Combustion Efficiency Calculation after Preheating

AIM :

To calculate the effect of heating temperature on AFT and combustion efficiency.

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 improves the energy efficiency of the system as a whole.

1.Effect of preheating on the adiabatic flame temperature:

For this problem, we are using the Quantity class method to create our product (Methane) and reactants (Oxygen, Nitrogen) and creating a mixture to determine the adiabatic flame temperature. As the pre-heating, the air is done in the reactants side, we are initializing the temperature of the quantity of the reactants with respect to the given pre-heating temperature and the final adiabatic flame temperature is calculated. The effect can be seen in the following plot which was generated in the current program

Reaction:

CH4 + 2*O2 + 7.52*N2 = CO2 + 2*H20 + 7.52*N2.

Here 2 instances A and B of the Quantity class has been created, A is for the fuel which is at 298k,1 atm and1mole, where B is used for the air which will take the pre-Heating temperature ranging from 298k-600k at 1 atm and for 9.62 moles (from above reaction total moles of air is 7.52+2 =9.52)

\"\"\"
EFFECT OF PRE_HEATING ON AFT 
programme by | Naseer Abdul |
\"\"\"
import cantera as ct 
import matplotlib.pyplot as plt

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

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

#Pre heating Temperature
T_min = 298
T_max = 600


B = ct.Quantity(gas)

for T in range (T_min,T_max):

		B.TPX = T,ct.one_atm,{\'O2\':0.21 , \'N2\':0.79}
		B.moles = 2/0.21

		M = A+B
		M.equilibrate(\'HP\',\'auto\')
		plt.plot(T,M.T,\'*\',color =\'green\')


plt.grid(\'ON\')
plt.title(\'EFFECT OF PRE_HEATING ON AFT\')
plt.xlabel(\'PRE_HEATING TEMPERATURE\')
plt.ylabel(\'AFT\')
plt.show()

Result:

From the graph, With pre-heating temperature, we can conclude that AFT is increased. This is because pre-heating helps to raise the device temperature which in turn leads to combustion efficiency through the use of less fuel. Preheating, therefore, helps to save fuel.

2.The effect of preheating temperature on combustion efficiency:

Combustion efficiency is a function of the context in which the combustion of fuel is used. The ratio of LHV-specific Q is calculated where Q-specific is the ratio of work done in Q with the fuel mass.

where LHV is the lower heating values for the which is given as 50 MJ/kg

Here we use a quantity class for generating and mixing our product reactants. In the following, we include the pre-heating temperature and the 1700 Kelvin exhaust as per the problematic statement. Then, the combustion efficiency is calculated.

\"\"\"
EFFECT OF PRE_HEATING IN COMBUSTION EFFICIENCY
programme by | Naseer Abdul |
\"\"\"

import cantera as ct 
import matplotlib.pyplot as plt


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

r = ct.Quantity(gas)
r.TPX = 298.15, ct.one_atm, {\'CH4\': 0.095, \'O2\': 0.1901 , \'N2\': 0.7148}
r.moles = 10.52


p = ct.Quantity(gas)
p.TPX = 298.15, ct.one_atm, {\'CO2\':0.095, \'H2O\':0.1901 , \'N2\': 0.7148}
p.moles = 10.52

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


prod = ct.Quantity(gas)
prod.TPX = 1700,ct.one_atm,{\'CO2\':0.095, \'H2O\': 0.1901, \'N2\': 0.7148}
prod.moles = 10.52
M_prod = prod.mass




Tpre_min = 298
Tpre_max = 600
LHV = 50e6


for T_pre in range (Tpre_min,Tpre_max):

		Air = ct.Quantity(gas)
		Air.TPX = T_pre, ct.one_atm, {\'O2\':0.21, \'N2\':0.79}
		Air.moles = 9.52
		M_air = Air.mass



		H_Air = Air.enthalpy_mass
		H_fuel = fuel.enthalpy_mass
		H_prod = prod.enthalpy_mass


		Air_to_fuel = M_air/M_fuel


		Q = M_air*H_Air + M_fuel*H_fuel - M_prod*H_prod
		Qsp = Q/M_fuel
		eff = Qsp/LHV
		plt.plot (T_pre,eff,\'*\',color =\'red\')

plt.xlabel(\'Pre-Heating Temperature[k]\')
plt.ylabel(\'combustion Efficiency\')
plt.grid(\'on\')
plt.title(\'EFFECT OF PRE-HEATING IN COMBUSTION EFFICIENCY\')
plt.show()

Result:

The graph shows that pre-heating temperature increases combustion. Preheating is dependent on the exhaust temperature following the combustion when the outlet temperature does not remain at a specific temperature, As the process continues, the outlet temperature (gas temperature) is continued to increase. The temperature rises at a certain point that damages the recuperator portion, which in turn affects the recovery performance. So it is right to assume a fixed furnace exit temperature.