Autocausticization by Borate in Burning Black Liquor Droplets, 2004 International Chemical Recovery Conference

Partial autocausticization with borate with the aim of eliminating bottlenecks at the recaust plant and lime kiln has been found to be technically feasible. To be able to get detailed data on the decarbonation mechanism we have used single droplet laboratory scale pyrolysis and combustion experiments where we followed the carbonate formation and destruction during the different combustion stages by analyzing the amount of carbonate in the residues of the experiments.

The results show that the amount of carbonate in pyrolysis residues can vary depending on the liquor and borate load. For one liquor there was no effect on the amount of carbonate with increased borate load, but for another the carbonate remained on a lower level with an increased borate load. Carbonate is rapidly formed during pyrolysis and an increasing borate loading can decrease the amount of carbonate. It is not clear if it is due to hindering the formation of carbonate or by the decomposition of the already formed carbonate. In our laboratory scale combustion experiments there was a narrow area of conditions used that gave a clear decomposition of the amount of carbonate in the residue. In smelt residues there was also seen a decrease in carbonate with increasing borate load. The effect of the Na2CO3 decomposition mechanism known from earlier should be elucidated in these experiments.

The autocausticization reactions in the recovery boiler does not only happen in the smelt bed or during char burning but partially already in-flight and all through the droplet combustion process.