Old MacDonald Had a Pyrolysis Doohickey

Mobile biofuel refineries provide sustainable energy for farms

A hundred years ago, threshing machines chugged from farm to farm across the plains, separating stalk from grain and turning raw crops into valuable commodities. By sharing the machine, farmers could boost productivity without owning the prohibitively pricey equipment. Today, that business model could work for a new product: biofuel.

Biofuel from farm waste is a promising alternative to oil, but it’s too expensive for any one farmer to make alone. Manufacturing cellulosic ethanol, the most common biofuel made from waste, costs up to $3 a gallon—going mostly to shipping raw materials and the enzymes used to break them down. Six years ago, Roger Ruan, a biosystems engineer at the University of Minnesota, began designing a smaller, simpler way to convert biomass to biofuel, using a process called pyrolysis.

The technique usually involves grinding biomass into a powder and heating it to break it into its chemical components, which are turned into fuel. The pulverizing ensures that the material heats evenly but is energy-hungry and slow. To eliminate the grinding step, Ruan added a microwave generator, which nukes chunks of organic material from the inside out. Off-the-shelf tech helps keep the price below that of conventional systems.

Ruan commissioned a Chinese factory to build a camper-size prototype, and this fall he’ll hitch it to a pickup and hit the roads of rural Minnesota to conduct a field study. “We’ve got the choir signed on,” says Linda Meschke, a Minnesota farmer who is lining up farms for the pilot study, “and a tentative congregation watching to see what happens next.”

How To Turn Trash Into Power

1. Turn up the heat. Biomass is sealed inside an oxygen-free chamber and heated with microwaves to 500F, breaking the material into solid and gas components.

2 Clean out the dustbin. The burned solids, called biochar, collect in a tray and can be used as fertilizer. The remaining mix of gases flows up into a condenser column.

3. Squeeze the gas. Half of the gas is condensed into a liquid bio-oil, which can substitute for heating oil or industrial petroleum.

4. Start again. The remaining gas is siphoned off and burned to produce electricity to power the system. For every pound of biomass, farmers get half a pound of bio-oil and a quarter pound each of biochar and combustible gas.