Note: Descriptions are shown in the official language in which they were submitted.
CA 02245322 1998-08-19
Procedure for Process Management in an Anode Furnace
The present invention relates to a procedure for process management in an
anode furnace,
from which an adjustable flow of flue gas is directed into at least one
combustion space.
In anode furnaces of this type, the gas flow is directed through a fire duct
past anodes held
under the exclusion of air and is thereby heated by the combusted anodes. In
the form of pre-
heated combustion air, the flow of gas is then directed through the fire zone
and heats up the
cold anodes as flue gas. In accordance with the annular furnace principle, the
fire zone moves
around the furnace which is made up of chambers arranged in a ring structure,
whereas the
anodes are held stationary. As such, the furnace may consist of individual
large chambers for
the anodes and have a single flow of flue gas, or it may be made up of a range
of adjacent
anode and fire ducts.
Such furnaces are used for the purpose of burning large quantities of carbon
anodes which are
used in the electrolysis process for manufacturing aluminium through the
reduction of
alumina.
The carbon anodes consist of petroleum coke and pitch which are subjected to a
heat-
treatment process in the furnace under the exclusion of oxygen. The procedure
involved in the
heat treatment process is such that the anode is initially heated up to about
1000 °C with a
defined temperature/time gradient and then cooled down according to the
sintering process.
The heat treatment process is undertaken in open or covered annular furnaces
which consists
of anode combustion spaces into which the uncombusted anodes are loaded and
sealed against
the ingress of oxygen by means of anthracite. The heat treatment process for
the anodes must
take place in a defined sequence in order to make it possible to determine the
quality of the
combusted anodes in advance. To this end, moving "fires" are arranged on the
furnace, these
fires consisting of mobile flue gas extraction, measurement, burner and
cooling ramps.
CA 02245322 1998-08-19
2
A predefined, time-dependent set point characteristic on the burner ramps and
the flue gas
extraction ramps is used for achieving a specified temperature/time
characteristic in the anode
ducts arranged between the fire ducts. Whilst the heat thereby created is
directed over the
anodes at differing points, a specified temperature/time characteristic is
established in the fire
ducts arranged between the anode ducts.
The temperature/time characteristic is set during the heating-up period by
means of a
controlled flow of flue gas drawn out of the fire zone. At the same time, the
temperature/time
characteristic can be set in the fire zone by controlling the supply of fuel.
The
temperature/time characteristic is regulated in the cooling zone by blowing in
cold air.
Starting from the temperature prevailing at the anode during the combustion
process, the
quality of the anode is determined by the temperature/time area in the
sintering range, which
is also referred to as the combustion index. A specific temperature/time
characteristic is
required in the fire ducts in order to achieve a defined combustion index. In
the plants which
are in service, a temperature program of this type is specified as a set point
and the plant
controllers follow this program as the set point as precisely as possible.
Heating of the anode causes the pitch which was used as a binding agent to
escape in the form
of a combustible gas. The consistency for the anodes is limited by having the
same quantity
of flue gas flowing through several anode ducts arranged in the flue gas flow
direction during
the heating-up phase. The gas evolution speed depends on the temperature at
the anode and
takes place over quite a large temperature interval. This means several
chambers in a row are
involved in the gas evolution. However, in certain cases this can lead to
uncontrollable
temperature fluctuations in the fire ducts, or alternatively if the
temperature is below the
ignition temperature, it can be expected that uncombusted hydrocarbons will be
emitted.
Furthermore, the status of the individual fire ducts worsens continuously over
time due to the
thermal stress, in other words the continuous heating up and cooling down.
This invariably
leads to an increase in flow resistance in the fire ducts, so the proportion
of infiltrated air
flowing in increases. This additional amount of air also influences the anode
temperatures
CA 02245322 1998-08-19
which have to be achieved, with the result that the desired temperature
characteristic is not
attained.
In addition, the burner bridges are switched off for a greater or lesser
amount of time during
the conversion, or situations arise in which not enough oxygen is present in
the fire duct so the
fuel is not completely combusted and consequently the set point temperatures
are not
achieved.
All these disruptions to the combustion process have the effect of making it
impossible to
prevent fluctuations in the combustion index, resulting in different levels of
quality in
individual anodes.
For anodes of the same composition, the quality of the combusted anode is
largely dependent
on the combustion temperature and combustion duration which were achieved. The
higher the
temperature and the longer the period during which it is maintained, the
better the quality.
However, a high combustion temperature and a long duration lead to increased
combustion
costs and the service life of the fire ducts is reduced as a result of the
higher thermal stresses.
