≈ 400–1,500 mg/Nm³
Raw HCl, MSW firing
Industries · Waste-to-energy
Waste-to-energy: emission control for MSW and RDF firing
Waste-to-energy flue gas is an acid-gas problem wearing a dust problem's clothes: raw HCl of roughly 400–1,500 mg/Nm³, SO₂ and HF alongside, dioxins and volatile heavy metals condensed on the finest particles. The reference train is semi-dry or dry sorbent injection plus activated carbon into a bag filter, whose dust cake doubles as the reaction bed.
250–400 °C
Dioxin de-novo window to avoid
0.1 ng I-TEQ/Nm³
Dioxin control level, widely applied
420–450 °C
Superheater metal temperature, conservative
01 — The duty
Why waste to energy emission control is chemistry first, dust second
On every other fuel on this site, particulate is the problem. Here it is the easy part.
Municipal solid waste and RDF are the most chemically hostile fuels in Arrow's industries range. The waste stream delivers chlorine from plastics and food salt, sulphur from gypsum and rubber, fluorine from treated textiles — and the flue gas that results carries raw acid-gas loads of roughly 400–1,500 mg/Nm³ HCl, 150–600 mg/Nm³ SO₂ and 5–20 mg/Nm³ HF at the sector's standard reference of 11 % O₂ dry, swinging hour by hour as the tipping-hall mix changes. Behind the acids come trace organics — dioxins and furans, controlled to the widely applied 0.1 ng I-TEQ/Nm³ level — and volatile heavy metals that leave the furnace as vapour and condense preferentially onto the finest fly-ash fractions, precisely the particles hardest to collect.
Particulate control alone therefore cannot make a WtE stack compliant; the train must remove gases, adsorb organics, and capture metal-enriched fines in one integrated system. That inversion drives every selection on this page: why the bag filter, not the ESP, anchors the reference train; why gas temperature must be managed through a specific forbidden window; why the boiler's own steel sets the steam cycle; and why the plant produces two ash streams with legally different fates. Full acid-gas system design sits on the flue-gas treatment page; this one sets the WtE duty that drives it.
02 — Ash & gas
Raw gas and residue properties for MSW firing
The specification the tipping hall writes, hour by hour.
| Property | Typical value | Design consequence |
|---|---|---|
| HCl, raw | ≈ 400–1,500 mg/Nm³ | Sizes the sorbent system; dosing follows composition swings |
| SO₂, raw | ≈ 150–600 mg/Nm³ | Competes with HCl for lime; stoichiometry set on both |
| HF, raw | ≈ 5–20 mg/Nm³ | Reacts readily with lime; glass-attack risk on some media |
| Fly-ash load at boiler exit | ≈ 1–5 g/Nm³ | Modest by solid-fuel standards — dust is not the sizing case |
| Dioxins/furans | Formed at trace level; re-formation risk 250–400 °C | Fast quench through the window; activated carbon injection |
| Volatile heavy metals (Hg, Cd, Pb, Zn) | Condense on sub-micron fines and carbon | High fine-particle capture; carbon adsorbs Hg |
| Chlorine in deposits | High — alkali and zinc chlorides | Superheater metal held to 420–450 °C conservative |
| Bottom ash | ≈ 15–25 % of waste input | Inert; metal recovery, aggregate reuse |
| APC residue | ≈ 2–5 % of waste input | Hazardous class — stabilisation and controlled disposal |
The last two rows carry a discipline the rest of the site does not need: segregation. Bottom ash from the grate is inert enough for processing and reuse in many jurisdictions. The air-pollution-control residue — reacted sorbent, activated carbon, condensed metals, adsorbed dioxins — is the concentrated end point of everything the train removed, and is handled as hazardous waste, typically stabilised before landfill. Conveying, storage and loadout are designed so the two streams cannot cross-contaminate, because one tonne of APC residue in a bottom-ash stockpile reclassifies the stockpile.
03 — The train
The reference train: sorbent injection into a bag filter
Semi-dry or dry lime, activated carbon, and a dust cake that does chemical work.
Why does a bag filter beat an ESP on waste-to-energy duty?
Because filtration through the dust cake is a second reaction stage. Every Nm³ of gas passes through a fixed bed of partly reacted lime and activated carbon held on the bags, so acid capture and dioxin adsorption continue after injection — lifting sorbent utilisation well above what in-flight contact alone achieves. An ESP drops the sorbent into hoppers on contact; the cake never forms.
The reference train reads, in gas order: combustion control to destroy organics at source; boiler surfaces bringing gas down toward the treatment window, passed quickly through 250–400 °C — the band where dioxins re-form catalytically on fly ash (de-novo synthesis), which is a residence-time rule for boiler and duct design, not a piece of equipment; then acid-gas capture, either a semi-dry absorber spraying lime slurry whose water evaporates completely to leave reactive solids, or dry sorbent injection (DSI) of hydrated lime or bicarbonate straight into the duct; activated carbon injection at roughly 50–150 mg/Nm³ for dioxins and mercury; and finally the bag filter at typically 130–160 °C — comfortably below the de-novo band — collecting fly ash, spent and unspent sorbent, and carbon in a single cake. Semi-dry buys better acid capture and gas cooling in one vessel; DSI buys simplicity and retrofit ease at higher sorbent consumption. The trade is worked per project on the flue-gas treatment page's basis.
