A NOx and SO 2 gas analyzer using deep-uv and violet light-emitting diodes for continuous emissions monitoring systems Ryoichi Higashi a,c, Yu Taniguchi a, Kozo Akao a, Kazuhiro Koizumi a, Noritomo Hirayama a and Yoshiaki Nakano b,c a Fuji Electric Co., Ltd., 1, Fuji-machi, Hino, Tokyo 191-852, Japan; b Department of Electrical Engineering and Information Systems (EEIS), Graduate School of Engineering, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan; c Department of Advanced Interdisciplinary Studies (AIS), Graduate School of Engineering, The University of Tokyo, 4-6-1, Komaba, Meguro-ku, Tokyo 153-894, Japan ABSTRACT A nitrogen oxides (NOx) and sulfur dioxide (SO 2 ) gas analyzer using deep ultraviolet (DUV) and violet lightemitting diodes (LEDs) is developed. The LEDs with wavelengths of 28 nm and 4 nm were alternately turned on to detect SO 2 and nitrogen dioxide (NO 2 ) absorption. Nitric oxide (NO) was converted to NO 2 with an ozonizer. In order to reduce water interference caused by water adsorption onto an inner surface of a gas flow cell, collimating optics reducing reflected lights were designed. As a result, less than 1% by full scale (%F.S.) of fluctuation, 2%F.S. of drift and.5%f.s. of water interference were achieved in -5 ppm concentration range. Conversion efficiency from NO to NO 2 was over 95%. Keywords: UV, LED, sensing, analyzer, NOx, NO 2, SO 2, gas, absorption 1. INTRODUCTION Gas sensing is one of the promising applications of DUV LEDs. For continuous emissions monitoring systems in industry, concentrations of NOx (NO + NO 2 ) and SO 2 in exhaust gas shall be continuously measured. Currently, non-dispersive infrared (NDIR) or non-dispersive ultraviolet (NDUV) gas analyzers are commonly used whose light sources are filaments or lamps. They require mechanical choppers for modulating light and wavelength filters for narrowing emission spectra of light sources. If they are replaced by LEDs, gas analyzers need no mechanical choppers or wavelength filters because the lights are turned on and off by injecting currents and spectral bandwidths are originally narrow. Recently, AlGaN based DUV LEDs have been researched and developed, whose wavelengths range from 21 nm to 36 nm. 1 5 Furthermore, DUV LEDs with wavelength longer than 24 nm are commercially available. These LEDs can be applied to NDUV gas analyzers based on absorption spectroscopy to measure NO 2 and SO 2 gas concentrations because their central emission wavelengths are selected at peak absorption wavelengths of NO 2 and SO 2. Degner et al. showed that the approach can resolve NO 2 and SO 2 concentrations. 6 However, there remain two issues. One issue is that further investigation of performances is necessary such as drift and water interference, which are indicators of gas analyzer performance. Another issue is that NO has maximum absorption wavelength of 226nm in room temperature in UV region which existing commercial UV LEDs do not cover. Therefore, direct absorption spectroscopy of NO is difficult and an alternative approach is necessary. In order to solve the issues, we designed and validated a UV LED based NO 2 and SO 2 gas analyzer with a careful output power monitoring and light collimating optics to realize low drift and low water interference. We also propose that NO is converted to NO 2 by reacting NO with ozone. NOx concentration, which is sum of NO and NO 2 concentrations, could be measured using the analyzer and the converter. Send correspondence to R. Higashi: E-mail: higashi-ryouichi@fujielectric.co.jp
Light source Gas inlet Gas outlet PD Window Gas flowing cell Window Figure 1. The schematic of a NDUV gas analyzer. UV light from a light source is absorbed by specific gas and transmitted light is detected by a photodiode. 2. NO 2 AND SO 2 CONCENTRATION MEASUREMENT This section describes a UV LED absorption based gas analyzer for NO 2 and SO 2. The method, experimental setup, and evaluation result of fluctuation, drift and water interference are discussed. 2.1 Method of NO 2 and SO 2 Gas Concentration Measurement by UV Light Absorption Fig. 1 shows a schematic of a NDUV gas analyzer with a light source, a gas flowing cell and a detector. Assuming there is homogeneous gas in the gas flowing cell, principle of absorption spectroscopy of specific gas is expressed by the Lambert-Beer law: I 1 (λ) = I (λ)exp[ σ(λ)cl], (1) where, λ is a wavelength of light, I (λ) is initial light intensity, σ(λ) is absorption cross section of the specific gas, c is concentration of the gas, L is the optical path length and I 1 (λ) is transmitted light intensity. This equation means that light is absorbed by the specific gas thorough the optical path and the amount of absorption is proportional to inverse exponential of the absorption cross section, concentration and optical path length. Practically, the optical path length is fixed and concentration reading is calibrated by flowing standard gas into the gas flowing cell. However, I (λ) varies by injection current and ambient temperature variations when LEDs are used as light sources. Therefore, light output monitoring is essential to stabilize concentration measurement. NO 2 and SO 2 absorptions are resolved using UV LEDs as light sources because they have different absorption spectra in UV wavelength. 