By-Shailesh Shambhuling Sakule

TE(MECH) DIV-BROLL NO-247

“ ANALYSIS OF GAS TURBINE INTEGRATED

COGENERATION PLANT”

Guided By-Prof. S.M.Narayankar

INTRODUCTIONWhat is Cogeneration?

Cogeneration or combined heat and power (CHP) system is defined as

generation of process heat and shaft work from a single fuel source to improve

energy utilization in a process industry.

Cogeneration is the “simultaneous production of electrical and thermal energy

from a single fuel.”

Gas turbine based cogeneration plant (GTCP) is one of the important

cogeneration plants with less pollution. Cogeneration is a thermodynamically efficient use of fuel.

BACKGROUND Cogeneration is a form of local or distributed generation as heat and power

production take place at or near the point of consumption. For the same output of useful energy, cogeneration uses far less fuel than does traditional separate heat and power production, which means lower greenhouse gas (GHG) emissions as fossil fuel use is reduced.

Several studies in India and other parts of the world, point to the sugar industry as a prime candidate for supplying low cost, non-conventional power via cogeneration. The different systems have been designed for electricity generation from all types of wastes. The major power outages in North America and Europe have resulted in focus on developing energy technologies like domestic scale micro CHP (combined heat and power) to reduce the reliance of the consumers on large generators and the grid.

The largest potential for increased utilization of cogeneration is in the industrial sector. In the United States, the industrial sector is responsible for approximately one third of the country‘s total energy consumption.

The industrial sector‘s direct GHG emissions account for 20 percent of the U.S. total, and an additional 9 percent of U.S. GHG emissions come from centrally generated electricity consumed in the industrial sector . Direct industrial emissions come from on-site combustion of fossil fuels and from non-energy related process emissions.

GAS TURBINE A gas turbine, also called a combustion turbine, is a type of 

internal combustion engine. It has an upstream

rotating compressor coupled to a downstream turbine, and

a combustion chamber in-between.

The basic operation of the gas turbine is similar to that of

the steam power plant except that air is used instead of

water. Fresh atmospheric air flows through a compressor

that brings it to higher pressure. Energy is then added by

spraying fuel into the air and igniting it so the combustion

generates a high-temperature flow

A simple cycle gas turbine can achieve energy conversion efficiencies

ranging between 20 and 35 percent. When waste heat is captured from

these systems for heating or industrial purposes, the overall energy cycle

efficiency could approach 80 percent.

TYPES OF GAS TURBINE COGENERATION SYSTEMa) Open-cycle gas turbine cogeneration systems

The air is delivered through a diffuser to a constant-pressure combustion chamber, where fuel

is injected and burned. The diffuser reduces the air velocity to values acceptable in the

combustor. There is a pressure drop across the combustor in the range of 1.2%. Combustion

takes place with high excess air. The exhaust gases exit the combustor at high temperature

and with oxygen concentrations of up to 15-16%.

The highest temperature of the cycle appears at this point; the higher this temperature is, the

higher the cycle efficiency is. The upper limit is placed by the temperature the materials of

the gas turbine can withstand, as well as by the efficiency of the cooling blades. With current

technology this is about 1300°C.

Fig . Open Cycle Gas Turbine Cogeneration System

The high pressure and temperature exhaust

gases enter the gas turbine producing

mechanical work to drive the compressor and

the load (e.g. electric generator).

The exhaust gases leave the turbine at a

considerable temperature (450-600°C), which

makes high-temperature heat recovery ideal.

This is affected by a heat recovery boiler of

single-pressure or double pressure, for more

efficient recovery of heat.

b) Closed cycle gas turbine cogeneration systems

In the closed-cycle system, the working fluid (usually helium or air) circulates in a closed

circuit. It is heated in a heat exchanger before entering the turbine, and it is cooled down

after the exit of the turbine releasing useful heat. Thus, the working fluid remains clean and

it does not cause corrosion or erosion.

Fig . Closed Cycle Gas Turbine Cogeneration system

Need for Cogeneration

Thermal power plants are a major source of electricity supply in India.

The conventional method of power generation and supply to the

customer is wasteful in the sense that only about a third of the primary

energy fed into the power plant is actually made available to the user in

the form of electricity (Fig).

In conventional power plant, efficiency is

only 35% and remaining 65% of energy is

lost. The major source of loss in the

conversion process is the heat rejected to the

surrounding water or air due to the inherent

constraints of the different thermodynamic

cycles employed in power generation.

Fig. Schematic of Gas turbine Integrated Cogeneration System

TARGETING OF GTCP FOR MINIMUM FUEL CONSUMPTION

An analytical methodology for energy integration of gas turbine and regenerator into

process plant is developed in this section for the minimum fuel consumption.

