Energy Density of Alternative Fuels

Why Petroleum, Diesel, and Coal are Here to Stay

Not all fuels are equal. The energy density of alternative fuels is particularly low. The sum of energy in a gallon of diesel, for example, is about 10 percent higher than the energy in a gallon of gasoline.  There is a causal correlation between energy density and fuel efficiency. The greater the fuel density, the more energy produced per unit of measure.

It is because of energy density that diesel engines travel about 30 percent farther per gallon than comparable gasoline engines. Fossil fuels with the highest energy density include No. 6 fuel oil, diesel, anthracite coal, and kerosene. Fossil fuels with mid-level fuel density include gasoline, propane, and sub-bituminous coal.

Fossil fuels with the lowest energy density include natural gas and brown coal.

Types of Biofuels

Again, the fuel density of each fuel is different. That holds true with respect to biofuels as well. There are two types of biofuel and categories within each type. First generation biofuels are one type and second generation fuels are the other.

First generation biofuels include biodiesel, green diesel, ethanol, propanol, biogas, bioethers, and solid biofuels. Biofuels of the first generation type also include vegetable oil, olive oil, sunflower oil, and fat. Second generation biofuels include biohydrogen, algae, cellulosic ethanol, methanol, and dimethylfuran.

Measuring Energy Density of Biofuels

There are two measures qualifying the value of a biofuel. The first is a combination of the life cycle and combustion emissions. The second measure of biofuel value is energy density. Energy density is important for a variety of reasons. Most importantly, energy density is a means of determining the actual price of a biofuel.

Price per gallon — as the diesel versus gasoline example illustrates — is indicative of volume price. Price per gallon, however, does not indicate the price of a fuel with respect to energy return. The price of fuel is different than the price of energy.

Liquid fuel energy is measured in British Thermal Units (Btu) per gallon. The energy density of alternative fuels is measured against gasoline. Gasoline is the standard by which all fossil and biofuels are measured. The energy density of fuels is typically expressed as gasoline gallon equivalent (GGE) or megajoules per kilogram. It can also be expressed in Btu per unit of measure.

According to the Physics Factbook, gasoline has an energy density of between 112,114 – 116,090 Btu/gal.

Biofuel Types, Their Energy Density, and their Gasoline Gallon Equivalents

Biofuels — also called “alternative” fuels — are made from feedstocks. Feedstocks are the fuel materials used to produce biofuels. There are a variety of feedstocks. Feedstocks for petroleum alternatives include corn, grains and agricultural waste.

Ethanol

The most common petroleum alternative/additive is ethanol. Ethanol produces 76,330 Btu/gal. In other words, the energy density of ethanol is just slightly more than half of that of gasoline. The energy density of ethanol is only about 40 percent of the energy density of diesel.

Megajoules per kilogram is another unit of measure by which to understand the fuel density of alternative fuels. Ethanol produces 30 megajoules/kilogram. In fact, animal fat produces more energy per kilogram. Even manure produces 15 megajoules per kilogram.

Propanol

Another alcohol fuel, propanol is used primarily as a fuel additive. Though it has a higher energy density than ethanol, its expense makes it unsuitable as an alternative fuel.

Propanol has an energy density of 34 megajoules/kilogram. Propanol has considerably less energy per kilogram than even olive oil. That means tremendous sums of propanol must burn to produce the the energy equivalence of gasoline.

Butanol

Like propanol, butanol is far too expensive to produce on a mass scale. At least for the purposes of powering vehicles, equipment, boilers, or furnaces, it is of no value. So, it is used as a fuel additive. Butanol does, however, have the highest energy density of all biofuels. But, at 36.6 megajoules/kilogram, it is still far below that of diesel and petroleum. Sunflower oil produces 39 megajoules/kilogram.

Biodiesel

Fats and oils are the feedstocks used to produce biodiesel. Biodiesel has an energy density of 38 megajoules/kilogram. That is considerably less than traditional diesel’s 45.

 Biomass Types, Their Energy Density, and their Gasoline Gallon Equivalents

The energy density of alternative fuels that fall into the biomass is the lowest of all types. Biomass, on average, has a much lower energy density than biofuels. And again, biofuel energy densities do not compare to those of traditional fossil fuels. However, biomass is used as a legitimate alternative to coal and fuel oil to fire furnaces and boilers.

• Wood – Of all biomass products, those made of wood have the highest energy densities. Still, wood has a fuel density of only between 16 – 21 megajoules/kilogram.

• Seeds – At 15 megajoules/kilogram, seeds have the second highest energy density of all biomass.

• Manure – Again, manure has an energy density of up to 15, though it can vary in density. The difference in density rates depends on moisture content.

• Bagasse – Like manure, bagasse has an very low energy density, never reaching more than 10 megajoules/kilogram.

Emissions from Biofuels and Biomass

But, it is not just the energy density of alternative fuels that is a concern. In the last half-decade, concerns about the excessive emissions produced by biofuels have arisen. According to several reports from internationally renowned organizations, biofuels are dirtier than fossil fuels.

Biofuels Dirtier than Fossil Fuels? 

“A new analysis conducted by the Ecofys Consultancy for the European Commission shows that biodiesel from palm oil can produce three times the emissions of conventional diesel oil. And, biofuel from soybeans can produce twice as many greenhouse gases as diesel.” This according to Grist.org.

It’s not just palm oil and soybeans that are the culprit either. The United Nations Intergovernmental Panel on Climate Change claim indirect emissions from biofuels are far greater those from fossil fuels. That means the total emissions from alternative fuels are higher than those of fossil fuels. Not only do alternative fuels not accomplish their intended purpose, they actually make matters worse.

Cleaning the Fuels We Trust

It is possible the alternative fuels experiment was a failure. So, the question remains, how do we lower emissions of the fuels we do have confidence in? The answer is simple. We must increase the efficiency of fossil fuels. There are a variety of solutions. The Rentar Fuel Catalyst, for example, improves fuel efficiency by between 3% and 8%.

It is clear, however, that biofuels are not the solution.

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