ASME TURBO EXPO 2003:

Land, Sea, and Air

JUNE 16-19, 2003 ATLANTA , GEORGIA , USA

GT 2003- 38977

Humid Air Injection Power Augmentation Technology has Arrived

Michael Nakhamkin / Energy Storage and Power Consultants, Inc.                              Robert Pelini / Struthers Wells Corporation

Manu I. Patel / Consultant

Abstract

This paper presents the latest information on Humid Air Injection (HAI) power augmentation technology for Combustion Turbine and Combined Cycle power plants. It describes:

a)       The summary of the latest activities on the implementation of HAI and Dry Air Injection (DAI) technologies including results of the validations tests conducted on the PG7241 (FA) combustion turbine, and findings of various CT-HAI implementation projects;

b)       The technical background including the latest CT-HAI and CT-DAI concepts resulting on the performance improvements and reduced emissions and

c)       The novel concept for humidification of the injected air that further reduces overall capital costs by 15%.

INTRODUCTION

It is well-known paradox that while for most electric power customers' power demands reach their peak during the summer, the power outputs of combustion turbine (CT) and combined cycle (CC) power plants are reduced due to high ambient temperatures. This motivated the recent trend for development and implementation of a number of power augmentation technologies. Extensive comparative analyses of the Humid Air Injection (HAI) power augmentation technology (CT-HAI and CC-HAI as applied to power augmentation of CT and CC plants, respectively) vs. competing alternatives, conducted by end-users, consultants and equipment manufacturers (Reference 1) demonstrated that the CT-HAI/ CC-HAI technology delivers the highest amount of power augmentation and has the best economics. It is relatively simple in its application for retrofits of existing and new CT and CC plants. For sites with a shortage of water resources the Dry Air Injection technology (CT-DAI and CC-DAI) could be the power augmentation technology of choice.

Some of the latest activities on implementation of the CT-HAI technology are as follows:

1. After exhaustive comparative evaluations of the HAI technology and other competitive power augmentation options, Tennessee Valley Authority (TVA) contracted Parsons Infrastructure and Technology Group for engineering and cost estimates of the HAI technology for application to CT and CC plants based on the GE 7121 (EA) for specific operating and site conditions and economics. It was concluded that, for the specified 90F (30C) summer peak ambient temperature, the conversion of the GE 7121 (EA) into CT-HAI increases the power from 75.9 MW to 103 MW and reduces the heat rate from 10,630 Btu/kWh (2,679 kcal/kWh) to 9,610 Btu/kWh (2,422 kcal/kWh) (Reference 2).  Installed specific costs at 90 F (30C) were estimated as $180/ incremental kW. It was, also, determined that, for 90 F (30C) ambient temperature, the conversion of a typical GE 7121 (EA) based CC plant into CC-HAI increases the power from 123.4 MW to 154.8 MW with the heat rate of 6,390 Btu/kWh (1,610 kcal/kWh).  Installed specific costs for the CC-HAI plant at 90 F (30C) were estimated as $160/ incremental kW.

2. Following these developments, TVA, then decided to demonstrate this concept on one of their operating PG7661 gas turbines at the Colbert Power Plant. The project conducted by TVA with support of consultants and equipment suppliers concluded that the retrofit of the PG7661 into CTHAI will result in approximately 30% power augmentation and 15% heat rate reduction as it relates to 95F (35C) ambient temperature (Reference 3). The power augmentation and heat rate reduction take place over the wide range of ambient temperatures. TVA estimated that specific incremental capital costs for conversion of the CT into CTHAI and CC into CCHAI are approximately $200/ incremental kW. The project had been in a well-advanced stage (RFP had been issued, turnkey proposals submitted and evaluated) but ultimately had been indefinitely postponed due to budgetary reasons.
3. As a part of the License Agreement, Calpine Corporation (a Licensee of the HAI/DAI technologies for application to Calpine owned CT and CC plants) conducted (in the September-October 2001) validation tests on the PG7241 (FA) at the Broad River Energy Center, Cherokee County, South Carolina, while operating with Humid Air Injection, Dry Air Injection, Stem Injection and no injection (Reference 4). Test results confirmed the projected power augmentation of 18.3MW when operating on the "dry" control with the humidity level of only 3% of the total airflow. This power augmentation is extrapolated to 22.3 MW while operating on "wet" control. No known operational limitations of the CT were exceeded. Extrapolation of the test results indicates that an increase in the humidity level to only 5% will result in the power augmentation of the tested CT of approximately 28 MW for "dry" operation and over 32 MW when the CT operates on the "wet" curve.
4. In 2002 after a long due diligence efforts the HAI/DAI technology had been licensed to a few power engineering/ service companies that will offer turnkey firm price retrofit projects for power augmentation of CT and CC plants.

