Torrefaction

Torrefaction is the roasting of wood or other biomass to create a product that (1) has increased energy density; (2) has characteristics that make it easy to handle and transport; and (3) is practical to co-fire in existing coal plants.

Torrefaction Process

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Energy Density

Torrefaction removes moisture and low energy volatiles from the roasted wood, producing a product that is more energy dense (more energy per unit of weight) than wood and almost as dense as coal.  The significance of this increased energy density is shown in the following comparison.

Transportation and Handing

Since torrefied wood has a higher energy density, units of torrefied wood are more economical to transport than units of green wood or wood pellets with similar total energy content.  Torrefied wood is also more durable than green wood or wood pellets because it (1) is moisture resistant and can be shipped in bulk open containers and stored uncovered and handled in a manner similar to coal, and (2) can withstand 1.5 to 2 times the crushing force of wood pellets.  Thus, when compared to green wood or wood pellets, torrefied wood can be transported cheaper, farther and with less environmental impact from transportation operations.

Co-Firing in Existing Coal Plants

Grinds Similar to Coal.  Torrefied material grinds similar to coal.   It can be ground in many existing facilities, easing its integration into existing coal facilities.  By comparison, wood pellets are difficult to grind, effectively prohibiting their use in most existing coal plants.

Burns Similar to Coal. While torrefied wood is a renewable, carbon neutral resource, it burns similar to coal.  Certain volatiles and other compounds (hemi-cellulose material contained in wood) are largely eliminated in the torrefaction process, making torrefied materials easier to co-fire in power plants originally designed to use only coal.  This change in the chemical composition of the wood not only increases the energy density, but also improves the manner in which the wood is burned in a coal gasifier, thus permitting more of the energy to be converted into electricity.  This makes the energy content of torrefied wood more valuable. By comparison, wood pellets contain volatile organic compounds that release smoke when burned.  These volatiles can create slag or ash in existing coal power plants and restrict the use of wood pellets in such plants.

No Upfront Capital Cost, Green Alternative

The integration of torrefied wood into existing coal power plants requires no upfront capital investment by the coal plant operator.  Thus, torrefied wood provides existing coal burning utilities the ability to produce green energy with no upfront cost.  Torrefied wood is a renewable resource under current EU law and the American Clean Energy and Security Act of 2009 (as passed by the House of Representatives earlier this year and currently pending in the Senate).  As a result, coal plant operators can obtain renewable energy and/or carbon trading credits by incorporating torrefied wood into their production process.