Elsevier

Biomass and Bioenergy

Volume 78, July 2015, Pages 164-174
Biomass and Bioenergy

Physical characteristics of AFEX-pretreated and densified switchgrass, prairie cord grass, and corn stover

https://doi.org/10.1016/j.biombioe.2015.04.018Get rights and content

Highlights

  • Three different biomass of three different sizes (2, 4 and 8 mm) were pretreated via AFEX-pretreatment method.

  • AFEX-pretreated biomass were densified by using novel compaction method.

  • The properties studied before and after storage revealed the possibility of long term storage of AFEX-densified biomass.

  • Bulk density of the biomass increased by 1.2–6 fold.

  • Highly durable biomass (pellet durability of 78–95%) were formed.

Abstract

The goal of the study was to evaluate and compare the physical properties of control, pretreated and densified corn stover, switchgrass, and prairie cord grass samples. Ammonia Fiber Expansion (AFEX) pretreated switchgrass, corn stover, and prairie cord grass samples were densified by using the comPAKco device developed by Federal Machine Company of Fargo, ND. The densified biomass were referred as “PAKs” in this study. All feedstocks were ground into three different grind size of 2, 4 and 8 mm prior to AFEX pretreatment and the impact of grinding on pellet properties was studied. The results showed that the physical properties of AFEX-PAKed material were not influenced by the initial grind size of the feedstocks. The bulk density of the AFEX-PAKed biomass increased by 1.2–6 fold as compared to untreated and AFEX-pretreated materials. The durability of the AFEX-PAKed materials were between 78.25 and 95.2%, indicating that the AFEX-PAKed biomass can be transported easily. To understand the effect of storage on the physical properties of these materials, samples were stored in the ambient condition (20 ± 2 °C and 70 ± 5% relative humidity) for six months. After storage, thermal properties of the biomass did not change but glass transition temperature decreased. The water absorption index and water solubility index of AFEX-treated and AFEX-PAKed biomass showed mixed trends after storage. Moisture content decreased and durability increased upon storage.

Introduction

Lignocellulosic biomass, for example corn stover, switchgrass, and prairie cord grass are widely available and can be converted through wide range of technologies to different forms of energy, chemicals, and materials [1], [2]. Efficient conversion of biomass into bioenergy relies heavily on the effective, consistent, and economical transport of biomass from field to the biorefinery. The high moisture content, irregular shape and size, and low bulk density of biomass (40–200 kg/m3) [3], [4] are significant challenges to the developing bio-economy. These properties can lead to high costs of biomass handling, transport, and storage [5]. To address this problem, biomass materials could be densified in a regional biomass processing facility located near the field and then transported to a centralized biorefinery for final processing to energy products [6], [7], [8], [9], [10], [11]. The concept of decentralized Regional Biomass Processing Depots (RBPDs) to feed a larger centralized biorefinery was first introduced by Carolan et al. [6] in 2007. Since then, this model has been extensively studied by various group of researchers. Studies have indicated that implementation of RBPDs in feeding large centralized biorefineries would help advance local economies by generating jobs; reducing the cost of biomass storage, handling, and transportation; and reducing greenhouse gas emissions [7], [8], [9], [11], [12].

