Following the destruction of Japan’s road network during World War II, expressway companies were established to construct and maintain roads independently of the Ministry of Land, Infrastructure, Transport and Tourism (MLIT).

The main priorities for these companies included construction of two long expressways as main arteries for the national road network run by Nippon Expressway Company (Nexco). The first expressway, the Meishin Expressway between Nagoya and Kobe, was fully completed in 1965. The second was the Tomei Expressway between Tokyo and Nagoya, fully completed in 1969.

After completion of the two basic expressways, Nexco began to extend their road expressway network throughout Japan, creating expressways such as the Hokuriku Expressway, the Sanyo Expressway and the Kyushu Expressway.

In 1959 the MEC (Metropolitan Expressway Company) was established for the Tokyo region. In 1962 the HEC (Hanshin Expressway Company) was established in Osaka to develop the expressway network. The purpose of both the MEC and HEC was to relieve congestion and to increase transportation capacity on the road network.

Nexco’s expressway tunnels
Japan is very mountainous. All Nexco Expressways required the construction of long mountain tunnels.

In the 1960s, Nexco’s expressway tunnels were constructed with limited budgets through difficult geological conditions.

In most instances either simple longitudinal ventilation or simple longitudinal ventilation with intermediate fresh air and exhaust shafts were used. These designs were chosen because they required the minimum cross sectional area of tunnels and were able to meet Nexco’s in-tunnel air specifications (T. Baba et al, 1979).

During the 1960s in Japan there was rapid and unexpected economic growth. This resulted in greater traffic volumes than had been expected. By the late 1960s residents near the Tennozan tunnel in the Meishin Expressway and the Nihonsaka Tunnel in the Tomei Expressway were sensitive to air quality.

Community concern in the late 1960s focused on the impact of human health at the Tennozan Tunnel while the concern at the Nihonsaka Tunnel focused on crop damages caused by black soot (especially oranges and/or green tea trees). In 1994 Panasonic installed the world’s first electrostatic precipitator (ESP) for purifying tunnel air in the Tennozan Tunnel (PIARC, 2008). And in 2000, Fuji Electric Company installed ESPs for removing soot at the Nihonsaka Tunnel.

Japanese tunnel ventilation expressway standards
In 1964 Nexco produced ‘The Manual for Designing Car Road Tunnel Ventilation’. This document is possibly the first standard for tunnel ventilation in Japan. Although this document was large (around 400 pages) it is clear that the main concerns were sufficient volume of ventilation air, the visibility index (VI) and carbon monoxide (CO) concentrations in the tunnels.

In 1975 the Japan Road Association (an extra governmental organisation of MLIT) published ‘The Road Tunnel Manual’ which included the tunnel ventilation standards for all roads including non-expressways. This was the first official publication on tunnel ventilation in Japan.

The 1975 manual focused upon several aspects of tunnel design: geometry, design, construction, ventilation, lighting, emergency facilities and maintenance/repair.

A number of parameters were considered with the 1975 standard: a CO concentration of 100ppM or less; VI of 50 per cent or more for the first and secondclass roads and 40 per cent or more for the third and fourth-class roads; how to calculate ventilation air volume; how to model the dispersion on tunnel exhaust including calculation of effective heights of stacks, exit velocities, stack optimization and ground level calculation.

It is not surprising that in 1975 there was no mention of ESPs in the manual as the technology had not yet been proven.

ESP trials
In 1975 Nexco and its associated companies began field-testing and development of ESPs at the Tsuburano Tunnel (T. Baba, 1991).

Early ESP trials focused upon improving VI values. In long tunnels this meant the construction of short bypass tunnels in which ESPs could be installed.

The use of ESPs in bypass tunnels allowed visibility to be improved without the need for an external ventilation point part way through the tunnel. The first ESP plant was installed at the Tsuruga Tunnel in 1980. At that time there was no Japanese standard for ESPs. Designs were determined on the basis of the specific needs of a project. Approval was based upon the successful liaison between the client and the ESP manufacturers.

In 1987 Nexco specified a tunnel ESP standard in Japan. This ‘standard specification for tunnel ESP systems’ formed part of the official documents of the ‘Common Specifications of Mechanical, Electrical and Communication’, (1987 version) by Nexco.

Under the Nexco standard, the required performance of ESP systems was specified as 80 per cent soot collection at 7m/sec.

The 1987 Nexco specification was designed to ensure sight improvement distances (VI) within tunnels but not for purifying tunnel air to be vented.

In 1994 the world’s first ESP system for purifying tunnel air exhausts was installed in the Tennozan Tunnel. There was no specification for the environmental performance of ESPs in the Tennozan tunnel and accordingly the same specifications for achieving visibility improvement were used.

