However, converting the woody biomass to hydrocarbons instead of alcohols could have key advantages. "Hydrocarbons derived from biomass are attractive because of their high energy density and compatability with existing energy infrastructure," said John Regalbuto, director of catalysis and biocatalysis at the US National Science Foundation, writing in Science. "If recent technological innovations result in competitive production costs, hydrocarbons rather than ethanol will likely be the dominant biofuel."

Several companies say that they will be ramping up production of hydrocarbons from biomass to commercial levels within the next couple of years. According to Regalbuto, there are a number of suitable processes, including microbial fermentation or liquid-phase catalysis of sugars from woody biomass; pyrolysis or gasification of woody biomass directly; and conversion of lipids from non-food crops such as algae.

"I think it's safe to say that all these processes, per unit of energy produced, will use less energy than the corresponding process to make ethanol," Regalbuto told environmentalresearchweb. "That's because hydrocarbons spontaneously separate from water, like oil and water salad dressing. One of the costliest steps in making ethanol is that you have to separate the 6 or 8% ethanol you get from fermentation from the water solution it's in. To recover the 6 or 8% of the stuff you want, you have to heat the whole stream up, and that's energy consuming and expensive. A simple gravity separation of the hydrocarbons and water means less energy consumed and a lower carbon footprint."

For comparison, producing ethanol from sugarcane makes about 8 or 9 units of energy, for every unit put in. "With corn ethanol, you only get out about 1.4 units of energy for every unit you put in," said Regalbuto. "With cellulosic gasoline, the number will be more like sugarcane ethanol, and probably a bit better, for the same feedstock."

With regards to pyrolysis and gasification processes, Regalbuto explains that it's a common misconception that these require a lot of energy since they occur at high temperature. "These reactions are actually autothermal; that is, the reactions are exothermic and give off enough heat that the reactor is heated up to a high temperature by the biomass itself," he said. "Both of these process can be used to generate power and heat instead of hydrocarbons, if that's what you're after. In the present context, we're aiming to replace foreign oil imports by making liquid transportation fuels."

What's more, gasification, pyrolysis and aqueous phase reforming are all water positive reactions – they produce more water than they consume. "In fact, this will cause a water problem of its own," said Regalbuto, "the extra water that is produced will have to be cleaned of any potentially harmful hydrocarbons that are left in small amounts in the water."

While it's too early to compare the different processes on cost, one company estimates that it can produce green gasoline, per amount of energy produced, for about 20% less than making ethanol from sugarcane. "This and other companies are optimistic enough about economics that they are proceeding with process development without depending on any subsidy," said Regalbuto.

"I will not be surprised if we [the US] are energy independent in two decades," he added. "Between lignocellulosic hydrocarbons and algae, which can be easily converted into hydrocarbons but which we don't yet know how to economically grow and harvest in large quantities, we will have the biofuels manufacturing capacity to replace all foreign oil imports. When light vehicles go electric or plug-in hybrid, we'll still need diesel and jet fuel for our planes, trains, boats, and trucks, and there will be plenty of biomass for this."

It's possible to produce hydrocarbons directly, rather than ethanol and other alcohols, from sugars derived from biomass by using genetically altered microorganisms instead of yeast. According to Regalbuto, companies Amyris and LS9 are near the point of commercializing such routes. Indeed Amyris plans to produce a replacement fuel for diesel, as well as speciality chemicals, from sugar cane, with initial commercialization planned for 2011.

Employing catalysis in place of fermentation can also convert biomass sugars into hydrocarbons. James Dumesic at the University of Wisconsin-Madison has developed a number of solid-phase catalysts for converting dissolved sugars into hydrocarbons, says Regalbuto. Virent Energy Systems in partnership with Royal Dutch Shell claims to be five to seven years away from commercial production of hydrocarbon fuels to the tune of 100 million gallons per year.

Pyrolysing woody biomass   heating it in the absence of oxygen   produces an intermediate product called biooil, which must be stabilized in a catalytic step. With this in mind, George Huber of the University of Massachusetts, Amherst and colleagues have developed a catalytic process to convert biomass into gasoline-range aromatics in a single step. Envergent, a joint venture between petroleum refiner UOP and biooil producer Ensyn, plans to commercialize its pyrolysis efforts for producing transportation fuels from biomass by 2011, making about 100 million gallons per year. KiOR, meanwhile, is working on biomass catalytic cracking for producing diesel and gasoline components from forestry or agricultural waste by 2011.

Gasification converts whole biomass into a mixture of carbon monoxide and hydrogen gases, which can then be used to synthesize hydrocarbons. Lanny Schmidt and colleagues at the University of Minnesota have created a process along these lines that works in a single small reactor using a catalyst. Choren Industries of Germany, meanwhile, is commercializing a biomass-to-liquids process based on gasification and the Fischer-Tropfsch synthesis process for syngas, which will produce diesel from ligno-cellulosic feedstocks from within a distance of about 50 km.

Algae and unsaturated plant oils can be converted into fuels by hydrotreating. Sapphire Energy has developed algae farms that can operate on non-arable land using nonpotable water. The company plans to produce 100 million gallons of gasoline per year by 2016. And ExxonMobil has partnered with Synthetic Genomics to produce hydrocarbon biofuels from algae in five to ten years.