INJECTING HUMIDIFIED AND HEATED COMPRESSED AIR TO MEET

PEAK POWER DEMANDS

Dr. Michael Nakhamkin and Boris Potashnik, ESPC and Ronald H. Wolk, WITS

Combustion Turbine Peak Power Augmentation at a Fraction of CT Costs

Additional megawatt-hours (MWH) can be obtained at low cost during peak demand periods from gas turbines and combined cycle power plants by injecting the supplementary air flow, which is externally compressed, humidified, and heated, into a combustion turbine (CT), up-stream of combustors. This novel approach is denoted as CT-HAI, (HAI is an acronym for Humidified Air Injection) for simple cycles and CC-HAI for combined cycles. It results in significant power augmentation over the whole range of ambient temperatures, but it is the most effective at high ambient temperature conditions when reduction in normal unit output is most severe. These new concepts are a further extension of the power augmentation method presented in Power Engineering, March 1999, pages 38-40. ESPC and Parsons Infrastructure and Technology Group are in the final stages of a project, commissioned by Tennessee Valley Authority, for evaluation of the application of the CT-HAI novel method to a number of 7EA General Electric combustion turbines at Gallatin and Johnsonville power plants. Therefore, for illustration purposes only, the performance, operational and cost data below are presented for these types of CTs. It is obvious, that this novel method could be applied to any combustion turbine or combined cycle plant as long as the compressor surge margins and other CTs constraints are properly recognized.

It was determined in the TVA study that, for 90 ºF ambient temperature (the specific temperature specified by TVA), the conversion of a typical GE7EA CT into CT-HAI increases the power from 75.9 MW to 103 MW and reduces the heat rate from 10630 Btu/kWh to 9350 Btu/kWh. Installed incremental specific costs (applied to an incremental power at 90 º F) were estimated at $150/ incremental kW. Moreover, it was determined that the CT-HAI concept allows maintaining the plant constant power over the whole range of ambient temperatures.

Though it is recognized that peak power augmentation is more relevant to CTs, the technology development team looked at this concept application to CC-HAI plants. It was determined that, for 90 ºF ambient temperature, the conversion of a typical GE 7EA based CC plant into CC-HAI increases the power from 123.4 MW to 141 MW with practically the same heat rate of about 7000 Btu/kWh. Installed specific incremental costs at 90 ºF for the CC-HAI plant are estimated at $220/ incremental kW.

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, it was determined that the incremental power comes almost instantaneously, the CT-HAI concept could be fully automated and remotely controlled, with minimum interference with CT operations.

The CT-HAI concept has significantly lower heat rates over a wide range of loads from the maximum power down the CT minimum power, by properly injecting the humidified air. This should significantly increase hours, when the plant is dispatched, thus improving the plant economics.

For all other equal considerations, the ability to generate an essentially constant amount of power over a wide ambient temperature range should significantly improve economics of CT-HAI and CC-HAI plants relative to CT and CC plants, respectively.

Background

For most electric power customers, power demands reach their peak during summer, when high ambient temperatures result in a reduction of the power output of CT and CC plants to the minimum. The simplified explanation for reduced power production is that lower inlet air density, a result of the high ambient temperature, reduces mass flow through a CT with a respective reduction of the power produced. Also, power consumption by the compressor is increased with increased inlet temperature. In the aforementioned article published earlier this year (see Power Engineering, March 1999, pages 38-40), we discussed the first version of the novel power augmentation concepts with the supplementary air flow injection. The 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 the wide range of ambient temperatures. The CT-AS and CC-AS concepts are based on power augmentation, with introduction of the supplementary compressed air flow from a storage facility, upstream of combustors, in order 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, which 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.

