Simulation of integrated first and second generation bioethanol production from sugarcane: comparison between different biomass pretreatment methods

Sugarcane bagasse is used as a fuel in conventional bioethanol production, providing heat and power for the plant; therefore, the amount of surplus bagasse available for use as raw material for second generation bioethanol production is related to the energy consumption of the bioethanol production process. Pentoses and lignin, byproducts of the second generation bioethanol production process, may be used as fuels, increasing the amount of surplus bagasse. In this work, simulations of the integrated bioethanol production process from sugarcane, surplus bagasse and trash were carried out. Selected pre-treatment methods followed, or not, by a delignification step were evaluated. The amount of lignocellulosic materials available for hydrolysis in each configuration was calculated assuming that 50% of sugarcane trash is recovered from the field. An economic risk analysis was carried out; the best results for the integrated first and second generation ethanol production process were obtained for steam explosion pretreatment, high solids loading for hydrolysis and 24–48 h hydrolysis. The second generation ethanol production process must be improved (e.g., decreasing required investment, improving yields and developing pentose fermentation to ethanol) in order for the integrated process to be more economically competitive.     

Weedy lignocellulosic feedstock and microbial metabolic engineering: advancing the generation of ‘Biofuel’

Lignocellulosic materials are the most abundant renewable organic resources (~200 billion tons annually) on earth that are readily available for conversion to ethanol and other value-added products, but they have not yet been tapped for the commercial production of fuel ethanol. The lignocellulosic substrates include woody substrates such as hardwood (birch and aspen, etc.) and softwood (spruce and pine, etc.), agro residues (wheat straw, sugarcane bagasse, corn stover, etc.), dedicated energy crops (switch grass, and Miscanthus etc.), weedy materials (Eicchornia crassipes, Lantana camara etc.), and municipal solid waste (food and kitchen waste, etc.). Despite the success achieved in the laboratory, there are limitations to success with lignocellulosic substrates on a commercial scale. The future of lignocellulosics is expected to lie in improvements of plant biomass, metabolic engineering of ethanol, and cellulolytic enzyme-producing microorganisms, fullest exploitation of weed materials, and process integration of the individual steps involved in bioethanol production. Issues related to the chemical composition of various weedy raw substrates for bioethanol formation, including chemical composition-based structural hydrolysis of the substrate, need special attention. This area could be opened up further by exploring genetically modified metabolic engineering routes in weedy materials and in biocatalysts that would make the production of bioethanol more efficient.     

Wet disk milling pretreatment without sulfuric acid for enzymatic hydrolysis of rice straw

Rice straw has recently attracted interest in Japan as a potential source of raw material for ethanol production. Wet disk milling, a continuous pretreatment to enhance the enzymatic digestibility of rice straw, was compared with conventional ball milling and hot-compressed water treatment. Pretreated rice straw was evaluated by enzymatic hydrolysis using Acremonium cellulase and characterized by X-ray diffraction and scanning electron microscopy. Glucose and xylose yields by wet disk milling, ball milling, and hot-compressed water treatment were 78.5% and 41.5%, 89.4% and 54.3%, and 70.3% and 88.6%, respectively. Wet disk milling and hot-compressed water treatment increased sugar yields without decreasing their crystallinity. The feature size of the wet disk milled rice straw was similar to that of hot-compressed water-treated rice straw. The energy consumption of wet disk milling was lower than that of other pretreatments. Thus, wet disk milling is an economical, practical pretreatment for the enzymatic hydrolysis of lignocellulosic biomass, especially herbaceous biomass such as rice straw.     

Alkaline-sulfite pretreatment and use of surfactants during enzymatic hydrolysis to enhance ethanol production from sugarcane bagasse

