Two-stage dilute-acid pretreatment of softwoods

Whole treechips obtained from softwood forest thinnings were pretreated via single-and two-stage dilute-sulfuric acid pretreatment. Whole-tree chips were impregnated with dilute sulfuric acid and steam treated in a 4-L steam explosion reactor. In single-stage pretreatment, wood chips were treated using a wide range of severity. In two-stage pretreatment, the first stage was carried out at low severity tomaximize hemicellulose recovery. Solubilized sugars were recovered from the first-stage prehydrolysate by washing with water. In the second stage, water-insoluble solids from first-stage prehydrolysate were impregnated with dilute sulfuric acid, then steam treated at more severe conditions to hydrolyze a portion of the remaining cellulose to glucose and to improve the enzyme digestibility. The total sugar yields obtained after enzymatic hydrolysis of two-stage dilute acid-pretreated samples were compared with sugar yields from single-stage pretreatment. The overall sugar yield from two-stage dilute-acid pretreatment was approx 10% higher, and the net enzyme requirement was reduced by about 50%. Simultaneous saccharification and fermentation using an adapted Saccharomyces cerevisiae yeast strain further improved cellulose conversion yield and lowered the enzyme requirement.     

Effects of operating parameters on countercurrent extraction of hemicellulosic sugars from pretreated softwood

In a previous study using a continuous countercurrent screw extractor for two-stage dilute-acid hydrolysis, which was focused on the effects of liquid-to-insoluble solids (L/IS) ratio, we demonstrated that by using low volumes of wash water soluble sugars can be recovered from first-stage pretreated softwood at high yields and also at high sugar concentrations. In this study, we investigated the effects of important operating parameters other than the L/IS ratio, such as the feed rates of water and pretreated biomass and the extractor inclined angle, on the performance of the extractor using first-stage pretreated softwood. As biomass and water feed rates increased at the same L/IS ratio, the recovery yield of soluble sugars decreased, probably owing to a reduced solids residence time in the extractor, which is related to the solid/liquid contact time. The sugar recovery yield was higher at a higher extractor inclined angle. This may be attributed to the effects of increased back mixing and a longer residence time for solids at a higher extractor angle. Countercurrent extraction was also carried out with other pretreated biomass having smaller particle sizes and poor drainage rates. The countercurrent screw extractor was found to be unsuitable for these fine materials due to the slow liquid drainage rate and filter-clogging problems. In a test for stability of soluble sugars in first-stage softwood hydrolysate, irrespective of the storage temperature and storage form, the sugar concentration slowly decreased with storage time. However, storage in slurry form showed higher sugar stability compared with that in liquor form at the same conditions.

     

Dilute acid hydrolysis of softwoods

Whole tree chips obtained from softwood forest thinnings were converted to ethanol via a two-stage dilute acid hydrolysis followed by yeast fermentation. The chips were first impregnated with dilute sulfuric acid, then pretreated in a steam explosion reactor to hydrolyze, more than 90% of the hemicellulose and approx 10% of the cellulose. The hydrolysate was filtered and washed with water to recover the sugars. The washed fibers were then subjected to a second acid im pregnation and hydrolysis to hydrolyze as much as 45% of the reamining cellulose. The liquors from both hydrolysates were combined and fermented to ethanol by a Saccharomyces cerevisiae yeast that had been adapted to the inhibitors. Based on available hexose sugars, ethanol yields varied from 74 to 89% of theoretical. Oligosaccharide contents higher than about 10% of the total available sugar appear to have a negative impact on ethanol yield.     

