ATMOSPHERIC AIR POLLUTION

SAMPLING

Prof JC (Koos) Engelbrecht

Associate Professor: Environmental Health

Faculty of Science

TSHWANE UNIVERSITY OF TECHNOLOGY

2013

SOURCE SAMPLING

Stack sampling An overview of sampling techniques

Main aim of this lecture:

After promulgation of the list of activities which result in atmospheric

emissions which have or may have a significant detrimental effect on the

environment (Government Gazette No. 33064 dated 31 March 2010), the

sampling of point (stack) sources was prioritised. Industries that are holders

of atmospheric emission licenses, are required to conduct stack sampling to

comply to emissions standards as prescribed in the Minimum Emission

Standards. Specific methodology is also prescribed as per Schedule A of the

Standards.

This lecture aims to provide summative information to interested and affected

parties on the basics of stack sampling and to provide guidance on an

overview of sampling techniques and equipment.

SOURCE SAMPLING

Stack sampling An overview of sampling techniques

ISOKINETIC

ISO - means the same

KINETIC – means moving energy

Misconceptions about

isokinetic sampling

• It is not a quick, simple exercise – fairly involved and complex, “almost an art”

• Time consuming – setting up of equipment, preliminary checks etc

• It is not related to occupational measurements – totally different discipline

• Specifically used for determining flue gas particulate / gas concentrations in a duct or stack

• Purpose designed equipment

ISOKINETIC SAMPLING

DEFINITION

Sampling at such a rate that the velocity and the

direction of the gas entering the sampling nozzle is

the same as that of the gas in the duct / stack at the

same sampling point.

Source sampling Stack (Isokinetic)

Reasons for sampling

• To determine the effectiveness of control equipment

• To demonstrate legal compliance (atmospheric emission license

conditions)

• To determine the specific source contribution to the receptor

environment

• To provide data for dispersion modeling

• To provide data for process efficiency

Particulate

stack sampling

(Isokinetic)

Stack sampling

Isokinetic sampling

Sampling train diagrammatic representation

Stack sampling

Isokinetic sampling equipment

particulates

Stack sampling

Isokinetic sampling

Sampling train for particulates

Stack sampling

Sampling train for gaseous pollutants (SO2) / PM

Stack sampling

Velocity measurement system

Stack sampling

Isokinetic sampling

Criteria for selection of the sampling location

• At the point of greatest interest (e.g. a stack)

• In a straight section op ducting

• 5 or more diameters downstream from any bend or flow disturbance

• 3 or more diameters upstream from any bend or flow disturbance

• Accessible to sampling personnel and equipment

• Availability of electricity, water and air if needed

• Safety of point of sampling

OFTEN AN IDEAL SITE CANNOT BE FIND

AND A COMPRIMISE MUST BE REACHED

Stack sampling

Isokinetic sampling

Exemplified structure of measuring hole

Stack sampling

Isokinetic sampling

Typical sampling point provisions

Stack sampling

Isokinetic sampling

Gas velocity profile

• Velocity of gas entering the sampling probe must be

equal to the gas velocity at the sampling position.

• The gas velocity must be measured continually and

sampling velocity adjusted accordingly

• The sampling needs to be conducted under so called

“Normal conditions” that is prescribed by legislation

as 10% O2, 273 Kelvin and 101,3 kPa

Stack sampling

Isokinetic sampling - Gas velocity profile

Sampling position - theory

Isokinetic sampling - Gas velocity profile

Sampling position - theory

At any point “r” from the center the velocity is found from:

U/Center = (1 – r/R)1/7

Also known as the1/7th law

Samples should therefore not be taken

from the central point of blocks (rectangular)

or rings (round) ducts, but from

representative positions in the duct.

Stack sampling

Isokinetic sampling

Over and under sampling

Stack sampling

Isokinetic sampling

Over and under sampling

Isokinetic sampling

Sampling probe positions

Circular ducting

Annual rings of equal area minimum of 10 sampling points

Isokinetic sampling particulates

Sampling position

Particle distribution in a duct

Isokinetic sampling

Sampling probe positions

Rectangular duct

Minimum of nine measurement in the center

of each envisaged rectangular

Stack sampling

Isokinetic sampling

Sampling position – bends and curves

Sampling must be conducted:

In a straight section of pipe

If bends or flow disturbances:

• 5 or more diameters downstream

• 3 or more diameters upstream

Stack sampling

Isokinetic sampling

Sampling position – bends and curves

Stack sampling

Isokinetic sampling

Angle between inlet tube and stream direction

Stack sampling

Isokinetic sampling

Sampling standards for particulates

Mostly ISO (International Standards Organisation) and EPA (United

States Environmental Protection Agency) are recommended in Schedule

A (Method for sampling and analysis) as methods to be used to confirm

legal compliance. Some examples are:

• ISO 9096 Stationary source emissions – Manual Determination of the

mass concentration particulate matter

• ISO 7935 Stationary source emissions – Determination of the mass

concentration of sulfur dioxide - Performance characteristics of

automated measuring method

• EPA Method 10 Carbon monoxide - NDIR

Other methods that the listed Schedule A may be used with the written

consent of the National Air Quality Officer

Source sampling

Sampling equipment

Isokinetic sampling

Particulates

Sampling nozzle

Isokinetic sampling

Particulates

Sampling head with

filter holder

Isokinetic sampling

Particulates

Filters – in stack fitration

Isokinetic sampling

Particulates

Filters and filter holders

Isokinetic sampling

Stack gas velocity - pitot tubes

Isokinetic sampling

Vacuum pump

Gaseous stack

sampling

GASEOUS SAMPLING PRINCIPLES

• Homogenous nature of gases – therefore not

necessary to sampling isokinetically because:

– Flue gas are presumed mixed thoroughly

– Very small molecules therefore not adversely

affected by gravity

– Properties of gases are such that a sample can be

taken at several predetermined points along the

diameter of the duct and still be representative

without having to sample isokinetically

Stack sampling

Sampling train for gaseous pollutants (SO2) / PM

Gas sampling

Bubblers

Gas washing bottles

Stack sampling

Gases

Sampling train

Associated errors that could

influence sampling accuracy

• Moisture

• Gas composition

• Nozzle, probe direction

• Leakages on test equipment

• Test equipment not calibrated

• Non-compliance to minimum requirements of relevant standard

• Not collecting enough particulate matter

• Unstable process conditions

CONTINIOUS

MONITORING

(Gases and particulates)

Stack sampling

Opacity monitoring

In line stack

ADDITIONAL REFERENCE MATERIAL

• Methods of Air Sampling and Analysis. Third Edition. James P Lodge.

Lewis Publishers. 1988.

• AIR POLLUTION. Volume III Measuring, monitoring and surveillance of

air pollution. THIRD EDITION. Arthur C Stern. Academic Press. 1976.

• AIR POLLUTION. Volume VII. Supplement to measuring, monitoring and

surveillance and engineering control of air pollution. THIRD EDITION.

Arthur C Stern. Academic Press. 1986.