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Guidelines for Air Emission Regulation

4. Calculation of outlet heights

4.1 Introduction

This chapter contains instructions on how to calculate stack or outlet heights in order to ensure adequate dilution of emissions into the atmosphere to comply with C-values.

In general, these calculations must be based on the OML model. This model and its use is described in more detail in section 4.2. Section 4.3 explains the concepts "source strength G" and "dispersion coefficient D" used in OML calculations. Section 4.4 provides instructions regarding calculations. Section 4.5 describes the use of the model in connection with odour emissions, and section 4.6 specifies some of the limitations applying to this model.

Calculations using the OML model may be carried out by the applicant installations themselves, or by consultants. The approving authority assesses whether such calculations are adequate.

4.1.1 Information about the OML model

For information on the OML model, refer to the National Environmental Research Institute⁴⁰ manuals and supporting materials regarding this particular model. The National Environmental Research Institute also maintains a website that features information of interest to OML model users⁴¹. Here, anyone interested can find information on current versions of the model and any problems reported, and they can download tips and help files.

A PC version of OML-Point (the guidelines model) is available from the Danish Ministry for Environment and Energy Shop, Miljøbutikken⁴². All other software is available from the National Environmental Research Institute.

Inquiries regarding the normal application of these models should be made to the Danish EPA. Other, more technical questions on the application and function of models should be directed to the National Environmental Research Institute⁴³.

The previous version of these Guidelines included a description of a method for calculating outlet heights without using a PC, the so-called nomogram method. Due to subsequent developments in general PC usage, this method is not included here.

4.2 The OML model

OML is an acronym for the Danish "Operationel Meteorologisk Luftkvalitetsmodel" (Operational Meteorological Air Quality Model), and is used to calculate required outlet heights by means of a computer software package. This package is available in two versions: OML-Point and OML-Multi.

OML-Point is used to carry out calculations for point sources that are assumed to be at one single geographical point, whereas OML-Multi can take into account the relative positions of many outlets. This means that OML-Multi is used in cases involving several outlets with some distance between the individual outlets. Both versions are based on a Gaussian plume model. They include a division by a factor of 1 million in the sense that if emissions are submitted in g/sec, the resulting ground level concentration will be indicated m g/m³.

4.2.1 Results from the model compared with C-values

The user must supply various information, such as a tentative outlet height. On this basis, the model calculates average concentrations for every hour of a reference year at a number of user-defined points – the so-called receptor points. As a measure for the concentrations, 99-percent fractiles are tabulated at all receptor points for each month of the reference year.

These calculated concentrations are compared to the C-values laid down in the C-value Guidelines, and repeated calculations using various outlet heights make it possible to determine an outlet height on the basis of the criterion that the 99-percent fractiles calculated must be less than or equal to the relevant C-values.

The C-values must be complied with for each month, and calculations must always be carried out for all 12 months of the year, even if emissions occur only for part of the year. C-values must be complied with everywhere outside of installation boundaries.

4.2.2 Data basis for OML calculations

Calculations using the OML model require information on source strength, volume flow, flue-gas temperature, inner and outer diameter of stacks, outlet height, topography, buildings in the immediate vicinity of the source, as well as time series meteorological data. Meteorological data for a reference year (Copenhagen Airport 1976) are supplied with the model. Section 4.3 features a more detailed description of the concepts of "source strength" and "dispersion coefficient", which can be used for preliminary estimates.

4.3 Source strength and dispersion coefficient

4.3.1 Source strength "G"

Basically, the source strength G is the maximum permissible emission of a given substance during one hour of operation, measured as mg/sec. G should be determined by means of one of the following approaches:

1. G may be determined on the basis of the limit value for emissions laid down in the terms of the relevant approval for the outlet in question, and on the basis of the maximum hourly flow rate during operation. G is calculated by multiplying the limit value for emissions, mg/normal m³, set in the approval with the maximum flow rate in the outlet measured in normal m³/sec.
      
2. In cases where no limit value for emissions has been set, the maximum hourly emission during normal operation is used instead. In several cases, for example, G may be determined on the basis of the amount of paint used at a surface-treatment plant, where all organic solvents are normally emitted into the atmosphere. The maximum hourly consumption will then form the basis for calculations of source strength.

Where devices intended to reduce pollution have been installed, and these devices mean that emissions from the installation in question are significantly lower than the limit values for emissions laid down in these Guidelines, the actual emissions may be used when calculating outlet heights, if a maximum hourly emission can be determined on the basis of the figures available on actual emissions. The approving authority and the relevant installation should consider whether the emission limits should be reduced instead.

4.3.2 Dispersion coefficient "D"

The dispersion coefficient D is a concept that may be useful in connection with preliminary estimates.

The dispersion coefficient is defined as the source strength, G, measured as mg/sec of the relevant substance, divided by the C-value in mg/m³ for the same substance.

Equation 4

D is measured in m³/sec and expresses the quantity of air that the discharged substance must be evenly mixed with every second in order to comply with the C-value.

If the dispersion coefficient is less than 250 m³/sec, the only requirement to the outlet is that it should be at least 1 m above roof level and directed upwards to ensure free dilution⁴⁴.

