Grass as a possible energy source
Ian Williams, of Dunedin, asked:
I recently left sacks of grass clippings in my car overnight. To my amazement, the next morning the interior of the car was unimaginably hot, the windows dripping with moisture, and the grass almost too hot to touch. Will grass ever be the renewable energy source of the future?
Paul Guy, a botanist at the University of Otago, responded:
Grass, like all other plant material, contains large amounts of stored solar energy in the form of cellulose, starch and sugars. We make good use of this stored energy when we digest starch and burn cellulose to cook our food and warm our homes. Other organisms take advantage of these polymers too. The grass clippings in the back of your car were inhabited by a range of fungi, yeasts and bacteria. Live plants have a range of sophisticated mechanisms to defend themselves against attack by invading organisms but once you mowed your lawn the dead grass became a free meal.
As the microbes decompose the clippings the solar energy stored in plant material is released and if it is not dissipated, heat can build up often with catastrophic consequences. Damp haystacks can catch fire and many old wooden ships went to the bottom as a result of damp produce catching fire. Even without the aid of moisture and microbes there is danger: fine dust in grain elevators composed of starch and cellulose, under the right conditions, needs only a single spark to cause a deadly explosion.
Can we harness this energy? The use of lawn clippings may be a way off yet but we have been making use of a giant grass for more than 100 years. Many years ago I visited a sugar refinery in northern Queensland. Trains shunted vast quantities of sugarcane stems into the refinery, the cane was crushed, juice was extracted and cooked. Huge bubbling vats of molasses, brown sugar and eventually table sugar made a big impression: overwhelming even for a sweet tooth such as myself.
The most remarkable part of the visit was to learn that the refinery was also powered by sugarcane. After the juice was extracted, the crushed cane was dried and then blown like lawn clippings into a huge furnace which heated the vats and produced electricity via steam to power the refinery. The abiding memory of the day was looking through a furnace portal and watching the burning fibres dance like fireflies in this massive chamber.
More recently, energy technologists have looked to grasses as a source of renewable energy. The grass seeds we call grain (wheat, barley, corn) and sugarcane are being fermented to produce bioethanol. Switchgrass and Miscanthus grass are being gown in fields, cut and burnt as a renewable solid fuel or treated to release sugars which, again, can be fermented to produce bioethanol. Like your lawn, these grasses regrow quickly and produce large amounts of biomass which is easy to harvest.
Send questions to:
AskAScientist,
PO Box 31035, Christchurch 8444 Or email:
questions@askascientist.net