The Thin Green Line passes on an editorial from today's SF Chronicle:
California should continue to lead the way in the fight against climate change by requiring cleaner-burning fuels in this state.
The state Air Resources Board is scheduled to vote today on whether to force refiners and distributors to reduce the "carbon intensity" of the transportation fuels they sell, starting in 2011. The so-called Low Carbon Fuel Standard represents a critical step toward this state's commitment to reduce overall emissions of heat-trapping gases by a third by 2020.
Passage of a California cleaner-fuels standard would intensify the pressure on Congress to make a national commitment to promote lower-carbon options to gasoline and diesel.
Holy moly, I never thought of this? It's brilliant! Let's just legislate that hydrocarbons should have less carbon! And tell the refiners to figure it out.
In all seriousness, assuming this is not just insane (which may be a poor assumption in CA) I presume they have something in mind here. Does anyone know what opportunity they see, because I sure don't. Here is why I am confused:
Basically transportation fuels are made up various hydrocarbon chains. The shortest is methane, CH4, then C2H6, then C3H8, etc. As the chains get longer, the molecule gets heavier (for example, CH4 is a gas at room temperatures; C3H8 is propane, which is a gas but a liquid under pressure in our BBQ tanks; C8H18 is octane and liquid at normal car operating temperatures.)
Motor fuel is a careful blend of many different molecules, and is actually frighteningly complex (the above just discusses straight chain forms, there are also rings and other shaped hydrocarbon molecules). There are literally hundreds of specs it has to meet, and several present difficult tradeoffs that must be carefully balanced. Trying to make one spec can easily put one out of another spec. So this is an optimization equation with a lot of constraints.
All things being equal, decreasing the carbon intensity of fuel basically means making it lighter, with shorter molecules. Why? Well, look at the molecular equations. Basically a straight chain hydrocarbon is C(x)H(2x+2). Shorter molecules get a higher ratio of their BTU's from combustion of hydrogen vs. larger molecules get a higher ratio of their BTU's from carbon.
So, it is correct that burning propane in a car vs. currently formulated gasoline will be less carbon intensive, with only the teeny tiny problem that most cars today cannot burn propane. Modern engines are carefully built to run most efficiently (valve design, cylinder pressure and size, air mixtures, fuel injection) on a certain range of gasoline, and that range is moderately narrow. And, besides the pure physics of engine design, lightening up motor fuels will create a variety of secondary problems -- for example, lighter fuels tend to have higher vapor pressures and volatility that can cause vapor lock in engines on warm days. Another way to reduce carbon intensity is to go from ring molecules (e.g. benzine) to straight chains of the same size, but this creates other problems, for example in maintaining octane numbers.
And speaking of unintended consequences, my understanding is that environmentalists like diesel engines, because the best diesel technologies today are far more efficient than gasoline engines. But diesel is a heavier, more "carbon intensive" fuel than gasoline. So is the carbon dioxide emissions from a heavier fuel in an engine that is more efficient less or more than a typical gasoline engine? Who knows, and the answer is probably "it depends" anyway.
Update: I think I have figured it out. The California legislature is going to mandate changing the size of the 2p valance shell, allowing more hydrogen molecules per given carbon molecule.