Cogeneration

By Engr. Shehroze Ali Baig

Cogeneration
Cogeneration, also known as Combined Heat and Power (CHP), is a
highly efficient method of generating electricity and useful thermal
energy (such as steam or hot water) simultaneously from a single fuel
source.

Integration of Power and Heat Generation:

In traditional power generation methods, such as Cogeneration systems are designed to capture
gas or steam cycles, a significant amount of and utilize this waste heat, thereby increasing
thermal energy is lost as waste overall efficiency and reducing energy losses.
Components and Operation

Prime Mover: Cogeneration systems typically use a gas turbine, reciprocating engine, or
steam turbine as the primary generator of mechanical energy from fuel combustion.

Heat Recovery: After electricity generation, the waste heat from the exhaust gases or
from cooling systems is captured.

Utilization: The recovered heat is used for various purposes, including:

• Heating buildings or facilities (district heating)

• Providing steam for industrial processes
• Preheating boiler feedwater
• Absorption cooling for air conditioning
Efficiency Benefits:

Cogeneration systems can achieve efficiencies of 70% to 80% or higher, compared to

around 35% to 45% for conventional power plants that do not utilize waste heat.
By maximizing the use of fuel energy for both electricity and heat production,
cogeneration reduces fuel consumption and lowers greenhouse gas emissions per
unit of useful energy produced.

Types of Cogeneration Systems:

• Combined Cycle: Integrates a gas turbine with a steam turbine, where the exhaust
heat from the gas turbine is used to generate steam for the steam turbine.
• Combined Heat and Power (CHP): Utilizes various configurations such as gas
engines, steam turbines, or combined gas-steam cycles to generate both electricity
and heat.
• Let us examine the operation of a process-
heating plant closely. Disregarding any heat
losses in the piping, all the heat transferred to
the steam in the boiler is used in the process-
heating units, as shown in Fig. 10–20.
Therefore, process heating seems like a
perfect operation with practically no waste of
energy.
• However, from another perspective, things
aren't as perfect. Furnaces operate at very
high temperatures, about 1400°C, which
means they have high-quality energy. But
when this energy is used to heat water and
make steam at around 200°C or lower, it's a
process with a lot of waste.
• This waste leads to a loss in the
potential for doing useful work, called
exergy. Using high-quality energy for
tasks that could be done with lower-
quality energy isn't a wise choice,
according to the laws of
thermodynamics.
• Industries that use large amounts of
process heat also consume a large
amount of electric power. Therefore, it
makes economical as well as
engineering sense to use the already-
existing work potential to produce
power instead of letting it go to waste
• The schematic of an ideal steam-
turbine cogeneration plant is shown
in Fig. 10–21.
• Let us say this plant is to supply
process heat Q . p at 500 kPa at a
rate of 100 kW. To meet this
demand, steam is expanded in the
turbine to a pressure of 500 kPa,
producing power at a rate of, say, 20
kW. The flow rate of the steam can
be adjusted such that steam leaves
the process heating section as a
saturated liquid at 500 kPa.
• Steam is then pumped to the boiler
pressure and is heated in the boiler
to state 3. The pump work is usually
very small and can be neglected.
Disregarding any heat losses, the
rate of heat input in the boiler is
determined from an energy balance
to be 120 kW
• Probably the most striking feature of
the ideal steam-turbine
cogeneration plant shown in Fig. 10–
21 is the absence of a condenser.
• Thus, no heat is rejected from this plant
as waste heat. In other words, all the
energy transferred to the steam in the
boiler is utilized as either process heat
or electric power. Thus, it is appropriate
to define a utilization factor u for a
cogeneration plant as
COMBINED GAS–VAPOR
POWER CYCLES

