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COMBUSTION SYSTEM EFFICIENCY
Contributing Members:
Kyle Bowen
Brandon Todd
Neal Patel
Danar Sampurno
Ivan Victoria
http://superiorboiler.com/mohawk.htm
2
Objectives
• To understand how furnaces and boilers utilize
• To model efficiency estimates with numerical methods
combustion
• To discuss how to optimize combustion systems.
• To understand the importance of the air/fuel ratio, and
how it relates to the system’s efficiency.
• To present future research possibilities
http://www.africoal.co.za/optimal-combustion-processes-fuels-excess-air/
% Excess Air that Power Plant boilers typically operate at for various fuels
http://www.sankey-diagrams.com/tag/heat-loss/
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Furnaces
• Furnaces and Boilers operate by inputting fuel
and air into the furnace and igniting it to release
energy. The figure to the left shows a typical
furnace
• Furnaces and boilers require large amounts of
fuel to operate.
• Many Furnaces and Boilers take in more air than
stoichiometric amounts of air to ensure complete
combustion
http://home.earthlink.net/~jschwytzer/gas_fired_furnace.gif
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Other Furnace Examples
http://www.onehourairconditioningcharlotte.com/furn
aces.html
http://www.directindustry.com/prod/seco-warwick-sp-z-oo/rotary-hearthfurnaces-16223-134754.html
http://www.enggpedia.com/chemical-engineeringencyclopedia/dictionary/thermodynamics/1778-furnace-types-classification-of-furnace
http://www.diychatroom.com/attachments/f17/63693d13582057
95-furnace-filters-turn-black-quickly-furnace-drawing.jpg
http://koppelservices.com/furnace
-repair-basics/
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Boilers
• Similar to furnaces, boilers operate by inputting
fuel and air to release energy to boil liquid. The
figure to the left shows an example of a boiler
• Boilers require large amounts of fuel to operate,
especially as the boiling point of the liquid
increases.
• These kind of systems are extremely important
to Chemical Engineers because distillation
columns are greatly important to chemical
plants.
http://www.hauserman-engineering.com/Gasification.html
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Other Boiler Examples
http://science.howstuffworks.com/transport/enginesequipment/steam2.htm
http://www.fapdec.org/boilers.htm
http://www.johnstonboiler.com/
http://www.aciindustries.com/boiler.htm
https://secure.sceg.com/enercom/library/furntune.asp
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Combustion System Efficiency
• The ratio of inputted heat energy to output work is very low
for combustion systems. Small increases in efficiency lead
to large fuel savings.
• Increasing the efficiency of the entire system allows for less
fuel consumption, which leads to an increased revenue.
• Gas fired furnaces are widely used, even though they are
significantly less efficient than electric furnaces.
http://www.rayteq.com/images/gas_vs_electric_figure1.png
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Air To Fuel Ratio
• Combustion of hydrocarbons requires O2 and are of this form:
https://www.thermalfluidscentral.org/encyclopedia/index.php/Basics_(Combustion)
• Air is the cheapest source of O2 however, as shown in the
pie chart, air is only 21% O2. When air is used, extra heat is
required to heat the remaining 79% of the components of
air as well
• Excess Air allows more complete combustion, but it also
requires more fuel in order to heat it.
http://pattiisaacs.files.wordpress.com/2011/12/air-composition-pie-chart2.jpg
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Air To Fuel Ratio
Improve Combustion System Efficiency, Bill Axon, pg. 43
Improve Combustion System Efficiency, Bill Axon, pg. 41
This graphic shows how the amount of fuel required
greatly increases as % excess air increases
The graph further illustrates how fuel efficiency
increases as stoichiometric Air/Fuel ratio is
approached
10
Air To Fuel Methodology
• In order to simulate efficiency savings, regression
techniques were used with the data from the table
to the left.
• Polynomial Regression and Plotting techniques
were used to create plots of the data to emphasize
various trends from the data.
