With the addition of ethanolamine into alkaline pulping liquor, delignification was improved due to the increase of cleavage of fi-O-4 ether linkages and decrease of condensation of lignin [156]. This ideal has been realized in pulping of olive wood trimmings [157]. Under the optimal conditions, i. e., 15% ethanolamine concen­tration, 7.5% soda concentration and liquid to solid ratio of 4 at 195°C for 30 min, pulp was produced with acceptable yield and viscosity.

In addition, ethanolamine can be used as a cooking agent at a high concen­tration without the addition of alkali, and this pulping process was applied to oil palm EFB, rice straw [158-160] and hesperaloe funifera [161]. A comparison of pulping of EFB with ethyleneglycol, diethyleneglycol, ethanolamine and dietha­nolamine suggested that pulp obtained by using ethanolamine exhibited the best properties [158]. With regard to yield, kappa number and brightness, the properties

Fig. 11.4 Process of solvents and by-products recovery stages of ethylene glycol fractionation [140] of EFB ethanolamine pulp were comparable to those of kraft pulp from holm oak or eucalyptus wood. In addition, this process can be operated under a lower solvent concentration, temperature and time, with reduced energy and immobilized capital costs.

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.

Partial pressure of the produced hydrogen is one of the important factors affecting the hydrogen yields. When converting reduced components such as the long chain fatty acids into hydrogen and volatile fatty acids the positive Gibbs free energy makes the system thermodynamically unfavorable. Also positive Gibbs free energy results from the conversion of acetate to hydrogen again making the system unfavorable (Eqs. 10.8, 10.9). So the system becomes extremely sensitive to biohydrogen formation.

n(LCFA) ! (n — 2)LCFA + 2 Acetate + 2 H2 AG° = +48kJ/mol (10.8)

CH3COOH + 2H2O! 4H2 + CO2 AG0 = +104.6kJ/mol (10.9)

Gas sparging can be a good solution for removing the produced hydrogen from the system but for large-scale operations this will raise the process costs. So researchers are working on membrane technologies which are a more efficient and cost effective solution for hydrogen removal from the gas mixture. But after a while on membrane surface biofilm formation could occur and this can favor the methanogens’ activity. Therefore, it is very important to find a technique to purify the hydrogen and to use it directly in fuel cell systems [88].

12.3.1 General Overview

The production of high added-value chemicals from biomass process streams, as lignin, is crucial in the integrated approach of multiple processes and multiple major products in the concept biorefinery [86]. Consequently, reaction and sepa­ration processes for the production of compounds from biomass, namely ligno — cellulosic, have been continuously the subject of applied research. Due to its structure and somewhat complex chemistry, lignin is one of the most fascinating targets of research in three essential modes [38]: one aiming to breakdown the tridimensional network for conversion to aromatic (or non-aromatic) chemicals (thermochemical processes, Figs. 12.4, 12.5); the other one intending to use the lignin functionalities to integrate it in more complex matrices or construct renewable polymers; and the third one, dealing with lignin as source of power (green fuels and syngas [87] Fig. 12.4).

For centuries people have been using microbes to their advantage, turning grapes into wine, milk into cheese, and cabbage into sauerkraut. People benefit from what microbes do naturally: They eat and digest organic compounds, changing the chemical makeup of one product and turning it into a completely different yet tasty food or drink. Now microbes, in form of biomining, are providing efficient helping hand for extraction of heavy metals from sub-graded ores and minerals (Fig. 14.7).

Biomining is the interaction between metals and microbes with the specific aim of converting insoluble metal sulfides to soluble metal sulfates. Bioleaching has been defined as the dissolution of metals from their mineral sources by certain naturally occurring microorganisms or the use of microorganisms to transform elements so that the elements can be extracted from a material when water is filtered through it. So, it is the application of microbial process in the mining industry for economic recovery on a large scale [7] (Fig. 14.8).

Fig. 14.7 Biomining

In short, biomining is a term that describes the processing of metal containing ores and concentrates of metal containing ores using microbiological technology. It is often called bioleaching.

