When you think of a gas turbine, your mind’s eye sees a big piece of equipment that doesn’t seem like it would be very flexible. And, generally it is not. However, with the growth of renewable energy and technological innovations, the business of power production and the equipment used to generate power are profoundly changing. There are several techniques and programs discussed in this section that help gas turbines become more flexible to the load demands, the environment and even the weather.
The objective of this system is to continuously monitor and automatically optimize the performance of the DLN combustion system as it responds to:
- Changes in seasonal ambient conditions
- Degradation of combustion hardware over its maintenance cycle
- Degradation of gas control valve calibrations
- Changes in fuel Wobbe index (composition or temperature) within limits specified in GE’s gas fuel specification (+/-5%)
The specific performance benefits achieved by the auto-tuning system are:
- Limiting wear and mechanical distress of the combustion hardware caused by combustor dynamic pressures
- Preventing lean blow out of the combustor
- Maintaining NOx & CO emissions within design or regulatory compliance limits
Based on the customer’s specific requirements and past operating history, different priority can be given to each of the above three factors. For example, a combined–cycle site which has an SCR and has previously experienced damaged combustion hardware may want to prioritize dynamics reduction over minimizing NOx because excess NOx emissions can be reduced by the SCR. However, simple-cycle peaking units subject to stringent emissions regulations must prioritize keeping NOx below 9 ppm ahead of minimizing dynamics. Prevention of lean blow out is given high priority in all instances.
The auto-tuning system can be installed on units having either lean-PM1 or rich-PM1 operating configurations; it can also be implemented on units that either do or do not use quaternary fuel.
Our field engineers have extensive experience in tuning and troubleshooting more than 80 GE F-class gas turbines. Additionally, we employ engineering staff that were involved in the development of the DLN-2.6 combustion system at GE. Our broad practical experience and in-depth design knowledge of the DLN-2.6 combustion system are incorporated into the auto-tuning system algorithms. This breadth and depth of knowledge are necessary for developing an auto-tuning system that can handle the wide range of combustion system behaviors seen throughout the GE 7FA fleet.
The auto-tuning system is comprised of four main components:
- Auto-Tuning Software Program
- Combustion Dynamic Monitoring System (CDMS)
- Continuous Emissions Monitoring System (CEMS)
- Turbine Controller (Mk V / VI)
Fast Start Program (Any Turbine Model)
Get going faster!
We can review the current configuration of site units and provide information on the issues, benefits and costs associated with each of these changes. Our team can also implement these changes including any associated mechanical, electrical or software changes.
The US Department of Energy defines fleet flexibility as the ability of the generation fleet to change its output (ramp) rapidly, start and stop with short notice and achieve a low minimum turn-down level. For “F” units, faster starts (increased flexibility) can be achieved by implementing some or all of the following changes:
- Modifying Fuel System to Obtain a Purge Credit Capability
- LCI (Load Commutated Inverter) Pre-Connect
- Ignition During Acceleration (Fire on the Fly)
- Eliminate Turbine Warm-Up
- Fast Sync
- Increased Acceleration Rate
- Increased Loading Rate (Simple Cycle)
Base Load Performance Optimization
Attaining each unit’s full potential
It is common for the OEM to sell a 7FA+e gas turbines with performance guarantees — both output and heat rate — that are significantly below the unit capability. The units get commissioned at these reduced performance levels and then, for hundreds of thousands of dollars, the OEM offers “uprate packages” that bring unit performance up to the level it was already capable of. These performance deficiencies can arise because the unit is either not operating at its maximum airflow or is firing below its rated firing temperature of 2420oF. Many 7FA+e gas turbines suffer from one or both of these base load performance deficiencies.
Our team offers analysis and testing that optimizes base load performance by bringing units up to their maximum airflow capability and rated firing temperature, while also maintaining acceptable combustion dynamics and emissions levels. We carry out preliminary performance analysis to determine current firing temperature and airflow. Onsite implementation involves controls modification to open IGVs and install new base load controls curves representing the rated firing temperature across the ambient temperature range. Testing is performed to benchmark the increases in output and efficiency relative to as-found conditions. Combustion tuning at base load is also performed to ensure the units meet emissions compliance limits while also achieving acceptable dynamics pressure levels.
Load Turndown Optimization
Extended Load Turndown in Mode 6
More operational flexibility, less unit cycling
Many 7FA DLN-2.6 turbines are commissioned with only 60 percent load turndown. We maximize turndown while also maintaining emissions compliance. Besides increasing operational flexibility, combined-cycle plants can avoid overnight shutdowns, reducing start/stop cycles and extending hardware life.
