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Article
Simultaneous Quantitative Detection of HCN and C2H2in
Combustion Environment Using TDLAS
Wubin Weng * , Marcus Aldén and Zhongshan Li
Citation: Weng, W.; Aldén, M.; Li, Z.
Simultaneous Quantitative Detection
of HCN and C2H2in Combustion
Environment Using TDLAS. Processes
2021,9, 2033. https://doi.org/
10.3390/pr9112033
Academic Editor: Albert Ratner
Received: 15 October 2021
Accepted: 10 November 2021
Published: 14 November 2021
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4.0/).
Division of Combustion Physics, Lund University, P.O. Box 118, SE-221 00 Lund, Sweden;
marcus.alden@forbrf.lth.se (M.A.); zhongshan.li@forbrf.lth.se (Z.L.)
*Correspondence: wubin.weng@forbrf.lth.se; Tel.: +46-46-222-3208
Abstract:
Emission of nitrogen oxides (NO
x
) and soot particles during the combustion of biomass
fuels and municipal solid waste is a major environmental issue. Hydrogen cyanide (HCN) and
acetylene (C
2
H
2
) are important precursors of NO
x
and soot particles, respectively. In the current
work, infrared tunable diode laser absorption spectroscopy (IR-TDLAS), as a non-intrusive in situ
technique, was applied to quantitatively measure HCN and C
2
H
2
in a combustion environment.
The P(11e) line of the first overtone vibrational band v
1
of HCN at 6484.78 cm
−1
and the P(27e)
line of the v
1
+v
3
combination band of C
2
H
2
at 6484.03 cm
−1
were selected. However, the infrared
absorption of the ubiquitous water vapor in the combustion environment brings great uncertainty
to the measurement. To obtain accurate temperature-dependent water spectra between 6483.8 and
6485.8 cm
−1
, a homogenous hot gas environment with controllable temperatures varying from 1100 to
1950 K provided by a laminar flame was employed to perform systematic IR-TDLAS measurements.
By fitting the obtained water spectra, water interference to the HCN and C
2
H
2
measurement was
sufficiently mitigated and the concentrations of HCN and C
2
H
2
were obtained. The technique was
applied to simultaneously measure the temporally resolved release of HCN and C
2
H
2
over burning
nylon 66 strips in a hot oxidizing environment of 1790 K.
Keywords:
hydrogen cyanide; acetylene; tunable diode laser absorption spectroscopy; combus-
tion/gasification; hot water interference; biomass/waste
1. Introduction
The combustion of biomass fuels and municipal solid waste is an important fossil-free
energy sector providing heat and power. However, the emission of nitrogen oxides (NO
x
)
and soot particles during combustion is seen as a major environmental issue. Hydrogen
cyanide (HCN) and acetylene (C
2
H
2
) are important precursors of NO
x
and soot particles,
respectively. To fully understand the NO
x
and soot particle formation process in various
combustion environments, reliable measurements of HCN and C
2
H
2
are essential. Con-
sidering that both HCN and C
2
H
2
will be consumed downstream, nonintrusive in situ
laser-based techniques are preferred.
Due to the lack of proper electronic transitions in the UV/visible spectral regions, the
detection of HCN in a combustion environment is mainly focused on using the absorption
lines in the infrared region. Sun et al. [
1
] demonstrated quantitative measurements of
HCN in premixed CH
4
/N
2
O/O
2
/N
2
flames using midinfrared polarization spectroscopy
(IRPS). The P20 line of the fundamental C–H stretching band at around 3248 cm
−1
was
selected. Using the same absorption line, Hot et al. [
2
] achieved quantitative in situ mea-
surements of HCN released from burning straw pellets at atmospheric pressure using
mid-infrared degenerate four-wave mixing (IR-DFWM). Goldman et al. [
3
] reported HCN
measurement in low-pressure flames using fiber laser intracavity absorption spectroscopy
(FLICAS), probing the first overtone vibrational band at around 1.5
µ
m. In addition, ex
situ measurements of HCN in flames were reported by Gersen et al. [
4
], using wavelength
Processes 2021,9, 2033. https://doi.org/10.3390/pr9112033 https://www.mdpi.com/journal/processes
Processes 2021,9, 2033 2 of 11
modulation absorption spectroscopy (WMAS) to probe the P(13) line of the first over-
tone, and Lamoureux et al., using pulsed cavity ring-down spectroscopy (CRDS) [
5
] and
continuous-wave CRDS [
6
] to probe the second and first overtone. Among the different
available techniques, tunable diode laser absorption spectroscopy (TDLAS) is competitive,
which is relatively less complex, more robust, and cost effective compared to the others. In
TDLAS measurement, a diode laser operates in a single longitudinal mode and provides
single-frequency emission with a narrow linewidth. The laser emission scans in wavelength
to resolve the atom/molecule absorption lines, and the atom/molecule concentration is
derived based on the Beer–Lambert law, well-known to measure various atoms [
7
] and
molecules [
8
,
9
] with high detection sensitivity and precision. In the present work, TDLAS
was developed for quantitative in situ measurement of HCN in a harsh biomass/waste
combustion environment by probing the first overtone ro-vibrational band of HCN at
1.5 µm (Figure 1).
