ASME Power
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March 30 - April 1, 2004

PWR2004-52095

DRAFT

Power Augmentation of Heavy Duty and Two-Shaft Small and Medium Capacity Combustion Turbines with Application of Humid Air Injection and Dry Air Injection Technologies

Abstract

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

• The summary of the latest activities on the implementation of HAI and DAI technologies including results of the validations tests conducted on the PG7241 (FA) combustion turbine, and findings of various CT-HAI implementation projects;
• The technical background including the latest CT-HAI and CT-DAI concepts resulting in the performance improvements and reduced emissions
• A novel concept for humidification of the injected air that further reduces overall capital costs by 15%.
• The novel approach for the power augmentation of two-shaft small and medium capacity CTs with application of HAI and DAI technologies. Two-shaft CTs are widely used for electric power generation, including distributed generation, as well as a variable-speed mechanical driving engine including driving natural gas (NG) pipeline compressors (PC).

INTRODUCTION

It is well-known paradox that while most electric power customers' power demands reach their peak during the summer, the power outputs of CT and 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. Conducted by end-users, consultants and equipment manufacturers (Reference 1) extensive comparative analyses of the HAI power augmentation technology (CT-HAI and CC-HAI as applied to power augmentation of CT and CC plants, respectively) vs. competing alternatives demonstrated that the CT-HAI/ CC-HAI technology delivers the highest amount of power augmentation with the lowest heat rate 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.        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 (Reference 2) that, for the 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).  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.        TVA then initiated the project for the retrofit of the PG7661 engines with the HAI technology. (Reference 3). The project had been in a well-advanced stage (RFP had been issued, turnkey proposals submitted and evaluated) but ultimately had been indefinitely postponed for budgetary reasons.

3.        As a part of its License Agreement, Calpine Corporation (a Licensee of the HAI/DAI technologies for application to Calpine owned CT and CC plants) conducted (during the September-October 2001) validation tests on the PG7241 (FA) at one of its facilities. (Reference 4). Test results confirmed the projected values of power augmentations with HAI and DAI technologies.

4.        In 2002 after extensive due diligence efforts the HAI/DAI technology had been licensed to a number of power engineering/ service companies that are offering 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.

Description of HAI/DAI concepts 

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. 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.

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.

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

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

As a part of EPRI's support of advanced humid air concepts, it sponsored significant test programs for combustors 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.

The major attractive features of the HAI technology are:

• It provides the most significant power augmentations and heat rate reduction within constraints established by OEMs
• It utilizes a standard industrial compressor and conventional heat and mass exchange equipment installed externally to a CT
• The humid air is injected into a CT at any point upstream of combustors (for example in the compressor discharge plenum) and could utilize existing provisions for the steam injection
• 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 4: Heat and mass balanced for the CT-DAI Plant Based on PG7241 (FA) combustion turbine.

POWER AUGMENTATION OF TWO-SHAFT SMALL AN MEDIUM CAPACITY COMBUSTION TURBINES WITH HAI/DAI TECHNOLOGY.

Two-shaft CTs are widely used for electric power generation, including distributed generation and, also, as a variable-speed mechanical engines used for marine and aircraft applications as well as for driving natural gas (NG) pipeline compressors (PC). The application of HAI and DAI power augmentation technologies to two-shaft CTs requires slightly different technical approaches as compared to single-shaft heavy-duty CTs at electric power generation facilities. In order to demonstrate generic technical issues and recommended solutions that are specific for applications of the HAI and DAI technologies to typically smaller size two-shaft CTs in a mechanical drive service, the paper describes the HAI/DAI technology application for power augmentation of the two-shaft Rolls Royce Allison (RRA) KC7 engine driving a PC, which is one of a frequent applications of this CT.

These specifics are as follows:

·         These CTs require special controls and HAI injection methods to operate both shafts within operational, performance and mechanical limitations, due to the fact that two- shaft CTs have both the Turbocompressor (TC) shaft (that is the high pressure (HP) turbine driving the integral CT's compressor) and a Power Turbine (PT) shaft mechanically driving a PC operating with variable a speed,

·         The power augmentation technology that provides the highest power augmentation for each CT (i.e. the highest boost to the pipeline productivity) should be the technology of choice, due to the fact that two-shaft CTs in the mechanical-drive service have single or limited number of unit applications (vs. a multi-unit approach typical for the electric power generation). It is different from the electric power generation applications, where one could apply a technology with a smaller power augmentation to a larger number of CT units for the same total power increase.

·         CTs driving pipeline compressors are frequently remotely located along a pipeline and therefore have particular requirements for a) simplicity, b) high reliability and availability and c) provisions for remote control.

·         Pipeline applications can provide the opportunity to use an NG expander, utilizing the pressure difference between a transmission line (approximately 500-1500 p.s.i.a) and distribution lines (200 p.s.i.a), for driving of the auxiliary compressor. That will enhance the net power increase.

While, as it was indicated in References 1-5, the HAI/DAI power augmentation of heavy-duty CTs requires injection of the humid or dry air into a CT at any point upstream of combustors, an analysis of specifics of two-shaft CTs indicated that this type if injection will result in over speeding of the TC shaft. RRA for the KC7 does not allow even relatively minor over speeding of this shaft and it could be conservatively assumed that this is the case for other two-shaft CTs. Optimization studies concluded that in order to address the problem of over speeding of the TC shaft, the humid or dry air should be injected into a two-shaft CT at two separate injection points - one is upstream of combustors (as is the case for heavy duty single-shaft CTs) and the other is between the HP turbine exhaust and the inlet to the PT. The injection flows into these two points had been optimized to maintain practically the same operating speed of the T C shaft and at the same time to provide a significant power augmentation of the PT driving the PC.

