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Steam-assisted biomass fractionation. II. fractionation behavior of various biomass resources
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Biomass and Bioenergy Vol. 14, No. 3, pp. 219±235, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0961-9534/98 $19.00 + 0.00 S0961-9534(97)10037-X
STEAM-ASSISTED BIOMASS FRACTIONATION. II. FRACTIONATION BEHAVIOR OF VARIOUS BIOMASS RESOURCES WOLFGANG G. GLASSER and ROBERT S. WRIGHT Virginia Tech, Blacksburg, Virginia, VA 24067, U.S.A. (Received 8 August 1994; accepted 1 October 1995) AbstractÐThe fractionation behavior following steam explosion of three biomass resources, yellow poplar (Liriodendron tulipifera) wood chips, peanut hulls (Arachis hypogaea), and sugar cane (Saccharum ocinarum) bagasse (three separate fractions, leaf, pith, and whole bagasse) were examined following steam explosion using a Stake II reactor. Component separation was evaluated using a range of dierent severity factors. Water solubles, alkali solubles, and an insoluble fraction were collected separately and subjected to component analysis. The latter addressed both chemical (component analysis by NMR and UV spectroscopy) and molecular structure (molecular weight determinations). The water soluble fraction consisted primarily of xylose (mono and oligo-saccharides) and water soluble lignin; the alkali soluble fraction contained most of the lignin and unidenti®ed alkali-soluble polysaccharides; and the residual ®ber was mostly cellulose. Whereas the hemi-celluloses could not be preserved as polymers, lignin had molecular weights (Mn) in excess of 3000 and dispersities in excess of 10, except for highly severe reaction conditions which produced lignins with lower molecular weight. The molecular weight of the cellulose declined steadily with reaction severity; that of lignin dropped abruptly at a severity of log R0 4.25 where homolytic depolymerization was indicated. Fraction behavior and fraction character were found to be highly dependent on the severity of the steam explosion treatment. # 1998 Elsevier Science Ltd. All rights reserved KeywordsÐWood; agricultural harvesting residues; peanut hills; sugar cane bagasse; steam-explosion; cellulose; lignin; pentoses; xylose; molecular structure.
1. INTRODUCTION
``Steam explosion'' refers to a treatment technology in which ligni®ed biomass is exposed to high pressure steam followed by sudden (explosive) decompression. This is recognized as a hydrolytic pretreatment prior to biomass sacchari®cation for ethanol production;1±6 it is recognized as a method capable of converting biomass into a ®brous pulp that is easily fractionated into a water soluble, an alkali soluble, and an insoluble fraction;7±11 and it is known as a technique for rendering cellulose degradable by cellulolytic enzymes1,4±6,12 as well as accessible to solvolysis by cellulose dissolving solvents.13±15 Steam explosion of biomass can be performed in batch reactors as well as continuously. The relationship between treatment conditions (steam pressure and residence time) and steam explosion eects has been described by Chornet and Overend.7,16 A ``severity factor'' has been de®ned which combines treatment type and temperature into a single parameter. Although extensively studied and 219
demonstrated on the pilot plant scale, steam explosion of biomass is not currently industrially practiced. However, the treatment of biomass with high pressure steam, followed by sudden decompression, is industrially practiced in the ``Masonite process'' which represents the basis for ®berboard production.16 The chemistry of the biomass fractions separated by water washing and alkali extraction from steam exploded biomass is also wellknown.14,16±18 The water solubles are known to consist of water-soluble lignin plus low molecular weight carbohydrates; the alkali solubles represent the bulk of the lignin; and the insoluble ®ber fraction consists primarily of cellulose. The latter is known to have greater accessibility to enzymes and solvents than corresponding non-steam treated materials; it has lower molecular weight, and it has increased crystallinity over natural cellulose.1,7,13,14,16±22 While there is a vast amount of information available on steam explosion, the separation of components following steam explosion
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using water and alkali, and the composition of the resulting fractions, no systematic studies have focused on the relationship between severity factor and fractionation behavior as well as fraction character using a continuous pilot scale steam exploder. This is the subject of the present investigation. A pilot and demonstration-scale steam explosion instrument, Stake Tech II reactor, manufactured by Stake Technology of Norval, Ontario, Canada, was available at Bioregional Energy Associates Ltd, of Floyd, Virginia.23 This instrument was operated in an experimental mode demonstrating feasibility of fractionation following treatment with a variety of severity factors. Individual samples measuring several hundreds of pounds were fractionated into their constitutive components by water washing and alkali extracting in a specially designed pilot laboratory capable of processing steam exploded biomass in hundred pound quantities in a process more similar to the countercurrent mass ¯ow operations of the pulp and paper industry than the typical laboratory experiments which have been used to describe fraction behavior of biomass in the past. This pilot fractionation laboratory made it possible to isolate fractions in sucient quantity to stimulate subsequent commercial interest.23,24 It was the objective of the present investigation to explore the fractionation behavior, and the fraction character, of three biomass sources using a range of practical severity conditions, and employing a continuous pilot scale steam explosion device.
2. EXPERIMENTAL
2.1. Materials 2.1.1. Yellow poplar (Liriodendron tulipifera) wood chips. These were clean, de-barked, pulp industry standard 3/4-in. wood chips acquired from the Turman Lumber Co., Radford, VA. The species was professionally con®rmed. This type of material is reasonably free ¯owing, contains about 50±55% moisture (dry basis), and lends itself very well to the material handling equipment existing with the Stake gun installation. It forms a very dense and reliable plug of material as the pressure seal in the Stake feeder.
2.1.2. Sugar cane bagasse (Saccharum spp, hybrids); leaf, pith, and whole bagasse. Sugar cane bagasse is a biomass valued at its corresponding fuel value. In the process of sugar production, bagasse may be harvested as a separate leaf fraction; as a mechanically separated pith fraction; or as intact ``whole'' bagasse. All three components were examined in this study. Shortly after harvest and collection these bagasse samples were air-lifted from Maui, HI to Washington, DC and then surface transported to Floyd, VA. These samples had retained their original moisture content and, despite the fact that they were steam exploded within four days of harvest, some surface mold developed in small localized areas. This material tended to be long, ®brous, and somewhat sticky. 2.1.3. Peanut hulls (Arachis hypogaea). Peanut hulls are the waste from the peanut shelling industry. The hulls for this project, ca 10 t total, were supplied by Birdsong Peanuts, Inc. of Franklin, VA. This material tended to be very dry (ca 10±15% moisture, dry basis) and dusty. The addition of water to the furnish to control dust and increase the moisture content to a level suitable for steam pretreatment was necessary. The shells were free ¯owing and worked very well with the equipment. The odor generated during the steam treatment was reminiscent of roasting peanuts. 2.2. Methods 2.2.1. Steam explosion. The Stake Tech system involves a co-ax feeder mechanism15 which enables the continuous feeding of a broad spectrum of feedstocks, of appropriate size and moisture content, into the compression zone of the feeder which forms a dense plug of the feedstock. This, in turn, constantly supplies the digester with material in a steady state±steady ¯ow (SSSF) manner against digester pressures as high as 3.0 MPa and temperatures as high as 2378C. The production capability of this digester with its reduced port (150 mm) feeder bore was dependent upon the feedstock material, but had a maximal range of up to ca 135 kg hÿ1 for certain types of paper waste and grasses, and up to ca 1000 kg hÿ1 for wood chips, dry basis. The equipment was ¯exible enough to have a turn down ratio of ca 7:1 for wood chips. The feedstocks which could not be fed as quickly into the digester had reduced turn down ratios.
