Week 4 - Combustion Efficiency Calculation after Preheating

Aim:To Perform Preheating in a furnace which is connected to recuperator and to calculate

1) Effect of preheating on the adiabatic flame temperature

2) Effect of preheating on the Combustion effieciency under fuel saving

Apparatus or Given data:

1)Methane and Air mxture is used to burn thee furnace

2)Prehating Temperatures

Tmin=298K

Tmax=600k

3) Furnace Exit or Exhaust temperature =1700k

4)LHV of combustion of Methane=50 MJ/KG.

Explanation:

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.

case 1:Effect of preheating on the adiabatic flame temperature

Python Code:

"""The effect of preheating on the adiabatic flame temperature"""

import cantera as ct 
import matplotlib.pyplot as plt 
gas = ct.Solution('gri30.cti')
#specifying the temp range
for Temp in range (298,600,5):
	Air = ct.Quantity(gas)
	Air.TPX = Temp,ct.one_atm,{'O2':0.21, 'N2':0.79}
	Air.moles = 2/0.21

	Fuel = ct.Quantity(gas)
	Fuel.TPX = 298.15,ct.one_atm,'CH4:1546'
	Fuel.moles = 1
	products = ct.Quantity(gas)
	products.TPX = 1700,ct.one_atm,{'CO2':0.095, 'H2O':0.1901, 'N2':0.7148}
	products.moles = 10.52

	Combustion = Air+Fuel
	Combustion.equilibrate('HP')
	print 
	plt.plot(Temp,Combustion.T,'+', color='red')
plt.show()

Resultant Plot:

observation:

In the above plot, we can observe that as the air inlet temperature increase it proportionally increases the Adiabatic flame Temperature it's because when we increase the air inlet temperature it leads to increasing the enthalpy of the air, in other words, the increases temperature of the air will proportionally increase the providing energy to the furnace. We are dealing with adiabatic conditions so the energy cant goes through boundaries so the supplied energy will ultimately lead to rising in the Adiabatic temperature of the air.

Case:2 Effect of pre-heat temperature on Combustion effieciency under fuel saving

Combustion Efficiency: It is the ratio of the sum of heat gains and losses to the total energy provided

ηcombustion=Qreactants-Qexhaustmf⋅LHV

percentage of fuel-saving = (mass of fuel with recuperator - mass of fuel without recuperator)/(mass of fuel without recuperator)

The python code for the calculation of combustion efficiency and percentage of saving fuel for stoichiometric combustion of one mole of methane

Python Code:

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 
H_fuel = fuel.h

products = ct.Quantity(gas)
products.TPX = 1700,ct.one_atm,{'CO2':0.095,'H2O':0.1901, 'N2':0.7148}
products.moles = 10.52
mass_products = products.mass
H_products = products.h 

air = ct.Quantity(gas)
air.TPX = 298.15,ct.one_atm,{'O2':0.21,'N2':0.79}
air.moles = 9.52
mass_air = air.mass 
H_air = air.h

Q = (mass_air*H_air) + (mass_fuel*H_fuel) -(mass_products*H_products)
LHV = 50e6
LHV = 50e6
# calculating efficiency without preheat condition
efficiency_STP = (Q/((mass_fuel)*(LHV)))*100

for T in range (298,600,5):
	air.TPX = T,ct.one_atm,{'O2':0.21,'N2':0.79}
	air.moles = 9.52
	mass_air = air.mass 
	H_air = air.h

	Q = (mass_air*H_air) + (mass_fuel*H_fuel) -(mass_products*H_products)
	LHV = 50e6
	efficiency = (Q/((mass_fuel)*(LHV)))*100

	saving = ((efficiency/efficiency_STP)-1)*100

	plt.plot(T,efficiency,'+',color = 'red')
	plt.plot(T,saving,'o',color = 'blue')

plt.xlabel('preheat_temp in (K)')
plt.ylabel('combustion_efficiency')
plt.legend(['combustion_efficiency','percentage_of_saving_the_fuel'])
plt.show()

Resultant Plot:

Observation:

The plot showing both combustion efficiency and the percentage of change in fuel for changing in preheat temperature.

It is better to maintain the standard output temperature this will indirectly keep the constraint for the maximum requirement of energy for that particular model because above the production of that energy will do no good for that model in the worst case it may fail also. If the output energy is maintained it also increases the life span of the recuperator so the maintaining of standard output temperature is good