Coal Combustion
- Lightning
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I copied and pasted this from science book online a number of months ago. I thought I would share it for us egg heads to take a look at. There is some interesting information here..
Nitrogen performs no useful office in combustion and passes through the furnace without change. It dilutes the air, absorbs heat, reduces the temperature of the products of combustion, and is the chief source of heat losses in furnaces.
Nitrogen is drawn into the furnace with the air. Its density is 0.9673 (Air = 1); its weight, at 32 degrees Fahrenheit and under atmospheric pressure, is 0.07829 pounds per cubic foot; each pound of air at atmospheric pressure contains 0.7685 pounds of nitrogen, and one pound of nitrogen is contained in 1.301 pounds of air.
Oxidizable Substance or Combustible
Chemical Symbol
Atomic or Combining Weight
Chemical Reaction
Product of Combustion
Oxygen per Pound of Column 1 Pounds
Nitrogen per Pound of Column 1. 3.32[23] × O Pounds
Air per Pound of Column 1. 4.32[24] × O Pounds
Gaseous Product per Pound of Column 1[25] + Column 8 Pounds
Heat Value per Pound of Column 1 B. t. u.
The descriptions for the lines below are listed above.
Carbon C 12 C+2O = CO2 Carbon Dioxide 2.667 8.85 11.52 12.52 14600
Carbon C 12 C+O = CO Carbon Monoxide 1.333 4.43 5.76 6.76 4450
Carbon Monoxide CO 28 CO+O = CO2 Carbon Dioxide .571 1.90 2.47 3.47 10150
In practice it is impossible to obtain perfect combustion with the theoretical amount of air, and an excess may be required, amounting to sometimes double the theoretical supply, depending upon the nature of the fuel to be burned and the method of burning it. The reason for this is that it is impossible to bring each particle of oxygen in the air into intimate contact with the particles in the fuel that are to be oxidized, due not only to the dilution of the oxygen in the air by nitrogen, but because of such factors as the irregular thickness of the fire, the varying resistance to the passage of the air through the fire in separate parts on account of ash, clinker, etc. Where the difficulties of drawing air uniformly through a fuel bed are eliminated, as in the case of burning oil fuel or gas, the air supply may be materially less than would be required for coal. Experiment has shown that coal will usually require 50 per cent more than the theoretical net calculated amount of air, or about 18 pounds per pound of fuel either under natural or forced draft, though this amount may vary widely with the type of furnace, the nature of the coal, and the method of firing. If less than this amount of air is supplied, the carbon burns to monoxide instead of dioxide and its full heat value is not developed.*
* Trouble is, we don't know how much CO is produced with varying amounts of oxygen supplied to the fuel.
An excess of air is also a source of waste, as the products of combustion will be diluted and carry off an excessive amount of heat in the chimney gases, or the air will so lower the temperature of the furnace gases as to delay the combustion to an extent that will cause carbon monoxide to pass off unburned from the furnace. A sufficient amount of carbon monoxide in the gases may cause the action known as secondary combustion, by igniting or mingling with air after leaving the furnace or in the flues or stack. Such secondary combustion which takes place either within the setting after leaving the furnace or in the flues or stack always leads to a loss of efficiency and, in some instances, leads to overheating of the flues and stack.
This paragraph above is a good description of what can happen with excessive secondary air.
The object of a flue gas analysis is the determination of the completeness of the combustion of the carbon in the fuel, and the amount and distribution of the heat losses due to incomplete combustion. The quantities actually determined by an analysis are the relative proportions by volume, of carbon dioxide (CO2), oxygen (O), and carbon monoxide (CO), the determinations being made in this order.
The variations of the percentages of these gases in an analysis is best illustrated in the consideration of the complete combustion of pure carbon, a pound of which requires 2.67 pounds of oxygen,[28] or 32 cubic feet at 60 degrees Fahrenheit. The gaseous product of such combustion will occupy, when cooled, the same volume as the oxygen, namely, 32 cubic feet. The air supplied for the combustion is made up of 20.91 per cent oxygen and 79.09 per cent nitrogen by volume. The carbon united with the oxygen in the form of carbon dioxide will have the same volume as the oxygen in the air originally supplied. The volume of the nitrogen when cooled will be the same as in the air supplied, as it undergoes no change.