• The combined cycle power

plant is a highly efficient
method of electricity
generation that integrates two
thermodynamic cycles: the
Brayton cycle (gas turbine) and
the Rankine cycle (steam
turbine).
Working of Combined Cycle Power Plant:
Gas Turbine Cycle (Brayton Cycle):
• Compression: Air is drawn into the compressor where it is compressed
to high pressure.
• Combustion: The compressed air is mixed with fuel and ignited in the
combustion chamber, generating high-temperature, high-pressure
exhaust gases.
• Expansion: The hot gases expand through the turbine, driving it to
produce mechanical power.
• Exhaust: The exhaust gases leave the gas turbine at very high
temperatures
Heat Recovery Steam Generator (HRSG):
• Purpose: The exhaust gases from the gas turbine are directed into the
HRSG.
• Heat Transfer: The HRSG utilizes the heat energy in the exhaust gases to
produce steam by heating water.
• Steam Generation: The steam produced in the HRSG is at high pressure
and temperature, suitable for driving a steam turbine.
Steam Turbine Cycle (Rankine Cycle):
• Expansion: The high-pressure steam from the HRSG expands through the
steam turbine, similar to a traditional steam turbine cycle.
• Mechanical Power: The expansion of steam drives the steam turbine,
which generates additional mechanical power.
• Condensation: After passing through the turbine, the steam is
condensed back into water in a condenser.
• Regeneration: Some plants use regeneration, where the extracted steam
from the steam turbine is used to preheat the feedwater entering the
HRSG, improving overall efficiency.
Gas Turbine
• Gas turbine cycles operate at much
higher temperatures compared to steam
cycles.
• In modern steam power plants, the
maximum temperature of the fluid
entering the turbine is around 620°C
(1150°F), while in gas turbine power
plants, it can exceed 1425°C (2600°F).
• Turbojet engines, used in aircraft,
operate even higher, with temperatures
over 1500°C at the burner exit.
High Temperature
• Higher temperatures mean that gas turbine
cycles have the potential for greater thermal
efficiency.
• This is because higher temperatures result in
more efficient conversion of heat into
mechanical energy. However, there's a
drawback: the exhaust gas from the gas
turbine exits at very high temperatures, usually
above 500°C (932°F).
• This high exhaust temperature limits the
potential efficiency gains because a significant
amount of heat energy is lost with the exhaust
gases.
• To mitigate this drawback,
techniques like regeneration can be
used. Regeneration involves
recovering some of the waste heat
from the exhaust gases to preheat
the air entering the combustion
chamber.
• While regeneration improves
efficiency to some extent, its impact
is limited compared to the inherent
challenge of high exhaust
temperatures in gas turbine cycles.
Fuel & Combustion
• Any material that can be burned to release
thermal energy is called a fuel. Most familiar
fuels are called hydrocarbon fuels.
• A chemical reaction during which a fuel is
oxidized, and a large quantity of energy is
released is called combustion.
• The oxidizer most often used in combustion
processes is air, for obvious reasons—it is free
and readily available.
Combustion
In order to start combustion reaction two things
are important
• Ignition temperature of fuel for e.g approximately
260°C for gasoline.
• the proportions of the fuel and air must be in the
proper range for combustion to begin. For
example, natural gas does not burn in air in
concentrations less than 5 percent or greater
than about 15 percent.
 It ensures that all the
Air–fuel ratio fuel is burned
completely with the
available air.

• A frequently used quantity in the analysis of

combustion processes to quantify the
amounts of fuel and air is the air–fuel ratio AF.
• It is usually expressed on a mass basis and is
defined as the ratio of the mass of air to the
mass of fuel for a combustion process (Fig).
That is,
HEATING VALUE
• It is the amount of energy released when fuel is burnt completely
in a steady flow state.
• The heating value is dependent on the phase of water in the
combustion process.
• If the water is in liquid form, then it has high heating value.
• If the water is in vapor/steam form, then it has low heating value.
Higher Heating value (HHV)
• The heat of combustion of fuels is expressed by the higher and
lower heating values (HHV and LHV).
• The higher heating value (HHV) or gross calorific value is defined
as the amount of heat released when fuel is combusted and the
products have returned to a temperature of 25°C.
• The heat of condensation of the water is included in the total
measured heat.
Lower Heating value (LHV)
• The lower heating value (LHV) is defined as the net calorific value
and is determined by subtracting the heat of vaporization of water
vapor (generated during combustion of fuel) from the higher
heating value.
• Same types of fuels can usually be compared according to their
HHV, whereas the different types of fuels are usually compared
according to their LHV.
• Because hydrogen contents of the different types of Engr fuels are
different from each other (e.g. oil and coal).
 HHV VS LHV

• Useful for understanding the total • More practical for systems where water
energy potential of a fuel, vapor does not condense, giving a true
especially in systems where picture of the energy available for work.
waste heat recovery is possible. • In practical combustion systems, such
• In systems designed to condense as internal combustion engines, gas
the water vapor and recover its turbines, and conventional boilers, the
latent heat, such as condensing water vapor produced during
boilers or some industrial combustion does not condense within
processes the system.
• The HHV is relevant because it • Thus, the energy in the vapor phase is
represents the maximum not recovered. LHV provides a more
possible energy that can be accurate measure of the usable energy
obtained from the fuel. in these contexts.
STIOCHEMETRIC MIXTURE
• A mixture of air and fuel that just contain sufficient oxygen
required to complete combustion of fuel.
• A mixture which has excess air is termed as weak mixture.
• A mixture which has deficiency of air is termed as rich mixture.
• (A/F)s = Mass of air / Mass of fuel
STIOCHEMETRIC MIXTURE
We know that in SI engine air and fuel mixture enters into the engine
cylinder and combust with the help of spark plug thus producing
power.
• Now the ratio of air to fuel in that mixture at which there is just
enough oxygen to completely burn the fuel is known as
STIOCHEMTRIC A/F RATIO
• STIOCHEMTRIC A/F RATIO is the air to fuel ratio that just contain
enough oxygen to burn the fuel completely.
MIXTURE STRENGTH:
• A standard way of indicating the composition of given mixture.
• It is defined as the ratio of stoichiometric of A/F ratio to actual A/F
ratio
Mixture Strength = (A/F)s / (A/F)a
• Mixture strength > 1 (fuel rich)
• Mixture strength < 1 (fuel lean)
• For complete or stoichiometric combustion, all carbon is burned
to carbon dioxide (CO2 ) and all hydrogen is converted into water
(H2O). These two complete combustion reactions are as follows:
Example
• A complete combustion of octane in oxygen is represented by the balanced
combustion equation. The balanced combustion equation is obtained by
making sure we have the same number of atoms of each element on both
sides of the equation. That is, we make sure the mass is conserved.

