Wood-fueled pickup trucks
A few weeks ago I gave a talk at the West Virginia Small Farms conference on “Farming with Less Fossil Fuel” (See slides, 4 Mb pdf). The speaker after me was Wayne Keith, from Springville Alabama, who talked about the wood-powered pickup truck that he drove to the conference. Was I ever up-staged!
Mr. Keith runs several of his trucks on wood and other biomass. Each truck has a gasifier in the back, which converts carbon-rich material — wood, switchgrass, junk mail, dead cats etc. — to a volatile mixture of carbon monoxide and hydrogen, called syngas. Syngas can replace gasoline as fuel for internal combustion engines. Mr. Keith’s old Ford F-250 gets about a mile per pound of sawmill waste.
The idea isn’t new. People started using gasifiers in the 1800s, and quickly realized that syngas could be used to run automobiles. When Europeans faced gasoline shortages during World War II some added gasifiers to their vehicles. Both the UN-FAO and FEMA offer online publications on replacing gasoline with syngas in internal combustion engines.
One of the nice things about the technology is that existing vehicles can be modified to run on syngas. Part of the reason given for the push for bioethanol production in the U.S. is that it is compatible with existing internal combustion engines. In contrast to the transition to electric or hydrogen-powered vehicles, existing machines could be modified, rather than replaced.
Let’s say that we want to run a vehicle on wood. There are at least three ways to do it:
- Use an on-board gasifier to make syngas for an internal combustion engine.
- Generate electricity for electric engines in a wood-fired power plant.
- Convert the wood to ethanol to burn in an internal combustion engine.
Which is the most efficient?
I’ll trust that Mr. Keith’s ‘mile-per-pound’ estimate is accurate for the first option. As a rule-of-thumb, wood-fired power plants consume 1.5 tons of wood per megawatt-hour (wet weight) and electric vehicles get about 3 miles per kilowatt-hour. That works out to exactly a mile per pound, wet weight (do the math), making Mr. Kieth’s pickup truck roughly as efficient as an electric vehicle. The pickup truck requires no batteries, and uses its original internal combustion engine. The biggest disadvantage of the gasifier is its bulk.
Theoretically, an ethanol plant can make about 50 gallons of ethanol per ton of wood (wet weight). A pickup truck that gets 20 miles per gallon of ethanol would get about 0.5 miles per pound of wood (do the math) — half the efficiency of the other options. The efficiency of wood ethanol is further compromised by the fact the processing demands acids or expensive enzymes to hydrolyze cellulose into sugar, and considerable energy for fermentation and distillation of sugars into ethanol. People did it during World War II, and they’ll probably try it again, thanks to US ethanol mandates, but I don’t think it makes much sense.
Hmmm… Electric tractors might be a neat idea, but maybe we should be trying to run more farm machinery on syngas.