Operational Experiences from Burning Soap in Recovery Boiler, 2010 TAPPI/PAPTAC International Chemical Recovery Conference

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Soap is extracted from black liquor in softwood based pulp mills. Soap can either be processed to tall oil by acidifying it with sulfuric acid or it can be mixed with black liquor and burned in recovery boiler. Many modern mills prefer to burn the soap because they have surplus of sulfur in their chemical cycle.

A modern recovery boiler in a pulp mill with mixed pine and eucalyptus pulp production had some difficulties in achieving good smelt reduction rate. Reasons for this were studied and improvements found by tuning the boiler operation. It was found out that soap burning in the boiler had remarkable effect on the furnace operation.

Black liquor mixed with soap seemed to burn faster and more intensively than black liquor without soap. Main reasons for this seemed to be lower viscosity and larger higher heating value with soap. No differences were found in results from black liquor droplet burning tests done for black liquor samples with and without soap.

Due to more intensive burning with soap, less carbon reached the lower part of the furnace and combustion took place too high in the furnace. Black liquor droplet size was hence needed to be increased when soap was burned. This was done by decreasing black liquor temperature and partly by increasing black liquor pressure.

In this paper, experiences from operation of the recovery boiler with and without soap in the black liquor are discussed. In order to find reasons for low reduction rate and differences in operation with and without soap firing, laboratory analyses and droplet burning tests for black liquor samples were done. In order to visualize the differences, CFD (computational fluid dynamics) modeling was also made.