9 gas turbine- cogeneration_3

7/28/2019 9 Gas Turbine- Cogeneration_3

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Dr. Walid Abdelghaffar



Dr. Walid AbdelghaffarThermal Equipment/Cogeneration

CogenerationCogeneration

IntroductionTypes of cogeneration systems

Assessment of cogeneration systemsEnergy efficiency opportunities




Background

Cogeneration first appeared in Europe

and in the U.S.A. during the early partsof the 20th century.

In addition to decreased fuelconsumption, cogeneration results in a

decrease of pollutant emissions.




What is Cogeneration?

Cogeneration is on-site generation andutilisation of energy in different forms

simultaneously by utilising fuel energy atoptimum efficiency in a cost-effective andenvironmentally responsible way.

Cogeneration systems are of severaltypes and almost all types primarilygenerate electricity along with making the

best practical use of the heat, which is aninevitable by-product.




The most prevalent example of cogeneration is the generation of electric

power and heat. The heat may be used for generating

steam, hot water, or for cooling through

absorption chillers. In a broad sense, the system, that

produces useful energy in several forms by

utilising the energy in the fuel such thatoverall efficiency of the system is veryhigh, can be classified as Cogeneration

System or as a Total Energy System.






Conventional utility power plants utilise the highpotential energy available in the fuels at the endof combustion process to generate electricpower.

However, substantial portion of the low-endresidual energy goes to waste by rejection to

cooling tower and in the form of hightemperature flue gases.

On the other hand, a cogeneration process

utilises first the high-end potential energy togenerate electric power and then capitalises onthe low-end residual energy to work for heatingprocess, equipment or such similar use.




IntroductionIntroduction

• Increased efficiency of energy conversion anduse

• Lower emissions, especially CO2

• Ability to use waste materials• Large cost savings

• Opportunity to decentralize the electricity

generation

• Promoting liberalization in energy markets

Benefits of Cogeneration / CHP)






Assessment of cogeneration systems

Energy efficiency opportunities




Type of Cogeneration SystemsType of Cogeneration Systems

•Steam turbine

•Gas turbine

•Reciprocating engine

•Other classifications:

- Topping cycle- Bottoming cycle





• Widely used in CHP applications

• Oldest prime mover technology

• Capacities: 50 kW to hundreds of MWs• Thermodynamic cycle is the “ Rankin cycle”

that uses a boiler

• Most common types•Back pressure steam turbine

•Extraction condensing steam turbine

Steam Turbine Cogeneration System





• Steam exits the turbine at a higher pressure

that the atmospheric

Back Pressure Steam Turbine

Fuel

Figure: Back pressure steam turbine

Advantages:

-Simple configuration-Low capital cost-Low need of cooling water-High total efficiency

Disadvantages:-Larger steam turbine-Electrical load and outputcan not be matched

Boiler Turbine

Process

HP Steam

Condensate LP

Steam





• Steam obtained byextraction from anintermediate stage

• Remaining steam is

exhausted

• Relatively high capitalcost, lower totalefficiency

• Control of electricalpower independent of thermal load

Extraction Condensing Steam

Turbine

Boiler Turbine

Process

HP Steam

LP SteamCondensate

Condenser

Fuel

Figure: Extraction condensing steam turbine





• Operate on thermodynamic “ Brayton cycle”• atmospheric air compressed, heated,

expanded

• excess power used to produce power

• Natural gas is most common fuel

• 1MW to 100 MW range

• Rapid developments in recent years• Two types: open and closed cycle

Gas Turbine Cogeneration System




Typical Heat-to-Power Ratios for EnergyIntensive Industries

1.92.51.5Paper

1.22.50.8Food

23.00.8Fertilizer

2.02.51.5Pharmaceuticals

3.14.51.1Breweries

AverageMaximumMinimumIndustry




Gas turbine based cogeneration system

This type of system works on the basic principle of Bryton cycle of thermodynamics.

Air drawn from the atmosphere is compressed and

mixed in a predetermined proportion with the fuel in acombustor, in which the combustion takes place. The flue gases with a very high temperature from the

combustor are expanded through a gas turbine, whichdrives electric generator and air compressor.

A portion of mechanical power is used for compressionof the combustion air: the balance is converted intoelectric power.

The exhaust flue gases from the gas turbine,

typically at a high temperature of 480-540 C, acts asa heat source from which the heat is recovered inthe form of steam or hot air for any desiredindustrial application.





Industrial gas turbine based power plants installed togenerate only electric power operate at the thermalefficiency of 25-35% only depending of type and size of gas turbine.

Aero derivative gas turbines operate at marginal higherefficiency than the conventional industrial heavy-dutymachines.

