Week 4.2 - Combustion Efficiency Calculation after Preheating

AIM:- a) To find the effect of Preheating of air on the Adiabatic Flame Temperature(AFT)

b) To identify the effect of Preheating on Fuel Saving and the variation of Combustion efficiency with respect to preheating temperature

Theoritical Background:

Recuperator

A recuperator is the heat Exchanger in which heat is transferred from the hot flue gases to the inlet air during combustion process. As shown in the above image rcuperator is connected to the furnace to recover some energy to heat the atmospheric air. Recuperator performance depends on the conditions of the fluid streams, thus assuming a constant effectiveness, or pressure drop is not accurate if flow conditions change greatly, such as if the flow rate halves during off-design.

Case 1: Combustion process of Methane in a furnace attached to Recuperator

Balanced Combustion Equation for Methane

import cantera as ct
import matplotlib.pyplot as plt

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

Air = ct.Quantity(gas)
Air.TPX =  298.15,ct.one_atm,{'O2':1,'N2':0.79}

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

T_min=298
T_max=600
T_ph = []
T_aft = []

# Changing the Preheat Temperature
for T_pre in range(T_min,T_max+1):
	Air.TPX = T_pre,ct.one_atm,{'O2':0.21,'N2':0.79}

	Air.moles=9.523
	Mix=Air+Fuel

	Mix.equilibrate('HP',solver='auto')
	T_aft.append(Mix.T)
	T_ph.append(T_pre)
	if (T_pre%10==0):
		plt.plot(T_pre,Mix.T,'*',color='red')

plt.plot(T_ph, T_aft, color='maroon')
plt.xlabel('Pre-Heat Temperature(K)')
plt.ylabel('Adiabatic Flame Temperature(K)')
plt.grid()
plt.show()

Output

From the above graph it can be observed that Preheated air causes increase in Adiabatic Flame Temperature(AFT). This will cause increase in the Temperature of Exhaust gases resulting in increase in the heat transfer at the recuperator. The preheating air temperature must be monitered as its increased value damages components of the system.

Case 2: Effect of Preheating on Fuel Saving and the variation of Combustion efficiency with respect to preheating temperature

Python code:

import cantera as ct
import matplotlib.pyplot as plt

gas = ct.Solution('gri30.cti')
N_Total = 10.52
N_CO2 = 1/N_Total
N_H2O = 2/N_Total
N_N2 = 7.52/N_Total

# Intitial conditions of Fuel
Fuel = ct.Quantity(gas)
Fuel.TPX = 298.15,ct.one_atm,'CH4:1'
Fuel.moles = 1
m_f = Fuel.mass
h_f = Fuel.enthalpy_mass

#Products

Exh = ct.Quantity(gas)
Exh.TPX = 1700, ct.one_atm,{'CO2':N_CO2,'H2O':N_H2O,'N2':N_N2}
h_p = Exh.enthalpy_mass

LVH = 5e7

saving = []
T_ph = []
eff = []

T_min = 298
T_max = 600
for T_pre in range(T_min,T_max+1):
	Air = ct.Quantity(gas)
	Air.TPX = T_pre,ct.one_atm,{'O2':0.21,'N2':0.79}
	Air.moles=9.52
	h_a = Air.enthalpy_mass

	A_F = (Air.mass/m_f)
	Q_sp = -((A_F+1)*h_p-A_F*h_a-h_f)
	eta = Q_sp/(LVH)

	if T_pre==298:
		eta_0=eta
	save = 100*(1-(eta_0/eta))
	saving.append(save)
	T_ph.append(T_pre)
	eff.append(eta)
	if (T_pre%10==0):
		plt.figure(1)
		plt.plot(T_pre,eta,'*',color='blue')

plt.figure(2)
plt.plot(T_ph,saving, color='purple')
plt.xlabel('Pre-Heat Temperature(K)')
plt.ylabel('Percentage Fuel saved')
plt.grid()

plt.figure(1)
plt.plot(T_ph,eff, color='green')
plt.xlabel('Pre-Heat Temperature(K)')
plt.ylabel('Combustion Efficiency')
plt.grid()

plt.show()

Output:

From the above graph it can be observed that the Pre-Heat air Temperature is directly proportional to the Combustion Efficiency. When the Preheated Air temperature increases less amount of heat needs to be supplied to reach the required exit temperature of the furnace.

From the above graph it can be observed that the Percentage of Fuel saving is more as the temperature of the Pre heated air increases. The process of preheating is very useful in conserving Energy.