CT-HAI Augments CT Power by 35%, Reduces Heat Rate and NOx Emissions by 12-14%

Figure 1 is a simplified heat and mass balance of the CT-HAI plant, which consists of the following major components:

1. a commercial combustion turbine with the provision to inject, at any point upstream of the combustor, the externally supplied humidified and preheated supplementary compressed air. Engineering and mechanical aspects of the air injection for CT-HAI concepts are similar to the steam injection for the power augmentation, which has accumulated significant operating experience
2. a supplementary compressor (consisting of commercial off-the-shelf compressor or standard compressor modules) to provide the supplementary air flow up-stream of combustors
3. a saturation column for the supplementary air humidification and preheating
4. heat recovery water heater and the saturated air preheater
5. balance-of-plant equipment and systems including interconnecting piping, valves, controls, etc

Figure 1 GE 7EA Combustion Turbine Performance at 90 ^oF with Externally Compressed, Humified and Heated Air

Performance characteristics for the CT-HAI plant are presented in Table 1 (for a range of ambient temperatures) and in Fig. 1 (for 90º F ambient temperature). The major features of this concept are summarized as follows (see Figure 1):

• the CT-HAI power is increased (vs. the CT), due to the injection of the humidified supplementary air flow up-stream of combustors
• the heat rate is reduced due to both the significant reduction of the power consumption by the supplementary air compressor and due to the utilization of exhaust heat.
• the supplementary air flow is humidified to 60% RH, which (for the same the maximum mass of fluid injected) reduces the mass of supplementary air flow required to achieve the desired level of power augmentation
• the compressor size, costs and power consumption are reduced; the latter is of particular importance, because this compressor operates simultaneously with the CT.
• the hot water required for the humidification can be produced in the aftercooler or by utilizing the exhaust gas heat.

Table 1

Comparison of 7EA CT and CT-HAI

Total NOx emissions (lbs/year) are typically the most restrictive operating permit for CT and CC plants. It is reasonable to expect that the CT-HAI concept, as compared to the CT, both operating at the same ambient temperature, will have approximately the same NOx ppmv emissions (if approximately the same fuel/air ratio is maintained). Still, CT-HAI has lower NOx emissions in lbs/kWh (by approximately 12 -14%, at 90 ºF) due to lower heat rates (see Table 1). Increasing the injected air temperature higher than 700F presented on Fig. 1 – all the subject of economics, could further reduce the CT-HAI heat rate. This leads to the very critical conclusion that for the same total permits for NOx emissions (lbs/year), the CT-HAI concept allows the production of approximately 12-14 % more kWh of electrical energy (as relates to 90 ºF). Though these particular numbers have been estimated for the particular CT and the 90 ºF ambient temperature case, the conclusions are generic for various temperatures and combustion turbine types.

As it was mentioned, the augmented power is delivered almost instantly with the start of the supplementary compressor.

The CT-HAI concept has significantly lower heat rates over a wide range of loads from the maximum power down the CT minimum power. An analysis demonstrated that by properly injecting the supplementary humidified air into the CT-HAI plant, it operates with significantly reduced heat rates (as compared to the CT) at loads well below the CT design point. This should significantly increase hours, when the plant is dispatched, thus improving the plant economics.

Quality of make-up water for the humidification has been the subject of very serious analyses, conducted by a number of experts. It was demonstrated that there is no need for the demineralizer and average industrial type quality water could be used as make-up.

CC-HAI Augments CC Power by 14%

Figure 2 is a simplified heat and mass balance of the CC-HAI plant. The additional equipment that is required for converting a CC plant to a CC-HAI plant is the same as that for the CT conversion. In order 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) and in Fig. 2 (for 90 F ambient temperature). 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). As a result CC-HAI results in the incremental power of 17.6 MW, which represents approximately 15% power augmentation as compared to the CC plant. Due to practically the same heat rate for CC-HAI, as for a CC plant, NOx emissions are expected to be the same.

