## 2 Three Periods of Peak Shaving  
The early produced 200 MW and 300 MW low-capacity units were mainly designed for the baseload condition, and the minimum power output at steady combustion in DPSW is only 60% to 70% of the rated power (\(P_{n}\)). Therefore, the peak shaving capacity is very limited and not suitable for frequent start/stop operations [22]. The later-developed 600 MW and 1000 MW high-capacity units come with a slightly better peak-shaving capability, and the minimum output power for steady combustion in DPSW could be reduced to 40% to 50% of \(P_{n}\). In accordance with the \"Implementation Rules for the Management of Auxiliary Services for Grid-Connected Power Plants in North China\", the basic peak-shaving standard for units directly managed by grid operators is 50% of \(P_{n}\), which means that the existing 600 MW and 1000 MW coal-fired units also have reached the practical operation limit during DPSW. However, if the combustion-supporting systems are installed, the minimum output power for steady combustion could be further brought down to 30% to 35% of \(P_{n}\). Based on the power generation characteristics for different load levels, the peak shaving procedure could be divided into three periods: RPS, DPSW, and DPSC as shown in Figure 1, where \(P_{\text{max}}\) is the maximum or rated output power; \(P_{\text{a}}\) is the minimum power output in RPS period; \(P_{b}\) and \(P_{c}\) are, respectively, the minimum power output for steady combustion in DPSW and DPSC. For the most part, the coal-fired units normally run in the RPS period with a comfortable operation condition, and the wear-and-tear loss is negligible. While in the DPSW and DPSC periods, the coal-fired unit is operated at a low-load condition with various operational risks and losses resulting, and its deep peak shaving capacity depends on several factors such as the combustion stability of boiler,  
Figure 1: Schematic diagram of peak shaving stages of coal-fired units. safety of hydrodynamic work conditions (SHWC), boiler auxiliary machine parameters, and operators' technical level, etc. [23; 24]. 1. Combustion stability of boiler as the most key factor is closely relevant to the metal fatigue pressure, coal quality, fineness and concentration of coal powder, hot wind speed, and temperature, etc. Generally, when output is lower than \(P_{a}\), the combustion conditions become worse, which is prone to cause accidents to coal-fired units because of flame failures;
2. SHWC cannot be ignored when the boiler is running at a low load level. When loads change rapidly, the growing thermal deviation of the circulating pipeline on the water-cooled wall will reduce the circulation flow speed, and further results in boiler circulation stagnation or reversion. The test shows that when the flow speed falls below the critical value at 30% \(P_{n}\), the SHWC cannot be guaranteed;
3. Boiler auxiliary machines, such as the coal pulverizing system, water pumps, and fans, are the auxiliary facilities for combustion. On account of boilers' large inertia, the coal pulverizing system is apt to blocking coals or suspending pulverizations while the boiler is running at low load level. Since the plasma ignition system must operate simultaneously with the coal pulverizing system and the ignition in DPSC is with a high furnace temperature, if there is too much coal powder in the furnace to be ignited timely, the accumulation of coal powder inside the furnace may cause deflagration. The expansion of flue gas from deflagration raises the furnace pressure suddenly and may further lead to the boiler explosion. Since the compressed air supply for the plasma burner contains water and oil, it may induce arcing failure or fire in the barrel of plasma igniter. Meanwhile, the probability of the pump and the fan failure is greatly raised;
4. During the low-load level operation, the monitoring, protection, and control systems may frequently alert warnings because the boiler with its auxiliary machines is operating near a critical stable point.