Chemical Recovery boilers are critical to the operation of kraft pulp mills. The purpose of a recovery boiler is to recover chemicals to reuse in the pulping process—and in the process produce steam to generate electricity. Without these boilers, a significant amount of chemical and resources would be wasted.
A waste material from the pulping process, called
black liquor, contains organic material from trees (
lignin, resins, etc.) and spent alkaline chemicals used to pulp the wood chips.
The black liquor is evaporated to remove water, which increases the fuel potential of the liquor. It is then injected into the recovery boiler for incineration. In the process the organic material is burned off, while the chemicals remain behind for recovery.
These remaining chemicals are mixed with water to form green liquor. The
green liquor is sent on for further processing to return it to its original form so it can be reused in the pulp mill
digesters to pulp more wood chips.
Our Thunder Bay Operations has one operational kraft mills (‘B’ Mill). As part of upgrading the facility and addressing local air quality issues around
odour and
particulate, a new state of the art ‘C’ Recovery Boiler was built in 2001. The new ‘C’ Boiler is a low odour recovery boiler, which utilizes the latest state-of-the-art evaporation, combustion and pollution control equipment.
Like a woodstove, boilers require airflow to ensure the fuel is burned efficiently. The old style recovery boilers used only two levels of combustion air. A large amount of material was not being completely burned off, resulting in the discharge of odorous compounds.
‘C’ Boiler utilizes 3 levels of combustion air to ensure that there is complete combustion of the black liquor, which results in no odour being generated.
As well, the older style recovery boilers used a direct contact evaporation system to evaporate water from the liquor prior to burning. This involved passing hot
flue gas from the boiler over the liquor to drive off the water so the liquor would be thick enough to burn efficiently. While this was an economical way to evaporate water, it also caused odour compounds to be discharged into the environment.
The ‘C’ Recovery Boiler utilizes “non-contact evaporation.” This means the liquor is evaporated in closed containers and the associated water and odour compounds are collected for treatment.
This has been a considerable improvement. The installation of ‘C’ Boiler has resulted in a 60% reduction of odour compounds being discharged from the facility.
‘C’ Boiler also utilizes the latest state of the art particulate control equipment including a high efficiency electrostatic precipitator. An
electrostatic precipitator is similar to electronic air filters located in houses and restaurants. The flue gas passes through a large chamber that contains electrically charged wires and plates. The particulate is charged with electricity and is attracted to large metal plates. The clean gas is then discharged to the atmosphere. When there is a sufficient buildup on a plate, a large rapper or hammer knocks the plate resulting in the heavy particulate falling to the bottom of the precipitator for collection. A precipitator is considered a dry system (unlike a
wet scrubber) and the resulting flue gas is dry which results in no visible emissions during warm weather. When the weather is cold, a vapour plume will be visible due to the hot air being discharged. The particulate collected from the precipitator is sent back to the mill for re-use and recovery.
Installation of this state of the art “C” Recovery Boiler and Electrostatic Precipitator resulted in the shut down of coal burning on site. This resulted in an additional 40% reduction in facility particulate.