In view of the significant growth in renewable energy generation, it has become crucial to flexibilise thermal power plant (TPP) operations. While flexibility can be achieved in a number of ways such as energy storage and demand-side management, flexibilising conventional generation plants that are already in operation is being seen as the optimal solution for the short term.
Backdrop
Flexibilisation is a combination of capital retrofits and operational modifications in power plants. It requires significant capital expenditure. The cost of retrofits depends on boiler technology, type of coal, etc., and can vary from plant to plant. The key considerations when choosing between operational measures and physical retrofits include the useful life of the power plant, its design – whether it is suitable for cycling – and the cost recovery mechanism. In most cases, a combination of capital retrofits and operational modifications is chosen to achieve flexibilisation. Training and skill development of employees are also critical for proper monitoring of the cycling impact. Some of the physical modifications to achieve flexibilisation are reduction in the mill size; increase in the number of mills; installation of advanced burners and indirect firing systems in boilers; use of advanced alloys (SA 213 T23, SA 213 TP347H, Super 304 H for tubes and SA 335 P91, SA 335 P92) for improved strength; installation of external steam preheating systems to reduce the start-up time; and elimination of thick-walled components such as drums. Operational modifications include laying down procedures for boiler ramp rates, controlling the temperature movements during plant start-up and shutdown, and undertaking rigorous inspection programmes for plant components.
Impact of cycling on major equipment
Cycling has an adverse effect on TPP components. It mainly affects thick-walled components operating above the creep (slow and continuous deformation due to high temperature even at constant load) limit such as steam turbines and boilers. Cycling also leads to increased wear and tear in pulverisers, frequent start and stop of high tension motors of fans, increased make-up, premature failure of high pressure heaters, and fouling of catalysts in selective catalytic reduction systems. The boiler water wall suffers from corrosion fatigue due to oxygen outage or high start-up oxygen, and chemical deposits on the wall. At present, thick-walled components, for example, boiler headers, are not designed for the combination of creep and fatigue damage (failure of material when subjected to repeated and/or fluctuating stress due to thermal cycling). As a result of cycling, cracking can be seen in thick-walled components such as turbine valves and casings, and boiler headers. In addition, with frequent start/stop and load variation, water and steam chemistry parameters are disturbed leading to corrosion. High temperature differential and hotspots from low steam flow during start-ups adversely impact boiler superheaters and reheaters. Meanwhile, feedwater heaters are affected by high temperature ramp rates during start-ups. In low pressure turbines, there occurs last-stage blade erosion due to moisture.
Mitigating the effect of cycling
Additional condition monitoring systems/sensors help mitigate the effect of cycling. These include turbine stress controllers, boiler stress monitoring systems, blade vibration monitoring systems, stator end winding vibration monitoring, rotor flux monitoring, partial discharge monitoring, and additional sensors for health monitoring. Further, the design of boilers and turbines needs to be improved to allow faster ramping and increased number of cycles. For older plants, a low cycling regime may be adopted. The replacement of fatigued/ worn-out components may further alleviate the adverse effects of flexibilisation. Analytics is also essential to achieve flexibilisation. The upgradation of the standard operating procedure, enhanced visibility of parameters, customised dashboards, predictive analytics and asset health index can help overcome the challenges of flexibilisation.
Conclusion
Currently, a large number of coal-based power plants are idle and facing uncertainty regarding their commercial viability. Flexibilisation provides tremendous opportunities for such plants. Through retrofits, advanced technologies and operational improvements, flexibilisation can help coal-based power plant owners move away from baseload operations and respond to changes in their operations. However, it requires a customised approach and station-specific conditions need to be studied to adopt the right strategy for flexible operations.
Renu Dahiya