Environment Agency
The main reasons for carrying out stack emission monitoring are to:
- comply with environmental legislation
- collect data for emissions inventory compilation
- calibrate continuous emission monitoring systems (CEMS)
- collect data for environmental impact assessments
- collect data to assess process efficiency and process control
- assess the performance of a pollution-control device (abatement system)
This document describes our overall approach to stack emission monitoring. It focuses on:
- the role of MCERTS
- different approaches to stack emission monitoring
- sampling strategy
- the hierarchy of different methods
To support this guidance, we have produced guidance on standards for:
The guidance on monitoring techniques and standards is classified by substance and meets our monitoring requirements under the environmental permitting regulations (EPR).
Applications for environmental permits must include proposals for monitoring emissions.Permits include conditions setting out suitable emission monitoring requirements. These will specify the:
- measurement methodology
- monitoring frequency
- reporting procedures
Wherever possible, you should monitor emissions using standards from recognised standards making organisations.
How this guidance relates to MCERTS
MCERTS is for instruments, monitoring and analyses. The scheme is built on proven international standards. It provides industries with a framework for choosing monitoring systems and services that meet our performance specifications. MCERTS brings together relevant standards into a scheme that manufacturers, operators, regulators and test houses can easily access.
The main reference documents that underpin MCERTS for stack emission monitoring are:
- this guidance document which covers choosing the correct monitoring approach
- standards for CEMS and automated batch samplers
- monitoring techniques and standards for periodic monitoring
- M1 sampling requirements for stack emission monitoring which covers the sampling requirements
International standard EN ISO/IEC 17025 covers the competence of testing laboratories. It describes the general requirements laboratories have to meet to demonstrate that they:
- operate a quality system
- are technically competent
- can generate technically valid results
To comply with MCERTS for stack emission monitoring, organisations need to meet the requirements of EN ISO/IEC 17025. They must also:
- use MCERTS certified personnel
- use Agency Method Implementation Documents
- carry out risk assessments
- use site-specific protocols (measurement plans)
- report results in a standard format
- get stack emissions monitoring samples analysed by laboratories accredited to the MCERTS performance standard for testing samples from stack emissions monitoring
The different approaches to monitoring stack emissions
There are 2 types of stack emissions monitoring:
Periodic measurements
This is where you carry out measurements at periodic intervals, such as once every 3 months. You withdraw the sample from the stack and either analyse it off-site or measure it on-site.
You may use:
- a manual technique where you extract a sample on-site and then analyse it in a laboratory
- an instrumental or automatic technique where you extract a sample and analyse it on-site using a transportable automated measuring system
You may obtain samples every 30 minutes to several hours.
CEMS
This is where you carry out automatic measurements continuously. You may carry out measurements in the stack (in situ) or use extractive sampling with an instrument permanently located at or near the stack.
The main elements of successful monitoring
Whichever monitoring approach you choose, you must follow this fundamental principle of sampling: the small amount of material you collect must be representative of the overall character of the material.
The number, and locations, of samples you need to make up a representative sample depends on how homogeneous the bulk material (the stack gas) is. If the stack gas is homogeneous, you may only need a few samples. If it is not homogeneous, you will need many more samples.
Our requirements for representative sampling are covered in M1 sampling requirements for stack emission monitoring. Because sampling uncertainty is often much greater than analytical uncertainty, it is crucial that you follow this guidance on sampling.
Isokinetic sampling for particulates
Due to the wide range of particle sizes that are normally present in process emission streams, you must sample isokinetically to make sure you obtain a representative sample of the particulate emission.
To perform isokinetic sampling, you need to calculate the required sampling flow rate. This is to make sure that the velocity of the gas entering the nozzle is the same as the velocity of the stack gas at the sampling plane. This takes into account the velocity of the gas in the stack at the sampling point and the effective diameter of the sampling nozzle.
Sampling flow rate = area of nozzle velocity of gas entering nozzle.
By comparing the velocity of the gas at the nozzle with the velocity of the stack gas at the sampling plane, you can determine the isokinetic ratio.
Isokinetic ratio (%) = (velocity at the nozzle velocity of stack gas) 100.
You can also check for isokinetic sampling compliance during monitoring by comparing the required sampling flow rate to the actual sampling flow rate.
Isokinetic ratio (%) = (actual sampling flow rate required sampling flow rate) 100.
If the mean actual isokinetic ratio during the sampling at the sampling plane differs by more than -5 to +15% the measurement is not valid. This is stated in the standard EN 13284-1 for determining the low range mass concentration of dust.
If the sampling velocity is less than the isokinetic ratio (usually expressed as a percentage), the actual volume sampled will be less than it should be.
At first sight, it would appear that you will underestimate the emission. However, because the sampling rate is too low, there is a divergence in flow around the sampling inlet. Small particles are able to follow the flow and a percentage of them will not be sampled. Larger particles are not able to follow the flow because of their greater inertia so more of them will enter the sampler.
So, a sub isokinetic sampling ratio will lead to a bias in the sampled particle-size distribution towards the larger particles. This could lead to you overestimating the particulate concentration, depending on the original size distribution.
Sampling at a rate higher than the isokinetic ratio will lead to a bias in the sampled particle-size distribution towards the smaller particles. This could lead to you underestimating the emission rate, depending on the original size distribution.
Sample conditioning
With the exception of in situ CEMS, the collected sample is usually conditioned in some way before it is analysed. It is important that any conditioning you carry out to make the gas compatible with the analysis method does not alter the substance being monitored.
This conditioning may occur:
- at the probe
- duri