Could LPG be the marine fuel for the future?

World LPG Association technical director Nikos Xydas. Credit: World LPG Association

One of perhaps the most known alternatives to marine fuel oil is liquefied natural gas (LNG). Some investment is being made in infrastructure but, requiring cryogenic technology, there are challenges in maintaining, handling and storing this fuel, which translate to added costs and investment levels.

One of the biggest issues with LNG, however, is emissions of unburnt methane, termed ‘methane slip’. Methane has a high greenhouse effect and is one of the six GHGs listed by the Intergovernmental Panel on Climate Change (IPCC).

Over 100 years, methane has a global warming potential of 28-36 times that of carbon dioxide as well as being a precursor to ozone, another GHG. If left uncontrolled, therefore, methane slip can cancel out the environmental advantage of using LNG instead of diesel of bunker fuel.

Other carbon-based fuels with lower emissions than fuel oil include methanol and ethanol but, although easy to handle and requiring lower investment in infrastructure, fuel prices are currently prohibitively high.

LPG offers a practical and immediate solution that could be quickly adopted because of existing infrastructure, bunkering options, availability and cost. It consists of propane, butane or a combination of these two gases, neither of which are GHGs listed by the IPCC.

The use of traditional LPG, which is obtained during the production of natural gas or during refining processes, offers the potential for immediate, rapid reduction in shipping emissions. 

LPG can offer an even lower emission long-term solution with its renewable twin, the renewable LPG, often called bioLPG (along with other fuels and technologies).

Renewable LPG can reduce carbon emissions by a further 80% compared with traditional LPG. Chemically, it is exactly identical and so offers a ‘drop-in’ carbon neutral replacement for the future if shipping operators choose the quick wins offered today with traditional LPG. It can be produced from renewable or bio feedstocks such as plant or vegetable waste material and is a by-product of most biodiesel production processes.

Another possible production path for renewable LPG is the conversion of biomethane and biogases, giving this fuel much potential for the future. Furthermore, research is underway into the production of renewable LPG (synthetic) from green hydrogen and sequestered carbon dioxide, which will provide a completely carbon-neutral fuel for the future.

Other future carbon-neutral fuels include biomethane and synthetic methane, which offer clear advantages as they can directly replace current high carbon fuels. However, production levels are too low and prices too high for them to make a viable solution for today.

Zero-carbon fuels include hydrogen and ammonia, but these also present challenges and require significant investment in terms of production and distribution infrastructure. 

Ammonia, specifically, if challenges are overcome in the future, could also be a drop-in replacement for LPG with minor modifications, in vessels that have already adopted LPG as a fuel. In other words, LPG also offers a path to a carbon-free fuel.

Dual-fuel engines, such as the ME-LGI from MAN Energy Solutions, offer added flexibility to reduce environmental impact today. Designed to operate with low-flashpoint liquids (LFL) such as LPG with any combination of propane and butane, they offer an easy pathway to convert to lower emission fuels, while ensuring there are no limitations on engine acceleration or instability in rough seas.

Offering the same fuel consumption and tuning methods as equivalent diesel engines, the ME-LGI uses high-pressure injection and operates on a minimum of 75% LPG or other LFLs and a maximum of 25% ethane.

Furthermore, these LPG engines employ the diesel principle, meaning they can maintain high efficiency even when operating in oil mode, giving operators flexibility and keeping them competitive even if LPG bunker is not available.

Throwing oils and toxic chemical elements in the fauna of the ocean.

LPG is already widely used in the marine sector and is gaining growing acceptance as a fuel alternative with two and four-stroke LPG engines already available. The first dual-fuel LPG powered very large gas carrier (VLGC) is already in operation and 55 more vessels are in the pipeline.

LPG engines have low maintenance costs and retrofit to LPG operation is much more economically viable (and produces a lower carbon footprint) than new builds.

The economic appeal of the fuel means that increasing numbers of both new and retrofitted vessels are due to come into operation soon. So, what economic advantages does LPG offer over other fuel alternatives today?

Firstly, there are already more than 1,000 storage facilities and terminals and more than 700 small carriers for ship-to-ship bunkering. As all LPG terminals could become potential supply points, the infrastructure investment for adopting this fuel is relatively small. LPG is already accessible in ports throughout the world and the cost and time to build new LPG terminals much lower than for LNG.

Using LPG as fuel allows for an even further optimised engine tuning with regards to compression ratio and layout of the combustion chamber, offering around 10% reduction in fuel cost compared with using traditional IMO compliant marine fuel oils and scrubbers. For LPG carriers, the use of LPG as a bunkering fuel further represents substantial savings in bunkering time.

Although there are many promising fuels and technologies for the future of the shipping industry, the need to address climate change, pollution and clean air is for today.

LPG as a marine fuel meets IMO emission limits without the need for scrubbers. It reduces sulphur oxide emissions by 97% compared with HFO or ULSFO and lowers nitrous oxide emissions by around 20%. Its use today will reduce GHG emissions by around 24% and lower the discharge of particulate matter by a remarkable 90%.

Furthermore, LPG is nontoxic, not only making it safer and easier to handle, but also protecting soil and water from harmful toxins, in case of accidental leaks or accidents at sea. This makes it an appealing option for use in protected and environmentally sensitive areas for all sizes and types of boats and engines.

There are several options to power the shipping industry of the future, but LPG is the only one that offers a clear environmental and economic advantage today without adding additional safety or environmental risk, such as is the case with methane slip from LNG.

LPG also offers a clear pathway to make the industry even more sustainable in the future, with drop-in renewable LPG, from carbon-neutral production processes and renewable LPG sources or the use of ammonia as fuel.

Indeed, technology and production paths leading to renewable LPG are advancing rapidly. In Europe, for example, it is anticipated that the entire market demand will be covered by production from renewable sources, i.e. renewable LPG, by 2050. 

With several operators already switching to LPG fuel, the existing supply chain infrastructure, low investment cost and reduced operating costs mean LPG is likely to be a key component in the wider industry in both the near and distant future.