From Black Liquor to Sludge: Converting a Recovery Boiler to a Bubbling Fluid Bed

Boiler conversion extends asset life to meet plant’s renewable energy needs.


Conversion of the plant’s unneeded recovery boiler as a bubbling fluidized-bed (BFB) power boiler provides an economic and environmentally sound solution to:

  • Burn paper mill sludge, wood waste and tires to displace other more expensive fossil fuels
  • Enable existing boiler assets to be fully utilized, such as the upper furnace, superheater, boiler bank and precipitator
  • Reduce landfill costs by utilizing mill sludge as a fuel source

Solution: Reduce Capital Costs of a New Power Boiler Island

Babcock & Wilcox (B&W) has extensive experience with boiler conversions and modifications, including extensive refurbishments of older recovery boilers to BFB conversion while reusing existing buildings and ancillary equipment.

B&W’s renewable solution – an open-bottom, BFB unit – addresses the project’s special concerns and is helping the customer achieve its goals of increased steam flow, fuel conversions and reduced

Unique Project Features and Challenges:

  • Utilizing bottom supported BFB hoppers reduced boiler weight to top steel through use of B&W’s innovative bed seal design
  • Auxiliary equipment and construction activities required insightful planning and procedures to accommodate demolition, installation of new equipment and start-up

Successful Outcomes:

B&W’s expert design was successful at converting the customer’s boiler to a BFB with an increased steam flow of more than 65 % and meeting emissions requirements while utilizing waste and opportunity fuels available at lower costs than traditional fossil fuels at the plant. 

This renewable project resulted in converting a once stranded asset into a valued asset at lower capital costs. The plant continues to serve as a model conversion facility.

Sideview of Recovery Boiler Converted to B&W’s Open-bottom BFB Design

Hover and click on the numbered image shown below to learn more!

Recovery Boiler

Ammonia/Urea Injectors

Ash Conveyors



Fluidized Bed

Fuel Chutes

Generating Bank

Load Burners

Overfire Air System

Refractory Line


Start-Up Burners

Steam Drum


BRB Sideview

Auxiliary Burner

Fluidized Bed

Fuel Chute

Open-bottom/Bubble Cap Air Distribution

Overfire Air System

Startup Burner

B&W’s open-bottom BFB furnace design provides complete debris removal with lower capital costs and maintenance expenses.

New Build

B&W Projects New Build
We consistently demonstrate the ability to successfully execute innovative solutions for our customers, from new equipment to large, complex full-scope engineering and construction projects.

Upgrades / Retrofits

Upgrades and Retrofits by B&W
Modifying, improving and maintaining existing equipment are vital considerations for the long-term viability and profitability of all operations.

Parts / Services

Babcock & Wilcox Parts and Service Support
An extensive network of worldwide regional service centers, field service engineers and technical support personnel are available to provide the customized service and aftermarket solutions for your requirements.