As India rapidly increases its renewable capacities in order to meet its intended nationally determined contribution (INDC) targets, managing the intermittency of renewable energy sources has become a key concern. The lack of adequate storage systems prevents renewables from providing 24×7 power. This has created the need for flexibilising conventional energy sources. While gas and hydropower plants are known for their flexible power generation capabilities, there is limited availability of cheap gas or storage-based hydro. Given the abundant coal reserves, flexibilisation of coal-based power plants can help ensure reliable power for safe and stable operation of the grid, until proper storage solutions are developed for renewables.
Flexibility measures
According to a report by the Central Electricity Authority (CEA) on flexible operation of thermal plants for integration of renewable generation, preparation of a coal-fired unit for flexible operation would require measures at all levels of operations and maintenance (O&M) and administration. These include raising awareness about flexible operation and the commercial impact of flexibilisation. Further, optimising the instrumentation and control system is the most cost-effective way to enhance power plant flexibility. A certain level of automation is also required. An effective control system should be in place to strengthen flexible operation, such as precise steam temperature control. The optimisation of control loops, that is, coal supply, drum level and air control, is a basic requirement. Apart from these, there is a need to implement mitigation measures to manage the consequences of flexible/cyclic operation. Some of these measures are reassessment of all O&M procedures, with a special focus on water and steam quality, preservation and lay-up procedures and maintenance strategies. Stable combustion is the key to maintaining minimum load operation. Besides, reliable flame detection for each individual burner, transparency regarding the coal quality and composition, optimisation of air flow management, operation with a limited number of mills and operation of the boiler protection system at low load are some of the other requirements.
In order to ensure fast and efficient start-up, plant operators should check start-up-related temperature measurements and replace the measuring equipment if required. Plants should improve the efficiency at part load and adopt a dynamic behaviour with measures like optimisation of the water-steam cycle as well as improvement in the performance of important equipment and components, such as feed water pumps.
Better quality of coal improves the combustion process. Therefore, blending, washing and online coal analysis should be undertaken to improve and monitor coal quality. Further, all processes should be automated, including start-up of fans and pumps, mill operation, steam temperature control and flue gas temperature control. Since each unit has a different plant layout, equipment design, efficiency in terms of operation practices and general condition of the machinery, a test run should be conducted before any intervention is made.
Flexibilisation challenges
Varying coal quality poses a major challenge to flexibilisation of coal-based power plants. Plant design is another challenge as all plants have been designed for baseload operations. Hence, the time taken for start-up is considerably higher. Further, most of the state utilities are yet to reduce the minimum load levels. Moreover, most of the coal-based power plants have long-term contracts with limited flexibility. Therefore, there is a need to incentivise flexibilisation through regulations or market signals. Another challenge is geographical concentration of renewable power and its transmission constraints. Significant transmission lines will be required for renewable energy evacuation. However, these lines will operate for 8-10 hours only. Low capacity utilisation will result in higher costs. Another major challenge is the training and development of plant personnel, who are trained to operate plants at high plant load factor (PLF) and in high efficiency mode. Moving towards flexibilisation would require adequate manpower training to enable a smooth transition.
Cycling impact
The flexibility of a power plant can be described as its ability to adjust the net power fed into the grid, its overall bandwidth of operation and the time required to attain stable operation when starting from a standstill condition. When there is shortage of power, a quick start-up of thermal power plants (TPPs) is required, whereas when there is surplus power, minimum stable generation is needed. In fact, to bring TPPs at minimum load is one of the biggest challenges. The provision for a technical minimum, which was kept at 70 per cent earlier, came down to 60 per cent and now stands at 55 per cent.
The CEA’s National Electricity Plan released in December 2016 analysed the hourly data of three years and estimated the all-India load profile and net load curve for the year 2021-22, based on the respective projected peak demand and energy requirements. In addition, high ramping up and ramping down requirements were observed, especially during peak and off-peak hours. The duck belly demand to peak demand ratio is 61 per cent, which will lead to partial loading and shifting, that is, cycling of conventional power plants and, hence, low PLFs. Other impacts of flexibilisation on the existing plants are expected to be increased forced outages and O&M cost, equipment lifetime reduction, poor heat rate and high auxiliary power. An average of 40 per cent annual reduction in fatigue life is expected with daily cycling operations and 25 per cent annual reduction in creep life.
Flexibilisation costs
Both regulated and market-based power systems can ensure appropriate investment in power plant flexibility measures based on the value of specific flexibility services. The costs involved in flexible operation of thermal generation units include both capex, which is a one-time expenditure incurred in the installation of equipment required to make the plant capable of low load operation, and opex, which is a recurring cost incurred due to increase in the O&M cost and decrease in efficiency.
The capex depends on the number and type of interventions required, which vary depending on the age of the unit, design type, size, coal quality, historical operation, maintenance, and other scope of work. The scope will also depend on the future flexibilisation regime the unit is expected to support, that is, low/ very low load, start/stop-daily/weekly shift operation. The factors impacting opex are increase in heat rate and auxiliary power consumption, increase in O&M due to reduced life of components, and increase in oil consumption on account of frequent start/stops. Thermal plant units are designed for baseload operations, but as the operation regime changes from baseload to load cyclic, the plant components deteriorate at a faster rate. This is observed in increased failure rate and frequent replacement of components. The life of components decreases with the increase in number of start-stop the unit undergoes in a year. As a result, the O&M cost is significantly higher in units operated on a daily or weekly start-stop basis.
Start-up of thermal power stations requires secondary fuel support, mainly in the form of oil. The quantity of oil consumption depends on the duration of start-up. During flexible operation, a large number of cold, warm or hot start-ups may be required. This increases the additional fuel cost. There are high system costs associated with flexibilisation due to suboptimal utilisation and variability, requirement of backup capacity and decrease in full load hours of capital-intensive despatchable power plants. Further, day-ahead forecast errors can cause unplanned intra-day adjustments of despatchable power plants and require operating reserves to respond within minutes to seconds, resulting in balancing costs. There are additional grid-related costs as renewable energy is located far from load centres, entailing investments in transmission.
Conclusion
At the technology level, some of the key issues pertain to HP temperature control, main steam and reheat pressure control, drum level control, combustion and mill tuning, and smooth switch-over of pumps and fans. Feasibility studies of boilers and turbines should be carried out based on test data and the ramping up and ramping down procedure should be automated. Large-scale renewable energy has been successfully integrated into grids world over. International cooperation is necessary, particularly from renewable energy-rich countries, particularly Germany, as they have managed to achieve low load operation of thermal units.
Policy and regulation have to play a major role in the integration of renewable energy to promote green energy, and ensure grid stability and reduction of the overall system operation cost (grid as well as generating units) in the long run by reducing the extent of cycling at coal-based generating stations. As the all-India PLF for despatchable generation is bound to reduce with renewable integration, the threshold PLF for fixed cost recovery may accordingly be reduced. Units catering to variable load requirements must be sufficiently compensated through special tariffs. Wider participation of fossil plants will be necessary for meeting the flexibility requirements. Also, participation of original equipment manufacturers is necessary. Training and capacity building of operators is also important to minimise the impact of cyclic and minimum load level operations.