With rising energy demands and consumption of fossil fuels, there is an imminent need of finding alternative, renewable sources of energy. An important candidate is hydrogen produced from organic waste, or “biomass” of plants and animals. Biomass absorbs, removes, and stores CO2 from the atmosphere, while biomass decomposition can also bring ways to negative emissions of greenhouse gases removal. But even though biomass heralds a way forward, there is still the question of the best way to maximise its conversion into energy.
To combat this, scientists led by Professor Hubert Girault at the EPFL’s School of Basic Sciences recently developed a new method for biomass photo-pyrolysis, which produces not only valuable syngas but also biochar of solid carbon that can be repurposed in other applications. The work got published in Chemical Science. The method uses flashlight pyrolysis using a Xenon lamp, commonly used for curing metallic inks for printed electronics. Girault’s group has also used the system in the last few years for other purposes, like synthesising nanoparticles. The lamp’s white flashlight works as a high-power energy source and releases short pulses that promote photo-thermal chemical reactions. The idea is to generate a powerful flashlight shot that the biomass can absorb and that can instantaneously trigger a photothermal biomass conversion into syngas and biochar.
This flashing technique was used on different sources of biomass: banana peels, corn cobs, orange peels, coffee beans, and coconut shells, all of which were initially dried at 105°C for 24 hours and then ground and sieved to a thin powder. The powder was then placed in a stainless-steel reactor with a standard glass window at ambient pressure and under an inert atmosphere. The Xenon lamp flashes and the whole conversion process is over in a few milliseconds. “Each kg of dried biomass can generate around 100 liters of hydrogen and 330g of biochar, which is up to 33wt.% of the original dried banana peel mass,” says Bhawna Nagar, who worked on the study. The method also had a positive calculated energy outcome of 4.09 MJ·per kg of dried biomass.
The end products – hydrogen, and solid-carbon biochar, are most valuable in this method. The hydrogen can be hence used as green fuel, while the carbon biochar, can either be buried and used as a fertilizer or be used to manufacture conductive electrodes. “The relevance of our work is further heightened by the fact that we are indirectly capturing CO2 stores from the atmosphere for years,” one of the researchers reiterates. “We have converted that into useful end products in no time using a Xenon flash lamp.
Currently, there exist two main methods for converting biomass into energy: gasification and pyrolysis. Gasification puts solid or liquid biomass at temperatures around 1000°C, converting it into gas and solid compounds; the gas is called “syngas” while the solid is “biochar.” Syngas, on the other hand, is a mix of hydrogen, methane, carbon monoxide, and other hydrocarbons, and those are what are used as “biofuel” to generate power. Biochar is quite often regarded as a solid carbon waste, but it can be used in agriculture applications if done right.