Proceedings
of the
Combustion
Institute
Proceedings of the Combustion Institute 30 (2005) 1611–1618
www.elsevier.com/locate/proci
Sensitive in situ detection of CO and O2
in a rotary kiln-based hazardous waste incinerator using
760 nm and new 2.3 lm diode lasers
V. Eberta,*, H. Teicherta, P. Straucha, T. Kolbb, H. Seifertb, J. Wolfruma
a
Physikalisch-Chemisches Institut, Universität Heidelberg, Im Neuenheimer Feld 253, D-69120 Heidelberg, Germany
b
Institut für Technische Chemie, Forschungszentrum Karlsruhe, D-76021 Karlsruhe, Germany
Abstract
CO and O2 are key combustion parameters closely linked to the process stoichiometry and most frequently determined by gas sampling. This limits time resolution and hinders the extraction of representative concentration data from inhomogeneous flue gas streams. Especially, batch-fired processes like
hazardous waste incineration in rotary kilns (RK) face fast and spatially confined CO fluctuations, making them difficult to optimize. To address this sensor deficiency, we used new 2.3 lm distributed-feedback
diode lasers (DFB-DL) accessing the CO-2m-band to develop fast, sensitive, and spatially integrating in
situ absorption spectrometers suitable for the harsh conditions in full-scale combustion processes. Spectrally multiplexing the 2.3 lm-DL with a 760 nm-DFB-DL for the O2-A-band we also developed a simultaneous in situ CO/O2 spectrometer, which is most interesting for control strategies requiring the coverage
of wide stoichiometry ranges. These new spectrometers were successfully tested for up to two weeks in a
3.5 MWth hazardous waste incinerator. The absorption path (l = 2.56 m, T = 800–1000 C) was located in
the post-combustion chamber right at the RK exit. Direct absorption spectroscopy enabled calibrationfree species detection. A new data acquisition system using a digital signal processor and voltage-controlled preamplifiers ensured 100% data throughput, enabled a real-time validity and transmission
evaluation of individual laser scans, and thus permitted a real-time transmission compensation and hence
an automatic dynamic range adaptation. This proved to be an effective method to avoid systematic errors
found in PC-based systems under these rapidly fluctuating combustion conditions. With 1 s acquisition
time we achieved for CO/O2 an optical resolution (1r) of 1.2 · 104/6 · 105 OD corresponding to detection limits of 6.5 ppm CO and 250 ppm O2. Sensitivity and time resolution of the spectrometer was high
enough to detect—even under fuel lean conditions—small periodic stoichiometry and CO changes caused
by the periodic fuel feed of the RK.
2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
Keywords: Absorption spectroscopy; In situ; Gas analysis; Diode laser; Combustion control
1. Introduction
*
Corresponding author. Fax: +49 6221 545050.
E-mail address: volker.ebert@pci.uni-heidelberg.de
(V. Ebert).
One of the most important indicators for combustion processes is the carbon monoxide level.
Aside from the health issues CO is an important
molecule for combustion control purposes. CO
1540-7489/$ - see front matter 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
doi:10.1016/j.proci.2004.08.224
1612
V. Ebert et al. / Proceedings of the Combustion Institute 30 (2005) 1611–1618
plays a central role in high temperature corrosion
and slagging of boilers in coal- or waste-fired
power plants, thereby limiting the plant life time.
Of most importance is its role as an stoichiometry
indicator, its concentration being related to the
combustion completeness and efficiency. Due to
the discontinuous feed of solid, viscous or liquid
fuels with highly variable calorific values batchfired processes, like hazardous waste incineration
in rotary kilns (RK), are plagued with large stoichiometry fluctuations, which generate strong
spatiotemporal CO gradients, that cannot be removed by flue gas cleaning. Hence, hazardous
waste incinerators (HWI) are often designed as
RK followed by a high-temperature post-combustion chamber (PCC).
