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Design of the heat recovery systems at the reconstructed pulp mill Östrand

Johan Ahlström ; Marcus Benzon
Göteborg : Chalmers tekniska högskola, 2015. 74 s.
[Examensarbete på avancerad nivå]

The aim with this master thesis project was to provide a reference for the reconstruction of the heat recovery systems for the future possible reconstructed pulp mill Östrand, owned by SCA and located in Timrå, Sweden. The principle was to design a heat exchanger network and a secondary heating system that are more efficient than those already proposed by consultants. To achieve this Pinch Analysis was used. The required data for this work were gathered during a one-month visit to Östrand. For the majority of the new process equipment, quotations from different equipment producers were used. However, in some cases, current factory data were used and scaled up to the future production levels. With the data gathered, the heat exchanger network proposed by Östrand was analysed first to provide a reference case that was later retrofitted to more energy efficient designs. More specifically, two new designs of the heat exchanger network were proposed. The first retrofit design was performed with the goal of using energy as efficiently as possible, but still creating a network that is possible to implement from a practical point of view. The second retrofit design was obtained by including more detailed practical constraints, according to inputs from plant engineers at SCA Östrand, which could have more chances to be implemented at the reconstructed mill. As a consequence of increased heat recovery, less steam is required for process heating. Since the minimum amount of steam produced is coupled to the amount of black liquor that has to be recovered, increased heat recovery translates into larger steam mass flow rate available for electricity production. This represents a first economic advantage, which should pay back the investment in extra heat exchangers required in the proposed designs compared to the reference prospected design. Additionally, a modified heat exchanger network requires a modified secondary heating system. This is based on a hot/warm water loop and is a common way in pulp mill to collect and deliver heat where direct heat transfer is not possible, e.g. due to large distance between equipment units, or due to different operation times which also requires water tanks to be used. After the two retrofit suggestions for the heat exchanger network were completed, the design of the secondary heating system was therefore investigated. Depending on the heat available at different temperature levels, different amount of hot water can be produced and the mass flow rates of the different segment of the water loop can be optimized. This translates in reality in optimizing the starting and ending temperatures of these segments, which are separated by a tank where water is stored to accommodate process operational flexibility. While such analysis should follow an economic principle, in this work this was conducted following the objective of recovering as much as excess heat as possible and a procedure called the “tank method” was followed, which was previously developed at the div. of Industrial Energy Systems and Technologies. In this thesis three suggestions regarding what to do with the excess heat are discussed: increased production of district heating, increased steam production for electricity production, and heat pumping. The economic result of excess heat utilization provides a second way to pay back the investment in extra heat exchangers required in the proposed designs compared to the reference prospected design. While a profitability analysis should be ultimately conducted to identify the best design solutions for the heat exchanger network and for the secondary heating systems, in this work only a discussion of the economic aspects is provided based on the results of the thermodynamic analysis. These results show that for the ambitious retrofit the electricity production is increased with 24.2 MW and 32.8 MW of excess heat is liberated. For the realistic retrofit, the electricity production is increased with 13.9 MW and 65.7 MW of excess heat is produced. Compared to the relatively small changes to the original design of the heat recovery systems which was the starting point of this work, the results are considered rather promising and show that there is room for Östrand to improve the design of the future plant.

Nyckelord: Pinch analysis, Pulp production, Process integration, Secondary heating system, the tank method

Publikationen registrerades 2015-10-23. Den ändrades senast 2015-10-23

CPL ID: 224716

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