Abstract
Greenhouse gas (GHG) generation is inherent in the production of aluminium by a technology that uses carbon anodes. Most of those GHG are composed of CO2 produced by redox reaction that occurs in the cell. However, a significant fraction of the annual GHG production is composed of perfluorocarbons (PFC) resulting from anode effects (AE). Multiple investigations have shown that tetrafluoromethane (CF4) can be generated under low-voltage conditions in the electrolysis cells, without global anode effect. The aim of this paper is to find a quantitative relationship between monitored cell parameters and the emissions of CF4. To achieve this goal, a predictive algorithm has been developed using seven cell indicators. These indicators are based on the cell voltage, the noise level and other parameters calculated from individual anode current monitoring. The predictive algorithm is structured into three different steps. The first two steps give qualitative information while the third one quantitatively describes the expected CF4 concentration at the duct end of the electrolysis cells. Validations after each step are presented and discussed. Finally, a sensitivity analysis was performed to understand the effect of each indicator on the onset of low-voltage PFC emissions. The standard deviation of individual anode currents was found to be the dominant variable. Cell voltage, noise level, and maximum individual anode current also showed a significant correlation with the presence of CF4 in the output gas of an electrolysis cell.
Highlights
In primary aluminium reduction, continuous efforts are taken by the industry to minimize the tonnes of CO2 equivalent produced per ton of aluminium
This paper investigates the correlation between specific cell variables and the level of CF4 emissions at the duct end of the electrolysis cell
It might be relevant to reconsider the lower limit of prediction of the model in the future to avoid being too close to the detection limit of the Fourier transformed infrared spectrometer (FTIR)
Summary
Continuous efforts are taken by the industry to minimize the tonnes of CO2 equivalent produced per ton of aluminium. These ‘high-voltage PFC’ emissions are well known in the industry and specific guidelines[2] exist to quantify the amount of gas generated during this event For this reason, smelters have optimised their process worldwide over the years and the total amount of PFC emissions from the aluminium industry has been significantly reduced between 1990 and 2010.3 by lowering the AE frequency and duration, another source of emissions has become more apparent in recent years, characterized as ‘low-voltage PFC’ emissions. Smelters have optimised their process worldwide over the years and the total amount of PFC emissions from the aluminium industry has been significantly reduced between 1990 and 2010.3 by lowering the AE frequency and duration, another source of emissions has become more apparent in recent years, characterized as ‘low-voltage PFC’ emissions This particular type of emission can occur for a significant period of time with little or no indication of misbehaviour in the electrolytic cell, for instance, an increase in cell the noise or voltage. Low-voltage PFC are important to take into consideration but, up to now, no available method other than real-time PFC monitoring exists to account for (Published online August 2, 2016)
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