Week 4 - Combustion Efficiency Calculation after Preheating
#Aim:1Using Cantera,to study the Effect of Preheating Air on Adiabatic flame temperature.
2.Effect of Pre heat temperature on Combustion efficiency.
Introduction:
Industrial furnaces are used for carrying out certain processes which requires heat. Heat in the furnace
is provided by (i) fuel energy, (ii) chemical energy, (iii) electrical energy or (iv) a combination of these
energies. Gases which are generated during the process leaves the furnace at a temperature which is
the inside temperature of the furnace and hence have a high sensible heat content. Sometimes the
exhaust gases carries some chemical energy, which raises the temperature of exhaust gases further due
to post combustion because of this chemical energy. The heat energy contained in the exhaust gases is
the waste energy since it gets dumped in the environment. However, it is possible to recover some part
of this energy if investments are made in waste heat recovery devices (WHRDs).
Methods for waste heat recovery include (i) transferring heat between exhaust gases and combustion air
for its preheating, (ii) transferring heat to the load entering furnaces, (iii) generation of steam and
electrical power, or (iv) using waste heat with a heat pump for heating or cooling facilities.
WHRDs work on the principle of heat exchange. During heat exchange the heat energy of the exhaust
gases gets transferred to some other fluid medium. This exchange of heat reduces the temperature of
the exhaust gases and simultaneously increases the temperature of the fluid medium. The heated fluid
medium is either recycled back to the process or utilized in the production of some utilities such as
steam or power etc.
The benefits of WHRDs devices are multiple namely (i) economic, (ii) resource (fuel) saving, and (iii)
environmental. The benefits of these devices include (i) saving of fuel, (ii) generation of electricity and
mechanical work, (iii) reducing cooling needs, (iv) reducing capital investment costs in case of new
facility, (v) increasing production, (vi) reducing greenhouse gas emissions, and (vii) transforming the
heat to useful forms of energy.
Heat exchangers are most commonly used to transfer heat from combustion exhaust gases to
combustion air entering the furnace. Since preheated combustion air enters the furnace at a higher
temperature, less energy must be supplied by the fuel. Typical WHRDS used for air preheating include
recuperators, furnace regenerators, recuperative and regenerative burners, passive air preheaters, shell
and tube heat exchangers, finned tube heat exchangers or economizers, rotary regenerator or heat
wheel, preheating of load, waste heat boilers, and heat pumps.
Recuperators:-
In a recuperator there is a direct transfer of heat. The two fluids (exhaust gases and the combustion air)
are separated by a heat transfer surface and they do not mix. The heat exchange takes place between
the exhaust gases and the air through metallic or ceramic walls. Duct or tubes carry the air for
combustion to be pre-heated, while the other side contains the waste heat stream.
Recuperators generally recover heat from the exhaust gases of a furnace of medium temperature or high
temperature and transfer it to incoming combustion air. Recuperators can be categorized by the relative
directions of gas flow such as (i) ‘in parallel-flow heat exchangers’ where both the gases flow in the
same general direction, (ii) ‘in counter flow exchangers’ where both the gases flow in opposite
directions, or (iii) ‘in cross-flow’ where the gases flow at right angles to each other. Counter flow heat
exchangers have the greatest effectiveness while the parallel flow arrangement has the lowest
effectiveness.
Recuperators can be based on the principle of heat transfer by radiation, convection, or
combinations. Recuperators are constructed out of either metallic or ceramic materials. Metallic r
ecuperators are used in applications with temperatures below 1050 deg C, while heat recovery at higher
temperatures is better suited to ceramic tube recuperators which can operate with hot side temperatures
as high as 1500 deg C and cold side temperatures of around 950 deg C.
#Coding Approach:
1.Required packages imported.
2.Instance object "Gas" is created that belongs to Solution class.
3.Three objects A,B,C of Quantity class created.(A class representing a specific quantity of a Solution. In addition to the properties which can be computed for class Solution, class Quantity provides several additional capabilities.)
4.Given LHV value is used,preheating temperature range set and all arrays to be used created.
5.For stoichiometric combustion of methane ,moles of Air and fuel are set.
6.Qout and specific Qout calculated.
7.At last Combustion efficiency and AFT found out.
8.Plot created at the end.
