Bioalcohols

The use of alcohol as a fuel is not a new concept. Henry Ford originally planned for his cars to run on ethanol, but the value of alcohol for drinking made it more expensive for use as a fuel than the newly discovered petroleum. The first four alcohols, (in order of carbon content) methanol, ethanol, propanol, and butanol, are of greatest interest for fuel use as their chemical properties make them useful in internal combustion engines.

Fuel Economy and Octane

One of the problems with alcohols is that they have lower energy densities than gasoline. The chart below illustrates the differences.

Fuel

Energy Density(megajoules/liter)

Average Octane (AKI rating/RON)

Gasoline

~33

85-96/90-105

Methanol

~16

98.65/108.7

Ethanol

~20

99.5/108.6

Propanol

~24

108/118

Butanol

~30

97/103

AKI - Anti-Knock Index: This octane rating is used in countries like Canada and the United States.

RON - Research Octane Number: This octane rating is used in Australia and most of Europe

The chart above also reveals average octane numbers for each fuel. Besides reducing knock, higher octane values are indicative of a fuel that burns slowly. In general, the slower a fuel burns, the more efficient it is to extract energy from it. Thus, a higher octane also reveals a more energy efficient fuel. The fact that alcohols have higher octane values than gasoline helps to offset some of the difference in energy density. The net result is that the loss of fuel economy (how far a car can travel on a volume of fuel) is not as drastic as it would be if the octane numbers were the same.

Biological Production versus Refining

It is true that any of the alcohols above can be generated from fossil fuels. However, it is easier and more efficient to derive these products from biomass or even carbon dioxide and water than to refine petroleum. It is also the case that petroleum-derived alcohols tend to be less pure (ethanol is contaminated with methanol for instance) and often cannot be purified through simple distillation.

Methanol

Methanol is closely related to methane. In fact, there is only one atom different between these two chemicals (Oxygen shown in blue, carbon in black, hydrogen in red).

image of a methane molecule image of a methanol molecule

Methane            Methanol



Though these molecules are remarkably similar, their properties could not be more different. To begin, methane is a gas at standard temperature and pressure. Methanol, on the other hand, is a liquid. Methane has an energy density of 55 MJ/kg while methanol has an energy density of only 23 MJ/kg at best. Burning methane produces twice the amount of carbon dioxide per kilogram as burning methanol does (2.74 kg CO2/kg methane versus 1.37 for methanol).

How can two molecules that are so similar act so different? The answer is in the oxygen atom. This highly electronegative atom (that means it like electrons) changes the structure of the bonds so much that the overall reaction energy for the molecule is changed. Of course, losing energy is not the only consideration. Methane makes a poor fuel because it is a gas, which makes it difficult to transport. On the other hand, methane isn't toxic, while methanol will cause blindness or even death in small quantities. Perhaps there is a better alcohol that has the positive properties of methane and methanol, with fewer of the negative properties.

Ethanol

Ethanol is standard drinking alcohol, so we know it isn't poisonous. Already we're off to a better start than methanol. It also has a higher energy density than methanol and is still a liquid, which makes it an attractive alternative. Ethanol is only one atom different than ethane. Like methane and methanol, the differences between these molecules are drastic.

image of an ethane molecule

Ethane

image of an ethanol molecule

Ethanol

Ethanol is a common additive to fuels in many countries. Globally, the average ethanol content in petrol is roughly 5.4%, though some countries supply 25% or even 100% ethanol for vehicle fuel. The world's largest producers of ethanol are the United States and Brazil. In the U.S., petrol contains an average of 10% ethanol. In Brazil, petrol has a minimum of 25% ethanol by law and as many as 17.3 million vehicles use 100% ethanol (called neat ethanol) as a fuel.

Ethanol has some benefits as a fuel or as a fuel additive. First, because it has a higher octane number than ethane and even many of the larger hydrocarbons, ethanol can be used to boost the octane of a fuel. Beyond that, it burns cleaner than most hydrocarbon fuels and, if created from biomass rather than petroleum, contains little or no contamination to damage vehicle parts or lead to smog.

The drawbacks to ethanol as a fuel, however, have prevented its widespread adoption. First of all, it takes about 1.5 times more ethanol than gasoline to get the same energy. That means you need a fuel tank 1.5 times larger if you want to travel the same distance on ethanol that you do on gasoline. Another problem with ethanol is that it is corrosive to the rubbers used in the gaskets and fuel delivery lines of older vehicles. A reformulation of rubber was necessary to ensure that these components do not fail in modern cars, particularly those running on 100% ethanol. Finally, ethanol absorbs water from the environment, which dilutes its concentration and makes it impossible to ship it through pipelines.

So, ethanol isn't the optimal standalone fuel, but it does seem to make for a great fuel additive. In addition, some inventive individuals have found ways to combine ethanol and gasoline combustion to improve fuel economy by up to 30%. At the Massachusetts Institute of Technology, the use of high compression engines that mix gasoline and ethanol (each stored separately onboard) to combat knock during high engine loads has led to a car that is 30% more fuel efficient than a standard gasoline engine and which avoids the high costs associated with diesel and hybrid technology. Though ethanol may never be a standalone fuel, its potential as a fuel supplement appears high so long as we remain a petroleum-based society.

Propanol

Propanol is the forgotten alcohol fuel, but for good reason. Propanol is the most difficult and expensive alcohol to produce. Because its energy gains over ethanol are minimal, the large scale production and use of this fuel is hard to justify.