By convention bioleaching has been divided into two approaches:

• Direct bioleaching

• Indirect bioleaching

Direct bioleaching entails an enzymatic attack by the bacteria on components of the mineral that are susceptible to oxidation. In the process of obtaining energy from the inorganic material the bacteria cause electrons to be transferred from iron or sulfur to oxygen. In many cases the more oxidized product is more soluble. It should be noted that the inorganic ions never enter the bacterial cell; the electrons released by the oxidation reaction are transported through a protein system in the cell membrane and then (in aerobic organisms) to oxygen atoms, forming water. The transferred electrons give up energy, which is coupled to the formation of adenosine triphosphate (ATP), the energy currency of the cell.

Indirect bioleaching, in contrast, does not proceed through a frontal attack by the bacteria on the atomic structure of the mineral. Instead the bacteria generate ferric iron by oxidizing soluble, ferrous iron; ferric iron in turn is a powerful oxidizing agent that reacts with other metals, transforming them into the soluble oxidized form in a sulfuric acid solution. In this reaction ferrous iron is again produced and is rapidly reoxidized by the bacteria. Indirect bioleaching is usually referred to as bacterially assisted leaching. In an acidic solution without the bacteria, ferrous iron is stable and leaching mediated by ferric iron would be slow. T ferrooxidans can accelerate such an oxidation reaction by a factor of more than a million.

Biomining is applied using four different engineered methods:

• Dump bioleaching

• Heap bioleaching

• Heap minerals biooxidation

• Stirred-tank bioleaching

• Minerals biooxidation

Dump bioleaching extracts copper from sulfide ores that are too low grade to process by any other method. This process has been used since the mid-1950s.

Heap bioleaching, which has been used since the 1980s, extracts copper from crushed sulfide minerals placed on engineered pads.

Heap minerals biooxidation pretreats gold ores in which the gold particles are locked in sulfide minerals, significantly enhancing gold recovery.

Stirred-tank bioleaching extracts base metals from concentrates of metal containing sulfide ores.

Stirred-tank minerals biooxidation enhances gold recovery from mineral concentrates in which the gold is locked in sulfide minerals [8].

Advantages

The advantages of biomining process over chemical leaching are:

(i) Biomining is a way to exploit low grade ores and mineral resources located in remote areas that would otherwise be too expensive to mine.

(ii) It is more environmentally friendly than the conventional (smelting) method, since it uses less energy and does not produce SO2 emissions. This also translates into profit, as the companies have to spend huge sums finding ways of limiting their SO2 emissions.

(iii) Less landscape damage occurs, since the bacteria grow naturally. Native bacteria can operate over a wide temperature range between 20 and 55°C. Other materials for the process are also natural such as air and water.

(iv) The bacteria breed on their own, i. e. they are self-sustaining. Since there is no need to pay for heating and chemicals required in a conventional operation, companies may be able to reduce the price of metal production by nearly a half.

(v) It is a less energy intensive process.

(vi) It is simpler and therefore cheaper to operate and maintain, as no technical specialist is needed to operate complex chemical plants [9].

(vii) Even the dumps left behind after traditional mining processes can be reprocessed to extract residual metal [10].

So, biomining is the process of extracting valuable metals from ores and mine tailings with the assistance of microorganisms. It is a green technology that can help mine valuable metals with minimal impact on the environment. It requires low energy, causes low gaseous emission and is not labor intensive.

