With environmental legislation now recognising the significance of mass emissions of particulates and the 2006 Euro 4 regulations requiring an 80% reduction for particulate matter (PM), one tunnel operator has been led, for the first time, to use electrostatic precipitators to help control emissions for health reasons.
Health effects of particulate emissions from road transport have been of interest for many years. Historically, particulate emissions from road vehicles have been controlled and progressively reduced through legislation based on particulate mass measurement. More recently, interest has extended into more specific characteristics of particle emissions such as size, number, surface area and composition.
Electrostatic precipitators (ESPs) have until now been installed in a limited number of projects in Japan and Norway where the tunnel ventilation system alone cannot meet visibility requirements within the tunnel and in a few cases the air quality goals. This has changed with the Ecotunnel project in Cesena in Italy, where ESPs have been added to help combat the problems of particle emissions for visibility.
Up until now these tunnels generally have had a high percentage of diesel vehicles or topography that does not suit exhaust stacks, e.g., mountainous terrain or underwater. In Norway they also have a problem with steel-studded tyres which creates a dust problem in their tunnels.
A Norwegian example
Most road tunnels in Norway are longitudinally ventilated, with air expelled at the portals naturally or induced by fans.
In 1989, Norway began a research program to determine the possibility of cleaning air in road tunnels. The key factor in the programme was the pollution emissions at the portals, the particulate component of which is partially caused by the widespread use of studded tyres in winter.
Norway’s road tunnel air treatment program has involved the use of electrostatic precipitators (ESPs). ESPs are designed to electrically charge dust particles and remove them from the air on collector plates. Three different types of layout for the ESPs have been used in Norwegian road tunnels: ESPs in a ventilation shaft connected to the tunnel; ESPs in a bypass area adjacent to the tunnel; and ESPs in the tunnel ceiling.
ESPs have been installed in seven road tunnels. The operating system is triggered automatically by equipment measuring carbon monoxide levels. A timer switch for operation on a peak traffic basis is used as an alternative.
ESPs have produced promising results in short-term tests under optimal conditions. Measurements immediately before and after ESP plates have shown a reduction in particles of 80-90%. While these tests produce an operational guarantee standard for the ESPs, they do not show a practical filtration effect in road tunnels or outcomes over a medium to longer term.
Case study in Japan
Work to develop ESPs to remove particles from tunnel air began in Japan to address poor visibility in tunnels. ESPs were installed for the first time anywhere in the world in the 2.1km Tsurga Tunnel in 1979.
The development of ESPs extended the range of longitudinal ventilation. The first long tunnel combining longitudinal ventilation and ESPs was the 11km Kan’etsu Tunnel in 1985. The Tunnel, still the longest road tunnel in Japan, combines longitudinal ventilation using intake and exhaust shafts and ESPs in bypass passages. This involves part of the air in the tunnel being diverted into the bypass passages to be treated.
Both Matsushita and Mitsubishi are leading manufacturers of ESP systems. Research by both companies has targeted improvement of particle collection efficiency and an increase in air speed through ESPs. The companies reported that testing has shown that for air speeds of up to 9m/sec an efficiency of 90% can be achieved. ESPs have been developed and installed that can operate at speeds of up to 13m/sec. At this speed however, the efficiency drops back to just over 80%.
The Italian example
In the Italian town of Cesena an urban tunnel named Ecotunnel has recently been constructed in an area with a high population density. The tunnel traverses the city and is a crucial link between the E45 motorway from Rome and SS 9 main road.
The design of the tunnel was presented to the Borough of Cesena and its population. In order to find a widely accepted solution, several proposals were presented: standard longitudinal ventilation system; the same, but with an end tunnel shaft; and the final agreed solution with electrostatic exhaust filtration system of polluted air.
Options considered
Longitudinal ventilation was the preferred option, its initial low cost and simplicity makes it one of the most popular forms of tunnel ventilation in use in Italy. In the Ecotunnel the two bores with unidirectional traffic and the relatively short tunnel length, of about 1600m, would have been ideal for such an application, even the emergency fire scenario would have been fully covered.
