2000 International Joint Power Generation …

1 Proceedings of Insert Conference Abbreviation: Insert Conference Name Insert Conference Date and Location Paper Number Here Combined Cycle heat recovery Optimization A. Ragland, Vogt-NEM W. Stenzel- EPRI solutions ABSTRACT The US electrical Power industry has changed from a regulated business where utilities were virtually guaranteed a rate of retum to an unregulated business where the market sets the price. As with any unregulated business, the price of a service or supply is determined by supply and demand, and the seller must be competitive in order to meet its desired revenue target. Profitability will largely be determined by the business' overall operating efficiency. As the electricity supply business becomes more competitive, it becomes more important to optimize fuel selection and plant design to achieve competitive electricity prices while providing a satisfactory financial return for owners and other participants.

2 Plant designs need to be adjusted to the specific project parameters to achieve optimum results. Fuel costs, yearly plant outputs, interest rates and other factors need to be considered when establishing plant designs. Four plant designs are compared using natural gas in this paper. This paper will present a view of the cost benefits achieved through heat recovery steam generator (HRSG) optimization. The many possible business and technology scenarios, and site specific nature of each project makes it difficult, time consuming and costly to effectively optimize fuel selection, generating unit selection, efficiency, capital cost, and return on investment. Using computer tools is very important to properly handle these complex analyses. The use of the SOAPP Combustion Turbine Software as an analysis/decision support tool for optimizing combined cycle plants is described herein.

3 INTRODUCTION Achieving financial improvements requires proper analysis of many requirements and parameters. Changing one input design parameter often results in modifications to other parameters. For example, changing the HRSG design changes efficiency, fuel cost, capital cost, debt financing, and emission rates. Often it is important to assess the impact of a range of inputs in the analysis. Combustion turbine performance has the primary impact on combined cycle plant efficiency. The next most important piece of equipment that impacts efficiency is the heat recovery steam generator. The HRSG parameters to optimize include steam pressures, temperatures, flows, pinch points, approach temperatures, and HRSG exit gas temperatures. HRSG BASIC DESIGN CONCEPTS Figure 1 is a typical cycle for a single pressure HRSG.

4 Multiple pressure HRSGs with duct firing and other capabilities can become much more complicated. This diagram shows the main gas, steam , and condensate flows, and the typical HRSG surfaces and steam drums. Flue gas from the combustion turbine enters the HRSG and is reduced in temperature by the superheater, reheater, dram evaporative surfaces, and economizer before it enters the stack. Condensate from the combined cycle condenser enters the deaerator, and flows through the economizer to the drum. steam from the drum flows to the superheater and then to the high pressure turbine. steam from the high pressure steam turbine flows through the reheater and then to the intermediate pressure turbine. Pinch points and approach temperatures are important HRSG design parameters.

5 Reducing these temperatures will increase cycle efficiency. However, optimization involves fairly complicated heat transfer calculations and steam cycle heat balances to avoid operational problems. Figure 2 provides a simple diagram showing pinch and approach temperatures: Proceedings of 2000 International Joint Power Generation Conference Miami Beach, Florida, July 23-26, 2000 IJPGC2000-15031 1 Copyright (C) 2000 by ASME[. - } Attemperator 4 ! o=-, fl : Attemlc, er ator [" '[" IP ~eam Hot Reheat ". " :'..- Colcl Reheat ~"."J ;" : ..: .~---d~ 4~ ~; ."~..,- ' ;.. , =v'e,r~:tl.;,, ",..I L', ",'~." "=l,',l ~i F n p To -k : .-..~y- ~, ;.~".: .. I =.'~..'=|. "." ..' ).:. ~ .. ".. ",., .. :..=~It~, ~." i.:" "-~i! _~-' ::'.:.-; eo,er~') ":" ;P ~''!'.]]]

