J. Andres Soria and Armando G. McDonald

13.1 Introduction

The use of biomass resources as a renewable feedstock for producing liquid fuels has been dominated by the biochemical conversion of glucose polymers (i. e. starch, cellulose) into small molecular weight alcohols, most notably ethanol. The production capacity of ethanol in the US alone has reached levels of 15 billion gallons per year [1], but it is met with a limitation on how much ethanol con­ventional gasoline engines can accept without modifications or causing mechanical damage, the so-called blend wall [2]. In addition, the displacement of feed and food grade corn and soybeans for the production of liquid biofuels has socio­economic implications that affect the long-term viability of this feedstock as a sustainable source of food and energy [3, 4]. To make ethanol a viable biofuel then, a new vehicle fleet, fuel processing and transportation infrastructure is needed, which will come at an elevated financial and policy cost.

An alternative is to develop the next generation of biofuels that are capable of being produced from non-food based biomass resources, and that maximize the use of ‘‘waste biomass’’, or biomass that has no established market under

J. Andres Soria

Agricultural and Forestry Experiment Station,

University of Alaska Fairbanks, Palmer, AK 99645, USA e-mail: jasoria@alaska. edu

J. Andres Soria

School of Engineering, University of Alaska Anchorage, Palmer, AK 99645, USA

A. G. McDonald (H)

Renewable Materials Program, College of Natural Resources, University of Idaho, Moscow, ID 83844-1132, USA e-mail: armandm@uidaho. edu

C. Baskar et al. (eds.), Biomass Conversion,

DOI: 10.1007/978-3-642-28418-2_13, © Springer-Verlag Berlin Heidelberg 2012 current economic activities. These include non-food agricultural crops, grasses, non-merchantable timber, tree tops and limbs — and demolition-derived biomass, among others. By creating a hydrocarbon-based fuels rather than alcohols, the new generation of liquid biofuels will have less limitations in its application, maxi­mizing the existing vehicle engine and logistics infrastructure, and have a greater chance of being fully adopted into the market without the need of mandates or subsidies.

Technologies capable of yielding high conversion rates and stable chemical platforms to produce these advanced biofuels are still in development, with the most promising pathways being those that employ thermochemical conversion techniques [5]. Thermochemical conversion has been successfully applied to biomass to yield new generation biofuels, particularly through pyrolysis [6-9] and gasification platforms with Fisher-Tropsch gas conversions [10].

In pyrolysis, high temperatures in excess of 500°C are used in an oxygen — deficient atmosphere to thermally breakdown the biomass polymers (cellulose, hemicellulose and lignin) into smaller oligomers and monomers which are con­densed from vapor phase into a liquid bio-oil [6-9]. Pyrolysis is capable of transforming hardwood and softwood species into a bio-oil with a maximum reported yield of 75 wt%, with a significant fraction of water and oxygenated species, which affect storage and processing [6, 7]. The prevalence of the oxy­genated fractions of bio-oil result in the need to hydrotreat it in order to improve the stability and functionality of the product [11]. Hydrotreating bio-oil using transition metal and zeolite catalysts has been successfully done [12-15], and continues to attract significant interest as the promising pathway toward new generation biofuels.

Pyrolysis does suffer from a variety of problems, most prominently being that the biomass feedstock needs to be dry and reaction conditions must be carefully controlled in order to have a repeatable, consistent product [16]. Although these are process engineering issues that are generally resolved at the pilot scale, some aspects are more difficult to resolve such as the high energy requirement to conduct the transformation, and the low product stability of the pyrolysis-generated bio-oil [16]. Potential solutions to these problems involve using proprietary catalytic processes that are exothermic in nature (i. e. KiOR pyrolysis plant in MS, USA), and using chemical stabilizers in an attempt to extend the shelf life and viability of the bio-oil prior to upgrading into the final liquid biofuel [17].

One alternative approach is to employ a different thermochemical processing technique, where biomass is liquefied under less severe conditions than pyrolysis, employing supercritical fluids [18]. A supercritical fluid is a compressible gas that has reached its critical temperature and critical pressure, at which point, differ­ences between the two phases disappear [19]. Specifically, supercritical methanol (SCM) allows for a net reduction of reaction times, improved solvent recovery, low char yield and selectivity of the reaction conditions through modification of pressure and temperature parameters [18, 20-24].

As it applies to biomass conversion, supercritical water [25], phenol [26], carbon dioxide [27] and methanol [18, 20, 24] have been used. Given the reaction conditions of moderate temperature (238°C) and pressure (8.1 MPa), SCM has been shown to elicit the highest conversion rates and stable compounds when compared to pyrolysis bio-oil [18]. Biomass liquefaction in SCM stems from the changes in density, specific weight, polarity and viscosity of methanol as it interacts with the biomass ultrastructure components namely carbohydrates, lignin and extractives in the SCM environment. In the supercritical state, cleaving of selective phenolic bonds occurs [24, 28], resulting in a system that depolymerizes the biomass and maintains these monomeric, oligomeric and polymeric structures in solution when returned to ambient conditions. The degree of depolymerization was found to be influenced by SCM density which can be manipulated by tem­perature, pressure and ratio of methanol to reactor vessel volume. This enables SCM batch systems process flexibility to tune for final product attributes, which is unrivaled by pyrolysis systems, leading to production of a bio-oil in excess of 90 wt% [24]. Furthermore, the process conditions at a modest 367°C resulted in 92% liquefaction.

The current study focuses on the use of methanol under supercritical conditions to liquefy wood from Alaskan softwood and hardwood as models for non­merchantable timber species, yielding a biochar and a liquid bio-oil product. The resultant bio-oils yields were determined and composition was determined by gas chromatography-mass spectrometry (GC-MS) analysis, while the solid biochar was characterized by Fourier transform infrared (FTIR) spectroscopy.