Torrefaction: What is that?

Improving biomass qualities for use in combustion.

A need for reducing carbon dioxide emissions has increased interest in using plant biomass for generating electricity and liquid fuels. The Renewable Portfolio Standard (RPS) in Michigan has created some urgency in evaluating renewable energy sources to replace coal and natural gas as energy sources. Several utilities are evaluating biomass to be blended with coal to meet the RPS requirements. But raw biomass is challenged by several disadvantages, including low energy density, high and variable moisture content, degradation during storage, and the expense of grinding to create small particles.

Torrefaction is a biomass pretreatment option that can overcome many of these challenges. It involves heating of raw biomass to temperatures ranging from 200 to 300°C (390 to 570°F) with low or no oxygen to produce a charcoal-like fuel. Torrefaction is driven by the combustion of volatile gases which were released from the biomass and subsequently recovered and recycled to fuel the process. The reduction of volatiles and moisture reduces the mass of the plant material while retaining 70 to 90 percent of the original energy content, which results in torrefied product characterized by higher energy density, improved storability and less costly shipping after pelletization.

Torrefied biomass is a renewable fuel that is high in energy density, low in moisture content, resistant to moisture absorption and readily mixable with coal at conventional, pulverized-fuel fired boilers at power plants. Furthermore, these qualities benefit existing power facilities by minimizing the amount of new capital investment required of power producers in order to utilize biomass.

Torrefaction has been recognized internationally as a promising development in improving the feasibility of renewable energy production. While there are several pilot-scale facilities operating currently, large commercial-scale facilities are not in production. Technological production hurdles continue to receive significant attention which will hopefully lead to commercialization in a limited time.

You may find more information about torrefaction by reading Torrefaction? What’s that? by USDA Forest Service’s Dana Mitchell and Tom Elder.

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