And this paragraph demonstrates the volume in = volume out rule..
Feel free to post comments..
Nitrogen performs no useful office in combustion and passes through the furnace without change. It dilutes the air, absorbs heat, reduces the temperature of the products of combustion, and is the chief source of heat losses in furnaces.
Nitrogen is drawn into the furnace with the air. Its density is 0.9673 (Air = 1); its weight, at 32 degrees Fahrenheit and under atmospheric pressure, is 0.07829 pounds per cubic foot; each pound of air at atmospheric pressure contains 0.7685 pounds of nitrogen, and one pound of nitrogen is contained in 1.301 pounds of air.
Oxidizable Substance or Combustible
Chemical Symbol
Atomic or Combining Weight
Chemical Reaction
Product of Combustion
Oxygen per Pound of Column 1 Pounds
Nitrogen per Pound of Column 1. 3.32[23] × O Pounds
Air per Pound of Column 1. 4.32[24] × O Pounds
Gaseous Product per Pound of Column 1[25] + Column 8 Pounds
Heat Value per Pound of Column 1 B. t. u.
The descriptions for the lines below are listed above.
Carbon C 12 C+2O = CO2 Carbon Dioxide 2.667 8.85 11.52 12.52 14600
Carbon C 12 C+O = CO Carbon Monoxide 1.333 4.43 5.76 6.76 4450
Carbon Monoxide CO 28 CO+O = CO2 Carbon Dioxide .571 1.90 2.47 3.47 10150
In practice it is impossible to obtain perfect combustion with the theoretical amount of air, and an excess may be required, amounting to sometimes double the theoretical supply, depending upon the nature of the fuel to be burned and the method of burning it. The reason for this is that it is impossible to bring each particle of oxygen in the air into intimate contact with the particles in the fuel that are to be oxidized, due not only to the dilution of the oxygen in the air by nitrogen, but because of such factors as the irregular thickness of the fire, the varying resistance to the passage of the air through the fire in separate parts on account of ash, clinker, etc. Where the difficulties of drawing air uniformly through a fuel bed are eliminated, as in the case of burning oil fuel or gas, the air supply may be materially less than would be required for coal. Experiment has shown that coal will usually require 50 per cent more than the theoretical net calculated amount of air, or about 18 pounds per pound of fuel either under natural or forced draft, though this amount may vary widely with the type of furnace, the nature of the coal, and the method of firing. If less than this amount of air is supplied, the carbon burns to monoxide instead of dioxide and its full heat value is not developed.*
* Trouble is, we don't know how much CO is produced with varying amounts of oxygen supplied to the fuel.
An excess of air is also a source of waste, as the products of combustion will be diluted and carry off an excessive amount of heat in the chimney gases, or the air will so lower the temperature of the furnace gases as to delay the combustion to an extent that will cause carbon monoxide to pass off unburned from the furnace. A sufficient amount of carbon monoxide in the gases may cause the action known as secondary combustion, by igniting or mingling with air after leaving the furnace or in the flues or stack. Such secondary combustion which takes place either within the setting after leaving the furnace or in the flues or stack always leads to a loss of efficiency and, in some instances, leads to overheating of the flues and stack.
This paragraph above is a good description of what can happen with excessive secondary air.
The object of a flue gas analysis is the determination of the completeness of the combustion of the carbon in the fuel, and the amount and distribution of the heat losses due to incomplete combustion. The quantities actually determined by an analysis are the relative proportions by volume, of carbon dioxide (CO2), oxygen (O), and carbon monoxide (CO), the determinations being made in this order.
