Breaking the 11 kWh/kg Al barrier
M. Dupuis, GeniSim
It is important when performing a design study to establish a de sign goal. For the study presented here, the design goal was to break the 11 kWh/kgAl cell energy consumption barrier, because this was judged to be an achievable
Introduction
The R&D design work presented in this article is the immediate fol
In [1], the second design is based on using a wider cell, an idea ini tially presented a year ago in this Journal [4]. That wider cell uses the Reversed Compensation Current (RCC) busbar concept first shown in [5]. The outcome of the study is a 650 kA, wider cell design op erating in thermal balance at 11.3 kWh/kgAl. Again in [1], the future work section proposed some ideas to further reduce the cell energy consumption and the present study tested those ideas as well.
Table 1: Design 1, Wider 650 kA cell using RCC busbar design
Amperage 
762.5 kA 
650 kA 
650 kA 
Nb. of anodes 
48 
48 
36 
Anode size 
2.6 x .65 m 
2.6 x .65 m 
2.6 x .86 m 
Nb. of anode studs 
4 per anode 
4 per anode 
12 per anode 
Anode stud diameter 
21.0 cm 
24.0 cm 
16.0 cm 
Anode cover thickness 
15 cm 
24 cm 
25 cm 
Nb. of cathode blocks 
24 
24 
24 
Cathode block length 
5.37 m 
5.37 m 
5.37 m 
Type of cathode block 
HC 10 
HC 10 
HC 10 
Collector bar size 
20 x 12 cm 
20 x 15 cm 
20 x 15 cm 
Type of side block 
HC3 
HC3 
HC3 
Side block thickness 
7 cm 
7 cm 
7 cm 
ASD 
25 cm 
25 cm 
25 cm 
Calcium silicate thickness 
3.5 cm 
6.0 cm 
6.0 cm 
Inside potshell size 
17.02x5.88m 
17.02 x 5.88m 
17.02 x 5.88m 
ACD 
3.0 cm 
2.8 cm 
2.8 cm 
Excess AIF3 
11.50% 
11.50% 
11.50% 




Anode drop (A) 
347 mV 
296 mV 
252 mV 
Cathode drop (A) 
118 mV 
109 mV 
109 mV 
Busbar drop (A) 
300 mV 
220 mV 
170 mV 
Anode panel heat loss (A) 
553 kW 
327 kW 
339 kW 
Cathode total lieat loss (A) 
715 kW 
499 kW 
482 kW 
Operating temperatur (D/M) 
968.9 °C 
967.0 °C 
966.5 °C 
Liquidus superheat (D/M) 
10.0 °C 
8.1 °C 
7.6 °C 
Bath ledge thickness (A) 
6.82 cm 
11.86 cm 
14.25 cm 
Metal ledge thickness (A) 
1.85 cm 
3.38 cm 
4.58 cm 
Current efficiency (D/M) 
95.14% 
94.80% 
94.90% 
Intenial heat (D/M) 
1328 kW 
832 kW 
804 kW 
Energy consumption 
12.85 kWh/kg 
11.3 kWh/kg 
11.0 kWh/kg 
Fig. 1: Topology of the 4 carbon blocks per anode,
3 stubs per carbon block new anode design
Fig. 2: Half anode model mesh
showing the copper insert in green
Fig. 3: Anode model temperature solution
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Design 1: Wider 650 kA cell using RCC busbar design
In [1], it was concluded that the 2 carbon blocks per anode, 2 stubs per carbon block anode design was limiting the possibilities to further reduce the anode voltage drop. For that reason, the current study developed a new anode model in order to test a new 4 carbon blocks per anode, 3 stubs per carbon block anode design. Fig. 1 presents the new anode model topology. This keeps the anode block surface
Fig. 4: Wider 650 kA model pre
dicted busbar voltage drop
Fig. 5: Wider 650 kA 8 risers RCC busbar network
Fig. 6: Wider 650 kA 8 risers
RCC busbar network
horizontal in order to simplify the model topology. A more detailed model will include extra features to minimize the carbon usage. As Table 1 indicates, the global anode length has remained unchanged with 4 carbon blocks of 1.3 x 0.43 metres for a total area of 2.6 x
0.86metres per anode. Since there are 3 stubs per carbon blocks, each stub is providing current to an almost square 0.433 x 0.43 metres carbon surface area. The new stub diameter is 16 cm.