The following sections summarize the latest HAI/DAI thermal cycles as applied to CT and CC plants and engineering equipment related developments targeting further improvements in economics, performance and operational characteristics.

Technology background, VALIDATION TESTS, LATEST IMPLEMENTATION ACTIVITIES 

Figure 1 is a simplified heat and mass balance of the CT-HAI plant based on the PG7241 (FA) combustion turbine, which is self-explanatory. The major attractive features of the HAI technology are:

• It provides the most significant power augmentations and heat rate reduction within constraints established by OEM
• It utilizes a standard industrial compressor and conventional heat and mass exchange equipment installed externally to a CT
• The humid air injection into a CT techniques could utilize existing provisions for the steam injection, but could be further simplified
• This concept requires only filtered and softened  make-up water to produce the humidified air of steam quality
• The augmented power is produced almost instantaneously

Figure 1: Heat and mass balanced for the CT-HAI PLANT based on PG7241 (FA) combustion turbine.

• NOx Emissions are significantly reduced due to the heat rate reduction as well as due to massive humidity levels.

Figure 2 presents the heat and mass balance for the CC-HAI plant based on the PG7142 CT that is similar to that presented on Figure 1 except that the humid air is created by mixing of steam, extracted from the steam turbine, with the supplementary airflow from the auxiliary compressor. This concept capitalizes on the fact that the bottoming cycle already has the steam available for the supplementary air humidification and on  available demineralized water resources.

Figure 2: Heat and Mass balance for the CC-HAI Plant based on PG 7241 (FA) Combustion Turbine

As a part of EPRI support of advanced humid air concepts, it sponsored significant test program for combustor operating on the humid air with various fuels. Textron (USA) and Aero Industrial Technologies ( Britain ), in parallel, conducted these tests on diffusion type test combustors. Figure 3 demonstrates some test results that indicate single digit measured NOx emissions even for relatively "dirty" diffusion type combustors operating on a typical natural gas (Reference 5). It could be concluded that the HAI technology could operate with much more operationally flexible diffusion type combustors (vs. DLN) and still have single-digit NOx emissions

Figure 3: Tested NOx Emissions with Humid Air Injection

The Figure 4 illustrates the CT-DAI concept applied to the same PG7241 (FA). This concept provides lower than HAI, but still very significant power augmentation (16 MW) without use of the water.

Figure 4: Heat and mass balanced for the CT-DAI Plant Based on PG7241 (FA) combustion turbine.

Tables 1 and 2 summarize the latest performance characteristics of HAI and DAI technology applied to CT and CC plants based on PG7241 (FA), respectively.

Table 1: Performance Characteristics of CT-HAI and CT-DAI Plants Based on PG7241(FA)

Table 2: Performance Characteristics of CC-HAI/CC-DAI Plants Based on PG7241 (FA)

There are a number of HAI concepts configurations; each of them could be preferable for specific situations. For example, Figure 5 presents an alternative CT-HAI configuration, similar to the CC-HAI configuration on Figure 2, where the supplementary airflow from the additional compressor and  the steam flow generated in the HRSG/OTSG(Once-Through-Steam generator) are mixed in a mixer and then the humid air is injected into a CT. This concept has only two additional components that are commonly used by end-users (HRSG and the auxiliary compressor) vs. three components in the configuration on Figure 1 (the water heater, the saturator -actually, a simple column commonly used for heat and mass exchange in variety of industrial applications- and the auxiliary compressor). This configuration could be preferable for cases where the HRSG is already installed, though this configuration still will have some typical for steam injection drawbacks like delays in the additional power due to the HRSG inertia and a need for the demineralized water with some associated additional variable and operating costs. The following section presents some the latest equipment related developments that further simplify and conventionalize this CT-HAI concept with significant cost reductions.

Figure 5: The HAI Configuration with Mixing of the Supplemental Airflow and Steam.

NEW ENGINEERING CONCEPT

Any steam or humid air stream injected into a CT must be of sufficient purity so that the blades of the CT will not be damaged.  The major concern here is nonvolatile or condensable matter, such as entrained solid particles that could melt in the combustor and deposit on the turbine blades.  When steam or humidified air is injected into the combustor of a CT to enhance power output, it must have a very low entrained solids content.  The specific limit for solids content depends upon the turbine design, the purity of the intake air, and the purity of the fuel, among other variables.  One rather strict solids concentration limit for CT-HAI is 0.5 p.p.m. solids by mass in the injection stream, which will be suitable for nearly all applications.

