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Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 344-348
Steam Explosion of Mixed Hardwood Chips as a Biomass Pretreatment Tor P. Schultz,' Chris J. Blermann, and Gary D. McGlnnis Mississippi Forest Products Utilization laboratoty, Mississippi State University, Mississippi State, Mississippi 39762
Mixed southern hardwood chips were pretreated by steam explosion. Reaction temperatures ranged from 167 to 235 O C , with reaction times of 0.5, 1.0, and 2.0 min. The highest reaction temperatures degraded up to twethirds of the hemicelluloses, with the remainder soluble in hot water. The cellulose content decreased only slightly at the higher reaction temperatures. The steam-explosion pretreatment did not increase the rate of acid hydrolysis of the cellulose. The apparent lignin content increased as the steam-explosion temperature increased. The lignin was partially extractable by a hot alkaline solution. No differences were observed with the addition of NaOH catalyst. Scanning electron microscopy showed that wood chips exploded under mild condiiions were only partially defibrated, while severe conditions shredded the fibers into many individual fragments.
Introduction Many countries have experienced petroleum supply problems in the past few years because of declining reserves, increased demand, and political problems in oilexporting nations. These events have created difficulties for those oil-importing countries which depend upon petroleum to supply many of their energy and chemical needs. A long-term solution would be to use renewable resources to produce many of the hydrocarbons required by a technically advanced society. Cellulose, lignin, and hemicellulose are major constituents of woody biomass, present in quantities of approximately 50,25, and 25%, respectively. Cellulose and lignin are also the most abundant organic compounds on earth. Cellulose, which is a polysaccharide composed entirely of glucose units, can be hydrolyzed and then fermented into ethanol. Ethanol in turn can be directly used as fuel or further converted into various chemicals via an ethylene intermediate. A variety of chemical and physical pretreatment methods have been developed for increasing the susceptibility of cellulose and lignocellulose materials toward acid and enzymatic hydrolysis. The ideal pretreatment should not only loosen the plant cell structure, but should use inexpensive chemicals and require simple equipment. In addition, the pretreatment should solubilize the lignin and hemicelluloses and decrease the crystallinity of the cellulose. Another important characteristic of a pretreatment, but one which has been largely ignored, is that the pretreatment needs to be effective with a wide range of biomass material (Emert, 1979). Finally, if large-scale alcohol units are ever to become economically feasible, some markets for the lignin and hemicellulose must be found, since these materials make up approximately 50% of plant material (Cowling and Kirk, 1976; Clark, 1969). Therefore, at some point in the process, these components need to be isolated and separated. One of the most promising pretreatments appears to be steam explosion. This process was originally developed by Mason in 1925 and has been extensively used in the manufacture of hardboard (Spalt, 1977). The commercial process involves first filling a vertical cylinder with wood chips. Once filled with chips, the cylinder is sealed and pressurized with saturated steam at pressures up to 1000 psig. The chips are permeated by the saturated steam and develop high internal pressures. When the bottom of the cylinder is opened, the wood chips are defibrated by the sudden decompression. This steam explosion not only 0 196-432 118311222-0344$0 1.5010
causes a physical change in the wood but also causes considerable chemical changes. In 1978, the Iotech Corporation, Ltd., of Canada started using this process for production of feed for ruminants. In view of the early results which showed digestibility of steam-exploded wood, Iotech decided to explore the use of this process as a method for pretreating aspen (Foody, 1980). Since Iotech first reported their initial findings, a few other investigators have also examined steam explosion as a biomass pretreatment for aspen. Basically, these investigators have found that the following chemical changes occur in steam-exploded aspen (Marchessault et al., 1980, 1981, 1982; Marchessault, 1981; DeLong, 1981; Foody, 1980): (1) The lignin is broken down into low-molecular-weight products (M, 400-8000) which retain the basic lignin structure and are moderately reactive. Since the lignin has been extensively depolymerized by cleavage of the @-aryl-ether bonds, it is soluble in alkaline solutions or certain organic solvents. (2) The hemicelluloses are partially broken down and are predominantly soluble in water. In addition, some degradation products are formed which apparently condense with lignin, increasing the lignin content. (3) The major effect of steam explosion is the large increase in the accessibility of the cellulose to enzymatic hydrolysis. Jurasek (1978) determined that the steam-explosion pretreatment resulted in an approximately tenfold increase in the susceptibility of aspen wood to enzymatic hydrolysis. In reviewing this past research, it should be kept in mind that aspen makes up only 3% of the woody biomass in the United States. More importantly, aspen has been shown to be a uniquely easy wood to pretreat. Millett et al. (1970) found that aspen treated with anhydrous ammonia had a digestibility coefficient of 50%,while the value for red oak was only 10%. In reviewing various physical and chemical pretreatments, Millett et al. (1976) and Lipinsky (1979) both noted that aspen had unusual susceptibility to many pretreatments. Iotech (Foody, 1980) steam-exploded other hardwoods, in addition to aspen, and reported that oak gave relatively low yields. Another problem is that it is unlikely that a biomass conversion plant would use only one type of wood as the feedstock. For economic reasons, a biomass plant would use the lowest-cost wood available in each particular region. In the southeastern United States, the feedstock source may likely be mixed hardwood chips obtained from the so-called trash species. 0 1983 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 2, 1983 345 e No Additive Insoluble in Refluxinq Water o 2% NaOH Additive
Table I. Number of Runs Made at the Different Reaction Times with Either No Additive or Sodium Hydroxide Additive total no. of steam explosion runs reaction time, min no additive NaOH additive
0.5 1.0 2.0
4
13
6 7
4
0
The autohydrolysis of southern pine (April et al., 1982; Casebier et al., 1969) and of aspen (Chau and Wayman, 1979; Lora and Wayman, 1978; Wayman and Chau, 1979; and Wayman, 1980) depolymerized and degraded the lignin and hemicelluloses. These results are very similar to what has been reported for steam explosion. The autohydrolysis pretreatment has also been shown to increase the biodegradability of various lignocellulosics by methane-producing organisms (Colberg et al., 1981). In view of these findings, the authors felt that an examination of the feasibility of pretreating mixed southern hardwood chips by steam explosion would be worthwhile. To ensure that the results would be meaningful, chips from mixed low-grade hardwoods which were obtained from a commercial wood yard were used.