As in the initial concept presented in [4], there is a 6 cm wide channel between the front and back carbon blocks to enhance elec trolyte flow and mixing. There is a smaller 2 cm channel between the 2
In [1], it was also recommended to further optimized the RCC busbar network in order to reduce even more the reported 220 mV busbar drop. As presented in Fig. 5 of [1], that RCC busbar network is using a total of 6 risers, 3 on the upstream side and 3 more offset on the downstream side. That network of 6 RCC busbars was replaced by a network of 8 risers RCC busbars, with 4 risers on the upstream side and 4 more offset on the downstream side, as presented in Fig. 4. As Table 1 shows, this much lowers the busbar voltage drop, predicted down by 50 mV to 170 mV.
Fig. 5 presents the global setup of the MHD model. Notice that
instead. Figs
kWh/kgAl, which is reaching but not breaking the 11.0 kWh/kgAl bar rier!
Design 2: 520 kA cell with 100% downstream side current extraction
The reduction of the cell energy consumption is always a compro mise to balance the cell’s lower internal heat generation (hence lower ohmic resistance components of the cell voltage) against the reduc tion of the cell heat loss. The dilemma comes from the fact that reduc ing the electrical resistance of cathode and the anode will also typi cally reduce the thermal resistance of those ohmic components. This means that limits to further reduction of the cell energy consumption exists both on the heat production side and on the heat dissipation side. For Design 1, the limitation is on the heat production side, as a
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wider cell dissipates less heat per unit production. But on the other hand this makes it more difficult to lower the electrical resistance of the ohmic components like the busbar.
Design 2 shows the opposite situation: lowering the ACD from
3.2to 2.8 cm automatically reduces the cell internal heat generation, but at 500 kA, it was not possible to find a way to operate the cell in thermal balance at a ‘reasonable’ cell superheat. So the cell amperage had to be increased to 520 kA in order to maintain the cell internal heat around 700 kW, as indicated in Table 2. Of course, this change of ACD and amperage means that the anode and cathode models must be rerun to analyze the impact of those two operational changes. These new results are also reported in Table 2 third column: first the anode drop increases by 10 mV from 238 to 248 mV, without significant change to the anode panel heat loss. Second, the cathode voltage drop increases by 3 mV from 125 to 128 mV, again without significant impact on the cathode heat loss, bit at the expense of a slight reduction of the cell superheat from 5.4 to 5.3 °C. Some people would argue that it is not possible to operate a cell at such a low cell superheat, yet the author remembers having deliberately designed the cell lining of the A310 cell so that it would operate a 6 °C of cell liquidus superheat. This cell operated quite well at that predicted superheat and predicted ledge thickness, although with a much greater bath volume than was usual at that time.
The second recommendation for this ‘500’ kA cell with 100% downstream side current extraction was shrink the cell
Table 2: Design 2, 520 kA cell with 100% downstream side current extraction
Amperage 
600 kA 
500 kA 
520 kA 
Nb. of anodes 
48 
64 
64 
Anode size 
2.0 x .665 m 
1.95 x .5 m 
1.95 x .5 m 
Nb. of anode studs 
4 per anode 
4 per anode 
4 per anode 
Anode stud diameter 
17.5 cm 
17.5 cm 
17.5 cm 
Anode cover thickness 
10 cm 
20 cm 
20 cm 
Nb. of cathode blocks 
24 
24 
24 
Cathode block length 
4.17 m 
4.17 m 
4.17 m 
Type of cathode block 
HC 10 
HC 10 
HC 10 
Collector bar size 
20 x 10 cm 
20 x 20 cm 
20 x 20 cm 
Type of side block 
SiC 
HC3 
HC3 
Side block thickness 
7 cm 
7 cm 
7 cm 
ASD 
28 cm 
30 cm 
30 cm 
Calcium silicate thickness 
3.5 cm 
6.0 cm 
6.0 cm 
Inside potshell size 
17.8 x 4.85 m 
17.8 x 4.85 m 
17.8 x 4.85 m 
ACD 
3.5 cm 
3.2 cm 
2.8 cm 
Excess AIF3 
12% 
12% 
12% 




Anode drop (A) 
318 mV 
238 mV 
248 mV 
