For deciding the technique to be adopted for removing the pollution from the gaseous emission of the industries, automobile, vehicles, etc. The most important work is to collect the samples of the emission of gases at source.
This collection of samples is also known as stack sampling at origin of pollutants. The emission inventories data’s are systematically collected to have the detailed information regarding the air pollution emission in the particular area.
The main objects of the sampling at source are:
1. To measure the quality and quantity of pollutant produce by the source.
2. To determine the quality and efficiency of control equipment’s under various conditions.
3. To determine the emission due to changes in raw materials and the processes in the industry.
4. For the application of the local control system at the place.
5. To collect data from a single pollutant or multiple pollutant source.
6. To know the nature of the pollutant source.
7. To determine the effect on the emission of the pollutant source in different wealth’s of the year.
8. To determine the effect in the emission in different zones/areas of the country.
Sampling Study Planning:
For the stack sampling, proper planning is required. The proper survey is done before planning. For the planning following points should be considered:
1. The planner should be familiar with the process and operation to determine the cycle operation.
2. All methods of the sampling should be well known.
3. Sample time because some sample undergoes cyclic changes.
4. Amount of sample required.
5. The site should be well surveyed before taking the sample because the source point must located clearly.
Selection of Sampling Location:
The selection of the sampling site is an individual judgment and requires the well experience of the field. The sampling point should be as far as possible from the disturbing influence such as elbows, bends, transition pieces, baffles or other obstructions. Researcher have shown that sampling point should be at a distance of 5 to 10 times diameters downstream from any obstructions and 3 to 5 times diameter upstream from similar disturbance.
Sampling Point Size:
For the collection of samples an opening has to be made. An extent of accommodating the probes, the size of the sampling may be made in the range of 7 to 10 cms in diameter. A flange may be riveted so that the opening may be closed during non-sampling period.
The samples should be collected at various places across the stack. This is essential because there may be change in temperature and velocity during the sampling.
These points where the samples are collected across the section are called Traverse Points. These points should be located at centre of each of a number of equal area in the selected cross section of the stack. The study show that the number of points should be taken as given in Table 8.1. The traverse points should be located as given in Fig. 8.1.
The success of the sampling depends on the conditions at which the sample was collected. It must be representative like waste water collection. It can be achieved by Isokinetic condition. This condition exists when the velocity in the stack V s equals the velocity at the top of the probe nozzle V n at the sample point.
This is important when the particle size is greater the 3 micro (sign). Figure 8.2 shows that when the velocity of gas in sampling nozzle is less than the gas velocity in the duct, portions of the gas stream approaching at a higher velocity are deflected.
Due to the gas stream the deflection of the light particles follow the deflected gas stream and do not enter in the probe. The heavier particles by virtue of the weight collected in the probe resulting a non-representative high concentration, of course, particles and the sample weight is in error on higher side. Similarly, when the velocity in the probe is higher than the access amount of lighter particles entering in the probe, this results the sample weight is on the lower side.
Determination of Velocity:
Measurement of velocity is very essential for stack sampling. This measurement is not only required for establishing the gas flow rate but also on fixing Isokinetic condition. The pilot-tube shown in Fig. 8.3 is a standard and most widely used instrument.
This shows a two opposite opening one made face upstream and the other facing downstream during the velocity measurement. This pilot-tube is kept at the traverse point. The velocity head is calculated from the detection in the Manometer.
The values are substituted in the equation:
where Kp, Cp and P and M are constant P and T are variable across the cross-section of the stack.
The velocity thus obtained is at stack condition and this has to be corrected to standard equation.
The value of the constants are:
Kp = 34.96
CP = pilot-tube coefficient 0.9
m = molecular weight
PS = stack pressure
Determination of Gas Composition:
At the field work the gas composition is determined by the Orsal apparatus. The gas is collected in the Orsat apparatus and analysed for the composition of CO2, O3 and CO in the same order and the remaining is assumed as Nitrogen.
Molecular weight of gas = (∑) MX BX
where MX = molecular weight of CO2, O3, M2 and CO.
Bx = represents percentage of gases.
Determination of Moisture Content:
The determination of moisture is done by the following methods:
(a) Wet bulb and dry bulb temperature technique
(b) Condenser technique
(c) Silica gel tube
The both first method are used when moisture content is less than 18% and dew point is less than 125° F and cannot be used for acid streams.
Determination of Temperature:
The temperature has to be measured across the cross-section of the stack at predetermined traverse points. The temperature probe is inserted into the stack and the readings are taken with the help of Pyrometer. Care should be taken in selecting the probe.
Procedure for Particular Matter Sampling:
The NEERI has given the procedure for the collection of particular matter sampling:
1. Determine the gas composition and correct to moisture content.
2. Determine the temperature and velocity at each traverse point.
3. Determine the empty weight thimble (W1).
4. Mark out the traverse points on the probe. The marks are properly fixed by tying with asbestos thread.
5. Check all points for leakages.
6. Determine the flow rate to be sampled under isokinetic condition.
7. Insert the probe at the traverse points 1, very close to the stack. Start the pump and adjust the flow so that the rotameter reads the predetermined valve.
8. Switch off the pump at the end of sampling time.
9. Read the vacuum at the dry gas meter (DGM) and temperature.
10. Move the probe to the subsequent traverse points by repeating the steps 5 to 8.
11. After completion of collection of sample, remove the probe and allow it to cool.
12. Remove the thimble carefully. Some of the dust would be adhered to the nozzle. These should be removed by tapping and transfer to the thimble.
13. Weigh the thimble with sample. The difference in weight gives the dust collected.
14. The volume of sample collected either given by dry gas meter (cu.m) or by the sampling rate given by rotameter multiplied by the sampling time.
15. Hence from (12) and (13) the emission rate can be calculated. This will give at DGM conditions. This is to be corrected for temperature and pressure so as to obtain for standard conditions.
Sample Recovery Tests:
After cooling, the outside of probe assembly is cleaned with cotton waste. Disconnect the nozzle. Remove the thimble and keep it in clean glass beaker. The particulate matter adhered to the inside walls of the nozzle, should be transferred carefully to the thimble. Weigh the thimble with sample (W2). The difference in weight (W1 – W2) will give the particulate matter collected.
Total volume of gas sampled; Vm @ meter condition.
Total volume of gas Pm/PS × Vm × Ts/Tm @ standard condition.
VS = sampled
Vm = m3 as given by DGH
Tm = Temperature @ DGH mm of Hg
Pm = Pressure @ DGH mm of Hg
Ps = Standard pressure 760 mm of Hg
Ts = Standard temperature 298°K
Flow rate @ = Vs × As cu.m/mt. @ stack condition
= Qsld@ std. condition (applying correction)
Total emission Qstd × 1440 m3/day per day
Total particulate matter emission per day
F × Qstd × 1440 gms/day
For the gaseous sampling different procedure requires as in the particulate sampling.
Following methods are used for the gaseous sampling:
3. Freeze out/condensation.
Whenever the flow rate and process operation are uniform and whenever analytical equipment of extreme sensitivity is available, it is possible to take a grab sample from the stack by one of the following methods:
(a) Use of evacuate container
(b) Purging (displacement of air)
(c) Inflation of flexible bag
(d) Use of syringe.
Whenever the source conditions, especially flow rate changes, with time, the sampling must be done proportionally so that sample is representative. This required the velocity measurement as explained and corrected to standard conditions.