Sugarcane bagasse is a by-product from the sugar and ethanol industry which contains approximately 70 % of its dry mass composed by polysaccharides. To convert these polysaccharides into fuel ethanol it is necessary a pretreatment step to increase the enzymatic digestibility of the recalcitrant raw material. In this work, sugarcane bagasse was pretreated by an alkaline-sulfite chemithermomechanical process for increasing its enzymatic digestibility. Na₂SO₃ and NaOH ratios were fixed at 2:1, and three increasing chemical loads, varying from 4 to 8 % m/m Na₂SO₃, were used to prepare the pretreated materials. The increase in the alkaline-sulfite load decreased the lignin content in the pretreated material up to 35.5 % at the highest chemical load. The pretreated samples presented enhanced glucose yields during enzymatic hydrolysis as a function of the pretreatment severity. The maximum glucose yield (64 %) was observed for the samples pretreated with the highest chemical load. The use of 2.5 g l^?1 Tween 20 in the hydrolysis step further increased the glucose yield to 75 %. Semi-simultaneous hydrolysis and fermentation of the pretreated materials indicated that the ethanol yield was also enhanced as a function of the pretreatment severity. The maximum ethanol yield was 56 ± 2 % for the sample pretreated with the highest chemical load. For the sample pretreated with the lowest chemical load (2 % m/m NaOH and 4 % m/m Na₂SO₃), adding Tween 20 during the hydrolysis process increased the ethanol yield from 25 ± 3 to 39.5 ± 1 %.     

Improved one‐step steam pretreatment of SO2‐Impregnated softwood with time‐dependent temperature profile for ethanol production

In the production of ethanol from lignocellulosic material, pretreatment of the raw material before enzymatic hydrolysis and fermentation is essential to obtain high overall yields of sugar and ethanol. Two‐step steam pretreatment results in higher ethanol yields from softwood than the standard one‐step pretreatment process. However, the difficulty with separation and washing of the material at high pressure between the two pretreatment steps is a major drawback. In this study, a new one‐step pretreatment procedure was investigated, in which the time‐temperature profile was varied during pretreatment. The efficiency of pretreatment was assessed by performing simultaneous saccharification and fermentation on the pretreated slurries. Pretreatment of SO₂‐impregnated softwood performed by varying the temperature (190–226°C), the residence time (5–10 min), and the mode of temperature increase (linear or stepwise), resulted in recovery of about 90% of the mannose and glucose present in the raw material. The highest ethanol yield, 75% of theoretical based on the glucan and mannan content of the raw material, was obtained at pretreatment conditions of 190°C for 12 min. Similar ethanol yields were achieved when running the pretreatment as one‐step (190–200°C), two levels of temperature, at shorter residence time (7 min), which results in lower capital costs for the process. © 2010 American Institute of Chemical Engineers Biotechnol. Prog., 2010     

Steam pretreatment of H(2)SO(4)-impregnated Salix for the production of bioethanol

In the bioconversion of lignocellulosic materials to ethanol, pretreatment of the material prior to enzymatic hydrolysis is essential to obtain high overall yields of sugar and ethanol. In this study, steam pretreatment of fast-growing Salix impregnated with sulfuric acid has been investigated by varying the temperature (180-210 degrees C), the residence time (4, 8 or 12 min), and the acid concentration (0.25% or 0.5% (w/w) H(2)SO(4)). High sugar recoveries were obtained after pretreatment, and the highest yields of glucose and xylose after the subsequent enzymatic hydrolysis step were 92% and 86% of the theoretical, respectively, based on the glucan and xylan contents of the raw material. The most favorable pretreatment conditions regarding the overall sugar yield were 200 degrees C for either 4 or 8 min using 0.5% sulfuric acid, both resulting in a total of 55.6g glucose and xylose per 100g dry raw material. Simultaneous saccharification and fermentation experiments were performed on the pretreated slurries at an initial water-insoluble content of 5%, using ordinary baker's yeast. An overall theoretical ethanol yield of 79%, based on the glucan and mannan content in the raw material, was obtained.     

Enzymatic hydrolysis of sugarcane bagasse and straw mixtures pretreated with diluted acid

Abstract

In Brazil, sugarcane biomass is generated in large amounts. Sugarcane bagasse and straw are considered as an important feedstock for renewable energy and biorefinery. This paper aims to study the generation of monosaccharides (C5 and C6) from sugarcane biomass via processing bagasse or straw and mixtures of both materials (bagasse:straw 3:1, 1:1 and 1:3). Samples were pretreated with sulfuric acid which resulted in approximately 90% of hemicellulose solubilization, corresponding to around 58 g L^{? 1} of xylose. Pretreated straw showed greater susceptibility to enzymatic hydrolysis in comparison to bagasse, as shown by glucose yields of 76% and 65%, respectively, whereas the mixtures showed intermediate yields. Thus, one strategy to balance sugarcane biomass availability and possibly increasing 2G ethanol production would be to use bagasse–straw mixtures in appropriate ratios according to market fluctuations. Untreated and pretreated samples were analyzed using X-ray diffraction, but there was no relationship to enzymatic hydrolysis.     

