1. Introduction
Recently, pollutants generated during the combustion of hydrocarbon-based fuels have caused serious problems related to air pollution, and hence, the regulations on emissions have been strengthened. Meanwhile, the development of high-efficiency combustion technology has been promoted to reduce fuel consumption. Nitrogen oxides (NOX) and sulfur oxides (SOX) formed during the combustion process are considered as fine dust precursors. Therefore, the development of a clean combustion technology that suppresses the generation of air pollutants, such as fine dust precursors in the combustion process, and that of high-efficiency combustion technology for improving the combustion efficiency, are being actively promoted.
Carbon monoxide (CO) produced during the combustion of hydrocarbon fuels is a colorless and odorless toxic gas, whose concentration increases in case of incomplete combustion. Moreover, it is a factor that directly affects the combustion efficiency. CO has a trade-off with NO
X: it decreases during complete combustion, while NO
X increases with the flame temperature [
1,
2]. Therefore, in order to optimize the combustion condition of a large combustion system, such as a power plant boiler and steel annealing, it is essential to measure the CO concentration in real-time. A combustion environment that requires measurement of CO concentration is a steel annealing system. In the case of the non-oxidation furnace (NOF) zone in a steel annealing system, the combustion environment is maintained in a deoxidation atmosphere to prevent the formation of oxide layers on the steel sheet surface. For this reason, it maintains a fuel-rich condition, generating considerable CO. In these processes, to increase the system efficiency, to comply with atmospheric regulations, and to maintain production quality, combustion control through continuous monitoring based on CO concentration measurement in real-time is required for technological applications.
Most of the devices for measuring CO concentration are contact-type, such as an electrochemical sensor; these devices are sampling methods that are insufficient for analyzing the overall internal circumstance in large-scale combustion systems. Moreover, an electrochemical method usually measures a high CO concentration with a higher dilution factor, but the dilution rate causes a large error. These methods have a long response time due to the sampling and analyzing process, and they need frequent replacement due to the persistence limitation of sensor sensitivity in harsh environments. Consequently, the existing methods have a limitation in measuring the CO concentration in a combustion environment in real-time.
Thus, optical technologies with a non-contact measurement method have been considered to measure CO concentration in a combustion environment. Among the typical optical measurement methods, tunable diode laser absorption spectroscopy (TDLAS), which exhibits high selectivity, durability, and fast response characteristics through a narrow linewidth, is mainly used in combustion environments [
3,
4,
5,
6,
7,
8,
9].
Various CO concentration measurement methods can be found in the literature. From 1990 to the beginning of 2019, CO concentration has been measured within the near-infrared wavelength range of 6410 cm
−1 to understand the phenomenon of the combustion reaction in coal-fired power plants [
5]. On the other hand, the mid-infrared wavelength range has been used to measure CO concentration under various combustion environment conditions: it has been measured in a mid-infrared wavelength region of 4300 cm
−1 [
10,
11] and another region of 2173 cm
−1 [
12]. After analyzing previous studies in detail, Chao et al. [
10] measured the CO concentration of a premixed ethylene/air flame using a distributed feedback (DFB) laser in the mid-infrared wavelength region, but the measurement was limited to fuel-lean conditions. Mihalcea et al. [
8] measured CO, CO
2, and CH
4 concentrations of a premixed methane/air flame by using a DFB laser in the near-infrared wavelength region, but increased the optical distance to ensure sufficient resolution. In addition, Spearrin et al. [
12] measured CO and CO
2 concentrations and temperature in a scramjet combustion chamber using quantum cascade (QC)-TDLAS in the mid-infrared wavelength region. Summarizing the previous studies, CO concentration has been measured at 6410 cm
−1 in the near-infrared and 4300 cm
−1 and 2173 cm
−1 in the mid-infrared regions. In the mid-infrared wavelength of 2173 cm
−1 region, the QC-TDLAS system has been used with a high resolution; however, it is large and suited better for a laboratory environment rather than field measurement environments because of the cooling problem of the laser. Meanwhile, a low resolution has been confirmed for the blended or interference absorption of H
2O in the near-infrared 6410 cm
−1 region. The mid-infrared 4300 cm
−1 region using a DFB laser has been considered suitable for field measurements, as there are no H
2O interference and cooling problems of the laser.
