iFluids Engineering & Consultancy WLL conducted Air dispersion Analysis for different relief case scenarios to assess the pollutant (H2S and CO2) levels as a result of cold vent release during pressure relief operations for Qatar Gas. Concentrations of pollutants have been compared to agreed Qatar Ambient Air Quality Standards outside of the project boundary and for exposure related to an OSHA’s 8-hour TWA day.
The maximum concentration of all pollutants in particular to H2S were found to be deposited closer to the stack and within project boundary. To learn more on H2S Zoning. Click Here The Carbon dioxide emission were found to be diluted rapidly and was found the impact to be negligible.
The H2S ground level concentration during emergency relief scenarios were found to be closer to ambient Qatar Ambient air quality guidelines. Understanding, that the scenarios studied are part emergency case scenarios and it is very rare to occur, and no individual stream carries more than 1000 ppm of H2S, it is recommended that toxic gas detectors shall be provided at the air inlet ducting at buildings within 30 m (100 ft) of the vent stack and spacing requirement shall be followed.
Therefore, it would be safe to consider the vent stack of 35 m height and 1 m diameter, do not cause any detrimental contribution for both workplace and environmental criteria to the local and regional environment
Qatar Gas is currently working on a CO2 Export Facilities projects to provide dehydration facility for moisture removal of CO2 rich off gas coming from the AGE absorbers at the Qatar Gas South plant as well as an export facility to deliver the dense phase CO2 at the QP designated point.
The CO2 rich stream will be treated using TEG as the dehydration technology to meet the required export specification at the QP Dukhan Oil field. Existing Acid Gas Compressors (3 compressor trains each with 4 stages) are used to handle CO2-rich streams from the AGE absorbers. TEG dehydration package will be installed in between 3rd stage and 4th stage of each compressor trains.
Compressor discharges (dense phase CO2) of all the three trains are combined in a common header and sent to the export facility. The CO2 export facility is designed for handling 120 MMSCFD of dense phase CO2 through 2 centrifugal pumps with a capacity of 60 MMSCFD each to export the dense phase CO2 to QP station A6.
Dry gas flare, CO2 release during relief valve popping-up and depressurization can lead to the flare flame-out and that requires support of additional assist gas to prevent such flare flameout. Hence, all the relief and vents associated with CO2 system which includes the existing compressors, injection facility and injection system are to be diverted to the new Vent Stack System.
Two main relief headers, namely Dry Gas and Wet Gas relief headers, are connected to the new Vent Stack system.
Scope of Work
The scope of work is to perform Vent dispersion study in order to verify if the given dimensions meets the Safe dispersion criteria.
The study methodology is to conservatively estimate ground level concentration from the emission source (Vent Stack) at the receptors within the project study area of 25 km by 25 km. The modelling assessment is to assess if the emissions from the proposed relief gases released through vent stack would meet the Qatar Ambient Air Quality Limits for H2S and in the absence of Qatar Ambient Air Quality limits for CO2 OSHA’s Permissible exposure limit is used.
It is also to be noted that ground level concentration (GLC) to the following shall imply:
- Ambient air quality would be typically applicable to areas outside of the plant area.
- Areas within the project boundary (battery limit) would be considered as workplace regulations. As a conservative assessment the Areas within the Project were to abide by the Ambient Air Quality limits.
This section provides the rationale for the dispersion model selection and a summary of the associated modelling approach and methodologies. The flat terrain and rural dispersion options were selected for the dispersion modelling was selected to assess both the long-term air quality impacts (five year) associated with emissions from the project area, a Ten -year (2010-2020) time series of hourly meteorology data from the project Area (upper air and surface airport data sets) and local surface data set meteorology monitoring stations, were used to prepare the input meteorology data.
The meteorological Data set in NOAA (National Oceanic Atmospheric Administration) format was used to extrapolate the wind profile of the project area. The dispersion model can make air quality concentration predictions for multiple emission scenarios using hourly average meteorology data sets, and can predict concentrations for various output averaging times, at many receptor locations.
Air dispersion Modelling
The Air dispersion modelling approach and the related assessment methodologies applied in this project are further described as follows:
Executing the dispersion modelling based on receptors incorporating variations in the local terrain was based on the criteria contained in the Model Guideline. The dispersion modelling domain was set to a square region extending 25 by 25 kilometer from the site center in each direction.
A ten-year hourly average meteorology data set, based on surface and upper air observations at the regional International Airport and Regional Air monitoring stations for both upper air and surface data were used for the years 2010-2020, was used in this assessment.
Each hour of the output meteorology data includes wind speed, wind direction, ambient temperature, surface heat flux, friction velocity, convective velocity scale, Monin-Obukhov length, surface roughness, Bowen ratio, surface albedo, wind speed reference level, and ambient temperature reference level.
The vent stack is modelled as a continuous source and the model predictions are directly compared with the air quality standards.
The purpose is to demonstrate the ground level concentration of the H2S and CO2 during the relief scenarios meets the ambient air quality standard and is being let at a safe height so as to cause minimum interference or hazard to workplace and regional environment.
Meteorology plays a major role in determining air quality changes downwind of industrial and nonindustrial emission sources. Regional meteorological dataset as established for the project was incorporated into the regional weather databased developed by NOAA (National Oceanic and Atmospheric Administration) for the five years 2010-2020. The Hourly meteorological data was used for the modelling assessment of the meteorological changes to assess the worst-case scenario occurring in the last five year which is assumed to resemble the general weather pattern in the area.
The Dispersion modelling assessment of ground-level concentrations (GLCs) for H2S and CO2 for the proposed vent stack height of 35 m above ground was conducted based on the maximum emissions criteria as per the design conditions associated with cold venting scenario at the vent stack. Modelling was performed with the U.S. EPA AERMOD model (version 19191), the results of which are determined to be conservative.
Based on the analysis for the worst-case criteria, the vent stack height of 35 m above grade and diameter of 1m as proposed by the design is deemed sufficient based on-air dispersion modelling. Of the relief case scenarios studies, impact of CO2 release is negligible and maximum ground level concentration of H2S is found to be within the project boundary.
The relief scenarios was categorized as Primary Case scenarios and secondary case scenario. The primary relief case scenarios consist of 1 design case of dry gas relief with maximum mass flow rate and high temperature and secondary relief scenarios are based on release from the blowdown valve and pressure safety valve.
The maximum ground level concentration of various scenarios studied were found to be closer to the vent stack and within the project boundary. Since the maximum concentration lies within project boundary and workplace criteria would apply rather than environmental ambient air quality standards. The workplace criteria as per OSHA (Occupational Safety and Health Administration) for H2S and CO2 are 70 and 900 mg/m3 respectively. The maximum concentration is well below the prescribed limit. The maximum concentration outside of the project boundary complies with the Qatar Ambient Air Quality Standards.
However, as per the Qatar Gas Loss Prevention Philosophy, toxic gas detectors shall be placed at appropriate location such that any H2S release from the vent stack during relief scenarios shall be detected immediately and appropriate action to be take. To further on the studies, vent gas dispersion studies shall be carried out to identify appropriate location of the H2S gas detectors
The vent gases do not cause any detrimental contribution for both workplace and environmental criteria to the local and regional environment. Therefore, it is safe to consider that Vent Stack height of 35 m and with diameter of 1 m is adequate for the safe dispersion at the present gas configuration and is acceptable to operate.