Unfortunately, current gas sensing techniques
generally employed in industrial incinerators are
mostly based on non-dispersive infrared absorption sensors that cannot detect these CO fluctuations, as they work only extractively, relying on
gas samples taken from the flue gas duct. As a result, CO values are significantly delayed by the gas
transport to the sensor and distorted by the gas filters needed to cool the gas and to remove dust,
soot, and excessive water. In addition, the spatial
CO gradients within the flue gas cause large systematic errors since the gas samples are not representative of the average gas composition across
the duct. O2, the natural CO-antagonist in most
combustors, may also serve as stoichiometry indicator, especially for fuel-lean conditions, when
CO may become too low to be detectible. But,
as for CO, O2 is mainly detected via slow and error-prone extractive paramagnetic sensors or
ZrO2-sensors which only probe locally.
These severe limitations of industrial CO/O2
sensing techniques prevent a reliable stoichiometry detection, hinder the development of efficient
combustion control strategies, and prevent a minimization of the spatiotemporal CO fluctuations.
Consequently, waste incineration processes have
to be run with high safety margins, which significantly limit process throughput and minimize
costs.
Accordingly, there is a large interest in in situ
CO/O2-sensors with fast response, high sensitivity, and the ability to determine representative
concentrations. Industrial applications require
such devices to be robust and inexpensive. One
promising technique to fulfill these complex
requirements is in situ laser absorption spectroscopy [1]. Especially, near infrared diode lasers
(NIRDL) show great potential for industrial
applications, as they possess an otherwise unavailable combination of spectroscopic and technical
properties. NIRDLs, mainly developed for the
telecommunication industries, have a large commercial market and have been extensively optimized. The growing interest in NIRDL-based
gas sensors can be deduced from an increasing
number of publications over the last few years
[1–21].
NIRDLs are frequently used to study CO [7–
10] due to their overlap with the 3m CO overtone
band. While these lasers are readily available
and rather inexpensive, the weak intensity of the
3m-band allows only moderate detection sensitivities. Commercial room-temperature 2.3 lm diode
lasers for the stronger 2m-band were not available
until recently, when research grade lasers have
been realized as Fabry–Perot structures (FPDLs). Although 2.3 lm FP-DLs enabled first
CO measurements in absorption cells or small laboratory flames [11–13,17], they generated many
spectroscopic difficulties making them unsuitable
for industrial applications.
The recently developed, complex gain-coupled,
distributed-feedback diode laser (CGC-DFB-DL)
[22] may resolve this dilemma, as we show here.
Using this new feedback principle a purely single-mode 2.3 lm room-temperature DFB-DL
could be fabricated that provides excellent side
mode suppression ratio (SMSR), good tunability,
and sufficient optical output power [14].
In this paper, we demonstrate the first in situ
CO detection in a HWI using 2.3 lm DFB-DLs
which is highly interesting for combustion control
applications needing a representative, high-sensitivity, high-speed in situ CO sensor. Further, we
realized the first simultaneous in situ CO/O2-detection based on 2.3 and 0.76 lm-DFB-DLs,
which is very promising for combustion control
systems that need to cover wide stoichiometric
ranges.
2. Experimental details
2.1. Absorption spectroscopy
The measurement principle used in this paper
is known as ‘‘direct absorption spectroscopy’’
(DAS). This method, described in detail elsewhere
[1–7], permits absolute and calibration-free measurements of absorber concentrations. Thus,
DAS is well suited for in situ gas analysis in harsh
environments where calibration is difficult. The
trade-off for DAS is of lower sensitivity compared
to double modulation techniques like wavelength
modulation spectroscopy (WMS) [15] and frequency modulation spectroscopy (FMS) [8] or
dual beam techniques [16].
The basic setup is rather simple: a diode laser
beam is directed through the measurement volume
onto a photo detector. The laser is continuously
and repetitively scanned over the absorption line
by linear laser current modulation, while the
detector signal, including all offsets and disturbances, is digitized. Great care has to be taken
to correct the in situ signal for various strong disturbances created in the measurement volume:
V. Ebert et al. / Proceedings of the Combustion Institute 30 (2005) 1611–1618
these are caused by broadband absorption, scattering by particles or beam steering and create
strong fluctuations in the transmission Tr (t) of
the measurement path. Further, there may be
background radiation E (t) that adds to the laser
radiation. Assuming a homogeneous medium,
the resulting signal can be described by BeerÕs
law [1]:
IðkÞ ¼ I 0 ðkÞ exp½SðT Þ gðk k0 Þ N L
TrðtÞ þ EðtÞ;
ð1Þ
where I (k), respectively, I0 (k) are the detected and
the initial laser intensity. The absorption signal is
described by the temperature-dependent line
strength S (T), the absorber number density N,
the absorption path length L, and the normalized
line shape function g (k k0), which is centered at
wavelength k0 [1]. To compensate for rapid transmission and emission variations, we developed
techniques to effectively separate them from the
molecular absorption by taking advantage of the
diode laserÕs fast wavelength tunability [1–7]. By
tuning the laser much faster than the disturbances
they can be assumed to be constant during the
wavelength scan and corrected during the fitting
process by removing the offsets first and than
dividing the scan through the baseline function.
