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What will be in tomorrow’s tanks?

2/26/2019 | Words: Lucie Maluck | Pictures: Shutterstock

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Hydrogen, methane, methanol, DME, OME, synthetic diesel – when it comes to combustible fuels we are spoilt for choice. But which of them offers the best prospects for the future? R&D specialists at MTU are currently working on a range of projects to establish which fuels are most economical and efficient and what the best engines for them will be.

The latest report from the Intergovernmental Panel on Climate Change (IPCC) reads like a final warning: This late in the day, climate targets agreed upon internationally are going to prove very difficult to achieve and they will only be achievable at all if we act with unswerving determination. And those determined efforts must include the decision to say goodbye to fossil fuels like coal, oil and natural gas because burning these resources has been a major factor in the increase of greenhouse gases in our atmosphere.

A future fueled by hydrogen and methane? 
The future lies with synthetically produced fuels that offer clean and climate-neutral combustion overall. One such fuel is hydrogen that can be produced from renewable energy sources using electricity. It can either be used directly or synthesized into methane using carbon dioxide. The big advantage of hydrogen and of synthetic methane is that they produce no, or at least significantly less, polluting emissions during combustion. In addition, they can also be produced using electricity generated by wind power and photovoltaic installations. However, if not used immediately, this power has so far proved difficult to store. This explains why, on especially sunny or windy days, there is often more power available than consumers need. An alternative way of storing that energy is to use it to produce fuel – that means transforming electrical energy into energy for powering engines.

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Electric power generated from renewable sources of energy is available in excess when there is plenty of sunshine or plenty of wind but this superfluous energy cannot be directly utilized. One possibility for storing it involves the electrolysis of water to generate hydrogen. Only oxygen is generated in addition to hydrogen. The hydrogen generated can either be used directly or CO2 can be introduced in a process of synthesis to produce gaseous fuels such as methane (power-to-gas) or liquid fuels such as methanol, DME or OME (power-to-liquid).

Methanol: An alternative to LNG
Engine specialists have taken this line of thought a step further because methanol can be derived from methane and, as a liquid fuel, it is much easier to store. The benefits of methanol are especially obvious for marine applications. “Until now, LNG (liquefied natural gas) has been seen as a possible future fuel for ships. But LNG can only be transported and stored in high-pressure tanks or at temperatures of minus 164°. It is difficult to maintain such temperatures over longer periods of time,” explained Dr Peter Riegger, MTU Director Research & Technology. Methanol could provide an alternative because, unlike LNG, it does not require complex infrastructural storage facilities and can therefore be integrated much more easily in marine vessels. 

But methanol is not the ultimate step in the ongoing fuel development process. Methanol can be used to produce diesel alternatives such as DME and OME. These are synthetic fuels that could also be used in slightly modified diesel engines. In this context, Fischer-Tropsch synthesis processes can also be used to synthetically produce diesel fuel that fully conforms to standards.

The main question currently facing MTU specialists is which fuel is most likely to prove most economic and most energy-efficient in the future. “If we use hydrogen to produce methane or methanol, then we lose energy in the process,” said Riegger. “Despite that, methanol could still prove to be the fuel of the future, particularly in marine applications because it is relatively simple to store and handle,” he added. However, the situation looks rather different when it comes to stationary engines for generating electricity. Here, infrastructure is by no means as important because existing natural gas grids can be extended. Consequently, for this scenario, hydrogen presents a more promising alternative.

“I believe that we will rely on a range of different fuels in future. Sole reliance on a single fuel is not a likely option,” said Riegger. 

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Renewable electric power generated from solar or wind power can be used to generate hydrogen (H2). In subsequent stages, this can be further processed to produce, for example, methane (CH4), methanol (MeOH), alternative fuels such as DME/OME (dimethyl ether / polyoxymethylene dimethyl ether) or diesel-type fuels (P-to-L). As the process chain advances, energy is released in the form of usable thermal energy that is then no longer available for drive purposes. Compatibility with diesel / gas engines increases as the process stages advance. In the initial stages of the process chain, fuel cells represent an alternative to combustion engines.

