Comparative study of SPORL and dilute-acid pretreatments of spruce for cellulosic ethanol production

Abstract

The performance of two pretreatment methods, sulfite pretreatment to overcome recalcitrance of lignocellulose (SPORL) and dilute acid (DA), was compared in pretreating softwood (spruce) for fuel ethanol production at 180 °C for 30 min with a sulfuric acid loading of 5% on oven-dry wood and a 5:1 liquor-to-wood ratio. SPORL was supplemented with 9% sodium sulfite (w/w of wood). The recoveries of total saccharides (hexoses and pentoses) were 87.9% (SPORL) and 56.7% (DA), while those of cellulose were 92.5% (SPORL) and 77.7% (DA). The total of known inhibitors (furfural, 5-hydroxymethylfurfural, and formic, acetic and levulinic acids) formed in SPORL were only 35% of those formed in DA pretreatment. SPORL pretreatment dissolved approximately 32% of the lignin as lignosulfonate, which is a potential high-value co-product. With an enzyme loading of 15 FPU (filter paper units) per gram of cellulose, the cellulose-to-glucose conversion yields were 91% at 24 h for the SPORL substrate and 55% at 48 h for the DA substrate, respectively.

Introduction

Second-generation bioethanol, produced from lignocellulosic materials, is a promising alternative to fossil fuel for vehicular transportation (Ragauskas et al., 2006, Carroll and Somerville, 2009). The benefits of cellulosic ethanol include, but are not limited to, reduced greenhouse gases emission, value-added utilization of agricultural and forest residues, enhancement of the rural economy, and improved national energy independence and security (Farrell et al., 2006, Scharlemann and Laurance, 2008). A typical process for cellulosic ethanol production consists of three steps: feedstock pretreatment to enhance cellulases accessibility to cellulose, enzymatic saccharification of the cellulose-to-glucose, followed by fermentation of the glucose to ethanol. One of the barriers to the commercialization of cellulosic ethanol is the lack of economical and effective technologies for the feedstock pretreatment. Pretreatment is a necessary operation required to achieve optimal bioconversion for all forms and types of lignocellulosic feedstocks to ethanol, but is particularly important for the more recalcitrant softwoods. The significance and importance of the pretreatment step cannot be overemphasized, as the effectiveness of the pretreatment affects the upstream selection of biomass, the yield of fermentable sugars, and the chemical and morphological characteristics of the pretreated substrate which in turn govern downstream hydrolysis/saccharification (Wyman et al., 2005).

An effective pretreatment method should be economical (in terms of both capital and operating costs) and effective for a variety of lignocellulosic biomass. Specifically, it should require minimal feedstock preparation and preprocessing prior to pretreatment, maximally recover all lignocellulosic components in usable forms with minimal formation of fermentation inhibitors, and produce a readily digestible cellulosic substrate that can be easily hydrolyzed with a low loading of enzymes. In the last several decades, research and development efforts have made significant progress in pretreatment technologies for lignocellulosic feedstocks (Lynd et al., 2002, Mosier et al., 2005, Wyman et al., 2005, Chandra et al., 2007, Hendriks and Zeeman, 2009). Many pretreatment technologies, such as lime, dilute acid, hot water, ammonia, steam explosion, and organosolv pretreatments (Nguyen et al., 2000, Pan et al., 2005a, Chandra et al., 2007, Sendich et al., 2008, Wingren et al., 2008, Gupta and Lee, 2009, Kim et al., 2009, Monavari et al., 2009, Sierra et al., 2009), have achieved varying levels of success.

