Combined Cycle Power Plant
Combined Cycle Power Plant.doc
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What is a Combined Cycle Power Plant? As opposed to a Simple Cycle Power Plant that is not very efficient as the heat from the turbine is "wasted" to the atmosphere, a Combined Cycle Power Plant ("CCPP") recovers this heat. CCPP's, also referred to as "Combined Cycle Cogeneration, is a natural gas (or diesel) turbine coupled with an electrical generator, together, referred to as a "genset." The exhaust heat from the gas turbine is directed to a waste heat recovery boiler ("WHRB") or heat-recovery steam generator ("HRSG"). The steam from the WHRB or HRSG where the steam is directed to a steam turbine generator where the steam is used to power the steam turbine genset. By capturing the waste heat of the gas turbine in a Combined Cycle Power Plant, and putting it to work, the overall thermal efficiency of the plant is increased. In a typical cogeneration plant, electric power is generated but some of the steam from the WHRB or HRSG is used for process heat. By diagram below, the combined-cycle power plant combines the Rankine (steam turbine) and Brayton (gas turbine) thermodynamic cycles by using heat recovery boilers to capture the energy in the gas turbine exhaust gases for steam production to supply a steam turbine as shown in the figure "Combined-Cycle Cogeneration". Process steam can be also provided for industrial purposes. Fossil fuel-fired (central) power plants use either steam or combustion turbines to provide the mechanical power to electrical generators. Pressurized high temperature steam or gas expands through various stages of a turbine, transferring energy to the rotating turbine blades. The turbine is mechanically coupled to a generator, which produces electricity. Steam Turbine Power Plants: Steam turbine power plants operate on a Rankine cycle. The steam is created by a boiler, where pure water passes through a series of tubes to capture heat from the firebox and then boils under high pressure to become superheated steam. The heat in the firebox is normally provided by burning fossil fuel (e.g. coal, fuel oil or natural gas). However, the heat can also be provided by biomass, solar energy or nuclear fuel. The superheated steam leaving the boiler then enters the steam turbine throttle, where it powers the turbine and connected generator to make electricity. After the steam expands through the turbine, it exits the back end of the turbine, where it is cooled and condensed back to water in the surface condenser. This condensate is then returned to the boiler through high-pressure feedpumps for reuse. Heat from the condensing steam is normally rejected from the condenser to a body of water, such as a river or cooling tower. Steam turbine plants generally have a history of achieving up to 95% availability and can operate for more than a year between shutdowns for maintenance and inspections. Their unplanned or forced outage rates are typically less than 2% or less than one week per year. Modern large steam turbine plants (over 500 MW) have efficiencies approaching 40-45%. These plants have installed costs between $800 and$2000/kW, depending on environmental permitting requirements. Combustion (Gas) Turbines: Combustion turbine plants operate on the Brayton cycle. They use a compressor to compress the inlet air upstream of a combustion chamber. Then the fuel is introduced and ignited to produce a high temperature, high-pressure gas that enters and expands through the turbine section. The turbine section powers both the generator and compressor. Combustion turbines are also able to burn a wide range of liquid and gaseous fuels from crude oil to natural gas. The combustion turbine’s energy conversion typically ranges between 25% to 35% efficiency as a simple cycle. The simple cycle efficiency can be increased by installing a recuperator or waste heat boiler onto the turbine’s exhaust. A recuperator captures waste heat in the turbine exhaust stream to preheat the compressor discharge air before it enters the combustion chamber. A waste heat boiler generates steam by capturing heat form the turbine exhaust. These boilers are known as heat recovery steam generators (HRSG). They can provide steam for heating or industrial processes, which is called cogeneration. High-pressure steam from these boilers can also generate power with steam turbines, which is called a combined cycle (steam and combustion turbine operation). Recuperators and HRSGs can increase the combustion turbine’s overall energy cycle efficiency up to 80%. Combustion (natural gas) turbine development increased in the 1930’s as a means of jet aircraft propulsion. In the early 1980’s, the efficiency and reliability of gas turbines had progressed sufficiently to be widely adopted for stationary power applications. Gas turbines range in size from 30 kW (micro-turbines) to 250 MW (industrial frames). Industrial gas turbines have efficiencies approaching 40% and 60% for simple and combined cycles respectively. The gas turbine share of the world power generation market has climbed from 20 % to 40 % of capacity additions over the past 20 years with this technology seeing increased use for base load power generation. Much of this growth can be accredited to large (>500 MW) combined cycle power plants that exhibit low capital cost (less than $550/kW) and high thermal efficiency. The capital cost of a gas turbine power plant can vary between $35000-$950/kW with the lower end applying to large industrial frame turbines in combined cycle configurations. Availability of natural gas-fired plants can exceed 95%. In Canada, there are 28 natural gas-fired combined cycle and cogeneration plants with an average efficiency of 48 %. The average power output for each plant was 236 MW with an installed cost of around $ 500/kW. Simple Cycle Power Plants (Open Cycle) The modern power gas turbine is a high-technology package that is comprised of a compressor, combustor, power turbine, and generator, as shown in the figure "Simple-Cycle Gas Turbine". In a gas turbine, large volumes of air are compressed to high pressure in a multistage compressor for distribution to one or more combustion gases from the combustion chambers power an axial turbine that drives the compressor and the generator before