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The limiting factors for the biomass ¯ow rates, in the cases involving long, stringy, nonuniform feedstocks, were determined by the capabilities of the existing materials handling equipment prior to the co-ax feeder inlet hopper, and by the adjustable position choke cone which controls the size of the inlet ori®ce into the digester proper. In the cases of fast, free ¯owing feedstocks, the co-ax feeder was limiting, from which point on there essentially never existed any problems. The residence time within the reactor was determined by the adjustment of the rotating speed (revolutions per minute or rpm) of the screw conveyer within the digester and calculated according to: residence time min : tr 28=rpm where 28 was the number of active ¯ights on the digester screw conveyor. This obviously depends upon zero back-¯ow or slippage of the biomass relative to the screw, which seems a reasonable assumption due to the relatively slow speeds of 3±43 rpm and the relatively low volume per ¯ight which reaches a maximum of ca 25±30% full under the conditions of maximum feed rate and maximum tr (minimum rpm). The brief dwell time in the discharge auger and chamber was not considered. For any given biomass and set of operating conditions, this equipment tended to produce such consistent and repeatable results, based upon the known reactor residence time tr, that the discharge time was considered inconsequential. Digester steam pressure was determined by the pressure controls of the Clayton model 200 steam generator and veri®ed by numerous pressure gauges, temperature gauges and thermocouples. Since steam explosion depends upon saturated steam for proper steam penetration and condensation within the biomass, temperature and pressure are dependent properties and one can be used to verify the other. Upon achieving SSSF, the reactor would experience pressure swings due to the periodic discharge of its contents through the timed discharge valve. The pressure swings were on the order of 135±200 kPa. During discharge, the thermocouple at the discharge end of the reactor rarely indicated as much as 18C variation due to the immense thermal inertia of the digester. Overall, the system was easy
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enough to control to enable highly repeatable production of the desired test samples. The discharge from the digester proceeded through a 150 mm diameter pipe approx. 30 m in length into the rear of a box trailer where it was collected until SSSF had been attained following any change in operating conditions or feedstock type. At that time, the discharge pipe was diverted in such a way that the ejected biomass would strike a de¯ection shield and deposit the sample into stainless steel sample drums for collection. There were no provisions for collecting the volatile components of the biomass in the steam. The sample drums were placed in 38C storage until they were fractionated. 2.2.2. Water washing. Upon receipt of steam exploded biomass, its dry mass equivalent was determined on a moisture balance. Through the experience of extracting dierent steam exploded materials (yellow poplar, sugar cane bagasse, peanut hulls and others), the maximum initial dry mass was determined to be 6.5±7.0 kg per batch. This was due to the size of the extraction vessel; 65 l, and the nature of de-®bered, steam exploded ®ber which tended to absorb liquid readily. This absorption resulted in heavier ®bers tending to compress in the bottom of the extraction vessel at low liquor to wood (L:W) ratios resulting in poor mixing. As a result a liquor to wood ratio of nearly 10:1 was utilized for some biomass types. The extraction vessel was a stirred and jacketed Pfaudler reactor. This vessel was lined with borosilicate glass and pressure rated to 345 kPa. Its agitator was a large anchortype which did not induce a great deal of shear to the ®bers while mixing. The water/®ber separation technique incorporated a series of three table-top Buchner funnels, each measuring 600 mm in diameter with a capacity of 75 l each. These were connected, through a shut-o valve, to a high capacity vacuum system. Steam exploded, hot water extracted ®ber was placed in the funnels after the temperature had been reduced to avoid damage to the funnels. The liquid fraction (strong liquor) was removed via the vacuum system and stored in any of three 65 l stainless steel ®ltrate receivers. From the receivers, the strong liquor was pumped back onto a fresh batch of steam exploded ®bers for a second extraction using the same liquid. Recycle was repeated until the carrying ca-
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pacity of the strong liquor attained a point of diminishing return. This liquor recycle substantially reduced the amount of fresh water required and it allowed for the concentration of the recoverable water soluble components in the liquor making the solids recovery from solution more economical. A pattern of processing six batches of 6.5 kg dry ®ber each was developed while using the same strong liquor. These same six ®ber batches received two additional water washings while recycling that weak liquor as well. The liquid/®ber separation could also be accomplished in a 16 kg capacity ®ber centrifuge. Both separation techniques resulted in a dry ®ber solids content following extraction of ca 25±35%. Recovery of the water-soluble polysaccharidic solids was accomplished in, essentially, a three step process involving bleaching for the partial removal or alteration of water-soluble lignin, ultra®ltration for molecular weight separation, and spray or freeze drying of the 6±8% solids content solution.24 A dry powder, primarily xylan, is the recovered product from the water extraction step. 2.2.3. Alkali extraction stage. The solids contents of the water extracted ®bers were determined, before they were returned into the Pfaudler for deligni®cation. The liquor to wood ratio was adjusted to 10:1, taking into account the water in the ®bers, with either aqueous sodium hydroxide (NaOH) or potassium hydroxide (KOH). The NaOH was used when the desired product was lignin. The KOH was used when the desired product was a spray-dried potassium lignate salt. In either case, the reactor was heated to ca 808C to solubilize lignin (predominantly) and other alkali soluble components. The extraction remained at temperature for one hour, was drained and then either centrifuged or ®ltered to separate the liquid fraction (strong liquor) from the ®bers. The strong liquor pH was adjusted by the addition of either more NaOH or KOH, and it was then recycled onto a new batch of water extracted ®bers. This recycle pattern was repeated for six batches before a signi®cant reduction in lignin removal capacity was noticed. The alkali extracted ®bers were water rinsed to remove residual alkali soluble material. The weak liquor was limited to 2±3 vol., and it was recycled over the same six batches of ®ber to increase the concentration of lignin in solution.
2.2.4. Lignin isolation. To collect the lignin from the liquors, the pH was lowered at ca 608C to a point where the lignin was no longer soluble and precipitation took place. The lignin was water washed to remove salts and dried via any one of several methods (vacuum oven, freeze drying, tray drying, air drying). If the desired product was potassium lignate the pH was carefully lowered to ca 7±8 before the mixture was pumped to the spray dryer. Spray-dried K-lignate was recovered as a free¯owing powder. The concentration of lignin in solution, in both the strong liquor and the weak liquor, was important to its harvesting. Whenever the concentration was on the order of 6±9% by mass, the lignin ¯occulated into large particles which had good ®lterability. The temperature at which precipitation took place was found to aect the size of lignin ¯occs, with good ®lterability occurring at ca 70±808C. 2.2.5. Solids content determination. These tests were performed with an O'Haus MB 200 moisture balance. Determinations were equivalent in accuracy to oven drying determination (overnight at 1058C) down to solids content as low as 20±21%. Solids contents below this level were to be checked against the oven drying method. The issue was one of thermal degradation to peripheral ®bers which would dry sooner than the bulk of the sample. 2.2.6. Analytical tests. Water soluble solids were analyzed with regard to lignin content by UV-spectrophotometry using 330 nm as wavelength and an absorptivity coecient of 12.02 l gmÿ1 cmÿ1. The analysis of hydrolyzable sugars was performed using dilute sulfuric acid, 1208C, and either gas chromatographic separation using N-TMS-imidizole as derivatization agent,25 or liquid chromatographic separation in accordance with the procedure of Kaar et al.26 Alkali-soluble lignin was recovered by precipitation with HCl followed by ®ltration. Cellulose molecular weights were determined by derivatization with phenyl isocyanate (``tricarbanilate'' procedure) according to Wallis et al.,27 and isolated lignins were analyzed following peracetylation.28 Molecular weight determinations were performed on a high pressure liquid chromatography system equipped with a refractive index concentration detector; a dierential viscometer with appropriate computer facilities and software capabilities; and with a series of
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Fig. 1. Mass ¯ow diagram for steam exploded ®ber extraction with simulated counter-current washing.
micro-spherogel columns having exclusion volumes of 100, 1000, and 10,000 A, and operating in THF solvent.29 3. RESULTS
3.1. General mass balance and fractionation considerations Chemical treatments of biomass in the laboratory usually involve repeated and exhaustive treatments of highly sorptive and swellable ®ber solids with aqueous ¯uids. Mass transfer equilibria are established which require the use of inordinately high ¯uid to solid (liquor-to-wood ®ber) mass ratios. While it is recognized that highly valid and reproducible results are obtained when ¯uid to solid ratios approach in®nity, commercial (and pilot plant) practicality dictates a more pragmatic approach. This involves less than exhaustive extraction with extensive extract recycle and countercurrent washing. The latter implies that fresh wash water be used only with the cleanest ®ber fraction, that which had been previously extracted with ``weak'' and ``strong'' extraction liquors. Biomass and ¯uid ¯ow using countercurrent operation is illustrated in Fig. 1. The accumulation of lignin solids in the extract of the alkaline extraction stage with number of recycles (Fig. 2) illustrates that there is only a minor loss in extraction eciency with liquor re-use. Strong liquor
rises to nearly 10% lignin content after ®ve recycles, and weak liquor reaches levels of nearly 4%. It is obviously much more practical, economical, and ecient to recover lignin from 10%-solutions than it is from 1%. 3.2. Mass balances of biomass solids and of fractions The fractionation of steam exploded yellow poplar wood chips into water-solubles, alkalisolubles and insoluble ®ber solids in relation to severity factor (Fig. 3) provides evidence for the eciency with which steam explosion assists in the fractionation of biomass into constitutive components. Between 10 and 20% of the biomass solids become soluble in water, less at low severity and most at severities ran-
Fig. 2. Lignin concentration in the alkaline strong liquor and counter ¯ow weak liquor in relation to number of recycles.
