Electric boats may be making headlines but, with diesel-powered watercraft still dominating, isn’t it time to consider greener fuel alternatives?
While there’s understandable excitement around the new generation of electric boats like X Shore, Candela and Delphia, the truth is that battery power isn’t going to replace the internal combustion engine in larger planing boats anytime soon. .
The physics just don’t add up due to the weight, bulk and cost of the batteries required. Depending on how you calculate it, one kilogram of diesel contains 30 to 100 times the energy of one kg of lithium-ion battery.
There is still a place for diesel-electric hybrid drivetrains that will allow boats to glide silently through the harbor, but the combustion engine is likely to remain the primary power source on larger boats.
Instead, the focus shifts from engines to the fuels they burn. Biodiesel made from vegetable oils is one possible solution, but scientists are also developing new forms of fuel that emit fewer greenhouse gases and don’t rely on them being extracted from the ground in countries with unsavory diets.
So what are the most promising diesel alternatives?
5 of the best alternatives to diesel
Hydrogen can be used in fuel cells to generate electricity or burned in a combustion engine. In both cases, the main emission is water rather than carbon dioxide. It can also be produced cleanly by electrolyzing water into its constituent elements.
However, it has a number of drawbacks. Electrolysis requires a lot of energy, so today the majority of commercially produced hydrogen is made by reacting natural gas (a fossil fuel) with steam to create hydrogen. , carbon monoxide and carbon dioxide.
It is also a very bulky gas with a relatively low calorific value which must be compressed to at least 350 bar and stored in high pressure tanks or frozen at -253c in liquid form to be transported on board a ship in quantities sufficient, both being energy intensive processes.
Advantages: Burns cleanly, non-toxic and efficient energy source
The inconvenients: Expensive to produce, highly flammable, odorless, leaks easily
Methanol has a number of advantages over hydrogen in that it is a clear liquid that does not need to be stored under pressure or at freezing temperatures and can be burned in conventional gasoline and diesel combustion engines.
However, its energy density is about half that of gasoline and therefore requires twice as much volume to store it. It is also toxic to humans and corrosive, requiring a number of fuel system adaptations.
On the plus side, it burns cleanly, producing water and carbon dioxide, but less particulate matter and sulphur/nitrogen oxides than diesel, making it an attractive option for boats.
Currently, the majority of methanol is produced from natural gas by reforming it with steam and distilling the resulting gas into methanol. However, it is also possible to produce it cleanly by fermenting biomass waste from wood.
Advantages: Liquid at normal temperature, simple conversion of diesel engines, lower emissions, not as explosive as gasoline
The inconvenients: Low energy density, toxic to humans, highly corrosive
Like hydrogen, ammonia is a flammable gas that can be burned in a combustion engine without creating carbon dioxide, but while hydrogen can only be stored under extreme pressures or temperatures, ammonia can be stored at room temperature at a relatively modest pressure of 10 bar or as a liquid at -33C.
It also has a higher energy density of 12.7 MJ/L compared to 8.5 MJ/L for liquid hydrogen. And because ammonia is used in the production of fertilizer, the infrastructure to produce, transport and store it already exists.
When ammonia burns, it produces water, nitrogen and nitrogen oxides, but no soot or CO2. The vast majority of ammonia is currently produced by converting natural gas or liquefied petroleum gas into hydrogen, which is then reacted with nitrogen to create ammonia.
However, if this hydrogen is created by solar or wind electrolysis, it can be considered a green and renewable fuel.
Advantages: No CO2 or soot when burning. Can be produced from renewable energy and can be stored as a gas or liquid under pressure
The inconvenients: Toxic in low quantities, production still consumes a lot of energy
Formic or methanoic acid is still being tested, but shows promise both as a fuel and as a means of carbon capture.
In theory, carbon dioxide can be converted to formic acid in the presence of a catalyst under considerable heat and pressure, although it is currently produced commercially for use in cleaning products by a chemical reaction between methanol and carbon monoxide.
As a fuel, it can be used to generate electricity directly in a formic acid fuel cell or as a hydrogen carrier. When converted to hydrogen in the presence of a catalyst, the only emissions produced are water and CO2, which can be captured and used to create more formic acid.
Being a liquid at room temperature, it is easier to store than hydrogen, but it is corrosive and susceptible to biodegradation to carbon monoxide and water, which can cause troublesome buildup in a vehicle’s fuel tanks. boat.
Advantages: Liquid at room temperature and can be converted to hydrogen. There is no emission of nitrogen oxides, sulfur oxides or soot
The inconvenients: Highly corrosive, subject to biodegradation
Liquefied natural gas (LNG)
Liquid natural gas is composed largely of methane and ethane (natural gas) which has been cooled in its liquid form to -162°C for ease of transportation and storage.
This requires considerable energy to cool it and insulated (but not pressurized) tanks to store it. In this form, it has an energy density of about 23 MJ/litre, half that of diesel.
It also burns cleaner than diesel with harmful NOX emissions reduced by up to 80% and a CO2 reduction of approximately 25%.
It is possible to create LNG cleanly from biomethane, but the majority of current production comes from traditional oil and gas drilling operations.
Advantages: Burns cleaner than diesel and is relatively easy to produce, store and transport
The inconvenients: Lower density than diesel, requires gas-specific engine, currently still a fossil fuel
Conclusion: which diesel alternatives are the most promising?
The number and variety of fuels tested by engine manufacturers, oil companies and scientists gives boat owners hope. It’s hard to predict which diesel alternatives are pushed and which are dropped, but in the medium term it will likely involve several different solutions.
There are already a number of hydrogen craft under development, and LNG and methanol are also being tested by the shipping industry. Hopefully at least one will form the basis of a greener future for all of us.
Energy Density Table of Alternatives to Diesel
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First published in the July 2022 issue of MBY. Thanks to MarQuip Exhausts for their help with this article.