Cathode drop (A) 
104 mV 
123 mV 
182 mV 
Busbar drop (A) 
311 mV 
134 mV 
85 mV 
Anode panel heat loss (A) 
449 kW 
292 kW 
295 kW 
Cathode total lieat loss (A) 
692 kW 
402 kW 
404 kW 
Operating temperatur (D/M) 
964.8 °C 
958.4 °C 
958.3 °C 
Liquidus superheat (D/M) 
11.8 °C 
5.4 °C 
5.3 °C 
Bath ledge thickness (A) 
6.36 cm 
11.84 cm 
11.83 cm 
Metal ledge thickness (A) 
1.76 cm 
3.48 cm 
3.46 cm 
Current efficiency (D/M) 
96.4% 
96.3% 
96.5% 
Intenial heat (D/M) 
1140 kW 
699 kW 
701 kW 
Energy consumption 
13.26 kWh/kg 
11.2 kWh/kg 
10.85 kWh/kg 
cell design parameter has not previously been optimized. At the same time, the thicker busbar sections further reduce the busbar voltage drop, which was already quite low due to the 100% downstream side current extraction consisting only of anode risers. The number was kept to 6 risers, but on second thoughts, again it might have been
Fig. 7: Wider 650 kA 8 risers RCC busbar network
Fig. 8: Wider 650 kA 8 risers RCC busbar network
Fig. 9: 520 kA 6 risers 100% downstream current
extraction cell with ECC busbar network,
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Fig. 10: 520 kA 6 risers 100% downstream current extraction
cell with ECC busbar network predicted voltage drop
Fig. 11: 520 kA 6 risers 100% down stream current extraction cell with ECC busbar network predicted BZ
Fig. 12: 520 kA 6 risers 100% downstream current extraction
cell with ECC busbar network: predicted
Fig. 13: 520 kA 6 risers 100% downstream current extraction cell with ECC busbar network: predicted metal flow field
better to increase that to 8 risers, as it is the cross section of the risers that prevented further shrinking of the cell
This drastic lowering of the busbar voltage drop obviously greatly contributes to lowering the cell power consumption. Another great advantage of reducing that component of the cell ohmic resistance is that it does not affect the cell heat balance. Figs
The present study achieves its design goal to break the 11.0 kWh/ kgAl barrier, as Table 2 proves. The new version of the 520 kA cell, with 100% downstream side current extraction, is predicted to con sume only 10.85 kWh/kgAl produced, well below the 11.0 mark.
Comparison of the two cell design options
The comparison exercise presented in [1] is repeated here, using the updated designs for both options. On the opex side, the discrepancy has increased to 0.15 kWh/kgAl, now that the two cell designs are operated at the same ‘minimum’ 2.8 cm of ACD. Clearly the busbar length requirement is what most differentiates the two cell designs which both favour of the 100% downstream side current extraction cell design option.
On the capex side, a very crude comparison was made in [1]. Based on the length of potroom(s) required for a smelter to produce 1 million tonnes of aluminium per year, that number of cells has not changed for the wider cell design option: at 95% current efficiency, a 650 kA cell produces 4.974 tonnes Al per day. So it needs 550 cells, and with a 7.5 metres of
Future work
It is the opinion of the author that further reduction of the cell energy consumption will probably have to come from recovering of some of the heat lost by the cell. At the recent ICSOBA conference in Hamburg, such a heat recovery system (HRS) was presented by EGA
[7].Fig. 14 from [7] presents the HRS concept, where heat can be extracted both from the cell exhaust gas and from the cell side walls. According to [7], 120 kW can on average be collected by the side wall heat exchangers. No data was provided in [7] for the conver sion efficiency, but in their TMS 2014 paper [8], Goodtech Recov ery Technology, that provided the HRS to EGA, was talking of only 10%, admittedly on the conservative side. Based on [9], it may be reasonable to assume 20% of conversion efficiency. On that basis, out of the 120 kW of cell heat loss collected, 24 kW can be converted
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Fig. 14: Heat recovery system (HRS) tested by EGA, extracted from [7] presentation
back into electrical energy, which for a 455 kA cell represents 53 mV or 0.165 kWh/kgAl.