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Fig. 3. Mass distribution of yellow poplar steam-exploded ®ber samples treated at various severity factors (log R0) and fractionated into water-soluble, alkali-soluble and insoluble ®ber solids. (Data are presented in cumulative manner).
ging between 4.2 and 4.3. Alkali-soluble solids amount to between 25 and 30% of total biomass solids, and they are highest at severities between 4.1 and 4.2. Insoluble ®ber solids (by dierence) range between 50 and 65% of total solids, and they are lowest at severities of
Fig. 4. Cumulative water- and alkali-soluble solids fractions of dierent biomasses steam treated at several severities. (A) on log R0 scale; (B) on linear R0 scale.
around 4.2. These results are generally consistent with numerous related studies.1,5,7,8 A comparison between the dierent biomass resources, yellow poplar, various sugar cane bagasse components, and peanut hulls (Fig. 4), reveals signi®cant dierences. Whereas yellow poplar shows relatively little variation in fractionation response at severity levels of between 3.9 (R0=8000) and 4.3 (R0=20,000), bagasse shows best fractionation behavior at a severity of 3.6 (R0=4000) and fraction yield declining rapidly as severity increases. Peanut hulls, by contrast, proved to be resistant to steamassisted fractionation over the entire severity factor range. The residual (insoluble) ®ber fraction always consisted of more than 75% of total solids, and the alkali-soluble fraction never exceeded 10%. This suggests that dierences in chemical, morphological, and anatomical structure between biomasses play an important role in determining steam explosion quali®cations. This is consistent with observations made with softwood chips which have generally required an acid catalyst for ecient pretreatment.3,30,31 Among the three resources tested, all three bagasse fractions were superior in their response to steam explosion to yellow poplar wood chips and peanut hulls (in this order). Mass balances with regard to constitutive components of the biomass (Figs 5 and 6)
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Fig. 5. Xylose mass distribution in relation to steam explosion severity (log R0). Biomass source: yellow poplar. (100% xylose corresponds to 15% xylose content of wood).
revealed dierent degrees of accountability. With yellow poplar, only 45±60% of total xylan could be accounted for by analyzing the water soluble and ®ber fractions in the severity factor range (log R0) 3.9±4.3 (Fig. 5). This suggests that close to half of all xylose-rich components escape detection, most probably as part of the alkali-soluble components (Fig. 6). A small and diminishing proportion of xylan is retained in the solid ®ber fraction.32
The numerical accounting of lignin is between 80 and 120% in the severity factor range (log R0) of 3.8±4.4 (Fig. 6). (These variations are explained with variations in the UV absorptivity coecient over the severity factor range, and are discussed below). The data reveal that approx. 10% of lignin is water-soluble, almost independently of the severity of treatment. Water-insoluble, alkali-soluble lignin components represent between about 50 and 80% of total available lignin; and the bal-
Fig. 6. Lignin mass distribution in relation to steam explosion severity (log R0). Biomass source: yellow poplar. (100% lignin corresponds to 24% lignin content of wood).
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ance is retained by the cellulose rich ®ber fraction (Fig. 6, which illustrates cumulative lignin amounts). A sharp rise in extractable lignin is seen above log R0 4.2. ``Best'' fractionation conditions in terms of separating constitutive components into distinct fractions apparently involve severities (log R0) of between 4.2 and 4.3 for a yellow
poplar chip furnish. These are dierent for other biomass resources. 3.3. Composition of water soluble solids Size exclusion chromotograms (SEC) of water soluble yellow poplar solids in the severity factor (log R0) range of 3.8±4.4 (Fig. 7) using both refractive index and U.V. detection
Fig. 7. Size exclusion chromatograms of aqueous extracts from yellow poplar wood chips steamexploded at dierent severity factors. Optical (UV) and RI-density on relative time scale of 1 hour.
Steam-assisted biomass fractionation. Part II
indicate a series of components eluting at dierent retention times. Components eluting ®rst, after between 20 and 30 min of retention time, are high molecular weight substances which are detected by both the RI and UV detector. The lower molecular weight substances, those in the elution time range of 35± 45 min, are detected only by the RI monitor. It is clearly apparent that a shift exists from the high to low molecular weight portion of the solids as severity factor increases. In addition, the response patterns of the high molecular weight, U.V.-absorbing components change with severity. Size exclusion chromatography is conducted in aqueous medium, and separations are subject to variations in relation to ionic strength as well as molecular weight factors. In order to eliminate charge eect-related separation details, the same water soluble solids were separated in a THF-based gel permeation chromatography system equipped with dierential viscosity detector. THF-solubility was achieved by peracetylation. GPC/DV allows the separation of components by size (i.e. hydrodynamic volume) and the computation of absolute molecular weights. The procedure has been described elsewhere.33 The results (Fig. 8) reveal that the high molecular weight fractions (those that were UV-absorbing by SEC, Fig. 7) are in the molecular weight range of 103±104, and these are distinctly separated from com-
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Fig. 8. Generic size exclusion chromatogram of water-soluble solids from yellow poplar on molecular weight scale.
ponents having molecular weights of between 102 and 103. These results suggest that the water-soluble solids consist of a series of low molecular weight mono and oligo-saccharides (which become more prevalent with higher severity factors); and of U.V.-absorbing higher molecular weight components that are present under all conditions of severity. These results are consistent with mass balance results suggesting a mixture of water-soluble lignin and carbohydrates in the water solubles fraction; and they are in accord with reports in the literature.34
Fig. 9. Absorptivity coecients of isolated (by acid precipitation) lignin from yellow poplar in relation to severity factor (log R0).
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Fig. 10. Typical 1H-NMR spectrum of acetylated steam exploded lignin from yellow poplar. Note the distinction of aliphatically vs aromatically bonded acetoxy groups.
Fig. 11. Relationship between acetoxy signals by 1H-NMR of acetylated yellow poplar lignins in relation to steam explosion severity.
Fig. 12. Molecular weight distributions of yellow poplar lignins (as acetates) for steam explosion severities ranging from log R0 3.7±4.4.
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Mass balance data (Fig. 3) revealed that the abundant alkali-soluble solids consist of both lignin and (water-insoluble) carbohydrates in varying proportion. The most practical use for the alkali soluble solids is as polyphenolic component following isolation by precipitation with acid.35 This is industrially practiced in several pulp mills using alkaline deligni®cation (kraft pulping). Acid-precipitated lignin can be ®ltered and separated by ®ltration; after drying it was obtained as free ¯owing, brown powder. The absorptivity coecient at 280 nm of acid-precipitated lignin from yellow poplar, isolated by acidi®cation of the strong liquor at elevated temperature, has values of between 18 and 20 l gÿ1 cmÿ1 as severity factor (log R0) rises from 3.8 to 4.4 (Fig. 9). The increase, however, is not gradual. Instead a sudden rise is observed from a lower to a higher plateau value at the severity of 4.2±4.25 (Fig. 9). This increase, which coincides with the increase in lignin accountability and yield (Fig. 6), must be related to changes in lignin's chemical structure which suddenly occur when severity rises above 4.2. An examination of the chemical structure of isolated, alkali-soluble/acidinsoluble lignins by 1H-NMR spectroscopy (Fig. 10) reveals that the changes in overall composition that may explain the sudden rise in UV-absorptivity coecient relate to the ratio of phenolic to aliphatic OH groups.35,36 When the signal representing phenolic acetoxy-protons is ratioed to the total acetoxyprotons (Fig. 11), a slight increase is recorded for the severity factor range (log R0) between 3.8 and 4.2, and a sudden increase is observed between 4.2 and 4.25. Total acetoxy-1H fraction, by contrast, remains constant for the entire severity factor range. These results suggest that the sudden changes in lignin's chemical structure that have caused a rise in UV absorptivity coecient at log R0 of 4.2 coincide with an increase in lignin accountability and yield (Fig. 6) and also with a sudden rise in phenolic OH content. Such an increase in phenolic OH content is indicative of depolymerization since lignin's premier intermonomer linkage (esp. in hardwood or syringyl-type lignins) is an alkyl±aryl ether linkage.37 When the molecular weights of alkali-soluble, acid-insoluble lignins are analyzed by GPC (as THF-soluble peracetates in
Fig. 13. Molecular weight distributions of sugar cane bagasse lignins (as acetates) for steam explosion severities ranging from log R0 3.9±4.25.
3.4. Composition of alkali solubles/acid insolubles
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lower yield; and a higher molecular weight than that isolated at higher severity (above 4.25). 3.5. Composition of ®ber fraction
Fig. 14. Weight average molecular weights of lignins (as acetates) from dierent biomass sources in relation to steam explosion severity (log R0).
accordance with earlier work),29 molecular weight patterns emerge that follow a consistent trend (Fig. 12). A more or less normal (but broad) distribution at low severity becomes increasingly bimodal (log R0 3.9±4.2) before resuming again a more normal distribution (log R0 4.25±4.4). This pattern is repeated in the bagasse lignin fractions (Fig. 13). A review of the numerical molecular weights of isolated lignins reveals weight average molecular weights (Mw) that decline precipitously at log R0 4.2 (Fig. 14). This sudden decrease is further evidence for a suddenly occurring depolymerization reaction involving alkyl±aryl ether bonds at a severity of 4.2. Lignin at lower severity has a lower UV absorptivity coecient; a lower phenolic hydroxyl content;
The combined glucose and galactose (because of less than baseline separation by HPLC) content of the ®ber solids isolated at a severity factor (log R0) of between 3.9 and 4.4 (Fig. 15) shows more or less constant values of between 40 and 50% for the steam exploded and water-washed ®bers, and a trend of increasing glucose contents, from 60 to 85% following alkali extraction in the severity factor range of between 3.9 and 4.3. Under extremely severe conditions, log R0 4.4, glucose content seems to decline. The xylose content of the ®ber solids declines to below 2% of both water and alkali extracted ®bers as severity rises to 4.3 (Fig. 16). The lignin content of the alkali-extracted ®bers (Fig. 17) declines from a Kappa number of 86 to below 50 as severity increases from 3.8 to 4.4. (A Kappa number of less than 15 is considered ``bleachable grade'' pulp.) The steepest decline in lignin content is experienced around the severity factor (log R0) range of 4.2. The molecular weight distributions of insoluble ®ber solids were determined by gel permeation chromatography in THF using fully carbanilated unbleached ®bers.27 The results indicate DPn values of between 200 and 1100, and DPw values between 1000 and 3000, for
Fig. 15. Glucose (and galactose) content of steam exploded ®bers from yellow poplar in relation to treatment severity and extraction condition.
