Operating and Design Modifications Improve HRSG Efficiency

During the technical sessions at the 2025 Western Turbine Users Inc. (WTUI) conference in Long Beach, CA, Jack Odlum, the Northwest Regional Manager at HRST Inc., outlined how operating and design modifications improve a heat recovery steam generator’s (HRSG) efficiency. His session focused on efficiency from two perspectives: operational efficiency or to ensure the HRSG is running reliably in an ever-changing market, and thermal efficiency from the classic heat-rate point of view.

Odlum said it’s surprising how even small changes impact HRSG efficiency. “Things like baffle gaps or tube fouling might seem like no big deal, but as they add up, it can mean lost MWs of production in larger HRSGs,” he said. “Not every efficiency-boosting modification needs to be something like adding an economizer surface or installing an upgraded duct burner. Sometimes, it’s as simple as keeping the HRSG in close to as-built conditions as possible. When it comes to operational efficiency, it’s more directed at running the plant well in an evolving power market, ensuring you can get online fast enough with as little damage to the boiler as possible.”

Determining inefficient operations depends on what the HRSG is doing. Odlum said for common power-plant applications, efficiency is characterized by heat rate. “Are we still getting the same MW output for the fuel we send to the combustion turbine and duct burners? If not, the bottom line suffers,” he said. In applications where the HRSG is used for cogeneration, lost efficiency is largely seen in a drop in steam production. How will this impact the steam host? Will auxiliary boilers be needed to make up for the lost production from the HRSG?

So how do you improve efficiencies? There is no one-size-fits-all component to improve HRSG efficiency; each has a specific set of needs, but generally speaking, Odlum said good gas baffling can be globally applied, and replacing aging components like penetration seals and expansion joints eliminates gas-side leakage. “Making sure superheater and reheater attemperator controls are stable and optimized for the most routine operating points will help. The less spray that is needed to control final steam temperatures, the more steam turbine generation capacity is available."

Investigating further could reveal that some HRSGs may benefit from added economizer sections with built-in bypass for certain loads, but again, this is not a one-size-fits-all.

In terms of the exhaust gas side, “The worst performance thieves in the gas path are generally baffling and tube fouling,” Odlum said. “Good baffling keeps the gas flowing through the finned tubes where the heat can be extracted. Bypass gaps have much less flow resistance, so a disproportionate amount of gas will miss the heating surface through these paths.”

He said that even when the baffling is good, you have to ensure the finned tubes' heat transfer surface is in good condition. “Fouling, such as rust or ammonia salt buildup, has a detrimental impact on heat transfer, which can be noticeable when it accumulates over years and across multiple bundles/pressure parts.”

HRSG Trends

Odlum said one trend he’s seeing out West and beyond in HRSG operation and design is cycling. “I came into the industry after the boom in the mid-00s, but the sites I visit often tell the same story: ‘We weren’t built to start this much’ and the variety of issues it leads to in the boiler. Newer HRSG designs seem to follow this, as things have been done to increase the fast start capability via innovative designs at the points weakest to cycling. This may mean placing the steam drum separation equipment outside the steam drum, allowing the drum itself to be smaller in diameter (and thus thinner shelled), or an attemperator being upgraded from a single probe style to a multi-nozzle radial injection type for better atomization.”

A second trend he sees is challenges with the size of newer turbines, like the H-class machines. “These scaled-up HRSGs are beginning to age and show their own unique problems.”

Regardless, Odlum said it’s important to stay updated on industry trends. “These conferences are a unique way to share lessons learned from the field that HRSG users can then apply to their boiler equipment to help ensure reliability and increase operational flexibility. Sometimes, changes to operational conditions or parameters may necessitate the creation of a detailed thermal model of the HRSG/boiler, and HRST can assist with that.”