Additional Low-cost Power can be Obtained from Gas Turbines and Combined-cycle Power Plants during peak demand periods by injecting supplementary air-externally compressed, humidified and heated-into a combustion turbine (CT) upstream of the combustors. This novel approach, denoted CT-HAI (HAI is an acronym for humidified air injection) for simple cycles and CC-HAI for combined cycles, results in significant power augmentation over the whole range of ambient temperatures, but is the most effective at high ambient temperature conditions when normal unit output reduction is most severe. These new concepts are a further extension of the power augmentation method presented in Power Engineering‘s March 1999 issue.
ESPC and Parsons Infrastructure and Technology Group are in the final stages of a project, commissioned by Tennessee Valley Authority, to evaluate the application of CT-HAI to a number of 7EA General Electric combustion turbines at the Gallatin and Johnsonville power plants. Therefore, for illustration purposes only, the performance, operational and cost data below are presented for these specific CTs. This method could be applied to any combustion turbine or combined-cycle plant, however, as long as the compressor surge margins and other CT constraints are properly recognized.
The comparative analysis of the CT-HAI method vs. other power augmentation alternatives proved that this method results in the highest power augmentation at lowest specific incremental costs. In addition, the incremental power gain comes almost instantaneously, and the CT-HAI concept can be fully automated and remotely controlled, with minimum interference in CT operations.
The March 1999 Power Engineering article demonstrated significant power augmentation and performance improvement for both simple CT-AS (AS is the abbreviation for air storage) and combined-cycle CC-AS plants, for a wide range of ambient temperatures. The CT-AS and CC-AS concepts are based on the introduction of supplementary compressed air from a storage facility, upstream of combustors, to approach the CT’s maximum expander power (usually achieved at low ambient temperatures). As stipulated in the article, compressor surge is one of a number of considerations that could potentially limit the supplementary air injection and correspondingly, limit the expander power. The performance, cost and engineering data presented in this paper for the CT-HAI and CC-HAI concepts are based on some tighter compressor surge limitations than those assumed in the previous article. These latest surge limitations are similar to those for the commercially offered steam injection Cheng cycle, applied to the same CT. Any further limitations, if recognized, will slightly affect the amount of the power augmentation, but with a limited effect on the merits of this technology.
Figure 1 is a simplified heat and mass balance of the CT-HAI plant, which consists of the following major components:
- A commercial combustion turbine that can inject, at any point upstream of the combustor, externally supplied humidified and preheated supplementary compressed air. Engineering and mechanical aspects of the air injection for CT-HAI concepts are similar to steam injection technologies for power augmentation, which have accumulated significant operating experience.
- A supplementary compressor (off-the-shelf compressor or standard compressor modules) to provide the supplementary air flow upstream of combustors.
- A saturation column for supplementary air humidification and preheating.
- A heat recovery water heater and saturated air preheater.
- Balance-of-plant equipment and systems including interconnecting piping, valves and controls.
These components provide the CT-HAI concept with several unique features. CT-HAI power is increased (vs. a conventional CT) due to the injection of the humidified supplementary air flow upstream of the combustors. The heat rate is reduced due to significantly reduced power consumption by the supplementary air compressor and due to exhaust heat utilization. The supplementary air flow is humidified to the extent that the air flow represents approximately 40 percent of the total injected flow, with water vapor accounting for the remaining 60 percent. To achieve the desired level of power augmentation (associated with a particular mass flow injected), the CT-HAI method reduces, by about 60 percent, the required amount of supplementary air flow, per incremental kW, as compared to a CT or to CT-AS. In turn, compressor size, costs and power consumption are reduced. The hot water required for the humidification can be produced in the aftercooler or by utilizing the exhaust gas heat.
Make-up water quality for the humidification has been the subject of very serious analyses, conducted by a number of experts. These analyses have demonstrated that there is no need for a demineralizer, and average industrial-quality water could be used as make-up.
Performance characteristics for the CT-HAI plant are presented in Table 1 for a range of ambient temperatures. For the TVA application, at a 90 F ambient temperature, the conversion of a typical GE7EA CT into CT-HAI increases net power output from 75.9 MW to 103 MW and reduces the heat rate from 10,630 Btu/kWh to 9,350 Btu/kWh. These values include power consumption by the supplementary air compressor and all other recognized auxiliary requirements. Moreover, the CT-HAI enables the plant to maintain constant power output over the whole range of ambient temperatures.
