Biofuels Revisited II

Ah. Here's the relevant quote from Den Beste. http://denbeste.nu/cd_log_entries/2002/09/Morepracticalproblems.shtml

Biomass: I'm a bit embarrassed that I forgot this one to begin with. Biomass is, at its core, an extremely roundabout form of solar power. The idea is to use farmland to grow greenery, and then to burn the greenery to generate energy, but depending on who is making the proposal the details can vary wildly. Ethanol as a fuel is one example of this, but it is exceedingly inefficient because it is based on corn and only utilizes the grain, and wastes most of the energy in that and uses none of the energy in the rest of the plant.
A more efficient form of biomass is methanol, which can be created from the entire plant. The most efficient form is to burn the entire plant in a big power facility, for instance as a substitute for coal in electrical generation. There are some engineering issues involved, such as the fact that the biomass has to be dried or somehow have its water content reduced substantially, but that's a detail of the process.
It's an attractive idea, but I'm not sure the numbers make sense. I'm not sure I believe it's possible to actually supply a significant portion of our current energy use this way. You're only talking about actually harvesting greenery from the fields once or at most twice per year, and you're only going to get a few tons of dried fuel per acre each time you harvest. The US uses about 60 million short tons (about 55 million metric tonnes) of coal per month, or about 650 million metric tonnes per year.
According to this page at Oak Ridge National Laboratory, anthracite and bituminous coal (which make up most of the coal we use) contain 27-30 gigajoules per metric tonne. Agricultural residues are 10-17 gigajoules per metric tonne (as a function of water content). As an approximation, that means about 2 tonnes of biomass would be needed to replace each tonne of anthracite. Are we actually capable of producing, collecting and transporting 1.3 billion tonnes of dried biomass per year? The US currently has about 200,000 square kilometers of irrigated land; can we generate seven thousand tons of biomass per square kilometer? Or even a tenth of that? Not easily, if it's possible at all.
One of the reasons we can produce that much coal is that it's concentrated; a given coal mine can produce millions of tonnes of coal with a relatively small amount of machinery. But biomass would be extremely spread out, and you'd need an impressive infrastructure investment for all the trucks to collect it and bring it to rail yards for transport to the power plants. And a non-obvious part of the problem is that right now our agricultural practices are using that same biomass, partly to stop soil erosion and partly to reduce fertilizer usage. (Also, a lot of it is used as animal feed.)