Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR MEASURING GASEOUS SPECIES BY DERIVATION
The present invention relates to a method and to a device for measuring the
quantity of chemical species contained in a high-temperature gas and
especially the
quantity of CO and/or CO2 contained in a gas output by a metal treatment
furnace, and
especially an electric arc furnace (EAF) or a basic oxygen furnace (BOF) or
converter.
The invention is aimed more particularly at providing a solution for the
continuous chemical analysis of the flue gases, collectively called off-gas,
from an
electric arc furnace, said off-gas being at high temperature (around 1800 C)
and laden
with dust (100 to 200 g/Nm3).
By continuously analyzing the off-gas of a furnace it is possible to obtain
information about the treatment process: material and energy balance, state of
the
chemical reactions inside the furnace, etc. The systems for analyzing the
composition of
the off-gas, especially that output by an electric arc furnace, must withstand
a
particularly hostile environment, firstly because of the high temperature of
the off-gas
(around 1800 C) and secondly because of the high dust concentration (100 to
200
g/Nm3), this dust also being very fine (down to 1 micron in size).
A first method, developed by the Applicant and known by the commercial name
ALARC AS (and described for example in U.S. Pat. No. 5,344,122) consists in
taking
samples of off-gas and analyzing these samples: a water-cooled sampling probe
is
placed in the gap that exists between the outlet of the furnace and the gas
exhaust duct
of the furnace so as to withdraw a sample and take it into a region where the
dilution
with ambient air is minimal. The sample thus has a composition representative
of the
chemical composition inside the furnace. The sample is filtered and then
conveyed via a
heated line (heated so as to prevent the temperature dropping below the dew
point of
water, and therefore preventing this water from condensing) to a dryer and
then to the
various analyzers used: infrared analyzers for measuring the carbon monoxide
and
carbon dioxide concentrations, thermal conductivity analyzers for measuring
the
hydrogen concentration, and electrochemical or paramagnetic cells for
measuring the
oxygen concentration.
However, such a system has a number of drawbacks: response time: to prevent
the filters and dryers from clogging up too rapidly, the withdrawal rate is
low. Since the
analyzers must be located in areas where the temperature conditions are stable
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(environmentally controlled box or room), the analyzers are often located
relatively far
from the sampling point, resulting in a large dead volume. Associated with a
low rate,
the response time of the analytical system is long, ranging from around 30
seconds to 3
minutes; and maintenance: with the large quantity of dust in the off-gas, the
filters are
rapidly saturated therewith. Likewise, inside the sampling probe, the mixture
withdrawn,
consisting of dust and locally condensed water, rapidly for-ms a sealed plug.
Unclogging
cycles are provided so as to unclog this orifice with compressed air or
nitrogen, but the
long-terni operation requires frequent maintenance (to change filters, to
clean or replace
the sampling probes, etc.) which, depending on the type of installation, is
restrictive to a
greater or lesser extent.
Another known method consists in using a coherent light beam emitted by a
laser source, and especially a diode laser whose wavelength can vary within a
certain
wavelength range (for example a TDL or tunable diode laser).
The measurement of the composition of a gas by spectroscopy, especially using
laser radiation, is based on the property of gas molecules to absorb radiation
at
characteristic wavelengths (defined by the absorption spectrum specific to
each
molecule of the gas).
U.S. Pat. No. 5,984,998 (or WO-A-99/26058) and CA-A-2158516 disclose a
laser radiation system for measuring the absorption spectrum of the off-gas in
the gap in
order to measure the CO and 02 concentrations of this off-gas. However,
certain
systems use a wavelength range lying in the middle of the wavelength range
corresponding to the infrared (also called the 'mid-infrared'). This has the
drawback of
requiring cryogenically cooled lasers -- apart from their high cost, these
instruments lack
flexibility and cannot be easily transported.
