[0001] The invention relates to a method to control a process for heating or melting a metal,
in particular aluminium, comprising heating said metal in a fuel-fired furnace wherein
a fuel is combusted with an oxygen containing gas, and measuring the concentrations
of carbon dioxide and oxygen in the furnace atmosphere.
[0002] The invention relates to the field of heating or melting of metals in fuel-fired
furnaces. Liquid or gaseous hydrocarbon containing fuels may be used. The heating
or melting process is carried out in rotary or reverberatory furnaces. The process
may be continuous or a batch process. The material to be melted, for example scrap
or ingots, is charged through large doors into the furnace. Typically a furnace is
charged two or more times during a process cycle.
[0003] During melting of aluminium, metal losses occur essentially due to the following
phenomena: A part of the losses originates from direct oxidation of the metal with
the furnace atmosphere. A second part of the metal losses comes from metal that is
entrapped between the metal oxides formed through direct oxidation.
[0004] The oxidation of aluminium is temperature dependent. The rate of oxidation increases
with increasing temperature, especially at temperatures above 780 °C the oxidation
increases rapidly.
[0005] The heat introduced into the melting furnace by burners is not uniformly distributed
over the whole metal surface and thus local overheating may occur. Such local overheating
leads to a local increase in the metal oxidation. Local overheating is more likely
to occur in reverberatory furnaces owing to the heat transfer characteristics of reverberatory
furnaces. But the problem of hot spots also exists in rotary furnaces.
[0006] There are various means used in industry to minimize the metal losses:
For example, in rotary furnaces the temperature of the metal charge and of the metal
melt is homogenized by rotation of the furnace in order to avoid overheating. In reverberatory
furnaces mechanical or electromagnetic stirrers are installed to get a more uniform
heat distribution within the furnace.
[0007] Another example for reverberatory furnaces is to optimise the timing for removal
of the dross layer on the metal melt by skimming. The dross layer comprises aluminium
oxide which has a high melting point. The dross layer will not melt further, but functions
as a heat insulator. If it is allowed to grow too much, it will insulate the metal
melt from the burner flame. The dross will be more heated and more metal will be oxidized.
[0008] Various attempts have been made to find control parameters to monitor how the oxidation
of the metal proceeds without opening the furnace door. Such a parameter would allow
to determine the correct time for skimming the dross, for stirring the metal melt
or for changing the burner power.
[0009] For reverberatory furnaces it is state of the art to measure the temperature in the
furnace or the temperature of the flue gases so that the burner power can be reduced
or other measures can be taken when a critical temperature is reached. Such measuring
devices are mainly used to protect the refractory, but they do not indicate the formation
of local hot spots where oxidation takes place.
[0010] An alternative is to submerge a thermocouple into the melted metal. However, this
is only a local indication and it does not give any information as to hot spots on
other locations. Monitoring of the temperature is thus not a sufficient means to monit
or how the metal oxidation proceeds.
[0011] In
US 2005/0103159 A1 it is suggested that in an aluminium melting process the measurement of the temperature
of the flue gases in combination with the determination of the concentration of carbon
monoxide or hydrogen in the flue gases gives an indication of the formation of aluminium
oxides.
[0012] However, there are disadvantages with that method. In a melting furnace carbon monoxide
and hydrogen are formed in an uncontrolled manner by gasification of organic contaminations
on the metal charge. Also, carbon monoxide and hydrogen are oxidized by air leaking
into the furnace. This makes the interpretation of the carbon monoxide concentration
uncertain.
[0013] Further, an industrial furnace can never be perfectly sealed. Thus, a significant
amount of air is always leaking into a furnace causing an excess of oxygen which may
oxidize any carbon monoxide or hydrogen formed in the furnace. The oxygen from the
leak air may also oxidize metal. This makes the use of the carbon monoxide con centration
even more uncertain.
[0014] The authors of
US 2005/0103159 A1 consider various interactions between the components of the furnace atmosphere and
the metal charge. The various modes of heat transfer do not lend themselves to an
easy solution by modeli ng, but a learning procedure, for example based on neural
networks, is proposed.