Propanol is not without its uses, however. The major use, at least in the automotive segment, comes from the drying properties of 2-propanol, which is better known as isopropyl alcohol or rubbing alcohol. It is known as a “gas dryer,” but 2-propanol actually keeps water in solution with gasoline, thereby preventing it from freezing in gas lines. You can also buy isopropyl alcohol in spray cans to de-ice your windshield.

Butanol

Butanol is more similar to gasoline than ethanol or methanol. This similarity is a consequence of its longer hydrocarbon chain, which means there is more carbon in relation to the single oxygen and thus the molecule is less polar. The diagram below shows only one version of butanol. Because there are four carbons, there are four possible structures to butanol.

image of a butane molecule

Butane

image

 

n-Butanol

 

The similarity of butanol to gasoline means that it can be used in a standard vehicle without the need for modifications. Also, because of its size, butanol has an energy density similar to that of gasoline. In fact, butanol is so close to gasoline in energy content that its octane rating nearly makes up for the difference in energy density. In other words, a liter of butanol will get your car about as far as a liter of gasoline, with the difference only being about 10%. It is also true that butanol only produces about 2.03 kg of carbon dioxide per kilogram of butanol while gasoline produces 3.3 kg of carbon dioxide per kilogram of gasoline.

With all of these advantages the natural question is “why has butanol not replaced gasoline?” The answer is three-fold. First, butanol actually produces more carbon dioxide than what arises just from burning it. The reason for this discrepancy is that producing the biomass, harvesting it, and processing it all requires energy which releases CO2. This input of energy needed just to produce butanol as raises questions about how efficient biofuels are, a topic addressed in detail in another article. Right now there is a great deal of research into using algae to produce butanol. Tulane University is leading the charge in this are as well as in the use of bacteria to produce butanol from cellulose.

The second reason that butanol has not replaced gasoline is that its health effects are not well understood. It is thought to behave similar to ethanol in the human body, but further research into its effects, especially when burned as a fuel, are required before it is considered safe.

Finally, butanol has not replaced gasoline because it is very difficult to produce. Until recently, butanol was not considered a viable biofuels because it tended to kill the organisms that produce it before they were able to create it in any great quantity. In order to prevent this, butanol has to be removed from solution as it is made. Until 2012, the removal of butanol away from the organisms producing it was an energy intensive and expensive process. Research out of the University of Illinois, however, may make the process simpler and allow for the efficient production of larger quantities of butanol. It remains to be seen how effective this process is on larger scales, but it may pave the way for butnaol to replace gasoline in the near future.

Understanding Carbon Dioxide and Carbon Fuels

In the sections above, mention is given to the amount of carbon dioxide that each alcohol produces and this is compared to the carbon dioxide production of an equivalent amount of the fossil fuel that most closely resembles, structurally, the alcohol. Comparing carbon dioxide in this way, however, can be misleading.

A better way to compare carbon dioxide production is in terms of energy produced per kilogram of carbon dioxide expelled. This is a better example because even though a quantity of ethanol produces less carbon dioxide than the same quantity of gasoline, it also produces less energy. The result is that it is necessary to burn more of the alcohol to get the same amount of energy as the fossil fuel. The chart below contains a few examples that help to clarify the point.

Fuel

Energy Density (MJ/kg)

Carbon Dioxide production (kg/kg)

Carbon Dioxide production (MJ/kg)

Carbon Dioxide* (kg/Equivalent)

Methanol

~21

1.37

~15

~3.6

Methane

~55

2.74

~20

N/A

Ethanol

~24.5

1.91

~13

~4.05

Ethane

52

2.93

~18

N/A

Butanol

36

2.37

~15

~3.02

Gasoline

~46

3.30

~14

N/A

* This measure compares an alcohol to its closest hydrocarbon equivalent. The value of carbon produced is arrived at by determining how many kilograms of alcohol are needed to derive the same amount of energy as the hydrocarbon. This number is multiplied by the CO2 production in kg/kg to determine how many kilograms carbon dioxide are produced for when energy production is kept the same.

What the chart above tells us is that alcohols, no matter how they are produced, are not likely to be viable alternatives to fossil fuels until they start to get larger. In other words, methanol and ethanol are not great fuel sources because they produce more carbon dioxide than their equivalent hydrocarbons for the same amount of energy. Butanol, however, begins to show a difference. Butanol produces LESS carbon dioxide than gasoline for the same amount of energy. If humans can overcome the challenges of producing butanol in large quantity and avoid impact on the food chain, then it may become a viable alternative to hydrocarbon fuels.

Long Chain Alcohols

The information in the chart above naturally brings about the question of why scientists don't just produce longer chain alcohols (5 or more carbons) and use those as fuel. Part of the answer to that question is related to the food supply as discussed in other articles. The other part relates to chemistry. Synthesizing long-chain alcohols is not easy. In fact, five carbon alcohols (called pentanols or amyl alcohols) are the longest scientists have been able to produce. Recently, however, a method of genetic engineering has revealed that bacteria may be the key to producing long-chain alcohols, which hold even more benefit and have greater energy densities than butanol.

Conclusion

In the short term, alcohol fuels will not replace hydrocarbon fuels. They will, however, be important additives to fuels for the foreseeable future. If science continues to progress and the problem of compromising the food chain can be solved, then alcohol fuels may provide an excellent alternative to fossil fuels that allow us to better balance how much carbon dioxide we put into the atmosphere with how much plants remove.