Here are a few links on this subject:

Co-Firing Biomass with Coal: Constraints & Role of Biomass Pretreatment 
(See 6.2.7 Torrefaction & 6.2.8. Torrefaction with pelletizing)

Fuels for Biomass Co-firing

Biomass Co-firing in Coal-Fired Boilers

Model Fuel Contract - Co-Firing Biomass with Coal

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