Steam hydrolysis of pine (Pinus radiata) sawdust

Biomass 8 (1985) 301-313

Steam Hydrolysis of Pine

(Pinus radiata) Sawdust

Jose M. Aguilera and Ricardo San Martin* Department of Chemical Engineering, Catholic University, PO Box 114-D, Santiago, Chile (Received: 2 June, 1984)

ABSTRACT Pine (Pinus radiata) sawdust was steam-exploded at 300 and 500 psig and at times varying from 30 s to 5 rain. Indexes used to monitor the process were: water solubility (WS), enzyme digestibility (ED), in vitro rumen digestibility (RD) and reducing sugars (RS). Kinetic data showed that, at 300 psig, degradation increases with time asymptotically to a maximum value. A t 500 psig a similar maximum is reached at times around I min, but the values o f parameters decrease thereafter. Increases in ED, WS and R S ranged from 3 to 7 times. Addition o f acicl as catalyst induced further improvements varying from about 8 to 13 times the original values. SEM work showed extensive disruption at the ultrastructural level. Key words: steam hydrolysis, pine sawdust, Pinus radiata, cellulose, hemi-

cellulose, lignin, digestibility.

INTRODUCTION A total of 34 million hectares, almost half of all continental land in Chile, is classified as forest land. Lumber is the main o u t p u t of timber utilization and 84% o f it corresponds to the exploitation of a single species: Pinus racliata. Approximately 300 000 tonnes of sawdust are produced yearly, of which 10% is presently used as fuelJ This forest biomass could be used as a renewable source of food and feed. However, the carbohydrates of whole-wood residues are resistant * Present address: Department of Chemical Engineering, University of California, Berkeley, USA. 301 0144-4565/85/$03.30- © Elsevier Applied Science Publishers Ltd, England, 1985. Printed in Great Britain

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to degradation by chemical, enzymatic and microbiological means due to a close physical and chemical association between cellulose, hemicellulose and lignin.2 Ruminants have a limited capacity for utilizing cellulosic by-products as energy sources, and efforts at increasing the nutritive value of these residues through chemical and physical treatments have been extensive.3 This is a very appropriate alternative for a less-developed country where the cereal supplies are insufficient to meet the nutritional needs of the human population and surpluses are unavailable for use in ruminant fattening rations or dairy compounds. Steam hydrolysis is a relatively simple and inexpensive pretreatment for wood degradation.4 Basically, it consists of loading the lignocellulosic material into a pressure chamber (batchwise or continually) for short periods of time (10 s-2 rain). Explosion is accomplished by the sudden release of pressure. During exposure of wood to high-temperature steam, acetic acid and other acids are formed that catalyse the decomposition of hemicelluloses and cellulose, cleave /3-ether linkages of lignin and chemically modify lignin and carbohydrates,s Extensive studies of steam hydrolysis of hardwoods and agricultural residues that have moderately high initial digestibilities have been performed by Iotech Corporation. 6 The objective of this work was to study the steam-explosion process for a softwood (Pinus radiata) and to evaluate the degradation accomplished by parameters representing in vivo digestibility. MATERIALS AND METHODS

Sawdust Pinus radiata sawdust was obtained from a local lumberyard. Particle

size was reduced by ball milling (B) and hammer milling (H) to sizes under 60 mesh (fraction 1) and over 60 mesh (fraction 2). Consequently, four samples (B1, B2, H1 and H2) were used in the steam-explosion tests. The moisture content was constant at 10-12%.

Equipment A stainless steel autoclave (Parr Instrument Co., Moline, Illinois) rated for 1000 psig maximum pressure was used as the reactor. Each sample

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was placed in an internal cylinder provided with a conical outlet to facilitate the discharge and avoid contact with the heated walls and with the condensate. Explosion was effected by opening a quickdischarge valve. A schematic diagram of the reactor is shown in Fig. 1.

