Novel process integration opportunities in existing kraft pulp mills with low water consumtion

Doctoral thesis, 2004

The economic opportunities for energy savings due to process integration of the evaporation plant have proven to be good in model mills with low water consumption. This is due to the fact that the temperature level of excess heat is higher compared with pulp mills with larger water consumption. The opportunities in existing pulp mills with low water consumption have, however, not been revealed earlier. The economic and environmental system consequences of process-integrated evaporation must also be compared with other competitive measures in order to establish the most beneficial measure. Given this background, the objective of this thesis has been to establish the economic and environmental potential for efficient use of excess heat in existing pulp mills. To establish this potential a new systematic approach is needed that reveals the practically maximum amount of excess heat in the system at the highest possible temperature level. Therefore a set of novel pinch curves, developed for retrofit situations, is applied in a case study. To gain access to the excess heat the heat exchanger network is retrofitted in a conceptually new way to produce the warm and hot water needed in the system with smaller temperature differences in the heat exchangers. To determine the most beneficial measure process-integrated evaporation is compared with elimination of pinch violations, heat pumping and district heating at different degrees of heat recovery. The results show that large amounts of excess heat should be available at usable temperatures in existing kraft pulp mills with low water usage. This excess heat can be accessed in practicable HENs to a reasonable investment cost through rearrangements of the HEN. In a total cost analysis and environmental evaluation it is established that process-integrated evaporation plant has, compared with heat pumping and district heating, great potentials. 16-79% of the steam demand in the evaporation plant can be reduced through process integration. Introducing process-integrated evaporation at one single kraft pulp mill could, depending on the fuel reduced and the marginal electricity producer, reduce 1.2-3.6% of the 2.0 Mton of fossil CO2 emissions from the pulp and paper industry in Sweden.