Sunday, August 26, 2012

Generalizing what biofuels do

Many people have thought of biofuel as the ideal way to replace petroleum fuels. Biofuels can be liquids, and in fact, "liquid fuel made from biomass" is the most common meaning of the term. Because the focus here is on liquid carbon, liquid forms are the biofuels of interest. Examples include ethanol and biodiesel.

Basic chemistry tells us that the amount of CO2 released when burning liquid fuels is essentially the same for a given amount of energy provided. However, the common view is that biofuels are inherently carbon neutral. That's because the CO2 released when the biofuel is burned is offset by the CO2 that was taken up from the atmosphere when the fuel's biomass feedstock was grown. Combine this fact with the fact that biofuels can be produced from home-grown biomass, and it's easy to see why biofuels have such enormous appeal as replacements for oil.

The LCA script

Lifecycle analysis (LCA) is a very popular form of environmental assessment. A good number of academics and others assert that it is the ideal way to examine greenhouse gas emissions impacts and so advocate LCA as a way to compute "carbon footprints."

This view has come to dominate energy policy discussions of ways to reduce GHG emissions from transportation fuels. In particular, nearly everyone involved in the issue says that biofuels should be evaluated according to their full lifecycle impacts. As the major UNEP report on Assessing Biofuels puts it, "Environmental and social impacts need to be assessed throughout the entire life-cycle."

However, biofuel LCA has become ever more complex and has resulted in models that are no longer even close to being scientifically verifiable. Therefore, no amount of new data that one puts into a lifecycle study can put an end to the disputes over LCA-based estimates of the net GHG impact of biofuels. This argument is made in the paper "Biofuels and Carbon Management" Climatic Change (2012), which then proposes a different approach to analysis.

Saturday, August 25, 2012

Liquid fuels and carbon dioxide

A most troubling aspect of the global climate problem is the carbon dioxide (CO2) emitted by petroleum consumption. Oil is still the world's largest source of energy, though it now runs second to coal as a direct source of anthropogenic CO2 emissions. Oil is a liquid and is by far the main feedstock for liquid fuels, i.e., hydrocarbon energy carriers such as gasoline, diesel, jet fuel, bunker oil and so on.

Carbon dioxide
Combustion of any liquid hydrocarbon fuel directly releases an amount of CO2 that varies only a little depending on the fuel. This is true for most carbon based liquids, not just strictly hydrocarbons. For example, burning ethanol directly releases only about 2% less CO2 than gasoline for a given amount of energy provided.

Therefore, in terms of direct emissions, changing the form of the liquid fuel, e.g., from gasoline to ethanol or from diesel to biodiesel, makes no appreciable difference in the amount of CO2 released to the atmosphere. 

Therefore, as long as society burns liquid fuels -- and we now consume enough of them to make oil the world's number one source of commercial energy -- we have what can be called a "liquid carbon" problem. I'll leave to others the tasks of thinking about how to replace liquid fuels, for example, with electric cars or hydrogen-powered vehicles, using energy carriers that don't contain carbon atoms. Meanwhile, society has a huge liquid carbon problem to solve.