Energy efficiency improvements of Mondi Dynäs paper mill : A survey of steam and condensate with improvement measures for increased condensate recovery

Abstract: This thesis has been performed at Mondi kraft pulp and paper mill located in Sweden, Kramfors Dynäs due to the low return rate of condensate. Currently, the return rate is only at 65 - 75 %. This is problematic as the demineralization process is operating on full capacity and thus requires a reduced make-up water need. The production of demineralized water is an expensive process due to large requirements of chemicals and pump-energy, but also due to the cost of raw water and maintenance. Therefore the purpose of the work was to investigate the possibility to increase the condensate return rate and thus reduce the need of make-up water. The investigation was performed during normal production rates to simplify the interpretations. The steam and condensate system was mapped and extracted flows were identified, measured and marked on map. The extracted flows were investigated in more detail and characterized as either consumed in a process or lost to sewer or atmosphere. In the work, both minor and major improvements were investigated. Minor improvements are accomplished with small intervention measures and could be done with small or no investments costs. Major improvements require large investments costs and further investigations. Six different processes were found to constitute the total production of make-up water. These are the free blowing of excess steam, soot blowing steam, paper machine 6, de-aerating process in the feed water tanks, digester process and steam- & condensate leaks along the pipe network. Improvements with the category of minor were found to reduce steam and condensate consumption by 22 - 23 ton/h. This implies a reduction by 30 - 32 % of the added make-up water, including an annual financial benefit of 3,3 - 3,9 MSEK. Additionally 0,41 - 0,82 MSEK could be gained annually by improving the insulation on the cylinder dryer heads. The make-up water would be further reduced if insulation jackets and inspection programs were implemented on equipments along the steam lines. Two major improvements are recommended. The first major improvement involves elimination of the free blowing of excess steam. This could be accomplished by drying the bark before combustion. The added make-up water would be reduced by 25 % and with the possibility to sell the dried bark during low steam production. The second major improvement involves the digester process, where the consumptions of medium pressure steam could be reduced by replacing steam with liquor-steam from the strong liquor accumulator. Medium pressure steam could instead be converted into electricity by the turbine.