~50 % moisture · LHV 7.2–7.5 MJ/kg
Bagasse as fired
Industries · Sugar & ethanol
Sugar & bagasse: emission control for bagasse-fired boilers
A bagasse fired boiler burns fuel at ~50 % moisture and 7.2–7.5 MJ/kg LHV, throwing a char-heavy, bimodal fly ash at raw loads around 6,000 mg/Nm³ @ 6 % O₂ dry. Arrow Energy's reference train for this duty is a two-stage Electrocyclone + ESP: 720 mg/Nm³ after the first stage, 24 mg/Nm³ at the stack on the design basis.
6,000 mg/Nm³ @ 6 % O₂ dry
Raw dust at train inlet (design basis)
24 mg/Nm³ @ 6 % O₂ dry
Two-stage train outlet (design basis)
120–200 days/yr
Campaign length
01 — The duty
Why bagasse boiler emission control is its own discipline
Half the fuel is water, half the fly ash is unburnt carbon, and the plant runs four to seven months a year.
Bagasse arrives at the boiler front essentially as it leaves the last mill: around 50 % moisture, LHV 7.2–7.5 MJ/kg — roughly a third of the heating value of bituminous coal. To release the same heat, a bagasse fired boiler moves far more fuel mass and far more flue gas per MW than a coal unit, and the fibrous fuel burns partly in suspension. The consequence at the boiler outlet is a raw particulate load of several g/Nm³ — Arrow's design basis for a 60 t/h unit uses 6,000 mg/Nm³ at 6 % O₂ dry — dominated not by mineral ash but by char: bagasse itself is low-ash (2–4 %), yet loss on ignition in the fly ash commonly runs 30–60 % because partially burnt fibre is lifted out of the furnace before it finishes combusting.
The second defining feature is the campaign. A sugar mill crushes for 120–200 days, then stops for the off-season. Every start pulls the collection train through the acid and water dewpoints; every stop leaves hygroscopic ash sitting in hoppers through months of humid off-season air. Equipment that tolerates cycling — and can be inspected and overhauled in a predictable annual window — outlasts equipment optimised only for steady state. This page sets out the fuel and ash properties, the recommended two-stage train, and the retrofit realities of the sector; the parent industries hub places sugar alongside Arrow's other fuels and processes.
02 — Ash & gas
Bagasse fly ash and flue gas: the numbers that size the train
A bimodal, char-heavy, abrasive dust in a wet, ESP-friendly gas.
| Property | Typical value | Design consequence |
|---|---|---|
| Fuel moisture, as fired | ~50 % | High gas volume per MW; wet gas at the collector |
| Fuel LHV | 7.2–7.5 MJ/kg | Large fuel and ash mass flow per unit of steam |
| Fuel ash content | 2–4 % | Low mineral ash; fly ash is mostly char |
| Fly-ash loss on ignition | 30–60 % | Ember risk; re-injection debate (below) |
| Particle-size distribution | Bimodal — coarse fibrous char ≥ 100 µm plus fine ash; d₅₀ 21 µm at ESP inlet on the design basis | Pre-collector takes the coarse mode; ESP sized for the fine mode |
| Ash silica content | High (soil-derived), abrasive | Wear allowances at bends, tubes, hopper throats |
| Fly-ash resistivity | ≈ 10⁸–10¹¹ Ω·cm | Inside the favourable ESP window; no conditioning needed |
| Flue-gas moisture | ≥ 20 % by volume | Natural resistivity conditioning; watch dewpoint at start-up |
| Gas temperature at collector | 150–180 °C design point | Below the 200 °C dry-ESP ceiling with margin |
Two entries deserve emphasis. The resistivity row is why bagasse is one of the most ESP-friendly fuels in the biomass family: the wet gas keeps a conductive moisture film on the ash, so the electrostatic precipitator runs at healthy corona power without the back-corona pathologies of dry, high-resistivity dusts. The LOI row is why the train cannot be a bag filter by default: a 30–60 % carbon fly ash includes glowing embers, and coarse burning char landing on filter media is a fire risk that plate-and-hopper equipment simply does not share.