Processes 2021, 9, x FOR PEER REVIEW 2 of 11
ex situ measurements of HCN in flames were reported by Gersen et al. [4], using wave-
length modulation absorption spectroscopy (WMAS) to probe the P(13) line of the first
overtone, and Lamoureux et al., using pulsed cavity ring-down spectroscopy (CRDS) [5]
and continuous-wave CRDS [6] to probe the second and first overtone. Among the differ-
ent available techniques, tunable diode laser absorption spectroscopy (TDLAS) is compet-
itive, which is relatively less complex, more robust, and cost effective compared to the
others. In TDLAS measurement, a diode laser operates in a single longitudinal mode and
provides single-frequency emission with a narrow linewidth. The laser emission scans in
wavelength to resolve the atom/molecule absorption lines, and the atom/molecule con-
centration is derived based on the Beer–Lambert law, well-known to measure various at-
oms [7] and molecules [8,9] with high detection sensitivity and precision. In the present
work, TDLAS was developed for quantitative in situ measurement of HCN in a harsh
biomass/waste combustion environment by probing the first overtone ro-vibrational band
of HCN at 1.5 µm (Figure 1).
Figure 1. Calculated absorbance of 1000 ppm HCN, 1000 ppm C
2
H
2
, and 10% H
2
O over 6400–6600
cm
−1
at 1200 K and atmospheric pressure with an optical path length of 0.34 m. The spectra data
from the HITRAN2016 database were adopted for spectral simulation.
Compared to HCN, more laser diagnostics have been developed for quantitative
measurements of C
2
H
2
in flames. The techniques include IRPS [10,11], coherent anti-
Stokes Raman scattering (CARS) [12,13], laser-induced fluorescence (LIF) [14], spontane-
ous Raman scattering (SRS) [15–18], and tunable diode laser absorption spectroscopy
(TDLAS) [19,20]. In a harsh combustion environment, the strong broadband interference
from the laser-induced fluorescence of polycyclic aromatic hydrocarbon (PAH) and the
laser-induced incandescence (LII) from soot particles suppresses the weak signal of C
2
H
2
LIF and spontaneous Raman scattering. Recently, Kim et al. [18] developed time-resolved
polarization lock-in filtering for background suppression. Techniques based on the prob-
ing of the IR absorption line seem to be more viable. To manage the simultaneous meas-
urement of HCN, the v
1
+ v
3
vibrational combination band at 1.5 µm (see Figure 1) was
selected.
However, in combustion environments, challenges arise from the strong absorption
of hot water lines at 1.5 µm, interfering with the HCN and C
2
H
2
absorption signal, as
Figure 1.
Calculated absorbance of 1000 ppm HCN, 1000 ppm C
2
H
2
, and 10% H
2
O over
6400–6600 cm
−1
at 1200 K and atmospheric pressure with an optical path length of 0.34 m. The
spectra data from the HITRAN2016 database were adopted for spectral simulation.
Compared to HCN, more laser diagnostics have been developed for quantitative
measurements of C
2
H
2
in flames. The techniques include IRPS [
10
,
11
], coherent anti-
Stokes Raman scattering (CARS) [
12
,
13
], laser-induced fluorescence (LIF) [
14
], spontaneous
Raman scattering (SRS) [
15
–
18
], and tunable diode laser absorption spectroscopy (TD-
LAS) [
19
,
20
]. In a harsh combustion environment, the strong broadband interference
from the laser-induced fluorescence of polycyclic aromatic hydrocarbon (PAH) and the
laser-induced incandescence (LII) from soot particles suppresses the weak signal of C
2
H
2
LIF and spontaneous Raman scattering. Recently, Kim et al. [
18
] developed time-resolved
polarization lock-in filtering for background suppression. Techniques based on the probing
of the IR absorption line seem to be more viable. To manage the simultaneous measurement
of HCN, the v1+v3vibrational combination band at 1.5 µm (see Figure 1) was selected.
However, in combustion environments, challenges arise from the strong absorption of
hot water lines at 1.5
µ
m, interfering with the HCN and C
2
H
2
absorption signal, as shown
in Figure 1. To minimize line interference, it is necessary to select suitable HCN and C
2
H
2
lines.
Processes 2021,9, 2033 3 of 11
Figure 2shows the high-resolution spectra of 1000 ppm HCN, C
2
H
2
, and 10% H
2
O
over 6481–6489 cm
−1
at 1200 K and atmospheric pressure with an optical path length of
0.34 m. The calculation was carried out using the HITRAN2016 [
21
] and HITEMP2010 [
22
]
databases. The interference from other typical species, such as CO, CO
2
, C
2
H
4
, and NH
3
,
that would be released from burning biomass/waste combustion was also considered.
Among these species, NH
3
has the strongest absorption at this wavelength, and the ab-
sorption spectrum of 1000 ppm NH
3
is also presented in Figure 2. In the present work,
the P(11e) line of the first overtone vibrational band v
1
of HCN at 6484.78 cm
−1
with a
line intensity (S) of 5.639
×
10
−21
cm/molecule at 296 K was selected, and the typical
absorption curve of this line is presented in the inset of Figure 2. At the same time, a
nearby C
2
H
2
absorption line, which is the P(27e) line of the v
1
+v
3
combination band at
6484.03 cm
−1
with a line intensity (S) of 8.113
×
10
−22
cm/molecule at 296 K, was used for
C2H2measurement.
Processes 2021, 9, x FOR PEER REVIEW 3 of 11
shown in Figure 1. To minimize line interference, it is necessary to select suitable HCN
and C
2
H
2
lines.
Figure 2 shows the high-resolution spectra of 1000 ppm HCN, C
2
H
2
, and 10% H
2
O
over 6481–6489 cm
−1
at 1200 K and atmospheric pressure with an optical path length of
0.34 m. The calculation was carried out using the HITRAN2016 [21] and HITEMP2010 [22]
databases. The interference from other typical species, such as CO, CO
2
, C
2
H
4
, and NH
3
,
that would be released from burning biomass/waste combustion was also considered.
