  This paper describes a new nonlinear dynamic model for a natural circulation
boiler. The model is based on physical principles, i.e. mass, energy and
momentum balances. A systematic approach is followed leading to new insights
into the physics of drum water level and downcomer mass flow. The model
captures fast dynamic responses that are necessary to describe the operation of
a boiler under highly variable load conditions. New features of the model
include (i) a multi-compartment model for the risers, (ii) a new model for drum
water level, and (iii) a new dynamic model for the flow of water in the
downcomers. Implications of the model for control system design are explored in
detail. Finally, the suggested improvements are validated in a sugar mill
boiler.