[0015] Thus, it is an object of the invention to provide a more simple method for monitoring
the metal oxidation in melting furnaces, in particular in aluminium melting furnaces.
[0016] This object is achieved by a method to control a process for heating or melting a
metal, in particular aluminium, comprising:
- heating said metal in a fuel-fired furnace wherein a fuel is combusted with an oxygen
containing gas,
- measuring the concentrations of carbon dioxide and oxygen in the furnace atmosphere,
wherein
- the theoretical concentration of oxygen in the furnace atmosphere is calculated on
the basis of said concentration of carbon dioxide,
- the difference between said theoretical concentration of oxygen and said detected
concentration of oxygen is determined and
- said process is controlled depending on said difference.
[0017] When oxygen and methane are combusted the following reaction will take place:
CH
4 + O
2 → 2 H
2O + CO
2
[0018] If methane and oxygen react at stochiometric amounts in an absolute tight furnace
without any leak air, the flue gases will contain only water and carbon dioxide. Thus
only CO
2 and H
2O would be analysed and the oxygen concentration would be 0 %.
[0019] This is an ideal situation. In an industrial furnace there is always air leaking
into the furnace diluting the furnace atmosphere and thus the CO
2 and H
2O concentration will be lower. On the other hand, since leak air contains 21 % oxygen
the oxygen content in the flue gases will be higher.
[0020] The concentrations of oxygen and carbon dioxide in the atmosphere of a combustion
process in an inert furnace where a hydrocarbon fuel is combusted with oxygen and
wherein leak air may introduce into the furnace, can be described by a straight line.
In case of a stochiometric combustion and that an analysing equipment analysing dry
gases is used this relation is given by
- (1)
%O2 = k*%CO2 + m
wherein
- %O2
- = the oxygen content in the furnace atmosphere
- %CO2
- = the carbon dioxide content in the furnace atmosphere
- k
- = constant, depending on the composition of the fuel and on the CO2 content in the furnace atmosphere without any leak air (in the case of combustion
of CH4 with pure O2: k = -0,21
- m
- = the oxygen content of the leak air, that is m = 0,21
[0021] For a man skilled in the art it is clear that what has been explained by using the
combustion of methane with oxygen as an example can be easily transferred to the use
of other fuels and other oxygen containing gas mixtures. Deviation from stochiometric
combustion will shift the line. Analysing wet gases, for example by using a laser
will also shift the line.
[0022] According to the invention the CO
2 concentration in the furnace atmosphere is measured. Then equation (1) allows to
calculate the theoretical O
2 concentration in the furnace atmosphere.
[0023] If a reducing substance, such as a metal or an organic material, is added to the
furnace some oxygen will be consumed and the oxygen content in the furnace will be
reduced. The O
2 concentration in the furnace atmosphere is measured and compared to the theoretical
O
2 concentration. The difference between both values is an indicator for the amount
of metal or material that has been oxidized.
[0024] The O
2 concentration and the CO
2 concentration can be determined by direct measurement or detection of the respective
concentrations within the furnace or, according to a preferred embodiment, the O
2 concentration and the CO
2 concentration are measured in the flue gas stream. More preferred a sample is taken
from the flue gas stream and then analyzed in order to determine the O
2 concentration and the CO
2 concentration.
[0025] If the reducing substance is aluminium it will react with oxygen according to the
following reaction
(2)
4 Al + 3 O2 → 2 Al2O3
[0026] Carbon dioxide will also react with aluminium according to
(3)
3 CO2 + 2 Al → Al2O3 + 3 CO
and the reaction
(4)
3 H2O + 2 Al → 2 Al2O3 + 3 H2
will also occur.
[0027] By simultaneous analysis of the concentrations of carbon dioxide, carbon monoxide,
and hydrogen the inventors could show that reactions (3) and (4) can be neglected
compared to reaction (2). It can be concluded that in case aluminium is charged into
the furnace the amount of oxygen deviating from the oxygen concentration calculated
from equation (1) is mainly consumed by reaction (2). Thus, the amount of aluminium
oxidized is proportional to the deviation of the measured oxygen content from the
oxygen content given by equation (1).
