Water Fraction Measurement in Marine Fuel Emulsions

Abstract

The proposal, from the International Maritime Organisation (IMO), to limit further the emissions from marine diesel engines came into effect in May 2005. This has considerable consequence for the management and operation of ship diesel plant. One method that has been shown to limit the emissions of NOx is the addition of quantities of water as an emulsion into the heavy fuel oil (HFO) before it is injected into the burners. This reduces the peak combustion temperature, improves atomization of the diesel fuel, and can reduce emissions by as much as 30 per cent. A key component for an efficient and cost-effective system is a method to monitor the water content to an accuracy sufficient to allow the mix to be adjusted to meet the needs of the varying engine loads. This paper briefly presents the environmental, legislative, and technical background. The principle aim is, however, to describe the experimental work examining the application of an in-line optical sensor. Laboratory tests on HFO, having a room temperature viscosity of 180 cSt, were undertaken at two nominal temperatures, 80 and 130°. These tests provide empirical evidence that an in-line optical monitor could determine water fraction within the emulsion to the accuracy requirement (better than 3 per cent) and over the operational water content range (15-33 per cent water to oil). A hypothesis is presented to explain the changes in the optical scattering characteristics of the oil/water emulsion with water content. Additional results are presented that demonstrate the use of two commercial viscometers to quantify the oil/water fraction. It was concluded that the measurement of emulsion viscosity can be related to water fraction but that the current instruments do not have the required resolution and have serious limitations due to their temperature sensitivity. A key requirement for further work is that the scattering properties of the emulsion be investigated in greater detail. In particular a test must be undertaken at temperatures in the region of 170°. Also, the instrument must be developed to cope with the wide variety of diesel fuels that a ship may take on at bunkering facilities around the world.