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Fig. 16. Xylose content of steam exploded ®bers from yellow poplar in relation to treatment severity and extraction condition.
severities ranging between 4.4 and 3.8 (Fig. 18). Virtually linear relationships (R2-values of 0.96 and 0.99) were established for the severity factor range between 3.9 and 4.4 (Fig. 18 insert). These relationships obey the following functions. hDPn i 3:908103ÿ 8:4102 log R0 ; R2 0:99 hDPw i 1:151104ÿ 2:46103 log R0 ; R2 0:96
Since microcrystalline cellulose has values of around 150 and 250 for DPn and DPw, respectively, unbleached cellulose from steam explosion lies well above that common for microcrystalline cellulose.32 The gradual decline in molecular weight with increasing severity is typical of a kinetically controlled (hydrolytic) reaction; and this is consistent with ®ndings elsewhere.16±18,22,34
Fig. 17. Relationship between Kappa number and steam explosion severity (log R0) for extracted yellow poplar ®bers.
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Fig. 18. Molecular weights of extracted yellow poplar ®bers in relation to steam explosion severity (log R0).
4. DISCUSSION
Steam explosion of biomass is known to be a hydrolytic pretreatment; one that increases enzyme and solvent accessibility of cellulose; one that renders biomass separable (fractionatable) into constitutive components; and one that raises the crystallinity of the cellulose component. The results of a pilot-scale study involving the steam explosion of yellow poplar wood chips, bagasse fractions, and peanut hulls in a continuous Stake II reactor were found, in general, to be consistent with these observations reported previously.14,16,18 With regard to cellulose, a steady decrease is recorded that allows predicting molecular weight on the basis of treatment severity. The resulting cellulose is virtually free of other carbohydrate components, but it retains 7± 10% lignin even under relatively severe conditions. Non-cellulosic carbohydrate components (branched hetero-polysaccharides) are eectively depolymerized resulting in extensive water and alkali-solubility. There was no indication for the potential of isolating xylose-rich polymeric components having more than 10 or 15 monomeric repeat units. This has been con®rmed in a recent xylan isolation study.24,32
Lignin is distributed between a water-soluble fraction (ca 10% of total lignin), alkalisoluble fractions, and insoluble substances. The alkali-soluble lignin fraction reveals a sudden change in chemical structure at a severity of log R0 4.2±4.25. UV280-absorptivity coecient rises from 18 to 22, phenolic OH-groups are liberated, yield increases, and weight average molecular weights decline precipitously at log R0 4.2. This behavior is consistent with the previously observed homolytic depolymerization of lignin under steam explosion conditions.38±40
5. CONCLUSIONS
The steam assisted fractionation of biomass by continuous steam explosion produces at best two useful polymer fractions, a 85±90% pure, partially hydrolyzed cellulose fraction in approx. 40±45% yield; and a 90±95% pure (alkali-soluble, acid-insoluble) lignin fraction in 15±20% yield. Molecular weight, yield, and functionality of the latter depend primarily on whether steam explosion severity is below or above the severity factor (log R0) threshold of 4.2.
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The hemicellulose fraction is divided between a water-soluble and an alkali-soluble fraction, and it is severely degraded. Dierent biomass sources respond very differently to treatment with high pressure steam. These dierences are not, or not only, related to diusivity, but they seem more in¯uenced by chemical, morphological, and anatomical features. AcknowledgementsÐFinancial support for this study was provided by the Center for Innovative Technology, Herndon, VA; by Bioregional Energy Associates, Ltd, of Floyd, VA; by the Hawaiian Sugar Planters Association (HSPA) of Aeia, HI; and by Birdsong Peanuts of Suolk, VA. Skilful technical assistance was provided by Mr Woodrow L. McKenzie, Ms Jody Jervis, Mr Marshall Beard, Mr J. S. Brown, Dr Rajesh K Jain, and Dr William E. Karr, and this is acknowledged with gratitude.
12.
13. 14.
15. 16.
REFERENCES 1. Focher, B., Marzetti, A. and Crescenzi, V. (eds), Steam Explosion TechniquesÐFundamentals and Industrial Applications. Gordon and Breach Science Publishers, Philadelphia, 1991. 2. Mamers, H. and Menz, D. N. J., Explosion pretreatment of Pinus radiata woodchips for the production of fermentation substrates, Appita, 1984, 37(8), 644± 649. 3. Clark, T. A. and Mackie, K. L., Steam explosion of the softwood Pinus radiata with sulphur dioxide addition. I. Process optimisation, Journal of Wood Chemistry and Technology, 1987, 7(3), 373. 4. Schwald, W., Breuil, C., Brownell, H. H., Chan, M. and Saddler, J. N., Assessment of pretreatment conditions to obtain fast complete hydrolysis on high substrate concentrations, Applied Biochemistry and Biotechnology, 1989, 20/21, 29±44. 5. Schultz, T. P., Rughari, J. R. and McGinnis, G. D., Comparison of the pretreatment of sweetgum and white oak by the steam explosion and RASH processes, Applied Biochemistry and Biotechnology, 1989, 20/21, 9±27. 6. Dekker, R. F. H., Steam explosion: an eective pretreatment method for use in the bioconversion of lignocellulosic materials. In Steam Explosion TechniqueÐFundamentals and Industrial Applications. Gordon Breach Science Publishers, Philadelphia, 1991, pp. 277±305. 7. Overend, R. P. and Chornet, E., Fractionation of lignocellulosics by steam-aqueous pretreatments, Phil. Transactions of the Royal Society London A, 1987, 321, 523±536. 8. Wallis, A. F. A. and Wearne, R. H., Fractionation of the polymeric compounds of hardwoods by autohydrolysis±explosion±extraction, Appita, 1985, 38, 432± 437. 9. Beltrame, P. L., Carniti, P., Visciglio, A., Focher, B. and Marzetti, A., Fractionation and bioconversion of steam-exploded wheat straw, Bioresources Technology, 1992, 39, 165±171. 10. MontaneÂ, D., SalvadoÂ, J., Farriol, X. and Chornet, E., The fractionation of almond shells by thermo±mechanical aqueous-phase (TM±AV) pretreatment, Biomass Bioenergy, 1993, 4(6), 427±437. 11. Kokta, B., Steam explosion pulping. In Steam Explosion TechniquesÐFundamentals and Industrial
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Applications, ed. B. Focher, A. Marzetti and V. Crescenzi. Gordon and Breach Science Publishers, Philadelphia, 1991, pp. 163±206. Sciaraa, F. and Marzetti, A., Enhancement of Wheat Straw digestibility of steam explosion pretreatment. In Steam Explosion TechniquesÐFundamentals and Industrial Applications, ed. B. Focher, A. Marzetti and V. Crescenzi. Gordon and Breach Science Publishers, Philadelphia, 1991, pp. 365±374. Samaranayake, G., Li, X. and Glasser, W. G., Solvent accessibility of steam exploded cellulose, Holzforschung, 1994, 48(Supplement), 69±71. Marchessault, R. H., Steam explosion: a re®ning process for lignocellulosics. In Steam Explosion TechniquesÐFundamentals and Industrial Applications, ed. B. Focher, A. Marzetti and V. Crescenzi. Gordon and Breach Science Publishers, Philadelphia, 1991, pp. 1±20. Stake Technology Ltd. The StakeTech System, Sales brochure, Stake Technology Ltd, 2838 Highway 7, Norval, Ontario LOP 1KO, Canada. Chornet, E. and Overend, R. P., Phenomenological kinetics and reaction engineering aspects of steam/ aqueous treatments. In Steam Explosion TechniquesÐ Fundamentals and Industrial Applications, ed. B. Focher, A. Marzetti and V. Crescenzi. Gordon and Breach Science Publishers, Philadelphia, 1991, pp. 21± 58. Excoer, G., PeÂguy, A., Rinaudo, A. and Vignon, M. R., Evolution of lignocellulosic components during steam explosion. Potential applications. In Steam Explosion TechniquesÐFundamentals and Industrial Applications, ed. B. Focher, A. Marzetti and V. Crescenzi. Gordon and Breach Science Publishers, Philadelphia, 1991, pp. 83±95. Bobleter, D., Bonn, G. and Prutsch, W., Steam explosion-hydrothermolysis, organosolv. a comparison. In Steam Explosion TechniquesÐFundamentals and Industrial Applications, ed. B. Focher, A. Marzetti and V. Crescenzi. Gordon and Breach Science Publishers, Philadelphia, 1991, pp. 59±82. Glasser, W. G., Research with biopolymers from steam-exploded biomass. In 6th Proceedings of the International Symposium Wood Pulp Chemistry. Melbourne, Australia, 1991, pp. 319±322. Goto, T., Tanahashi, M., Harada, H. and Higuchi, T., X-ray diraction analysis of main components of wood and evaluation for crystallinity of cellulose component, Mokuzai Kenkyu Shiryo, 1985, 21, 102±107. Teejaer, R., Gravitis, J., Erins, P., Pugulis, J., Kulkevics, A., Gromov, V. S. and Lippmaa, E., Changes in the phase structure and conformation state of wood components during explosive autohydrolysis, Doklai Akademie Nauk SSSR, 1986, 288(4), 932±935. Yamashiki, T., Matsui, T., Saitoh, M., Okajima, K. and Kamide, K., Characterisation of cellulose treated by the steam explosion method. Part I. in¯uence of cellulose resources on changes in morphology, degree of polymerisation, solubility and solid structure, British Polymer Journal, 1990, 22, 73±82. Glasser, W. G. and Wright, R. S., Research and pilot studies on chemicals from hardwoods, Final Technical Progress Report on Research Project No. MAT-87-031-01 to Virginia's Center for Innovative Technology, Herndon, VA, June 1993. Kaar, W. E., Jain, R. K., Sealey, J. E. and Glasser, W. G., Xylan I. isolation options from biomass on pilot scale, (in preparation). Pierce, 1989 Handbook and General Catalog.]6, 1989, Method, 162. Kaar, W. E., Cool, L. G., Merriman, M. M. and Brink, D. C., The complete analysis of wood polysac-