Microbial production host selection for converting second-generation feedstocks into bioproducts

Background  

Increasingly lignocellulosic biomass hydrolysates are used as the feedstock for industrial fermentations. These biomass hydrolysates are complex mixtures of different fermentable sugars, but also inhibitors and salts that affect the performance of the microbial production host. The performance of six industrially relevant microorganisms, i.e. two bacteria (Escherichia coli and Corynebacterium glutamicum), two yeasts (Saccharomyces cerevisiae and Pichia stipitis) and two fungi (Aspergillus niger and Trichoderma reesei) were compared for their (i) ability to utilize monosaccharides present in lignocellulosic hydrolysates, (ii) resistance against inhibitors present in lignocellulosic hydrolysates, (iii) their ability to utilize and grow on different feedstock hydrolysates (corn stover, wheat straw, sugar cane bagasse and willow wood). The feedstock hydrolysates were generated in two manners: (i) thermal pretreatment under mild acid conditions followed by enzymatic hydrolysis and (ii) a non-enzymatic method in which the lignocellulosic biomass is pretreated and hydrolyzed by concentrated sulfuric acid. Moreover, the ability of the selected hosts to utilize waste glycerol from the biodiesel industry was evaluated.     

Alkali‐based AFEX pretreatment for the conversion of sugarcane bagasse and cane leaf residues to ethanol

Sugarcane is one of the major agricultural crops cultivated in tropical climate regions of the world. Each tonne of raw cane production is associated with the generation of 130 kg dry weight of bagasse after juice extraction and 250 kg dry weight of cane leaf residue postharvest. The annual world production of sugarcane is ～1.6 billion tones, generating 279 MMT tones of biomass residues (bagasse and cane leaf matter) that would be available for cellulosic ethanol production. Here, we investigated the production of cellulosic ethanol from sugar cane bagasse and sugar cane leaf residue using an alkaline pretreatment: ammonia fiber expansion (AFEX). The AFEX pretreatment improved the accessibility of cellulose and hemicelluloses to enzymes during hydrolysis by breaking down the ester linkages and other lignin carbohydrate complex (LCC) bonds and the sugar produced by this process is found to be highly fermentable. The maximum glucan conversion of AFEX pretreated bagasse and cane leaf residue by cellulases was ～85%. Supplementation with hemicellulases during enzymatic hydrolysis improved the xylan conversion up to 95–98%. Xylanase supplementation also contributed to a marginal improvement in the glucan conversion. AFEX‐treated cane leaf residue was found to have a greater enzymatic digestibility compared to AFEX‐treated bagasse. Co‐fermentation of glucose and xylose, produced from high solid loading (6% glucan) hydrolysis of AFEX‐treated bagasse and cane leaf residue, using the recombinant Saccharomyces cerevisiae (424A LNH‐ST) produced 34–36 g/L of ethanol with 92% theoretical yield. These results demonstrate that AFEX pretreatment is a viable process for conversion of bagasse and cane leaf residue into cellulosic ethanol. Biotechnol. Bioeng. 2010;107: 441–450. © 2010 Wiley Periodicals, Inc.     

A strain of <Emphasis Type="Italic">Meyerozyma guilliermondii</Emphasis> isolated from sugarcane juice is able to grow and ferment pentoses in synthetic and bagasse hydrolysate media

The search for new microbial strains that are able to withstand inhibitors released from hemicellulosic hydrolysis and are also still able to convert sugars in ethanol/xylitol is highly desirable. A yeast strain isolated from sugarcane juice and identified as Meyerozyma guilliermondii was evaluated for the ability to grow and ferment pentoses in synthetic media and in sugarcane bagasse hydrolysate. The yeast grew in xylose, arabinose and glucose at the same rate at an initial medium pH of 5.5. At pH 4.5, the yeast grew more slowly in arabinose. There was no sugar exhaustion within 60 h. At higher xylose concentrations with a higher initial cell concentration, sugar was exhausted within 96 h at pH 4.5. An increase of 350 % in biomass was obtained in detoxified hydrolysates, whereas supplementation with 3 g/L yeast extract increased biomass production by approximately 40 %. Ethanol and xylitol were produced more significantly in supplemented hydrolysates regardless of detoxification. Xylose consumption was enhanced in supplemented hydrolysates and arabinose was consumed only when xylose and glucose were no longer available. Supplementation had a greater impact on ethanol yield and productivity than detoxification; however, the product yields obtained in the present study are still much lower when compared to other yeast species in bagasse hydrolysate. By the other hand, the fermentation of both xylose and arabinose and capability of withstanding inhibitors are important characteristics of the strain assayed.     

