Instrumentation - NOx
June 4, 2006

Introduction

NOx in the form of NO2 and NO is of primary importance in the area of combustion and the proper operation of a fuel fired boiler, furnace, etc.  In sulphuric acid plant application NOx is a concern in both metallurgical and regeneration acid plants.  NOx can be produced in the upstream metallurgical process or in the regeneration furnace.  Measurement of NOx is important to determine how much is entering the contact section and how much is leaving in the stack.  Emission of NOx in the stack can lead to a poor stack appearance and may exceed local emission limits.

Principle of Operation

Ultraviolet Light Absorption - This technique is based on the differential absorption of UV-visible light by the sample stream at two different wavelengths.  Radiation from a selected light source passes through the sample and into the photometer, where a semi-transparent mirror splits it into two beams.  The measuring beam passes through an optical filter which excludes all wavelengths except one absorbed strongly by the sample constituent under analysis.  The reference beam is filtered to a wavelength which the sample absorbs weakly or not at all.  Each beam then falls on a photodiode which converts the light signal to an electrical current proportional to the light intensity of the beam.

A logarithmic amplifier, in turn, provides a voltage proportional to the negative logarithm of the photodiode currents.  The differential of the amplifier outputs provides a single output linearly proportional to the concentration of the measured constituent in the sample, in accordance with Beer's law.  The split-beam technique is inherently accurate and interference-free because the wavelengths are examined simultaneously.  Adverse sample conditions, such as particle matter turbidity and fluctuations in light intensity, produce an equal effect on the intensity of the energy at both wavelengths and, therefore, cancel out.

Process gas is drawn into the analyzer where the nitrogen dioxide (NO2) in the sample is analyzed directly by the difference in light absorption by NO2 at two different wavelengths (436 nm measuring wavelength and a 578 nm reference wavelength).  Since nitric oxide (NO) is essentially transparent in the visible light and ultraviolet, it first must be quantitatively converted to NO2.  The process gas sample can be reacted oxygen at 5 atmosphere pressure in the sample cell.  The reaction is both rapid and reproducible.

Instruments