How to use an Electrostatic precipitator in a tunnel

The upper portion of the diagram shows a tunnel with a change in slope half way along the tunnel. Electrostatic precipitator units are installed under the ceiling and arranged to treat some, but not all, of the air flowing along the tunnel. Precipitators remove almost all particles from this air. The lower portion shows the effect of filtering on the smoke and particles in the tunnel. The upper black line shows the increasing particle concentration along an unfiltered tunnel and the lower lines, what happens when 1/3 or 1/2 of the air is cleaned at each filter station.

Unless it is possible to simply blow the tunnel exhaust from the ends of a tunnel, the most efficient way to clean particles and smoke from the inside of a tunnel is to install electrostatic precipitators along the ceiling in a way which matches the rate of production of the smoke. This is what the Japanese reported some years ago and which is now confirmed by emerging Norwegian experience.

Although electrostatic precipitators do not remove gasses like carbon monoxide and nitrogen dioxide, they are very efficient at removing the particles which cause the main annoyance inside the tunnel and which are harmful to those outside the tunnel. Carbon monoxide and nitrogen dioxide have a 'short term' effect but the effect of particulate matter is cumulative.

The PIARC (World Road Association) 1995 rules for road tunnels note:

"For assessment of the toxicity of the exhaust gases, by tradition the carbon monoxide CO is taken as the leading gas.

To assess the comfort in the tunnel atmosphere concerning visibility and odor nuisance the diesel- smoke concentration is the leading substance"

According to both the Japanese and the Norwegians, there are two ways in which precipitator technology can make a useful contribution to tunnel design and construction. Ultimately, the solution selected is determined by what the designer needs to do.

· If the only consideration is the protection of the environment, then a precipitator installation in the exhaust structure or stack is appropriate. The idea is similar to the filter on the end of a cigarette, but in this case, it is effective in protecting health. The Japanese did this in a large tunnel near a national shrine.(the TENOZAN tunnel)

· If in-tunnel conditions are bad due to heavy diesel traffic and ventilation volumes are determined by particle levels (visibility) and/or by the related adverse health impacts, then progressive filtration along the tunnel should be used.

Progressive filtration is also appropriate when there are concerns about both the internal conditions and the external impacts. The concentration of particles in the tunnel exhaust is determined mainly by the location of the final filter.

Progressive filtration also makes it possible to significantly reduce the amount of air required to be supplied in smoky or dusty tunnels, the flow being determined by the need to remove either carbon monoxide or nitrogen dioxide. This reduction is what reduces the cost of ventilation.

By use of roof mounted filter modules along the length of the tunnel, particle levels are controlled and can be held to almost any desired level. Although each unit only treats a portion of the total air flow (usually between 1/3 and 2/3), by matching the removal capacity to the local particle production rate at maximum traffic flow, the particle concentration can be held constant. At times of lower traffic flow individual units can be switched off as required. As volumetric flow drops the proportion of the total volume passing through the filter can increase, further improving the degree of removal.

Whatever happens, particles removed by the filters do not pollute the air inside or outside the tunnel.

In designing a filtration system, the variables involved are the desired outcome for particle concentration, local particle production rate, efficiency of removal in the filter (effectively over 95%), proportion of flow treated and distance between stations. The fans incorporated in the precipitators take over most if not all the air movement in the tunnel and jet fans are retained only for emergency use or under stopped traffic conditions.

The diagram shows the impact of different spacing of units and of variations in production rate caused by grade changes in the tunnel. The trend lines show the particle profile in an unfiltered tunnel and with precipitators treating 1/3 and 1/2 of the ventilation volume at each pass. It is similar to the recently opened Stroemsas Tunnel near Drammen in Norway and to a number of tunnels in Japan. It may be possible to achieve the same effect in the M5 East. It is worth looking at.

The diagram also shows the impact of a terminal vertical discharge which may be needed in urban areas like Darling Harbour or Lane Cove to achieve dispersal of remaining exhaust components.

The experience of Stroemsas Tunnel is summed up by Sigurd Wiljugrein, of XENOLITH AS, a consultant for the Norwegian Roads Authority on the construction of the Stroemsas and other filtered tunnels in Norway:

"Increasing evidence of the harmful impacts of particle pollution makes control of this insidious form of pollution essential. It is no longer possible to use an argument of additional cost against the use of precipitators, at least in urban areas, as the experience of the Stroemsas tunnel shows these costs to be, at the most, marginal. With careful design, it should be possible to use precipitator installation to reduce running and, in some cases, capital costs, while at the same time providing a superior in tunnel environment".