Phase locked averaging of the wavelength scans
is used for further noise reduction. Finally, to calculate the absorber number density the absorption
line area is extracted using a fitting algorithm.
2.2. CO and O2 spectroscopy
Diode laser-based CO detection has been performed (Fig. 1) on the fundamental band at
1613
4.5 lm [23] and on the 2m- (2.3 lm) [11–14,17]
and 3m-bands (1.5 lm) [7–10]. The strong 1m-lines
frequently used for basic research are only accessible via cryogenically cooled lead salt DL [18]
or sparsely available quantum cascade DL [23].
The 3m-band lasers combine low cost and excellent
availability with near perfect fiber-coupled DLpackages. However, the 3m-lines are more than
1000 times weaker than 1m and plagued with
strong interfering H2O lines. Further, the
1.56 lm lasers are designed to deliver a minimized
wavelength chirp, which is contradictory to the
gas sensor needs.
A good compromise between CO line strength
and H2O/CO2 interference is the 2m-band at
2.3 lm, which is about 60· stronger than the 3mband. Synthetic absorption spectra (Fig. 2) calculated using HITRAN96/HITEMP data [24,13]
reveal that some CO lines are virtually H2O/CO2
interference free. The R30 line at 2302 nm (Fig.
2) with 1782 cm1 ground state energy is well suited for higher temperatures, while the R18 line
(2315 nm) is good for lower temperatures and
simultaneous H2O detection. Diode laser-based
O2 sensing is only possible via the O2-A-band at
760 nm [3,16] using vertical cavity lasers or as in
this paper via non-telecom DFB-DLs.
2.3. 2.3 lm DFB diode lasers
DFB-DLs unlike Fabry–Perot types are a near
perfect choice for gas phase spectroscopy, combining continuous tuning with spectral purity
and stability. However, their complicated and
expensive production largely prevents the development of devices outside the telecom wavelengths. Recently, the first commercial samples
Fig. 1. HITRAN line strength data of the CO-2n- and 3n-bands, the O2-A-band and the interferences CO2 and H2O.
1614
V. Ebert et al. / Proceedings of the Combustion Institute 30 (2005) 1611–1618
Fig. 2. Simulated CO absorption spectra for a typical
gas composition of a combustion process.
Laser current tuning is mainly caused by a
temperature change by current-induced ohmic
heating. However, for kHz modulation frequencies, the temperature response is successively delayed by the thermal mass of the laser module,
so that the dynamic current tuning, dm/dI(t), becomes increasingly damped and non-linear. This
leads to a severe reduction of the scan depth
and a significant asymmetry of the absorption
line in time space, which furthermore depends
on the position of the absorption line within the
scan. As higher modulation frequencies are
needed for an effective disturbance suppression,
there is a trade-off necessary between suppression
effectiveness and scan width reduction, i.e., a
complete coverage of pressure broadened absorption lines. Due to the importance of precise
dynamic tuning data for calibration-free measurements and stability, we realized a high-resolution
measurement of dm/dI(t) as a function of the
modulation frequency from 500 Hz to 5 kHz
[14]. This range is most important for measurements under the harsh conditions in industrial
combustion systems. Compared to the static current tuning this laser showed a strong decrease in
the tuning coefficient of 50% even at rather low
frequencies of 500 Hz. At 5 kHz, the tuning span
was reduced by 66% of the static value and
showed a strong non-linearity across the scan
with about 30–40% variation between maximum
and minimum value of dm/dI(t).
of room-temperature 2.3 lm-GaSb-CGC DFBDLs could be fabricated. Based on new lateral
metal gratings [22], they eliminate crucial manufacturing steps and allow cost-effective piece-bypiece processing with precise wavelength tailoring
for each individual laser.
