Combustion Efficiency Calculating after Preheating

Objective :

In this challenge, we are going to calculate the combustion efficiency of Recuperator.

• Calculate, the effect of preheating Temperature on the Adiabatic Flame Temperature.
• Calculate, the effect of preheating temperature on combustion efficiency.

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 improves the energy efficiency of the system as a whole.

1)Calculate The Adiabatic Flame Temperature:

The effect of preheating Temperature(298 K to 600 K) on The Adiabatic Flame Temperature.

1.1)Adiabatic Flame Temperature:

To obtain the final temperature after complete fuel burn inside the combustion chamber. Here we\'re using different preheating temperature, which is the effect on Final temperature.

1.2)General Equation

CH4 + 2(O2+3.72 N2)=CO2 + 2H2O +7.52 N2

Calculate mole fraction of species

Fuel Mole Fraction = CH4 = 1

Air Mole Fraction = O2 = 0.21, N2 = 0.79.

Products Mole Fraction = CO2 = 0.095

H2O = 0.1901

N2 = 0.7148

For finding Moles of Air = (No. of moles of species/Mole fraction of species)

For N2 = (7.52/0.79)=9.52

For O2 = (2/0.21) = 9.52

For Products mole fraction

For CO2 = (1/0.095) = 10.52

For H2O = (2/0.1901) = 10.52

For N2 = (7.52/0.7148) =10.52

Writen code In cantera for Adiabatic Flame Temperature

\'\'\'
	Adiabatic Flame Temperature using Preheat temperature
\'\'\'

import cantera as ct
import matplotlib.pyplot as plt
gas=ct.Solution(\'gri30.cti\')

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

#air
A = ct.Quantity(gas)
for T_pre in range (298,600):
	A.TPX = T_pre, ct.one_atm,{\'o2\':0.21, \'n2\':0.79} # Mole fraction in g
	A.moles= (2/0.21)  #mole fraction of O = 0.21, Number Moles Of O = 2

	\'\'\'
	Mixture of fuel
	\'\'\'
	M=A+B
	M.equilibrate(\'HP\')
	
	#ploting between pre heated temperature and Adiabatic Flame Temperature
	if T_pre% 10 is 0:
		plt.plot(T_pre,M.T,\'*\',color=\'red\')
		print(M.T)
plt.xlabel(\'Preheat Temperature (K)\')
plt.ylabel(\'Adiabatic Flame Temperature (K)\')
plt.title(\'Preheating Temperature Vs Adiabatic Flame Temperature\')
plt.show()

Result:

In this method, we have used the product as Methane(CH4) and reactants as Air(O2 + N2). As we have seen above figure when preheating Temperature increases as well as Adiabatic Flame Temperature increases because the preheating temperature helps to preheat the air before the fuel mixing and it helps to the getting high temperature before entering into the furnace. This way, the combustion becomes a very less amount of energy being used. So decreasing fuel usage. Yes, Does The recuperator help to reduce fuel usage.

Does preheating help with fuel-saving?

Yes, Does The recuperator help to reduce fuel usage and save fuel because to preheat the air before the fuel mixing and it helps to the getting high temperature before entering into the furnace. This way, the combustion becomes a very less amount of energy being used. So decreasing fuel usage

Effect of Pre-Heating Temperature On Combustion Efficiency:

1) Write the formula for combustion efficiency?

Combustion efficiency is the measurement of fuel burned and being utilized in combustion processes.

`eta_(combustion) =(Qout)/(mf*LHV)`

here,

mf=mass fuel

LHV= Lower Heating Value of Fuel

Qout = Mass fuel * Enthalpy fuel + Mass air * Enthalpy air - Products mass * products enthalpy.

In this section again we were using products and reactants to form them into mixtures. Here we were including pre-heating temperature, and providing the exhaust temperature 1700K, and calculated the combustion efficiency that effect of pre-heating temperature.

\'\'\'
	Combution efficiency using different preheating temperature
\'\'\'
# genrnel equation CH4+2(O2+3.72N2)=CO2+2H2O+7.52N2
import cantera as ct
import matplotlib.pyplot as plt

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

# Define Fuel CH4
fuel=ct.Quantity(gas)
fuel.TPX=298.15,ct.one_atm,\'CH4:1\'
fuel.moles=1
fuel_mass=fuel.mass

#Define products (Co2 + H2O + N2)
products=ct.Quantity(gas)
products.TPX=1700,ct.one_atm,{\'CO2\':0.095,\'H2O\':0.1901,\'N2\':0.7148}    # Mole fraction = (No.of moles specified species/Total No. of Moles of species)
products.moles=(1/0.095)  # moles = (No.of moles of species/Mole fraction of species)
products_mass= products.mass

# Define Air (O2 + N2)
# Define preheating Temperature in range of 298 to 600
for T in range (298,601):
	air=ct.Quantity(gas)
	air.TPX=T,ct.one_atm,{\'O2\':0.21,\'N2\':0.79}  # Mole fraction = (No.of moles specified species/Total No. of Moles of species)
	air.moles=(2/0.21)  # moles = (No.of moles of species/Mole fraction of species)
	air_mass=air.mass

	# calculate enthalpy
	H_air=air.enthalpy_mass
	H_fuel=fuel.enthalpy_mass
	H_products=products.enthalpy_mass

	\'\'\'
		Calculate efficency
		Combution Efficiency = Q/(fuel_mass * LHV)
		Q= fuel_mass*H_fuel + air_mass*H_air - products_mass*H_products
	\'\'\'
	LHV=50e6
	Q = fuel_mass*H_fuel + air_mass*H_air - products_mass*H_products
	Qsp= (Q/fuel_mass)
	efficency = (Qsp/LHV)
	
	if T % 10 is 0:
	# plot between Preheating temperature and efficiency
		plt.plot(T,efficency,\'*\',color=\'red\')
		plt.pause(0.01)
		print(T,efficency)
		plt.xlabel(\'Preheating Temperature (K)\')
		plt.ylabel(\'Combustion Efficiency \')
		plt.title(\'Preheating Temperature VS Combustion Efficiency\')
plt.show()

Result:

In the above plot, we observe the same phenomenon when compared to the previous plot. When the pre-heating-temperature increases as well as increases combustion efficiency. There is a 10% increase in combustion from 0.34 to 0.44 for pre-heating temperature from 298 to 600 K and assume the lower heating value of 50e6 (j/kg)

As the preheating temperature depends on the exhaust temperature after combustion and providing less temperature at exhaust when compared to the Adiabatic Flame Temperature. if we will get a high temperature at the exhaust and it may affect on the Recuperator. if we get less temperature at the exhaust which effects on the combustion processes which means does fuel not completely burn, the final resulting loss of efficiency.

Is it right or wrong to assume a fixed furnace exit temperature? Explain your answer

As we have seen recuperator figure at above, there is the exhaust temperature helps to preheat the air before fuel mixing, but these process to run again and again so that the exhaust temperature increases to high and it may affect on the recuperator and it will damage. So we need to provide certain exhaust temperature, it\'s may help to doesn\'t to exceed the preheating temperature as we specified exhaust temperature. which may help to prevent the damage of the recuperator. Yes, It\'s right to assume a fixed furnace exit temperature.