Blowdown a new target for methane slip reductions

While engine makers make progress on methane slip from combustion chambers, MAN Energy Solutions is additionally targeting the blowdown of recirculated fuel when gas operation is stopped. Fuel supply system blowdown is the venting of pressurised methane from the engine and piping downstream of the gas valve train directly to the atmosphere. It is a routine procedure conducted before gas purging with nitrogen.

In a white paper titled Managing methane slip on ME-GI installations published earlier this year, the company stated it is reintroducing its already developed system which was introduced with its first design iteration of the ME-GI engine technology but which at the time never saw adequate market interest. As more focus has now come on emissions reductions, vessel owners and operators have started focusing on downstream emissions as well, posting the required demand for MAN ES to re-introduce their system in an updated version.

The gas return system has reported the capability to capture up to 95% of the gas emitted during blowdown. The gas can then be stored in a buffer tank and supplied to downstream consumers such as boilers or auxiliary engines rather than being emitted from the vent mast. The system will be offered as an optional feature for new ME-GI engines.

Currently, MAN guarantees a methane slip in the range of 0.2-0.28 g/kWh for its ME-GI engines across loads of 25-100%. Research quantifying emissions continues with the white paper providing results from two different engine sizes: 10G95ME-C10.5-GI (single engine on a container ship) and the smaller 5G70ME-C10.5-GI (twin engines on an LNG carrier).

Data was obtained from CEAS engine data reports and real engine operating profiles. Of the total exhaust emission profile, both engines displayed a methane content of 6-7%, calculated by Global Warming Potential. Looking at the overall vessel emissions, including both exhaust emissions and what is ventilated from the supply system, the dual engines LNG carrier, displayed a methane content of approximately 17%. For the container vessel, it was 11%.

MAN notes that the ME-GI provides owners with specific fuel oil consumption 7% less than that of a low-pressure two-stroke engine and 32% less than a four-stroke genset.

ME-GI engine exhaust methane slip is only 16% of that from a low-pressure two-stroke engine (ME-GA) and only 6% of that of a four-stroke. Even with these merits research continues to reduce total methane slip. As engine exhaust values are already negligible, MAN has, with the latest technological offer, turned its sight towards the supply systems and vessel integration for further reductions. For the ME-GA engine, high-pressure exhaust gas recirculation is the technology of choice, and oxidation catalysts such as IMOKAT II are the choice for four-strokes, along with optimized design features such as cylinder skip firing and reduction in crevice volumes.

Low-emission engines
Meanwhile, Wärtsilä recently introduced a new ultra-low emissions version of its Wärtsilä 31DF engine which can further reduce methane emissions on a 50% load point by up to 56%. On a weighted average, this new technology can reduce methane emissions by 41% more than the standard Wärtsilä 31DF engine.

The new version, which is applied on one of the four engines on board Wasaline’s Aurora Botnia ferry, has already helped the Finnish-Swedish ferry operator further reduce the Aurora Botnia’s methane emissions by 10%. As part of the EU co-funded Green Ray and SeaTech projects, Wärtsilä piloted the ultra-low emissions concept onboard the Aurora Botnia with results verified through an independent study conducted in December 2022 by VTT, the Technical Research Centre of Finland.

Researchers from VTT studied methane slip from two of the Wärtsilä 31DF engines onboard – one in standard configuration and the other piloting the new combustion concept. These medium-speed 4-stroke engines have 8 cylinders, with a power of 550 kW per cylinder. Both engines were studied under five engine load conditions while the vessel operated on its normal route between Vaasa, Finland and Umeå, Sweden.

The study showed that overall methane emissions were lower than what has been reported by previous onboard studies with similar-sized low-pressure dual-fuel engines. Both engines emitted less methane at higher engine loads, especially compared to the lowest engine load of 10%. At engine loads of 50–90%, the new combustion engine produced 50–65% less methane compared to the standard engine, and at the lower loads (with higher absolute methane levels), the difference between the engines was even higher. At 10% load, the engine with the new combustion concept produced methane emission below 4 g/kWh compared to the over 12 g/kWh of the standard engine. At higher engine loads, the emissions from the new combustion concept were below 2 g/kWh.

