APPENDIX H AIR DISPERSION MODELLING REPORT BY PROJECT MANAGEMENT LTD. (REF. CHAPTER 11 AIR QUALITY AND CLIMATIC FACTORS)

Transcription

1 RP.0001 A Environmental Impact Statement 15 th April 2005 APPENDIX H AIR DISPERSION MODELLING REPORT BY PROJECT MANAGEMENT LTD. (REF. CHAPTER 11 AIR QUALITY AND CLIMATIC FACTORS) S:\Projects\190900\ Centocor\EIS\ RP-0001 A.doc

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3 CONTENTS 1. EXECUTIVE SUMMARY 3 2. INTRODUCTION 4 3. AIR DISPERSION METHODOLOGY Emissions Modelled Modelling Approach Meteorological Data Building Downwash Terrain Source Data Receptors 8 4. EMISSION INVENTORY AND SOURCE DATA Mass Emission Rates Source Physical Parameters 9 5. MODELLING RESULTS Stack Height Determination Nitrogen Oxides DISCUSSION OF MODEL RESULTS Nitrogen Dioxide CONCLUSIONS 14 APPENDIX 1 Isopleth Charts of the Predicted Ground Level Pollutant Concentrations N:\101050\22 ENVIRONMENTAL\310 AIR QUALITY\ rp-0003\ RP-0003.doc 2

4 1. EXECUTIVE SUMMARY PM carried out modelling analysis of predicted emissions from the proposed Centocor Biologics Ireland plant at Ringaskiddy. An air dispersion model was used to determine the ground level concentrations of nitrogen oxides due to the emissions of the combined heat and power plant and the steam boilers. Predicted ground levels were evaluated against air quality standards. Emissions from the site have been modelled at worst-case emission rates, although in all cases the actual emissions from the plant are likely to be significantly less than these levels. The results of the assessment have shown that the maximum short term (1-hour) ground-level nitrogen oxide concentrations occur to the north east of the facility, while the long term (annual) maximum ground level concentration also occur to the north east of the facility. In summary, it was determined that the minimum appropriate stack height is 30 metres above ground level and it was demonstrated that emissions to atmosphere from the proposed Centocor site will not result in ground level concentrations of pollutants exceeding any applicable air quality standard limit values. N:\101050\22 ENVIRONMENTAL\310 AIR QUALITY\ rp-0003\ RP-0003.doc 3

5 2. INTRODUCTION PM carried out an air quality modelling analysis of emissions from the proposed Centocor facility at Ringaskiddy, Co. Cork. The main emission points on the site were modelled; namely emissions from the combined heat and power plant (CHP) and the steam boiler. The ground level concentration of these emissions was calculated over a 3 km radius around the site. The site is set in an established industrial area, bordered by a mix of residential and agricultural uses. Emissions of nitrogen oxides were modelled and the predicted ground level concentrations were evaluated against the Air Quality Standard Regulations. A number of scenarios were run to determine the impact of different operational conditions on the maximum ground level concentrations predicted by the model. Emission rates used in the model represent the maximum permitted emission rate, and consequently the worst-case ground level concentrations. All calculations are based on 5-years of meteorological data, which was collected at Cork Airport meteorology station, which is considered to be representative of the meteorological conditions experienced at the site. N:\101050\22 ENVIRONMENTAL\310 AIR QUALITY\ rp-0003\ RP-0003.doc 4

6 3. AIR DISPERSION METHODOLOGY 3.1 Emissions Modelled The model was used to calculate the ground level concentrations of three groups of Nitrogen Oxides (NO X ). The CHP plant and the steam boiler are powered by mains natural gas. The sulphur content of the gas is negligible and thus the potential for the generation of significant emissions of sulphur dioxide does not arise Nitrogen Oxides The major emission points for nitrogen oxides on the site are the CHP plant boilers. The CHP plant consists of a gas turbine coupled to a waste heat recovery boiler with a backup stand-alone steam generation boiler. The operational details of the CHP plant are detailed in Chapter 11 of the main EIS document. Table 1 Emission Scenarios Modelled Scenario Scenario 1 Scenario 2 Scenario 3 Description CHP operating in Open Cycle mode, generating electricity only (emission through CHP bypass stack) CHP operating in Closed Cycle mode, generating steam and electricity (emission through CHP main stack) CHP operating in Closed Cycle mode, generating steam and electricity, with supplementary steam generation by backup boiler (emission through CHP bypass stack and boiler stack) The emission rates which are input to the model are in the form of NO X, thus giving predictions of ground level NO X concentrations. The plant emissions are mainly in the form of NO, but the air quality standards apply only to NO 2. NO 2 is formed in the atmosphere through the oxidation of NO. As there is no long-term site specific monitoring data available for the site for the conversion of NO to NO 2 the NO 2 to NO X ratio is assumed to be 0.75, as recommended by the USEPA guideline on air quality models Emission Rate Assumptions Maximum pollutant concentrations for the CHP and steam boiler emission points were used in the model and the model is therefore conservative as the various emission points will not always discharge at maximum emission concentrations. 1 USEPA, 40 CFR Ch. I ( Edition) Appendix W To Part 51 Guideline On Air Quality Models N:\101050\22 ENVIRONMENTAL\310 AIR QUALITY\ rp-0003\ RP-0003.doc 5

