Markus Antonietti, Arne Thomas and Maria Titirici discuss the hydrothermal carbonization of biomass - is it a solution to the CO₂ problem?

Biomass can be converted to a form of carbon much like coal

The global atmospheric concentration of carbon dioxide (CO₂) has increased markedly as a result of human activities since the industrial revolution. The inevitable impact on the world climate, recently announced by the IPCC (Intergovernmental Panel on Climate Change, see link below), has prompted intensive discussions from politics, industry and the scientific community about how to treat the CO₂ problem.

Energy storage and saving and a more responsible usage of the remaining energy resources is unavoidable. Replacing parts of the energy production system by biomass schemes (bioethanol, -diesel or -gas) or the artificial sequestration of CO₂ (in the ocean or into underground geological formations) are also under discussion. All these methods can lower future CO₂ emission, but cannot compensate for past and current emissions of CO₂ from fossil fuels. However, the IPCC report claims that anthropogenic warming and sea level rise will continue for centuries due to the timescales associated with climate processes and feedback, even if greenhouse gas concentrations were to be stabilized immediately. It would therefore be highly desirable to not only slow down further CO₂ emissions, but also to invert the current development by sequestering the atmospheric CO₂ released by many years of industrialization.

The biggest carbon converter, with the highest efficiency to bind CO₂ from the atmosphere, is biomass. Interestingly, the removal of just 8.5% of freshly produced biomass from active ecosystems would compensate for all the CO₂ liberated from oil. Coal formation from biomass is one of the natural sinks that has been active in the past on the longest scale. Natural coalification of biomass takes place on a timescale of some hundred (peat) to hundred million (black coal) years. Due to its slowness, it is usually not considered in renewable energy exploitation schemes or as an active sink in CO₂ cycles. Nevertheless, it is obvious that carbon fixation into coal is a lasting effort, as brown or black coal are practically not biodegradable. Thus, turning coal formation into an active element of carbon sequestration schemes would be very meaningful, but requires the acceleration of the underlying coalification processes.

Hydrothermal carbonization (HTC) can be such a process (see the reference below): HTC describes heating carbohydrate sources in water, e.g. biomass dispersions, in a closed reaction vessel for 4-24 h to temperatures around 200 °C. Upon dehydration of the carbohydrates, carbon with a chemical composition similar to brown coal is observed. Thus, HTC is an extremely simple, cheap and easily scalable process. Furthermore, it was shown that HTC of carbohydrates can yield interesting carbon micro- and nanostructures, such as carbon microspheres. Using suitable additives, carbon nanocables and fibres, porous carbon architectures and metal-carbon composite materials can be generated.

A variety of nanostructures and porous architectures can be made

In CO₂ conversion schemes, HTC has a number of other practical advantages: once activated, HTC is a spontaneous, exothermic process, it liberates up to a third of the combustion energy stored in the carbohydrate throughout dehydration. Furthermore, HTC inherently requires wet starting products or biomass, as effective dehydration only occurs in the presence of water, plus the final carbon can be filtered easily from the reaction solution. This way, complicated drying schemes and costly isolation procedures can be avoided. In addition, most of the original carbon stays bound to the final structure. Carbon structures produced by this route-either for deposit or materials use-are therefore the most CO₂-efficient.

Thus, the carbonization of biomass from fast growing plants can be an efficient process for removing atmospheric CO₂. For a negative atmospheric CO₂ balance, the generated carbonaceous materials have to be deposited on a large scale, and potential carbon landfills may lay the foundations for chemical starting materials of the next century. Another quite attractive application with immediate impact is the use of such carbons as water- and ion-binding components to improve soil quality. This is a chemical process that is also found in nature, and 'carbonaceous soil' is the largest active carbon sink on earth. Instead of clearing the rainforest for questionable palm oil production, such a 'carbon-reinforced rainforest' would produce at least 10 times the energy, but stored in carbon, while also being CO₂-benign for the climate and supporting biodiversity at the same time.

In that sense, HTC can be seen as much more than just a technique for making carbon-rich substances.

Read the full Opinion article in issue 6 of New Journal of Chemistry.