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"Twenty years ago, someone in China might drink beer from a barrel out of a mug," explains materials science and engineering professor James W. Evans, "Now they're going to drink out of an aluminum can."
World demand for aluminum is expected to continue rising, but aluminum production consumes enormous amounts of energy and generates significant greenhouse gas emissions. By outfitting an aluminum smelting plant with wireless sensors, Evans and his team, in collaboration with mechanical engineering professor Paul Wright, are exploring ways to reduce emissions while increasing efficiency. A project of CITRIS (Center for Information Technology Research in the Interest of Society), the effort demonstrates how information technology can reduce energy usage, hazardous emissions and overall environmental impact.
Aluminum production relies on an energy-intensive process called electrowinning, which extracts pure aluminum from alumina derived from mined bauxite. Electrowinning takes place in 3- by 10-meter electrolytic cells, which may number in the hundreds in a large plant. Each cell receives one megawatt or more of continuous DC, like charging a huge battery. Chemical reactions inside the cell free the elemental aluminum, which collects in a molten pool at the bottom of a 960°C salt electrolyte bath, where it is siphoned out.
Inside each cell, a solid salt crust naturally forms above the hot bath. Whenever this crust is breached, the cell emits greenhouse gases, including hydrogen fluorides (HFs) and perfluorocarbons (PFCs). The crust is broken intentionally and re-sealed with alumina during routine maintenance, but the greater environmental threat occurs when a crust cracks spontaneously and goes undetected. "Fairly frequently a cell will have a hole in its crust that the operator doesn't know about," Evans explains, "This allows atmospheric air to contact the molten salt electrolyte, which causes excess emissions until the problem is discovered."
Historically, the industry has let this pollution happen rather than expending the effort to inspect each pot. Monitoring never progressed past visual inspection because the pots' enormous electrical currents always made extra wiring a safety hazard.
"These pots cost $2 million to $3 million dollars each, and they were never instrumented, except for the one measurement of voltage," Evans notes. "My son's a pilot, and I asked him what kind of plane you could buy for that much. He said you could get a slightly used Learjet. You'd find all sorts of gauges in the cockpit of one of those, but these cells are still from the era of the Sopwith Camel."
Berkeley researchers Evans and Wright installed wireless temperature gauges on aluminum processing pots at an experimental plant. These "motes" transmitted readings to a central laptop via low-power FM, allowing the operator to see the pots' real-time status. Breaks in the crust showed up as sudden temperature jumps inside the pots' gas exit ducts; after the breaks were patched, temperatures returned to normal. Using this simple system, plant operators can fix breaks when they happen rather than let HFs and PFCs leak undetected for hours or days.
To keep things low maintenance, some motes ran off energy scavenged by thermoelectric generators (TEGs) rather than batteries that would need replacing. The TEGs generated current from the temperature differential between hot duct gases and the outside air. A finned metal heat sink radiated heat away on the cool side to boost power.
Such sensors now let researchers study and refine electrowinning in ways that have been impossible throughout the more than 100-year history of the process. "We have a real opportunity here," says Evans, "Using wireless sensors, we can finally instrument these things properly and figure out how to save energy. Current aluminum cells are only 40% to 45% efficient."
Wireless monitoring could improve other industries, Evans says, whether or not they have electric safety issues. The promise of streamlining large operations by bringing in a laptop and slapping on some magnetically attached sensors is irresistible. Evans, Wright and their team have experimented with wireless monitoring for copper production and are considering motes for wine barrels, industrial looms and paper mills.
"CITRIS has launched many projects for civil applications, like security, transportation and HVAC," explains Wright, "But industrial applications can also be improved. In fact, solving the power and environmental problems from industrial processing may be even more crucial for a green future in society. These issues grow hotter by the day."
Find this article at:
http://www.coe.berkeley.edu/labnotes/0307/aluminum.html
