Millipore Thermal Oxidiser Emissions Dispersion Modelling Impact Assessment

Transcription

1 Millipore Thermal Oxidiser Emissions Dispersion Modelling Impact Assessment Issue No EPA Export :19:52:54

2 Project Title: Report Title: Millipore Thermal Oxidiser Emissions Project No: Report Ref: Status: Client Contact Name: Client Company Name: Issued By: Mairead Creedon Clean Technologies Centre URS Ireland Iveagh Court 6-8 Harcourt Road Dublin 2 Ireland Tel: (0) Fax: (0) Document Production / Approval Record Issue No: 2 Name Signature Date Position Prepared by Klara Kovacic 25/07/06 Project Manager Checked by Ian Marnane 25/07/06 Project Director Approved by Gerard Kelly 25/07/06 Project Director Document Revision Record Issue No Date Details of Revisions 1 24/07/06 Draft 2 25/07/06 EPA Export :19:52:54

3 LIMITATION URS Ireland Limited (URS) has prepared this Report for the sole use of Clean Technologies Centre in accordance with the Agreement under which our services were performed. No other warranty, expressed or implied, is made as to the professional advice included in this Report or any other services provided by us. This Report may not be relied upon by any other party without the prior and express written agreement of URS. Unless otherwise stated in this Report, the assessments made assume that the sites and facilities will continue to be used for their current purpose without significant change. The conclusions and recommendations contained in this Report are based upon information provided by others and upon the assumption that all relevant information has been provided by those parties from whom it has been requested. Information obtained from third parties has not been independently verified by URS, unless otherwise stated in the Report. COPYRIGHT This Report is the copyright of URS Ireland Limited. Any unauthorised reproduction or usage by any person other than the addressee is strictly prohibited. EPA Export :19:52:54

4 CONTENTS Section Page No 1. INTRODUCTION Background Methodology MODEL INPUTS, OUTPUTS AND ASSESSMENT CRITERIA The ADMS Dispersion Model Model Inputs Model Outputs Assessment Criteria RESULTS AND DISCUSSION Results Discussion APPENDIX A - ADMS MODEL Page i EPA Export :19:52:54

5 1. INTRODUCTION 1.1. Background The Clean Technology Centre in Cork has contracted URS Ireland Ltd. to carry out air dispersion modelling of emissions of VOCs from a new Thermal Oxidiser (TO) at the Millipore Ireland facility in Carrigtwohill, Cork. The existing TO will continue in operation hence the modelling study considers simultaneous emissions from both TOs. URS employed the ADMS dispersion modelling software tool for modelling emissions of VOCs from the proposed and the existing thermal oxidiser units at Millipore The model was set up to calculate ground level concentrations for each hour of meteorological data for a regular grid of receptors covering the site and the surrounding area. The model was run three times as follows: 1. Normal operational releases from both TOs covering a distance of up to 1 km to the north, south, east and west of the site. This is based on a small grid spacing of approximately 40 metres for determination of ground level concentrations close to the site; 2. Normal operational releases from both TOs covering a distance of approximately 5 km to the north, south, east and west of the site. This is based on a larger grid spacing of approximately 200 metres for determination of concentrations further from the site; 3. Bypass releases, where untreated waste gases containing solvents are released to atmosphere through the main TO stack. Release at 15 % LEL of butanol was assumed. A distance of approximately 5 km to the north, south, east and west of the site was modelled. In first two scenarios the VOC concentration in the release was estimated by Millipore at 100 mgc/m 3. In the third scenario butanol concentration in the release was estimated at 6,363 mg/nm 3 (i.e. 15 % of the LEL value of butanol). A mixture of VOCs will be present in the waste gases treated by the TO, however for the purposes of the assessment it is assumed that all VOCs are present in the form of butanol, as this is VOC in the waste gases which has the lowest guideline values (as reported in Section 2.4). In order to complete as accurate an assessment as possible a detailed modelling study was completed by URS, including the impacts of factors such as buildings and local topography on the dispersion of the released material. The overall methodology is detailed in Section 1.2. Model inputs and assessment criteria are described in Section 2, while the results are presented in Section 3. Page 1 EPA Export :19:52:54

