## 3 Results and Discussion
### Tar Removal  
The results of the destruction of the tar components in the biomass producer gas simulator are presented in Figure 5. They suggest that the catalysts are activated even though the gas temperature is much lower than required by classical catalytic processes. In the presence of the Rh-based catalyst, all three tar components can be completely removed from the processed gas at the highest discharge energy. In the case of the Ni catalyst, only naphthalene disappeared while traces of toluene and  
Figure 4: Pulse energy of the nanosecond dielectric barrier discharge calculated according to formula (1) over the full frequency range of the NPG-15/2000 pulse generator. Gas mixture CH\({}_{4}\) (2%):CO\({}_{2}\) (9%):H\({}_{2}\) (12%):CO (18%):N\({}_{2}\) (42%):H\({}_{2}\)O (17%). No tar added. Initial temperature 250 \({}^{\circ}\)C.  
Figure 3: Effect of catalyst on the current pulse in the DBD reactor. Gas mixture CH\({}_{4}\) (2%):CO\({}_{2}\) (9%):H\({}_{2}\) (12%):CO (18%):N\({}_{2}\) (42%):H\({}_{2}\)O (17%). No tar added. Initial temperature 250 \({}^{\circ}\)C. Pulse repetition rate 3 kHz. benzene remained. Non-thermal plasma alone can destruct benzene, toluene, and naphthalene, but with lower efficiency than in a reactor containing catalysts. As can be seen in Figure 5, the Rh-based catalyst exhibits higher activity in these conditions compared to the Ni catalyst. This is in agreement with an experiment of [PERSON] et al. [37] showing that the Ni/Al\({}_{2}\)O\({}_{3}\) had a lower tar reforming activity than an Rh/LaCoO\({}_{3}\)/Al\({}_{2}\)O\({}_{3}\) catalyst. When concentrations are presented as logarithms (Figure 6), it can be seen that they are typical for a first order reaction, with the difference that the time parameter is replaced by energy. This means that tar removal depends exponentially on the discharge energy and is driven by radical linear termination [4]. Most of the radicals are formed via electron collisions with molecules of the main gas components, i.e., N\({}_{2}\), CO\({}_{2}\), H\({}_{2}\), CO, H\({}_{2}\)O, and CH\({}_{4}\). From all resulting radicals, only O, H, and OH play an important role in tar decomposition. As was proven by [PERSON] et al. [4], the dominant mechanism of tar removal in biomass producer gas treated by non-thermal plasma is oxidation by O radicals. [PERSON] et al. [40] claim that also H radicals highly contribute to the naphthalene decomposition. The same conclusion may be drawn from an analysis of the results presented in Figure 5. One can  
Figure 5: Influence of catalyst on the concentration of (**a**) benzene, (**b**) toluene, (**c**) naphthalene. Error bars represent standard deviation. notice that the concentration of naphthalene decreases faster than the two other tar components, and toluene removal is a little bit faster than benzene. It is worth noting that the rate constant of the reaction of O radicals with naphthalene at 250 \({}^{\circ}\)C is almost 2 and 4 times higher than for toluene and benzene, respectively [41]:  
\[\text{Naphthalene + O}\rightarrow\text{products, k}=4.13\times 10^{-12}\text{ cm}^{3}\text{/molecule s}, \tag{3}\]  
\[\text{Toluene + O}\rightarrow\text{products, k}=9.27\times 10^{-13}\text{ cm}^{3}\text{/molecule s}, \tag{4}\]  
\[\text{Benzene + O}\rightarrow\text{products, k}=5.11\times 10^{-13}\text{ cm}^{3}\text{/molecule s}. \tag{5}\]  
Reaction of naphthalene with H radicals is even faster than with O atoms and its rate constant at 250 \({}^{\circ}\)C is much higher than for toluene and benzene [41]:  
\[\text{Naphthalene + H}\rightarrow\text{products, k}=3.83\times 10^{-9}\text{ cm}^{3}\text{/molecule s}, \tag{6}\]  
Figure 6: Logarithm of the concentrations of benzene, toluene, and naphthalene (**a**) without catalyst, (**b**) with Rh-based catalyst and (**c**) with Ni-based catalystm DBD reactor without catalyst. C—temporal concentration, \(C_{0}\)—initial concentration. (7)  
(8)  
However, neither O nor H radicals are leading in the tar decomposition reactions.