Las
Vegas, NV, December 9, 2003
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
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Michael Nakhamkin Energy Storage and Power
Consultants, Inc. |
Robert
Pelini Struthers
Wells Corporation |
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Manu I. Patel Consulting Engineer |
Ronald H. Wolk Wolk
Integrated Technical Services |
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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:
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 (30°C) were estimated as $180/ incremental kW. It was, also,
determined that, for 90°F (30°C) 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 (30°C) 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.
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 PG7241 (FA) Combustion Turbine.
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)
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
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:
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:
·
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 psia) and distribution lines (200 psia), 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 TC 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 CTs 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 ppm 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 hot
water generated in the Heat Recover Unit (HRU) in a counter-flow direction, 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.
·
Relative to the previously considered humidification concepts
based on the saturator and HRU, the total OTPSG concept is simpler and reduces
specific capital cost for the overall incremental system by approximately exceed
15% on the $/kW basis
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, or others 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, the 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.
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