Improve Combustion System Efficiency, Bill Axon, pg. 41
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MATLAB Program For Following Slide
clear
% Fuel savings by cutting back 50% excess air to 10%
%x= Furnace exit gas temperature (Farenheit)
%y= Fuel Savings (%)
x=[2600 2400 2200 2000 1800 1600 1400 1200 1000 800 600 400];
y=[81.1 59.5 45.3 35.3 27.7 21.8 17.1 13.2 9.98 7.25 4.91 2.88];
%Initialization
N=1;
i=1;
P = polyfit(x,y,N);
yp=polyval(P,x);
erri=(yp-y).^2;
err(1,i)=sqrt(sum(erri(:)))/length(y);
%While loop optimizes the degree of polynomial used for the fitting.
%Loop terminates whenever the sum of the squared errors increases or when
%the degree is 3.
while (N<3)
N=N+1;
i=i+1;
ypold=yp;
P = polyfit(x,y,N);
yp=polyval(P,x);
erri=(yp-y).^2;
err(1,i)=sqrt(sum(erri(:)))/length(y);
a=abs(err(i)/err(i-1));
if a>1, yp=ypold;N=N-1;break,end
end
% Plots the data
figure(2)
plot(x,y,'oblack',x,yp,'.blue')
xlabel('Furnace Exit Temperature, x (F)')
ylabel('Fuel Savings , y (%)')
legend('Data',['N=',num2str(N)],2)
title('Fuel Savings by cutting from 50% to 10% Excess Air')
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Fuel Savings
• As the exit temperature of the Combustion system
increases, fuel savings from decreasing excess air
greatly increase. This is fitted by a cubic polynomial
(N=3).
• This shows that the Air/Fuel ratio is extremely
important to the efficiency of high temperature
combustion systems.
• For low temperature furnaces, the there is still
beneficial fuel savings for decreasing excess air.
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MATLAB Program For Following Slide
clear
%x=Percent of Excess Air you are operating at (%)
x=[15 20 30 40 50 70];
%y= rows are percent fuel savings achieved by cutting back to 10% excess
%the multiple columns are values at different Furnace Exit Gas Temperatures
y=[7.44 14.9 29.8 44.6 59.5 89.3;...
4.41 8.82 17.6 26.5 35.3 52.9;...
2.73 5.45 10.9 16.4 21.8 32.7;...
1.25 2.5 4.99 7.49 9.98 15];
%Initialization
n=length(y(1));
N=0;
i=0;
error=[];
a=0.5;
yp=y;
%The while loop optimizes the degree of the polynomial used to fit the data
%It calculates the sum of the squared errors for a given degree and
%compares it to the error of the previous degree. Will not exceed cubic
while (N<3)
N=N+1;
i=i+1;
for h=1:n
ypold(h,:)=yp(h,:);
P=polyfit(x,y(h,:),N);
yp(h,:)=polyval(P,x);
error(i)=sum((yp(h,:)-y(h,:)).^2);
if i>1
a=error(i)-error(i-1);
end
if a>=1,yp=ypold; break, end
end
end
%Plot of of the X values (% excess air) vs the Y values (Fuel
savings in %)
%Plot displays regressions for multiple temperatures as well as data
points
plot(x,yp(1,:),'-black',x,yp(2,:),'--g',x,yp(3,:),'.r',x,yp(4,:),':blue'), hold on
plot(x,y(1,:),'xblack',x,y(2,:),'og',x,y(3,:),'.r',x,y(4,:),'+blue')
xlabel('% Excess Air from which to cut back to 10%'), ylabel('Fuel
Savings in Percent')
title('Fuel Savings versus Excess Air at various temperatures')
legend('2400 F','2000 F', '1600 F','1000 F',2)
%display degree of polynomial
N
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Fuel Savings
• As Furnace Exit Gas Temperature increases,
the effect of cutting back on excess air
increases dramatically.
• For lower temperature exit gases (i.e. 1000 F),
the fuel savings for cutting back on excess air
is less pronounced and less important to
overall efficiency
• For higher temperature exit gases (i.e. 2400 F),
the fuel savings are dramatic for decreasing
excess air
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MATLAB Program For Following Slide
clear
% Available heat is a function of fluegas temperature and 50% excess air
%x= Furnace exit gas temperature (Fahrenheit)
%y= available heat (%)
x=[300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800
1900];
y=[84.02 81 77.93 74.82 71.67 68.48 65.24 61.97 58.66 55.31 51.93 48.51 45.05
41.56 38.03 34.47 30.87];
%Initialization
N=1;
i=1;
P = polyfit(x,y,N);
yp=polyval(P,x);
erri=(yp-y).^2;
err(1,i)=sqrt(sum(erri(:)))/length(y);
%While loop optimizes the degree of polynomial used for the fitting.
