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

30 2. THE MARINE BOILER PLANT
The oil system is a bit more complicated, and we will not show any diagram hereof.
The burner is a pressure atomising burner. The specific burner on the test boiler has a
relatively large turndown ratio of 1:5.7. At fuel flows below the minimum allowable
fuel flow there is not enough pressure to allow for proper atomising. In such case
the combustion is incomplete which leads to more pollution, poor utilisation of the
fuel energy, risk of carbon deposits in the furnace and flue gas pipes. In such case, the
combustion air flow could be increased giving a higher excess air number, but at the risk
of cooling the flame which also gives poor combustion, more CO2 and NOx emissions
and less efficiency.
The air flow supplied to the combustion in the test boiler is adjusted by a damper after
the fresh air fan. The amount of air is controlled electronically and determined as a
function of the current wanted fuel flow. This function is not treated in the present
work. The maximum fuel flow is 154 kg
h and the minimum fuel flow is 27kg
h .
The sensors on the boiler for measuring pressure and temperature are all standard equipment
and so is the water level sensor. However, the water level sensor is interesting as
we will see that it limits the speed of the water level feedback loop. It is based on
the capacitive measurement principle where an electrode rod is submerged in the water
acting as one plate of a capacitor, and the boiler drum walls act as the other plate.
The water acts as the dielectric. When the water level changes, the capacitor changes
as the dielectric between the plates changes. This change is detected and converted
into an output signal. The change in capacitance is proportional to the change in level.
Although accurate and placed in a protection tube such, measurement is very sensitive.
This also means that the chaotic behaviour of the boiling water level is detected
by this device. As the noise on the water level has no specific frequency this makes
it difficult to detect whether changes of the water level are due to changes caused by
the shrink-and-swell phenomenon or simple bubbles breaking loose from the water surface,
or the raised surface towards the flue gas pipes collapsing creating wave motions
on the surface. Other devices for measuring liquid surfaces are available also for pressurised
vessels containing boiling substances. However, these have not been available
throughout the project.