Document Type

Dissertation - Open Access

Award Date

2017

Degree Name

Doctor of Philosophy (PhD)

Department / School

Agricultural and Biosystems Engineering

First Advisor

Muthukumarappan Kasiviswanathan

Keywords

bioenergy, bioethanol, biomass densification, enzymatic hydrolysis, lignocellulosic biomass, pyrolysis

Abstract

Lignocellulosic biomass poses significant challenges during handling, transportation, and storage due to its low bulk density. Densification involves conversion of the low bulk density biomass into a highly compacted product which helps in improving the handling, transporting, and storage obstacles associated with biomass logistics. Besides the logistical challenges, the recalcitrant nature of the lignocellulosic biomass makes it even more challenging during the enzymatic hydrolysis. The carbohydrate components, cellulose and hemicellulose are not readily accessible by the enzymes during the hydrolysis process due to the presence of lignin. Pretreatment is the process to convert the native recalcitrant biomass in the form, which is effective to enzymatic hydrolysis. Numerous pretreatment technologies have been extensively studied on different lignocellulosic biomass using physical, chemical, and biological methods. Ammonia Fiber Expansion (AFEX™) is a promising pretreatment method, which involves treating the biomass with liquid ammonia at moderate temperature and pressure. The impacts of AFEX™ pretreatment include cellulose decrystallization, hemicellulose hydrolyzation, and lignin depolymerization. Due to these alterations, the cellulose and hemicellulose components can be easily accessed by the enzymes during the hydrolysis step, resulting in increased sugar yields. To address the logistical issues faced by the large-scale biorefineries, a concept called “Regional biomass processing depots” (RBPD) was developed. RBPDs involve procuring, pretreating, and densifying low density lignocellulosic feedstocks on a distributed scale to minimize the logistical challenges and carbon footprint. To make the RBPDs successful, it is imperative to understand the impacts of different preprocessing operations on the physical qualities of the densified products and the product yields. The increased lignin availability after AFEX™ pretreatment helps in better binding of the fibers during the densification process to produce well compacted products. Although, the densification produces compacted products, it is imperative to examine the effects of densification on the biomass conversion process. Hence, this study was designed to study the impacts of AFEX™ pretreatment and densification on the densified products quality and the product yields from the densified products. The lignocellulosic biomass corn stover, prairie cord grass, and switchgrass were selected for this research. The objectives of this research are: to understand the compression behavior of the AFEX™ pretreated biomass, to study the impacts of AFEX™ pretreatment and densification on the physical qualities of the densified products and sugars yields through enzymatic hydrolysis, and to understand the impacts of AFEX™ pretreatment and densification on the fast pyrolysis behavior. Five different researches were conducted and the brief summary of the individual studies is given below: The objective of the first study was to understand the effect of (AFEX™) pretreatment on the compression behavior of selected lignocellulosic biomass. Size reduced (2, 4, and 8 mm) untreated and AFEX™ pretreated samples were moisture adjusted (8, 12, 16, and 20% wb) and were compressed using a single pelleting unit. AFEX™ pretreated corn stover with moisture content of 20% at screen size of 2 mm produced pellets with 21% higher unit density compared to untreated corn stover pellets. AFEX™ pretreated prairie cord grass and switchgrass with 20% moisture content at a screen size of 2 mm produced pellets having 25% and 21% higher unit density. The decrease in hammer mill screen size and the increase in moisture content and applied pressure increased the pellet unit density. Data obtained from the compression experiments were fitted with different compaction models. The Kawakita and Ludde model exhibited high degree of accuracy in all the samples. The constant value ‘1/b’ in Kawakita and Ludde model represents the yield strength of the compacts, and the results showed that the AFEX™ pretreatment made the biomass easier to compress. Lower values of yield strength were obtained at high moisture content signifying that AFEX™ pretreated biomass at high moisture content leads to onset of deformation at relatively low