Soaking Pretreatment of Corn Stover for Bioethanol Production Followed by Anaerobic Digestion Process

The production of ethanol and methane from corn stover (CS) was investigated in a biorefinery process. Initially, a novel soaking pretreatment (NaOH and aqueous-ammonia) for CS was developed to remove lignin, swell the biomass, and improve enzymatic digestibility. Based on the sugar yield during enzymatic hydrolysis, the optimal pretreatment conditions were 1?% NaOH?+?8?% NH₄OH, 50°C, 48?h, with a solid-to-liquid ratio 1:10. The results demonstrated that soaking pretreatment removed 63.6?% lignin while reserving most of the carbohydrates. After enzymatic hydrolysis, the yields of glucose and xylose were 78.5?% and 69.3?%, respectively. The simultaneous saccharification and fermentation of pretreated CS using Pichia stipitis resulted in an ethanol concentration of 36.1?g/L, corresponding only to 63.3?% of the theoretical maximum. In order to simplify the process and reduce the capital cost, the liquid fraction of the pretreatment was used to re-soak new CS. For methane production, the re-soaked CS and the residues of SSF were anaerobically digested for 120?days. Fifteen grams CS were converted to 1.9?g of ethanol and 1337.3?mL of methane in the entire process.     

Oxidative Lime Pretreatment of Alamo Switchgrass

Previous studies have shown that oxidative lime pretreatment is an effective delignification method that improves the enzymatic digestibility of many biomass feedstocks. The purpose of this work is to determine the recommended oxidative lime pretreatment conditions (reaction temperature, time, pressure, and lime loading) for Alamo switchgrass (Panicum virgatum). Enzymatic hydrolysis of glucan and xylan was used to determine the performance of the 52 studied pretreatment conditions. The recommended condition (110°C, 6.89 bar O₂, 240 min, 0.248 g Ca(OH)₂/g biomass) achieved glucan and xylan overall yields (grams of sugar hydrolyzed/100 g sugar in raw biomass, 15 filter paper units (FPU)/g raw glucan) of 85.9 and 52.2, respectively. In addition, some glucan oligomers (2.6 g glucan recovered/100 g glucan in raw biomass) and significant levels of xylan oligomers (26.0 g xylan recovered/100 g xylan in raw biomass) were recovered from the pretreatment liquor. Combining a decrystallization technique (ball milling) with oxidative lime pretreatment further improved the overall glucan yield to 90.0 (7 FPU/g raw glucan).     

Lime pretreatment of crop residues bagasse and wheat straw

Lime (calcium hydroxide) was used as a pretreatment agent to enhance the enzymatic digestibility of two common crop residues: bagasse and wheat straw. A systematic study of pretreatment conditions suggested that for short pretreatment times (1–3 h), high temperatures (85-135°C) were required to achieve high sugar yields, whereas for long pretreatment times (e.g., 24 h), low temperatures (50–65°C) were effective. The recommended lime loading is 0.1 g Ca(OH)₂/g dry biomass. Water loading had little effect on the digestibility. Under the recommended conditions, the 3-d reducing sugar yield of the pretreated bagasse increased from 153 to 659 mg Eq glucose/g dry biomass, and that of the pretreated wheat straw increased from 65 to 650 mg Eq glucose/g dry biomass. A material balance study on bagasse showed that the biomass yield after lime pretreatment is 93.6%. No glucan or xylan was removed from bagasse by the pretreatment, whereas 14% of lignin became solubilized. A lime recovery study showed that 86% of added calcium was removed from the pretreated bagasse by ten washings and could be recovered by carbonating the wash water with CO₂ at pH 9.5.     

