Enzymatic hydrolysis of biomass at high-solids loadings – A review
TL;DR: In this article, the authors provide a consolidated source of information on studies using high-solids loadings in enzymatic hydrolysis, including a brief discussion of the limitations, such as a lack of available water, difficulty with mixing and handling, insufficient mass and heat transfer, and increased concentration of inhibitors, associated with the use of high solids.
Abstract: Enzymatic hydrolysis is the unit operation in the lignocellulose conversion process that utilizes enzymes to depolymerize lignocellulosic biomass. The saccharide components released are the feedstock for fermentation. When performed at high-solids loadings (≥15% solids, w/w), enzymatic hydrolysis potentially offers many advantages over conversions performed at low- or moderate-solids loadings, including increased sugar and ethanol concentrations and decreased capital and operating costs. The goal of this review is to provide a consolidated source of information on studies using high-solids loadings in enzymatic hydrolysis. Included in this review is a brief discussion of the limitations, such as a lack of available water, difficulty with mixing and handling, insufficient mass and heat transfer, and increased concentration of inhibitors, associated with the use of high solids, as well as descriptions and findings of studies that performed enzymatic hydrolysis at high-solids loadings. Reactors designed and/or equipped for improved handling of high-solids slurries are also discussed. Lastly, this review includes a brief discussion of some of the operations that have successfully scaled-up and implemented high-solids enzymatic hydrolysis at pilot- and demonstration-scale facilities.
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TL;DR: An overview on the diversity of biomass, technological approaches and microbial contribution to the conversion of lignocellulosic biomass (LCB) into ethanol can be found in this paper.
Abstract: Bioethanol is one of the most promising and eco-friendly alternatives to fossil fuels, which is produced from renewable sources. Although almost all the current fuel ethanol is generated from edible sources (sugars and starch), lignocellulosic biomass (LCB) has drawn much attention in recent times. However, the conversion efficiency as well as ethanol yield of the biomass differs greatly with respect to the source and nature of LCB, primarily due to the variation in lignocellulosic content. Two major polysaccharides in LCB, namely, cellulose and hemicellulose firmly link to lignin and form a complex lignocellulosic network, which is highly robust and recalcitrant to depolymerization. For this reason, generation of ethanol from LCB requires a complicated conversion process that has made it commercially non-competitive. As attempts to exploit LCBs into commercial ethanol production, recent research efforts have been devoted to the techno-economic improvements of the overall conversion process, in addition to screen out promising feedstocks. This review paper presents an overview on the diversity of biomass, technological approaches and microbial contribution to the conversion of LCB into ethanol.
536 citations
TL;DR: In this paper, the authors present an overview on the first and second generation bioethanol production with a particular attention to the potential of various biomass sources, technological approaches, role of microorganisms and factors affecting ethanol production process.
Abstract: Bioethanol is an attractive biofuel having potential for energy security and environmental safety over fossil fuels. To date, numerous biomass resources have been investigated for bioethanol production, which can broadly be classified into sugars, starch and lignocellulosic biomass. However, conversion of biomass into ethanol varies considerably depending on the nature of feedstock, primarily due to the variation in biochemical composition, and so, only a few feedstocks have been exploited commercially. In recent years, the conversion process of biomass has been improved significantly, even though most of these achievements are yet to be implemented in commercial facility. All the major steps in a typical conversion process, particularly fermentation of sugars that is the common step for all biomass, are greatly influenced by microorganisms. A traditional yeast, Saccharomyces cerevisiae, and a bacterial species, Zymomonas mobilis, are widely used in the ethanol fermentation technology. Many factors affect ethanol production process, and the final yield is directly associated with the optimum conditions of these attributes. This review paper presents an overview on the first and second generation bioethanol production with a particular attention to the potential of various biomass sources, technological approaches, role of microorganisms and factors affecting ethanol production process.
526 citations
TL;DR: This review presents an overview of the initial targeted chemical processing stages for conversion of lignocellulosic biomass to platform molecules that serve as intermediates for the production of carbon-based fuels and chemicals.
374 citations
TL;DR: In this article, the authors investigated the inhibition of phenolic compounds on cellulase during enzymatic hydrolysis using vanillin as one of the typical lignin-derived phenolics and Avicel as cellulose substrate.
