Document ID: EPA-HQ-OW-2008-0667-0621
Agency: epa
Document Type: Supporting & Related Material
Title: 
Posted Date: 2011-04-20T04:00Z

MEMORANDUM

Tetra Tech, Inc.
10306 Eaton Place, Suite 340
Fairfax, VA 22030
phone	703-385-6000
fax	703-385-6007

DATE: 		January 15, 2009

TO:			Paul Shriner and Jan Matuszko, EPA
	
FROM:		Peter Sherman, Tetra Tech and John Sunda, SAIC

SUBJECT:	Combined Cycle Power Generation Under 316(b) Phase II 

This memo presents information regarding the combined cycle electric power production systems, including information on how combined cycle systems could reduce demand for cooling water and the number of combined cycle systems potentially subject to the Phase II rule.  

Cooling Water Use at Combined Cycle Power Plants

Combined Cycle electric power generating systems use a two-stage power generation process.  The most common arrangement is a configuration where electricity is produced by two combustion gas-driven turbine-generators with the hot exhaust gas from these turbines then being used to produce steam in a single heat-recovery steam generator (HRSG).  The steam from the HRSG produces electricity using a single steam-driven turbine generator.  The overall thermal efficiency of combined cycle generating units is 50% to 60%.  Conventional coal-fired plants have a typical thermal efficiency of 36% to 40%. 

Since the combustion turbine component does not require cooling water and the steam power generation component produces only about one-third of the total electric energy generated, combined cycle plants should require about a third as much cooling water (i.e., 67% reduction) as a similar capacity conventional steam-driven power plant (Micheletti 2002).  Thus, replacing a conventional steam generation system with a combined cycle generation system of the same power generation capacity could reduce cooling water flow by up to 67%.  The data in the EPRI study presented below appears to be reasonably consistent with this estimate.

According to an EPRI report (2002) addressing water and sustainability, combined cycle electric power facilities withdraw approximately 54% to 60% less water than comparable facilities that do not use combined cycle technology.  In the study, EPRI determined the water withdrawal and water consumption rates per unit of power generation for several categories of facilities (See Table 1 below).  Although this study focused on water consumption (loss due to evaporation), it also developed water withdrawal data ranges for steam-based and combined cycle facilities using several types of cooling water systems.  For the facilities using once-through cooling systems, the combined cycle system water withdrawal rates were approximately 38%-40% of the water withdrawal rates of those facilities using a single cycle (steam) power generation process.  Similarly, for the facilities using cooling towers, the combined cycle system water withdrawal rates were approximately 38%-46% of the water withdrawal rates of those facilities using a single cycle power generation process.  The facilities using combined cycle technology and cooling towers had the lowest overall cooling water withdrawal rate.

Table 1.  EPRI Report Data on Average Water Withdrawal and Consumption Based on Cooling Design
                                     Fuel
                                  Prime Mover
                                Cooling System
                          Water Withdrawal (gal/MWh)
                          Water Consumption (gal/MWh)
Fossil/biomass/waste
Steam
Once through
20,000 to 50,000
~ 300
Fossil/biomass/waste
Steam
Cooling pond
300 to 600
300-480
Fossil/biomass/waste
Steam
Cooling towers
500 to 600
~ 480
Nuclear
Nuclear steam
Once through
25,000 to 60,000
~ 400
Nuclear
Nuclear steam
Cooling pond
500 to 1100
400  -  720
Natural gas/oil
Combined cycle
Once through
7500 to 20,000
~ 100
Natural gas/oil
Combined cycle
Cooling towers
~ 230
~ 180
Natural gas/oil
Combined cycle
Not specified
~ 0
~ 0
Coal/petroleum residuum
Combined cycle
Cooling towers
~ 380
~ 200
Source: Water & Sustainability (Volume 3): U.S. Water Consumption for Power Production -- The Next Half Century, EPRI, Palo Alto, CA: 2002. 1006786

Currently, most combined cycle generating systems use natural gas or fuel oil as the fuel source (typical new facilities use natural gas).  The use of natural gas also results in the lowest air emissions of commonly used fuels.  The 2009 EIA Energy Outlook (early release) projects that natural gas (55%) and renewable sources (22%) will provide most of the 255 gigawatts of generating capacity projected to be added between 2007 and 2030.  A new technology in the developmental stage called Integrated Coal Gasification Combined Cycle (IGCC) uses coal as the fuel source.  These systems first convert coal to a gas and then use the gas to generate electricity in a similar manner as conventional combined cycle systems.  Due to the energy requirements of the additional processes, the thermal efficiency of IGCC systems is not quite as high as for conventional combined cycle, but may reach up to 50%.  Similarly, flow reduction isn't quite as high - IGCC systems use 20-50% less water compared to a conventional (non-combined cycle) coal power station of similar size (CCSD 2008).  

