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Tuesday, November 22, 2011

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"...overall fuel saving by the proposed Aluminium and methanol coproduction, relative to the conventional processes, would be 53.3%." Read more below...

Extract from:
M. Halmann, A. Frei, and A. Steinfeld

Mineral Processing & Extractive Metall. Rev., 32: 247–266, 2011
Copyright © Taylor & Francis Group, LLC
ISSN: 0882-7508 print/1547-7401 online
DOI: 10.1080/08827508.2010.530723


The carbothermic reduction of alumina can be represented by overall (simplified) net reaction as follows:

Al2O3(s) + 3C(s) = 2Al(g) + 3CO(g)               (1)

The current industrial production of aluminium is based on the electrolytic Hall–Héroult process [discovered in 1886], which is characterized by the large energy consumption, about 15 MWh / tonne Aluminium (International Aluminium Institute 2009) and by the release of perfluorocarbon (PFC) and vast amounts of greenhouse gases, reaching 7 kg CO2-equiv / kg Aluminium in modern plants.

Figure 3: Equilibrium distribution vs. temperature for the system Al2O3 + 3C at 10-4 bar.
Under a vacuum of 10-4 bar (Figure 3), with an onset of Aluminium production already at 1600 K, the formation of Al2O and Al4C3 is calculated to be significantly suppressed.

At 1800 K, the equilibrium reaction is represented by Equation (10):

Al2O3 + 3C = 1.99Al(g) + 2.99CO(g) + 0.006Al2O3(g) + 0.006C(gr)       (10)

A preliminary estimation of the energy demand of the process under these conditions was made in a two-step calculation. The hypothetical heat input in the reaction of Equation (1) to 2Al(g) + 3CO(g) at T=1800K and 1 bar was calculated to be 1605 kJ / mol Al2O3. The minimum work of expansion from 1 bar to p=10-4 bar was 699 kJ / mol Al2O3. The total theoretical energy input would be 0.0427 GJ / kg Aluminium, comparable to that of about 0.045 GJ / kg Aluminium in modern smelters by the Hall–Héroult process (International Aluminium Institute 2009). In case CO would be submitted to water-gas shift to form syngas for methanol synthesis, this could result in the production of 1.056 kg methanol / kg Aluminium. In conventional methanol production via methane steam reforming (MSR), the total fuel consumption is 0.0445 GJ / kg methanol, thus requiring 0.047 GJ to produce the above amount of methanol (Frank et al. 2009).

Therefore, the theoretical overall fuel saving by the proposed Aluminium and methanol coproduction, relative to the conventional processes, would be 53.3%. The required heat for the process may be supplied by concentrated solar energy, induction furnaces, or electric discharges.

In case concentrated solar energy would be used for process heat, the total theoretical energy demand for pumping work would be only 0.0129 GJ / kg Aluminium.

If the combustion of fossil fuel is used for process heat, the calculated CO2 emission would be 3.32 kg CO2 / kg Aluminium, resulting in 53% avoidance of greenhouse gas emission relative to modern Hall–Héroult plants (International Aluminium Institute 2009). Emissions would be further decreased when applying concentrated solar energy as the source of process heat.