MOBE: Final Report: Modelling and Optimization of biomass-based Energy production

The present report is the documentation of the work in the PSO-project MOBE,
“Modelling and Optimization of biomass-based Energy production”. The aim of
the project is to develop better control methods for boilers in central power plant
units, so the plant will achieve better controllability with respect to load changes.
in particular focus is on the low load operation near and below the Benson point.
The introduction of the report includes a description of the challenges the central
power stations see in the modern electricity market where wind power delivers
a signigicant prioritized production, and thus, in connection with consumption
variations, contributes to the load requirements of the central units.
The report documents the work on development of a common simulation platform
for the partners in the project and for future model work. The result of this is
an integration between the DTU simulation code DNA and Matlab. Other possible
tools are suggested.
The modelling work in the project has resulted in preliminary studies of time
constants of evaporator tubes, an analysis that shows that Ledinegg-instabilities do
not occur in modern boilers even at low load, development of a validated evaporator
model that can be coupled to tools for control system development, and an analysis
of two different configurations af the low load system of Benson boilers.
Based in a validated power plant model different control strategies have been
studied. Because constraints on control signals and temperature gradients are dominating,
it is recommended to use model predictive control. It is demonstrated, how
such a simulator can handle large low gradients without violating the constraints.
By switching between different linearized models the whole load range may be
covered.
The project indicates that Model predictive control can improve the control in
low low significantly. This should be studied further in future projects by realistic
tests. At first these should be done with a simulator based on first principles
and after this on a real boiler. The model work in MOBE has made the basis for
an applicable model but this should be developed further to better understand the
evaporator process and its coupling to the rest of the plant, particularly reheat.
The models developed in the project are based on several simplified assumptions.
These assumptions will have to be evaluated in future work by more focus on the
real operation of Benson boilers with different fuels.