Experiments Initially, 28 runs were performed to study the effect of pressure and reaction time. After loading the cylinder with 120-150 g of sawdust, the autoclave was closed and the steam accumulator was brought to 700 or 1100 psig, depending on whether 300 or 500 psig, respectively, were needed in the reactor. The bringing of the reactor to working pressure was accomplished in less than 10 s. Explosions were performed at 1, 2 and 5 min of reaction time and also at 30 s for 500 psig. Solids from the explosion were trapped in a canvas bag and oven-dried at 80°C. Later, three extra explosion tests were performed on sample H2 using 0.4% (w/w) sulphuric acid as catalyst, a pressure o f 500 psig and retention times o f 20, 30 and 60 s.

Analyses All analyses were run in duplicate. The original sawdust was analysed for proximate composition. Moisture content of the samples was deter-

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mined by weight difference before and after drying in an oven at 80°C for 18 h and pH was measured in a 10% (w/w) dispersion in distilled water. Cellulose, hemicellulose and lignin were determined following procedures for forage fiber analysis. 7 Water solubility of wood was determined as r e c o m m e n d e d by ASTM 8 and reducing sugars by the AOAC m e t h o d 20.035. 9 To determine in vitro enzyme digestibility, the following procedure was adopted: 2 g of sample (particle size < 4 2 5 / a m ) were mixed with 40 ml of acetic acid-sodium acetate buffer (0.1 M) to control pH at 4.8. Flasks were incubated for 1 6 h at 50°C with 1 0 m l of an enzyme solution, consisting of 200 mg o f Takamine developmental cellulase TV concentrate (Miles Laboratories, Elkhart, Indiana) dissolved in 100 ml o f buffer. In vitro digestibility is expressed as percentage total solubles after enzymatic treatment. In vitro digestibility with rumen was determined by a procedure developed by Tilley and Terry. 1°

Scanning electron microscopy (SEM) Selected dry samples were m o u n t e d on metal stubs using silver conducting paint. After coating with gold-palladium, the specimens were examined with a JEOL JSM-25 S11 scanning electron microscope at 5 kV. RESULTS AND DISCUSSION Chemical composition o f pine sawdust and selected physicochemical properties related to its digestibility are presented in Table 1. Cellulose and hemicelluloses amounted to almost 70% o f the total dry weight while the lignin content was 24.8%. These results compare well with reference values from the literature, although that for lignin is in the lower range of what is expected for a softwood (e.g. 27 + 2%). 2 Hardwoods not only have less lignin (e.g. 20 -+ 4%) than softwoods but also the lignin contains syringyl precursors in addition to the guaiacyl precursors that are almost the exclusive components o f softwood lignins. The parameters selected to assess the extent o f degradation of sawdust are water solubility (WS), enzyme digestibility (ED), reducing sugars (RS) and in vitro rumen digestibility (RD). WS and RS are used to monitor all chemical changes induced by hydrolysis, while ED and

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TABLE 1 Chemical Composition and Physicochemical Properties of Pine (Pinus radiata) Sawdust

Component

Moisture Nitrogen Ash Crude fiber Cellulose Hemicellulose Lignin Property Water solubility Reducing sugars Enzyme digestibility Rumen digestibility

Per cent {dry basis) 11.8 0.1 0,7 73,0 50.0 18.5 24.8 2-8 1.9 5.8 3.5

RD are more specific for cellulose activity and relate to physical modifications at the ultrastructural level. All of them are referred to hereafter as degradation indexes. WS, ED, RS and RD values of the raw material are also shown in Table 1. The low values reported, typical of softwoods, are a result of the type of lignin and the stronger association existing between lignin and cellulose compared to hardwoods. Reported values of enzyme digestion of untreated sawdusts using commercial cellulase vary between 0-4% for softwoods, 9% for aspen and 25% for alfalfa, used as the control, am Similarly, s o f t w o o d s are barely digested at all by rumen (RD < 5%) while hardwoods exhibit a rather broad range, from a low of 2% to a high of 33% for aspen. 12 Changes in the values of WS, ED and RS induced by the steamexplosion process are shown in Fig. 2(a) for 300 psig, and in Fig. 2(b) for 500 psig. There are important differences in the kinetics at these two pressure levels. At 300 psig (approximately 218°C) the degradation indexes increase continuously and asymptotically to a constant maximum value. When a pressure of 500 psig (approximately 243°C) is used.

