Coordinated Feedwater Heater Energy Control to Achieve Higher Peak Load Generation Reduced NO_X Emissions

摘要

The recent shift in operating profile for many coal fired plants to overnight minimum load or frequent shutdowns causes furnace slag to shed temporarily lowering steam temperatures on subsequent operation. With the load variability of intermittent renewable energy sources, a premium value is placed on higher peak generation capability from conventional coal fired plants. The high pressure feedwater heater energy control is a means to achieve design steam temperatures, improve unit heat rate and increase peak load generation capability. High pressure feedwater heater energy control can be a vital component of a multivariable steam temperature, load generation and NO_x emissions control system. Conventional fossil fired steam power plants apply the Rankine cycle to maximize power plant efficiency while meeting power generation requirements. At design time the cycle is analyzed combining information from the turbine, boiler, and subsystems to design the specific Rankine cycle configuration of feedwater heaters. The turbine extraction points determine the final feedwater heater temperature as a function of turbine steam flow. The boiler heats the feedwater to generate superheated steam targeting the design pressure and temperature. Drum style steam power plants have fixed surface areas for each function of the steam process; economizer for water preheating, evaporator for steam generation, superheater for superheating steam and reheater for reheating steam. Surface areas designs achieve cycle steam temperatures for specified steam flows and fuels. Modifications to a boiler section surface area or changes in fuel quality can result in an energy distribution imbalance that challenge the attainment of design steam temperatures and load generation. Application of in-furnace controls to redistribute energy may have adverse consequences on NO_x emissions and boiler efficiency. A high pressure feedwater heater energy control provides an additional element of control to attain design steam temperatures, redistribute boiler energy, increase load generation and respond to electrical grid dispatch. Achieving design steam temperatures improves cycle efficiency. The system provides the additional benefit of increased peak MW generation up to 7% depending on cycle design and equipment limitations for both drum type and once-thru type boilers by increasing the steam flow through the intermediate and low pressure steam turbines. generation capability can be significantly increased through high pressure feedwater heater energy control with little, if any, consequence on the unit heat rate. The estimated potential increase in load generation is up to 7% depending on cycle design and equipment limitations. For units that do not have a steam temperature deficiency, including once through units, the efficiency impact of cooler feedwater is mitigated by the increased boiler efficiency and reheat steam flow through the LP turbine. High pressure feedwater heater energy control is a viable and effective additional element of regulation for superheat and reheats steam temperatures and peak load generation. For units operating with low superheat steam temperature, the potential improvement in cycle efficiency through achieving design steam temperatures increases Rankine cycle efficiency. High pressure feedwater heater energy control can be a vital component of a multivariable boiler, load generation, emissions, and steam temperature control system. With the additional advantage of increased sustainable MW generation and reduced furnace NO_x emissions, the potential benefits are substantial. The increased utilization of intermittent renewable energy sources and the shift in energy supply favoring gas turbines places new burdens on the electrical grid dispatch of coal fired units. Coal fired units must now operate to very low minimum loads with the capability to ramp load quickly and sustain high peak loads. The application of high pressure feedwater heater energy control provides significant potential benefits and should be integrated with a multivariable boiler/turbine control for regenerative Rankine cycles with reheat.