PRODUCTION OF SECOND-GENERATION BIOFUELS

Biofuels are renewable fuels that are derived from biological materials including plants, microorganisms, animals, and wastes (Aro, 2016). Biofuels are the most favorable alternative for fossil fuels to tackle down the growing problem of global warming, environmental issues, and energy demand. Current research and developments are focused on biofuels such as bioethanol, biobutanol, biomethane, biohydrogen, and biodiesel. All of these biofuels except biodiesel are produced by the process of microbial fermentation of suitable carbohydrate feedstocks. Biofuels can be categorized in to first, second, and third-generation biofuels based on the feedstock utilized for conversion. First-generation biofuels are produced directly from edible food-based feedstocks which have several social, economical, and environmental issues such as increasing food price, and deforestation for farming, etc. Therefore, the production of first-generation biofuels in the future cannot be produced on a large scale without threatening food supplies (Barros-Rios et al., 2016; Ghosh et al., 2017; Tomei and Helliwell, 2016). Second-generation biofuels are produced from lignocellulosic feedstocks which do not compete with food supply. Second-generation biofuels are socially, economically, and environmentally sustainable due to the utilization of non-edible lignocellulosic biomass which are the most promising carbohydrate sources for the production of biofuels (Cheng et al., 2011; Ghosh et al., 2017).

Enzymatic hydrolysis of lignocellulosic biomass is an effective and environment- friendly method for the conversion of lignocellulosic materials into reducing sugars. The enzyme-based conversion of lignocellulosic biomass into bioenergy products such as bioethanol, biobutanol, and biohydrogen consists of four major steps which are:

  • 1. Pretreatment of lignocellulosic feedstock to make it accessible to enzymes.
  • 2. Enzymatic hydrolysis of cellulose and hemicelluloses to fermentable sugars.
  • 3. Fermentation of released sugars (during pretreatment and fermentation) into bioenergy products such as ethanol, butanol, hydrogen, and methane. Simultaneous saccharification (hydrolysis of the pretreated substrate) and fermentation can also be combined in a single bioreactor.
  • 4. Recovery of bioenergy products (Nguyen et al., 1999).

This review deals with recent developments in the enzymatic hydrolysis of pretreated lignocellulosic feedstocks (Table 5.1).

TABLE 5.1

Enzymatic Hydrolysis of Different Lignocellulosic Biomass

Source of Enzyme

Enzymes

Lignocellulosic

Substrate

Hydrolysis (Total Reducing Sugars) Yield

References

Aspergillus

nidulans

AKB-25

Cellulase, xylanase, and p-glucosidase

Pearl millet stover

64.77%

Kumar et al. (2016b)

Talaromyces stipitatus MTCC 12687

Cellulase, xylanase, and p-glucosidase

Parthenium

hysterophorous

biomass

734 mg/g

Bharti et al. (2018)

MAPS Enzymes Private Ltd. Aspergillus niger ADH-11

Cellulase. hemicellulases, and p-glucosidase

Sugarcane

bagasse

614 mg/g

Patel et al. (2017)

Aspergillus

oryzae

Cellulase, xylanase, and p-glucosidase

Saccharum

spontaneum

631.5 mg/g

Chandel et al. (2011)

Commercial

enzyme

Cellulase, xylanase, and p-glucosidase

Rice hulls

428 mg/g (90%)

Saha and Cotta (2007)

Commercial

enzyme

Cellulase-immobilized magnetic nanoparticles

Napier grass

42%

Ladole et al. (2017)

Aspergillus

fumigates

(CWSF-7)

Cellulase + commercial xylanase

Pennisetum

grass-DG

Pennisetum

grass-HNG

478.7 mg/g 483.3 mg/g

Mohapatra et al. (2018)

Penicillium roqueforti ATCC10110

Cellulase, xylanase

Sugarcane

bagasse

662.34 mg/g

de Almeida Antunes Ferraz et al. (2018)

Streptomyces

flavogriseus

AE64X

Streptomyces

flavogriseus

AE63X

Cellulase, xylanase

Arundo donax, Populus nigra, and Panicum virgatum

In a range of 82-86%

Pennacchio et al. (2018)

Inonotus

obliquus

Cellulase, xylanase, and p-glucosidase

Wheat straw Rice straw

130.24 mg/g 125.36 mg/g

Xuetal. (2018)

Thermomyces

hmuginoosus

VAPS-24

Xylanase

Rice bran

126.89 mg/g

Kumar et al. (2017)

Enzymatic Hydrolysis of Lignocellulosic Biomass for Biofuels Production

Second-generation biofuels are derived from carbohydrates available in the cell wall of plants. The lignocellulosic plant cell wall consists mainly of three types of polymers that are cellulose, hemicelluloses, and lignin which are interlinked to each other in a hetero-matrix. Middle lamella is consists of pectic substances. The ligno- cellulosic plant cell wall is complex and resistant to degradation. The carbohydrates present in the lignocellulosic plant cell wall are broken down into reducing sugars either by acid hydrolysis of enzymes. Cellulose and hemicelluloses are hydrolyzed into fermentable sugars by cellulolytic and hemicellulolytic enzymes. Ligninolytic enzymes assist the release of reducing sugars by degrading the lignin content of the pretreated lignocellulosic feedstock. The conversion of lignocellulosic materials into reducing sugars is the main factor for the high cost of ethanol production; the step of enzyme production costs up to 40% of total production expenses during ethanol production from lignocellulosic biomass (Chandra et al., 2010; Kumar et ah, 2016a). Cost-effective enzymatic hydrolysis of lignocellulosic feedstocks can lower the production cost of bioenergy products. To decrease the cost of saccharification different approaches such as high solid loadings of the substrate, recycling of enzymes for the reutilization, immobilization of enzymes, simultaneous saccharification, and fermentation have been utilized. Enzymatic hydrolysis of lignocellulosic biomass is greatly affected by various substrate and enzyme related factors.

 
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