Carbonization off-gas treatment: afterburner, boiler, quench, baghouse, scrubber
The standard train for carbonization off-gas is afterburner, waste-heat boiler, quench, baghouse, then wet scrubber: burn CO and tars at 850-1100 C first, recover heat, then cool, dedust and de-acidify. The order cannot be swapped. This page gives each unit's job, temperature window, common failure causes and a duty-checklist your operators can copy.
The conclusion first: burn it, recover it, then clean it
The standard configuration order for carbonization off-gas is: afterburner, waste-heat boiler, quench tower, baghouse, wet scrubber. First burn out carbon monoxide, tars and combustible volatiles at 850-1100 C, then recover the sensible heat, then cool and dedust the gas, then remove acid gases before the stack. This order cannot be rearranged.
Why? Gas leaving the carbonization zone (450-550 C) is not clean flue gas. It is a hot stream loaded with CO, tar mist and combustible volatiles. No dedusting or scrubbing equipment downstream can fix odour or CO exceedances - only high-temperature oxidation can. And heat recovery must sit before the quench: once the heat is dumped into quench water, it is gone for good.
Compressed into one sentence, the engineering logic is: arrange the train from the highest temperature to the lowest, and from the hardest pollutant to the easiest. High-temperature combustion goes first, ambient scrubbing goes last, heat recovery and dust removal sit in between. This sequence is common practice on activated-carbon carbonization lines worldwide; projects differ only in the sizing and selection of each unit.
The gas load itself is set by the carbonization furnace: higher output and wetter feed mean a heavier off-gas duty, so the treatment train must be designed for the worst case. Furnace selection and mass balance are covered on the carbonization furnace page; this page deals only with how to treat the gas.
What each unit does, its temperature window and common failure causes
| Unit | Problem it solves | Typical temperature window | Common causes of exceedance |
|---|---|---|---|
| Afterburner | Oxidises CO, tars and combustible volatiles; kills odour | 850-1100 C | Burn temperature not reached, residence time too short, insufficient combustion air |
| Waste-heat boiler | Recovers sensible heat; raises steam for the activation plant | About 900 C class inlet, outlet per duty | Fouled heating surfaces, dew-point corrosion, scaling on the water side |
| Quench tower | Drops the gas below what the filter media can withstand | Outlet below the media's continuous rating (per media and duty) | Blocked nozzles, unstable water flow and pressure, insufficient cooling |
| Baghouse | Captures dust and carbon fines | Upper limit set by filter media rating | Overtemperature bag burn, condensation blinding, failed cleaning and rising differential pressure |
| Wet scrubber | Removes SO2, HCl and other acid gases | Ambient section | Loss of control of liquor pH, fouled packing, blocked spray levels |
Why the order cannot be swapped: three underlying rules
- **Combustion comes first.** The afterburner leads the train. CO and tars have exactly one practical route out - high-temperature oxidation: 850-1100 C with a second-scale residence time (commonly designed at no less than 2 seconds, subject to the permit). Skip the afterburner and the stack will smell, full stop; dedusting and scrubbing can do nothing about CO.
- **Recover heat before you throw it away.** The waste-heat boiler must precede the quench. Gas leaving the afterburner still carries several hundred degrees of sensible heat; quenching it directly is money down the drain. The recovered steam can join the activation steam network - steam activation itself consumes 1.5-2.2 kg of steam per kg of carbon (duty-dependent), so recovery directly offsets that fuel cost. The full utilities account is in our energy consumption per tonne breakdown.
- **Dedust before you de-acidify.** The baghouse must precede the wet scrubber. Dusty gas fed straight into a scrubber fouls the packing within months as solids bond with the circulating liquor. Dedust first and the scrubber sees clean, cool gas - stable packing, stable spray levels, lower alkali consumption.
Duty checklist: six checks operators can copy today
Watch the afterburner temperature
Confirm every shift that the afterburner holds 850-1100 C. Below 850 C, check combustion air and burners first - do not push under-burnt gas downstream.
Check boiler differential pressure and stack-out temperature
Rising pressure drop usually means fouled heating surfaces; blow back in time. Abnormally rising exit temperature means heat transfer is degrading - check fouling before the water side.
Watch the quench outlet temperature
It must sit below the filter media's continuous rating. If it climbs, check nozzles for blockage and the pump for lost pressure before blaming instruments.
Log baghouse differential pressure
A slow periodic rise is a normal cleaning cycle; a sudden spike means moisture blinding, a sudden drop suggests a torn bag - leak-test immediately.
Test scrubber liquor pH
Top up alkali per procedure when pH drifts low; wide swings point to an unstable acid-gas load - go back and verify afterburner combustion.
Walk the stack and smell
The crudest and most sensitive instrument on site. Odour at the stack almost always means the afterburner is not burning out - return to check one and the residence time.
The most common commissioning mistake
Sizing the train to pass acceptance only: firing natural gas to push the afterburner above 850 C for the inspection, then switching to self-produced gas or throttling combustion air in normal operation, letting the temperature run below 850 C - odour and CO bounce straight back, and retrofitting costs far more than the fuel saved. Design the combustion stage for the worst case (wettest feedstock, highest output, dirtiest raw material), not for the day of inspection.
How this connects to the rest of the plant
Off-gas treatment is not an isolated chapter; it ties into three areas. First, refractories: the afterburner and hot ducting operate above 850 C continuously, and their lining belongs to the same materials system as the carbonization zone - a wrong selection shows up as hot shell and frequent repairs; see the carbonization lining page. Second, operating cost: steam from the waste-heat boiler offsets activation steam directly, so the train is both an environmental and an energy-saving asset. Third, investment decisions: if you are still weighing self-carbonization against buying char, the capex, footprint and running cost of the off-gas train belong in the three-year cash account - see build carbonization or buy char.
Keep this order in mind and any supplier quotation becomes easy to audit: is the train sequence right, are the temperature windows generous enough, and does every unit have matching instrumentation and a checking routine? Three yeses mean the system will hold up; miss any one and life after acceptance will not be easy.
Related notes
Send the raw material, target tonnage and finished spec. You get a configuration, a utility schedule and a budget range — not a brochure.