sPeCIAL ArABAL - ALUMINIUM-Nachrichten – ALU-WEB.DE

sPeCIAL ArABAL
ALUMINIUM INdUstry IN the GULf
promoted in the Venturi
reactor. Pitch loaded fines
are then collected by a dust
collector (bag filter) and are
reintroduced into the anode
paste recipe. The clean
gases are then released to
the atmosphere (Fig. 1).
The pitch loaded fines
are then recycled and used
into the recipe for the anode
manufacturing process:
there is neither waste
stream nor product loss.
The global capture
efficiency of dry scrubbing for PAH
is about 95%; this efficiency is higher
for heavy PAH (up to 99%).
The dry scrubbing system philosophy is
based on the ability of PAH to condensate:
this allows to be adsorbed on the fines coke
surface. So, the efficiency is better with heavy
PAH that condensate easily. For the lighter
PAH, mainly emitted from the cooler as
noted above, the efficiency of this technology
is limited: if there is no condensation, no
adsorption will occur and the pollutants pass
through the bag filter without being captured.
The condensation depends, obviously, of the
ambient temperature: in cold countries the
capture efficiency of the dry scrubber is better
(up to 98%).
For this reason new technologies based on
oxidation reaction are used in order to improve
light PAHs treatment efficiency.
rtO – an effective method
of destroying PAhs
The Regenerative Thermal Oxidation (RTO)
is based on the oxidation principle. PAHs are
mainly composed by C-H bonds which are
easily cracked by oxidation according to the
RedOx reaction (combustion reaction):
C a H 2b + (a+b/2) O 2 → aCO 2 + bH 2 O + Heat
All PAHs are converted into carbon dioxide
and water vapour.
The RTO is a compact equipment composed
of three main elements:
• The combustion chamber where the oxidation
reaction occurs. The temperature in the
combustion chamber is about 850-900 °C.
• The ceramics beds, which are used as heat
exchangers. The aim is to store heat and to
recover if for preheating the inlet gas flow, and
thus save energy.
• The valve box: two valves for inlet and
outlet ducts, and one valve for purge for each
chamber.
The raw gas (1) is introduced via the valve
Fig. 2: RTO – schematic diagram
box into the tower B, and pre-heated passing
through the ceramics (5) to achieve the
required temperature for the combustion of
PAH (at least 850 °C). The residence time in
the combustion chamber (6) is about 1-2 sec,
this time will do to destroy PAH. The combustion
of PAH generates heat which reduces
the consumption of the burner (7). The clean
gas is conveyed from the combustion chamber
into the second heat exchanger A, and is then
released through the exit stack (9). The third
heat exchanger C is in purge mode in order to
remove the remaining raw gas residues (Fig. 2).
eolios at Qatalum paste plant –
an innovative solution for
optimum emission performances
At the Qatalum paste plant, the highly concentrated
fumes, which are collected from the
paste cooler and which contain mainly water
vapour and light PAH fractions, are specifically
treated by the RTO unit. To avoid condensation
of tar in the duct, it is necessary to
heat up the fumes with hot air in a mixing
pot, situated on the paste cooler. After mixing,
diluted fumes enter into the RTO unit at a temperature
of 150 °C. After the RTO treatment,
the clean gas is rejected to the atmosphere.
All others suction points (mixer, vibrocompactor
area, etc.) are treated by the dry scrubbing
system (Fig. 3).
Paste mixer
Paste cooler
Calcined
petroleum coke +
liquid pitch
+ Water
A set of FID (Flame Ionisation Detector)
measurements were performed in order to
estimate the relationship between RTO temperature
and destruction efficiency. The gas
sample is continuously sampled into the FID
which measures the concentration of total
volatiles organic compounds.
The FID measurements done at the Qatalum
plant show that higher temperature combustion
creates an improved RTO efficiency
due to the increase of organic compounds
destruction:
RTO combustion
temperature
RTO efficiency
840 °C 91.2%
870 °C 96.2%
The PAH sampling and analysis, done in the
common stack, confirm the high efficiency of
the optimised combination of a dry scrubbing
system and RTO with a level of emissions
never reached before for such installation,
and which is now considered as a new benchmark
in the Gulf at 1 mg/Nm 3 of PAH 16 according
to the NS 9815 Norwegian Standard.
Environmental emissions are becoming
more and more stringent, especially for carcinogen
substances such as PAHs from pitch
vapour. Eolios, which combines dry scrubbing
and RTO, allows to operate the green anode
plant while maintaining the PAH’s emissions
at stack below such stringent requirements.
Eolios leads to lower operating costs and
a smaller carbon footprint than alternative
technologies such as full RTO, for instance.
The first industrial implementations of
Eolios at the Moesjen paste plant and recently
at the Qatalum paste plant confirm the performance
and benefits associated with this
technology.
■
RTO
Stack
Coke Fines Injection
Dry scubbing system
Others suctions point
(anode forming area
mixer....)
Fig. 3: Eolios system at the Qatalum paste plant
ALUMINIUM · 9/2013 55