Biofuels research has largely focused on woodchips from mature poplar trees, as these contain the highest concentration of sugar essential for making ethanol and chemicals. This biofuel production route is not cost-competitive: the trees are harvested just once every decade or so, most of the poplar parts are wasted and the resulting fuel is deemed costly when the relatively low prices of oil are considered.

If poplar were planted close together like an agriculture crop, and whole trees were harvested on a much quicker cycle, it could make sense from a cost perspective and offer a short return on investment — and be more attractive for farmers.

University of Washington researchers hope to improve the economic viability of poplar by demonstrating production of younger poplar trees that could be harvested more frequently — after only two or three years — instead of the usual 10 to 20-year cycle. The trees are planted close together in the coppicing approach and are cut so that more branches grow from the stump after each harvest.

The team planted trees in Jefferson, Oregon in the spring of 2012 and harvested less than two years later before The researchers harvested poplar trees approximately the size shown. (Credit: Chang Dou/University of Washington)the leaves had fallen. The collected wood materials were subjected to processes that use high temperature, pressure and enzymes to yield sugar which can be fermented to produce ethanol, acetic acid, lactic acid and other valuable chemicals. Leaves are naturally low in sugar content and were observed to contain other chemicals that undermined the sugar-releasing process. At a commercial scale, the researchers recommend removing the leaves and using them for animal feed or other purposes.

A high-heat pyrolysis process was also tested to convert whole poplar trees directly to bio-oil. Inclusion of leaves did not adversely affect the quality of the resulting bio oil, indicating that at industrial scales, producers could save time and money by not separating leaves from branches to achieve similar quality oil.

The researchers say that coppice poplar is likely the best balance of cost and reliability for Pacific Northwest growers to produce biofuel.

“Our research proved that poplar coppice can be a good option to meet the cheap, high-volume criteria of biofuel feedstock,” said Chang Dou, a doctoral student in the university’s Bioresource Science and Engineering program. “Our findings are significant for the future biofuel industry, and the ultimate goal is to make poplar coppice biofuel a step closer to the pump.”

The video shows the speed with which coppice, or short-rotation, poplar can be harvested.