7 3.2 Modelling Approach Dispersion modelling has been carried out using the AERMOD computer model developed by the US EPA. The model consists of mathematical algorithms which simulate the transport and dispersion of air pollutant emissions downwind of a source. AERMOD is a steady-state plume model which represents the plume as having a normal (Gaussian) distribution in both the horizontal and vertical directions for the stable boundary layer (SBL) and the horizontal direction for the central boundary layer (CBL), but a bi-gaussian probability density function for vertical distribution in the CBL. The model predicts average concentrations over 1-hour, 24-hour and annual periods and percentiles thereof. The model allows for the effects of general plume rise, stack tip downwash and building downwash. The choice was made to use AERMOD model instead of the older ISC model as the ISC model is in the process of being replaced as the preferred model by the USEPA. The short-term version of ISC (ISCST3) has several theoretical deficiencies, including a poor characterisation of building downwash and terrain features. AERMOD was developed by the USEPA in response to these deficiencies and is generally regarded as yielding a more theoretically accurate treatment of atmospheric dispersion. AERMOD is increasingly used in Ireland for the assessment of air quality impact and has previously been accepted by the Irish EPA for such assessments. 3.3 Meteorological Data The meteorological data required by the dispersion model is wind speed, wind direction, Pasquill-Gifford stability category, boundary layer height and ambient temperature. The stability category and boundary layer height are used to characterise the turbulence within, and the height of the lower levels of the atmosphere. Extremely unstable conditions can cause plume looping and elevated concentrations close to the stack. Under stable conditions elevated concentrations can occur due to the emissions being trapped below the boundary layer. Neutral conditions, characterised by cloudy skies and strong winds, are most favourable for dispersion due to the mechanical mixing of the lower atmosphere. The wind direction determines the direction in which the plume is blown, and for a particular stability, higher wind speeds will result in reduced plume rise so causing the plume to reach ground level closer to the stack with elevated emission concentrations. The boundary layer height determines the total vertical distance over which the plume may spread. Five years (1999, 2000, 2001, 2002, 2004 * ) of meteorological data for Cork Airport was used. This is considered to be representative of meteorological conditions experienced at the proposed site of the Centocor facility. The data obtained consists of hourly values of wind speed, wind direction, air temperature, stability category and mixing height. Wind direction is converted to a flow vector (the * 2003 data was not used, as it was not a complete data set N:\101050\22 ENVIRONMENTAL\310 AIR QUALITY\ rp-0003\ RP-0003.doc 6

