2.2.1 Composting and Anaerobic Digestion

Composting is a biological process in which organic waste such as food waste, manure, garden waste, trimmings, etc., are converted into a humus-like substance by microorganisms under aerobic conditions. Composting is a robust, low-cost, and low-tech option that can handle a variety of organic wastes varying in composition and moisture content. There are three major designs used in composting: static piles, vessels, and windrows. Windrow composting is usually considered the most cost-effective composting option. The organic material is layered into long piles and thoroughly mixed by turning machines. The temperature within windrows can reach 65°C; hence composting can also act as a biological drying process. Therefore, composting is frequently used in combination with biogas production in areas with high livestock density, where surplus nutrients cause considerable problems (Luo et al., 2013). Compost is used as a soil amendment and slow-release organic fertilizer. During the composting process, a large part of the carbon in the organic waste feedstock is metabolized to carbon dioxide, which is usually not captured for recycle into a useful bioproduct, but it provides a low-tech method to reduce the volume of waste streams and provide a useful product.

Compost is one of the best sources of stable organic matter from which new humus can be formed in degraded soils. An estimated 45% of European soils have low organic matter content, principally in southern Europe, but also in areas of France, the UK, and Germany. Standards on the use and quality of compost exist in most countries (eg, Organic Farming Regulation or eco-labels for soil improvers). The standards differ substantially, partly due to differences in national and international soil policies.

In AD processes, organic material is converted in the absence of oxygen into raw biogas (methane plus carbon dioxide) and unmetabolized solids, or digestate, which can be used as fertilizer. After purification, the methane component of biogas (ie, biomethane) is equivalent in energy content to fossil natural gas, and can be used as fuel or as a source of biobased hydrogen or syngas. The carbon dioxide component rejected during the biomethane purification process is generally exhausted to the atmosphere and not captured for reuse. Codigestion facilities are typically agricultural anaerobic digesters that simultaneously accept two or more input materials. Feedstock for AD can include a range of organic materials, including biodegradable waste, such as grass clippings, food waste, sewage, and manure. Anaerobic digesters can also be fed with dedicated energy crops, such as Silphium perfoliatum or silage maize, for biogas production, although this is generally considered to be controversial due to land use issues. Lignocellulosic biomass is not the preferred feedstock for AD, because most anaerobes are unable to degrade lignin. However, the carbohydrate content of plant cell walls is generally useable by AD microbial consortia.

2.2.2 Preprocessing Technologies

Important issues in the handling and delivery of biomass include low bulk and energy densities and instability in storage, that necessitate conditioning or preprocessing operations prior to introduction into a conversion process. In addition, biomass delivered to the conversion process must possess physical and chemical properties that fall within the design specifications of the biomass conversion process, which vary from one conversion process to another. Preprocessing technologies of potential interest include—but are not necessarily limited to—operations such as drying, milling, fractionation and separation, blending, densification, and torrefaction.

2.2.3 Pretreatment: Physical, Chemical, and Biochemical

In connection with biocatalytic conversion processes, the term pretreatment is used for a number of single or combined physical, chemical, and biological processes that are used to efficiently convert lignocellulosic biomass, which is usually recalcitrant to enzymatic hydrolysis, into an activated form that can be cost=effectively hydrolyzed enzymatically to produce C5 and C6 sugars in aqueous solution. Thus, the ultimate pretreatment technology would be a low-cost, sustainable process that renders lignocellulosic biomass completely accessible to enzymes and/or microorganisms that efficiently and completely hydrolyze all carbohydrate polymers present in plant cell walls and avoids the formation of toxic degradation products that negatively impact downstream biological or chemical processing, and thereby process yield and profitability.

Pretreatment technologies for biocatalytic conversion technologies can be broadly divided into two categories: pretreatment and fractionation.³ The goal of fractionation is to separate biomass into its three main components: hemicellulose, cellulose, and lignin. The quality of the manufactured cellulose can be of similar quality as obtained by conventional pulping processes. Although many research groups have tried to find a more economically attractive application for lignin, it is usually used as fuel for the conversion of cellulose- and hemicellulose-derived sugars into value-added products, which can be energy-intensive. Hemicellulose is commonly treated as a secondary sugar stream from lignocellulosics-based biorefineries. This is often due to the lack of efficient and robust fermentation processes for the conversion of C5 sugars to bioethanol or other product. The quality of the hemicellulose stream, as defined by its sugar composition and the level of sugar degradation byproducts, depends on the specific biomass pretreatment employed. In this regard, optimized pretreatment and fractionation schemes are necessary to ensure the maximum exploitation of the biomass carbohydrates.

