Introduction
For around 15 years adsorptive cabin air filters have been used in cars to remove noxious gases from incoming external air by adsorption onto activated carbon. Quality standards are given by the automobile manufacturers in their performance specifications. These requirements include limits of adsorption capacity and the times of breakthrough for selected test gases.
The testing method for cabin air filters is described in several standards, for example ISO TS 11155-2: Road vehicles – Air filters for passenger compartments – Test for gaseous filtration. Testing consists mainly of the measurement of breakthrough curves of several single gases in humid air (50% relative humidity) as a carrier gas through the cabin air filters at 23°C. Test gases are often toluene or nitrogen dioxide. The testing concentrations given in the standards vary between 30 ppmV and 80 ppmV. The capacity of each filter type for single noxious gases could be calculated by mass balances using the breakthrough curves. The results allow a comparison between different filter types.
The influence of temperature and relative humidity on the adsorption capacity of cabin air filters has been investigated in several studies (for example Heschel and Ahnert [1], Zhang et al. [2], Sager and Schmidt [3], Sager and Schmidt [4]). Studies concerning the influence of filter ageing due to particles and already adsorbed gases are more rarely published (Bittermann [5], Klijn et al. [6], Ilgen [7]).
In order to get information about the filter performance at the end of the product lifetime, filters used in cars were tested with nitrogen dioxide as the test gas. In recent years the environmental relevance of nitrogen oxides has increased, because unlike other noxious gases, ambient exposure concentrations have not reduced significantly. The test results were compared with those of new filters. The used filters were provided by dealer's garages, with their history and mileage unknown. All filters tested were of one type for the VW Golf VI from a German filter manufacturer.
In addition different methods of artificial filter ageing were applied to the same cabin air filter types. In order to simulate filter ageing under field conditions, filters were loaded with particle test aerosol A2 (ISO 12103, silicon dioxide), diesel exhaust gas and ambient air. The aim was to carry out the artificial loading until the pressure drop increase was comparable to that of the filters used in cars (10-60% in relation to new filters). For experimental reasons this demand could not be fulfilled completely. In order to identify whether particles or gaseous components of diesel exhaust gas or ambient air are controlling the ageing process, additional loading tests were conducted. Filters were loaded with almost particle-free diesel exhaust gas or ambient air.
Afterwards the loaded filters were also tested with the test gas NO2. The resulting breakthrough curves were compared to the ones of new and used filters. First conclusions were drawn concerning the development of the filter capacity during lifetime for the test gas NO2 and the suitability of the loading with different aerosols to simulate filter ageing.
Experimental
The breakthrough experiments with the test gases NO2 in humid air were carried out in a filter test stand. The test arrangement is shown in Figure 1. Nitrogen dioxide is supplied from a canister by a mass flow controller to a flow of conditioned air at 23°C and 50% relative humidity. In order to obtain the required NO2 vapour pressure, the canister has to be heated. The pipes and mass flow controller for the NO2 supply are insulated to avoid condensation. The test gas air mixture passes through the cabin air filter to be tested. The pressure drop Δp is measured. Upstream and downstream of the filter, samples are taken and analysed with regard to the nitrogen dioxide and the nitrogen monoxide (NO) concentrations with two NOx-analysers (type AC31M of ansyco, Karlsruhe, D). The results are recorded and evaluated as breakthrough curves in relation to the experimental time. The adsorption capacity is calculated from the breakthrough curve by a mass balance. Further test conditions were a NO2 input volumetric content c1 of 4 ppmV, a face velocity of about 0.13 m/s and an experimental breakthrough time of 90 min.
For artificial ageing of new filters with the particle aerosol A2, the experimental set-up of Figure 1 was used. Instead of the test gas, the aerosol is supplied to the test facility. The silicon dioxide aerosol A2 is generated by a rotating brush aerosol generator (RBG 1000, Co.: Palas, Karlsruhe, Germany). Filters are loaded up to a pressure drop of 140 Pa. The initial pressure drop of new cabin air filters is around 30 Pa.