8 direction toward which the emission moves) by adjusting the direction by 180 degrees. The effect of landuse is incorporated into the Meteorological dataset. 3.4 Building Downwash Air streams blowing across buildings can become disrupted, with turbulent eddies occurring downwind in the building wake. If an emission point is sufficiently close to a building, then the plume may become entrained in the turbulent eddies of the building wake. This entrainment can cause plume downwash resulting in elevated emission concentrations close to the emission point. The Good Engineering Practice (GEP) stack height is the minimum height required by a stack in order to avoid structural or building wake-effect induced downwash. Downwash brings pollutants closer to ground level at a shorter downwind distance giving the worst-case scenario for a particular site. The GEP heights and relevant building dimensions were evaluated following USEPA guideline procedures for determining, in each of the 36 wind direction sectors (10 o width/sector), if the building has potential to cause downwash of stack emissions. The stack modelled is subject to downwash and, as a result, direction specific building dimensions were calculated. The AERMOD model interprets the influence zone of each building for a given wind direction using the Building Profile Input Program (BPIP). All of the main proposed buildings on the site were included in the modelling analysis, including warehouses, office/labs area, production building, etc. 3.5 Terrain It was necessary to input a terrain height for each of the receptors on the grid. Terrain heights have been digitised from the ordnance survey of Ireland map of the area surrounding the site. The terrain file included terrain data for a 3 km by 3 km grid centred on the site. Survey data was used for the terrain in the immediate vicinity of the site (1 km). 3.6 Source Data Emissions data and discharge parameters have been provided by CHP manufacturers (Cummins). The emissions have been modelled at the maximum anticipated emission levels, although in all cases the actual emissions from the plant are likely to be significantly less than these levels. The stack discharge parameters and emissions data is summarised in Section 3. The output from each emission point was treated as a point source in the model. Details of the locations and characteristics of the source were inputted to the model using the site layout plan as a template and using information obtained from operations personnel. These details include; stack diameter, height of stack, volumetric flow or velocity and temperature of exhaust gases (heat release). The emission rates were modelled at maximum emission limits to establish the worstcase scenario for air quality impact, while the plant operates within the anticipated requirements of the IPPC licence conditions. N:\101050\22 ENVIRONMENTAL\310 AIR QUALITY\ rp-0003\ RP-0003.doc 7

9 3.7 Receptors The model was set up to examine the impact of emissions on the area surrounding the facility using a series of receptors. A receptor is a location at which the model will calculate maximum ground level concentrations. A polar co-ordinate receptor grid system was established with its centre on the facility. Receptors were placed at 500 m intervals over a 3 km by 3 km grid centred on the site. A detailed rectangular 1km by 1km grid was established around the facility with receptors placed at 100m intervals. This detailed grid allowed greater resolution in determining the maximum ground levels concentrations in the areas immediately adjacent to the facility. N:\101050\22 ENVIRONMENTAL\310 AIR QUALITY\ rp-0003\ RP-0003.doc 8

10 4. EMISSION INVENTORY AND SOURCE DATA 4.1 Mass Emission Rates The emission values and the corresponding emission rates used in the model are listed below. Table 2 Emission limit values and mass flows Emission Point Emission Concentration Emission Rate* mg/nm 3 g/s Nitrogen Oxides CHP Bypass Stack CHP Main Stack Steam Boiler Stack * The emission rate has been uncorrected from STP to emission conditions. 4.2 Source Physical Parameters The physical parameters of the emission sources are listed below. The model was run at various stack heights as part of the stack height determination exercise. Table 3 Physical Parameters of the Emission Sources Modelled Emission Point Stack Height Diameter Temperature Flow Rate mogl M K Nm 3 /h CHP Bypass Stack CHP Main Stack Steam Boiler Stack 25, 30, ,850 25, 30, ,850 25, 30, ,000 N:\101050\22 ENVIRONMENTAL\310 AIR QUALITY\ rp-0003\ RP-0003.doc 9

11 5. MODELLING RESULTS The ground level concentrations across various averaging periods are summarised in the tables below. An isopleth chart of the predicted ground level concentrations are presented for nitrogen oxides in Appendix A. 5.1 Stack Height Determination There are three main stack emission points from the proposed facility from which oxides of nitrogen will be emitted. These main emission flues will be coupled together in one structure. It is assumed for the purposes of this assessment that the emission from the stacks will be continuous. As the plant will be constructed by a turnkey contractor on a design and build basis, the following stack discharge parameters are based on typical parameters supplied by specialised gas turbine suppliers. In all cases the worst case parameters, i.e. those that would lead to the highest ground level concentrations, are used in the analysis. Although other more simple modelling tools (such as Screen3) are available for the purpose of conducting screening analyses, it was decided to use AERMOD for the screening analysis as this model will be used for a detailed analysis. The screening analysis was carried out using a 3 km square grid as it was found in an initial analysis that the maximum concentration occurred within 3 km of the stack for all stack heights and meteorological conditions. The stack height was assessed at 5m intervals from 25 m to 35 m. The maximum hourly average ground level NOx concentrations were calculated for the range of stack heights for operation of the CHP plant in the closed cycle modes, with the backup boiler in operation. To relate the NOx levels calculated to the Air Quality Standards, a 75 % conversion of NOx to NO 2 is assumed. The trend in concentrations gives an indication as to general plume behaviour as the stack height varies. The results are shown in Figure 1 on the following page. The proposed 99.8th percentile (not to be exceeded for more than 18 hours per annum) limit value is also indicated on Figures 1 for reference. As can be seen in Figure 1 the maximum ground level concentration decreases steadily as the stack height increases. Although the concentrations arising from stacks > 35 metres are lower again, the greater visual impact of these stacks, given the elevated nature of the site, may not be justified as the 30m stack is likely to disperse the stack emissions sufficiently to prevent any adverse air quality impact. The detailed assessment will therefore be carried out on the 30m stack. N:\101050\22 ENVIRONMENTAL\310 AIR QUALITY\ rp-0003\ RP-0003.doc 10

