Special Focus: Hydrogen Mobility and Transportation

R. LAURSEN, ABS, Copenhagen, Denmark

Hydrogen (H₂) is seen as a viable long-term fuel solution in the maritime industry, but it will require significant research and development to reach a comparable level of technological readiness. Methane (CH₄) and methanol (CH₃OH), for example, are proven marine fuels and many ships have been using them for years. However, these fuels also contain carbon in its molecular form. H₂ can serve as a cleaner option in applications where storage space is not a hinderance and where it is possible to store H₂ in pressurized bottles.

This article will focus on the use of methanol and liquefied natural gas (LNG) as a means to store H₂ onboard, and the use of new developments in pre-combustion techniques to crack the fuel into carbon and H₂. The H₂ can then be fed directly into either a combustion engine, a fuel cell or perhaps a combination of both. Some engine makers are also exploring the idea of mixing green H₂—H₂ produced using renewable electricity—with a fossil fuel stream to enable existing vessels to achieve partial decarbonization.  

Storing H₂. In long-distance shipping, H₂ would likely need to be stored in a liquid form to give the ships enough range for global trade. However, the volume of H₂ takes up 4.5 times more space than marine diesel fuel, which makes it difficult to build a business case because the H₂ fuel storage would lead to a significant loss of cargo capacity (FIG. 1).

 

H₂ can also be stored within other materials, such as metal hydrides. This storage method binds H₂ to metal alloys in porous and loose form by applying moderate pressure and heat. Subsequently, H₂ is extracted by removing the pressure and heat. Most methods of material-based H₂ storage require a steady supply of metallic or complex organic compounds, which can be difficult or expensive to achieve onboard the ships of a long-distance shipping merchant fleet.  

Fuels such as marine gasoil (MGO), LNG and methanol have more favorable storage conditions than H₂, with storage technology that is well-proven. One way around the storage issue is to store the H₂ in either methanol or in LNG onboard ships where the H₂ can then be reformed, or cracked, into carbon and H₂.

The makeup of H₂. A detailed look at H₂ reveals several factors to consider. H₂ has a high energy content per weight ratio, 40 MJ/kg vs. 120 MJ/kg which is three times that of gasoline. At the same time, it has a very low energy density per volume. Combining those factors, a volume onboard is required that is 4.5 times greater than diesel fuel—additionally, the H₂ must be kept cold (–253°C). To maintain H₂ at that temperature requires the H₂ to be stored in spherical tanks to minimize the heat ingress. Additionally, thick insulation is needed to further lower the heat ingress. 

H₂ stored in these tanks will be boiling, and vapor is generated. Any vented vapor is lost fuel. Further, H₂ is also lately being regarded as an indirect greenhouse gas (GHG) that is up to 11 times more potent than carbon dioxide (CO₂). So, the tanks must be equipped with a reliquefication system to reliquefy the H₂ vapor. The complexity and energy consumption of equipment operating at –253°C adds significantly to the cost and operational cost of the ship. 

The spherical tanks, the insulation and the reliquefication system take up space, adding to the higher volume challenge. So, in reality, H₂ takes up something like 6–8 times more space than fuel oil. 

If the chemical properties of H₂ are also examined, it is a small molecule that easily escapes if there is a leak. In fact, it can penetrate even a helium-tested, gastight system over time. So, designing the containment system is another challenge.  

On the upside, if it escapes, H₂ is very buoyant and diffuses very quickly. On the downside, H₂ is highly flammable with a wide flammability range. When mixed with air, ignition can occur with as little as 4% H₂ to as much as 75% H₂—for comparison, the flammability range for methane is between 5% and 15%. 

Generating power with H₂. Since H₂ can generate power from either internal combustion or the oxidation process that takes place in a fuel cell, there are several good reasons to support using H₂ directly in an internal combustion engine (ICE) or in fuel cells.  

The energy to ignite H₂ is very low and the heat released when burning H₂ is very high compared to other fuels. So, combustion efficiency can become very high when an engine (or other power converter) is fully optimized. The resulting exhaust does not contain sulfur oxides (SOx), CO₂ or carbon monoxide (CO), and the particulate matter (PM) in the exhaust will be significantly reduced. When H₂ is used in a fuel cell, the nitrogen oxides (NOx) emissions and PM are also eliminated. 

H₂ burned in combustion engines may develop NOx, depending on the combustion principles, well-known measures can remove NOx from combustion exhaust, such as selective catalytic reduction (SCR) and exhaust gas recirculation (EGR) technologies.  

Fuel cell technology is also becoming more mature, the cost is being reduced and reliability is constantly improving. Within the near future, the industry should expect to see fuel cells being ordered and used in ships—first for electricity production, replacing onboard ICE generator capacity or in a hybrid setup, and secondly to generate propulsion power in some ship applications. Several small, short-distance ships have piloted the technology, but fuel cells are not anticipated to replace ICEs in the long-distance shipping fleet in the near term.  

