Chemical recovery boilers are critical to the operation of kraft pulp mills. The purpose of a recovery boiler is to recover chemicals for reuse in the pulping process and to generate steam for electricity. Without recovery boilers a significant amount of chemicals and resources would be wasted.
The waste pulping solution, called
black liquor, contains the organic material from trees (
lignin, resins etc) and the spent alkaline chemicals used to pulp the wood chips. The black liquor is evaporated to remove water, which increases its fuel potential. After evaporation, it is injected into a recovery boiler.
The organic material is burned off while the chemicals remain behind for recovery. The remaining chemical is mixed with water to form
green liquor. The green liquor is sent on for further processing called “causticizing” to return it to its original form (
white liquor) for reuse in the pulp mill digester.
Prior to black liquor being burned in a recovery boiler, it needs to undergo a chemical reaction called “oxidation.” Oxidation is required to stabilize the
sulphur compounds that are present in the liquor. This process results in reduced odour being discharged to the environment.
Traditionally oxidation occurred by either pumping or blowing air directly through the black liquor. This converted the unstable, highly
odourous sulfide compounds into more stable non-odourous thiosulfate. Unfortunately, traditional oxidation systems robbed the liquor of its heating value as fuel, and were not very effective at stabilizing the liquor. To address these shortcomings, the mill worked with the local supplier of oxygen to develop a new high-efficiency oxidation system called Total Heat Recovery (THR).
The THR system uses pure oxygen to stabilize the liquor. The pure oxygen is extremely efficient at converting the sulfide to thiosulfate and actually raises the heating value of the black liquor. This new process has improved the overall chemical recovery efficiency at the mill while reducing odour compounds. This application was the first of its kind in the world and is now being used at a number of other mills.
As a result of the new THR system, we have seen a 25% drop in
total reduced sulphur (TRS) emissions from the facility.