The variations of the percentages of these gases in an analysis is best illustrated in the consideration of the complete combustion of pure carbon, a pound of which requires 2.67 pounds of oxygen,[28] or 32 cubic feet at 60 degrees Fahrenheit. The gaseous product of such combustion will occupy, when cooled, the same volume as the oxygen, namely, 32 cubic feet. The air supplied for the combustion is made up of 20.91 per cent oxygen and 79.09 per cent nitrogen by volume. The carbon united with the oxygen in the form of carbon dioxide will have the same volume as the oxygen in the air originally supplied. The volume of the nitrogen when cooled will be the same as in the air supplied, as it undergoes no change.
And this paragraph demonstrates the volume in = volume out rule..
Feel free to post comments..
- Hambden Bob
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Wow ! Lee,my knuckle-dragging mind started to ache while reading this one ! The Scientific Breakdown of Coal Combustion can be an interesting animal,to say the least...... That's the Great Thing about 'Der Coal Board-We cover from Mild to Wild,from Mongo to Einstein as far as this Heat Animal goes ! I damn near had to put on my Coal Shirt and bust out the Little Orphan Annie Secret Decoder to sift through this one !! Nice Job posting this one up !
- Lightning
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Thanks Bob!
Ya know, I always thought that the hydrocarbons were baked out in the first couple hours after shake and load. I may have to re-think this some. Just a little while ago, I went and revved up the fire a little and noticed my water coils were soaked with condensation to the point of dripping after the fire got going good. This would be a pretty good indicator that there is more than just Carbon burning. The only way for condensation to form is if there is water vapor being produced. It's been 18 hours since last tending, been running low and slow till I revved it up just a short time ago. I'm guessing there are several different hydrocarbons in coal, the most volatile ones bake out first, the rest must bake out at higher temperatures or more slowly. This makes even better reason to have some, not allot, of secondary air throughout the burn cycle.
Thoughts?
Ya know, I always thought that the hydrocarbons were baked out in the first couple hours after shake and load. I may have to re-think this some. Just a little while ago, I went and revved up the fire a little and noticed my water coils were soaked with condensation to the point of dripping after the fire got going good. This would be a pretty good indicator that there is more than just Carbon burning. The only way for condensation to form is if there is water vapor being produced. It's been 18 hours since last tending, been running low and slow till I revved it up just a short time ago. I'm guessing there are several different hydrocarbons in coal, the most volatile ones bake out first, the rest must bake out at higher temperatures or more slowly. This makes even better reason to have some, not allot, of secondary air throughout the burn cycle.
Thoughts?
Last edited by Lightning on Sat. Nov. 28, 2015 5:57 pm, edited 1 time in total.
- Hambden Bob
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- Coal Size/Type: Rice-A-Roni ! / Nut
- Other Heating: Pro-Pain Forced Air
In my simple thoughts,using all of the products of gassification,without having the consumption heat go up the chimney,would be the primary,efficient goal. Probably why Mr.Sherrick and the Boys do so well with their baseburners. The longer you can keep the heat absorbing into the stove mass,and then exhausting,makes the process right. If the design of your Coal Burning Appliance doesn't allow for that,and the needed set of controls for as close to complete combustion without roaring the whole deal out the flu,the efficiency goes to pot. And this is only a Knuckle-Draggers opinion.
- SMITTY
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I wish someone would invent an AFFORDABLE gas analyzer for permanent mounting on our stoves / boilers. Then you could fine tune to perfection every single time.
I know in the automotive world, those analyzers are BIG $$$$$$$$. One of the tools I wished I had, but still don't after all these years.
I know in the automotive world, those analyzers are BIG $$$$$$$$. One of the tools I wished I had, but still don't after all these years.
-
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My eyes started going crazy reading that
I figure when a load gets completely turned to ash and I can put my hand on the stove pipe when the barrel is at 550*F, I'm ahead of the game
I figure when a load gets completely turned to ash and I can put my hand on the stove pipe when the barrel is at 550*F, I'm ahead of the game
- Lightning
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I agree, but knowing what's going on in there is just so much more fun!scalabro wrote:My eyes started going crazy reading that
I figure when a load gets completely turned to ash and I can put my hand on the stove pipe when the barrel is at 550*F, I'm ahead of the game
- Rob R.