During the ethanol fractionation process, the effect of severity parameter on crystallinity of lignocellulosic material is not fully defined. Under mild conditions, the degradation of carbohydrates mainly occurred at the amorphous region, resulting in the removal of hemicelluloses and amorphous cellulose, but cellulose in the crystalline region is resistant to degradation. This was supported by the comparative analysis of solid state CP/MAS 13C NMR spectra of the treated and the untreated Miscanthus x giganteus [18]. Pan et al. [19] reported that the crys­tallinity of cellulose increased with increased severity of ethanol fractionation pretreatment of Lodgepole pine, suggesting that cellulose in amorphous region was more easily degraded than that in crystalline region. In another investigation on Pine (Pinus radiata) fractionation by formic acid, a decrease of crystallinity after the treatment was also shown [38]. However, a more serious severity was capable of disrupting the crystallinity of cellulose, resulting in the decrease of CrI, as reported in the ethanol fractionation of Buddleja davidii [39]. The degradation results in cellulose with a decreased degree of polymerization (DP) and a narrow molecular weight distribution. In addition, it has also been found that crystalline cellulose dimorphs (Ia/Ib) are converted into para-crystalline and amorphous type.

Carbohydrates in lignocellulosic materials undergo decomposition under acidic conditions during the auto — or acid-catalyzed ethanol fractionation process. Car­bohydrates are first hydrolyzed into oligosaccharides and monosaccharides, and the resulting monosaccharides further dehydrate to generate furfural (from pen­toses) and hydroxymethylfurfural (HMF) (from hexoses). Furfural and HMF undergo further degradation to form levulinic acid and formic acid, respectively. In addition, the products, i. e., furfural, HMF and levulic acid, tend to condense and form polymers such as humins [40]. The contents of furfural and HMF increase with increased severity parameter. But the overall effect of severity is minor due to the low yield of these products. At a high temperature and a high pressure, water can act as an agent for the degradation of carbohydrates [41]. These effects con­tribute a lot in the hot compressed water and dilute acid treatment of woody biomass. However, they are reduced largely by ethanol fractionation because of the elimination of strong acid and the high water content [40].

The effect of temperature in the reaction rate and yield of vanillin production from lignin oxidation is shown in Fig. 12.9d for the range 372-414 K: higher initial temperatures led to higher vanillin yields in a shorter reaction time; however, the vanillin degradation is also higher. In fact, for pH 14, the temperature has an important effect on the rate of vanillin degradation as shown in Fig. 12.10c: at 414 K, for 40 min of reaction time about 20% of the initial vanillin was consumed while at 393 K the decrease was only about 4%. This effect is even clearer for long reaction times.

The work of Mathias, Fargues and Rodrigues [36, 116, 118, 149] allowed calculating the activation energies (Ea) for the vanillin production and oxidation:

29.1 and 46.0 kJ/mol, respectively. The kinetic constant for vanillin production can be expressed as:

3502

k = 1.376 x 101exp — (l/mol)175min—1. (12.2)

For vanillin oxidation, the following kinetic law was found for pH >11.5:

— rv = k'[O2][Cv] (12.3)

with,

k’ = 4.356 x 106exp^ —^7—^ (l/mol. min). (12.4)

Other authors reported the kinetic laws and activation energy for vanillin and syringaldehyde production from hardwood lignin [112, 113]. Similar Ea values were found for vanillin and syringaldehyde, 70.5 and 62.6 kJ/mol, respectively [113]. However, the rate constant for syringaldehyde production is higher than that for vanillin, as depicted in the kinetic laws for syringaldehyde (Sy) and vanillin (v) [113]:

rsy = 1.3 x 105exp——— — [O2][OH—]L4[L] (12.5)

rv = 5.4 x 105exp ^ — 8479^ [O2][OH—]19[L] (12.6)

For cost effective phyto-extraction, it is essential to create stabilizing plants which produce high levels of root and shoot biomass, high tolerance and resistance for heavy metals. This can be done by mycorrhizal association.

Mycorrhizal association: It is a symbiotic association between a fungus and the roots of a plant. The fungus colonizes the host plants’ roots, either intracellularly or extracellularly. This mutualistic association provides the fungus with relatively constant and direct access to carbohydrates, such as glucose and sucrose supplied by the plant. The carbohydrates are translocated from their source (usually leaves) to root tissue and on to fungal partners. In return, the plant gains the benefits of the mycelium’s higher absorptive capacity for water and mineral nutrients (due to comparatively large surface area of mycelium: root ratio), thus improving the plant’s mineral absorption capabilities. These fungi have a protective role for plants rooted in soils with high metal concentrations. The trees inoculated with fungi displayed high tolerance to the prevailing contaminant, survivorship and growth in several contaminated sites. This was probably due to binding of the metal to the extramatricial mycelium of the fungus, without affecting the exchange of beneficial substances.