Our first spectroscopic characterization
showed that these lasers provide excellent spectral
purity (40 dB SMSR), good tunability, and sufficient optical output power (1 mW) at low driving
currents (45 mA). Using a gas cell (l = 75 cm,
79 vol.% CO in air at 1 bar, 293 K) and temperature tuning (DT = 18 K) the laser over five CO
absorption lines between 2300 and 2305.8 nm we
realized the first CO detection with a 2.3 lm
DFB-DL. After removal of the amplitude modulation, the resulting spectrum (Fig. 3) coincides
well with a HITRAN96 simulation. Temperature
tuning extracted from the line positions was very
linear with a quasi-static tuning coefficient of
0.423 cm1/K (0.224 nm/K). The static current
tuning coefficient determined with a 10 cm airspaced etalon yielded 2.45 · 102 nm/mA
(4.6 · 102 cm1/mA).
All spectrometers were tested at the 3.5 MWth
hazardous waste incinerator THERESA [25] at
the Forschungszentrum Karlsruhe (Fig. 4). THERESA is comprised of a rotary kiln (RK, 8.4 m
long, 1.4 m diameter) for solid, highly viscous
and liquid wastes heated by one 1.5 MWth burner,
followed by a post-combustion chamber (PCC,
Fig. 3. Measured cell absorption spectrum (black),
HITRAN simulation (grey) of 79% CO in air recorded
by laser temperature tuning. Quasi-static temperature
tuning of the 2.3 lm DFB-DL (line with circles).
Fig. 4. Schematic of the hazardous waste incinerator
THERESA comprised of a RK and a post-combustion
chamber. The CO/O2 measurement location (black
arrow) is located in the PCC right at the kiln exit.
2.4. Process and spectrometer setup
V. Ebert et al. / Proceedings of the Combustion Institute 30 (2005) 1611–1618
1.9–2.5 m diameter, 12 m height) that is equipped
with two 1 MWth burners.
The following setup was used to realize in situ
CO/O2 measurements in this highly dynamic
batch-fired combustion process: the measurement
path of the spectrometer was 2.5 m long and located right at the RK exit in the lower part of the
PCC. This location minimized signal delays while
simultaneously avoiding complications due to the
temperature gradients within the RK. Thermal
expansion movements of the furnace were compensated for by mounting the spectrometer optics
directly onto the furnace walls (Fig. 5). Optical access to the PCC was possible via flanges attached
to steel pipes (l = 80 cm, /inside = 100 mm) to cross
the furnace insulation. The collimated laser beam
was directed into the PCC through Brewster windows (/ = 50 mm), which were purged with intentionally ‘‘leaky’’ window gaskets, taking
advantage of the reduced process pressure. This
ensured sufficient window transmission over the
two-week measurement period. The transmitted
light was collected with a 3 in. spherical goldcoated mirror, filtered with dielectric filters
(FWHM = 10 nm) to remove background radiation, and softly focused onto an InGaAs detector
(/ = 1 mm, cut-off 2.6 lm). Analogous to our previous studies on multi-species detection [3], we
spectrally multiplexed a 760 nm DFB-DL with
the 2.3 lm DFB-DL to enable the first simultaneous in situ 2m-CO/O2 detection. The two-color
laser beam was separated after passage of the measurement volume exploiting the CO filters high
reflectivity for 760 nm radiation and directed on
silicon and InGaAs detectors.
An automatic alignment system [3,7] based on
a motor driven #–u-mirror mount and a line-byline search algorithm was used for the initial
spectrometer setup and to compensate for load
dependent thermal distortions of the combustion
chamber. Even under difficult conditions we obtained perfect alignment within a few minutes.
Fig. 5. Optical setup of the in situ CO-spectrometer
based on the 2.3 lm DFB-DL.
1615
Excellent noise immunity was ensured by isolating
the weak modulated laser radiation from the luminous background using phase-sensitive detection.