Abstract: Lignin-derived phenolic compounds are universal in the hydrolysate of pretreated lignocellulosic biomass. The phenolics reduce the efficiency of enzymatic hydrolysis and increase the cost of ethanol production. We investigated inhibition of phenolics on cellulase during enzymatic hydrolysis using vanillin as one of the typical lignin-derived phenolics and Avicel as cellulose substrate. As vanillin concentration increased from 0 to 10 mg/mL, cellulose conversion after 72-h enzymatic hydrolysis decreased from 53 to 26 %. Enzyme deactivation and precipitation were detected with the vanillin addition. The enzyme concentration and activity consecutively decreased during hydrolysis, but the inhibition degree, expressed as the ratio of the cellulose conversion without vanillin to the conversion with vanillin (A
0
/A), was almost independent on hydrolysis time. Inhibition can be mitigated by increasing cellulose loading or cellulase concentration. The inhibition degree showed linear relationship with the vanillin concentration and exponential relationship with the cellulose loading and the cellulase concentration. The addition of calcium chloride, BSA, and Tween 80 did not release the inhibition of vanillin significantly. pH and temperature for hydrolysis also showed no significant impact on inhibition degree. The presence of hydroxyl group, carbonyl group, and methoxy group in phenolics affected the inhibition degree. Besides phenolics concentration, other factors such as cellulose loading, enzyme concentration, and phenolic structure also affect the inhibition of cellulose conversion. Lignin-blocking agents have little effect on the inhibition effect of soluble phenolics, indicating that the inhibition mechanism of phenolics to enzyme is likely different from insoluble lignin. The inhibition of soluble phenolics can hardly be entirely removed by increasing enzyme concentration or adding blocking proteins due to the dispersity and multiple binding sites of phenolics than insoluble lignin.
151 citations
TL;DR: Different process configurations of the main technological steps (enzymatic hydrolysis, fermentation of hexose/and or pentose sugars) are discussed and their efficiencies are compared and the potential for use of immobilized biocatalysts is considered.
148 citations
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01 Jan 2007
TL;DR: A number of pretreatment technologies are under development and being tested in pilot scale for lignocellulose, which is the largest known renewable carbohydrate source as mentioned in this paper, but these are not readily accessible to enzymatic hydrolysis and require a pretreatment, which causes an extensive modifi cation of the lignosic structure.
Abstract: The economic dependency on fossil fuels and the resulting effects on climate and environment have put tremendous focus on utilizing fermentable sugars from lignocellulose, the largest known renewable carbohydrate source. The fermentable sugars in lignocellulose are derived from cellulose and hemicelluloses but these are not readily accessible to enzymatic hydrolysis and require a pretreatment, which causes an extensive modifi cation of the lignocellulosic structure. A number of pretreatment technologies are under development and being tested in pilot scale. Hydrolysis of lignocellulose carbohydrates into fermentable sugars requires a number of different cellulases and hemicellulases. The hydrolysis of cellulose is a sequential breakdown of the linear glucose chains, whereas hemicellulases must be capable of hydrolysing branched chains containing different sugars and functional groups. The technology for pretreatment and hydrolysis has been developed to an extent that is close to a commercially viable level. It has become possible to process lignocellulose at high substrate levels and the enzyme performance has been improved. Also the cost of enzymes has been reduced. Still a number of technical and scientifi c issues within pretreatment and hydrolysis remain to be solved. However, signifi cant improvements in yield and cost reductions are expected, thus making large-scale fermentation of lignocellulosic substrates possible. © 2007 Society of Chemical Industry and John Wiley & Sons, Ltd
957 citations
TL;DR: A number of pretreatment technologies are under development and being tested in pilot scale for lignocellulose, which is the largest known renewable carbohydrate source as discussed by the authors, but these are not readily accessible to enzymatic hydrolysis and require a pretreatment, which causes an extensive modification of the lignosic structure.
Abstract: The economic dependency on fossil fuels and the resulting effects on climate and environment have put tremendous focus on utilizing fermentable sugars from lignocellulose, the largest known renewable carbohydrate source. The fermentable sugars in lignocellulose are derived from cellulose and hemicelluloses but these are not readily accessible to enzymatic hydrolysis and require a pretreatment, which causes an extensive modification of the lignocellulosic structure. A number of pretreatment technologies are under development and being tested in pilot scale. Hydrolysis of lignocellulose carbohydrates into fermentable sugars requires a number of different cellulases and hemicellulases. The hydrolysis of cellulose is a sequential breakdown of the linear glucose chains, whereas hemicellulases must be capable of hydrolysing branched chains containing different sugars and functional groups. The technology for pretreatment and hydrolysis has been developed to an extent that is close to a commercially viable level. It has become possible to process lignocellulose at high substrate levels and the enzyme performance has been improved. Also the cost of enzymes has been reduced. Still a number of technical and scientific issues within pretreatment and hydrolysis remain to be solved. However, significant improvements in yield and cost reductions are expected, thus making large-scale fermentation of lignocellulosic substrates possible. © 2007 Society of Chemical Industry and John Wiley & Sons, Ltd
942 citations
"Enzymatic hydrolysis of biomass at ..." refers background in this paper
...DONG Energy has free-fall reactors in a 849 variety of sizes for research and development purposes (400 L) and has successfully scaled one 850 up to a capacity of 11,000 L [6, 64]....
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...However, the plant is capable of 908 handling up to 40% (w/w) solids in any of its process streams [6, 64]....
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TL;DR: Inhibition of enzyme adsorption by hydrolysis products appear to be the main cause of the decreasing yields at increasing substrate concentrations in the enzymatic decomposition of cellulosic biomass.
