• Energy Research

Abstract Lignocelluloses are often a major or sometimes the sole components in different waste streams from various sources such as industries, forestry, agriculture and municipalities. It represents an as-of-yet untapped source of fermentable sugars for significant industrial use. Many physico-chemical, structural and compositional factors hinder the hydrolysis of components present in the biomass to sugars and other organic compounds that can later be converted into fuels. During the past few years, a large number of chemical pretreatment methods including lime, acid, steam explosion, sulfur dioxide explosion, ammonia fiber explosion, ionic liquid and others have been developed for efficient pretreatment of biomass. Many pretreatment methods have shown high sugar yields i.e. more than 90% of the theoretical yield from lignocelluloses. In this review, we discuss various chemical pretreatment processes, feasibility of the processes at industrial scale in terms of the mechanisms involved, advantages, disadvantages and economic assessment. It is not possible to define the best pretreatment method as it depends on many factors such as type of lignocellulosic biomass, process parameters, environmental impact, economical feasibility, etc. However, some of these chemical pretreatments have disadvantages such as formation of inhibitory compounds especially furfural and 5-hydroxyl methyl furfural (HMF).

Lignocellulosic ethanol has been considered as an alternative transportation fuel. Utilization of hemicellulosic fraction in lignocelluloses is crucial in economical production of lignocellulosic ethanol. However, this fraction has not efficiently been utilized by traditional yeast Saccharomyces cerevisiae. Genetically modified S. cerevisiae, which can utilize xylose, has several limitations including low ethanol yield, redox imbalance, and undesired metabolite formation similar to native xylose utilizing yeasts. Besides, xylose uptake is a major issue, where sugar transport system plays an important role. These genetically modified and wild-type yeast strains have further been engineered for improved xylose uptake. Various techniques have been employed to facilitate the xylose transportation in these strains. The present review is focused on the sugar transport machineries, mechanisms of xylose transport, limitations and how to deal with xylose transport for xylose assimilation in yeast cells. The recent advances in different techniques to facilitate the xylose transportation have also been discussed.

Abstract Electricity generation from biomass has captured a lot of attention these days. Many countries have inclined to start large-scale research projects so that the microbial fuel cells could be installed to fulfill the power requirements of domestic as well as industrial sectors. The chemical energy stored in the algal biomass can be harnessed for sustainable production of fuels and other value-added products. Bioelectricity production using algae seems to be a wise approach to extract energy from sunlight in an economic and sustainable manner. It is achieved through integration of photosynthesis with microbial fuel cell (MFC). Algae have been used commonly in MFCs to reduce oxygen at cathode or as a substrate for bacteria. However, sufficient electric current can also be generated at anode, where cytochromes help indirect shuttling of electrons generated in photosystem II of the algal cells and can be called as photosynthetic algal microbial fuel cell (PAMFC). Despite being environmental friendly, low efficiency makes these neoteric systems unviable. Hence, a good understanding is needed for the bioelectrochemical mechanisms working behind the electron transfer from algae to electrode. Oxygen is also a limiting factor among different variables viz. pH, substrate loading rate etc., affecting the fuel cell performance. The present review addresses the mechanism of electron transfer in algae and algae to electrode and the factors affecting the performance of PAMFC.

Abstract Paddy straw is an abundantly available lignocellulosic biomass in Southern and South-Eastern Asia, which remains in surplus. The surplus straw is either burnt in the farms or dumped openly to get the farms evacuated for next crop, which not only creates the environmental pollution but also loses the potential energy source. It is a vital source of renewable energy and is considered to mitigate the global dependency on fossil fuels by producing sustainable biofuels. Anaerobic digestion is one of the biochemical conversion routes, which utilizes paddy straw for biogas production, and reduces the environmental pollution by preventing straw burning and generating carbon-neutral biofuel. This review paper is focused on anaerobic digestion process as one of the most cost-efficient alternatives for utilization of surplus paddy straw to produce biogas as gaseous biofuel. However, anaerobic digestion of paddy straw faces challenges of relatively low methane yield, high retention time, and instability of anaerobic digestion system due to its recalcitrant structural composition, unbalanced nutrients, and lack of efficient inoculum. These challenges could be conquered by different strategies viz pretreatment of paddy straw, co-digestion with other organic matters, selection of effective inoculum sources, and optimization of process parameters. Thus, the present review highlights the environmental impact and biomethane potential of paddy straw, major challenges and their solutions to enhance the biodigestibility, and factors responsible for process stability. Lastly, few approaches to attain the process sustainability by adopting efficient supply-chain logistics and future advanced technologies have also been discussed.

