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cycle or in other processes within the plant. However, there is a limit on how much hot water can be used for the plant processes. Excess heat may also be used in the Kalina cycle to produce electricity. Large plant owners can consider building a cogeneration facility. There is also an opportunity to recover heat from coke calciner afterburner waste gas streams at 1,900°– 2,000°F (1,040°–1,090°C) to produce steam or electricity, but this is a business case and economic decision rather than an R&D need. Smelting Electrolysis – Pots There is good potential for heat recovery in this area. The sidewalls and endwalls represent a 15%–20% heat loss while the off gases represent about a 15% energy loss. There is a band around the circumference of the pot about 500 mm high where the heat flux is 10–15 kW/m2 and temperatures range from 570°–1,020°F (or 300°–550°C) (with a normal temperature around 842°F or 450°C). Higher temperatures are achieved when production is pushed. It would be possible to attach a thermoelectric device on the wall, which is covered with a steel shell. Accessibility depends on the age of the pots and geometry. Some smelters are more accessible than others—for example, Warwick smelting pots are difficult to access. Some trials on heat recovery have been done in other countries; results are proprietary. The pots operate with a crust or “ledge” a few centimeters thick. A heat sink is needed to maintain the crust; however, too much cooling can result in a ledge that is too thick, reducing the pot’s capacity. Alcoa wants to recover heat while keeping a thermal balance in the pot. There may be some additional benefits from heat recovery if hot air generated by natural convection around the pots is able to be captured and removed, such as a more stable operation and improved work environment. Thermoelectric (TE) devices are considered an emerging technology. With ZTs1 hopefully approaching 1.5, there may be more opportunities for TE device application. It is assumed that the technology will continue to improve. There may be an opportunity for applied R&D that focuses on managing the heat sink; scaling and transforming modules to workable devices; process integration and infrastructure development; power management and quality of power; handling thermal expansion, attach panels, and durability; and overall control of the system, as well as cold side heat recovery considerations. Off gases from the pot are 212°F (or 100°C) after the scrubbers and only 212°–266°F (or 100°–130°C) before the scrubbers. Alcoa is interested in capturing waste heat before the scrubbers because there are issues with build-up and particulate fouling. Make-up air is used to prevent fugitive emissions and the duct system needs to be open for periodic maintenance. Cleaning the gases remains a challenge. The fouling problem is covered in available literature. An Installation in Europe utilizes a heat exchanger on exhaust gas stream to recover heat from the aluminum smelting cells or reactors, commonly known as pots. A heat exchanger (gas to water) upstream of the scrubber is used to get hot water to supply heat to a district heating system at a location in Iceland. As of July 2010 the system has been operating for a few months [Martin Fleer 2010]. The industry is monitoring improvements to the organic Rankine cycle using a portfolio approach in which there is initial tolerance for long payback periods of up to 5 years, with efforts to identify improvements 1 ZT is an indicator of thermodynamic efficiency of the TE device and is a convenient figure for comparing the potential efficiency of devices using different materials. Values of ZT equaling one are considered good, and values of at least three to four are considered essential for thermoelectric to compete with mechanical generation and refrigeration in efficiency. Industrial Waste Heat Recovery Page 9PDF Image | Industrial Waste Heat Recovery: Potential
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