ASME TURBO EXPO 2000:

Land, Sea, and Air

                                May 8-11, 2000 Munich, Germany

2000-GT 0596

INJECTING HUMIDIFIED AND HEATED AIR TO MEET PEAK POWER DEMANDS

Abstract

The method for combustion turbine (CT) power augmentation denoted as CTHAI, (HAI is an acronym for Humidified Air Injection) allows production of additional megawatt-hours (MWH) at low cost during peak demand periods by injecting externally compressed, humidified, and heated air into a CT upstream of the combustors. Energy Storage and Power Consultants (ESPC) and Parsons Infrastructure Group (Parsons) conducted a study for Tennessee Valley Authority (TVA) on the power augmentation of the new PG7121EA combustion turbine by implementing the CTHAI concept.  Results of the study demonstrated significant power augmentation and heat rate reduction over the whole range of ambient temperatures, and it was shown to be the most effective at high ambient temperature conditions when reduction in normal unit output is severe (References 1 and 2). Following these developments, TVA, after exhaustive evaluations and analyses of the CTHAI power augmentation technology and other competitive power augmentation options, decided to demonstrate this concept on one of their operating PG7661 gas turbines at the Colbert Power Plant.

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 a CT plant 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 affected by increased inlet temperature. The CTHAI demo unit will be the unit designated as Colbert 4. The Colbert plant has a total of eight (8) operating units, and TVA has twenty-eight (28) combustion turbines (CTs) of this type, which have been in operation since the late 1970s. The importance of this project, vs. the project based on the new PG7121EA is in the fact that it demonstrates the effectiveness of the retrofit of relatively old, inefficient CTs, which practically have not been dispatched by TVA, into higher power and more efficient CTHAI plants. The CTHAI project is in the advanced stages and turnkey proposals for the conversion of GE PG7661 into the CTHAI concept are scheduled to be delivered by mid-January 2000. This paper presents a summary of the project developments.

Performance Characteristics of the CTHAI Plant based on PG7661 

The heat and mass balance for the CTHAI plant based on PG7661, operating at TVA-specified conditions, which are 95^oF (35^oC) ambient temperature, 60% relative humidity and 464 ft. (141 meters) elevation, is presented in Figure 1.  This figure also shows that the CTHAI plant consists of the following major components:

·         Commercial combustion turbine (for this demo project, the existing PG7661 gas 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 CTHAI concepts are similar to the steam injection concept for power augmentation, which has accumulated significant operating experience.  Potential points of the humid air injection are also similar to the steam injection points: compressor discharge plenum, combustor cylinder, etc.

·         Supplementary compressor, with inlet air filter (consisting of commercial off-the-shelf compressor or standard compressor modules) to provide the supplementary air flow up-stream of combustors

·         Saturation column for the supplementary air humidification and preheating

·         Heat recovery unit (HRU),  consisting of water heater and the saturated air heater

·         Balance-of-plant equipment and systems including interconnecting piping, valves, controls, etc.

·         Modeling of the CTHAI concept is based on the PG7661 performance model and performance characteristics of all incremental components - the compressor, saturator, HRU, pumps, etc., as quoted by the component suppliers.

Expected performance characteristics of the CTHAI plant at full power augmentation, based on PG7661 and the CT at four ambient temperatures 40 ^oF, 59 ^oF, 95 ^oF and 105 ^oF, are presented in Table 1. The definition of full power augmentation is the operation with injection of the maximum allowable humid airflow. This flow could be limited by many factors.  The most important are the compressor surge (affected by the total injected mass) and the combustion stability and emissions (affected by the humidity level). The specified parameters (the injected flow and humidity level) are presumed safe for the injection into the CT at 95 ^oF (35^oC). Based on information provided by General Electric (GE), concern lies with the total moisture up-front of the combustors, which is the moisture in the CT's compressor inlet air plus the moisture in the injected humid air. For the retrofit of old CTs, one shall consider the actual equipment performance and conditions deterioration for proper assessment of the maximum allowable power augmentation with the CTHAI concept. An analysis of all these factors indicated that the maximum allowable electric generator power of 71 MW at 95 ^oF (35^oC) is the limiting factor for  the maximum power augmentation of the CTHAI plant. As it was stated above, the HAI equipment and systems are optimized for the 95 ^oF(35^oC) operation, and operations at other ambient temperatures are considered as off-design operations.