Should collected fly ash be re-injected into the furnace?
Re-injection returns the heat in unburnt char — worth on the order of 1 percentage point of boiler efficiency at 30–60 % LOI — but recirculating fines can roughly double the dust load the collection train must handle. It is a legitimate trade, provided the collector is sized for the re-injected load and the re-injected stream is taken from the coarse, char-rich first-stage catch, not the fine ESP ash.
A two-stage train makes the debate tractable, because it splits the ash. The pre-collector hopper holds the coarse, high-carbon fraction — the only fraction worth burning again — while the ESP hoppers hold fine, low-value ash that should go to disposal or soil amendment. Re-injecting everything, from a single collector, sends incombustible fines around the loop repeatedly and pays double for the privilege: added dust load and added erosion.
03 — The train
Recommended train: Electrocyclone + ESP, two stages
The worked design basis for exactly this duty.
DESIGN BASIS — ILLUSTRATIVE CALCULATION, NOT A GUARANTEE · all concentrations @ 6 % O₂ dry
- Boiler
- 60 t/h bagasse-fired
- Raw dust at train inlet
- 6,000 mg/Nm³
- Electrocyclone first stage, 88 %
- 720 mg/Nm³ at ESP inlet
- Four-field ESP, 96.67 % on remaining load
- 24 mg/Nm³ at stack · train total 99.60 %
- One ESP field out of service (n−1)
- 57 mg/Nm³
- ESP for this duty
- SCA 75.7 s/m · 3,888 m² collecting area · inlet d₅₀ 21 µm · 177 kW absorbed
The reasoning behind the split: bagasse fly ash is bimodal, so give each mode the collector that suits it. The Electrocyclone — cyclonic separation with electrostatic augmentation, rated to 400 °C — removes the coarse fibrous char and most of the 5–20 µm middle at 88 % on the design basis, quenching embers and taking the abrasive silica-rich fraction out at the most wear-tolerant point in the train. The ESP then works on a 720 mg/Nm³ fine-ash load instead of 6,000 mg/Nm³ of mixed everything. Because required collecting area scales with [ln(1/P)]² under Matts-Öhnfeldt, relaxing the ESP's required efficiency from 99.60 % to 96.67 % cuts its specific collecting area by roughly 60 % — the difference between a compact four-field unit at 75.7 s/m and a five- or six-field machine, with the capital, plot length and absorbed power that follow.
The n−1 line matters for a campaign plant: at 57 mg/Nm³ with one field down, the mill keeps crushing through a field fault and repairs at the next planned stop — no mid-campaign shutdown to stay under a limit in the tens of mg/Nm³. Applicable stack limits vary by country and permit vintage: CONFIRM: current particulate limit for bagasse-fired boilers in the target jurisdiction, with reference O₂.
04 — Cogeneration
Campaign operation and the export-power arithmetic
Emission control as the enabler of the surplus-bagasse business case.
Bagasse is roughly 28–30 % of cane mass, so a mill crushing ~100 t/h of cane raises about 30 t/h of bagasse. The design-basis boiler raises 60 t/h of steam, and to do so fires roughly 25–30 t/h of bagasse — on the order of 55–60 MW of fuel heat at 7.2–7.5 MJ/kg; a modern high-pressure cogeneration cycle converts the surplus beyond mill steam demand into export power. Every efficiency measure that stretches bagasse — economizer retrofit worth +3.1–4.0 percentage points, excess-air trim to λ ≈ 1.15 worth another +0.7–0.8 on units running high excess air — converts directly into saleable bagasse or saleable kWh. The emission-control train is what lets the boiler that burns it keep its operating permit while doing so, and its own absorbed power (177 kW for the design-basis ESP, 120–140 kW for an Electrocyclone) is parasitic load subtracted from export — one more reason the two-stage train, which minimises total collecting area, earns its place.
What does start-stop campaign operation do to an ESP?
Each start-up drags the casing through the water and acid dewpoints while the gas is at its wettest, condensing moisture onto cold plates and insulators; each 120–200-day campaign ends with hygroscopic, char-laden ash sitting in hoppers for months. The countermeasures are procedural and cheap: hopper heaters and insulator purge air energised before light-off, hoppers emptied completely at shutdown, and internals inspected in the off-season window.
