This morning's email included a link to a Business Week article about a slate of new, non-traditional ethanol projects, including several based on the gasification of biomass. One of the great things about gasification is its ability to handle a wide variety of feedstocks, extending beyond oil, coke and coal to include, in this case, yard waste, wood chips and citrus peels. Although the article isn't very clear in describing the path from feed to gasification to end product, it suggests that some of these projects will gasify biomass to make hydrogen, ethanol and methanol. This has me scratching my head, because once you have the synthesis gas, or "syngas", that a gasifier produces, you can make essentially any hydrocarbon you want, particularly the long, straight-chain hydrocarbons of diesel fuel. Why use this expensive process to make challenging and problematic fuels, instead of one that is 100% compatible with current energy systems?
One of the biggest problems plaguing alternative fuels is infrastructure and end-use compatibility. Ethanol cannot be shipped through petroleum products pipelines, so it gets to market via costlier truck, rail and barge routes. Methanol faces similar constraints, and is a neurotoxin, as well. The logistical challenges facing hydrogen are even more daunting. Then, once these fuels reach a retail facility, the only current option for their use is in specially modified gasoline engines--or in the case of ethanol in low-volume blends with gasoline, in which the lower energy content of ethanol reduces fuel economy and vehicle range. This hardly sounds like the way to maximize the energy benefit of biomass.
Synthetic diesel fuel is another story. Compression ignition engines are typically 30% more efficient than the spark-ignition ones used with gasoline, and the properties of ultra-clean diesel from the Fischer-Tropsh synthesis process allow them to run optimally, with lower pollution than from petroleum diesel. If European-style diesel cars, with state-of-the-art particulate cleanup technology, take off in the US, demand for this kind of fuel will grow rapidly. It would also blend nicely with biodiesel, which still can't be used year-round in many northern markets, because of its poor low-temperature properties.
In terms of greenhouse gas emissions, since every carbon atom in the biomass gasification feed will ultimately result in a molecule of CO2 emitted to the atmosphere--without sequestration at the gasifier--the differences in overall emissions for the various fuel options described above depend on the efficiency of transportation and end use. Diesel handily beats hydrogen and alcohols on both counts, unless the hydrogen is feeding a fuel cell. All of this suggests that we may need to rethink our definition of biofuels to encompass any fuel from biological sources, not just those that chemically resemble current-generation biofuels, such as ethanol from corn or cane.
Biomass-to-diesel looks like a promising way to tap the environmental and energy security benefits of biofuel, even though its product is hard to distinguish from diesel made from non-renewable feedstocks. However, before climbing on the "BTL" bandwagon, we should withhold judgment until some of these plants have been built and run for a while. The solids-handling end of the gasification business can be tricky, particularly when dealing with material of inconsistent quality and characteristics. That could turn out to be a much bigger challenge for biomass gasification than the molecular engineering process that sits on the back end of these facilities, but which has been proven in over 80 years of application to oil and coal.