Nowadays, natural gas, oil, and coal are still the main sources of energy used to meet our energy demands. However, burning large amounts of fossil fuels leads to the emission of high amounts of greenhouse gas emissions, which influence global warming and climate change. Therefore, great attention has been paid to renewable sources of energy, such as solar, wind, hydroelectric, geothermal, and bioenergy to satisfy the current demand for energy while ensuring our energetic independence.
Bioenergy is one of the renewable sources of energy that uses biomass to produce, for example, transportation fuels, electricity, heating, or even cooling. Biomass is a term commonly used to characterize renewable organic material that can be obtained from plants and animals, and includes for example forestry and crop residues, manure, animal fats, and municipal solid waste.
Biomass can be converted into solid (e.g. charcoal), liquid (e.g. bioethanol, biodiesel), or gaseous (e.g. biogas, syngas, hydrogen) biofuels by means of chemical, biochemical, and thermochemical processes. Chemical methods use transesterification and esterification reactions, with the help of chemicals and catalysts to generate, for example, biodiesel, while biochemical processes refer to utilization of microorganisms for anaerobic digestion and fermentation processes. Thermochemical methods include drying, torrefaction, combustion, gasification, pyrolysis, and hydrothermal carbonization (HTC).
HTC is a thermochemical process in which the biomass is mixed with water and heated inside of a reactor at temperatures between 180-250ºC. The pressure inside of the vessel will increase autogenously and at the end of the process, it is possible to obtain a solid fraction (hydrochar) and liquid wastewater (L-HTC). This method is particularly attractive since it allows the conversion of various types of biomass with an elevated moisture content into a wide variety of high-quality products. Although this process has several known advantageous, studies have also shown the L-HTC can contain dissolved nutrients and organic compounds, that are harmful to the environment if directly discharged into water bodies.
Therefore, this project titled 'Utilization of liquid by-products from hydrothermal carbonization (HTC) for the production of value-added products utilizing purple phototrophic bacteria and anaerobic digestion' was funded by the Estonian Research Council (MOB3JD1234) and it aims to investigate the possibilities of recycling and reutilizing the L-HTC produced during the hydrothermal carbonization of lignocellulosic biomass. To this end, the L-HTC will be processed sustainably, utilizing purple phototrophic bacteria and anaerobic digestion in order to generate valuable and commercial products such biogas, biomethane, and hydrogen. The scenario proposed in this project will also allow the identification of the optimal operating parameters to improve the quality of the end-products, while reducing the production of inhibitory products.