FRACTIONATION OF LIGNOCELLULOSIC BIOMASS FOR CELLULOSIC ETHANOL AND CHEMICAL PRODUCTION

Abstract

A process is defined for the continuous steam pretreatment and fractionation of low lignin lignocellulosic biomass to produce a concentrated cellulose solid stream that is sensitive to enzymatic hydrolysis. Valuable chemicals are recovered by fractionating the liquid and vapor stream composed of hydrolysis and degradation products of the hemicellulose. Cellulosic derived glucose is produced for fermentation to biofuels. A xylo-oligosaccharides rich liquids fraction is recovered that can be converted to value added products including ethanol.

Claims

1. A continuous process for fractionation of lignocellulosic biomass having a lignin content of less than 12%, comprising the steps of: a) exposing the lignocellulosic biomass to steam and an acid catalyst in a reaction vessel at a preselected temperature and a preselected reaction pressure, for a preselected exposure time, and at a selected pH value for removing a hemicellulose fraction of the lignocellulosic biomass and activating a cellulose fraction of the lignocellulosic biomass to obtain a prehydrolyzed lignocellulosic biomass; wherein the pH value is adjusted using an acid catalyst, wherein the acid catalyst is all or in part acetic acid released from the breakdown of the hemicellulose fraction of the lignocellulosic biomass, wherein a severity index of 3.5 to 4.0 is maintained during the exposing step, the severity index being calculated according to the equation: Log Severity Index=Log[Exp {(Temperature C.100)/14.75}Retention Time (min)], and wherein the steam pretreatment is carried out for less than 90 min at a pH value of 3.0 to 4.0; b) purging liquid condensate and vapor generated during the exposure step to remove and collect a first liquid stream with water soluble compounds and a first vapor stream with volatile chemicals; c) extracting and removing from the prehydrolysed lignocellulosic biomass, under pressure and prior to explosive decompression, a hemicellulose degradation stream containing solubilized degradation byproducts of hemicellulose created in the exposing step, wherein acetic acid in the first vapor stream is collected, or acetic acid in the first vapor stream and in the hemicellulose degradation stream is collected; d) rapidly releasing the reaction pressure after the extracting step to afford explosive decompression of the prehydrolyzed lignocellulosic biomass into fibrous solids, vapor and condensate; e) collecting the vapor and condensate from the explosive decompression for separation and recovery of byproducts; and f) refining the hemicellulose degradation stream by vacuum evaporation, solvent extraction, adsorption, or ion-exchange precipitation to increase the concentration and to simultaneously remove volatile compounds and acid catalysts.

2. The process of claim 1, wherein the preselected temperature is 170 C. to 205 C.

3. The process of claim 1, wherein the exposing step is carried out at the reaction temperature of 170 C., the reaction pressure of 689 kPa (100 psig), and for the time interval of 25-85 minutes.

4. The process of claim 1, wherein the Severity Index is maintained at 3.6.

5. The process of claim 1, wherein a Severity Index of 3.8 to 4.1 is maintained during the exposing step.

6. The process of claim 5, wherein the Severity Index is maintained at 4.0.

7. The process of claim 6, wherein the exposing step is carried out at the reaction temperature of 205 C., the reaction pressure of 1620 kPa (235 psig), and for the time interval of 8 minutes.

8. The process of claim 1, wherein the process is carried out in a pretreatment exposing system and volatile compounds are removed continuously by venting the pretreatment exposing system.

9. The process of claim 1, wherein solubilized byproducts of hemicellulose degradation created in the exposing step are extracted and removed from the solid portion both before and after explosive decompression, with or without the addition of an eluent.

10. The process of claim 1, wherein the lignocellulosic biomass is pre-steamed prior to the exposing step with steam for 10 to 60 min at a temperature of up to 99 Celsius to remove air and adjust a moisture content of the lignocellulosic biomass to between 30 and 60%.

