Operational Flexibility in Pulp Mill Steam Production at Off-design Heat Loads

Abstract

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 the operational limits of the steam production units as well as the heat load variations over the year. Large variations in combination with the retrofit that causes off-design loads in the steam production system will influence the flexibility of the steam system. Minimum boiler load limits will then be a greater constraint on operation since the average load of boilers is moved closer to the minimum for longer periods of time. 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 the operational limits. The multi-period approach suggested in this paper considers this operational flexibility associated with different design choices. A case study based on a Kraft pulp mill with a recovery boiler and a bark boiler shows the benefit of properly modelling the varying heat demand. Numerical results are presented that compares the results of the multi-period model with that of a conventional annual average approach. Differences in design decisions, energy balances and economic performance are demonstrated and discussed.