Beyond cellulose and lignin, trees contain small amounts of extractives such as fatty acids, resin acids, and pitch compounds. Collectively known as tall oil, these materials are by‑products of the kraft pulping process. When reacted with sodium hydroxide, they form tall oil soap, which is typically washed from pulp and sent to the recovery system. If not collected, this soap is burned in the recovery boiler for its calorific value.
In many jurisdictions, mills recover and acidulate tall oil soap, creating a valuable commodity. Fatty acids can be converted into renewable diesel, resins serve specialized industrial uses, and pitch is burned for energy. Historically, Canadian mills used tall oil as a substitute for bunker oil in lime kilns, but interest declined with the shift to cheaper natural gas and the pine beetle epidemic in British Columbia. Today, economic pressures and shifts in fibre supply are reshaping the industry.
Now, oil refineries are increasingly interested in liquid hydrocarbons with lower carbon intensities. While Canadian mills may not match the volumes of southeastern U.S. producers, tall oil represents a new revenue stream outside of traditional pulp markets. Soap recovery technologies are well established, but unmanaged soap can accumulate in liquor tanks, leading to scaling, downtime, and spikes in boiler load.
Mills that have historically or are currently collecting tall oil should consider performing a soap inventory to assess whether soap is becoming an issue or a potential opportunity. Updating this balance periodically helps guide decisions on recovery and conversion, especially when fibre furnish changes. With the last industry‑wide review dating back to the pre‑pine beetle era, it is time to revisit tall oil production capacity in Canada. This will identify regions where tall oil recovery is once again viable, and help mills turn a process challenge into a green fuel commodity.
FPInnovations is trialling a thermal infrared camera for helping to identify the volume of accumulated soap in black liquor storage tanks. Soap has a higher thermal conductivity than black liquor, so as it accumulates, it produces an intermediate thermal layer that is cooler than the black liquor but hotter than the air above. Although both handheld and fixed cameras have been used by mills, this is the first application for continuous automated detection. This technology has entered its second trial under real mill conditions at a member company mill. The purpose of the trial is to validate the methodology and gather data on the durability and effectiveness of this approach. If the current trial is successful, the project team is looking for a second mill at which to test this technology and would be open to hearing from interested members.
For more information, please contact Matthew Tomkins, Senior Research Engineer, at matthew.tomkins@fpinnovations.ca.
Matthew posing with a junction box and camera. A black liquor storage tank is in the background.