Generating energy from banana peel

In the 1985 science fiction film back to the future, a flamboyant inventor turns his car into a plutonium-powered time machine and travels back and forth in time. During a visit to the year 2015, he updates his engine so that it now needs any kind of matter to generate energy – even a carrot or two thrown into the “tank” will do.

Well, 2015 has passed us by. Fusion-powered vehicles are still beyond the horizon. And we continue to hope for new and better ways to get clean energy from renewable sources. Like carrots, or maybe bananas – which has indeed been achieved by a research group at the Swiss Federal Institute of Technology in Lausanne ( Chemical Sciences2022).

Their version of the banana split involved splitting biomass—banana peel, orange peel, coconut shells—by flashes of light from a xenon lamp.

Attraction of hydrogen

But before we get into this innovative approach, a few words about what makes hydrogen an attractive energy source. Storing large amounts of energy in a modest space is an essential requirement and hydrogen has an impressive energy storage capacity. When classifying fuels in terms of their energy value (also called heating value), the decisive elements are carbon, hydrogen and oxygen. Hydrogen has an energy value that is seven times that of carbon.

In the combustion of wood, carbon and hydrogen are oxidized in a heat-generating reaction, the end products of which are carbon dioxide and water. The first is a greenhouse gas that contributes to global warming. Burning hydrogen only gives us water and heat. A smarter way to use the energy in hydrogen would be to generate electricity with it. This is achieved in a fuel cell with a proton exchange membrane where, in the presence of a metal catalyst, a hydrogen molecule is split into protons and electrons, with the electrons providing the current output.

Transport vehicles

Such fuel cells are now being used in some parts of the world to power some light passenger vehicles. Unlike electric cars, hydrogen cars have a refueling time of only about five minutes. Commercially available hydrogen-powered cars have fuel tanks that can carry 5-6 kg of compressed hydrogen, with each kilo providing a range of about 100 km (and emitting nine liters of water, mostly as steam).

The limited popularity of hydrogen as a fuel is due to production and distribution restrictions. It is safer to handle than household cooking gas.

Industrial quantities of hydrogen gas are used in processes such as the production of ammonia for fertilizers. More than 90% of the world’s hydrogen is produced from fossil fuels.

This brings us back to the search for alternative energy sources that do not burden the environment. Biomass is a collective name for organic waste material of plant and animal origin. It is a rich source of both hydrogen and carbon – our dried banana peel has a hydrogen content of 5% and 33% is carbon. A key goal of all climate change mitigation protocols is to capture as much carbon as possible — don’t let it turn into gas.

The Swiss group uses pyrolysis, which breaks down organic material with small bursts of intense heat under inert conditions.

Irradiation flashes from a xenon lamp provide the heat – a total of 15 milliseconds of irradiation is enough to heat the system to 600 degrees Celsius and dissolve a kilogram of banana peel powder – releasing 100 liters of hydrogen gas.

This brief burst of photothermal energy also produces 330 grams of biochar, a solid residue rich in carbon.

It should be noted that if the biomass had been burned, gaseous carbon would have escaped as carbon monoxide and carbon dioxide. Pyrolysis ensures that carbon remains sequestered as a solid.

Benefits of biochar

Biochar has other uses as well – aside from storing carbon, biochar has several uses in agriculture.

Agricultural residues such as rice husk are an important source of biomass and the biochar it forms has significant mineral content. Adding it to the soil enriches the plant nutrients.

The porous nature of biochar makes it suitable for remediation – the adsorption of toxic substances in contaminated soils – reducing the potency of contaminants in the soil ( Annal Agric. science, 2019).

Biomass, be it from banana peels, tree bark or poultry manure, thus improves air quality and adds value to agricultural products – while setting that zero-emission car in motion.

(The article was written in collaboration with Sushil Chandani who works in molecular modeling.


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