special - Alu-web.de

TECHNOLOGY
ample in the proposed
exhaust gas system – is
not appropriate in light
of the temperature
level after the recuperator.
The effect of the
exhaust gas system can
now be estimated now
that all the fuel-heated
thermo-process units
have been considered: in total, from the recycling
furnace, the holding/casting furnace,
the heated vertical magazine and the billet
heating furnace an exhaust gas heat content
of 335 kWh th /t Al is obtained, at a temperature
of approx. 250 °C with adiabatic mixing.
Only 30 kWh th /t Al are needed for preheating
the scrap supplied to the recycling furnace
to 100 °C and drying it. Now, instead of storing
the scrap coming from within and outside
in heaps, a scrap bunker heated by the energy
flow from the exhaust gas system should be
provided. A possible approach would be to
set up a system of circulating ‘pick-and-place’
charging troughs: in a cyclic system these
would be charged with incoming scrap, parked
automatically in the heated bunker, and as
necessary or after an appropriate dwell time,
removed for charging the recycling furnace.
Possible result
For the foundry and extrusion plant production
sector various approaches have been discussed
for reducing energy demand:
• spatial combination of the casthouse and
the extrusion plant
• construction of an exhaust gas system for
preheating the scrap to 80 °C
• beginning homogenisation at 300 °C
instead of from room temperature
• beginning billet heating at 300 °C instead
of from room temperature
• using recuperator burners in the ageing
furnace.
The measures described above lead to an energy
saving of 245 to 265 kWh th /t Al , which
corresponds to 22% (Table 2). With an average
industrial natural gas price (EU 28) of
0.039 €/kWh th [6] and the assumed annual
production of 32,000 tonnes, in the casthouse
and extrusion plant energy costs between
305,000 and 330,000 euros can be saved.
Thermo-process plant Saving (kWh th /t Al )
Optimised energy demand
(kWh th /t Al )
Two-chamber hearth furnace 30 630 – 670
Casting/holding furnace none 20 – 40
Homogenising furnace 100 95 – 105
Billet heating furnace 105 – 125 70 – 90
Ageing furnace 10 65 – 75
Thermal energy demand opt. 245 – 265 880 – 980
Table 2: Thermal energy demand in the continuous casting plant and the
extrusion plant, after optimisation
there are a series of measures which could be
implemented when planning a new plant. Nevertheless
in individual cases higher investment
costs have to be allowed for, which become
economical only if longer amortisation times
are acceptable. Alternatively, only (even)
higher energy costs would produce the necessary
incentive for action. The first approach is
to be preferred – lower energy needs reduces
demand and lowers the price.
At the same time it is clear that to make
use of the potentials whose existence is not
in doubt, requires interdisciplinary collaboration
between various fields of engineering
science. However, it must be admitted that at
the locations existing today, restrictions are in
force which enable only some of the measures
described.
For the sake of completeness, there is an
important aspect which should not be left unmentioned:
the above considerations imply
that between the recycling furnace and the
ageing furnace some 32,000 tonnes of aluminium
a year are processed without losses.
This, of course, is not the case. In fact, for the
actual sale of 32,000 tonnes of profiles as a
rule more than 42,000 tonnes of aluminium
have to be charged into the recycling furnace.
In the form of combustion loss, overfill losses,
billet offcuts, billet and extrusion discards,
profile scrap and rejects, it is usual for more
than 30% of the material originally charged to
be returned back into the cycle. That fact often
is the first to justify the operation of a casthouse
in combination with an extrusion plant.
To that extent, improving productivity also has
high potential for reducing energy demand.
References
[1] Valder, G.: Ermittlung des Energieeinsparpotenzials
und Bestimmung von CO 2 -Produktbenchmarks
bei der Herstellung stranggepresster Halbzeuge aus
Sekundäraluminium, Dissertation RWTH Aachen,
2011, S. 120
[2] Quinkertz, R.: Optimierung der Energienutzung
bei der Aluminiumherstellung, Dissertation RWTH
Aachen, 2002, S. 78
[3] Hajek, A.: Entwicklungen auf dem Rohstoffmarkt,
Forum für Zukunftsenergien e.V., 23. Sitzung
am 16. März 2005
[4] VDMA: Leitfaden Energieeffizienz von Thermoprozessanlagen,
Eigenverlag VDMA, 3/2009, S. 19
[5] Gauvain, M. et al.: Otto Junker successfully commissions
a new type of high-efficiency billet heating
unit at Sapa Offenburg, (Article published also
in German language), International ALUMINIUM
Journal, 5/2012, S. 62-66;
[6] Destatis: Daten zur Energiepreisentwicklung,
Statistisches Bundesamt, Wiesbaden, https://www.
destatis.de/.../EnergiepreisentwicklungPDF_
5619001.pdf, 11/2012, S. 22
Authors
Dr.-Ing. Günter Valder heads the Thermal Process
Plant division at Otto Junker GmbH, Simmerath.
Prof. Dr.-Ing. Herbert Pfeifer is head of the Institute
for Industrial Furnace Construction and Heat Technology
at RWTH Aachen.
Summary
The analysis of potentials for saving thermal
energy was carried out on the assumption of a
‘greenfield’ investment. It has been shown that
Fig. 7: Example of a typical ageing furnace
48 ALUMINIUM · 3/2013