W0-A-01/33200 discloses a system for analyzing the off-gas using a TDL
operating in the wavelength range corresponding to the infrared near the
visible (called
the 'near-infrared') allowing measurements by laser absorption spectroscopy of
the
various constituents: CO, CO2, 02, H20, etc. One of the advantages of this
type of
instrument and method is that low-power diode lasers are used, which ennit
radiation in
wavelengths close to those intended in general for telecommunication, and
conveyed in
optical fibers, said fibers, tailored to such wavelengths, being available to
bring, without
appreciable loss, the radiation output by the diode laser right to the off-gas
duct or gap.
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The radiation then passes through the off-gas duct or gap, is partly absorbed
by the
molecules that it is desired to analyze, and is received by a receiver.
This particularly effective system does, however, under certain circumstances,
prove to be difficult to use when the off-gas to be analyzed has a high dust
content: for
example, it is very quickly observed that, during operation of an electric arc
furnace, the
light signal received by the receiver located at the gap becomes, after a few
minutes,
too low to be interpreted. Thus, application WO-A-01/033200 proposed placing a
screen
over at least part of the width of the off-gas duct, acting as a deflector and
preventing
the stream of dust-laden off-gas from attenuating the light radiation too
greatly. The
drawback of such a system is the insertion of a fitted part that is
permanently present in
the off-gas duct where the temperature is around 1500 C WO-A-02/090943
describes a
similar solution, which has the same drawbacks.
The problems inherent in a measurement based on a light beam emitted by a
diode laser passing through the off-gas duct at the gap of an electric fumace
may be
summarized thus: loss of signal: when the concentration of dust particles
becomes too
high, their scattering (the particles are approximately spherical, with a
dianneter of the
order of the wavelength of the laser) attenuates the transmitted intensity of
the laser,
and the recovered signal has an amplitude such that the signal/noise ratio is
too low for
this signal to be exploitable; species measured: in the near-infrared and at
temperatures
around 1500 C, flot ail the lines of the chemical species it is desired to
measure are
exploitable. This is because, in order to be able to determine a species
accurately,
without interference from another species, the absorption line that
characterizes this
species must be sufficiently separate from the characteristic lines of the
other chemical
species likely to be present in the off-gas. The variation in temperature
affects the
distribution and the intensity of the absorption peaks: the wavelengths used
at room
temperature for measuring a given gas can in general no longer be used at
other
temperatures. For example, for wavelengths in the near-infrared, the
absorption lines
characteristic of CO2 can no longer be measured accurately above about 200 C.
The
CO2 concentration therefore cannot be measured directly in the gap, where the
temperatures reach 1400 to 2000 C using laser radiation in the near-infrared.
In the
case of an oxygen concentration measurement for example, this problem is
aggravated
by the low emission power of commercially available diode lasers in the range
of
wavelengths in question: with a high dust content, the transmitted power is
too low to
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provide a reliable signal; and measurement accuracy: two phenomena upset the
accuracy of a direct measurement in the gap.
Firstly, the presence of dilution air, which is entrained by the hot gas via
this opening
and which cools said gas, while causing combustion of the carbon monoxide
leaving the
furnace. Knowing that the concentration measurement given by the diode laser
is the
absorption averaged over the path taken by the radiation, the composition of
the dilution
air and its effects have an effect on this calculation. The measurement is
therefore less
representative of the atmosphere in the furnace. Secondly, the temperature
conditions
also disturb the accuracy of the measurement: at high temperature, the water
absorption
lines are omnipresent and greatly confuse the measurement and increase the
uncertainty.
According to a first aspect, the invention aims to measure, in particular and
preferably, the CO and CO2 concentrations, and optionally the 02 and H20
concentrations, in the off-gas output from a furnace with a response time of
less than 10
seconds, usually around 5 seconds, making it possible in particular to control
the
furnace in real time by overcoming the aforernentioned drawbacks.
Another aspect of the invention relates to the blocking of the gas sampling
lines
due to dust in the off-gas, as explained above.