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charides using HPLC, Journal of Wood Chemistry and Technology, 1991, 11, 447±463; 465±477, and 479±494. Evans, R. and Wallis, A. F. A., Comparison of cellulose molecular weights determined by HPSEC and viscometry. In Proceedings of 4th International Symposium on Wood Pulping Chemistry. (IWSPC), Paris, France, Vol. 1, 1987, pp. 201±205. Glasser, W. G., Barnett, C. A., Muller, P. C. and Sarkanen, K. V., The chemistry of several novel bioconversion lignins, Journal of Agriculture and Food Chemistry, 1983, 31(5), 921±930. Glasser, W. G., DaveÂ, V. and Frazier, C. E., Molecular weight distribution of (semi-) commercial lignin derivatives, Journal of Wood Chemistry and Technology, 1993, 13(4), 545±559. Wayman, M., Tallevi, A. and Winsborrow, B., Hydrolysis of biomass by sulphur dioxide, Applied Biochemistry and Biotechnology, 1988, 17, 33±42. Wayman, M. and Parekh, S. R., SO2 prehydrolysis for high yield ethanol production from biomass, Applied Biochemistry and Biotechnology, 1988, 17, 33±42. Winkle, S. C. and Glasser, W. G., Chemical cellulose from steam-exploded wood by peracetic acid treatment, Journal of Pulp and Paper Science, 1995, 21(2), J37±J43. Himmel, M. E., Tatsumoto, K., Oh, K. K., Grohman, K., Johnson, D. K. and Chum, H. L., Molecular weight distribution of aspen lignins estimated by universal calibration. In LigninÐProperties and Materials. eds. W. G. Glasser and S. Sarkanen. ACS Symposium Series. 1989, 397, pp. 82±99.
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34. Debzi, E. M., Excoer, G., Toussaint, B. and Vignon, M., Steam explosion treatment of wood: eects of pressure and time on cellulose behavior. In Steam Explosion TechniquesÐFundamentals and Industrial Applications, ed. B. Focher, A. Marzetti and V. Crescenzi. Gordon and Breach Science Publishers, Philadelphia, 1991, pp. 141±161. 35. Glasser, W. G. and Kelley, S. S., Lignin. In Encyclopedia of Polymer Science and Engineering, Vol. 8. John Wiley & Sons, Inc., New York, 1987, pp. 795±852. 36. Glasser, W. G., Glasser, H. R. and Morohoshi, N., Simulation of reactions with lignins by computer (SIMREL). VI. interpretation of primary experimental analysis data (``Analysis Program''), Macromolecules, 1981, 14(2), 253±262. 37. Glasser, W. G., Barnett, C. A. and Sano, Y., Classi®cation of lignins with dierent genetic and industrial origins, Journal of Applied Polymers Symposium, 1983, 37, 441±460. 38. Tanahashi, M., Wood Research and Technical Notes (in Japanese) 1983, 18, pp. 34±65. 39. Tanahashi, M. and Higuchi, T., Steam explosion process for wood and its development. Kami Pa Gikyoshi 1985, 39, pp. 118±127. 40. Tanahashi, M., Characterization and degradation mechanisms of wood components by steam explosion and utilization of exploded wood, Wood Research, 1990, 77, 49±117.
Biomass and Bioenergy Vol. 14, No. 3, pp. 219±235, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0961-9534/98 $19.00 + 0.00 S0961-9534(97)10037-X
STEAM-ASSISTED BIOMASS FRACTIONATION. II. FRACTIONATION BEHAVIOR OF VARIOUS BIOMASS RESOURCES WOLFGANG G. GLASSER and ROBERT S. WRIGHT Virginia Tech, Blacksburg, Virginia, VA 24067, U.S.A. (Received 8 August 1994; accepted 1 October 1995) AbstractÐThe fractionation behavior following steam explosion of three biomass resources, yellow poplar (Liriodendron tulipifera) wood chips, peanut hulls (Arachis hypogaea), and sugar cane (Saccharum ocinarum) bagasse (three separate fractions, leaf, pith, and whole bagasse) were examined following steam explosion using a Stake II reactor. Component separation was evaluated using a range of dierent severity factors. Water solubles, alkali solubles, and an insoluble fraction were collected separately and subjected to component analysis. The latter addressed both chemical (component analysis by NMR and UV spectroscopy) and molecular structure (molecular weight determinations). The water soluble fraction consisted primarily of xylose (mono and oligo-saccharides) and water soluble lignin; the alkali soluble fraction contained most of the lignin and unidenti®ed alkali-soluble polysaccharides; and the residual ®ber was mostly cellulose. Whereas the hemi-celluloses could not be preserved as polymers, lignin had molecular weights (Mn) in excess of 3000 and dispersities in excess of 10, except for highly severe reaction conditions which produced lignins with lower molecular weight. The molecular weight of the cellulose declined steadily with reaction severity; that of lignin dropped abruptly at a severity of log R0 4.25 where homolytic depolymerization was indicated. Fraction behavior and fraction character were found to be highly dependent on the severity of the steam explosion treatment. # 1998 Elsevier Science Ltd. All rights reserved KeywordsÐWood; agricultural harvesting residues; peanut hills; sugar cane bagasse; steam-explosion; cellulose; lignin; pentoses; xylose; molecular structure.
1. INTRODUCTION
``Steam explosion'' refers to a treatment technology in which ligni®ed biomass is exposed to high pressure steam followed by sudden (explosive) decompression. This is recognized as a hydrolytic pretreatment prior to biomass sacchari®cation for ethanol production;1±6 it is recognized as a method capable of converting biomass into a ®brous pulp that is easily fractionated into a water soluble, an alkali soluble, and an insoluble fraction;7±11 and it is known as a technique for rendering cellulose degradable by cellulolytic enzymes1,4±6,12 as well as accessible to solvolysis by cellulose dissolving solvents.13±15 Steam explosion of biomass can be performed in batch reactors as well as continuously. The relationship between treatment conditions (steam pressure and residence time) and steam explosion eects has been described by Chornet and Overend.7,16 A ``severity factor'' has been de®ned which combines treatment type and temperature into a single parameter. Although extensively studied and 219
demonstrated on the pilot plant scale, steam explosion of biomass is not currently industrially practiced. However, the treatment of biomass with high pressure steam, followed by sudden decompression, is industrially practiced in the ``Masonite process'' which represents the basis for ®berboard production.16 The chemistry of the biomass fractions separated by water washing and alkali extraction from steam exploded biomass is also wellknown.14,16±18 The water solubles are known to consist of water-soluble lignin plus low molecular weight carbohydrates; the alkali solubles represent the bulk of the lignin; and the insoluble ®ber fraction consists primarily of cellulose. The latter is known to have greater accessibility to enzymes and solvents than corresponding non-steam treated materials; it has lower molecular weight, and it has increased crystallinity over natural cellulose.1,7,13,14,16±22 While there is a vast amount of information available on steam explosion, the separation of components following steam explosion
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using water and alkali, and the composition of the resulting fractions, no systematic studies have focused on the relationship between severity factor and fractionation behavior as well as fraction character using a continuous pilot scale steam exploder. This is the subject of the present investigation. A pilot and demonstration-scale steam explosion instrument, Stake Tech II reactor, manufactured by Stake Technology of Norval, Ontario, Canada, was available at Bioregional Energy Associates Ltd, of Floyd, Virginia.23 This instrument was operated in an experimental mode demonstrating feasibility of fractionation following treatment with a variety of severity factors. Individual samples measuring several hundreds of pounds were fractionated into their constitutive components by water washing and alkali extracting in a specially designed pilot laboratory capable of processing steam exploded biomass in hundred pound quantities in a process more similar to the countercurrent mass ¯ow operations of the pulp and paper industry than the typical laboratory experiments which have been used to describe fraction behavior of biomass in the past. This pilot fractionation laboratory made it possible to isolate fractions in sucient quantity to stimulate subsequent commercial interest.23,24 It was the objective of the present investigation to explore the fractionation behavior, and the fraction character, of three biomass sources using a range of practical severity conditions, and employing a continuous pilot scale steam explosion device.
