Energy consequences in minimum effluent market kraft pulp mills.

Application: This information can help mills when condsidering water reuse, in order to not increase their energy demand while doing so.

Many environmental objectives in pulp and paper mills focus on water closure. In order to close the water loops, the problem of accumulation of non-process elements (NPE) has to be controlled. There are many ways of closing the water loops and there are several processes either available or under development to remove NPE. In this study, we evaluated the combined effects of water loop closure and the introduction of new processes on energy consumption by looking at four different model mills with different closures and processes used to remove NPE. The NPE removal processes we evaluated included pre-evaporation of bleach plant effluents, chip pretreatment, and combinations thereof. They are part of the group of processes discussed for this purpose, and they have a significant impact on the energy demand.

Earlier studies show that excess heat can be made available if the secondary heat system is designed differently from common practice today. If this excess heat is used for evaporation, the total heat demand can be reduced by almost 15%. The same approach is used here. First, we evaluated a process-integrated mill for excess heat at 70[degrees]C-100[degrees]C. This excess heat is made available by redesigning the secondary heat system. Through process modifications and a redesign of the evaporation plant, excess heat becomes available. Using that excess heat results in a net reduction in the total live steam demand for the plant. When doing this for the four minimum effluent mills, mill closure can be achieved without increasing the live steam demand even when including pre-evaporation (a large energy consumer). Based on the approximate cost for making these changes, the payback period in the different minimum effluent mills varies between 3 and 5 years. S!

Wising is a Ph.D. student and Berntsson is professor in the Department of Heat and Power Technology, Chalmers University of Technology, 412 96 Goteborg, Sweden; Asblad is with CIT Industriell Energianalys AB, 412 88 Goteborg, Sweden; email Wising at [e-mail].