EP-A-0462898 teaches a method of taking a sample and analyzing it, using a
water-cooled sampling probe placed in the gas exhaust duct of the furnace so
as to
draw off a sample into a region where the dilution with air does flot corrupt
the
measurement. The sample thus has a composition representative of the chemical
composition inside the furnace. The sample is filtered and then conveyed via a
heated
line (heated so as to prevent the temperature falling below the dew point of
water) as far
as means for extracting this water vapor, and then to the analyzers. These are
those
commonly used, namely infrared analyzers for carbon oxides, thermal
conductivity
analyzers for hydrogen, and electrochemical or paramagnetic cells for oxygen.
The problems inherent in a sampling system followed by conventional analyzers
are the following: response time: to prevent the filters and dryers from
clogging up too
rapidly, the withdrawal rate is low.
Since the analyzers must be located in areas where the temperature conditions
are
stable (environmentally controlled box or room), the analysis bay is often
located
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relatively far from the sampling point, resulting in a large dead volume. With
the low rate,
the response time of the analytical system is considerable (between 30 seconds
and 3
minutes); and maintenance: with the large quantity of dust in the off-gas, the
filters are
relatively rapidly saturated therewith. Likewise, inside the sampling probe,
the mixture
withdrawn, consisting of dust and locally condensed water, rapidly forms a
plug that
blocks off the gas passage. Unclogging cycles are provided, by blowing
compressed air
or nitrogen, but the long-term operation requires frequent maintenance (to
change
filters, to clean or replace the sampling probes, etc.) which, depending on
the type of
installation, is restrictive to a greater or lesser extent.
The method according to the invention is characterized in that a portion of
the
gas to be analyzed is taken off, its temperature is lowered down to less than
300 C,
preferably down to a temperature of 200 C or below, so as to obtain a gas with
a
temperature between 300 C, preferably 200 C, and room temperature, and then at
least
the quantity of CO and/or CO2 present in this gas is measured by means of the
coherent
light signal that is emitted by a diode laser through said gas and recovered
upon
emerging from said gas.
The coherent light beam may be reflected in a known manner using a mirror and
sent back through the gas to be analyzed, or else recovered directly upon
emerging
from the gas.
It is conveyed via an optical fiber and/or converted directly into an
electrical
signal, in a manner known per se.
According to the invention it is thus possible to measure a single species,
whatever the species, but also several species and especially a species chosen
from
CO and/or CO2 and/or 02 and/or H20. It is also possible to measure the
temperature of
the gas in the gap directly using a diode laser by measuring the adsorption of
two Unes
of any one species within the range of wavelengths continually scanned within
the
wavelength range of the TDL, or else by using a temperature sensor, in a
manner
known per se, preferably with the aid of a diode laser emitting in the near-
infrared,
preferably including the 1581 nanometer wavelength.
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In accordance with one aspect of the present invention, there is provided a
method for measuring the quantity of chemical species contained in a dust-
laden high-
temperature gas from a gas output of a metal treatment furnace, the method
comprising
the steps of: (a) withdrawing a portion of the dust-laden high-temperature gas
from the
gas output of the metal treatment furnace; (b) adjusting the temperature of
the dust-
laden high-temperature gas to be within the range of 20 C and 300 C, inclusive
to form
a dust-laden cooled gas; and (c) measuring the quantity of chemical species
present in
the dust-laden cooled gas by means of a coherent light signal that is emitted
by a diode
laser through an optical path extending through said dust-laden cooled gas and
recovered upon emerging from said dust-laden cooled gas, wherein dust is
present in
the dust-laden high temperature gas and the dust-laden cooled gas in an amount
of 100
to 200 g/Nm3 and the optical path has a distance of from 5 to 50 cm.
