How adjustable peak firing can improve your plant’s bottom line

Depending on your plant’s power purchase agreement, adding peak-firing capability to simple- and/or combined-cycle gas turbines can provide significant economic benefit—provided the peak-fire capability is used strategically at times of high demand to avoid unnecessary maintenance.

Recall that traditional peak firing increases the firing temperature a fixed, incremental amount above the rated baseload value. This increase can equate to at least a 2.5% bump in output above baseload for the latest GE units not suffering compressor or turbine performance degradation. Because an increase in firing temperature boosts NOx production, the degree of incremental firing achievable above baseload may be constrained by NOx emissions limits.

Adjustable peak firing can be a valuable tool where emissions exceed allowable limits before the gas turbine reaches its standard peak-firing limit. It allows you to increase load and take advantage of periods of high electricity prices while holding emissions within permit limits. This capability is especially meaningful for merchant plants equipped with SCRs.

When enabled, the adjustable peak-firing option from Turbine Technology Services Corp, included with TTS’s Dynaflex Performance™ toolkit, allows operators to incrementally increase output in steps of 0.1 to 0.2 MW. Note that the TTS adjustable peak option maintains an upper firing-temperature limit equal to the standard peak-firing temperature for the unit.

However, certain unit configurations or NOx permits may enable some DLN turbines using the adjustable peak tool to achieve incremental outputs above those possible when limiting the upper firing temperature to that recommended by the OEM.

Two scenarios illustrating the value of the TTS adjustable peak option follow:

    • A plant with 7FA DLN2.6 gas turbines restricted to NOx emissions of 9 ppm on a 30-day rolling average basis is allowed to release 10 ppm NOx on a 1-hr rolling average. Typical baseload NOx emissions for this owner’s units are about 8 ppm.

Enabling the TTS variable peak-firing solution on these units would allow the user to increase output above baseload by up to 2.5% while still maintaining NOx emissions at just below the 10-ppm, 1-hr limit. When used strategically, this capability would significantly increase revenue without putting the 30-day rolling-average limit of 9 ppm at risk.

    • In many locations, DLN gas turbines in combined-cycle systems use an SCR in the HRSG to reduce NOx emissions from 9 ppm to low-single-digit levels—2 to 5 ppm is typical. For these units, emissions permits put specific limits on NOx at the stack exit. Thus, use of variable peak firing on SCR-equipped units may allow operators to deliver more than a 2.5% increase in incremental output provided the plant has adequate ammonia injection capability.

Your interest in peak firing piqued, the first question you have might be: “What would I have to do to implement the peak-firing option at my plant?” Simple answer in general terms:

    • Verify your plant-specific air permit allows such modifications and obtain state approval.

    • Make some control-system logic modifications.

    • Modify the HMI screen to allow an operator to select adjustable peak fire.  

    • Make the combustion tuning adjustments necessary to install peak-firing capability.

Your second question might well be: “Who has done this and what were the results?”

One experience is shared in the case history below.

Case study

Rock Springs Generation Facility, equipped with four simple-cycle 7FA.03 gas turbines (744 MW), operates in the PJM capacity market and has NOx emissions limits of 9 ppm (30-day rolling average) and 10.5 ppm (hourly average). Cogentrix Energy Power Management LLC operates the Rising Sun (Md) facility for owner The Carlyle Group.

General Manager Ralph Jones, who has been at the facility since 2008, said that the plant began life as a joint venture between Old Dominion Electric Co-op (ODEC) and Essential Power LLC, an IPP. Jones was an Essential Power employee (operations side) before Carlyle acquired the facility and remembers being challenged by management to decrease operating expenses and potentially increase unit capacity.  

During that period the units required remote tuning seasonally, possibly more frequently, to control NOx or combustion dynamics. GE would perform remote tunes when requested, adding significant unplanned expenses. In many instances, a remote “tuner” was not available to immediately tune the engines. This caused the operator to reduce load to maintain compliance or correct combustion dynamics.

In 2014, Jones continued, plant staff was searching for a solution that would minimize having the units remotely tuned for a minor emissions or combustion dynamics issue. Turbine Controls and Excitation Group (TC&E), Yarmouth, Maine, was invited to the plant and John Downing made some small adjustments at baseload to PM1 and PM3, which were effective. But this would not be a viable solution over the long term because of its cost, so Downing and his team developed logic and an operator interface screen that enabled so-called Manual Adjust Fine Tuning (MAFT) through the DCS.

Using MAFT allows operators to adjust fuel flow by ±1% on PM1 or PM3 gas valves with a small set of written instructions. This way, operators can make adjustments when emissions are trending toward noncompliance to avoid a NOx excursion while also providing a means to suppress combustion dynamics within certain frequencies/tones. The method was very successful and minimized the number of remote tunings required and kept the units at baseload.

During the plant’s first decade, give or take a few years, the recommended way to increase peak output was by using GE’s peak-fire product. Jones described it as the equivalent of an on/off toggle switch. When a unit approached the emissions limit in “Peak On” mode, the operator would have to flip the switch to “Peak Off,” thus missing the commercial opportunity.

In 2015, Rock Springs asked TTS if it could provide an adjustable peak-firing system for its two gas turbines that would maintain combustion dynamics and NOx emissions during peak-fire operations. The design of the system would allow for simple “on the fly” adjustments by the operator. The TTS solution was implemented and delivered 4 MW of summer peak-load capacity per engine and worked in complete harmony with MAFT.

Fast forward to September 2018: The Carlyle Group purchased from ODEC the remaining two units at the site and hired TTS to install its adjustable peak-firing solution on those engines. The benefit was 3.5 MW of summer peak-load capacity each, bringing the total for all four units to 15 MW. In the PJM capacity market the 15-MW increase translates to significant financial gain.

One final note: TTS Senior Systems Engineer Mitch Cohen, well known to readers of CCJ ONsite for his many valuable editorial contributions over the years, was onsite at Rock Springs to analyze fuel-flow curves and other data required to develop the successful adjustable peak-firing logic modifications. Jones concluded his interview with the editors saying the project went off “without a hitch” and offered the owners an excellent return on their investment.

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