Algal Fuels and Massive Scales

Guest post by John Goetz

I keep an active watch of the news for progress being made in the areas of renewable and alternative energy sources. One area that has caught my eye is algal fuel (biofuel produced by algae). One company that has been in the news lately is Sapphire Energy, which claims to be able to produce ASTM compliant 91-octane biogasoline. Sapphire Energy says their technology “requires only sunlight, CO₂ and non-potable water – and can be produced at massive scale on non-arable land”.

I am not trying to pick on any one solution or Sapphire Energy in particular. I simply wondered how massive a scale of CO2 and non-arable land is needed to make a noticeable dent in our gasoline demand.

First, how much CO2 do we need? The IPCC guidelines for calculating emissions require that an oxidation factor of 0.99 be applied to gasoline’s carbon content to account for a small portion of the fuel that is not oxidized into CO2. To calculate the CO2 emissions from a gallon of fuel, the carbon emissions are multiplied by the ratio of the molecular weight of CO2 to the molecular weight of carbon, or 44/12. Thus, the IPCC says the CO2 emissions from a gallon of gasoline = 2,421 grams x 0.99 x (44/12) = 8,788 grams = 8.8 kg/gallon = 19.4 pounds/gallon.

Now let’s assume Sapphire Energy simply reverses the process and consumes the CO2 to produce gasoline. In other words, we take 19.4 pounds of CO2 out of the atmosphere for every gallon of gasoline we produce. This seems like is a nice “carbon neutral” process.

What is the cubic volume of atmosphere required to make 1 gallon of gas? Let’s assume for the moment an efficiency factor of 100%, meaning our process will consume 100% of the atmospheric CO2 it is fed. This is unrealistic, but it is unrealistic on the “optimistic” side. According to the EPA, one cubic meter of CO2 gas weighs 0.2294 lbs. At an atmospheric concentration level of 385ppm, one cubic meter of atmosphere contains 0.000088319 lbs of CO2. Thus, 19.4 / .000088319 = 219658 cubic meters (yes, I am ignoring the atmospheric density gradient as one moves from the ground upward, but hang with me). This equates to roughly 4553 gallons of gasoline per cubic kilometer of air.

According to the US Energy Information Administration, US gasoline consumption is currently averaging (4-week rolling) 9.027 million barrels of gasoline per day, or about 379 million gallons (42 gallons per barrel). Thus, to completely replace US gasoline consumption, Sapphire Energy would need to “scrub”, at 100% efficiency, just over 83000 cubic kilometers of air per day. Certainly there is plenty of air available – this volume represents less than 0.02% of the volume of air in the first 1 km of atmosphere. Nevertheless, it is an enormous amount to process each day.

Of course, Sapphire Energy’s near-term goals are much more modest. As CEO Jason Pyle told Biomass Magazine, “the company is currently deploying a three-year pilot process with the goal of opening a 153 MMgy (10,000 barrel per day) production facility by 2011 at a site yet to be determined.” Using my fuzzy math above, that equates to a minimum of 92 cubic kilometers of air a day. Still seems like a lot.

So where will all of the CO2 come from?

Presumably the answer is coal-fired power plants. But let’s see if that makes sense. According to Science Daily, the top twelve CO2-emitting power plants in the US have total emissions of 236.8 million tons annually, or 1.3 billion pounds per day. Now, if that can be converted completely to gasoline, it would amount to 67 million gallons per day, or roughly 1/6 of the daily gasoline consumption.

(Science Daily refers to the twelve as the “dirty dozen,” which I found somewhat humorous given that CO2 is colorless and odorless, and is presumably needed to sustain some forms of life. But then again, so is dirt.)

Sounds great, except that a lot of land is needed to grow all that algae. According to Wikipedia, between 5,000 and 20,000 gallons of biodiesel can be produced per acre from algae per year. Assume for the moment that biogasoline can be produced at the same rate per acre. If we attempted to produce 67 million gallons of gasoline from our “dirty-dozen” every day, we would need between 1.2M and 4.9M acres of land to do this on. The low-end of the scale puts the area needed at more than that of Rhode Island. The high-end adds in Connecticut.

I kind of doubt there is that much land around each of the dirty dozen facilities. This means the gas would have to be sent by pipeline to a giant algae field. Given our ability to pipe oil and natural gas all over the place, sending CO2 across the country via pipeline is probably doable. There may also be plenty of unused or abandoned land (think abandoned oil fields) available to produce the gasoline. Nevertheless, the production scale and transportation logistics required to make this a viable alternative do indeed look massive.

So while the technology holds promise at the micro-scale, it remains to be seen what can actually be done at a scale that matters.

Talk among yourselves.