The major concern though was the tunnel location in a highly populated area. Various complementary solutions were evaluated:
Longitudinal Ventilation with Stacks (LVS) – The principle alternative studied was the possibility of collecting the polluted air near the tunnel exit via a ventilation stack. Several studies showed that a proper stack location, height (of about 20m) and air exhaust velocity (about 25m/s) gave a substantially lower fall down concentration to the environment. Unfortunately this was not acceptable to the people due to ascetics.
Longitudinal Ventilation with Stacks and Filtration system (LVSF) – An electrostatic filtration system was subsequently proposed and accepted for each bore, including a ARPA (Local Health Government Board) monitoring on the environmental conditions.
To highlight the cleanliness of the system it was decided to use it as an experiment to learn, check and evaluate the results.
The ventilations system
Flakt Woods has supplied the complete ventilation system for the tunnels. Each of the Ecotunnel unidirectional rectangular section bores has two carriageways with a gradient varying from –3.7/+0.9%.
Considering a horizon of at least 15 years calculations have been developed. Today’s data (increased in % up to 2020) are 2,300 private cars per hour, 60% petrol and 40% diesel, 210 commercial vans per hour, all diesel, 210 heavy lorries including buses (30t) per hour. Traffic speed is 60km/hr on a normal carriageway, and 80km/hr in the overtaking carriageway.
For normal, congested and stopped traffic conditions calculations have been made using appropriate emission data and pollutant levels.
For each bore the fresh air requirement to cover all scenarios, including fire, is 250m3/s. To induce such a volume of air it’s necessary for each bore to supply 14 booster fans, reversible type 1000mm diameter, 900N thrust, made of stainless steel EN 1.4435/4404/4432 (AISI 316L) and equipped with 27kW motors. The jet fans are fully tested and comply with EN 12101-3 F 400/2. In normal conditions 10 off are needed and 4 off in emergency.
Near to each portal Flakt Woods calculated the provision of a large ceiling section of 64m2 with dampers made of stainless steel EN 1.4435/4404/4432 (AISI 316L) equipped with actuators. Dampers comply with EN 12101-3 and are certified for F 400/2. In the ventilation room (one for each bore) there are 2 off large axial fans, 2240mm diameter, air volume 125m³/s at 1650Pa total pressure, equipped with 330kW motors suitable as per EN 12101-3 for F 250/2 and inverter regulated.
The filtration system
The filtration system used has been developed with the University of Technology of Graz (A) and using the facility of a pilot plant in Austria’s Plabutsch tunnel. The basic equipment is the Electrostatic Precipitator (EP) modified with the latest technology. This leads to a completely new generation combining the advantages of an EP and mechanical filter. It is called ECCO (Electrostatic Charged Contact) and is ATEX certified.
There are two plants (one at each bore exit) and the main technical data are:
• Air volume = 250m3/s per plant
• Width = 19m
• Height = 4.8m
• Working voltage = 14kV
• Power = 18.3kW
• Pressure drop = 250Pa
• Gas cleaning = no
Principle of operation
The particles must be charged so they have to pass the ionizer blades between the grounded electrodes. The particles will be charged in the high intensity field created by high voltage alternating current. An industrial high voltage power supply with pulse mode is used to get the best efficiency, high intensity electrostatic fields and effective control systems. For the discharge electrodes, Polygonal-Sinus-Perforation (PSP) blades made of stainless steel are used. This guarantees a long lifetime with the highest intensity for charging the particles. The charging of particles is similar as in a standard EP The main and important difference is the collecting of particles not realised between collector plates.
The new collector is a filter media placed in an electrostatic field between a high voltage grid and a grounded grid. The charged particles must have a contact with the filter; so the particles will be collected at the surface and inside the filter.
During the operation the filter media is not removed and can be cleaned with an automatic process depending on: 1) high voltage current = an increased value shows a filter clogging, 2) pressure drop = an increased value shows a filter clogging, 3) fixed time between cleaning= a fixed time can be set up irrespective to the other parameters. To reduce pressure drop there are diffusers at the end.