Overproduction of metabolites, high product yield and process economics are greatly influenced by the media composition used for growth and fermentation. The main purpose of this study is to enhance the ethanol production through statistical tool of response surface methodology (RSM) by optimizing media components for the growth and fermentation of thermotolerant isolates Kluyveromyces marxianus NIRE-K1 and NIRE-K3. Five different salts were used in the Face-centered Central Composite Design (FCCD), with the responses of biomass formation and ethanol production for growth and fermentation, respectively. Yeast extract and K2HPO4 were found to be the key media components for the growth and fermentation which is revealed from their interaction in both the yeast isolates. Further studies on batch fermentation kinetics using the optimized values of the medium composition for K. marxianus NIRE-K1 and NIRE-K3 resulted in final ethanol concentration of 17.73 (86.27% of theoretical ethanol yield) and 19.01 g l−1 (94.12% of theoretical ethanol yield), respectively. An increase in the ethanol yield and productivity by 11.36, 10.42% and 2.0, 2.7% was revealed in NIRE-K1 and NIRE-K3, respectively, as compared to our previous study.

An initiative has been taken to develop different solid, liquid, and gaseous biofuels as the alternative energy resources. The current research and technology based on the third generation biofuels derived from algal biomass have been considered as the best alternative bioresource that avoids the disadvantages of first and second generation biofuels. Algal biomass has been investigated for the implementation of economic conversion processes producing different biofuels such as biodiesel, bioethanol, biogas, biohydrogen, and other valuable co-products. In the present review, the recent findings and advance developments in algal biomass for improved biofuel production have been explored. This review discusses about the importance of the algal cell contents, various strategies for product formation through various conversion technologies, and its future scope as an energy security.

Abstract Paddy straw is one of the largely produced crop residues obtained after harvesting the rice crop. Environmentally unsustainable disposal of paddy straw such as uncontrolled digestion and stubble burning leads to health threats to living beings and climate change via large greenhouse gas emissions. Presence of high hexose (C6) and pentose (C5) sugars in paddy straw makes it potentially valuable source for ethanol production through hydrolysis of polysaccharides into simple sugars followed by fermentation. Utilizing these fermentable sugars of paddy straw is not only an environmentally sustainable management of paddy straw but it also generates renewable and carbon neutral energy. Simultaneous saccharification and fermentation (SSF) is one of the well-known techniques, which enhances the ethanol productivity and yield by reducing process time and preventing feedback inhibition of cellulases, respectively. The present review article focuses on the availability and ethanol potential of paddy straw in the earlier part. Further, bioprocessing of paddy straw into ethanol using SSF, environmental sustainability, economic evaluation, key challenges and solutions for ethanol production are addressed in the later part.

Thermophilic anaerobic digestion (TAD) technology has been adopted worldwide mainly due to it being a pathogen-free process in addition to the enhanced biogas yield and short hydraulic retention time (HRT). Taking the high metabolic rate of the thermophilic microbial community with highly efficient enzymatic systems into consideration, thermophiles are being widely explored as efficient inocula for lignocellulosic biomass (LCB) degradation and improved biomethane production. The advantages of TAD over mesophilic anaerobic digestion (MAD), including improved kinetics, efficient degradation of organic matter, and economic and environmental sustainability, make it one of the best strategies to be operated at moderately high temperatures. This review sheds light on the relevant role of thermophilic microorganisms as inocula in the anaerobic digestion of organic matter and factors affecting the overall process stability at high temperatures. Further, the discussion explains the strategies for enhancing the efficiency of thermophilic anaerobic digestion.

A yeast strain Kluyveromyces sp. IIPE453 (MTCC 5314), isolated from soil samples collected from dumping sites of crushed sugarcane bagasse in Sugar Mill, showed growth and fermentation efficiency at high temperatures ranging from 45 degrees C to 50 degrees C. The yeast strain was able to use a wide range of substrates, such as glucose, xylose, mannose, galactose, arabinose, sucrose, and cellobiose, either for growth or fermentation to ethanol. The strain also showed xylitol production from xylose. In batch fermentation, the strain showed maximum ethanol concentration of 82 +/- 0.5 g l(-1) (10.4% v/v) on initial glucose concentration of 200 g l(-1), and ethanol concentration of 1.75 +/- 0.05 g l(-1) as well as xylitol concentration of 11.5 +/- 0.4 g l(-1) on initial xylose concentration of 20 g l(-1) at 50 degrees C. The strain was capable of simultaneously using glucose and xylose in a mixture of glucose concentration of 75 g l(-1) and xylose concentration of 25 g l(-1), achieving maximum ethanol concentration of 38 +/- 0.5 g l(-1) and xylitol concentration of 14.5 +/- 0.2 g l(-1) in batch fermentation. High stability of the strain was observed in a continuous fermentation by feeding the mixture of glucose concentration of 75 g l(-1) and xylose concentration of 25 g l(-1) by recycling the cells, achieving maximum ethanol concentration of 30.8 +/- 6.2 g l(-1) and xylitol concentration of 7.35 +/- 3.3 g l(-1) with ethanol productivity of 3.1 +/- 0.6 g l(-1) h(-1) and xylitol productivity of 0.75 +/- 0.35 g l(-1) h(-1), respectively.