Flow diagram of the consolidated process biorefinery scheme. Click to enlarge.

… to make bioethanol as a fuel additive or alternate fuel, a nonsugary or nonstarchy feedstock had to be brought into reality. Lignocellulosic bioethanol turned out to be a promising answer, against all scientific and socio-political debates; however, marketwise, it encountered stiff competition with existing fossil fuels, as well as sugar- or starch-derived ethanol. Hence, to make cost-competitive lignocellulosic ethanol as a high-volume−low-value fuel chemical, it was essential to counterbalance production logistics by valorizing various process intermediates, which could make the entire business model attractive to find a payback for investors.

Various bottlenecks in lignocellulosic ethanol process were already well-documented and many of them have been counteracted in various ways. Pilot-plant-level data were also scanty and did not answer all bottlenecks. It was also obvious that the lignocellulosic ethanol plant should have a biorefinery mode, with multiple products as a zero-waste technology.

—Ghosh et al.

As presented in their paper, the team used a commercial cellulase enzyme mix from Advanced Enzyme Technologies, Ltd., India and the commercial solid acid catalyst Indion 130. Kluyveromyces that had been isolated from dumping sites of crushed bagasse in a local sugar mill was used as a fermenting yeast.

They hydrolyzed the sugarcane bagasse with dilute sulfuric acid and steam, varying the acid concentration (0.25%−1% (v/v)), solid:liquid ratio (1:8−1:15), temperature (120−140 °C), and holding time (90−180 min) in different combinations to determine the optimal settings to extract the maximum amount of pentose sugar and minimum toxicants. The resulting pentose stream was neutralized by overliming, clarified via filtration, and processed for further utilization.

The acid-hydrolyzed biomass was then subjected to enzymatic saccharification. The enzyme dosage was optimized using various enzyme concentrations (3%−9% (w/w)), saccharification times (1−18 h), and solid:liquid ratios (1:8−1:15) in different combinations, at a fixed cooking temperature of 50 °C and pH 4.5. Polyethylene glycol with a molecular weight of 6000 (PEG 6000) was added under varying concentrations, based on the presence of acid-insoluble lignin (0.05− 0.25 g/g of lignin) during optimized enzymatic saccharification.

The hexose stream was passed through a 30 kDa ultrafiltration system (TFF) to separate cellulase. The cellulase-rich retentate was recycled during the next saccharification batch. The hexose-rich permeate was used for ethanol fermentation. Ethanol was recovered from fermentation broth via conventional distillation techniques.

Biphasic catalytic conversion of furfural was optimized in a Parr reactor with the pentose stream and methyl isobutyl ketone (MIBK) in 1:1 ratio and 30% w/w Indion 130 catalyst under different temperatures (120−190 °C) and reaction times (30−120 min) with autogenous pressure. Higher affinity of furfural toward MIBK over aqueous xylose stream enabled easy product recovery. Furfural was down-streamed by normal distillation.

The lignin-rich residual biomass remaining after enzymatic saccharification was pelletized and then gasified in a custom-made gasifier.

In this process, we have not concentrated the sugar broth to get higher ethanol titer; instead, we have channelized each stream as such (with very little adjustment) to get a real-time material balance. Our previous publications confirmed the high gravity fermentation capability of the thermophilic yeast. We strongly advocate the use of our wild strain Kluyveromyces sp. IIPE453 for ethanol production over Saccharomyces sp. (wild as well as genetically modified) or any other pentose fermenting strain, because of its high adaptive nature with various types of biomass hydrolysate and saccharified broth in the presence of inhibitors, enzyme, as well as PEG 6000, and process advantages that are due to high-temperature reaction. An integrated biorefinery would drastically reduce the cost of feedstock, including its transportation logistics from farm to factory, because of its availability in the premises of the mill. Bagasse as lignocellulosic biomass could be utilized more efficaciously than simple co-generation or electricity production by the prescribed process looping.

—Ghosh et al.

Resources

• Debashish Ghosh, Diptarka Dasgupta, Deepti Agrawal, Savita Kaul, Dilip Kumar Adhikari, Akhilesh Kumar Kurmi, Pankaj K. Arya, Dinesh Bangwal, and Mahendra Singh Negi (2015) “Fuels and Chemicals from Lignocellulosic Biomass: An Integrated Biorefinery Approach” Energy & Fuels doi: 10.1021/acs.energyfuels.5b00144