25 ECCO NEW ELECTROSTATIC PRECIPITATOR PILOT PLANT AT PLABUTSCH TUNNEL IN GRAZ Heinz Aigner Aigner GmbH Dieselstraße 13, A4623 Gunskirchen email: heinz.aigner@aigner.at Abstract We have been researching and developing a filtration system for road tunnels together with the Technical University of Graz since 1991. The first result of this was an electrostatic precipitator based on a 2stage filter technology with an ioniser and a collector made of parallel plates. During this research work we found a new possibility of collection of dust particles the ECCOsystem. ECCO means Electrostatic Charged Contact. This also contains an ionising part, but for the second stage a special collector without voltage was developed. Electrostatic precipitators (E.P.) are used in road tunnels, but there is significant uncertainty as to their effectiveness, depending on local conditions and the precise equipment used. A pilot plant was thus set up within the Plabutsch tunnel in Graz, in order to determine whether EP s are suitable for road tunnel applications. In particular, the initial investment costs, running costs and maintenance interval for a typical installation were studied. Electrostatic Precipitators can handle only dust particles e.g. diesel soot or tyre wear particles, as well as products of road abrasion. Gaseous components were not investigated in these tests. 1. ASSEMBLY OF PILOT PLANT 1.1. Location ventilation room Raach shaft north shaft south 64 m H=24 m H=9 m 455 m pilot plant 1% 1 1% 2 1911 Plabutsch tunnel 9755 m 3 1947 1427 1% 4 5.5% 1947 1916 ventilationsection length e of section Figure 1 Location of pilot plant
26 1.2. Technical data Tunnel exhaust air will be taken of the outlet shaft (figure 1) in the south of the tunnel. The pilot plant consists of the duct system (Ø 63 mm), filter unit, radial fan and cleaning equipment (figure 2). Airflow max. 15 m³/h High voltage 121 kv Current max. 8 ma Figure 2 pilot plant The dust concentration will be recorded upstream and downstream of the filter at the same time, velocity and pressure drop will be recorded too as oneminute mean values. 2. PRINCIPLE OF OPERATION 2.1. 2stage electrostatic precipitator As contaminated air enters the E.P. it must pass by spiked ionizer blades supported between grounded electrodes. The DC voltage creates a high intensity field wherein the particulate matter in the air becomes electrically charged. The charged particles then pass a collector plate section made up of a series of equally spaced parallel plates. Each alternate plate is charged with the same polarity as the particles, which repel, while the interleaving plates are grounded, which attract and collect. Positive and negative ionisation is possible. Figure 3 shows positive charging. discharge electrode (positive) collecting plates (negative) airflow + + Figure 3 2stage electrostatic precipitator
27 2.2. ECCO system The particles must pass also the ioniser blades between grounded electrodes. The particles will be charged as shown in figure 3 in the high intensity field created by the DC high voltage. The ECCO collector is a mesh filter and the charged particles must have a contact with the filter. So the particles will be collected at the surface and inside the filter. To reduce the risk of flashover, negative ionisation can be used. discharge electrode (positive) airflow + grounded electrodes (negative) ECCO collector Figure 4 ECCO principle of separation 2.3. Cleaning of the ECCO filter system Standard E.P. must be washed in offline mode. The complete time for cleaning including the drying of the filter cells need several hours. At this time the filter cannot be used. The waste water includes the whole dust and must be cleaned. The ECCO collector has no voltage with the advantage that it is possible to clean it in online mode during operation. The collector could be cleaned with an automatic pneumatic system or also a wash system (depending of filter material). With a high pressure nozzle or pressed air ( figure 6) dust will be blown away of the filter. At the other side the dust will be collected with a nozzle (figure 5) and separated in a usual dust filter. The ionising part should be washed. There is only a small amount of dust and the time intervals for cleaning are longer. A water treatment with recirculated wash water can be used. 3. RESULTS 3.1. Measuring instruments Figure 5 Nozzle for dust collection Figure 6 High pressure nozzle
28 3.1. Measuring instruments TEOM Series 14 (Rupprecht & Patashnik). A filter element vibrates together with the tapered element at a frequency of several hundred Hertz. With increasing dust load on the filter element the frequency of the oscillating sensor diminishes. The electronic system continually registers these changes in frequency (every 2 s) and calculates the overall mass concentration. Aerosol spectrometer Grimm 1.15. A realtime optical method (laser light scattering). When particles pass the laser beam they scatter light. These signals are collected by a photo diode detector and classified by multichannel pulse height analyzer for size classification. STE 12 (Ströhlein). A gravimetic method with glass fibre plane filter. The mass of suspendend particles was gravimetrically determined with a microbalance after drying for 1 hour at 15 C. We have compared the different systems to obtain more confidence in the results. In partcular, the laser technology always required a second system to determine weight factors for each dust. 3.2. Efficiency Different systems are compared, as there are: standard electrostatic filter unit XT special tunnel filter cell ECCO filter system One of the goals in development of filter systems is to have a small filter to reduce costs for rock excavation and installation. This means higher velocities in E.P. as common in standard applications. Standard equipment cannot really be used because the efficiency reduces rapidly with higher velocity. This leads to the development of special tunnel filter cells with optimised distance between the collector plates, length of collector and high voltage. The efficiency of these units is about 85 to 9 %. The problem of high velocity is shown in figure 7. Collected dust can be blown away if the thickness of the dust layer is increased and the velocity high. 3 25 c before filter [µg/m³] c after E.P. [µg/m³] efficiency [%] 1 9 8 2 7 6 15 5 4 1 3 5 2 1 7.12.1 : 7.12.1 2:24 7.12.1 4:48 7.12.1 7:12 7.12.1 9:36 7.12.1 12: 7.12.1 14:24 Figure 7 unintended cleaning