Figure 2 GE 7EA Combined Cycle Performance at 90 ^oF with Externally Compressed, Humidified and Heated Air

Table 2

Comparison of 7EA CC and CC-HAI

CT-HAI and CC-HAI Specific Incremental Capital Costs are Approximately 50% of

Those for CT and CC, Respectively

Incremental costs to convert a CT into a CT-HAI plant (almost the same for the conversion of CC into CC-HAI) are as follows:

• costs of the combustion turbine modifications, if required, to provide for the compressed air injection upstream of combustors
• compressor train cost
• saturation column cost
• heat recovery water heater and the saturated air preheater costs
• 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-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. The connection to the CT is similar to the steam injection. A significant consideration is proper integration of the supplementary compressor controls with the control system of the CT.

Estimated specific incremental cost for equipment and systems (with over 70% of cost based on commercial quotations) required for the conversion of the Frame 7 EA combustion turbine CT plant into the CT-HAI plant is approximately $150/ incremental kW (as it relates to the incremental power at 90 ºF, see Table 1). This compares favorably with approximately $400/kW specific cost for a turnkey installation of similar combustion turbine (at 90 ºF ambient temperature). Converting a GE 7EA-combined cycle plant to a CC-HAI plant will cost approximately $220/incremental kW, which actually presents the same fraction of specific costs for CC plants operating at the same temperature.

Conclusions

The CT-HAI method allows the increasing of power output of CT plants over the whole range of ambient temperatures, practically keeping it constant and approximately equal to the maximum power output quoted by an OEM for the lowest ambient temperatures. For example, the CT-HAI plant based on the 7EA CT during summer peak hours (with 90 ºF ambient temperature) could increase power by 27 MW, which is more than 35 % increase. At a higher than 90 ºF specified summer peak ambient temperature the power increase will be higher, with corresponding improvements in economics of this method.

The CT-HAI heat rate is reduced by 12-14% (as related to the total power output) as compared to the CT at the same 90F temperature. Increasing the supplementary humidified air temperature in the preheater, if economically justified, could further reduce the heat rate.

The CT-HAI concept has significantly lower heat rates over a wide range of loads from the maximum power down the CT minimum power, by properly injecting the humidified air. This should significantly increase hours, when the plant is dispatched, thus improving the plant economics.

Also, due to reduced heat rates, the CT-HAI can produce 12-14% more kWhs for the same NOx permits in lbs/year. The incremental power is delivered almost instantaneously. The plant could be fully automated and remotely controlled.

The CC plant, based on the same CT, converted to CC-HAI, could have power increased by approximately 15%, due to the considerations above.

Estimated specific incremental capital costs for conversion of a CT into CT-HAI and CC into CC-HAI are approximately $150/ incremental/kW and $220/ incremental kW, respectively, as estimated for the power augmentation at 90 F operations. These specific costs are approximately 50% of costs for the purchase of a new CT or CC plant, respectively. CC-HAI net heat rate is practically the same as compared to the CC plant.

O&M of CT-HAI and CC-HAI plants are expected to be lower than 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 practically off-the-shelf components and can be provided skid mounted by a number of service providers.

TVA’s economic analysis of the CT-HAI economics strongly indicated that aforementioned features of CT-HAI result in significantly better economics as compared to other considered alternatives.

Authors

Dr. Michael Nakhamkin is the president and founder of Energy Storage and Power Consultants, Inc. (ESPC), P. O. Box 1337, Mountainside, N. J. 07092 (e-mail espcinc@espcinc.com)

Boris Potashnik, Consulting Engineer, ESPC

Ronald H. Wolk is the principal of Wolk Integrated Technical Services (WITS), 1056 Hyde Ave., San Jose, CA 95129 (e-mail ronwolk@aol.com)

References

1. "Compressed Air Inflates Gas Turbine Output," Power Engineering. March 1999.

2. Method of Operating a Combustion Turbine Power Plant at Full Power at High Ambient Temperatures or at Low Air Density Having Compressed Air Storage." US Patent # 5,934,063.

3. "Method of Operating a Combustion Turbine Power Plant at High Ambient Temperature or at Low Air Density Using Supplemental Compressed Air." US Patent Application Allowed #09/281.776.

This information was also presented in an article in the November 1999 issue of Power Engineering.