What Are the Options When Incinerator NOx Limits Get Stricter?

Starting from existing DeNOx equipment: the conditions and limits of each option, and the order in which to choose

01 · REGULATORY PRESSURE

What changes for a plant in Korea when emission limits get stricter?

More than the concentration limit changes. For incinerators in Korea, four pressures act through separate channels: concentration, total load, charges and permits.

• Concentration: These are the permissible emission standards in the Enforcement Rule of the Clean Air Conservation Act, Annex 8. Values differ by the type, capacity and installation date of the incineration facility, so the current limit for a given facility should be checked directly in Annex 8. In the amendment effective January 2020, NOx limits were tightened by about 28% on average across all emission facilities (the degree of tightening varies by facility).^[1]
• Total load: Under the Special Act on the Improvement of Air Quality in Air Control Zones, sites within an air control zone that emit NOx, SOx or dust above a certain scale (Class 1–3 sites) are allocated an emission cap separate from the concentration limits.^[2] Even when the concentration limit is met, the cap can become the binding constraint first if operating hours or throughput increase.
• Charges: NOx has been subject to emission charges since 2020. Even within the permissible limit, emissions above a certain concentration incur a basic charge based on the amount emitted, and exceeding the limit incurs an excess charge. However, total-load management sites that have been allocated a NOx cap are exempt from the NOx basic charge, and instead pay a cap-exceedance penalty if they exceed the cap.^[11] For this reason, keeping a margin is often more cost-effective than barely meeting the limit.
• Permits: For sites subject to an integrated environmental permit under the Act on the Integrated Control of Pollutant-Discharging Facilities, limits for each emission facility are written into the permit conditions.^[3] Permit conditions and permitted emission limits are reviewed periodically, and again whenever the facility is modified.

On top of this comes stack TMS (tele-monitoring system) management. A TMS 30-minute average is the mean of six consecutive 5-minute values,^[4] so even when the average concentration is within the limit, short spikes can push a 30-minute average up. The goal is twofold: lower the average and reduce fluctuation.

02 · EXISTING EQUIPMENT

What should be checked before looking at the options?

The first thing to check is whether DeNOx equipment is already installed. The same tightening calls for different measures at sites with and without it.

EXISTING DENOX EQUIPMENTHow do the options change if DeNOx equipment is already installed?

Figure 1. An incinerator with existing SNCR and SCR (left) compared with one that has only a baghouse and no DeNOx equipment (right) (concept illustration)

Left: a plant with in-furnace injection (SNCR) and a catalytic reactor (SCR). Right: a plant with only a baghouse and no DeNOx equipment.

Existing DeNOx equipment (SNCR or SCR)

Supplement to close the gap

• Options: SNCR upgrade, SNCR optimization + downstream polishing, catalytic filter bags
• Check first: why it is at its limit (injection location and rate, temperature, catalyst deactivation)

No existing DeNOx equipment

New-build alternatives: choosing a method

• Options: high-dust or tail-end SCR, ceramic candles, catalyst module above the baghouse, ozone oxidation + wet scrubbing
• If a baghouse is in service: catalytic filter bags are also a candidate
• Check first: flue gas temperature, installation space

Source: Based on Micro-one in-house technical data · AI-assisted concept illustration · actual shapes and dimensions differ

With existing equipment, the task is to supplement it

If SNCR or SCR is already installed, the first candidates are measures that close the shortfall rather than a switch to new equipment. What needs checking is why the existing equipment has reached its limit. The right supplement depends on whether the problem lies in the reagent injection location and rate, a temperature mismatch or catalyst deactivation.

Without equipment, a new-build alternative is needed

An incinerator without DeNOx equipment has to choose a method from scratch. Rather than looking at efficiency alone, first map out which equipment sits in the current flue gas train and in what order. The system layout is covered in How Is an Incinerator Flue Gas Treatment System Arranged?. In trains with little space or a low flue gas temperature, the options narrow sharply.

03 · OPTIONS

What are the options, and what are the conditions and limits of each?

NOx reduction options for incinerators fall broadly into eight types. The table below compares the conditions each method requires and the limits it runs into. Efficiency figures vary widely with site conditions, so they are left out of the table.

Four representative methods from the table are summarized below with illustrations. The cards show at a glance where each method reduces NOx and what it requires.

NOx REDUCTION OPTIONSHow do SNCR, SCR, catalytic filters and ozone oxidation differ?

1SNCR

The temperature window is everything

• Ammonia or urea is injected into the hot zone inside the incinerator to reduce NOx without a catalyst.
• The reaction temperature window is roughly 870–1,150°C depending on the reagent (ammonia 870–1,100°C, urea 900–1,150°C);^[5] outside it, efficiency drops or unreacted ammonia is left as slip.
• Because temperature swings with waste composition, a common upgrade is adding injection points at several levels.
• Adding more reagent raises ammonia slip, which has to be managed as well.