11. The process of claim 1, wherein the lignocellulosic biomass is selected from the group consisting of miscanthus, switchgrass, corn cob, prairie grass, sorghum straw, corn stover, and wheat straw.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Other objects and advantages of the invention will become apparent upon reading the detailed description and upon referring to the drawings in which:

[0045] FIG. 1 shows a process diagram of the continuous pretreatment unit proposed in the example.

[0046] FIG. 2 shows the total percentage recovery of cellulose and hemicellulose produced during the fractionation of corncobs.

[0047] FIG. 3 illustrates the susceptibility of pretreated corncob cellulose to enzymatic hydrolysis i.e. cellulose to glucose conversion.

[0048] FIG. 4 shows hydrolysis and fermentation results using pretreated corncobs produced at pilot scale (2.5 metric tons, 17% consistency).

[0049] FIG. 5 shows the total percentage recovery of cellulose and hemicellulose produced during high pressure fractionation of corncobs.

[0050] FIG. 6 shows the total percentage recovery of cellulose and hemicellulose produced during low pressure fractionation of corncobs.

[0051] FIG. 7 shows hydrolysis and fermentation results using pretreated corncobs produced at pilot scale and low pressure.

[0052] FIG. 8 shows the total percentage recovery of cellulose and hemicellulose in solid and liquid fractions produced over the fractionation of Bagasse.

[0053] FIG. 9 shows hydrolysis and fermentation results using pretreated bagasse produced at pilot scale and high pressure;

[0054] FIG. 10 illustrates the susceptibility of pretreated cellulose from concob (Example 3) to enzymatic hydrolysis (cellulose to glucose conversion) and fermentability of hydrolyzed cellulose (glucose to ethanol conversion);

[0055] FIG. 11 illustrates the susceptibility of pretreated cellulose from bagasse (Example 4) to enzymatic hydrolysis (cellulose to glucose conversion) and fermentability of hydrolyzed cellulose (glucose to ethanol conversion); and

[0056] FIG. 12 illustrates the relative proportion of the solids, solubles and volatiles streams obtained by hemicellulose autohydrolysis during steam pretreatment of low lignin biomass at different severity indexes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0057] Before explaining the present invention in detail, it is to be understood that the invention is not limited to the preferred embodiments contained herein. The invention is capable of other embodiments and of being practiced or carried out in a variety of ways. It is to be understood that the phraseology and terminology employed herein are for the purpose of description and not of limitation.

[0058] The abbreviations used in the figures have the following meaning: [0059] C., temperature in degree Celsius [0060] ms, millisecond [0061] DM, Dry matter [0062] SI, Severity Index [0063] t.sub.90%, Time to reach 90% of maximum theoretical cellulose to glucose conversion.

Pretreatment of Lignocellulosic Biomass

[0064] This invention is a new process for fractionating lignocellulosic biomass with a lignin content below 12% by weight on a dry matter basis and an acetyl group content of 3-6% by weight on a dry matter basis, in particular a process for fractionating the lignocellulosic biomass into two main commercially valuable components, a cellulose-rich solids fraction and a xylo-oligosaccharides-rich liquids fraction (solution). The cellulose-rich component is valuable for many purposes, since it can be more easily hydrolyzed to glucose and in turn more easily fermented to ethanol or other biofuels than in previous processes.

[0065] A preferred aspect of the invention is a continuous process for the pretreatment of these types of lignocellulosic biomass solely by autohydrolysis, in the absence of any acid catalyst, thereby minimizing the amount of residual acid in the pretreated biomass. In particular, all those types of biomass are treated in accordance with the invention by using exactly the same steam pretreatment conditions to achieve highly digestible cellulose. This not only makes it possible to use the same process equipment for different types of biomass, thereby significantly lowering the capital cost for processing plants intended to treat different biomasses, but also allows for the treatment of a mixture of different biomasses, thereby providing much wider access to a larger amount of biomass sources.

