Flexibility to seasonal demand variations in pulp mill steam production: The effect of steam savings leading to off-design heat loads

This paper focuses on the steam production in a chemical pulp mill that is retrofitted to reduce its process heating demand. A multi-period optimization model for design decisions is proposed that takes into account operational limits of the boilers and variations in heat demand. Large variations in combination with the retrofit cause off-design loads that affect the flexibility of the steam system. The minimum boiler load limits will be a greater constraint on operation when the average load of the boilers is moved closer to the minimum for longer periods of time. As shown in this paper, a conventional approach that considers fixed annual averages of process parameters therefore risks leading to sub-optimal solutions because of neglecting the variations in heat demand and inaccurately modeling the operational limits. The multi-period approach suggested in this paper considers operational flexibility associated with different designs. A case study based on a Kraft pulp mill with a recovery boiler and a bark boiler shows the benefit of this approach. Four scenarios for heat savings and lignin prices are analyzed. Numerical results are presented that compares the solution of the multi-period model with that of a conventional annual-average approach. Differences in designs, energy balances and economic performance are demonstrated.