Furthermore, during the manufacture of anodes, it is also necessary to bear in
mind that the
electrolysis process for obtaining aluminium is dependent on which anode
displays the
shortest service life during the electrolysis, because the shortest service
life of an anode
determines the service life of the electrolysis cells. Consequently, during
the manufacture of
anodes, care must be taken to ensure that all anodes which are to be used in
an electrolysis
process should have as even a level of quality as possible. This is why the
most similar
possible combustion indices should be achieved during different combustion
processes.
The purpose of the present invention is to provide a procedure for combustion
process
management which guarantees an optimum combustion process to the extent that
the
combusted anodes display as consistently even a level of quality as possible
in order to
minimise the combustion costs and the costs of electrolysis by having the same
anode quality.
CA 02245322 1998-08-19
4
Beyond the scope of state-of the-art measurements of flue gas temperatures and
setting the
burner and flue gas volumes in accordance with the required temperature/time
characteristic,
this problem is solved by means of the following process steps:
~ measuring the flue gas temperature in each combustion space as the ACTUAL
temperature,
~ registering the oxygen content or the hydrocarbons,
~ setting the power of the burners in accordance with the fire duct
temperature and/or the
available oxygen content in the fire duct,
~ comparing the ACTUAL temperature in the combustion space with a specified
set point
temperature and/or
~ determining the amount of oxygen available in the fire duct by direct
measurement or by
means of an empirical model involving the supplied quantity of fuel and the
equivalent
quantity of fuel from the pitch gas evolution as well as the flow of flue gas,
~ determining a deviation index as a time integral of the temperature
deviation,
~ continuously modifying the set point characteristic in order to zero the
deviation integral at
the end of the process management and/or
~ determining a combustion index or a deviation integral in each anode duct
and
~ controlling the ACTUAL temperature in order to zero the deviation integral
at the end of
the process management.
Further advantageous configurations of the invention derive from the
subordinate claims.
The procedure in accordance with the present invention specifies two stages
with which the
consistency of the anodes and thereby the combustion index can be improved and
the energy
consumption for the combustion process can be reduced.
In the first stage of the procedure, it is assumed that the combustion index
is directly
dependent on the fluctuations in the fire duct temperature. The important
factor here is not a
specific temperature characteristic, but rather the equivalent temperature
characteristic,
namely the one which guarantees the flow of heat to the anodes. As a first
degree of
approximation, it is possible to assume that the conduction of heat remains
approximately
constant during a limited temperature interval. This means it is possible to
accept a deviation
CA 02245322 1998-08-19
from the rated curve, in other words away from the set point temperature
characteristic,
provided the time integral of the deviation is returned to zero at the end of
the combustion
process by an opposing temperature deviation of corresponding duration and
magnitude away
from the set point.
5
This method provides a means of measuring the lack of or surplus heat flow to
the anode
within the combustion space, so a deviation from the set point temperature can
be deliberately
allowed to arise. For example, if an attempt is made to maintain a specified
temperature
characteristic during the heating-up phase, the volumetric flow of the flue
gas must be reduced
when the evolved pitch gas is combusted. This combustion situation leads to a
lack of oxygen
in the fire duct, resulting in an inadequate combustion temperature and the
liberation of
uncombusted hydrocarbons.
In accordance with the present invention, this undesirable situation is
avoided by increasing
the flow of flue gas before this state of affairs arises, thereby achieving
the ignition
temperature and thus providing enough oxygen for combustion. It has also
proved
advantageous to permit the excess temperature during the heating-up phase and
not to regulate
it, thereby allowing the temperature to set itself to an appropriate value
independently. This
ensures that combustion is complete and there are no harmful emissions of
uncombusted
hydrocarbons.
As part of this procedure, the deviation index from the known set point
temperature is to be
measured and compensated for during the continuation of the process management
in order to
guarantee an even consistency in all combusted anodes. The fire duct
temperature is reduced
by supplying smaller quantities of flue gas and/or fuel during the
continuation of the
combustion process.
A further combustion situation arises if there is insufficient oxygen in a
fire duct because the
burners do not have their own air supply. Instead, the air required for
combustion which is
supplied to the burners is the pre-heated air from the cooling zone which is
drawn into the fire
zone. However, this means more fuel has to be used in this combustion
situation in order to
CA 02245322 1998-08-19
6
achieve the set point temperature. Nevertheless, it is also possible that this
set point is not
achieved despite the fact that the burner is operated at full power. Instead,
the fuel leaves the
fire duct without being combusted, thereby leading to the liberation of
harmful hydrocarbons.
Assuming an adequate supply of oxygen, a specific quantity of fuel is required
within a
specific temperature interval in order to achieve a specified temperature
gradient, and
consequently it is possible to draw conclusions about the efficiency of
combustion by
comparing these measurable values, in particular with regard to the necessary
burner power.
