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Making the most out of nature’s resources: combining technologies means higher bioenergy yields

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Posted July 28, 2014

Sustainable energy sources are by all means on great demand in our energy-dependent world. Sun, wind and water are certainly the most environmentally neutral, but biomass remains a natural and time-proven resource, and efficient biofuel technologies are on-the-rise.

Biochar – a useful agricultural material, a biofuel and an environmentally-friendly carbon sink. Credit: ClimateTechWiki

Biochar – a useful agricultural material, a biofuel and an environmentally-friendly carbon sink. Credit: ClimateTechWiki

 

The so-called bioenergy can be produced from basically any organic matter – some of the extreme examples including landfill fumes or animal and even human waste. However, while few may object to producing energy from waste, food shortage in large areas of the world clash with the use of food crops for biofuel, such as ethanol or gas.

Researchers at The German Leibniz Institute for Agricultural Engineering Potsdam-Bornim (ATB) have been developing a combination of bioenergy technologies to get the most out of non-food biomass, such as straw, grasses, rice hulls or corn cobs, which are otherwise difficult to utilize in raw form and have low energy yields.

The research was recently published in JoVE, The Journal of Visualized Experiments. Along with an instructive video, the scientists describe how traditional anaerobic digestion of straw, which produces valuable biogas, can be combined with a rather young but already much talked about technique – hydrothermal carbonization.

Organic material tends to keep much of its original state after biogas production is completed – in fact, digested straw looks and feels just like pre-digestion feedstock. It also keeps much of its carbon-bound energy, which could potentially be used as a fuel itself. However, as 80% of this material is purely water, it is neither reasonable to burn it directly nor to spend valuable time and energy on the drying process.

Raw straw feedstock, anaerobically digested straw and biochar produced after hydrothermal carbonization. Image courtesy of the researchers.

Raw straw feedstock, anaerobically digested straw and biochar produced after hydrothermal carbonization. Image courtesy of the researchers.

Carbonization of straw, on the other hand, uses temperature and pressure to produce a material much the same as charcoal, which has been created by nature from biomass through millions of years. Except the production of biochar takes days or weeks AND the end product can be used not only as fuel, but as a valuable soil fertilizer and a long-term carbon sink.

Compared to raw organic waste, biochar is largely hydrophobic, therefore it is not only useful as a solid biofuel, but it also resists biodegradation. Returning this carbon to the soil would mean, firstly, that the better part of atmospheric CO2, which has been absorbed into the vegetation, will remain there, generating a “negative carbon footprint”. Secondly, biochar is known to deliver important nutrients to the soil, as well as to improve its other qualities, such as water retention. In such a way, poor, sandy or otherwise infertile areas could benefit immensely from this waste-derived material.

Most importantly, the researchers demonstrate that by combining the two techniques (digestion and carbonization) it is possible to achieve the maximum bioenergy yields. That is, the energy that would be wasted as the unused straw decomposes is now “fixed” in a solid and stable state (biochar), and all the biofuels produced in the process contain more energy combined than if either method was used on its own.

The large scale production of such biofuels is yet to be evaluated, especially from an economic point of view. However, this could become an excellent tool to make the most out of unused organic matter, reduce global carbon footprint and produce valuable biomaterials all at the same time.

Written by Egle Marija Ramanauskaite

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