Reforming H₂ onboard vessels. Options for onboard reforming or cracking the different fuels into H₂ fuel are presented in the following section—three different examples of industry reformer options will be explained, each of which has its own benefits. 

The first example is developed by the company Helbio, which is offering a heat integrated wall reactor that is available for either LNG or liquified petroleum gas (LPG), and cracks these chemicals into H₂ and CO₂. For both, the fuel must be mixed with steam and fed to the reforming reactor that produces most of the H₂. In the reactor, a large portion of the H₂ comes from the steamed water, which means that water acts as a fuel in this process. 

The heat required for the process is produced by catalytic combustion using a fraction of the fuel, a process that eliminates any open flames and hot spots in the reactor. Some CO is also generated, which goes into the reformer outlet. The CO is then converted in the water-gas-shift (WGS) reactors, where CO reacts with water to produce additional H₂ and CO₂. In the end, the H₂ goes through a pressure swing absorption (PSA) unit that purifies the H₂ to the desired level. 

The CO₂ emissions can be directly emitted into the atmosphere; however, if the CO₂ emissions must be collected in a tank system, to minimize the storage space requirement, the CO₂ will need to be turned into liquid CO₂ using an onboard liquefaction system. This can present spacing issues: for (roughly) each ton (t) of LNG being reformed, 3 t of CO₂ is generated and space must be found onboard for the CO₂ tank and liquefaction system.  

The second example is the methanol reformer–membrane purification method from e1-marine. This process begins by mixing water with methanol to create the blend required for the process. The blend is then pumped into a heat exchanger to cool the product (H₂), preheat the blend and obtain optimal thermal efficiency for the generator. 

The pre-heated blend then flows into the reactor, also called the “hot box,” where it is converted into a vapor and then directed into a catalytic reactor to convert the blend into a synthetic gas. The final step is H₂ purification using a membrane purifier. Also, for this process, CO₂ must be either emitted or stored onboard in CO₂ tanks.  

The third example is an option from Rotoboost that uses a liquid catalyst to convert LNG (methane) into H₂ and solid carbon. The conversion of methane into H₂ gas and solid carbon is conducted via thermo-catalytic decomposition (TCD) process, also referred to as methane pyrolysis with catalyst. In this process, the methane molecule is cracked with the help of heat. The H₂ is released as H₂ gas and carbon in solid form. This reaction is possible to achieve with heat energy alone, but the introduction of a catalyst lowers the required temperature significantly, making the process less energy consuming.  

The heat can be produced via different methods, including:  

• Combustion of a small side stream of the feedstock gas  
• Combustion of a small fraction of the produced H₂ gas. 

Naturally, the combustion of vaporized LNG is the most feasible and energy-efficient solution onboard marine vessels with the technology now available. Looking forward, when CO₂ emissions limits will be even more stringent, it will be possible to start using a fraction of the produced H₂ for process heating.  

The catalyst forms a vital part of the process due to its impact on fuel consumption and the economics. The molten catalyst is molten metal alloy that is heated to a reaction temperature of 600°C–800°C.  

When the methane molecule is split into H₂ and carbon, the H₂ gas will continue its flow forward or upwards in the molten media. In a conventional dry methane conversion process, the remaining solid carbon would normally attach itself to the surface of the catalyst and eventually fully cover the catalyst surface area, blocking it from functioning. When the catalyst is liquid metal, it has no surface to which carbon can attach. The carbon particles remain as solid particles flowing in liquid metal. The density difference between molten metal and carbon particles makes the carbon particles float up on top of the molten media.  

This process produces solid carbon that occupies only 1/6 of the space compared to liquid CO₂. Prismatic tanks can be used, which are more volume efficient than bullet-shaped type C tanks needed for liquid CO₂.  

In the future, the solid carbon is expected to be sold for the production of graphite, a valuable market commodity that is mostly needed now in the manufacture of different steel grades, batteries and fuel cells.  

One of the risks with the storage of solid carbon is that if dust is dispersed, it can develop into an explosible dust-air mixture. It is therefore vital that storage systems are kept tightly closed—controlling the humidity can be a means to mitigate the dust-air mixture. It is also important that the carbon is kept away from open flames, hot surfaces and other sources of ignition, to take precautionary measures against static discharge. 

Takeaway. Reforming feedstock fuel requires both heat for the process to run and the cracking of the fuel molecule into H₂ and carbon. This reduces the effective energy output from the base fuel that is stored onboard; however, this is the price that must be paid for the removal of the carbon. To find the most feasible solution, a thorough study must be conducted that considers the lost cargo effect from the storage of liquid H₂, compared with feedstock fuel that is going to be reformed, the space and equipment for storage of the carbon, while considering the possible income from selling solid carbon.H₂T

About the author

RENÉ LAURSEN serves as a Director of Sustainability for ABS, leading the ABS Copenhagen Sustainability Center. In his current role, he supports ship owners in selecting the best technologies and fuel mixes for their fleet.