"Impossible!" you say. "Even wind and solar have carbon emissions from their manufacturing, and biofuels are carbon neutral at best. How can a fuel be carbon negative?" But listen to people working on gasification and terra preta, and you'll have something new to think about.
We've mentioned terra preta before: it's a human-made soil or fertilizer. "Three times richer in nitrogen and phosphorous, and twenty times the carbon of normal soils, terra preta is the legacy of ancient Amazonians who predate Western civilization." Although we don't know how it was made back then, we do know how to make it now: burn biomass (preferably agricultural waste) in a special way that pyrolisizes it, breaking down long hydrocarbon chains like cellulose into shorter, simpler molecules. These simpler molecules are more easily broken down by microbes and plants as food, and bond more easily with key nutrients like nitrogen and phosphorus. This is what makes terra preta such good fertilizer. Because terra preta locks so much carbon in the soil, it's also a form of carbon sequestration that doesn't involve bizarre heroics like pumping CO2 down old mine shafts. What's more, it may reduce other greenhouse gases as well as water pollution: according to Biopact, a network that promotes biofuels and biomass energy,
Char-amended soils have shown 50 - 80 percent reductions in nitrous oxide emissions and reduced runoff of phosphorus into surface waters and leaching of nitrogen into groundwater. As a soil amendment, biochar significantly increases the efficiency of and reduces the need for traditional chemical fertilizers, while greatly enhancing crop yields. Experiments have shown yields for some crops can be doubled and even tripled.
As it happens, the process of burning/pyrolisizing agricultural char is also a way to produce energy. MIT Professor Amy Smith, a recipient of the prestigious MacArthur "genius award," gave a TED Conference talk in 2006 on using agricultural char as fuel in developing countries. It works because the chemical reactions that break down the long hydrocarbon chains also give off hydrogen gas, methane, and various other burnable fuel gases. (As well as tars and non-useful gases like CO2.) This is gasification. The fuel gas can be burned for heat, or if it's pretty clean (that is, if the tar levels are low), it can be used to power an engine.