Among these species, NH
3
has the strongest absorption at this wavelength, and the ab-
sorption spectrum of 1000 ppm NH
3
is also presented in Figure 2. In the present work, the
P(11e) line of the first overtone vibrational band v
1
of HCN at 6484.78 cm
−1
with a line
intensity (S) of 5.639 × 10
−21
cm/molecule at 296 K was selected, and the typical absorption
curve of this line is presented in the inset of Figure 2. At the same time, a nearby C
2
H
2
absorption line, which is the P(27e) line of the v
1
+ v
3
combination band at 6484.03 cm
−1
with a line intensity (S) of 8.113 × 10
−22
cm/molecule at 296 K, was used for C
2
H
2
measure-
ment.
Figure 2. Calculated absorbance of 1000 ppm HCN, C
2
H
2
, NH
3
, and 10% H
2
O over 6481–6489 cm
−1
at 1200 K and atmospheric pressure with an optical path length of 0.34 m. The spectra data from the
HITRAN2016 and HITEMP2010 databases were used for spectral simulation. Spectra with high-
resolution at 6483.8–6485 cm
−1
are shown in the inset.
However, using the selected HCN and C
2
H
2
lines, it is still impossible to completely
avoid the interference of the hot water lines, as shown in the inset in Figure 2. Moreover,
the HITRAN2016 and HITEMP2010 databases present different water absorption spectra.
To further minimize the interference with the HCN and C
2
H
2
measurement, a systematic
investigation of the water line in the 6483.8–6485 cm
−1
spectral range is needed. In the
present work, a homogenous hot gas environment provided by a laminar flame burner
covering temperatures from 1120 to 1950 K was employed to obtain the temperature-de-
pendent, hot-water-line absorption spectra. The accurate water absorption data were used
to mitigate the interference from the hot water lines, and the time-resolved concentration
of HCN and C
2
H
2
at 5 mm above burning nylon 66 strips was quantitatively and simulta-
neously measured.
2. Experimental Setup
Figure 2.
Calculated absorbance of 1000 ppm HCN, C
2
H
2
, NH
3
, and 10% H
2
O over 6481–6489 cm
−1
at 1200 K and
atmospheric pressure with an optical path length of 0.34 m. The spectra data from the HITRAN2016 and HITEMP2010
databases were used for spectral simulation. Spectra with high-resolution at 6483.8–6485 cm−1are shown in the inset.
However, using the selected HCN and C
2
H
2
lines, it is still impossible to completely
avoid the interference of the hot water lines, as shown in the inset in Figure 2. Moreover,
the HITRAN2016 and HITEMP2010 databases present different water absorption spectra.
To further minimize the interference with the HCN and C
2
H
2
measurement, a systematic
investigation of the water line in the 6483.8–6485 cm
−1
spectral range is needed. In the
present work, a homogenous hot gas environment provided by a laminar flame burner
covering temperatures from 1120 to 1950 K was employed to obtain the temperature-
dependent, hot-water-line absorption spectra. The accurate water absorption data were
used to mitigate the interference from the hot water lines, and the time-resolved concen-
tration of HCN and C
2
H
2
at 5 mm above burning nylon 66 strips was quantitatively and
simultaneously measured.
2. Experimental Setup
The temperature-dependent absorption spectra of water vapor were collected in hot
gas environments with temperatures ranging from 1120 to 1950 K. The hot gas environ-
ments were provided by laminar flames anchored on a multijet burner. The details of the
burner were described by Weng et al. [
23
]. As shown in Figure 3a, the burner comprises
of a jet chamber, a co-flow chamber, and a burner head. A methane–air–oxygen mixture
was introduced into the jet chamber and evenly distributed into 181 jet tubes. At the outlet
of the jet tubes, Bunsen-type premixed flames were stabilized to generate hot flue gas
Processes 2021,9, 2033 4 of 11
containing hot water vapor. At the same time, a mixture of nitrogen and air used as the
co-flow passed through the co-flow chamber and finally mixed with the hot gas product
from the jet flames. After this mixing, a hot gas environment with a high degree of unifor-
mity and a size of about 85
×
45 mm was generated in the burner head region, as shown in
Figure 3a. The temperature of the hot gas can be adjusted from 1120 to 1950 K using the
flame conditions listed in Table 1. The temperature at 5 mm above the burner outlet, where
hot water absorption was investigated, was measured using two-line atomic fluorescence
(TLAF) thermometry with indium atom. The details of the temperature measurement
technique were described by Borggren et al. [
24
]. The temperature was evenly distributed
in a region of about 70
×
40 mm
,
, with a narrow transition zone on the edge [
23
,
24
]. The
water concentration in different hot gas environments was obtained based on chemical
equilibrium calculation. Five mass flow controllers (BRONKHORST HIGH-TECH BV,
Ruurlo, Netherlands) were used to control the gas flow rate. A flow stabilizer was placed
35 mm above the burner outlet for flow stabilization. Moreover, the hot gas environment at
1790 K provided by flame F7 was used for burning nylon 66 strips. As shown in Figure 3c,
500 mg nylon 66 strips were carried by two ceramic rods and placed in the center of the
hot flue gas.
