The rapidly burgeoning share of renewable energy in the power mix is increasingly necessitating the incorporation of flexible coal-based power plants that are operationally and commercially viable. India’s renewable energy capacity (including large hydro) currently stands at 164 GW, accounting for 40 per cent of the installed capacity. Moreover, the country plans to augment its renewable capacity to 500 GW by 2030, driven by private investment and government stimulus.
It has also become essential to add hydro generation and low-cost renewable power to reduce the average power cost. At the same time, the rising renewable energy-based capacity is characterised by variance, uncertainty, intermittency and slow ramp-up rate and has to be balanced with firm power to maintain consistent supply. It is specifically important to develop flexible capacity to avoid difficulties in load frequency control, non-availability of transmission network, need for ancillary services, etc. Hence, the power sector is witnessing a shift from a consistent addition of renewable energy-based installed capacity to a regulatory environment, emphasising round-the-clock supply and flexible thermal power plants (TPPs) and power plants that complement renewable energy capacity. Flexibilisation of TPPs is a unique process and involves extracting information about the plant’s status and checking for the degradation level. The data that is gathered is sent for processing and analysis.
Digital tools for flexibilisation
Digitalisation can be effectively used for flexible operations of conventional power plants. Some advanced digital tools that may be deployed alongside the existing control system include tuning of existing control loops, advanced process control (APC) software, asset monitoring system, pattern recognition software, fatigue monitoring system and combustion control techniques.
Boiler fatigue monitoring system can help optimise boiler fatigue, which aims to calculate fatigue of individual boiler component and, in turn, can help in early detection of deviations. It has numerous advantages such as improving transparency in relation to impact of operating mode on residual life. It can detect and prevent high-wear operating modes. It helps in the optimum selection of moments and intervals for requisite overhaul and inspection. It can enhance the safety and reliability of the power plant and can help with better utilisation of spare component material. It can also help in cost-effective in-service monitoring and analysis. Devices that can remotely determine creep and low-cycle fatigue of critical components and calculate lifetime and number of cycles to crack initiation and can also store data to draw trends and comparisons, can help achieve this objective.
Temperature optimisation is needed to achieve the maximum steam temperature, while adhering to material limits. It can improve the plant efficiency because of higher steam temperatures and reduction in reheater regulation. This can be achieved via robust, easy to measure and adaptive state space controllers with inbuilt observers and by using the entire control range through to injection into saturated steam, wherever it is needed.
In large parts, there is substantial potential for embedding IoT and digitalising closed loop processes, given that seamless control of their operation will help maintain the minimum base load without causing wear and tear to the equipment. APC for power plants has become increasingly sophisticated over the past 20 years as it helps the plant to operate automatically, using real-time data from thousands of sources and thereby, eliminate the reliance on gauges and manpower to do that task. APC solutions can be used to control gas turbine and combined-cycle power plants too. However, modern gas turbines often operate at limits of their material capabilities and are already closely controlled to ensure that they do not exceed these limits. Another process area with a potential for improvement is load response, that is, start-up and ramping, given the low capability of these units. Other closed loop process that can be optimised by integrating sensors and digitalising them include coal supply, drum level, pressure systems, temperature systems, air control systems, etc.
Increasing flexibility of TPPs also endows them to cater to sudden increase in demand and ramp up generation accordingly, in a bid to cater to peak demand seamlessly. This is specifically achieved by digitalising the process of steam temperature optimisation as it allows an increase of steam temperature set points in order to move to design limits, while simultaneously expanding the plant ramp rates. Furthermore, steam temperature optimisation and a high degree of flexibilisation ensuing from such an optimisation provides TPPs access to more competitive energy markets that are willing to pay a premium for instant supply during peak hours.
Artificial intelligence (AI)/machine learning (ML)-based data analytics technologies can help by enabling better forecasting and scheduling of power demand through advanced analytics and technologies and better asset management techniques. These measures will consequently ensure viable flexible power availability that has minimal grid-volatility and simultaneously, reduces operations and maintenance (O&M) costs.
Elongating/enhancing the asset life and reducing O&M costs despite rising flexibilisation of asset utilisation is another benefit of digitalising the plant. APC systems help keep these expenses under control by ensuring that the plant always operates within certain limits. During start-up, for example, if temperature gradients within the boiler can be limited, thermal stresses can be reduced. Maintaining tight control of conditions during start-up and shutdown, and when a plant is ramping, through APC solutions helps in extending the lifetime of plant components. In addition, the data collected during each cycle can be processed by AI/ML to build up a historical picture of the component health that can be used to predict equipment failure before it happens. Effectively, it will keep unplanned downtime to a minimum and allow maintenance personnel to schedule needed repairs at optimum times. Correspondingly, the digitalisation of plant components enabling real-time data flow will also help the power plant develop meticulous models, simulating to determine its operation in the near future as well as medium-term. These models are also known as digital twins, given that their virtual version is homologous to their physical version.
NTPC Limited is at the forefront of pioneering pilots in India in the sphere of flexibilisation and some of these pilots concern integrating digital systems in order to automate and optimise oil consumption and reduce start-up time. Similarly, the utility has deployed condition monitoring for boiler fatigue monitoring and turbine life monitoring. In boilers, monitoring of the condition of thick wall pressure parts is important. The software available for monitoring increased stresses caused from cyclical operation helps NTPC in identifying the extent of wear and tear. In turbines, part-load operations may lead to steam temperature changes, especially hot reheat temperature, which develops stress in the equipment. Therefore, solutions for calculating equivalent operating hours are being considered for flexible operation.
Recently, some researchers used genetic algorithms that can provide holistic solutions for complex problems to optimise the steam temperature and boiler operations after accounting for the coal flow, steam flow, heat reduction spray flow, etc. Using these algorithms led to coal saving of 1.9 grams per kWh.
The way forward
The flexibilisation of TPPs involves several challenges such as a reduction in plant efficiency and increased risk of failures owing to wear and tear from repeated changes in capacity utilisation. In other words, flexibilisation will lead to higher O&M costs and lower revenue accretion given that renewable energy will be cheaper. However, monitoring, digitalisation, IoT, analytics and automation can help tackle these challenges to an extent by optimising processes. Automation and APC solutions are cost-effective and proven measures to help flexible operations. These solutions are also effective in part-load operations of units, efficiency and reliability improvement.
Notably, some studies project that there will be net load swings of 80 GW or more in 2023-24, when India is expected to have a renewable penetration of more than 175 GW. Therefore, it is vital for power generation stakeholders to collaborate and develop a roadmap that ensures that TPPs improve their ramping levels and flexibilisation capabilities and transition to their role of providing firm power. In addition, the roadmap needs to ensure that their commercial viability remains unharmed, regardless of flexibilisation.