A combined laser/Peltier-element driver stabilized laser current and temperature. A digital signal generator modulated the laser current with a
triangular 1 kHz signal. The photodiode signals
were preamplified, digitized by fast ADCs, averaged and finally stored for offline evaluation. To
extract the absorption line area and simultaneously correct for transmission/emission fluctuations we used a Levenberg–Marquart-Algorithm
with approximated Voigt line shapes [26] and
background polynomials up to 3rd order. Line
areas were converted into concentrations applying
the extended BeerÕs law (Eq. (1)), the ideal gas law
and taking into account the dynamic tuning coefficient of the laser and the process gas temperatures measured with thermocouples.
Spatial inhomogeneities—mainly in temperature—along the beam path are another important
issue for in situ measurements, as they affect line
strength and shape. To be able to apply the simplifications of Eq. (1), we experimentally verified
the temperature variations along the beam path
using suction thermocouples. Within the PCC
the temperature was typically flat to less than
±50 K. Further, we minimized the effect of changing temperatures on the measured CO mixture
fraction to 2.8% per 100 K temperature variation
by use of the R30 CO line, which shows a broad
maximum in S (T)/T at 1150 K.
In the beginning, we used a PC-based data
evaluation system based on a 5 Ms/s 12 bitADC-Board (NI 6110E) and custom-designed
Labview code under Win98. Due to Win98/Labview limitations only 1–10% of all scans in a given
time could be captured. This low duty cycle generated a statistical noise penalty factor of 3–10.
To overcome system throughput limitations
associated with this PC-based data acquisition
(DAQ), a fast monolithic real-time DAQ unit
based on a digital signal processor (DSP) and a
microcontroller was developed [27]. This system
captures and averages 100% of the scans even at
kHz modulation frequencies and simultaneously
performs a real-time evaluation of individual
scans before the averaging process. It contains
all electronics necessary to control lasers and Peltier elements, to generate the modulation signals
and to readout detector and process control signals. It provides a four channel low-noise analog
front end with an electrically variable amplifier
gain of ±20 dB and individual 5 Ms/s 12 bitADCs. The 100% data throughput improves the
optical resolution by up to a factor of 10 or the
temporal resolution by a factor of up to 1000 to
the ultimate limit, the laser scan frequency.
Utilizing variable amplifier gain and real-time
evaluation capabilities of the DSP we developed
an advanced data acquisition that can cope with
1616
V. Ebert et al. / Proceedings of the Combustion Institute 30 (2005) 1611–1618
the quite severe and highly dynamic transmission
fluctuations found in batch-fired processes. The
transmission fluctuations in the incinerator were
as large as a factor of 2 within a millisecond and
3–4 orders of magnitude within a second, and thus
even faster and stronger than in our previous measurements in full-sized power plants [7].
The custom-designed DSP software simultaneously rejects clipped, excessively distorted or
attenuated absorption profiles in real-time. The
percentage of the rejected scans depends largely
on the temporal and dynamic behavior of the
transmission fluctuations. To remove only unusable scans and avoid excessive rejection quota,
we largely compensated the transmission variations before the scan digitization by a new
closed-loop gain adaptation.
This function adapts in real-time for each single wavelength scan the gain of the analog voltage-controlled preamplifiers according to the
results of a rapid transmission assessment of the
preceding scan performed by the DSP. The gain
control loop reacts stepwise within 5–10 ms, i.e.,
5–10 scans. This proved in most cases fast enough
to deal with the transmission fluctuations in this
incinerator. But even if the gain adaptation speed
is too small compared to the speed of the transmission changes, excessively distorted scans will
still be rejected by the scan evaluation before the
averaging procedure.
This compensation not only ensured low rejection quota, often below the 1% region, it simultaneously matches the analog signal to the dynamic
range of the AD converter, and thus enhanced the
resolution compared to the PC-based DAQ. Further, it avoided overmodulation of the front-end
amplifier as well as saturation of the ADCs. All
these problems created significant systematic errors in the PC-based DAQ, since invalid absorption scans were incorporated in the scan averages.
