Words Matter

I don’t watch many ads on TV, but I happened to see this one near the end of a football game last week. It was one of those public relations ads from Exxon Mobile telling what a wonderful job they were doing developing biofuel from algae. Someday this fuel will be used to power planes, ships and trucks “and cut their emissions in half.”

Wait a minute; something is not right here! Biofuels are carbon-based. When they burn, they emit mostly carbon dioxide. The only way to cut emissions in half is for the same amount of fuel to produce twice the energy as traditional diesel or jet fuel. That involves a factor called energy density.

Now biofuel is generally good. It comes from plants that extract CO₂ from the atmosphere. When biofuel burns it releases the same CO₂ back into the atmosphere, so it is considered to be carbon neutral. 

The downside is that first generation biofuels, such as ethanol, are produced from edible crops. They require a lot of space to grow and a lot of energy to plant, harvest and transport.

Biofuels processed from algae are a second-generation biofuel, similar to fuel made from the inedible and discarded part of a food crop, such as cornhusks or switch grass. The advantage is that food and fuel do not compete with each other, as when ethanol is made from corn.  And the production consumes “small amounts of energy while producing considerable fuel output.”

Other advantages are that algae production is efficient, in that, it can be grown “in almost any climate thanks to the open- or closed-tank approaches that are available today. As long as we can provide this natural product with enough sunlight to create photosynthesis, then it has the capability to grow quickly.” It is a renewable energy source that can be refined into a variety of products, and it can yield almost five times more fuel per acre than sugar cane or corn.

Two major disadvantages according to the same source are that it currently requires about 20% more energy to grow than it yields and production cost is still higher than the alternatives.

Exxon Mobile confronts the skeptics by citing partnerships with several major universities and a private firm to use gene technology and other research to build on the advantages and overcome many of the disadvantages. It looks promising, but they still “face some significant technical hurdles before biofuels production from algae will be possible at a significant commercial scale.”

The question remains about the claim of half the emissions. All fuel from wood and manure to jet fuel has a characteristic called energy density. “Energy density is the amount of energy stored in a given system or region of space per unit volume or mass.”  Gasoline has an energy density of about 45 megajoules per kilogram (MJ/kg). “Ethanol produces 30 megajoules/kilogram.” For that reason, a car will go farther on a gallon of pure gasoline than it will on a gallon of ethanol or any mix of ethanol and gasoline. Adding ethanol at the pump reduces MPG.

This 2011 paper from Stanford gives the energy density of algae-based biofuel at 20-25 MJ/kg, about half that of gasoline. Even if the research at Exxon Mobile’s partners could double it to equal that of gasoline or diesel, it would still be far short of their claim of cutting the emission of planes, ships and trucks in half. 

Deeper reading at the Exxon Mobile site gives the answer: “If key research hurdles are overcome, algal biofuels will have about 50 percent lower life cycle greenhouse gas emissions than petroleum-derived fuel.” The saving is not in emissions but in the energy expended during the entire production and delivery process.

The claim in the ad is inconsistent with that explanation thus is really inaccurate. Perhaps the advertising firm misunderstood, or the company just dumbed it down for football fans. In either case, I love it when I’m right!