5-26: Syngas fermentation for liquid biofuels

The village scale pyrolysis project at Cornell University is a multi-disciplinary collaborative effort to demonstrate that sustainable management of agricultural waste biomass in rural agricultural communities can  improve soil fertility and energy independence while resulting in a negative net balance of carbondioxide released to the atmosphere. The project involves pyrolysis of biomass, production of bio char for amendment of agricultural soil, production of synthesis gas, and syngas fermentation into biofuels. Here, we present our approach to optimize the syngas fermentation with Clostridium ljungdahlii, a homoacetogenic bacterial species known for its good ethanol production capability. The organism is grown in a two-stage continuous flow fermentation system with gas-recycling in both stages and cell recycling in the second stage. Different growth media compositions and growth parameters are tested. The experiments are being conducted with a synthetic syngas mix (60% CO, 35% H₂) and 5% CO₂). The general approach is to run the fermentation in  2 stages: In the fist stage (growth stage, continuously stirred tank reactor with 1L working volume) the pH is controlled at 5.5, allowing C. ljungdahlii to grow fast while producing mainly acetic acid. In the second stage (production stage, a bubble column with 4L working volume), the pH is about one unit lower, which, in combination with the high amount of acetate pumped from the first stage, triggers solventogenesis (ethanol production, conversion of acetate into ethanol). Results regarding ethanol concentration, production rates and syngas conversion efficiencies will be presented.