Cellulosic biofuels

Currently there are two primary feedstocks for the production of renewable biofuels: sugar from sugar cane (primarily used in Brazil) and starch from corn (the source of most US-based ethanol).

Corn ethanol’s lack of scalability means that it will not be able to satisfy our fuel needs in the medium term. However, it is useful as a stepping stone by mitigating many of the early technological and capital risks associated with cellulosic ethanol and helping develop the infrastructure necessary for cellulosic ethanol.

Switchgrass, sorghums and miscanthus-like grasses as well as certain trees, such as poplar and willow are the most likely feedstocks to satisfy liquid fuel requirements in the long run. Other promising feedstocks are
• waste: municipal sewage and even municipal solid waste,
• excess forest product that is currently unused.

The most critical factor regarding cellulosic biofuels is land efficiency (tons of biomass per acre and hence gallons of fuels produced per acre – or more accurately, miles driven per acre). V. Khosla believes biomass yields per acre will improve 2-4 times from today’s norms by 2030.

This increase in yield will come from genetic optimization, as well as improvement of harvesting, storage and transport processes. Increasing yields while decreasing inputs will also come from a combination of:
1. Crop rotation, such as:
• 10 year energy and row crop rotation, which would improve the carbon content of the soil and decrease the need for inputs;
• Cover crops such as grasses, legumes or small grains between regular crop production periods
2. Polyculture plantation, since many processes can accept a mixture of biomass types
3. Perennials as energy crops, which require less nutrients because of their extensive roots and improve soil carbon since they do not need to be replanted each year
4. Better agronomic practices, such as no-till or minimum till farming

Regarding the food vs. fuel debate, it is worth noting that unless we dramatically reduce carbon emissions and stop global warming, millions of acres of land will be dislocated from their current uses and must be figured into the “net land use” equation.

Equally important, ethanol is compatible and complementary to other petroleum use reduction technologies like hybrids and plug-in electric hybrid cars. The high cost of hybrids and plug-in hybrids will limit their penetration in the coming two decades. In contrast, flex-fuel vehicles (FFV’s) capable of running on either gasoline or ethanol for a marginal cost of only $35 per car could see their penetration greatly increase. Moreover, as biofuel penetration grows, engines should be optimized for biofuels. Engines designed for ethanol first will operate at much higher compression ratios and thus get far more mileage per gallon of ethanol.

Source: V. Khosla