Sustainable power generation is arguably the greatest challenge facing the shipping industry today. Ever-increasing fuel costs and rising environmental demands during a period of global economic uncertainty threaten the very existence of all but the most efficient options within the industry. While conventional residual and distillate fuel oils can be expected to continue as the dominant fuels for the world’s merchant fleet for the foreseeable future, the rising cost of bunkering and, more recently combustion, makes the consideration of alternative fuels for marine power generation crucial.
Lloyd’s Register is actively engaged with industry in evaluating a range of potential or alternative marine fuels so that industry may rise to and succeed in achieving a sustainable future, not least in marine power generation.
Natural gas
There has been widespread recent interest in the use of natural gas as a marine fuel,particularly when stored cryogenically as liquefied natural gas (LNG). In terms of emissions, the relative advantages associated with the clean combustion of natural gas compared to conventional marine fuel oils are by now well known, including the total elimination of SOx emissions, the almost total reduction of particulate matter and between 80–90% less NOx emissions. Reductions of around 20% in CO2 emissions are also realised on board. However, although such reductions may well represent a saving for the marine industry, it may not be evident when considering the impact of LNG production, storage, delivery and combustion as a whole.
While the use of natural gas is well established as a fuel for highly reliable multi-megawatt shore power generating stations, its use as a marine fuel is not without challenges.
The relatively low volumetric energy density when liquefied at a temperature of –162 °C compared to conventional marine fuel oils means that the storage volume required for the cryogenic tanks, typically ‘Type C’ tanks, will be somewhere between 2.5 to three times greater than the volume required for storage of marine diesel oil and even greater for compressed natural gas (CNG). The introduction of prismatic cryogenic storage tanks should see that storage volume fall to approximately twice the volume required for marine diesel oil. A further challenge for the industry is the lack of worldwide availability of LNG at the dockside. If LNG is to offer an alternative to conventional marine fuel oils, for ships trading internationally, worldwide availability of LNG of appropriate and consistent quality will be required. Although the future of LNG as an alternative fuel for ships trading internationally remains to be seen, there is no doubt its suitability for ships operating frequently in Emission Control Areas (ECAs) such as short sea shipping, inshore and inland shipping with frequent bunkering opportunities at one, or a limited number, of established bunkering facilities. Lloyd’s Register is currently engaged in a number of LNG-related projects including the ropax ferry Viking Grace and the inland waterway chemical tanker Argonon. While clearly not a sustainable fuel in the truest sense of the word, with global oil reserves in decline, the prospect of natural gas meeting ever-increasing power demands ashore and at sea while bridging the gap between conventional fuel oils and the renewable or carbon-neutral fuels required for a truly sustainable future is clearly an option. To facilitate the safe use of natural gas onboard ships Lloyd’s Register has already published corresponding Rules and Regulations and is currently working alongside the marine industry in the development of the International Code for Gas-Fuelled Ships (IGF Code) and in several related national and international initiatives aimed at ensuring the safety of gas-fuelled ships.
Biofuels
Unlike their first generation counterparts, second and third generation biodiesels offer the potential for a realistic and sustainable alternative, or at least a supplement, to conventional marine fuel oils. Although of lower volumetric energy density than conventional marine diesel oils, biodiesel compares favourably with LNG and is significantly greater than CNG. However, unlike LNG or CNG, storage tanks for biodiesel are likely to be comparable in size and in structure to conventional marine fuel oil storage tanks.
While first generation biofuels are produced using feedstocks, water or land resources traditionally associated with food production, second and particularly third generation biofuels do not compete with either. Second generation biofuels make use of non-edible plants, crop waste or biomass. While third generation biofuels offer the prospect of large-scale production of carbon neutral fuels using land and water totally unsuitable for crop plant or food production.
One of the most promising developments is the use of algae to produce oils with molecular structures similar to the petroleum and refined hydrocarbon products in use today, offering the potential for manufacturing a range of fuels including gasoline, diesel fuels and jet fuels with the same specifications. Such algae is theoretically capable of producing over 2,000 gallons of oil per acre per year – compared to first generation feedstocks such as soybeans which yield approximately 50 gallons per acre per year – and in doing so consume atmospheric carbon dioxide.
In addition to sea trials of algae-derived biodiesel, Lloyd’s Register is also evaluating the use of other alternative fuels currently manufactured from hydrocarbon feedstocks but potentially capable of carbon neutral production using biofeedstocks including methanol and dimethylether (DME).
Hydrogen
In the longer term hydrogen would appear to be the ultimate clean fuel or more accurately ‘energy carrier’. If used to supply fuel cell generators the prospect of true zero emission power generation (no SOx, NOx, CO2 or PM [particulate matter]) and extremely high electrical efficiency (60–70%) could be realized. However before it may be considered as a practical alternative to conventional marine fuel oils a number of challenges have yet to be overcome. The volumetric energy density of hydrogen, even when compressed or liquefied, is extremely low. Compressed to 250 bar the energy density is less than a tenth of that of conventional marine diesel oils and when liquefied, at a temperature of approximately –250°C, still only a quarter suggesting the need for prohibitively large storage tanks. A further challenge is that the energy necessary for compression may represent as much as 5–20% of energy content of the hydrogen, depending on the storage pressure, and as much as 30– 40% for liquefaction.
Such inefficiencies currently make the use of hydrogen as an alternative to conventional marine fuel oils unlikely in the short term for all but niche applications such as vessels with very low power demands, and/or very short trading routes or operating in very environmentally sensitive areas such as harbours or inland waterways. For hydrogen to replace conventional marine fuel oils as a truly zero emission fuel, it will need to be produced using established or sustainable production processes that are currently the focus of worldwide research and development (R&D) including electrolysis, photo-electrolysis, high temperature decomposition, photo-biological production and thermo-chemical splitting. In addition to ‘green’ production hydrogen storage technology will need to improve. R&D worldwide is currently focused on the development of hydrogen storage technologies such as organic hydrides and carbon nanostructures which, if realised, will almost certainly revolutionise our industry and all other transportation sectors too.
Lloyd’s Register is monitoring the development of enabling technologies necessary for the use of hydrogen as an alternative to conventional marine fuel oils, particularly hydrogen storage and hydrogen generation technologies. The challenges facing the marine industry in striving for sustainable marine power are significant. For its part, Lloyd’s Register aims to support the industry by facilitating, without promoting, the use of alternative fuels through the provision of timely and appropriate Rules, Regulations, guidance and advice.
Source: Lloyd's Register, Horizons Edition