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
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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
This is the cause for: • Deposits, which grow at the walls • Formation of heavy corrosion by acids of SO3 and Fluor Finally the pressure drop of the cooling tower, and consequently for the whole FTC, starts to increase continuously. Consequences for the fabric filter Fabric filters are designed and calculated through the so called “filter surface load”. The value of the filter surface load should be favourable in a range of < 1,0 m3 / m 2 and minute. For maintenance purposes, the filter can be operated in a (n-1) chamber mode. During this period, the value will be in a range of 1,5 or up to 2,0 for a short period of time.
Fig. 2 Cooling tower arrangement improve the situation, and can only achieve a temporary stabilization. Consequences for the Cooling Tower The cooling tower is designed as a direct current cooler. The off gases are guided to the top of the cooling tower (gas inlet) and leave the cooling tower at the bottom. Water spray lances are installed in the top level of the cooling tower. Most cooling towers in older plants have only 1 to 3 water lances and are equipped with less water nozzles. Based on today’s knowledge the generated drops of water are too big. Figure 2 shows a typical cooling tower arrangement. The cooling tower uses the principle of the enthalpy of water evaporization. The design criteria include a dedicated exhaust gas volume and a target “cooled” outlet gas temperature. This finally results in the size of the cooling tower due to the stretch necessary
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for evaporization. If the exhaust volume now increases above the design values, the cooling tower will become overloaded. Under normal operation conditions, production staff will not recognize it, because the outlet temperature of e.g. 105 °C is still reached by the controls. The evaporization itself is a nonstationary process, which needs time in relation to the size of the droplets. For higher exhaust gas volumes, the average dwell time within the cooling tower decreases significantly and as a result, the off gases at the outlet of the cooling tower are chilled, but no longer dry. For the lifecycle of the FTC, this is of vital significance. The high kinetic energy of the non-evaporized water droplets at the outlet leads to wet walls and ducts.
This implicates that the fabric filters are designed on a conservative level, and are consequently more robust in overload situations compared to the cooling tower or the fans. The cooling tower and fans reach their technical limits at a very early stage, and are therefore unable to facilitate futher volume flow increases. The fabric filters allow the precipitation of the adsorbant alumina including all attached pollutants, even in an over load situation. But due to the described bottlenecks in the furnace and the cooling tower, the exhaust gases start becoming loaded with higher concentrations of pollutants, such as soot and condensed tars. The resulting situation can only be compensated by higher throughput capacity of alumina, which will exponentially increase the abrasive wear of the FTC. The increased volume flow implies further disadvantages. The pressure loss between the inlet and the outlet of the fabric filters is increased quadratic
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and further feeds the vicious cycle with regard to all influences caused by reduced volume flow. The differential pressure value of the fabric filter moves fast towards the threshold limit, for the pulse cleaning system of > 22 mbar. The filter bags are physically stressed and start to burst partially. When the pulse cleaning system is no longer able to fully clean the filter bags and the differential pressure stays at high values, the technical limit is exceeded. As a final consequence, the filter bags have to be changed in much shorter time intervals where nominative usage rates are in the range of 3 years plus. Figure 3 shows the used and destroyed filter bags. Fig. 4 Fan system in operation which will further increase the abrasive wear of the FTC.
possible, and every subsequent malfunction disturbs the furnace production immediately.
Consequences for the fans Fig. 3 Used and destroyed filter bags Consequences for the filtration of harmful components The continuous overload of the FTC by exceeded volume flows finally results in a negative overall situation for the main task of the FTC, which is the efficient cleaning of the exhaust gases. Exceeded volume flows reduce the average reaction time between the adsorbant alumina and the pollutant. Some important adsorbant reactions have to take place in intermixed reactor chambers prior to the final precipitation in the filtration cake. Especially the probability of contacts between the aerosols -conditioned in the cooling tower- and the alumina will be reduced intensively due to insufficient dwell times. Some unsteady adsorbtion processes are not yet finished, while the fume gases already penetrate into the filtration cake. As already mentioned earlier, this can also be compensated by higher throughput capacities of alumina,
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The FTC will now operate at much higher pressure losses. The fans can only compensate this with a decreased volume flow. To ensure the desired flow, spare fans or spare capacities have to be operated in parallel to reach the necessary volume flow. An operation mode of n-1 is no longer
Systematic maintenance, including turn- down of fans becomes impossible. Figure 4 shows a fan system in operation.
