How Is an Incinerator Flue Gas Treatment System Arranged?
Where SNCR, reactors, baghouses and SCR sit, what each one does, their pros and cons, and the order to check them
01 · PROCESS FLOW
In what order is incinerator flue gas treated?
The typical order is furnace and waste heat boiler, reactor, baghouse, SCR (if needed), wet scrubber (if needed) and stack, with the SNCR reagent injected into the hot zone of the furnace and boiler. Flue gas is not cleaned in one step: dust, acid gases and NOx fall in turn as the gas passes each unit. Each unit treats different substances, so one cannot do another's job.
The main factor that sets the order is flue gas temperature. Korea's management standard for waste treatment facilities requires general incinerators to keep the combustion chamber outlet at 850°C or higher and the combustion gas residence time at 2 seconds or longer, with separate values for some facility types and sizes.[5] This hot gas gives up heat in the waste heat boiler and cools further through the reactor and baghouse. The same Korean standard requires combustion gas entering the first dust collector to be kept at 200°C or lower (250°C for facilities below 2 t/h), so reactors and baghouses usually operate at or below that temperature.[5]
A1234B5678Figure 1. Numbers follow the flue gas path; A and B are units outside the gas path. Auxiliary units such as economizers, gas heaters and fans are omitted. No. 2 SNCR exists only at some plants; its injectors are not drawn, so only its location is marked. This is a concept illustration, and actual size and layout vary by plant.
| No. · Unit | What it does |
|---|---|
| AFeed crane · feeding system | Moves waste from the pit to the hopper and into the furnace. It is not part of the flue gas path. |
| 1Incinerator (furnace) | Burns the waste and produces flue gas. Korea's standard calls for a combustion chamber outlet of 850°C or higher and a gas residence time of 2 seconds or longer.[5] |
| 2Selective non-catalytic reduction (SNCR) | Injects reagent into the hot zone of the furnace to reduce NOx. It reacts without a catalyst at about 870–1,150°C.[1] Found only at plants that install it. |
| 3Waste heat boiler | Recovers heat from the hot flue gas to make steam, lowering the gas temperature accordingly. |
| 4Semi-dry reactor (SDR) | Sprays hydrated lime slurry to neutralize acid gases such as hydrogen chloride, hydrogen fluoride and sulfur oxides.[3][4] |
| BHydrated lime & activated carbon silo (DR) | Stores powdered hydrated lime and activated carbon and feeds them into the flue gas. Injecting the dry sorbent as a powder is the DR method. Activated carbon adsorbs mercury so it is captured in the dust collector.[4] |
| 5Baghouse (fabric filter) | Filter bags capture dust, reaction products and activated carbon.[3] |
| 6Selective catalytic reduction (SCR) | Uses a reagent and catalyst to convert nitrogen oxides (NOx) into nitrogen and water.[2] |
| 7Wet scrubber | Washes remaining acid gases out of the flue gas with a scrubbing liquid. |
| 8Stack | Releases the treated flue gas to the atmosphere. |
Source: Based on Micro-one in-house technical data · AI-assisted concept illustration
No. The SNCR reaction needs a high temperature of about 870–1,150°C,[1] while flue gas entering the baghouse is kept at 200°C or lower under Korea's standard.[5] SNCR is therefore placed in the hot zone of the furnace and boiler, and downstream of the dust collector NOx is handled by catalytic SCR.
02 · EQUIPMENT ROLES
What does each unit remove, and how?
SNCR handles NOx; the reactor handles acid gases such as hydrogen chloride (HCl), hydrogen fluoride (HF) and sulfur oxides (SOx); the baghouse handles dust and reaction products; and SCR handles the NOx that SNCR could not remove. The table below summarizes the position, target pollutants and method of each of the four units.
| Unit | Position in the process | Target | Method |
|---|---|---|---|
| SNCR | Hot zone of the furnace and waste heat boiler | NOx | Injects ammonia or urea solution at about 870–1,150°C to reduce NOx to N₂ and H₂O without a catalyst[1] |
| SDR / DR | After the boiler, before the baghouse | Acid gases such as HCl, HF and SOx | Neutralizes them with sprayed hydrated lime slurry (SDR, semi-dry) or dry sorbent injection (DR, dry)[3][4] |
| Baghouse | After the reactor | Dust, reaction products, fly ash | Captures them by filtration through filter bags[3] |
| SCR | After the baghouse (before it at some plants) | NOx (the remainder SNCR could not remove) | Passes reagent-dosed flue gas through a catalyst bed to reduce NOx; typical operating temperature about 250–427°C[2] |
SNCR injects reagent at high temperature without a catalyst
SNCR (selective non-catalytic reduction) reduces NOx to nitrogen (N₂) and water (H₂O) by injecting a reagent such as ammonia or urea solution into a high-temperature zone, with no catalyst.[1] The U.S. EPA states that the reaction occurs at 1,600–2,100°F (about 870–1,150°C), and that in SNCR the combustion unit itself, such as the furnace or boiler, serves as the reaction chamber.[1] Because no separate reactor is needed, its capital and operating costs are among the lowest of the NOx control methods.[1]
Figure 2. Cross-section concept of SNCR, in which reagent from the tank is pumped to wall nozzles in the hot zone of the furnace. Waste burns on the grate, and the combustion gas rises, meets the reagent in this zone and then passes to the waste heat (WH) boiler. Injection points and the number of nozzles depend on the furnace design and temperature profile.