%Loop terminates whenever the sum of the squared errors increases or when
%the degree is 3.
while (N<3)
N=N+1;
i=i+1;
ypold=yp;
P = polyfit(x,y,N);
yp=polyval(P,x);
erri=(yp-y).^2;
err(1,i)=sqrt(sum(erri(:)))/length(y);
a=abs(err(i)/err(i-1));
if a>1, yp=ypold;N=N-1;break,end
end
% Plot x vs y
figure(2)
subplot(1,2,1)
plot(x,y,'oblack',x,yp,'-blue')
xlabel('Temperature, x (F)')
ylabel('Available Heat, y (%)')
legend('Data',['N=',num2str(N)])
title('% Available Heat as function of Fluegas
Temperature')
%Plot for extrapolation
subplot(1,2,2)
fplot(@(x) P(1)*x^3+P(2)*x^2+P(3)*x+P(4),[200 2500]);
xlabel('Temperature, x (F)')
ylabel('Available Heat, y (%)')
legend('50% Excess Air')
title('% Available Heat as function of Fluegas
Temperature')
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Furnace Losses
•
As the figure to the left shows, there are several
areas of heat loss in a furnace.
•
The most prominent of these sources are sensible
heat losses from exiting Fluegas.
•
The Fluegas losses can be minimized by adjusting
temperature and operating excess air.
Improve Combustion System Efficiency, Bill Axon, pg. 42
http://www.vesma.com/tutorial/furnaces.htm
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Available Heat Methodology
• In order to view trends in available heat based
upon excess air and fluegas temperature,
regression techniques were used with the data
from the table to the left.
• Polynomial Regression and Plotting techniques
were used to create plots of the data to emphasize
various trends from the data.
Improve Combustion System Efficiency, Bill Axon, pg. 41
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Available Heat Output
The Fluegas temperature has a dramatic effect on the amount of
useful heat output the system produces. Thus it is incredibly
important to minimize Fluegas temp in your system.
This regression plot gives is extrapolated from 0 F to 2500 F to allow
estimate the amount of available heat for a system with different
fluegas temperatures.
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MATLAB Program For Following Slide
clear
%x=Percent of Excess Air you are operating at (%)
x=[0 5 10 25 50];
%y= Percent of Available heat as percentage of gross heat input
%the multiple columns are values at different Fluegas Temperatures
y=[86.14 85.93 85.72 85.08 84.02;...
79.68 79.19 78.71 77.25 74.82;...
65.91 64.85 63.79 60.61 55.31;...
58.64 57.29 55.93 51.85 45.05];
%Initialization
n=length(y(:,1));
N=0;
i=0;
error=[];
a=0.5;
yp=y;
%The while loop optimizes the degree of the polynomial used to fit the data
%It calculates the sum of the squared errors for a given degree and
%compares it to the error of the previous degree. Will not exceed cubic
while (N<3)
N=N+1;
i=i+1;
for h=1:n
ypold(h,:)=yp(h,:);
P=polyfit(x,y(h,:),N);
yp(h,:)=polyval(P,x);
error(i)=sum((yp(h,:)-y(h,:)).^2);
if i>1
a=error(i)-error(i-1);
end
if a>=1,yp=ypold; break, end
end
end
%Plot of of the X values (% excess air) vs the Y values (Fuel
savings in %)
%Plot displays regressions for multiple temperatures as well as data
points
plot(x,yp(1,:),'-black',x,yp(2,:),'--g',x,yp(3,:),'.r',x,yp(4,:),':blue'), hold on
plot(x,y(1,:),'xblack',x,y(2,:),'og',x,y(3,:),'.r',x,y(4,:),'+blue')
xlabel('% Excess Air'), ylabel('Available Heat (% of gross heat
input)')
title('Available Heat as function of % Excess Air')
legend('300 F','600 F','1200 F','1500 F')
%display degree of polynomial
N
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Effect of Temperature on Available Heat Output
• For low Fluegas temperatures, the
available heat is roughly constant for all
% excess air
• As Fluegas Temperature increases,
losses in output heat are greater as %
excess air increases.