The development of pretreatment technologies for lignocellulose is driven by the production of second-generation bioethanol. Therefore pretreatment methods are the subject of intensive research activities (Mosier et al., 2005; Yang and Wyman, 2008; Gírio et al., 2010; Goh et al., 2011; Shi et al., 2011; Panagiotopoulos et al., 2012; Chiesa and Gnansounou, 2014).

One example of a pretreatment strategy is dilute acid hydrolysis, which is probably the oldest pretreatment option available. Dilute acid pretreatment processes typically employ a dilute (eg, 0.1%) strong acid, such as sulfuric acid, high temperature (eg, 150–200°C), and a fairly short residence time (eg, 10 minutes) to break the lignin seal and nearly completely hydrolyze hemicellulose into its constituent sugars. The net effect is to increase the surface area of cellulose fibers that is accessible to subsequently added cellulase enzymes. This approach ultimately results in increasing the concentration and amount of fermentable sugars from a given amount of input biomass.

There are a variety of physical and physio-chemical methods that have advantages and disadvantages (Linde et al., 2008; Hendriks and Zeeman, 2009; Alvira et al., 2010; Goh et al., 2011; Lai et al., 2014; Reisinger et al., 2014; Papa et al., 2015). In general, ammonia fiber explosion, wet oxidation, and liquid hot water are more suitable for agricultural residues and dedicated herbaceous energy crops, while steam explosion is applicable for both herbaceous and woody biomass. Hardwoods are typically less recalcitrant to dilute acid pretreatment than softwoods because their hemicelluloses are composed of acetylated xylans that rapidly hydrolyze in water at elevated temperatures. Another reason for the recalcitrance of softwood species is the presence of a more heavily crosslinked lignin in softwood cell walls relative to hardwoods and herbaceous species. Hardwoods also contain lower contents of readily fermentable C6 sugars in their hemicellulose than softwoods, as do agricultural residues and herbaceous crops (Gnansounou, 2008). This is because softwood hemicelluloses contain higher amounts of the C6 sugars, mannose and galactose, while hardwood and herbaceous species hemicellulose contains primarily C5 sugars, mainly xylose and arabinose. Furthermore, in acid pretreatment conditions pentose sugars are more easily degraded to byproducts, such as short-chain organic acids and furan compounds. Thus, the preferred pretreatment should facilitate high hemicellulose sugar recovery yields, while maintaining an acceptable downstream cellulose enzymatic hydrolysis rate and glucose yield.

Pretreatment processes are frequently classified as biological, physical, chemical, physicochemical, and solvent-assisted. Most chemical/biochemical pretreatments result in sugars in aqueous solution and a solid fraction. However, sugar degradation products are often also produced during pretreatment, which can negatively affect subsequent purification and/or conversion steps. The types and relative amounts of the sugars produced depend on process conditions and feedstock composition. Table 2.1 shows selected process conditions for various pretreatment methods as well as advantages and disadvantages of the methods.

2.2.4 Saccharification of Cellulose and Hemicellulose

The use of hydrolytic enzymes to degrade cellulose to fermentable sugar is generally considered more effective than the use of concentrated mineral acids, because enzymes are highly specific and can work at mild process conditions, for example, 45–50°C and pH 4.8–5 for cellulases derived from the filamentous fungus, Trichoderma reesei. In addition, the mild chemical conditions employed for enzymatic processes obviate the need for extremely expensive, corrosion-resistant materials of construction that are required when using concentrated mineral acids. Concentrated mineral acids also degrade sugars to unfermentable products, which results in yield losses.

Saccharification of lignocellulosic material requires the use of several enzymes with complementary activities: endoglucanase, which attacks regions in the interior of linear cellulose chains; exoglucanases or cellobiohydrolases, which hydrolyze cellobiose units from the ends of cellulose chains; and β-glucosidase, which converts cello-oligosaccharides and cellobiose into glucose. There are also several complementary enzymes that attack hemicelluloses, including glucuronidase, xylan acetylesterase, xylanase, β-xylosidase, galactomannase, and glucomannase. Filamentous fungi, such as Trichoderma viride, T. reesei, and Trichoderma longibrachiatum, are the principal producers of commercially available cellulases. These commercial products are cocktails containing mixtures of several enzyme types. Prior to their use in the production of second-generation biofuels, cellulases were included in detergent formulations and employed in the manufacture of “stonewashed” jeans.

