Combustion of Soap in Kraft Recovery Boilers

Keijo Salmenoja, Ari Kankkunen, Petri Pynnönen

Research output: Chapter in Book/Report/Conference proceedingConference article in proceedingsScientificpeer-review


At softwood kraft pulp mills soap generally separated from black liquor and converted into tall oil. Tall oil is a valuable by-product for softwood mills and is usually sold to tall oil refineries. However, soap acidulation brings from 3 to 5 kg/ADt of sulfur into the recovery cycle in the form of sulfuric acid (H2S04). This additional sulfur has to be removed from the recovery cycle and is typically realized by purging the recovery boiler precipitator ash. This in turn requires replacement of lost sodium (Na) with caustic (NaOH). If the purging need is high, this will result in high additional costs. In some cases the mill does not want to invest in soap acidulation partly due to the costs and partly due to the S/Na balance. In some cases precipitator ash purging is not allowed, too. Generally, some soap is always burnt along with as-fired black liquor due to the solubility of resin and fatty acids. Therefore, all the soap cannot be recovered from black liquor and the amount of soap in as-fired liquor is typically around 1% of the dry solids flow. In some cases, however, the mill does not want to invest into the tall oil plant due to the S/Na balance issues or due to environmental reasons or for some other reasons. In these cases, all soap must be burnt in the recovery boiler. This paper presents a special mill case, where existing tall oil plant was taken out of operation and all soap is presently burnt in the recovery boiler. What made it challenging was the high amount of soap, ca. 6-7% of black liquor dry solids flow. A new dedicated soap burning system was developed and commissioned, based on soap spraying studies on laboratory scale. A new type of soap burner was developed according to the tests. The newsystem was successfully commissioned and started up in 2015 and is presently in continuous operation at the mill. Paper summarizes the development process from lab-scale tests into full-scale plant system and presents the most important constraints of the soap burning system.
Original languageEnglish
Title of host publication2016 Pulping, Engineering, Environmental, Recycling, Sustainability Conference Proceedings
Subtitle of host publicationPeers
PublisherTAPPI Press
Number of pages9
ISBN (Electronic)978-1-59510-251-5
Publication statusPublished - 2016
MoE publication typeA4 Conference publication
EventTAPPI PEERS Conference - Jacksonville, United States
Duration: 26 Sept 201628 Sept 2016


ConferenceTAPPI PEERS Conference
Abbreviated titlePEERS
Country/TerritoryUnited States


  • SOAP


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