Source: Based on Micro-one in-house technical data · AI-assisted concept illustration
Reactors neutralize acid gases with a sorbent
A reactor brings an alkaline sorbent such as hydrated lime into contact with the flue gas, turning acid gases into solids such as calcium salts so they can be removed.[4] A semi-dry reactor (SDR) sprays hydrated lime slurry so the reaction takes place as the water evaporates, while a dry reactor (DR) injects powdered sorbent into the duct.[3][4] Both produce dry solid reaction products that are captured downstream in a baghouse or electrostatic precipitator.[3] Some systems also inject activated carbon to reduce mercury; the mercury adsorbed on the carbon is captured in the downstream dust collector.[4] A spray dryer or dry sorbent injection combined with a fabric filter is described as a system that can control dioxins/furans (CDD/CDF), dust and heavy metals along with acid gases.[4]
Figure 3. Flue gas enters the top of the SDR tower, passes through the slurry spray zone and leaves through a duct on the lower side of the tower. Blowing powdered sorbent from the hydrated lime silo on the right into this duct to react with the gas is the DR method, and the duct continues to the next unit, the baghouse. Layout, injection points and sorbents vary by design.
Source: Based on Micro-one in-house technical data · AI-assisted concept illustration
Baghouses filter out dust and reaction products
A baghouse is a fabric filter dust collector that captures dust by passing flue gas through filter bags. In an incinerator, both the solid reaction products from the reactor and the fly ash are captured in the baghouse.[3] Fabric filters are often used in combination with acid gas control and flue gas cooling.[4] Another type of dust collector is the electrostatic precipitator (ESP); the two are compared in Electrostatic Precipitator (ESP) vs. Baghouse: What Is the Difference?. How filter bags capture dust is covered in Depth Filtration vs. Surface Filtration: What Is the Difference?, and bag materials in How Do You Choose the Right Filter Bag Material for Your Process?.
Figure 4. On the left, a baghouse with rows of filter bags hanging in each compartment; on the right, an enlarged cross-section of a single bag. When dust-laden flue gas reaches the outside of the bag, dust builds up on the surface and is filtered out, while clean gas passes inside and leaves through the top outlet duct. The number of compartments and bags varies by unit.
Source: Based on Micro-one in-house technical data · AI-assisted concept illustration
SCR reduces the remaining NOx in a catalyst bed
SCR (selective catalytic reduction) reduces NOx to nitrogen and water by passing reagent-dosed flue gas through a catalyst bed.[2] The optimum operating temperature is given as about 250–427°C, though it depends on the catalyst type and gas composition.[2] Removal efficiency is typically 70–90%.[2] The catalyst lets SCR reach higher efficiency at lower temperatures than SNCR, but it brings catalyst management and space requirements. One common design stacks the catalyst in layers of honeycomb modules with narrow channels for the gas to pass through.
Figure 5. On the left, where catalyst modules sit inside the SCR reactor; on the right, an enlarged view of part of the honeycomb catalyst. Flue gas travels along the fine honeycomb channels, and NOx reacts with the reagent on the catalyst surface of the channel walls. This is a concept illustration; actual channel geometry and module layout vary with the catalyst and the unit.
Source: Based on Micro-one in-house technical data · AI-assisted concept illustration
Wet scrubbers wash out remaining acid gases with a scrubbing liquid
In this configuration, the wet scrubber is the last treatment unit the flue gas passes through. Nozzles inside the tower spray an alkaline scrubbing liquid that dissolves and removes sulfur oxides and other acid gases left in the gas.[3] In packed towers, layers of packing such as Pall rings let the liquid flow over the packing surface, increasing the contact area and time with the gas. The U.S. EPA describes wet systems on an SO₂ basis, with removal efficiency above 90%, higher than semi-dry or dry systems.[3] A mist eliminator at the tower outlet removes droplets carried by the gas.[3] The trade-off is a wet waste stream whose disposal adds to operating costs.[3] Some plants add it as a final step after the reactor and baghouse.
