Quench tower (off-gas quencher)

01

Set inlet and outlet conditions

US patent 6,149,137 (Method and apparatus for quenching hot flue gases) explains that hot flue gases from thermal oxidation processes "frequently require cooling to their adiabatic saturation temperature" before further processing in a packed absorber or venturi scrubber. It gives hot flue gas temperatures "typically in the range of 1400 to 2000 degrees F"; adiabatic saturation temperature, the minimum reachable by water evaporation with no other heat loss, varies with pressure and moisture but "is largely a function of the temperature of the incoming hot flue gases", about 150–195°F for that inlet range. [1]

EPA's Air Pollution Control Cost Manual (Section 6, wet scrubbers for particulate matter) notes that when hot gas passes through a wet scrubber, water evaporates, humidifying and cooling the gas; outlet conditions are taken as saturated for design and evaporation is modelled as adiabatic saturation, with the dry air mass flow constant. It says "for PM applications, wet scrubbers are generally limited to a temperature range of (50°F to 700°F) due to evaporation", high-temperature gas reduces droplet formation in a venturi, so the gas must be cooled first with a quencher spraying water into it, and quenchers increase capital and operating costs only marginally. [2]

Editorial calculation: 1400–2000°F is about 760–1093°C and 150–195°F about 66–91°C, so the quench takes the gas down by roughly 694–1002 °C (Editorial calculation); ask the supplier for the water evaporated per hour at your flow and the outlet temperature guarantee. [1]

Give the supplier the gas source, flow and composition, inlet temperature and pressure, dust and acid gas loading, required outlet temperature or saturation, downstream equipment, water source, recycle and blowdown, allowable nozzle or venturi pressure drop, materials and lining for corrosive condensate, emergency water supply on loss of quench water, instrumentation, and space and height limits.

02

Choose spray nozzle or venturi quench

US 6,149,137 says conventional quench systems use spray nozzles in a duct or spray contactor vessel ("typically, numerous nozzles are connected to the water source"), the nozzles taking a pressure drop of about 80–100 psi to make fine drops for rapid cooling; their drawbacks are substantial valving and piping, nozzles that corrode, erode and plug with solids in the "hot and usually corrosive recycled spray water", and the need for a high-pressure pump. Its system mounts a venturi in a quench vessel and raises the water level to submerge the downcomer when the venturi pressure drop alone does not cool the gas to adiabatic saturation. [1]

Editorial note: ask the supplier which quench principle it proposes for your dirty, corrosive gas, how nozzles or venturi are kept from plugging, and what happens on loss of quench water, since downstream equipment may not survive inlet temperature.

03

Check the gas temperature the US incinerator rule ties to dioxin limits

Where a quench cools gas from a hazardous waste incinerator in the United States, the federal emission rule links the dioxin and furan limit to gas temperature. Under US EPA's 40 CFR 63.1219, an existing incinerator equipped with a waste heat boiler or dry air pollution control system must not emit dioxins and furans in excess of 0.20 ng TEQ/dscm, corrected to 7 percent oxygen, or alternatively 0.40 ng TEQ/dscm provided the combustion gas temperature at the inlet to the initial particulate matter control device is 400 °F or lower, based on the average of the test run average temperatures. [3] The rule states that operation of a wet particulate matter control device is presumed to meet the 400 °F or lower requirement. [3] Editorial calculation: 400 °F is about 204 °C ((400 − 32) ÷ 1.8).

For existing incinerators with neither a waste heat boiler nor a dry air pollution control system, the rule sets 0.40 ng TEQ/dscm. For new sources it sets 0.11 ng TEQ/dscm with a waste heat boiler or dry air pollution control system, and 0.20 ng TEQ/dscm without either, all corrected to 7 percent oxygen. [3] The rule counts a dry system followed by a wet system as a dry system, and a wet system followed by a dry system as not a dry system. [3]

Editorial note: if the quench feeds a fabric filter or electrostatic precipitator rather than a wet scrubber, write the gas temperature the quench must hold at that device inlet into the order, and ask the supplier how it is held at minimum and maximum gas flow.

04

Ask how the nozzle layout was checked for outlet temperature and wall wetting

In a 2024 article by Zumao Chen, the engineering firm Becht describes using computational fluid dynamics (CFD) modelling to set the number, layout and orientation of spray nozzles. In a quench section with an internal diameter of 1 m, three rows of six independently operated nozzles gave an average exit gas temperature of 547 K against a design requirement of 573 K, a local maximum of 650 K at the exit, and a maximum wall temperature of 639 K against a 1000 K limit. [4]

In a second project, nozzles in the third pass of an incinerator quench cooled flue gas from 2000°F to 1000°F ahead of a spray dryer absorber. Moving the nozzles away from the sidewalls and placing one row on the rear wall cut the water striking the wall tubes from 14.2% to 4.4% of the spray; the two designs had different targets (820°F initial, 1000°F final), so their water flows are not directly comparable. [4]

Editorial note: ask the supplier how its nozzle layout was checked for an even outlet temperature and for spray reaching the walls.

05

Trace the quench system

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 quench tower quotation needs values for your gas and plant. 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, water and downstream data together

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

Gas: [source; flow; composition; inlet T and P]
Contaminants: [dust; acid gases]
Outlet: [temperature; saturation]
Downstream: [scrubber; absorber; fan]
Water: [source; recycle; blowdown; filtration]
Design: [spray or venturi; pressure drop]
Materials: [shell; lining]
Safety: [loss-of-water protection]

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