Research engineers have found a way to make ship engines meet IMO Tier III NOx limits with a fuel-water emulsion, potentially eliminating the need for expensive and bulky aftertreatment
The new technique, presented by MAN Energy Solutions at CIMAC World Congress, involves emulsifying fuel with at least the same amount of water. Previous tests with emulsion fuels were limited to around 30% water due to combustion difficulties, but the latest tests deploy a pilot injection of pure diesel fuel to ignite the emulsion.
Trials on Mitsui Engineering & Shipbuilding’s 4S50ME-T test engine revealed that at a high engine load a ratio of around 1:1 of water to fuel was required. For low loads from 25-50% a ratio of around 1.7:1 is needed. MAN said it expected the result to be applicable across engines of other sizes.
The authors conclude that fuel-water emulsions could enable Tier III compliance on an engine tuned to meet Tier II limits. Capital cost was also attractive, they said.
“An initial preliminary business case study suggests that the addition of a secondary fuel injection system and auxiliary components needed for water/fuel on a marine two-stroke diesel engine could be made significantly less expensive than the systems needed for either selective catalytic reduction [SCR] or exhaust gas recirculation,” the authors wrote. They added that space requirements would be significantly less, especially compared to SCR.
In the engine test a mixing system from ENH Engineering was used to distribute the diesel into the water. The system consists of three pumps for water, diesel and emulsifier, flow meters and a mill that introduces large shear stress through counter-rotating plates with several small teeth.
However, operating cost remains challenging because of the volume of water needed (and the impact on on-board water production and storage) as well as the fuel penalty associated with emulsions. The operating cost of a Tier III solution based on emulsion is estimated to be lower than for SCR – because of urea consumption - but slightly higher than for EGR.
The system has only been tested for 60 hours so far, making it difficult to draw conclusions on possible long-term operational or component lifetime problems. Other issues that will need longer testing times include fuel and emulsifier compatibility, water quality influences and fuel supply system compatibility.
The study was part of a wider project in which MAN was also able to show that it could emulsify methanol fuel, used in its ME-LGIM engines, to meet Tier III NOx limits.
Tier III NOx limits currently only apply in North American emission control areas. From 2021, two further areas will be introduced in the North Sea and the Baltic Sea. Enginebuilders expect shipowners to order more Tier III than Tier II engines by 2022.