Combined use of H2SO4 and SO2 impregnation for steam pretreatment of spruce in ethanol production

Fuel ethanol can be produced from softwood through hydrolysis in an enzymatic process. Prior to enzymatic hydrolysis of the softwood, pretreatment is necessary. In this study, two-step steam pretreatment employing dilute H₂SO₄ impregnation in the first step and SO₂ impregnation in the second step, to improve the overall sugar and ethanol yield, was investigated. The first pretreatment step was performed under conditions of low severity (180°C, 10 min, 0.5% H₂SO₄) to optimize the amount of hydrolyzed hemicellulose. In the second step, the washed solid material from the first pretreatment step was impregnated with SO₂ and pretreated under conditions of higher severity to make the cellulose more accessible to enzymatic attack, as well as to hydrolyze a portion of the cellulose. A wide range of conditions was used in the second step to determine the most favorable combination. The temperatures investigated were between 190 and 230°C, the residence times were 2, 5, and 10 min; and the SO₂ concentration was 3%. The effect of pretreatment was assessed by both enzymatic hydrolysis of the solids and by simultaneous saccharification and fermentation (SSF) of the whole slurry, after the second pretreatment step. For each set of pretreatment conditions, the liquid fraction was also fermented to determine any inhibitory effects. Ethanol yield using the SSF configuration reached 66% of the theoretical value for pretreatment conditions in the second step of 210°C and 5 min. The sugar yield using the separate hydrolysis and fermentation configuration reached 71% for pretreatment conditions of 220°C and 5 min.     

Optimization of steam explosion to enhance hemicellulose recovery and enzymatic hydrolysis of cellulose in softwoods

A combination of Douglas fir heartwood and sapwood chips were steam pretreated under three conditions as measured by the Severity Factor (log R_o), which incorporated the time, temperature/pressure of pretreatment. By adjusting the steam pretreatment conditions, it was hoped to recover the majority of the hemicellulose component as monomers in the water-soluble stream, while providing a cellulosic-rich, water-insoluble fraction that could be readily hydrolyzed by cellulases. These three conditions were chosen to represent either high hemicellulose sugar recovery (low severity [L], log R_o=3.08), high-enzyme hydrolyzability of the cellulosic component (high severity [H], log R_o=4.21), and a compromise between the two conditions (medium severity [M], log R_o=3.45). The medium-severity pretreatment conditions (195°C, 4.5 min, 4.5% SO₂ logR_o=3.45) gave the best compromise in terms of relatively high hemicellulose recovery after stream pretreatment and the subsequent efficiency of enzymatic hydrolysis of the water-insoluble cellulosic fraction. The percent recovery of the original hemicellulose in the water-soluble fraction dropped significantly when the severity was increased (L-76.8%, M-64.7%, and H-37.5%). However, the ease of enzymatic hydrolysis of the cellulose-rich, water-insoluble fraction increased with increasing severity (L-24%, M-86.6%, and H-97.9%). Although more severe pretreatment conditions provided optimum hydrolysis of the cellulosic component, less severe conditions resulted in better recovery of the combined hemicellulose and cellulosic components.     

Two-step steam pretreatment of softwood with SO2 impregnation for ethanol production

Two-step steam pretreatment of softwood was investigated with the aim of improving the enzymatic digestibility for ethanol production. In the first step, softwood was impregnated with SO₂ and steam pretreated at different severities. The first step was performed at low severity to hydrolyze the hemicellulose and release the sugars into the solution. The combination of time and temperature that yielded the highest amount of hemicellulosic sugars in the solution was determined. In the second step, the washed solid material from the optimized first step was impregnated once more with SO₂ and steam pretreated under more severe conditions to enhance the enzymatic digestibility. The investigated temperature range was between 180 and 220°C, and the residence times were 2, 5 and 10 min. The effectiveness of pretreatment was assessed by both enzymatic hydrolysis of the solids and simultaneous saccharification and fermentation (SSF) of the whole slurry after the second pretreatment step, in the presence of antibiotics. For each pretreatment combination, the liquid fraction was fermented to determine any inhibiting effects. At low severity in the second pretreatment step, a high conversion of cellulose was obtained in the enzymatic hydrolysis step, and at a high severity a high conversion of cellulose was obtained in the second pretreatment step. This resulted in an overall yield of sugars that was nearly constant over a wide range of severity. Compared with the one-step steam pretreatment, the two-step steam pretreatment resulted in a higher yield of sugar and in a slightly higher yield of ethanol. The overall sugar yield, when assessed by enzymatic hydrolysis, reached 80%. In the SSF configuration, an overall ethanol yield of 69% was attained.     