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the degradation indexes increase very rapidly to a maximum value reached at about 1 min o f reaction time, and decay afterwards. The total m a x i m u m degradation achieved, as measured by the various indexes, is similar for both pressures. ED increases between 3 and 5 times, while WS changes by a factor of about 5. RS increases nearly 3 times at 300 psig and up to 7 times at 500 psig. There seems to be an effect of pressure on RS at small particle sizes, since the m a x i m u m values attained at 500 psig were almost double those at 300 psig. Other-

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wise, no major differences are detected on the effect of particle size or milling, as is also reported by lotech. 6 The production of acid, measured by the pH of the solid residue, as a function of pressure for sample H1 is shown in gig. 3. Acid production is faster and the final pH lower at 500 psig than at 300 psig. This coincides with the higher initial rates of degradation achieved at the higher pressure. The pH due to acid formation is maximum at about 2.0 min and then becomes constant.

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TABLE 2

Properties of Steam-exploded Pine Sawdust (H2) Using H2SO4 as Catalyst Time (s)

Water solubility

Enzymatic digestibility

Reducing sugars

20 30 60

27.4 26.8 29.8

29.9 29.7 35.9

17.4 17.3 22.9

Increases in catalyst (acid) concentration are known to improve the yield o f sugars from cellulose} 3 Addition of 0-4% sulfuric acid to aspen sawdust results in almost a 40% higher glucose yield after steam explosion. 14 Pinesaw dust steam-exploded at 500 psig using a similar a m o u n t of catalyst presents improvements in the degradation indexes of the same order or higher, compared to those without acid addition (Table 2). Optimum conditions appear to occur at around 60 s where

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the values o f WS, ED and RS are 29.8, 35.9 and 22.9%, respectively. These figures represent increases of 8.1, 7.8 and 12.7 times over the values of the raw materials. The positive response of pine sawdust to acid may be due to the low content of acetyl groups present in softw o o d hemicelluloses, is the acetyl group being the precursor of acetic acid which catalyses steam hydrolysis. Further improvements in the order of 50% could be expected if the m e t h o d for addition of acid (spraying) was changed to soaking, since penetration of the acid into the fibers is critical in order to achieve a uniform catalytic effectJ 4 In vitro rumen digestibilities of selected samples are presented in Table 3. Again, RD values o f the raw materials are low, varying between 3.0 and 4.7%. Particle size appears to have an effect on enzymatic-type tests performed on untreated samples, since fraction 1 showed higher ED and RD than fraction 2. Selected steam-exploded samples present RD values ranging from 16.5 to 20.3% and for the acid catalysed sample (500 psig, 30 s) a value o f 28-6%. These figures represent increases in RD TABLE 3 In vitro Rumen Digestibility of Selected Samples

Sample a

In vitro rumen dry matter digestibili~

(~) Sawdust

H1 H2 B1 B2 Steam-exploded B1/300/5 B2/30015 B1/500/1 H2/500/0.5 H1/500/1 Steam-(acid )-exploded H2/500/0.5

3.5 3.0 4.7 3.3 18-5 16.5 20.2 19.2 20.3 28.6

a Steam exploded samples individualized by sample type/pressure/time.

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Fig. 4. Scanning electron micrographs of pine sawdust, sample M2: (A) Untreated sample; (B) untreated sawdust after enzymatic attack. Arrow shows pitting on the surface; (C and D) sample M2 steam-exploded at 500 psig and 30 s. Arrows point at aggregates, possibly formed by molten lignin. Magnifications: 300x, except Fig. 4(D), 1000×.

