Possibilities to Extend Uninterrupted Operation Periods of Kraft Recovery Boilers

ABSTRACT
All the pulp mills are looking for different ways to generate more production from existing equipment. One of the most
important operating factors to reach this target is the mill’s total availability. Although, the capacity of kraft recovery
boilers has increased rapidly during recent years and the nominal furnace loading has increased significantly, the
availability of recovery boilers has stayed at a very high level. Typically, the availability of the kraft recovery boiler is
close to 100%. Thus, the availability of the recovery boiler has seldom been limiting mill’s total availability. If the
availability cannot be increased, then another possibility to raise pulp production is to increase the number of annual
operation days. One way of increasing the annual operation days is to extend the operation periods between scheduled
shutdowns i.e., reducing the time spent in the shutdowns. Traditionally, 12 months of operation between scheduled
shutdowns has been the standard in the pulping industry. Encouraged by the high availability numbers of modern recovery
boilers, mills are pushing the boiler suppliers to extended operation periods up to 18 months. Some mills are even targeting
at 24-month long operation periods.
What is really limiting the length of the recovery boiler operation periods between shutdowns? Are the limiting factors
related to safety, perceptions, mechanical constraints, process issues, fouling and plugging, corrosion, local legislation,
or recommendations from recovery boiler committees? This paper reviews the most important limiting factors and how
to mitigate the effects on operation periods. The paper also studies the possibilities to extend uninterrupted operation
periods of kraft recovery boilers. However, limitations originating from the legislation has generally set the maximum
length of the periods and cannot thus be overlooked.
Keywords: Kraft recovery boilers, operation, availability, shutdown 

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