According to another aspect of the invention, the aim of the latter is to
provide an
effective system for automatically unclogging the sampling probes for taking
dust-laden
gas samples, and especially one that is applicable to the system described in
the
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abovementioned patent application. Combined with a pneumatic unclogging device
is a
moving part that removes, during each unclogging operation, the dust that has
built up
in the probe. This type of unclogging operation gets round the problem of the
accretion
of dust and water that attaches to the walls of the probe and that is flot
removed by
blasting with compressed air. The maintenance operations carried out on the
probe are
therefore greatly reduced and sampling is available throughout the heat.
The essential part of these unclogging nneans consists of a rod with at least
two
fins that can be rotated, for example by rneans of an air cylinder, so as to
sweep
substantially the entire inner wall of the probe in which these fins move. The
rotation is
accompanied by a blast of compressed air (either at the same time or
afterwards) which
expels the dust accretions on the wall.
Preferably, in this unclogging system (in order to draw off the minimum amount
of dust while still taking a sample from a region representative of the
atmosphere in the
furnace), the end of the sampling probe will be beveled and the probe placed
so as to
draw off, preferably countercurrently, the flow of off-gas. The orifice via
which the gas is
conveyed is thus protected from being directly splashed, for example with
slag, thereby
preventing this end from becoming blocked.
More particularly, this other aspect of the invention relates to a system for
unclogging a probe of axial symmetry for taking samples from a gas stream
containing
impurities.
The system according to this aspect of the invention is characterized in that
it
comprises a part that can rnove about the axis of symmetry of the probe and
can
remove the impurities that have built up on the internai wall of said probe by
relative
rotation of the part and/or of the probe about the axis.
According to a preferred embodiment, this system is characterized in that it
includes additional pneumatic unclogging means using compressed air.
The invention will be more clearly understood with the aid of the following
exemplary embodiments, given by way of nonlimiting example, in conjunction
with the
figures which show:
FIG. 1, a schematic view of an electric furnace of the EAF type;
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FIG. 2, a schematic view of the method and device for implementing the
invention;
FIG. 3, a detailed view of the system for measurement in the off-gas, the
temperature of which has been lowered;
FIG. 4, a schematic view of the system for cleaning the optics;
FIG. 5, a detail of FIG. 1;
FIG. 6, a diagram showing the principle according to the invention of the
unclogging of the sampling probe; and
FIG. 7, a view of the sampling probe according to the invention.
FIG. 1 is a diagram of an electric arc furnace EAF 1 in the lower part of
which
lies the molten metal 2, near the electrodes 3 that are surrounded by an
atmosphere 4
of off-gas extracted via the duct 5. To allow the roof of the furnace to be
maneuvered in
various ways, the duct 5 is separated from the duct 7, which extends it, by a
gap 6
between the two. It is within this gap that the sampling system of FIG. 2 is
placed.
In FIG. 2, a gas sample is taken off from the duct 10 at the outlet of the
furnace,
from a gas stream representative of the atmosphere in the furnace, but flot
contaminated with dilution air, by means of a sampling probe 11 cooled by
water 12,
with a withdrawal rate higher than that of the withdrawal probes of the prior
art. The
probe 11 has a larger diameter and may optionally contain a mechanical
unclogging
system. The gas taken off by the probe 11 at a temperature of about 1500 C is
cooled
by flowing through the cooled probe 11 into the line 13 and into the chamber
14 on
either side of which the optical heads of the diode laser are attached. The
entire system
--probe 11, line 13 and chamber 14¨ has a geometry (diameter, length) that
depends
on the material used and on its capacity for heat exchange with the cooling
(water), in
such a way that the temperature of the off-gas, when it enters the chamber 14,
does flot
exceed 300 C, preferably 200 C. The distance between the emitting optic 22 and
the
receiving optic 23 is reduced to a few tens of centimeters (from 1 to 100 cm,
preferably
5 to 50 cm and ideally 10 to 15 cm, representing the diameter of the chamber
14). The
off-gas is withdrawn, for example, by a Venturi system 18 supplied with a
fluid,
preferably compressed air 19 de-oiled beforehand in order to prevent dust
accretion
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downstream of the blowing. The analyzed gas sample is discharged via the line
20 and
the pipe 21 into the duct 10.
