If you need to develop a simulation model to test different control technics for biomass combustion one of the options is to use TRNSYS together with Type 210 component. TRNSYS itself is a pre-programmed component based simulation platform where you build you system by putting together different components (pumps, tanks, boilers, etc.). There are wide range of control solutions like PID, temperature differential, simple temperature hysteresis, etc. Type 210 is a component made to simulate pellet stoves and boilers. It allows to simulate heat and mass ballance, as well as emissions of CO. There are numerous articles available on internet on Type 210 including mathematical description, please read if it is ok for your task.