[0066] The inventors discovered that using steam pretreatment without the addition of any acid catalyst and without controlling the amount of acetic acid released in the pretreatment step, when carried out at autohydrolysis conditions, will result in a cellulose of equal digestibility for biomasses of such diverse content as corncobs (8:1:7, cellulose:lignin:hemicellulose) and bagasse (1.8 :1:1.3), as long as the lignin content of the biomass is below 12% and the biomass has an acetyl group content of 3-6% by weight on a dry matter basis, the latter being important for a successful operation at autohydrolysis conditions.

[0067] The intention of steam explosion pretreatment of lignocellulosic biomass is generally to create a solids fraction with easily digestible cellulose. For the generation of ethanol from lignocellulosic biomass, the primary goal is to maximize the cellulose recovery and accessibility. Separate recovery of the hydrolysis products of the hemicellulose portion of the biomass (mainly xylose, xylo-oligosaccharides and degradation products) is possible, but generally of little interest, since the hemicellulose hydrolysis products recovered include sugar degradation products that are inhibitory to fermentation to ethanol, or interfere with upgrading of the xylo-oligosaccharides into valuable downstream products. In existing processes for the production of ethanol from lignocellulosic biomass the hemicellulose breakdown and degradation products are often removed together during pretreatment to reduce their inhibitory effect on downstream cellulose hydrolysis and glucose fermentation. Although purified xylo-oligosaccharides can be of value as starting compounds in chemical and pharmaceutical products, the cost of treating the recovered stream of hemicellulose breakdown and degradation products to obtain a purified xylo-oligosaccharides stream render this option uneconomical. Thus, in other known processes for the production of ethanol from lignocellulosic biomass the hemicellulose breakdown and degradation products are first removed together during pretreatment and later combined with the cellulose hydrolysis products for co-fermentation with the cellulose hydrolysis products. Although the ethanol yield from fermentation of the hemicellulose hydrolysis products is only a fraction of that from the cellulose hydrolysis products, co-fermentation results in a better overall economic result than upgrading the hemicellulose hydrolysis and degradation products into starting compounds for chemical and pharmaceutical products.

[0068] The inventors have now developed a continuous process for fractionation of lignocellulosic biomass which allows for fractionation of the products obtained from autohydrolysis of lignocellulosic biomass not only into digestible cellulose and hemicellulose hydrolysis and breakdown products, but into a cellulose-rich solids fraction, a xylo-oligosaccharides-rich liquid fraction and a vapor fraction containing inhibitors to fermentation.

[0069] During testing of different steam explosion pretreatment process options, the inventors discovered, first, that autohydrolysis be carried out using the same general treatment conditions for lignocellulosic biomass with a lignin content of less than 12% as long as the acetyl-group content is 3-6% by weight on a dry matter basis and, second, that fractionation of the hemicellulose in the biomass can be improved in such a manner to render the creation of purified xylo-oligosaccharides economical.

[0070] The inventors discovered that the hemicellulose originally in the lignocellulosic biomass in solid form decomposed during steam pretreatment into a solids fraction, a solubles fraction and a volatiles fraction, the solubles fraction including a monomeric fraction (mainly xylose monomers) and an oligomeric fraction (mainly xylo-oligosaccharides). The inventors observed that the relative proportion of each fraction was dependent on the severity index of the steam explosion pretreatment. In particular, the inventors found that, as illustrated in FIG. 12, the proportion of each fraction varied non-linearly with the severity index. In particular, the degradation of hemicellulose into inhibitory volatiles accelerated above a severity index of about 3.7, the conversion of the solids fraction into solubles and volatiles slowed down above a severity index of about 4, the proportion of the recoverable solubles decreased above a severity of about 4 and the proportion of the monomeric portion derived from the starting solids remained static above a severity index of about 4. Thus, the inventors found that by choosing a severity index of about 4 for the autohydrolysis of the biomass, three different optimization goals that were previously not considered in combination can be achieved simultaneously in relation to the hydrolysis of the hemicellulose fraction of the biomass. The inventors discovered that relative maximization of the hemicellulose solids breakdown, maximization of the proportion of xylo-oligosaccharides generated, and relative minimization of the hemicellulose degradation can all be achieved simultaneously if the hemicellulose fraction of the biomass is subjected to autohydrolysis by steam pretreatment at a severity index of about 4.