Figure 5 illustrates a simplified heat and mass balance of the power augmented KC7 with DAI. Injection of airflows of 1 lbs/sec and 2.8 lbs/sec into the CT upstream of combustors and upstream of the PT, respectively, resulted in the gross power output of the CT increasing from 5 MW to 6.3 MW (approximately 25%) while maintaining practically constant speed of the TC shaft (the HP turbine-driven integral compressor). The net power augmentation, after subtracting of the 0.650 MW power required for the motor-driven auxiliary compressor, is 0.77 MW, i.e. a net increase of approximately 15%. This power augmentation will boost the NG supply to customers with associated enhancement of economics. The heat and mass balance on Figure 5 presents only one of a number of operating scenarios and could be easily adjusted to reflect specific site conditions and equipment and system requirements and limitations.

Figure 5: Power Augmentation of RRA KC7 with Dry Air Injection

Figure 6: Power Augmentation of RRA KC7 with Humid Air Injection

The modeling of the power augmentations of the RRA's KC7 had been performed with the technical support of RRA personnel, who provided major thermal cycle parameters and the performance data for the CT (to accurately build the performance model of the CT), as well as critical mechanical and operational limitations. These results are consistent with heavy-duty applications, where the HAI technology significantly augments power and reduces heat rate as compared with the DAI if consistently applied to the same CT. The HAI concept, though relatively simple and practical, requires humidification of the airflow, which, in turn, requires an available water source. This typically is not a problem for power generation applications, but could be a problem for pipeline applications, which emphasize simplicity and remote control options.

The concepts presented on Figures 5 and 6 illustrate generic technical solutions for applications of HAI and DAI technologies to two-shaft CTs- they provide a method of significant power augmentation of a PT driving a PC without violation of operating parameters of the TC shaft.

Numerous comparative analyses of various power augmentation efforts performed for a variety of heavy duty CTs indicated that the HAI and DAI technologies provide the highest power augmentation. This conclusion should be valid for pipeline applications.

For the particular NG pipelines applications, it looks that the DAI concept is preferable due to its absolute simplicity, higher reliability and availability, and that it can be easily controlled remotely. The HAI power augmentation concept could the concept of choice if a larger power augmentation is required and water is available.

The estimated net power augmentation of KC7 could be further enhanced if the NG expander could be used for driving of the auxiliary compressor. In this case, the power augmentation of KC7 with HAI and DAI technologies will be approximately equal and estimated at 25% net. This further increases attractiveness of the simpler DAI technology.

The economics of the power augmentation of pipeline CT s are driven by economics of the increased NG supply though the pipeline and sales to customers. The achievement of 25-27% power augmentation could result in approximately 15% increase in the pipeline compressor productivity and corresponding NG sales. For each particular application the effectiveness of the power augmentation should be evaluated based on specific pipeline operations and economics. Estimated specific costs for the power augmentation of pipeline CTs are expected to be 30-50% higher than for large capacity power generation turbines (estimated to be under $200/kW, see references). However, they need to be evaluated against the specific costs of small CTs, which are minimum 30-50% higher than those of larger CTs.

NOVEL APPLICATION OF THE ONCE-TROUGH PARIAL STEAM GENERATOR.

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 HAI is 0.5 p.p.m. solids by mass in the injection stream, which will be suitable for nearly all applications.

Previous HAI concepts considered auxiliary air humidification by using a one of two alternative methods: a) air humidification in a saturator by the flowing in e counter-flow direction hot water, generated in the Heat Recover Unit (HRU) or by b) mixing of the auxiliary air with the steam produced by a conventional Once- Through Steam generator (OTSG). Continuous efforts to further improve the HAI concept resulted in the development of the novel concept presented in Figure 7. This novel concept is based on the use of the Once-Through Partial Steam Generator (OTPSG). OTPSGs are based on 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 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.

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 p.p.m.  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 HAI concept with OTBPS and air-steam mixer has the following advantages as compared to a conventional OTSG:

·         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 normally required 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.

Figure 7: The HAI Concept Schematic with Once-Through Partial Steam Generator

Conclusions

The following is the summary of the paper:

·         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 is either a standard (typical industrial compressors supplied by Cooper Turbocompressors, Ingersoll Rand, etc.) and/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.

·         The Humid Air Injection and Dry Air injection power augmentation technologies could be successfully applied for power augmentation of two-shaft combustion turbines driving pipeline compressors.  The developed methods are based on the two separate injection points, which with proper selection of the injection flows practically maintain operating parameters of the turbocompressor shaft independent of ambient air temperature and provide a significant power augmentation of the combustion turbines driving pipeline compressors. These methods could be conceptually applied to practically any two-shaft CT, being adjusted for the CT specific features and restrictions.

·         The HAI and DAI power augmentation methods applied to KC7 could increase power by 22% and 15 %, respectively, with approximately 15% and 12% increase in the pipeline compressor NG flow delivery.

·         Though the HAI power augmentation concept results in higher power augmentation, it is believed that DAI concept is preferable, due to its simplicity, capacity for being remotely controlled and expected high reliability and availability.

·         If the NG expander is utilized for the auxiliary compressor driving the HAI and DAI power augmentations are approximately equal to each other at about 25%.

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.