• Note we often can balance the C and H for complete combustion by

inspection.
• The amount of oxygen is found from the oxygen balance. It is
better to conserve species on a monatomic basis as shown for the
oxygen balance.
• In most combustion processes, oxygen is supplied in the form of
air rather than pure oxygen.
• Air is assumed to be 21 percent oxygen and 79 percent nitrogen on
a volume basis.
• For ideal gas mixtures, percent by volume is equal to percent by
moles. Thus, for each mole of oxygen in air, there exists 79/21 =
3.76 moles of nitrogen.
• Therefore, complete or theoretical combustion of octane with air
can be written as
Percent Excess Air
• In most cases, more than theoretical air is supplied to ensure
complete combustion and to reduce or eliminate carbon
monoxide (CO) from the products of combustion.
• The amount of excess air is usually expressed as percent excess
air
Stoichiometric analysis by volume
• Consider the combustion equation of hydrogen:
2H2 + O2 2H2O
• This tells us that
(i) Hydrogen reacts with oxygen to form steam or water
(ii) Two molecules of hydrogen react with one molecule of oxygen to give
two molecules of steam or water.
• i.e. 2 volumes H2 + 1 Volume O2 2 volumes H2O
• It should be noted from equation that total volume of the reactants is 2
volumes H2 + 1 Volume O2 = 3 volumes H2O
• The total volume of the product is only 2 volumes. There is therefore a
volumetric contraction on combustion
Stoichiometric analysis by Mass
• Consider the combustion equation of hydrogen:
2H2 + O2 2H2O
2(2 x1) + (2 x 16) 2(2x1+16)
36 kg Reactant = 36kg Product
• Since oxygen is accompanied by nitrogen if air is supplied for the combustion, then
the nitrogen should be included in the equation. As nitrogen is inert as far as
chemical reaction is concerned, it will appear on both sides of the equation.
2H2 + O2 + 3.76 N2 2H2O + 3.76 N2
Practice problem
• Write the combustion equation for complete combustion of octane
with 20 percent excess air and Calculate the stoichiometric A/F ratio
and actual A/F of Octane

C8H18 + 12.5 (O2 + 3.76 N2) 8CO2 + 9H2O + 47N2

• With 20 % excess air (The Actual combustion equation will become)

C8H18 + 1.2 [12.5 (O2 + 3.76 N2)] 8CO2 + 9H2O + 0.2 12.5 O2 + 1.2[47N2]
 Mass of Air
Stoichiometric A/F ratio =
Mass of fuel
12.5 (2 𝑋16+3.74 𝑋 2 𝑋14)
• Stoichiometric A/F ratio = = 15.05
(8 X12 + 18 X1)

Mass of Air
Actual A/F ratio =
Mass of fuel
1.2 𝑋12.5 (2 𝑋16+3.74 𝑋 2 𝑋14)
• Actual A/F ratio = = 18.063
(8 X12 + 18 X1)
Balancing the Combustion Equation
• One kmol of octane (C8H18) is burned with air
that contains 20 kmol of O2, as shown in Fig.
Assuming the products contain only CO2,
H2O, O2, and N2, determine the mole number
of each gas in the products and the air–fuel
ratio for this combustion process.

C8H18 + 20 (O2 + 3.76 N2) xCO2 + yH2O + zO2 + wN2

• Note that the coefficient 20 in the balanced equation above represents the number of moles
of oxygen, not the number of moles of air. The latter is obtained by adding 20 3.76 75.2
moles of nitrogen to the 20 moles of oxygen, giving a total of 95.2 moles of air. The air–fuel
ratio (AF) is determined from Eq. 15–3 by taking the ratio of the mass of the air and the mass
of the fuel,
 Mass of Air (NM)air
Actual A/F ratio = =
Mass of fuel NMc +NMH2

20 𝑋4.76 𝑋 29
Actual A/F ratio = = 24.2 KG
(8 X12 + 9 X2)
• That is, 24.2 kg of air is used to burn each kilogram of fuel during
this combustion process