Mathematical Model of Natural Circulation Biomass Boilers during Start-Up

A natural circulation biomass boiler is a highly dynamic system, especially during start-up where the entire system is initially cold and at rest. During the start-up procedure, swelling inside the steam drum is often experienced due to the evaporation and rapid volumetric expansion of water. A mathematical model describing the dynamics of a natural circulation evaporator is presented. The boiler system is divided into subsystems where the mass, momentum and energy equations are formulated and applied. The complex geometry of the boiler is simplified and discretized in one dimension. The model captures the transient evaporation of water and the dynamic instabilities affiliated with the phase transition. During the phase transition to the two-phase regime, the steam produced displaces water. The rapid volumetric expansion of water causes mass to be transferred to the steam drum, resulting in high water levels. It is concluded that in order to model the dynamic behaviour of a natural circulation boiler with a steam drum, the subcooled region and the phase transition needs to be considered, since the phase transitions in the evaporator tubes result in rapid swelling inside the steam drum.