## 3 Results and Discussion
For example, the TDL and suction pyrometer temperatures (Figure 2a) follow the trend of the FO flow (Figure 2b), since at fuel-lean combustion, there is a practically linear dependence between the equivalence ratio proportional to fuel flow and the temperature. The difference between the TDL and suction pyrometer temperatures was about 60 K for most of the shown data points and was about 40 K in  
Figure 5: Online measurements of T (TDL, UV absorption, Suction pyrometer, TC) (**a**), H2O (TDL), Plasma power and Flows of process air and hydrogen (**b**), NO (UV absorption) and NO and NO2 (Probe) (**b**) during the Air plasma heating and H2 combustion. Position number in the legend (pos nnn) shows the distance in cm from the beginning of the ECF to the measurement position. general during the FO combustion, which is very good considering the uncertainties of the methods. We note that the laser-based and optical methods report the path- averaged values, while standard probes report the data from a point. For other fuels or plasma heating, the difference between the TDL and pyrometer measurements was around 100 K; the somewhat larger difference in Figure 3a between 8:00 and 13:00 is because the pyrometer was located closer to the wall, the temperature of which is lower than that of the gas. The TDL and UV NO absorption temperatures were, in general, within 50 K; the TDL temperature was higher than the NO UV one. Accounting for the heat losses of gas between the TDL and NO UV positions, 2 m, might improve the agreement. The TDL diagnostics is more responsive to the fast variations in gas temperature than the suction pyrometer detection, for example, the fuel feeding interruption results in the immediate decline of the TDL temperature, while the suction pyrometer requires some minutes to register the decline, see Figure 6 showing online measurements of T (TDL, suction pyrometer, TC) and H\({}_{2}\)O (TDL) during the FO combustion. The reason for the above is that the TDL measures the gas temperature, while the suction pyrometer measures the temperature of the thermocouple tip, which might be affected by the radiation from combusting fuel, particles, gas products, and walls. Figures 2b, 3b and 5b include the TDL H\({}_{2}\)O measurements; there is a quite clear and intuitively correct correlation between the H\({}_{2}\)O measurements, and all the variations in the air and fuel flows discussed above. Furthermore, the H\({}_{2}\)O measurements show distinct variations of the type III coming from the rapid periodical fluctuations of the H\({}_{2}\) flow rate, see Figure 3b. Later in the campaign, these rapid fluctuations in the H\({}_{2}\) flow were reduced by narrowing the range between the upper and lower limits of the flow controller settings for a given flow rate. The measured and calculated H\({}_{2}\)O volume fractions for H\({}_{2}\) combustion shown in Figure 3b are in excellent agreement with each other. The same can be said about all the H\({}_{2}\) combustion/plasma heating experiments made in the tests. The agreement between the measured and calculated H\({}_{2}\)O volume fraction for FO combustion is also very good, better than 8%. The TDL H\({}_{2}\)O sensor's ability to accurately detect fluctuations in the air and fuel flows offers interesting opportunities for the implementation of the technique for diagnostics and the control of industrial processes. For example, the TDL H\({}_{2}\)O measurements can be used to control the overall air-fuel equivalence ratio and determine the degree of the air leakages. Figure 6: Online measurements of T (TDL, Suction pyrometer, TC) and H\({}_{2}\)O (TDL) during the FO combustion (Day 2). The NO mole fractions obtained using the NO UV absorption and the probe measurements correlate well with each other and the variations in the air and fuel flows.