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Document Type

Dissertation - University Access Only

Award Date

2010

Degree Name

Doctor of Philosophy (PhD)

Department

Agricultural and Biosystems Engineering

First Advisor

Kasiviswanathan Muthukumarappan

Abstract

Petroleum provides more than 40% of the total energy in the US. With limited reserves and production, imports continue to grow and have reached over 70% of total petroleum consumption at a cost of $400 billion/year. According to the National Research Council in 2000, the goal of the biobased industry is to provide at least 10%of liquid fuels by the year 2020 and to provide50%of liquid fuels by the year 2050. In the last three decades researchers have been focusing on alternate fuel resources to meet the ever increasing energy demand and to avoid dependence on crude oil. Lignocellulosic materials are the most abundant renewable resources on earth. Biomass generally contains 60-70% of cellulose and hemicellulose,which are not readily available for enzymatic hydrolysis, and hence, the pretreatment step, one of the most expensive steps in the conversion of biomass to biofuel, becomes inevitable. The purposes of pretreatment are to open up the biomass structure, to reduce the cellulose crystallinity, and to increase the surface area and porosity. Pretreatment methods using physical, chemical, and biological principles are under various stages of investigation. To date no perfect biomass pretreatment method is available to produce biofuels from biomass on a large scale.
Extrusion, a well-known process/technique in the snack food, feed, and plastic industries, causes physio-chemical changes when the feed material pass through the extruder. Advantages of extrusion are highs hear, rapid heat transfer, mixing, moderate temperature, short residence time, and adaptability to process modification - all in a continuous operation. Extrusion pretreatment does not require washing and conditioning steps, as are required in dilute acid and alkali pretreatment methods; in addition, no potential fermentation inhibitors were reported from earlier studies. A viable continuous pretreatment method might be found through extrusion. The hypothesis of this study is that high shear coupled with temperature might disturb the biomass cell-wall structure, thereby increasing the access of hydrolic enzymes to cellulose. Com stover, switchgrass, big bluestem, and prairie cord grass were selected for this study based on national and regional importance. The objectives of the current study are: to understand the influence of extruder parameters such as compression ratio, barrel temperature, screw speed and biomass parameters such as moisture content and particle size, to optimize the pretreatment parameters for maximum sugar recovery, and to compare the formation of byproducts and torque requirements of the selected biomasses.
The second most expensive input in biomass conversion is enzyme loading. The effectiveness of pretreatment could be masked by favorable enzymatic hydrolysis conditions. The initial trials were conducted to select a suitable enzyme combination (cellulase & ß-glucosidase, multienzyme & ß-glucosidase) and its ratio. Corn stover was pretreated in a single screw extmder with five screw speeds (25, 50,75, 100, and 125rpm) and five barrel temperatures (25, 50, 75, 100, and 125°C). The other biomasses were also pretreated using a single screw extrader at various screw speeds (100,150, and200rpm) and barrel temperatures (50-200°C). A cellulase and p-glucosidase combination at a ratio of 1:4 resulted in a higher sugar recovery from the selected biomasses than from the multienzyme and P-glucosidase combination. A maximum glucose and combined sugar recovery of 75 and 61, 38.7 and 28.2,57.5 and 57.6, and 41.1 and 44.6%, respectively, from corn stover, switchgrass, big bluestem, and prairie cord grass were recorded at different barrel temperatures and screw speeds. Statistical analysis revealed that the barrel temperature and the screw speed had a significant influence on sugar recovery.
Screw compression ratio, screw speed, and barrel temperature are the important factors which influence sugar recovery from the biomass. In addition, the biomass moisture content and particle size are also important. The compression ratio has a direct impact with shear development within the extruder barrel. The extruder barrel temperature facilitates the melting or softening/plasticizing of the feed. Screw speed is responsible for the rate of shear development and the mean residence time of the feed. Moisture content plays a role in thermal softening by utilizing the barrel temperature and rate of shear development; particle size affects the rate of heat transfer, resistance offered for conveyance, and melting/thermal softening. In order to select a suitable screw compression ratio, the selected biomasses were pretreated at three different barrel temperatures (50, 100, and 150°C) and three different screw speeds (50, 100, and 150 rpm) using two different screw compression ratios (2:1, 3:1) while varying the moisture content (15, 25, 35, and 45%). From statistical analysis, it was found that a screw compression ratio of 3:1 resulted in a higher sugar recovery than for a ratio of 2:1 for the selected biomasses. The highest glucose and combined sugar recoveries of 89.9 and 91.3,45.2 and 44.5, 55.2 and 65.4, and 61.4 and 65.8%, respectively, were obtained from corn stover, switchgrass, big bluestem, and prairie cord grass pretreated at different pretreatment conditions. Statistical analysis revealed that all the independent variables considered in this study had a significant effect on torque requirements for the selected biomasses. Among the independent variables considered, moisture content, screw speed, and temperature had a negative effect on torque requirement for all the biomasses. Switchgrass required the highest torque followed by corn stover, prairie cordgrass, and big bluestem during the extrusion pretreatment.