With recovery of heat in exhaust flue gases in a waste

heat recovery boiler (WHRB) or heat recovery steamgenerator (HRSG) to generate the steam, overall plantefficiency of around 85-90% is easily achieved.

As an alternative, the heat of exhaust flue gases can

also be diverted to heat exchanger to generate hot wateror hot air (District Heating purpose in foreign countries)instead of generating steam. The Figure shows aschematic of Gas Turbine based cogeneration system.




Gas turbine based cogeneration systemwith supplementary fired WHRB





Compared to steam turbine based cogeneration system,the gas turbine based cogeneration system is ideal for thechemical process industries where the demand of steam is

relatively high and fairly constant in comparison to that of steam.

Gas turbine based cogeneration system gives a better

performance with clean fuels like natural gas, or non-ashbearing or low ash bearing liquid hydrocarbon fuels likeNaphtha, High speed Process Consumer diesel, etc. Though high ash bearing hydrocarbon based fuels like fueloil, crude oil or residual fuel oil can also be fired in the gasturbines, but with some inherent problems like frequentcleaning of gas turbine, more maintenance and spares,etc.





•Open Brayton cycle:

atmospheric air atincreased pressure tocombustor

Open Cycle Gas Turbine

Air

G

Compressor Turbine

HRSG

Combustor

Fuel

Generator

Exhaust

Gases

Condensatefrom Process

Steam to

Process

•Old/small units: 15:1

New/large units: 30:1

•Exhaust gas at 450-

600 oC

•High pressure steamproduced: can drivesteam turbine

Figure: Open cycle gas turbine cogeneration





•Working fluid circulates ina closed circuit and doesnot cause corrosion or

erosion•Any fuel, nuclear or solar

energy can be used

Closed Cycle Gas Turbine

Heat Source

G

Compressor Turbine

Generator

Condensate

from Process

Steam to

Process

Heat Exchanger

Figure: Closed Cycle Gas Turbine Cogeneration System





•Used as direct mechanical drives

Reciprocating Engine Cogeneration

Systems

Figure: Reciprocating engine cogeneration system (UNESCAP, 2000)

•Many advantages:operation,

efficiency, fuelcosts

•Used as directmechanical drives

•Four sources of usable waste heat





• Supplied fuel first produces powerfollowed by thermal energy

• Thermal energy is a by product used forprocess heat or other

• Most popular method of cogeneration

Topping Cycle





Bottoming Cycle

• Primary fuel produces high

temperature thermal energy

• Rejected heat is used to generate

power

• Suitable for manufacturing processes




Assessment of Cogenerationssessment o ogenerat on

SystemsSystems

• Overall Plant Heat Rate (kCal/kWh):

Ms = Mass Flow Rate of Steam (kg/hr)

hs = Enthalpy of Steam (kCal/kg)

hw = Enthalpy of Feed Water (kCal/kg)

• Overall Plant Fuel Rate (kg/kWh)

Performance Terms & Definitions

)(

)(

kW Output Power

hwhs x Ms −

)(

)/(*

kW Output Power

hr kgnConsumptioFuel

A f C it t




Assessment of Cogenerationssessment o ogenerat on

SystemsSystems

• Steam turbine efficiency (%):

Steam Turbine Performance

Gas Turbine Performance

• Overall gas turbine efficiency (%) (turbine

compressor):

100)/(

)/( x

kgkCalTurbinetheacrossdrop Enthalpy Isentropic

kgkCalTurbinetheacross Drop Enthalpy Actual

100)/()/(

860)( x

kgkCalFuelof GCV xhr kgTurbineGas for Input Fuel

xkW Output Power




Assessment of Cogeneration Assessment of Cogeneration

SystemsSystems

• Heat recovery steam generator efficiency(%):

Ms = Steam Generated (kg/hr)

hs = Enthalpy of Steam (kCal/kg)

hw = Enthalpy of Feed Water (kCal/kg)

Mf = Mass flow of Flue Gas (kg/hr)t-in = Inlet Temperature of Flue Gas (0C)

t-out = Outlet Temperature of Flue Gas (0C)

Maux = Auxiliary Fuel Consumption (kg/hr)

Heat Recovery Steam Generator (HRSG)

Performance

100)]/([)]([

)( x

kgkCalFuelof GCV x M t t Cp x M

hh x M

auxout in f

wss

+−

−




Energy Efficiency OpportunitiesEnergy Efficiency Opportunities

Gas Turbine Cogeneration System

Gas turbine – manage the following parameters:

• Gas temperature and pressure

• Part load operation and starting & stopping• Temperature of hot gas and exhaust gas

• Mass flow through gas turbine

• Air pressure



Dr. Walid Abdelghaffar

9 gas turbine- cogeneration_3

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