[0028] In a preferred embodiment the invention utilizes this insight to control an aluminium
melting process. The content of oxygen in the furnace atmosphere is detected several
times and the relative amount of aluminium oxide is determined from the difference
between the detected oxygen concentration and the theoretical oxygen concentration.
This information is used to regulate and / or control the melting process, for example
by changing the burner power.
[0029] What has been described above with respect to the melting of aluminium can be generalized
to the heating and / or melting of other metals. The oxidation reactions (2), (3)
and (4) could then be formulated as:
(5)
Me + ½O2 → MeO
(6)
Me + CO2 → MeO + CO
(7)
Me + H2O → MeO + H2,
[0030] Reaction (5) would be dominating as long as free O
2 is present in the furnace atmosphere. For example the invention could be applicable
for the heating of steel or steel alloys..
[0031] According to the invention there is no need to detect the concentration of carbon
monoxide or hydrogen in the furnace atmosphere or in the flue gases or to measure
the flue gas temperature. Preferably the heating or melting process is controlled
without using the temperature of the flue gases or the temperature in the furnace.
It is further preferred that the control of the melting process is not based on carbon
monoxide measurements or on measurements of the hydrogen content in the furnace atmosphere
or in the flue gases. It is especially advantageous to base the control of the heating
or melting process on the difference between the theoretical O
2 concentration and the measured O
2 concentration, only.
[0032] In a preferred embodiment the oxygen concentration and the carbon dioxide concentration
are continuously detected. In the beginning when the temperature in the furnace is
low so that no metal is oxidized the measured oxygen concentration will be essentially
equal to the theoretical oxygen concentration calculated from the measured CO
2 concentration. With increasing temperature, at least at some local spots metal will
be oxidized.
[0033] Preferably said furnace is heated by one or more burners. Further it is preferred
to measure the amount of fuel supplied to the burner(s). If the fuel flow is measured
the absolute amount of CO
2, for example the mass of CO
2 in kg, can be calculated from the chemical reaction equation. Further, that information
allows to calculate the absolute amount of O
2 which has been consumed by oxidation of the metal in the furnace. That is, the absolute
difference, for example in kg, between the theoretical oxygen content and the measured
oxygen content can be given.
[0034] Preferably the amount of oxidized metal, for example aluminium, is calculated using
the absolute amount of oxygen consumed in that oxidation reaction and the formula
weight of the metal oxide, for example the formula weight of alu minium oxide Al
2O
3. As described above the metal may also be oxidized by H
2O and CO
2 but the inventors could show that the oxidation with oxygen is dominating in an industrial
furnace.
[0035] Preferably the oxygen and the carbon dioxide concentration are detected in the flue
gases. A flue gas analysis provides a direct information on the composition of the
atmosphere within the furnace. For practical reasons it is preferred to determine
the oxygen and the carbon dioxide content in the furnace atmosphere from a measurement
in the flue gas duct.
[0036] The measurement of the oxygen concentration can be carried out by any equipment for
analyzing oxygen. In a preferred embodiment a laser, especially a diode-laser, is
used to analyze the oxygen concentration.
[0037] When a deviation between the theoretical and the measured oxygen concentration is
monitored some metal must have been oxidized. According to the invention the so determined
metal oxidation rate is used to control the heating or melting process. In a preferred
embodiment the heating or melting process is controlled by changing the power of the
burner or of the burners which are used to heat the furnace and its charge.
[0038] According to another preferred embodiment the amount of oxygen supplied to the burner
is changed in order to influence the heating or melting process. For example, it may
be switched from oxygen burners to air burners or vice versa.
[0039] In a further embodiment several charges of metal are melted in said melting furnace
and for each charge the difference between the theoretical and the measured oxygen
concentration is determined. These data are then stored, for example in a computer
memory. By varying different process parameters or exchanging part of the furnace
equipment and recording new curves, these new curves can be compared to the stored
curves. The comparison of the new curves with the stored curves allows to further
optimize the heating or melting process. In addition these data can be used for training
of personnel operating the furnace.
