UFZ-Report 06/2012 - Energietage

Biomass is a readily available renewable energy source which has received increasing attention due to rising prices of fossil fuels and the urgent need to mitigate anthropogenic global warming. The conversion of biomass into gaseous and liquid biofuels by microorganisms can be considered as a way to gain safe and sustainable energy which does not contribute to a further build up of carbon dioxide in the atmosphere (McKendry 2002). The most biological processes converting biomass into liquid biofuels, hydrogen or biogas are anaerobic. Changes in stoichiometry and kinetic of anaerobic digestion processes are reflected by heat production rates in real-time. The combination with other monitoring tools either on-line (e.g. gas emmisions, pH etc.) or offline (intermediates) allows the separation of stoichiometric and kinetic information using thermokinetic models. Forming the enthalpy balance of a reactor allows the determination of the metabolic heat production rate in an easy way. This approach was already successfully demonstrated at different scales for aerobic bioprocesses [1] and the principle should also be applicable to anaerobic digestion processes. Since with any scale-up the ratio of the heat producing volume to the heat exchanging surfaces increases and therefore the accuracy of the heat measurement improves this calorimetric principle might be particularly advantageous for anaerobic digestion processes as they are typically performed in large tanks. The gas production is a further easily on-line monitored variable. For exploring the information content of the calorimetric signal the heat production rate as well as the gas emission (calorespirometry) of Clostridium acetobutylicum was monitored in real time. This test system was chosen because Clostridia contribute to the complex biogas process and Clostridium acetobutylicum has been used for production of butanol for decades. Butanol may be used as a fuel alternative to ethanol in combustion engines. Thus, Clostridium acetobutylicum being a well-studied bacterium some information on kinetics, stoichiometry, genetics, and proteomics are available from literature. The ratio of the on-line signals heat production rate to gas evolution shows clearly differences between different productions states (e.g. acetogenisis and solventogenisis). Thermodynamic calculations show some similarities to other anaerobic processes. This opens opportunities to monitor and control such processes. However, the recently available calorimeter types are not perfectly suited for controlling anaerobic processes at technical scale. This challenges the development of tailor-made calorimeter. The recent achievements and potential development directions will be discussed.