Steam hydrolysis o f pine (Pinus radiata) sawdust

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J. M. Aguilera, R. San Martin

in the order of 4-6 times in the first case and almost 10 times for the acid-aided explosion, with respect to the original values. A very good correlation (r 2 = 0-989) is found between the results of the cellulase assay (ED) and the rumen test (RD). The predicted RD value for the acid catalysed sample (500 psig, 1 min) is 35.9%, equivalent to 12 times the RD of the raw material. Changes in the ultrastructure of sawdust effected by various treatments are shown in Fig. 4. Apparently, there are two fundamental modifications at the fiber level. First, the tightly held structure of sawdust (Fig. 4(A)), impervious to enzymatic attack (Fig. 4(B)), is completely obliterated by the steam-explosion process (Fig. 4(C)), facilitating solubilization of components and enzymatic attack. 16 Secondly, the limitation to further degradation is a layer of lignin formed during the process, that shields hidden inner fibers. Previous SEM work on steam-exploded aspen showed the presence of molten lignin under similar processing conditions. Formation of lignin droplets is apparent in Fig. 4(C). Folding and shrinkage of cell walls due to the absence of hemicelluloses is also evident (Fig. 4(D)). Future work will include treatments to solubilize the lignin that limits degradation, and a kinetic study of steam degradation of pine sawdust.

ACKNOWLEDGEMENTS Tha authors acknowledge the analytical work of Ms Wanda Villablanca and the pilot plant assistance of Mr Jorge Cea. Dr Jorge Garrido, of the Institute of Biological Sciences, performed the SEM work. This research was supported by grant No. 628-82 from the Fondo Nacional de Tecnologia.

REFERENCES 1. Morales, R. (1984). Recursos forestales y su utilizaci6n. In: Recursos renovables chilenos, eds J. M. Aguilera et al., Chile, Ediciones Universidad Cat61ica, pp. 73-8. 2. Goldstein, I. S. (1981). Organic chemicals from biomass, Boca Raton, CRC Press.

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3. Satter, L. D., Baker, A. J. & Millett, M. A. (1981). Increasing the nutritive value of wood and forest products through chemical and physical treatments. In: Upgrading residues and by-products for animals, ed. J. T. Hubers, Boca Raton, CRC Press, pp. 61-76. 4. Muzzy, J. D., Roberts, R. S., Fieber, C. A., Faass, G. S. & Mann, T. M. (1983). Pretreatment of hardwoods by continuous steam hydrolysis. In: Wood and agricultural residues, New York, Academic Press, pp. 351-68. 5. Marchessault, R. H., Malhotra, S. L., Jones, A. Y. & Perovic, A. (1983). The wood explosion process: characterization and uses of lignin/cellulose products. In: Wood and agricultural residues, New York, Academic Press, pp. 401-13. 6. Foody, P. (1980). Optimization of steam explosion pretreatment. Final Report by Iotech Corp., US Dept. of Energy, Contract No. DE-AC02-79 ETZ 3050. 7. Goering, H. K. & Van Soest, P. J. (1970). Forage fiber analysis. In: Agricultural handbook, No. 379, Washington, US Department of Agriculture. 8. American Society for Testing and Materials (1981). Method D-1110-56, Part 22, Pennsylvania, ASTM. 9. Association of Official Analytical Chemists (1980). AOACMethods of Analysis, 13th edn. 10. Tilley, J. M. A. & Terry, R. A. (1963). A two-stage technique for the in vitro digestion of forage crops. J. British Grassl. Soc., 18, 104. 11. Baker, A. J., Millett M. A. & Satter, L. D. (1975). Wood and wood-based residues in animal feeds. In: Cellulose technology research, ACS Symp. Set. 10, Washington, DC, American Chemical Society, Part 6, pp. 75-86. 12. Millett, M. A., Baker, A. J., Feist, W. C., Mellenberger, R. W. & Satter, L. D. (1970). Modifying wood to increase its in vitro digestibility. J. Anita. Sci., 31, 781-8. 13. Harris, J. F., Saeman, J. F. & Locke, E. G. (1963). The chemistry of wood, ed. B. L. Browning, New York, John Wiley & Sons, p. 555. 14. Foody, P. (1982). Steam explosion as a pretreatment for biomass conversion. Final Report by Iotech Corp. to Midwest Research Institute, Contract No. IB-1-9343-1. 15. Sj6strom, E. (1981). Wood chemistry: fundamentals and applications, New York, Academic Press. p. 65. 16. Bender, F., Heaney, D. P. & Bowden, A. (1970). Potential of steamed wood as feed for ruminants. Forest Prod. J., 20 (4), 36-41.