Combined ensiling and hydrothermal processing as efficient pretreatment of sugarcane bagasse for 2G bioethanol production

Background

Ensiling cannot be utilized as a stand-alone pretreatment for sugar-based biorefinery processes but, in combination with hydrothermal processing, it can enhance pretreatment while ensuring a stable long-term storage option for abundant but moist biomass. The effectiveness of combining ensiling with hydrothermal pretreatment depends on biomass nature, pretreatment, and silage conditions.

Results

In the present study, the efficiency of the combined pretreatment was assessed by enzymatic hydrolysis and ethanol fermentation, and it was demonstrated that ensiling of sugarcane bagasse produces organic acids that can partly degrade biomass structure when in combination with hydrothermal treatment, with the consequent improvement of the enzymatic hydrolysis of cellulose and of the overall 2G bioethanol process efficiency. The optimal pretreatment conditions found in this study were those using ensiling and/or hydrothermal pretreatment at 190 °C for 10 min as this yielded the highest overall glucose recovery yield and ethanol yield from the raw material (0.28–0.30 g/g and 0.14 g/g, respectively).

Conclusion

Ensiling prior to hydrothermal pretreatment offers a controlled solution for wet storage and long-term preservation for sugarcane bagasse, thus avoiding the need for drying. This preservation method combined with long-term storage practice can be an attractive option for integrated 1G/2G bioethanol plants, as it does not require large capital investments or energy inputs and leads to comparable or higher overall sugar recovery and ethanol yields.

     

Continuous Ethanol Fermentation of Pretreated Lignocellulosic Biomasses,Waste Biomasses,Molasses and Syrup Using the Anaerobic,Thermophilic Bacterium Thermoanaerobacter italicus Pentocrobe 411

Lignocellosic ethanol production is now at a stage where commercial or semi-commercial plants are coming online and, provided cost effective production can be achieved, lignocellulosic ethanol will become an important part of the world bio economy. However, challenges are still to be overcome throughout the process and particularly for the fermentation of the complex sugar mixtures resulting from the hydrolysis of hemicellulose. Here we describe the continuous fermentation of glucose, xylose and arabinose from non-detoxified pretreated wheat straw, birch, corn cob, sugar cane bagasse, cardboard, mixed bio waste, oil palm empty fruit bunch and frond, sugar cane syrup and sugar cane molasses using the anaerobic, thermophilic bacterium Thermoanaerobacter Pentocrobe 411. All fermentations resulted in close to maximum theoretical ethanol yields of 0.47–0.49 g/g (based on glucose, xylose, and arabinose), volumetric ethanol productivities of 1.2–2.7 g/L/h and a total sugar conversion of 90–99% including glucose, xylose and arabinose. The results solidify the potential of Thermoanaerobacter strains as candidates for lignocellulose bioconversion.     

Production of bioactive polysaccharides by Inonotus obliquus under submerged fermentation supplemented with lignocellulosic biomass and their antioxidant activity

The effect of lignocellulose degradation in wheat straw, rice straw, and sugarcane bagasse on the accumulation and antioxidant activity of extra- (EPS) and intracellular polysaccharides (IPS) of Inonotus obliquus under submerged fermentation were first evaluated. The wheat straw, rice straw, and sugarcane bagasse increased the EPS accumulation by 91.4, 78.6, and 74.3 % compared with control, respectively. The EPS and IPS extracts from the three lignocellulose media had significantly higher hydroxyl radical- and 2,2-diphenyl-1-picrylhydrazyl radical-scavenging activity than those from the control medium. Of the three materials, wheat straw was the most effective lignocellulose in enhancing the mycelia growth, accumulation and antioxidant activity of I. obliquus polysaccharides (PS). The carbohydrate and protein content, as well as the monosaccharide compositions of the EPS and IPS extracts, were correlated with sugar compositions and dynamic contents during fermentation of individual lignocellulosic materials. The enhanced accumulation of bioactive PS of cultured I. obliquus supplemented with rice straw, wheat straw, and bagasse was evident.     