The economics of biomass saccharification is sensitive to the cost of cellulase enzymes. The low specific activity of cellulases requires large amounts of enzyme to effectively saccharify the cellulose in biomass in a reasonable amount of time. In addition, unproductive binding of the enzymes to biomass increases the amount of enzyme required and therefore operational costs in the process. Addition of surfactants can be useful to decrease nonproductive binding of cellulases to lignin and polysaccharide–lignin complexes (Cui et al., 2011).

Enzyme companies and others have made significant progress toward reducing enzyme cost by streamlining enzyme production and formulation processes, inclusion of new enzymes in cellulase enzyme cocktails, and increasing the enzyme specific activity using enzyme engineering strategies. For example, with funding from the U.S. Department of Energy (US DOE), Genencor, Novozymes, and DSM each have dramatically reduced the cost and improved the quality of cellulase enzyme cocktails for use in cellulosic biomass conversion processes. Genencor has launched their Accellerase Duet product produced by a genetically modified strain of T. reesei, and which contains both cellulases and xylanases. Similarly, Novozymes has released their CELLIC product family (CTEC2 and CTEC3, and HTEC). These new cellulase products have been designed especially for use in the production of second-generation bioethanol. Dyadic is another US company that is producing cellulase enzyme cocktails for industrial use. Fig. 2.2 describes the improvement of the commercial blends.

Feedstock composition and the specific requirements of downstream processes determine the preferred pretreatment strategy. The types and relative amounts of sugars present in the feedstock are a function of the plant species, but the availability of those sugars for downstream conversion processes are a function of how well the pretreatment and enzymatic saccharification processes perform. Hence, the preferred combination of pretreatment and conversion technology depends on a number of factors and therefore no one-size-fits-all technology option currently exists. The ideal pretreatment process should result in high yields of fermentable sugars from hemicellulose and cellulose, avoid the generation of toxic compounds and yield losses due to sugar degradation, minimize enzyme usage and associated cost, and require little or no energy or chemical inputs. The following section describes biochemical conversion technologies, as well as their connection to pretreatment processes.

2.2.5 (Bio)-Catalytic Production of Bioethanol and Various Chemicals

Bioethanol has become the most commonly used biofuel worldwide. Bioethanol can be blended with gasoline or used in the synthesis of ethyl tertiary-butyl ether, which is used as an antiknocking agent in gasoline. Both compounds serve as oxygenates in gasoline, and were first used in the US to improve air quality in densely populated urban areas.

Besides its use as a transportation fuel blending agent, ethanol is also considered a potent platform chemical for the synthesis of other value-added chemicals. Catalytic conversion of bioethanol has been successfully applied to the production of important chemicals such as ethylene, n-butanol, diethyl ether, etc. (Posada et al., 2013). Important bulk chemicals that can be derived from ethanol are shown in Fig. 2.3. The conversion of ethanol to bulk chemicals is still in its infancy, but this family of chemical conversions is probably one of the most promising near-term pathways to a biobased economy. Moreover, the conversion of ethanol to other chemicals would allow for the maintenance of existing downstream processing infrastructure. Ethanol production can be used as a proxy for other products derived from cellulosic sugars via biochemical conversion routes (eg, butanol, furan derivatives, etc.) because the equipment for purification and upgrading of those products is similar to that used for ethanol.

For the production of second-generation biofuels and biobased chemicals, hydrolysis and fermentation steps can be combined in simultaneous saccharification and fermentation (SSF) of hexoses or simultaneous saccharification and cofermentation (SSCF) of both hexoses and pentoses. Consolidated bioprocessing (CBP) is a system in which cellulose enzyme production, substrate hydrolysis, and fermentation take place simultaneously in the same reactor vessel using a fermentation organism that produces the enzymes that saccharify the cellulose. CBP offers the potential for lower biofuel production costs due the process integration, which eliminates the capital and operating costs associated with the separate production of enzymes and ensures higher efficiencies than separate hydrolysis and fermentation (SHF) or SSF (van Zyl et al., 2013). The Saccharomyces cerevisiae strain developed by the Mascoma Corporation is the best CBP organism engineered so far, as this strain can convert several cellulosic substrates to ethanol with addition of minimal exogenous enzymes (Mcbride, 2010). Various recombinant microorganisms have demonstrated impressive saccharification potential (Ha et al., 2011), although their use for large-scale industrial processes still requires fine-tuning of the entire process. The (simultaneous) fermentation of mixed C5 and C6 sugar hydrolysates is still one of the most important challenges in the production of biofuels and biobased chemicals.