exhausting to atmosphere. In this way, the combustion gases in a gas turbine power the turbine directly, rather than requiring heat transfer to a water/steam cycle to power a steam turbine, as in the steam plant. The latest gas turbine designs use turbine inlet temperatures of 1,500C (2,730F) and compression ratios as high as 30:1 (for aeroderivatives) giving thermal efficiencies of 35 percent or more for a simple-cycle gas turbine. How to calculate overall thermal efficiency of combined cycle power plants – a sample CCGT presented Calculating or predicting the overall performance of a combined cycle power plant, specifically a combined cycle gas turbine (CCGT) power plant is sometimes difficult for most design engineers. Your favorite energy technology expert again comes to the rescue – Engineer Marcial T. Ocampo – has derived the following equation to guide the design engineer and project finance modeler or business development engineer in predicting the overall thermal efficiency of the combined cycle. Here’s the step by step derivation: Let Total Energy Input = Total Energy Output = 100%. Then let’s assume that the proportion of the energy input going to the gas turbine (GT) and to the waste heat recovery steam generator (SG) is around: GT = 1/3 = 33.33% SG = 1 – 1/3 = 2/3 = 66.67% The energy input to the steam generator is then split into 85% (up to 90%) being recovered as steam energy in the boiler while the balance of 15% (or 10%) is lost to the atmosphere (smoke stack, radiation and convection loss). The steam energy is then captured in the steam turbine with an efficiency of 37% and the balance (63%) is lost to the cooling water in the condenser. The mechanical energy captured in the GT and ST shafts are then coupled in a clutch (100% efficiency) to drive finally the electric generator (EG) with mechanical to electrical conversion efficiency of 98% (balance of 2% lost to heating in the generator windings and generator cooling system). The final formula is thus: (with correction for GT efficiency per suggestion of George – see his comments) OE = (GT x GTE + SG x BE x STE) x ME x GE where OE = overall energy efficiency, % of fuel energy input (as GHV or LHV) GT = 1/3 = 33.33% (assumption) GTE = gas turbine efficiency = say 90% – 95% (the reader please comment or advice) = function of heat loss from GT due to conduction, convection and radiation from GT casing, any heat loss due to friction resulting in higher GT exhaust is captured, however, by the heat recovery steam generator SG = 1 – GT = 1 – 1/3 = 2/3 = 66.67% (balance) BE = waste heat recovery boiler efficiency = 85% (up to 90%) = function of exit flue gas temperature (energy lost), gas turbine exhaust temperature (energy input), boiler design, flue gas composition (fuel, excess air) STE = steam turbine efficiency = 37% (up to 40%) = function of steam inlet temperature, steam exhaust pressure, condenser vacuum pressure, cooling system, steam turbine design, steam quality ME = mechanical drive shaft and clutch efficiency = 100% = function of drive shaft design, clutch system, bearing lubrication, mechanical design, windage losses (air drag) GE = electric generator efficiency = 98% = function of generator design, voltage, windage losses (air drag), type of gas cooling (hydrogen, air), bearing lubrication Putting all together now, the predicted overall thermal efficiency of the combined cycle power plant is: OE = (33.33 x 95% + 66.67% x 85% x 37%) x 100% x 98% = 51.58% For instance, if we raise the gas turbine efficiency from 95% to 98%, the resulting overall efficiency would be raised to OE = (33.33 x 98% + 66.67% x 85% x 37%) x 100% x 98% = 52.56% For instance, if we raise the boiler efficiency from 85% to 90%, the resulting overall efficiency of the combined cycle power plant is raised to: OE = (33.33 x 98% + 66.67% x 90% x 37%) x 100% x 98% = 53.77% Further raising the steam turbine efficiency to 40% will result in a much higher efficiency of: OE = (33.33 x 98% + 66.67% X 90% X 40%) X 100% X 98% = 55.53% I guess this provides an upper limit of what a CCGT could deliver, around 56%. The only way to go higher than this is to improve further the gas turbine efficiency beyond 98%, boiler efficiency beyond 90%, raising the steam turbine efficiency beyond 40% and optimizing the proportion of energy output thru the gas turbine (currently 1/3) and the steam generator (balance of 2/3). The author has developed an state of the art project finance model for a 25-year economic life CCGT and may be requested thru this link: The model has the following capability: 1) Given the all-in capital cost (EPC, installation and erection, taxes and duties, project development, regulatory costs, working capital, interest during construction), O&M costs (variable and fixed O&M, recurring regulatory costs, property taxes, ROW and land lease, property insurance, business interruption insurance, etc) and electricity tariff (industries, distribution utilities, national grid, wholesale electricity spot market) ==> it determines the maximum price of the natural gas fuel needed to meet the 15% p.a. DCF IRR for equity investment. 2) Alternatively, if the capital cost, O&M costs, electricity tariff and fuel cost (natural gas, gas oil, bunker oil) are fixed, the model will calculate the net present value and project IRR (return of investment or ROI from the project cash flow), NPV and equity IRR (return on equity or ROE from the equity cash flow) or the more stringent financial analysis tool of bankers today –> the NPV and dividends IRR (return on dividends from dividends cash flow). Other variations of the model objective include determining the maximum capital cost given the O&M cost, tariff and fuel cost to meet the 15% equity IRR hurdle rate for making equity investments. The model is currently capable of analyzing several CCGT machines / models from various manufacturers for a number of plant locations with specific type of cooling system (sea water once thru, lake water once thru, river water or deep well cooling tower, and dry cooling with radiator). By using case switches, the model will go over each pair of sensitivity (machine and plant location/type of cooling) and calculate the fuel cost needed to meet 15% equity IRR. It uses goal seek to set the equity cash flow NPV to zero for each of sensitivity pair.