Kraft Recovery Boilers – Liquor System
“Strong” black liquor enters the recovery boiler containing 65-85% dry solids. The recovery boiler uses strong black liquor as its main fuel and during normal operation functions with black liquor as the only fuel. The cooking chemicals flow out of the recovery boiler as molten salts, or smelt, and are turned into green liquor in the dissolving tank. This green liquor is sent to the recausticizing process for conversion to white liquor. This module describes the components and flow of a recovery boiler liquor system as well as the combustion process of black liquor.
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Learning Objectives
• Describe why strong black liquor is burned in a kraft recovery boiler
• Describe the components of black liquor
• Identify the causes of black liquor variability
• Identify the effects of black liquor variability
• Identify and describe the key components of a recovery boiler liquor system
• Describe the flow of black liquor to the recovery boiler
• Describe the combustion process of black liquor
• Identify and describe the safety hazards around the kraft recovery boiler liquor system and typical safety systems used
Specs
| Course Level | Intermediate |
| Languages | English, Portuguese, French, Polish, Russian |
| Compatibility | Audio, Video |
| Based on: | Industry Standards and Best Practices |
Key Questions
Why does black liquor need to be heated?
Concentrated black liquor is quite viscous. Heating the liquor is required so that it can be pumped and sprayed into the boiler.
What happens to black liquor in a recovery boiler?
Black liquor is burned in a recovery boiler in order to recover heat and cooking chemicals.
What will happen if a solids level below 60% is detected at the liquor ring header?
If low solids is detected in black liquor flowing to the boiler, the liquor is diverted back to the mix tank.
What is mixed with black liquor at the mix tank?
Dust and ash from the sootblower hoppers and the electrostatic precipitator, and sometimes make-up saltcake is added to the black liquor in the mix tank.
What can cause variations in the viscosity of black liquor?
Some of the conditions that affect the viscosity of black liquor include, solids concentration, wood species, final kappa, and the time and temperature cooking profile.
Sample Video Transcript
The Kraft recovery boiler air system consists of three levels of air ports: Primary, secondary, and tertiary that allow the heated air to enter the boiler for combustion. The strong black liquor is sprayed into the boiler between the secondary and tertiary ports through the liquor gun ports located at about 15 feet above the furnace bottom. First, the liquor droplets dry as they fall through the hot combustion gases, then the easily volatilized components are released from the liquor droplets and burned. Next, the remaining char which is mostly carbon and the sodium compounds falls unto the smelt bed where the char burns off leaving the smelt. The smelt then flows out of the bottom of the furnace and into the dissolving tank to become green liquor completing the recovery boiler processing of the strong black liquor.
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