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This is why I often harp about blowing combustion air through ash on a stoker. Some people crank the air way up in an effort to eliminate all unburned coal from the ash...even if they were successful (they are not) the stack losses from the excess air are large.Nitrogen performs no useful office in combustion and passes through the furnace without change. It dilutes the air, absorbs heat, reduces the temperature of the products of combustion, and is the chief source of heat losses in furnaces.
- confedsailor
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I asked at work if we still had our Fyrite analyzers from when we had a steam plant. Sadly they went the way of the dodo. I have a wonderful book from Babcock & Wilcox that goes into all of that to excruciating detail.SMITTY wrote:I wish someone would invent an AFFORDABLE gas analyzer for permanent mounting on our stoves / boilers. Then you could fine tune to perfection every single time.
I know in the automotive world, those analyzers are BIG $$$$$$$$. One of the tools I wished I had, but still don't after all these years.
-
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When the hydrogen in coal burns you get water vapor which condenses on a cool enough surface. Usually in the upper part of a chimney.Lightning wrote:Thanks Bob!
Ya know, I always thought that the hydrocarbons were baked out in the first couple hours after shake and load. I may have to re-think this some. Just a little while ago, I went and revved up the fire a little and noticed my water coils were soaked with condensation to the point of dripping after the fire got going good. This would be a pretty good indicator that there is more than just Carbon burning. The only way for condensation to form is if there is water vapor being produced. It's been 18 hours since last tending, been running low and slow till I revved it up just a short time ago. I'm guessing there are several different hydrocarbons in coal, the most volatile ones bake out first, the rest must bake out at higher temperatures or more slowly. This makes even better reason to have some, not allot, of secondary air throughout the burn cycle.
Thoughts?
- Lightning
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- Coal Size/Type: Pea Size - Anthracite
Or on water coils which are cold in relation to the environment around them. I'm thinking there are many complex hydrocarbon's present in fresh coal, even though there isn't a huge percentage of them.franco b wrote:When the hydrogen in coal burns you get water vapor which condenses on a cool enough surface. Usually in the upper part of a chimney.
Thanks for contributing..
- lsayre
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I found a few websites that list C₂₄₀H₉₀O₄NS as the nominal chemical formula for"high grade" anthracite.
http://freepages.school-alumni.rootsweb.ancestry. ... acite.html
http://www.purdue.edu/discoverypark/energy/assets ... -Oct08.pdf
http://freepages.school-alumni.rootsweb.ancestry. ... acite.html
http://www.purdue.edu/discoverypark/energy/assets ... -Oct08.pdf
- Lightning
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Awesome find Larry! It states that the hydrocarbon volatile matter needs 1700 degrees to bake out completely. This is fantastic information. Meaning that it's possible to have volatile gases burning thru the entire burn cycle if the fresh load isn't heated to this temperature immediately following tending. Neato.
It's likely the lightest most unstable ones bake out first, then heavier more complex hydrocarbons bake out later. With this information, I will alter my routine slightly and not rev the fire up so hot after loading and I'll be more conscious of allowing secondary air thru the burn. This will make better use of the hydrocarbon heat value of the fuel load.
It's likely the lightest most unstable ones bake out first, then heavier more complex hydrocarbons bake out later. With this information, I will alter my routine slightly and not rev the fire up so hot after loading and I'll be more conscious of allowing secondary air thru the burn. This will make better use of the hydrocarbon heat value of the fuel load.
Last edited by Lightning on Sun. Nov. 29, 2015 4:54 pm, edited 1 time in total.
- Lightning
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And Rob, I totally agree with your statement earlier. It's better to loose some coal off the end of the grate than to push excessive amounts of combustion air thru it just to carry heat out the chimney riding on nitrogen.