Biofuel Chemistry: How they Burn?

In a previous article we talked about the combustion of hydrocarbons and detailed not only how they burned in ideal conditions, but also how they burned in real-life settings. In this article, we do the same thing for biofuels, starting with alcohols.

Burning Alcohol

Ethanol, methanol, butanol, and other alcohols are all flammable. Interestingly, both alcohols and gasoline don't burn in their liquid forms. That is to say, the liquid forms of these molecules are relatively difficult to burn, why is that?

The answer has to do with oxygen (molecular oxygen). Combustion requires oxygen and only gasoline or alcohol exposed to oxygen can burn. In the liquid form, both substances are packed tight enough to prevent too much oxygen from entering the liquid. This is why gasoline burns better when sprayed into an engine by a fuel injector. It is also why the surface of ethanol burns and not the entire liquid contents. So, both fuels need to be aerosolized in order to burn efficiently.

When ethanol is burned in the presence of oxygen, we get an equation that looks something like this.

formula of ethanol combustion

Combustion of Ethanol

When butanol is burned, we see this.

formula of butanol combustion

Combustion of Butanol

These equations are, of course, the ideal relationships. Because alcohol contains very little sulfur, there is very little sulfur dioxide produced when it is burned and thus little sulfuric acid. However, alcohol is not a “clean-burning fuel.” Alcohol, and any fuel derived from recently living plant and animal matter, contains larger amounts of nitrogen. This means that biofuels produce more nitric oxide and other nitrogen compounds when burned. In some cases, this not only offsets the savings from not producing sulfur compounds, but actually worsens the long term trend for acid rain.

Alcohols are not immune to inefficient combustion issues either. The inefficient combustion of alcohol produces carbon monoxide, formaldehyde, ammonia, benzene, and other toxic chemicals. In some studies, the combustion of ethanol actually produced more formaldehyde, a toxic and carcinogenic compound, than burning gasoline. Ultimately, burning ethanol does produce less carbon monoxide because the fuel itself supplies some oxygen in addition to what is found in the atmosphere. The benefits in terms of carbon monoxide are less with butanol because the ratio of oxygen to carbon is lower in butanol. In ethanol, the ratio is one oxygen molecule to two carbon molecules or 1:2. In butanol, the ratio is 1:4.

We can do a bit of analysis to determine how much byproduct ethanol produces in comparison to gasoline. This comparison will only be for the actual combustion step and does include production, distribution, etc.

When a kilogram of gasoline is burned, we get approximately 33 megajoules of energy and somewhere on the order of 3.09 kilograms of carbon dioxide. When a kilogram of ethanol is burned, we get 20 megajoules of energy and about 1.91 kilograms of carbon dioxide. However, remember that we get 20/33 times the amount of energy from ethanol or only about 0.61 times the amount of energy. This means we need to burn 1.64 kilograms of ethanol to get the equivalent amount of energy, which brings us up to 3.14 kilograms of carbon dioxide. So, ethanol produces MORE carbon dioxide than gasoline.

For butanol, about 2.37 kilograms of carbon dioxide are produce per kilogram burned. Because it returns somewhere on the order of 29 MJ/kilogram, we only need about 1.14 kilograms to get the same energy content as gasoline. Thus, for the same energy content, butanol produces only about 2.7 kg of carbon dioxide, putting it ahead of gasoline and well ahead of ethanol.

Burning Biodiesel

There are two types of biodiesel that are generally used when calculating combustion, the C19 and the C20 chain lengths. We will focus just on C19 (nineteen carbons in the chain) to keep things simple. The equation for ideal combustion of biodiesel looks like this.

formula of ideal biodiesel combustion

Ideal Combustion of Biodiesel

If we run the math on this equation, we find that biodiesel produces about 2.52 kilograms of carbon dioxide for every kilogram of fuel burned (2.59 if we use C20). This compares very favorably to petrodiesel, which produces 3.17 kilograms of carbon dioxide per kilogram of fuel. If you figure in the reductions in sulfur emissions, then biodiesel seems to be faring quite well (at least better than ethanol). Of course, we cannot forget to include the energy conversion, which is about 38 MJ/kg for biodiesel and about 43 MJ/kg for petrodiesel. Thus, we need about 1.13 times as much biodiesel to get the same amount of energy and will therefore produce around 2.86 kg of carbon dioxide.

The biggest problem with biodiesel is the production of nitrogen compounds, namely nitric oxide, which are poisonous in and of themselves and contribute to the production of acid rain. In a strange twist of irony, it has been shown that varying the temperature in a diesel engine can reduce the production of nitrogen compounds only to increase the production of soot. Thus, there is a tradeoff that seems to greatly limit the utility of biofuel in reducing overall emissions.

Bottom Line

Biofuels are a mixed blessing. On the one hand they are renewable and can reduce emissions of carbon dioxide when properly selected. On the other hand, they tend to increase emissions of other poisonous or harmful chemicals and this is to say nothing of the impact that growing these fuels has. Of course, as our energy demands grown and our ability to extract petroleum stabilizes or declines, we are likely to see more and more biofuel substituting for fossil fuel, which means engine manufacturers better be paying close attention to how biofuels burn!