Week 4 - Combustion Efficiency Calculation after Preheating

Aim:

To calculate the effect of preheating on the adiabatic flame temperature.

To determine the variations in combustion efficiency as a function of preheating temperature.

Theory:

Recuperator

The recuperator is a special purpose counter-flow energy recovery heat exchanger positioned within the supply and exhaust air streams of an air handling system, or in the exhaust gases of an industrial process, in order to recover the waste heat. Generally, they are used to extract heat from the exhaust and use it to preheat air entering the combustion system. In this way they use waste energy to heat the air, offsetting some of the fuel, and thereby improves the energy efficiency of the system as a whole.

CASE 1 

The programme involves the instances of fuel and air by using the class called Quantity class.

The no of moles of fuel ie CH4 = 1

The no of moles of air that contains O2 and N2 =9.52

Mole fraction of O2=0.21

Mole fraction of N2=0.79

The fuel is kept at STP condition with a temperature 298.15k wile temperature of the air being varied with preheating condition and the pressure is being defined constant.

A mixture is created by summing up fuel and air and then it is equilibrated with constant enthalpy and pressure.

Code and Results

"""
Effect of preheating on AFT
"""
import cantera as ct 
import matplotlib.pyplot as plt 

#creating gas instance
gas=ct.Solution('gri30.cti')

#Creating fuel and air intances
fuel=ct.Quantity(gas)
fuel.TPX=298.15,ct.one_atm,'CH4:1'
fuel.moles=1

air=ct.Quantity(gas)

#Defining preheating condition
T_min=298
T_max=600

#Calculating temperature for preheating condition
for T in range(T_min,T_max):
	air.TPX=T,ct.one_atm,{'O2':0.21,'N2':0.79}
	air.moles=9.52


	mixture=air+fuel
	mixture.equilibrate('HP','auto')
	plt.plot(T,mixture.T,'-o',color='brown')

plt.xlabel('Effect of preheating')
plt.ylabel('AFT')	
plt.title('AFT vs Effect of preheating')
plt.show()

The AFT tends to increase with preheating temperature, this due to the facts that as the preheating condition enhances the combustion and therefore releases more heat and results in higher AFT.

CASE II

Combustion efficiency is a measure of how effectively the heat content of a fuel is transferred into usable heat.

Efficiency=Q_sp/LHV

Q_sp=heat specific

LHV=low heat value (its value is 50MJ/KG)

Q_sp=Q/mass of fuel

where Q is work done

Q = (mass of air*enthalpy of air)+(mass of fuel*enthalpy of fuel)-(mass of products*enthalpy of products)

In this case, the product's(exit temperature) is assumed 1700 k 

Code and Results

"""
Effect of preheating on AFT
"""
import cantera as ct 
import matplotlib.pyplot as plt 

gas=ct.Solution('gri30.cti')

fuel=ct.Quantity(gas)
fuel.TPX=298.15,ct.one_atm,'CH4:1'
fuel.moles=1
mass_fuel=fuel.mass

prod=ct.Quantity(gas)
n_tot=1+2+(2*3.76)
n_co2=1/n_tot
n_h2O=2/n_tot
n_N2=7.52/n_tot
prod.TPX=1700,ct.one_atm,{'CO2':n_co2,'H2O':n_h2O,'N2':n_N2}
prod.moles=n_tot
mass_prod=prod.mass

LHV=50e6

T_min=298
T_max=600

air=ct.Quantity(gas)



for T in range(T_min,T_max):
	air.TPX=T,ct.one_atm,{'O2':0.21,'N2':0.79}
	air.moles=9.52
	mass_air=air.mass

	H_products=prod.enthalpy_mass
	H_air=air.enthalpy_mass
	H_fuel=fuel.enthalpy_mass

	air_fuel_ratio=mass_air/mass_fuel

	Q=(mass_air*H_air)+(mass_fuel*H_fuel)-(mass_prod*H_products)
	Qsp=Q/mass_fuel
	eff=(Qsp/LHV)*100


	
	plt.plot(T,eff,'-o',color='brown')

plt.xlabel('Preheating Temperature')
plt.ylabel('Combustion Efficiency')	
plt.title('Effect of preheating vs Combustion Efficiency')
plt.grid('ON')
plt.show()

From the above figure, it can be said that combustion efficiency increases linearly with preheating temperature.

The extra amount of heat available in combustion requires less energy thereby results in increase inefficiency.

Assuming products temperature is not a wise decision as in reality, the exit temperature depends on various factor and also produces  different species having their own intermediate reaction and hence also the temperature is also certain