Processes 2021, 9, x FOR PEER REVIEW 4 of 11
The temperature-dependent absorption spectra of water vapor were collected in hot
gas environments with temperatures ranging from 1120 to 1950 K. The hot gas environ-
ments were provided by laminar flames anchored on a multijet burner. The details of the
burner were described by Weng et al. [23]. As shown in Figure 3a, the burner comprises
of a jet chamber, a co-flow chamber, and a burner head. A methane–air–oxygen mixture
was introduced into the jet chamber and evenly distributed into 181 jet tubes. At the outlet
of the jet tubes, Bunsen-type premixed flames were stabilized to generate hot flue gas
containing hot water vapor. At the same time, a mixture of nitrogen and air used as the
co-flow passed through the co-flow chamber and finally mixed with the hot gas product
from the jet flames. After this mixing, a hot gas environment with a high degree of uni-
formity and a size of about 85 × 45 mm was generated in the burner head region, as shown
in Figure 3a. The temperature of the hot gas can be adjusted from 1120 to 1950 K using the
flame conditions listed in Table 1. The temperature at 5 mm above the burner outlet, where
hot water absorption was investigated, was measured using two-line atomic fluorescence
(TLAF) thermometry with indium atom. The details of the temperature measurement
technique were described by Borggren et al. [24]. The temperature was evenly distributed
in a region of about 70 × 40 mm
,
, with a narrow transition zone on the edge [23,24]. The
water concentration in different hot gas environments was obtained based on chemical
equilibrium calculation. Five mass flow controllers (BRONKHORST HIGH-TECH BV,
Ruurlo, Netherlands) were used to control the gas flow rate. A flow stabilizer was placed
35 mm above the burner outlet for flow stabilization.
Moreover, the hot gas environment
at 1790 K provided by flame F7 was used for burning nylon 66 strips. As shown in Figure
3c, 500 mg nylon 66 strips were carried by two ceramic rods and placed in the center of
the hot flue gas.
Figure 3. Schematic of the multijet burner (a), the TDLAS setup (b), and the measurement of HCN
and C
2
H
2
above burning nylon 66 strips (c). PD: photodiode. (Reproduced from Weng et al. [25]
Copyright 2020 Elsevier.)
Table 1. Summary of the flame conditions, with corresponding temperature measured at 5 mm above the burner outlet
and water concentration in the gas product.
Flame Case
Gas Flow Rate (SLM) Fuel/O
2
Equiva-
lence Ratio ϕ
Gas Product
Temperature (K)
H
2
O in Gas
Product (%)
Jet-Flow Co-Flow
CH
4
Air O
2
N
2
Air
F1 2.95 19.20 2.09 6.84 7.09 0.78 1950 15
F2 2.66 17.34 1.89 10.83 7.74 0.74 1750 13
F3 2.47 12.23 2.58 18.97 8.90 0.70 1550 11
Figure 3.
Schematic of the multijet burner (
a
), the TDLAS setup (
b
), and the measurement of HCN and C
2
H
2
above burning
nylon 66 strips (c). PD: photodiode. (Reproduced from Weng et al. [25] Copyright 2020 Elsevier.)
Table 1.
Summary of the flame conditions, with corresponding temperature measured at 5 mm above the burner outlet and
water concentration in the gas product.
Flame
Case
Gas Flow Rate (SLM) Fuel/O2Equivalence
Ratio φ
Gas Product
Temperature (K)
H2O in Gas
Product (%)
Jet-Flow Co-Flow
CH4Air O2N2Air
F1 2.95 19.20 2.09 6.84 7.09 0.78 1950 15
F2 2.66 17.34 1.89 10.83 7.74 0.74 1750 13
F3 2.47 12.23 2.58 18.97 8.90 0.70 1550 11
F4 2.28 11.89 2.26 22.69 9.83 0.67 1390 9
F5 2.09 10.90 2.07 26.50 10.66 0.63 1260 8
F6 1.71 8.91 1.69 26.92 10.25 0.60 1120 7
F7 2.66 17.34 1.89 18.60 0.00 0.96 1790 13
A schematic of the optical setup of the TDLAS system is presented in Figure 3b. The
laser beam was produced by a distributed feedback (DFB) diode laser (Butterfly, Toptica)
Processes 2021,9, 2033 5 of 11
controlled by a combined laser diode and TEC controller (ITC4001, Thorlabs, LD current
1 A). The laser operates at temperatures of 5–45
◦
C to set it at a wavelength range of
1547–1551 nm and power of approximately 40 mW. In the presented work, the operating
temperature was set to 36.4
◦
C and the laser wavelength at around 6485 cm
−1
. By ramping
up the driving current to between 0.15 and 0.25 A, the laser wavelength scanned over
2 cm
−1
at 100 Hz. The linewidth of the laser emission was below 1 MHz (3.3
×
10
−4
cm
−1
),
which could well-resolve the absorption lines, such as water lines that have a FWHM of
about 0.072 cm
−1
in a combustion environment. Using a fiber splitter, the laser was split
into two parts. The one with 25% of the initial power was used as the reference beam,
and the remaining portion was used for measurement, which efficiently eliminated the
measurement uncertainty caused by laser energy fluctuation. The power of the reference
beam was monitored by an IR photodiode (InGaAs, detecting wavelength 0.9–2.6
µ
m,
PDA10D2, Thorlabs), i.e., PD1 in Figure 3b. The measurement beam was guided through
the hot gas 5 mm above the burner outlet, or 5 mm above the burning nylon 66 strips (see
Figure 3c), four times using two silver-coated concave mirrors (f= 100 mm,
D= 50 mm
,
Thorlabs) to achieve a total optical path length of about 34 cm. The laser was finally
collected by another IR photodiode (InGaAs, detecting wavelength 1.2–2.6
µ
m, PDA10D-
EC, Thorlabs), i.e., PD2. A 632.8 nm laser beam provided by a HeNe laser was used as the
alignment beam. Using the signal from PD1 and PD2, the number density (N) of HCN and
C2H2was derived based on Beer–Lambert law:
Abs =−ln(I(v)−Is)/Tr
I0(v)−Is0 =S(T)·g(v−v0)·N·L(1)
where Abs is the absorbance,
I0(v)
is the initial laser intensity obtained by the reference
photodiode (PD1),
I(v)
is the intensity of the laser after the absorbing obtained by the
measurement photodiode (PD2), vis the laser frequency,
Is0
and
Is
are the wavelength-
independent signals originating from the detector dark current, Tr is the transmission of the
laser with soot or other particles in the flame, S(T) is the absorption line strength,
g(v−v0
)
is the area normalized shape function, and Lis the optical path length. The detector dark
current signal was obtained when the laser was blocked. In the clean, hot flue gases
provided by the laminar flames, Tr was 1. In sooty environments, ln(Tr) was determined
by spline interpolation of the deviation between the measured and the calculated H
2
O
absorbance at five wavelengths (6484.07, 6484.59, 6484.93, 6485.42, and 6485.79 cm
−1
).
