Definition
Honeycomb catalyst
A honeycomb catalyst is an extruded ceramic SCR catalyst block with parallel square channels. It gives high surface area but needs careful ash, pitch and cleaning management.
- Subject
- SCR and SNCR
- Also known as
- honeycomb SCR catalyst, extruded catalyst
A honeycomb catalyst is a monolithic extruded ceramic block containing a dense grid of parallel square channels through which flue gas flows. The active material is commonly vanadium pentoxide with tungsten or molybdenum promoters on a titanium dioxide carrier, although catalyst chemistry varies with fuel, sulphur content, mercury oxidation duty and allowed ammonia slip.
Honeycomb is the most common form of SCR catalyst in utility boilers, waste-to-energy plants, biomass units, gas turbines with HRSGs and many industrial fired heaters. Blocks are loaded into steel modules, the modules form a catalyst layer, and reactors usually carry several layers with space left for future additions or replacement.
Strengths and weaknesses
| Strength | Weakness |
|---|---|
| High geometric surface area per unit volume | Channels can plug with fly ash or large-particle ash |
| Low pressure drop when clean | Brittle ceramic needs careful handling during installation |
| Mature supplier base and predictable DeNOx performance | Cleaning access is limited once channels are blinded |
| Wide pitch range for different dust loads | Small pitch increases pluggage risk on high-dust duty |
The square channels give good contact between flue gas, ammonia and catalyst surface, but the same geometry makes the inlet face vulnerable. A few blocked cells are not important; broad blinding across the top layer causes gas channelling, higher local velocity through the remaining open channels, rising pressure drop and loss of NOx reduction efficiency.
Pitch selection
Pitch is the centre-to-centre spacing between adjacent channels. Smaller pitch gives more active area in the same reactor volume, so it is attractive where the gas is clean. Larger pitch sacrifices some surface area to keep dusty gas moving.
| Duty | Typical pitch logic |
|---|---|
| Gas turbine HRSG or clean gas | Fine pitch, often about 3.5 to 4.5 mm |
| Coal high-dust SCR | Medium or large pitch, commonly about 5 to 7.4 mm |
| Biomass and WtE | Larger pitch or a sacrificial guard layer because ash is sticky and irregular |
| Tail-end SCR after particulate control | Smaller pitch can be used because dust loading is low |
Pitch is not selected in isolation. It is tied to reactor velocity, allowable pressure drop, ammonia mixing quality, expected ash size distribution, layer spacing, access for inspection and the economics of catalyst replacement.
Fouling and maintenance
Honeycomb catalyst fouling usually appears as inlet-face masking, individual channel blockage, edge build-up around module frames, or hard deposits caused by ammonium bisulphate and sticky ash. The practical symptoms are increasing reactor differential pressure, rising ammonia slip at the same NOx outlet target, uneven outlet NOx profiles and a loss of margin during load changes.
Plants inspect honeycomb layers during outages using visual checks, pressure-drop trending, catalyst sampling and activity testing. Plugged blocks may be vacuumed, washed, replaced as individual blocks, or replaced as complete modules depending on deposit hardness and remaining catalyst activity.
Acoustic-cleaning relevance
Sonic horns and steam sootblowers are installed above or between catalyst layers to keep ash from settling on the inlet face. Acoustic cleaning is most useful against dry, friable masking deposits and early-stage LPA accumulation. It cannot restore catalyst that is chemically poisoned, sintered or already plugged with fused material, but it can slow the pressure-drop rise that forces early replacement.
For Sylio-style acoustic cleaning, the important design questions are horn frequency, line of sight to the catalyst face, layer spacing, hot-zone material selection and whether the ash is dry enough to detach under cyclic acoustic pressure. On sticky biomass or WtE duty, acoustic cleaning is normally part of a broader design that may include larger pitch, guard layers, LPA screens and tighter ammonia-injection control.
Field evaluation notes
Honeycomb catalyst condition is usually judged by a combination of pressure drop, activity testing and physical inspection. A reactor can still meet the outlet NOx target while losing margin, because operators compensate with more reagent or accept higher ammonia slip. The practical warning signs are a faster pressure-drop slope after load changes, white or tan masking across the top layer, uneven module-to-module ash patterns and outlet NOx stratification that follows gas-flow maldistribution.
Inspection should distinguish between loose face dust, bridged channel mouths, deep channel pluggage and chemical deactivation. Loose face dust may respond to online cleaning or outage vacuuming. Deep pluggage often needs block replacement, especially when particles are wedged into small-pitch cells. Chemical poisoning from alkalis, arsenic, phosphorus or heavy metals is a catalyst-life issue, not a cleaning issue. That distinction matters because acoustic cleaning can protect open gas passages but cannot restore lost active sites.
Design reviews should check superficial velocity, pitch, module sealing, bypass gaps, layer spacing, access lanes, ash particle size and upstream flow distribution. A well-cleaned honeycomb layer still performs poorly if ammonia is badly mixed or if gas bypasses the module frames.
Related terms
Explore the subject
Related terms
5 terms
- Selective Catalytic ReductionSCR reduces NOx by injecting ammonia upstream of a catalyst. Temperature, mixing, catalyst condition and ash control determine performance.
- Plate catalystPlate catalyst uses coated metal plates with open gas channels. It is often selected for high-dust SCR service where pluggage risk matters.
- Corrugated catalystA corrugated catalyst uses corrugated fibre-reinforced sheets coated with active material. Lighter than honeycomb, particularly common on tail-end SCR and marine duty.
- Catalyst layer and moduleAn SCR catalyst module is a steel-framed cassette holding multiple catalyst elements. Modules are stacked into layers; layers are stacked into the SCR reactor.
- Catalyst pluggageCatalyst pluggage is the physical blockage of SCR catalyst channels by large-particle ash, popcorn ash or ammonium-salt deposits. It causes delta P rise and gas-flow maldistribution.
References