Green methanol for mtu marine engines

"Green methanol has the potential to shape the energy transition in marine propulsion systems," says Denise Kurtulus. She heads up the mtu marine propulsion business at Rolls-Royce's Power Systems division and has long been convinced that methanol can be a real game changer. Why?  •

• Sustainability: Green methanol, which is produced by synthesizing CO2 and green hydrogen, can power ships in a climate-neutral way. It is true that CO2 is released when methanol is burned, regardless of whether it is green or not. However, in the case of green methanol, the CO2 has already been removed from the air during production. In addition to e-methanol, which is produced from synthesis, this also applies to bio-methanol, which can be produced   from biomass.   The carbon cycle keeps the CO2 content of the atmosphere in balance. In addition, methanol burns cleaner than conventional fossil fuels and therefore produces less sulphur oxides, nitrogen oxides and soot particles (particulate matter).  
• Tank volume: Methanol has only around 50 percent of the energy density of diesel. This means that the tank volume must be twice as large as that of conventional diesel engines. However, this is still the best alternative among the "green" fuels. The volume of ammonia is comparable to that of methanol, while that of hydrogen is considerably higher.  
• Easy storage and handling: Methanol does not need to be refrigerated like LNG, as it is liquid at ambient temperature and pressure. This simplifies storage and reduces costs for special tanks and equipment. Compared to hydrogen or LNG, methanol is easier to handle and requires less complex infrastructure.
• Safety: Methanol is biodegradable and dissolves in water, which means there is less environmental damage in the event of an accident than with fossil fuels or ammonia, for example. Methanol is also less explosive than LNG or hydrogen.  
  

Rolls-Royce engineers are already testing methanol combustion with a technology demonstrator on the test bench. They are working in parallel on concepts for single-fuel methanol engines that only burn methanol in the Otto process and on dual-fuel engines. These are compression-ignition engines that can run on both diesel and methanol. The single-fuel methanol engine is being developed as part of the publicly funded MeOHmare project.  

The development of methanol engines is therefore a challenge

"In a few months' time, we will be testing the first mtu methanol engine on the test bench," explains Dr. Johannes Kech, who heads up the development of mtu methanol engines at Rolls-Royce. He can hardly wait for the moment when the first engine is running and the developers can see how the combustion process proves itself in a full engine and interacts with the specially developed turbocharging system. Together with his team, he has had to solve a number of challenges over the past few months. And he still has just as many ahead of him.  

"Developing a methanol engine is no trivial matter," explains Kech. He is particularly concerned with two issues when it comes to a pure single-fuel methanol engine: injection and mixture formation. "There are simply no injectors for methanol engines on the market for our power class that have a long enough service life," explains Kech.    

It is also complicated to have a homogeneous fuel-air mixture in the combustion chamber that can burn cleanly. This is because the lower lubricity of methanol means that the fuel cannot be shot into the combustion chamber at 2000 bar or more pressure like diesel. However, the lower the pressure, the more difficult it is to create a homogeneous mixture in the combustion chamber. "Put simply, the finest possible droplets of methanol must be present throughout the combustion chamber so that they evaporate quickly in order to ensure a homogeneous and ignitable mixture at the spark plug," says Kech.

But both Johannes Kech and Denise Kurtulus are convinced that the effort will be worth it. After all, green methanol can not only be used to power ships in a climate-neutral way. Burning methanol in the engine also produces far fewer nitrogen oxides and particles than running engines on diesel fuel. A complex exhaust gas aftertreatment system as with diesel engines will not be necessary. Any hydrocarbons produced by unburned or incompletely burned fuel can be converted using a catalytic converter. "In shipping, there is no way around methanol," says Kurtulus.  

Methanol dual-fuel engines as a bridging technology

"In the long term, we see pure methanol single-fuel engines as the best solution," she says. At the same time, however, she recognizes the concern of many customers that green methanol will not be widely available in the medium term. For this reason, the Rolls-Royce engineers have also turned their attention to dual-fuel engines. "We see dual-fuel engines primarily as a bridging technology until green methanol is available across the board," says Kurtulus. "However, the length of this bridge varies depending on the application," says the expert. Yachts that call at ports all over the world will certainly rely on flexible dual-fuel propulsion for longer than ferry or tugboat operators who know their ports and therefore their fuel infrastructure.  

However, the development of a dual-fuel methanol engine is not easy either. Here, Kech and his colleagues are more in the comfort zone - after all, this is also about self-igniting engines in which diesel is injected into the combustion chamber.   But the aim here is to get by with as little diesel as possible. What's more, the existing diesel injectors now have to inject much smaller quantities of diesel. At the same time, the combustion chamber of the engine must not be optimized for methanol combustion, as the engine must continue to function in pure diesel mode. Compromises must therefore be made that limit the substitution rate and/or the efficiency of the engine.

There is no such thing as a "silver bullet"

However, both Denise Kurtulus and Johannes Kech are certain that the development effort is important. Both are convinced that there will not be just one single solution for environmentally friendly ship propulsion systems in the future. "There is no such 'silver bullet'," they emphasize. Instead, various solutions based on renewable energies are needed. One of these solutions will be mtu methanol engines. And they are not alone in this conviction. The Danish shipping company Maersk is already relying heavily on methanol. Not only has the company already put the first methanol-powered container ship into operation, it has also ordered over 20 more such ships. And as demand increases, so does the production of green methanol.  

"When the time comes, we will be ready"

"It will be a while before there is enough green methanol in the world to convert entire fleets of ships to methanol propulsion," says Kurtulus.   Experts assume that "But when the time comes, we will be ready," says the marine expert.