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The production of biofuels from synthesis gas that utilizes a wide variety of biomass is an emerging concept, particularly with the focus on biomass based economy. Biomass is converted to synthesis gas via gasification, which involves partial oxidation of the biomass at high temperature. This route of ethanol or liquid biofuel production has the advantage of utilizing the entire biomass including the lignin content. Though the technology is yet to establish, there is a major breakthrough in understanding the microbial route of synthesis gas conversion. Acetogenic microorganisms such as Clostridium ljungdahlii, Clostridium aceticum, Acetobacterium woodii, Clostridium carboxidivorans and Clostridium autoethanogenum have already been reported for converting synthesis gas to ethanol and other organic acids including acetic acid.
The present research focused on microbial fermentation of synthesis gas into platform chemicals, specifically ethanol and acetic acid. Special emphasis has been laid on isolation and identification of most efficient syngas fermenting mixed bacterial consortium and enhancing the synthesis gas bioconversion into important platform chemicals (ethanol and acetic acid) by performing process and medium optimization as well as scale-up studies in higher volume bioreactor. With this aim, screening of nineteen anaerobic strains through an enrichment protocol helped to develop an efficient consortium TERI SA1 capable of utilizing syngas for ethanol and acetic acid production. Four different types of media including (ATCC 1754 PETC, DSMZ 640, modified PBM and PBM) were tested for maximum metabolites production using mixed consortium TERI SA1. PBM medium being most cost-effective and contained limited amount of nutrients resulted in the maximum amount of ethanol production (0.61 g/L), albeit accompanied by significant acetic acid (1.6 g/L) concentration and was considered as the most appropriate among the four media used for this study. Furthermore, two approaches were simultaneously used for isolation and molecular characterization of individual strains from anaerobic consortium TERI SA1 involving 16S rRNA sequencing of culturable bacterial isolates as well as making 16S rRNA gene library of total community DNA. Based on similarity search with NCBI database selected positive clones as well as strains isolated by culturable method (ASH051 and ASH052) were found most closely related with acetogenic microorganisms Clostridium scatalogenes and Clostridium drakei. However, comparative study indicated that mixed consortium TERI SA1 was most efficient in comparison to isolated individual bacterial stains ASH051 and ASH052 with the maximum ethanol and acetic acid production achieved upto 0.6 g/L and 1.65 g/L respectively. Therefore, mixed consortium TERI SA1 was considered as best candidate for bioconversion of syngas into ethanol and volatile fatty acids in comparison to individual bacterial isolates ASH051 and ASH052.
Physiological and operational parameters were optimized for enhanced ethanol and acetic acid production. The optimized value of operational parameters i.e. initial medium pH, incubation temperature, initial syngas pressure and agitation speed were 6.0 ± 0.1, 37 â°C, 2 kg/cm2 and 100 rpm respectively. Under these conditions ethanol and acetic acid production by the selected mixed consortium were 1.54 g/L and 0.8 g/L respectively. However, semi-continuous fermentation studies under optimized conditions further enhanced ethanol and acetic acid production up to 2.2 g/L and 0.9 g/L respectively. Further, medium optimization study was carried out in order to investigate the effect of three nitrogen sources on metabolite formation from synthesis gas bioconversion: (yeast extract (0.0–2.0 g/L), ammonium chloride (0.0–1.5 g/L) and corn steep liquor (0.0-10 g/L). Optimized parameters enhanced the production of volatile fatty acids upto 3.9 g/L, which indicated an increase of around 172% over previous experimental studies using mixed consortium TERI SA1. However, final ethanol concentration remained at 0.3 g/L. This was due to the additional nitrogen sources provided for bacterial cell growth which increased growth period of bacterial cell and thus resulted in increased concentration of volatile fatty acids. VFAs, in turn can be utilized as a renewable feedstock and building block for the production of various important biofuels and biochemicals for industrial uses.
Finally, a 10 L horizontal prototype was designed specifically for syngas bioconversion, to enhance ethanol and acetic acid production. The reactor was designed horizontally to increase the contact area between the gaseous components (CO and H2) and the liquid medium inside the reactor. The experiment was performed in such a way that nearly one third (3 L) of the reactor volume was filled with the medium, and the rest (7 L) of the reactor headspace was pressurized with syngas. The efficiently designed 10 L horizontal bioreactor operated under semi-continuous fermentation conditions enhanced final ethanol and acetic acid concentrations up to 1.43 g/L and 6.7 g/L respectively. This indicated 467 % and 171 % respective improvement in the product concentrations from previous small volume serum bottle nitrogen source optimization studies. From these bases, currently designed horizontal prototype can be further modified to two stage bioreactor, which is reported for maximum bioethanol production in extant syngas fermentation studies. In addition, after successful investigations in 10 L bioreactor an efficient 100 L prototype can be designed for pilot scale fermentation studies in higher volume bioreactor
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