Table 1 shows that at 95 ^oF (35^oC), which is typical for TVA's summer peak conditions, the CTHAI concept increases the CT power from approximately 49 MW to approximately 65 MW, i.e. by 30%. This power augmentation is complemented by the heat rate reduction from 11,800 Btu/kWh (12,500 kJ/kWh) to 10,250 Btu/kWh (10,800 kJ/kWh) i.e. by 15%. The table also shows that the CTHAI concept results in a significant power augmentation and heat rate reduction over the wide range of ambient temperatures.

Figure 1.   Heat and mass balance of CTHAI Plant at Design Condition.

Table 1

Performance Characteristics of the CTHAI Plant Based on PG7661 and PG7661 at Various Ambient Temperatures

 The total NOx emissions (lbs/year) permit is very often the most restrictive operating permit.  It is reasonable to expect that the CTHAI concept, as compared to the CT, both operating at the same ambient temperature will have the same NOx ppmv emissions (due to approximately the same fuel/air ratio). Still, it should be expected that the CTHAI plant will have lower NOx emissions in lbs/kWh - proportional to fuel consumption reduction (heat rates reduction) as it is shown in Table 1. This leads to the very critical conclusion that for the same NOx emissions (lbs/year), the CTHAI concept allows the production of approximately 15% more kWh of electrical energy (as related to operations at 95 ^oF ambient temperature). In principle these conclusions are generic for the CTHAI concept regardless of CT type and operating temperature. Tests of combustors operating with the humid air (Reference 3) demonstrated very significant NOx reduction from 150 ppmv for "dry" operations to single digits for the humid air operations. These findings are applicable to the CTHAI concept and are expected to further reduce NOx emissions.

Part Load Operations. Start-up

Part load performance characteristics of the CTHAI plant for operation at 95 F ambient temperature are presented in Fig.2.  It demonstrates, for the same loads, a significant CTHAI efficiency improvement as compared to the CT. These finding lead to very important conclusion that it is more efficient to operate the CTHAI concept not only when power augmentation is needed, but also at loads well below the CT design point power.

The following HAI start-up conditions of the HAI equipment and systems are identified as typical and guiding:

·         The HAI equipment and systems start-up for power augmentation when the CT operates at the full load

·         The HAI plant start-up when the CT is operating at a minimum stable loads (typical minimum load limited by emissions).

Figure 2. CTHAI Plant Part-Load Performance Characteristics at 95 F Ambient Temperature

Engineering and Capital Costs

Incremental costs to convert a CT into a CTHAI plant 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 unit cost

·         costs of interconnecting piping, valves, electrical and controls for the overall system integration

The presented power augmentation concept is relatively simple in terms of engineering and construction. The preliminary engineering indicated that the supplementary compressor and saturator are off-the-shelf standard equipment and could be delivered to the site fully assembled and skid mounted (compressor) by a number of OEMs contacted by ESPC. The connection to the CT is similar to the steam injection. The significant consideration is the integration of the supplementary compressor controls with the control system of the CT.

Estimated specific incremental cost for equipment and systems required for the conversion of the PG7661 into the CTHAI plant is approximately $200/ incremental kW (as it relates to the incremental power at 95 F, see Table 1). This favorably compares with approximately $400/kW specific cost for a turnkey installation of similar combustion turbine (at 90 F ambient temperature).

O&M of CTHAI plants are expected to be lower, as compared to the CT, because incremental equipment and systems are relatively simple, proven and low maintenance components.

Conclusions

The project findings lead us to the following conclusions:

·         Retrofit of the PG7661 into CTHAI will result in approximately 30% power augmentation and 15% heat rate reduction as it relates to 95F ambient temperature operations.

·         Power augmentation and heat rate reduction take place over the wide range of ambient temperatures.

·         Estimated specific incremental capital costs for conversion of a CT into CTHAI and CC into CCHAI are approximately $200/ incremental/kW as estimated for the power increase at 95 F.  These specific costs are small fractions as compared to the cost for purchasing a new CT, and are significantly lower than other power augmentation alternatives.

·         The heat rate reduction will contribute for much more frequent dispatching of the CTHAI plants by TVA as compared to original PG7661 plants.

References

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

2.        "Humidified Air Injection Raises Peak Turbine Output," Power Engineering, November 1999.