Recovery boiler

File:EarlyTomlinsonRB.gif

Recovery Boiler

A recovery boiler is a crucial component in the chemical recovery process of a kraft pulp mill. It is designed to burn black liquor, a byproduct of the pulping process, to generate steam and recover chemicals for reuse in the pulping process. This article provides an in-depth look at the design, operation, and significance of recovery boilers in the pulp and paper industry.

Design and Components

A recovery boiler consists of several key components that work together to achieve its dual purpose of energy generation and chemical recovery:

Furnace

The furnace is the primary chamber where black liquor is combusted. It is lined with water-cooled tubes that absorb heat from the combustion process to generate steam. The furnace operates at high temperatures to ensure complete combustion of the organic material in the black liquor.

Superheater

The superheater is a set of tubes located in the upper part of the boiler. It heats the steam produced in the furnace to a higher temperature, increasing its energy content and efficiency for use in turbines.

Economizer

The economizer is a heat exchanger that preheats the feedwater entering the boiler using the flue gases exiting the furnace. This process improves the overall efficiency of the boiler by reducing the energy required to convert water into steam.

Air System

The air system supplies the necessary oxygen for combustion. It typically includes primary, secondary, and tertiary air supplies to ensure complete combustion and control of emissions.

Smelt Spouts and Dissolving Tank

The smelt spouts are located at the bottom of the furnace and direct the molten smelt into a dissolving tank. The smelt contains inorganic chemicals that are recovered and recycled back into the pulping process.

Operation

The operation of a recovery boiler involves several critical steps:

1. Black Liquor Combustion: Black liquor is sprayed into the furnace, where it is combusted. The heat generated is used to produce steam. 2. Steam Generation: Water in the boiler tubes absorbs heat from the combustion process, converting it into steam. 3. Chemical Recovery: The inorganic chemicals in the black liquor are recovered as smelt, which is dissolved in water to form green liquor. This is further processed to regenerate white liquor for the pulping process. 4. Emission Control: Modern recovery boilers are equipped with systems to control emissions of particulates, sulfur compounds, and nitrogen oxides.

Importance in the Pulp and Paper Industry

Recovery boilers are vital for the economic and environmental sustainability of kraft pulp mills. They allow for the efficient recovery and reuse of chemicals, reducing the need for fresh chemical inputs. Additionally, the energy generated from burning black liquor contributes significantly to the energy needs of the mill, often making it energy self-sufficient.

Challenges and Innovations

Recovery boilers face challenges such as corrosion, fouling, and emissions control. Innovations in materials, design, and control systems continue to improve their efficiency and longevity. Advanced monitoring and control technologies are being developed to optimize combustion and reduce emissions.

Also see

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