Solid biomass fuels come in many shapes and types. They are all of organic origin and the vast majority are plant derived. This means that they display at least some of the fibrous structure of plants. Coal is also plant material, although highly metamorphosed after tens of millions of years in geological deposits exposed to varying levels of pressur and heat, which has removed almost all signs of this fibrous origin on a macroscopic level. So when coal is processed in a mill, it behaves much like . mineral, hard and brittle and with no apparent fibrous structure. Coal mills are, thus, designed to process hard and brittle materials.
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