There are two broadly adopted gas turbine technologies – open cycle gas turbines (OCGTs) and combined cycle gas turbines (CCGTs). The OCGT type is low on efficiency but relatively fast in response. When paired with steam turbines to form CCGT plants, they have a higher overall thermal efficiency, but take much longer to reach full load than is required for applications such as peaking, renewable energy balancing or standby reserve. If deployed for part-load operations, with renewable energy variation, the efficiency will fall drastically. However, it is important to note that almost the entire fleet of gas power equipment in India, which is idle or stranded, is of the CCGT type. These plants have been rendered uneconomical because they cannot compete on the merit order against coal plants for baseload application, nor are they flexible enough to provide quick ramp rates as required for renewable energy balancing. Further, if started/stopped frequently, they need to be overhauled in a shorter period. However, design and operation changes can help improve the flexibility of gas turbine technologies so that they can increasingly contribute to accommodate the increased variability of renewable energy sources.
A closer look at the technology trends in the gas-based power generation market and operations and maintenance (O&M) best practices….
Increased flexibility of gas-based power turbine technologies
In the case of CCGT power plants, flexibility is restricted by the heat recovery steam generator (HRSG), the steam turbine and the balance of plant (IEA, 2018). Design and operational changes to these units can improve their overall flexibility. For instance, traditionally HRSG components are thick-walled, requiring a longer time to warm up. Replacing these with thin-walled components makes the system better suited to handling sudden changes in temperature and, in turn, for starting up in a shorter time. Similarly, component thickness can be reduced in the steam turbine to enable quicker start-up.
Flexible operation can also be achieved by adding a bypass so that waste heat from the gas turbine is not captured; or by adding a steam bypass from the HRSG to the condenser, which enables a CCGT plant to be operated as an OCGT plant.
Some gas turbine vendors have developed new plant designs, which offer a higher degree of flexibility. One of these is the internal combustion engine (ICE). Unlike gas turbine plants, ICE plants are modular in construction and operation. Thus, a 200 MW plant may consist of 10 engines of 20 MW each and can be scaled up in modules to meet the growing demand. Each of these engines can be operated separately, or together with other engines, to suit varying demand, with the highest thermal efficiency at any plant load. Each engine (or the entire plant) can be started instantly and ramped up to full load in two to five minutes from a standby mode when there is a need, whether due to a sudden spike in demand or a sharp drop in renewable energy generation (wind failure, cloud cover, etc.). They can be stopped instantly when renewable energy generation picks up again, to allow its full absorption without curtailment. And this start/ stop sequence can be done any number of times without a maintenance penalty. These characteristics of rapid response and instant despatchability make ICE an excellent balancing foil for variable renewable energy.
O&M best practices
While rapid start-up significantly improves the operational flexibility of a plant, the costs associated with start-ups include more frequent maintenance and additional fuel consumption. In view of these challenges, adopting efficient O&M strategies is essential to ensure efficient and uninterrupted operations.
The key O&M practices should include optimising the usage of available cheap domestic gas to maximise generation, and doing a stringent review of operating parameters during start-up, operation and shutdown. Further, utilities should adopt strict monitoring of water chemistry and preservation procedures and strive to reduce auxiliary power consumption. Training and succession planning of O&M executives should be strengthened and procurement should be done through global tendering and reverse engineering.
The renovation and modernisation of control and instrumentation systems should also be undertaken, using state-of-the-art technology. Refurbishment of hot gas path components can be done through the non-original equipment manufacturer route.
The plants can be operated using high speed diesel due to its higher thermal efficiency and lower cost compared to naphtha. On-site compressor blade coating should be undertaken to avoid erosion.
The maintenance should be based on equivalent operating hour (EOH) on recommendations by the OEM. Regular inspections and overhauling should be carried out by manpower service providers in India and supervision by the utility and the OEM. There should be annual maintenance contracts. Opportunity-based combustor chamber inspection and offline compressor washing should be undertaken to improve efficiency. Usage of filter wraps can help prevent dust ingress in the air intake system and cleaning of removed air intake filters and wraps should be undertaken for reuse. Painting of all the equipment after every major inspection/overhaul can not only improve the aesthetic appearance but also prevent corrosion.
Further, heat recovery steam generators hot spot identification and result analysis with infrared thermography can help in timely identification of duct cracks and flue gas leakages. Dry ice cleaning of waste heat recovery boiler (WHRB) tubes can remove deposits from their external surface. During unit shutdown, air curtains (plastic) should be provided in the gas turbine filter house to prevent dust ingress and ensure higher life and less choking of filters. Humidity control in gas turbines, WHRB flue gas path, steam turbines and generators can help protect from corrosion. By filling 10pH demineralised (DM) water, followed by nitrogen capping during a long shutdown, can promote better preservation during shutdown, so that the layer of magnetite (Fe3O4) remains stable.
Moreover, the best operation practices can include having a monthly full-speed no-load trial of gas turbines, bringing steam turbines on turning gear on a weekly basis, and carrying out capacity test of units in operation. In addition, monitoring of gas turbine compressor efficiency is also vital.
Emerging requirements
There is a growing need for conversion of baseload gas stations to peaking stations in view of the increasing renewable generation, and gas-based generating stations can play a vital role for meeting this peak demand. Emerging requirements demand increased inspections and more repair work. There should be ready availability of spares and service providers. Other O&M best practices can include having inspection of LP rotors after every 16,000 EOH, and have test synchronisation of turbines every six months, even without schedule. The uncertainty in getting schedule is leading to inability to plan inspections and overhauls, which should be avoided. Utilities should increasingly opt for long-term service agreements with OEMs, and have residual life assessment of gas turbine components and WHRB. Positioning of nitrogen generators and dehumidifiers can help achieve targeted humidity levels.
While it is reassuring to see that India is going ahead with its aspirational renewable energy targets, if the country is to realise its vision, it must strive to include all sources of technology in its electricity mix. Going ahead, ensuring gas availability, followed by the best O&M practices and technological upgrades, can help provide optimised solutions to meet the challenges in gas-fired power stations and thereby ensure plant safety and availability.
Nikita Gupta