At these wavelengths, the absorption intensity is the lowest and the least sensitive to
temperature. Several rounds of iterative calculation were performed to obtain Tr. At the
beginning, an initially estimated concentration was used for Tr calculation. Based on the Tr
value, the H2O concentration was determined through fitting between the calculated and
measured water spectrum and used for another round of Tr value calculation.
3. Results and Discussion
The absorbance spectra over 6483.8–6485 cm
−1
, obtained from the hot flue gas at
1260 K provided by flame F5, is shown in Figure 4a. It was attributed to the 8% H
2
O in
the hot flue gas. Other major species, such as CO
2
, were excluded due to their negligible
absorption cross-section at this wavelength. The absorption spectrum of the 8% H
2
O at
1260 K was simulated using HITEMP2010 (Figure 4b). All water lines were recognized
as hot lines. The spectral data, including transition wavenumber (v), line intensity (S)
at 1260 K, lower-state energy (E), and the vibrational and rotational quantum numbers
for the upper and lower states of typical transitions labeled a to f in Figure 4b (obtained
from HITEMP2010) are summarized in Table 2. Compared with experimental results,
the simulation can approximately predict strong lines, but the discrepancy is large, for
instance, at the wavelength where the HCN (6484.78 cm
−1
) and C
2
H
2
(6484.03 cm
−1
) lines
are located.
Processes 2021,9, 2033 6 of 11
Processes 2021, 9, x FOR PEER REVIEW 6 of 11
lower-state energy (E), and the vibrational and rotational quantum numbers for the upper
and lower states of typical transitions labeled a to f in Figure 4b (obtained from
HITEMP2010) are summarized in Table 2. Compared with experimental results, the sim-
ulation can approximately predict strong lines, but the discrepancy is large, for instance,
at the wavelength where the HCN (6484.78 cm
−1
) and C
2
H
2
(6484.03 cm
−1
) lines are located.
Figure 4. Absorbance spectra of H
2
O measured in the hot flue gas (T = 1260 K) provided by flame
F5 (a), and the calculated one of 8% H
2
O at 1260 K using HITEMP2010 (b). The chosen lines for HCN
and C
2
H
2
are marked with vertical bars.
Table 2. Spectra data of the six typical H
2
O transitions indicated in Figure 4b obtained from HITEMP2010. Transition
wavenumber (v), line intensity (S) at 1260 K, lower-state energy (E), and the vibrational and rotational quantum numbers
for the upper (′) and lower (″) states are presented.
Tran. v (cm
−1
) S (cm/Molecule) (1260
K) E (cm
−1
) v′ v″ J′ Ka′ Kc′ J″ Ka″ Kc″
a 6483.840 2.91 × 10
−24
5076.349 2 0 1 1 0 0 9 1 9 10 3 8
b 6484.212 4.30 × 10
−24
6108.281 0 3 1 0 1 0 17 5 13 18 5 14
c 6484.411 1.05 × 10
−23
1282.919 0 2 1 0 0 0 8 1 7 9 3 6
d 6484.744 4.08 × 10
−24
4837.700 1 2 1 1 0 0 8 2 6 9 2 7
e 6485.234 6.73 × 10
−24
4738.634 0 2 1 0 0 0 19 3 17 20 3 18
f
6485.580 1.75 × 10
−23
3360.600 2 0 0 0 0 0 14 4 11 15 5 10
By measuring HCN and C
2
H
2
in a combustion environment containing hot water va-
por, a total absorbance contributed by HCN, C
2
H
2
, and H
2
O was obtained. To derive the
concentrations of HCN and C
2
H
2
, the H
2
O absorption needs to be subtracted. As can be
seen, the absorption spectrum resulting from the simulation using the HITEMP2010 data-
base was not sufficiently accurate. Therefore, in the presented work, the temperature-de-
pendent absorption spectra of hot water over 6483.8–6485 cm
−1
was experimentally meas-
ured. In the measurement, hot flue gas environments of 1120–1950 K were provided by
the flames F1–F6 (cf. Table 1). Using the TDLAS system, the absorbance was measured
under each flame condition. The measured H
2
O absorbance spectra are shown in Figure
5, where the absorbance was corrected to the value with an identical H
2
O number density,
Figure 4.
Absorbance spectra of H
2
O measured in the hot flue gas (T= 1260 K) provided by flame F5
(
a
), and the calculated one of 8% H
2
O at 1260 K using HITEMP2010 (
b
). The chosen lines for HCN
and C2H2are marked with vertical bars.
Table 2.
Spectra data of the six typical H
2
O transitions indicated in Figure 4b obtained from
HITEMP2010. Transition wavenumber (v), line intensity (S) at 1260 K, lower-state energy (E), and the
vibrational and rotational quantum numbers for the upper (0) and lower (”) states are presented.
