Keywords

Ultraviolet, Photometers, Absorption, Process ConULTRAVIOLET ABSORPTION SPECTROSCOPY, NITRIC OXIDE, NO_X

Abstract

The accurate analysis of nitrogen oxides at trace levels is of increasing importance in our society, due to its implications on photochemical smog. This has led to tighter regulations on emissions, and more stringent demands on the analytical performance of emissions monitoring systems. To meet these requirements, improved means for the detection of low levels of NO_x are required.

The use of ultraviolet absorption spectroscopy as a means for the selective determination of low levels of NO_x has been proposed and used for a number of years. In many cases, it provides better accuracy than other analytical methods and at lower cost. One drawback has been its inability to match the low concentration performance and rapid response of chemiluminescence analyzers.

The speed of response and low detection limit requirements has been addressed in a new analyzer configuration. Unlike other ultraviolet absorption analyzers, this device is able to give extremely rapid response to changes in gas concentration with no associated reduction in signal-to-noise ratios. As well, the device uses a novel means to convert NO₂ to NO, which displays high conversion efficiencies, long life, and reduced costs to end-users. In fact, the NO_x converter becomes an integral part of the system, capable of providing an important analytical measurement itself. Data is presented to demonstrate the efficiency of the converter, the speed of response of the analyzer, and the accuracy of the measurement.

Introduction

The measurement of NO_x emissions levels is of primary importance due to the adverse effects which result from elevated concentrations of these species in ambient air. These effects include acidic deposition on natural resources, increased levels of photochemical smog with the associated impacts on public health, and reduced yields of crops. Of particular importance is the role of NO_x in formation of ozone in the troposphere. This reaction proceeds through a basic photochemical cycle in which Nitrogen Dioxide (NO₂) is photo-dissociated into NO and an oxygen atom, and the resultant oxygen atom reacts with atmospheric oxygen to produce ozone:

NO₂ +hv -> NO + O
O + O₂ -> O₃

Complex chain reactions between the NO_x species, ozone and hydrocarbons yield what has been referred to as ozone smog or photochemical smog. Recent data implies that various unsaturated hydrocarbons (ethylene, propylene) act as triggers for unusually rapid increases in photochemical smog [1] in the presence of tropospheric ozone.

The US Environmental Protection Agency (EPA) has taken several steps to reduce NO_x emissions and the time period from 2003 to 2007 marks a transition line for many US industries to comply with recent NO_x emissions standards. The recently finalized NO_x State Implementation Plan (SIP) Call substantially limits summer season NO_x emissions in 22 states in the Eastern US. The California SCAQMD has long been known for its stringent emissions regulations, but recently Texas has promulgated even more stringent regulations, with a significant focus on small to mid-size sources.

With more stringent regulations comes the requirement for improved monitoring systems for the measurement of low-level NO_x in stack gases. While chemiluminescent analyzers have the required sensitivity, issues related to the accuracy, reliability and system complexity have been raised [2,3,4]. Ultraviolet absorption spectroscopy has also been used, and has been proposed [5] as an alternative to EPA method 7E for NO_x. While traditional ultraviolet absorption analyzers have had difficulties meeting the low detection limits required for trace NO_x analysis, a novel “No Moving Parts” spectrometer is presented herein which can meet these requirements. Also presented is an improved data processing methodology which allows for fast response times and a novel, highly efficient method for converting NO₂ to NO.

Spectrometer Design

The spectrometer used was of a dual-beam multi-wavelength design, as shown schematically in Figure 1. The light sources used in this device are hollow cathode lamps, chosen specifically for their narrow emission lines at specific and reproducible wavelengths. The measure and reference detectors are photomultiplier tubes (PMT) chosen specifically for their excellent photometric accuracy and linearity [6]. Fixed filters in front of each light source are used to isolate specific emission lines.