A simultaneous comparison of both data evaluation strategies clearly proved the advantages of
the rapid DSP-based scan selection and gain
adaptation. The DSP-based DAQ completely
avoided the systematic errors, which showed up
before as spike like CO excursions, that sometimes
were up to a factor of 100 higher than the average
CO value. The useful application range of in situ
absorption spectroscopy could thus be extended
even further to full-scale pulsed batch
combustors.
fuel lean conditions, similar to those conditions
requiring high safety margins to minimize CO
peaks.
Figure 6 shows typical absorption scans (the
CO-2m-R30- and the O2-R17Q18/R19R19 lines)
of the dual-species version after transmission/
emission correction, clearly indicating the capability for a simultaneous in situ CO/O2 detection.
Both absorption profiles are averages of 1000
wavelength scans captured within 1 s at 1 kHz
modulation frequency using the DSPs 100% data
throughput. The averaged scans were transferred
to a PC via a serial port, as the rather complex final fitting process was still realized on a PC. With
1 s measurement time this reduced the temporal
separation between two concentration values to
3.4 s. A future implementation of fast data transfer protocols and DSP-based fitting routines will
remove this bottleneck and allow real-time concentration data with millisecond time resolution.
The optical resolution of the spectrometer was
deduced from the fit residuals. For the R30 line in
Fig. 6 (equivalent to 160 ppm CO) we determined
an optical resolution, i.e., one standard deviation
(1r) of the residuum, of ±1.2 · 104 OD. At
900 C this corresponds to an excellent 1r detection limit of 6.5 ppm CO or a detectivity of
16.5 ppm m s1/2. Compared to our previous CO
measurements based on 2.3 lm FP-DLs and PCbased data acquisition [17] we thus achieved a
15-fold resolution improvement.
For the O2 lines in Fig. 6, the optical resolution
was ±6 · 105 OD. Even though this value is
obviously dominated by discrepancies in the line
shapes caused by absorption contributions from
outside air which have a different line width this
is the best optical resolution we have achieved
with DAS in a full-scale technical combustor to
date. The unknown temperature profile in the
3. Results
A 2.3 lm-CO- and a dual-species CO-2.3 lm/
O2-760 nm-spectrometer were successfully tested
at the THERESA waste incinerator for periods
of up to two weeks. Various solid and liquid fuels
were fired under different stoichiometric conditions. CO detection limits were determined under
Fig. 6. In situ signals of the R30 CO line (left) and two
O2 lines (right), corrected for transmission, emission and
amplitude modulation and compared with the fitted
Voigt line shape (dashed lines). Fit residuals shown
underneath (horizontal dashed lines indicate 2r values of
the standard deviation).
V. Ebert et al. / Proceedings of the Combustion Institute 30 (2005) 1611–1618
1617
Fig. 7. Periodic CO fluctuations (left) with ±17 ppm amplitude (±10 ppm for 10 s average) measured at the RK exit. A
FFT of the fluctuations (right) over a 2 h period shows same frequency as the feeding cycle of the incinerator.
air-purged optical access tubes also hampered the
absolute O2-evaluation so that only relative O2
changes could be extracted. Based on HITRAN
calculations we approximate that 50% of the spatially integrated absorbance is caused by hot gases
in the PCC and thus estimate an 1r-O2 detection
limit of 250 ppm respectively 600 ppm m, which
indicates at least a 4· improvement to our previous studies [2]. N2 or steam purging will remove
both problems in the future. The residual noise
floor in Fig. 6 indicates that resolutions of about
±1.5 · 105 for O2 (±2.5 · 105 for CO) become
available in that case. By combining the current
setup with our digital fast scanning WMS technique [15], the resolution may be improved further
to below ±1 · 105 OD, so that even in industrial
combustors CO/O2 detection limits in the subppm/sub-100 ppm range seem feasible.
At the kiln exit we currently found for lean
conditions (kburner = 1.5) in average 100 ppm CO
and for stoichiometric conditions in a previous
study with 2.3 lm FP-DLs [17] 1000–2400 ppm.
The first measurement within the kiln, based on
1.5 lm-DLs [17], revealed consistently higher
and quickly pulsating CO values of up to
5000 ppm, which could be nicely anti-correlated
with the O2 fluctuations, that were simultaneously
measured with a 760 nm DFB-DL.
