Week 4.2 - Combustion Efficiency Calculation after Preheating

AIM: To find out the combustion efficiency and adiabatic flame temperature when a recuperator is connected to a furnace for recovering

some energy( methane and air is burned in the furnace).

OBJECTIVE:

• Effect of preheating on the adiabatic flame temperature.
• How the combustion efficiency is affected with preheating.
• plotting the effect of preheating temperature on combustion efficiency.

THEORY:

• A recuperator is a type of heat exchanger that has separate flow paths for each fluid throughout its passages and heat is transfered through the separating walls.
• Most reciperators operate as counter flow heat exchangers.
• Generally these are used to extract heat from the exhaust and use it to preheat air entering the combustion system.
• They use waste energy to heat the air,saving some of the fuel,therby improving the energy efficiency of the system.
• In more detail,the recuperator transfers some of the waste heat in the exhaust to the compressed air,thus preheating it before entering the fuel burner stage. Since the gases have been preheated, less fuel is needed to heat the gases upto the turbine inlet temperature,thus the recuperator can make a gas turbine or heat engine significantly more efficient.

fig. recuperator having inlet(13, containing hot gases)

Python code for finding the effect of preheating temperature vs Adiabstic flame temperature.

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

gas=ct.Solution('gri30.cti')
Fuel=ct.Quantity(gas)
Fuel.TPX=298.15,101325,{'CH4':1}
Fuel.moles=1

air=ct.Quantity(gas)
# preheating temperature
T_preh=np.linspace(298,600,25)
AFT=[]
for i in T_preh:
	air.TPX=i,101325,{'O2':0.21,'N2':0.79}
	air.moles=9.52
	M=Fuel+air
	M.equilibrate('HP','auto')
	AFT.append(M.T)
plt.plot(T_preh,AFT,'*',color='blue',label='cantera')
plt.xlabel('preheating Temperature')
plt.ylabel('adiabatic flame temperature')
plt.grid('on')
plt.title('preheating temperature vs AFT')
plt.show()

output

Fig.1 grapgh showing preheating temperature vs adiabatic flame temperature

• As shown in the figure, it is found that as the preheat temperature increases the AFT increases.
• As AFT increase ,then there will be less fuel consumed to heat the incoming fluid to desired temperature.

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

gas=ct.Solution('gri30.cti')
fuel=ct.Quantity(gas)
fuel.TPX=298.15,101325,{'CH4':1}
fuel.moles=1


LCV=50000000  #j/kg
products=ct.Quantity(gas)
products.TPX=1700,101325,{'CO2':0.095,'H2O':0.1901,'N2':0.714}
products.moles=10.52

# preheating temperature for loop
T_ph=np.linspace(298,600,25)
efficiency=[]
fuel_saving=[]

for i in T_ph:
	air=ct.Quantity(gas)
	air.TPX=i,101325,{'O2':0.21,'N2':0.79}
	air.moles=9.52
	Q=(air.mass*air.h+fuel.mass*fuel.h-products.mass*products.h)/fuel.mass
	n=(Q/LCV)*100
	efficiency.append(n)
	#saving of fuel
	f_s=1-((efficiency[0]/efficiency[-1]))
	fuel_saving.append(f_s)
    
plt.plot(T_ph,fuel_saving,'*',color='green')
plt.xlabel('preheating temperature(k)')
plt.ylabel('fuel saving')
plt.title('preheat temperature vs fuel saving')
plt.grid('on')
plt.show()

plt.plot(T_ph,efficiency,'*',color='blue')
plt.xlabel('preheating temperature(k)')
plt.ylabel('combustion efficiency(n)')
plt.title('preheat temperature vs combustion efficiency')
plt.grid('on')
plt.show()

Fig.2

• In the above figure shown,the fuel saving percentage increases as preheat temperture increases.
• The maximum is found at preheat temperture of 600k.

combustion efficiency= actual heat release/theoretical heat release(lower heating value of fuel)

Fig.3 Preheat temperture vs combustion efficiency

• It is right to assume a fixed furnace exit temperature.If it varies then the calculation of combustion efficiency might be difficult.
• Moreover it is assumed to be a steady flow device.So, the exit temperature should have a fixed temperature.