[0071] Moreover, the inventors found that the individual hemicellulose fractions generated during pretreatment can be captured separately during pretreatment, rather than separated in downstream steps. In particular, the inventors discovered that a vapor phase containing mostly inhibitory volatiles can be captured by purging the volatiles during pretreatment as well as right after explosive decompression. Volatiles recovered by purging included a high proportion of degradation products not desirable in purified xylo-oligosaccharides and inhibitory to cellulose hydrolysis and/or fermentation. The solubles fraction of the hydrolyzed hemicellulose can be captured separately by purging condensate during pretreatment as well as right after explosive decompression, liquid extracting the solids fraction during pretreatment after purging of the condensate and combining the purge streams and the liquid extracted solubles into a combined solubles stream including mainly xylose monomers and oligomers as well as a minor portion of degradation products. The inventors have found that by separately capturing the volatiles, the condensate and the liquid extracted solubles, the majority of degradation compounds created during autohydrolysis of the biomass is separately captured in the vapor stream with volatiles, while only a minor portion of degradation products is still found in the combined solubles stream, making it possible to render the combined solubles stream into a purified, xylo-oligosaccharides rich liquid fraction by a cost efficient evaporation step for driving out the remaining degradation products.

[0072] On the basis of this research, the inventors have developed a continuous pretreatment process incorporating the findings relating to the influence of the severity index of the autohydrolysis pretreatment on the composition of the hemicellulose hydrolysis products mix with the findings relating to the fractionation of the hemicellulose hydrolysis products during pretreatment. This pretreatment process allows for the continuous fractionation of lignocellulosic biomass into a cellulose-rich fraction, a xylo-oligosaccharides-rich fraction and a vapor fraction containing inhibitors. This is achieved by first obtaining biomass, or a mixture of biomasses, having a lignin content of less than 12% and an acetyl group content of 3-6% and then subjecting this biomass to steam explosion pretreatment at a severity index of about 4 to maximize autohydrolyis of the hemicellulose fraction in the biomass and maximize a proportion of xylo-oligosaccharides generated, while minimizing hemicellulose degradation. During the steam pretreatment exposure time, liquid condensate, cooking liquids and vapor generated during autohydrolysis are captured in a first liquid stream with hemicellulose sugars free of lignin and water soluble compounds, and a first vapor stream with volatile chemicals. After the autohydrolysis step, the prehydrolyzed biomass still under pressure is liquid extracted to obtain a liquid stream containing hemicellulose sugars free of lignin and hemicellulose degradation components. After this liquid extraction step, the reaction pressure is rapidly released to afford explosive decompression of the extracted, prehydrolyzed lignocellulosic biomass into fibrous solids, vapor with hemicellulose degradation components and volatile chemicals inhibitory to fermentation, as well as condensate containing mostly hemicellulose sugars free of lignin. Vapor and condensate generated during the explosive decompression are separately captured as a second vapor stream and a second liquid stream. The first and second liquid streams are then combined with the liquid hemicellulose degradation stream into a liquids fraction which is subsequently subjected to evaporation of the hemicellulose degradation products for separation and recovery of a xylo-oligosaccharides rich solution.