In the CT-HAI concepts considered until now, the generation of the injection-quality humid air required make-up softened, potable water for the basic concept (Figure 1) with a water heater, the saturator with a moisture separator, and a superheater. The alternative concept (Figure 5) with a once-through steam generation (OTSG) and a steam-air mixer requires demineralized make-up water

The Alternative Concept with Once-through Steam Generation.  In the case of the OTSG concept, demineralized water is fed to an OTSG (actually a combination of an economizer, boiler, and superheater) where it is evaporated and superheated (Figure 5).  All solids in the boiler feedwater (BFW) are either entrained in the steam or deposited on the heat transfer surfaces of the once-through boiler.  Purity of the steam is controlled strictly by control of the BFW purity.  As steam is formed, the very low concentration of solids in the BFW becomes progressively higher until, as the last of the water is evaporated, the solution becomes concentrated and potentially corrosive.  If the water is not deaerated, there is also the potential for oxidation.  To mitigate these problems, once-through boilers generally are made of high-alloy tubes such as Alloy 800.  The economic drawbacks of this concept are obvious:  the variable and fixed operating costs are high due to the need for demineralized water, and the cost of the OTSG is higher than it would otherwise be if a lower alloy construction could be used.

The Basic Concept with Air Saturator and Heat Rejection Unit (HRU).  To avoid the drawbacks of the OTSG concept, the basic concept was developed, as depicted schematically in Figure 1.  In this system, aerated water is heated by circulation through the HRU's water heater coils.  As the hot water is injected into the saturator and contacted with air, it evaporates to form the humidified air.  As water is evaporated, dissolved solids in the unvaporized circulating water are concentrated.  The concentration of total dissolved solids (TDS) is limited to acceptable levels by continuous or periodic blowdown of water from the saturator (typically 20% of the water feed rate), and make up water is fed to the system to balance the outflow of water in the humidified air and the blowdown streams.

The humidified air leaving the saturator will carry off small droplets of water containing dissolved solids, but the mass flow of droplets exiting the saturator will usually be on the order of 0.05% of the humidified airflow.  If the blow down rate is adjusted to produce a TDS concentration of, say, 1000 p.p.m. in the saturator, then the solids carried out by the droplets in the humidified air will amount to 0.5 ppm by mass in the humidified air stream, which is sufficiently pure for almost all applications.  The humidified air leaving the saturator is significantly superheated (200F) before being injected into the CT combustor to avoid any chance of small droplets of water entering a CT.

The heating of aerated water in the HRU to over 400ºF requires an oxidation-resistant alloy for all the wetted parts in the saturator, water tubes, and circulating system.  Austenitic stainless steels (304, 316, etc.) have acceptable oxidation resistance, but they are not suitable for use in the water heater because of their lack of resistance to chloride stress corrosion cracking (SCC), which particularly affects austenitic stainless steels.  Duplex stainless steels such as Alloy 2205 have reasonable resistance to chloride SCC, and these are recommended for the saturator, and water circulating equipment, but duplex alloys cannot be used for the water heater tubes because of embrittlement problems if exposed to temperatures above about 885ºF.  The ASME code does not permit the use of ferritic stainless steels or duplex stainless steels for design temperatures above 600ºF because of this reason.  Since a CT-HAI water heater is not always in service, the tubes could be heated by the hot turbine exhaust to temperatures exceeding 1100ºF and must be capable of withstanding this "idle mode" condition.   

The requirements for oxidation resistance, chloride SCC resistance, and no irreversible deterioration of physical properties at temperatures exceeding 1100ºF generally require the use of a costly high nickel alloy, such as Alloy 625 or Alloy G, for the water heater tubes.

The comparative cost analysis shows that the basic concept with HRU and the saturator is more economical than the alternative concept with the OTSG, air-steam mixer and demineralized water. Still, while the basic concept succeeds at eliminating the need for demineralized water, it still requires expensive alloys and it adds an extra component (the saturator) into the system.