7FA gas turbines in 2-on-1 combined-cycle configurations incur increased life cycle costs if subject to load demand that requires one unit to be cycled offline on a frequent basis. One way to reduce the need for unit cycling is to enable the individual gas turbines — and, in turn, the combined cycle block as a whole — to turndown to a lower load.
Earlier vintage DLN-2.6 combustion systems were designed to operate with a “lean-PM1” split schedule in Mode 6, the low emissions (NOx and CO) operating mode. With lean-PM1 operation, load turndown in Mode 6 is typically in the range of 50-60%, depending on ambient temperature, part load control curve configuration and the thoroughness of DLN tuning at the low end of Mode 6. Below this load range, combustors with lean-PM1 split schedules will start to generate increased CO emissions, pushing them out of emissions compliance.
More recently, GE developed a new operational configuration employing “rich-PM1” split schedules. In Mode 6, rich-PM1 operation enables the combustion system to turndown to lower loads than the lean-PM1 system without experiencing increases in CO emissions. Typically, units employing rich-PM1 operation can achieve nominally 40% load turndown before CO emissions begin to increase.
Older units with lean-PM1 split schedules can be modified to operate with rich-PM1 split schedules and thereby achieve increased load turndown. This operational modification involves making changes to control logic but does not involve any hardware adjustments. In addition, complete retuning of the combustion system throughout the Mode 6 load range must be performed.
Peak Fire Operation
Extra power when you really need it
Depending on their power purchase agreement, gas turbine operators may receive payment based on their maximum generation capacity, or they may need to hold a certain amount of generation in reserve as a percentage of their maximum capacity. For these reasons, adding peak firing capability to both simple and combined cycle units can bring economic benefits, even if the peak fire capability is not utilized in normal operation.
For a 7FA+e, peak firing is nominally 35oF in firing temperature above base load. This increase equates to a peak firing temperature of 2455oF for a unit operating at its rated base load firing temperature of 2420oF. A 35oF increase in firing temperature is sufficient to achieve at least a 2.5 percent increase in output above base load for units which are not suffering from compressor or turbine performance degradation. For units operating at or below 9 ppm at base load, NOx emissions at peak load are expected to be less than 15 ppm.
Implementation of peak firing requires control system logic modifications, HMI modifications (to select peak fire) and combustion tuning necessary to install peak firing capability. Base and peak load operating data are acquired, using station instrumentation, to document the increase in output from peak firing. Combustion tuning will be performed to achieve the optimal balance of NOx emissions and dynamics resulting from peak firing.
Adjustable Peak Firing
More revenue, less emissions
A variation on the standard peak firing option is the adjustable option. This will allow operators to incrementally raise and set load at any point between the base load curve and the standard peak load curve. This capability allows the operator to increase load to take advantage of periods of high electricity prices while staying within the maximum allowable NOx emissions. This mode is especially useful for merchant plants with simple cycle units or with combined cycle units without SCR's.
The adjustable peak option is enabled from the HMI. Load will be adjusted manually, using the speed/load raise and lower buttons on the main control screen. The modified peak option will have a maximum firing temperature limit which will not exceed the standard 7FA peak firing temperature, which is 2455oF, 35oF above the rated firing temperature of the unit.
Implementation of peak firing again requires control system logic modifications, HMI modifications (to select peak fire) and combustion tuning necessary to install peak firing capability.
Cold Weather Optimization
More power, safe dynamics
To provide cold weather dynamics margin, 7FAs are commissioned with sharply de-rated firing temperature and output at ambient below 59°F. This de-rating often goes beyond what is required to achieve safe dynamic margin and offers the opportunity to appreciably improve output and heat rate on cold days.
We can undertake a review of the current unit settings and advise on how to increase output while maintaining safe dynamic margins and then make the control system modifications necessary to implement these changes.
Fogging/Wet Compression Integration and Optimization
Ensuring Optimum Output with Power Augmentation
The addition of a fogging or wet compression system to a DLN unit increases unit output and also impacts firing temperatures and DLN system operation.
When any power augmentation system is added to an existing unit, it’s important to undertake a review of the temperature control curves and make the necessary adjustments to ensure optimal operation of the unit under all possible operating configurations.
Having TTS undertake these calculations and implement the correct control strategies for each possible operating scenario will ensure that the unit complies with emissions and combustor dynamic requirements while always operating at the rated firing temperature.