Pall rings (packing)Figure 6. Cross-section concept of a wet scrubber: flue gas enters from the duct on the left and, as it rises through the tower, passes scrubbing liquid sprayed from several nozzle levels and layers of Pall ring packing. The circle on the right is a close-up of the packing; the liquid flows over its surface and enlarges the contact area with the gas. A tank and pump at the bottom recirculate the liquid. Packing type and number of layers vary by unit.
Source: Based on Micro-one in-house technical data · AI-assisted concept illustration
03 · PROS AND CONS
What are the strengths and limits of each unit?
SNCR is simple and low-cost but less efficient; SCR is highly efficient but carries a heavier burden of catalyst management and pressure drop. Reactors and baghouses handle acid gases and dust, not NOx, so their pros and cons should be judged within that role. The efficiency figures below are general ranges from U.S. EPA sources; actual values depend on design and operating conditions.
| Item | SNCR | Reactor (SDR/DR) | Baghouse | SCR |
|---|---|---|---|---|
| Target | NOx | Acid gases such as HCl, HF and SOx | Dust, reaction products | NOx |
| Main strengths | No catalyst or separate reactor needed; relatively low capital and operating costs[1] | Simpler than wet systems, with relatively low water use and waste handling (SO₂ removal basis)[3] | Captures reaction products and dust in one unit and is easy to combine with acid gas control and cooling[4] | High NOx removal, typically 70–90%[2] |
| Main limits | Typical efficiency 30–50%[1]. At low load the optimum temperature window shifts upstream[1]. Excess reagent causes ammonia slip, and ammonium sulfate can plug and corrode downstream equipment[1] | Sorbent must be supplied continuously and reaction products disposed of. In semi-dry systems, residual moisture can plug the baghouse, so the outlet temperature is kept above saturation[3] | Filter bags are consumables; service life ranges from 1 year at the short end to 5 years at the long end, typically 2–3 years, depending on gas temperature and fluctuations, acid gases and moisture, dust properties and loading, cleaning interval and pressure, and differential pressure control | Catalyst bed pressure drop is high, so the fan may need upgrading[2]. When the catalyst deactivates through poisoning, plugging or erosion, removal falls and ammonia slip rises[2] |
Reactor removal efficiency depends on the method. EPA SO₂ removal data report about 80–90% for semi-dry systems and about 50–60% for dry calcium-based sorbents, so semi-dry systems are generally more efficient.[3] These figures are for SO₂, however, so HCl and HF removal in an incinerator must be designed separately based on sorbent type, feed rate and flue gas temperature.
04 · LAYOUT LOGIC
Why this order, and where does SCR go?
Units are placed to follow the flue gas as it cools step by step, and the point where site decisions differ most is the SCR position. SNCR needs the hottest zone, reactors and baghouses react and filter at lower temperatures, and SCR again needs gas above a certain temperature. So the required temperature falls and then rises again within one system.
Before the baghouse, the catalyst is exposed to dust and acid gases
If SCR is placed before dust removal, the catalyst is in direct contact with the dust and acid gases in the flue gas. The EPA lists the causes of catalyst deactivation as poisoning of active sites, thermal sintering at high temperature, plugging by ammonium-sulfur salts or particulate matter, and erosion by high gas velocity.[2] In incinerator flue gas, which is high in dust and acid gases, these factors can govern catalyst life.
Figure 7. Flue gas carrying dust and acid gases passes the catalyst bed first and then goes to the baghouse. The red labels are the catalyst deactivation factors cited by the EPA that relate to this layout (poisoning, plugging and erosion); they do not mark individual layers. This is a concept illustration; unit size and layout vary by plant.
Source: Based on Micro-one in-house technical data · AI-assisted concept illustration
After the baghouse, reheating may be needed
The EPA explains that placing SCR after dust and sulfur removal (the cold side) reduces the load on the catalyst, but the flue gas must be reheated, which can raise operating costs significantly.[2] Because Korean incinerators keep the baghouse inlet at 200°C or lower,[5] gas leaving the baghouse often falls short of the typical SCR operating temperature (about 250°C or higher).[2] The actual layout is therefore decided by looking at the catalyst type and flue gas temperature together.
Figure 8. Flue gas leaving the baghouse is cool and often below the temperature SCR requires, so a heater may be needed before the SCR to reheat it. Reheating can raise operating costs (OPEX). This is a concept illustration; unit size and layout vary by plant.
