LAYPERSONS TECHNOLOGY OVERVIEW
The EnPot system gives smelter operators ability to smooth out energy use and production in the normal operating window, the dynamic control it brings enables a smelter for the first time to significantly vary its electricity use.
There is also another significant bonus of being able to insulate the pots in a smelter, and that is to delay the solidification of the molten liquid in the case of a serious power failure. The smelter will have up to twice the time available to prepare for a shutdown. This can be a significant cost saver when trying to restart a smelter, as shutdowns are serious, and can even be catastrophic.
The EnPot system should also reduce insurance risk and therefore business interruption premiums. It could even enable the decommissioning of fossil fuel and nuclear power stations in many counties.
Dr Pretesh Patel gives a laypersons overview of the Technology behind EnPot.
The process of making aluminium is a chemical process that requires an electric current as a catalyst. Basically in laymen’s terms, electric current is run through a molten pot of metal oxide and electrolytes, and aluminium drops out the bottom.
The chemical process not only makes aluminium but also produces heat as a by-product, a lot of heat. The heat balance of the pot is critical, as a pot is made from a steel shell, but uses as a buffer a solid ledge of a compound called Cryolite. If the heat is too great it will melt the cryolite ledge and then dissolve the sidewall lining and steel shell which will ultimately lead to cell failure.
If on the other hand the temperature drops too much, then the liquid material will solidify and effectively freeze the pot. All smelters are built with a temperature window, which is very narrow, usually about 30°c, which is equivalent of + or – 5% in amperage terms. So to step up production you have to remove heat faster, and to slow production down, you have to insulate to keep the pot from cooling.
This has been impossible before now.
Key aspects of the EnPot system include:
- Air based, heat exchanger units are custom designed to be retrofitted to any existing pot design including pots with fins, without interrupting production
- Each exchanger design is site specific and custom designed requiring full assessment of pot design, operations and performance as part of the design process
- Retrofittable to existing pot designs, including pots with fins etc or can be integrated into new pot designs
- Design is optimised to achieve maximum heat removal from the critical zones when amperage is increased and maximum insulating capability when amperage is decreased.
- Sidewall coverage is maximised. The more coverage the greater the control over the pots heat balance
- Control of the air flow through the exchangers, controls the heat transfer coefficient (HTC) at the shell wall.
- All EnPot systems are designed to ensure there is no impact on any current operational practices.
- Use of air as the heat transfer medium ensures ease of installation and operation, whilst eliminating any explosion risks that liquid mediums may pose.
- Air based, heat exchanger units are custom designed to fit any cell. Suction fans are used to draw air through the exchanger units. Control of the air flow through the exchangers, controls the heat transfer coefficient (HTC) at the shell wall.
The air temperatures exiting the EnPot units range from 120 to 180oC. There is the potential of recovering and re-using this waste heat. A range of options exist for use of the heat recovered from the sidewalls. The end use however of the recovered heat will be based on a number of factors including:
Options for use of the collected energy include at least:
EnPot enables opening of the operating window for the pots, allowing heat loss from the pot shell to be regulated and controlled from the sidewall of the cell. This allows stable ledge profile to be maintained under varying load conditions. The amperage can be increased and decreased beyond design limits for extended periods of time (months). Allowing smelters to dynamically control operations based on market conditions. The figure below illustrates the potential operating window compared to that which currently exists.
As the industry has matured, the size of new cells has continued to increase, from 40-50kA in the middle of last century to 350kA and more in modern smelters. Through this period of cell size/amperage increase, the proportion of the total energy demand, converted to heat and subsequently lost to the environment has not changed significantly. While cells have physically grown in length, with corresponding increases in energy input and heat generation, the ratio of heat generated to cell surface area has increased. As a result, the ability to adequately balance heat, and to maintain a stable ledge throughout the life of the cell, has often been compromised. This presents a very real limit to the up-scaling of new cells, and the further capacity creep of existing cells.
Operational measures taken to manage this issue, particularly in the cell sidewall region, typically involve local cooling of the worst affected regions, using compressed air impinging directly on the cell wall. Without such control measures the potential exists for ledge erosion and ultimately shell failure. Most cell technologies, including state of the art 300-350kA cells do require air cooling on certain regions from time to time, and some employ continuous cooling. A number of alternative means for sidewall heat removal have been proposed, and in some cases patented. Most of the concepts put forward have been based on the use of cooling medium (usually air) on a once through basis, without regard for collection or recovery of the energy transferred from the cell wall, and with low thermal efficiency in terms of air usage and heat transfer. The work of the LMRC addresses these shortcomings and offers a practical technology to achieve cell wall temperature control with the potential for energy recovery.