Energy Efficient Evaporation in Future Kraft Pulp Mills

Doktorsavhandling, 2002

In the future, the technical and economic conditions for energy savings in the pulp and paper industry will change. More excess heat will be available in future mills above 80°C than in existing mills, increasing the potential for process integration, and governmental actions for reducing CO2 emissions and the deregulation of electricity markets will change the economic conditions for investments aimed at saving energy. Evaporation plants are some of the largest energy consumers in the pulp and paper industry, but also some of the most flexible unit operations. The objective of this thesis has therefore been to study the possibilities to save energy in future kraft pulp mills by integrating the evaporation plant with the rest of the process. The emphasis has been on quantifying the energy savings possible and understanding how they are influenced by different parameters on several system levels. Conventionally designed evaporation plants as well as evaporation plants that use excess heat and/or deliver low-pressure steam to the steam network have been simulated for different model mills. Using the simulation results, the energy efficiency and economics for the different designs have been compared at a wide range of conditions. The reductions in CO2 emissions have also been calculated. The work presented in this thesis has been performed within a Swedish research program called the Eco-cyclic Pulp Mill. The simulation results show that the live steam demand in the evaporation plant can be lowered by 22-93% in the model mills evaluated if the evaporation plant is process integrated. They also show that the greatest reductions are achieved if the evaporation plant is designed to use both MP steam and excess heat. In general, the results from the economic parameter studies show that the process integrated evaporation designs also are interesting from an economic perspective. They are also favored by a low electricity price (unless condensing power is produced) and a high fuel price, since this increases the benefit of saving fuel. It is further concluded that all process integrated designs reduce CO2 emissions; the size of the reduction, however, depends on how the saved fuel is used and how the marginal electricity production in society is generated.