How Do Modern HRSG Designs Optimize Plant Energy Efficiency?Follow article
Heat recovery steam generators (HRSGs) play a vital role in power plant efficiency. Thus, people have progressively engineered design improvements so that they operate with lower energy consumption. Here are some of the considerations that can make noticeable differences when aiming to reach that goal.
Incorporate Austenitic Stainless Steel Into the Design
Gas turbines become more efficient as their combustion temperatures rise. Thus, the turbines that use energy most effectively require the HRSGs to withstand super-heated steam. However, making strategic selections for the tube material and length can prevent oxidation and reduce the likelihood of fouling.
Conventional steels used in HRSGs typically show oxidation on the steam side of the tube. However, many modern HRSG designs include austenitic stainless steel in the hottest parts of the HRSG to prevent that effect.
There’s also a preference for longer HRSG tubes. Many of the largest HRSGs are also the most cost- and energy-efficient. However, rather than having welded joints in straight tube sections, newer HRSGs more often have longer, joint-free tubes.
Austenitic Stainless Steel in Action
Several years ago, a power plant in Northern France achieved 62.22% combined cycle fuel efficiency, setting a new record. Austenitic stainless steel featured prominently in the HRSG designs used at the site.
For example, the superheater and reheater sections of the HRSGs featured that material instead of carbon steel. The austenitic stainless steel also included niobium (Nb) for increased stabilization.
Moreover, the HRSGs included TP347H stainless steel in the superheater and reheater tube headers and connectors. That example shows that the ideal approach is to assess when using austenitic stainless steel is most appropriate while simultaneously choosing other complementing materials to get the desired results.
Ensure the HRSG Design Supports Using Analytics
Enhancing power plant efficiency often means depending on analytics to learn more about the causes behind any decreased performance. That may mean verifying that the HRSG’s design supports connected sensors.
A 2020 McKinsey & Company report profiled how the power-generation industry must adapt with the help of technology to stay relevant in a changing marketplace. One of the options suggested in that document was applying sound-monitoring sensors to detect leaks in HRSG tubes. Then, responsible parties can address those issues before they cause performance decreases.
These solutions generally function by continuously monitoring equipment noises. When sounds surpass a predetermined threshold for an extended period, the appropriate parties receive alerts. People initially used such sound-monitoring options for conventional boilers and have done so for several decades.
However, the solution more recently became a viable choice for HRSGs, too. That’s because it alerted people to leaks faster than conventional methods could. By getting warnings of unusual changes and sounds, plant managers can prevent the economic and productivity-related costs of emergency HRSG shutdowns.
Install Sound Sensors on Attemperator Valves
Another potential leak-detection application for HRSGs involves using acoustic sensors on the attemperator valves to warn of water leaking through closed valves. If water drips from the attemperator’s nozzle during times of low steam flow, mechanical failures could occur when the liquid touches hot steam tubes and pipes.
Then, those unexpected breakdowns would have a progressively detrimental impact on overall efficiency. Plus, leaks could negatively affect the overall energy used.
Understand the Link Between Maintenance and HRSG Efficiency
The most innovative HRSGs still require users to carry out periodic maintenance. For example, deposit buildup on the finned tubes of an HRSG causes fouling. Severe cases can cause unplanned shutdowns and wasted money.
Backpressure is a common fouling side effect. When left unaddressed, this problem can cause turbine trips. It also hurts a company’s profitability. Statistics suggest that businesses experience $100,000 annual reductions in their bottom lines for every half-inch of backpressure on an F-class HRSG.
Gas-side fouling can reduce an HRSG’s heat absorption effectiveness. One practical way to stay abreast of energy-efficiency changes related to fouling is to use thermal modelling and develop a procedure to track performance degradation over time. The thermal model can also help professionals verify the difference in heat absorption before and after cleaning occurs.
The traditional approach is to schedule gas-side cleaning to happen during annual shutdowns. However, that system does not give power plant workers the data they need to spot inefficiencies before they become severe.
Investigate Predictive Maintenance Opportunities
Using maintenance best practices for energy efficiency also involves assessing which HRSG components have higher-than-average failure rates. Issues with the steam traps can decrease energy efficiency after getting stuck open or closed.
One study found that 18% of a chemical manufacturer’s steam traps could fail in a year. In that case, the problem causes up to $16,000 of wasted energy per trap. However, sensors exist that can interpret the sounds of traps periodically opening to release condensate. People can then view data from all the traps on a centralized dashboard, allowing them to look into signs of trouble quickly.
That’s one example of predictive maintenance applied to HRSGs, but it’s not the only option. When General Electric opened its Topolobampo III power plant in Mexico, that facility’s setup included two HRSGs. GE designed that equipment to work with an industrial real-time data platform. It let people immediately clarify how current operating conditions differed from historical performance.
Design the HRSGs to Start Faster and Look for Process Delays
People who design HRSGs are also interested in ways to make them start faster after shutdowns. One expert from Vogt Energy presented a talk on the subject concerning drum-type HRSGs. They noted that cold starts consume seven times more drum life than warm ones. A suggested way to address that problem was to install a stack damper to help keep the HRSG warm during shutdowns.
However, a relatively new product from Siemens called the DrumPlus HRSG reveals other design decisions that allow for faster starts. First, the water-steam separation occurs in two stages, with the later one happening in bottles outside the steam drum. The dual-stage functionality reduces the steam drum’s diameter and wall thickness.
This design also contributes to better power plant efficiency. For example, an average F-class HRSG generates approximately 20 megawatt-hours (MWh) in the first 40 minutes after startup. However, this model creates almost six times more energy in the same period.
Check for Preventable Process Slowdowns
Some power plant managers also realize that taking a closer look at how the broader system operates can reduce startup times. More specifically, people can scrutinize operating procedures and identify characteristics that cause delays.
In one case, personnel at a plant took that approach and saw that the necessity of using steam turbine seals prevented bypassing steam to the condenser earlier in the process. They deal with that problem by finding an earlier steam source using an available let-down station. That change caused an approximate 20-minute reduction in the startup timeframe.
Better Power Plant Efficiency Requires a Dedicated Approach
While there is no universally guaranteed way to boost efficiency through HRSG design, the best approaches may involve choosing different materials, ensuring the design supports analytics sensors or sticking to a more regular maintenance schedule.
People are more likely to get optimal results by choosing the goals they want to meet. How does the company define energy efficiency, and where does room for improvement currently exist? Confirming those all-important details allows moving forward with approaches that are most likely to cause the desired results.