Two-Stage Fractionation of Corn Stover Using Aqueous Ammonia and Hot Water

Hot water and aqueous ammonia fractionation of corn stover were used to separate hemicellulose and lignin and improve enzymatic digestibility of cellulose. A two-stage approach was used: The first stage was designed to recover soluble lignin using aqueous ammonia at low temperature, while the second stage was designed to recover xylan using hot water at high temperature. Specifically, the first stage employed a batch reaction using 15 wt.% ammonia at 60 °C, in a 1:10 solid:liquid ratio for 8 h, while the second stage employed a percolation reaction using hot water, 190–210 °C, at a 20 ml/min flow rate for 10 min. After fractionation, the remaining solids were nearly pure cellulose. The two-stage fractionation process achieved 68% lignin purity with 47% lignin recovery in the first stage, and 78% xylan purity, with 65% xylan recovery in the second stage. Two-stage treatment enhanced the enzymatic hydrolysis of remaining cellulose to 96% with 15 FPU/g of glucan using commercial cellulase enzymes. Enzyme hydrolyses were nearly completed within 12–24 h with the remaining solids fraction.     

Optimization of SO2-catalyzed steam pretreatment of corn fiber for ethanol production

A batch reactor was employed to steam explode corn fiber at various degrees of severity to evaluate the potential of using this feedstock as part of an enzymatically mediated cellulose-to-ethanol process. Severity was controlled by altering temperature (150–230°C), residence time (1–9 min), and SO₂ concentration (0–6% [w/w] dry matter). The effects of varying the different parameters were assessed by response surface modeling. The results indicated that maximum sugar yields (hemicellulose-derived water soluble, and cellulose-derived following enzymatic hydrolysis) were recovered from corn fiber pretreated at 190°C for 5 minutes after exposure to 3% SO₂. Sequential SO₂-catalyzed steam explosion and enzymatic hydrolysis resulted in a conversion efficiency of 81% of the combined original hemicellulose and cellulose in the corn fiber to monomeric sugars. An additional posthydrolysis step performed on water soluble hemicellulose stream increased the concentration of sugars available for fermentation by 10%, resulting in the high conversion efficiency of 91%. Saccharomyces cerevisiae was able to ferment the resultant corn fiber hydrolysates, perhydrolysate, and liquid fraction from the posthydrolysis steps to 89, 94, and 85% of theoretical ethanol conversion, respectively. It was apparent that all of the parameters investigated during the steam explosion pretreatment had a significant effect on sugar recovery, inhibitory formation, enzymatic conversion efficiency, and fermentation capacity of the yeast.     

Dilute-acid hydrolysis of sugarcane bagasse at varying conditions

Sugarcane bagasse, a byproduct of the cane sugar industry, is an abundant source of hemicellulose that could be hydrolyzed to yield a fermentation feedstock for the production of fuel ethanol and chemicals. The effects of sulfuric acid concentration, temperature, time, and dry matter concentration on hemicellulose hydrolysis were studied with a 20-L batch hydrolysis reactor using a statistical experimental design. Even at less severe conditions considerable amounts (>29%) of the hemicellulose fraction could be extracted. The percentage of soluble oligosaccharides becomes very low in experiments with high yields in monosaccharides, which indicates that the cellulose fraction is only slightly affected. For the sugar yields, acid concentration appears to be the most important parameter, while for the formation of sugar degradation products, temperature shows the highest impact. It could be demonstrated that the dry matter concentration in the reaction slurry has a negative effect on the xylose yield that can be compensated by higher concentrations of sulfuric acid owing to a positive interaction between acid concentration and dry matter contents.     