The dual-beam design was chosen to provide exceptional baseline stability. In this device, a separate reference and measure path is maintained for each measurement wavelength employed. The light emitted from the lamp is split into two separate beams through the use of a beam splitter. One beam passes through “clean’ air and impinges on the reference photomultiplier tube (PMT) while the other beam passes through the sample and impinges on the measure photomultiplier tube. Data are acquired from the two PMT’s simultaneously. Any drift in the light source is seen on both the reference and measure path. Taking the ratio of the signal strength as observed on reference and measure paths eliminates the effects of light source drift. This methodology allows for common-mode rejection of both short-term noise and long-term drift in the light sources, resulting in exceptional baseline stability. It is this stability which prompted Henderson to write [7] “Accurate measurement of the absorption coefficient at different wavelengths is best made with a double-beam spectrometer”.

FIGURE 1 – M921 SERIES SPECTROMETER

Hollow cathode lamps were chosen as the light sources for the analyzers for a number of reasons. They emit characteristic emission lines that are determined solely by the materials used for construction of the cathode. The center wavelengths of the emission lines are known to high accuracy and precision, and due to the nature of the discharge in these lamps, the center wavelength does not drift due to temperature, pressure or any other external influence. The emission band is a fundamental property of the material. For example, a cadmium emission line which is used for the measurement of NO occurs at precisely 214.441 nanometers. The bands are also extremely narrow, which contributes to the linearity of the absorbance measurement These narrow precise and reproducible emission lines are instrumental in achieving high resolution, wavelength accuracy and wavelength precision in the spectrometer, independent of variations in the filter bandpass.

For additional information on the design of UV spectrometers, and factors that influence their performance, the reader is directed to papers presented at previous ISA Meetings [8,9]. The spectrometer described herein has been used in a range of applications, some of which are now presented.

Data Processing

The analyzer uses two fixed filters in front of two emission sources. Both lamps are on at the same time, and there is no mechanical chopper in the system. The light intensity from each lamp is modulated by varying the current to the lamp, and thereby varying the light intensity. When one lamp is being pulsed, the other lamp is being “simmered” at a low base current. This simmering effect keeps the lamp warm and prevents transients which can occur when the lamp is first turned on. At the same time, the low simmering current preserves the lamp life.

At first glance, this methodology would appear to be a source of non-linear response, in that two different wavelengths of light are incident on the detector at the same time. However, the unique way in which the data is processed prevents this from occurring. Three separate measurements are taken, one when lamp 1 is pulsed and lamp 2 is being simmered, one when lamp 2 is pulsed and lamp 1 is being simmered, and one when both are being simmered.

It can be shown that the measured transmission (as defined as the ratio between the measured intensity on the sample path as to the measured intensity on the reference path) can be given by:

From Equation 1, we note that the actual intensity of the light source cancels out in numerator and denominator. This indicates that the optical configuration and modulation scheme allows for common-mode noise rejection for light source variations. A similar equation applies for the transmission at the second wavelength. Similar equations may be derived to show that effects of variations in detector sensitivity, and optical path transmission can also be reduced by common-mode rejection.

Thus the mechanical design of the spectrometer and the data processing scheme has been optimized to reduce the largest source of noise (light source variations) and many sources of drift (detector drift and optical path variations due to window fouling).

In addition to these obvious benefits, the analyzer employs an adaptive filter to dramatically reduce noise while not impacting speed of response. This adaptive filter is based on a solid statistical model, and is NOT a predictive filter. The adaptive filter is designed such that it can not overshoot as predictive filters can.

Data Processing

The difficulty in measuring low levels of NO_x is compounded by the fact that the measurement of two species (NO and NO₂) is required. Since the molar extinction coefficient of NO₂ is three times less than that of NO, the accuracy of the NO₂ measurement is three times worse than that of the NO measurement on a UV spectrometer. The reported NO_x is the sum of the NO and NO₂, so the noise in the reported NO_x is strongly affected by the noise in the measured NO₂. For this reason, there is a distinct advantage to converting the NO₂ to NO prior to the measurement of NO_x in the ultraviolet.