This rule may only be applied to a limited number of outlets within a single installation; the exact number depends on the size of the relevant installation.

Example:

4.4 Calculation using OML

4.4.1 One or more outlets and one or more substances

Table 10
Which method should be used?

Note the following regarding this model:

The OML model calculates concentrations in the surroundings – not outlet heights. When OML calculations are carried out, these calculations must be repeated using a different outlet height each time until a height (or set of outlet heights) in compliance with the C-value has been successfully determined.

OML-Point is best suited for simple outlet conditions with only one point source. OML-Point may be used in situations involving several outlets, but it will provide results that err on the conservative side, whereas OML-Multi will give more realistic results. This is because OML-Point treats outlets as though they were situated at one single geographical location, whereas OML-Multi is capable of superimposing concentrations from different sources. As a rule of thumb, use of OML-Multi should be considered in situations where two separate outlets are located at a distance of more than two outlet heights from each other.

In cases involving outlets from comfort ventilation with concentrations less than the recommended limit values for indoor climate issued by the Working Environment Authority, and with dispersion coefficients (as defined in section 4.3) less than 250 m³/sec, the only requirement to the outlet is that it should be at least 1 m above roof level⁴⁵ and directed upwards or designed in some other environmentally friendly manner, so that the air emitted from the outlet can disperse freely. In those special cases where an installation emits substances with identical toxicological properties, these substances must be considered one single substance in these calculations. Section 4.4.2 specifies how calculations are carried out in such cases.

The supporting material accompanying the OML model may provide helpful information and guidance when specifying input data for model simulations. This material is also available on the Internet⁴⁶.

These Guidelines do not include detailed instructions on how to calculate environmental contributions from sources other than point sources. It should, however, be noted that OML-Multi includes methods to perform calculations for the so-called area sources, i.e. sources where emissions may be assumed to be evenly distributed within a rectangular area.

4.4.2 Substances with identical effects

In special cases, where an installation emits different substances within the same substance group with identical toxicological effects and health-related C-values (see section 3.1.7), outlet calculations must be carried out on the basis of the total substance emission involved.

When substances have different C-values you may choose either an approach (‘the C_r method") where the corresponding C-value is calculated as a weighted average value, C_r, or a more technical method ("the C₁ method"), where source strengths are normalised in a manner which makes it possible to apply a C-value of 1 mg/m³.

4.4.2.1 The C_r method

The C_r method is exact when substances are emitted from one single outlet, but it is unnecessarily conservative in scenarios where emissions occur from several different outlets situated at some distance from each other or of varying heights. The C_r method is also exact in cases where substances have identical C-values.

The C_r method with a weighted average value requires the calculation of a C_r value in accordance with the definition in section 3.1.7. This is followed by an OML calculation. In this OML calculation, emissions of all the substances with identical toxicological properties are included as if only one substance were involved.

4.4.2.2 The C₁ method

The C₁ method will always be exact, but the ground level concentration concentrations calculated will be fictitious and must be compared with a C-value of 1 mg/m³.

According to the C₁ method, the source strength of each substance must be normalised according to its relevant C-value prior to the outlet calculation. This is done as follows:

Equation 5

The OML calculation using OML-Multi is carried out taking into account all outlets and all substances with identical toxicological effects. If more than one substance is emitted from a single outlet, a total normalised source strength must be calculated for this outlet, as specified below in Equation 6. The result of the entire OML calculation must be compared to a C-value of 1 mg/m³.

Equation 6

Example of application of the C₁ method for two substances with identical effects, A and B

4.5 Application of the OML model for odour emissions

The OML model can also be used to calculate outlet heights in connection with odour emissions. The source strength is the product of the odour-emission concentration⁴⁸ stated as odour units/normal m³ and the maximum permissible air quantity (normal m³/sec). This concentration must be determined in accordance with current methods applying to odour-emission measurement. In order to take into account the fact that assessments of odour ground level concentrations are normally based on an averaging period of 1 minute rather than the 1 hour used in the OML model, the source strength must be corrected using a factor of 7.8. In practice, this means that the emission must be multiplied by a factor of 7.8 and divided by a factor of 1,000,000 in the model. The result is the odour ground level concentration stated directly in odour units/m³.

4.6 Exceptions

4.6.1 Heavy gases

Note that the OML model cannot be applied to gases that are significantly heavier than the surrounding air. In calculations of plume rise, the relevant parameter is the bulk density of the gas mixture, not the molecular weight of individual pollution components. Consequently, the model should not be used in cases involving flue gas at low temperatures (in practice, the limit may be set at -5°C).

4.6.2 Wet flue gases

The OML model cannot be expected to produce reliable results when applied to flue gases with excessively high moisture contents (e.g. in connection with forage drying plants or certain types of flue gas scrubbers). There may be an absence of lift in the plume, and the plume may also shed drops.

It is difficult to quantify problems involving wet smoke plumes, and such problems are not addressed in the OML model. The National Environmental Research Institute is currently establishing a knowledge bank for methods to counteract problems with very wet flue gases. Further information is available from the National Environmental Research Institute and on the Internet⁴⁹.

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