2. EXPERIMENTAL
2.1. Materials 2.1.1. Yellow poplar (Liriodendron tulipifera) wood chips. These were clean, de-barked, pulp industry standard 3/4-in. wood chips acquired from the Turman Lumber Co., Radford, VA. The species was professionally con®rmed. This type of material is reasonably free ¯owing, contains about 50±55% moisture (dry basis), and lends itself very well to the material handling equipment existing with the Stake gun installation. It forms a very dense and reliable plug of material as the pressure seal in the Stake feeder.
2.1.2. Sugar cane bagasse (Saccharum spp, hybrids); leaf, pith, and whole bagasse. Sugar cane bagasse is a biomass valued at its corresponding fuel value. In the process of sugar production, bagasse may be harvested as a separate leaf fraction; as a mechanically separated pith fraction; or as intact ``whole'' bagasse. All three components were examined in this study. Shortly after harvest and collection these bagasse samples were air-lifted from Maui, HI to Washington, DC and then surface transported to Floyd, VA. These samples had retained their original moisture content and, despite the fact that they were steam exploded within four days of harvest, some surface mold developed in small localized areas. This material tended to be long, ®brous, and somewhat sticky. 2.1.3. Peanut hulls (Arachis hypogaea). Peanut hulls are the waste from the peanut shelling industry. The hulls for this project, ca 10 t total, were supplied by Birdsong Peanuts, Inc. of Franklin, VA. This material tended to be very dry (ca 10±15% moisture, dry basis) and dusty. The addition of water to the furnish to control dust and increase the moisture content to a level suitable for steam pretreatment was necessary. The shells were free ¯owing and worked very well with the equipment. The odor generated during the steam treatment was reminiscent of roasting peanuts. 2.2. Methods 2.2.1. Steam explosion. The Stake Tech system involves a co-ax feeder mechanism15 which enables the continuous feeding of a broad spectrum of feedstocks, of appropriate size and moisture content, into the compression zone of the feeder which forms a dense plug of the feedstock. This, in turn, constantly supplies the digester with material in a steady state±steady ¯ow (SSSF) manner against digester pressures as high as 3.0 MPa and temperatures as high as 2378C. The production capability of this digester with its reduced port (150 mm) feeder bore was dependent upon the feedstock material, but had a maximal range of up to ca 135 kg hÿ1 for certain types of paper waste and grasses, and up to ca 1000 kg hÿ1 for wood chips, dry basis. The equipment was ¯exible enough to have a turn down ratio of ca 7:1 for wood chips. The feedstocks which could not be fed as quickly into the digester had reduced turn down ratios.
Steam-assisted biomass fractionation. Part II
The limiting factors for the biomass ¯ow rates, in the cases involving long, stringy, nonuniform feedstocks, were determined by the capabilities of the existing materials handling equipment prior to the co-ax feeder inlet hopper, and by the adjustable position choke cone which controls the size of the inlet ori®ce into the digester proper. In the cases of fast, free ¯owing feedstocks, the co-ax feeder was limiting, from which point on there essentially never existed any problems. The residence time within the reactor was determined by the adjustment of the rotating speed (revolutions per minute or rpm) of the screw conveyer within the digester and calculated according to: residence time min : tr 28=rpm where 28 was the number of active ¯ights on the digester screw conveyor. This obviously depends upon zero back-¯ow or slippage of the biomass relative to the screw, which seems a reasonable assumption due to the relatively slow speeds of 3±43 rpm and the relatively low volume per ¯ight which reaches a maximum of ca 25±30% full under the conditions of maximum feed rate and maximum tr (minimum rpm). The brief dwell time in the discharge auger and chamber was not considered. For any given biomass and set of operating conditions, this equipment tended to produce such consistent and repeatable results, based upon the known reactor residence time tr, that the discharge time was considered inconsequential. Digester steam pressure was determined by the pressure controls of the Clayton model 200 steam generator and veri®ed by numerous pressure gauges, temperature gauges and thermocouples. Since steam explosion depends upon saturated steam for proper steam penetration and condensation within the biomass, temperature and pressure are dependent properties and one can be used to verify the other. Upon achieving SSSF, the reactor would experience pressure swings due to the periodic discharge of its contents through the timed discharge valve. The pressure swings were on the order of 135±200 kPa. During discharge, the thermocouple at the discharge end of the reactor rarely indicated as much as 18C variation due to the immense thermal inertia of the digester. Overall, the system was easy
221
enough to control to enable highly repeatable production of the desired test samples. The discharge from the digester proceeded through a 150 mm diameter pipe approx. 30 m in length into the rear of a box trailer where it was collected until SSSF had been attained following any change in operating conditions or feedstock type. At that time, the discharge pipe was diverted in such a way that the ejected biomass would strike a de¯ection shield and deposit the sample into stainless steel sample drums for collection. There were no provisions for collecting the volatile components of the biomass in the steam. The sample drums were placed in 38C storage until they were fractionated. 2.2.2. Water washing. Upon receipt of steam exploded biomass, its dry mass equivalent was determined on a moisture balance. Through the experience of extracting dierent steam exploded materials (yellow poplar, sugar cane bagasse, peanut hulls and others), the maximum initial dry mass was determined to be 6.5±7.0 kg per batch. This was due to the size of the extraction vessel; 65 l, and the nature of de-®bered, steam exploded ®ber which tended to absorb liquid readily. This absorption resulted in heavier ®bers tending to compress in the bottom of the extraction vessel at low liquor to wood (L:W) ratios resulting in poor mixing. As a result a liquor to wood ratio of nearly 10:1 was utilized for some biomass types. The extraction vessel was a stirred and jacketed Pfaudler reactor. This vessel was lined with borosilicate glass and pressure rated to 345 kPa. Its agitator was a large anchortype which did not induce a great deal of shear to the ®bers while mixing. The water/®ber separation technique incorporated a series of three table-top Buchner funnels, each measuring 600 mm in diameter with a capacity of 75 l each. These were connected, through a shut-o valve, to a high capacity vacuum system. Steam exploded, hot water extracted ®ber was placed in the funnels after the temperature had been reduced to avoid damage to the funnels. The liquid fraction (strong liquor) was removed via the vacuum system and stored in any of three 65 l stainless steel ®ltrate receivers. From the receivers, the strong liquor was pumped back onto a fresh batch of steam exploded ®bers for a second extraction using the same liquid. Recycle was repeated until the carrying ca-
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pacity of the strong liquor attained a point of diminishing return. This liquor recycle substantially reduced the amount of fresh water required and it allowed for the concentration of the recoverable water soluble components in the liquor making the solids recovery from solution more economical. A pattern of processing six batches of 6.5 kg dry ®ber each was developed while using the same strong liquor. These same six ®ber batches received two additional water washings while recycling that weak liquor as well. The liquid/®ber separation could also be accomplished in a 16 kg capacity ®ber centrifuge. Both separation techniques resulted in a dry ®ber solids content following extraction of ca 25±35%. Recovery of the water-soluble polysaccharidic solids was accomplished in, essentially, a three step process involving bleaching for the partial removal or alteration of water-soluble lignin, ultra®ltration for molecular weight separation, and spray or freeze drying of the 6±8% solids content solution.24 A dry powder, primarily xylan, is the recovered product from the water extraction step. 2.2.3. Alkali extraction stage. The solids contents of the water extracted ®bers were determined, before they were returned into the Pfaudler for deligni®cation. The liquor to wood ratio was adjusted to 10:1, taking into account the water in the ®bers, with either aqueous sodium hydroxide (NaOH) or potassium hydroxide (KOH). The NaOH was used when the desired product was lignin. The KOH was used when the desired product was a spray-dried potassium lignate salt. In either case, the reactor was heated to ca 808C to solubilize lignin (predominantly) and other alkali soluble components. The extraction remained at temperature for one hour, was drained and then either centrifuged or ®ltered to separate the liquid fraction (strong liquor) from the ®bers. The strong liquor pH was adjusted by the addition of either more NaOH or KOH, and it was then recycled onto a new batch of water extracted ®bers. This recycle pattern was repeated for six batches before a signi®cant reduction in lignin removal capacity was noticed. The alkali extracted ®bers were water rinsed to remove residual alkali soluble material. The weak liquor was limited to 2±3 vol., and it was recycled over the same six batches of ®ber to increase the concentration of lignin in solution.