2SCR

Its character depends on location

• Reagent and NOx react on a catalyst, so higher efficiency than SNCR can be expected even at lower temperatures.
• The catalyst has a defined operating temperature range, and ammonia slip and poisoning must be managed.^[6]
• High-dust (before dust collection): If the gas is within the catalyst temperature range, it can run without reheating, but dust reaches the catalyst.
• Tail-end (after dust collection and acid gas treatment): The catalyst stays clean, but the gas must be reheated.

3Catalytic filter

Filtration and DeNOx in one place

• The filter medium that captures dust also carries the catalyst.
• Ceramic candle versions are offered for hot gas,^[7] while fiber-type versions have the same shape as conventional filter bags and fit into a baghouse.^[8][9]
• A catalyst module above the baghouse keeps the filters as they are and adds a catalyst module in the upper section of the baghouse.
• They require a baghouse in service, with matching dimensions and operating temperature.

4Ozone oxidation + wet scrubbing

An alternative for low-temperature trains

• Ozone oxidizes poorly soluble NO, via NO₂, to highly soluble N₂O₅, which is then absorbed in the scrubbing water.^[10]
• NO has low solubility and is poorly removed by a conventional scrubber alone, so an oxidation step is needed.
• With flue gas below 100°C and a scrubber configured to absorb soluble NOx, NOx can be reduced by up to 80–95%. Without these conditions, these figures cannot be expected.
• The system pairs an ozone oxidation system with a downstream wet scrubber.

Source: Based on Micro-one in-house technical data · AI-assisted concept illustration · actual shapes and dimensions differ

04 · SELECTION CRITERIA

By what criteria, and in what order, should the options be chosen?

There are five criteria. An option that fails any one of them drops off the list.

• Space: Check whether there is room for a reactor or additional equipment, and a route to connect to existing piping and ducts. Without space, the options left are those that work within existing equipment.
• Capital cost: Look beyond the cost of new equipment and include the downtime and the scope of modification.
• Operating cost: Reagent, reheating fuel, ozone generation power, catalyst replacement and fan power from rising pressure drop make up the annual cost.
• Reheating: The more the catalyst sits where gas temperature has dropped, as in tail-end SCR, the greater the reheating burden.
• Equipment compatibility: The method to be installed must accommodate the existing baghouse tubesheet and cage dimensions, loads and operating temperature range.

Run the checks in this order

Temperature comes first because temperature conditions decide the method itself. If the furnace has an SNCR temperature window, SNCR is a candidate; if the boiler outlet gas falls within the catalyst temperature range (roughly 250–430°C), high-dust SCR; if the baghouse operating temperature falls within the catalyst activity range, a catalytic filter; and if the train runs below 100°C with a wet scrubber, ozone oxidation. Tail-end SCR is considered on the assumption of reheating.^[6] Looking at efficiency tables without knowing the temperature leads to comparing methods that do not fit.

Source: Based on Micro-one in-house technical data · concept summary

05 · FIBER-TYPE CATALYTIC FILTER

Where can a fiber-type catalytic filter be considered?

A catalytic filter bag is a filter bag made from catalyst-loaded filter media: it filters out dust and reduces NOx on the catalyst. Where an existing baghouse is in service and the tubesheet and cage dimensions match, DeNOx capability can be added within the scope of replacing the filter bags and adding a reagent injection system. Compared with methods that require new equipment or a reactor, the smaller burden on space and modification scope is the reason to consider this approach.

The conditions to check beforehand are equally clear. First, the operating temperature must fall within the catalyst activity range. Second, the baghouse tubesheet and cage dimensions must match the new filter bags. Third, there must be a location for reagent injection with enough mixing distance. Fourth, if the gas carries many poisoning components, catalyst life must be assessed separately. If all of these conditions are met, the option goes on the list; if any one fails, compare it with the other options in the previous section.

RETROFIT WITH CATALYTIC FILTER BAGWhat changes when only the filter bags are replaced?

Figure 3. A baghouse with conventional filter bags (left) compared with one fitted with a reagent injection system and catalytic filter bags (right) (concept illustration)

Left: a baghouse with conventional filter bags (cream). Right: the same baghouse with a reagent injection system added and the bags replaced with catalytic filter bags (light gray).

Conventional filter bags

Dust capture

• The baghouse filters out dust
• NOx is treated in separate equipment

With catalytic filter bags

Dust capture + NOx reduction

• Filter bags replaced with catalytic filter bags (light gray bags)
• Reagent injection system added (tank, pump, injection piping)

Check before applying: baghouse in service, tubesheet and cage dimensions, operating temperature

Source: Based on Micro-one in-house technical data · AI-assisted concept illustration · actual shapes and dimensions differ

Only a handful of catalytic filter bag suppliers, such as Gore, Topsoe and Micro-one, have delivered to customer sites and built up an operating track record in this field. In 2024, Micro-one delivered 3,300 CataFill™ catalytic filter bags to the Kanding incinerator in Taiwan. Whatever the product, whether it suits site conditions needs case-by-case review.

FAQ

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