The off-season is the sector's structural advantage: a guaranteed multi-week outage every year. Arrow schedules electrode alignment checks, rapping-system overhaul and transformer-rectifier testing into it, so the train enters each campaign at full field count — and the n−1 margin above is held in reserve for genuine faults rather than spent on deferred maintenance.
05 — Retrofit reality
Replacing wet scrubbers: the water and effluent argument
Most bagasse retrofits start from an existing scrubber, not from a green field.
A large share of the installed base still runs wet scrubbers, fitted in an era of limits in the hundreds of mg/Nm³. Three pressures are retiring them. First, performance: on fine char a scrubber struggles below roughly 100–150 mg/Nm³ without venturi pressure drops that punish fan power, while permits are moving toward tens of mg/Nm³. Second, water: continuous make-up in a sector where water rights and effluent discharge are increasingly the binding constraint on mill expansion. Third, the effluent itself — an ash slurry needing clarification, sludge handling and disposal, all of which disappear when collection goes dry.
The retrofit pattern Arrow applies is the two-stage train in the scrubber's footprint envelope: the Electrocyclone needs only 60–65 % of an equivalent-duty ESP's plot, which is often what makes a dry conversion fit inside an existing, congested boiler-house layout at all. Where a serviceable mechanical collector already exists, the staging logic still applies — the Electrocyclone replaces or follows it as first stage, and the ESP is sized against the pre-collected load. Ash handling converts from slurry to dry conveying at the same time, which is usually where the operating-cost case closes.
Arrow's anonymised reference classes on this duty include a 170 t/h bagasse-fired sugar mill in Thailand and a 230 t/h sugar mill in Colombia; comparable project summaries, stated without customer names, are collected under references. Guaranteed figures are stated per project after the technical assessment, on a stated basis — mg/Nm³, reference O₂, dry or wet, and load range.
FAQ
Engineering questions, answered
What is the best dust collector for a bagasse fired boiler?
For stack limits in the tens of mg/Nm³, a two-stage train — a mechanical or hybrid pre-collector followed by an electrostatic precipitator — is the reference choice. On Arrow's design basis for a 60 t/h bagasse boiler, an Electrocyclone takes 6,000 mg/Nm³ down to 720, and a four-field ESP finishes at 24 mg/Nm³ at 6 % O₂ dry.
Can an ESP handle bagasse fly ash?
Yes, and the conditions are favourable: bagasse flue gas carries 20 % or more moisture by volume, which holds ash resistivity in the workable 10⁸–10¹¹ ohm-cm window, so back-corona is rarely the problem it is on cement or coal duty. The real design drivers are the heavy char load, the coarse-plus-fine size distribution, and glowing ember carryover.
Why are sugar mills replacing wet scrubbers with ESPs?
A wet scrubber converts an air-emission problem into a water one: continuous make-up water, an ash slurry to clarify and dispose of, and a saturated plume. It also struggles below roughly 100–150 mg/Nm³ on fine char. A dry Electrocyclone + ESP train reaches 24 mg/Nm³ on the design basis with no process water and dry, handleable ash.
Does fly ash re-injection make sense on a bagasse boiler?
It recovers heat from unburnt char — loss on ignition in bagasse fly ash is commonly 30–60 % — and can return on the order of 1 percentage point of boiler efficiency. The price is a dust load to the collector that can roughly double. If re-injection is planned, the collection train must be sized for the re-injected load, not the once-through one.
What dust emission level can a bagasse boiler achieve?
On Arrow's illustrative design basis, a two-stage Electrocyclone + ESP train delivers 24 mg/Nm³ at 6 % O₂ dry from a 6,000 mg/Nm³ raw load, and 57 mg/Nm³ with one ESP field out of service. Guaranteed figures are stated per project after the technical assessment, on a stated basis of mg/Nm³, reference O₂, dry or wet, and load range.
Send us your plant data
Fuel, boiler capacity, gas flow, current emission and the limit you must meet. An Arrow engineer replies with a technical assessment basis — not a brochure.