The loading of new filters with diesel exhaust gas is carried out in a separate experimental rig, see Figure 2. The diesel exhaust gas is produced by a four cycle twin diesel engine (16 kW, 1248 cm3) which is part of a commercially available emergency power unit. Loading was carried out for one hour with a flow rate of 200 m3/h. Former studies had shown that variation of the loading time duration of 1-8 hours had no influence on the subsequent breakthrough tests. The increase of the pressure drop over the filter by the loading was considerably higher (>>100%).
In order to obtain diesel exhaust gas with a considerably reduced particle fraction upstream of the cabin air filter, a HEPA filter medium was used. Apart from that loading conditions remained the same (one hour loading at 200 m3/h). Visual inspection of the cabin air filter after loading showed that the particle fraction was clearly reduced, but not completely eliminated. The pressure drop increase due to the loading of about 25% supports this evaluation. It is significantly lower than the one achieved by loading with the untreated diesel exhaust gas.
In order to realise the loading with the ambient air, the same test facility as used for the diesel exhaust gas (see Figure 2) was employed. Two new filters received 200 m3/h of ambient air for two weeks. Loading resulted in an increase in pressure drop of around 10% (3 Pa). For the reduction of the particle fraction during filter loading with ambient air, a HEPA filter upstream of the cabin air filter was used once more. The resulting pressure drop increase was again about 10%.
The particle diameter characteristics of the loading aerosols A2 and diesel exhaust gas are different. Figure 3 shows the fractional number distribution of the two aerosols weighted by the logarithmic class width of the respective measuring instrument. N is the particle number and dp is the particle diameter. A2 is measured with an aerodynamic particle sizer (APS 3321, Co.: TSI, Shoreview, Mn, USA). The measured aerodynamic diameter is converted to the geometric equivalent diameter using the density of A2. The particle diameters of the diesel exhaust gas are measured with a scanning mobility particle sizer (SMPS 3080/DMA 3081, Co.: TSI, Shoreview, Mn, USA). The equivalent diameter therefore represents an electric mobility diameter. The mode of the diesel aerosol is located at 0.1 μm; the one of A2 at 0.7 μm. The particle distribution of the ambient air in urban areas in principle consists of three modes; the nuclei mode, the accumulation mode and the coarse particle mode. The size distribution of the accumulation mode corresponds approximately to the one of the diesel aerosol, whereas the distribution of the coarse particle mode matches the one for A2 (see also Kievit [8], Qi et al. [9]).
Results
Firstly, the capacity and the performance of new cabin air filters were tested. In Figure 4, the breakthrough curves of NO2 through new filters are shown (black line, bold). The values of the volumetric content measured downstream of the cabin air filter, c2, related to the ones measured upstream, c1, are shown as a function of experimental time. Furthermore the breakthrough of the sum parameter nitrogen oxide (NOx = NO2 + NO) is depicted (black line, dashed). The breakthrough curves are averaged, and the error bars display the standard deviation. The breakthrough curves for NO2 and NOx are different, which means that even though only NO2 is supplied downstream of the filter, NO is present too. NO2 is reduced to NO by the activated carbon as catalyst. The carbon is either oxidised to CO or oxo-functionalised groups are generated on the inner surface of the activated carbon which could result in the formation of CO2 (Heschel and Ahnert [1]). The breakthrough of NO2 after 90 minutes experimental time is about 12%; the one for NOx is around 44%.
In addition, Figure 4 shows the breakthrough curves for NO2 and NOx of five cabin air filters used in cars (coloured lines). The five selected filters represent the large number of used filters that were tested. Filters used in cars exhibit very different breakthrough behaviour subject to the loading history. Depending on the environment in which the car was driven the loading varies strongly. The loading history of the used filters is unknown. However, compared with new filters the efficiency of all the filters that were used in cars has clearly declined. The breakthrough of one of the filters even reaches 100%. Furthermore the fraction of NO2 which is converted to NO clearly decreased. The breakthrough curves of NO2 and NOx through filters used in cars are very similar.