12 Ground Level Concentration Concentration (mg/m3) Air Quality Standard (NO2) Stack Height (m) Figure 1 Results of Stack Height Determination (ambient levels in blue) Table 4 Summary Results of Stack Height Assessment Stack Height AQS Average Background* Max Ground Level NOx Max Ground Level NO 2 ** m µg/m 3 µg/m 3 µg/m 3 µg/m * Background concentration taken as twice maximum daily ambient concentration measured. ** The assumed conversion of NOx to NO 2 is 75 % N:\101050\22 ENVIRONMENTAL\310 AIR QUALITY\ rp-0003\ RP-0003.doc 11

13 5.2 Nitrogen Oxides Emissions of nitrogen oxides were modelled at the maximum emission rates. In the assessment of the results it is assumed that 75 % of the NO X is converted to NO 2 in the atmosphere, this is a conservative approach and is used in the absence of longterm monitoring results for the area. The model predicts the short-term (1-hour) maximum concentration will occur to the north-west of the site in the areas of Ballybricken Point. Table 5 Predicted Ground Level Nitrogen Oxide and Nitrogen Dioxide Levels Averaging Period Scenario AQS percentile of Hourly NOx Concentrations µg/m percentile of Hourly NO 2 Concentrations Annual Average NOx µg/m µg/m Annual Average NO 2 µg/m N:\101050\22 ENVIRONMENTAL\310 AIR QUALITY\ rp-0003\ RP-0003.doc 12

14 6. DISCUSSION OF MODEL RESULTS The modelling results demonstrate that atmospheric emissions from the proposed plant do not result in ground level concentrations (GLCs) of pollutants which exceed applicable air quality standard (AQS) limit values. 6.1 Nitrogen Dioxide The maximum predicted GLCs of NO 2 are well below their respective AQS limit values. Scenario 2 represents the normal operational scenario for the CHP plant and shows that the maximum predicted hourly and annual NO 2 GLCs are less than 20 % and 6 % respectively of the AQS limit values. N:\101050\22 ENVIRONMENTAL\310 AIR QUALITY\ rp-0003\ RP-0003.doc 13

15 7. CONCLUSIONS A stack height determination resulted in the stack height of 30 meters being selected as the lowest appropriate stack height. Lower levels resulted in poor dispersion and while higher stacks improved dispersion the visual impact is likely to be too high. The stacks at 30 m provide adequate dispersion while minimising visual impacts. All of the emissions were modelled at maximum emission levels. The plant is likely to operate with lower emission rates. The maximum ground level concentrations represent the worst-case of maximum emission rate and meteorological conditions with poor dispersion characteristics. These weather conditions occur less than 10 to 15% of the hours in any one year In summary, the air dispersion model study has demonstrated that emissions to atmosphere from the proposed facility will not result in ground level concentrations of pollutants exceeding any applicable air quality standard (AQS) limit values. The statutory AQS limit values have been designed for the protection of human health and the environment. Therefore atmospheric emissions from the proposed plant are not predicted to have any significant adverse impact on ambient air quality and human health and the environment. N:\101050\22 ENVIRONMENTAL\310 AIR QUALITY\ rp-0003\ RP-0003.doc 14

16 EIS Air Dispersion Modelling Report Appendix A 15 th April 2005 APPENDIX A ISOPLETH CHARTS OF THE PREDICTED GROUND LEVEL POLLUTANT CONCENTRATIONS N:\101050\22 ENVIRONMENTAL\310 AIR QUALITY\ rp-0003\ RP-0003.doc Page 1 of 4

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