Catalyst substrate for SCR (honeycomb catalyst)

01

Set the channel pitch and cell density for the gas

EPA's SCR cost manual (Chapter 2, 2019) defines catalyst pitch for honeycomb and metal plate catalysts as the width of a cell plus the wall thickness. For a given flow, a wider pitch lowers the gas velocity in the channels and keeps ash from depositing and bridging over cells, but it reduces surface area per unit volume and can require more catalyst volume to meet NOx and ammonia-slip limits. For high-dust coal-fired arrangements it gives a honeycomb pitch of about 7–9 mm, compared with 3–4 mm for gas-fired boilers, and about 4–7 mm where the catalyst sits after a hot-side electrostatic precipitator (low-dust), with 3 or 4 catalyst layers in high-dust designs and 2 in low-dust designs. [1]

Cormetech, a maker of titanium dioxide based ceramic honeycomb SCR catalysts for power, refinery, industrial and petrochemical plants, states that its catalyst formulation and cell pitch are customized for each flue gas application to balance NOx reduction, ammonia slip and pressure loss, and that the honeycomb is installed in steel frames or canisters for handling and replacement. [2]

Diesel-exhaust substrates are specified by cell density instead. Sefu's SCR catalyst page lists cylindrical honeycomb ceramic substrates from Ø143.8 × 76.2 mm to Ø330 × 152.4 mm at 300 or 400 cells per square inch (cpsi), and describes the catalyst as a metal or ceramic carrier with vanadium-based, manganese-based, rare-earth oxide, molecular sieve or titanium dioxide active material. [3]

Editorial calculation: for square cells, cell density ≈ (25.4 / pitch in mm)² cells per square inch. A 7–9 mm pitch is therefore about 8–13 cpsi, 3–4 mm about 40–72 cpsi, while 300 and 400 cpsi correspond to pitches of about 1.47 and 1.27 mm. The diesel substrates are about 23–50 times denser than high-dust honeycombs and about 4–10 times denser than gas-fired ones, which is why the two are not interchangeable. [1] [3]

Give the supplier the gas source (coal, gas or oil boiler, turbine, engine, kiln, heater), flow, dust loading and ash type, SO2/SO3, inlet NOx and target outlet NOx or removal, allowable ammonia slip and SO2-to-SO3 conversion, allowable pressure drop, temperature range at all loads, reactor or canister dimensions, number of layers, pitch or cell density wanted, and quantity.

02

Match the material and temperature window to the duty

EPA gives operating temperatures of 480–800 °F (250–430 °C) for metal-oxide SCR catalysts, with NOx removal rising to a maximum at about 370–400 °C and falling above 400 °C; above the specified range N2O formation increases and the catalyst sinters. It describes the base-metal formulation of vanadium, titanium and tungsten introduced in the late 1970s, metal oxides such as TiO2, ZrO2, V2O5 and SiO2 added later to broaden the temperature range, and zeolites for 360–540 °C service. Thermal sintering — a permanent loss of activity from a change in pore structure — can begin at 230 °C depending on catalyst composition. [1]

The same chapter names calcium oxide, magnesium oxide, potassium, sodium, arsenic, chlorine, fluorine and lead as catalyst poisons, with poisoning the main cause of deactivation, and ammonia-sulfur salts, fly ash and particulates as causes of blinding, plugging and fouling. It reports vendor-guaranteed operating lives of 8,000–24,000 hours for coal-fired boilers and greater than 32,000 hours with oil and natural gas, and lists current demands on coal-fired catalysts as 90% design NOx removal, 2 ppm ammonia slip and SO2 oxidation below 1%. [1]

For mobile and small-engine duty, Sefu claims denitration conversion of at least 98%, ammonia leakage of 5 ppm or less and a start temperature of 200 °C or less for its SCR catalyst; these are the supplier's statements, not tested results. Its cordierite honeycomb catalyst substrate page gives a maximum continuous working temperature of 1,200 °C for the cordierite body, and a VOC-oxidation version with walls under 0.2 mm, compressive strength above 10 MPa and heat shock temperature of 900 °C. [3] [4]

Ask for the formulation family (vanadia–tungsta–titania, zeolite or other), guaranteed NOx removal, ammonia slip, SO2 oxidation and life at your conditions, tolerance to the poisons in your gas, the activity test method used for audit samples, and whether the catalyst can be washed or regenerated.

03

Choose the catalyst body type

EPA describes SCR catalyst configurations as ceramic honeycomb and pleated metal plate (monolith) designs in fixed beds, with high surface area per volume; pellets have more surface area but plug more easily, which limits them to clean-burning fuels such as natural gas. Catalyst elements are placed in frames to form modules, quoted as about 1 m × 2 m in area and 1 m high, which are stacked in layers and hoisted into the reactor by crane. [1]

04

Compare catalyst substrate suppliers

This company lists SCR honeycomb catalysts and cordierite honeycomb catalyst substrates. Its published pages show what it offers, not tested performance.

05

Trace the parts of a honeycomb SCR catalyst

This is a functional reference built from the sources cited; it is not one maker's design.

06

Fill the specification fields with values for your gas

A catalyst substrate quotation needs values for your gas and reactor. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.

07

Send the gas, performance and reactor data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Gas: [source; flow; temperature range; dust; SO2/SO3; poisons]
Performance: [inlet NOx; removal; NH3 slip; SO2 oxidation; pressure drop]
Geometry: [pitch or cpsi; module or canister size; layers]
Material: [titania-based; zeolite; cordierite or metal carrier]
Life: [guaranteed hours; regeneration]
Supply: [quantity; frames; test samples]

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