6 1.2. Methodology A detailed dispersion modelling scenarios were developed including a range of specific data, such as: Meteorological data for Cork Airport for 2005; Local topographical data; Impact of site buildings. URS has modelled releases for a full year of hourly meteorological data. For the normal release scenarios, the VOC concentration is taken as 100 mgc/m 3. As a conservative estimate the volumetric flow rate is assumed as 15,000 m 3 /hr for the existing TO and 29,000 m 3 /hr for the proposed TO (although in practice the flow is likely to be lower than this value, hence VOC mass emissions will be lower). These figures give a conservative VOC mass flow value of 1.5 kgc/hour for the existing TO and 2.9 kgc/hour for the proposed TO. For the third scenario, as a conservative approach continuous release in bypass mode was modelled for hourly data for the whole year. The butanol concentration is taken as 6,363 mg/m 3. The volumetric flow rate is assumed as 12,000 m 3 /hr for the existing TO and 25,000 m 3 /hr for the proposed TO. These figures give butanol mass flow value of 76.4 kg/hour for the existing TO and kg/hour for the proposed TO. The predicted ground level concentrations are compared to available guideline values for butanol as reported in Section 2.4. The assessment has been completed giving general consideration to the recommendation in the Draft UK Guidelines for the Preparation of Dispersion Modelling Assessments for Compliance with Regulatory Requirements an Update to the 1995 Royal Meteorological Society Guidance. Further information on the model inputs and outputs are presented in Sections 2 and 3. Page 2 EPA Export :19:52:54

7 2. MODEL INPUTS, OUTPUTS AND ASSESSMENT CRITERIA 2.1. The ADMS Dispersion Model URS has employed the ADMS dispersion model in the assessment. This is an advanced quasi-gaussian dispersion model developed in the UK by Cambridge Environmental Research Consultants. It is used throughout Europe for dispersion modelling for regulatory purposes and is regularly used by URS Ireland in modelling studies completed for IPPC licensed facilities in Ireland. A more detailed description of the model is included in Appendix A Model Inputs Meteorological Data Prevailing weather conditions can have a significant impact on ground level concentrations of compounds released to air from a stack. Wind speed and direction, in particular, impact the location and magnitude of the maximum ground level concentrations. A range of meteorological parameters are monitored by Met Éireann at their synoptic monitoring stations, including wind speed, wind direction, temperature, rainfall, cloud cover and humidity. There are significant variations between different stations in Ireland hence it is important that a station is chosen which is representative of the area under investigation. URS has employed meteorological data from the Met Eireann synoptic monitoring station at Cork Airport as this is considered representative of the site area. The meteorological related inputs required by the model are: Month; Time of day; Temperature; Wind speed; Wind direction; and Cloud cover. Average values of the temperature, wind speed, and cloud cover for 2005 are presented below. Cloud cover is measured in oktas, 0 to 8 shows the fractions, in oktas, of the celestial dome covered by all clouds. Note that the model employs hourly data for all parameters listed above, the data provided below is for summary purposes only. Page 3 EPA Export :19:52:54

8 Table 2.1: Summary of Cork Airport meteorological data used in the ADMS 3 model Year Wind Speed (m/s) Temperature ( o C) Cloud Cover (Oktas) Figure 2.1: Wind rose for Cork Airport for (knots) Wind speed (m/s) Topography Local topography can have a significant impact on the dispersion of released materials. The ADMS model is capable of including topographical data, if required. There are two parameters which can be employed in the model to describe local topography, as detailed below. Surface Roughness This parameter is specified in all modelling assessments. Surface roughness describes the degree of ground turbulence caused by the passage of winds across surface Page 4 EPA Export :19:52:54

9 structures. Ground turbulence is greater in urban areas than in rural areas, for example, due to the presence of tall buildings. The area surrounding the Millipore site includes some trees and low walls, with no significant groups of buildings or crops. Based on visits to the site a surface roughness value of 0.3 metres has been chosen which is typical of agricultural areas and is considered to represent typical surface roughness in the site area. Complex Terrain The presence of steep hills (known as complex terrain) in the vicinity of a site can effect dispersion of emissions. A gradient of 1:10 or greater is normally taken as the criteria for inclusion of terrain in a modelling assessment. The topography in the vicinity of the site is generally flat, with the exception of a steep hill (gradient > 1:10) approximately 1 to 1.5 kilometres to the north of the site. While it is considered that local topography will have little impact on dispersion of stack emissions it has been included in the model to allow as accurate an assessment as possible of the impact of the release. There is also a smaller hill to the southwest of the site. The topography in the vicinity of the site is presented in Figure 2.1. Page 5 EPA Export :19:52:54

10 Figure 2.1: Local topography in the area of the site (contours in metres) Existing TO Proposed TO Building Effects The main effect which buildings can have on pollutant dispersion is to entrain released pollutants into the cavity (the region in the immediate leeward side) of the building, which is isolated from the flow and in which a reversal of flow can actually occur. For modelling building effects, buildings or structures should be incorporated into the model if the building height is greater than 40 % of the stack height. The buildings which are considered to have a significant impact on pollutant dispersion at the Millipore site are represented in the schematic presented in Figure 2.2, which also illustrates the site boundary and the approximate locations of the nearest sensitive receptors (two local houses, one of which appears to operate as a guesthouse). Page 6