The CT-HAI concept has significantly lower heat rates over a wide range of loads. By properly injecting the supplementary humidified air into the CT-HAI plant, significantly reduced heat rates (as compared to the CT) can be achieved at loads well below the CT design point. This should significantly increase hours when the plant is dispatched, thus improving plant economics.
Total NOx emissions (lb/year) are typically the most restrictive operating condition for CT and CC plants. When operating at the same ambient temperature and approximately the same fuel/air ratio, it is reasonable to expect that a CT-HAI plant’s NOx ppmv emissions will be about the same as those of a conventional CT plant. Still, CT-HAI has lower NOx emissions in lb/kWh (by approximately 12 to 14 percent at 90 F) due to lower heat rates (Table 1). Increasing the injected air temperature higher than 700 F could further reduce the CT-HAI heat rate if economics are favorable. This leads to the very critical conclusion that for the same permitted NOx emissions (lb/year), the CT-HAI concept allows the production of approximately 12 to 14 percent more kWh of electrical energy.
Though it is recognized that peak power augmentation is more relevant to CTs, humidified air injection is also beneficial for combined-cycle plants. Figure 2 is a simplified heat and mass balance of the CC-HAI plant at 90 F ambient temperature. The additional equipment required for converting a CC plant to a CC-HAI plant is the same as that for the CT conversion. To minimize changes to the standard bottoming cycle, the water heating for the humidification can be partially performed in the aftercooler and the additional recuperator coils could be optional.
Performance characteristics for CC-HAI are presented in Table 2 for a range of ambient temperatures. Though the injection of the supplementary humidified air increases the combustion turbine exhaust temperature, the power augmentation for the CC-HAI concept is lower than for the CT-HAI concept. This is because the bottoming cycle power is reduced due to the reduction of the available exhaust heat (used for the water heating for the humidification of the supplementary airflow). For a 90 F ambient temperature, the conversion of a typical GE 7EA based CC plant into CC-HAI increases the power from 123.4 MW to 141.0 MW, a 17.6 MW power gain that represents approximately 15 percent power augmentation as compared to the CC plant. The heat rate stays relatively constant around 7,000 Btu/kWh; as a result, NOx emissions in the CC-HAI plant are expected to be the same as those for a CC plant.
Incremental costs associated with converting a CT into a CT-HAI plant (almost the same for the conversion of CC into CC-HAI) include:
- costs for combustion turbine modifications, if required, to provide for the compressed air injection upstream of the combustors;
- compressor train cost;
- saturation column cost;
- heat recovery water heater and the saturated air preheater costs; and
- costs of interconnecting piping, valves and controls for the overall system integration.
These power augmentation concepts are relatively simple in terms of engineering and construction. Preliminary engineering analysis indicated that the main components-supplementary compressor and saturator-are off-the-shelf standard equipment. They could be delivered to the site, fully assembled, pre-piped and fully tested, by a number of suppliers contacted by ESPC. Connection to the CT is similar to procedures used for steam injection. A significant consideration is proper integration of the supplementary compressor controls with the CT control system.
The estimated specific incremental cost for equipment and systems (with more than 70 percent of the cost based on commercial quotations) required for the conversion of a Frame 7EA combustion turbine CT plant into a CT-HAI plant is approximately $150/incremental kW at 90 F. This compares favorably with the $400/kW approximate cost for a turnkey installation of similar combustion turbine (at 90 F ambient temperature). Converting a GE 7EA combined-cycle plant to a CC-HAI plant will cost approximately $220/incremental kW, which also represents a fraction of specific costs for CC plants operating at the same temperature.
Operations and maintenance costs for CT-HAI and CC-HAI plants are expected to be lower than for CT or CC plants, respectively, because incremental equipment and systems are relatively simple, proven and low maintenance components. The equipment and systems required for the conversion of CT and CC plants into CT-HAI and CC-HAI plants are based on off-the-shelf components and can be provided skid-mounted by a number of service providers.
- “Compressed Air Inflates Gas Turbine Output,” Power Engineering, March 1999.
- “Method of Operating a Combustion Turbine Power Plant at Full Power at High Ambient Temperatures or at Low Air Density Using Compressed Air Storage,” U.S. Patent Application #09/110.672.
Dr. Michael Nakhamkin is the president and founder of Energy Storage and Power Consultants, Inc. (ESPC).
Boris Potashnik is a consulting engineer with ESPC.
Ronald H. Wolk is the principal of Wolk Integrated Technical Services (WITS).