Potential of Agricultural Residues and Hay for Bioethanol Production

Production of bioethanol from agricultural residues and hays (wheat, barley, and triticale straws, and barley, triticale, pearl millet, and sweet sorghum hays) through a series of chemical pretreatment, enzymatic hydrolysis, and fermentation processes was investigated in this study. Composition analysis suggested that the agricultural straws and hays studied contained approximately 28.62-38.58% glucan, 11.19-20.78% xylan, and 22.01-27.57% lignin, making them good candidates for bioethanol production. Chemical pretreatment with sulfuric acid or sodium hydroxide at concentrations of 0.5, 1.0, and 2.0% indicated that concentration and treatment agent play a significant role during pretreatment. After 2.0% sulfuric acid pretreatment at 121 degrees C/15 psi for 60 min, 78.10-81.27% of the xylan in untreated feedstocks was solubilized, while 75.09-84.52% of the lignin was reduced after 2.0% sodium hydroxide pretreatment under similar conditions. Enzymatic hydrolysis of chemically pretreated (2.0% NaOH or H2SO4) solids with Celluclast 1.5 L-Novozym 188 (cellobiase) enzyme combination resulted in equal or higher glucan and xylan conversion than with Spezyme(R) CP- xylanase combination. The glucan and xylan conversions during hydrolysis with Celluclast 1.5 L-cellobiase at 40 FPU/g glucan were 78.09 to 100.36% and 74.03 to 84.89%, respectively. Increasing the enzyme loading from 40 to 60 FPU/g glucan did not significantly increase sugar yield. The ethanol yield after fermentation of the hydrolyzate from different feedstocks with Saccharomyces cerevisiae ranged from 0.27 to 0.34 g/g glucose or 52.00-65.82% of the theoretical maximum ethanol yield.     

Oxidative Lime Pretreatment of Dacotah Switchgrass

Oxidative lime pretreatment increases the enzymatic digestibility of lignocellulosic biomass primarily by removing lignin. In this study, recommended pretreatment conditions (reaction temperature, oxygen pressure, lime loading, and time) were determined for Dacotah switchgrass. Glucan and xylan overall hydrolysis yields (72 h, 15 FPU/g raw glucan) were measured for 105 different reaction conditions involving three different reactor configurations (very short term, short term, and long term). The short-term reactor was the most productive. At the recommended pretreatment condition (120 °C, 6.89 bar O₂, 240 min), it achieved an overall glucan hydrolysis yield of 85.2 g glucan hydrolyzed/100 g raw glucan and an overall xylan yield of 50.1 g xylan hydrolyzed/100 g raw xylan. At this condition, glucan oligomers (1.80 g glucan recovered/100 g glucan in raw biomass) and xylan oligomers (25.20 g xylan recovered/100 g xylan in raw biomass) were recovered from the pretreatment liquor, which compensate for low pretreatment yields.     

Oxidative lime pretreatment of high-lignin biomass

Lime (Ca[OH]₂) and oxygen (O₂) were used to enhance the enzymatic digestibility of two kinds of high-lignin biomass: poplar wood and newspaper. The recommended pretreatment conditions for poplar wood are 150°C, 6 h, 0.1 g of Ca(OH)₂/g of dry biomass, 9 mL of water/g of dry biomass, 14.0 bar absolute oxygen, and a particle size of −10 mesh. Under these conditions, the 3-d reducing sugar yield of poplar wood using a cellulase loading of 5 filter paper units (FPU)/g of raw dry biomass increased from 62 to 565 mg of eq. glucose/g of raw dry biomass, and the 3-d total sugar (glucose + xylose) conversion increased from 6 to 77% of raw total sugars. At high cellulase loadings (e.g., 75 FPU/g of raw dry biomass), the 3-d total sugar conversion reached 97%. In a trial run with newspaper, using conditions of 140°C, 3 h, 0.3 g of Ca(OH)₂/g of dry biomass, 16 mL of water/g of dry biomass, and 7.1 bar absolute oxygen, the 3-d reducing sugar yield using a cellulase loading of 5 FPU/g of raw dry biomass increased from 240 to 565 mg of eq. glucose/g of raw dry biomass. A material balance study on poplar wood shows that oxidative lime pretreatment solubilized 38% of total biomass, including 78% of lignin and 49% of xylan; no glucan was removed. Ash increased because calcium was incorporated into biomass during the pretreatment. After oxidative lime pretreatment, about 21% of added lime could be recovered by CO₂ carbonation.     