[0040] According to another embodiment the invention is used to monitor the combustion of
organic contaminants on the metal charge. For example, if the metal charged into the
furnace is contaminated by organic matter, such as oil, lacquer, or plastics, the
se materials are evaporated and combusted and oxidized inside the furnace. This oxidation
will also create a difference between the calculated and the measured oxygen in the
flue gas or in the furnace atmosphere. The oxidation of the organic matter can then
be studied in the same way as the oxidation of the metal. When oxidation of organic
matter is detected, it can be controlled by adding excess oxygen to the furnace.
[0041] The evaporation of organic matter dominates at the beginning of the process at temperatures
below 500 °C, especially between 400 and 500 °C.The oxidation of the metal dominates
later in the process when the metal is at higher temperatures, especially above the
melting point of the metal. In case of aluminium the oxidation increases at temperatures
above the melting point at 660 °C and it may increase rapidly at temperatures above
about 780 °C. In case of iron, the oxidation starts to be significant above 900 °C.
Therefore, the invention shows either oxidation of organic matter or oxidation of
metal, but not the two at the same time. For the man skilled in the art it is obvious
at what part of the process it is of interest to study oxidation of organic matter
and at what part of the process it is of interest to study oxidation of the metal.
[0042] The invention has several advantages compared to the state of the art technology.
The inventive method provides a signal showing the oxidation rate of a metal, in particular
of aluminium, that is independent from the amount of leak air entering the furnace.
[0043] Thus the inventive method is more reliable than methods based on flue gas temperature
measurements or based on carbon monoxide measurements. The invention provides a method
which is very appropriate for industrial furnaces, in particular for rotary furnaces
and reverberatory furnaces used for heating or melting of metals. The user of the
invention will be able to have a better process control and hence will be able to
decrease the aluminium losses and to get a higher metal yield. Further, the inventive
method is easy to implement. The invention is in particular useful to control a process
for melting aluminium.
[0044] The invention as well as further details and preferred embodiments of the invention
are disclosed in the following description and illustrated in the accompanying drawings
in which the figures show:
- figure 1
- an aluminium melting furnace with the equipment to carry out the inventive control
method and
- figure 2
- the on-line flue gas analysis measured with the arrangement according to figure 1.
[0045] Figure 1 shows an aluminium melting furnace 1 of the rotary type. The aluminium melting
furnace 1 has been charged with aluminium scrap 2. Melting furnace 1 is heated with
an oxy-fuel burner 3 which can be supplied with fuel, oxygen and / or air. The amount
of fuel, oxygen and air provided to burner 3 is regulated by flow control valves 4
and can be measured by flow measurement means 5.
[0046] Burner 3 generates a burner flame 6 which heats the aluminium charge 2. The flue
gases 7 which are produced during the heating and melting of charge 2 leave the furnace
1 through a flue gas duct 8.
[0047] Flue gas duct 8 is provided with an oxygen analyzer 9 and a carbon dioxide analyzer
10. Oxygen analyzer 9 and CO
2 analyzer 10 provide signals 11, 12 which are proportional to the concentration of
oxygen and carbon dioxide in the flue gases 7. These signals are sent as input to
a process computer 13.
[0048] From the flow measurement means 5 process computer 13 further receives input signals
14, 15, 16 proportional to the measured flow of fuel, oxygen and air, respectively.
Any of the data 11, 12, 14, 15, 16 can be shown on a computer monitor 17. Computer
monitor 17 is also used to visualize the analysis of the data 11, 12, 14, 15, 16.
[0049] Process computer 17 calculates from the data 11, 12, 14,' 15, 16 a signal 1 8 which
is used to control the melting process by varying the flow of fuel, oxygen, and /
or air supplied to burner 3. These calculations are made on-line and can be shown
on computer monitor 17 in a real time graph.
[0050] By variations of the pressure within aluminium melting furnace 1 or by opening the
door of aluminium melting furnace 1 the amount of leak air entering the furnace 1
will change. CO
2 analyzer 10 continuously measures the CO
2 concentration in the flue gas stream 7. The measured values are sent to process computer
13 and are recorded. For example, every minute one measured value is recorded. By
using equation (1) process computer 13 calculates the theoretical oxygen concentration
for every measured CO
2 value. Thus, for every minute a measured CO
2 concentration and the corresponding theoretical O
2 concentration is recorded.
