Autocausticization Reactions in Burning Single Droplets of Boron Containing Black Liquors, 2003 Fall Technical Conference

Partial autocausticization with the aim of eliminating bottlenecks at the recaust plant and lime kiln has been found to be technically feasible. The reactions have been reported to mainly happen between the inorganic sodium carbonate and metaborate. It has although not been elucidated to what extent the reaction happens in the different stages of combustion.

In this paper we have looked at the reaction efficiencies during the initial stages of black liquor combustion. We report the effect of borate on the general combustion characteristics of two black liquors and on the carbonate formation and destruction during pyrolysis and combustion of single black liquor particles.

The results from the experiments with black liquors doped with borate for achieving partial (up to 50%) autocausticization showed that autocausticization reactions can happen already during pyrolysis and char burning stages. The two liquors showed to have different autocausticization efficiency, one of the liquors had clear autocausticization during the pyrolysis stage whereas the reaction was more efficient during char burning and in the smelt for the other liquor.

The borate addition changed the combustion properties to the same degree or less as changes in raw material in mills pulping campaigns can. This it seems reasonable that the use of borate for partial autocausticization can occur without major adjustments to the recovery boiler firing strategy.