Cleaning of filtration system
Tests on the PSP ionizer showed that there is no significant build up of dust, but after some weeks there is the possibility of a build up of carbon on the tips of the PSP blades. To avoid this the ionizer is washed on a weekly basis: this takes only a limited time. The washing is via a fresh water supply or can be re-circulated by cleaning the wastewater with a Slope bed filter. The system is automatic, and the wastewater is collected in a wastewater tank.
Between the washing cycle the wastewater is cleaned with the bed filter and then goes into the clean water tank. The control system ensures water loss is automatically replaced. To improve the cleaning efficiency a detergent is added.
The main advantage of the PSP versus the other traditional E.P. collectors is that the collector can be dry-cleaned.
Standard electrostatic collector plates must be washed, which means it is necessary to stop the filtration system and wait until the E.P. has dried, typically this means a break of several hours before it is possible to use it again.
The newly collected dust is collected in the filter media placed between two grids in an electrostatic field. Depending on high voltage or pressure drop or pre set fix time between the wash cycles, the cleaning of the filter media starts. During the cleaning cycle the filter is moving (rotating filter type) and will be cleaned using a high-pressure fan.
The dust is then separated in a dust filter with an automatic process and collected in dustbins. The latter ones have an automatic level control which give a signal when they are full. This reduces the exposure to heavy metal dust, particles and reduces the risk of inhalation of particle material.
It is possible to clean the filter during operation. Each module of ECCO filter has a damper, which is closed during normal operation. During cleaning these open and this operation is repeated for each module.
Efficiency
The first approach is to use a standard test to measure and define the efficiency. ASHRAE 52.2-1999 and EN 779 are using the same dust composition and the used filter system can achieve 94% average synthetic dust weight filtration.
In road tunnels the dust consists of very fine particles (mainly from diesel soot with particle size < 1 micron) and quite different from the ASHRAE & EN dust test. So the filter system shows in this case a peak value of 90% and a mean value of 85%.
The size distribution and efficiencies are shown in Table 1 (p43).
The PM’s values are used for environmental behaviour but nowadays they are also for filtration too. International standards define the inhalation and thoracic fractions of dust. Table 2 (p43) gives an idea of values in thel air in the Plabutsch tunnel.
To complete the explanation, the diesel soot consists of particles less than 1 micron with typical highest number at 0.1 micron. Ultra fine diesel particles can cause cancer and are cities’ main pollution problem.
The particles with nano dimensions have a large surface but proportionally a less mass concentration, this is the reason why it is better to count the particles in order to determine the risk.
Control system
The filtration system works automatically: normal operation, cleaning, high voltage are controlled by a PLC and a bus system. It has been decided to have a Control Centre (CC) where all the main system parameters are collected; Tunnel air quality (CO, NOx, opacity); Fire (fibre laser system); SOS (emergency points); TVCC (cameras); Ventilation (fan operation parameters); ECCO filtration (high voltage, current, filtration level, cleaning requirements).
Field PLC’s (one per each ventilation room) receive the input signals and send them via an optical close loop to the CC It is worth paying attention to the filtration system, even if the operations are fully automatic the application of such equipment requires particular care. A dedicated course to the engineers of the CC will be undertaken in due time.
Nevertheless the risk of out of service or a malfunction of operation is still possible so an ISDN has been installed connecting the manufacturer of the system in Austria and Fläkt Woods – that provides a real time check on the main parameters and a quick response if necessary.
Conclusions
New ECCO filters can substantially reduce the PM’s and the cost of this technology is nowadays acceptable. During the second half of this year the tunnel will be open and a local survey by the University of Technology of Graz is already foreseen, carrying out measurements with and without filtration system in order to determine the efficiency and prove the benefit of adopted solution.
The Local Health Board will participate and share knowledge of the experience.
Jet fan ECCO collectors Dust is separated in a filter automatically and collected in dustbins The Ecotunnel jet fans are reversible type 1000mm diameter, 900N thrust made of stainless steel Table 1 Table 2