The sannpling and analysis system described in the case of an electric arc
furnace can be applied to any furnace off-gas exhaust system (without being
limited to
an electric furnace).
FIG. 3 shows a detail of the chamber 14 of FIG. 2 and of the optics for the
diode
laser system used. The diode laser emitting coherent laser radiation is flot
shown in
FIG. 3: the radiation arrives via the optical fiber 30 at its end 31, which
sends the
radiation onto the lens 27, inside the sleeve 28 then to the inside 16 of the
chamber 14
and then inside the sleeve 28; the parallel beam 32 is focused by the lens 27
onto the
receiver 26 and the signal is sent into the fiber 25.
FIG. 4 is an exploded view of a system for cleaning the optics and of the
ducts
placed at the optics so as to ensure they are kept clean. A line for supplying
inert gas,
for example nitrogen, argon, helium or any species whose presence is
controlled and
therefore will flot disturb the measurement to be carried out, includes an
injection ami
44 for injecting the inert (or other) gas into the optic carried by the
support 45 and
through which the laser beam 41 passes, while another arm 43 prevents the
cylindrical
tube placed around the beam 41 to protect it from dust from being blocked.
This
cleaning system may if necessary be applied in the chamber 14, but also
directly in the
gap 6 (FIG. 1) or in the duct 10 (FIG. 2), in which case the measurement would
be
carried out directly in the gap according to the systems of the prior art,
with a distance
between the ends of the two tubes 45 on either side of the duct 10 (defining a
'free' path
for the laser beam in the dust-laden atmosphere of the duct 10) which must in
no case
be greater than 30 cm in order to ensure lasting operation of the system. The
cleaning
gas flow rate is generally constant during a heat and increased between heats
in order
to expel any dust.
The laser signal may either be conveyed near the furnace by means of an
optical
fiber, while the optical signal received by the optical sensor 23 after having
passed
through the off-gas is converted into an electrical signal by this sensor and
transmitted
via a coaxial cable to the central control unit, where it is reconverted into
an optical
signal and then transmitted via an optical fiber to the central control unit.
The optical
heads 22, 23, which are placed on either side of the analysis chamber, easily
withstand
the temperature differences, and the accumulation of dust and splashes. All
the
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ennission electronics (diode laser, etc.) and signal processing electronics
are placed at a
substantial distance (usually around 30 meters) from the furnace, without this
affecting
the response time.
If desired, it is also possible to produce the laser signal near the analysis
chamber. In this case, a protection device is needed (or even a cooled casing
so as to
get round the problem of temperature variations). The noise, which is
superimposed on
the diode laser signal and may be generated by the propagation of the signal,
is
eliminated, this being advantageous if it is desired to measure compositions
having low
concentrations of gaseous species.
Another advantage of the measurement system according to the invention is that
it is unnecessary to remove the moisture from the gas sample before taking the
measurement: it is therefore unnecessary, as in the systems of the prior art,
to use a
drying system. By reducing the optical path to a few tens of centimeters (1 to
100 cm,
preferably 5 to 50 cm and ideally 10 to 15 cm) it is possible to achieve
satisfactory signal
transmission despite a high dust concentration. Filters are therefore
unnecessary in the
path of the sampled gas and the dead volume is therefore reduced.
Another advantage of the invention is that it is possible to vary the gas
withdrawal rate from the off-gas duct. In conventional systems, too high a
withdrawal
rate saturates the filters and dryers. The use of a Venturi system and the
absence of
filters allow a higher withdrawal rate and therefore a shorter analysis
response time.
An essential advantage of the invention is that in particular it allows the
CO2
concentration of the off-gas output by an electric furnace to be measured.