Control Curve Evaluation and Design
Matching Control Curves to New/Modified Hardware
The upgrade and/or modification of hot section, combustion or compressor components can impact firing temperature and DLN system operation.
When any significant modification is made to an existing unit, it is important to undertake a review of the temperature control curves and make the necessary adjustments to ensure optimal operation of the unit. Having TTS undertake these calculations will ensure that the unit complies with emissions and combustor dynamic requirements while always operating at the rated firing temperature.
Plant Performance Assessment and Evaluation
Find the Missing Megawatts
The importance of maintaining optimum plant operation in a deregulated electric market is well understood. This is especially true in combined cycle plants where the complexity of the systems creates opportunities for significant underperformance and output loss. This can be caused by unexpected component performance degradation, inaccurate control instrumentation, conservative output and heat rate guarantee, faulty fuel calibrations, leaking valves and more. Identifying the cause of adverse performance operation and taking appropriate action can significantly improve overall performance and profitability.
The TTS team develops a software model of the plant which allows for identification of the optimum operating conditions and scenarios. Actual operating data can then be loaded into the model and actual operation can be compared to the optimum to identify shortfalls. Where significant underperformance is identified, additional analysis of the model allows for identification of the source of the underperformance. Rehabilitation strategies can then be identified and executed. The success of these strategies can then be assessed using the model.
Common issues causing underperformance include:
- Valve Leakage:
HRSG valve leakage can be hard to detect and can cause plant underperformance. A plant model can quickly identify this type of issue based on the differences in calculated pressure and temperature values around a correctly operating valve and actual measured values.
- Incorrect Fuel Flow Meter Calibration and Measurement:
This is a very common problem. The plant model can identify the amount of this error and remedial action can be taken if it is found to be significant.
- Incorrect Power Measurement Using Rotating Watt-Hour Meter or Digital Meter:
Again, a common problem that the model can identify and quantify.
- Exhaust Thermocouple Bias Issues:
The control system is designed to manage thermocouple biasing. However, variations in thermocouple materials can lead to errors in the bias and underperformance, which can be identified by the model.
- Compressor Flow Capacity Loss from Compressor Fouling or Damage:
The loss of compressor flow due either to fouling or to compressor damage will result in a loss of plant output. The model can quantify these losses and help identify the cause of the losses.
These are just a few examples of plant equipment issues that can impact plant performance. Of course there are many more. Our team offers the expertise to identify and quantify any deficiency.
Plant Performance Testing and Consulting
Ensuring that you are getting what is promised
TTS can provide the expertise and equipment to undertake a full unit (ASME PTC 22) or overall plant performance test (ASME PTC 46) as required for unit acceptance following commissioning or on other occasions when a complete performance test is required.
Our team can also support customers in developing requirements for plant testing and acceptance prior to purchase of equipment. There are a number of areas that information should be passed to the purchaser of plant equipment that later might reveal hidden problems or can be quite useful during your plant commissioning and start-up. For example, these areas can include factory test data, certain control input data and selected measurements planned for your plant.
The following are just a few of the subject areas where we can give you guidance:
- What data should you have up front on a contract signing?
- What should be in your plant’s test procedure, tolerances and selected measurements used during testing?
- Testing your plant: is control setup properly following the best procedures and expert witnessing?
- Plant operation: normal practices, washes, fuel contaminants, etc.
- Guarantee results: corrections, shortfalls and settling on liquidated damages.
These are just a few examples of areas to consider prior to purchase. Detailed initial review will ensure that important commissioning and guarantee procedures are not vaguely specified. This will also ensure real, verifiable and enforceable performance guarantee criteria which translate into increased value for the owner.
Life Cycle AGP Upgrade Evaluation 7F .03 to 7F .04
Does This Upgrade Make Sense for You?
The OEM is currently offering an upgrade from the existing 7F .03 unit configuration to an “advanced gas path” .04 configuration. They are highlighting the benefits associated with this upgrade but this is not a suitable or desirable upgrade for many users. TTS can assist customers in assessing this upgrade as it applies to their plant from both a commercial and technical perspective.
Commercial considerations vary depending on whether the owner is operating in a regulated or unregulated environment, on unit operational configuration and utilization, etc.
Technical considerations include the impact of the upgrade on the overall plant design. In particular, an HRSG evaluation verifies that the AGP upgrade does not result in the HRSG being pushed outside of its design parameters and analyzes the impact of resulting exhaust flow and temperature changes on the ST.
Correction Curve Development
Need Correction Curves?
In cases where a complete set of correction curves are not available, we can generate these curves allowing for expedited guarantee testing and performance testing.