Optimization of steam pretreatment of SO2-impregnated corn stover for fuel ethanol production

In this study, corn stover with a dry matter content of 20% was impregnated with SO₂ and then steam pretreated for various times at various temperatures. The pretreatment was evaluated by enzymatic hydrolysis of the solid material and analysis of the sugar content in the liquid. The maximum overall yield of glucose, 89% of the theoretical based on the glucan in the raw material, was achieved when the corn stover was pretreated at 200°C for 10 min. The maximum overall yield of xylose, 78%, was obtained with pretreatment at 190°C for 5 min.     

Pretreatment of switchgrass by ammonia fiber explosion (AFEX)

The effects of ammonia fiber explosion (AFEX) pretreatment of switch grass using its major process variables are reported. The optimal pretreatment conditions for switchgrass were found to be near 100°C reactor temperature, and ammonia loading of 1:1 kg of ammonia: kg of dry matter with 80% moisture content (dry weight basis [dwb]) at 5 min residence time. Hydrolysis results of AFEX-treated and untreated samples showed 93% vs 16% glucan conversion, respectively. The ethanol yield of optimized AFEX-treated switchgrass was measured to be about 0.2 g ethanol/g dry biomass, which is 2.5 times more than that of the untreated sample.     

Extrusion Pretreatment of Pine Wood Chips

Pretreatment is the first step to open up lignocellulose structure in the conversion of biomass to biofuels. Extrusion can be a viable pretreatment method due to its ability to simultaneously expose biomass to a range of disruptive conditions in a continuous flow process. Extruder screw speed, barrel temperature, and feedstock moisture content are important factors that can influence sugar recovery from biomass. Hence, the current study was undertaken to investigate the effects of these parameters on extrusion pretreatment of pine wood chips. Pine wood chip at 25, 35, and 45?% wb moisture content were pretreated at various barrel temperatures (100, 140, and 180?°C) and screw speeds (100, 150, and 200?rpm) using a screw with compression ratios of 3:1. The pretreated pine wood chips were subjected to standard enzymatic hydrolysis followed by sugar and byproducts quantification. Statistical analyses revealed the existence of significant differences in sugar recovery due to independent variables based on comparing the mean of main effects and interaction effects. Pine wood chips pretreated at a screw speed of 150?rpm and a barrel temperature of 180?°C with a moisture content of 25?% resulted in a maximum cellulose, hemicellulose, and total sugar recoveries of 65.8, 65.6, and 66.1?%, respectively, which was about 6.7, 7.9, and 6.8 fold higher than the control (unpretreated pine chips). Furthermore, potential fermentation inhibitors such as furfural, hydroxyl methyl furfural, and acetic acid were not found in any of the treatment combinations.     

Surfactant-Assisted Acid Pretreatment of Sugarcane Tops for Bioethanol Production

Sugarcane tops is one of the largest biomass resources in India and in tropical countries such as Brazil in terms of surplus availability. Conversion of this feedstock to ethanol requires pretreatment to make it more accessible for the enzymes used in saccharification. Though several pretreatment regimens have been developed for addressing biomass recalcitrance, very few seem to be promising as an industrial process. A novel hybrid method involving use of mild acid and surfactant was developed which could effectively remove lignin and improve the sugar yield from sugar cane tops. Operational parameters that affect the pretreatment efficiency (measured as yield of sugars) were studied and optimized. Changes in structural properties of the biomass were studied in relation to the pretreatment process using scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier Transform Infrared (FTIR) analysis, and the changes in chemical composition was also monitored. The biomass pretreated with the optimized novel method could yield 0.798?g of reducing sugars per gram of pretreated biomass upon enzymatic hydrolysis.     

