I am not sure if wood fibre is the most efficient material for a CHP. At least for the Kauttua CHP of the Kauttua paper mill it says this:
The fuel used in the boiler includes various wood, paper and plastic-based recycled trade and industry fuels suitable for energy use. Peat is also used.
Also this conference paper shows how other materials than fibers can be used (i.e. black liquor and other process waste).
But wood fibre can indeed be bark, or whatever is left from trees after the best bits are taken for the production of pulp and paper.
It is nonetheless a good step in recycling energy and heat!