STACK SAMPLING AND ANALYSIS OF AIR POLLUTANTS
Stack sampling poses a problem due to the varying composition of pollutants in the flue gas thereby making the process of obtaining a representative sample difficult. The important factors in obtaining a representative sample are:
- selection of the sampling site and
- number of sampling points required
- The sampling site should be located at least eight stack or duct diameters downstream and tro diameters upstream from any source of flow disturbance such as bends, fittings or constrictions. The gas stream in a stack is normally under turbulent flow conditions and any flow disturbance causes non-uniform and unstable gas flow profiles along with non-uniform particle concentration patterns.
The above problems can be minimized by providing proper distance so that adequate mixing may occur. Sometimes it is not possible to ensure uniform flow. Hence, multiple samples are used to acquire a representative sample.
- Actual sampling must be performed at a number of points in the stack.
- Two sets of reading, at right angles should be taken at the same plane for circular stacks.
- The traverse points required is at least six or the number of points depends on location of upstream and downstream disturbances.
- For rectangular stacks, the sampling site is located by calculating the equivalent diameter using the equation: Deq = 4 * Cross-sectional area of flow / Wetted perimeter
Other problems associated with stack sampling are:
- High temperature
- Collection of additional parameters like
- moisture content
- flow rate of gas
- composition of flue gas
Accurate measurements of ALL these factors is essential for valid sampling.
Stack sampling is carried out by diverting part of the gas stream through a sampling train. The sampling train consists of a nozzle, a sampling probe, particulate collection devices, a flow measuring device and a prime mover such as a vaccum pump or an ejector.
The devices used for collection of particulates in gases in stack sampling are:
- Filtration devices
- Devices for wet or dry impingement
- Devices for impaction, electrostatic and thermal precipitation
- Devices for collecting particulates
- Adsorption, Absorption and freeze-out devices for collecting gases.
The technique used for sampling particulate laden gases is called “isokinetic technique”. Under isokinetic conditions, the static pressure at the tip of the probe is equal to the static pressure in the free stream and hence the sampling velocity is equal to the free stream velocity. Under these conditions, the flow pattern in front of the probe is not disturbed.
The process for obtaining a gas sample from a stack is similar to that used in sampling particulates. However, the sampling is easier as it is not necessary to sample under isokinetic conditions.
- Gas sample is withdrawn from stack at a constant rate independent of the flow rate in the stack.
- Precautions to be followed in gas sampling are as follows:
- Particulate matter must be filtered upstream of collection system to:
- prevent downstream line plugging
- minimize loss of gaseous pollutants due to reaction with particulates on cooling
- To minimize amount of particulates that are pulled into sampling line, the probe must be pointed downstream.
- If a straight probe is used, it should be fitted with a filter such as glass or pyrex wool.
- Moisture present in stack gases can condense in the sampling line and dissolve gaseous constituents of interest.
- To prevent losses due to condensation the sampling line should be heated.
- The preferred material for the probe is usually stainless steel and teflon is preferred in some special applications.
- The rate and duration of sampling are important in determining the amount of constituent gas collected and it depends on the technique used for collection.
ANALYSIS OF AIR POLLUTANTS
- Air quality measurements are done by continuous automatic analysers.
- Conventional laboratory techniques for for analysis of discrete samples is done for spot checking.
- For measurement of gaseous pollutants procedures are physical and chemical principles of measurement
- In chemical methods, the pollutant being measured undergoes chemical transformation and the product is analysed using an appropriate chemical technique.
- In wet chemical analysis, the chemical is absorbed in a liquid for a specific time and then treated with a reagent causing a formation of another product indicated by a change of colour. The intensity of colour is related to the concentration of the original pollutant.
- In physical methods of measurement, a physical property of the pollutant is exploited – such as the ability of the gas to absorb infrared radiation. Its concentration is given by amount of radiation absorbed.
- The most common methods for measuring atmospheric SO2 are based on colorimetry, iodimetry or turbidimetry.
- The West and Gaeke colorimetric procedure is the reference or standard method. In this method, SO2 from a measured quantity of air is absorbed in a solution of sodium tetrachloromercurate. This forms stable and non-volatile dichlorosulphitomercurate complex. This is reacted with formaldehyde and para-rosaniline to yield a magenta coloured para-rosaniline sulphonic acid product. Photometric methods are used to detect the colour intensity of this acid. This is proportional to the concentration of SO2. EDTA is added to prevent interferences due to Iron and other heavy metals.
- The automatic instruments for monitoring sulphur dioxide are based on conductometric, colourimetric and flame photometric principles. In the conductometric method the sampled air containing SO2 is passed through a dilute solution of H2O2 and dilute sulphuric acid. SO2 is oxidised to H2SO4 with an increase in electrical conductivity of the solution which is proportional to the concentration od SO2 in the sample. However, acidic gases like HCl give positive errors while NH3 interferes negatively.
- Coulometry is used for automatic monitoring of SO2. In this process, SO2 is drawn continuously through an electrolytic cell containing and acidified bromine solution and two sets of electrodes. SO2 in the air sample is oxidised by bromine causing a reduction in the concentration of bromine. This causes a potential difference between the indicator electrode and the reference electrode. The current flow is a measurement of the SO2 concentration in the air stream.
- Flame photometric analyser works on the principle that when an air stream containing sulphur is ignited in a hydrogen rich flame, a characteristic flame emission spectrum is produced at 394 μm. This wavelength is monitored by a narrow band pass filter and a photomultiplier tube. The amount of light emitted is proportional to concentration of sulphur within the flame.
- In the electrochemical method, the SO2 gas diffuses through a semi permeable membrane and a thin electrolyte layer to get absorbed at the sensing electrode where it undergoes an electrochemical reaction. The current generated is proportional to the SO2 concentration. Electrochemical analysers are sensitive and stable for SO2 monitoring. Moreover, they are simple, portable, have a low cost and give immediate results.
- Infrared and ultraviolet spectrophotometry are based on selective absorption of light at a given wavelength by SO2. Degree of absorption is proportional to SO2 concentration.
- Spectrophotometric analysers
- require small particle filtration
- water removal
- removal of sulphuric acid mist
- are bulky and difficult to transport
Because of the above reasons, spectrophotometric analysers are least advantageous for field application.