Pretreatment of Reed by Wet Oxidation and Subsequent Utilization of the Pretreated Fibers for Ethanol Production

Common reed (Phragmites australis) is often recognized as a promising source of renewable energy. However, it is among the least characterized crops from the bioethanol perspective. Although one third of reed dry matter is cellulose, without pretreatment, it resists enzymatic hydrolysis like lignocelluloses usually do. In the present study, wet oxidation was investigated as the pretreatment method to enhance the enzymatic digestibility of reed cellulose to soluble sugars and thus improve the convertibility of reed to ethanol. The most effective treatment increased the digestibility of reed cellulose by cellulases more than three times compared to the untreated control. During this wet oxidation, 51.7% of the hemicellulose and 58.3% of the lignin were solubilized, whereas 87.1% of the cellulose remained in the solids. After enzymatic hydrolysis of pretreated fibers from the same treatment, the conversion of cellulose to glucose was 82.4%. Simultaneous saccharification and fermentation of pretreated solids resulted in a final ethanol concentration as high as 8.7 g/L, yielding 73% of the theoretical.     

Effects of hemicellulose and lignin on enzymatic hydrolysis of cellulose from dairy manure

This study focused on the effect of hemicellulose and lignin on enzymatic hydrolysis of dairy manure and hydrolysis process optimization to improve sugar yield. It was found that hemicellulose and lignin in dairy manure, similar to their role in other lignocellulosic material, were major resistive factors to enzymatic hydrolysis and that the removal of either of them, or for best performance, both of them, improved the enzymatic hydrolysis of manure cellulose. This result combined with scanning electron microscope (SEM) pictures further proved that the accessibility of cellulose to cellulase was the most important feature to the hydrolysis. Quantitatively, fed-batch enzymatic hydrolysis of fiber without lignin and hemicellulose had a high glucose yield of 52% with respect to the glucose concentration of 17 g/L at a total enzyme loading of 1300 FPU/L and reaction time of 160 h, which was better than corresponding batch enzymatic hydrolysis.     

Effect of sulfuric and phosphoric acid pretreatments on enzymatic hydrolysis of corn stover

The pretreatment of corn stover with H₂SO₄ and H₃PO₄ was investigated. Pretreatments were carried out from 30 to 120 min in a batch reactor at 121°C, with acid concentrations ranging from 0 to 2% (w/v) at a solid concentration of 5% (w/v). Pretreated corn stover was washed with distilled water until the filtrate was adjusted to pH 7.0, followed by surfactant swelling of the cellulosic fraction in a 0–10% (w/v) solution of Tween-80 at room temperature for 12 h. The dilute acid treatment proved to be a very effective method in terms of hemicellulose recovery and cellulose digetibility. Hemicellulose recovery was 62–90%, and enzymatic digestibility of the cellulose that remained in the solid was >80% with 2% (w/v) acid. In all cases studied, the performance of H₂SO₄ pretreatment (hemicellulose recovery and cellulose digestibility) was significantly better than obtained with H₃PO₄. Enzymatic hydrolysis was more effective using surfactant than without it, producing 10–20% more sugar. Furthermore, digestibility was investigated as a function of hemicellulose removal. It was found that digestibility was more directly related to hemicellulose removal than to delignification.     

Steam pretreatment of douglas-fir wood chips

Douglas-fir sapwood and heartwood were impregnated with SO₂ and steam exploded at three severity levels, and the cellulose-rich, water-insoluble component was enzymatically hydrolyzed. The high-severity conditions resulted in near complete solubilization and some degradation of hemicelluloses and a significant improvement in the efficiency of enzymatic digestibility of the cell ulose component. At lower severity, some of the hemicellulose remained un hydrolyzed, and the cellulose present in the pretreated solids was not readily hydrolyzed. The medium-severity pretreatment conditions proved to be a good compromise because they improved the enzymatic hydrolyzability of the solids and resulted in the recovery of the majority of hemicellulose in a monomeric form within the water-soluble stream. Sapwood-derived wood chips exhibited a higher susceptibility to both pretreatment and hydrolysis and, on steam explosion, formed smaller particles as compared to heartwood-derived wood chips.