The 1987 Nexco standard was revised in 1996 and 2006. A summary of these revisions appears in Table 1.

Following the introduction of ESPs for environmental improvement in the Tennozan tunnel in 1994 there was a series of projects in Japan where ESPs were installed in the exhaust sections of tunnel ventilation systems (R. brandt, 2009). Examples include the 1997 Shimoneseki ventilation station in Kanman tunnel, the 1998 Tennozan west ventilation station, the 1999 Moji ventilation station in Kanmon tunnel and the 2000 Nihonsaka ventilation station.

Installation of the ESPs for suspended particle matter (SPM) removal for the above tunnels was in accordance with the Nexco specifications. In no instance was the effect on ground level concentrations used as the basis of the specifications for the ESPs. The basis for using the technologies is that the SPM removal occurred, not that SPM removal resulted in a change to ground level concentrations.

Construction of metropolitan expressways
The first metropolitan expressway was partially opened in Tokyo in 1962. There was, and remains, an emphasis on creating radial ring roads. Even today, road networks are still under construction to create further outer ring roads.

The use of expressway tunnels became more common in the 1990s. Typically major urban road tunnels used transverse or semi transverse ventilation systems. Generally such systems do not require air to be purified because of the continual fresh air being supplied by a transverse or semi transverse ventilation system. However, in areas outside the tunnel ventilation stacks, community concerns about the effects of exhausted air were being raised.

These concerns were also applied to both longitudinally ventilated tunnels and those with ventilation stations.

In 2002 the MEC constructed its first urban tunnel with ESP systems for purifying tunnel exhaust to the atmosphere. A selection of projects used ESPs for external air quality management (R. Brandt, 2009): the 2002 Asukayama Ventilation Station (MEC) the 2003 Midoribashi Ventilation Station (Nagoya Expressway), the 2006 Kitamachi Ventilation Station (Tokyo Metropolis), the 2008 Jujo & Yamashina Ventilation Station (HEC) and the 2009 Yumeshima Ventilation Station (MLIT).

The performance of each of these installations was assessed on a project by project basis. The impact of installation and use of the ESP technology on ground level concentrations was not calculated. It is not expected that this will occur in the future because studies and calculations demonstrate the technology has no measurable effect.

It is expected that increased concerns about the effects of NO2 by the community will promote the further development and use of NO2 removal technologies this century.

Denitrification systems
In recent years there has been increased concern about the health effects of NO2 from tunnel exhaust.

1970s to 1980s
In the 1970s and 1980s Japan experienced significant issues with photochemical smog. Concern was widespread and there were issues with acid rain. There was a demand that NOx (nitrogen oxides) should be removed from tunnel exhausts as part of a campaign to manage photochemical smog and acid rain in urban areas.

1990 – Denitrification Committee
In 1990 MLIT convened a committee, ‘The Committee for Surveying NOx Decrease in Metropolitan Area’, to develop denitrification systems capable of removing 80 per cent or more of NOx from tunnel air.

1991 to 1992
The MLIT entered into arrangements with six private companies to develop NOx removal technologies. These companies tested their equipment at the Ohi ventilation sites in MEC. The six Japanese companies were: Panasonic, Kawasaki Heavy Industries (KHI), Mitsubishi Heavy Industries (MHI), Ebara, Hitachi Zosen and Kobelco. Each company used a different technique to remove the NOx. Each company achieved 80 per cent efficiency or better NOx removal.

1995
In 1995 MLIT announced a tender for a low concentration denitrification test plant that attained an 80 per cent of NOx removal at 44m3/s of gas flow (S. Yoshida et al, 2007) The successful bidder was Kobelco.

NOx Removal – 1997 to 1999
Following the successful operatons of the Kobelco test plant with 80 per cent NOx removal efficiency, MLIT announced that the field trial was a success.

NOx Removal 1997
In 1997 the committee investigating how to decrease the NOx levels in the metropolitan area confronted a series of issues.
• The committee concluded that it was technically possible to construct full scale plants for NOx removal.
• The cost for constructing these full scale plants in both initial capital cost and ongoing expenses was grossly disproportionate to the environmental benefit achieved.
• The environmental standards of Japan did not (and do not) specify NOx limits. NO2 limits are set in a zone between 0.04ppM and 0.06ppM. The committee decided that NO2 is more harmful to the human body than NO as the ACGIH regulation value in the USA for NO was 25ppM or less.
• It was considered that if the committee concentrated only on the NO2 removal the cost might be more realistic, however it was unclear what the removal specification should be.

