special - ALUMINIUM-Nachrichten – ALU-WEB.DE

SPECIAL
ALUMINIUM SMELTING INDUSTRY
be erected simultaneously and independently.
The DDS system may integrate a SO 2 scrubber
on the top, which enables it to comply
with more stringent emission limit values. The
available reagent choice is between an alkaline
solution or seawater. This DDS / SO 2 technology
will be in operation around the middle of
2013 at a smelter in Europe.
The AHEX is integrated to the DDS, upstream
of the filter stage. The hot gas, containing
the condensable fumes, is cooled inside
multiple water-cooled steel tubes in the
AHEX, where it enters the cooling tubes from
the top. The fumes are mixed with alumina
in the plenum upstream of the tubes inlets.
The hot fumes include condensable tar components,
which during the gas cooling condense
on the alumina surface. Simultaneously HF
and to some extent SO 2 is adsorbed. Due to
the efficient mixing of alumina and gas inside
the heat exchanger tubes, the AHEX FTC absorbs
more than 95% of the HF and tar on
the alumina. The efficient collection of tar
aerosols on the alumina particles reduces the
risk of tar depositing on the heat exchanger
surfaces. In addition the injected abrasive
alumina particles will clean the surfaces of
possible deposits, as demonstrated in the earlier
trials in the ME, which were the basis for
this patented design.
Control or elimination of fouling of heat
exchanger surfaces has been the main driver
behind Alstom’s development of this new fire
tube heat exchanger. Alstom has long-term
experience with fire tube heat exchangers on
similar or more difficult flue gases, such as
from Fe/Si- and Si-metal furnaces. Over the
last three years this technology has also been
proven for potgas in full-size demonstration
units (EHEX, MHEX, IHEX) at Alcoa Mosjøen
in Norway.
The adsorption process is enhanced by the
even gas / particle distribution, relatively long
retention time and short mixing length within
the confined space of the multiple parallel
tubes. The dry process of the novel AHEX
FTC, allows the gas to be cooled to temperatures
below 105 °C, possibly even below 80
°C. This allows for further condensation of
PAH and improved cleaning efficiency. After
leaving the heat exchanger the cooled gas enters
directly into the dry scrubber, where the
main part of the injected alumina is separated
into the filter hopper and re-circulated directly
back to the heat exchanger inlet. Primary
alumina is injected into the filter compartment
and collects on the bags in a final polishing
stage to adsorb any trace components of
tar fumes and HF. Through an overflow device
in the filter hopper the re-circulated or spent
alumina leaves the system to be sent to the
pots. The new AHEX FTC can efficiently handle
a larger variation of the flue gas flow than
today’s systems. There is no need to re-circulate
the gas, as is common for the conditioning
tower-based FTC.
The heat energy recovered in the AHEX
may be used or disposed to the environment.
One example of efficient use is in district heating,
another to use it for seawater desalination.
Electricity production is also possible by
deploying an Organic Rankine Cycle (ORC)
machine. For the AHEX plant at Mosjøen, the
hot water will be used for both district heating
and for driving an ORC for electricity production.
During the cold season, an extension
from the plant’s (and thus also the town’s) district
heating system to the AHEX is planned.
Validation of the AHEX FTC concept
The full scale AHEX concept is demonstrated
at the existing Alcoa Mosjøen FTC, which Alstom
delivered. This includes six filter compartments
downstream of the conditioning
tower. One compartment is retrofitted with
the AHEX heat exchanger. Thus the gas bypasses
the existing conditioning tower and
flows directly into the top of the heat exchanger
and further on to one filter compartment,
which operates on gas from the heat exchanger
only. This compartment is therefore
conveniently benchmarked with the other
five compartments which run on flue gas from
the conditioning tower. The measurements on
the gas from these compartments are references
in the full-scale validation of the AHEX
performance. The ingoing water temperature
to the AHEX is usually 60 °C and the outgoing
is 80-90 °C. The inlet gas temperature
normally varies between 160 and 190 °C and
the corresponding outlet gas temperature
reads 90-100 °C.
The heat recovered in the AHEX heats up
the 50% glycol water mixture to about 90 °C.
This fluid flows in a closed loop between the
AHEX and the heat delivering heat exchanger.
Here it is normally cooled down to about 60
°C. The heat flow is calculated from measuring
the fluid mass flow and corresponding temperatures
in and out of the AHEX, deploying
a specific heat value of approx. 3,300 kJ/kgK
for the heat transfer fluid. The heat transferred
to the fluid is in the range of 0.8 to 1 MW.
This indicates a total heat recovery potential
of about 5 MW for the complete anode bake
plant at Alcoa Mosjøen.
A 50% higher gas flow is estimated to flow
through the AHEX compartment, compared
to the remaining compartments. The reason
for the higher gas flow to the AHEX compartment
is the lower pressure drop across the
AHEX compared to the conditioning tower.
The gas flow is estimated within ± 10% accuracy
assuming a gas specific heat value
The installed full scale demo AHEX FTC at Alcoa
Mosjøen
of about 0,37 Wh/Nm 3 . This is based on the
fact that the heat absorbed in the fluid will
be equal to the heat recovered from the gas
(neglecting the small heat loss to the environment).
The total gas flow to the remaining
compartments is measured in a venturi duct.
To validate that there is no excessive dust
deposits on the AHEX tubes, a heat transfer
coefficient is calculated from the measured
data and divided by a theoretically calculated
heat transfer coefficient from the literature.
The stable quota curve indicates that the heat
transfer coefficient is
not degrading due to
e. g. excessive dust de-
Schematic diagram of AHEX, the combined heat
exchanger and tar condensation system
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