Capabilities of biogas-producing microorganisms are underestimated
Researchers at the Leibniz Institute for Agricultural Engineering in Potsdam, Germany, have shown that anaerobic microorganisms can use complex organic pollutants for biogas production. Phenols, furans, aldehydes and ketones, which are frequently found in liquid by-products of thermochemical conversion of biomass, can easily and efficiently be degraded into bio-methane. This provides the basis for an efficient and sustainable integration of carbonization processes such as pyrolysis and hydrothermal carbonization (HTC) into bio-refinery concepts. The results have now been published in the renowned scientific journal "Bioresource Technology".
The production of materials and chemicals from renewable resources usually bases on biological or thermochemical processes. The latter possess the advantage of very high reaction rates. At temperatures of 250°C and above, even complex organic compounds that are recalcitrant to biologic degradation like lignin are rapidly decomposed. However, thermochemical processes are highly unselective in their product pattern. Besides the target products a range of more or less problematic organic compounds is formed. This decreases the product yield and causes additional costs for the waste water treatment.
Researchers of ATB's junior research group APECS have put focus on the liquid by-products of thermochemical biomass conversion. This includes waste waters from HTC as well as pyrolysis. Both processes are highly flexible both in their feedstocks and products and are therefore expected to fulfil important roles in future bio-refinery concepts. However, until now a major problem is: These processes form waste waters that contain various environmental hazardous substances such as phenols, furans, aldehydes and ketones.
The main product of pyrolysis and HTC is biochar, a carbon and energy-rich solid material, which can be used as fuel, but also for a range of further applications from soil amendment to carbon electronics. By-products of pyrolysis are the so-called syngas, which can be used as a fuel, and a condensate composed of volatile compounds, for which no satisfying type of use exists today. In the HTC, a process liquid is formed which contains a wide variety of organic and mineral compounds. Thus, both waste waters from HTC and pyrolysis require an effective treatment before they can be released into the environment.
The Potsdam researchers report on the successful anaerobic biological conversion of water-soluble pyrolysis condensates in laboratory tests. The condensates were obtained from pyrolysis of solid digestate, which is a by-product of biogas production, at temperatures between 330°C and 530°C. Large parts of the organic compounds contained in these condensates could be degraded and transformed into bio-methane. After the biological treatment, the analysed toxic components 5-HMF, phenol, furfural, catechol and guaiacol were removed below detection limit. Only cresol remained detectable, but was still degraded by 10 to 60 %. The temperature at which the condensate was produced had strong impact: the higher the pyrolysis temperature the less organic compounds were degraded. In detail, increasing the temperature from 330 to 530°C decreased the overall degradation efficiency, expressed as the chemical oxygen demand (COD), from 57 to 37 %.
"Our results indicate the range of synergistic options to combine thermochemical processes like pyrolysis and HTC with biogas production", project leader Jan Mumme emphasizes the added value of these integrated systems. "In addition to the production of biochar, energy can also be obtained in form of biogas", adds junior scientist Tobias Hübner. "Recently, high research activity is seen concerning the integration of thermochemical and biological processes following bio-refinery concepts. With our research results we want to contribute to a better economic performance and a higher sustainability of these systems", concludes Mumme.
The use of HTC waste water for biogas production was demonstrated by the APECS researchers earlier in 2013. In another joint study with Fraunhofer Institute for Chemical Technology ICT Pfinztal, quantification of individual substances in the HTC liquid by NIR showed promising results that could lead to an advanced control of biomass conversion and, thus, to higher efficiency.