Fuel of ChoiceThe fuel we choose is based in part on cost in dollars and cents. Some of our choices are heating oil, wood, liquefied petroleum gas (LPG), and coal. In terms of cost, how do these compare? Back to the flip, how efficient are these fuels in maximizing carbon dioxide greenhouse gas production? For simplicity’s sake, we will make a few assumptions. Then we will do the chemistry. Then we will do the math.
If you are not “into” chemistry or math you can jump down to the bottom of the page to the subheading Evaluation.
AssumptionsWe will assume the coal converts completely to the dioxide, with no monoxide. Wood will be considered as pure cellulose, a quite good approximation. The unit chemical formula for cellulose is (C₆H₁₀O₅).
Liquefied petroleum is generally a mixture of condensed propane and butane gases. But they are so similar, we will choose only one component, propane (C₃H₈) for our calculations.
The ChemistryWood (cellulose) burns according to the equation,
2 C₆H₁₀O₅ + 11 O₂ → 6 CO₂↑ +10 H₂O↑ + heatLPG (propane) burns according to the equation,
2 C₃H₈ + 10 O₂ → 6 CO₂↑ + 8 H₂O↑ + heatCoal burns according to the equation,
C + O₂ → CO₂↑We won’t consider the chemistry of heating oil. It resembles that of LPG moderately closely.
The MathIn order to make a comparison of the various fuels, a standard quantity of each needs to be agreed upon. We choose to use one kilogram of each fuel. Energy is given in kilocalories. The volume of carbon dioxide greenhouse gas each burning produces is given in liters.
Moles CO₂ from CoalLet’s begin with the coal or carbon. The atomic weight of the element carbon (which is the same as the molecular weight in the case of coal) is 12. One kilogram of coal thus contains 1000/12 = 83 atomic weights (molecular weights or “moles”) of carbon.
This means that, since one carbon plus one molecule of oxygen produces one molecule of carbon dioxide, the result of burning 83 molecular weights of carbon is the production of 83 molecular weights of carbon dioxide greenhouse gas. How many liters is that? At room temperature, a gas occupies 22.4 liters. Multiplying that time 83 gives 1859 liters. Burning 1 kilogram of coal produces 1859 liters of carbon dioxide.
Moles CO₂ from WoodThe molecular weight of cellulose, if it is viewed in terms of one unit of the total structure, namely C₆H₁₀O₅, is 242 grams. This means one kilogram of cellulose amounts to 4.1 moles. Since burning 2 moles of cellulose produces 8 moles of carbon dioxide, burning 4.1 moles of cellulose produces 16.4 moles of carbon dioxide. Burning 1 kilogram of wood produces 367 liters of carbon dioxide.
Moles CO₂ from LPGThe propane component of LPG has a molecular weight of 44 grams. One kilogram of the gas thus amounts to 22.7 moles. 2 moles of butane burn to produce 6 moles of carbon dioxide. One kilogram of propane thus burns to produce 68.1 moles of carbon dioxide, which amounts to 1525 liters. Burning 1 kilogram of LPG produces about 1525 liters of carbon dioxide (the actual figure, taking the presence of butane into effect, is less).
Kilocalories per Kilogram CoalThe Engineering Toolbox reference informs us that 1 ton of coal produces 28,000,000 BTUs of heat. By simple conversions, we get,
28,000,000 BTU x 0.252 kilocalories / BTU = 7,100,000 kilocalories
1 ton = 2000 lbs. x 454 grams per lb. / 1000 grams per kilogram = 908 kilograms
Therefore, 7,100,000 kilocalories / 908 kilograms = ?
Answer: 7,800 kilocalories per kilogram of coal
Kilocalories per Kilogram WoodOne pound of wood generates 8,000 BTUs. The conversion is calculated,
8,000 BTUs per lb. x 0.252 kilocalories per BTU / 0.454 kilograms per pound = ?
Answer: 915 kilocalories per kilogram of wood
Kilocalories per Kilogram LPGLPG produces 91,300 BTUs per gallon burned. Its density is approximately 0.55 grams per milliliter. A gallon = 3.8 liters. The weight of a gallon of LPG is approximately,
3.8 liters x 1000 milliliters per liter x 0.55 grams per milliliter = 2,100 grams per gallon LPG = 2.1 kilograms LPG
91,300 BTUs per gallon / 2.1 kilograms per gallon LPG = 43,500 BTUs per kilogram LPG
43,500 BTUs per kilogram LPG x 0.252 kilocalories per BTU = ?
Answer: 11,000 kilocalories per kilogram of LPG
Kilocalories per Kilogram Heating OilWe recall there are 139,000 BTUs in 8.2 lbs. of heating oil.
(139,000 BTU / 8.2 lbs.) x 0.252 kilocalories per BTU x 0.454 kilograms per pound = ?
Answer: 1,940 kilocalories per kilogram of heating oil
Kilocalories Heat per Liter Carbon Dioxide All FuelsHeating Oil:
139 pounds of CO2 are released for every 1,000,000 BTU heat produced. We first convert the BTUs to kilocalories. Then we convert the pounds to liters. The results will be expressed as kilocalories heat per liter carbon dioxide.
252,000 kilocalories / 32,100 liters CO₂ = ?
Answer: 7.9 kilocalories heat / liter CO₂LPG:
The numbers are essentially identical for LPG (propane).
Answer: 7.9 kilocalories heat / liter CO₂Wood:
A German web site concerned with carbon dioxide greenhouse gase emission (see references) tells us that wood produces 110 kilograms CO₂ per gigajoule of energy produced. Unfortunately, this appears intended to refer to an industrial site where electricity is generated from wood. Still, it should give us an approximate answer.
There are 239,000 kilocalories in 1 gigajoule.
110 kilograms CO₂ = 110,000 grams CO₂ (110,000 grams CO₂ / 44 grams per mole) = 2,500 moles CO₂
2,500 moles CO₂ x 22.4 liters per mole = 56,000 liters CO₂
br> 239,000 kilocalories / 56,000 liters = ?
Answer: 4.3 kilocalories heat / liter CO₂Coal (anthracite):
228.6 pounds of CO₂ are released for every 1,000,000 BTUs of heat produced.
Skipping some steps, we get, 252,000 kilocalories / 52,800 liters CO₂ = ?
Answer: 4.8 kilocalories heat / liter CO₂
Carbon Dioxide Greenhouse Gas – An EvaluationEnergy produced per kilogram of fuel, listed from best to worst is: LPG, coal, heating oil, and wood.
The energy produced per liter of carbon dioxide greenhouse gas, from most to least, is: LPG and heating oil (tied), coal, and (believe it or not) wood.
Yes, wood is worst of all!
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- North American Combustion Handbook
- U.S. Energy Information Administration: Frequently Asked Questions
- Engineering Toolbox Com (BTUs per unit of fuel)
- Volker Quaschning (Data for burning wood)
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