One of the largest air purification systems in the world, installed for a road tunnel in Hong Kong, will provide residents with cleaner air around tunnel portals
Air Purification Systems (APS) have been applied to many road tunnel projects around the world in order to maintain tunnel air quality and/or reduce tunnel emissions. However, less than ten projects use APS to remove both dust and nitrogen dioxide (NO2) or mono-nitrogen oxides (NOx).
A new road tunnel is being constructed in an urban district of Hong Kong. Eight APS plants will be installed in three ventilation buildings of the tunnel. To satisfy the environmental requirements, an electrostatic precipitator (ESP) to remove dust has been installed. To further improve visibility and air quality, equipment for removing NO2 is also provided in the project. In case of fire, the APS is bypassed and the tunnel ventilation system will control and effectively discharge smoke to ensure the safety of tunnel users. The purification system provides added benefit to the surrounding environment as well; the APS produces cleaner outdoor air for local residents around the tunnel portals and ventilation buildings. The proposed APS is one of the largest in the world. WSP | Parsons Brinckerhoff is the designer for all the mechanical and electrical (M&E) systems of the project, including the APS. This tunnel project employs Building Information Modeling (BIM).
In designing an APS for a road tunnel, the major challenges and technical requirements are:
- Ensuring the Efficiency of the APS
The APS consists of an electrostatic precipitator (ESP) and a de-nitrification system (De-NO2) for the removal of dust particulates and NO2 in the tunnel exhaust air stream. With consideration of the current technology and the long term operation of the APS, the APS shall achieve at least 80 percent efficiency for removing the dust particulates and NO2 from the airflow stream. - Selection of De-NO2 Filter
To avoid excessive replacement work, the lifetime of the De-NO2 filter shall be longer than 3 years under a tunnel operation environment. To achieve a comparative tender price, the filter shall be available on the open market. - Fire and Life Safety Features in APS
Safety interlocks of the APS with personal access doors and motorized fire dampers connected to the smoke extraction system shall be provided. - Control and Monitoring of the APS
Adequate heat detection, particle detection, and NO2 detection will be provided to monitor the performance of the ESP and De-NO2 systems. - Civil, Mechanical, and Electrical Provisions for APS
Sufficient provisions shall be reserved for the APS, especially for the regeneration plant. Civil provisions include space for the plant and the associated delivery route, sump pit for the regeneration plant, lifting provision for the equipment. Mechanical and electrical provisions include water supply for the cleaning of the ESP, drainage system for the wastewater produced by the APS, high voltage power supply system for the ESP, control and monitoring system, etc. - Avoid Converting Air Pollution to Water Pollution
Wastewater is mainly generated from the ESP cleaning system. Reducing the amount of wastewater from the ESP is one of the key considerations in the design (refer to Network Issue 77, February 2014, “Wastewater Reduction in Road Tunnel Air Purification System”).
It is essential that the major parameters of the APS are specified in the tender document and that a sufficient number of market players can join the competition. WSP | Parsons Brinckerhoff has considered all features and spatial constraints and developed a design that is feasible for various suppliers to bid for the project, thereby providing fair market opportunities while meeting local environmental requirements.
