Multivariate data analysis (MDA)

In order to study the effects of applied voltages on the emissions of the diesel fuels, a data matrix with 20 criteria (different diesel engine emissions from Table 8-1excluding C6H6, iso-C4H8, H3COCH3, (CH3)3COCH3 and SO2) and six variables (applied voltages) was submitted to Visual PROMETHEE software to perform PROMETHEE-GAIA ranking analysis.

This procedure is a mathematical method, which has been used in different environmental problems (Lim et al., 2005, Al-Shiekh Khalil et al., 2005, Ayoko et al., 2004). It has the capability of providing the interrelationships among a given data matrix in a brief but precise manner to aid in conceptualising the data set (Kelly et al., 2005). The goal of this process is to provide a summary of the necessary information for ranking and interpretation patterns in the entire data set matrix (Lim et al., 2007).

In PROMETHEE-GAIA analysis, first, criteria are ranked by PROMETHEE based on preference functions and weighting conditions of the variables. The preference functions of objects were selected as the V-shape in Visual PROMETHEE software, which is more suitable for quantitative values in environmental applications (Gunawardena et al., 2012). Moreover, preference functions of objects are modelled as minimum since lower emissions (which show the engine is producing fewer pollutants) are favoured and all components receive an equal weighting in software.

On the other hand, GAIA as a visual complement to PROMETHEE provides a principal component analysis-like biplot, which can be used for object–variable, variable–variable and object–object relationships (Gunawardena et al., 2014). In PROMETHEE-GAIA analysis, a data pre-treatment method will be developed by PROMETHEE before GAIA analysis and the PROMETHEE outcome will be used by GAIA to develop the GAIA biplot (Espinasse et al., 1997). The results of PROMETHEE-GAIA analysis have been shown in Figure 8.18.

From this figure, U values are found to be positive for variables such as 13.5 kV and 12 kV and they are negative at 0 V, 7.5 kV and 9 kV, and applied voltage of 10 kV can be considered as a kind of threshold. As was seen before, the plasma was not so efficient as to make a lot of changes at 7.5 kV and 9 kV and the similar behaviour of these two voltages with the state of no plasma (0 V) has also been confirmed from this figure. Furthermore, the length of each criteria vector represents its influence by the variables (Islam et al., 2013). Very short criteria vectors (CO2, C7H8, C3H6, CH4, NH3, 1, 3-C3H6, CH3OH, H2O, etc.) indicate that changing the applied voltages showed little to no effect on these pollutants. Therefore, it can be concluded that the NTP technique does not affect these species so much. On the other hand, NTP was found to be affecting NO_X (including NO, NO₂ and N₂O), THC (especially because of C₂H₄), CO and, to some extent, aldehydes. Generally, the

Chapter 8: Influence of non-thermal plasma after treatment technology on regulated and unregulated emissions from a diesel engine 181

criteria that are close to each other with the angle of ±45° are correlated, while the anti-correlated ones are in reverse directions with the angle of 135−225°, and approximately, criteria in orthogonal directions have no or less relationship (Islam et al., 2013, Espinasse et al., 1997). For example, we can find a reverse relationship between NOx and NO with NO2 from the figure, due to the oxidation of NO to NO₂ by non-thermal plasma, as is expected. One interesting finding is an anti-correlation between CO with NO and NOx. We can conclude the occurrence of some reactions inside the plasma, which can reduce NO concentration while increasing CO concentration simultaneously.

These reactions can be relevant to the hydrocarbons. Existence of hydrocarbons in diesel exhaust can promote the NO to NO2 conversion, while they produce a considerable amount of CO during NO conversion (Shin and Yoon, 2003). Also, we can see THC is correlated to C2H4. It means that among different hydrocarbons, the reduction of THC by plasma is more relevant to the C2H4 concentration reduction.

Figure 8.18: The GAIA plot of the effect of different applied voltages on different gaseous emissions

8.4 CONCLUSION

Investigation of more than twenty different species of exhaust emissions at six different applied voltages has revealed that the NTP technique can strongly affect regulated and unregulated diesel engine emissions. In particular, NTP showed a good potential for NO_x and hydrocarbon reduction.

On the other hand, the total concentration of Aldehydes has been increased by increasing the applied

182 Chapter 8: Influence of non-thermal plasma after treatment technology on regulated and unregulated emissions from a diesel engine

voltage. Moreover, a continuous increase in CO concentration has been found during the experiments while the CO₂ concentration did not change considerably. It revealed that, during our experiments, hydrocarbons and soot are removed and converted to CO more than CO₂.

Furthermore, principal component analysis (PCA) is considered to facilitate the interpretation of our data. From this analysis, CO2, C7H8, C3H6, CH4, NH3, 1, 3-C3H6, CH3OH, H2O, etc. with very short criteria, are found not to be affected so much by varying the applied voltages. In contrast, NTP affected NOX (including NO, NO2 and N2O), THC (especially because of C2H4), CO and to some extent aldehydes with longer criteria vectors. The reverse relationship between NOx and NO with NO2 concentration under plasma condition is confirmed from PCA analysis as well. Further to this, an anti-correlation between CO with NO and NOx has been observed. This can be relevant to the existence of hydrocarbons in diesel exhaust, which can promote the NO to NO2 conversion while producing a considerable amount of CO.

The results of this study could be related to formation of nucleation mode particles which has been observed in previous chapters at high voltage levels. As it has been observed in this study, the concentration of some components such as acetaldehyde and formaldehyde increased at high voltage levels. Therefore, further research is required to find out any relationship between the presented gaseous emissions and other species such as polycyclic aromatic hydrocarbon with the formation of particles in nucleation mode.