Experimental investigation of nanosecond pulsed discharges for CO2 recycling

Renewable energy sources (RES) must gradually replace fossil fuels in order to reduce the emissions of carbon dioxide into the atmosphere. Unfortunately, in the absence of efficient energy storage technologies, the fluctuations of RES prevents their widespread utilization. Potentially C-neutral ways to store renewable energy include converting CO2 into compounds with greater enthalpies through endothermic processes. This strategy is defined as power-to-fuel and a possible implementation relies on non-equilibrium discharge plasmas – also called non-thermal plasmas (NTP) –. In the plethora of electrical discharges as sources of non-thermal plasmas, nanosecond pulsed discharges (NPDs) represent a promising niche. Their intrinsic transient behaviour due to the presence of fast-rising high-voltage pulses –alongside the possibility of operating them at atmospheric pressure– make them compelling for the generation of non-equilibrium plasmas, and consequently for their applications. In the thesis, the efficiency of CO 2 splitting and dry reforming of methane is investigated as a function of the temporal separation of the pulses. Two pulsing schemes are used: i) the continuous mode in which the ns pulses are equally spaced and their temporal separation cannot be shorter than 250 µs, ii) the burst mode that consists of packets of temporally close ns pulses (down to 10 µs) and a fast repetition rate of the packets (bursts). In both processes, the efficiency of the burst mode is significantly higher than the continuous mode. The relevance of these results lies in the possibility to achieve different performances in terms of conversion and efficiency by feeding the same energy into the system, only acting on the temporal distribution of the nanosecond pulses. The collisional energy transfer laser-induced fluorescence (CET-LIF) technique was adopted to track the variation of the gas consumption in between the nanosecond pulses in the CO 2 splitting experiment. The time-resolved measurements showed that while most of the energy is consumed in the first nanoseconds of the discharge, the conversion rises only after an order of microsecond delay, during which the system remains in a sort of quasi-‘metastable’ state. These findings indicate the presence of delayed dissociation mechanisms mediated by CO2 excited states rather than direct electron impact. The importance of the pulse sequence manifests when the subsequent pulses take place during the quasi-‘metastable’ condition. In this case, the energy required to achieve high values of conversion (around 70 % after a few microseconds) is significantly lower. The thesis’ last chapter will detail a chemical looping experiment that was carried out in collaboration with the University of Gent. A pre-reduced oxygen scavenger material was placed downstream of the plasma zone during a CO2 splitting process. The average conversion raised by about a factor of three compared to the absence of the plasma-material synergy.