6 Peak absorption wavelengths are around 4 nm for NO 2 and 28 nm for SO 2. At 28 nm, NO 2 has absorption that is one tenth of weaker than that of SO 2. UV LEDs have typically a few ten nanometers of linewidths. Therefore, NO 2 and SO 2 absorptions are resolved using UV LEDs whose wavelengths of 4 nm and 28 nm, with SO 2 absorption at 28 nm corrected by NO 2 absorption. 2.2 Experimental Setup Fig. 2 shows the experimental setup of the NO 2 and SO 2 analyzer. The light source is an LED array in a can package. It contains two chips of LEDs of wavelength at 28 nm and a single chip of LED at 4 nm. The chips are closely aligned within 1 mm distance in order to collimate and focus lights by the same optics and control temperature of the chips by the same Peltier module. The lights from LEDs are collimated by a lens and divided into reflected lights and transmitted lights by a fused silica window. The reflected lights are focused into a silicon photodiode (Si-PD) to monitor outputs. The transmitted lights are through the gas flowing cell and focused into another Si-PD to measure gas absorptions. Collimating and focusing lights realize that the two PDs detect the lights from LEDs within the same solid angle. This is important to improve long term stability of output monitoring because angular distribution variations of light emissions from LEDs are caused by injection current and ambient temperature variations or degradation of LEDs. The gas flowing cell is a stainless steel pipe with the length of 3 mm and the inner diameter of 24 mm. The top and bottom surfaces are sealed by CaF 2 windows to transmit UV lights. The inner surface is finished
PD Standard gas N 2, NO 2, SO 2 LED array Gas inlet Gas outlet PD Lens Beam splitter Gas flowing cell Figure 2. The experimental setup. by electro-chemical-buffing, which prevents gas molecules from adsorbing the surface. The size of the inner diameter is determined so that collimated lights do not reflect the inner surface. This is effective to reduce water interference in concentration measurement. When water vapor adsorb the inner surface, UV light reflectivity changes. 7 If the inner surface is used as a light guide, the reflectivity variation causes incident power change at the absorption detection PD. Thus, even though water itself has no absorption cross section at 28 nm and 4 nm, adsorbed water cause water interference. The 28 nm LEDs and the 4nm LED are alternately turned on and off in every 12 ms to resolve NO 2 and SO 2 gas absorption. Each pulse width is.6 ms. Peak optical outputs are controlled around 1 mw by constant injection currents. The can package is temperature controlled by the Peltier module at 3 degrees of Celsius. Photocurrents at PDs are converted into voltages by trans-impedance amps and the voltages are integrated. Each normalized transmission is calculated by the integrated voltage at the absorption detection PD divided by that at the output monitoring PD. Using N2, NO 2 and SO 2 standard gases, zero point and span point concentrations are calibrated in advance. NO 2 and SO 2 standard gas concentrations are 5 ppm by volume. The concentration from to 5 ppm is a typical target range of continuous emissions monitoring systems applied to exhaust gas in industry. Fluctuations, drifts and water interferences in concentration measurements are evaluated in unit of percent by full scale (%F.S.). 2.3 Results and Discussions Fig. 3 shows the result of simultaneous concentration measurements and fluctuation evaluation. Concentration readings averaged for 3 s are obtained every 1 s. Note that 1% of NO 2 concentration readings are subtracted from SO 2 concentration readings in real time because NO 2 has absorption at 28 nm. NO 2 and SO 2 concentrations are resolved and simultaneously measured owing to the subtraction. Each fluctuation, which is variation of readings in every 1 s for a few hundreds of seconds, is lower than 1%F.S. Therefore, this analyzer has sufficiently low fluctuations or signal to noise ratio for continuous emissions monitoring systems. Fig. 4 shows the result of zero point drift evaluation for more than 15 hours. Light output variations or degradations are compensated by monitoring and drifts are within +/- 2%F.S. This performance is acceptable for continuous emissions monitoring systems. However, there remains one day cycle variations. The cause is that ambient temperature variation changes angular distribution of LED lights or light axes and output compensation may be incomplete. Fig. 5 shows the result of water interference. Water vapor saturated at 2 degrees Celsius is flown. Both interferences in NO 2 and SO 2 measurement are lower than.5%f.s., which is sufficiently low as gas analyzers for continuous emissions monitoring systems. Residual water interference may be caused because water vapor adsorbed on windows light transmissions were reduced. In this evaluation, water vapor concentration equaled to saturated vapor at 2 degrees Celsius. However, exhaust gas contains more water vapor because gas temperature is typically higher than 25 degrees Celsius and water vapor is saturated. Therefore, exhaust gas needs to be extracted and cooled to remove water vapor in advance.
