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What is biocoal?

Biocoal is a solid fuel made from biomass by heating it in an inert atmosphere. The result is either charcoal, or if the process temperature is mild, a product called torrefied wood. Charcoal and torrefied wood can be called by common name biocoal.

Compared to untreated biomass biocoal has several advantages. It has high energy content, uniform properties and low moisture content. Biocoal can be used in coal fired power plants, which have difficulties with other biomass based fuels, such as wood chips.

If biocoal raw material originates from sustainably managed forests, the product is CO2 neutral. The growing new tree generation captures the same amount of CO2 from atmosphere that is released in the manufacture and combustion of biocoal.

Biocoal can be made from nearly all kind of organic materials. Wood is the most important raw material, but also straw, peat bones and even manure can be used.

What is biocoal? - Photo: Juha Rikala

Biocoal is made with a carbonization (also called pyrolysis) process, where biomass is heated in the absence of oxygen. Heating causes thermal degradation, producing various gases and solid material. If the highest temperature of carbonization is above 400 °C, the solid material is charcoal, and contains mostly pure carbon. If the temperature remains between 200 and 300 °C, the solid product is called torrefied wood, and the process torrefaction.

Carbonization of biomass produces various gaseous products; part of these can be condensed into liquid called pyrolysis oil. The non-condensable gases contain mainly carbon dioxide, carbon monoxide and hydrogen. Both non-condensable gases and pyrolysis oil can be used as fuel. Non-condensable gases are often burned in the charcoal making process, and are not recovered for later use. Pyrolysis oil is instead often recovered. It is becoming an important fuel, and in future it may be a major source of biomass based transport fuels.

Temperature is most important process condition in carbonization. Most biocoal properties, such as heating value and ash content depend on temperature. Also yield depends on temperature. In mild torrefaction, where temperature is 230 °C, the yield can be over 90 % percent. Increasing temperature to 900 °C decreases the yield to 25 %.

High process temperature also improves heating value of the fuel. The charcoal heating value can be even better than coal heating value, because charcoal contains less ash. Table 1. shows heating values for untreated wood, charcoal, coal and torrefied wood.

Heating value of torrefied wood is generally only slightly better than that of native wood. In practice the energy density of torrefied wood is still significantly better compared to chipped forest residue, since moisture content of wood fuel can be circa 50 % after harvesting. Moisture content of torrefied wood is much lower, which improves energy density.

Table 1. Heating values of some wood based solid fuels and coal.

   
Processing temperature (°C)
Heating value (MJ/kg)
Untreated wood      18
Torrefied wood   230   18,5
      250   19
      280   22
Charcoal   > 300   28-33
Coal      15-31


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