Where does the electrostatic precipitator still appear? Upstream, as a de-duster: taking the 1–5 g/Nm³ boiler-exit ash out before the chemical stage, so the bag filter cake is sorbent-rich rather than ash-diluted and the fly ash — sometimes separately manageable — stays out of the hazardous APC residue. On retrofits of older plants an existing ESP is commonly retained in exactly this role while a new sorbent-plus-baghouse stage is added behind it.
04 — Corrosion limit
Chlorine sets the steam cycle: the 420–450 °C rule
The most important emission-control decision in a WtE plant is made in the superheater.
Chlorine does not only make HCl. Alkali and zinc chlorides condense on convective surfaces, and above roughly 450 °C tube-metal temperature their deposit chemistry strips the protective oxide from conventional superheater steels — active oxidation — at rates that can consume a tube bank in a few years. Conservative WtE practice therefore holds final superheater metal to 420–450 °C, which caps steam temperature around 400–430 °C and costs one to two percentage points of cycle efficiency against a biomass unit. Designs that push higher pay for it in Inconel cladding or accept shortened tube life; the honest engineering position is that on high-chlorine fuel, tube metallurgy and steam-cycle economics are one decision, taken together at the boiler design stage.
The corrosion logic reaches the back end too. Downstream of acid capture, gas must stay above the residual acid dewpoint everywhere — casing cold spots, dead duct legs, standby lines — because a bag filter casing that condenses HCl-bearing moisture corrodes from the inside out. Insulation standards, heat tracing at hoppers and penetrations, and disciplined start-up and shutdown sequences (preheating before waste feed, purging before cool-down) are specified with the same seriousness as the collectors themselves.
05 — Compliance
Performance basis, monitoring and references
Continuous multi-component compliance, stated on a defined basis.
What emission levels must a waste-to-energy plant meet?
WtE permits regulate dust, HCl, SO₂, HF, NOx, CO, TOC, mercury, other heavy metals and dioxins simultaneously, on short averaging periods and continuous monitoring, referenced to 11 % O₂ dry. Numerical limits differ by jurisdiction and permit vintage — CONFIRM: current emission limit set for the target jurisdiction, all species, with averaging periods — so the train is designed against the project's actual limit table, not a generic one.
What can be said generically: a properly sized sorbent-injection-plus-bag-filter train, behind good combustion and with carbon injection, is the configuration with which modern plants meet single-digit mg/Nm³ dust and the 0.1 ng I-TEQ/Nm³ dioxin level, and it is the configuration Arrow offers as the WtE reference. Continuous emission monitoring closes the control loop: stack HCl feeding back to sorbent dosing, cleaned-gas dust monitors watching for bag failure by compartment.
Arrow's scope on WtE and RDF projects spans the boiler pressure parts, the sorbent stage, the bag filter and the ash-handling split, with the acid-gas system engineered per project. Anonymised summaries of comparable flue-gas-treatment projects are collected under references. Guaranteed figures are stated per project after the technical assessment, on a stated basis: mg/Nm³ per species, 11 % O₂ dry, and waste-composition envelope.
FAQ
Engineering questions, answered
Why do waste-to-energy plants use bag filters instead of ESPs?
Because the bag filter's dust cake is a working reaction bed: flue gas passes through a fixed layer of lime and activated carbon on every bag, finishing the acid-gas and dioxin capture the injection stage started. An ESP collects the sorbent instead of using it, giving markedly lower utilisation. ESPs still appear as upstream de-dusters ahead of the chemical stage.
How are dioxins controlled in a waste-to-energy plant?
Three layers: complete combustion, rapid gas cooling through the 250–400 °C window where dioxins re-form catalytically on fly ash (de-novo synthesis), and activated carbon injected at roughly 50–150 mg/Nm³ then captured in the bag filter, where the carbon-laden cake adsorbs residual dioxins. The widely applied performance level is 0.1 ng I-TEQ/Nm³.
What acid gas loads does MSW combustion produce?
Typical raw-gas values are HCl around 400–1,500 mg/Nm³, SO₂ around 150–600 mg/Nm³ and HF around 5–20 mg/Nm³, at 11 % O₂ dry — driven by plastics, food waste and gypsum in the fuel, and swinging hour to hour with waste composition. Sorbent dosing control must follow those swings, usually on upstream and stack HCl analyzers.
Why are WtE superheaters limited to 420–450 °C metal temperature?
Chlorine chemistry. Above roughly 450 °C tube metal temperature, chloride-laden deposits drive accelerated corrosion of conventional superheater alloys, which is why conservative WtE designs hold final superheat lower than an equivalent biomass or coal unit, accepting 1–2 percentage points of cycle efficiency as the price of tube life.
What is the difference between bottom ash and APC residue?
Bottom ash — roughly 15–25 % of waste input by mass — is the grate discharge: inert, commonly processed for metal recovery and aggregate use. APC residue — a further 2–5 % — is the bag filter and sorbent catch: reacted lime, carbon, condensed heavy metals and adsorbed dioxins, classified as hazardous in most jurisdictions and stabilised before disposal. They must never be mixed.
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