Emulsified light distillate fuel for low-NOx large engines
Anders Andreasen, senior research engineer at MAN Diesel & Turbo in Denmark, describes how emulsifier development enables water-emulsified light distillate fuel for reducing nitrous oxide emissions in large marine engines.
The marine industry is estimated to account for approximately 90% of all goods-transport globally. Despite being the most carbon and energy friendly transportation method available today, international shipping still contributes around 2.7% of global carbon emissions (from human sources). Furthermore, due to the very high combustion temperatures and pressures inside modern marine engines, shipping’s contribution of nitrogen oxides (NOx) to global emissions has been shown to be relatively large. This is also, in part, due to the industry’s generally poor up-take of exhaust-gas after-treatment methods, such as the catalytic methods known from the automotive industry, for example, the three-way catalytic converter and selective catalytic reduction (SCR).
As a result, IMO is introducing stricter regulations worldwide that include reduced limits for NOx emissions and a progressively lower sulphur-content in fuel in order to reduce SOx and particulate emissions. Additionally, IMO has designated sulphur emission control areas (SECAs) with even tougher legislation with more yet to come. Apart from IMO regulation, other national, regional and local regulation, as well as taxation and incentive schemes, are driving emission reduction. Furthermore, customer demand in conjunction with the shipping industry’s aim to be seen as an environmentally responsible industry are sparking an interest in emission reduction that extends beyond regulatory requirements.
The method of adding water to fuel, termed water-in-fuel emulsion, or WIF, prior to injection into the combustion chamber of direct-injection diesel engines is an effective way of reducing flame temperature that, in turn, suppresses NOx formation. WIF has been tested in the past on large, two-stroke diesel engines but most experience is limited to the use of residual fuel oil that inherently emulsifies a moderate amount of water due to its distinct, physical properties. On the other hand, distillate fuel oils will become more widely used, especially in the IMO-designated SECAs that include some of the heaviest concentrations of shipping traffic. This is due to the limited availability of residual fuel oils that meet the strict sulphur regulations within these areas. In order to create stable WIF emulsions with distillate fuels, a suitable emulsifier is required.
This has served as the main motivation for establishing a collaborative project between MAN Diesel & Turbo’s Marine low-speed business unit and Danisco, with the main objective of finding an emulsifier that facilitates the emulsification of large quantities of water (up to 100% added volume) into distillate fuels, while still allowing safe and reliable operation of the engine without any significant deterioration in engine stability. Other project objectives include the mapping of the emission reduction potential of WIF using distillate fuels, as well as investigating the potential synergy effects of combining WIF with exhaust-gas recirculation (EGR), another effective NOx reduction method. The project has been supported financially by the Danish EPA.
An exhaustive laboratory screening phase tested a wide variety of commercial and custom-made emulsifiers, and even included combinations of different emulsifiers. Testing used state-of-the-art experimental methods within fields such as rheology, confocal laser scanning microscopy for the evaluation of emulsion homogeneity, water-droplet-size distribution by nuclear magnetic resonance, viscosity measurements, and measurement of both static and dynamic interfacial tension. Finally, and probably most importantly, visual inspection of emulsion stability and the tendency for sedimentation and even phase separation were checked. The screening process identified four potential emulsifier candidates, of which the two best-performing ones were tested on the MAN B&W Diesel 4T50ME-X research engine at MAN Diesel & Turbo’s Diesel research centre in Copenhagen.
The purpose of this was to test the emulsifiers in a realistic, scaled-up environment closely resembling conditions on a real production engine installed onboard a ship. Testing was performed using a neat gas oil and an additional volume of 70% water. Engine stability was evaluated using data acquired from several hundred revolutions for all cylinders. A statistical analysis showed no variation, either in maximum cylinder pressure nor in the injection period from cycle-to-cycle and cylinder-to-cylinder, compared to running on pure fuel. An optimisation study showed that it was possible to reduce the emulsifier amount from around 1% (on the basis of fuel) to about 0.2%, again without any deterioration in performance and stability.
Finally, tests confirmed that it was possible to shut down the engine, including the fuel circulation pumps in the fuel system, for an hour and still successfully restart. Emission measurements showed that up to 60% reduction in NOx was feasible with water contents approaching 90% volume added. It was also demonstrated that for moderate water contents (24% volume water added), WIF has a favourable NOx-SFOC trade-off giving up to 5g/kWh NOx reduction for 1g/kWh SFOC penalty. At low engine-load and moderate water content, reductions in both SFOC and NOx have been achieved simultaneously.
The next step for the project is to test the emulsifier on a production engine on the test bed, as well as on sea trials running on gas oil. The first tests are scheduled for October 2011 where the emulsifier will be tested on a MAN B&W 6S80ME-C9 engine (23,000kW at MCR ) on the test bed at Hyundai Heavy Industries in Ulsan, Korea. The engine is to be installed in a 4,500TEU container ship for Maersk Line. The engine is equipped with waste-heat recovery technology, including power-turbine and steam-turbine installations.
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