1. Syngas fermentation for liquid biofuels
Hanno Richter and Largus T. Angenent
Cornell University, Biological & Environmental Engineering, Ithaca, NY 14853
ABSTRACT
THEORETICAL STOICHIOMETRIES
Lab scale syngas fermentation
From carbonmonoxide:
6 CO + 3 H2O CO2 + 1 CH3CH2OH (-218 kJ/mol ethanol)
From carbonmonoxide and hydrogen:
2 CO + 4 H CH3CH2OH + H2O (-138 kJ/mol ethanol)
From carbondioxide and hydrogen:
2 CO2 + 6 H CH3CH2OH + 3 H2O (-98 kJ/mol ethanol)
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 are presented.
Process optimiziation
effluent
Pumps
Growth
medium
Microbubbles, required
for efficient gas transfer
pH control
Stage II
Stage I
Kinetics of 2-stage fermentation
2-stage fermentation schematics
Stage 1
Stage 2
Table: parameters achieved in stage 2 of continuous syngas fermentation
OD600 12
Ethanol conc. in effluent (mM)
229
Acetate conc. in effluent (mM) 4
Molar Ratio ethanol/acetate 57
% of fed CO consumed 99
% of fed H2 consumed
% CO, % H2 and % CO2 in exhaust gas
4, 5, 90
Rate of CO consumption (mmol / Liter and hour) 23
Rate of H2 consumption (mmol / Liter and hour) 11
Rate of ethanol production per Liter and hour
2 mmol (95 mg)
% carbon from CO found in ethanol* 17
*Max. theoretical possible carbon recovery per CO consumed is 43% C, with our gas mix
ACKNOWLEDGEMENT
Funding provided by Yossie Hollander
and the Foundation de Fondateurs.