According to
the invention, means (cooled probe, line length, chamber, etc.) are provided
that allow
the gas temperature to be lowered down to less than 300 C, preferably down to
200 C
or below. This allows the CO2 content to be measured in addition to that of
CO. Of
course, it is also possible at this temperature to nneasure the concentration
of other
species, such as CO, H20, 02 (and optionally the temperature of the gases,
which
would be of little interest here, given that it has been modified beforehand).
Preferably, the temperature of the gas in the analysis chamber is now only of
the
order of a hundred degrees (from around 20 C to about 200 C depending on the
withdrawal rate). The shorter optical path also allows diode lasers of lower
emitting
power to be used.
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The gas temperature is simply measured using a thermocouple. However, it is
possible, as mentioned above, to use the measurements made on at least two H20
lines
and to deduce the temperature therefrom by calculation (using an algorithm
known per
se). The temperature may thus be measured in real time, which allows the gas
composition measurement to be refined.
It is possible with the system of the invention to measure the 002, CO, H20
and
02 species simultaneously. The CO2 concentration is measured at a temperature
below
300 C, preferably between 20 C and 200 C, using an absorption line at a
wavelength
different from that used for measuring the CO concentration. However, these
two
wavelengths may be achieved by the same laser source, the wavelength of which
is
modulated (using a TDL whose tunable wavelength can vary substantially over a
wavelength range that is regularly scanned over the entire range thanks for
example to
a sawtooth contrai signal). The two wavelengths used are preferably located in
the
region of 1581 nnn. These two absorption peaks possess the property of being
relatively
separate and of sufficient amplitudes. A simultaneous measurement of the CO
and CO2
content of the composition using the same equipment is therefore possible. To
measure
the oxygen and water content would require different equipment, since the
wavelengths
are too far from the usable wavelengths for CO and CO2 (the scanned wavelength
range is limited).
The abovementioned wavelengths were chosen so as to limit the interference
between species according to the conventional composition of the off-gas in an
electric
arc furnace (in which CO (15-20% on average, peaks at more than 40%), CO2 (20-
25%
on average), H2 (10% on average), H20 (20% on average), N2 and 02 (variable
amounts
depending on the air intake) are present).
The following description of FIGS. 5, 6 and 7 relates more especially to the
aspect of the invention concerned with the unclogging of the sampling probe
101.
The probe 101 takes a gas sample 112 into a region where the decomposition is
representative of the atmosphere in the furnace. For example, in an electric
arc furnace,
the optimum region for taking the sample lies in the region called the gap
113, close to
the center of the gas stream 112, undiluted by the incoming air 114, 115
before the
bend 111 and before the cooled jacket 110. The combustible gases in the off-
gas are, at
this point, net yet burnt by the dilution air 114, 115.
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To withstand the high temperature (around 1600 C at least), the probe 101 is
water-cooled, by water flowing in the cavity 102 placed concentrically around
the region
106 through which the gases 112 in the probe 101 flow. Accretions of dust on
the
internai wall of the probe, which have to be removed, are shown at 103.
The moving mechanical part consists of a rod 105 fastened to which are one or
more fins 104. This part 104, 105 is rotated by an air cylinder 124 so that
the entire wall
of the probe is cleaned by the passage of the fins (which in the case shown in
FIG. 6
make a rotation of 180 about the axis 105). The fins are flot necessarily
continuous
over the entire length of the rod.
Compressed air is injected at 125 and 126 at the top of the probe after or
during
the rotation of the fins so as to expel the dust accretions such as 103 that
might adhere
to the fins 104. The unclogging cycle may be repeated several times (a half-
turn, or a
quarter-turn on one side more than on the other side in the present example).
The gases from the probe are taken off via the orifice 123. A purge of
compressed air or nitrogen may also be effected via this orifice. The cooling
water
circulates in the probe via the orifices 121 and 122.
The off-gas is taken off at 126 at the base of the probe (in FIG. 7) via the
beveled opening 120, preferably directed countercurrently with respect to the
gas 112.