On the cell exhaust gas heat recovery, it would be far more ef ficient to first increase the gas exhaust temperature. This could be achieved by reducing the area of the hood openings and also by insu lating the hoods and the fume plate so as to decrease the gas exhaust flow rate and to keep more of the anode panel heat loss in the ex haust gas. Also at the ICSOBA conference, the author presented a model that was developed to study the impact of such design changes
[10]on the cell hooding system heat balance and the cell hooding HF capture efficiency. If the equivalent amount of heat collected on the cell side walls can be collected from the gas exhaust and convert ed into electrical energy, we are talking about a potential of 100 mV or about 0.31 kWh/kg of cell energy consumption reduction due to HRS having only 20% conversion efficiency.
Discussion and conclusions
Contrary to the author’s expectations one year ago, the present study has produced cell designs that have reached, and in the second case broken, the 11.0 kWh/kgAl cell energy consumption barrier. At
10.85kWh/kgAl, at the time of writing this paper, the author ran out of design idea to further reduce that number without involv ing HRS. Even with heat recovery, the very low conversion rate of such low grade heat energy into electrical energy means that HRS has the potential to further decrease the cell energy consumption by another 0.31 kWh/kgAl, down to about 10.54 kWh/kgAl. This is still
0.54kWh/kgAl away from the next barrier to be broken, the 10.0
kWh/kgAl barrier.
Clearly, it would be far more efficient to use that captured, low grade heat energy to preheat the alumina and/or the anodes, or maybe use it to reduce the fuel consumption in the anode baking furnace. In such a case the 0.31 kWh/kgAl of cell energy consumption reduction becomes 1.55 kWh/kgAl, plenty to break the 10.0 kWh/ kgAl barrier and even to start dreaming of breaking the next bar rier!
References
[1]M. Dupuis, Very low energy consumption cell designs: the cell heat bal ance challenge, TMS Light Metals, 2018, to be published.
[2]M. Dupuis, A new aluminium electrolysis cell busbar network concept, Proceedings of 34th International ICSOBA Conference, Québec City, Canada, 3 – 6 Oct. 2016, Paper AL39, Travaux No. 45,
[3]M. Dupuis, Low energy consumption cell design involving copper inserts and innovative busbar network layout, TMS Light Metals, 2017,
[4]M. Dupuis and B. Welch, Designing cells for the future – wider and/or even higher amperage?, ALUMINIUM, 93 (1/2), 2017,
[5]M. Dupuis, A new aluminium electrolysis cell busbar network concept, Proceedings of 33rd International ICSOBA Conference, Dubai, UAE, 29 Nov.
– 1 Dec. 2015, Paper AL21, Travaux No. 44,
[6]M. Dupuis, Presentation of a New Anode Stub Hole Design Reducing the Voltage Drop of the Connection by 50 mV, VIII International Congress & Exhibition
[7]A. AlJasmi and A. AlZarouni, Findings from operating a pot with heat re covery system, Proceedings of 35th International ICSOBA Conference, Ham burg, Germany, 2 – 5 Oct. 2017, Paper AL29, Travaux No. 46,
[8]Y. M. Barzi, M. Assadi and H. M. Arvesen, A novel heat recovery technol ogy from an aluminum reduction cell side walls: experimental and theoretical investigations, TMS Light Metals, 2014,
[9]T. C. Hung, T. Y. Shai and S. K. Wang, A review of organic rankine cycles (ORCs) for the recovery of
[10]M. Dupuis, M. Pagé and F. Julien, Development of an Autodesk CFD Based 3D Model of a Hall Héroult Cell Hooding System and HF Capture Efficiency, Proceedings of 35th International ICSOBA Conference, Hamburg, Germany, 2 – 5 Oct. 2017, Paper AL07, Travaux No. 46,
Author
Dr. Marc Dupuis is a consultant specialized in the applications of math ematical modelling for the aluminium industry since 1994, the year when he founded his own consulting company GeniSim Inc. (www.genisim.com). Before that, he graduated with a Ph.D. in chemical engineering from Laval University in Quebec City in 1984, and then worked ten years as a research engineer for Alcan International. His main research interests are the devel opment of mathematical models of the
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