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In the last few decades, there has been a huge world-wide interest in production of fuels and chemicals from biomass due to concerns such as rapid fossil fuel depletion and climate change. Over 500 million tons of agro-residues are generated in India every year, out of which 150–200 million tons is estimated to be surplus. A large fraction of these residues can be used for production of ethanol and other chemicals. Pretreatment, consisting of breaking down the complex biopolymers into fermentable sugars, is one of the most significant steps for bioconversion of lignocellulosic biomass.
The present work focused on utilization of rice straw and sweet sorghum bagasse, two important agro-residues available in India, with the overall objective of increasing the recovery of sugars and reducing the fermentation-inhibiting compounds such as acetic acid, furfural and phenolics. A two-stage pretreatment process was optimized for ash and lignin removal and high polysaccharide recovery from the biomass. For rice straw, dilute sulphuric acid pretreatment was performed in the first stage for hemicellulose fractionation while alkalineperoxide delignification was carried out during the second stage to obtain a cellulose-rich residue for subsequent enzymatic saccharification. For sweet sorghum bagasse, steam treatment was investigated in addition to dilute acid hydrolysis. The effect of various process parameters such as temperature, acid concentration, reaction time, particle size, and substrate concentration were investigated for dilute acid hydrolysis. Also, an attempt has been made to develop a Toxicity index (TI) for quantifying the efficacy of pretreatment and fermentability of sugars and for correlating it with the combined severity (CS) of acid hydrolysis. Furthermore, the unburnt carbon separated from sugar cane bagasse fly ash has been investigated as a low cost adsorbent for removal of degradation compounds from the dilute acid hydrolysate.
For rice straw, the optimum conditions for dilute acid hydrolysis were obtained as: 1% (w/w) H2SO4 concentration, 60 min reaction time and a reaction temperature of 121°C at 10% substrate concentration. This was followed by alkaline-peroxide delignification with 1.5% NaOH and 0.5% H2O2 at 121°C for 60 min, which lead to >90%pure cellulose and ~95% delignification (lignin removal). Total sugars yield from the two-stage chemical pretreatment followed by enzymatic saccharification yielded 36.5 g sugars/100 g dry rice straw, corresponding to 59.1% of the theoretical maximum sugar recovery. For sweet sorghum bagasse, two-stage chemical pretreatment (dilute acid hydrolysisalkaline peroxide delignification)-enzymatic saccharification and steam treatment-enzymatic saccharification, resulted in 40.95 g and 53.1 g sugars/100 g dry bagasse, corresponding to 54.1% and 70.1% of the theoretical maximum sugar recovery respectively. In comparison to Pichia stipitis NCIM 3497, Debaryomyces hansenii sp. was found to be a better microorganism for ethanol production from the hydrolysate/condensate from steam treatment. Based on the range examined in this work, 10% substrate loading in the CS range of 1.64-1.93 was found to be optimal (C5 yield > 22.3 g/ 100 g bagasse, TS/TDC > 8). Bagasse particle size below 180 μm resulted in a high ratio of total sugars to total degradation compounds. Detoxification of acid hydrolysate obtained for sweet sorghum bagasse with 1% (w/w) activated carbon dosage of modified deashed carbon (AC5), resulted in maximum furfural removal of 62.2%. Except for phenolics, the performance of AC5 carbon was comparable to commercial activated carbon (AC1) and unmodified unburnt carbon (AC2). It was observed that TI should be lower than 0.1 in order to obtain good ethanol yield.
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