Addressing the Lifecycle Maintenance Challenges of Natural Gas Plants

The deluge of distributed energy resources and renewables that has flooded grids in recent years has created unprecedented complications for natural gas plants' asset lifecycle management plans. In the heyday of combined-cycle gas turbines (CCGTs), stops and starts were infrequent, and operational flexibility was not the imperative it has since become.

These lifecycle management challenges don't just apply to aging natural gas plants, either. Companies that invested in CCGTs less than a decade ago are facing tough decisions about the future of relatively new plants. Here's a look at how two combined-cycle power plants (CCPPs)—one in southern Germany, the other in northern Italy—addressed challenges both technical and commercial with varying degrees of success.

Debate Over Mothballing a Modern Plant

The German example is a classic case of how an adverse commercial and regulatory environment can affect even modern, highly flexible CCGTs. The plant in question began operating in 2011; with a fuel-conversion efficiency of 60.4 percent, it's one of the most efficient gas-fired power plants in the world, according to a press release from the plant. Yet its owner has applied several times to mothball the plant, because it's not adequately compensated for the number of hours it's called upon to run.

There is a cruel irony here. Germany has been heavily promoting investment in renewable energies, especially wind and solar power, and around a third of its power is now provided from these sources. This modern, flexible, and high-efficiency CCPP is the perfect load-balancing partner for the nation's variable renewables.

Unfortunately, the legal environment forces the owner to provide this backup service at prices that do not cover the plant's costs. Yet the grid operator has refused the owner permission to mothball the plant, because of its criticality to system stability. This is partially because Germany plans to close all its nuclear power plants by 2022, according to Deutsche Welle.

There is hope in sight. The German government has established a commission to examine the role of coal-fired generation, Clean Energy Wire reports. Coal has such a high share of the market that the nation will not meet its 2020 greenhouse gas emissions targets, despite the high penetration of renewables, CLEW continues.

The commission's conclusions may, however, be pre-empted by the skyrocketing price of emissions allowances in the European Union Emissions Trading System (ETS), according to Sandbag. At current levels, carbon prices work in favor of gas and to the detriment of coal.

Resurrection of an Ineffective, Aged Asset

The Italian example concerns a gas-fired CCPP called Chivasso, located on the outskirts of the industrial city of Turin. The power market in this region has changed significantly because of the rise of renewables and the arrival of a high-voltage cable from France, creating a glut of electricity generation capacity, according to POWER. In 2013, the plant's owner—the utility A2A—decided to take it out of commission, because it was no longer competitive in the prevailing business environment, mainly due to its low operational flexibility, POWER reports.

But then came a turning point. Engineers analyzed the plant's technical parameters and its commercial environment and concluded that a combination of new gas turbine components, a modern control system, and cloud-based asset performance management software could revitalize the plant. A new type of combustor on the gas turbines improved response times and reduced emissions. Turndown performance and ramp rates were transformed.

For the operators of older CCPPs like Chivasso, asset lifecycle management has become a complex and urgent issue. Not only are there maintenance challenges to consider, but asset managers must also decide whether investments in improving the operational efficiency of these plants—to improve their standing in the merit order—are worthwhile.

For example, upgrading the combustion systems on the gas turbines will not only reduce emissions in general, but also keep emissions within limits at lower turndown loads. In the case of machines whose rotors are nearing the end of their lives, there are a number of options: install a new rotor, implement a rotor-life-extension program, or fit a refurbished rotor. In some cases, though, the best option may be to shut down the plant for good.

Prevention Is Better Than Identifying a Cure

These aren't the only factors that affect the natural gas lifecycle, however. Oftentimes, preventative maintenance can go a long way toward enhancing not only an asset's health, but also its ability to rapidly cycle. Here are a few more examples of flexibility measures that can make or break a plant's future, as operating regimes change.

Steam turbines can suffer accelerated wear because of repeated, excessive thermal expansion of their rotors in a deep-cycling mode. One way to reduce this is to install a blanketing system that will keep the equipment warm if it's going to be offline for hours before the next start.

Asset managers can apply a similar strategy to the heat recovery steam generator (HRSG), which can be damaged by repeated thermal expansion, causing tube cracks and failure of the water steam cycle. One way to reduce thermal shocks is to install an ambient air-injection system. Another is to install a damper to prevent the flow of cold night air through the equipment, slowing down the cooling rate.

Such strategies can not only reduce maintenance needs, but also make startup faster, perhaps creating the opportunity to take advantage of a demand peak that the plant might otherwise have missed.

Backing Up Operations With Battery Storage

Another option to boost operational flexibility is to install utility-scale battery storage, the cost of which is falling steeply, to provide a buffer during startup. In such a configuration, the battery would take up the load while the CCGT starts up. A large, non-agile plant could then become dispatchable as a peaker.

This could be an economic option, given the rest of the equipment already in place, such as step-up transformers and tie-ins to transmission grids.

Ultimately, owners of aging and modern CCPP assets need to think flexibly and creatively, so that they can respond to the uncertainties of the energy transition—rather than clinging to past business models that are no longer relevant.