N 2 NO 2 SO 2 N 2 12 1 8 6 4 2-2 5 1 15 2 Time (s) N 2 NO 2 SO 2 N 2 5 4 3 2 1-1 -2-3 -4-5 5 1 15 2 Time (s) Figure 3. Simultaneous concentration measurements of NO 2 and SO 2. The right graph is a magnification of the left graph in vertical axis around zero points. Arrows above the graphs mean times for which each standard gas is flown. 12 1 8 6 4 2 5 1 15-2 Time (hour) 1 8 6 4 2-2 -4-6 -8-1 5 1 15 Time (hour) Figure 4. Drifts of zero point measurement of NO 2 and SO 2. The right graph is a magnification of the left graph in vertical axis around zero points. NO 2 and SO 2 of 5 ppm are flown at around 4 and 147 hours to check span variations. N 2 Water vapor N 2 2 1.5 1.5 -.5-1 -1.5-2 1 2 3 4 5 Time (s) Figure 5. Water interference of NO 2 and SO 2 concentration measurements. Arrows above the graphs mean times for which each gas is flown.
O 2 NO+NO 2 Ozonizer Reactor Thermal Decomposition pipe Gas Analyzer Figure 6. The experimental setup to evaluate conversion efficiency from NO to NO 2 by mixing ozone. 4 4 Gas Concentration Readings (ppm) 35 3 25 2 15 1 5-5 1 2 3 4 Provided Ozone Concentration (ppm) NO 2 NO Gas Concentration Readings (ppm) 35 3 25 2 15 1 5-5 1 2 3 4 Provided Ozone Concentration (ppm) NO 2 NO Figure 7. The results of the conversion experiment from NO to NO 2, without decomposition (left) and with decomposition (right) by the thermal decomposition tube. Dotted lines are virtual lines in the case that conversion efficiencies are 1%. 3. CONVERSION FROM NO TO NO 2 BY OZONE When NO gas and ozone gas are mixed, NO is converted to NO 2, following a chemical reaction; Furthermore, excess ozone reacts NO 2 ; NO + O 3 NO 2 + O 2. (2) 2NO 2 + O 3 N 2 O 5 + O 2. (3) When NOx (NO + NO 2 ) concentration is measured by sum of original NO 2 and converted NO 2 concentrations by the NDUV gas analyzer, an issue is that the reaction described in the equation (3) is undesirable because N 2 O 5 has no absorption cross section at wavelength of 4 nm. Fig. 6 shows the experimental setup to solve the issue and evaluate conversion efficiency from NO to NO 2. A thermal decomposition tube, which is made of stainless pipe with a heater is added after reaction of sample gas and ozone. The tube will decompose ozone to oxygen and N 2 O 5 to NO 2. Ozone is generated from oxygen in air by using a discharging ozone generator. NO and NO 2 gas whose concentrations are known are provided and mixed with ozone at a reactor made from jointed fluoro-plastic tubes. After the decomposition tube, gas concentrations are measured by a NDIR gas analyzer. Fig. 7 shows the results. The concentrations before reactions are 2 ppm for NO and 165 ppm for NO 2. Ozone concentration is variable. When the thermal decomposition tube is not heated, concentration readings of NO 2 decrease as excess ozone concentration increases. When the thermal decomposition tube is heated around 3 degrees of Celsius, concentration readings of NO 2 does not decrease as excess ozone concentration increases and small concentration of NO is detected. Therefore, decomposition of N 2 O 5 effectively works even though some NO 2 or N 2 O 5 are decomposed to NO. Conversion efficiency is over 95% if excess ozone concentration is appropriately controlled, for example, 5% of span gas concentration of NO.
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