In recent years, incidents involving black liquor recovery boilers have occurred in kraft pulp mills in B.C. Some of the incidents resulted in serious worker injuries, but all had the potential to do so.

What is the potential risk?

Black liquor recovery boilers (BLRBs) are complex and hazardous pieces of equipment essential to the operation of kraft pulp mills. BLRBs recover and regenerate chemicals used for the digestion of wood chips in the kraft pulping process. Hazards associated with BLRBs include:

• Steam and hot water being released from piping
• Explosions in the furnace resulting from water leaks or uncombusted black liquor reacting with molten smelt (melted pulping chemicals at approximately 900°C)
• Molten smelt runoff that can burn workers on contact
• Explosions in dissolving tanks from uncontrolled molten smelt reacting with water

The first two — risk of water and steam leaks from piping, and the reaction with molten smelt inside the furnace — appear to be well understood by industry. These aspects of recovery boilers, involving pressure piping, are regulated by Technical Safety BC (TSBC).

Recent incidents show that other hazards may not be fully understood and/or effectively managed. These hazards, which may be outside TSBC’s jurisdiction, include:

• Upset conditions, including slag falls or smelt-water reactions, which can lead to hot molten smelt being ejected
• Molten smelt accumulations in the furnace, which can lead to uncontrolled smelt flow and explosions in dissolving tanks

Who is at risk?

Black liquor recovery boilers present a hazard to all workers at kraft pulp mills due to the potential severity of explosions. In 1963, seven workers were killed in Squamish, B.C., when a recovery boiler exploded. With respect to the key hazards outlined above, the workers at greatest risk are:

• Workers assigned to power/recovery operations in kraft pulp mills. These workers are at increased risk because they regularly work near recovery boilers and the associated hazardous equipment.
• Workers assigned to conduct routine inspection and maintenance activities in the field. Often referred to as field engineers or spoutmen, these workers are most at risk from molten smelt ejections and explosions of the dissolving tank. They frequent locations near the sources of upset events, such as on the smelt spout deck or around the dissolving tank. They also routinely perform periodic high-risk activities such as clearing smelt spouts on operating furnaces to ensure the continuous flow of molten smelt to the dissolving tank.

How can this risk be reduced in the workplace?

There are many opportunities to reduce the risks associated with BLRBs. Key considerations include the following points.

General recovery boiler safety considerations:

• Operate and maintain boilers and equipment in accordance with established industry standards and relevant regulatory requirements, including TSBC requirements. Stay current by ensuring periodic reviews (see the first three listings under “Where can I find more information?”).
• Maintain good control of liquor (and thus smelt) chemistry. The properties and behaviour of smelt change, depending on composition. Maintaining good control of liquor chemistry will minimize the risk of upset conditions. These include plugged smelt spouts, hazardous smelt accumulations, and smelt ejection.
• Ensure adequate process control. Include provisions for effectively detecting hazardous conditions when they develop. Examples of such conditions include:
  • Smelt accumulations in a furnace
  • The introduction of uncombusted black liquor
  • Slag accumulations in upper furnace
  • Water leaks
• Ensure clear policies and procedures are in place for responding to plugged smelt spouts and other hazardous conditions. The potential for hazardous molten smelt accumulations to be behind the plug and the potential for an uncontrolled heavy smelt runoff must be considered. Specific procedures should be established for responding to situations when all smelt spouts have plugged with a full char bed, as this presents a risk of heavy smelt runoff.
• Inspect BLRBs and include the use of non-destructive inspection techniques to identify cracking in tubes. Repair or replace tubes, when applicable.
• Ensure there are shutdown criteria for operations personnel specifying when it is unsafe to continue operating the recovery boiler and when they should cool and water-wash the furnace bed.
• Ensure safety-critical equipment and controls are regularly inspected, tested, and maintained in good operating condition. For example, regularly inspect and test the emergency shutdown system and dissolving tank explosion dampers.
• Periodically review the hazards and risks of recovery boilers. Research available technologies and practices for maintaining process control and for reducing hazards and risks in accordance with the hierarchy of controls. (See section 5.55 of the Occupational Health and Safety Regulation for more information.) For example, remote monitoring technologies to detect and assess hazardous upset conditions have improved over time.

Specific worker safety considerations:

• Establish safe work practices for workers to safely work on and around recovery boilers. Follow regulatory requirements for performing maintenance work on energized equipment and establish procedures for responding to upset conditions.
• Provide appropriate personal protective equipment to workers. Standard coveralls or fire-resistant coveralls may not offer effective protection against severe smelt splashes. There are protective clothing and equipment available that have been tested and are rated for protection against smelt splashes from recovery boilers.
• Establish emergency response and evacuation procedures relevant to potential recovery boiler incidents. Conduct emergency drills at least once a year, as required by section 4.14 of the OHS Regulation.
• Conduct first aid assessments and include provisions for the treatment of injuries associated with chemical and heat burns.
• Adopt, where practicable, control measures that reduce the frequency of workers’ exposure to the risks associated with molten smelt splash or ejection, heavy smelt runoff, and dissolving-tank explosions. For example, recovery boiler manufacturers have developed automated smelt spout runners and spout-cleaning robots.

Where can I find more information?

The following groups and industry resources provide guidance around hazards, risks, and best practices for operating recovery boilers and for the pulp and paper industry in general. This guidance may help employers to comply with regulatory requirements specific to B.C.

• Black Liquor Recovery Boiler Advisory Committee — Guidelines and Recommended Practices
• Pulp and Paper Technical Association of Canada (PAPTAC)
• Technical Association of the Pulp and Paper Industry (TAPPI)
• The National Board of Boiler and Pressure Vessel Inspectors — Black Liquor Recovery Boilers: An Introduction

See also the following sections of the OHS Regulation:

• Part 4, General Conditions — Section 4.14, Emergency procedures
• Part 5, Chemical Agents and Biological Agents — Section 5.55, Type of controls