     

Evaluation and Characterization of Forage Sorghum as Feedstock for Fermentable Sugar Production

Sorghum is a tropical grass grown primarily in semiarid and drier parts of the world, especially areas too dry for corn. Sorghum production also leaves about 58 million tons of by-products composed mainly of cellulose, hemicellulose, and lignin. The low lignin content of some forage sorghums such as brown midrib makes them more digestible for ethanol production. Successful use of biomass for biofuel production depends on not only pretreatment methods and efficient processing conditions but also physical and chemical properties of the biomass. In this study, four varieties of forage sorghum (stems and leaves) were characterized and evaluated as feedstock for fermentable sugar production. Fourier transform infrared spectroscopy and X-ray diffraction were used to determine changes in structure and chemical composition of forage sorghum before and after pretreatment and the enzymatic hydrolysis process. Forage sorghums with a low syringyl/guaiacyl ratio in their lignin structure were easy to hydrolyze after pretreatment despite the initial lignin content. Enzymatic hydrolysis was also more effective for forage sorghums with a low crystallinity index and easily transformed crystalline cellulose to amorphous cellulose, despite initial cellulose content. Up to 72% hexose yield and 94% pentose yield were obtained using modified steam explosion with 2% sulfuric acid at 140 °C for 30 min and enzymatic hydrolysis with cellulase (15 filter per unit (FPU)/g cellulose) and β-glucosidase (50 cellobiose units (CBU)/g cellulose).     

纤维素超临界水预处理与水解研究

利用超临界水解工艺进行生物质废弃物(秸秆)能源转化, 使其主要成分纤维素在超临界水中快速水解为低聚糖, 为其进一步葡萄糖转化和乙醇发酵解决技术瓶颈. 其中纤维素在超临界水中的溶解是预处理与水解过程的限速步骤. 研究表明, 反应温度达到380 ℃及以上时, 纤维素可迅速溶解并进行水解, 液化比例可达100%; 在374～386 ℃范围内反应温度对纤维素的转化率有明显作用, 低聚糖和六碳糖的总产率在临界点附近出现最大值. 超临界条件下, 低聚糖和六碳糖转化率在较短反应时间内出现峰值, 而后随反应时间的延长快速下降, 固液比对于纤维素的低聚糖和六碳糖转化也有显著影响. 最优水解条件研究显示, 在380 ℃, 40 mg纤维素/2.5 mL水条件下反应16 s可获得最大的低聚糖产率, 为29.3%, 在380 ℃, 80 mg纤维素/2.5 mL水条件下反应18 s可获得最大的六碳糖产率, 为39.2%.     

Tailoring Wet Explosion Process Parameters for the Pretreatment of Cocksfoot Grass for High Sugar Yields

The pretreatment of lignocellulosic biomass is crucial for efficient subsequent enzymatic hydrolysis and ethanol fermentation. In this study, wet explosion (WEx) pretreatment was applied to cocksfoot grass and pretreatment conditions were tailored for maximizing the sugar yields using response surface methodology. The WEx process parameters studied were temperature (160–210 °C), retention time (5–20 min), and dilute sulfuric acid concentration (0.2–0.5 %). The pretreatment parameter set E, applying 210 °C for 5 min and 0.5 % dilute sulfuric acid, was found most suitable for achieving a high glucose release with low formation of by-products. Under these conditions, the cellulose and hemicellulose sugar recovery was 94 % and 70 %, respectively. The efficiency of the enzymatic hydrolysis of cellulose under these conditions was 91 %. On the other hand, the release of pentose sugars was higher when applying less severe pretreatment conditions C (160 °C, 5 min, 0.2 % dilute sulfuric acid). Therefore, the choice of the most suitable pretreatment conditions is depending on the main target product, i.e., hexose or pentose sugars.     

