Combustion #2 – The Ratio of Weights and Calculating Emissions

To figure out how much carbon you’re dumping into the air, you need to examine the molecular weight of the fuel and the carbon content of the fuel.  For energy calculations, the following atomic weights are pretty much all you need to know:

Molecular Weights

  •  C             12
  • H                1
  • H2             2
  • O              16
  • O2            32
  • CO           28
  • CO2         44
  • H20         18
  • CH4         16
  • C2H4       28
  • C2H6O    46
  • S               32
  • NO           30
  • NO2         46
  • SO2          64
  • C8H18    114 (typical gasoline)

Don’t forget to refer to the Combustion #1 post to get heat of combustion values for various fuels.

Note that carbon burned in the air will preferentially create carbon dioxide and release heat.  Observe that a molecule of carbon has a weight of 12, whereas a molecule of CO2 has a weight of 44.  Therefore, burning 1 unit of carbon (it can be pounds, tons, etc) will generate 3.7 units of carbon dioxide.

Let’s take a look at the combustion analysis for natural gas and oil.

For our purposes, we will assume that natural gas consists of only CH4 (methane.)  In fact, natural gas often contains some butane, propane. ethane, nitrogen and other elements.

The combustion equation is pretty simple.  The corresponding molecular weights are to the right in parentheses.

CH4 + 2O2 = CO2 + 2H2O     (16 lb + 64 lb = 44 lb + 36 lb)

This tells us that burning 16 pounds of natural gas will generate 44 pounds of CO2.  The 36 pounds of water vapor can be ignored because it is benign, which is why hydrogen is such a great fuel.

The weight ratio of natural gas to carbon dioxide is 2.75.  So burning a ton of natural gas will release 2.75 tons of carbon dioxide.

You may recall from my earlier post that burning a pound of methane results in a release of 23,875 Btu.  From this we can conclude that methane delivers 8,682 Btu heat per pound of CO2 generated.

Let’s see how that compares with a generic fuel oil.  The combustion equation (ignoring the hydrogen products) and pertinent molecular weights are:

2 C14H30 (396) combines with 43 O2 => 28 CO2 (1,232) + 30 H2O

We observe that the weight ratio of fuel to CO2 is 3.1.  So burning a ton of fuel oil will release 3.1 tons of carbon dioxide.  You may find it surprising that the carbon dioxide emissions are not that much higher than natural gas, since natural gas is widely cited as a “cleaner” form of energy than oil.

What we need to keep in mind is that fuel oil delivers less heat per pound than natural gas.  If we figure a fuel oil energy content of 20,000 Btu per pound, we see that fuel oil delivers only 6,430 Btu heat per pound of CO2 generated.

To put this another way, 100,000 Btu of heat will result in CO2 emissions of 11.5 pounds if you are burning natural gas, 15.6 pound if burning fuel oil.  That difference is significant.

Coal is even more striking.  While the carbon content of coal varies, let’s figure it to be about 75% by weight.  It has negligible other combustibles, though there is other stuff in there like sulfur and mercury that can cause problems.

Well, we know the ratio of carbon to carbon dioxide is 3.7 to 1.  So with 75% carbon content, burning a ton of coal will emit about 2.8 tons of carbon dioxide (75% of 3.7.)

Coal contains roughly 13,500 Btu per pound, so coal delivers about 4,820 Btu heat per pound of CO2 generated.  And 100,000 But of heat from coal will result in CO2 emissions of roughly 20.8 pounds of CO2.  Let’s summarize this:

Carbon Dioxide Emissions per 100,000 Btu Liberated Heat

  • Hydrogen                  0
  • Natural Gas          11.5 pounds
  • Generic Fuel Oil  15.6 pounds
  • Generic Coal        20.8 pounds

So there you have it.  Once you know the chemical makeup of your particular fuel, calculating the carbon emissions is pretty simple.

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One comment on “Combustion #2 – The Ratio of Weights and Calculating Emissions

  1. BIlly says:

    This is going to be very useful for me thank you very much for posting

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