Week 4.2 - Combustion Efficiency Calculation after Preheating

AIM:-

Combustion efficiency Calculation after Preheating

Case:-1

Effect of preheating on adiabatic Flame Temperature (Change in temperature from 298K to 600K)

Case:-2

• Does preheating help with fuel savimg.
• Formula for combustion efficiency

CODE and Description:-

Let us under stand what is recuperator:-

A 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 improve the energey efficiency of the system as a whole.

Case:-1

"""
Code for the effect of preheating on AFT for methane
"""
import cantera as ct
import matplotlib.pyplot as plt
import numpy as np

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

#CH4 + 2(O2+3.76N2) = CO2 + 2H2O +7.52N2

#Fuel
B= ct.Quantity(gas)
B.TPX=298.15, ct.one_atm,'CH4:1'
B.moles=1

#Air
A = ct.Quantity(gas)
Temp = np.linspace(298, 600, num=21)

for T_pre in Temp:

	A.TPX=T_pre, ct.one_atm,'O2:0.21, N2:0.79'
	A.moles = 9.52

	#For molar balance
	M=A+B 
	M.equilibrate('HP')

	print('Value of AFT=',M.T)
	plt.plot(T_pre, M.T, 'o',color = 'blue')

plt.grid()
plt.xlabel('PreHeating Air Temperature')
plt.ylabel('Adiabatic Flame Temperature')
plt.title('Effect of preheating on AFT')
plt.show()

By the above plot we can clearly tell that the AFT is increasing linearly with respect to the Preheating of Air.

Case 2:-

This program is for checking the efficiency or energy gains

import cantera as ct
import matplotlib.pyplot as plt
import numpy as np

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

#CH4 + 2(O2+3.76N2)= CO2 + 2H2O +7.52N2

#Air
A = ct.Quantity(gas)
A.TPX = 298.15, ct.one_atm,{'O2':0.21,'N2':0.79}
A.moles = 9.52
mass_A = A.mass
H_A = A.enthalpy_mass

#Fuel
B= ct.Quantity(gas)
B.TPX=298.15, ct.one_atm,'CH4:1'
B.moles=1
mass_B = B.mass
H_B = B.enthalpy_mass

LHV = 50e6



products = ct.Quantity(gas)
products.TPX=1700, ct.one_atm,{'CO2':1/10.52, 'H2O':2/10.52, 'N2':7.52/10.52}
products.moles = 10.52
mass_products = products.mass
H_prodduts  = products.enthalpy_mass

M_Air_Fuel= mass_A/mass_B

#Combustion efficiency at STP
eff_stp = (M_Air_Fuel*H_A+H_B-(M_Air_Fuel+1)*H_prodduts)/(LHV)
print("Combustion Efficiency as STP=",eff_stp)

#PreHeat Temperature
preheat_T=[]

#Combustion Efficiency with respect to varying preheat temperature
Efficiency = []

#Fuel Saving
Fuel_Saving = []


for T in range(298,608):

	A.TPX=T, ct.one_atm,{'O2':0.21, 'N2':0.79}
	A.moles = 9.52
	mass_A=A.mass  
	H_A=A.enthalpy_mass
	M_A_fuel = mass_A/mass_B

	eff = (M_Air_Fuel*H_A+H_B -(M_Air_Fuel+1)*H_prodduts)/(LHV)
	Fuel_Savings = abs(1-(eff_stp/eff))
	
	Fuel_Saving.append(Fuel_Savings)
	Efficiency.append(eff)
	preheat_T.append(T)


print("Combustion Efficincy at varied PreHeating Conditions =", Efficiency)
print("PreHeating Temperature(K)=",preheat_T)
plt.plot(preheat_T,Efficiency)
plt.plot(preheat_T,Fuel_Saving,color='red')
plt.xlabel('preheat_T')
plt.ylabel('Efficiency')
plt.legend(['Combustion Efficiency','Fuel Saving'])
plt.title('Combustion Efficiency with PreHeating',size=20)
plt.grid(True)

print("Fuel Saved=",Fuel_Saving)

plt.show()

By the above plot we can see that the combsution efficiency is increased with the temperature and also there is fuel, initially the fuel saving is negligible amount but as the Air temperature is increased thefuel savings is also increased.

At the initial temperature (298.15 K) the fuel saved is 0.00015 kg but at the high temperature (608 K) the fuel saved is 0.232 kg, which is approximately 200 times more saving of fuel wrt preheat temperature.

The values are noted from the report of the executed program

Similarly the combustion efficiency at 289.15K is 0.338 and at 608K is 0.447.

So we can coclude that the Preheating of Air using the recuperator helps to save the fuel and increase the efficiency.

Let us calculate the Combustion Efficiency

Combustion Efficiency:-

`eta=((A/F)*Hunderset(air)``+Hunderset(fuel)`-`(A/F+1)*Hunderset(pro)/LHV`

Here,

A/F = Mass of Air/Fuel

H_air = Air Enthalpy

H_fuel = Fuel enthalpy

H_products = Product Enthalpy

LHV = Lower Heat Value of Fuel

Fuel Saving:-

Fuel Saving = 1- `etaunderset(STP)`/`etaunderset(preheat`

The exit temperature is directly proportional to the preheat temperature of the air. So the The hotter the exit air the more fuel is saved.