Optimisation of Marine Boilers using Model-based Multivariable ...

Abstract
Traditionally, marine boilers have been controlled using classical single loop controllers.
To optimise marine boiler performance, reduce new installation time and to minimise
the physical dimensions of these large steel constructions a more comprehensive and coherent
control strategy is needed. This research deals with the application of advanced
control to a specific class of marine boilers combining well known design methods for
multivariable systems.
This thesis presents contributions for modelling and control of the one-pass smoke tube
marine boilers as well as for hybrid systems control. Much of the focus has been directed
towards water level control which is complicated by the nature of the disturbances
acting on the system as well as by low frequency sensor noise. This focus was
motivated by an estimated large potential to minimise the boiler geometry by reducing
water level fluctuations. Strategies for achieving such goal, based on model predictive
control, are suggested while pressure control is achieved by using a multivariable
control setup.
The thesis further directs attention towards control of the boilers in load situations requiring
on/off switching of the burner. A new strategy for handling such situations
based on a generalised hysteresis control approach is presented. The solution is optimal
according to a specific performance and the new method prove promising results
compared to traditional methods and existing optimisation based methods (finite horizon
model predictive control). In the thesis the pressure control is based on this new
method when on/off burner switching is required while the water level control is handled
by a model predictive controller.
The thesis is presented as a collection of eight papers accompanied by a short introduction
of the marine boiler plant, motivation, background, summary and conclusions.
The first paper (A) discusses modelling and control of a novel turbocharged burner unit.
The derived model is compared with measurement data, and a control strategy for the
burner is suggested.
In the second paper (B) a model of the marine boiler is presented along with application
of linear quadratic Gaussian control to control pressure and water level in a test boiler.
In the third paper (C) a thorough analysis of the marine boiler control properties is
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