Power station operations Millmerran power station feed water dissolved oxygen control

Abstract

[Abstract]: This project investigated the operation of a coal fired power station. The project developed and analysed some operational control system engineering problems faced by Millmerran power station. The main area covered in this project is on feed water dissolved oxygen control. This involved feeding of oxygen gas at required amounts into the boiler water to protect boiler tubes from corrosion effects. Millmerran power plant uses a supercritical once through boiler which operates at extremely high temperatures of about 540 degree celcius and pressure of 24 Mega Pascal which can accelerate corrosion of the boiler tubes. Oxygen injection process, which is known as oxygenated treatment performed well in high purity feed water with a pH ranging from 8.0 to 8.5. The OT process control of Millmerran power plant utilized an advanced proportional integral and derivative controller (A-PID) which regulates the opening and closing of the servo control valve. The control process encountered some stability problems during load changes due to the effect of large dead time. The dead time was mainly the time between injecting oxygen gas and detecting the dissolved oxygen in the feed water. Dead time of the OT process of Millmerran power plant ranges from 12 to 15 minutes. Oxygen is injected at the polisher outlet and detected downstream at the economizer inlet. In this dissertation, simulation models for the behavior of the advanced PID controller and a servo DC motor under various time delays were carried out using simulink and matlab software. The simulation models showed that by varying the process dead times, the process variable was becoming stable. This was only experienced during small time delays of less than 6 minutes. To obtain a quick and more stable response of the OT process control, the dead time can be reduced by simply moving the injection point closer to the oxygen sensor. This will help in maintaining the protective oxide layer hence reducing chances of oxide exfoliation which can lead to boiler tube failures and power plant shut down. A proper OT control will help protect the boiler hence saving millions of dollars for the power plant.