Elsevier

Industrial Crops and Products

Volume 61, November 2014, Pages 382-387
Industrial Crops and Products

Torrefaction of Conservation Reserve Program biomass: A techno-economic evaluation

https://doi.org/10.1016/j.indcrop.2014.07.030Get rights and content

Highlights

Abstract

The Conservation Reserve Program (CRP), which was initiated to prevent soil erosion, provides a large amount of cellulosic biomass that is potentially useful for bioenergy production. We investigated the effects of torrefaction conditions on the physicochemical properties of CRP biomass using an elemental analyzer, a thermogravimetric analyzer, and a calorimeter. Results suggest that the upgraded biomass is a hydrophobic, high-energy density, and low-moisture-content material. The study on biomass polymer composition showed how polymer components changed with processing conditions. The polysaccharides in biomass were degraded significantly at 300 °C, suggesting that processing conditions should be managed properly for sugar or energy recovery. Our economic analysis suggested that the processing cost for a torrefaction plant with an annual capacity of 100,000 tons of CRP biomass is $16.3 per ton of feedstock. Further analysis of the effects of torrefaction on the biomass supply chain suggested that processing could save pelletization and transportation costs.

Introduction

Cellulosic biomass from agricultural residues has become an important energy source because its use for biofuels production does not compete with food production; however, overuse of this biomass could cause a decrease in soil quality, and agricultural crop production could then be affected if the residue is not left for soil amendment (Lal, 2009). The Conservation Reserve Program (CRP) began in 1985 as an effort to prevent soil erosion and enhance groundwater recharge from highly erodible lands. About 30 million acres of CRP land prevent 0.3 million tons of nitrogen and 50,000 tons of phosphorous annually from flowing into river or lakes (USDA, 2012). About 50 million tons of dry biomass could be harvested annually from CRP land, indicating great potential for bioenergy production (Perlack et al., 2005). A recent study suggested that CRP biomass is a potential bioenergy feedstock if appropriate management practices are applied (Lee et al., 2013). Compared with conversion of CRP land for starch-based agricultural production such as corn and soybean, direct use of the CRP land for cellulosic biomass production would avoid carbon debt according to a recent analysis (Gelfand et al., 2011). Therefore, CRP biomass, the mixed grass from the CRP land, becomes a competitive feedstock because it does not compete with food production and could minimize soil erosion. Assuming that 20% of the total amount of CRP biomass is harvested for bioenergy production and all other biomass is left for land conservation, more than 2 million tons of cellulosic ethanol (as a representative biofuel) could be produced annually, which is equal to 5% of the 2022 cellulosic biofuels objective (16 billion gallons) made by the Energy Independence and Security Act of 2007 (EISA, P.L. 110-140) (Schnepf, 2011).

Although recent biomass-processing techniques have proven effective in biomass conversion, the production cost of developing cellulosic biofuel remains high. Biomass upgrading through torrefaction shows great potential to benefit both the supply chain and downstream processing units (Batidzirai et al., 2013, Chin et al., 2013, Ciolkosz and Wallace, 2011). The torrefaction of biomass is basically a thermal process conducted in the temperature range of 200–300 °C under anaerobic conditions atmospheric conditions (Van der Stelt et al., 2011). Biomass moisture content (MC) is reduced in the initial drying process and biomass is partially degraded. Studies have shown that torrefaction enhanced the properties of different biomass materials (Couhert et al., 2009, Ren et al., 2012). Torrefaction is being applied to bioenergy production in thermal–chemical and biochemical platforms, and the enhanced properties after torrefaction were reported to improve the efficiency of biomass gasification and conserve chemical energy (Prins et al., 2006). Energy consumption was reported to be lower for torrefied biomass than for untorrefied biomass in the production of cellulosic ethanol (Chiaramonti et al., 2011). Torrefaction also improves biomass properties by increasing hydrophobicity. Most agricultural wastes, including grass biomass, show significant hydrophilicity, which results in problems during biomass storage, transportation, and processing; for example, biomass easily absorbs moisture, which results in decreased energy density. More importantly, hydrophilic biomass needs much more water to reduce viscosity of the slurry, resulting in increased energy consumption in the subsequent separation process. In addition, moisture absorption during storage causes fungi formation that could decrease the quality of feedstock (Rentizelas et al., 2009), whereas torrefaction provided microbial-resistant biomass, which reduces storage cost (Medic et al., 2012). Thus, torrefaction offers great potential for the biomass processing chain.

In this paper, we report the first study of CRP biomass enhancement through torrefaction. Changes in CRP biomass were investigated through different techniques. To integrate the torrefaction unit into the biomass processing system, an economic evaluation is critical for commercial application. We conducted our technical analysis including the results of mass and energy balances. Following the analysis of torrefaction unit, we analyzed how torrefaction affected related biomass processing units such as transportation, grinding, and pelletization.

Section snippets

Materials

The CRP biomass was harvested in 2012 from Valley Falls, Kansas, and field-dried to reduce the MC to about 20%. The biomass was then stored in plastic bag at 4 °C. All chemicals used in this study were from Sigma–Aldrich, Inc. (St. Louis, MO).

Torrefaction

The torrefaction experiments were conducted using a Parr 4570 pressure reactor with a Parr 4848 temperature controller (Parr Instrument Co., Moline, IL). CRP biomass was cut to about 10 cm in length before loading. After biomass loading, the reactor was

Mass loss

The effects of torrefaction temperature and time on the dry mass loss of CRP biomass were investigated; results are shown in Fig. 2. Previous reports showed that biomass MC significantly affected the dry mass recovery after torrefaction and almost 50% (wet base) of biomass lost (Van der Stelt et al., 2011). In this study, the mass loss was up to 35% at 300 °C because the CRP biomass has a relatively low MC (about 20%) after a field dry. As shown in Fig. 2, the dry mass loss increased as

Conclusions

CRP biomass has potential to be sustainably used for bioenergy production. Biomass torrefaction upgrades biomass properties by increasing energy density, reducing MC, reducing particle size, increasing hydrophobicity, and increasing brittleness for easier grinding. The study on biomass composition, especially polymer composition, suggested that the polysaccharides in biomass were converted to other high-carbon-content materials. Preserving energy content (e.g., cellulose and lignin) and

Acknowledgment

This is contribution number 14-010-J from the Kansas Agricultural Experiment Station.

References (33)

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