Source: Based on Micro-one in-house technical data · AI-assisted concept illustration
In a split arrangement where the baghouse handles dust and SCR handles NOx, two sets of equipment need installation space and capital, and their pressure drops add up, increasing the fan load.
05 · CHECK ORDER
In what order should units be selected and checked?
Causes are generally easiest to find if you first identify which parameter is closest to its limit, then check temperatures, and then inspect the units responsible for that parameter in system order. Because the condition of an upstream unit changes the load on downstream units, a problem seen at the stack may originate upstream.
- Step 1, identify the problem parameter: From Korea's stack TMS (continuous emission monitoring) 30-minute averages (the mean of six consecutive 5-minute values),[6] determine which of dust, NOx and acid gases has the least margin to its limit. How 30-minute averaging affects filter bag selection is explained in the earlier column on depth and surface filtration.
- Step 2, check temperatures: Check the temperatures at the furnace outlet, boiler outlet, reactor outlet, baghouse inlet and SCR inlet for the same time period. The SNCR temperature window, reactor outlet temperature, the 200°C baghouse inlet limit and the SCR activation temperature all depend on these values. You can check in advance how much margin the baghouse operating temperature has above the sulfuric acid dew point with the acid dew point calculator.
- Step 3, inspect equipment: Inspect in system order, starting with the unit responsible for the problem parameter. The table below shows where to look first for each symptom.
- Step 4, compare options: Compare adding a unit, upgrading an existing unit and combined treatment in the existing baghouse in terms of installation space, capital cost, pressure drop and operating cost.
| Symptom on site | Check first | Why |
|---|---|---|
| NOx fluctuates with load changes | Flue gas temperature at the SNCR injection point | At low load, the optimum temperature window shifts upstream.[1] |
| Downstream plugging or corrosion, rising ammonia slip | Reagent injection rate and reaction temperature; for SCR, catalyst activity | Unreacted ammonia can form ammonium sulfate that plugs or corrodes downstream equipment,[1] and slip also rises as catalyst activity declines.[2] |
| HCl or SOx rising | Sorbent feed rate, reactor outlet temperature, capture of reaction products | Acid gases are removed only when they contact the sorbent and turn into solids, and the reaction products are captured downstream.[3][4] |
| Rising baghouse differential pressure after the reactor | Reactor outlet temperature and filter bag condition | In semi-dry systems, residual moisture can build up damp solids that plug the baghouse.[3] |
| NOx removal falling after SCR | Catalyst inlet temperature, catalyst plugging or poisoning | When catalyst activity falls, removal drops and ammonia slip rises.[2] |
FAQ
Frequently asked questions
Why is SNCR reagent injected inside the incinerator?
Because the reaction needs a high temperature of about 870–1,150°C without a catalyst. That temperature is found in the hot zone of the furnace and boiler, while flue gas entering the baghouse is kept at 200°C or lower in Korea, so the SNCR reaction does not occur downstream. In SNCR, the furnace and boiler themselves act as the reaction chamber.
What is the difference between SDR and DR?
They differ in how the sorbent is introduced. An SDR (semi-dry) sprays hydrated lime slurry so the reaction occurs as the water evaporates, while a DR (dry) injects powdered sorbent into the duct. Both produce dry solid reaction products that are captured in the downstream baghouse.
Why is the baghouse placed after the reactor?
Because it must capture the solid reaction products from the reactor along with the fly ash. Fabric filters are often combined with acid gas control and flue gas cooling, so placing them after the reactor is common.
Should SCR go before or after the baghouse?
Neither is always better; it depends on flue gas temperature and dust and acid gas conditions. Placed before, the catalyst is exposed to dust and poisons; placed after, reheating is needed and can raise operating costs significantly.
- U.S. EPA, "Air Pollution Control Technology Fact Sheet: Selective Non-Catalytic Reduction (SNCR)," EPA-452/F-03-031.
- U.S. EPA, "Air Pollution Control Technology Fact Sheet: Selective Catalytic Reduction (SCR)," EPA-452/F-03-032.
- U.S. EPA, "Air Pollution Control Technology Fact Sheet: Flue Gas Desulfurization (FGD) - Wet, Spray Dry, and Dry Scrubbers," EPA-452/F-03-034.
- U.S. EPA, "AP-42, Chapter 2.1 Refuse Combustion," 1996.
- Korea Ministry of Government Legislation, Korea's Wastes Control Act Enforcement Rule, Annex 11, "Management Standards for Waste Disposal or Recycling Facilities" (amended Aug. 31, 2026; in Korean).
- Korea Environment Corporation, CleanSYS glossary, "30-minute (average) data" (in Korean).