Effect of chip size on steam explosion pretreatment of softwood

Although considerable progress has been made in technology for converting lignocellulosic biomass into ethanol, substantial opportunities still exist toreduce production costs. In biomass pretreatment, reducing milling power is a technological improvement that will substantially lower production costs for ethanol. Improving sugar yield from hemicellulose hydrolysis would also reduce ethanol production costs. Thus, it would be desirable to test innovative pretreatment conditions to improve the economics by reducing electrical power of the milling stage and by optimizing pretreatment recovery of hemicellulose, as well as to enhance cellulose hydrolysis. The objective of this study was to evaluate the effect of chip size (2–5, 5–8, and 8–12 mm) on steam-explosion pretreatment (190 and 210°C, 4 and 8 min) of softwood (Pinus pinater).     

离子液体[BMIM]Cl预处理对微晶纤维素酶解的影响

以微晶纤维素为研究对象, 设计了离子液体1-丁基-3-甲基咪唑氯盐(1-butyl-3-methylimidazolium chloride, [BMIM]Cl)预处理微晶纤维素Avicel的实验方法以实现纤维素的高效酶解糖化. 在[BMIM]Cl中Avicel完全溶解, 经水洗沉淀得到再生纤维素, 回收后的离子液体可重复利用. 预处理后底物酶解的可溶性糖转化率在24 h时高达94.65%, 较之同样条件下未经预处理底物的酶解糖转化率(48.57%)有飞跃性提升. 进一步考察了离子液体预处理对纤维素结构及形态的影响, 结果表明: [BMIM]Cl预处理后Avicel氢键减弱; 结晶度明显下降, 结晶型态由纤维素I型转变为纤维素II型; 由规整的平行排布转变为疏松有孔的无序形貌. 正是离子液体预处理引起的纤维素微观与宏观结构性质的显著改变使得再生后纤维素酶解的可溶性糖转化率大幅提高.     

生物质半纤维素稀酸水解反应*

半纤维素是木质纤维素类生物质中第二大组分,半纤维素的高效、低成本转化是实现木质纤维素类生物质转化工艺实用化的一个技术关键。稀酸水解技术被广泛应用于水解生物质半纤维素,其对半纤维素糖的转化率高,得到的糖可进一步发酵生产燃料乙醇等。半纤维素还可直接水解制低聚糖等功能性食品和糠醛等化工产品。本文综述了半纤维素稀酸水解反应的研究进展。介绍了半纤维素的基本结构特征,解析了稀酸催化半纤维素水解的反应机理及反应网络,评述了半纤维素水解过程中反应条件等对目标产物的影响,并总结了半纤维素稀酸水解动力学模型。在此基础上,对今后半纤维素稀酸水解反应的研究方向与水解产物的利用进行了展望。     

Pretreatment of corn stover with acidic electrolyzed water and FeCl3 leads to enhanced enzymatic hydrolysis

Pretreatment has been viewed as the most efficient strategy for lignocellulosic biomass-to-fermentable sugars conversion. In this study a novel pretreatment with acidic electrolyzed water (AEW) and FeCl₃ was proposed and tested to deconstruct the recalcitrance of corn stover and enhance its subsequent cellulose-to-sugar conversion. The effects of AEW pH and FeCl₃ concentration on hemicellulose degradation were investigated, and the results showed the highest hemicellulose removal (93.40 %) and recovery (93.04 %) were achieved at AEW pH 2.30 and FeCl₃ concentration 0.05 mol/L. Further research on the properties of AEW solutions with FeCl₃, including their pH, ORP, and DO revealed the synergistic effects of strong acidity and high oxidizing capacity of the solution could boost hemicellulose breakup and enhance the enzymatic hydrolysis of cellulose (92.00 %) by removing most of hemicellulose and increasing the accessibility and digestibility of cellulose. Therefore, these studies prove AEW coupled with FeCl₃ pretreatment is an effective and promising approach in biomass-to-biofuel process.