Dilute-acid pretreatment (DA), one of the most investigated pretreatment methods, is conducted typically using sulfuric acid at high temperature (160–200 °C) (Schell et al., 2003, Lloyd and Wyman, 2005, Wyman et al., 2005). DA enhances the digestibility of lignocellulose mainly by dissolving hemicellulose and partially pre-hydrolyzing cellulose. DA can achieve satisfactory levels of cellulose saccharification for agricultural residues and some hardwood species, but is not effective for softwoods. The low pH value of the dilute-acid process causes serious equipment corrosion problems. In addition, high temperature and low pH lead to formation of significant amounts of fermentation inhibitors, notably furfurals from hemicellulosic sugars. Furthermore, dilute-acid pretreatment causes extensive condensation of lignin, which diminishes the commercial value of lignin for co-products development (Shevchenko et al., 1999, Nguyen et al., 2000).

Sulfur dioxide (SO₂)-catalyzed steam explosion is another acidic pretreatment at milder conditions (weak acid and short reaction time), which reduces equipment corrosion and minimizes the generation of fermentation inhibitors. The process has been extensively studied by Zacchi’s and Saddler’s groups (Galbe and Zacchi, 2002, Chandra et al., 2007) and is one of the most investigated pretreatments for woody biomass, including softwoods. Feedstock is first treated using gaseous SO₂ and then subjected to steam explosion. The process works well with agricultural residues and some hardwoods, but performs less satisfactorily with softwoods. Two-stage steam explosion can improve the enzymatic digestibility and overall sugar recovery from softwoods, but the advantages are partially outweighed by increased energy consumption and operation cost (Galbe and Zacchi, 2002). The toxicity of gaseous SO₂ is another concern in application. Furthermore, the mild SO₂ treatment is unable to sulfonate or dissolve lignin significantly. The lignin is extensively condensed during the subsequent steam explosion (Shevchenko et al., 1999), a key factor imparting recalcitrance of steam exploded softwood substrate (Pan et al., 2005b).

Recently, we developed and reported a novel pretreatment method, sulfite pretreatment to overcome recalcitrance of lignocellulose (SPORL), for robust conversion of woody biomass including softwood to sugars (Wang et al., 2009, Zhu et al., 2009a). The pretreatment consists of a short chemical treatment of feedstock followed by mechanical size reduction (fiberization). Wood chips or other feedstocks first react with a solution of a sulfite salt (e.g., Na, Mg, or Ca) at 160–180 °C and pH 2–4 for about 30 min, and are then fiberized (size-reduced) using a disk mill to generate fibrous substrate for subsequent saccharification and fermentation. SPORL produced readily digestible substrates and excellent recovery of fermentable hemicellulose sugars with low amount of fermentation inhibitors. Energy consumption for post-SPORL size reduction of wood chips was about 30 Wh/kg, equivalent to those consumed for size reduction of agricultural biomass. In addition, direct pretreatment of commercial wood chips afforded a low liquid-to-wood ratio (<3) (Zhu et al., 2009), which can lead to considerable thermal energy savings and produce a concentrated hemicellulose sugar stream. Because the lignin dissolved in SPORL pretreatment hydrolysate is sulfonated (lignosulfonate), it has a variety of commercial applications within the established market. It should be pointed out that SPORL pretreatment is compatible with SO₂-catalyzed steam explosion. One can conduct sulfite-catalyzed steam explosion by using sulfite in addition to SO₂ as catalysts to significantly improve cellulose enzymatic saccharification, especially for softwoods.

To further understand the fundamentals of the SPORL pretreatment, in this study, we systematically compared SPORL with the DA for the pretreatment of spruce, a softwood that is notoriously more recalcitrant than other types of biomass (hardwood and herbage) to bioconversion processes. The experimental flowchart is summarized in Fig. 1. The chemical changes of cell-wall components were investigated before and after the SPORL and DA pretreatments. The enzymatic digestibility of the pretreated substrates was evaluated. Mass balances of carbohydrates in the two pretreatments were established. The structural changes of lignin and carbohydrate during the pretreatments were examined using NMR spectrometry. Additionally, the formation of fermentation inhibitors during the pretreatments was compared. The fermentability of the spent pretreatment liquors of the SPORL and DA was evaluated using an in vitro ruminal fermentation assay.