Black liquor

Black liquor is the spent cooking liquor from kraft pulping after it has dissolved lignin and other organic material from wood chips. It contains water, organic fuel value and inorganic pulping chemicals. After concentration in evaporators, it is burned in a recovery boiler to generate steam and recover sodium and sulphur chemicals.

Weak black liquor from the pulp mill has too much water to burn efficiently, so it is concentrated to heavy black liquor before firing. The recovery boiler dries droplets, burns the organics and forms a molten smelt of inorganic chemicals at the furnace bottom. That smelt is dissolved and returned to the chemical recovery cycle.

Operating implications

Black-liquor firing combines combustion, heat recovery and chemical recovery in one unit. Important variables include solids content, viscosity, spraying pattern, sulphidity, heating value, carryover, char-bed condition and smelt-water explosion risk. Deposit formation in the superheater, generating bank and economiser can reduce availability and raise tube temperature.

Safety context

Recovery boilers have strict safety practices because water entering molten smelt can cause violent explosions. Changes to cleaning, washing, drains, sootblowing or instrumentation must respect mill safety procedures and BLRBAC guidance where applicable.

Acoustic-cleaning relevance

Sonic horns may be considered for selected dry ash or dust deposit zones outside the smelt-sensitive lower furnace, but application requires conservative review. Sylio-style cleaning is relevant only where non-contact online deposit control reduces sootblower dependence or manual cleaning without introducing water or impact risks.

Operating variables

Black liquor firing depends on dry-solids content, liquor temperature, viscosity, spray pattern, droplet size, sulphur chemistry and furnace air staging. Higher dry solids usually improve boiler efficiency but also change char-bed behaviour and heat release. Poor atomisation can create carry-over, unstable combustion and deposits in the upper furnace or convective pass.

Recovery boilers are designed to recover inorganic chemicals and generate steam, so black-liquor combustion is both an energy process and a chemical-recovery process. Operators monitor liquor solids, firing temperature, char-bed condition, smelt flow, steam temperature, TRS emissions, carry-over and fouling rates.

Safety and cleaning relevance

Water and smelt interaction is a severe hazard in recovery boilers, making leak detection, BLRBAC guidance and disciplined washing procedures critical. Acoustic cleaning may have relevance in dry convective passes or downstream dust-control areas, but it must never be treated as a substitute for recovery-boiler safety controls. Where deposits are dry and friable, horns can reduce fouling between sootblower cycles; where deposits are molten, sticky or chemically bonded, process chemistry and sootblowing strategy dominate.

Operating evidence

Deposit observations should be recorded with liquor solids, firing rate and sootblower operation. In recovery boilers, cleaning decisions must always remain subordinate to leak response and smelt-water safety procedures.

Sampling and trend evidence

Useful black-liquor records combine fuel and recovery data rather than treating the liquor as a simple boiler fuel. Operators normally track dry-solids percentage, firing temperature, liquor heater performance, spray-gun condition, char-bed stability, steam generation, carry-over, sootblower use and TRS excursions. When fouling increases, the cause may be a liquor solids change, a spray pattern fault, air-system imbalance or a chemistry shift rather than a cleaning-device fault. Outage inspections should note where deposits are dry, where they are sticky, and whether they are linked to a particular burner elevation or gas turn. That evidence is what determines whether acoustic cleaning has a suitable dry deposit target or whether the issue belongs to combustion and chemical-recovery control.