[0073] The preferred process of the invention includes the steps of obtaining lignocellulosic biomass having a content of less than 12% lignin by weight in the dry matter and an acetyl group content of 3-6% by weight in the dry matter, exposing the optionally ground, lignocellulosic biomass to steam at 170 C. to 220 C. at 100 to 322 psig for 2 to 300 minutes without the use of mineral acid catalysts. The term lignocellulosic biomass in this context is meant to cover both a specific biomass as well as a mixture of biomasses, as long as the lignin and acetyl group content are as required. The pretreatment preferably includes the continuous purging of volatile and liquid compounds. The exposing step preferably steam treats the biomass to a temperature and retention time with translates into a Severity Index of 3.9 to 4.1, most preferably about 4, the Severity Index being calculated according to the equation:

Severity Index=LogExp[(Temperature C.100)/14.75]Retention Time (min).

[0074] Steam pretreating corncobs at a severity index of 4 leads to a final pH of 3.5 to 4.0 of the pretreated biomass, while the same severity index leads to a final pH of 4.5 to 5 with bagasse biomass.

[0075] The process also includes liquid extraction of the steam pretreated fibres under pressure with/or without eluent addition to remove water soluble hemicelluloses, acids and hemicellulose and cellulose breakdown and degradation products. As an option, these compounds, which are inhibitors of downstream hydrolysis and fermentation may be extracted during pretreatment, after pretreatment, or both during and after pretreatment. The extraction of the soluble compounds from the pretreated fibers preferably results in 4% to 10% xylose based sugars consisting of polymers (xylan), monomers and xylo-oligosaccharides remaining in the prehydrolysis fibers.

[0076] The extracted fibers, also referred to as prehydrolysate, are then separated from the gaseous reaction products in a cyclone separator and collected at the bottom of the separator, shredded and diluted to a desired consistency and subsequently transported to the enzymatic hydrolysis step.

[0077] The collected prehydrolysate is then shredded, diluted with water to 10-30% consistency and then reacted with cellulase enzymes to produce glucose. The glucose rich solution is readily utilized in the subsequent fermentation step where an organism converts the glucose into ethanol.

EXAMPLE 1

Autohydrolysis Pretreatment Process

[0078] In the following example, reference numbers refer to features of the pretreatment system and process streams, as shown in FIG. 1.

[0079] Continuous steam explosion pretreatment of lignocellulosic biomass is carried out in a steam explosion pretreatment system. In this example the biomass is corncobs.

[0080] Corncobs 10 are received, stored, cleaned, ground (0.5 to 1 cm3 particle size) and fed through a V shaped hopper and screw auger (not shown). The corncob moisture is adjusted to 50% DM.

[0081] Prepared corncobs are pre-conditioned by preheating them with live steam 20 at atmospheric pressure, in a holding bin or preheating and conditioning container 30 to a temperature of about 95 C. for about 10-60 minutes. Air and steam are vented through an air vent 35 from the preheating and conditioning container 30.

[0082] Preheated corncobs are compressed in a first modular screw device 40 to remove air 50 through an air vent and inhibitory extracts 5. The corncobs are then fed into a pressurized upflow tube 70.

[0083] Pressurized saturated steam at a temperature of 205 C. is injected upstream of and/or directly into the upflow tube 70 by direct injection 60 and/or indirect injection of steam 61 in a jacketed section of the upflow tube until the desired cooking pressure is reached.

[0084] Corncobs are moved through the upflow tube with the aid of a screw conveyor/mixer (3 min) and are discharged into a pretreatment reactor 80.

[0085] Corncobs are continuously discharged from the pretreatment reactor 80 to a second pressurized modular screw device 100 after a residence time of 5 min at 205 C. in the pretreatment reactor 80. This results in a treatment severity index of 4).

[0086] During the residence time, condensate and cooking liquids collected at the bottom of the pretreatment reactor are purged through a purge discharge control valve 95.

[0087] Pretreated corncobs are washed with water eluent under pretreatment pressure. Hot water 90 is added to dilute the pretreated corncobs as the fiber is discharged from the pretreatment reactor. Further hot water is also added along the pressing device 100 to reach a ratio of about 6:1 wash water:corncobs and to achieve a greater extraction of hemicellulose. The extracted hemicellulose solution 110 is collected and concentrated to the desired dryness for further applications.

