(174f) Plasma Soot Removal System for Diesel Exhaust Gas Consist of Grid Electrodes

Diesel engines are being increasingly used worldwide due to their higher thermal efficiency, longer durability, increased reliability, and lower operating costs compared with gasoline engines. In return for these benefits, they emit 2?20 times more nitrogen oxides and roughly 30?100 fold higher particulate matter than gasoline engines. The particles present in diesel exhaust are composed of a center core of elemental carbon and adsorbed organic compounds [1]. These particles have a large surface area, which makes them an excellent medium for adsorbing organics. In addition, their small size makes them highly respirable and able to reach the deep lung. Many of the organic compounds present on the particles are individually known to have mutagenic and/or carcinogenic properties [2, 3].

Several techniques have been developed for decreasing particulate emissions from diesel engines such as engine modification, fuel additives, alternative fuels, and after-treatment systems. Mentioned approaches are in fact complementary and must be followed simultaneously [1, 4].

One of the most effective and commonly after-treatment devices is non-thermal plasma systems, which are used extensively since give high removal efficiency, low-pressure drop and compact construction [5]. The present work is focused on presenting and developing efficient method using self-consistent non-thermal plasma system consist of grid electrodes, which can be applied for eliminating of fine particulate matter from diesel exhaust gas. Experimental

For the purpose of this research, numerous removal efficiency measurements have been performed for various geometric and electric parameter settings under laboratory and real condition.

The carbon black particles (in range of 0.4-1.2 µm) were considered in this study to simulate soot particle behaviors in the system [6]. Loadings depend on experiment ranged from 0.8 to 5.0 g/m³. The average residence time in the system was about 0.32 second and was kept constant for all tests presented here.

The concentration of particles is measured at downstream of the system with a particle counter capable of measuring the number of particles in real time. The net removal efficiency was calculated then as ε = (1 -C_on/ C_off) ×100 where C_on and C_off were the concentrations at the system outlet when the voltage was turn on and off, respectively. This measuring strategy allows relating the obtained fractional efficiencies exclusively to the electric and geometric setting.

Experimental investigations were performed with a laboratory scale plasma system Equipped with 16 parallel electrodes with a width of 50 mm, a height of 40 mm and an interelectrode spacing of 8 mm. Parallel electrodes act as corona discharge and collecting electrodes simultaneously.

To perform investigation on the effect of electrode geometry on removal efficiency, six kinds of electrodes consist of plane electrode (A0), punched electrodes (A1, A2) and grid electrodes (B, C, D) used. For the improvement of particle removal efficiency under short residence times and also increase the performance of system for ultra-fine to submicron particle elimination, a corona type precharger employed and located before of the parallel electrodes. The mentioned precharger consists of 13 needle electrodes (in two rows) made of stainless steel and a flat electrode in dimension of 15×65 mm made of aluminum. Minus 3.2 kV DC was applied to the needle electrodes through 2 MΩ resistors which results in a high concentration of negatively charged ions near the needles tip. Results and Discussion

The results of research carried out to optimize electrode configuration using six kinds of electrodes indicate that the removal efficiency obtained with grid electrodes was higher in respect of plane and punched electrodes probably due to the stronger electric field at the sharp-edges of any grid causing the increase in charge of particles.

In addition the results indicate that the removal efficiency of grid electrodes, will be increase with hole size decreasing. Nevertheless, the increase in removal efficiency, slows down with decrease hole size probably as a result of the particle re-entrainment on electrodes and the homogenization of the electric field which reduces the field strength at the electrodes surface. Base this phenomenon the removal efficiency of Type-C and D electrodes are almost identical despite of the fact that the hole size of Type-D is smaller considerably.    

Experimental results on the variation of removal Efficiency of electrode C with applied voltage and soot concentration (from 0.8 to 5.0 g/m³) demonstrate that the removal efficiency weakly affected by soot concentration probably due to increase contribution of fine particles in the lower concentration. The results also showed drop in removal efficiency, slows down with increase in applied voltage and decrease in soot concentration.

Investigation of removal efficiency of electrode C under a constant soot concentration (1 g/m³), for the case of with and without precharger indicate that the use of precharger cause increase of removal efficiency.

Other investigations, which performed to determine the effect of operation Time, showed that when precharger was not employed, the removal efficiency had no obvious variation with time up to 35 min and it was kept stable at around 82%. However, it diminishes gradually after this time due to the contamination of the electrodes and the particles re-entrainment [7].

For the case of with precharger, removal efficiency was kept stable at around 94% up to 20 min of operation time and then dropped sensibly during the test period. In this case, it reduces gently to the value of without precharger case.

In the last step, experimental results of fabricated system consist of Type-C electrodes, under real condition, indicated that the removal efficiency about 83% was achieved with Minibus Benz Model O508 exhaust gas operating in steady-state condition and in the present of the precharger. The dissimilarity occurred between the results obtained from laboratory and real conditions may be due to the difference condition in diesel exhaust such as temperature, humid and soot chemical and physical property. Conclusion

Presented results suggest that, the non-thermal plasma system consist of grid electrodes can be considered to apply for efficient elimination of fine particulate matter from diesel exhaust gas. However, it seems that some modification must be done for improvement of removal efficiency under real condition.

It should be noted that experimental investigations for developing effective method for sequential regeneration of mentioned system and removal of deposited soot are underway. Reference

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