Research articlePhysicochemical characterization, modelling and optimization of ultrasono-assisted acid pretreatment of two Pennisetum sp. using Taguchi and artificial neural networking for enhanced delignification
Introduction
The fast diminution of global fossil fuel reserves and increasing price of crude oil has become one of the most daunting challenges that are encountered by the modern world. Adding to it, are the concerns of global warming due to increase in consumption of fossil fuel, which has made it essential for search of renewable alternatives that can bring the promise of energy security and environmental protection (Charles et al., 2007). Biofuels from plant biomass has become one of the important alternative energy sources which is not only renewable but is also carbon neutral. It has been estimated that by 2050, the use of biofuels must increase upto four folds which would lead to a reduction of worldwide CO2 emissions by 50% (Wang et al., 2016). Bioethanol production from different plant sources such as sugary, starchy and lignocellulosic biomasses holds great promise to be used as biofuels. Among these plant based sources, the sugary and starchy substrates can be easily processed for bioethanol production. However, use of food crops is discouraged as this will lead to food crisis. On the other hand, lignocellulosic biomass such as agricultural and forest residues, woody and herbaceous energy crops is expected to find its application as a promising source of biofuel as these do not interfere with the food source of humans (Pan et al., 2014). Recent researches have indicated that, global bioethanol production will be headed by lignocellulosic biomass which produces 1.5 × 1010 tons/year of biomass with a conversion efficiency of 60% (Chen and Peng, 2013). Among the lignocellulosic biomass, substrates from grasses could be a promising source of biofuel. The polysaccharides present in grass biomass can be converted to glucose which is subsequently fermented to alcohol.
Grass as lignocellulosic biomass have several advantages like high production, have good suitability for low quality land, cause lower environmental impacts and have low lignin content (Cha et al., 2014). However, there are certain constraints for industrial bioethanol production from grass biomass which include collection of the raw material and its complicated method of conversion to ethanol (Vucurovic et al., 2009). Bioethanol production from biomass requires enzymatic hydrolysis and fermentation. Nevertheless, lignocellulose biomass is usually complex and highly resistant to enzymatic hydrolysis because of its recalcitrance. Therefore pretreatment is needed to increase the hydrolysis rates. Various pretreatment methods have been reported for delignification of biomass using acids, alkalis, ionic solvents, flux treatments, high pressure steam explosion and ultrasonication pretreatment, etc which are applied either in single or in combination. Among these processes acid pretreatment has the advantage such as disruptions of covalent, hydrogen and van der Waals forces which leads to low requirement of enzymes in the hydrolysis step (Maurya et al., 2015). The intention of pretreatment is to open or moderately break up its highly resistant lignin-carbohydrate complex and make the cellulose more accessible for further hydrolysis or saccharification. Subsequently, during saccharification process cellulose degrading enzyme convert cellulose into simpler sugars, primarily glucose, which can then be utilised by the microorganisms for fermentation into bioethanol (Dias et al., 2013, Healey et al., 2015). Apart from acid, ultrasonication pretreatment had also been reported to play a positive impact on the pretreatment and subsequent conversion of biomass. The amalgamation of ultrasonic energy and acid pretreatment permits the destruction of the recalcitrant lignocellulosic structure, promotes the solvation and fractionation of biomass components and finally helps in increase of equilibrium yields of sugars (Ravindran and Jaiswal, 2015). Besides the ultrasonication pretreatment on the lignocellulosic biomass in an aqueous media produces cavitation. The cavitation leads to decomposition of water molecule into free radical that leads to cleavage in lignin and xylan networks (Subhedar and Gogate, 2014). Simultaneously, the collapse of cavities with the neighbouring cavities also causes pyrolysis of the molecules trapped inside these cavities (Badve et al., 2014).
Modelling and optimization are the most important tools in a biological process to increase the efficiency of the process (Betiku and Taiwo, 2015). Taguchi orthogonal array (OA) statistical design is quick and effective in the simultaneous study of many factors, with enhanced process performance, giving high yield and improved stability (Taguchi, 1986). On the other hand data analysis tools based on ANNs are now used successfully for solving biotechnological complex problems for modelling and optimization (Balaraman and Soundar, 2005). Thus, the present investigation was aimed at screening and optimization of pretreatment with different acids and different pretreatment parameters using Taguchi L16 design for efficient delignification and enhanced cellulose exposure of two Pennisetum sp. Further optimized results obtained in Taguchi experiments were validated using ANN model by using genetic algorithm as the global optimization procedure. The dilute acid showing the best response was subjected to ultrasound assisted acid pretreatment for enhanced delignification which was also optimized for different parameters. Further, untreated (native), acid pretreated and the combined pretreated biomasses (acid and ultrasonicated) were analysed by FTIR (Fourier transform infrared spectroscopy), SEM (Scanning electron microscopy) and autoflurosence analyses for assessment of structural changes.
Section snippets
Materials
Two grass verities of Pennisetum sp (Hybrid Napier grass (variety CO-3) [HNG] and Denanath grass [DG] used as lignocellulosic biomass for acid pretreatment were collected from the Directorate of Research for Women in Agriculture (DRWA), Bhubaneswar, Odisha during the month of March 2015. The collected substrates i.e the plant biomass except the roots were brought to the microbiology laboratory of Department of Biotechnology, College of Engineering and Technology, Bhubaneswar and were processed
Results and discussion
Experiment on screening of pretreatment with different acids (H2SO4, HCl, H3PO4 and H2NO3) on both the grass (DG and HNG) biomass was optimized employing Taguchi orthogonal array for soaking time, concentration of acid and temperature. The pretreatment with the acid under autoclaving condition has been denoted as AA (acid + autoclaving) in rest of the sections. The acid showing the highest cellulose exposure with the maximum removal of hemicellulose and lignin was further used for a combination
Conclusion
The present study investigated the screening of different acids in low concentrations for pretreatment of two Pennisetum sp. (DG and HNG) for enhanced delignification and maximum cellulose exposure. Dilute HCl was found to be the best acid for pretreatment of the two grass varities showing 33.0% and 31.8% delignification along with simultaneous cellulose yield of 56.5% and 46.6% for DG and HNG respectively. Statistical optimization of acid pretreatment of DG and HNG by Taguchi OA confirmed that
Acknowledgement
Authors are grateful to the authorities of college of engineering and technology, BPUT, Bhubaneswar for providing necessary laboratory facilities to carry out this experiment and to Dr. Anil Kumar, Senior scientist-DRWA, Bhubaneswar for kindly providing the two grass varieties.
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