Liquor Cycle Chloride Control Restores Recovery Boiler Availability

Kraft recovery boilers operating today are experiencing higher fouling rates due to increased chloride content within the liquor cycle. It has been shown that excessive chloride content in recovery boiler ash results in increased fouling rates on the boiler’s convective surfaces.(1, 2) Tighter liquor cycles and more efficient precipitator ash collection have resulted in higher chloride content in the liquor being sent to recovery boilers. By purging a small portion of the precipitator ash, the Alabama River Pulp and Alabama Pine Pulp Companies of Claiborne, Alabama have taken dramatic steps to restore the availability of their recovery boilers to design levels. Their liquor chloride control strategy has allowed both boilers to run between planned outages without unscheduled cleanings or loss of steam temperature. The following examination of this strategy will describe operation at the mills before and after changes were made in the liquor cycles, and will compare the impacts of ash dumping to operational cost savings. Introduction Liquor and ash chemistry is critical within the Kraft process for maintaining recovery boiler availability. As such, it has been the subject of intensive study within the pulp and paper industry. Studies have shown that ash fouling and gas path plugging within recovery boilers are directly related to the melting properties of the ash in the boiler.(1,2) It has also been shown that chloride and potassium concentrations within the liquor cycle are the most significant factors affecting ash melting points. Therefore, by reducing the chloride and potassium levels within the liquor cycle, ash melting properties are made less conducive to fouling. The result is improved cleanliness within the boiler, which translates into greater availability with less performance degradation. In most cases, efforts to maintain boiler performance by reducing ash chloride levels have been undertaken on units with very high ash chloride concentrations. High chloride concentrations are the result of high chloride sources such as wood supply or makeup chemicals. Environmental improvements within the last decade that have led to reduced loss of process chemicals have also resulted in an increase in non-pulping deadload chemicals, including chlorides, within the liquor stream. After experiencing increased fouling and plugging within their recovery boilers, Alabama River Pulp (ARP) and Alabama Pine Pulp (APP) undertook a joint program to reduce their liquor chloride levels and thereby restore recovery boiler performance. By adopting a strategy of periodically purging precipitator ash and by switching to lower chloride caustic, ash chloride levels were reduced from around 4.5% to 1.5% or less by weight (8.8% mole to 2.8% mole Cl/(Na+K)). This change has allowed both of the mills to restore operation from an eight week run between cleanings to the scheduled annual outages without steam temperature degradation or draft loss. This has allowed the mills to reduced the downtime on the units from five unscheduled outages to no unscheduled outages.