Preliminary results on optimization of pilot scale pretreatment of wheat straw used in coproduction of bioethanol and electricity

The overall objective in this European Union-project is to develop cost and energy effective production systems for coproduction of bioethanol and electricity based on integrated biomass utilization. A pilot plan reactor for hydrothermal pretreatment (including weak acid hydrolysis, wet oxidation, and steam pretreatment) with a capacity of 100 kg/h was constructed and tested for pretreatment of wheat straw for ethanol production. Highest hemicellulose (C5 sugar) recovery and extraction of hemicellulose sugars was obtained at 190°C whereas highest C6 sugar yield was obtained at 200°C. Lowest toxicity of hydrolysates was observed at 190°C; however, addition of H₂O₂ improved the fermentability and sugar recoveries at the higher temperatures. The estimated total ethanol production was 223 kg/t straw assuming utilisation of both C6 and C5 during fermentation, and 0.5 g ethanol/g sugar.     

Pretreatment of Rice Straw by a Hot-Compressed Water Process for Enzymatic Hydrolysis

Hot-compressed water (HCW) is among several cost-effective pretreatment processes of lignocellulosic biomass for enzymatic hydrolysis. The present work investigated the characteristics of HCW pretreatment of rice straw including sugar production and inhibitor formation in the liquid fraction and enzymatic hydrolysis of pretreated material. Pretreatment was carried out at a temperature ranging from 140 to 240 °C for 10 or 30 min. Soluble oligosaccharides were found to constitute almost all the components of total sugars in the liquid fraction. The maximal production of total glucose at 180 °C and below accounted for 4.4–4.9% of glucan in raw material. Total xylose production peaked at 180 °C, accounting for 43.3% of xylan in raw material for 10-min pretreatment and 29.8% for 30-min pretreatment. The production of acetic acid increased at higher temperatures and longer treatment time, indicating more significant disruption of lignocellulosic structure, and furfural production achieved the maximum (2.8 mg/ml) at 200 °C for both 10-min and 30-min processes. The glucose yield by enzymatic hydrolysis of pretreated rice straw was no less than 85% at 180 °C and above for 30-min pretreatment and at 200 °C and above for 10-min pretreatment. Considering sugar recovery, inhibitor formation, and process severity, it is recommended that a temperature of 180 °C for a time of 30 min can be the most efficient process for HCW pretreatment of rice straw.     

Optimization of dilute-acid pretreatment of corn stover using a high-solids percolation reactor

We have previously demonstrated that pretreatment of corn stover with dilute sulfuric acid can achieve high digestibility and efficient recovery of hemicellulose sugars with high yield and concentration. Further improvement of this process was sought in this work. A modification was made in the operation of the percolation reactor that the reactor is preheated under atmospheric pressure to remove moisture that causes autohydrolysis. This eliminated sugar decomposition during the preheating stage and led to a considerable improvement in overall sugar yield. In addition, liquid throughput was minimized to the extent that only one reactor void volume of liquid was collected. This was done to attain a high xylose concentration in the hydrolyzate. The optimum reaction and operating conditions were identified wherein near quantitative enzymatic digestibilities are obtained with enzyme loading of 15 FPU/g glucan. With a reduced enzyme loading of 5 FPU/g glucan, the enzymatic digestibility was decreased, but still reached a level of 92%. Decomposition of carbohydrates was extremely low as indicated by the measured glucan and xylan mass closures (recovered sugar plus unreacted) which were 98% and 94%, respectively. The data obtained in this work indicate that the digestibility is related to the extent of xylan removal.     

Impact of sulfuric acid pretreatment of durian peel on the production of fermentable sugar and ethanol

After harvesting season, large amounts of durian peels were produced and uselessly disposed of by combustion or landfilling leading to environmental pollution and human health hazards. Proper management of these wastes is necessary to reduce not only an environmental problem but also to create value-added products. Herein, we optimized sulfuric acid pretreatment to promote enzymatic saccharification of durian peels and convert fermentable sugars to bioethanol. Three pretreatment parameters were optimized based on Response Surface Methodology (RSM), including acid concentration (0.5%–3.5%), temperature (60–140 ​°C), and time (20–100 ​min). At optimal pretreatment condition using 2.75% H₂SO₄, at 127.14 ​°C for 74.13 ​min, 0.53 ​g/g-biomass of reducing sugars were produced, which is 1.88 folds higher than the untreated durian peel. The pretreatment liquor and biomass hydrolysate were analyzed by Gas Chromatograph-Mass spectrometer (GC-MS), and fermentation inhibitors, i.e. acetic acid, furfural, and furan methanol, were identified in those fractions. Due to pretreatment at the optimal condition, a higher yield of reducing sugar was observed, and the production of ethanol from the pretreated biomass was 5.70 ​g/L (equivalent to 87.43% of theoretical yields). These findings demonstrated the potential of using durian waste in the biorefinery concept to achieve a concept of the green economy.     

