Torrefaction. What is it?

Torrefaction is the method where biomass is heated in a low oxygen environment to a point where chemical changes start occurring. At the right temperature and residence time, the end product resembles coal in color and physical characteristics. It is able to burn alongside coal, or co-fire, in a coal power plant without the plant needing to perform any costly modifications. The torrefaction process breaks the bonds inherent in green and dry biomass that make it undesirable for co-firing with coal. Let alone green or dry biomass being far less energy dense that coal, burning green or dry biomass in a coal-fired plant without costly modifications will cause gumming up the grinders and interior of the boilers, making the plant less efficient, and requiring more frequent shutdowns for cleaning and repairs. Another benefit of torrefied biomass is its hydrophobic nature, such that after pelleting it can be stored and transported just like coal.

Both woody biomass and energy crops, such as switchgrass, prairie grass, bamboo, and eucalyptus, can be raised specifically for torrefaction. Some types of biomass will grow where no food crops will grow, so previously unusable land could become productive.

There are many factors that determine the final energy content of torrefied biomass. The most important are the type of biomass being torrefied, as each type of biomass has its own energy characteristics, and the percent dry solids lost during the torrefaction process. With the right biomass type, the torrefier can be tailored such that the final product can have any desired energy content between 6,400 Btu/lb – 10,700 Btu/lb (14.9 GJ/MT – 25 GJ/MT).

How does one torrefy biomass?
Torrefying biomass is the process of heating the biomass in a low oxygen environment. You have done this at home if you have a wood stove or fireplace “insert”. Once you get the fire going, you can close the damper such that very little oxygen is getting into the stove. The wood is still creating heat, but it will turn black and become crispy. By cutting off the oxygen you don’t have to load the stove up as often and the heat given off is much more mild than a raging fire. Konza controls the torrefaction reaction much better than this to make sure the biomass isn’t burned up. The biomass is never actually caught on fire, but the high temperature in the torrefaction reactor forces the reaction.

The torrefaction reaction drives off part of the biomass solids to create torrefaction gas that can be burned as energy in the torrefier system. Depending on the type of biomass used and its starting moisture content, the torrefaction gas can supply over 75% of the energy necessary to dry and torrefy the product, making the process very efficient.

Who uses it?
Since it has been just recently that torrefaction production on a commercial scale has been possible, no one is using torrefied biomass for anything much more than laboratory testing. However, there is much demand by coal-fired power plants, especially those in Europe, as requirements for using renewable fuels are increasing. Once the viability of steady production is proved and torrefied material’s satisfactory testing in the coal plants has been completed, we see the demand skyrocketing.

Theoretically any industry that uses coal will be able to use torrefied biomass. It can be used for power production, as cooking coal to make steel, in chemical plants, and other industrial applications. Although not many people have coal burners in their homes anymore, they do still exist. Torrefied biomass could be substituted in these situations. A quick search on Google will result in many many different uses for coal, most of which torrefied biomass could be a substitute.

 

Learn more
Thompson Dryers engineer, Becky Long will be speaking at the Biomass Carbonization & Torrefaction Summit, April 16th in Atlanta, Georgia. Becky will speak on rotary drying technology for industrial production of biocoal.