Renewable methane for marine applications
Headed jointly by MTU and the DVGW (German Technical and Scientific Association for Gas & Water) Research Unit at the Engler-Bunte Institute of the Karlsruhe Institute for Technology, the ‘MethQuest Project’ currently involves 27 partners who are working on methods of generating fuels from renewable energy sources for use in engines. One area of their activities involves MTU researchers looking at two new engine concepts for marine applications. “We are working on two different projects. One focuses on an Otto engine concept and the other involves a flexible-fuel engine concept with direct injection,” explained project leader, Dr Manuel Boog. 

Otto engines have been around for a long time because it is easy to burn gas in Otto engines. The problem with this concept is that the gas is never entirely combusted and uncombusted methane can escape. The phenomenon is called ‘methane slip’. “Methane is more damaging to the atmosphere than CO2. Consequently, the potential of gas engines to achieve substantial reductions in greenhouse gases is not exploited,” said Boog. One of the aims of the MethQuest Project is to develop a ‘methane oxicat’ (catalytic converter) to neutralize the negative effects of methane. The problem is that methane requires high temperatures for oxidation in the exhaust gas tract. In the engine, such temperatures are only present upstream of the turbocharger turbine. The catalytic converter therefore needs to be located here. However, this has a seriously negative influence on engine dynamics and MTU engineers are therefore working on an electrically assisted turbocharging concept that will counteract these disadvantages. 

Gas engine with no methane slip
The second gas-engine concept under investigation for marine applications, the flexible-fuel, direct-injection concept, involves the development of a completely new combustion process. “Here, just like in a diesel engine, the air is first compressed in the combustion chamber. The main source of energy, gas, is then introduced and a small quantity of diesel is injected at the same time to ignite the gas,” explained Boog. The advantage of this process is that the gas is almost entirely combusted and the unwanted occurrence of methane slip remains negligible. “We have already demonstrated that this combustion process works in a different, publicly sponsored project called ‘FlexDi’,” added Boog. The concept has the additional benefit that the combustion process involved also means that methanol can be used to power engines without complications. One of the challenges remaining to be solved here is the development of a suitable high-pressure gas system as the injection concept means the gas needs to be highly compressed and heated for injection.

“We will run trials with both concepts and the pressurized gas supply system and will then decide which is the most promising for further development in the context of the drive system overall,” said Boog. Both concepts aim to produce engines that deliver comparable power and performance to diesels but with a significant reduction in environmentally negative emissions.

Hydrogen and the combustion engine
In the context of the ‘MethPower’ research project, MTU is also working on the development of engine concepts for stationary gas engines. “We want to establish which engine will allow us to generate electricity most efficiently,” said Project Leader Dr Michael Thoma. This project also involves the development of two engine concepts that will ultimately be compared with each other. One is a hydrogen-powered engine. “Hydrogen can be produced from superfluous electric power by electrolysis. It therefore makes sense to use it in our engines,” explained Thoma. Just as with the natural gas engine, the MethPower Project employs the combustion of hydrogen using the Otto process. A spark plug is used to ignite the hydrogen/air mixture. However, hydrogen burns much faster than natural gas. “That is a challenge we will deal with over the coming months,” declared Thoma. 

Power-to-gas
Together with its other aims, the MethPower Project is working on the continued development of an engine that will run on natural gas and/or methane. “The MTU portfolio has included these engines for quite some time. What we are now seeking is to incorporate the engine in a system based on fuel generation and exhaust gas utilization,” said Thoma. The CO2 generated during combustion could be extracted directly from the engine exhaust and this CO2 could be used to produce new methane. “The idea is particularly interesting in the context of microgrids,” said Thoma. In this configuration, the gas engine is integrated in a network of power generators and storage devices. The network could be extended by including a ‘power-to-gas’ installation that would reduce the battery requirement. “The power-to-gas installation would work in conjunction with a gas storage vessel, like a large battery. It would use renewable electrical energy to produce methane that could be converted back to electricity if required whenever no sun or no wind is available,” explained Thoma. 

At the end of the process, R&D specialists will have to decide which concept will provide the best balance between energy generation and energy consumption and thus deliver the greatest efficiency.

Solutions for the future
Synthetic fuels and new engine concepts all have one thing in common – they all make a major contribution to minimizing CO2 emissions and promoting moves toward the more responsible use of energy. “That is our aim and our duty as a responsible company. We aim to develop drive solutions for the future and we commit to playing an active role in that process,” said Dr Peter Riegger in summary.

The content of the stories reflects the status as of the respective date of publication. They are not updated. Further developments are therefore not taken into account.

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