Fig. 5 Dual phase spray nozzle system
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Fig. 6 “Symmetrical” design of the duct system
Solutions for a solid state upgrade
Cooling tower
Main targets for a solid state upgrade of an FTC are:
The cooling tower has to contain the stretch necessary for evaporization. Existing cooling towers have to be checked to establish if this stretch is available. Sometimes it is possible to increase the performance of evaporation by upgrading the lance system to a dual-phase nozzle technology with finest droplets. Figure 5 indicates such a spray nozzle system, consisting of 5 nozzles per lance.
1. A maximum reliability to operate 24-7-365 without any major interruptions. 2. Spare capacity and redundancy for maintenance works to be performed without production stop, operation with n-1 chambers to ensure 1. 3. A continuous high quality performance of anode production with minimum energy consumption and minimum emission. It is also essential to perform a complete combustion of the volatiles inside the furnace. This can be ensured by an advanced state-of-the-art firing system, providing the FTC can provide the adequate volume and flow for all operation situations at the furnace.
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In other cases, it might be advisable to renew the cooling tower by size and technology to eliminate the first major bottleneck. Duct systems and Dampers The duct system guides the gases through the FTC. Existing Duct Systems and Dampers have to be checked carefully. Non-optimized
Fig. 7 Condition of a duct system with accumulated deposits layouts with multiple bends and elbows lead to high pressure losses. The same applies for special types of dampers. A proper design of the duct system is the basic task to minimize these pressures losses. A redesign of the damper system, can improve the performance of the FTC tremendously. Figure 6 shows a “symmetrical” design to optimize flows and pressure losses. Finally, in existing plants the duct system has to be inspected for
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In other applications it is possible to raise the filter chambers, and to prolong the filter bags. The maximum length is limited by the efficiency of the pulse jet cleaning system, and the maximum physical load on the filter bags during the cleaning cycle. Filter bags of up to 6m are feasible, longer bags need to be examined with care. Figure 8 shows the inner part of the filter chamber. If all these solutions are not viable, the amount of filter chambers has to be increased. In general 2 main topics have to be considered: â&#x20AC;˘ Production must be possible even with n-1 chambers in operation, meaning with 1 chamber isolated. â&#x20AC;˘ The change of one complete set of filter bags for one chamber should be possible during one production shift. Conclusion Fig. 8 Inner part of the filter chamber deposits. A complete cleaning of the duct system, dampers, and the cooling tower will also minimize pressure losses and improve the flow rates and performance of the FTC. Figure 7 shows the condition of a duct system with accumulated deposits. Filter chambers The adaptation of existing filter chambers is strongly dependant on the existing infrastructure and the existing design. The process requirements for a redesign of an existing filter chamber are easy to formulate, but not as easy to realize. The future value of the filter surface load should be favourable in a range of <1,0m3/m2 and minute. High gas flow rates in a range of 18 m/s in the ducts and reactors should be reduced in steps to less than 1 m/s in the area of the
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filter bags. The separation of heavy particles (agglomerates) needs to be executed prior to the filtration cake. In this area, gas speeds of < 2m/s are obligatory. The gas flow onto the filter bags should be in a wide area from bottom to top. Technical solutions which contain a horizontal flow generate an early tear and wear of the fabrics by partial overloads, and in parallel inactive areas inside the filter chambers.
If all these aspects are technically realized the desired performance for higher production will be available. As a positive outcome, the dosing of fresh alumina can now be tuned and minimized. If the right balance is found between fresh and recirculated alumina, the wear and tear in all aspects of the FTC will be minimized and the adsorbtion ratio, respectively the cleaning effect is maximized. This leads to minimum emissions in the clean gases which are vented to the atmosphere. At the end, a detail assessment will be conducted to show, if or how, a retrofit or partial renewal of equipment is feasible.
As a first approach, disturbing installations should be eliminated. Second, the filter chamber should Authors be fully furnished with a maximum Dipl.-Ing. D. Maiwald; possible amount of filter bags. In Dr.-Ing. F.Heinke; Dipl.-Ing.(PE) D. Di Lisa; addition, a pre-separation chamber Innovatherm should be designed and installed to Prof.-Dr. Leisenberg ensure the staggered deceleration of GmbH & Co KG, the gas Fig. 5 speed. Dual phase spray nozzle systemButzbach, Germany
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Integrated Technology Firing and Fume Treatment for Anode Baking Furnaces
ProBake Advanced Firing Systems Lowest energy consumption Total pitch burn Higher quality consistency
innovatherm 06/2013
ProClean Fume Treatment Technology Higher adsorbtion ratios Lower emissions Higher reliability
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