Why iron could be a suitable future marine fuel
Meeting upcoming emission targets for shipping has brought about a search for new marine fuels and systems. Recently, a feasibility study was carried out investigating such a completely different and CO2-free alternative fuel: iron powder. Plans for a self-propelled demonstrator are underway.
Within the International Maritime Organization (IMO) context, the maritime sector is aiming for a fifty per cent reduction of CO2 emissions in 2050 compared to 2008. On top of that, the EU has set its own “Green Deal” zero CO2 emissions targets for shipping by 2050.
In anticipation of these climate goals, the search for CO2-free marine fuels and systems had already started. Various studies are currently being conducted which investigate alternative fuels, such as bio-(m)ethanol, ammonia and hydrogen; some of them are already being implemented.
These all have their pros and cons, reason to continue the search for a breakthrough technology. A bottleneck for all alternative fuels is the money required for research and implementation, as well as giving them a fair chance to compete with fossil fuels. The first steps to meet both these requirements are taken, such as the recent proposal from global ship owners associations to IMO to form an international innovation financing fund, which is fed by a surcharge on the fossil fuel price.
Ship owner associations propose an innovation financing fund, fed by a surcharge on the fossil fuel price
Iron as a CO2-free fuel
Like some other metals, iron can burn and produce heat. The iron powder used for this purpose has a grain size comparable to the thickness of a hair. Eindhoven University of Technology has been researching “metal fuels” as a circular energy source for some time and sees iron as a promising option for three areas of application: power plants, the process industry and shipping.
It is an abundant element in the earth’s crust and has a relatively high volumetric energy density. The contained energy can be released rapidly by burning the iron powder, this without producing any CO2 and hardly any NOX and Particulate Matter (PM) emissions. Thus, iron powder meets the requirements for a future marine fuel.
Capturing and reducing rust with green hydrogen can create a CO2-free cycle
By capturing and reducing the resulting iron oxide (better known as rust) with green hydrogen, a complete CO2-free cycle can be created (as illustrated with the figure below). A major problem with renewable energy sources such as sun and wind is the fluctuation in supply and a non-matching demand. These can now be solved by using a surplus of energy to recycle the iron oxide powder and reduce it to metallic iron powder, thereby completing the cycle.
Renewable metal power (courtesy of McGill University).
In addition to being CO2-free and circular, the use of iron powder is a practical and relatively safe option for both storage and transport. Iron offers to be an energy carrier that can work in applications with high power demands and conversion speeds, properties that are desired in the shipping sector.
High specific mass
A disadvantage of iron as a fuel for ships is the relatively high specific mass of iron powder and the increase in the weight of the iron oxide that is produced during combustion. As a result, a ship will lie deeper and deeper during the voyage.
After the iron oxide is reduced, the recycled iron powder retains the same volume as the iron oxide powder from which it was formed. This is because the iron oxide recycling process leaves pores where the iron-bound oxygen used to be. In the study on which this article is based, only an initial estimate was made of the technical and economic feasibility. To this end, an inventory was made of all questions that arose during an analysis of the required systems on board and ashore.
Prototype ship in 2030
In order to find out whether iron offers a realistic CO2-free alternative, we have delved deeper into the technical and economic feasibility. Although the use of iron as a fuel for ships was concluded as technically feasible, additional measures are also needed to make it truly feasible, that is, not only technically, but also economically and operationally.
To this end, various recommendations were made in consultation with all consortium parties, starting with technological follow-up research. Based on this, a step-by-step plan was drawn up that should ultimately lead to the order of a prototype ship that uses iron as a fuel in 2030.
A step-by-step plan should lead to a prototype ship that uses iron as a fuel in 2030
Development of most of the required systems for iron as a marine fuel can initially be based on known technologies. For transport of the iron and captured iron-oxides, well known technologies are highly suitable, such as two-phase pneumatic transport. Although the storage of iron powder proves to be possible ashore under atmospheric conditions, it is still unknown what the effect of the salt and humid environment at sea is.
In order to make iron fuel a circular fuel, and to limit the amount of iron oxide particles emitted to the atmosphere, a minimum of 99.99 per cent must be collected. Existing filter technologies can be used for this.
NOX practically absent
In order to convert the combustion heat to mechanical power, a heat engine is required. A well-known heat engine is the (Rankine) steam engine, using a boiler, a steam turbine and a condenser. The boiler design will have to be adjusted compared to fossil fuel boilers, to include the capture of the heavy oxide particles. This can be compared with the techniques that are currently used in coal-fired boilers, but with the necessary adjustments.
Although combustion of fossil fuels is accompanied by the production of NOX, the production of NOX for iron combustion is practically absent. At Eindhoven University of Technology, theoretical research is being conducted into the formation mechanisms of NOX during the burning of iron. This shows that although NOX is formed, its formation under practical conditions is nearly zero. Measurements on laboratory and also larger setups confirm this conclusion.
Although NOX is formed, its formation under practical conditions is nearly zero
Burning iron to produce heat
In order to convert heat into rotational energy, intermediate steps are required, whereby steam first came into the picture as the old familiar. However, since the 1980s, steam has hardly been used for propulsion of ships, with the exception of nuclear naval vessels. In the meantime, technology did not stand still, such as in the field of materials. This meant that everything was discussed again in this study and that innovative concepts such as high pressure supercritical steam and supercritical CO2 cycles were also mapped to be developed for maritime applications.
Burning iron along with the production of steam is already demonstrated on laboratory scale – see the picture at the top – making the Technological Readiness Level (TRL) in the order of 2-3. However, a 100-kW installation – see the picture below – is being developed by the Metal Power consortium (consisting of TU Eindhoven, SOLID, Enpuls, Uniper, Nyrstar, EMGroup, HeatPower, Romico Engineering Solutions and Metalot and subsidised by the Province of Noord-Brabant). This will soon bring the TRL to 4-5, as this demonstrator will produce steam for the Bavaria brewery process of Royal Swinkels Family Brewers.