Therefore, in the present study, the mid-infrared 4300.7 cm−1 region was selected for measuring CO concentration in a Liquefied Petroleum Gas (LPG)/air flame. Most of the experiment was performed under a fuel-rich condition with a high CO concentration. The present study aimed to determine the detection limit of CO concentration measurement in the combustion gas using the TDLAS method. The experimental results were compared with the theoretical values.
4. Results and Discussion
This study aimed to measure various CO concentrations in a quartz gas cell at room temperature and the real combusted gas under the fuel-rich condition and to confirm its detection limit when the TDLAS method was used.
Figure 4a shows the results of the light absorption signal obtained by measuring CO concentrations of 0.1%, 0.25%, 0.5%, 1%, and 2% at room temperature. The light absorption signal reflected well the tendency of the light absorption signal area, which increased with the CO concentration, as shown in Equation (5).
Figure 4b shows a comparison of the integrated absorbance obtained experimentally by analyzing various CO gases through the TDLAS method at room temperature and that obtained theoretically by simulation of the Voigt line-shape function [
16]. At CO concentrations of 0.1%, 0.25%, 0.5%, 1%, and 2%, both the experimental and theoretical integrated absorbances were well-fitted.
However, the result analyzed at above 2% CO concentration had a large error. The peak absorbance values derived via Equation (12) were 0.582 and 0.777 for CO concentrations of 1.5% and 2%, respectively. An error in the concentration analysis occurred when the peak absorbance was 0.958 at a CO concentration of 2.5%.
Table 2 shows the measurement results of CO concentration in the preliminary experiment. Summarizing the results of the preliminary experiment, the concentration range was measured from 0.101% to 2.021% under room temperature and atmospheric pressure at a path length of 111 cm. Under most experiment conditions, except for where the CO concentration was 2.5%, the peak absorbance was calculated to be below 0.8, and the maximum error rate was 1.60%. At a CO concentration of 2.5%, the peak absorbance was 0.958, and the concentration measurement failed because of the absorption signal of optically thick conditions for calculation.
According to the measurement results of CO concentration in the laboratory environment, an experiment was conducted on the CO concentration measurement of the exhaust gas produced by the combustion reaction under fuel-rich conditions.
Figure 5a shows that the experiment was performed at 0.05 intervals of the equivalence ratio ranging from 1.15 to 1.50, and the CO absorption signal increased as the equivalence ratio increased from 1.15 to 1.50. Considerable unburned CO was generated with an increase in the fuel ratio under the fuel-rich condition. Therefore, the increase in the CO absorption signal was expected to match the general trend [
20].
Figure 5b shows the CO absorption signal measured at an equivalence ratio of 1.15 in the combustion environment.
As the water vapor and soot particles generated from the LPG/air flame affected the signal distortion, the integrated absorbance obtained by applying Voigt fitting to the CO absorption signal was compared with the experimentally obtained integrated absorbance without fitting. Likewise, the integrated absorbance derived through Voigt fitting was analyzed for the CO concentration signal throughout the remaining equivalent ratio from 1.20 to 1.50.
Figure 6a shows a comparison between the integrated absorbance of CO absorption signals obtained from TDLAS and that obtained from the simulation of the Voigt line-shape function in the combustion environment. The experimental and theoretical values with increasing equivalence ratio were derived using Equation (5) and Voigt simulation [
16]. When the equivalence ratio was increased from 1.10 to 1.50, the black and red dots in
Figure 6a indicated the integrated absorbance measured via TDLAS and that measured by Voigt fitting, respectively. The integrated absorbance compensated by Voigt fitting almost coincided with the experimental integrated absorbance. Moreover, the integrated absorbance obtained from TDLAS under the conditions of equivalence ratio ranging from 1.15 to 1.50 almost coincided with that obtained through Voigt simulation.
The results obtained in the combustion environment were tested under steady-state conditions.
Table 3 shows the results of the CO concentration measured in the exhaust gas produced under the fuel-rich condition. The CO concentrations at equivalence ratio ranging from 1.15 to 1.50 were measured at 0.495% and 6.139%. The peak absorbance ranged between 0.064 and 0.787, with equivalence ratios of 1.15 and 1.50. When the equivalence ratio increased to 1.55, the peak absorbance was expected to exceed 0.8, according to the increasing tendency. As confirmed in the preliminary experiment, if the equivalent ratio increases beyond 1.55, the absorption signal corresponding to the condition of optically thick is expected to be generated. The absorbance analysis was practically impossible at the equivalence ratio of 1.55. Therefore, the concentration detection limit was confirmed at 6.139% with a path length of 190 cm, based on the exhaust gas temperature of 733 K.