Optimization of pretreatment conditions is one of the most important stages in the development of an efficient and economical pretreatment method; accordingly, the pretreatment conditions such as barrel temperature, screw speed, moisture content, and particle size of the selected biomasses were optimized using a central composite rotatable design adopting response surface methodology. The current study was undertaken to investigate the influence of biomass particle size on sugar recovery and to optimize the parameters such as extruder barrel temperature (45-225°C), screw speed (20-200 rpm), moisture content (10%-50%), and particle size (2-10 mm) for maximum sugar recovery from different biomasses. An experiment consisting of 36 treatment combinations based on a central composite rotatable design was developed using Design Expert. Statistical analyses confirmed that all the independent variables considered had a significant effect on sugar recovery. The following optimum pretreatment conditions for com stover- 180°C barrel temperature, 150 rpm screw speed, 20%wb moisture content, and 8 mm particle size resulted in a maximum glucose, xylose, and combined sugar recovery of 85.7, 87.5, and 86.3%, respectively. The optimum conditions for switchgrass pretreatment were: 176°C barrel temperature, 155 rpm screw speed, 20% wb moisture content, and 8mmparticle size resulted in maximum glucose, xylose, and combined sugar recovery of 41.4, 62.2 and 47.4%, respectively. The following optimum pretreatment conditions for big bluestem- 180°C barrel temperature, 150 rpm screw speed, 20% wb moisture content, and 8 mm particle size resulted in maximum glucose, xylose, and combined sugar recovery of 71.3, 78.5, and 58.9%, respectively. The optimum conditions for prairie cord grass pretreatment was as follows; 90°C barrel temperature, 65rpmscrew speed, 20%wbmoisture content, and8mmparticle size resulted in maximum glucose, xylose, and combined sugar recovery of 48.3,77.8 and 56.9%, respectively. The differences in sugar recovery among the selected biomasses might be due to the inherent characteristics including chemical composition. No furfural and HMF were found in any of the pretreatment conditions. This study revealed that using a feedstock size of about 8mm for bioethanol production can reduce the energy cost of biomass size reduction to a greater extent, which is the need of the hour. Switchgrass, com stover, and big bluestem required the torque values of 165.2,70.5, and 99.4 N-m, respectively, for their optimized sugar recovery pretreatment conditions such as barrel temperature (176-180°C), screw speed (150-155 rpm), moisture content (20% wb), and particle size (8mm). The prairie cord grass torque requirement was 96.8 N-mat optimized pretreatment conditions such as barrel temperature of 90°C, screw speed of 65 rpm, moisture content of 20% wb and particle size of 8mm. Switchgrass required the highest torque among the biomasses studied a result which was in agreement with a previous study. A quadratic polynomial model was proposed to predict glucose, xylose, and combined sugar recovery from biomasses and the torque requirement of the selected biomasses, which had high F and values along with low p value.
A maximum of 86,48, 59, and 57% sugar recovery, respectively, from corn stover, switchgrass, big bluestem, and prairie cord grass was achieved from the above extension pretreatment, thus showing that there is still room to improve the sugar recovery to near quantitative. In general, alkali pretreatments result in less degradation of the sugar than acid pretreatments; moreover the addition of acid would lead to corrosion in the extruder. Therefore, the current study proposed to use alkali soaking and extension in sequence as an effective pretreatment to achieve the goal. In order to evaluate the combined effect of alkali soaking and extension, biomass (2,4, 6, 8, and 10 mm) was soaked indifferent alkali concentrations (0.5, 1.0, 1.5,2.0, and 2.5 %w/v NaOH) for30min at room temperature and then extended using a lab scale single screw extruder at various barrel temperatures (45-225°C) and screw speeds (20-200 rpm). The experimental design was developed in Design Expert using a central composite rotatable design, resulting in 36 treatments. Statistical analysis confirmed that all the independent variables considered had a significant effect on sugar recovery. The optimum pretreatment conditions for com stover were the following: 133°C barrel temperature, 85 rpm screw speed, 1.65% alkali concentration, and 8mm particle size for maximum glucose, xylose, and combined sugar recovery of 91.8, 82.3, and 90.0%. The optimum pretreatment condition for switchgrass was 180°C, 118rpm, 2% alkali concentration, and 6 mm particle size resulted a maximum glucose, xylose and combined sugar recovery of 90.5, 81.5, and 88%. For big bluestem, the optimum pretreatment condition of 90°C, 155 rpm, 2.0% alkali concentration, and 4 mm particle size resulted in a maximum glucose, xylose, and combined sugar recovery of 90.1, 91.5, and 89.9%.The optimum pretreatment condition for prairie cord grass was l14°C, 122rpm, 1.70%alkali concentration, and 8 mm particle size recorded a maximum glucose, xylose and combined sugar recovery of 86.8, 84.5, and 82%. In addition, acetic acid was found to be less than the inhibition level in most of the pretreatment conditions. No furfural and HMF were found in any of the pretreatment conditions. The maximum glucose, xylose, and combined sugar recoveries were 5.0,2.5, and 4.3; 4.7, 5.0, and 4.3; 4.4, 2.7, and4.0; 4.3, 2.5, and3.4 times higher than the control sample of com stover, switchgrass, big bluestem, and prairie cord grass, respectively. The proposed quadratic model to predict the sugar recovery had a high F and values with a low p value, and adequately represented the relationship among the independent variables. In general, alkali soaking and extension improved the sugar recovery to about90%for switchgrass, big bluestem, and prairie cord grass. These results show that extension is a viable continuous biomass pretreatment method which can be explored at industrial scale.

Library of Congress Subject Headings

biomass energy
biomass energy industries
bioreactors
extrusion process

Description

Includes bibliographical references (305-333)

Format

application/pdf

Number of Pages

365

Publisher

South Dakota State University

Rights

Copyright © 2010 Karunanithy Chinnadurai. All rights reserved

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