Introduction to Startup Schemes of FPPs

There are two kinds of startup schemes currently used in FPPs [1]. One is the constant pressure startup scheme, in which the boiler operates at constant supercritical pressure after the coolant is pressurized to this point. The other is the sliding pressure startup scheme, in which the boiler operates with variable pressures and the pressure increases with the generation output. 

The plant system of the constant pressure supercritical fossil-fired boiler is shown in Fig. 5.1. The startup bypass system includes a flash tank, pressure-reducing valves, and bypass valves. First, a minimum feedwater flow is established in the furnace prior to the firing of the boiler to prevent overheating of the tube walls. During the cold cleanup mode of operation, the flow is bypassed from the inlet of the primary superheater to the flash tank, until the water chemistry is brought to a predetermined level and the boiler firing starts. 

After this firing is initiated, and the temperature of the steam leaving the primary superheater reaches about 300°C, the steam from the flash tank is led to the condenser. When the flash tank pressure becomes high, the saturated steam from the flash tank can be used to roll the turbine. After this, the turbine is ramped to about 7% load. At about 20% unit load, the startup bypass operation is shifted to once-through operation. When the temperatures become stabilized, the feedwater flow and the turbine load are increased slowly to 100% of full load value. 

The system configuration of the constant pressure supercritical boiler is complicated and pressure ramp-up operation is required. The startup valves experience a 

The plant system of the sliding pressure supercritical fossil-fired boiler is shown in Fig. 5.2. It requires a steam-water separator, a separator drain tank, drain valves, and recirculation pumps. A minimum flow rate is maintained through the furnace walls by using a recirculation pump to add a recirculating flow to that provided by a boiler feedwater pump. The water leaving the furnace is passed to the steam-water separators. The water from the separators is collected in the drain tank and routed back to the economizer inlet via the boiler recirculation pump. 

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