29 For this reason it is recommend not to design to high air velocities or to long intervals between filter washing. The efficiency of the ECCO filter is up to 9 % with a mean value about 85 % and shown in figure 8. The efficiency also depends on the used material in the ECCO collector.the pressure drop is higher than with an E.P. with collector plates. Depending of the velocity through the filter, the pressure drop for an ECCO filter is between 15 and 25 Pa. Because the ECCO collector is also a mechanical filter, it is possible to handle all particle sizes. The efficiency would be even greater if larger particles are contained in the exhaust air. 3 c before filter [µg/m³] 1, c after filter [µg/m³] efficiency [%] 9, 25 8, 2 7, 6, 15 5, 4, 1 3, 5 2, 1,, 7.3.2 11:29 7.3.2 11:32 7.3.2 11:35 7.3.2 11:38 7.3.2 11:41 7.3.2 11:44 7.3.2 11:47 7.3.2 11:49 7.3.2 11:52 Figure 8 ECCO system The best result was obtained with the combination of ECCO with the XT tunnel cell. This gave an efficiency of up to 95 %, since the benefits of both systems are additive. The diagram shows that the efficiency is higher if the inlet concentration increases, since the outlet concentration remains more or less constant. 3 1 9 25 2 c before E.P. [µg/m³] c after E.P. [µg/m³] efficiency [%] 8 7 6 15 5 4 1 3 5 2 1 13.12.1 18:43 13.12.1 19:55 13.12.1 21:7 13.12.1 22:19 13.12.1 23:31 Figure 9 ECCO with XT tunnel cell
21 As mentioned previously, the size of the filter plant is very important. The construction of the filter units needs frames and so the velocity in the filter is not the same as in the crosssection of the tunnel. Table 1 compares the possible airflows. Table 1 Airflow m³/s per m² Airflow in m³/s for 1 m² cross section Eficiency [%] Standard E.P. XT ECCO ECCOXT 75 3,5 5,6 6,4 8 3, 4,8 6,4 85 2,6 4,2 6,4 4,6 9 3,8 4,2 Table 1 shows that a standard E.P. as used in some tunnels requires more than 6% room compared to special equipment designed tunnel air filtration. 3.3. Particle analysis The analysis of heavy metals as Pb, Cu, Co, Ni, Cd, Sb showed no significant contents. The samples were also analysed for DME (dieselmotoremission) as elemental Carbon. During the measurement the range was from 13 to 18%. The efficiency for DME was about 6%. The particle size analysis showed that about 3% are smaller than 2,5 µm. About 6% are between 2,5 and 1 µm, and 1% bigger than 1µm. (These results are from the measurement with the Grimm instrument and will be confirmed with an impactor shortly). 4. INVESTMENT AND RUNNING COSTS The initial investment costs depend on the wishes of the customer and governmental regulations, which can be different in different countries. The running costs consist of power supply for E.P. and fans, water or pressed air for filter cleaning and for maintenance. 4.1. Investment The price factor to compare ECCO with an E.P. with parallel collector plates is about 1,3 for aluminium and 2,2 for stainless steel higher than the ECCO filter. This factor includes only the filter cells and no other equipment. 4.2. Running costs The power consumption of an E.P. is very low because the current is only a few ma. The pressure drop across the filter results in higher fan capacity. The figures are calculated for an airflow of 1 m³/s.
211 Power consumption of E.P.: ( for 12 hours a day and 365 days a year) 12264 kw/a 12.3, 1 kw =,36 Power consumption of fan: It depends of the pressure drop of the filtersystem (ECCO 15 Pa, XT 5 Pa) 5 Pa,625 kw/m³s 195 kw/a 7, 15 Pa,1875 kw/m³s 585 kw/a 2.1, 25 Pa,3125 kw/m³s 975 kw/a 3.5, The calculation is made with 1 hours running a day with full capacity, 6 days a week and 52 weeks in a year. Cleaning of filter: For pressed air or water for the wash process are necessary about 4, a year. Maintainance: It would be recommended to make minimum one service in a year. The costs depends on different factors and could only calculated for a plant. For an estimate calculate about 7., for the ECCO filter and 11., fot the XT filter. 5. CONCLUSION Electrostatic precipitators are a good solution for separating the dust from road tunnel exhaust air. Various negative statements about the efficiency of E.P. are probably based on excessive promises, wrong lay outs and use of standard products. ECCO has a slightly reduced efficiency compared to E.P. with parallel plates, but many benefits for the user. It is cheaper, easier in construction, has longer lifetime and simpler maintenance. Both systems can be used for tunnel application. It would be of significant interest to develop a method for determining dust concentration to high accuracy. 6. ACKOWLEDGEMENTS The author would like to acknowledge the financial support of the Austrian federal ministry of traffic, innovation and technology, ASFINAG, Province of Upper Austria, Styria and Salzburg, who sponsored the pilot plant. Many thanks also to the Technical University of Graz and ÖSBS Leoben, who undertook all measurements reported here. Especially to Dr. Hannes Rodler (TUGraz) and DI Toni Schuster (ÖSBS) for there exceptionally effort.