6 ~ .. :':." ..:-. Feed - ":':;'~ t"'-:: .: : " ' L,.:-" : ::; Figure 1: Basic HRSG Cycle I~/a~: ,-,-111, S~m-a Out .. ,'.'..:~ ..% r~l~- .. ~'~ "." "-'--',.. ,.'d .~. _:-.,,.; ..:,, -'.~e~ ~ " ~4:. '.'~ .- .. ~',~:." .. Ouilet ~ L'. " %';= Temp r ".~. " To= I , . "-. :ii;.;, ".. " .. % ;)e~~~rheate r ~orator Eo0nomize r.. - ..). !~!: Figure 2: Pinch and Approach Points HRSG BASIC DESIGN CONCEPTS The HRSG design first begins with the combined cycle systems engineer employed by the either the owner or the EPC. The systems engineer begins with a desire to meet the plant's required net electrical output (MW) and heat rate (BTUs/MW). The first decision to be made is the choice of gas turbine size. As a general guideline, the gas turbine will represent 66% of the plant's electrical output assuming that the HRSG does not employ a duct burner.

7 The remaining output will be supplied by the steam turbine. For example, if the plant's requirement is 250 MW, then 165 MW would be supplied from the gas turbine and 85 MW would be supplied by the steam turbine. 60 Hz gas turbines currently available in the 165 MW class range are the GE's Frame 7FA, Siemens Westinghouse's 501F, Siemens' , and ABB's GT24. If it is desired that the steam turbine supply an additional percentage of the plant's output, this can be increased up to 50% if a duct bumer is added A steam turbine that supplies 50% of the output to a combined cycle plant using one of the aforementioned gas turbines would increase the plant's overall output from 250 MW to 335 MW. With the plant's size and gas turbine choice made, the combined cycle systems engineer proceeds next to determine the plant's heat rate.

8 This becomes a tradeoff between capital cost and efficiency. The lowest cost option would be a one pressure level, non reheat HRSG resulting in relatively poor heat rate. To obtain better heat rates with associated higher costs the following ranking should be used: 2 Copyright (C) 2000 by ASMEC osts HRSG Type heat Rate Low 1 Pressure Level, non reheat High Low to medium 2 Pressure Levels, non reheat Medium 3 Pressure Levels, non reheat Medium Medium to high 3 Pressure Levels, reheat High 3 Pressure Levels, reheat Low Optimizing pinch point and approach temperatures follows the selection of the pressure level. Decreasing the pinch point and approach temperature results in higher efficiency, but higher capital cost. OPTIMIZATION METHODOLOGY Most combined cycle optimizations should follow the steps summarized below: Determine the plant goals; , the amount and value of the Power , emission limits, fuel availability and costs, transmission requirements and/or limitations, forecasted Generation load schedules, target electricity market price and/or other requirements and goals.

9 Identify new plant requirements for meeting the Generation goals. Develop the initial design, operation parameters, capital costs, schedules and economics. Obtain the necessary equipment and construction bids. Select the best option(s) based on economics and other factors. Determining the Generation and business goals is the most important first step because this will usually lead to the identification of the most competitive plant configuration. Often, there is a tendency to begin these studies with a previous unit or a manufacturer's design to keep engineering costs low, or because of short schedules. However, this can lead to over looking the best options for optimized efficiency and plant costs, which leads to higher financial returns. OPTIMIZATION EXAMPLE The combined cycle plant options addressed in this paper are typical process steam supply and electrical Generation cases.

10 The selection of the performance, fuel cost, plant cost and economic parameters are based on a conservative approach. Actual project values will probably differ considerably. Four cases are developed. Table 1 shows the common design inputs for the selected 4 cases. Table 1. Common Design Input UI' Model Number Number ot CT's Perf P0mt Dry Bulb "l'emp - F PerI-Pomt Wet Bulb Temp - F Capacgy Factor - % Book Lile i'ax Lile Commercial Operating Year Base Year Capital Costs Esc Rate - %/yr o&M Costs Esc Rate - %/yr .. steam Price Esc Rate - %/yr Prunary Fuel Type / Cost Secondary Puel Type / Cost Primary / Secondary Fuel Esc Rate' %/yr GE PG7241(FA)-60 Hz 66 57 85 20 20 2002 1999 Natural Gas / $ US/MBtu No. 2 Fuel Oil / $ US/MBtu 3 Copyright (C) 2000 by ASMEThe following Table 2 shows the selected design cases with different pinch and approach points.