One new cabin air filter was artificially aged with A2. The breakthrough curves for NO2 and NOx through this filter (see Figure 5) are even slightly better than those through new filters. In interpreting this result, it should be taken into account that it is drawn from a single test. However, the extensive loading with A2 up to a pressure drop of 140 Pa over the filter does not change the efficiency of the filter concerning NO2 or the conversion rate to NO. Consequently, the normal ageing of cabin air filters with regard to NO2 separation could not be generated by loading with A2.
The capacity of cabin air filters artificially aged with diesel exhaust gas is nearly depleted, see Figure 6. The breakthrough curves of NO2 and NOx through the two filters loaded with untreated diesel exhaust gas (blue lines) almost coincide, thus the catalytic reduction to NO is nearly completely constricted. In contrast, the breakthrough of the two filters loaded with diesel exhaust gas with reduced particle fraction (red lines) is clearly lower and a catalytic reduction of the NO2 to NO is observable. The particle fraction of the diesel exhaust gas seems to have a strong influence on the filter performance regarding nitrogen oxides.
The tests with ambient air loading support these results concerning the influence of the particle fraction on the filter ageing process. Figure 7 shows the breakthrough curves for NO2 and NOx of the two filters loaded with ambient air; one with original ambient air (AA-c) and one with a reduced particle fraction (AA-rp). Both filters (AA-c and AA-rp) show a reduced capacity compared to new filters, but the decrease in capacity of the AA-rp filter is clearly lower than that of the AA-c filter. In addition, at the beginning of the breakthrough tests the formation of NO has almost diminished for both filters. Only during the tests were perceptible amounts of NO recorded. The time when NO is detected is reached more rapidly for the AA-rp filter and the amount of NO formed is larger. Apparently, it is the particle fraction that influences, not only the adsorption capacity for NO2, but also the ability for reducing NO2 to NO. However, artificial ageing with ambient air at the given experimental conditions does not influence the filter characteristics as strongly as diesel exhaust gas loading does.
Conclusion and outlook
Cabin air filters in different states – new, used in cars and artificially aged with silicon dioxide, diesel exhaust gas and ambient air – were subjected to breakthrough tests with NO2 as the adsorptive gas. As expected, cabin air filters already used in cars exhibit a lower adsorption capacity for nitrogen oxides and with it a clearly higher breakthrough compared to new filters. At the same time the amount of NO2 which is catalytically transformed to NO on the activated carbon decreases. The filter performance of used cabin air filters varies considerably, which is plausible when considering different ambient conditions while driving, loading histories and changing intervals. Further tests are planned to investigate a correlation between the pressure drop over the used filter and filter deterioration, which the completed tests indicate.
The simulation of cabin air filter ageing in cars with silicon dioxide, diesel exhaust gas and ambient air showed the influence of fine particles on the separation process of nitrogen oxide. While the filter loading with silicon dioxide particles in the micrometer range does not impair the NOx separation, the loading with diesel exhaust gas and ambient air containing fine particles in the nanometer range clearly deteriorates the filter performance. The breakthrough of NOx increases considerably and the catalytic conversion of NO2 to NO decreases. If the particle fraction of the diesel exhaust gas or the ambient air is almost removed, then the effects of the respective loading are reduced. These findings suggest that there is an important role for the fine particle fraction on the filter ageing process, in addition to the adsorptive loading with gaseous compounds. Currently, the activated carbon pellets of the filters loaded with diesel exhaust gas or ambient air are being microscopically investigated in order to locate the places of deposition for the aerosol particles. Furthermore, the loading of cabin air filters with an aerosol (air/graphite) with a mean diameter in the nanometer range is in preparation.
In summary, it is possible to simulate the ageing process of cabin air filters during their life cycles with regard to NO2 separation capacity, by using diesel exhaust gas or ambient air as the loading aerosol. Artificial ageing with untreated diesel exhaust gas leads to a limited filter performance with regard to NO2 separation.
Acknowledgments
The authors thank the German Federal Ministry of Economics and Technology for financial support from within the industrial cooperative research and development (IGF) agenda, based on a decision of the German Bundestag. The project was opened by the IUTA e. V., Duisburg, and organised by the AiF (IGF-Project No. 16793N).
References
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