Chemical refineries are great at converting petroleum into gasoline and the building blocks of plastics and other consumer goods. But when it comes to sustainable starting materials, such as wood chips, corn stalks, or other plant "biomass," refineries are too inefficient to make the process commercially viable. Researchers have now given that efficiency a major boost, perhaps enough of one to allow us to leave petroleum behind.
Biochemistry. A portable biorefinery for pyrolysis oil production.
There are plenty of ways to convert biomass into useful fuels and chemicals. But each has drawbacks. Yeast and other microbes can ferment plant sugars into ethanol, a gasoline additive. But only moderate amounts of ethanol can be added to gasoline without requiring engine modifications. Algae readily produce bio-oils, but the technology remains costly and requires too much land and fresh water to make an impact on the market. A third route, known as pyrolysis, heats dried and ground biomass to about 550˚C in an oxygen-depleted chamber (so the biomass doesn't burn), producing a mixture of gases, liquids, and a gray, carbon-rich solid called coke. When the gases cool and condense, they combine with the liquids to form a mixture of oils. These oils are cheap: It costs only $1 to make oil through pyrolysis that has the same energy content as a gallon of gasoline. But they must be further chopped into smaller hydrocarbons before they are suitable for industrial use. In addition, oxygen-rich acids in the oil make it corrosive, so it can't be used in conventional engines and storage containers.
Engineers have worked to tackle these problems with pyrolysis oils by adding a second treatment step, where the oils react with hydrogen over catalysts called zeolites. The hydrogen replaces oxygen in the acids and other compounds in the bio-oils and makes them less corrosive, and the zeolites break the large hydrocarbons into compounds such as toluene and benzene that are commonly used building blocks for a large number of industrial chemicals. The problem is that coke and other substances made by pyrolysis can gum up zeolites. Only 20% of the pyrolysis oils are converted to useful chemicals. Most of the rest winds up as coke, carbon monoxide, and carbon dioxide.
In hopes of improving the efficiency, George Huber, a chemical engineer at the University of Massachusetts, Amherst, and colleagues tested several combinations of zeolites (hundreds are known) and reaction conditions and found one standout. They split the second treatment step in two. First, they reacted their pyrolysis oils with hydrogen over a ruthenium and platinum catalyst, which stripped out much of the oxygen from the acids and added hydrogen. This made a mix of stable compounds that were less likely to form coke when they were processed in the second step over the zeolites. It also allowed the zeolites to convert 60% of the longer hydrocarbon chains into five key chemical starting materials: benzene, toluene, xylene, propylene, and ethylene. Together, these compounds represent five of the seven key starting materials (the others are methanol and 1,3-butadiene) that form the basis of the $400-billion-a-year petrochemical industry. The process can also be tailored to produce more of individual chemical building blocks, which in the future could allow chemical companies to produce the most valuable building blocks at any given time, the team reports in the 26 November issue of Science.
Robert Brown, who directs the Bioeconomy Institute at Iowa State University, Ames, says the new work is noteworthy because chemical companies have many decades of experience in using heat and catalysts to convert petroleum into a wide variety of commodity chemicals. "There is a notion that thermochemical processing is a mature technology," with little room for improvements, Brown says. "Huber's work demonstrates that there is potential for many advances," as the technology is applied to biomass, he says. Huber says he has formed a start-up company, Anellotech, that plans to commercialize the technology, first with a small pilot plant, followed by a commercial demonstration facility.