The flexible operation of existing coal-based power plants is crucial to ensure reliable power supply and maintain stable electricity grids while maximising generation from renewable energy sources. While there are many options of flexible power generation for integrating renewables, coal-fired plants are the best option considering factors such as availability, proportion and cost.
In the near future, thermal power plants (TPPs) are expected to operate at an average minimum load of 40 per cent. For this, TPP units that have so far operated as baseload plants will require overhauling. Pilot tests conducted at various plants demonstrated that Indian plants are capable of flexibilisation.
The Central Electricity Authority (CEA) has brought out a comprehensive report on the flexibilisation of coal-fired power plants, which highlights the operating procedure, challenges, retrofits and roadmap for achieving 40 per cent load operation. As per the report, flexible operation of coal power plants can be made technically feasible through upgrading and tuning controls. Further, the report has devised a phasing schedule based on the age of units and inputs received from major original equipment manufacturers (OEMs).
Power Line takes a look at the key aspects covered in the report…
Key requirements of flexibilisation
To assess their flexible capability, thermal units need to undergo testing for safety, security and stability parameters and a quantification of their available flexible power, ramp rate, etc. These tests are also essential for assessing the need for plant-specific retrofits. The low load tests should be conducted after a careful study of the unit and the test targets should be decided in consultation with the OEM.
The technical measures will depend on the levels of minimum load operation. Lower load operation, such as 40 per cent, will require measures such as automation/optimisation of controls, proper flame detection systems, optimised combustion processes, stable minimum mill operation, reassessment of O&M practices, etc. The Indian power sector has a large fleet of subcritical coal-fired units with a capacity of 500 MW or less, which are considered suitable for flexibilisation. Supercritical units are suitable for load ramping/flexible operation.
Studies conducted
Several flexibility studies have been conducted in association with national and international partners at central, state and private plants. The flexible operation tests have been conducted at 40 per cent load and higher ramps at number of stations in collaboration with OEMs. Observations from pilot studies show excessive fluctuations in steam temperatures, making steam temperature control difficult as fuel flow rate and feed water flow rate decrease. There are also high drum level swings during ramping, flame disturbance during ramping and at minimum thermal load, occasional furnace pressurisation, chances of stalling of primary air fans at low loads, and low boiler flue gas exit temperature leading to acid corrosion. With 3 per cent ramp commands, the actual ramp rate achieved in the full load range from technical minimum loading to full load was only 1.3 per cent due to manual milling system operation.
Flexibilisation challenges and possible solutions
The pilot studies carried out in the Indian power stations revealed deviations/ damage during flexible operation like high exhaust hood temperatures, high steam seal temperatures and high rate of change of metal temperatures. There are many commercial issues such as hight cost of retrofits, increased O&M costs, heat rate degradation costs, increased equivalent forced outage rate, which would need to be compensated by central and state regulators. There is low operator confidence in flexibilisation due to the operation of TPPs as baseload plants. Low load operation of units at 40 per cent calls for added precautions. Negligence and poor O&M practices can lead to catastrophic equipment failure. Hence, operator training is essential for the implementation of flexibilisation.
To achieve minimum load operation, it is important to sustain stable combustion by manipulating the firing rates, maintaining even temperature distribution within the different zones of the boiler, managing the coordination between the boiler and turbine, and reducing the number of mills in service. Another limitation with low load operation is the improper sizing of many control valves for low load or low flow operation, which can cause poor control response and sometimes result in hunting of valves. All these control valves must be checked for correct operation at low load and necessary modifications should be made. Additionally, for cyclic operation, review and modifications will be required for the alarms and protection logic. Moreover, when a baseload unit is converted to operate in flexible mode, the operator’s view of the process needs to be modified to include the actions that may be needed during the particular operating regime.
Prerequisites for flexibilisation and modifications required
Prerequisites for flexibilisation include ensuring all auto loops are available and fine-tuning the coordinated master control to minimise the deviation of parameters. The attemperator system and control valves should be set tightly and provide a fast response to the changing system demand, and dirty air flow test should be conducted at regular intervals to evaluate partially plugged coal pipes and burners. Further, burner tilts should be operational in full range in auto mode, air heater air leakages and other tramp air should be minimised, and turbine stop and control valves should be inspected with respect to valve position in the control room, among others.
To achieve greater flexibility, it is not necessary to retrofit the entire plant but only certain subsystems that are most effective in tackling plant flexibility. The technical measures required will depend on the levels of minimum load operation to be adopted. Operation at 55-50 per cent load may only require a reassessment of O&M practices, maintenance of critical components, and automation/optimisation of controls. However, lower load operation may require additional measures such as proper flame detection systems, efficient measures to optimise the combustion process, stable minimum mill operation and use of steam coil air preheaters. Temperature measurements are crucial to optimise the startup and shutdown procedures. The technical solutions are primarily aimed at ensuring safety, reducing the detrimental impact of flexible operation on the life of the unit, and achieving flexibility at the lowest cost.