Tran. v(cm−1)S(cm/Molecule)
(1260 K) E(cm−1)v0v”J0Ka0
Kc0
J”Ka”
Kc”
a 6483.840 2.91 ×10−24 5076.349 2 0 1 1 0 0 9 1 9 10 3 8
b 6484.212 4.30 ×10−24 6108.281 0 3 1 0 1 0 17 5 13 18 5 14
c 6484.411 1.05 ×10−23 1282.919 0 2 1 0 0 0 8 1 7 9 3 6
d 6484.744 4.08 ×10−24 4837.700 1 2 1 1 0 0 8 2 6 9 2 7
e 6485.234 6.73 ×10−24 4738.634 0 2 1 0 0 0 19 3 17 20 3 18
f 6485.580 1.75 ×10−23 3360.600 2 0 0 0 0 0 14 4 11 15 5 10
By measuring HCN and C
2
H
2
in a combustion environment containing hot water
vapor, a total absorbance contributed by HCN, C
2
H
2
, and H
2
O was obtained. To derive
the concentrations of HCN and C
2
H
2
, the H
2
O absorption needs to be subtracted. As can
be seen, the absorption spectrum resulting from the simulation using the HITEMP2010
database was not sufficiently accurate. Therefore, in the presented work, the temperature-
dependent absorption spectra of hot water over 6483.8–6485 cm
−1
was experimentally
measured. In the measurement, hot flue gas environments of 1120–1950 K were provided
by the flames F1–F6 (cf. Table 1). Using the TDLAS system, the absorbance was measured
under each flame condition. The measured H
2
O absorbance spectra are shown in Figure 5,
where the absorbance was corrected to the value with an identical H
2
O number density,
4.7
×
10
17
molecule/cm
3
(10% at 1500 K). The absorbance data are available in the Supple-
mentary Material. The water spectrum has a similar profile at different temperatures, but
the absorption coefficient significantly increased with temperature.
Processes 2021,9, 2033 7 of 11
Processes 2021, 9, x FOR PEER REVIEW 7 of 11
4.7 × 10
17
molecule/cm
3
(10% at 1500 K). The absorbance data are available in the Supple-
mentary Material. The water spectrum has a similar profile at different temperatures, but
the absorption coefficient significantly increased with temperature.
Figure 5. Measured H
2
O absorbance spectra between 6483.8 and 6485.8 cm
−1
at temperatures of 1120,
1260, 1390, 1550, 1750, and 1950 K. The absorbance was corrected to a value with an identical H
2
O
number density, 4.7 × 10
17
molecules/cm
3
(10% at 1500 K). The typical six absorption peaks are la-
beled in the figure. The chosen lines for HCN and C
2
H
2
are marked with vertical bars.
Six relatively strong hot water lines in the measured spectra are labeled at their re-
spective peaks, P1–P6, as shown in Figure 5. The peak value as a function of temperature
is plotted in Figure 6a. Nearly all linearly increased with temperature, but at different
rates (e.g., the values at P2 and P3). Figure 5 shows that the relative intensity between the
water lines with respective peaks at P2 and P3 changed significantly with temperature.
The ratio of P3 to P2 as a function of temperature between 1000–2000 K is shown in Figure
6b, fitted by a quadratic equation, and expressed by:
R = 1.1505 × 10
−6
× T
2
− 0.0047 × T + 5.4049 (2)
Therefore, the measured water absorption spectrum of these two lines between
6484.1 and 6484.6 cm
−1
can be used to evaluate the temperature. The uncertainty was esti-
mated to be approximately ±6% according to the equation fitting and the uncertainty re-
garding the TLAF measurement (about ±3%). However, in harsh environments with une-
ven temperature distribution or soot particles, additional uncertainty will be introduced.
Figure 5.
Measured H
2
O absorbance spectra between 6483.8 and 6485.8 cm
−1
at temperatures of
1120, 1260, 1390, 1550, 1750, and 1950 K. The absorbance was corrected to a value with an identical
H
2
O number density, 4.7
×
10
17
molecules/cm
3
(10% at 1500 K). The typical six absorption peaks are
labeled in the figure. The chosen lines for HCN and C2H2are marked with vertical bars.
Six relatively strong hot water lines in the measured spectra are labeled at their
respective peaks, P1–P6, as shown in Figure 5. The peak value as a function of temperature
is plotted in Figure 6a. Nearly all linearly increased with temperature, but at different rates
(e.g., the values at P2 and P3). Figure 5shows that the relative intensity between the water
lines with respective peaks at P2 and P3 changed significantly with temperature. The ratio
of P3 to P2 as a function of temperature between 1000–2000 K is shown in Figure 6b, fitted
by a quadratic equation, and expressed by:
R= 1.1505 ×10−6×T2−0.0047 ×T+ 5.4049 (2)
Processes 2021, 9, x FOR PEER REVIEW 8 of 11
Figure 6. Absorbance of H
2
O at line peaks P1–P6 (as labeled in Figure 5) (a), and the ratio of P3 to P2 (b), as a function of
temperature.
Since the absorption of HCN and C
2
H
2
at 6484.78 and 6484.03 cm
−1
limited interfer-
ence with these two water lines, in the HCN and C
2
H
2
measurement, the gas temperature
can be determined using the absorption profile of these two water lines, and based on the
temperature, the absorption spectrum of the H
2
O mixed with HCN and C
2
H
2
can be ob-
tained through interception of the absorption spectra in Figure 5. Temperature can also
be used to calculate the line intensity of HCN and C
2
H
2
, which is temperature-dependent.