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##Program for this challenge:
"""
Combustion Efficiency and AFT Calculation after Preheating
"""
import cantera as ct
import numpy as np
import matplotlib.pyplot as plt
Gas=ct.Solution('gri30.xml')
A=ct.Quantity(Gas) # A for Air
B=ct.Quantity(Gas) # B for Fuel
C=ct.Quantity(Gas) # C for Exhaust gases/combustion products
n=11
AFT=[0]*n #Adiabatic flame temperature array of zeros
Qout=[0]*n #Heat used in useful work,array
Qout_specific=[0]*n #Heat used in useful work per kg,array
Heat_gain=[0]*n #Heat gain or combustion efficiency ,array
LHV=50000000 #Lower Heating value J/Kg
Preheating_temp=np.linspace(298.15,600,n)
for j in range(n):
A.TPX=Preheating_temp[j],ct.one_atm,{'O2':0.21,'N2':0.79} #Temperature,pressure and mole fraction set.
A.moles=9.52 #Moles of A defined
Enthalpy_of_air=A.enthalpy_mass #SPecific Enthalpy
print(Enthalpy_of_air)
B.TPX=298.15,ct.one_atm,{'CH4':1} #Temperature,pressure and mole fraction set.
B.moles=1 #Moles of B defined
Enthalpy_fuel=B.enthalpy_mass
C.TPX=1700,ct.one_atm,{'CO2':1,'H2O':2,'N2':7.52} #Temperature,pressure and mole fraction set.
C.moles=10.52 #Moles of C defined
Enthalpy_of_Exhaust_gases=C.enthalpy_mass
Mass_of_Air=A.mass
print(Mass_of_Air)
Mass_of_methane=B.mass
print(Mass_of_methane)
Mass_of_Exhaust_gas=C.mass
print(Mass_of_Exhaust_gas)
Qout[j]=Mass_of_Air*Enthalpy_of_air+Mass_of_methane*Enthalpy_fuel-Mass_of_Exhaust_gas*Enthalpy_of_Exhaust_gases #Heat used by Useful work
Qout_specific[j]=Qout[j]/Mass_of_methane #Specific Heat
Heat_gain[j]=Qout_specific[j]/LHV
M=A+B #Both Air and Fuel mixed
M.equilibrate('UV') #To set a gas mixture to a state of chemical equilibrium(UV indicates -constant volume)
AFT[j]=M.T
print(Qout)
print(Qout_specific)
print(Heat_gain)
print(AFT)
plt.plot(Preheating_temp,AFT)
plt.title('Variation of AFT with Preheating_temp')
plt.show()
plt.plot(Preheating_temp,Heat_gain)
plt.title('Variation of Heat gain/Combustion Efficiency with Preheating_temp')
plt.show()
#Effect of preheating on the Adiabatic flame temperature:
From the plot above ,it can be seen that with Preheating or increase in air tempeature going for combustion,the AFT increases linearly as well.
Means for same amount of fuel and air,more heat is generated inside furnace.Air temperature is increases so as its Enthalpy also increases.Air and fuel mixed in a better way now,resulting in more heat generation for useful work.
Determining AFT temperature for a particular furnace is important,for making it sure that the furnace we are designing is going to take the heat generatd insie it or not,what kind of material need to be used and how much should be the temperature of preheated air.
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#Effect of pre-heat temeprature on combustion efficiency:
Yes with preheating fuel consumption decreases.
Graph shown below is taken for reference,this is prepared after some research.(by someone else)
#Formula for combustion efficiency:
Combustion efficiency= `Q_(out)/(LHV)`
`Q_(out)`=Useful work done
LHV=Lower heating value
From the plot it can be observed,that with the increase in preheat temperature combustion efficiency also increases.Combustion efficiency increases linearly with preheat temperature.
See the plot we can only use 34% energy of fuel with 298.15Kelvin temeprature and it only reach maximum to 44% with 600 kelvin temperature.
The rest energy is being wasted and remain untilised in exhaust gases.Therefore waste heat recovery device is very important for better utilization of energy of fuel.Here it is recuperator which is basically a heat exchanger.
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#Is it right or wrong to assume a fixed furnace exit temperature?
Ans.Here we have assumed fixed furnace exit temperature which is 1700 Kelvin.
Assume that is correct for minimum value of pre heat temperature (that is 298.15 Kelvin).With that we got 34 % combustion efficiency.
But with the increase in pre heat temperature till 600 kelvin we have kept exhaust gas temperature constant 1700 kelvin.So combustion efficiency increased from 34 to 44 % and varied linearly also.
So considering constant exhaust gas exit temperature is wrong.
Because with the increase in preheat temperature ,we saw AFT also increases ,so Exhaust gas temperature would also increase and we will get more combustion efficiency than this case and it may not vary linearly too.
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