Flexible Operation of High Efficiency Coal Power Plants to Ensure Grid Stability When Intermittent Renewables Are Included

Abstract

The introduction of intermittent renewables such as wind and solar has created a major integration problem that has to be addressed to ensure operational stability for power generation grid systems. While there is some expectation that large scale batteries may provide a solution, this still appears to be far from commercial viability. Consequently, in most cases coal fired power generation has needed to adapt to a new operating regime so that there is adequate energy system stability. This includes being able to achieve, at low minimum load, rapid ramp rates and cycling together with fast start-up. Flexible operation can have significant impact on a coal power plant with most components being affected. This is because of the increase in thermal and mechanical fatigue stresses in the different parts of the plant, which together with corrosion, differential expansion and other effects, often occurring in synergy, reduce the life time of many plant components. There has been considerable development work undertaken to counter these adverse effects while balancing the grid, including new technologies, processes and skills. There are several means to achieve low minimum load combustion, with the critical need to maintain stable combustion. These include the need to understand and mitigate the technical limitations to low burning rates, such as fire stability, flame monitoring, and minimising unburned coal and CO emissions. Fire stability itself depends on many factors, such as changes in firing rate or fuel quality, inaccurate fuel/air ratio or uneven coal flow. Another impact to consider is the effect of low load operation on downstream NOx control systems and connected equipment. Measures for achieving minimum load include ensuring coal quality, air/fuel flow optimisation and coal fineness, operation with a reduced number of mills or smaller mills, indirect firing, thermal energy storage for feedwater heating, tilting burners, reliable flame scanners and economiser modifications. Start-up procedures are complex and expensive as they usually require auxiliary fuel such as gas or oil, during burners the ignition period. Start-up times in power plants can be shortened through application of reliable ignition, turbine integration, reduced thickness of walls in boiler and turbine design, external heating of boiler thick wall components, measures in the turbine (sliding pressure, advanced sealings, steam cooling of the outer casting), proactive cleaning of boiler deposits plus more effective instrumentation and control. Measures to improve high ramp up rates include exploring mill storage capacity, use of a dynamic classifier instead of a static one; measures in the turbine such as opening of throttled main steam valves, condensate throttling, thermal storage for feedwater heater bypass and HP stage bypass, lower thicknesses of pressure parts, and increased number of headers. Preservation during standby periods is important including targeted plant chemistry management of the boiler and turbine. These and other issues are considered and examples presented.