Back to Capabilities

Acid gas scrubbing

Acid gas scrubbing, the stage that decides whether a thermal permit holds.

Burn a chlorinated, fluorinated, or sulfur-bearing feed and the halogens leave the combustion chamber as hydrogen chloride, hydrogen fluoride, and sulfur oxides. The scrubber is what removes them before the stack. On PCB, dioxin, and PFAS work it is not an accessory to the oxidiser, it is half the reason the system is permittable.

HCl, HF, SOx

Acid gases the stage is sized to remove

Wet / dry / semi-dry

Three selectable scrubbing routes

Stack-verified

Performance proven by permitted stack testing

Where it sits

Downstream of the oxidiser, upstream of the stack.

A thermal remediation train separates contaminant from soil in the desorber, destroys it in the thermal oxidiser, then has to deal with what destruction produces. Organic molecules containing chlorine or fluorine cannot simply disappear: the carbon leaves as carbon dioxide and the halogen leaves as acid gas. Sulfur-bearing feeds add SO2 and SO3.

The gas train therefore runs oxidiser, rapid quench, acid gas scrubbing, then particulate removal in a baghouse before the stack. Quench speed matters as much as scrubber capacity on chlorinated work, because slow cooling through roughly 400 to 200 degrees Celsius is the window in which dioxins can reform after they were already destroyed.

Selection

Wet, dry, or semi-dry is a site decision, not a preference.

Wet scrubbers spray an alkaline solution, typically sodium hydroxide or lime slurry, through the gas stream. They give the highest acid gas removal and handle high, variable halogen loading well, at the cost of water supply and a liquid effluent that itself needs management.

Dry scrubbers inject powdered lime or sodium bicarbonate into the duct, and the reaction products are collected by the downstream baghouse as a solid. There is no effluent, which is decisive on remote sites and in cold climates. Semi-dry designs sit between the two, trading water use against removal efficiency. The right answer follows the feed chemistry, the halogen loading, the permitted stack limits, and whether the site can accept a liquid stream at all.

Verification

The scrubber is only as good as the stack record.

Removal efficiency is established by stack testing under a permitted operating envelope, then held by continuous emissions monitoring that proves the unit stayed inside that envelope for the whole treatment period. Hydrogen chloride, sulfur dioxide, particulate, carbon monoxide, and oxygen are the channels regulators normally want to see.

That record is what closes a file. A treatment program that cannot show the gas train stayed in envelope is exposed no matter how good the destruction efficiency looked on the day of the test.

Specifying the stage

Interrogate the scrubbing stage when:

The feed contains chlorinePCBs, chlorinated solvents, and pesticide residues all produce hydrogen chloride on combustion. Acid gas scrubbing is mandatory, and the quench ahead of it has to be fast enough to shut the dioxin reformation window.

The feed contains fluorinePFAS-impacted material produces hydrogen fluoride. Removal and materials of construction both need to be specified for HF, not assumed from an HCl design.

The site cannot take a liquid effluentRemote, Arctic, or water-constrained sites point toward dry or semi-dry scrubbing, where reaction products leave as a solid with the baghouse dust.

Stack limits are tightWhere the permit sets low HCl or SO2 limits, scrubber sizing and reagent stoichiometry drive the design, not the oxidiser rating.

The unit has to mobiliseRoad-transportable emissions control has to be engineered as part of the train. Reagent supply, water supply, effluent handling, and commissioning stack tests all belong in the mobilisation plan.

Project evidence

Where the gas train was the hard part.

PCB gold mine, a Canadian firstOn-site thermal destruction of PCB-impacted material at a mine site, the first program of its kind in Canada, with the full gas train operating under permit.

Vietnam, legacy dioxin residuesThermal treatment of legacy dioxin residues under the main contractor, one of the few programs executed at this scale in Southeast Asia.

NATO PFAS test bedSelected as a NATO PFAS test bed selection for PFAS-impacted military-base soil. Mobile high-temperature destruction sized to AFFF-driven contamination profiles.

The Hub

Contamination data by jurisdiction

The Hub maps known contamination across North America and sets out the criteria and permitting each jurisdiction applies to thermal treatment.

Open the map

We'll reply within 1 business day.

Frequently asked

What is acid gas scrubbing?

Acid gas scrubbing is flue gas treatment that neutralises the acid gases formed when chlorinated, fluorinated, or sulfur-bearing material is combusted, principally hydrogen chloride, hydrogen fluoride, and sulfur oxides. It sits between the thermal oxidiser and the stack and is the stage that makes a thermal system permittable on those feeds.

What is the difference between a wet scrubber and a dry scrubber?

A wet scrubber sprays an alkaline solution, usually sodium hydroxide or lime slurry, through the gas stream, dissolving and neutralising the acids. A dry scrubber injects powdered lime or sodium bicarbonate into the duct, and the solid reaction products are captured by the downstream baghouse. Wet gives the highest removal and handles variable loading; dry produces no liquid effluent, which often decides remote and cold-climate sites.

Why does PFAS need acid gas scrubbing?

Because destroying a per- or polyfluoroalkyl substance breaks carbon-fluorine bonds, and the fluorine leaves the chamber as hydrogen fluoride. Without a scrubbing stage specified for HF, the destruction step simply moves the problem into the stack gas. Nelson's PFAS work remains at pilot and validation stage, and gas train design is a central part of that validation.

How is scrubber performance verified?

By stack testing against the permitted limits, supported by continuous emissions monitoring of hydrogen chloride, sulfur dioxide, particulate, carbon monoxide, and oxygen through the treatment period. The monitoring record, not the design drawing, is what a regulator accepts.

Does the scrubber prevent dioxin formation?

Not on its own. Dioxins can reform as chlorinated gas cools slowly through roughly 400 to 200 degrees Celsius, which is upstream of most of the scrubbing work. Rapid quench immediately after the oxidiser is what closes that window; the scrubber then removes the acid gases that remain.

Can acid gas scrubbing be mobilised to a remote site?

Yes. Emissions control is engineered as part of Nelson's road-transportable thermal trains, which have been deployed in the Canadian Arctic, on Pacific islands, and in tropical Southeast Asia. Reagent and water supply, effluent handling, stack height, and commissioning tests are planned with the rest of the mobilisation.

Related

Thermal oxidiser systems

Acid gas scrubbing, glossary

Thermal desorption soil remediation

PFAS thermal destruction

Dioxins and furans

PCBs