[0051] Oxygen analyzer 9 continuously measures the O
2 concentration in the flue gases. The measured values are also stored every minute
in the process computer 13.
[0052] If there is no oxidation in the furnace 1 the measured oxygen value and the theoretical
oxygen value should be equal. However, the furnace 1 contains aluminium and when this
aluminium starts to oxidize, the oxidation of aluminium will consume some of the oxygen
in the furnace atmosphere. The measured oxygen concentration will then be lower than
the theoretical oxygen concentration. The difference between both values is an indication
of aluminium oxidation. This difference is also calculated and stored in process computer
13.
[0053] With flow measurement means 5 the fuel flow to burner 3 is determined and stored
in the same process computer 13. Using the fuel flow data the difference between the
measured and the theoretical oxygen concentration can be calculated into mass units,
that is into kg oxygen. The amount of consumed oxygen in kg is also stored in the
same computer program 13.
[0054] Assuming that this amount of oxygen has reacted with aluminium to form aluminium
oxide the mass of aluminium that is oxidized can be calculated. These data are also
stored in process computer 13.
[0055] All these data - CO
2 concentration, measured and theoretical O
2 concentration, difference between measured and theoretical O
2 concentration, amount of oxidized aluminium - can be visualized during the melting
process by computer screen 17 (see figure 2). Computer screen 17 displays the measured
and calculated values as value versus time graphs. From the screen 17 the operator
can thus see how many aluminium is oxidized every minute and he can use this information
to optimize the melting process.
[0056] Figure 2 shows a typical graph recorded by process computer 13. At 14:50 a rapid
increase in aluminium oxidation is detected and this information is used to control
the melting process by changing the burner power.
[0057] The inventive method is independent of leak air into the furnace, since the influence
of leak air variations is compensated by repeating the calculation according to equation
(1) for every measurement - in the example above, every minute. Of course the data
can be calculated more or less frequent than every minute.
1. Method to control a process for heating or melting a metal (2), in particular aluminium,
comprising:
- heating said metal (2) in a fuel-fired furnace (1) wherein a fuel is combusted with
an oxygen containing gas,
- measuring (9, 10) the concentrations of carbon dioxide and oxygen in the furnace
atmosphere,
characterized in that
- the theoretical concentration of oxygen in the furnace atmosphere is calculated
on the basis of said concentration of carbon dioxide,
- the difference between said theoretical concentration of oxygen and said detected
concentration of oxygen is determined and
- said process is controlled depending on said difference.
2. Method according to claim 1, characterized in that said oxygen and said carbon dioxide concentration are continuously measured.
3. Method according to any of claims 1 or 2, characterized in that said furnace (1) is heated by one or more burners (3) and that the amount of fuel
supplied to the burner(s) (3) is measured.
4. Method according to claim 3, characterized in that the absolute value of said difference between said theoretical concentration of oxygen
and said measured concentration of oxygen is calculated based on said measured concentrations
of oxygen and carbon dioxide and on said measured amount of fuel.
5. Method according to claim 4, characterized in that the amount of oxidized metal is calculated.
6. Method according to any of claims 1 to 5, characterized in that said measured oxygen concentration and / or said measured carbon dioxide concentration
are detected by means of a laser.
7. Method according to any of claims 1 to 6, characterized in that said fuel is combusted with a gas containing more than 21 % oxygen, preferably more
than 50 % oxygen, preferably more than 90% oxygen.
8. Method according to any of claims 1 to 7, characterized in that said process is controlled by changing the power of said burner(s) (3).
9. Method according to any of claims 1 to 8, characterized in that several charges of metal (2) are melted in said furnace (1) and that for each charge
the difference between said theoretical concentration of oxygen and said measured
concentration of oxygen is determined.
10. Method according to claim 9, characterized in that for at least two different charges the respective differences between said theoretical
concentration of oxygen and said measured concentration of oxygen are compared.
11. Method according to any of claims 1 to 10, characterized in that sa id metal is contaminated with organic substances, that said organic substances
are at least partly oxidized with said oxygen and that the amount of oxidized substances
is monitored.