Some feedstocks, like wood chips, are pretty uniform. Each little piece of wood is of the same material, and machines can make all the pieces a similar size. Wood chips, grass, and similar uniform feedstocks are nearly ideal for turning waste into fuel through the gasification process.
Municipal solid waste, though, is another story. Yes, you can use tires and diapers as feedstock, but they're very different materials which require very different treatment before they can begin the gasification process. For one thing, there's more sorting to do to extract the usable trash at a transfer station than there is to get the wood waste at a pulp mill. More sorting and prep work means more cost.
That's not the only problem with the preparation required for garbage in the gasification process. Many waste materials, including municipal solid waste and some biomass feedstocks like grass or corn, have a higher moisture content. These items would have to be dried before being fed into the hopper for gasification.
There's a potential problem at the other end of the process, too: Municipal waste would have a higher ash content. Wood chips are a very efficient feedstock, with only a bit -- maybe 2 percent -- ending up as ash. The rest of its mass is converted to gas (think of a log in the fireplace). Any plant processing municipal solid waste like tires and diapers into syngas would have to have the facilities to deal with the leftover ash.
And, like every new fuel source from electricity to hydrogen, enough syngas needs to be created to allow it to be widely adopted. The nation's largest trash hauler, Waste Management, has spoken with Coskata about a partnership, yet widespread use of garbage-powered cars is still 3 to 5 years in the future. But if those ethanol-spewing bacteria have their way, your car could be a garbage-powered car like Doc's DeLorean soon enough -- no bananas or crazy wires required.
For more information about garbage-powered cars, synthetic gas and other related topics, follow the links on the next page.