Abstract: Working at high solids (substrate) concentrations is advantageous in enzymatic conversion of lignocellulosic biomass as it increases product concentrations and plant productivity while lowering energy and water input. However, for a number of lignocellulosic substrates it has been shown that at increasing substrate concentration, the corresponding yield decreases in a fashion which can not be explained by current models and knowledge of enzyme-substrate interactions. This decrease in yield is undesirable as it offsets the advantages of working at high solids levels. The cause of the 'solids effect' has so far remained unknown. The decreasing conversion at increasing solids concentrations was found to be a generic or intrinsic effect, describing a linear correlation from 5 to 30% initial total solids content (w/w). Insufficient mixing has previously been shown not to be involved in the effect. Hydrolysis experiments with filter paper showed that neither lignin content nor hemicellulose-derived inhibitors appear to be responsible for the decrease in yields. Product inhibition by glucose and in particular cellobiose (and ethanol in simultaneous saccharification and fermentation) at the increased concentrations at high solids loading plays a role but could not completely account for the decreasing conversion. Adsorption of cellulases was found to decrease at increasing solids concentrations. There was a strong correlation between the decreasing adsorption and conversion, indicating that the inhibition of cellulase adsorption to cellulose is causing the decrease in yield. Inhibition of enzyme adsorption by hydrolysis products appear to be the main cause of the decreasing yields at increasing substrate concentrations in the enzymatic decomposition of cellulosic biomass. In order to facilitate high conversions at high solids concentrations, understanding of the mechanisms involved in high-solids product inhibition and adsorption inhibition must be improved.
596 citations
"Enzymatic hydrolysis of biomass at ..." refers background or result in this paper
...[10] coined the term solids effect to describe the persistence of a measured 123...
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...[10] said, the inhibition caused by non729 productive binding of the enzyme to lignin does not seem to have as large of an effect at low 730...
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...linear with increasing initial solids content because yield (percent conversion) decreases with 120 initial solids content (slope is a function of substrate type, pretreatment, and enzyme loading, 121 among other things) [10]....
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...Lastly, cellulase adsorption was investigated as a possible source of the solids effect [10]....
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...[10] next attempted to quantify the effects of water on the hydrolysis 660 reaction....
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TL;DR: Experiments conducted at 2%–40% (w/w) initial DM revealed that cellulose and hemicellulose conversion decreased almost linearly with increasing DM, and a decrease in ethanol yield at increasing initial DM.
Abstract: To improve process economics of the lignocellulose to ethanol process a reactor system for enzymatic liquefaction and saccharification at high-solids concentrations was developed. The technology is based on free fall mixing employing a horizontally placed drum with a horizontal rotating shaft mounted with paddlers for mixing. Enzymatic liquefaction and saccharification of pretreated wheat straw was tested with up to 40% (w/w) initial DM. In less than 10 h, the structure of the material was changed from intact straw particles (length 1-5 cm) into a paste/liquid that could be pumped. Tests revealed no significant effect of mixing speed in the range 3.3-11.5 rpm on the glucose conversion after 24 h and ethanol yield after subsequent fermentation for 48 h. Low-power inputs for mixing are therefore possible. Liquefaction and saccharification for 96 h using an enzyme loading of 7 FPU/g.DM and 40% DM resulted in a glucose concentration of 86 g/kg. Experiments conducted at 2%-40% (w/w) initial DM revealed that cellulose and hemicellulose conversion decreased almost linearly with increasing DM. Performing the experiments as simultaneous saccharification and fermentation also revealed a decrease in ethanol yield at increasing initial DM. Saccharomyces cerevisiae was capable of fermenting hydrolysates up to 40% DM. The highest ethanol concentration, 48 g/kg, was obtained using 35% (w/w) DM. Liquefaction of biomass with this reactor system unlocks the possibility of 10% (w/w) ethanol in the fermentation broth in future lignocellulose to ethanol plants.
520 citations
"Enzymatic hydrolysis of biomass at ..." refers background or methods or result in this paper
...[29] developed a reactor for use in pretreatment and enzymatic hydrolysis 836 processes with a total volume of 280 L....
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...[29] resulted in no significant differences in cellulose conversion over 841 the tested range, so energy input for mixing is significantly reduced as compared to vertically 842 oriented stirred tank reactors....
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...Reactor Design for Enzymatic Hydrolysis at High Solids 810 Several groups studying the use of high-solids loadings for enzymatic hydrolysis have 811 embraced a horizontal orientation of the reactor [6, 29, 62, 63]....
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...757 One of the highest solids loadings in enzymatic hydrolysis reported to date is 40% (w/w) 758 [29, 51]....
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...This result is not unusual, as most studies 642 performed at high-solids loadings sacrifice conversion for a more concentrated glucose product 643 [10, 29, 41]....
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506 citations
"Enzymatic hydrolysis of biomass at ..." refers background in this paper
...However, it has recently been shown that 98 hydrolysis rates of cellulase and β-glucosidase are greatly impacted by hemicellulose-derived 99 products, like xylose, xylan and xylo-oligomers [14-16]....
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...higher solids loadings may be reduced by the higher concentration of xylooligomers produced 491 with the addition of xylanases, as has recently been shown [15]....
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