High-energy boron-based fuels promise cleaner, greener space rockets


In a move that could lead to carbon-free space launches, scientists at the University of California, Riverside are experimenting with borane ammonia (NH₃BH₃) as a substitute for conventional carbon-based chemical rocket fuels.

Besides its everyday use in eye drops and detergents, boron has a fascinating history in aerospace.

In the 1950s, the US Air Force and Navy were looking for ways to build faster and longer aircraft. One of the main obstacles to achieving this goal is fuel.

Ideally, the best fuel would be hydrogen, but even when cooled in cryogenic liquid, it still takes up far too much space, is extremely difficult and dangerous to handle, and poses all sorts of logistical problems. Conventional aviation fuels based on hydrocarbons refined from fossil fuels are much more convenient because the hydrogen is bonded to carbon atoms, making them much more compact and manageable.

Unfortunately, fuels like kerosene were still too bulky and contained too little energy per unit for the US military’s need for supersonic jets capable of flying halfway around the world. To overcome this, engineers wanted to create a new fuel that used another element to store hydrogen, but could be cheap enough and easy enough to use to be practical.

The result was a new family of fuels based on boron, which sits right next to carbon on the periodic table. These hydro-boronated compounds, or boranes, were developed under the code name Project Zip, hence their nickname “zip fuels”.

At first, these fuels held great promise, producing nearly twice the heat energy of conventional fuels. Planes like the XB-70 Valkyrie strategic bomber, the XF-108 Rapier long-range interceptor, the BOMARC missile and others were designed to use them, and there were plans to convert the jet engines already in use to burn boranes.

But borane fuels had major problems. Because they are solid at room temperature and burnt at higher temperatures, they had to be mixed with hydrocarbon fuels. Worse, they produced sticky waste that clogged engines and could only be removed with great effort. As a result, the projects were canceled in 1959.

Today, borane in the form of ammonia borane is used in fuel cells to power electric vehicles, where hydrogen is released through catalysts.

Now a team at UC Riverside, led by Prithwish Biswas, is looking for a way to use boranes to power rockets capable of putting satellites into orbit.

The key to this is developing a better understanding of how boranes burn and how to get them to do so efficiently, without the need for base fuel or catalysts, which could lead to a more environmentally friendly alternative. environment to hydrocarbon fuels and possibly cheaper space launches.

Unlike hydrocarbons, boranes have a complex combustion process as they change into a number of different compounds when burned. By turning borane into nanoparticles and burning it with oxidizers like potassium perchlorate (KClO₄) or ammonium perchlorate (NH₄ClO₄), the goal is to modify the combustion process and release more energy faster .

“This is analogous to the use of catalytic converters to enable the complete combustion of hydrocarbons,” said Pankaj Ghildiyal, Ph.D. in chemistry at the University of Maryland. raised. “Here we were able to create more complete combustion of the chemicals and increase the energy of the whole reaction by using the chemistry of the oxidant itself, without the need for a catalyst.”

Because the fine particles used in the study tend to degrade in humid environments, the next step is to find a way to give them a protective coating for storage.

“We have determined the fundamental chemistry that powers this combination of fuel and oxidant,” Biswas said. “Now we can’t wait to see how it performs on a large scale.”

The research has been published in The Journal of Physical Chemistry C.

Source: University of California, Riverside


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