Butanol Production from Micro-algae by Combining Dark Fermentation and ABE Fermentation Equipped with Gas Sparging

Abstract

Micro-algae Chlorella sp. as the third-generation biomass for biofuel production was used to evaluate the production of butanol via two-stage dark fermentation and solventogenesis process. Batch dark fermentation was carried out to investigate the effects of initial algae concentration (20, 40, 60, 80, 100, 120 g-VS/L), initial pH (5 and 7) , Temperature (35 ºC and 55 ºC), and inoculums (mesophilic mixed-cultures and thermophilic mixed-cultures) on formation of volatile fatty acids (VFAs). The optimal effect of thermophilic mixed-cultures fermentation for 72 h at 55 ºC, initial pH 7 was investigated. The highest butyric acid production from initial algae concentration at 80 g-VS/L was produced of 10.67 g/L which is a precursor for butanol generation in the solventogenesis stage. Thermolysis pretreated microalgae 60 g-VS/L with BA medium can produce butyric acid 11.42 g/L higher than without pretreatment, which produced hydrogen production of 224.43 mL-H2 (46.76 mL-H2/g-VS). Production of volatile acid in 2 L CSTR was operated at 9 conditions. The condition of the highest product is both hydrogen production (119-124 mL-H2/ g-VS or 1,190-1288 mL-H2/L-reactor.d) and butyric acid of 5.23-5.49 was investigated on 1.35 L working volume at operation of 6-day HRT from microalgae pretreated 50 g-VS/L along with 10 g-sucrose/L by thermophile mixed culture source at 55 °C. The optimal condition from 2 L CSTR reactor was operated under scale up to 10 L CSTR reactor at 5 L working volume. The production of hydrogen yield of 161-189 mL-H2/g-VSadd and hydrogen production rate of 1,938-2,269 mL-H2/L-reactor with the production of acetic acid propionic acid and butyric acid produced during the experiment were in the range of 5.25±0.14 g/L, 0.76±0.02 g/L and 7.50±0.15 g/L, respectively was investigated. biogas production 23.4 L-biogas/d of this stage can be used as a concept for gas stripping of butanol product recovery in second CSTR fermentation. Although, flow rate 3 L/min or 4,320 L/day was used as stripping gas recovery. However, gas stripping recovery can used as circulation system of gas flow with flow rate control to get the desired flow rate value and this process can reduce the cost of operating the system. The total tags data of this sample is 172,245 with 280 observed species. The main bacterial in the fermentation of mixed culture were Comamonas spp. (33%), Lysinibacillus macrolides (23%), Advenella sp. (13%), Dysgonomonas mossii (9%), Proteiniphilum spp. (4%), Acinetobacter sp. (3%), Clostridiales spp. (3%), Acetobacter pasteurianus (2%), and Caproicproducens spp. (2%). Two ABE fermentation bacteria of Clostridium butylicum TISTR1032 and Clostridium beijerinckii ATCC10132 was investigated at difference initial concentration of 20, 40, 60, and 80 g-VS/L. Highest butanol concentration of 10.17 g/L was obtained from Clostridium beijerinckii ATCC10132 at 40 g-VS/L initial glucose concentrations. The initial concentration 40 g-VS/L of microalgae pretreatment with TYA nutrient addition could provide butanol 2.30 g/L which is higher than untreated microalgae. In addition, by studying the B/G ratio from the Taguchi experimental design, the combination of 10 g/L butyric acid with 20 g/L glucose yielded the highest butanol yield at 3.67 g/L from the 15% inoculation concentration, which butyric acid concentrations up to 14 g/L can inhibit for this fermentation system. In addition, Butanol production yield from CSTR with semi-continuously fed 20 g-VS/L glucose on TYA medium at HRT 2 day was constant in the range of 5.51 g/L, 38% hydrogen concentration with yield of 74 mL-H2/g-VS (740 mL-H2/L/d). Using stripping gas having 70% CO2 and 30% H2 at 3 L/min and using sparging time for 48 hours, the stripping gas efficiency 43.86% can calculated after removal of butanol from fermenter with gas stripping while 75.69% efficiency of condensation. Recovered butanol was obtained highest concentration of 45.17g/L during the first 4-hr gas stripping from the fermented of 5.51 g/L butanol concentration. This stripping process is suitable when fermentation process provides high solvent production yield. In addition, using rich butyric acid dark fermentation effluent with 20 g-VS/L glucose could be capable of covering 5.54 g/L butanol with hydrogen production 117.68 mL-H2/g-VS in the early stage of ABE fermentation. Acid content was increased and consequently inhibited growth and activity of Clostridium beijerinckii. COD balance of substrate from microalgae in 10 L CSTR system from two-stage fermentation was investigated. The last production in the form of solution and hydrogen at the final stage can be produced from g-COD of the substrate in the system as 45.79%.