The absorbance spectrum obtained in the plume 5 mm above burning nylon 66 strips
in the hot flue gas environment (F7), at 1790 K with a residence time of 20 s, is shown in
Figure 7 using black scattering circles. The absorption was attributed to H
2
O, HCN, and
C
2
H
2
. Using the aforementioned method, the H
2
O absorption between 6484.1 and 6484.6
cm
−1
was fitted through the interception of the absorption spectra at the same wavelength
in Figure 5, with minimizing deviation. After the fitting process, the entire H
2
O absorption
spectrum was obtained, i.e., the mega dot line in Figure 7. Based on Equation (2), the gas
temperature was determined to be 1180 K. After subtracting the simulated H
2
O absorb-
ance from the raw absorbance, the absorption spectrum of HCN near 6484.78 cm
−1
was
obtained. By fitting using the parameters of the HCN line in the HITRAN2016 database
and a Voigt line shape, represented by the blue dot line in Figure 7, the concentration of
HCN was derived. The simulated H
2
O and HCN absorption was subtracted from the raw
absorbance, and the remaining absorbance of near 6484.03 cm
−1
, attributed to C
2
H
2
absorp-
tion, was fitted using the parameters of the C
2
H
2
line in the HITRAN2016 database and a
Voigt line shape (Figure 7) to obtain the concentration. In this case, the concentration of
HCN and C
2
H
2
where the measurement uncertainty originated from the absorbance
curve-fitting process was determined to be 820 ± 190 and 1170 ± 340 ppm, respectively.
The large uncertainty in the spectrum fitting may originate from the uneven temperature
of the gas plume and unrecognized species.
Figure 6.
Absorbance of H
2
O at line peaks P1–P6 (as labeled in Figure 5) (
a
), and the ratio of P3 to P2 (
b
), as a function of
temperature.
Therefore, the measured water absorption spectrum of these two lines between 6484.1
and 6484.6 cm−1can be used to evaluate the temperature. The uncertainty was estimated
Processes 2021,9, 2033 8 of 11
to be approximately
±
6% according to the equation fitting and the uncertainty regarding
the TLAF measurement (about
±
3%). However, in harsh environments with uneven
temperature distribution or soot particles, additional uncertainty will be introduced.
Since the absorption of HCN and C
2
H
2
at 6484.78 and 6484.03 cm
−1
limited interfer-
ence with these two water lines, in the HCN and C
2
H
2
measurement, the gas temperature
can be determined using the absorption profile of these two water lines, and based on
the temperature, the absorption spectrum of the H
2
O mixed with HCN and C
2
H
2
can be
obtained through interception of the absorption spectra in Figure 5. Temperature can also
be used to calculate the line intensity of HCN and C
2
H
2
, which is temperature-dependent.
The absorbance spectrum obtained in the plume 5 mm above burning nylon 66 strips
in the hot flue gas environment (F7), at 1790 K with a residence time of 20 s, is shown
in Figure 7using black scattering circles. The absorption was attributed to H
2
O, HCN,
and C
2
H
2
. Using the aforementioned method, the H
2
O absorption between 6484.1 and
6484.6 cm
−1
was fitted through the interception of the absorption spectra at the same
wavelength in Figure 5, with minimizing deviation. After the fitting process, the entire
H
2
O absorption spectrum was obtained, i.e., the mega dot line in Figure 7. Based on
Equation (2), the gas temperature was determined to be 1180 K. After subtracting the
simulated H
2
O absorbance from the raw absorbance, the absorption spectrum of HCN
near 6484.78 cm
−1
was obtained. By fitting using the parameters of the HCN line in
the HITRAN2016 database and a Voigt line shape, represented by the blue dot line in
Figure 7, the concentration of HCN was derived. The simulated H
2
O and HCN absorp-
tion was subtracted from the raw absorbance, and the remaining absorbance of near
6484.03 cm
−1
, attributed to C
2
H
2
absorption, was fitted using the parameters of the C
2
H
2
line in the HITRAN2016 database and a Voigt line shape (Figure 7) to obtain the concentra-
tion. In this case, the concentration of HCN and C
2
H
2
where the measurement uncertainty
originated from the absorbance curve-fitting process was determined to be 820
±
190 and
1170 ±340 ppm, respectively. The large uncertainty in the spectrum fitting may originate
from the uneven temperature of the gas plume and unrecognized species.
Processes 2021, 9, x FOR PEER REVIEW 9 of 11
Figure 7. The absorbance spectrum obtained in the plume 5 mm above burning nylon 66 strips in
the hot flue gas environment (F7), at 1790 K with a residence time of 20 s (black scattering circle)
and the fitted one (green solid line), reflects the absorbance spectrum of 11% H
2
O at 1180 K obtained
from our database (magenta dot line), along with the 820 ppm HCN (blue dot line) and 1170 ppm
C
2
H
2
(red dot line) using the HITRAN2016 database.
The measured concentrations of HCN and C
2
H
2
in the plume of the burning nylon 66
strips as a function of residence time are shown in Figure 8. After the first 16 s of preheat-
ing, the release of HCN and C
2
H
2
almost simultaneously began, and lasted over 20 s. The
maximum concentration of HCN was determined to be 4000 ± 820 ppm at 26 s, and the
maximum concentration of C
2
H
2
was determined to be 3800 ± 480 ppm at 22 s. During the
gas release process, the temperature dropped from about 1220 to 1000 K, similar to the
observation of the volatile release of burning biomass particles [2,26].
Figure 8. Variation in the HCN and C
2
H
2
concentrations in the plume 5 mm above burning nylon
66 strips in the hot flue gas at 1790 K provided by flame F7, and the local temperature obtained
based on Equation (2). (HCN: magenta circle, C
2
H
2
: green square, temperature: black diamond).
4. Conclusions
Quantitative and simultaneous measurement of HCN and C
2
H
2
in a combustion en-
vironment, using TDLAS at around 6484 cm
−1
, was developed in the presented work. The
P(11e) line of the first overtone vibrational band v
1
of HCN at 6484.78 cm
−1
and the P(27e)
line of the v
1
+ v
3
combination band of C
2
H
2
at 6484.03 cm
−1
were selected. It was discovered
that the main challenge of accurately measuring HCN and C
2
H
2
was the interference of
Figure 7.
