Minimum load reduction in a combined cycle equipped with once-through evaporator

Minimum load reduction in a combined cycle equipped with once-through evaporator

De Francesco, Luca (TU Delft Mechanical, Maritime and Materials Engineering)

Klein, S.A. (mentor)

Delft University of Technology

2017-10-13

The increasing share of renewables in power production is demanding conventional plants to take on additional tasks. It is estimated that the cyclical and unpredictable nature of renewables will create under- and over-capacity periods that the system will need to accommodate. Given their wider load range and faster load changes, combined cycles can complement renewables very well; therefore, energy industry is investing many resources to make them more flexible.
Sloecentrale is a modern combined cycle thermal power plant located in Vlissingen area; its Heat Recovery Steam Generator is equipped with once-through technology, characterized by the absence of a drum and by a continuous flow path from the economizer to the superheater. When operating in once-through mode, the water flow in the line is adapted to the heat input in the evaporator, in order to guarantee a full evaporation and prevent liquid flowing into the superheater. Below a certain GT load (minimum load), the flue gas is not able to fully evaporate the water-steam mixture and the HRSG is switched to non once-through mode; in this mode, a small separator separates steam from water and send it to the superheater. Given the higher efficiency at part-load and the faster load changes in once-through mode, the goal is to extend the load range operating in once-through mode by a reduction of the minimum load. A steady-state Matlab model of the once-through evaporator has been developed, returning the flow distribution in the heat exchanger and other relevant quantities. The model is based on mass and energy conservations, and takes into account the local heat flux along each tube as well as the non-uniform phase separation occurring in the junctions. The inputs of the model (flue gas and water-steam flow properties) have been computed by simulating the plant operations by a Thermoflex thermodynamic model. Both the models have been validated against real process data from Sloecentrale. By an analysis of the flow distribution at minimum load, the main limitation to the once-through load range extension has been located. Finally, some potential solutions to overcome the limitations have been proposed and simulated, and their technical feasibility has been assessed.

http://resolver.tudelft.nl/uuid:1ac7a982-4b68-48ae-b98c-e96697c06daf

2022-10-13

Student theses

master thesis

© 2017 Luca De Francesco