Energy efficiency of the secondary heating at the Stora Enso Skoghall mill

Abstract

In this study the potential cost reductions associated with reduced utility steam use in the secondary heating system at the Skoghall mill are investigated. Areas of special interest are: A) Balancing the supply and demand of hot water, B) Control strategy of the producers of hot water and C) Possibilities of replacing utility steam with secondary heat. Hot water producers within the Cooking plant, Evaporation plant, Bleaching plant and the Recovery boiler in the mill are examined to determine if the production of water from each producer is variable or not. The marginal cost for producing water is determined for the variable producers using efficiency data and utility steam cost. Data spanning over the past year for the individual water production to and the fill level in the four different buffer tanks of the secondary heating systems are then analysed to identify imbalances in supply and demand of water. In particular variable producers supplying hot water even when the corresponding buffer tank is full, something which may lead to increased costs, is examined. Furthermore to find the cost minimal solution to balance supply and demand a priority order of producers and some consumers of hot water is established. Any discrepancies compared to the current priority lists are determined to find potential savings. Finally some processes with a heat demand at a relatively low temperature level, which is currently satisfied using utility steam, is studied briefly. If instead supplying the heat demands with excess heat from the secondary heating system, the potential cost reductions are calculated. The conclusions are that approximately 0.3 million SEK per year can be saved by better balancing supply and demand alone, in this case by implementing an automatic control on one hot water producer. On the other hand there is a potential to reduce the costs by up to 1.3 million SEK annually if the cost-minimal control strategy identified in this study is used. On top of this, two processes are found in which utility steam use possibly can be replaced with excess secondary heat. If so the yearly potential savings are 3.2 and 0.5 million SEK respectively for the processes.