Coal-based power continues to dominate the Indian power generation segment. These plants account for over 63 per cent of India’s total installed generation capacity. However, the declining plant load factors (PLFs) are a cause for concern. The PLF of coal-based plants has declined significantly in the past few years, from 77.81 per cent in 2009-10 to 60.91 per cent in 2018-19. They are facing challenges such as coal shortages, falling demand vis-à-vis capacity and regulatory pressures to meet the new environmental norms. Also, increasing power demands from load despatch centres, frequent outages, prolonged periods of low load operations, varying ramp rates, and operations beyond design limits have impacted plant efficiency. These issues are expected to increase in the near future with the increasing penetration of highly intermittent renewable power.
Thus, there is a significant scope for improvement in the underperforming power plants. These challenges can be overcome by adopting O&M best practices. O&M best practices can ensure reliability, safety and economy of the power plant by reducing accidents and outages, rectifying deficiencies, and replacing damaged components.
O&M best practices
Turbine maintenance includes a series of activities to assess the actual condition of the machine and restore it to the desired condition. The service time of a turbine can be subdivided into three phases, the initial service period, the main service period and the last period when the life expenditure increases due to increasing material fatigue.
There are several factors that influence turbine outages. These include material selection, manufacturing, erection and commissioning. These could be a result of human failure and are mostly time independent. There are also certain time-dependent factors such as wear and tear, erosion and corrosion of components, distortions and deposits, which mainly cause component stressing.
Turbine maintenance could be routine or capital maintenance. Typically, an effective maintenance strategy would include a multi-stage process wherein the study of operational history and failure history along with a pre-overhaul survey, is undertaken in the first stage. In stage 2, TPPs identify the scope of work, corrective measures and spare requirements. The next stage involves planning and dismantling, while the fourth stage is for carrying out checks during dismantling. The scope is reviewed and corrections are carried out in stage 5, leaving the overhaul and performance comparison to the last stage.
One of the key O&M challenges for turbines is to balance baseload operations in the face of increasing renewables. For achieving this, power plant operators need to ensure a high level of flexibility and agility. This can be achieved through utilisation of better control systems, equipment upgrades and the adoption of O&M best practices such as condensate throttling for rapid response. Another O&M practice for improved safety is the use of wearable technology for remote communications. Data-driven fault detection, which uses real-time utility data, can also be used to simulate problems. Further, predictive maintenance technologies can be adopted for improving overall plant efficiency.
The boilers in thermal power plants (TPPs) experience several issues like molten ash deposition, mill fires, boiler tube failures, slagging and fouling, ash, slag and sludge build-up on sloped walls and hoppers, which greatly impair boiler efficiency. Some of the standard techniques to address the problem of slagging and fouling are proximate analysis, ultimate analysis, heating value, ash elemental analysis, hardgrove grindability index, and fusion temperature and chemical analysis of ash. Also, conventional indices are used for determining the slagging potential like the silica ratio and the base acid ratio. However, these conventional techniques and indices do not include ash content and temperature parameters, and are based on bulk properties of ash instead of specific particle properties. These indices are developed mainly for stoker-fired boilers and conditions, and are not applicable for pulverised fuel boilers.
The new specialised techniques for addressing slagging and fouling are thermo gravimetric analysis (study of mass with change in temperature), thermo mechanical analysis (study of physical properties of material with change in temperature) and scanning electron microscope analysis, which involves high resolution images of samples by casting a focused electron beam over the sample and detecting the secondary or backscattered electron signal.
Another O&M practice for addressing the issue of ash, slag and sludge build-up on sloped walls and hoppers is sonic soot blowing. O&M measures for molten ash deposition are mill tuning, checking boilers for burner tilts and possible flame impingement on boiler walls, frequent soot blowing and correction of the air flow. Molten ash deposition is related to poor burner operation, mill problems, poor coal quality and other factors. For avoiding coal dust ignitions and explosions inside the mill, measures such as steam inerting (before starting, after switch off and during mill discharge), automatic safety procedures for mill starting and switch off, automatic mill discharge after every emergency trip, and monitoring and protection of critical mill parameters can be taken.
Other O&M strategies include the deployment of model-based control systems, which make predictions on the basis of historical data and help in faster stabilisation of parameters during load changes. However, the existing control systems work on error philosophy and parameter excursions are difficult to control in such systems.
Plant efficiency can also be improved by equipping operating units with modified/augmented technology. Auxiliary power consumption can be reduced by upgrading pumps, motors, drives, power supplies, etc. Further, design changes can help uprate 110 MW units to 120 MW, and 200/210 MW units to 216/225 MW. For boilers, the replacement or augmentation of pressure parts, retrofitting/modification of air pre-heaters, augmentation of the milling system, renovation of firing systems, renovation of the draft system, and retrofitting of electrostatic precipitators can be undertaken. Further, obsolete control and instrumentation (C&I) systems can be modernised using state-of-the-art technology. Also, smart soot blowing systems can be installed, which enable complete furnace monitoring.
In the past, the focus was on increasing availability and reliability while keeping the maintenance costs stable. Today, the priority has shifted to reducing the maintenance costs while maintaining power availability and reliability. Also, TPPs now need to move from higher efficiency at full load to optimum efficiency over load range, and from larger unit sizes to smaller flexible unit sizes. Flexibility could be dynamic, or operational. Dynamic flexibility requires high operational gradient, short start-up minimum, nominal load and short minimum downtime. Meanwhile, operational flexibility requires high starting number and load cycles at reduced lifetime consumption, lowest possible minimum load at high efficiency and uniform, and high efficiency curve across the load. These requirements present further O&M challenges.
In sum, O&M best practices provide optimised solutions to meet the challenges in the power sector, ensure plant safety and availability, and increase the flexibility of assets with a minimum maintenance budget. These solutions are customised and cannot be standardised. A tailor-made approach is essential to improve the performance of various units.