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A Boiler: The Explosive Potential of a Bomb
Acoustic Emission Examination of Metal Pressure Vessels
Anatomy of a Catastrophic Boiler Accident
Austenitic Stainless Steel
Basic Weld Inspection - Part 1
Basic Weld Inspection - Part 2
Black Liquor Recovery Boilers - An Introduction
Boiler Efficiency and Steam Quality: The Challenge of Creating Quality Steam Using Existing Boiler Efficiencies
Boiler Logs Can Reduce Accidents
Boiler/Burner Combustion Air Supply Requirements and Maintenance
Carbon Monoxide Poisoning Preventable With Complete Inspection
Combustion Air Requirements:The Forgotten Element In Boiler Rooms
Creep and Creep Failures
Description of Construction and Inspection Procedure for Steam Locomotive and Fire Tube Boilers
Ensuring Safe Operation Of Vessels With Quick-Opening Closures
Environmental Heat Exchangers
Factors Affecting Inservice Cracking of Weld Zone in Corrosive Service
Failure Avoidance in Welded Fabrication
Finite Element Analysis of Pressure Vessels
Fuel Ash Corrosion
Fuel Firing Apparatus - Natural Gas
Grain Boundaries
Heat Treatment - What Is It?
How to Destroy a Boiler -- Part 1
How to Destroy a Boiler -- Part 2
How to Destroy a Boiler -- Part 3
Identifying Pressure Vessel Nozzle Problems
Inspection, Repair, and Alteration of Yankee Dryers
Inspection, What Better Place to Begin
Laminations Led to Incident
Lay-up of Heating Boilers
Liquid Penetrant Examination
Low Voltage Short Circuiting-GMAW
Low Water Cut-Off Technology
Low-Water Cutoff: A Maintenance Must
Magnetic Particle Examination
Maintaining Proper Boiler Inspections Through Proper Relationships
Microstructural Degradation
Miracle Fluid?
Organizing A Vessel, Tank, and Piping Inspection Program
Paper Machine Failure Investigation: Inspection Requirements Should Be Changed For Dryer Can
Pipe Support Performance as It Applies to Power Plant Safety and Reliability
Polymer Use for Boilers and Pressure Vessels
Pressure Vessel Fatigue
Pressure Vessels: Analyzing Change
Preventing Corrosion Under Insulation
Preventing Steam/Condensate System Accidents
Proper Boiler Care Makes Good Business Sense:Safety Precautions for Drycleaning Businesses
Putting a Stop to Steam Kettle Failure
Quick Actuating Closures
Quick-Actuating Door Failures
Real-Time Radioscopic Examination
Recommendations For A Safe Boiler Room
Recovering Boiler Systems After A Flood
Rendering Plants Require Safety
Repair or Alteration of Pressure Vessels
Residential Water Heater Safety
School Boiler Maintenance Programs: How Safe Are The Children?
Secondary Low-Water Fuel Cutoff Probe: Is It as Safe as You Think?
Short-Term High Temperature Failures
Specification of Rupture Disk Burst Pressure
Steam Traps Affect Boiler Plant Efficiency
Steps to Safety: Guide for Restarting Boilers After Summer Lay-Up
Stress Corrosion Cracking of Steel in Liquefied Ammonia Service - A Recapitulation
Suggested Daily Boiler Log Program
Suggested Maintenance Log Program
System Design, Specifications, Operation, and Inspection of Deaerators
Tack Welding
Temperature And Pressure Relief Valves Often Overlooked
Temperature Considerations for Pressure Relief Valve Application
The Authorized Inspector's Responsibility for Dimensional Inspection
The Effects of Erosion-Corrosion on Power Plant Piping
The Forgotten Boiler That Suddenly Isn't
The Trend of Boiler/Pressure Vessel Incidents: On the Decline?
The Use of Pressure Vessels for Human Occupancy in Clinical Hyberbaric Medicine
Thermally Induced Stress Cycling (Thermal Shock) in Firetube Boilers
Top Ten Boiler and Combustion Safety Issues to Avoid
Typical Improper Repairs of Safety Valves
Wasted Superheat Converted to Hot, Sanitary Water
Water Maintenance Essential to Prevent Boiler Scaling
Water Still Flashes to Steam at 212
Welding Consideration for Pressure Relief Valves
Welding Symbols: A Useful System or Undecipherable Hieroglyphics?
What is the Best Welding Process?
What Should You Do Before Starting Boilers After Summer Lay-Up?
Why? A Question for All Inspectors

Black Liquor Recovery Boilers - An Introduction

David Parrish
Factory Mutual Research Corp.

Winter 1998  

Category: Operations 


Summary: The following article is a part of National Board Classic Series and it was published in the National Board BULLETIN. (4 printed pages)



The sodium sulfate or kraft pulping process was patented in 1884 and accounts for over 80% of all pulp produced in North America. This process would not be economically viable without the black liquor recovery boiler (BLRB). The purpose of the BLRB is to reclaim the spent pulping chemicals in the "black liquor" by the process of combustion and to capture the heating value as steam to generate electricity and supply process steam demands.

