Biodiesel from Sugar Cane

I was intrigued by a story in yesterday's MIT Technology Today concerning a company that is applying biotechnology to convert Brazilian sugar cane to diesel, instead of ethanol. Amyris apparently intends to buy existing mills and convert them to produce hydrocarbons instead of alcohol. It has started up a demonstration-scale facility for this process near São Paolo. With so many other firms pursuing next-generation biofuels from cellulose or algae, tinkering with the most efficient current means of producing ethanol might seem an odd thing to do, but that efficiency is precisely the reason for choosing this pathway. Amyris sees an opportunity to produce a much better transportation fuel than ethanol at a cost low enough to compete with petroleum products, even if oil prices don't return to the levels we saw last year.

Energy efficiency and high energy returns on energy invested are essential to producing competitive biofuels in a way that avoids the trap the US corn ethanol industry fell into in 2008. Ethanol producers didn't benefit nearly as much from last year's high oil prices as they--and their investors--expected, because the rising cost of the large energy inputs required to make corn ethanol rose in tandem with the price of the fuels it was supposed to displace. This is an example of what some analysts call the Law of Receding Horizons. After factoring in the cost of natural gas-based nitrogen fertilizer, diesel-powered cultivation and harvesting, and gas-fueled distillation, the relatively small energy surplus created wasn't worth enough to make the operation profitable, even at the highest oil price in history.

Amyris's concept breaks out of this trap in several ways. First, by starting with sugar cane in the tropics, it avoids the large energy inputs associated with crop fertilizer. The article points out two other key benefits: Brazilian sugar/ethanol mills are net energy producers, not consumers, by virtue of capitalizing on the energy content of the waste left over from the grinding and fermentation process. In addition, while the ethanol produced by traditional fermentation is water soluble, requiring a lot of energy to separate the two, the molecules produced by the company's tailored microbes are not; the diesel precursors separate from water at little additional energy penalty.

The advantages of this approach continue after production, because of the properties of the fuel. Although it is possible to build engines that capitalize on ethanol's high octane and other properties to deliver fuel economy that nearly matches gasoline, the vast majority of the ethanol produced today will be burned either as a 10% blend in conventional cars or as a higher mix in flexible-fuel vehicles that must still be able to operate reliably on gasoline. That precludes the modifications that would compensate for ethanol's 33% lower energy content, compared to petroleum gasoline. Producing biodiesel instead of ethanol puts the fuel into engines that can take full advantage of its environmental properties, while yielding a roughly 30% fuel efficiency gain versus gasoline--and thus roughly twice the fuel economy of ethanol. Amyris claims that its biodiesel would be fully compatible with petroleum diesel, creating a significant advantage over biodiesel produced from soybeans, canola (rapeseed), and other vegetable oils. These so-called FAME biodiesels can normally only be used in blends of less than 5-10% in petro-diesel, to protect the sensitive fuel injection mechanisms of modern diesel engines.

There's no free lunch, of course. Part of diesel's advantage comes from its higher energy content, compared to either gasoline or ethanol, and the energy in the quantity of cane that would produce 100 gallons of ethanol could only yield around 60 gallons of diesel. However, when you burn these fuels in real cars--such as the VW Jetta that is available in both gasoline and diesel versions--the ethanol would take you around 1,650 miles, while the smaller quantity of diesel would be good for nearly 2,000 miles. That 20% improvement results from the higher efficiency of compression ignition engines over spark ignition.

This idea looks clever for another reason. Brazil has become a large exporter of ethanol, but the world's biggest ethanol market is protected by an import tariff designed mainly to recover the $0.45/gal. US ethanol blenders' credit. Meanwhile, the EU, which uses little ethanol, but where half of all new cars run on diesel, has just imposed an anti-dumping tariff on biodiesel imported from the US. That creates an opening for Brazilian biodiesel produced from this process to compete into a market that can't get enough diesel fuel. All that remains is for Amyris to demonstrate that the additional capital and operating costs associated with converting ethanol mills to produce diesel are small enough to preserve the big advantage they start with by choosing the world's most efficient biofuel source.