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Improving Excess Air Control
•
•
•
•
Below figure is a pressure balanced regulator
Offers improved control over furnace environment
Can maintain constant furnace temperature
Can maintain Air/Fuel Ratio
Improve Combustion System Efficiency, Bill Axon, pg. 43
• Above figure is a mechanically linked valve
control
• Difficult to maintain constant excess air content
Improve Combustion System Efficiency, Bill Axon, pg. 44
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Improving Excess Air Control
The figure to the left is an Electronic Mass-flow control system
Improve Combustion System Efficiency, Bill Axon, pg. 44
Pros:
• highest level of control over composition and furnace
conditions
• Compensates for Ambient Temperatures and other system
variances
• Little human input or knowledge required
Cons:
• Very expensive
• May be unnecessary for qualified human valve operator
http://community.cengage.com/Chilton/cfsfilesystemfile.ashx/__key/CommunityServer.Blogs.Components.WeblogFiles/davids_5F00_blog/2068.gasoline.direct.fuel.injectors.jpg
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Furnace Pressure On Efficiency
• Furnace Pressure affects heat losses.
• With a lower (negative) furnace pressure than
external temperature, colder external air infiltrates
the furnace as portrayed by the graph.
• A greater (positive) furnace pressure increases the
amount of heat lost due to hot gas forced out of the
furnace.
http://www.nwfpa.org/nwfpa.info/component/content/article/50-process-heat/252-furnace-pressurecontrollers
http://www.nwfpa.org/nwfpa.info/component/content/article/50-process-heat/255-reduce-air-infiltration-in-furnaces
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Furnace Pressure On Efficiency
•
The overall effect of cold air infiltration and hot air loss is
increased costs.
•
The figure to the left depicts how the operating costs of an
experimental furnace at positive and negative furnace
pressures
•
The graph shows that air infiltration due to negative pressure
causes prices to increase much quicker than gas loss due to
positive pressure.
•
For optimal conditions, furnace pressure should be as close to
zero as possible. Valves to control the pressure inside the
furnace would be beneficial to maintain an economically optimal
pressure inside the furnace.
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Why is this important to Chemical Engineers?
http://csd-new.newcastle.edu.au/simulations/distillation.html
http://www.quarkology.com/12-chemistry/92-production-materials/92Asynthetic-polymers.html
http://www.indiamart.com/bioenergyengineering/distillation-column.html
http://poweryoung.org/energy.cfm@page=oil_home-basics
http://www.tutorvista.com/content/physics/physics-ii/fission-andfusion/fractional-distillation.php
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Why is this important to Chemical Engineers?
• One of the most important units in any plant is the
distillation column
• Distillation columns require heat to boil the input
mixture. This is typically done with some sort of
combustion boiler
• Improved efficiency with the combustion system
will lead to lower energy and fuel costs and thus
increase the revenue of the plant.
http://www2.emersonprocess.com/en-us/news/pr/pages/1308-mol.aspx
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Areas for Future Research
• Optimal excess air for different fuel types at
different temperatures
• Quantitative analysis on the effect of
boiler/furnace pressure or Heat Output
• Analysis of how mixing the boiler liquid can
increase the heat transfer from combustion
• Research in how furnace shape and design can
decrease the necessity for excess air.
• Economic cost analysis of improving technology
for combustion systems vs. fuel savings
http://www.fundamentalform.com/html/doe_report-1.html
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Conclusion
• The air to fuel ratio is incredibly important to combustion system efficiency and
any attempt to minimize excess air within a process can lead to increased
revenue
• Numerical Methods can be applied to better understand and predict trends and
give estimates for projected savings from process improvements
• For Chemical Engineers, these systems are greatly important to distillation
columns in plants and refineries. An understanding of combustion efficiency
improvements can be a great asset to a chemical engineer.
http://www.ivaonline.com/IMG/jpg/Sealed_Quench_Furnace_MKe_MKg_with_open_antechamber_zoom.jpg