In the present study, we could detect similar
quasi-periodic CO fluctuations with amplitudes
of 30 ppm and 1/30 Hz periodicity (Fig. 7), thanks
to the high sensitivity and time resolution of the
2.3 lm DFB-CO-spectrometer. However, the
anti-correlated O2 signals were not detected and
are probably below our O2 detection limit. The
precise frequency of the CO oscillations was determined via a FFT on a two hour set of CO data.
The resulting spectrum (Fig. 7) shows a single,
pronounced peak at 0.033 Hz. A comparison with
other process parameters reveals that these fluctuations coincide nicely with the feed frequency of
the solid fuel into the RK. Using the new
2.3 lm-CO-spectrometer it is therefore possible
to detect in real-time the small fluctuations in
the combustion stoichiometry caused by the introduction of new fuel into the process.
4. Summary and conclusion
Two calibration-free absorption spectrometers
for the sensitive and rapid in situ CO/O2-detection
in harsh combustion environments have been realized for the first time. New 2.3 lm- and 760 nm
DFB-DLs were successfully used to enable a
simultaneous CO/O2-detection right behind the
kiln exit in a hazardous waste incinerator. Direct
absorption spectroscopy in combination with
DSP-based real-time transmission/emission correction as well as automatic alignment techniques
were deployed. A 1 s measurement time allowed
optical resolutions in the 104–105 OD range,
which permitted 1r-detection limits of 6.5 ppm
CO and 250 ppm O2. Hence, small periodic CO
and stoichiometry fluctuations could be detected
in the incinerator even under fuel lean conditions.
The CO and the CO/O2 spectrometers are therefore powerful tools for active combustion control
strategies to optimize full-scale batch-fired combustors, even if a wide stoichiometry range has
to be covered. Since the spectrometer is easily
expandable to other species[1–3,7] or other
parameters like gas temperatures [3] or gas residence times [6] DSP-enhanced in situ absorption
spectrometers will become very valuable tools
for advanced combustion control applications.
Acknowledgments
We thank J. Koeth for providing the 2.3 lm laser. V.E. thanks J. Fleming for his valuable assistance. Contributions of the THERESA control
team and the financial support by the state of Baden-Württemberg within the COSI-Project is
acknowledged.
1618
V. Ebert et al. / Proceedings of the Combustion Institute 30 (2005) 1611–1618
References
[1] V. Ebert, J. Wolfrum, in: F. Mayinger, O. Feldmann (Eds.), Optical Measurements. Springer Verlag, Heidelberg, 2001, pp. 227–265.
[2] V. Ebert, J. Fitzer, I. Gerstenberg, K.-U. Pleban,
H. Pitz, J. Wolfrum, M. Jochem, J. Martin, Proc.
Combust. Inst. 27 (1998) 1301–1308.
[3] V. Ebert, T. Fernholz, C. Giesemann, H. Pitz, H.
Teichert, J. Wolfrum, H. Jaritz, Proc. Combust.
Inst. 28 (2000) 423–430.
[4] E. Schlosser, T. Fernholz, H. Teichert, V. Ebert,
Spectrochim. Acta 58/11 (2002) 2347–2359.
[5] E. Schlosser, J. Wolfrum, B.A. Williams, R.S.
Sheinson, J.W. Fleming, V. Ebert, Proc. Combust.
Inst. 29 (2002) 353–360.
[6] E. Schlosser, J. Wolfrum, L. Hildebrandt, H.
Seifert, B. Oser, V. Ebert, Appl. Phys. B 75 (2–3)
(2002) 237–247.
[7] H. Teichert, T. Fernholz, V. Ebert, Appl. Opt. 42
(2003) 2043–2051, and references therein.
[8] J.J. Nikkari, J.M. Di Iorio, M.J. Thomson, Appl.
Opt. 41 (2002) 446–452.
[9] L. Upschulte, D.M. Sonnenfroh, M.G. Allen, Appl.
Opt. 38 (1999) 1506–1512.
[10] R.M. Mihalcea, D.S. Baer, R.K. Hanson, Appl.
Opt. 36 (1997) 8745–8752.
[11] J.-C. Nicolas, A.N. Baranov, Y. Cuminal, Y.