The conversion of NO₂ to NO is common practice on chemiluminescence-based analyzers, and numerous converters have been used. These include direct thermal converters where an oven is run at extremely high temperatures to convert NO₂ to NO, catalytic converters containing various materials (such as carbon) to chemically reduce the NO₂ to NO at elevated temperatures, and reactive converters containing Molybdenum which can operate at lower temperatures.

In this work, a unique means was used to convert the NO₂ to NO. A zirconium oxide oxygen sensor was used as the thermal catalytic converter. The unique (and patent pending) approach has the advantage of using the oxygen sensor (which is required in most applications) as the NO_x converter. The result is that the converter is added into the system at no extra cost, since an oxygen sensor was also required.

Typical data from a converter efficiency test are presented in Figure 2, where a range of NO₂ concentrations (balance air) were run through the converter, and the resultant NO_x measured as NO. Typical converter efficiencies of 98.5 to 99.5 % were observed in all cases.

FIGURE 2 – ZIRCONIA CONVERTER EFFICIENCY TEST RESULTS

ACCURACY / REPEATABILITY TESTING

Numerous experiments have been performed to determine the accuracy and the repeatability of the measurements using the new UV bench and the zirconia thermal converter. Typical results are presented below.

In one set of experiments, an Environics gas dilution system was programmed to vary the NO_x concentration (as NO₂) in a reproducible manner over a 24 hour period. The analyzer was spanned at 10 ppm NO_x, and the results recorded. These results are presented in Figure 3 below. In general, the NO_x accuracy was approximately +/- 0.1 ppm NO_x over the entire 24 hour period.

FIGURE 3 – NO₂ GAS RUN REPEATABILITY TESTING

A similar set of tests were performed to determine if there was any memory effects on the NO_x converter. In such tests, the same NO_x gas run was performed with increasing then decreasing concentrations. The accuracy and reproducibility were determined. Typical results are presented in Figure 4 and in Table I. Again, the data shows excellent accuracy, repeatability, conversion efficiency and no signs of hysteresis.

TABLE 1 – NO₂ GAS RESULTS WITH Zr CONVERTER AND UV ANALYZER

FIGURE 4 – NO₂ GAS RUN REPEATABILITY TESTING

Conclusions

A zirconia oxygen analyzer can be used as a thermal converter to convert NO₂ to NO, and has been shown to have high conversion efficiencies. When used with a novel UV photometer which employs a sophisticated data processing methodology, exceptionally accurate NO_x measurements can be performed in the 0 – 10 ppm NO_x range, with none of the problems typically associated with chemiluminescence based devices. The system has no moving parts, and has been shown to provide reliable long-term data for low NO_x applications.

References

1. Ozone Smog – The Latest Science. Texas AQS 2000 conference www.ghasp.org/taqs.html
2. Worthington, Bill and von-Hoersten, Henning., “Novel differential ultra-violet resonance absorption gas analyzer for NO_x measurement in continuous Emissions monitoring systems”, ISA Analysis Division , Denver, 2002.
3. Harris, Phil. “Simultaneous determination of SO₂ and NO_x concentrations by ultraviolet spectrophotometry”, ISA Analysis Division, New Orleans, 1997
4. Folsom, A.B., and Courtney, C.W., “ Accuracy of Chemiluminescent Analyzers Measuring Nitric Oxide in Stack Gases”, Journal of the Air Pollution Association, Vol 29, No 11, 1979.
5. PRE 5 - Determination of Nitrogen Oxide Emissions from Stationary Sources (Ultraviolet Instrumental Analyzer Procedure), www.epa.gov/ttn/emc/prelim.html
6. Bennet, H. E., “Accurate methods for determining photometric linearity”, Applied Optics, 5:1265, 1966.
7. Henderson, Brian, “Optical Spectrometers”, Handbook of Optics II, edited by M. Bass et al., McGraw Hill, New York, , 1995
8. Adam H. and Harris P., “The design and practical application of UV process photometers”, L’analyse Industriel, Paris, France, 1996.
9. Adam H. and Harris P., “The design and practical application of UV process photometers Part II Otpical Components and Configuration”, ISA 96, Chicago, Oct 1996