Sodium Borohydride as a Source of Maritime Fuel

Hi all,

I am a student at the Antwerp Maritime Academy and I am researching the public perception of Sodium Borohydride (NaBH4), a hydrogen carrier, as a possible maritime fuel. You will be given a short explanation of NaBH4 in the questionnaire. This questionnaire is completely anonymous and your answers will be used in my thesis. This survey will take about 10 minutes to complete.

Thank you for your cooperation!

Link to survey: Microsoft Forms https://forms.office.com/Pages/ResponsePage.aspx?id=J7Cp6NHcJkeCKqnlgTOsi0uzRU-twjpAreELJdkhontUOUhXTTZSRzgxNDhYWkxCVUw5WjdJWUQ5Ny4u

 

Well, it's a hypergolic rocket fuel additive and a reasonable hypergolic rocket ignitor along with other boranes and lithium X hydrides. There's a lot of stuff it really doesn't like, such as aluminum, acids, hydrocarbons, and even fire suppressant chemicals which are sometimes found on boats. Marine fuels are not generally known for their high purity, and control of this stuff beyond laboratory scale is going to require a massive investment. Reconstituting the fuel on board seems unlikely. Replacing a solvent mixture (a high ph alcohol mix, presumably) would drastically lower the specific energy content and present all kinds of hazards since any spill at all will probably catch fire immediately. And in the US, there are export restrictions on the stuff, which would be a nuisance for a proposed fuel. It has all the charm of a second-hand Russian nuclear reactor from the 60's, but you have to refuel it each voyage.

 

Your feedback is much appreciated. There is already a vessel under construction in the Port of Amsterdam that will have NaBH4 as its fuel source, so it's not unthinkable for it to become mainstream in the future.

 

Any information on the design of its fuel polishing system? I'd love to know how they will change filters on a hypergolic fuel aboard a boat. How are they dealing with the CO2 problem? It has a strong affinity for CO2 and can be used as a scrubber chemical. You need to either purge the air from the tanks or polish the fuel to remove the CO2 contamination. How are they dealing with phase-change problems. The crystalline form changes from a dehydrate to an anhydrate at 36 degrees C. You don't want polyphase materials in your fuel process. All doable, I suppose, but I'd like to see it in action at scale.

And I guess I should ask exactly what form the fuel will take. What are its chemical components, and what is its specific cyclic hydrogen storage capacity and its cyclic hydrogen storage density?

 

Doing a bit of digging here, there are decently stable, non-hypergolic alkaline solutions of sodium borohydride. Obviously, the rocket people aren't interested in those, so I had to look elsewhere. Potentially, a 15 wt% solution of sodium borohydride can liberate 3 wt % of hydrogen. But using any real machine, like a catalytic reactor at modest temps and pressures, it will be less than that - let's call it 2.5% by weight. Furthermore, we can subtract the self-consumption needed to power the process, and 2 wt% net isn't a bad guess. If we now shunt this to a PEM type fuel cell at about 55% efficiency, we get 0.55 x 142kJ/g(h) x 2% = 1.56 kJ(electric)/g(fuel). With MFO, we get 44kJ/g(MFO), an efficiency of 45% or so to the shaft (including parasitic losses as I did for the hydrogen gas generator), and 97% for the generator. These are pretty loose estimates, but it doesn't really matter. The result is 0.45 x 44kJ/g(MFO) x 0.97 x 100% = 19.2 kJ/g(fuel). So you need more than 12 times the weight of sodium borohydride fuel if done with a decently stable solution. Yes, you can reclaim it, but you need to look at a port facility and ask how you are going to increase the tank farm by a factor of 12 and how you will increase the port's fuel barge fleet capacity by a factor of 12. That's going to be a problem. Even if you make the fuel twice as good, you still have the same problem.

I also want to push back on the toxicity issue. I found two references that claimed the stuff was nontoxic. It is acutely toxic, as are most boron-containing substances. This includes any and all byproducts of boranes that might be encountered anywhere in the fuel process. https://www.sigmaaldrich.com/US/en/...aNJvTY43jvOwgvQgqe6GJW13_8gGKGQ0uqHJEOrEUd3mO