Industrial Applications of Tunable Diode Laser Absorption Spectroscopy

Tunable diode laser absorption spectroscopy (TDLAS) utilizes the absorption phenomena to measure the temperature and species concentration. The main features of the TDLAS technique are its fast response and high sensitivity. Extensive research has been performed on the utilization of diode laser absorption spectroscopy for the system monitoring and its control. The TDLAS technique gives self-calibrations to reduce the noise such as particles and dusts because the laser wavelength is rapidly modulated at kHz rates. In addition, two dimensional (2D) temperature and concentration distributions can be obtained by combining computed tomography (CT) with TDLAS. The TDLAS applications have been extensively studied with great progress. This chapter largely focuses on the engineering fields, especially the practical industrial applications

Simultaneous two cross-sectional measurements of NH3 concentration in bent pipe flow using CT-tunable diode laser absorption spectroscopy

Urea Selective Catalytic Reduction (urea SCR) system is widely used for diesel engine to reduce the emission of NOx by NH3 which is provided by a hydrolysis of urea water. Concentration distribution of NH3 in an exhaust pipe is an important factor for improvement of the SCR efficiency and prevention of NH3 slip and urea deposit. Therefore, it is necessary to measure two-dimensional (2D) concentration of NH3 in detail. The purpose of this study is to develop the real-time two cross-sectional measurements technology of NH3 concentration using the computed tomography-tunable diode laser absorption spectroscopy (CT-TDLAS). Theoretical NH3 concentration distribution which was reconstructed by CT agreed to CFD results and quadruple pipe’s results showed good resolution by 14th order reconstruction. Therefore, this method has enough resolution and accuracy for measuring the concentration distribution of NH3. And this method was employed in a bent pipe model demonstrated a urea SCR system. The experimental results of two cross-sectional 2D concentration of NH3 show differences of the concentration distribution of NH3 each cross-section and flow pattern like swirl flow. It was found that CT-TDLAS was an effective method to measure concentration distribution of NH3 and observe characteristics of flow. In addition, observing flow pattern enable to validate CFD results, and it helps to improve efficiency of after treatment system

Pulverized coal combustion application of laser-based temperature sensing system using computed tomography : Tunable diode laser absorption spectroscopy (CT-TDLAS)

The investigation of combustion phenomena in pulverized coal flames is significant for combustion optimization related to energy conservation and emission reduction. Real-time two dimensional (2D) temperature and concentration distributions play an important role for combustion analysis. The non-contact and fast response 2D temperature and concentration distribution measurement method was developed in this study. The method is based on a combination of computed tomography (CT) and tunable diode laser absorption spectroscopy (TDLAS). The accuracy evaluation of developed 32-path CT-TDLAS demonstrated its feasibility of 2D temperature measurement. 32-path CT-TDLAS was applied to CH4 and 5 kg/h coal combustion fields for 2D temperature measurement. The time-series 2D temperature distribution in coal combustion furnace was measured using 32-path CT-TDLAS measurement cell with kHz time resolution. The transient temperature field of combustion flame directly reflects the combustion mode and combustion stability. The measurement results demonstrate its applicability of CT-TDLAS to various types of combustor, especially the combustion fields with coal and ash particles. CT-TDLAS method with kHz response time enables the real-time 2D temperature measurement to be applicable for combustion analysis

Feasibility of Controlling Gas Concentration and Temperature Distributions in a Semiconductor Chamber with CT-TDLAS

The feasibility to control the gas concentration and temperature distributions in a semiconductor process chamber by measuring them was investigated. Gas concentration and temperature distributions for various flow rates were measured with the computed tomography-tunable diode laser absorption spectroscopy (CT-TDLAS). The infrared absorption spectra of multiple laser paths passing through the measured area were collected and the distributions of methane concentration and temperature in the chamber were reconstructed with the computed tomography (CT) calculations. The measured results indicated that the distributions can be independently controlled by measuring with the CT-TDLAS and adjusting the flow rates and the susceptor temperature

Validation of real-time 2D temperature measurement method using CT tunable diode laser absorption spectroscopy

Two dimensional (2D) temperature and concentration distribution plays an important role for the combustion structure and the combustor efficiency in engines, burners, gas turbines and so on. Recently, as a multi-species measurement technique with high sensitivity and high response, tunable diode laser spectroscopy (TDLAS) has been developed and applied to the actual engine combustions. With these engineering developments, transient phenomena such as start-ups and load changes in engines have been gradually elucidated in various conditions. In this study, the theoretical and experimental research has been conducted in order to develop the non-contact and fast response 2D temperature and concentration distribution measurement method. The method is based on a computed tomography (CT) using absorption spectra of water vapor at 1388 nm. The computed tomography tunable diode laser spectroscopy (CT-TDLAS) method was employed in engine exhausts to measure 2D temperature distribution. The measured 2D temperature shows a good agreement with the temperature measured by a thermocouple. The temporal and spatial resolutions of this method have also been discussed to demonstrate its applicability to various types of combustor

Applications of TDLAS based multi-species hydrocarbon measurement using a wide scanning range DFG laser

Tunable diode laser absorption spectroscopy (TDLAS) is a widely used hydrocarbon gas sensing method in many fields. However, the short scanning range limits its application where multi-species detection is necessary. In this paper, a laser system based on TDLAS using a difference frequency generation laser was applied for the investigation of the hydrocarbon gases produced in the coal pyrolysis process and engine exhaust. The coal sample was heated up to 623 K and the recorded spectra were analyzed by the comparison with the pure hydrocarbon spectra database. A least-squares fitting was performed to quantitatively determine the concentration of each component of the mixture. Totally nine different hydrocarbons were identified and the R2 values close to 1 indicate that the variance between measured and fitted data was small. The spectra of engine exhaust were recorded and analyzed using the same method. Hydrocarbon from C3–C8 and a small amount of methane and ethene were identified. The concentration variation with time was observed