Cost of flexible power
The cost of undertaking flexibilisation in a power plant depends on the level of automation in the plant, coal quality, age and size of units, type of machine, component material composition and design philosophy, maintenance philosophy, operational expertise and practices adopted and extent of cyclic operation – depth, breadth and frequency. The required capital investment for retrofits in a unit may be around Rs 60-70 million. For very old units that have not upgraded their plant control and instrumentation system, the capital investment will go up to about Rs 300 million, depending on the retrofit.
To compensate for the costs of flexibilisation, it has been recommended that the cost of modification/retrofit will form a part of the capital investment, and will be recoverable through a fixed part of the tariff over a period of five to seven years. Any increase in O&M expenses will have to be compensated as part of the fixed tariff. Units running at technical minimum load below 55 per cent can also be compensated through an increase in energy charge rate to account for heat rate deterioration. In cases where a unit has the capability for flexible operation, but may not have the opportunity or schedule to do so, any increase in O&M costs may not be considered as part of the fixed cost, whereas the increased capital cost for retrofitting will continue to be recovered. Any variable costs associated with flexibilisation should be reimbursed for the cycling period through an increase in the tariff.
Modifications required
Rather than retrofitting the entire plant, retrofitting only certain subsystems of power plant can be the most effective way of ensuring plant flexibility. The technical measures depend on the levels of minimum load operation to be adopted. The operation at 55-50 per cent load may only need reassessment of O&M practices, maintenance of critical components, automation/ optimisation of controls. However, lower load operation will require additional measures like proper flame detection systems, efficient measures to optimise combustion process, stable minimum mill operation and use of steam coil air preheaters.
Controls play a pivotal role in the operation of coal-fired power plants. They allow a smooth transition between different operating loads and ensure stable operation by adjusting all relevant process variables. The control system monitors and controls critical parameters like temperature, pressure inside the boiler, feed-water mass flow in the water-steam circuit, loading of coal mills and turbine valve positions. Based on the specific needs of the plant, control modifications can be adopted for improving the flexibility. These include drum level control, flue gas temperature control and automated start/stop of mills. Meanwhile, ramp rate improvements and stable minimum load operation strongly depend on a stable and optimised combustion. These can be achieved by burner tilt control, furnace/differential pressure control, furnace pressure control upgrade, among others. Further, reducing the wall thickness increases the permissable temperature change rate, which translates into a faster start-up by increasing the ramp rate. Further, predictive digital solutions can be used to optimise several parameters to reduce boiler/turbine start-up time.
Roadmap
The report has outlined a roadmap for flexibilisation, which includes a preliminary phasing plan based on the age of units and inputs received from major OEMs. As per the report, thermal capacity commissioned after January 2016 will have to deploy advanced digital controls and features to enable faster adoption of the 40 per cent load following operational regime. These units, totalling 101 units, should be brought under the purview of the flexibilisation operating regime first.
In the pilot phase, 11 units from the central, state and private sectors commissioned between January 2016 and December 2022 are proposed for refurbishment. It is estimated that the refurbishment will be completed over a period of one year, followed by the performance evaluation and rectification period of six months. The experience gained in the pilot phase will be useful for future planning.
Units commissioned between January 2012 and December 2015 will be taken up in the second phase and units commissioned between January 2001 and December 2011 will be considered in the third phase, to be completed over one and a half years. In the last phase, the remaining units commissioned up to December 2000 will be refurbished within one year. (See Table for details)
A minimum estimated time period of eight years may be required to make these units compliant with flexibility up to 40 per cent load and achieve higher ramp rates. The complete refurbishment work is estimated to end by December 2030. For funding some of the units in the pilot phase, INDC technology development and transfer/ fund mobilisation or government funding like the Power System Development Fund may be utilised.
The way forward
The existing hydropower, pumped storage and gas-fired generation capacities with high peaking performance are not adequate to meet the balancing requirements. Therefore, the existing thermal resources available for flexibilisation should be utilised first in a safe and secure manner before adopting newer options such as battery storage systems on a large scale. Two shift operation of old thermal generating units is being carried out in some parts of the country in a piecemeal manner to meet the generation load balance. There is a need for careful planning of two-shift operation across the country.
Going ahead, the availability of training simulators must be ensured for the training of operators at 40 per cent low load operation. Regulation should be introduced for 40 per cent minimum technical load operation of thermal generating units, along with suitable regulatory mechanisms at the central and state levels for compensation. Further, a study should be initiated for finding the possibility of two-shift operation of existing thermal generating units as per the grid requirement. Lastly, the design of new thermal units should allow for two-shift operation on a regular basis. Overall, the utilities should conduct detailed studies/tests and cost-benefit analyses to find the most optimal solutions to improve the flexibility of plants as the measures required are plant specific and depend on the level of flexibilisation. This should be done in consultation with the OEM or main plant manufacturer.
Nikita Gupta
