|Trees growing in the Maine woods - is torrefaction in their future?|
In torrefaction, raw wood or other biomass is typically heated to between 400 and 600 degrees F, a temperature selected to roast but not burn the material. (Green coffee beans undergo a similar process when they are roasted.) Wood and other raw plant material typically contains a large amount of water; this water is driven off by the torrefaction process, as are other volatile chemicals found in the raw material. Heating also partially breaks down natural biopolymers in the wood (like cellulose, hemicellulose, and lignin), releasing even more volatiles. The material left behind is solid, dry, and blackened: torrefied biomass.
Torrefied biomass has some advantages over raw wood as a fuel. Because the volatile chemicals have already been driven off through torrefaction, the combustion of the biomass may have lower environmental emissions when it is finally used. (A life-cycle analysis would take into account the volatiles driven off during torrefaction.) Torrefied wood generally has a higher energy density than raw wood (some sources suggest 30% more), so it can be more cost-effective to transport and ship over longer distances. This could open up more distant markets for forest products. Torrefied biomass can also be mixed with coal to produce electricity in existing coal-fired power plants; this use, along with its dark color, has led some to call it “bio-coal”.
While torrefaction has been around for over a century, it is still in a relatively early phase in commercial use, particularly in the U.S. What role will torrefied biomass play in the future of energy resources and forest products?