Ethanol from lignocellulosic materials by a simultaneous saccharification and fermentation process (SFS) with Kluyveromyces marxianus CECT 10875

Simultaneous saccharification and fermentation (SSF) process for ethanol production from various lignocellulosic woody (poplar and eucalyptus) and herbaceous (Sorghum sp. bagasse, wheat straw and Brassica carinata residue) materials has been assayed using the thermotolerant yeast strain Kluyveromyces marxianus CECT 10875. Biomass samples were previously treated in a steam explosion pilot plant to provide pretreated biomass with increased cellulose content relative to untreated materials and to enhance cellulase accessibility. SSF experiments were performed in laboratory conditions at 42 °C, 10% (w/v) substrate concentration and 15 FPU/g substrate of commercial cellulase. The results indicate that it is possible to reach SSF yields in the range of 50–72% of the maximum theoretical SSF yield, based on the glucose available in pretreated materials, in 72–82 h. Maximum ethanol contents from 16 to 19 g/l were obtained in fermentation media, depending on the material tested.     

Production of xylanase under solid-state fermentation by <Emphasis Type="Italic">Aspergillus tubingensis</Emphasis> JP-1 and its application

The production of extracellular xylanase by a locally isolated strain of Aspergillus tubingensis JP-1 was studied under solid-state fermentation. Among the various agro residues used wheat straw was found to be the best for high yield of xylanase with poor cellulase production. The influence of various parameters such as initial pH, moisture, moistening agents, nitrogen sources, additives, surfactants and pretreatment of substrates were investigated. The production of the xylanase reached a peak in 8 days using untreated wheat straw with modified MS medium, pH 6.0 at 1:5 moisture level at 30 °C. Under optimized conditions yield as high as 6,887 ± 16 U/g of untreated wheat straw was achieved. Crude xylanase was used for enzymatic saccharification of agro-residues like wheat straw, rice bran, wheat bran, sugarcane bagasse and industrial paper pulp. Dilute alkali (1 N NaOH) and acid (1 N H₂SO₄) pretreatment were found to be beneficial for the efficient enzymatic hydrolysis of wheat straw. Dilute alkali and acid-pretreated wheat straw yielded 688 and 543 mg/g reducing sugar, respectively. Yield of 726 mg/g reducing sugar was obtained from paper pulp after 48 h of incubation.     

An effective chemical pretreatment method for lignocellulosic biomass with substituted imidazoles

Lignocellulosic biomass is the most abundant naturally renewable organic resource for biofuel production. Because of its recalcitrance to enzymatic degradation, pretreatment is a crucial step before hydrolysis of the feedstock. A variety of pretreatment methods have been developed and intensively studied to achieve optimal yield without imposing significant adverse impact on the environment. Herein, we present a novel chemical pretreatment method using substituted heterocycles with low temperature and short residence time requirements. 1‐Methylimidazole (MI) is a precursor to some imidazolium‐based ionic liquids. In this study, its potential utilization as a biomass pretreatment agent is being investigated for the first time. At mild conditions, such as 25°C for 5 min at ambient pressure, a substantial increase in the hydrolysis rate throughout the entire course of conversion for cellulose substrate was obtained. Furthermore, the pretreatment effectiveness of MI on both untreated and steam‐exploded lignocellulosic biomass including loblolly pine, switchgrass, and sugarcane bagasse has been studied and MI was found to be an efficient delignifier. Remarkable rate enhancement was also observed for the non‐woody lignocellulosic substrates after a short period of MI pretreatment at ambient conditions. The mechanism of MI pretreatment is explored through analysis of cellulose physical properties including crystallinity index, degree of polymerization, accessibility, and lignin dissolution quantification. © 2014 American Institute of Chemical Engineers Biotechnol. Prog., 31:25–34, 2015     

Suitability of some local agro-industrial wastes as carrier materials for cyanobacterial inoculant

Survival and nitrogenase efficiency ofNostoc commune andN. austinii were evaluated monthly in four carrier materials (sugarcane bagasse, wheat straw, wheat bran and peat) at 10, 30 and 40 °C. Survival, as well as nitrogenase activity, of both species was much better in peat, followed by wheat bran, sugarcane bagasse than in wheat straw at 10 and 30 °C up to three months, the activity ofN. commune being better thanN. austinii. None of the materials tested was found to be superior to peat as carrier ofNostoc species but the results indicated that wheat bran and sugarcane bagasse can be used as inoculant carriers with relative success. Storage of inoculants in these carriers is feasible at 30 °C up to three months.     