pressure. The second study was intended to study the effects of AFEX™ pretreatment, feedstock moisture content, hammer mill screen size, compressive load on sugar recovery from corn stover, prairie cord grass, and switchgrass. Pellets were produced using a single pelleting unit from untreated and AFEX™ pretreated biomass. Then the pellets were subjected to enzymatic hydrolysis to determine the glucose and xylose yields. A significant increase in the glucose and the xylose recoveries was noted in all the feedstocks after AFEX™ pretreatment. Statistical analysis showed that only the screen size was significant (p0.05) in the case of untreated feedstocks and for the AFEX pretreated feedstocks all the selected factors were not significant (p>0.05). These results indicate that the larger screen size AFEX™ pretreated samples can be densified to increase the bulk density of the feedstocks without affecting the sugar yields. The blending effects of the AFEX™ pretreated corn stover and switchgrass was investigated in third study. AFEX™ pretreated corn stover and switchgrass were blended (25:75, 50:50 and 75:25 percent on dry weight) and compressed at different applied pressures. The impacts of blending ratio, screen size, and compressive pressure were studied on pellet unit density, pellet hardness, specific energy consumption for pellets and on the sugar yields. A single pelleting unit was employed the pellets produced from AFEX™ pretreated samples reached their maximum pellet unit densities at an applied pressure of 94.8 MPa. The pellets produced from the small screen size sample at a higher applied pressure required more force to break. Besides, blend with higher proportion of AFEX™ pretreated corn stover produced harder pellets (711 N). Specific energy consumption for the pellets production varied from 11.4 to 57.9 kW h t−1, and due to low bulk density of switchgrass, blends with a higher proportion of switchgrass consumed more energy. Pelleting and biomass blending had no significant effects on sugar yields of the AFEX™ pretreated corn stover and switchgrass samples. The effects of AFEX™ pretreatment, moisture content (5,10, and 15 % wb), particle size (2, 4, and 8 mm), and extrusion temperature (75, 100, and 125 °C) on pellet bulk density, pellet hardness, and sugar recovery from corn stover, prairie cord grass, and switchgrass were investigated in the fourth study. Pellets were produced using a laboratory-scale extruder. AFEX™ pretreatment increased the pellet bulk density for all the biomass. Maximum pellet hardness of 2342.8, 2424.3, and 1298.6 N was recorded for AFEX™ pretreated corn stover, prairie cord grass, and switchgrass, respectively. Glucose and xylose yields of AFEX™ pellets were not affected by the extruder barrel temperature and the screen size. The results obtained showed that low temperature and large particle size biomass can be employed for AFEX™ pretreated biomass without compromising sugar yields. The fifth study was intended to study the effects of AFEX™ pretreatment and densification on the fast pyrolysis product yields. Untreated and AFEX™ pretreated feedstocks were moisture adjusted and were densified using a single screw extruder and ComPAKco densification technique. Results of the thermogravimetric analysis showed the decrease in the decomposition temperature of the all the feedstocks after AFEX™ pretreatment indicating the increase in thermal stability. Loose and densified feedstocks were subjected to fast pyrolysis in a lab scale reactor and the bio-char and bio-oil yields were recorded. Bio-char obtained from the AFEX™ pretreated feedstocks exhibited increased bulk and particle density compared to the untreated feedstocks. The properties of the bio-oil were statistically similar for the untreated, AFEX™ pretreated, and AFEX™ pretreated densified feedstocks. Based on the bio-char and bio-oil yields, the AFEX™ pretreated feedstocks and the densified AFEX™ pretreated feedstocks exhibited similar behavior. Hence, it can be concluded that densifying the AFEX™ pretreated feedstocks could be a viable option in the biomass processing depots to reduce the transportation costs and the logistical impediments without affecting the product yields.

Library of Congress Subject Headings

Biomass energy.
Biomass conversion.
Hydrolysis.
Pyrolysis.
Lignocellulose--Biotechnology.
Corn stover.
Spartina.
Switchgrass.

Description

Includes bibliographical references (pages 161-182)

Format

application/ pdf

Number of Pages

206

Publisher

South Dakota State University

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Rights Statement

In Copyright