2.2.4. Lignin isolation. To collect the lignin from the liquors, the pH was lowered at ca 608C to a point where the lignin was no longer soluble and precipitation took place. The lignin was water washed to remove salts and dried via any one of several methods (vacuum oven, freeze drying, tray drying, air drying). If the desired product was potassium lignate the pH was carefully lowered to ca 7±8 before the mixture was pumped to the spray dryer. Spray-dried K-lignate was recovered as a free¯owing powder. The concentration of lignin in solution, in both the strong liquor and the weak liquor, was important to its harvesting. Whenever the concentration was on the order of 6±9% by mass, the lignin ¯occulated into large particles which had good ®lterability. The temperature at which precipitation took place was found to aect the size of lignin ¯occs, with good ®lterability occurring at ca 70±808C. 2.2.5. Solids content determination. These tests were performed with an O'Haus MB 200 moisture balance. Determinations were equivalent in accuracy to oven drying determination (overnight at 1058C) down to solids content as low as 20±21%. Solids contents below this level were to be checked against the oven drying method. The issue was one of thermal degradation to peripheral ®bers which would dry sooner than the bulk of the sample. 2.2.6. Analytical tests. Water soluble solids were analyzed with regard to lignin content by UV-spectrophotometry using 330 nm as wavelength and an absorptivity coecient of 12.02 l gmÿ1 cmÿ1. The analysis of hydrolyzable sugars was performed using dilute sulfuric acid, 1208C, and either gas chromatographic separation using N-TMS-imidizole as derivatization agent,25 or liquid chromatographic separation in accordance with the procedure of Kaar et al.26 Alkali-soluble lignin was recovered by precipitation with HCl followed by ®ltration. Cellulose molecular weights were determined by derivatization with phenyl isocyanate (``tricarbanilate'' procedure) according to Wallis et al.,27 and isolated lignins were analyzed following peracetylation.28 Molecular weight determinations were performed on a high pressure liquid chromatography system equipped with a refractive index concentration detector; a dierential viscometer with appropriate computer facilities and software capabilities; and with a series of
Steam-assisted biomass fractionation. Part II
223
Fig. 1. Mass ¯ow diagram for steam exploded ®ber extraction with simulated counter-current washing.
micro-spherogel columns having exclusion volumes of 100, 1000, and 10,000 A, and operating in THF solvent.29 3. RESULTS
3.1. General mass balance and fractionation considerations Chemical treatments of biomass in the laboratory usually involve repeated and exhaustive treatments of highly sorptive and swellable ®ber solids with aqueous ¯uids. Mass transfer equilibria are established which require the use of inordinately high ¯uid to solid (liquor-to-wood ®ber) mass ratios. While it is recognized that highly valid and reproducible results are obtained when ¯uid to solid ratios approach in®nity, commercial (and pilot plant) practicality dictates a more pragmatic approach. This involves less than exhaustive extraction with extensive extract recycle and countercurrent washing. The latter implies that fresh wash water be used only with the cleanest ®ber fraction, that which had been previously extracted with ``weak'' and ``strong'' extraction liquors. Biomass and ¯uid ¯ow using countercurrent operation is illustrated in Fig. 1. The accumulation of lignin solids in the extract of the alkaline extraction stage with number of recycles (Fig. 2) illustrates that there is only a minor loss in extraction eciency with liquor re-use. Strong liquor
rises to nearly 10% lignin content after ®ve recycles, and weak liquor reaches levels of nearly 4%. It is obviously much more practical, economical, and ecient to recover lignin from 10%-solutions than it is from 1%. 3.2. Mass balances of biomass solids and of fractions The fractionation of steam exploded yellow poplar wood chips into water-solubles, alkalisolubles and insoluble ®ber solids in relation to severity factor (Fig. 3) provides evidence for the eciency with which steam explosion assists in the fractionation of biomass into constitutive components. Between 10 and 20% of the biomass solids become soluble in water, less at low severity and most at severities ran-
Fig. 2. Lignin concentration in the alkaline strong liquor and counter ¯ow weak liquor in relation to number of recycles.
224
W. G. GLASSER and R. S. WRIGHT
Fig. 3. Mass distribution of yellow poplar steam-exploded ®ber samples treated at various severity factors (log R0) and fractionated into water-soluble, alkali-soluble and insoluble ®ber solids. (Data are presented in cumulative manner).
ging between 4.2 and 4.3. Alkali-soluble solids amount to between 25 and 30% of total biomass solids, and they are highest at severities between 4.1 and 4.2. Insoluble ®ber solids (by dierence) range between 50 and 65% of total solids, and they are lowest at severities of
Fig. 4. Cumulative water- and alkali-soluble solids fractions of dierent biomasses steam treated at several severities. (A) on log R0 scale; (B) on linear R0 scale.
around 4.2. These results are generally consistent with numerous related studies.1,5,7,8 A comparison between the dierent biomass resources, yellow poplar, various sugar cane bagasse components, and peanut hulls (Fig. 4), reveals signi®cant dierences. Whereas yellow poplar shows relatively little variation in fractionation response at severity levels of between 3.9 (R0=8000) and 4.3 (R0=20,000), bagasse shows best fractionation behavior at a severity of 3.6 (R0=4000) and fraction yield declining rapidly as severity increases. Peanut hulls, by contrast, proved to be resistant to steamassisted fractionation over the entire severity factor range. The residual (insoluble) ®ber fraction always consisted of more than 75% of total solids, and the alkali-soluble fraction never exceeded 10%. This suggests that dierences in chemical, morphological, and anatomical structure between biomasses play an important role in determining steam explosion quali®cations. This is consistent with observations made with softwood chips which have generally required an acid catalyst for ecient pretreatment.3,30,31 Among the three resources tested, all three bagasse fractions were superior in their response to steam explosion to yellow poplar wood chips and peanut hulls (in this order). Mass balances with regard to constitutive components of the biomass (Figs 5 and 6)
Steam-assisted biomass fractionation. Part II
225
Fig. 5. Xylose mass distribution in relation to steam explosion severity (log R0). Biomass source: yellow poplar. (100% xylose corresponds to 15% xylose content of wood).
revealed dierent degrees of accountability. With yellow poplar, only 45±60% of total xylan could be accounted for by analyzing the water soluble and ®ber fractions in the severity factor range (log R0) 3.9±4.3 (Fig. 5). This suggests that close to half of all xylose-rich components escape detection, most probably as part of the alkali-soluble components (Fig. 6). A small and diminishing proportion of xylan is retained in the solid ®ber fraction.32
The numerical accounting of lignin is between 80 and 120% in the severity factor range (log R0) of 3.8±4.4 (Fig. 6). (These variations are explained with variations in the UV absorptivity coecient over the severity factor range, and are discussed below). The data reveal that approx. 10% of lignin is water-soluble, almost independently of the severity of treatment. Water-insoluble, alkali-soluble lignin components represent between about 50 and 80% of total available lignin; and the bal-
Fig. 6. Lignin mass distribution in relation to steam explosion severity (log R0). Biomass source: yellow poplar. (100% lignin corresponds to 24% lignin content of wood).
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W. G. GLASSER and R. S. WRIGHT
ance is retained by the cellulose rich ®ber fraction (Fig. 6, which illustrates cumulative lignin amounts). A sharp rise in extractable lignin is seen above log R0 4.2. ``Best'' fractionation conditions in terms of separating constitutive components into distinct fractions apparently involve severities (log R0) of between 4.2 and 4.3 for a yellow
poplar chip furnish. These are dierent for other biomass resources. 3.3. Composition of water soluble solids Size exclusion chromotograms (SEC) of water soluble yellow poplar solids in the severity factor (log R0) range of 3.8±4.4 (Fig. 7) using both refractive index and U.V. detection
Fig. 7. Size exclusion chromatograms of aqueous extracts from yellow poplar wood chips steamexploded at dierent severity factors. Optical (UV) and RI-density on relative time scale of 1 hour.
Steam-assisted biomass fractionation. Part II
indicate a series of components eluting at dierent retention times. Components eluting ®rst, after between 20 and 30 min of retention time, are high molecular weight substances which are detected by both the RI and UV detector. The lower molecular weight substances, those in the elution time range of 35± 45 min, are detected only by the RI monitor. It is clearly apparent that a shift exists from the high to low molecular weight portion of the solids as severity factor increases. In addition, the response patterns of the high molecular weight, U.V.-absorbing components change with severity. Size exclusion chromatography is conducted in aqueous medium, and separations are subject to variations in relation to ionic strength as well as molecular weight factors. In order to eliminate charge eect-related separation details, the same water soluble solids were separated in a THF-based gel permeation chromatography system equipped with dierential viscosity detector. THF-solubility was achieved by peracetylation. GPC/DV allows the separation of components by size (i.e. hydrodynamic volume) and the computation of absolute molecular weights. The procedure has been described elsewhere.33 The results (Fig. 8) reveal that the high molecular weight fractions (those that were UV-absorbing by SEC, Fig. 7) are in the molecular weight range of 103±104, and these are distinctly separated from com-
227
Fig. 8. Generic size exclusion chromatogram of water-soluble solids from yellow poplar on molecular weight scale.
ponents having molecular weights of between 102 and 103. These results suggest that the water-soluble solids consist of a series of low molecular weight mono and oligo-saccharides (which become more prevalent with higher severity factors); and of U.V.-absorbing higher molecular weight components that are present under all conditions of severity. These results are consistent with mass balance results suggesting a mixture of water-soluble lignin and carbohydrates in the water solubles fraction; and they are in accord with reports in the literature.34
Fig. 9. Absorptivity coecients of isolated (by acid precipitation) lignin from yellow poplar in relation to severity factor (log R0).
