The integration of renewable energy sources into the energy mix has posed significant challenges for traditional thermal power plants (TPPs). In the future, TPPs are expected to operate at an average minimum load of 55 per cent. In this regard, the Central Electricity Authority (CEA) has notified the Flexible Operation of Coal-based Thermal Power Generating Units, 2023. It has also notified the phasing plan for achieving 40 per cent minimum technical load.
Hence, TPPs will need to adapt and be tuned to meet the new load demands in an effective and efficient manner. From baseload demand, they will need to adapt to meet dynamic grid requirements, including fluctuating loads, rapid start-ups and frequent shutdowns. This demands robust flexibilisation strategies.
Flexibilisation improves the operational efficiency of TPPs, ensures grid stability and extends the lifespan of plant equipment. These strategies provide several benefits, including enhanced grid support, better economic performance and prolonged equipment life. By enabling TPPs to respond effectively to fluctuating grid demands, flexibilisation complements renewable energy sources and ensures a stable power supply. Due to part load operations, utilities will be forced to operate units at lower efficiency and may therefore be required to undergo modernisation of these plants to improve heat rate at lower minimum loads.
CEA’s phasing plan
As per the CEA’s 2023 regulations, the 55 per cent minimum load and 2 per cent ramp rate operating requirement have to be implemented by all thermal generating units (central/state/private) within one year of the notification of the regulation. Power plants are required to implement measures, as per the phasing plans set by the respective power plant owners, to operate thermal units at 40 per cent minimum load with the following ramp rates: 1 per cent per minute for 40-55 per cent and 55-40 per cent loads; 2 per cent per minute for 55-70 per cent and 70-55 per cent loads; and 3 per cent per minute for 70-100 per cent and 100-70 per cent loads.
The CEA has also notified the phasing plan for achieving a 40 per cent minimum technical load. The plan is structured as follows: the pilot phase, covering 10 units with a total capacity of 5,580 MW, is scheduled from May 2023 to March 2024. Phase I includes 91 units totalling 51,080 MW and spans from July 2024 to June 2026. Phase II, comprising 100 units with a capacity of 46,825 MW, is planned from July 2026 to June 2028. Phase III will involve 101 units with 37,215 MW between July 2028 and December 2029. Phase IV, encompassing 191 units with 55,767 MW, is set for January 2030 to December 2030.
In the pilot phase, 10 units from the central, state and private sectors will undergo refurbishment. As of September 2024, the implementation of the phasing plan is delayed. The 10 units, totalling 5,580 MW under the pilot phase, are at different stages of execution.
BTG flexibilisation strategies
In the context of flexibilisation, boilers, turbines and generators (BTGs) must operate efficiently at varying loads while maintaining safety and minimising emissions. One key strategy is the enhancement of combustion control systems. Advanced systems ensure the precise mixing of fuel and air, which is critical for maintaining combustion efficiency during load fluctuations. Technologies such as low-load combustion systems enable boilers to operate safely at reduced capacities without risks such as flame instability or increased emissions.
Managing thermal stress is another vital aspect. Boilers undergo considerable thermal cycling during start-ups, shutdowns and load changes. Innovations in boiler materials, such as creep-resistant alloys, and the implementation of fast-acting spray systems reduce temperature gradients, preventing thermal fatigue and prolonging equipment life. Slag and ash management systems, including advanced soot blowers, further ensure stable operations by mitigating fouling and slagging. In the Indian context, where high ash content coal is commonly used, it is essential to maintain minimum flue gas velocity in the system to avoid the accumulation of ash within boiler/ducts.
Flexibilisation of turbines involves optimising their design and operations to efficiently handle variable loads. Modern control systems play a pivotal role in turbine flexibilisation. Condensate throttling is a proven measure for primary control to enable the fast increase of turbine power in case of grid frequency deviations.
Predictive maintenance tools also contribute by identifying potential issues early and reducing downtime. Low-load operation capabilities are another critical improvement. Adjustments in blade designs and seal clearances help minimise efficiency losses when operating at lower steam flows.
Generators must adapt to frequent load changes and transient conditions without compromising efficiency or reliability. Enhancements in rotor dynamics have proven essential for managing variable operations. Flexible rotor designs and improved damping systems help generators withstand rapid load changes while reducing vibration and mechanical stress. Additionally, strengthened insulation and windings protect against damage caused by transient conditions. Voltage and frequency regulation are equally important. Modern excitation systems allow for rapid adjustments to voltage and frequency, ensuring stability during fluctuations in grid demand. Coupled with dynamic grid support systems, these improvements enable generators to contribute to ancillary services like reactive power compensation. Reducing the minimum load levels for generator operation is another critical focus area.
In addition to the above strategies, artificial intelligence and machine learning algorithms integrated into plant operations facilitate predictive maintenance, adaptive load handling and seamless coordination between components. Installing digital tools for online condition monitoring and damage assessment can provide valuable insights for focused maintenance, helping determine the frequency of maintenance and component replacement.
Challenges and the way forward
Flexible operations present a range of challenges, particularly at the boiler level. Rapid ramping and frequent load fluctuations below design limits can cause significant temperature variations and thermal stress, leading to issues such as creep, fatigue, irreversible damage, high boiler tube leakage, material property degradation, and difficulties associated with drum level control. These problems contribute to increased forced outages, reduced availability and reliability and a shortened plant lifespan. Low-load conditions can also cause poor combustion, flame instability and low coal pipe velocity, resulting in choking, high furnace exit gas temperatures and eco-steaming, all of which negatively affect efficiency and emission compliance. Supercritical and ultra-supercritical boilers experience further complications, such as a sharp increase in evaporator metal temperature during wet-to-dry changeovers, which impacts other boiler parameters.
Turbines face their own set of challenges, including higher rotor train vibration, increased ovality, reduced casing hardness, a greater risk of low pressure (LP) blade failure and severe blade deposition. Excessive exhaust hood spray can cause LP blades to erode, exacerbating operational difficulties. Other concerns include the potential for high-pressure casing cracks, LP turbine blade fluttering and damage to steam valve internals.
As the energy sector evolves, embracing flexibility is both a necessity and an opportunity to improve operational efficiency and sustainability. While challenges remain, the benefits of flexibilisation in terms of efficiency, reliability and sustainability make it a vital strategy for the future of thermal power generation.
Akanksha Chandrakar