## 4 Characteristics of Flame and Its Combustion
### Ignition Temperature  
Generally, spontaneous ignition and forced ignition are two ignition ways in hydrothermal combustion. Spontaneous ignition temperature (SIT) refers to the minimum fuel inlet temperature that ensures spontaneous ignition, which means it is actually material temperature. To avoid confusion, the temperature of the heating wire that arouses the flame is defined as the forced ignition temperature (FIT). Based on the preheated reactants, ignition temperature can determine the power consumption of a system and whether the salt-deposition blockage can be overcome or not. The operating and geometric parameters of the reactor are two kinds of factors that affect the ignition temperature. Lower incident velocity, longer residence time and the use of volumetric reactors have been proved to be feasible measures to lower the ignition temperature. The correlation between SIT and fuel concentration is unobvious. [PERSON] studied the relationship between methanol concentration and SIT, and found that the SIT was insensitive to the change of mass fraction. Similarly, with other operation conditions keeping the same, although the mass fraction of methanol increased from 0.14 to 0.22, the SIT was stable at about 480 \({}^{\circ}\)C [24]. [PERSON] et al. measured the SIT of the methanol hydrothermal flame under different operation conditions, and drew an ignition map regarding the relation between SIT and mass fraction [52]. From the map, we can tell that the SIT varies from 500 to 550 \({}^{\circ}\)C and the increase of fuel concentration has no significant effect on the decrease of SIT, which is inconsistent with [PERSON]'s works [48]. Comparing the conditions of two experiments, [PERSON] thought that the SIT was sensitive to the structure of the reactor as well. The specially designed structure of the reactor could tremendously reduce SIT. The ignition experiments of 16 wt% methanol conducted in WCHB-3 [27] sharing the same nozzle as the TWR showed that SIT in WCHB-3 was in the range of 410-430 \({}^{\circ}\)C, while SIT in TWR fluctuated between 470 and 490 \({}^{\circ}\)C, and that there was a gap in heat dissipation between WCHB-3 and TWR due to different ways and distances of entering the reaction chamber for fuel and oxidant, and the cooling water in WCHB-3 also affected the temperature field. [PERSON] et al. found that the TWR with a tubular mixer and a post-mixing zone could start the combustion when the reactant temperature was as low as 150 \({}^{\circ}\)C [74]. Then they further studied the effects of four diverse tubular mixers on ignition by measuring the axial temperature of cross-sections and the TOC of effluent products [30], and found out that it was the Mixer-1 that first generated the flame, followed by the Mixer-3, while the Mixer-2 and Mixer-4 failed to produce the hydrothermal flame. From the perspective of free radicals, they considered that there was a connection between the reaction rate and the dissipation of free radicals. The two key factors affecting flame formation were high temperature and high velocity. High temperature promoted the production of free radicals, then reflux and post circulation enhanced the collision between newly generated free radicals, thereby reducing the ignition delay time. Mixer-3 and Mixer-4's failures were due to a shorter residence time led by high flow velocity, insufficient accumulation of free radicals, and the recirculation and back-mixing. Besides SIT, in order to ignite relatively cold materials, some institutions have also begun to study forced ignition with heating wire to determine the relationship between FIT and fuel temperature. [PERSON] tested a new type of hot surface ignition device on WCHB-4 for the ignition of ethanol-oxygen turbulent diffusion hydrothermal flame at room temperature [23].