The design of the BLRB is attributed to G.H. Tomlinson and the first unit having water walls was constructed in 1934 by Babcock and Wilcox. A contemporary BLRB is similar to a large industrial watertube boiler and may be either two drum design (popular for operating pressures less than 900 psi) or single drum (popular for pressures above 900 psi). The single drum design has the advantages of removing the drum from the corrosive flue gases and eliminating potential leakage from rolled tube joints. The combustion gases are highly corrosive, particularly at temperatures necessary for operation above 900 psi. Above 900 psi, special corrosion protection (such as composite tubes, chromizing, metal spray coating and stainless weld overlay) is provided for furnace tubes. The furnace of a BLRB is generally taller than a utility or industrial watertube boiler of similar steam generating capacity, due to the sticky nature of the combustion gases.

The pulp production capacity of a mill determines the size of a BLRB. For each ton of pulp, about 3,000 lbs of dry solids are generated. A typical small BLRB will process 750,000 lbs/day dry solids and be rated for 300 psi MAWP and 140,000 lbs/hr steam. A typical large BLRB will process 6,000,000 lbs/day dry solids and be rated for 1500 psi MAWP and 850,000 lbs/hr steam. Replacement values are on the order of $18,000,000 for the smaller BLRB and $40,000,000 for the larger BLRB. The daily revenue generated by a mill could be $200,000 for the smaller and $1,000,000 for the larger BLRB. These high values require exceptional attention to the operation and maintenance of BLRBs to assure optimum return on investment.

BLRBs have auxiliary fuel (gas or oil) burners located near the floor to raise the temperature for initiating combustion of the black liquor and to stabilize combustion if upsets occur. Auxiliary burners may also be provided higher in the furnace to supplement heat input during times of limited liquor availability or other upsets. The potential for fuel gas explosions in BLRBs is similar to that of any large watertube boiler, and explosion experience has been greatly improved by application of fuel combustion control systems. Auxiliary fuel combustion control systems equivalent to the requirements of NFPA 8502, standard for the prevention of furnace explosions/implosions in multiple burner boilers, are recommended. Generally, auxiliary fuel is shut off once firing of black liquor has become self-sustaining and stabilized.

Similar to any boiler, dry firing is a potential risk. Application of low-water protection schemes has greatly reduced the incidence of BLRB dry firing. Provision of either two fully independent low-water fuel cutoff devices, or a system providing similar reliability, is typically recommended.

Combustion of the black liquor reduces the sulfur compounds to sulfide and recovers the inorganic chemicals in a molten form. Black liquor, with at least 58% solids, is sprayed into the furnace. The remaining water is vaporized and partial combustion occurs during travel to the furnace floor. Combustion air is limited at the floor to promote a reducing atmosphere. The partially combusted liquor accumulates as a porous char bed. Molten smelt accumulates on the floor (protected from overheating by refractory or by solidified smelt) to a level of several inches before flowing out water-cooled smelt spouts to the smelt dissolving tank.

It is the pool of molten smelt, at a temperature of 1700°F - 1800°F (925°C - 980°C) that presents the unique BLRB hazard. As with any high-temperature molten material, mixing water can result in an expanding vapor explosion. The rapid vaporization of one pound of water can release as much energy as a half- pound of TNT. A smelt-water explosion generates a shock wave that causes severe local damage when the wave contacts the furnace. Although much study has been done, the only known method to avoid a smelt-water explosion is to prevent smelt-water contact. A challenging task in a water-cooled vessel! In addition to boiler water, several other water sources exist. If the water content of the black liquor exceeds 42%, an explosion can result. Condensate may enter through a faulty sootblower system or a faulty steam coil air heater. Water can be introduced from external water wash hoses, from the black liquor piping water wash system or from faulty liquor heat exchangers.

The Black Liquor Recovery Boiler Advisory Committee (BLRBAC) has developed a recommended system to avoid or mitigate smelt-water explosions. This system is known as the Emergency Shutdown Procedure or ESP. If water is known or suspected to be entering a BLRB furnace, the operator is trained to actuate the ESP which simultaneously sounds an emergency evacuation alarm, stops all fuel flow to the furnace, minimizes combustion air to the char bed, shuts off all water sources to the furnace and opens a group of "rapid drain" valves that drain all but the last 8 feet of water from the BLRB. This 8 feet of water is intended to prevent overheating of the floor and lower wall tubes. The drain is completed in about 20 minutes, and the superheater vent valve is then opened to more rapidly relieve the pressure (stored energy) remaining in the BLRB.

BLRBAC has been notified of over 150 BLRB explosions in North America. In most incidents, BLRB damage required days or even weeks to repair. In the worst incidents, damage was extensive, taking months to repair, and injury and death of operating personnel has resulted. The primary source of water entering a BLRB furnace is from boiler pressure part leaks. In addition to semi-annual or annual fireside inspections, annual inspection of pressure parts is recommended to locate existing leaks and identify potential leaks so proactive measures can be taken to prevent smelt-water contact. During this annual inspection it is also recommended all safety systems be tested - ESP, low-water protection, black liquor and auxiliary fuel firing systems. Also recommended is periodic determination that operators continue to know not only how to operate the BLRB on a daily basis, but also to recognize various upset conditions, know what corrective actions are appropriate, be able to recognize when water is entering the furnace and how to actuate the ESP system.

With some 280 BLRBs currently operating in North America and new ones under construction, the challenge of safe operation will be with us for years to come.



Editor's note: Some ASME Boiler and Pressure Vessel Code requirements may have changed because of advances in material technology and/or actual experience. The reader is cautioned to refer to the latest edition of the ASME Boiler and Pressure Vessel Code for current requirements.


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