EVALUATION OF 3D MEASUREMENT USING CT-TDLAS

In order to satisfy the requirements of high quality and optimal material manufacturing process, it is important to control the environment of the manufacturing process. Depending on these processes, it is possible to improve the quality of the product by adjusting various gases. With the advent of the TDLAS (Tunable laser absorption spectroscopy) technique, the temperature and concentration of the gases can be measured simultaneously. Among them, CT-TDLAS (Computed tomography-tunable diode laser absorption spectroscopy) is the most important technique for measuring the distributions of temperature and concentration across the 2-dimensional planes. In this study, suggest a 3-dimensional measurement to consider the irregular flow of supplying gases. Used the SMART (simultaneous multiplicative algebraic reconstruction technique) algorithm among the CT algorithms. Phantom data sets have been generated by the using Gaussian distribution method. It can be shown expected temperature and concentration distributions. The HITRAN database in which the thermo-dynamical properties and the light spectra of H2O are listed were used for the numerical test. The relative average temperature error ratio in the results obtained by the SMART algorithm was about 3.2% for temperature. The maximum error was 86.8K

CURRENT DENSITY EFFECTS ON PLASMA EMISSION DURING PLASMA ELECTROLYTIC OXIDATION (PEO) ON AZ91D-MAGNESIUM ALLOY

The effect of bipolar pulse mode current ratio on plasma behavior was investigated in PEO on AZ91D Mg-Alloy. Two cases of current ratio including 1.20 and 0.88 were applied to the sample. Plasma emission behavior was studied using plasma images and plasma emission measured by photodetector and Intensified Charged-Couple Device (ICCD) camera. The current ratio of greater than 1 shows the continuous increase and then stabilization in emission intensity with a gradual increase in voltage throughout the PEO process. In contrast, the current ratio of less than 1, a sudden drop in plasma emission intensity with voltage was found after 786s. Therefore, PEO process can be divided into two regimes, arc regime and soft regime, before and after voltage drop respectively. Results of measured spectra show that a soft regime does not have atomic or ionic excitation during PEO process. It is demonstrated that the growth of porous layer during PEO can be controlled, which is benefit for the protective oxide coating of sample

Two-dimensional temperature measurement in a high temperature and high pressure combustor using CT-TDLAS with a wide scanning laser at 1335-1375nm

Tunable diode laser absorption spectroscopy (TDLAS) technology is a developing method for temperature and species concentration measurements with the features of non-contact, high precision, high sensitivity, etc. The difficulty of two-dimensional (2D) temperature measurement in actual combustors has not yet been solved because of pressure broadening of absorption spectra, optical accessibility, etc. In this study, the combination of computed tomography (CT) and TDLAS with a wide scanning laser at 1335-1375nm has been applied to a combustor for 2D temperature measurement in high temperature of 300-2000K and high pressure of 0.1-2.5MPa condition. An external cavity type laser diode with wide wavelength range scanning at 1335-1375nm was used to evaluate the broadened H2O absorption spectra due to the high temperature and high pressure effect. The spectroscopic database in high temperature of 300-2000K and high pressure of 0.1-5.0MPa condition has been revised to improve the accuracy for temperature quantitative analysis. CT reconstruction accuracy was also evaluated in different cases, which presented the consistent temperature distribution between CT reconstruction and assumed distributions. The spatial and temporal distributions of temperature in the high temperature and high pressure combustor were measured successfully by CT-TDLAS using the revised spectroscopic database

Application of 2D temperature measurement for coal-fired furnace using CT-TDLAS

The measurement of temperature and species concentrations in combustion fields is very significant to develop the high-efficient combustion technologies for energy conservation and emission reduction. There are various measurement technologies including contact and non-contact measurement. Tunable diode laser absorption spectroscopy (TDLAS) technology is a proven non-contact method to detect the temperature and species concentrations by absorption measurement. To enable two-dimensional (2D) representation of temperature and species concentrations in combustion fields, the TDLAS technology is usually combined with computed tomography (CT). The latter is however considerably new in combustion research, especially in solid fuels reaction environment. In this paper, a 32-path, 2D CT-TDLAS system for temperature measurement in a pilot scale, coal-fired furnace was developed. The accuracy of CT algorithm to reconstruct 2D temperature distributions in different laser-paths arrangements was first analysed using SSD (sum of squared difference) and ZNCC (zero-mean normalized cross-correlation) by comparing to 2D temperature distribution of a full scale coal-fired furnace simulated using computational fluid dynamics (CFD). The accuracy was improved by 32-path reconstruction. The study was then progressed to investigate its accuracy for measurement in a simple CH4-air burner configuration with rounded and rectangular cells as well as sensitivity for flame shift detection whereby the reconstructed temperature distribution was compared to temperature measured using thermocouple. It is verified that this CT reconstruction was feasible for various measurement areas, even if the center of flame was shifted. Finally, a 32-path, 2D CT-TDLAS system with rectangular structure cell was developed and applied for a temperature measurement in a TNB Research’s pilot scale coal-fired furnace. 2D temperature distribution in coal-fired furnace was reconstructed accroding to the experimental results. It is demonstrated the potential of CT-TDLAS for online 2D temperature measurement for actual applications