There are currently two IGCC plants operating in the US and both have been in operation since the mid 1990s and thus are included in EPA's Phase II plant database.  These are the Tampa Electric Power Station's 250 MW Polk I IGCC plant that went online in 1996 (EIA Plant ID #7242) and the Wabash River Repower IGCC project that went online in 1995 (EIA Plant ID #1010).  IGCC is a relatively new technology and, as such, has presented issues of reliability, cost, and water pollution in early applications.  It is too early for this to be considered a mainstream technology. 

Single-cycle power plants using gas, oil or coal as a primary fuel can be retrofitted with a heat recovery steam generator (HRSG) that makes combined-cycle operations possible.  Typically, combined-cycle retrofitting involves retiring the original boiler and replacing it with a gas turbine.  The original steam turbine and feedwater heaters are usually retained.  Such retrofitting enables a power producer to increase its electrical output without building entirely new facilities because of the increased thermal efficiency of combined cycle technology.  Impediments to such retrofits include the capital costs, as well as the higher cost (relative to coal) and availability of natural gas as fuel. 

Use of Combined Cycle Power Generation in Phase II

The information presented in Tables 2 and 3 below summarize available DOE EIA data for steam generating units at Phase II facilities.  The generating capacity data summarized is for generating units that use steam as a prime mover in the generating process and generating units that are coupled with such units.  These include the following prime mover codes:

 ST - steam turbine that is not combined cycle;
 CT - combustion turbine component of combined cycle;
 CA - steam turbine component of combined cycle;
 CS - combined cycle generator driven by both steam turbine and combustion gas turbine.  

In this analysis, total combined cycle generating capacity is calculated by summing the nameplate capacity for all operating facility generating units with prime movers CS, CA, and CT.  The generating capacity for the steam turbine-only category is calculated by summing the nameplate capacity for all facility generators with prime mover ST.  The generating unit nameplate capacity is obtained from the EIA 2006 generator database.

A review of the 2006 EIA generator data indicates that 44 (8%) of the population of 525 facilities in the Phase II database operate combined cycle generating units.  Table 2 presents the estimated number of facilities in the Phase II universe that operate various combinations of the different types of steam generating units.  The estimated numbers are extrapolated from data for the population of 525 facilities in the database to the total universe of 536 facilities using facility sample weights.  The combined cycle component of those facilities with both types of steam generating units ranges from 22% to 95%.  The 2006 EIA generator database indicated that the steam generating units for six of the plants currently included in the Phase II database (Table 2 weighted total = 7) were permanently retired.  It is possible that the cooling water withdrawal rate for these plants may no longer be large enough (i.e., >50 MGD) for them to be considered as in-scope Phase II facilities.

                 Table 2. Estimated Number of Facilities in the Phase II Universe by Steam Generation Type
                             Steam Generation Type
                        Estimated Number of Facilities
                              Steam Turbine Only
                                      484
                              Combined Cycle Only
                                      20
                     Both Steam Turbine and Combined Cycle
                                      25
                        No Steam (mostly gas turbines)*
                                       7
                          Total Universe - All Plants
                                      536
                 *These plants operated steam turbine generating units in 2000 and the plants are still in operation but have since retired all steam turbine generation units.

Table 3 lists the population of facilities in the Phase II database that operated combined cycle generating units in 2006.  The table presents total combined cycle generating system nameplate capacity for generating units with the prime movers CT, CA, and CS in the 2006 EIA generator database.  Table 3 also shows the proportion of total steam generating capacity (sum of ST, CT, CA, and CS) that is comprised by the combined cycle generating system.
                 Table 3. Population of Facilities in the Phase II Database with Combined Cycle Generating Units
                                       
References

Micheletti, W. Burns, J. Emerging Issues and Needs in Power Plant Cooling Systems.
www.netl.doe.gov/publications/proceedings/02/EUW/Micheletti_JMB.PDF

CCSD. Cooperative Research Centre for Coal in Sustainable Development. IGCC Power Generation Fact Sheet. Accessed on December 19, 2008 at website: http://www.ccsd.biz/factsheets/igcc.cfm

Wabash River Energy Ltd. Wabash River Coal Gasification Repowering Project - Final Technical Report. August 2000.  Accessed at website:
http://www.netl.doe.gov/technologies/coalpower/cctc/cctdp/bibliography/demonstration/pdfs/wabsh/Final%20_Report.pdf

U.S. Department of Energy (U.S. DOE). Energy information Administration (EIA). Form EIA-860 Database (2006). Generator Database.