Pulp & paper: ESPs for recovery boilers, power boilers and lime kilns

A kraft recovery boiler burns concentrated black liquor to recover pulping chemicals and steam, and in doing so generates a dust found nowhere else in solid-fuel firing: saltcake fume. Sodium compounds vaporise in the char bed, then condense in the upper furnace into particles with a mass mode around 0.5–1 µm — an order of magnitude finer than boiler fly ash — at loads of roughly 10–30 g/Nm³, dominated by Na₂SO₄ with sodium carbonate and chloride. The electrostatic precipitator that collects it is not disposing of a waste: its catch is returned to the liquor cycle as make-up chemical, which makes the recovery boiler ESP a unit operation in the mill's chemistry as much as its emission control.

The same mill typically runs two or three other collection duties — a bark- or hog-fuel power boiler whose fly ash behaves like the general biomass case, and a lime kiln closing the calcium loop. Each has its own gas, dust and sizing basis, worked through below. This page belongs to Arrow's industries series; the quantitative headline for pulp and paper is a single ratio: recovery fume needs a specific collecting area of 100–150 s/m where 60–80 s/m suffices on the power boiler next door — roughly twice the precipitator per unit of gas, for reasons that are pure particle physics.

On the recovery boiler the ESP is effectively without competition, and the train is the precipitator itself, sized generously. Fabric filtration is disqualified by the sticky, hygroscopic cake — blinded media, not cleaned media, is the realistic outcome — and wet scrubbing would put the recovered chemical into a water stream the liquor cycle then has to evaporate again. Arrow's recovery basis is a multi-field ESP at 100–150 s/m with sectionalised T/R supplies so a field fault de-rates rather than trips the unit, casing and hopper heating against condensation, and the bottom arrangement chosen with the mill: wet-bottom, sluicing saltcake directly into liquor, or the now more common dry-bottom with drag-chain conveyors delivering dry saltcake to the mix tank. The n-plus-margin logic matters doubly here because a recovery boiler outage stops the entire fibre line.

Chloride and potassium enrichment adds a control dimension the power boiler never sees. NaCl and KCl are the most volatile components of the fume, so they concentrate in the finest particles — and because the ESP catch is recycled to the liquor, chloride recirculates and builds up in the cycle unless a purge exists. Many mills therefore treat part of the precipitator catch, or dust from the last field where enrichment is highest, as the chloride purge point. That process decision reaches back into ESP design: field-by-field dust segregation, separate conveying from the outlet field, and hopper arrangements that let the mill choose per campaign what is recycled and what is purged.

The bark-fired power boiler is the general biomass duty wearing mill colours — char carryover, embers, moderate resistivity — and takes the staged train described on the biomass power page: an Electrocyclone first stage removing the coarse char fraction (and quenching embers), then a compact ESP, or a bag filter where limits are in single digits and pre-collection has removed the fire risk. The lime kiln, whose fine calcium-carbonate dust returns to the lime circuit, is collected in either an ESP or a bag filter downstream of gas cooling; the hygroscopic, potentially cementitious dust pushes the same condensation-avoidance disciplines as the recovery unit.

The figures on this page — 0.5–1 µm fume, 10–30 g/Nm³ loads, SCA 100–150 s/m recovery versus 60–80 s/m power boiler — are indicative duty envelopes, not guarantees. Recovery ESP sizing in particular is driven by measured fume properties and by the mill's short-term averaging rules, since liquor firing rate and char-bed condition swing fume load on time scales of minutes. Applicable particulate limits for recovery boilers, power boilers and lime kilns vary by jurisdiction and permit vintage: CONFIRM: current particulate limits for recovery boiler, power boiler and lime kiln stacks in the target jurisdiction, with reference conditions.

Arrow supplies new precipitators, and rebuilds and upgrades existing recovery and power-boiler ESPs — internals, T/R sets, controls and casing-heating systems — within the annual mill shutdown window. Anonymised project summaries on comparable duties 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 liquor or fuel firing range.