Energikonsekvenser av slutning av CTMP- fabriken vid Skoghalls bruk

Abstract

This diploma work is a study of the consequences for energy and the production when a closure is done at Stora Enso’s CTMP-mill in Skoghall. Chemi-thermomechanical pulp (CTMP) is produced at the mill by refining chemically impregnated pre-heated woodchips. Deposition of organic compounds will be forbidden in Sweden by year 2005, thus it is of major interest to reduce the amount of COD (Chemical Oxygen Demand) to the wastewater treatment. By using a surplus of excess heat at low temperatures it would be possible to pre-evaporate the wastewater and return condensate to the plant and replace freshwater. The pre-evaporated wastewater will then be evaporated and burnt with black liquor in the sulphate pulp mill. This would reduce the deposition of organic compounds. To be able to study the impact on the energy situation of a closure, and the consequences this will have for the production a simulation model has been created in WinGEMS. A reference case was created from today’s conditions of production with an average of the production of 780 ADt/d. A number of proposals for how a closure could be made have been identified and a couple of proposals for how the energy situation could be better without a closure. The different alternatives of closure have been investigated by detailed simulation and the results is that they have less consequence for the production than expected. The temperature of the wastewater increases with the fraction of condensate from the pre-evaporation, which replaces freshwater. This could reduce the costs of the investment cost of the pre-evaporation plant, due to a higher temperature of the wastewater, which will reduce the number of evaporation effects. Simulations of alternatives for saving energy show that there are possibilities to achieve an increased production of steam and an increased temperature of the warm freshwater to the plant. The results of the diploma work show how the energy situation changes for the CTMP plant, but not for the Skoghall plant as a whole. They are however possible to use in an expanded study where the sulphate pulp mill and the paper mill are included.