Power Modulation of Aluminium Reduction Cells – Operational Constraints and Process Limits

Abstract

Aluminium reduction cells are operated traditionally with an energy input as constant as possible. This is to reduce the variability of the process and simplify process monitoring and detection of abnormal situations. Recent advances in control systems have decreased the number and magnitude of process excursions keeping a vast majority of reduction cells within optimum performance. Reorientation in energy generation towards volatile renewable sources and changes in marketing mechanisms in Germany have led to an elevated and volatile electricity price resulting in an incentive to overcome the concept of constant energy input.

This thesis postulates theoretical aspects and boundaries of both possibility and magnitude of power modulation. These theories are compared with results found in experiments simulating aspects of power modulation undertaken on industrial reduction cells as well as during practical operation of a whole smelter with continuous power modulation. Key results of these investigations are: 1. Theoretical boundaries of 0.728MWh were verified. 2. Side ledge shows a distinctly different behaviour if in contact with molten bath or molten metal. Previous considerations always assumed equilibrium when discussing side ledge. Experimental results show that there are significant dynamics influencing the shape of side ledge. 3. Cells show a significant capability of shedding additional heat in less than a day, stabilizing operation shortly after increased energy inputs. 4. Top cell cover contributes to 50% to physical changes in the cell induced by energy imbalances.

Basic approaches for modelling and incorporating effects of power modulation into process control algorithms are derived from experimental results. Changes in process efficiency found during continuous modulation are evaluated with regards to the economical impact on smelter operation showing that power modulation is a valid approach for reducing production costs. Other scenarios for generating revenue based on flexible smelter operation are highlighted.

To extend the capabilities for power modulation, engineering solutions available today, such as shell heat exchangers and dampers to control off-gas volume are discussed with respects to their ability to vary and control heat loss from a cell. It is shown, that shell heat exchangers are the only applicable system to actively vary heat loss.