How Much Carbon Dioxide From Biodiesel

Biodiesel puts CO₂ in the Atmosphere -- But How Much?

Biodiesel is made from vegetable oil or animal fat. In the U.S., most of it is made from soybean oil. This makes it a "renewable" or "alternative" liquid transport fuel.

The carbon dioxide released by burning biodiesel in your vehicle was originally taken out of the air by soybean plants. So in some sense biodiesel is "carbon neutral" and doesn't contribute to increasing atmospheric CO₂ as much as the regular diesel made from fossil petroleum.

To produce one gallon of biodiesel takes about 18 kg of soybeans. But the soybeans are only part of the soybean plants that produced them. The soybean harvest makes up only about one-quarter of the biomass of the soybeans the farmer grew. So the total biomass grown to make a gallon of biodiesel is about 75 kg.

The soybean plants made themselves out of air, using the energy in sunlight to fix CO₂ into carbohydrates and lipids. Growing that 75 kg of soybean plants took about 175 kg of CO₂ out of the air. (The plants spat the O₂ out again, and replaced it with the lighter element hydrogen.)

So that gallon of biodiesel started off as 175 kg of CO₂. To produce the soybeans, crush them to separate their oil, manufacture the oil into biodiesel, and transport the products among these operations took some energy derived from fossil fuels. Assuming this energy came half from natural gas and half from liquid fuels, these fuels contained 700 grams of carbon. When they were burned they released about 2.5 kg of CO₂. (True, the transportation fuels could have been biodiesel -- the impact of such substitution is left to the student as an excercise.)

Where the Carbon Goes

So about 177 kg of CO₂ is embodied in the gallon of biodiesel, and all of it will be released as the biodiesel is produced and used. Here is where it goes:

• About 30 kg of plant residue is left in the field. Eventually it decomposes and releases 110 kg of CO₂. This is more than 60% of the 177 kg of CO₂ we started with.
• Soybean oil makes up only about 18% of the soybean, by weight. After the oil is extracted what is left is "soybean meal", a valuable protein-rich animal feed. The Soybean meal from our one-biodiesel-gallon's worth of soybeans weighs about 15 kg and contains about 13 or 14 kg of carbon. Eventually it will be digested by animals, broken down, and converted back into CO₂ (it may have to go through several animals to finally be CO₂ again). That will yield about 53 kg of CO₂, about 30% of what we started with.
• When the soybean oil is converted into biodiesel there is some glycerine produced as a byproduct. When that glycerine is eventually broken down it will release another 850 grams of CO₂ (less than 1%).
• The fossil energy consumed in the growing, transporting, and processing to get biodiesel to the vehicle's fuel tank produces about 2.5 kg of CO₂ (about 1.5%).
• Finally we get to the biodiesel. One gallon of biodiesel has about 2.8 kg of carbon in it, and when it is burned it will produce about 10 kg of CO₂. that is about 5.5% of the CO₂ fixed by the soybeans and burned to make them into fuel.

Efficiency of Biodiesel

So in one sense we used agricultural and industrial ingenuity to convert 700 grams of carbon in the form of fossil fuels into 2700 grams of carbon in biodiesel. That is like multiplying that fossil carbon 3.8 times!

On the other hand, the process was shockingly inefficient, turning 50 kg of carbon fixed by plants into 2.7 kg of carbon in liquid fuel form. Only 5.5% of the total fixed carbon ended up powering a vehicle!

Moral

It all depends on how you look at it.

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environment energy climate change global warming alternative energy biodiesel