Chestnut Shell as Unexploited Source of Fermentable Sugars: Effect of Different Pretreatment Methods on Enzymatic Saccharification

Chestnut shell (CS) is an agronomic residue mainly used for extraction of antioxidants or as adsorbent of metal ions. It also contains some polysaccharide that has not been considered as potential source of fermentable sugars for biofuel production until now. In this study, the effect of different pretreatment methods on CS was evaluated in order to obtain the greatest conversion of cellulose and xylan into fermentable sugars. Hot acid impregnation, steam explosion (acid-catalysed or not), and aqueous ammonia soaking (AAS) were selected as pretreatments. The pretreated biomass was subjected to saccharification with two enzyme cocktails prepared from commercial preparations, and evaluation of the best pretreatment and enzyme cocktail was based on the yield of fermentable sugars produced. As AAS provided the best result after preliminary experiments, enhancement of sugar production was attempted by changing the concentrations of ammonium hydroxide, enzymes, and CS. The optimal pretreatment condition was 10 % ammonium hydroxide, 70 °C, 22 h with CS at 5 % solid loading. After saccharification of the pretreated CS for 72 h at 50 °C and pH 5.0 with a cocktail containing cellulase (Accellerase 1500), beta-glucosidase (Accellerase BG), and xylanase (Accellerase XY), glucose and xylose yields were 67.8 and 92.7 %, respectively.     

Evaluation and Optimization of Organic Acid Pretreatment of Cotton Gin Waste for Enzymatic Hydrolysis and Bioethanol Production

This paper investigates the efficiency of the organic acids on the pretreatment of an industrially generated cotton gin waste for the removal of lignin, thereby releasing cellulose and hemicellulose as fermentable sugar components. Cotton gin waste was pretreated with various organic acids namely lactic acid, oxalic acid, citric acid, and maleic acid. Among these, maleic acid was found to be the most efficient producing maximum xylose sugar (126.05?±?0.74 g/g) at the optimum pretreatment condition of 150 °C, 500 mM, and 45 min. The pretreatment efficiency was comparable to the conventional dilute sulfuric acid pretreatment. A lignin removal of 88% was achieved by treating maleic acid pretreated biomass in a mixture of sodium sulfite and sodium chlorite. The pretreated biomass was further evaluated for the release of sugar by enzymatic hydrolysis and subsequently bioethanol production from hydrolysates. The maximum 686.13 g/g saccharification yield was achieved with maleic acid pretreated biomass which was slightly higher than the sulfuric acid (675.26 g/g) pretreated waste. The fermentation of mixed hydrolysates(41.75 g/l) produced 18.74 g/l bioethanol concentration with 2.25 g/l/h ethanol productivity and 0.48 g/g ethanol yield using sequential use of Saccharomyces cerevisiae and Pichia stipitis yeast strains. The production of bioethanol was higher than the ethanol produced using co-culture in comparison to sequential culture. Thus, it has been demonstrated that the maleic acid pretreatment and fermentation using sequential use of yeast strains are efficient for bioethanol production from cotton gin waste.     

