Understanding the Requirements for Stack Emission Monitoring

The principle of particulate matters (PMs) monitoring, as one of many determinants of stack emission, is very simple. A known volume of stack gas is made to pass through a weighed filter paper, where the particulate matter gets deposited. The final weight of the paper is measured. The concentration of the particulate is simply computed by dividing the weight of particulates deposited by the volume of the gas that is allowed to pass through the filter.

But in practice, the job is not as simple. It is complicated with the mix of iso-kinetic sampling condition, the proper location point in the stack, appropriate numbers of traverse points, oxygen correction of the final result, and so on.

Air quality issues are perceived as an increasing priority for industry today. The air around us contains a number of substances, which may impair the health of humans, animals as well as plants; and the various air pollutants adversely affect the human health to different degrees. The major man made sources of the pollutants are often identified with stationary sources like, smoke stacks of power plants, manufacturing facilities, waste incinerators etc. Any pollution control program can be effective only if emission of the pollutants is controlled at the source itself, which in turn makes it imperative to have a very accurate, highly reliable and easily usable monitoring system. Stack emission monitoring is a fairly routine event at US and European process plants (where stack emission monitoring is applicable). Sampling and testing for many determinants of stack emission can be carried out, including:

• Particulate Matter

• Oxides of Nitrogen

• Sulphur Dioxide

• Carbon Monoxide

• Carbon Dioxide

• Dioxins and Furans

• Polycyclic Aromatic Hydrocarbons

• Total Hydrocarbons

• Particulate and Vapor Phase Metals

• Halides and Halogens

Industries that imply to stack emission monitoring compliance (not limited to):

• Pulp & Paper Industry

• Chemical Manufacturing Plants

• Steel Plants

• Non-Ferrous Smelters

• Cement Plants

• Thermal Power Stations

• Mining

• Waste Incinerators

• Printing Shops

• Waste Management

• Painting Facilities

• Petroleum Refining

• Petrochemical Industry

A stack testing program can last a few days or even weeks, depending on the needs. Typically, stack testing is performed by external contractors. Sample analyses are either done at the plant using in-house facilities or via a contract laboratory (for extractive samples). Many industries are having continuous monitoring instruments along with the spot testing facilities. Usually spot and continuous monitoring are conducted on the parameters as below:

Spot test: Dust, SO2, NOx, NH3, Hg, Heavy metals (usually As, Cd, Cr, Co, Cu, Mn, Ni, Pb, Sb, TI and V), Dioxins and Furans (D&F), PCB, PAH, HCl, HF etc

Continuous test: Dust, SO2, NOx, O2, CO, CO2, Hg, VOC etc.

The best practice of stack monitoring includes a step-by-step procedure for selecting an approved contractor, selecting the proper testing method, preparing a testing protocol, conducting the testing, analyzing the results, and preparing appropriate reports. Process stream parameters and recommended data are to be monitored during the testing period.

Environmental Monitoring Standards and Methods – Selection of Standards for Emission Monitoring

Standard reference methods are essential for the effective measurement and control of air pollution. Such standards are developed at National, European and world-wide level. USEPA, ISO, CEN, VDI, JIS are some world recognized organizations that prescribe test methods. Following is a chart showing common methods of stack emission monitoring:




Concentration and Mass Emissions of


US EPA Method 5 & 17


BS EN 13284-1: 2002


BS ISO 9096:2003

Flue Gas Velocity & Temperature


BS EN 13284-1: 2002


BS EN 14385:2005

Metals, Mercury

US EPA Method 29


BS EN 13211: 2001

Dioxins and Furans

BS EN 1948 parts1-3: 2006


US EPA Method 23

VOCs (Total and speciated)

BS EN 12619:1999


BS EN 13256:2001


BS EN 13649:2002


BS EN 14792:2005


ISO 10849:1996


BS EN 14791:2005


BS ISO 6069 pt4.4:1993

O2, CO, CO2

BS EN 14789:2005


ISO 12039:2001


BS EN 14790:2005


US EPA Method 4


Online FTIR

HCl, HF and NH3

BS EN 1911 parts 1-3: 1998


BS ISO 15713:2006


US EPA Method 26-26A


Online FTIR

Particle size distribution measurement

US EPA Method 201a

Trace micro pollutants such as PAHs

and PCBs

BS ISO 11338-1:2003


BS EN 1948 parts1-3: 2006

Iso-kinetic condition

The objective of PM sampling from stack is to collect a sample as representative as possible of what is present in the stack gas. This becomes difficult as the velocity of gas is not uniform throughout the stack. It is the maximum in the center and almost zero at walls. This velocity distribution causes particle size distribution in the stack, i.e. smaller particles try to move towards the center of the stack and the larger particles move towards walls. Therefore, it becomes very important to collect samples of stack gas at various points across the diameter of the stack. This is known as traversing the diameter of stack and the points of sample collection are known as traverse points. Samples are collected at each traverse point maintaining flow rate at collecting nozzle same as the velocity at that traverse point. This phenomenon is known as iso-kinetic condition.

