powertalk
Dual-fuel has no long-term future
Dual-fuel technology is establishing a role in current propulsion concepts but a lesson from the past offers a warning for the future, says David Bricknell*
ore than a century and a half ago, the steam ship Great Britain was a pioneering dual-fuelled ship, with both wind and coal driving it forward. When it was launched in 1843 it was the largest ship afloat and, for the first time, combined an iron hull and a screw propeller to become the first iron steamer to cross the Atlantic.
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But it was competing against ships that were either sail or steam, yet it was neither the best sailing vessel nor the best steam ship. In 1852 and under new ownership it was given a new engine and a more efficient propeller to operate to Australia, but it relied more on sail than steam to save money. Finally, in 1882, the engine was removed to make it competitive and it became a pure sailing cargo ship – ironically, to carry coal. That was an early demonstration that fuel flexibility comes at a price. While many today have expressed the view that dual-fuel is the only answer in today’s world, there are both benefits and drawbacks. On the plus side, if your ship can carry both liquid fuel and gas fuel then you have no concerns about fuel availability if there is poor gas supply infrastructure.
Where a ship’s area of operation is variable and uncertain, operators may experience the marine equivalent of the ‘range anxiety’ experienced by drivers of electric cars and worry about whether they will be able to refuel en-route; in these instances, the ability to revert to liquid fuel is comforting. However, compared to a single fuel engine, there will be an efficiency penalty for the low pressure dual-fuel engine while running on gas and a considerable efficiency penalty when running on liquid, since dual-fuel engines run with a BMEP of 19-21 bar and conventional diesels run at 25-28 bar. Add to this the space and weight penalty from including two sets of fuel tanks with the consequent loss of earning capacity, plus the significantly higher initial cost of the engine and its gas fuel system, and it is difficult to see how the ship can be competitive over its lifespan. Meanwhile, the LNG supply chain is developing, although some areas are rather better covered than
104 I Marine Propulsion I April/May 2014 20 per cent load 100 per cent
This general comparison illustrates how dual fuel options, whether high pressure or low pressure, cannot match pure gas-fuelled systems at most load levels (credit: Brycheins)
others; availability and demand will tend to develop alongside one another. Where routes are fixed – ferry services, for instance – or where the area of operations is constrained, such as for coastguards and coastal trades, then an infrastructure for security of fuel supply can be implemented and pure-gas fuelled ships are then highly attractive. From a technical point of view, by the end of 2013 the marine industry had largely removed, or developed solutions for, any obstacles that stood in the way of gas becoming the dominant marine fuel within a generation. Gas-engines, both two- and four-stroke, are being developed and released at an increasing pace by all the major manufacturers. Gas-fuelled ship designs and their accompanying safety rules are also being developed and built in increasing numbers. However, although liquid fuel engines are now available to meet the stringent emission targets of IMO Tier III and EPA Tier 4 without off-engine after-treatment, they cannot take advantage of lower fuel costs of gas. Gas fuelled engines are available in Otto-cycle (spark-ignited single-gas and low-pressure dual-fuel) and diesel-cycle (high-pressure dual fuel) and both cycles apply to both two- and four-stroke engines. Which engine technology is right for your ships depends upon their duty and area of operations.
During the 2000s, increasingly tight emissions regulations for NOx, and for particulates challenged engine designers to improve liquid combustion technology. Initially, combustion chamber shape,
improved turbochargers and higher-pressure
injectors (to reduce particulates and improve combustion) were adopted and then the Miller- Cycle was introduced (to reduce combustion temperature thus NOx). Most recently, two-stage turbocharging has become available to improve Miller-Cycle engine power and efficiency. Marine Propulsion has regularly covered such developments as exhaust gas recirculation, which some enginebuilders are using to meet NOx limits from liquid fuel without off-engine after- treatment. But others seem reluctant to follow, instead focusing their efforts on gas-fuelled engines that will meet the limits while, most importantly, taking advantage of very low gas fuel prices in most parts of the world.
So the growing expectation in the marine industry and within the major engine manufacturers is that gas LNG will increasingly and rapidly become a significant fuel throughout the world and for many ship types. The battle is now beginning as to which technology will win: Otto- or diesel-cycle. Dual-fuel engines do not have a long-term role in this scenario. Like Great Britain, ships that fit them will be overtaken by their rivals as gas becomes the new steam. MP
*David Bricknell is the owner and principal of Brycheins, an independent design and engineering consultancy. He has over 40 years’ experience and was formerly vice president for systems, product strategy and business development for Rolls-Royce
www.mpropulsion.com COMPARATIVE EFFICIENCY FOR COMPETING GAS TECHNOLOGIES gas – spark ignition
HP DF on gas or liquid
LP DF on gas LP DF on liquid
increasing efficiency
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