Upgrade of existing Fume Treatment Plants to cope with higher anode production requirements
Fig. 1: FTC in operation Introduction Aluminium Smelters are designed and built for a nominal start-up capacity. Once the ramp-up of all pots is finished, the production output is raised by continuous increase of the amperage. Other facilities of the Smelter, like the anode plant, have to adsorb these changes in production. Higher aluminium production output enforces higher production of anodes. Usually, the green anode plant is designed with some spare or extra capacity for maintenance purposes. In addition, the baking furnace can adsorb some geometrical changes of the anode, and with the help of a stateof-the-art system, higher production rates are possible by shorter fire cycles or implementation of a fourth burner ramp. What is most often not considered is the performance of the fume treatment plant. Existing plants have been designed for a nominal exhaust
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gas volume and a target exhaust gas temperature. Future expansion of the FTC is not foreseen. To accommodate higher production of anodes, an increase in exhaust gas volume is necessary. The specific volume per ton of produced anodes is in the range of 5.200 Nm3/t. The exhaust volume can not be boosted at will, as the main fans are designed for 100 % capacity. Other parts of the fume treatment plant also become affected. These effects and possible solutions will be outlined in this paper. General FTCs are designed for a maximum flow rate of the exhaust gases. The flow rates are limited by design, the gas speeds are in the range of 15 to 20m/sec. Higher flow rates can be temporarily realized by the use of stand-by fans, or over drive of the existing frequency driven fans. These
ANODE PLANT TECHNOLOGY
flow rates lead to higher gas speeds. The high gas speeds increase the entire pressure drop of the FTC. The effective pressure and volume flow for the anode baking furnace is realized by a much higher energy input at the fans. Higher maintenance, and a potential risk of production loss, are the logical consequence. Figure 1 shows a typical FTC in operation. The situation will get worse, if the produced gas volume is just sufficient for the production of anodes, but the remaining oxygen is not sufficient for the efficient combustion of the volatiles. The result will be more deposits of unburned tars and volatiles in the ducts. This will be the starting point of a vicious cycle, where the pressure drop continues to rise, and the effective gas volume continues to decrease. Eventually the FTC enters into a dramatic situation of operation. The operation needs to be extended for continuous supervision and extensive maintenance sessions, which will not