Using chipped wood is just one of the sources for the biomass used to produce energy. More from Gary's link above -
There are five basic categories of material:
•Virgin wood, from forestry, arboricultural activities or from wood processing
•Energy crops: high yield crops grown specifically for energy applications
•Agricultural residues: residues from agriculture harvesting or processing
•Food waste, from food and drink manufacture, preparation and processing, and post-consumer waste
•Industrial waste and co-products from manufacturing and industrial processes.
Three of these are categories are waste products that would otherwise got to landfill.