Multiscale modeling of biomass pretreatment for optimization of steam explosion conditions

Abstract

Steam explosion is the most cost-effective and widely used method for pretreating lignocellulosic materials. Unlike other hydrothermal pretreatment technologies, it is characterized by physical tearing effects. The severity factor cannot represent the effects of instantaneous decompression but it can depict the effects of high-temperature cooking. Using the theories of heat transfer, momentum transfer, and brittle fracture mechanics as bases, we analyze physical tearing in instantaneous decompression. Moisture content of materials w_opt is optimized by maintaining temperature T (T↔w_opt), and chip size d is optimized with discharge port area A (A∝d²). The chip size, moisture content of materials, and discharge port area are then incorporated into the severity factor. The enriched severity factor and the relationship among operation parameters, material property parameters, and equipment parameters serve as engineering references in designing the equipment and selecting the conditions for steam explosion technology. Such equipment and conditions are suitable for use under any given set of materials, products, and treatment targets.

Introduction

Since the early investigations of Mason (1929), steam explosion has been considered an effective method for disrupting different lignocellulosic plant materials into major plant components, such as cellulose, lignin, and hemicellulose (Saddler et al., 1993). Lignocellulosic biomass is heated at elevated temperatures with high pressure steam, and after a few minutes, subjected to explosive decompression, thereby physically and chemically modifying the biomass (Cantarella et al., 2004). Hemicellulose is hydrolyzed, lignin is solubilized, and cellulose is made more accessible to cellulose enzymes (Cantarella et al., 2004).

On the basis of decades of research, scholars developed steam explosion technology into a general biorefining technology platform. A series of novel processes based on steam explosion technology have been successfully established; these processes include clean pulping, cleaning and degumming hemp, and preparing humic acid (Chen and Li, 2002). Wheat straw was pretreated by steam explosion to produce cellulose pulp (Montane et al., 1998). Textile and composite materials were synthesized with steam exploded short staple flax fibers (Kessler et al., 1998). Chen (2009) extended steam explosion technology to develop bio-based production of chemicals from biomass, including furfural, levulinic acid, xylitol, xanthan gum, oxalic acid, lactic acid, and ethylene. The authors also prepared bio-based materials, including cellulose acetate, carboxymethyl cellulose, and ecological plate, and produced bio-based energy fuels, such as ethanol, butanol, hydrogen, and biodiesel (Chen (2009)).

As one of the most economical and efficient biomass pretreatment technologies (Alfani et al., 2000, Ballesteros et al., 2006, Talebnia et al., 2010, Zheng et al., 2009, Kumar et al., 2009, Costa Sousa et al., 2009), steam explosion was initially mistaken as a purely chemical process. The severity factor or modified severity factor of hydrothermal pretreatment processes is often used to describe the severity of the steam explosion process, as shown in Eqs. (1)–(6) in Table 1.

Two conclusions emerge from the effects of steam explosion on straw structure: (1) At high-temperature cooking, hemicellulose is degraded, lignin is solubilized, and cellulose binding is reduced. (2) Under instantaneous decompression, superheated water flashes into steam and steam volume abruptly expands. The impact force generated by flashing and volume expansion destroys cell structure. In this stage, materials are torn into small pieces; fiber bundles are separated from one another and their structures loosen, thereby re-distributing lignin; and cellulose is fully exposed.

The severity factor currently used represents the relationship among time, temperature, and acid concentration. Although it enables the easy comparison of experimental results to facilitate process design and operation, it does not represent all the factors that affect pretreatment efficiency (Hosseini and Shah, 2009). Unlike other hydrothermal pretreatment technologies, steam explosion is an effective fractionation technology that is characterized by physical tearing effects, which enhance lignocellulose defibration and increase specific surface area. Because the severity factor originates from hydrothermal pretreatment processes, it cannot represent the effects in the instantaneous decompression stage, but depicts those in the high-temperature cooking stage.

Hosseini and Shah (2009) developed a model based on the diffusion of steam into biomass, taking particle size and process time into account (see Eq. (6)). Although the mass transfer perspective dictates that chip size be considered first, Eq. (6) still represents the effects of high-temperature cooking on steam explosion. However, the equation does not depict the physical tearing effects of instantaneous decompression.

Physical tearing is affected by numerous factors such as chip size, material bulk density, material loading coefficient, and moisture content (Chen et al., 1999, Chen and Liu, 2007). The performance determinants of a pressure tank also considerably influences steam explosion results; these determinants include the height-to-diameter ratio of the tank, discharge port area (Chen et al., 1999), and buffer tank volume. Holding pressure (temperature) is another important factor.

In this paper, we analyze physical tearing in instantaneous decompression on the basis of the theories of mass transfer, heat transfer, and momentum transfer, as well as on brittle fracture mechanics. We consider particle size, moisture content, and discharge port area in the analysis. An enriched and comprehensive severity factor represents the effects of high-temperature cooking on steam explosion and the physical tearing effects of instantaneous decompression. Such representations enable the comprehensive interpretation of the mechanism of steam explosion technology, and serve as engineering reference in designing the equipment and selecting the conditions for steam explosion technology.