So, Mycorrhizal Association enhances plant growth on severely disturbed sites, including those contaminated with heavy metals and plays an important role in metal tolerance and accumulation [34, 35].

Ethanol can be produced in two different ways, either by Direct Microbial Con­version (DMC) [180] or by Simultaneous SSF process. Novel bioreactors con­sisting of more than one bioreactor along with genetic recombination techniques are being developed at laboratory and pilot scale to improve the yield and pro­ductivity of bioethanol [25, 102]. Thermophilic fermentation seems to be a promising technique [122]. Additionally, the use of supercritical CO2 as a pre­treatment option has increased the ethanol yield by 70% [207].

9.7.13 Boiethanol Refinery

The conversion of by-products into value added products under a biorefinery concept may further reduce the associated process costs with additional energy in the form of fuels, heat, and electricity such as formation of xylitol from xylose, methyl fuorate from furfural and plastic from hydroxylmethyl furfural. Never­theless, estimation of greenhouse gas emissions of these products as they are shaped into marketable products is required. The main technological issues have been summarized recently by Kumar [102]. Prasad [144] described the pros and cons of various pretreatment options for ethanol production from lignocellulosic biomass. Moreover, the availability of the feedstock and related logistics influence the effectiveness of bioethanol technology [180].

Cellulosic materials can be partially or totally hydrolyzed in acetone solution with the addition of small amounts of acidic catalysts. The hydrolysis process can be operated at 145-228°C with 70-100% acetone [162]. In a high concentration acetone solution, the formation of stable complexes with sugars can prevent the degradation of the material. The produced lignin and sugars were claimed to be commercially useful products. By using acetone fractionation process, wood or delignified pulps can be converted into saccharified feedstock to produce pento­sans and hexosans followed by sugars. It has been patented that lignocellulosic material can be cooked at 180-200°C with 60-70% (v/v) acetone containing 0.02-0.25% phosphoric, sulfuric or hydrochloric acids as a catalyst [163]. After the fractionation, a high purity of glucose fraction was obtained with the pre­dominately cellulosic material, whereas mixed pentose and hexoses were produced when applying the whole wood as a feedstock.

Acetone pulping of wheat straw [164-166] and Eucalyptus [167] has also been reported. For instance, a treatment using a temperature of 180°C, an acetone concentration of 40%, a cooking time of 60 min and 1,750 beating revolutions,
resulted in pulp with similar or even better properties than those for soda pulp. It was claimed that the advantages that the process was less contaminating since the acetone was easy to be recovered and that the dissolved liquor rich in lignin had great potential use in the production of new materials. In addition, acetone has been used in mixtures with formic acid [168], ethanol [169] and the mixtures of them [170]. Furthermore, oxygen delignification can be modified to oxygen — acetone delignification process. For instance, oxygen delignification of cotton­wood in acetone/water solutions (60/40, v/v) was evaluated with respect to pulping conditions as well as delignification kinetics [171, 172].

Recently, acetone organosolv fractionation of wheat straw has been studied to produce sugars and lignin [139]. The optimal conditions, i. e., 50% acetone for 1 h at 205°C, resulted in 82% hemicelluloses degradation, 79% delignification and 93% cellulose recovery. It has been shown that the acetone process improves the enzymatic hydroablity. After the fractionation pretreatment, a high glucose con­version yield up to 87% was achieved as compared to 16% for the untreated wheat straw. In another report, Pinus radiata D. Don was subjected to acetone pre­treatment. A higher ethanol yield of 99.5% was achieved under the pretreatment conditions below: 50% acetone, pH 2.0, 195°C and 5 min [138].