Rice husk: dust collection for the most abrasive biomass ash

Rice husk is the outlier of the biomass family, and one number explains why: ash content of 18–22 %, against 2–4 % for bagasse and 1–3 % for wood chip. Burn a tonne of husk and roughly 200 kg of ash comes out; burn a tonne of bagasse and 30 kg does. That ash is 85–90 % silica — amorphous when combustion is controlled — hard, angular and relentlessly abrasive. The fuel itself is otherwise convenient: dry as milled at around 10 % moisture, LHV about 12–14 MJ/kg, free-flowing, and produced exactly where steam is wanted, at rice mills whose cogeneration boilers typically run 10–30 t/h.

The collection problem therefore has three faces at once. Mass: per MW fired, a husk boiler generates five to ten times the fly ash of a bagasse unit of the same duty, so hoppers, conveyors and collection stages are sized against a fundamentally heavier load. Wear: silica at duct velocities erodes cyclone tubes, bend outer walls and fan blades on time scales that surprise operators arriving from other fuels. Electricity: low-alkali silica ash sits toward the high-resistivity end of the ESP window and must be sized accordingly. Set against all three is an opportunity none of Arrow's other industries offer: the ash itself is a product, covered in Section 05.

The staging logic that is advantageous on bagasse is close to mandatory on husk, because the first stage does double duty: it removes most of the mass and it removes it at the equipment best able to be sacrificial. Arrow's recommended train for rice-mill cogeneration is an abrasion-rated multicyclone — thick-walled or lined tubes, conservative velocities — or, where finer first-stage capture pays, the Electrocyclone at 85–95 % stage collection with service temperature to 400 °C and near-zero moving-part maintenance; followed by an electrostatic precipitator sized for high-resistivity silica ash where the applicable limit sits in the tens of mg/Nm³.

The ESP sizing deserves its own paragraph, because husk ash is not bagasse ash electrically. With little alkali and a drier gas — flue-gas moisture around 8–15 % by volume against 20 %+ on bagasse — resistivity trends toward 10¹¹ Ω·cm, the threshold where the collected layer stops conducting its charge away and back-corona begins. The countermeasures are conventional and effective when applied deliberately: operate at the temperature where the resistivity curve is lowest for the measured ash, keep the gas's own moisture in play, add SCA margin, and use energisation controls — intermittent charging, high-frequency supplies — that hold useful corona power on a marginal dust. What is not acceptable is transplanting a bagasse-basis ESP onto husk duty and expecting the same outlet: resistivity is measured from the actual ash and the machine sized to the measurement. Where an existing under-performing ESP is on site, this re-basis is done as an upgrade rather than a replacement — assessment through Arrow's ESP upgrade scope.

Downstream, the heavier catch changes ash handling: hoppers, rotary valves and conveyors are sized for a mass flow five to ten times the equivalent bagasse plant, with wear-resistant conveying components, and — anticipating Section 05 — with the coarse first-stage catch and fine ESP ash kept separable.

At 85–90 % amorphous silica, well-combusted husk ash is a pozzolan for blended cement and concrete, a foundry and insulation material, and — the high end of the chain — feedstock for precipitated silica production. Arrow's scope covers both ends of that chain: controlled-combustion rice husk burner systems engineered to produce reactive amorphous ash rather than merely dispose of husk, and silica extraction plants that convert the ash to precipitated silica. The collection train is the bridge between them: it must deliver ash dry, unmixed with other streams, consistent in carbon content, and traceable by stage — which is one more argument for the two-stage split, since first-stage and ESP catches differ in size and carbon and may command different grades.

The commercial consequence runs backwards through the whole design. Where ash has a sale price, the gas-cleaning investment is no longer justified by compliance alone: better collection means more recovered product, drier handling preserves grade, and the boiler, collectors and extraction plant are optimised as one system. That whole-system view — burner, boiler, collection train, ash plant — is how Arrow prefers to take on rice-husk work, and anonymised summaries of comparable projects 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.