11 Figure 2.2: Scaled schematic of main site buildings, site boundary and local sensitive receptors Grid Coordinates, metres (North) Existing TO Proposed TO Emissions Data Grid Coordinates, metres (East) Details of the emissions data employed in the modelling assessment are presented in Table 2.2 and Table 2.3. Table 2.2: Data describing emission points for normal release modelling Vent Height (meters) Diameter (meters) Flow (Nm 3 /hr) Emission Rate of VOC (kgc/hr) Temperature ( C) Existing TO Proposed TO , , Page 7

12 Table 2.3: Data describing emission points for bypass release modelling Vent Height (meters) Diameter (meters) Flow (Nm 3 /hr) Emission rate of butanol (kg/hr) Temperature ( C) Existing TO Proposed TO , , Model Outputs In all three scenarios, the model was set up to calculate ground level concentrations for each hour of meteorological data for a regular grid of receptors covering the site and the surrounding area. In the first and third scenario, the grid of approximately 2,600 receptors covers a distance of approximately 1 km to the north, south, east and west of the site. The grid spacing is 40 metres. In the second scenario, the grid of approximately 2,600 receptors covers a distance of approximately 5 km to the north, south, east and west of the site. The grid spacing is 200 metres. The scenario based on the 5 km coverage is purely for the purposes of estimating the impact over a 5 km area as required for IPPC licence applications. However, the maximum concentrations occur in close proximity to the site and hence the results for the 1 km grid includes the area of highest modelled ground level concentrations. The grid spacing for the 5 km grid are larger than the 1 km grid hence the resolution of the 1-km grid is higher, therefore the 5 km grid does not adequately characterise concentrations close to the site when compared to the 1 km grid. The results in the assessment (see Section 3) present the maximum concentration for each scenario. Using this value for comparison with the assessment criteria (see Section 2.4) provides a conservative estimate of the impact of the release Assessment Criteria The criteria used to assess normal and bypass release from the Millipore thermal oxidisers are listed below: Occupational Exposure Limit Divided by 40 Page 8

13 The Occupational Exposure Limit (OEL) divided by a factor of 40 for n-butanol is 7,575 µg/m 3. Odour Threshold Values Defined By The American Industrial Hygiene Association The lowest n-butanol odour threshold referenced by the AIHA was identified at 364 µg/m 3, however the upper end of the detection threshold is reported at 33,330 µg/m 3, hence the overall range is large. The mean odour threshold is reported at 3,636 µg/m 3. For the purposes of this assessment the lowest value of 364 µg/m 3 is assumed. UK Environmental Assessment Levels Environment Agency Guidance document IPPC H1 includes guideline values for ambient air quality in the form of Environmental Assessment Levels (EALs), which are listed in Appendix D of the EA H1 guidance note. The 1-hour EAL for n-butanol is 15,400 µg/m 3. Page 9

14 3. RESULTS AND DISCUSSION 3.1. Results The average, maximum and 98 th percentile ground level concentrations off site for each of the scenarios are presented in Table 3.1. Results are presented assuming the release is 100 % butanol for all three cases. Assuming 100 % n-butanol is highly conservative as in practice the n-butanol content of the waste gas will be small. For the two scenarios where normal release occurs the model calculates ground level concentration of VOC in µgc/m 3. This is converted into butanol in µg/m 3 by multiplying the concentration of VOC (in µgc/m 3 ) by the molecular weight of butanol (74) and then dividing by the weight of carbon per mole of butanol (48). The OEL/40, the odour threshold value (OTV) and the UK Environmental Assessment Levels (EAL) for butanol are also presented in Table 3.1. Table 3.1: Modelled ground level concentrations Concen tration Normal release (1 km grid) Normal release (5 km grid) Bypass release (1 km grid) Annual Average Concentration µgc/m 3 as VOC µg/m 3 as butanol Maximum 1-hour concentration µgc/m 3 as VOC µg/m 3 as butanol 98th percentile 1-hour concentration 1 µgc/m 3 as VOC µg/m 3 as butanol ,127 4,346 OEL/40 7,575 OTV 364 EAL 15,400 1 Used for comparison with the Odour Threshold Value (OTV). Page 10

15 Note that the results above are for off-site concentrations only. The maximum on site concentration for the normal release scenario is 82.1µgC/m 3 as VOC (130 µg/m 3 as butanol). Maximum on site concentration for the bypass release is 9,851 µg/m 3 as butanol. The assessment of the on-site concentrations is considered health and safety issue, which is outside of the scope of this report. Figure 3.1: Contour plot of maximum 1-hour ground level VOC concentrations (µgc/m3) for normal release for 2005 meteorological data, for distances up to 1 km from the site Grid Coordinate, Metres (North) Grid Coordinate, Metres (East) Page 11

16 Figure 3.2: Contour plot of maximum ground level butanol concentrations (µg/m 3 ) for bypass release based on 2005 meteorological data, for distances up to 1 km from the site Grid Coordinates, Metres (North) Existing TO Proposed TO Grid Coordinates, Metres (East) Page 12

