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Fireside behavior of black liquors containing boron.

Application: Initial testing shows that mills may benefit from boron to debottleneck the caustication segment of their recovery boiler operations.


Partial autocaustization using boron has become an interesting option for debottlenecking the recausticizing segment in the chemical recovery cycle of kraft pulp mills. Several full-scale trials in North America have shown that the autocaustization concept using heron works efficiently in recovery boilers and, thus, gives significant benefits in the causticization area. However, more information is needed of the detailed influence of boron on the operation of the recovery boiler. The presence of sodium metaborate in the black liquor will change the recovery furnace chemistry. Besides changing the smelt bed composition by replacing a part of the alkali carbonate with trialkali borate, the boron will also influence other furnace processes.

This paper presents initial findings concerning the influence of metaborate addition on liquor burning properties, smelt bed melting behavior, and fireside deposit characteristics in superheaters. The information is based on multiphase chemical equilibrium calculations of the furnace processes and results obtained by using the Abe Akademi single droplet burning test facility. The presence of boron lowers the melting temperature range of the smelt and the carry over liquor residue particles. Larger boron additions also changed the burning behavior of the black liquor studied. The characteristic swelling during the devolatilization stage decreased, resulting in somewhat longer burning times. The measured release of carbon and the formation of NO and SO2 from the burning droplet did not change significantly.

More work is required to test the validity of the conclusions to a larger number of liquors. The interaction between boron compounds and potassium and chlorine also needs further study to establish more generally the expected changes in the smelt and superheater deposit melting properties.

Hupa, Forssen, and Backman are with Abo Akademi University, Process Chemistry Group, FIN-20500 Turku, Finland. Stubbs and Bolton are with Borax Europe Limited, Guildford GU2 5RQ, U.K. Contact Hupa by email at
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Title Annotation:Recovery boilers: summary of peer-reviewed material
Author:Bolton, Roland
Publication:Solutions - for People, Processes and Paper
Date:Mar 1, 2002
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