If a burner then falls below a lower limit of efficiency, the procedure in
accordance with the
present invention dictates that the quantity of fuel is reduced until the
available oxygen is
completely combusted at that burner power level, thereby preventing the lower
efficiency
threshold of the combustion process from being violated. Attainment of the set
point
temperature is thus consciously dispensed with since this combustion situation
only arises in
the heating-up phase and can be corrected during the continuation of the
combustion process
in accordance with the present invention.
The measurements of oxygen content and/or hydrocarbons as volatile
constituents of the flow
of flue gas demand a considerable complexity of apparatus, so consequently
these values are
estimated in the procedure in accordance with the present invention without
additional
measuring sensors being required.
To this end, the burner power levels and the position of the flue gas slide
valves are defined as
manipulated variables in the control system. In addition to these, the low
pressure level in the
combustion channel must be measured, with the result that these values permit
a simple and
easy method of registering parameters which can be used for estimating the
total fuel quantity
and the volumetric flow of the flue gas with the required degree of accuracy.
The parameters
obtained in this manner must be brought into relationship with one another so
that these
normalised values permit conclusions to be drawn relating to the fuel load in
the fire duct as a
measure of the free oxygen content.
CA 02245322 1998-08-19
7
In order to estimate the volatile constituents in the flow of flue gas, it is
necessary to include
the combustible constituents of the anode, since these constituents lead to an
increase in
temperature in the gas evolution zone, the gradient of which represents a
measure of the fuel
quantity. The volatile constituents present in the form of a quantity of fuel
in the fire duct are
thus made up of hydrocarbons, sulphur gases and the combustible constituents
of the anode.
This empirical data makes it possible to calculate a quantity of fuel which is
equivalent to the
quantity of fuel actually supplied to the burners. Thus, without requiring
additional
measuring instruments, it is possible to determine the quality of fuel present
in the entire fire
duct, and consequently the available oxygen as well.
The conventional furnace management system involves a rise in temperature
followed by a
period during which this temperature is maintained. The thermal inertia of the
anode ducts
means the anode duct wall only reaches the fire duct temperature after a
considerable time lag.
The present invention makes beneficial use of this thermal inertia to the
extent that it is
possible to achieve a dynamic optimisation of the temperature characteristic,
because the
temperature of the first holding phase is raised above the set point
temperature without
leading to overheating and thus irreparable damage to the anode duct wall. The
temperature
can then be gradually lowered in the second and following holding phases or it
can be
maintained in the region of the set point temperature, with the effect that
the deviation integral
of the fire duct temperature or the anode temperature between the actual
temperature and the
set point temperature is reduced to zero during the management of the process.
Consequently, given that the heat transfer is the same, this first stage of
the procedure
guarantees that the heat flow to the anode is the same at the end of each
combustion process,
even if each combustion process was subject to different interim deviations.
Although the aforementioned steps in the procedure improve the consistency of
the anodes
and therefore their quality as well, there is nevertheless no guarantee that
the correct
combustion index is actually achieved on each anode.
CA 02245322 1998-08-19
8
The temperature at the anode is registered by means of the second stage of the
procedure in
accordance with the present invention, because the heat transfer alters as a
function of the
wear on the flameproof material during the combustion process.
In order to keep the wear on anode thermocouples as low as possible, the
second stage of the
procedure can be supplemented to the effect that a dynamic adaptive model of
the thermal
behaviour is created. For this purpose, the basic structure is assumed to be
that obtained from
the identification of the combustion process, namely by means of measuring the
temperatures
in the two fire ducts between which the anode duct is arranged, as well as by
measuring the
anode temperature. The parameters are automatically adapted by the fact that
all anode ducts
are equipped with thermocouples up to a temperature of approx. 500 °C.
This time interval is
sufFcient to adapt the basic model to the specific dynamics of heat transfer
in an anode duct
by means of the measured temperatures. The anode thermocouples can be removed
following
identification at low temperature. The further anode temperatures are then
obtained from a
model calculation.
Once these values have been obtained, it is possible to calculate a combustion
index and/or
the deviation index for each anode duct. Once the dynamic adaptive model for
heat transfer to
the anode is available, it is possible to modify the set point characteristic
of the fire duct
temperature so as to achieve the optimum combustion index at the end of the
combustion
process.
Since the fire and anode ducts are arranged in an alternating succession, each
anode duct is
located between two fire ducts which means it is heated from two sides.
Equally, each fire
duct is also supplied by two anode ducts. Consequently, an iterative
optimisation method must
be employed in order to find the specific temperature for each fire duct which
guarantees the
lowest maximum temperature. Since both outer fire ducts only supply one anode
duct each, it
is possible to solve the optimisation problem.
CA 02245322 1998-08-19
9
The present invention has been described with regard to preferred embodiments.
However,
it will be obvious to persons skilled in the art that a number of variations
and modifications
can be made without departing from the scope of the invention as described
herein.
