Stack emissions monitoring requires defined flow conditions at the sample location. These flow conditions are required to measure pollutants that exist in a particulate phase. They are also required to measure velocity, which is used to calculate pollutant mass emissions.
As well as selecting the correct location, it is also important to ensure that sampling facilities are suitable. This includes appropriate access to the sample location and a large enough work area for the sampling equipment and sampling personnel.
If a suitable location and / or sampling facilities are not available, it will mean that sampling of pollutants cannot be done in compliance with the required sampling methods. This means that the uncertainty associated with the results is greatly increased. In these circumstances meaningful results from stack emissions monitoring cannot be achieved.
Designing new plants
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 all concerned if the sampling location does not comply with the sampling requirements because it is either located in the wrong place or the sampling platforms are too small and do not allow access to the stack.
Permit applications (and Variations for plant extensions) require the operator to assess the monitoring arrangements against the requirements of Environment Agency (EA) Technical Guidance Note M11 (this guidance note is available from www.mcerts.net. It provides guidance on European Standard 152592). It is essential that this process is carried out at the plant design stage. Once this has been done it is strongly recommended that advice and confirmation of the monitoring arrangements is sought from the EA.
If the monitoring arrangements (i.e. sample location and facilities) at a newly built plant (or newly built extension to a plant) do not meet the requirements of EA TGN M1 and BS EN 15259, it is possible that the EA would not grant the applicant a permit to operate.
Installing a suitable sample location
The sample location must be positioned such, that there are stable flow conditions, which allow representative samples of the stack gas emission to be taken.
In summary, for newly built plant the sampling location should be located according to the recommendations contained in Table 1.
Table 1 Recommended location for meeting sample location requirements:
|General location of sampling plane||The sampling plane should be situated in a length of straight duct (preferably vertical) with constant shape and constant cross-sectional area. Where possible, the sampling plane should be as far downstream and upstream from any disturbance, which could produce a change in direction of flow (e.g. bend, fan or a partially closed damper).|
|Location of sampling plane in straight section||The sample plane criteria are usually met in sections of duct with five hydraulic diameters of straight duct upstream of the sampling plane and two hydraulic diameters downstream. If the sampling plane is to be located near the top of the stack outlet then the distance from the top should be five hydraulic diameters (making a straight length of 10 hydraulic diameters).|
By meeting the recommendations in Table 1 the flow stability requirements summarised in Table 2 should be met.
Table 2 Flow-stability criteria:
|Angle of gas flow||£±15° from stack longitudinal axis|
|Flow direction||No local negative flow|
|Minimum velocity||Dependant on the method used (for Pitot tubes a differential pressure larger than 5 Pa)|
|Gas velocities variations||Ratio of highest to lowest local gas velocities less than 3:1|
Sampling facilities for gas concentrations only
The flow-stability criteria do not have to be met when sampling gas concentrations, providing the gases are well mixed at the sample location. However, in practice, meeting the recommended locations given in Table 1 will help satisfy the requirements for gases.
Sampling facilities for CEMs
When designing a plant that requires CEMs to be installed, access and facilities are required to enable calibration by periodic monitoring, routine maintenance and functional checks to be done.
As CEMs require calibration by periodic monitoring the access and facilities should as a minimum comply with the requirements given for particulates and gases respectively.
Measurement ports should be considered at the design stage of new plants, as installing ports after a plant is built may be difficult and costly (or impossible if protective linings are present).
In summary the access ports:
- Must be big enough for the insertion and removal of the equipment used. It is recommended that access ports have a minimum diameter of 125mm, except on stacks smaller than 0.7m diameter. 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 manoeuvred. A working height of approximately 1.2m to 1.5m is recommended.
The sample location must be situated where it is possible to erect suitable working platforms. Ideally, the measurement site should be easily and safely accessible via stairs.
Measurements of velocity and of pollutants in a particulate phase require a sufficiently large working area around the stack, so that specified measurement points in the stack can be sampled with the appropriate length sample probe. In summary:
- The platform surface area should not be less than 5m2
- The minimum length in front of the access port shall be 2m or the length of the probe (which includes nozzles, suction/support tubes and associated filter holders) plus 1m (whichever is the greater)
- The platform should be wide enough to prevent sampling equipment extending beyond the platform
- Where necessary hoists or lifts should be provided to transport sampling equipment.
- The use of the sampling equipment should not be impeded by guard fences or other structures.
By ensuring that monitoring arrangements are considered at the design stage of a new plant it should be possible to ensure that stack emission monitoring measurements are both reliable and compliant with legislation.