## 3 Results and Discussion  
As an example, Figures 2-5 show real-time optical, laser-based, and conventional measurements of the process parameters in the experiments for heating up the process gas by FO, H2, N2 plasma, and Air plasma + H2, respectively. Figures 2a, 3a, 4a and 5a show the temperature measured inside the ECF using the TDL and UV NO absorption techniques at P1 and P2 and suction pyrometer at P3 and the TC measurement of the process air before the mixing/combustion chamber. Figures 2b, 3b, 4b and 5b demonstrate the volumetric flow rate of the coflow air and the fuel, plasma power, and, when available, the TDL H2O measurements and the H2O calculations made based on the air and fuel flows. Figures 2c, 3c, 4c and 5c present the NO and NO\({}_{2}\) concentrations measured by the optical and probe diagnostics. Due to the large amount of data shown in the figures, it is reasonable to start with describing the features of the air flow and temperature that are common for all the measurements. The flow rate of the process air, being on average 2180 m\({}^{3}\)/h, has variations of type I and II, see Figures 2b, 3b, 4b and 5b. The I type of the variations manifests itself by the rapid decrease and increase in the air flow by \(\sim\)5% occurring every 21 min. Thistype of variation is due to the regular switching of the process air flow from one to another gas-fired pebble heater. The variations of the II type are periodic with a period of several minutes coming from air flow control system. The process air temperature is measured by a bare TC is about 1230 K; the TC temperature shows regular increases in the temperature of 7-8 K matching the I type of variations; reducing the process air flow through the pebble heaters increases the air temperature. The II type of the variations is not noticeable in the TC temperature measurement, since the magnitude of the II type variations is several times lower than that of the I type. The TDL  
Figure 2: Online measurements of T (TDL, Suction pyrometer, TC) (**a**), H\({}_{2}\)O (TDL) and flows of process air and FO (**b**), NO (UV absorption) and NO and NO\({}_{2}\) (Probe) (**c**) during the combustion of FO (Day 1). Position number in the legend (pos nnn) shows the distance in cm from the beginning of the ECF to the measurement position. and suction pyrometer data, Figures 2a, 3a, 4a and 5a, reflect the variations of the I type by showing the temperature increases by several K at times when the switching of the process air flow from one to another gas-fired pebble heater occurs. The II type of the variations is not resolved by the TDL measurements. The suction pyrometer measurements have better sensitivity than the TDL ones to this type of variation, see Figure 5a. The temperature measured using the UV NO absorption does not show increases in this type, we remind that the method uses the precalculated spectra simulated with the temperature step of 20 K.  
Figure 3: Online measurements of T (TDL, UV absorption, Suction pyrometer, TC) (**a**), H\({}_{2}\)O (TDL) and flows of process air and hydrogen (**b**), NO (UV absorption) and NO and NO\({}_{2}\) (Probe) (**c**) during the combustion of hydrogen. Position number in the legend (pos nnn) shows the distance in cm from the beginning of the ECF to the measurement position. UV absorption T averaged over 100 s; UV absorption NO is fitted using fixed T = 1480 K.  
Figure 4: Online measurements of T (UV absorption, Suction pyrometer, TC) **(a)**, Plasma power and flow of process air (**b**), NO (UV absorption) and NO and NO\({}_{2}\) (Probe) **(c)** during the N\({}_{2}\) plasma heating. Position number in the legend (pos nnn) shows the distance in cm the beginning of the ECF to the measurement position. Apart from the small variations of the II type, the temperature of the process air was constant. The temperatures measured using the UV NO absorption, TDL, and suction pyrometer, in general, reflect the effects of the process air heating due to fuel combustion and/or mixing with hot plasma gases.