The firing system consists of an air-cooled reciprocating grate using the coarse biomass fractions of the wood panel production process as main fuel. Examples include bark, wood chips, coarse trim-mings. System can be equipped with various injection nozzles for combustion of dust and granulate fractions from screening, sawing and sanding. Generating hot thermal oil and providing hot gas to the dryer system can be controlled independently of each other in order to meet individual peak load demands. Recirculation of hot gas from downstream of the thermal oil heater is applied to optimize the combustion process and to allow best operational flexibility of the energy system.