100-kW experimental iron to steam setup.
For shipping, the application of this technology is still in the conceptual phase, however SOLID is developing plans to use this 100-kW system for a self-propelled demonstrator.
Back to steam?
A solution to convert the heat from the burnt iron to shaft power is one that several maritime engineers may have or have had experience with in the past; the Rankine steam cycle. This technology is still widely used, for example in large-scale nuclear and coal-fired power stations. However, on smaller scales, such as for shipping, it cannot compete with the compactness and hardly with the efficiency of the diesel engine.
This famous Rankine cycle, currently operating at pressures of up to 100 bar, can achieve efficiency of up to 44 per cent, including a boiler efficiency of ninety per cent. This creates a challenge on the path of developing iron as a fuel.
To investigate the feasibility of iron as a marine fuel, from both a shipbuilding and economic point of view, the 176-metre long 1700 TEU container vessel Rijnborg of Royal Wagenborg was chosen as a benchmark ship (built by Royal IHC and delivered in 2007). Based on the same performance, such as sailing speed and distance, the relatively high mass of the iron powder, and in particular of the captured iron oxide powder, appears to play a crucial role in the resulting load capacity to deadweight ratio of the ship. This can be illustrated by the figure below.
Energy density for various carbon-based and carbon-free fuels.
It shows the packaging-corrected energy density versus the specific energy for different types of fuels, such as fossil, NH3, methanol, hydrogen, batteries and iron. Pure iron appears on the far upper side of the graph, it contains a large amount of energy per volume. Yet, as explained earlier, we are dealing with iron oxide reduced to iron powder and not with pure iron.
In comparison with fossil fuels, such as marine diesel oil (MDO) and also methane, iron powder contains less energy, both per kilogramme and per m3. However, the differences are limited compared to other alternative fuels. Due to this lower energy density, just as with other alternative fuels, this must be taken into account in the ship design and the operational profile to be chosen.
An estimate of the effect of the energy properties of iron powder relative to heavy fuel oil (HFO) was made for the Rijnborg, which revealed that it would have to be extended by eighty to ninety metres (fifty per cent longer). Given these numbers, the following conclusions were drawn:
- Sailing profiles must be critically examined to determine an optimal bunker size; bunker intervals will have to be adjusted accordingly.
- For the time being, the technology seems to be the most suitable for ships with a high deadweight to displacement ratio, such as inland vessels and bulk carriers.
- The required bunker quantity can be limited by optimising the efficiency of the entire system. For this, both the boiler and the power cycle must be designed with the emphasis on high efficiency.
- In addition to improving efficiency, compactness of the boiler and energy conversion system must also be as high as possible.
Supercritical CO2 as a promising alternative for steam
With regard to the last two conclusions in particular, further research into alternative power cycles opposed to steam has been carried out. Supercritical steam and supercritical CO2 (sCO2) have emerged as major contenders for use in shipping.
sCO2 in particular has emerged as a process with great potential for use in shipping. The system could be up to ten times as compact and up to a few per cent more efficient than a traditional steam cycle. But if you compare sCO2 with supercritical steam at 300 bar, the latter might just win in terms of efficiency. This technique is still at an early stage of development and has therefore not been included in more detail in this feasibility study.
In any case, it is recommended to conduct further research into this. If this technology reaches the technical implementation stage, it is expected that the position of iron as a fuel will be considerably strengthened.
In the MIIP project “Iron as fuel for ships”, the economic feasibility of a ship powered by iron as a fuel was also investigated. A case study conducted on the basis of three ships of increasing size – 67, 176 and 399 metres, including the aforementioned Rijnborg – shows that the price of iron powder is dominant in the operational costs. And, as expected, the bigger the ship, the more interesting the case.
In the feasibility study, the costs of iron powder have been split into investment and operational costs, comparable to buying and recharging batteries. The costs of recycling (re-charging) iron oxide powder to iron powder with hydrogen are largely determined by the hydrogen price. It is expected that the global market for iron powder will gradually increase the implementation of iron as a fuel, closely followed by the infrastructure for recycling.
As a result, it can also be expected that the costs thereof will fall considerably. The case studies and in particular the analysis of the payback time of the iron-fed ship energy systems also confirm that, with the current transport prices of containers, both the capital and operational costs can be recovered.
Certainly, when a policy for a level playing field vis-à-vis fossils that is deemed necessary for that purpose is realised at a global level, iron powder as a fuel must not be missing from the range of future-proof fuels.
Finally, a road map analysis has shown that a ten-year period for ordering an iron-fuelled prototype ship, while challenging, can be considered feasible. However, much of the required technology, with some bandwidth, is still at a relatively low TRL level.
To finance the necessary research, support from industry is at least as important as writing high-quality research proposals. This support can first be found by publishing the results of this research on a large scale. But publication alone is not enough: seeing is believing! SOLID has therefore taken the first steps for a project with a sailing demonstrator.
Publication alone is not enough: seeing is believing!
Fair and equal level playing field
To encourage and support the implementation of innovative and promising fuels such as iron, we strongly recommend that not only ship owners associations, but also global governments take measures to make the use of fossil fuels less attractive. In our view, expanding the research budgets aimed at the development of alternative fuels requires top priority.
Finally, the research team concluded that achieving CO2- and NOX-free emission targets by shipping simply requires a paradigm change, which means that to save the world from the point of view of climate change, nothing can remain the same as before!
This article was published in SWZ|Maritime’s February 2020 issue. The authors of the article are: