The current forage handling equipment in the cellulosic ethanol industry is severely limited by the low bulk density of baled and ground biomass. Low bulk density contributes to flowability problems and lack of maximizing trailer capacities. Biomass pelleting process can improve the bulk density and flowability characteristics of forages. The objectives of this research were to evaluate: (I) the energy requirements of grinding sorghum stalks, corn stover, wheat straw, and big bluestem through two different screen size openings, (2) the energy requirements of pelleting forages from the two grind sizes, (3) the physical properties of pelleted biomass, and (4) the costs associated with biomass processing, transportation, and storage. The two mill screen size openings (3.2 and 6.5 mm) were found to have significantly different energy consumptions for grinding step from each other. All four forage types, except for big bluestem and corn stover, were also found to have significantly different energy consumptions for grinding. Production rate through the 6.5 mm screen was almost three times higher than that of the 3.2 mm screen (average of 181.4 vs. 68 kg/h). Hammermill screen size opening (i.e., grind size) was found to have significant effects on energy consumption for pelleting process. The four forage types were also found to have significantly different energy consumptions from each other, except for big bluestem versus wheat straw (P=0.1192). Particle length for the 3.2 mm grind ranged from 0.15 to 0.18 cm, while the 6.5-mm grind ranged from 0.20 to 0.31 cm. Pelleting increased bulk density from 99.96 to 160.02 kg/m(3) for raw biomass grinds to 499.30 to 701.13 kg/m(3) for pelleted biomass. Pellet durability ranged from 93% to 98%. A cost analysis indicated that it would take roughly $22 extra per metric ton for the transportation, pre-processing, and storage of pelleted cellulosic biomass than corn grain. This cost is still almost half that of the cost for baled biomass.

Agricultural residues and energy crops are considered potential feedstocks for bioethanol production because of their high availability and energy potential as well as relatively low cost. Previous studies have shown that pelleting biomass feedstocks could increase their bulk density, thus increasing ease of handling and decreasing cost of handling and transportation. The pelleting process has also been shown to have a positive impact on the sugar yield of biomass. However, the effects of the pelleting process on biomass structure have not yet been studied. Therefore, the objective of this study was to investigate the impact of dilute acid pretreatment and the pelleting process on biomass structure of cellulosic materials, including crystallinity index (CrI,%) measured by the x-ray diffraction (XRD) method, structure of constituents and chemical changes determined by Fourier transform infrared spectroscopy (FTIR) and solid-state cross-polarization/magic angle spinning (CP/MAS) 13C NMR spectroscopy, morphological structure determined by scanning electron microscopy (SEM), and thermal properties determined by thermogravimetric analysis (TGA). Wheat straw, big bluestem, corn stover, and photoperiod-sensitive sorghum were used for this study. Pelleting did not have a significant effect on the pattern of FTIR spectra and solid-state 13C NMR spectra of biomass. XRD analysis showed that biomass crystallinity increased after dilute acid pretreatment and the pelleting process. Based on SEM analysis of biomass, dilute acid pretreatment and pelleting enhanced the removal of the softened surface region of biomass. TGA analysis showed that the decomposition temperature of pelleted biomass was slightly higher than that of corresponding unpelleted biomass, indicating that the pelleted biomass was more thermally stable than the unpelleted biomass.

Pelleting of biomass can increase their bulk density and thus improve storability and reduce transportation costs. The objective of this research was to determine the effects of the pelleting conditions on chemical composition and fermentable sugar yield of the biomass. Corn stover, wheat straw, big bluestem, and sorghum stalks were used for this study. Dilute sulfuric acid was used for biomass pretreatment. Accellerase 1500™ was used for cellulose hydrolysis. Effects of mill screen size, die thickness, and L/D ratio of die on chemical compositions and sugar yield were determined. Glucan content of the biomass was positively affected by die thickness and negatively affected by mill screen size. Opposite trend was observed for xylan content. Wheat straw pellets had the highest sugar yield (92.5–94.1%) and big bluestem pellets had the lowest sugar yield (83.6–91.1%). Optimum pelleting condition is 6.5 mm screen size and 44.5 mm die thickness.

Photoperiod-sensitive sorghum, as a competitive biomass for ethanol production, was investigated to develop an integrated process for improving ethanol yield. Response surface methodology was employed to study the relationship between pretreatment variables (including temperature, sulfuric acid concentration, and reaction time) and cellulose recovery, as well as efficiency of enzymatic hydrolysis (EEH) in the solid part. Recovery yield decreased and EEH increased as the pretreatment temperature, acidic concentration, and reaction time increased. A model was successfully developed to predict total glucose yield with a maximum value of 82.2%. Conditions of co-fermentation were also optimized, and the optimal ethanol yield was obtained with constant-temperature simultaneous saccharification and fermentation at 38°C. Acetate buffer at a concentration of 50mM was found helpful for increasing efficiency of enzymatic hydrolysis, as well as ethanol yield. The maximum ethanol yield was 0.21g ethanol per dry mass at the conditions of 38°C, 0.05g yeast/L, and 50mM acetate buffer. A complete cellulose balance was provided for the whole process.

Photoperiod sensitive (PS) sorghum, with high soluble sugar content, high mass yield and high drought tolerance in dryland environments, has great potential for bioethanol production. The effect of diluted sulfuric acid pretreatment on enzymatic hydrolysis was investigated. Hydrolysis efficiency increased from 78.9 to 94.4% as the acid concentration increased from 0.5 to 1.5%. However, the highest total glucose yield (80.3%) occurred at the 1.0% acid condition because of the significant cellulose degradation at the 1.5% concentration. Synchrotron wide-angle X-ray diffraction was used to study changes of the degree of crystallinity. With comparison of cellulosic crystallinity and adjusted cellulosic crystallinity, the crystalline cellulose decreased after low acidic concentration (0.5%) applied, but did not change significantly, as the acid concentration increased. Scanning electron microscopy was also employed to understand how the morphological structure of PS sorghum changed after pretreatment. Under current processing conditions, the total ethanol yield is 74.5% (about 0.2 g ethanol from 1 g PS sorghum). A detail mass balance was also provided.

Conversion of cellulosic biomass such as agricultural residues to biofuels offers major economic, environmental, and strategic benefits. Sorghum is one of the important grain crops in US. It represents a renewable resource that is currently grown on six to ten million acres in the U.S. However, at present, there is not enough scientific information and knowledge about the use of sorghum stover for biofuel production. The objective of this research was to evaluate and characterize sorghum biomass as a feedstock for ethanol production. Five types of sorghum biomass, including brown midrib (bmr) sorghum, forage sorghum, grain sorghum, photosynthesis sorghum, and sweet sorghum, were characterized and used for ethanol production. Pretreatment with dilute acid was used to increase fermentable sugars yield. The effect of sulfuric acid concentration, treatment temperature, and residence time on fermentable sugars yield were studied. AccelleraseTM 1000 was used to hydrolyze cellulose into glucose at 50 oC and pH 4.8 for 96 h.

Densification of biomass feedstocks, such as pelleting, can increase bulk density, improve storability, reduce transportation costs, and make these materials easier to handle using existing handling and storage equipment for grains. The objectives of this research were to study (1) the physical properties of pellets made from corn stover, sorghum stalk, wheat straw, and big bluestem, (2) the effect of moisture content on bulk density, true density, and durability of the biomass pellets, and (3) the effect of hammer mill screen size and die thickness on bulk density, true density, and durability of the pellets. Biomass pelleting can significantly improve the bulk density from 47 to 60kg/m3 for biomass grinds to 360 to 500kg/m3 for biomass pellets. Of the four types of biomass pellets, wheat straw pellets had the highest bulk density value of 495.8kg/m3, and sorghum stalk pellets had the lowest bulk density value of 365.2kg/m3. An increase in moisture level resulted in a decrease in bulk density and true density of the pellets. The effect of moisture content on durability of the pellets made from corn stover, wheat straw, and big bluestem showed a similar trend; the maximum durability value was 96.8% at the equilibrium moisture content (EMC) range of 9% (d.b.) to 14% (d.b.) for corn stover and wheat straw, and 9% (d.b.) to 11% (d.b.) for big bluestem. A further increase in EMC value resulted in a decrease in pellet durability. For sorghum stalk pellets, the durability value increased initially with increased EMC and reached a maximum of 89.5% at EMC values between 14% (d.b.) and 16% (d.b.). Use of a larger hammer mill screen size (from 3.2mm to 6.5mm screen openings) resulted in increases of bulk density, true density, and durability of biomass pellets, but not in significant levels. Use of a thicker die size (from 31.8mm to 44.5mm in thickness) resulted in significant increase of bulk density, true density, and durability of biomass pellets.