[0088] The pressurized washed corncobs are then flashed into a cyclone 120. The solids, i.e. purified cellulose, collected at the bottom of the cyclone separator and are subjected to further processing i.e. shredded and then diluted with fresh water to the desired consistency for hydrolysis and fermentation.

[0089] The gaseous components are collected, condensed and fed to a condensate tank 130. Any gaseous emissions from the pretreatment reactor, the cyclone separator and other parts of the steam gun setup are collected and treated in an environmental control unit (not shown). Cleaned gases are exhausted to atmosphere from the environmental control unit.

[0090] In order to simulate this new process, steam explosion pretreatment of corncobs was followed by batch washing at pilot scale with a 97% recovery of cellulose (FIG. 2).

[0091] Extracted cellulose from the pilot scale pretreatment was highly susceptible to enzymatic hydrolysis. 80% of the maximum theoretical cellulose to glucose conversion was achieved in 60 h. 90% conversion of the 17% consistency slurry was reached in 95 h, using only 0.23% load of commercial cellulases product (FIG. 3).

[0092] In FIG. 3, the dashed line represents the trend of eleven enzymatic hydrolysis experiments carried out at three different scales (i.e. 1 kg, 300 kg and 2500 kg). These enzymatic hydrolysis experiments were carried out at 17% consistency, 50 C. and pH 5.0. The pH adjustment chemical used was aqueous ammonia (30%). Commercially available lignocellulolytic enzyme was used at a load of 0.23% weight/weight on incoming cob feedstock.

[0093] Samples of the continuously pretreated corncobs were hydrolyzed and fermented in a 2.5 metric tonne batch hydrolysis and fermentation trial (FIG. 4). The results were in accordance with the lower scale pilot and the laboratory scale results (FIG. 3). A concentration of 100 g/L glucose was reached at t 90% i.e. 100 hours hydrolysis of 17% consistency slurry, using only 0.23% load of commercial cellulase product.

[0094] The fermentability of the hydrolyzed cellulose was high. A concentration of 4.9% alcohol was reached in 20 hours (FIG. 4).

[0095] In FIG. 4, hydrolysis was carried out at 50 C., pH 5.0 and 0.23% enzyme load. Fermentation was carried out at 33 C., pH 5.3, using industrial-grade C6-fermenting yeast. Hydrolysis and fermentation pH adjustment was carried out using aqueous ammonia (30%). Grey circles indicate glucose concentration. Black squares indicate ethanol concentration.

[0096] The production of soluble xylo-oligosaccharides was equivalent to 12% of the weight of raw corncobs processed at pilot scale. 63% of the original content of corncobs hemicellulose was converted to volatile degradation products (FIG. 2). 66% of these volatiles were flashed off during the step of explosive decompression.

[0097] 81% of the hemicellulose remaining in the corncobs prehydrolysate after autohydrolysis was collected through the prehydrolysate water washing step. The resulting lignin free solution contained dissolved solids of which 87% were sugars, including 63% of xylo-oligosaccharides (w/w) on a dry matter basis.

EXAMPLE 2

High Pressure Pretreatment of Corncobs

[0098] Steam explosion pretreatment of corncobs was carried out in a steam explosion pretreatment system pressurized with saturated steam at a temperature of 205 C. No acid was added to the corncobs during the heating step. The corncob moisture was adjusted to 60% DM. The overall retention time of corncob pretreatment was 8 min e.g. 3 min in the up flow tube, 5 min in the pretreatment reactor at pH 3.8. Corncob acidification resulted from the release of acetic acid from hemicellulose breakdown.

[0099] Pretreated corncobs were water washed.

[0100] Cellulose extraction from corncobs was carried out at pilot scale with a percentage recovery of 98% (FIG. 5).

