Instrumentation - SO2 Concentration
August 21, 2001

Introduction Principle of Operation Installation Instruments

Associated Links

---

Introduction

SO2 concentration measurements are often done in a number of locations throughout an acid plant. 

The most common SO2 measurement is in the stack allowing the plant operator to stay within emission the permitted limits.  The SO2 concentration can range from 0 to 250 ppm SO2 for double absorption plants to as high as 0 to 3000 ppm SO2 in single absorption plants.

SO2 measurements may also be done at the inlet of a gas cleaning system.  This measurement tells the acid plant operator the gas strength that is coming from the metallurgical operation.

Another common location for SO2 measurements is at the blower discharge in metallurgical or regeneration acid plants.  After gas cleaning, the SO2 concentration is unknown as well as the O2:SO2 ratio.  Air dilution may be required to achieve the desired conversion in the converter.  On-line continuous measurement of SO2 and O2 may be required to control the amount of dilution air being added to the process.

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.

Installation

An instrument that draws a gas sample from the main process stream is preferred over an in-line instrument because of the problem of maintaining the instrument.  If the instrument is in-line it often cannot be removed during operation for maintenance, calibration or repair without a complete plant shutdown.

The experience at one plant with an SO2 analyzer installed at the discharge of the blower was that the vibration from the blower caused the reflecting mirror at the opposite side of the duct to go out of alignment such that the instrument failed to operate properly.

Instruments

Manufacturer	Type	Range	Model No.	Remarks
Lear Siegler	Optical Spectroscopy (Light Absorption)		SM8100	Vibration free installation required
Rosemount	Ultraviolet Light Absorption	0-30 ppm Autoranging up to 2000 ppm	ETL 9100	Quartz halogen source No moving parts
Rosemount	UV 'Transflectance'	0-50 ppm 0-5000 ppm	Model 890	Pulsed UV Lamp Operating Temp: 0 to 40°C Power: 350 W
SICK	Light Absorption		GM 21
SICK	Light Absorption		GM 30
AMETEK	Ultraviolet Light Absorption	SO2: 0-100 ppm O2: 2-25% wet	4600	Meets all known EPA requirements Sample Flow: 5 SCF/h Instr. Air: 2 SCFM field unit               2 SCFM probe Electrical: 2 kVa Sample taken to analyzer
AMETEK		SO2: 0-500 ppm NOx: 0-200 ppm	PDA-6010	Instr. Air: 35 SCF/h (60-100 psig) Electrical : 2 kVa Sample taken to analyzer