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Most published material on thermal oxidisers, spelled thermal oxidizer in North America, treats them as industrial air pollution control equipment for manufacturing exhaust. In contaminated site remediation the oxidiser does something more consequential: it is the single component that determines whether a contaminant is destroyed or merely relocated. This page covers how it works, how it is sized, and how its performance is proven to a regulator.
Where it sits in the treatment train
In a soil remediation train, contaminated material is heated in a thermal desorption drum until the contaminant volatilizes. At that moment the contaminant has been separated from the soil, but it still exists. It is now a concentrated vapour moving under negative pressure toward the next stage.
The thermal oxidiser receives that vapour and destroys it. Held above its destruction temperature, with excess oxygen and enough turbulence to prevent any portion of the stream from short-circuiting, the organic molecules break apart and reform as carbon dioxide, water, and, where halogens are present, acid gases that the downstream acid gas scrubbing stage captures.
This is why the oxidiser, not the desorber, is the component a technical reviewer should interrogate first. A large desorption drum paired with an undersized secondary combustion chamber produces impressive throughput numbers and incomplete destruction. Throughput is a commercial metric. Destruction efficiency is the environmental one.
The Nelson mobile fleet
Every Nelson mobile plant is a thermal desorption unit paired with a direct-fired secondary combustion chamber. The oxidiser on each unit is rated to 1,100 °C. The normal operating setpoint for petroleum hydrocarbon soil is 870 °C (1600 °F), which is where the standard block flow and the published destruction figures sit. The setpoint is raised toward the 1,100 °C rating for chlorinated compounds, PCBs, dioxins, and PFAS, where bond energy demands it. Residence time is held at two seconds minimum in every configuration.
Unit
Throughput
Operating envelope
Residence time
RS-945
50 to 70 t/hr
Primary to 500 °C · oxidiser to 1,100 °C
2 sec minimum
RS-45
35 to 50 t/hr
Primary to 500 °C · oxidiser to 1,100 °C
2 sec minimum
RS-40
30 to 45 t/hr
Primary to 500 °C · oxidiser to 1,100 °C
2 sec minimum
RS-20
15 to 25 t/hr
Primary to 500 °C · oxidiser to 1,100 °C
2 sec minimum
Nominal design envelopes. Final operating parameters are fixed at pilot or commissioning against soil moisture, contaminant loading, and the site remediation criteria. Full mobile fleet specifications, including fuel flexibility, mobilisation footprint, and emissions control, are published on the capabilities page.
Where the oxidiser sits in the train
Design fundamentals
TemperatureSet by the hardest bond on the site. Hydrocarbon vapour is destroyed at 760 to 870 °C. Chlorinated compounds and PCBs need 1,100 °C or above. PFAS requires a sustained band above 1,100 °C because the carbon-fluorine bond is the strongest single bond in organic chemistry. The chamber is designed for the worst case present, never the average.
Residence timeOne to two seconds at temperature for most constituents, with two seconds the standard requirement for PCBs and dioxins. Residence time is a function of chamber volume against gas flow rate, which is why chamber sizing cannot be separated from the desorber's off-gas volume.
TurbulenceAchieved through burner placement, chamber geometry, and induced swirl. Without it, a portion of the gas stream traverses the chamber faster than the design residence time and exits partially treated. Stack testing detects this; nothing upstream will.
Excess oxygenMaintained above stoichiometric requirement and monitored continuously. Oxygen deficiency produces products of incomplete combustion, which are frequently more hazardous than the parent compound.
The thermal oxidiser burner
The burner holds the chamber at setpoint and supplies the flame front that drives oxidation. It fires on natural gas, propane, or diesel depending on what the site can supply, which in remote deployment is a logistics decision made months before mobilisation.
The specification that matters most in remediation, and the one least discussed, is turndown ratio. Contaminant loading in the vapour stream is not constant. It rises and falls continuously as the excavation moves through hot spots and clean zones, and as feed moisture varies. A burner with a narrow turndown range either overshoots, wasting fuel and stressing refractory, or drops below setpoint during high-loading periods, which is precisely when destruction efficiency matters most. Wide turndown, tied to real-time chamber thermocouple feedback, is what keeps DRE stable across a working day.
Configurations
Exhaust passes through ceramic media beds that store heat and pre-warm the incoming stream, recovering up to 95 percent of thermal energy. Excellent for continuous, clean, predictable industrial exhaust. Rarely the right answer for mobile soil remediation: the beds are heavy to transport, slow to bring to setpoint, and vulnerable to fouling from the particulate and variable moisture that soil treatment inevitably produces.
A metallic heat exchanger transfers energy from exhaust to inlet, typically recovering 50 to 70 percent. Lighter and more tolerant of dirty streams than an RTO, but exchanger surfaces still require managed cleaning intervals when chlorides are present.
No heat recovery in the chamber itself; energy is recovered downstream where the gas is cleaner. Tolerant of particulate, variable moisture, and unpredictable loading, and it reaches setpoint quickly after a shutdown. This is the configuration Nelson uses across the mobile fleet, because remediation off-gas is dirty and variable by nature.
Verification
Destruction-and-removal efficiency compares the mass of a principal organic hazardous constituent entering the system with the mass leaving in the stack. Regulators commonly require 99.99 percent for hazardous constituents and 99.9999 percent, six nines, for PCBs and dioxins.
DRE is established during commissioning stack testing under a permitted operating envelope: chamber temperature, residence time, oxygen concentration, and feed rate. Once established, continuous emissions monitoring records prove the unit stayed inside that envelope for every hour of treatment. The closure package a regulator receives is the stack test plus the CEMS record plus the treated-soil confirmation sampling, submitted together.
One detail decides more stack tests than any other: the quench rate. Chlorinated compounds can reform dioxins during slow cooling between roughly 400 and 200 degrees Celsius. A correctly performing oxidiser followed by a slow quench will still fail. Rapid quench through that window is a design requirement, not an optimisation.
In the field
See the full mobile fleet and process flow
Where this sits
This page explains the method. The contaminant pages set out what the regulator expects for a specific problem, and the capabilities overview sets out the plant, crew, throughput, and verification behind the work on site.
Back to the problem
Dioxins and furansVapour-phase destruction requirements and the stack evidence a regulator expects.
PCBsDestruction and removal efficiency against a principal organic hazardous constituent.
HydrocarbonsWhere oxidation sits downstream of desorption on a production hydrocarbon site.
Talk to the contractor
Tell us four things about the site and a Nelson thermal specialist will come back with an honest read on treatability, throughput, and whether our mobile fleet is the right fit. No obligation, and no pressure if it is not.
Frequently asked
Related
Thermal Desorption, the process