3. Results and discussions
3.1. Experiments without catalysts
Experiments without catalysts (mode 1) are conducted first. The experimental conditions are as following: the input power
of plasma is 14.4 kW; the mole ratio of CH4/CO2 is 4/6; the discharge gases include Ar (1.9 m3/h, inlet I) and N2 (2.5 m3/h, inlet II); the flow rate of the feed gases introduced into the reactor from inlet III varies from 2 m3/h to 5 m3/h. The
experimental results are shown in Fig. 2.
As CO2 reforming of CH4 is a highly endothermic reaction, the reforming reaction in mode 1 is supposed to be a thermochemical process. Define V ¼ P/F, where, F is the total flux of CH4 and CO2 in m3/h; P is the power of plasma in kW. Obviously, V expresses the average energy provided to the feed gases, implying the reaction temperature or the reaction driving force. When the flux of the feed gases increases from 2 m3/h to 5 m3/h, the V falls down gradually, which makes it reasonable that the conversions of CH4 and CO2 decrease from 97% and 76% to 52% and 34%, respectively. However, there is no significant change for the selectivities of H2 and CO in the present experimental operation region, which verifies our previous experimental results again [21]. Note that, there are few side-reactions taking place in the reforming process by thermal plasma. Understanding the changes of specific energy and energy conversion efficiency is very important. With the increase of the feed gases flux from 2 m3/h to 4 m3/h, the SE decreases from 421.5 kJ/mol to 322.8 kJ/mol, and the ECE increases from 41% to 47%, respectively. And when the feed gases flux continues to increase, the SE goes up and the ECE falls down. The similar phenomena can be found in many chemical engineering processes. It is suggested that appropriate conversions of feed gases (or V) make for lower energy consumption or higher energy conversion efficiency. In other words, there seems to be a critical point of V for this process. More energy provided to the reaction enables the reaction with higher temperature, leading to more heat loss and lower energy conversion efficiency. Instead, the reaction is difficult to be initiated if the V is excessively low. Generally, there is an optimum match between the total feed gases flux and the discharge power of plasma to obtain the highest energy conversion efficiency of the thermal plasma reforming process.