With its eye on the LNG carrier market, MAN Energy Solutions demonstrated its first two-stroke, low-pressure Otto-cycle engine, targeted for delivery in early 2022
Up until about 10 years ago, steam turbines powered by steam generated from boil-off gas (BOG) were the propulsion power of choice for LNG carriers. While steam turbines still power a large portion of the fleet, a new crop of two-stroke, slow-speed, dual-fuel engines – underpinned by reliquefaction technology – have emerged as the preferred prime movers for today’s generation LNG carriers, cutting fuel consumption, improving emissions and lowering cargo transportation costs.
In 2016, Teekay’s 174,000 m3 Creole Spirit became the first LNG carrier equipped with MAN Energy Solutions’ two-stroke, slow-speed, high-pressure, Diesel-cycle, dual-fuel MAN B&W ME-GI engine, equipped with a Burckhardt compressor and partial reliquefaction system. Fuel consumption for the M-type, electronically controlled and gas-injected engine was estimated to be 100 tonnes per day – significantly less than the 125 to 130 tonnes per day for a comparable four-stroke, dual-fuel, diesel-electric propulsion system.
In 2017, SK Shipping’s SK Audace became the first LNG carrier fitted with two-stroke, low-pressure, dual-fuel X-DF engines from Winterthur Gas & Diesel (WinGD).
Based on the lean-burn, Otto-cycle combustion principle, in which fuel and air are premixed and burned at a relatively high air-to-fuel ratio, the two-stroke X-DF engines can operate on low loads of gas and are IMO Tier III-compliant out of the box, emitting low NOx emissions and virtually no sulphur oxides or particulate matter.
With Otto-cycle, lean-burn technology gaining significant traction in the LNG carrier sector – where more than 175 vessels are on order – MAN Energy Solutions set out to develop its own low-pressure, two-stroke, dual-fuel engine offering four years ago.
Initial results of its engine R&D were released in mid-March to the press during a ceremony live-streamed from MAN Energy Solutions’ Copenhagen Research Centre. An Otto-cycle variant of the company’s ME-GI engine, the ME-GA will be well suited for LNG carriers, since the vessels use BOG that can be fed directly into the engine via a low-pressure compressor. Similar in price to MAN Energy Solutions’ two-stroke, dual-fuel Diesel-cycle ME-GI engine, the ME-GA reduces the need for certain additional surrounding equipment, lowering the capex for shipowners.
This stems from the relative high cost of the fuel gas supply system (FGSS), including the high-pressure compressor, for boosting the pressure of the BOG to the 300-bar injection pressure needed for the ME-GI – an investment only used for the LNG carrier application.
As its pre-mixed combustion results in low NOx emissions, the ME-GA engine is inherently Tier II and Tier III compliant in gas-operation mode. To fully utilise its dual-fuel potential in Tier III areas, the engine is being offered with exhaust gas recirculation (EGR).
“We initiated this ME-GA project in late 2017 when we recognised a strong market desire for a lower-cost alternative to the ME-GI engine, driven primarily by the LNG carrier market,” said MAN Energy Solutions chief sales officer Wayne Jones, OBE. “Crucially, this new supplement to our dual-fuel portfolio continues our mission to decarbonise shipping and further the maritime energy transition to sustainable fuels.”
MAN Energy Solutions senior vice president and head of two-stroke promotion and customer support, Bjarne Foldager, noted the intense effort made by the engine designer to develop its Otto-cycle engine entrant: “It’s taken a team of 150 colleagues, and hundreds of tests and operational hours, to get the engine to this important stage. In doing so, we have drawn on the invaluable experience gained from developing our successful ME-GI concept. Similarly, our proprietary EGR system also plays an important role in the ME-GA set-up. Not only does it deliver NOx-compliance, it also helps maintain control of the ignition process and reduces fuel consumption by 3 and 5% in gas and fuel-oil modes, respectively.”
“This new supplement to our dual-fuel portfolio continues our mission to decarbonise shipping”
In November 2020, MAN Energy Solutions announced that it would offer its proprietary EGR system as an emissions solution for the ME-GA. EGR is a NOx-emissions-reduction technique that ensures IMO Tier III-compliance in diesel mode for the ME-GA.
The engine designer reports that EGR will enable the ME-GA to reduce specific gas consumption by about 3%, and specific fuel-oil consumption by 5%. It will also significantly reduce methane slip by 30 to 50% and improve the stability of the Otto-cycle combustion process. EGR will enable the ME-GA to meet Tier III requirements in both fuel oil and gas modes, without additional aftertreatment.
The ME-GA EGR solution is a high-pressure system, which can be integrated into existing engineroom designs, and the EGR unit itself does not change the engine footprint. Its design-similarity to that of ME-C engines’ EGR systems will lower its price point, since the supply chain and components are already matured.
The volume requirements of the ME-GA EGR system are also significantly lower with, for example, less pipework required than for low-pressure EGR solutions.
With Tier III compliance in gas mode, the engine meets all current and upcoming NOx emission regulations with the addition of EGR.
MAN Energy Solutions aims to start testing the first commercial ME-GA design by the end of 2021, with the first engine delivery following in early 2022.
LPG dual-fuel retrofits
MAN Energy Solutions reports that its portfolio of two-stroke, dual-fuel engines has accumulated over 1.6M operating hours from the 155 engines (6.3 GW) currently in service running on alternative fuels, LNG, LPG, ethane and methanol. It expects the option of retrofitting to dual-fuel engines to increasingly become a necessity.
BW LPG is in the midst of retrofitting 15 of its very large gas carriers (VLGCs) to operate on LPG dual-fuel technology under a US$130M programme. The sixth, seventh and eighth vessels in the programme – BW Volans, BW Var and BW Balder – are currently in China’s Yiu Lian Dockyard in Shenzhen, China undergoing retrofit with dual-fuel, Diesel-cycle, LPG-burning ME-C-LGIP engines.
World LPG Association (WLPGA) technical director Nikos Xydas sees increased adoption of LPG as a marine fuel in Asia. Speaking as a panellist at Riviera Maritime Media’s Green fuels, green engines, the role of LPG and of hydrogen fuel cells: what’s right for Asia? webinar, My Xydas noted LPG as a marine fuel produces 99% less SOx, 15% less CO2, 10% less NOx and 90% less particular matter (PM) than compliant fuels and does not suffer from methane slip, as is the case of LNG-fuelled vessels.
Currently, there are four VLGCs using LPG as a marine fuel. In total, refenced Mr Xydas, there are 79 retrofit and newbuilding LPG-fuelled vessels on the orderbook.
Held in April, the webinar was supported by the Methanol Institute and the World LPG Association, and was part of Asia Maritime & Offshore Webinar Week, supported by the Hong Kong Shipowners Association and Intermanager.