Black Liquor Recovery Boilers

Introduction

• Definition of Black Liquor: Mixture of organic matter and inorganic chemicals from the pulping process.
• Purpose of Black Liquor Recovery Boiler: To burn volatile organic materials in black liquor for heating value and recover inorganic chemicals.

Key Functions of Recovery Boiler

• Burn black liquor: Combustion of black liquor to generate energy.
• Recover inorganic chemicals: Collection of inorganic compounds as molten smelt.
• Generate superheated steam: Steam produced for various applications.

Combustion Process

• Furnace Section:

  • Combustion occurs here, requiring black liquor and air.
  • Liquid sprayed into the furnace with forced air system.
  • Self-sustained combustion is the norm.
  • Auxiliary fuel used for start-up, shut-down, and assisting marginal combustion.

• Furnace Design:

  • Rectangular box with openings for liquid and air.
  • Lower furnace: Liquid sprayed in, combustion air enters, leading to charbed smelt formation and drainage.
  • Upper furnace: Resistance time for complete combustion and heat absorption.

Recovery of Inorganic Chemicals

• Molten Smelt:

  • Composed of Na2S and Na2CO3.
  • Requires a reducing, oxygen-deficient environment.

• Chemical Recycling:

  • Dust (Na2SO4 and Na2CO3) collected in an electrostatic precipitator and returned to the boiler.
  • Bi-reduction efficiency is critical to the process.

Steam Generation Process

• Feed Water Heating:

  • Water enters through economizer; steam generation occurs in the generating bank.
  • Two drum design: hot water flows down to the furnace headers and is distributed evenly.
  • Separated vapor saturated steam is further heated in the superheater.

• Steam and Water Circulation:

  • Generating bank is a natural circulation loop.
  • Hot feed water enters steam drum; vapor separates from water flow.

Modern Recovery Boiler Design

• Shift from two-drum to single-drum designs since 1984 due to size and pressure considerations.
• Super Heater Design: Several banks with various arrangements for steam flow.
• Economizers: Increase fluid temperatures subjected to thermal stress.

Auxiliary Systems

• Auxiliary Fuel System:

  • Contains burners and igniters, flame safety systems, and controls.
  • Hearth and load burners for startup, shutdown, and extra steam generation.

• Soot Blowers: Use steam jets to remove deposits from heat transfer surfaces.

• Electrostatic Precipitators: Remove dust from flue gas, recycle recovered dust back to black liquor.

Final Concentration Steps

• Direct Contact Evaporator: Cools flue gas and increases black liquor concentration from 50% to 65% solids.
• Types of Evaporators: Cascade evaporators and cyclone evaporators.

Conclusion

• Importance of Black Liquor Recovery Boilers in the pulping process and chemical recovery.
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