The absorbance spectrum obtained in the plume 5 mm above burning nylon 66 strips in the
hot flue gas environment (F7), at 1790 K with a residence time of 20 s (black scattering circle) and the
fitted one (green solid line), reflects the absorbance spectrum of 11% H
2
O at 1180 K obtained from
our database (magenta dot line), along with the 820 ppm HCN (blue dot line) and 1170 ppm C
2
H
2
(red dot line) using the HITRAN2016 database.
Processes 2021,9, 2033 9 of 11
The measured concentrations of HCN and C
2
H
2
in the plume of the burning nylon
66 strips as a function of residence time are shown in Figure 8. After the first 16 s of
preheating, the release of HCN and C
2
H
2
almost simultaneously began, and lasted over
20 s. The maximum concentration of HCN was determined to be 4000
±
820 ppm at 26 s,
and the maximum concentration of C
2
H
2
was determined to be 3800
±
480 ppm at 22 s.
During the gas release process, the temperature dropped from about 1220 to 1000 K, similar
to the observation of the volatile release of burning biomass particles [2,26].
Processes 2021, 9, x FOR PEER REVIEW 9 of 11
Figure 7. The absorbance spectrum obtained in the plume 5 mm above burning nylon 66 strips in
the hot flue gas environment (F7), at 1790 K with a residence time of 20 s (black scattering circle)
and the fitted one (green solid line), reflects the absorbance spectrum of 11% H
2
O at 1180 K obtained
from our database (magenta dot line), along with the 820 ppm HCN (blue dot line) and 1170 ppm
C
2
H
2
(red dot line) using the HITRAN2016 database.
The measured concentrations of HCN and C
2
H
2
in the plume of the burning nylon 66
strips as a function of residence time are shown in Figure 8. After the first 16 s of preheat-
ing, the release of HCN and C
2
H
2
almost simultaneously began, and lasted over 20 s. The
maximum concentration of HCN was determined to be 4000 ± 820 ppm at 26 s, and the
maximum concentration of C
2
H
2
was determined to be 3800 ± 480 ppm at 22 s. During the
gas release process, the temperature dropped from about 1220 to 1000 K, similar to the
observation of the volatile release of burning biomass particles [2,26].
Figure 8. Variation in the HCN and C
2
H
2
concentrations in the plume 5 mm above burning nylon
66 strips in the hot flue gas at 1790 K provided by flame F7, and the local temperature obtained
based on Equation (2). (HCN: magenta circle, C
2
H
2
: green square, temperature: black diamond).
4. Conclusions
Quantitative and simultaneous measurement of HCN and C
2
H
2
in a combustion en-
vironment, using TDLAS at around 6484 cm
−1
, was developed in the presented work. The
P(11e) line of the first overtone vibrational band v
1
of HCN at 6484.78 cm
−1
and the P(27e)
line of the v
1
+ v
3
combination band of C
2
H
2
at 6484.03 cm
−1
were selected. It was discovered
that the main challenge of accurately measuring HCN and C
2
H
2
was the interference of
Figure 8.
Variation in the HCN and C
2
H
2
concentrations in the plume 5 mm above burning nylon
66 strips in the hot flue gas at 1790 K provided by flame F7, and the local temperature obtained based
on Equation (2). (HCN: magenta circle, C2H2: green square, temperature: black diamond).
4. Conclusions
Quantitative and simultaneous measurement of HCN and C
2
H
2
in a combustion
environment, using TDLAS at around 6484 cm
−1
, was developed in the presented work.
The P(11e) line of the first overtone vibrational band v
1
of HCN at 6484.78 cm
−1
and
the P(27e) line of the v
1
+v
3
combination band of C
2
H
2
at 6484.03 cm
−1
were selected.
It was discovered that the main challenge of accurately measuring HCN and C
2
H
2
was
the interference of water vapor absorption in the combustion environment. To eliminate
the influence, accurate knowledge of the hot water lines was important. However, the
simulation using the most prevailing databases, HITRAN2016 and HITEMP2010, was
inconsistent with the experimental measurement. Thus, an important step was carrying out
a comprehensive investigation of the temperature-dependent water absorption in various
hot gas environments at temperatures ranging from 1120 to 1950 K. Based on the measured
absorption spectra of H
2
O at different temperatures, the HCN and C
2
H
2
absorption spectra
were resolved after H
2
O absorption subtraction, and the concentrations of HCN and C
2
H
2
were calculated through the spectral fitting using the HITRAN database. The technique
was applied to simultaneously measure the temporally resolved release of HCN and
C
2
H
2
over burning nylon 66 strips in a hot oxidizing environment at 1790 K. In the hot
plume 5 mm above the stripes, the maximum concentration of HCN was detected to be
4000
±
820 ppm at the residence time of 26 s, and C
2
H
2
was detected to be 3800
±
480 ppm
at 22 s.
Supplementary Materials:
The following are available online at https://www.mdpi.com/article/
10.3390/pr9112033/s1, The supplementary material includes the measured hot water absorption
spectra between 6483.8 and 6485.8 cm−1at 1120, 1260, 1350, 1550, 1750, and 1950 K.
Author Contributions:
Conceptualization, W.W. and Z.L.; methodology, W.W. and Z.L.; investiga-
tion, W.W.; resources, W.W.; data curation, W.W.; writing—original draft preparation, W.W.; writing—
review and editing, Z.L.; visualization, W.W.; project administration, Z.L.; funding acquisition, M.A.
All authors have read and agreed to the published version of the manuscript.
Processes 2021,9, 2033 10 of 11
Funding:
The work was financially supported by the Swedish Energy Agency (KC-CECOST, 22538-4
biomass project), the Knut & Alice Wallenberg Foundation (COCALD KAW2019.0084), European
Research Council ERC Advanced Grant TUCLA 669466) and the Swedish Research Council (VR).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Conflicts of Interest: The authors declare no conflict of interest.
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