Methodology proposed by Varghese and Bandyopadhyay for energy integration of fired

heater with overall process is appropriately modified to apply for integration of gas turbine

and regenerator into process plant.

a) Targeting GTCP at minimum power to heat ratio (Rmin)

 The minimum flue gas flow rate can be targeted graphically by matching flue gas line drawn

from flue gas temperature at gas turbine exit with process GCC(grand composite curve) as

shown in Fig.

The minimum mass flow rate of flue gas

(mgmin) is given by following equation:

mgmin =

Tgto= gas turbine exit temperatureQp &Qk= required process heat dutyCpg= specific heat capacity at constant pressureTk=Shifted Temperature

 

where, Qk and Tk are coordinates of one of the vertexes from process plant GCC and Cpg is specific

heat capacity at constant pressure of flue gas.

The maximum of mass flow rate of flue gas calculated for each vertex represents the minimum mass

flow rate of flue gas (mgmin). Mass balance of flue gas flow rate can be written as:

ma+mf=mg

b) Targeting GTCP at critical regeneration limit (Rc) Heat availability increases with increasing mass flow rate in flue gas at any

pressure ratio. This increased heat in flue gas, above the minimum required value,

can be utilized through regeneration. By increasing flow rate of flue gas,

compressed air can be heated maximum up to flue gas temperature at gas turbine

exit with minimum approach temperature i.e. (Tgto - ΔT). Increasing flue gas flow

rate beyond a critical limit (mg at Rc) increases the stack losses and thereby,

deteriorates overall energy efficiency of the system.

PERFORMANCEOverall Plant Performance

Overall Plant Heat Rate (kCal/kWh)

Where,

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)

Overall Gas Turbine Efficiency (%) (Turbine & Compressor)

ADVANTAGES & DISADVANTAGES 

ADVANTAGES-

There are several advantages to using a gas power plant to generate electrical power as compared to

other systems. 

i) Gas turbine power plants can be started up and run at full capacity in only 10 to 20 minutes, making

them well suited as backup plants for utility companies that require additional electricity immediately.

ii) Increased reliability. System is independent of the grid and therefore immune to grid-level blackouts.

iii) Gas turbine systems also require much less water than steam power plants.

iv) Increased efficiency. CHP systems act as energy multiplier which:

saves energy

reduces carbon emissions by up to 30 percent

v) Very high power-to-weight ratio, compared to reciprocating engines.

DISADVANTAGES-

Gas turbine power plants have disadvantages as well

i) The power needed to drive the compressor reduces the net outputs,

consuming more fuel to do the same amount of work.

ii) Only suitable where there is a need for both electricity and hot water on

site.

iii) Heating demand must be continuous

iv) High frequency noise from the compressor is objectionable.

v) It has low thermal efficiency.

vi) cogeneration system might take 6-18 months to construct.

APPLICATION 

i) Gas turbine power plants are used to supply peak loads in steam or hydro-plants.

 

ii) They are used as standby plants for hydro-electric power plants.

 

iii) Gas turbines are used in jet, aircraft and ships.

 

iv) Gas turbine power plant is useful in marine applications.

v) These are useful in power generation systems.

CONCLUSION A methodology is proposed to integrate gas turbine and regenerator with process plant directly

at minimum fuel consumption. In addition to this, thermodynamic analysis of GTCP with

regeneration is presented on gas turbine pressure ratio versus power to heat ratio diagram.

The ultimate choice concerning the installation and operation of a cogeneration system is neither

a simple nor easy one.

There are many factors that affect such a decision and each of these must be considered before an

educated decision can be made.

REFERENCES[1] Y.S.H. Najjar, Gas turbine cogeneration systems: a review of some novel cycles,Appl. Therm. Eng. 20 (2) (2000) 179e197.[2] T. Heppenstall, Advanced gas turbine cycles for power generation: a criticalreview, Appl. Therm. Eng. 18 (9e10) (1998) 837e846.[3] A. Poullikkas, An overview of current and future sustainable gas turbinetechnologies, Renew. Sustain. Energy Rev. 9 (5) (2005) 409e443.[4] F.J.L. Renirie, Some asfects influencing the design of gas turbine co-generationplants, Resour. Conserv. 7 (1981) 267e286.[5] W.F. Kenney, Energy Conservation in the Process Industries, Academic Press,1984.[6] http://en.wikipedia.org/wiki/Cogeneration[7] J. Manninen, X.X. Zhu, Optimal gas turbine integration to the process industries,Ind. Eng. Chem. Res. 38 (11) (1999) 4317e4329.[8]http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=4237791&url=http%3A%2F%2Fieeexplore.ieee.org%2Fxpls%2Fabs_all.jsp%3Farnumber%3D4237791