Plastics get a new lease on life as electricity generators

All photos cour­tesy of Yiannis Levendis.

Here are two prob­lems our planet needs help with:

1. Get­ting rid of all the trash we pile on top of it
2. Making elec­tricity for us in a healthy, sus­tain­able way

Mechan­ical and Indus­trial engi­neering pro­fessor Yiannis Lev­endis has it cov­ered. He recently filed a patent for a reactor that effi­ciently turns plas­tics and other polymer-​​based waste prod­ucts into electricity…or, if you’re into this sort of thing, carbon nanotubes.

One day sev­eral years ago, Lev­endis set a Sty­ro­foam cup on fire. As you might expect, it burned quickly, cre­ating a thick black cloud of smoke and leaving behind a tiny pile of soot and tar. Poly­mers like poly­styrene are long chains of carbon and other atoms, arranged in dif­ferent unique pat­terns depending on the mate­rial. Each atom is con­nected by a chem­ical bond, which stores poten­tial energy. When he burned the cup, those bonds broke quickly and the energy was released as heat. The black soot was simply the new embod­i­ment of those same carbon atoms, now taking var­ious forms including poly­cyclic aro­matic hydro­car­bons, or PAHs, the pol­lu­tants released during tra­di­tional waste incineration.

Each year, the United states uses 330 mil­lion bar­rels of petro­leum to pro­duce 30 mil­lion tons of post-​​consumer plas­tics, Lev­endis told me. What if we could take those two num­bers and roll them together so that the waste plas­tics we pro­duce become the fuel we need to heat our homes and power our lights? Lev­endis real­ized the pos­si­bility was there, simply because of the energy stored between the carbon atoms. If he could figure out a way to turn that energy into some­thing useful without gen­er­ating the black smoke cloud, he would be on the right track.*

The word “burn” implies the pres­ence of oxygen in the envi­ron­ment, since com­bus­tion is the high-​​heat reac­tion between oxygen and a fuel (be it a wooden log, petro­leum, or a Sty­ro­foam cup). But if you heat that solid fuel to very high tem­per­a­tures in an inert atmos­phere such as nitrogen, you merely get the gaseous ver­sion of the fuel. No com­bus­tion takes place because there is no oxygen around to react with. The process is called pyrolytic gasi­fi­ca­tion and it is the cen­tral fea­ture of Lev­endis’ reactor.

Like many other crea­tures, the reactor eats carbon-​​based mate­rials and releases a stream of gas. The gas is then mixed with air, ignited and burned into a beau­tiful blue/​purple flame — no black smoke, no pile of soot. Since each type of plastic con­tains a unique mix of atoms, the ulti­mate gas pro­duced will con­tain dif­ferent levels of things like methane, ethane or pen­tane. The tech­nology for turning these gases into light or elec­tricity has been around for a while, as evi­denced by all the old gas lights on Beacon Hill, and else­where. Our aver­sion to such tech­nology stems from our unwanted reliance on finite fuel sup­plies. But there’s a whole bunch of plastic in the world that simply gets thrown into a land­fill or burned, gen­er­ating pollutants.

In the last decade, the cost of nat­ural gas has fluc­tu­ated between $2.5 and $14 per giga­joule of internal energy. Lev­endis’ cur­rent reactor uses a feeding mech­a­nism that requires the use of post-​​consumer plastic pel­lets which are not exactly cheap. With this design, the cost is about $10 per giga­joule. But he and his team are working on new feeding methods, which would allow them to directly use a pallet of plastic bot­tles, cups, bags, you name it. This design would bring the cost down to $1 per giga­joule. And, Lev­endis reminded me, none of these num­bers incor­po­rate the value of using wastes instead of virgin fuel and thus cleaning up the environment.

*At this point I need to insert a small dis­claimer: Lev­endis was doing all this the­o­rizing with two people I actu­ally knew quite well. I worked at a small start-​​up com­pany called Nano-​​C before coming to North­eastern. I dis­tinctly remember standing in the office back at The C, as we affec­tion­ately called it, talking to Hen­ning Richter about a project he was working on with a North­eastern pro­fessor, turning trash into carbon nan­otubes. I thought that was pretty darn intriguing, but I never learned more until I met Lev­endis last month. The person who founded Nano-​​C, Jack Howard, was a chem­ical engi­neer at MIT for many years and before his passing in 2007, he was crit­ical to the the­o­ret­ical process that would allow the trio (Lev­endis, Righter and Howard), along with grad­uate stu­dent Chuanwei Zhuo, tourn waste plas­tics into energy and carbon nanotubes.