## 3 Dry-Low Emission Gas Turbine
### Dry-Low Emission Gas Turbine Working Principle  
The DLE gas turbine has been popular since the 1970s, as the regulation of NOx reduction was tightened. Its working principle follows the Brayton cycle as defined in Section 2.2.1. The combustor performs the heating process in an isobaric condition from point 2, delivering heat to increase the temperature of high-pressure gas until the turbine inlet temperature is at point 3. In the combustion process, emission production is the function of temperature. Therefore, the DLE gas turbine operates at a lower temperature than the conventional one to produce lower emissions. Commonly, a conventional combustion occurs at operating temperature range from 3400 \({}^{\circ}\)F (1871 \({}^{\circ}\)C) to 3599 \({}^{\circ}\)F (1927 \({}^{\circ}\)C) [64]. Meanwhile, the DLE gas turbine operates under 2800 \({}^{\circ}\)F (1538 \({}^{\circ}\)C) to trim the emissions to a single digit of NOx [65]. The combustion can be controlled by adjusting the air and fuel composition, as illustrated in Figure 14.  
Refer to Figure 14, DLE gas turbine operates in fuel-lean conditions by implementing the lean-premixed (LPM) technique, mixing the fuel and air at the baseload to produce a lean mixture before entering the combustion chamber. The premixing prevents local \"hot spots\" that can accelerate a significant formation of the NOx [66; 67]. Commonly, the stoichiometric mixture of gas turbine varies between 1.4 and 3.0 [64]. The reason is that when the mixture of air and fuel is below a factor of 1.4, it will produce an intensely hot flame that will rapidly increase NOx formation. In contrast, the combustion becomes unstable when the mixture exceeds 3.0. For this reason, the air supplied is twice higher as the actual air needed to produce a lean condition that can lower the combustion temperature. Hence, the production of thermal NOx can be limited. Even though the DLE gas turbine significantly reduces the NOx, it is difficult to maintain the CO production that increases with the decreasing firing temperature. Therefore, controlling the air and fuel ratio is crucial in DLE application. The DLE combustor has a different air and fuel system configuration compared to the conventional type, as shown in Figure 15. The main fuel valve injects approximately 97% of the total fuel in the premixing chamber. A pilot fuel valve is added to inject fuel directly into the combustion chamber to maintain stability in rich burn conditions [68]. Therefore, the combustor's size is more prominent because of the additional pilot valve  
Figure 14: Air/fuel ratio effect to flame temperature and NOx emission. and the premixing chamber that contains a large quantity of air supplied of approximately 50-60% of the combustion airflow. Since the lean-burn operation is adopted in DLE gas turbines, the tendency to flame out is sometimes unavoidable. Therefore, the DLE gas turbine has a specific operating region to maintain healthy operating conditions. The combustion that is too lean will affect the chemical reaction to spend longer than the residence time. Hence, the burner fails to maintain the flame, which leads to LBO occurrence. Avoiding flame extinction can be achieved through the air or fuel staging [69]. Performing air staging can be done by reducing the airflow and decreasing the mixture strength in the combustion chamber to stabilize the combustion. On the other hand, the fuel staging approach can be carried out by axial or radial methods. For the axial approach, the fuel is injected into two zones, utilizing the products from the first combustion zone to be mixed with the air and fuel to the next combustion zone to maintain the lean operation. The use of pilot light or fuel reduction can be implemented for the radial approach. The number of fuel staging depends on the operating range; the common number of the stages used is two or three, as illustrated in Figure 16.  
For example, the implementation of fuel staging was used in a typical DLE combustor by General Electric named Dry Low NOx-1 (DLN-1).