Technically this issue is complex because of factors including oxidation rates of NO within the tunnel, the effect of ozone on oxidation rates produced by the EPs and the external oxidation of NO.

The committee undertook a technical comparison comparing NO2 removal with NOx removal. The analysis included:
• Modelling of diffusion from a hypothetical exhaust stack at a hypothetical tunnel ventilation station.
• Modelling of the impact on inhabitants in an area around a hypothetical ventilation station in order to evaluate the effect of NOx concentrations with the use of such technologies.
• The ratio of NO to NO2 used at the inlet of the air purification system was 90 per cent NO and 10 per cent NO2.
• For NOx removal the purification ratios were 80 per cent of NOx and 80 per cent of NO2. The impact of these purification ratios was modelled for both NOx and NO2 concentrations at ground level.
• For NO2 removal, a range of removal values were applied as part of a simulation for likely ground level concentrations.
• As a result of the simulations it was established that the performance of a 90 per cent removal of NO2 was comparable with an 80 per cent removal of NOx (NO and NO2) (S. Yoshida et al, 2007). The results of the simulation case study concluded that less emphasis should be placed on NO removal.

• It was determined that the most efficient method of ensuring the protection of health was to focus on NO2 removal. NO2 removal required approximately half the size plant to a NOx removal plant, the energy consumption was approximately one fifth and the cost was approximately half. Importantly, the ongoing operational cost with a focus on NO2 removal were one fifth the cost of the projected NOx removal alternative.

As a result of these investigations MLIT concluded that NOx denitrification for tunnels in Japan should be abandoned. It said that the purpose of tunnel denitrification is limited to the impact upon the area proximate to the tunnel ventilation stacks and should not be calculated in terms of its effect on the total urban area. It was decided that the most prudent form of denitrification was NO2 denitrification.

2000
In 2000 MLIT announced two tenders for the tunnel denitrification test plants. The new specifications were 90 per cent removal of NO2 at 22m3/s of gas flow. These were known as ‘Tokyo Pilot Plants’.

2001 to 2003
In 2001 two successful bidders were announced being Panasonic and KHI, both with NO2 adsorption technology (S. Yoshida et al, 2007). The two types of NO2 denitrification were then evaluated. In 2002 to 2003, testing of both the NO2 absorption and NO2 adsorption plants was performed.

March 2004
MLIT announced that the Tokyo Pilot Plants had been successfully finished with preferable results.

2004
MEC announced two tenders for the NO2 denitrification plants of the Shinjuku Line tunnel in the Central Circular Route. The first tender was with a gas flow of 1,672m3/s and the second with 1,837m3/s (PIARC, 2008 and S. Yoshida et al, 2007). The NO2 removal ratio of 90 per cent was the same on both tenders. The successful bidders were Panasonic in the first work section and Nishimatsu with absorption in the second work section.

December 2007
The first work section with four ventilation stations was constructed by Panasonic and was in daily operation of between three and 17 hours per day depending upon the operational hours of the ventilation ducts. Five Nishimatsu ventilation stations opened in March 2010.

2009
In 2009 Nexco announced a tender for a NO2 denitrification plant in the Shintomei expressway. The specifications to be achieved are 180m3/s gas flow with 90 per cent NO2 removal. In July 2009 Panasonic was successful in that bid.

Conclusions
Originally electrostatic precipitators were used to ensure in-tunnel visibility. Subsequently they have been used in conjunction with NO2 denitrification equipment in urban road tunnels where local NO2 levels are an issue, to remove particles to assist with meeting external air quality objectives. The electrostatic precipitators also protect the NO2 removal process from particles.

Currently the performance of ESPs and NO2 removal equipment is described by the efficiency of the removal of suspended particles and NO2 for a given volume of air. There has been no project in Japan where the effect on ground level concentrations of either NO2 removal or soot removal has been used to develop the specifications of the air cleaning technology.

The availability of denitrification technology in Japan does not of itself mean that it is used on every project. For example the Jujo and Yamashina ventilation stations of 2008 in HEC do not require denitrification despite the fact that they postdate the plants on the Central Circular Route (2007).

The use of ESPs and/or denitrification equipment is not mandated in Japan and decisions to use ESPs or ESPs with denitrification appear to be made on the basis of local factors. Where air purification is specified under new tunnel construction plans, such technology is usually adopted.

Typically a decision about air purification and the type of technology used is political and involves dialogue between citizens, politicians, and clients. There is no official standard on emissions from exhaust stacks. A policy on emissions from ventilation stacks is not expected.


Surak, Iran ESP – all photos courtesy of KGD Development Shaft electrostatic precipitator Table 1: Specification revisions of tunnel ESP systems Gyeongju, South Korea