Comparing the results obtained in the laboratory environment at room temperature with those obtained in the combustion environment in
Table 3, it was difficult to measure the CO gas concentration above 2% in the laboratory environment, but that in the combustion environment could be measured up to 6.139%. The peak absorbance in Equation (12) indicated that the detection limit was proportional to the concentration, path length, pressure, the peak value of the Voigt line-shape function, and line strength, which is a function of temperature. It could be confirmed that the CO line strength in the combustion environment in
Figure 1b was lower than that at room temperature in
Figure 1a. As shown in Equation (8), as the temperature increased, the line width increased due to the Doppler-broadening effect, which lowered the peak value of the Voigt line-shape function. According to Equation (12), the lower line strength created a margin for other variables, such as concentration, under the same peak absorbance condition. For this reason, the CO concentration measurement range increased in the combustion experiment as compared to that in the preliminary experiment. These results indicated that the path length was a recessive variable than the temperature to affect the detection limit. Comparing the results of the two experiments, the concentration detection range was increased, although the path length increased in the combustion environment experiment. It was expected that the CO concentration measurement range could be reduced because of the increase in the path length, according to Equation (12). However, the effect of temperature was dominant, which made the concentration detection range wider over the disadvantageous path-length condition.
Hence, the prediction of temperature, path length, and concentration in the measurement environment are important to judge whether it is measurable. In the case of the NOF zone of a steel annealing system, the internal temperature of the combustion furnace was generally 1273.15 K or higher, and the operation was performed at an equivalence ratio range of 1.10–1.40. Therefore, the CO concentration range between 1% and 2.5% was considered important in the NOF zone.
Figure 6b shows the results of simulating the detection limit range under various temperature and CO concentration conditions at 1 atm. The Y-axis of the figure indicates the peak absorbance normalized by path length, which is an independent variable that is not coupled with other variables, except for peak absorbance; the lower peak absorption limit was set at 10
−3.
From the simulation results, the detection limit range of the concentration could be expected according to the temperature and path length. As described above, as the internal temperature of the NOF zone operated at 1273.15 K or more, the detection limits for measuring the CO concentration range could be predicted as follows. Based on the target temperature of 1273.1–1673.15 K, the peak absorbance was determined from 0.03 to 0.6, and it contained CO concentration ranging from 1% to 10% when the path length was 300 cm. According to these results, the detection limit of the CO concentration could be wider under the higher-temperature condition than that under the lower-temperature condition. In conclusion, when the actual NOF zone condition was 1% or more of the CO concentration at a temperature ranging from 1273.15–1673.15 K, the simulation result obtained under the condition of 300 cm path length was suitable for the detection limit range of DAS measurement with high resolution.
5. Conclusions
In this study, the CO concentration in the combustion environment was measured using TDLAS. The detection limit of the CO concentration was analyzed under fuel-rich conditions. The wavenumber required to obtain an optical absorption signal of CO without interference from other combustion products at the desired measurement temperature was confirmed by HITRAN.
The experiment was divided into two categories. First, it was performed to measure various CO concentrations under room temperature in a laboratory environment. Second, the combustion system was designed to produce fuel-rich conditions. Fuel-rich conditions were formed at equivalence ratios of 1.15 and 1.50. As a result of the laboratory environment measurement, the detection limit of the CO concentration measured from 0.101% to 2.021% ranged peak absorbance between 0.038 and 0.777.
According to the results of the combustion environment, the CO concentration was measured as 0.495% at an equivalence ratio of 1.15 and 6.139% at a maximum equivalence ratio of 1.50. The detection limit in the combustion environment was analyzed at a path length of 190 cm and an internal temperature of 733 K. The peak absorbance was derived to range between 0.064 and 0.787, which were included between 10−3 and 0.80, when the equivalence ratio was varied from 1.15 to 1.50.
To determine the optimum detection limit range to be measured in various environments, the optimum measurement conditions according to temperature and concentration were derived through Voigt simulation. In this study, the detection limit was considered to be an accurate indicator for high CO concentration measurement in a fuel-rich environment, such as the NOF zone in the steel annealing system. The CO concentration measurement using TDLAS is expected to make an essential contribution to optimal combustion control.