## 4 Lean Blowout
### Lean Blowout Behaviour  
Lean blowout (LBO) is a phenomenon of flame extinguishment when the combustion occurs in very lean conditions and exceeds the LBO limit. The LBO limit is considered the lowest equivalence ratio that can carry on the flame [88]. The flame-out will occur accordingly once the combustion reaches that limit. The characteristic of LBO can be clearly explained by NOx and CO trend over the combustion temperature corresponding to the equivalence ratio as depicted in Figure 19. The red zone in the graph shows the LBO limit, which falls into a particular range. The blue zone represents the operating range of lean-burn combustion, which is applied in the DLE gas turbine to maintain a healthy operation. Figure 19: NOx and CO emission to characterize LBO behavior. Figure 18: Gas turbine trips cause in GDC plant. The equivalence ratio, where the LBO limit is located, is not a fixed number, and keeps changing due to various factors related to the operating conditions, such as the velocity field that influences the turbulence levels, ambient air, and the temperature and pressure of the combustion chamber as reported in [89]. The flame becomes unstable when the air velocity is high as the turbulence level corresponding to the Reynolds number also increases, as represented in Figure 20. This graph shows the stability loop of the gas turbine combustion for a given inlet pressure and temperature. The limit between the operability region and blow-out varies due to the amount of air mass flow and fuel-air ratio. The equivalence ratio at LBO will increase along with the air mass flow as reported in [90]. Similarly, increasing the fuel flow rate will shorten the residence time of the droplets inside the flame preheat zone, which increases the blow-out limit [91]. The fuel composition also significantly affects the LBO limit. According to [92], fuel with high hydrogen content produces a lower equivalence ratio. Hence, it will extend the lean stability limit and lower the possibility of flame to blow-out. A study by [90] evaluated the effect of the composition ratio in blended fuel of methane and ethane to LBO limit. The result showed that the LBO occurs at a higher equivalence ratio when the ratio of propane in the fuel increases. Further, methane dilution with carbon dioxide and nitrogen increases the LBO equivalence ratio. The percentage of pilot fuel also significantly impacts the LBO limit changes. According to [90], the increase in pilot fuel percentage decreases the equivalence ratio of LBO. Furthermore, the swirl strength and physical mixing of fuel and air will also influence the LBO limit [82,88,93]. The LBO limit increases with the rise of swirl intensity, as reported in [94]. Similarly, the swirl cup's geometry also significantly affects the limits, and the limits will decrease with the airflow of swirlers for dual-axial swirl cups. In contrast, the opposite happens for dual-radial swirl cups. The risk of the flame blowing out is also affected by the fluctuation of power demand [95]. During deceleration, power reduction is achieved by decreasing the fuel flow, affecting the turbine's lower gas temperature and velocity. The shaft rotational speed subsequently turns slower, which results in the compressor not rotating at a similar speed to the turbine. Hence, the mass flow rate of incoming air drops gradually, decreasing the equivalence ratio that gains the LBO occurrence. Further, the decrease in the equivalence ratio reduces the resistance of flame turbulence, raising the level of turbulence [96]. Hence, the flame is quenching, and a local blowout might happen. The high turbulence flame is  
Figure 20: Combustion operability for gas turbine based on air fuel ratio over the air mass flow. generally found near the fuel nozzle and the shear layers at the Inner Recirculation Zone (IRZ). Following that, the flame is driven towards the low-turbulence locations, which increases the residence time. Thus, the flame might reignite the exhausted non-burnt gases along the shear layers.