Recovery boiler

A recovery boiler is a crucial component in the pulp and paper industry, used primarily to recover chemicals and energy from black liquor, a byproduct of the kraft process. The recovery boiler plays a vital role in the chemical recovery cycle, enabling the recycling of chemicals and the generation of steam and electricity.

History

The development of the recovery boiler was a significant advancement in the kraft process, allowing for more efficient chemical recovery and energy production. The first recovery boilers were developed in the early 20th century, with significant improvements in design and efficiency occurring over the decades.

Function

The primary function of a recovery boiler is to combust black liquor, which contains organic material and inorganic chemicals. The combustion process recovers the inorganic chemicals as a smelt, which is then processed to regenerate the cooking chemicals used in the kraft process.

Combustion Process

Black liquor is sprayed into the recovery boiler through specialized nozzles, such as the one shown in

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A black liquor nozzle

. The liquor is burned in the furnace, releasing heat that is used to produce steam. This steam can be used to generate electricity or provide process heat for the mill.

Chemical Recovery

During combustion, the inorganic chemicals in the black liquor are reduced to a molten smelt, which is collected at the bottom of the boiler. This smelt is then dissolved in water to form green liquor, which is further processed to regenerate white liquor, the active cooking chemical in the kraft process.

Energy Production

The recovery boiler is a significant source of energy for a pulp mill. The heat generated from burning black liquor is used to produce high-pressure steam, which can drive turbines to generate electricity. This process is illustrated in the

Net heating value of black liquor

.

Environmental Impact

Recovery boilers help reduce the environmental impact of the kraft process by recycling chemicals and reducing the need for fresh chemical inputs. They also reduce emissions by efficiently burning organic material in black liquor.

Challenges and Innovations

Recovery boilers face challenges such as corrosion, fouling, and the need for efficient combustion. Innovations in materials, design, and control systems continue to improve the efficiency and longevity of recovery boilers.

Also see

• Kraft process
• Black liquor
• Pulp mill
• Chemical recovery

References

• Smook, G. A. (1992). Handbook for Pulp & Paper Technologists. Angus Wilde Publications.
• Adams, T. N., & Frederick, W. J. (1988). Kraft Recovery Boiler Physical and Chemical Processes. American Paper Institute.