Abstract
Increased utilization of industrial surplus heat can make significant contributions towards reaching
energy efficiency and emissions reduction goals. The off-gas from metal production smelters can
contain large amounts of thermal energy, and conversion to electric power often appears an enticing
prospect. However, the practical potential for exploitation can be significantly reduced from plant
processes that are designed considering surplus heat as a waste product to get rid of. This typically
makes the heat accessible only at reduced temperatures.
The HighEFF research centre for industrial energy efficiency has studied technologies, applications,
and cases for surplus heat utilization since its start in 2016. Heat-to-power conversion has been explored
in several cases provided by the partner industries from – among others – Norwegian aluminium
manufacturers. Centre research activities also include novel production processes and modifications,
which has side effects providing very different conditions and constraints for energy recovery. One such
process modification is off-gas recirculation, mainly developed to increase concentration of CO2 in the
off-gas to improve conditions for CO2 capture in the future, but which also will alter off-gas temperature
and recoverable heat as a side effect. This could improve the potential for energy recovery.
In this work, the potential for energy recovery is evaluated and compared in four cases – one
representing a current aluminium process, and three future process scenarios with flue gas recycling.
The simulated heat-to-power conversion is done by applying an organic Rankine cycle (ORC)
optimization model to each case. The results indicate significant benefits to energy recovery in the
recycling cases. In the case with the highest recycling rate and flue gas temperature, the potential for
electric power production increases by 270 % compared to the present-day case. In addition, the reduced
work of the main exhaust fans in the recycling cases brings further energy savings on the system level
equivalent to 25–50 % of the ORC power output, further increasing overall energy efficiency. From
this, some potential synergies between process design, heat-to-power, and thermal integration of other
technologies such as CO2-capture are discussed
energy efficiency and emissions reduction goals. The off-gas from metal production smelters can
contain large amounts of thermal energy, and conversion to electric power often appears an enticing
prospect. However, the practical potential for exploitation can be significantly reduced from plant
processes that are designed considering surplus heat as a waste product to get rid of. This typically
makes the heat accessible only at reduced temperatures.
The HighEFF research centre for industrial energy efficiency has studied technologies, applications,
and cases for surplus heat utilization since its start in 2016. Heat-to-power conversion has been explored
in several cases provided by the partner industries from – among others – Norwegian aluminium
manufacturers. Centre research activities also include novel production processes and modifications,
which has side effects providing very different conditions and constraints for energy recovery. One such
process modification is off-gas recirculation, mainly developed to increase concentration of CO2 in the
off-gas to improve conditions for CO2 capture in the future, but which also will alter off-gas temperature
and recoverable heat as a side effect. This could improve the potential for energy recovery.
In this work, the potential for energy recovery is evaluated and compared in four cases – one
representing a current aluminium process, and three future process scenarios with flue gas recycling.
The simulated heat-to-power conversion is done by applying an organic Rankine cycle (ORC)
optimization model to each case. The results indicate significant benefits to energy recovery in the
recycling cases. In the case with the highest recycling rate and flue gas temperature, the potential for
electric power production increases by 270 % compared to the present-day case. In addition, the reduced
work of the main exhaust fans in the recycling cases brings further energy savings on the system level
equivalent to 25–50 % of the ORC power output, further increasing overall energy efficiency. From
this, some potential synergies between process design, heat-to-power, and thermal integration of other
technologies such as CO2-capture are discussed