Alkali (NaOH) Pretreatment of Switchgrass by Radio Frequency-based Dielectric Heating

Radio-frequency (RF)-based dielectric heating was used in the alkali (NaOH) pretreatment of switchgrass to enhance its enzymatic digestibility. Due to the unique features of RF heating (i.e., volumetric heat transfer, deep heat penetration of the samples, etc.), switchgrass could be treated on a large scale, high solid content, and uniform temperature profile. At 20% solid content, RF-assisted alkali pretreatment (at 0.1 g NaOH/g biomass loading and 90°C) resulted in a higher xylose yield than the conventional heating pretreatment. The enzymatic hydrolysis of RF-treated solids led to a higher glucose yield than the corresponding value obtained from conventional heating treatment. When the solid content exceeded 25%, conventional heating could not handle this high-solid sample due to the loss of fluidity, poor mixing, and heating transfer of the samples. As a result, there was a significantly lower sugar yield, but the sugar yield of the RF-based pretreatment process was still maintained at high levels. Furthermore, the optimal particle size and alkali loading in the RF pretreatment was determined as 0.25–0.50 mm and 0.25 g NaOH/g biomass, respectively. At alkali loading of 0.20–0.25 g NaOH/g biomass, heating temperature of 90^oC, and solid content of 20%, the glucose, xylose, and total sugar yield from the combined RF pretreatment and the enzymatic hydrolysis were 25.3, 21.2, and 46.5 g/g biomass, respectively.     

Ammonia fiber explosion treatment of corn stover

Optimizing process conditions and parameters such as ammonia loading, moisture content of biomass, temperature, and residence time is necessary for maximum effectiveness of the ammonia fiber explosion process. Approximate optimal pretreatment conditions for corn stover were found to be temperature of 90°C, ammonia: dry corn stover mass ratio of 1∶1, moisture content of corn stover of 60% (dry weight basis), and residence time (holding at target temperature), of 5 min. Approximately 98% of the theoretical glucose yield was obtained during enzymatic hydrolysis of the optimal treated corn stover using 60 filter paper units (FPU) of cellulase enzyme/g of glucan (equal to 22 FPU/g of dry corn stover). The ethanol yield from this sample was increased up to 2.2 times over that of untreated sample. Lowering enzyme loading to 15 and 7.5 FPU/g of glucan did not significantly affect the glucose yield compared with 60 FPU, and any differences between effects at different enzyme levels decreased as the treatment temperature increased.     

Kinetics of ethanol production from carob pods extract by immobilizedSaccharomyces cerevisiae cells

Kinetics of ethanol production from carob pods extract by immobilizedS. cerevisiae cells in static and shake flask fermentation have been investigated. Shake flask fermentation proved to be a better fermentation system for the production of ethanol than static fermentation. The optimum values of ethanol concentration, ethanol productivity, ethanol yield, and fermentation efficiency were obtained at pH range 3.5–6.5 and temperature between 30–35°C. A maximum ethanol concentration (65 g/L), ethanol productivity (8.3 g/Lh), ethanol yield (0.44 g/g), and fermentation efficiency (95%) was achieved at an initial sugar concentration of 200, 150, 100, and 200 g/L, respectively. The highest values of specific ethanol production rate and specific sugar uptake rate were obtained at pH 6.5, temperature 40°C, and initial sugar concentration of 100 g/L. Other kinetic parameters, biomass concentration, biomass yield, and specific biomass production rate were maximum at pH 5.5, temperature 30°C, and initial sugar concentration 150 g/L. Under the same fermentation conditions non-sterilized carob pod extract gave higher ethanol concentration than sterilized medium. In repeated batch fermentations, the immobilizedS. cerevisiae cells in Ca-alginate beads retained their ability to produce ethanol for 5 d.     

Carob pod: A new substrate for citric acid production by Aspergillus niger

The production of citric acid from carob pod extract byA. niger in surface fermentation was investigated. A maximum citric acid concentration (85.5 g/L), citric acid productivity (4.07 g/L/d), specific citric acid production rate (0.18 g/g/d), and specific sugar uptake rate (0.358 g/g/d) was achieved at an initial sugar concentration of 200 g/L, pH of 6.5, and a temperature of 30°C. Other kinetic parameters, namely, citric acid yield, biomass yield, specific biomass production rate, and fermentation efficiency were maximum at pH 6.5, temperature 30°C, and initial sugar concentration 100 g/L. The external addition of methanol into the carob pod extract at a concentration up to 4% (v/v) improved the production of citric acid.