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

4 1. INTRODUCTION
what will be referred to as the water/steam part or drum of the boiler. In the top left side
of the boiler the steam is let out and in the top right side feed water is injected.
A mixture of air and residual fuel is injected into the furnace where it is ignited and
burned. A combination of thermal radiation and convection distributes the heat to the
furnace jacket and heats the surrounding water. The gas from the combustion (flue gas)
leaves the furnace through the flue gas pipes and contributes to heating of the water and
steam by thermal convection through the pipe walls.
The WHR boiler is placed in the funnel of the ship. The design sketched in Figure 1.1
is a water tube boiler, especially designed for heat recovery from diesel engine exhaust
gas. These boilers use forced circulation. The water inlet is taken from the bottom of
the oil-fired boiler. The water is then heated by the engine exhaust gas, and the mixture
of steam and water exiting the boiler is injected in the top of the oil-fired boiler.
1.3 State of the Art and Related Work
The steam boiler technology is over 200 years old and constitutes a complicated multivariable
nonlinear process. Nevertheless steam boilers are controlled with controllers
whose background theory is based on SISO (single input single output) processes. This
is general for both industrial and marine steam boilers. The steam pressure is controlled
using PID, control in some cases supplemented by a feedforward from the steam flow.
The water level is controlled using what is known as single, two or three element control
[Pedersen et al., 2003]. Single element control makes use of feedback from the water
level only. Two element control adds a feedforward action from the measured steam
flow to the feedback law. Finally three element control adds to the feedback a separate
loop which has the purpose of continuously adding the same amount of feed water to
the boiler as the amount of steam leaving by measuring both flows. The type of water
level controller depends on the specific boiler type. However, the water level feedback
controller is normally pure proportional action. For small boilers both the water level
and pressure can be controlled using a hysteresis controller supplying an on/off control
signal to the fuel valve or feed water pump rather than a continuous control signal.
Advanced model-based control has evolved much during the last century and has shown
potential in many industries. However, even though model-based control has had success
in other industries it has had difficulties being adopted in the marine steam boiler
industry. Therefore, there is a potential for improvements which is also backed by
economical and competitive incentives:
• A better controller for the water level can allow for more compact boilers with
reduced water and steam volumes.
• Using new signals can allow for better combined operation of oil-fired and WHR
boilers, which again allows for more compact boilers.