Biomass densification through various types of pelleting process has been found to be effective in increasing the initial bulk density of biomass from 40 to 200 kg/m3 to final bulk density of 600–800 kg/m3 [13], [14]. The biochemical conversion process of lignocellulosic biomass into ethanol is affected by several factors, primarily by the recalcitrant nature of the lignocellulosic biomass and by the accessible surface area of exposed cellulose [15]. Therefore, pretreatment is needed for cellular disruption and to further enhance the accessibility of carbohydrates to cellulolytic enzymes. Numerous pretreatment techniques have been studied. Pretreatment technologies can be broadly classified into four major categories: a) physical, b) chemical, c) solvent fractionation, d) biological [16]. Ammonia Fiber Expansion (AFEX), a physio-chemical pretreatment method, increases the surface accessibility, enhances cellulose de-crystallization and hemicellulose de-polymerization, and reduces lignin recalcitrance leading to glucose yields of up to 98% of theoretical [17]. Despite this, the bulk density of the AFEX-pretreated feedstocks remains fairly low and thus such pretreated feedstocks would need to be densified for effective handling, transportation, and storage. Various densification processes and their impact on the pellet characteristics have been studied [5], [18], [19], [20]. But the most of the established densification processes are energy-intensive and thus are costly [5], [21]. Therefore, to lower the net cost of feedstock delivery, cost-effective biomass densification processes are critically needed. In this study, a novel densification device referred to as the ComPAKco densification system, developed by Federal Machine (Fargo. ND) was used to compact the AFEX-pretreated switchgrass, corn stover, and prairie cord grass of different grind sizes (2 mm, 4 mm, and 8 mm). Unlike the commercial ring die pelleting units, this system uses a gear and mesh system to compress the biomass [1]. The system operates at relatively low temperatures (ambient to 60 °C) and pressures with low energy input (180 HP motor to produce 7–8 tons/h). Whereas, the traditional ring/die pelleting units uses 400 HP motor to process 4–5 tons/hr and temperature of the feedstock could reach as high as 180–200 °C due to high friction [5], [21]. Additionally, no binder is needed during the densification of AFEX-pretreated feedstocks. Hence, RBPD using integrated AFEX-pretreatment and ComPAKco densification could minimize the transportation and processing cost of the feedstocks for large scale bio-refineries. The briquettes/pellets produced by using ComPAKco system post-AFEX pretreatment are referred to as the “PAKs” throughout the article. Rijal et al. [1] studied the effect of densification on biochemical conversion of these PAKs by subjecting to fermentation process and their result showed that densification process did not counteract the effect of original pretreatment. Their study showed that AFEX pretreatment may enable the use of low-pressure, low-temperature densification in producing the biomass pellets. However, it is important to study the physical characteristics of the pellets as they are critical in determining their long term storability and their economic viability when transporting the densified biomass materials. These physical properties, which include moisture content, water activity, bulk density, true density, thermal properties, glass transition temperature, and pellet durability, were measured using 2 mm, 4 mm, and 8 mm grind size control samples as well as, AFEX-pretreated, and AFEX-densified PAKs of switchgrass, corn stover, and prairie cord grass. Switchgrass, corn stover and prairie cord grass are seasonal harvest biomass materials. After harvest, they must be stored, sometimes for long periods, until they are used. Thus, it is important to have biomass with consistent physical and chemical characteristics during long storage periods. Our literature review suggested that only few studies have been reported on the physical characteristics of the AFEX-pretreated and densified feedstocks. Hoover et al. [22] reported that highly dense and durable pellets can be produced from the AFEX-pretreated corn stover and Campbell et al. [23] indicated that low moisture content (<20%) AFEX-pretreated corn stover and wheat straw can produce high-quality pellets. In contrast to our present study, both of the studies have used a flat-die mill for pellet formation. From several studies, it has become apparent that the densification process, feedstocks types, and feedstock composition plays an important role in the quality of densified feedstocks [18]. Hence, the overall objectives of this study were to firstly, evaluate the effect of AFEX pretreatment and the novel densification process by examining and comparing the physical properties of the control (untreated biomass), AFEX-pretreated biomass, and AFEX-PAKed switchgrass, corn stover, and prairie cord grass. And secondly, to examine the physical properties of AFEX-pretreated and AFEX-PAKed switchgrass, corn stover, and prairie cord grass after storing in ambient condition (20 ± 2 °C and 70 ± 5% relative humidity) for six months.

Section snippets

Sample preparation

Switchgrass, corn stover, and prairie cord grass samples were collected from a local farm in Brookings, South Dakota and ground to size of 2, 4, and 8 mm using a hammer mill (Speedy Jr, Winona Attrition Mill Co, MN). Samples were then sent to Michigan State University for AFEX pretreatment using a high pressure stainless steel 5 gallon vessel (Parr Instruments; Moline, IL, USA) in a batch process. Pretreatment parameter for individual feedstocks was optimized (data not shown) according to the

Moisture content and water activity

The effect of AFEX-pretreatment and densification post-AFEX pretreatment on the moisture content and water activity (Aw) of the samples before and after storage are presented in Table 2. There were no significant differences in moisture content and water activity among the biomass grind sizes (2, 4 and 8 mm) for all three types of biomass. The initial moisture content of all three biomass were in the range of 6.8–7.1%, wb. The AFEX pretreatment and densification post-AFEX pretreatment did not

Conclusions

The ComPAKco novel densification process resulted in the production of highly densified pellets (bulk densities increased by 1.2–6.2 times depending on grind size and the type of biomass) of switchgrass, corn stover, and prairie cord grass. The study showed that the AFEX-pretreated and AFEX-densified switchgrass, corn stover, and prairie cord grass can be stored for long period of time without having any alterations in major physical properties. Desirable characteristics such as low moisture

Acknowledgment

We would like to acknowledge the valuable suggestions given by Dr. Farzaneh Teymouri, MBI International regarding procedures to characterize the compacted biomass. We also thank Mr. Charles Donald for doing the AFEX pretreatment for this project. This study was funding through US Department of Energy, Golden, CO, Grant number DE-FG36-08GO88073.

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