Pretreatment of Switchgrass
As one of the most widely studied cellulosic ethanol raw materials, there are numerous studies on pretreatment of switchgrass. These include acid pretreatments [124-127], hydrothermal [128, 28], ammonia [129, 130], lime [131], and ionic liquids [132]. Then there are a number of excellent recent studies, which compare different pretreatment methods [133-136, 132, 137].
Kim and coworkers recently reported their results of a detailed study on a comparison of five pretreatment techniques on three varieties of common switchgrass [133]. In this study, compositions and enzymatic digestion efficiencies for three cultivars of switch — grass Alamo, Dacotah and Shawnee, grown and harvested at different locations and seasons were studied. Saccharification yields of switchgrass processed by different pretreatment technologies (AFEX, dilute sulfuric acid, liquid hot water, lime, and soaking in aqueous ammonia) were compared in regards to switchgrass genotypes and harvest seasons.
According to this investigation, untreated switchgrass varieties did not exhibit any significant differences in sugar yields. Without pretreatment, sugar yields were only 14-16% for glucose and 4% for xylose. All pretreatments improved both rates and yields of the hydrolysis. Dacotah switchgrass resulted in lower glucose yields than Alamo and Shawnee at a statistically significant level (p < 0.05). The difference in glucose yields between Dacotah and the other two switchgrass varieties seemed greatest at the initial stage of the hydrolysis, and started to decrease as the hydrolysis progressed. Glucose yield from the pretreated Dacotah at 1 h of hydrolysis was only half of that from the other two varieties pretreated at the same conditions, especially for AFEX, DA, and LHW pretreatments. Despite the fact that Dacotah was pretreated at a more severe condition than Alamo for AFEX and SAA pretreatment, Dacotah gave a lower sugar yield than Alamo. Glucose yield from pretreated Dacotah after 168 h was 5-20% lower than the other two varieties. Alamo consistently resulted in the highest glucose yield among the samples for all pretreatment technologies. It should be noted that Alamo is a lowland variety while the other two are upland culti — vars. Unlike the glucose yields, there was no consistent trend in xylose yields between the pretreated switchgrass cultivars. Xylose yield was the lowest for Dacotah for AFEX, DA, LHW and SAA treatments, while Shawnee resulted in the lowest xylose yield for lime pretreatment [133].
In general, Kim and coworkers’ results indicate that spring-harvested Dacotah is more recalcitrant to pretreatment and enzymatic conversion than fall harvests of the other two varieties. Although the higher cellulose content in Dacotah harvested in the spring slightly increases its potential biofuel yield, that potential is more difficult to realize due to its higher recalcitrance. The differences in enzymatic digestibility between the fall-harvested lowland Alamo and upland Shawnee were much less than the difference found between the Shawnee and Dacotah, which are both upland ecotypes with different harvest seasons. The enzymatic hydrolysis results suggest that not only compositions, but also reactivity of switchgrass to pretreatment and enzymatic hydrolysis is strongly dependent on harvest season, with later harvests being more difficult to hydrolyze than early harvests. In a similar study on AFEX pretreatment, Bals et al. also reported lower sugar yields from later harvests than from early harvests of switchgrass [138]. In their study they showed that October harvest of CIR switchgrass required more severe pretreatment conditions than July harvest to achieve the similar extent of hydrolysis. These experiments show the importance of selecting the correct harvesting window in all varieties in the production of cellulosic ethanol through the aqueous-phase saccharification path [133, 138].