The first step is to separate the garbage, or municipal solid waste, at a transfer station. Glass and metal won't work in the gasification process and should be recycled anyway; things like wood chips, grass, tires and diapers will work. "If it gets really hot and melts, that's not a good substance," says Wes Bolsen, Chief Marketing Director at Coskata, Inc. "If it gets hot and turns into gas, like wood in a fireplace, that's good." Any carbon-based substance will fit the bill as a feedstock.
Once the garbage has been sorted, it can begin gasification. The feedstock is heated -- but not burned -- under pressure until it decomposes. Imagine the compost bin in your backyard, which has organic matter inside. As it is, those old lettuce leaves and carrot tops get quite hot and decompose to make compost. Now imagine the bin has no vents, so there's no oxygen to cause combustion, and even more heat and pressure are applied. The same decomposition process would occur, but much more intensely. The decomposition process breaks apart the carbon bonds in the old veggies and turns into something like natural gas -- syngas.
You could stop right here, after the gasification process has made syngas. Some experimental vehicles, like the Honda Civic NGV or even the city of Toronto's new fleet of garbage trucks, run on compressed natural gas. Syngas could probably be used as a fuel for these cars. But what about the millions of cars already on the road that can't use syngas? Coskata adds a couple steps to its process to make ethanol from the syngas that can be easily introduced to nearly every vehicle driving down the road today without changing a thing about the cars.
Once the syngas has been created at Coskata's plant, it's fermented from a gas to a liquid by bacteria in an aqueous environment -- which really means bugs that live in tanks of water. These patented strains of bacteria breathe in syngas (CO, CO2, and H2) and eliminate ethanol (C2). All that's left to do is distill the ethanol from the water. The ethanol is engine-ready, and the water goes back to the bugs with very little being wasted in the process.
And now that we know how fuel can be made from garbage, let's find out why we'd bother to use it.