The New Concept:  Once-Through Boiler with Partial Steam Generation (OTBPS).  The schematic diagram of the new concept is shown in Figure 6. This concept was developed to avoid drawbacks of both basic and alternative concepts and is new for this application. Once-Through Partial Steam generators are a well-proven technology, having been used extensively in enhanced oil recovery applications to generate high-pressure steam at 80% quality from softened high-TDS feedwater. In this new CT-HAI application, potable water is deaerated, and therefore may be heated in carbon steel or chrome-moly tubes, which are not susceptible to chloride SCC.  (Generally, chrome-moly will be used to withstand oxidation during the 1100ºF "idle mode" situation.)  Instead of completely evaporating the BFW as in the traditional OTSG, the BFW is only partially evaporated (typically to about 80% steam quality), with the remaining unevaporated BFW separated out, and all or a portion of it discarded as blowdown.  The blowdown rate is controlled so as to limit the TDS concentration in the water.

Figure 6: CT-HAI with Once-Through Partial Steam Generator

A steam/water separator vessel with mist eliminator is used to provide steam with about 0.05% entrained droplets, essentially the same entrained droplet concentration as in the humidified air in the CT-HAI saturator concept.  Both the concentration of TDS in the droplets and the droplet entrainment rate are equivalent to what can be achieved in a saturator, so the steam will have the same typical solids concentration of 0.5 ppm.  This injection-quality steam flow is mixed with air, further diluting the solids concentration and producing a steam-air mixture of the same basic composition and equal or lower solids concentration as what is achieved with a saturator.  This air-steam mixture is superheated before injection into the CT combustor, exactly as in the saturator concept.

The CT-HAI concept with OTBPS and air-steam mixer has the following advantages:

Relative to the basic concept with the saturator and HRU:

·         The total CT-HAI specific capital cost for the overall incremental system are reduced by approximately exceed 15% on the $/kW basis

·         The saturator is eliminated with associated simplification of engineering, construction and maintenance aspects.

·         The tube material for the once-through partial steam generator may be chrome-moly material instead of the much more expensive Alloy 625 or Alloy G tubes in the water heater required for the saturator system.  This material cost savings at least offsets the cost of adding a deaerator and steam separator for the once-through partial steam generator, possibly permitting further capital savings.

·         The feed pumps for the once-through steam generator may be of conventional BFW pump materials, while the BFW piping and valves may be of carbon steel.  The pumps, piping, and valves in the saturator system must be of a material providing both oxidation and chloride SSC resistance.  Possibly a ferritic stainless steel will be adequate; but probably a duplex stainless steel will be needed, as has been proposed for use in the saturator.  In either case, the use of a once-through partial steam generator with deaerated water will eliminate materials suitability concerns and reduce capital costs for pumps, piping, and valves.

Relative to the demineralized water fed OTSG with air-steam mixer concept:

·         The demineralized water system is eliminated and is replaced by a simple water softener.  This saves substantially in both the capital cost and the operating cost for full-time engineering supervision for a demineralized water system.

·         The tube material for the once-through boiler with partial steam generation may be chrome-moly material, which is much less expensive than the higher alloys required for the tubes of a demineralized water once-through boiler/superheater.  This tube material cost savings more than offsets the cost of the deaerator that is used with the partial steam generator, so that capital costs for the heat recovery system can be reduced.

Conclusions

The CT-HAI technology had been installed on a commercial PG7241 (FA) combustion turbine. Validation tests confirmed all projected major performance characteristics: power augmentation and heat rate reduction.

CT-HAI and CC-HAI technologies application projects conducted by various end-users, consultants and equipment suppliers (based on a variety of combustion turbines) demonstrated power augmentations ranging from 15% to 25% with 6-15% heat rate reductions.

The CT-HAI system is simple and external to a CT. The required additional equipment either a standard one (industrial compressors supplied by Cooper Turbocompressors, Ingersoll Rand, etc.) or conventional heat and mass exchange components provided by Struthers Wells and others.

Detail engineering and cost estimate efforts confirmed specific costs ranging from $150/kW to $180/kW.

Developed by Struthers Wells CT-HAI concept with OTBPS further simplifies the CT-HAI system and reduces specific costs by approximately 15%.

CT-HAI should provide single digit NOx emissions even with diffusion type combustors. The sponsored by EPRI humid air combustor tests on diffusion type combustors resulted in a single digit NOx emissions.

There are selected licensees commercially offering CT-HAI and CC-HAI power augmentation projects.

References

1.        R. Hall, D. Bradshaw, TVA "Advanced Combustion Turbine Cycles Meet the Needs of the Utility of the Future, Power-Gen, 1999.

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

3.        Injecting Humidified and Heated Air to meet peak Power Demands, 2000-GT-0596.

4.        Air Injected Power Augmentation is Validated By Fr7FA Peaker Tests, GTW, March-April, 2002.