[0101] 59% of the incoming hemicellulose was recovered after high pressure pretreatment of corncobs. 52% of incoming hemicellulose was collected into the xylo-oligosaccharides solution (FIG. 5). The resulting lignin free solution contained 89% sugars, including 66% of xylo-oligosaccharides (w/w) on a dry matter basis.

EXAMPLE 3

Low Pressure Pretreatment of Corncobs

[0102] Steam explosion pretreatment of corncobs was carried out in a steam explosion pretreatment system pressurized with saturated steam at a temperature of 170 C. No acid was added to the corncobs during the heating step. The corncob moisture was adjusted to 50% DM.The overall retention time of corncobs pretreatment was 85 min e.g. 15 min in an up flow tube, 70 min in a pretreatment reactor at pH 3.8. Corncob acidification resulted from the release of acetic acid from hemicellulose breakdown.

[0103] Pretreated corncobs were water washed.

[0104] Cellulose extraction from corncobs was carried out at pilot scale with a percentage recovery of 98% (FIG. 6).

[0105] 51% of incoming hemicellulose was recovered after low pressure pretreatment of corncobs. 43% of incoming hemicellulose was collected in the xylo-oligosaccharides solution (FIG. 6). The resulting lignin free solution contained 88% sugars, including 65% of xylo-oligosaccharides (w/w) on a dry matter basis.

[0106] After explosive decompression, the solid fraction from high or low pressure pretreatment i.e. purified cellulose was collected at the bottom of cyclone separator, shredded and then diluted with fresh water up to 17% consistency.

[0107] Extracted cellulose from high and low pressure continuous pilot scale pretreatment of corncobs was highly susceptible to enzymatic hydrolysis. Digestibility of cellulose pretreated at high and low pressure was similar (FIG. 3). 80% of the maximum theoretical cellulose to glucose conversion was achieved in 60 h. 90% conversion of the 17% consistency slurry was reached in 95 h, using only 0.23% load of commercial cellulases product (FIG. 3).

[0108] In FIG. 3, the dashed line represents the trend of six duplicate enzymatic hydrolysis experiments carried out at three different scales (i.e. 1 kg, 300 kg and 2500 kg) with cellulose extracted at high or low pressure. These enzymatic hydrolysis experiments were carried out at 17% consistency, 50 C. and pH 5.0. The pH adjustment chemical used was aqueous ammonia (30%). Commercially available lignocellulolytic enzyme was used at a load of 0.23% weight/weight on incoming cob feedstock.

[0109] At pilot scale (2.5 metric tonne fed batch hydrolysis and fermentation trial, FIG. 7) a concentration of 100 g/L glucose representing 91% conversion of the cellulose was reached after 100 hours hydrolysis of a 17% consistency slurry from low pressure pretreatment.

[0110] In FIG. 7, hydrolysis was carried out at 50 C., pH 5.0 and 0.23% enzyme load. Fermentation was carried out at 33 C., pH 5.3, using industrial grade C6-fermenting yeast. Hydrolysis and fermentation pH adjustment was carried out using aqueous ammonia (30%).

[0111] Grey circles indicate glucose concentration. Black squares indicate ethanol concentration.

[0112] Fermentability of the hydrolyzed cellulose was evaluated by adding enough C6-industrial grade commercial yeast to reach a concentration of 10.sup.8 yeast cells per gram hydrolysate at 33 C., pH 5.3 when 90% of the maximum theoretical cellulose to glucose conversion was reached. pH adjustment was carried out with aqueous ammonia (30%) prior to yeast addition.

[0113] Fermentability of the hydrolyzed cellulose was high. A concentration of 4.9% alcohol was reached in 20 hours (FIG. 7).