Rouillard, C. Alibert, Appl. Opt. 37 (1998)
7906–7911.
[12] J. Wang, M. Maiorov, J.B. Jeffries, D.Z. Garbuzov,
J.C. Connolly, R.K. Hanson, Meas. Sci. Technol.
11 (2000) 1576–1584.
[13] J. Wang, M. Maiorov, D.S. Baer, D.Z. Garbuzov,
J.C. Connolly, R.K. Hanson, Appl. Opt. 39 (2000)
5579–5589.
[14] V. Ebert, C. Giesemann, J. Koeth, H. Teichert, in:
Laser Appl. to Chem. and Environm. Anal., OSA
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
Tech. Dig. (Opt. Soc. of Am., Wash./DC), TuF-98
(2004).
T. Fernholz, H. Teichert, V. Ebert, Appl. Phys. B
75 (2–3) (2002) 229–236, and refs. therein.
P. Vogel, V. Ebert, Appl. Phys. B 72 (2001)
127–135.
H. Teichert, T. Fernholz, J. Koeth, H. Seifert, J.
Wolfrum, V. Ebert, in 16. Externes TECFLAM
Seminar 2002, Karlsruhe, ISBN-Nr. 3-926751-25-8
pp.
30–42
(German).
Available
from:
<www.gasanalysis.org>.
E. Wagner, M. Tacke (Eds.), Proceedings of the 5th
International Symposium on Gas Analysis by Tunable
Diode Lasers, Freiburg, Germany, 1998, VDI-Berichte 1366, 1998.
V. Ebert (Ed.), Gas Analysis with Near Infrared Diode
Lasers, Special Issue of TM-Technisches Messen,
Oldenbourg Verlag, 68, 9/10, 2001, in German.
A.A. Mantz, A.I. Nadezhdinskii (Eds.), Proceedings
of 3rd International Conference on Tunable Diode
Laser Spectroscocpy, Zermatt, Switzerland, Spectrochim. Acta, 58A, 11, 2002.
Proceedings of 3rd Conference on Applications and
Trends in Optical Analysis Technology, Düsseldorf,
Germany, 2002, VDI-Berichte Nr. 1667, 2002, (in
German).
D. Gollub, M. Fischer, M. Kamp, A. Forchel,
Appl. Phys. Lett. 81 (2002) 4330–4331.
A. Kosterev, F.K. Tittel, R. Köhler, C. Gmachl, F.
Capasso, D.L. Sivco, A.Y. Cho, S. Wehe, M.G.
Allen, Appl. Opt. 41 (2002) 1169–1173.
L.S. Rothman, C.P. Rinsland et al., J. Quant.
Spectrosc. Radiat. Transfer 60 (1998) 665–710.
H. Dittrich, L. Malcher, H. Seifert, VDI-Berichte
1629 (2001) 397–402 (in German).
E.E. Whitting, J. Quant. Spectrosc. Radiat. Transfer
8 (1968) 1379–1384.
V. Ebert, Rev. Sci. Instr. 2004, in preparation.
Comments
Gus Nathan, University of Adelaide, Australia. Did
you validate the effect of particle scattering by simultaneous measurement with a third line, tuned away from
any absorption bands?
Reply. The effect of particles on the in situ absorption
signals has been described in the text and in our previous
publications, as referenced in our paper. Particles generate
spectrally broadband losses along the absorption path
and reduce the total optical power which can be directed
from the laser to the detector. However, the shape of the
absorption line itself and even the baseline of the absorption signal will not be changed, since the laser is intentionally scanned much faster than typical changes in the
particle density. Thus the broadband absorption effects
generated by particles can be corrected for by a simple
multiplicative term in the Lambert Beer Law Eq. (1).
d
Franz Winter, Vienna University of Technology, Austria. Along your optical path you may experience in
large-scale combustors a wide range of CO concentrations. How will this affect your quantitative data
evaluation?
Reply. Any absorption spectrometer will integrate
the absorption signal over the entire absorption path
and thus provides a path-averaged CO concentration
only. This is an important feature of our in situ absorption spectrometers and a quite useful property, as it
avoids the pitfalls of common, locally acting sensors
and provides a much more representative CO signal than
any single-point measurement.