Experimental Procedure Unscreened, mixed hardwood chips, not sorted by size, were obtained from a wood yard. The length of the chips varied from about 0.1 to 15 cm. The chips were placed in 30-gal plastic bags with a small amount of formaldehyde added to inhibit decay and were stored at 40 O F . Approximately 82% by weight of the mixed chips waa actual wood chips. The remainder consisted of small twigs, leaves, bark, and other material. Examination of the wood chips showed a variety of woods present, with the majority made up of oak and gum species. Pine chips accounted for about 1% of the wood present. A l-ft3Masonite pilot gun reactor, located in Laurel, MS, was used for this study. The reactor was equipped with a pressure gauge and a digital thermometer. The rated maximum pressure of the reactor was 620 psig, which should give a temperature of 255 "C. However, due to heat losses, the maximum temperature reached was only 235 "C. Prior to steam exploding the wood, bags containing about 3 L of green chips (approximately 2.0 kg green, or 1.2 kg dry weight) were weighed. Sodium hydroxide powder (2% based on the green weight of the wood) was added as a catalyst to some of the samples. Before running the reactor, a weighed bag of chips was opened and the chips were added to the reactor. The reactor was sealed and the steam was gradually added so that the reactor reached the desired temperature in 1 min. Additional steam was then added, as necessary, to keep the temperature as stable as possible for the reaction time of 0.5,1.0, or 2.0 min. At the end of the reaction, the contents were blown out the bottom of the reactor and through a cyclone. For those runs in which the reaction presssure was less than 550 psig, the pressure was increased to 550 psig during the last few seconds of the run. This ensured that sufficient pressure was available to force the wood chips through the blow-out valve and the cyclone. Table I lists the number of runs made at the different reaction times. After steam explosion, the exploded fiber was air-dried and ground in a Wiley mill with a 2-mm screen. The grinding procedure was necessary to mix the fiber thoroughly. Analyses of samples which were not ground showed wide differences between duplicate samples due
Insoluble in Refluxing 2% NaOH
0 No Additive 2% NaOH Additive
e
e
REACTION TEMPERATURE, OC
Figure 1. Percent of steam-exploded fiber insoluble in refluxing water or in refluxing 2% NaOH vs. reaction temperature. The wood chips were steam exploded for 1 min.
"1 04 160
180
i
xx)
220
240
REACTION TEMPERATURE,S:
Figure 2. Cellulose and hemicellulose content of fiber which had been steam exploded for 1 min.
to the many different types of wood used. The ground fiber was stored in a cold room and analyzed by the methods reported below. Duplicate analyses were performed, with the average reported. Fiber samples selected for study by electron microscopy were air-dried and mounted on aluminum specimen stubs with silver conducting paint. The fiber samples were then coated with gold/palladium on a cooled-stage, sputter coater, The samples were examined on a Hitachi Model HHS-2R scanning electron microscope with a 20-kV acceleration voltage. The following analytical tests were run on the ground air-dried fiber samples. 1. Percent insoluble in refluxing water. Three grams of fiber was placed in 100 mL of boiling water for 1 h and then filtered and washed. 2. Percent Insoluble in Refluxing 2% NaOH. Three grams of fiber was boiled in 100 mL of 2% NaOH for 1 h, then filtered and washed. Figure 1 shows the results of the water and base extraction tests for the 1-min runs. 3. Total Klason Lignin. The insoluble lignin was determined by the Klason procedure, with the soluble lignin measured by UV at 205 nm. The results from the 1-min reactions are given in Figure 3. 4. Holocellulose Content. The holocellulose content was determined by the acid chlorite method (Browning, 1967).
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Ind. Eng. Chem. Rod. Res. Dar.. Vol. 22. No. 2. 1983 Nol\ddilii 0 2XkOHbdii
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