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Fig. 10. Typical 1H-NMR spectrum of acetylated steam exploded lignin from yellow poplar. Note the distinction of aliphatically vs aromatically bonded acetoxy groups.
Fig. 11. Relationship between acetoxy signals by 1H-NMR of acetylated yellow poplar lignins in relation to steam explosion severity.
Fig. 12. Molecular weight distributions of yellow poplar lignins (as acetates) for steam explosion severities ranging from log R0 3.7±4.4.
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Mass balance data (Fig. 3) revealed that the abundant alkali-soluble solids consist of both lignin and (water-insoluble) carbohydrates in varying proportion. The most practical use for the alkali soluble solids is as polyphenolic component following isolation by precipitation with acid.35 This is industrially practiced in several pulp mills using alkaline deligni®cation (kraft pulping). Acid-precipitated lignin can be ®ltered and separated by ®ltration; after drying it was obtained as free ¯owing, brown powder. The absorptivity coecient at 280 nm of acid-precipitated lignin from yellow poplar, isolated by acidi®cation of the strong liquor at elevated temperature, has values of between 18 and 20 l gÿ1 cmÿ1 as severity factor (log R0) rises from 3.8 to 4.4 (Fig. 9). The increase, however, is not gradual. Instead a sudden rise is observed from a lower to a higher plateau value at the severity of 4.2±4.25 (Fig. 9). This increase, which coincides with the increase in lignin accountability and yield (Fig. 6), must be related to changes in lignin's chemical structure which suddenly occur when severity rises above 4.2. An examination of the chemical structure of isolated, alkali-soluble/acidinsoluble lignins by 1H-NMR spectroscopy (Fig. 10) reveals that the changes in overall composition that may explain the sudden rise in UV-absorptivity coecient relate to the ratio of phenolic to aliphatic OH groups.35,36 When the signal representing phenolic acetoxy-protons is ratioed to the total acetoxyprotons (Fig. 11), a slight increase is recorded for the severity factor range (log R0) between 3.8 and 4.2, and a sudden increase is observed between 4.2 and 4.25. Total acetoxy-1H fraction, by contrast, remains constant for the entire severity factor range. These results suggest that the sudden changes in lignin's chemical structure that have caused a rise in UV absorptivity coecient at log R0 of 4.2 coincide with an increase in lignin accountability and yield (Fig. 6) and also with a sudden rise in phenolic OH content. Such an increase in phenolic OH content is indicative of depolymerization since lignin's premier intermonomer linkage (esp. in hardwood or syringyl-type lignins) is an alkyl±aryl ether linkage.37 When the molecular weights of alkali-soluble, acid-insoluble lignins are analyzed by GPC (as THF-soluble peracetates in
Fig. 13. Molecular weight distributions of sugar cane bagasse lignins (as acetates) for steam explosion severities ranging from log R0 3.9±4.25.
3.4. Composition of alkali solubles/acid insolubles
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231
lower yield; and a higher molecular weight than that isolated at higher severity (above 4.25). 3.5. Composition of ®ber fraction
Fig. 14. Weight average molecular weights of lignins (as acetates) from dierent biomass sources in relation to steam explosion severity (log R0).
accordance with earlier work),29 molecular weight patterns emerge that follow a consistent trend (Fig. 12). A more or less normal (but broad) distribution at low severity becomes increasingly bimodal (log R0 3.9±4.2) before resuming again a more normal distribution (log R0 4.25±4.4). This pattern is repeated in the bagasse lignin fractions (Fig. 13). A review of the numerical molecular weights of isolated lignins reveals weight average molecular weights (Mw) that decline precipitously at log R0 4.2 (Fig. 14). This sudden decrease is further evidence for a suddenly occurring depolymerization reaction involving alkyl±aryl ether bonds at a severity of 4.2. Lignin at lower severity has a lower UV absorptivity coecient; a lower phenolic hydroxyl content;
The combined glucose and galactose (because of less than baseline separation by HPLC) content of the ®ber solids isolated at a severity factor (log R0) of between 3.9 and 4.4 (Fig. 15) shows more or less constant values of between 40 and 50% for the steam exploded and water-washed ®bers, and a trend of increasing glucose contents, from 60 to 85% following alkali extraction in the severity factor range of between 3.9 and 4.3. Under extremely severe conditions, log R0 4.4, glucose content seems to decline. The xylose content of the ®ber solids declines to below 2% of both water and alkali extracted ®bers as severity rises to 4.3 (Fig. 16). The lignin content of the alkali-extracted ®bers (Fig. 17) declines from a Kappa number of 86 to below 50 as severity increases from 3.8 to 4.4. (A Kappa number of less than 15 is considered ``bleachable grade'' pulp.) The steepest decline in lignin content is experienced around the severity factor (log R0) range of 4.2. The molecular weight distributions of insoluble ®ber solids were determined by gel permeation chromatography in THF using fully carbanilated unbleached ®bers.27 The results indicate DPn values of between 200 and 1100, and DPw values between 1000 and 3000, for
Fig. 15. Glucose (and galactose) content of steam exploded ®bers from yellow poplar in relation to treatment severity and extraction condition.
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Fig. 16. Xylose content of steam exploded ®bers from yellow poplar in relation to treatment severity and extraction condition.
severities ranging between 4.4 and 3.8 (Fig. 18). Virtually linear relationships (R2-values of 0.96 and 0.99) were established for the severity factor range between 3.9 and 4.4 (Fig. 18 insert). These relationships obey the following functions. hDPn i 3:908103ÿ 8:4102 log R0 ; R2 0:99 hDPw i 1:151104ÿ 2:46103 log R0 ; R2 0:96
Since microcrystalline cellulose has values of around 150 and 250 for DPn and DPw, respectively, unbleached cellulose from steam explosion lies well above that common for microcrystalline cellulose.32 The gradual decline in molecular weight with increasing severity is typical of a kinetically controlled (hydrolytic) reaction; and this is consistent with ®ndings elsewhere.16±18,22,34
Fig. 17. Relationship between Kappa number and steam explosion severity (log R0) for extracted yellow poplar ®bers.
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Fig. 18. Molecular weights of extracted yellow poplar ®bers in relation to steam explosion severity (log R0).
4. DISCUSSION
Steam explosion of biomass is known to be a hydrolytic pretreatment; one that increases enzyme and solvent accessibility of cellulose; one that renders biomass separable (fractionatable) into constitutive components; and one that raises the crystallinity of the cellulose component. The results of a pilot-scale study involving the steam explosion of yellow poplar wood chips, bagasse fractions, and peanut hulls in a continuous Stake II reactor were found, in general, to be consistent with these observations reported previously.14,16,18 With regard to cellulose, a steady decrease is recorded that allows predicting molecular weight on the basis of treatment severity. The resulting cellulose is virtually free of other carbohydrate components, but it retains 7± 10% lignin even under relatively severe conditions. Non-cellulosic carbohydrate components (branched hetero-polysaccharides) are eectively depolymerized resulting in extensive water and alkali-solubility. There was no indication for the potential of isolating xylose-rich polymeric components having more than 10 or 15 monomeric repeat units. This has been con®rmed in a recent xylan isolation study.24,32
Lignin is distributed between a water-soluble fraction (ca 10% of total lignin), alkalisoluble fractions, and insoluble substances. The alkali-soluble lignin fraction reveals a sudden change in chemical structure at a severity of log R0 4.2±4.25. UV280-absorptivity coecient rises from 18 to 22, phenolic OH-groups are liberated, yield increases, and weight average molecular weights decline precipitously at log R0 4.2. This behavior is consistent with the previously observed homolytic depolymerization of lignin under steam explosion conditions.38±40
5. CONCLUSIONS
The steam assisted fractionation of biomass by continuous steam explosion produces at best two useful polymer fractions, a 85±90% pure, partially hydrolyzed cellulose fraction in approx. 40±45% yield; and a 90±95% pure (alkali-soluble, acid-insoluble) lignin fraction in 15±20% yield. Molecular weight, yield, and functionality of the latter depend primarily on whether steam explosion severity is below or above the severity factor (log R0) threshold of 4.2.
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The hemicellulose fraction is divided between a water-soluble and an alkali-soluble fraction, and it is severely degraded. Dierent biomass sources respond very differently to treatment with high pressure steam. These dierences are not, or not only, related to diusivity, but they seem more in¯uenced by chemical, morphological, and anatomical features. AcknowledgementsÐFinancial support for this study was provided by the Center for Innovative Technology, Herndon, VA; by Bioregional Energy Associates, Ltd, of Floyd, VA; by the Hawaiian Sugar Planters Association (HSPA) of Aeia, HI; and by Birdsong Peanuts of Suolk, VA. Skilful technical assistance was provided by Mr Woodrow L. McKenzie, Ms Jody Jervis, Mr Marshall Beard, Mr J. S. Brown, Dr Rajesh K Jain, and Dr William E. Karr, and this is acknowledged with gratitude.
12.
13. 14.
15. 16.
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