17 Figure 3.3: Maximum 1-hour ground level VOC concentration (µg/m 3 ) for distance up to 5 km from the site, based on 2005 meteorological data Grid Coordinates, Metres (North) Grid Coordinates, Metres (East) 3.2. Discussion The results for the normal release in Table 3.1, which are based on assumed 100 % butanol in the waste gas stream indicate no significant off-site impact, with the reported maximum concentrations being below the assessment criteria detailed in Section 2.4. As this is a worst-case scenario and assumes maximum emissions from both TOs throughout the year, the actual impact is likely to be lower. The results also indicate no likely significant off-site odour impact. The results for the bypass release also indicate that there would be no significant off-site impact, as the reported maximum concentration is below the assessment criteria of the Page 13

18 OEL/40 and the UK EAL value. The 98 th percentile concentrations exceed the lower quoted odour threshold value from Section 2.4. The presented odour threshold value is the lowest reported threshold value identified for butanol. The mean odour threshold value of 147,000 µg/m 3 (see Section 2.4) is above the reported concentrations in Table 3.1, hence based on this value there would be no odour impact. It should also be noted that the model assumes continuous release over the year, which will result in overestimation of the ground level concentration as the actual release period would be short with production being ceased in the event of a bypass occurring. The assessment also assumes both TOs are in bypass simultaneously, which again is highly conservative. In practice it is not expected that the reported concentrations would result in a significant odour impact. The results clearly indicate that the maximum concentrations occur in close proximity to the site, with on-site worst-case concentrations being higher than worst-case off-site concentrations. Therefore the use of a 1-km grid is considered more appropriate in determining maximum ground level concentrations, with a 5 km grid not being used for the bypass release scenario. Page 14

19 Appendix A - ADMS Model

20 Appendix A ADMS Model Information The ADMS model is an advanced modelling system which requires a variety of input data to ensure realistic predictions of ground level concentrations due to stack emissions. Required inputs include building dimension data, terrain data, surface roughness data, source data (Stack height, diameter, flow rate, emission rates for each compound to be modelled), meteorological data and receptor data. This assessment considers the impact of releases of substances from the TO stacks. No account is taken of the effect of any other fugitive or accidental releases. Air dispersion models are used for calculating air pollution concentrations given information about the pollutant emissions and the nature of the atmosphere (factors affecting the dispersion and dilution in the atmosphere). The resultant pollutant concentrations can be compared with air quality standards, objectives or guidelines. ADMS is a Windows based programme, which requires inputs on a number of tabs for a variety of different parameters. An example of one of the tabs is presented in the screenshot below. This is the screen where the source parameters are described. The impact of a release on the environment will be dependent on many factors, including: the rate of release of each substance;

21 other release characteristics, such as release location, release velocity and the temperature of the released material; the physical properties of the released substance (such as its physical form or particle size); the chemical properties of the released substances; the nature of the receiving medium, particularly its dispersive and transfer characteristics and how these vary with time; ambient concentrations of released substances already present in the environment; the locations of receptors in the environment sensitive to the released substances; and the degree of sensitivity of these receptors to enhanced concentrations of released substances. To quantify these effects and to establish the predicted ground level concentrations of species emitted from on-site sources, URS has undertaken detailed air dispersion modelling for the site. The selected model for use in this assessment is ADMS3, produced by Cambridge Environmental Research Consultants (CERC). This model is a new-generation model, which represents local meteorological conditions in a more technically correct way than the older models that utilise semi-empirical stability classes. The main features of ADMS3 are: all on-site sources can be modelled together in the same run, to provide an integrated assessment of the whole site; site-specific hourly sequential meteorological data is used in the modelling assessment to provide worst-case ground level concentrations for realistic conditions; meteorology is treated in a more comprehensive way than in early dispersion models, using the Monin-Obukhov length instead of the semi-empirical stability classes; worst-case conditions can be modelled e.g. adverse combinations of meteorology and emissions, which could result in pollution episodes; effects such as steep terrain, coastline and building effects can be taken into account; model outputs can be calculated for a wide range of averaging periods and percentiles, allowing direct comparison with all relevant ambient air pollutant standards and objectives.

22 The ADMS3 model takes a range of parameters including stack dimensions, emission conditions and representative meteorological data, and calculates the maximum concentrations at specified intervals from the emission source using sequential computer algorithms. It is generally considered that air dispersion models are conservative models, over-predicting ground level concentrations. All results quoted in this report are the maximum values predicted by the model, and therefore in the opinion of URS represent the worst case. The use of an advanced model such as ADMS3 rather than a simpler screening model is considered the best available analytical technique and enables the incorporation of terrain and building effects on dispersion (if required).

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