Optimization of organic acid pretreatment of wheat straw

The objective of this study was to determine the effectiveness of different organic acids (maleic, succinic, and oxalic acid) on enzymatic hydrolysis and fermentation yields of wheat straw. It was also aimed to optimize the process conditions (temperature, acid concentration, and pretreatment time) by using response surface methodology (RSM). In line with this objective, the wheat straw samples were pretreated at three different temperatures (170, 190, and 210°C), acid concentrations (1%, 3%, and 5%) and pretreatment time (10, 20, and 30 min). The findings show that at extreme pretreatment conditions, xylose was solubilized in liquid phase, causing an increase in cellulose and lignin content of biomass. Enzymatic hydrolysis experiments revealed that maleic and oxalic acids were quite effective at achieving high sugar yields (>90%) from wheat straw. In contrast, the highest sugar yields were 50–60%, when the samples were pretreated with succinic acid, indicating that succinic acid was not as effective. The optimum process conditions for maleic acid were, 210°C, 1.08% acid concentration, and 19.8 min; for succinic acid 210°C, 5% acid concentration, and 30 min; for oxalic acid 210°C, 3.6% acid concentration, and 16.3 min. The ethanol yields obtained at optimum conditions were 80, 79, and 59% for maleic, oxalic and succinic acid, respectively. © 2016 American Institute of Chemical Engineers Biotechnol. Prog., 32:1487–1493, 2016     

Efficient bioconversion of rice straw to ethanol with TiO2/UV pretreatment

Rice straw is a lignocellulosic biomass that constitutes a renewable organic substance and alternative source of energy; however, its structure confounds the liberation of monosaccharides. Pretreating rice straw using a TiO(2)/UV system facilitated its hydrolysis with Accellerase 1000(?), suggesting that hydroxyl radicals (OH·) from the TiO(2)/UV system could degrade lignin and carbohydrates. TiO(2)/UV pretreatment was an essential step for conversion of hemicellulose to xylose; optimal conditions for this conversion were a TiO(2) concentration of 0.1% (w/v) and an irradiation time of 2 h with a UV-C lamp at 254 nm. After enzymatic hydrolysis, the sugar yields from rice straw pretreated with these parameters were 59.8 ± 0.7% of the theoretical for glucose (339 ± 13 mg/g rice straw) and 50.3 ± 2.8% for xylose (64 ± 3 mg/g rice straw). The fermentation of enzymatic hydrolysates containing 10.5 g glucose/L and 3.2 g xylose/L with Pichia stipitis produced 3.9 g ethanol/L with a corresponding yield of 0.39 g/g rice straw. The maximum possible ethanol conversion rate is 76.47%. TiO(2)/UV pretreatment can be performed at room temperature and atmospheric pressure and demonstrates potential in large-scale production of fermentable sugars.     

Hydrolysates of lignocellulosic materials for biohydrogen production

Lignocellulosic materials are commonly used in bio-H₂ production for the sustainable energy resource development as they are abundant, cheap, renewable and highly biodegradable. In the process of the bio-H₂ production, the pretreated lignocellulosic materials are firstly converted to monosaccharides by enzymolysis and then to H₂ by fermentation. Since the structures of lignocellulosic materials are rather complex, the hydrolysates vary with the used materials. Even using the same lignocellulosic materials, the hydrolysates also change with different pretreatment methods. It has been shown that the appropriate hydrolysate compositions can dramatically improve the biological activities and bio-H₂ production performances. Over the past decades, hydrolysis with respect to different lignocellulosic materials and pretreatments has been widely investigated. Besides, effects of the hydrolysates on the biohydrogen yields have also been examined. In this review, recent studies on hydrolysis as well as their effects on the biohydrogen production performance are summarized. [BMB Reports 2013; 46(5): 244-251]