EFFECT OF PREHEATING WITH A HELP OF RECUPERATOR ON COMBUSTION EFFICIENCY USING CANTERA

Aim : To calculate the effect of preheating the inlet air on Adiabatic Flame temperature, Cycle efficicency and Fuel savings

Theory :

The products of combustion having high temperature expand in turbines(energy harvesting unit) and cool down to certain temperature. The efficiency of turbine never reaches 100% so after expansion there is some amount of work or heat that can be extracted from these product gases. This heat is used for heating fresh air before entering the combustion chamber . A heat exchanger or recuperator is used for this purpose.

Rough sketch :

When preheated air is used for combustion instead of normal air, with same amount of fuel the internal energy entering the chamber is higher than in normal case. This increased energy is expected to raise the inside temperature. The exhaust temperature would increase and would lead to inlet air temperature being hotter than before.

This would lead to heat accumulation in the combustion chamber. To, overcome this it is recommended that the heat output of the reaction be same even after the preheating, this balancing would give constant temperature of exhaust gas , Here the exhaust temperature is assumed constant which is 1700 K

Effect of preheating on AFT :

The balanced reaction of combustion of methane is

It consists air having 21% of oxygen and 79% of nitrogen on mole fraction basis. The total moles of air required are 9.52.It has 2 moles of oxygen and 7.52 moles of nitrogen. The inlet air is assumed to be preheated from temperatures 298 K to 600 K .

Mass conservation and Energy conservation :

The heat released from reaction has to be kept constant while using preheating. so, for calculating efficiency the amount of heat which should be kept constant is to be calculated

This heat value is kept constant by lowering the mass flow rate of fuel and accordingly mass flow rate of air , to maintain the Air-fuel ratio. In this case heat loss is neglected i.e system is assumed to be adiabatic

The combustion efficiency can be represented using

Fuel savings is calculated using

Code :

import cantera as ct
import matplotlib.pyplot as plt
import math
''' 
Program to calculate effect of preheating on Adiabatic flame temperature using a Recuperator

CH4 + 2(O2+ 3.76N2) -------> CO2 + 2H2O + 7.52N2 (Stoichiometric reaction)

Author : Harshith Rajamahanti

'''
gas = ct.Solution('gri30.cti')
phi=1 # Equivalance ratio

# Finding the mole fraction of the species
n_moles_total=7.52+2+1
x_CO2=1/n_moles_total
x_H2O=2/n_moles_total
x_N2=7.52/n_moles_total

# Fuel

Fuel=ct.Quantity(gas)
Fuel.TPX=298.15,ct.one_atm,{'CH4':1} # Fuel is at STP
Fuel.moles=1
Fuel_m=Fuel.mass # in kg
Fuel_h=Fuel.enthalpy_mass # in joule/kg


# Incoming Fresh air is heated by the Exhaust flue gases 

T_min=298
T_max=600
T_exhaust=1700 
LHV=50e6 # joule/kg

# Exhaust

Exhaust=ct.Quantity(gas)
Exhaust.TPX=T_exhaust,ct.one_atm,{'CO2':x_CO2,'H2O':x_H2O,'N2':x_N2}
Exhaust.moles=n_moles_total
Exhaust_h=Exhaust.enthalpy_mass

T_AFT=[]
T_preheating=[]
efficiency=[]
Fuel_saving=[]

for temp in range(T_min,T_max+1):

	# Air

	Air=ct.Quantity(gas)
	Air.TPX=temp,ct.one_atm,{'O2':0.21,'N2':0.79}
	Air.moles=9.52/phi
	Air_m=Air.mass
	Air_h=Air.enthalpy_mass

	# Mixture

	Mixture=Air+Fuel
	Mixture.equilibrate('hp','auto')
	T_AFT.append(Mixture.T)
	T_preheating.append(temp)

	# Air fuel ratio

	AF=Air_m/Fuel_m

	# Calculating Specific heat output

	Q_specific=Fuel_h + (AF*Air_h) - ((1+AF)*Exhaust_h)

	# Calculating efficiency

	eff=(Q_specific/LHV)*100
	efficiency.append(eff)

	if (temp==298):
		Q_nopreheat=Q_specific

	# Calculating percentage of fuel saving by preheating of air

	Fuel_sav=(1 - (Q_nopreheat/Q_specific))*100
	Fuel_saving.append(Fuel_sav)

print('Adiabatic flame temperature for Methane-air mixture for ' + str(temp) + ' K' + ' of preheated air temperature is ' + str(Mixture.T) + ' K')
print('Efficiency without preheating(T=298 K) is ',efficiency[0],'%')
print('Efficiency with preheating(T=600 K) is ',efficiency[len(efficiency)-1],'%')
print('Fuel saving after preheating is ',Fuel_saving[len(Fuel_saving)-1],'%')

plt.figure(1)
plt.plot(T_preheating,T_AFT,color='red')
plt.xlabel('Preheated temperature of Air in K')
plt.ylabel('Adiabatic flame temperature in K')
plt.title('Effect of preheating of air on Adiabatic flame temperature')
plt.grid(which='major')
plt.minorticks_on()
plt.grid(which='minor', alpha=0.2)
plt.show()

plt.figure(2)
plt.plot(T_preheating,efficiency,color='green')
plt.xlabel('Preheated temperature of Air in K')
plt.ylabel('Efficiency')
plt.title('Effect of preheating of air on Efficiency')
plt.grid(which='major')
plt.minorticks_on()
plt.grid(which='minor', alpha=0.2)
plt.show()

plt.figure(3)
plt.plot(T_preheating,Fuel_saving,color='blue')
plt.xlabel('Preheated temperature of Air in K')
plt.ylabel('Adiabatic flame temperature in K')
plt.title('Effect of preheating of air on Fuel saving')
plt.grid(which='major')
plt.minorticks_on()
plt.grid(which='minor', alpha=0.2)
plt.show()

Code Explanation :

1. Fuel is represented by Quantity class and the temeparature, pressure and mole fraction of fuel is provided. Fuel is at STP condition
2. Mass of the fuel and specific enthalpy of fuel at STP is also calculated
3. Total number of moles and mole fraction of product species are calculated
4. exhaust gas is also represented by Quantity class and the temperature, pressure and mole fraction of fuel is provided.
5. The exhaust gas temperature is fixed and is equal to 1700 k
6. Exhaust gas specific enthalpy is also calculated
7. Now varying the air temperature from 298 K (which is no preheating ) to 600 K in a for loop Adiabatic flame temperature is to be calculated by taking Mixture of air and fuel using equilibrate
8. Specific heat release due to combustion reaction , efficiency anf fuel saving percentage is calculated
9. Later the results are plotted on a graph

Output :

Conclusions :

1. With increase in preheating temperature of air Adiabatic flame temperature also increases
2. With increase in preheating temperature of air Efficiency also increases
3. With increase in preheating temperature of air fuel efficiency increases
4. Preheating temperature depends on the temperature of products after combustion , if the output temperature goes on increasing as the cycle proceeds could leads to the damage of recuperator. So, it is theoretically correct to assume a fixed furnace exit temperature if the input fuel Quantity or the Work extracted is not constant