Results also indicated that the upgraded engine technology had great potential to reduce overall emissions (including both CO2 and methane). Following the positive results, Wärtsilä launched the new ultra-low emissions version of the Wärtsilä 31DF engine to the commercial market last year.

The new engine technologies being developed will help shipowners futureproof their vessels against potentially tightening global requirements. From 2025, the FuelEU Maritime regulation will require ships calling EU ports to gradually reduce their well-to-wake greenhouse gas (GHG) intensity, including methane slip. The IMO is expected to introduce a goal-based marine fuel standard regulating the phased reduction of the marine fuel GHG intensity, also including methane slip, from 2027.

VCR narrows the gap
In the comparison between low-pressure and high-pressure dual-fuel engines, two things stand in favour of high-pressure, Diesel-cycle concepts: better performance in diesel mode and lower methane slip when using LNG. A new technology from WinGD aims to both close that gap and improve the case further for low-pressure engines, and there are early signs that some operators are seeing the value in variable compression ratio (VCR) technology.

VCR is the result of a decade-long development project between WinGD and Mitsui E&S DU Co, formerly Diesel United. It enables an engine’s compression ratio to be adapted for the first time on marine engines, optimising combustion depending on fuel type, ambient conditions, engine load and other parameters. In early tests, WinGD was encouraged by efficiency improvements in Diesel mode, but in subsequent tests, improvements in both gas efficiency and methane slip abatement have been noted.

In the most recent tests, using VCR reduced fuel consumption and CO2 emissions by 7.7% when running on diesel. In LNG mode, overall GHG emissions (including CO2 and methane) were reduced by 4.6% compared to a gas consumption reduction of 3.1%, highlighting the outsized impact VCR has in reducing methane slip, beyond the emissions saved by lower fuel use.

WinGD recently reported that it has received around 40 orders for VCR technology since it was introduced in June last year. Among the orders are engines for nearly 20 LNG carriers – the biggest vessel segment for low-pressure dual-fuel engines – but also pure car and truck carriers and bulk carriers. Given the promising test results, WinGD believes that interest could extend into the container segment as well.

According to WinGD GM Application & Technical Sales, Marcel Ott, the orders indicate demand from shipowners that will only rise as methane slip is costed into ship operation through carbon pricing, and while fuel supply flexibility necessitates peak efficiency on both LNG and diesel fuels. With several LNG carrier newbuilds expected to be confirmed in the coming months, Ott is optimistic there will be further orders for VCR to announce shortly.

There are further factors in VCR’s favour that may make it a popular addition to X-DF engines. Efficiency benefits while using LNG are highest at part-load operation, WinGD claims, making it well-suited to modern operating profiles such as slow steaming, providing the largest benefits at engine loads used most often. And it retains both the capex and opex competitiveness of low-pressure engines: the hydraulic solution has no impact on engine footprint or installation requirements and does not need additional maintenance between overhauls.

WinGD claims that its own evaluation of system-wide energy consumption and emissions show the benefits of deploying X-DF engines with VCR technology. On a 174,000 cbm LNG carrier with a typical operating profile, a configuration including two 5X72DF-2.2 engines with VCR outperformed not only conventional low-pressure engine systems but also high-pressure engine arrangements in terms of LNG consumption, air pollution, greenhouse gas emissions, electrical power demand for auxiliary machinery and fuel costs.

But Ott believes VCR can go further than improving the business case for low-pressure engines in the LNG carrier segment. He says: “We’ve seen on the testbed that VCR brings diesel efficiency in line with our own high-pressure single-fuel diesel X-engines. Diesel efficiency is important for operators that want full flexibility to choose between fuels, rather than using the cargo they are carrying, and we believe that VCR could tip the business case even in segments that might traditionally have opted for high-pressure dual-fuel engines.”