The typical equipments used for iso-kinetic samplings are –

• Apex Instrument

• ES (Environment Supply Co.)

• TCR Tecora

• SHC 500

• Westech(M9096), etc.

Few common brand instruments for Gas analyzer –

• Horiba (PG 500)

• Testo (Testo 350)

• Kane May (KM 9106)

Photo: a typical cement kiln stack arrangement.

Importance of good sampling locations and measurement ports

The importance of the location of equipment and sampling facilities is paramount in stack emission monitoring. Stack emission measurement requires defined and stable flow conditions at the sample location. This allows the velocity and concentration of the measured component in the stack emission to be determined. If suitable sampling facilities are not available it will mean that sampling of pollutants cannot be done in compliance with the required sampling methods. This also implies that the uncertainty associated with the results is greatly increased. Under this circumstance, meaningful results from stack emissions monitoring cannot be achieved.

The location requirements for measuring gas concentrations are less demanding than for particulate matters, as variations in velocity profiles tend not to affect the homogeneity of the gas concentration. In practice, meeting the requirements for particulate matters will satisfy the requirements for gases.

It is essential that designers of new plants remember to take account of stack emission monitoring at the plant’s design stage. Once a plant is built it is extremely difficult, if not impossible, to retrofit appropriate sampling facilities. It is extremely frustrating for everyone concerned, if the sampling location does not comply with the sampling requirements when it is either located in the wrong place or the sampling platforms are too small and do not allow access to the stack. Summarizing, the sampling point:

• Should be within a straight duct / stack section. It should meet 5D downstream and 2D upstream for a duct or 5D downstream and 5D upstream for a stack (D = stack/ duct diameter).

• Must be large enough for the insertion and removal of the equipment used. It is recommended that access ports have a minimum diameter of 125mm; Length of sampling port is 100 mm. For small stacks a smaller port may be appropriate.

• Must be installed at a suitable height to the platform, so that the equipment can be maneuvered. A working height of approximately 1.2m to 1.5m is recommended.

• Must be able to bear at least 400 kg point load. Must have handrails with 1m height for safety purpose.

• The platform surface area should not be less than 6m2: length of 3m in front of the sampling port and width of 2m.

• The platform should be wide enough to prevent sampling equipment extending beyond the platform.

• Where necessary, hoist or lift should be provided to transport sampling equipment.

• The use of the sampling equipment should not be impeded by guard fences or other structures.

• Drop zone shall be marked below the elevated position, and be kept free.

When designing a plant that requires continuous emission monitors (CEMs) to be installed, access and facilities are required to enable calibration by periodic monitoring, routine maintenance and functional checks.

Monitoring report:

The stack emission monitoring report should include the following:

• A narrative description of the process tested. The specific source identification should be clearly stated in the report.

• An explanation of test objectives.

• A summary of the source testing results.

• Description of sampling and analytical procedures, with explanation of any deviation from standard reference methods.

• Documentation of calculation procedures.

• An appendix including all laboratory reports.

• An appendix including all of the contractors’ field notes/data.

• An appendix of equipment calibration data.

• An appendix summarizing all of the process monitoring data.


To expect preciseness in results, strict compliance to requirements for stack emission monitoring along with following up appropriate measuring procedures is a must, as the values of stack emission parameters are expressed in mg, ppm or even more tiny units like µg, nanogram. A third party organization rendering services of stack emission monitoring should be accredited, like to ISO/IEC 17025:2005 for MCERTS (Monitoring Certification Scheme).

Useful References:

1) The National Physical Laboratory Website (www.npl.co.uk)

2) Website of MCERTS (www.mcerts.net)


To cite this article, please use following information:

(use the given format or any standard citation format)

Mohiuddin, M., Understanding the Requirements for Stack Emission Monitoring, ChE Thoughts 2 (2), 21-24, 2011.


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