EXAMPLE 4

High Pressure Pretreatment of Bagasse

[0114] Steam explosion pretreatment of Bagasse was carried out in a system pressurized with saturated steam at a temperature of 205 C. No acid was added to the bagasse fibers during the heating step. The overall retention time of the bagasse fibers during pretreatment was 8 min e.g. 3 min in the up flow tube and 5 min in the pretreatment reactor at pH 4.8. Bagasse acidification resulted from the release of acetic acid from hemicellulose breakdown.

[0115] Pretreated Bagasse was water washed.

[0116] Cellulose extraction from the pretreated and washed Bagasse mash was carried out at pilot scale with a percentage recovery in the solid fraction of 95% (FIG. 8).

[0117] 72% of the incoming hemicellulose was recovered after pretreatment of Bagasse. 63% of the incoming hemicellulose was collected in the xylo-oligosaccharides solution (FIG. 8). The resulting lignin free solution contained 85% sugars, including 62% of xylo-oligosaccharides (w/w) on a dry matter basis.

[0118] Extracted cellulose from pilot scale pretreatment of Bagasse was highly susceptible to enzymatic hydrolysis. 80% of the maximum theoretical cellulose to glucose conversion was achieved in 110 h (FIG. 9).

[0119] In FIG. 9, hydrolysis was carried out at 50 C., pH 5.0, using commercially available lignocellulolytic enzyme product at a load of 0.3% weight/weight on incoming cob feedstock. Fermentation was carried out at 33 C., pH 5.3 using an industrial-grade C6-fermnenting yeast.

[0120] A concentration of 54 g/L glucose representing 80% conversion of cellulose was reached after 110 hours of hydrolysis of a 13% consistency slurry, using only a 0.3% load of commercial cellulase.

[0121] Fermentability of the hydrolyzed cellulose was evaluated by adding enough C6-industrial grade commercial yeast to reach a concentration of 10.sup.8 yeast cells per gram hydrolysate at 33 C., pH 5.3. The time needed to reach the maximum theoretical cellulose to glucose conversion was determined. pH adjustment was carried out with aqueous ammonia (30%) prior to yeast addition.

[0122] The fermentability of the hydrolyzed cellulose was high. A concentration of 2.6% alcohol was reached in 20 hours (FIG. 9). This is equivalent to a glucose to ethanol conversion yield of 95%.

[0123] The achieved high degree of cellulose digestibility and cellulose to glucose conversion rates of cellulose derived from bagasse biomass subjected to pretreatment solely by autohydrolysis and without the addition of any acid catalyst was surprising. Numerous prior art references cited below teach the use of acid to improve hemicellulose hydrolysis during pretreatment for biomass having a low inherent acetic acid content. To date, it was not recognized in the art that due to the delicate interplay between the higher amount of hemicellulose breakdown achieved with added acid catalyst and the inhibitory effects of the breakdown products and the catalyst on the downstream cellulose hydrolysis and glucose fermentation processes, the use of acid catalyst for biomass with low acetic acid content is not always advantageous and may in fact lead to lower ethanol yields for certain lignocellulosic biomasses. The inventors have now surprisingly discovered that autohydrolysis without the addition of any acid catalyst can be carried out on lignocellulosic biomass of <12% lignin content and an acetyl group content of 3-6% weight/weight in the dry matter, with satisfactory ethanol yield and even higher ethanol yield compared to processes using added acid catalyst in the pretreatment step. In fact, as can be seen from FIGS. 10 and 11, optimal pretreatment conditions of corncob biomass and bagasse biomass with respect to the production of highly digestible cellulose and ethanol were found to be at exactly the same severity index and both without the addition of any acid catalyst. As is apparent from these Figures, the fastest time for digesting cellulose prehydrolysates was obtained with severity index of 4.0 SI in both cases, which is very surprising in view of the significant differences in lignin and acetyl group content of bagasse and corncob. The class of lignocellulosic biomasses of <12% lignin content and an acetyl group content of 3-6% weight/weight in the dry matter includes corncob, sugar cane bagasse, switchgrass, prairie grass, sorghum bagasse, corn stover, and wheat straw.

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