Mechanism of Top Heat Loss from Aluminium Smelting Cells

Abstract

Cell ventilation is one of the critical requirements in an aluminium smelting cell for removal of cell emissions, and to provide cooling to the cell components. The heat loss from the top of the cell, which is up to 55% of the total heat loss, is primarily exhausted to the scrubbing duct at around 130°C, which is of low quality and cannot be efficiently recovered. To improve heat recovery, the temperature of the exhaust gas must be increased to higher than 200°C, which requires a substantial reduction in the draught suction rate without adversely increasing fugitive emissions, cell temperature, or disruption of the mechanism of heat loss. Many aspects of the cell ventilation are not well understood, or have been reduced to predictive flow/pressure/temperature relationships. In particular, the interactive effects of hooding tightness, air velocity distribution, duct inlet positions, cell draught, thermal conductivity and radiative emissivity are unknown. The focus of this thesis is to contribute in resolving these issues. Three computational models were developed and validated for this investigation. An in-depth study on the mechanism of the top heat loss was performed using two CFD 1/8 cell slice models, and the paths of the top heat loss and their interactions were studied according to an analogue of Kirchhoff's law to develop a resistance network 'R-model'. The consequences of hooding tightness on the top heat loss and fugitive emissions from the cell were investigated for a range of cell draughts, and the range of draught reduction for maximum top heat recovery was determined. The potential for recovery of top heat loss was studied, including draught reduction in the main exhaust duct without alteration to the ventilation system and also using a newly devised hot fume capture ducting system at locations near the feeder holes. The maximum contribution to top heat recovery was found with an 85% reduction in the normal draught rate using the devised hot gas duct system. This is enables 38% of the total top heat loss to be recovered at a temperature of more than 300°C, with efficient capture of the hot fume gases directly from the feeder holes. A potential for equivalent energy saving of 0.17 kWh/kgAl, can be found at a practical application of the recovered heat, contributing to huge savings in the cost of transporting the iii extracted gas, and minimising the subsequent environmental emissions at low degree of hooding tightness, which is not possible with the present ventilation system. The approximately 1241 GWh per year equivalent energy in uncontrolled top heat loss from the air under the hoods for a typical smelter can be reduced by 43%, giving an annual energy saving of US$4.38 million. An equivalent saving in cell voltage by 0.053V can be made with practical work extraction of the captured energy at 17% thermal efficiency. There is further potential to increase the energy recovery by use of an optimum crushed bath anode cover with higher thermal conductivity.