precipitator (ESP) vs. Baghouse: What Is the Difference?. In plants such as waste incinerators, a combination is also used depending on site conditions: SNCR provides first-stage reduction in the combustion zone, and a downstream SCR trims the remainder. The right configuration depends on the target concentration, flue gas temperature and site.
06 · SELECTION
In what order should selection and checks be done?
Start from the target efficiency and narrow down by temperature, space and dust conditions, and the options shrink quickly.
- Target concentration and required efficiency: Work back from current emission concentrations and the applicable limits (concentration and cap) to the reduction needed. If the required reduction is within 30–50%, SNCR is also a candidate; if higher, consider SCR or a combination of both.
- Flue gas temperature profile: Measure whether temperatures at possible injection points fall within the SNCR temperature window, and whether reheating to SCR reaction temperature is needed downstream. Check load-swing periods too.
- Installation space and ductwork: Check the footprint and access for the reactor, reheating equipment and reagent storage and injection equipment.
- Dust and catalyst poisons: Check alkali metals, chlorine, sulfur compounds and dust concentration to decide where to place the catalyst.
- Reagent and permitted slip: Choose urea solution or aqueous ammonia to suit storage and safety conditions, and set the acceptable ammonia slip.
- Cost comparison: Compare capital and operating costs (reagent, reheating, catalyst replacement) with emission charge and cap burdens over the same period.
- Checks in operation: Regularly check ammonia slip, pressure drop and changes in efficiency. If slip rises or efficiency falls, check the injection location, catalyst condition and temperature first.
Deciding on the temperature at average load alone. Temperatures during startup, shutdown and low-load operation must also be checked.
FAQ
Frequently asked questions
Why is SNCR less efficient than SCR?
Because it relies on high temperature alone, without a catalyst. The temperature range in which the reaction occurs is narrow, and efficiency depends heavily on how well the reagent mixes with NOx. EPA data put typical field efficiency at 30–50%.[5]
Why does SCR need a reactor and reheating?
To match the temperature range in which the catalyst works and to provide enough residence time. If the reactor sits after the dust collector, the flue gas has cooled and must be reheated; if it sits before, the catalyst is exposed to dust and poisons.[7]
Should I use urea solution or aqueous ammonia?
It depends on site conditions. Ammonia is generally cheaper than urea, but urea is widely used in large boilers for its handling safety and spray dispersion. Decide based on storage and safety conditions and reaction temperature.[5]
In Korea, are cap-managed facilities exempt from the basic emission charge?
Yes. Pollutants under cap management are exempt from the basic emission charge. However, exceeding the cap incurs a cap-exceedance penalty and a smaller allocation the following year.[3]
Can SNCR and SCR be used together?
Yes. Depending on site conditions, SNCR can provide first-stage reduction in the combustion zone, with SCR downstream removing the remaining NOx. Capital and operating costs are higher than for either method alone, so compare the options against the target concentration.
- Korea Ministry of Government Legislation, Korea's Clean Air Conservation Act Enforcement Decree, Annex 4, "Criteria for Calculating Excess Emission Charges" (in Korean)
- Seoul Shinmun, "Cap management of workplace air pollutants cuts emissions 38.8% in five years" (in Korean), October 20, 2020
- Etoday, "Air pollutant cap system expanded nationwide, applied to 799 workplaces" (in Korean), October 20, 2020
- Korea Ministry of Government Legislation, Korea's Clean Air Conservation Act Enforcement Rule, Annex 8, "Permissible Emission Standards for Air Pollutants" (in Korean)
- U.S. EPA, "Air Pollution Control Cost Manual, Section 4.2 Chapter 1 Selective Noncatalytic Reduction", 2002
- U.S. EPA, "Air Pollution Control Technology Fact Sheet: Selective Catalytic Reduction (SCR)"
- U.S. EPA, "Air Pollution Control Cost Manual 7th Edition, Chapter 2 Selective Catalytic Reduction", 2019