Apart from bioethanol, a number of platform chemicals and advanced biofuels can be produced from sugars derived from lignocellulosic materials. The US DOE identified the top 12 chemicals potentially derived from biomass in 2004 (Werpy et al., 2004). The list was extended in 2010 to include alcohols, acids, amino acids, and terpenes (Bozell and Petersen, 2010). Table 2.2 lists chemicals that can be derived from sugars either through fermentation, for example, C4 acids or chemical conversion, for example, furan derivatives produced via catalyzed dehydration of sugars.

Succinic acid, a C4-dicarboxylic acid platform chemical, has received increasing attention for the production of new polyesters with good mechanical properties combined with full biodegradability. Succinic acid is widely used, especially for the production of renewable polyesters. Succinate concentration as high as 110 g/L has been achieved from glucose by the rumen organism Actinobacillus succinogenes (Liu, 2000). It can also be produced by Anaerobiospirillum succiniciproducens using glucose, lactose, sucrose, maltose, or fructose as carbon sources. The reduction of the carboxylate groups is one of the available routes to 1,4-butandiol (BDO), a building block for the production of various polyesters.

Several other organic acids are also considered to be potential platform molecules. Lactic acid is probably the most prominent. It can dimerize and subsequently form lactide, an intermediate for polymerization to the biodegradable plastic, polylactic acid. The monomer acid is typically produced by glucose fermentation using Lactobacillus delbrueckii (Datta, 2006). Luo et al. (1997) indicated that using the SSF strategy for lactic acid production offers the advantage that enzymatic hydrolysis and lactic acid fermentation work at similar process conditions (eg, temperature and pH). Other organic acids may be a suitable platform for specialty chemicals or niche markets, but the large volumes of waste products produced (ie, gypsum) are problematic and require the development of a more efficient process design.

Other biobased chemicals and advanced fuels obtained through the fermentation of sugars include alcohols higher than ethanol, diols, microbial lipids, and advanced hydrocarbons. Some of these are described below.

Biobutanol has a higher energy density and is less hydrophilic than ethanol, both of which make it a better blending agent for gasoline than ethanol. It can be produced by acetone-butanol-ethanol (ABE)-type anaerobic fermentation organisms (Atsumi et al., 2008). Clostridium beijerinckii ATCC 55025 can utilize hexoses and pentoses simultaneously to produce butanol (Liu et al., 2010). The main problem associated with the industrial production of biobutanol is the high energy required for purifying the product from fermentation liquors at low butanol concentrations. The combination of fermentation with pervaporation has been proposed to separate butanol during fermentation and increase the process efficiency (Qureshi et al., 2001). Gevo and Butamax have successfully converted cellulose-derived isobutanol produced via a fermentation process into isobutylene and paraffinic kerosene (jet fuel) via well-understood chemical processes.

2,3-Butandiol (BDO) is a promising compound with three stereo-isomer forms. It can be used both as liquid fuel and as a platform chemical. Both the levo- and dextro-isomers of BDO are chiral components for asymmetric synthesis reactions. Various microrganisms, namely Klebsiella pneumoniae (Ma et al., 2009), Klebsiella oxytoca (Cheng et al., 2010), Paenibacillus polymyxa (Gao et al., 2010), and Enterobacter aerogenes (Zeng, 1991), have been tested for the production of BDO by using various glucose sources. BDO can be converted to 1,3-butadiene, an intermediate for the synthesis of rubbers, polyesters, and polyurethanes, or to methyl-ethyl-ketone, a solvent and commonly used fuel additive.

Among polyalcohols, xylitol, a natural noncaloric sweetener with anticariogenic properties, has been considered as the main alternative to ethanol produced from C5 sugars. It is currently produced from the nickel-assisted hydrogenation of xylose. Sorbitol can also be biotechnologically produced by the bacterium Zymomonas mobilis, which uses fructose and glucose (Silveira, 2002).

Microbial lipids (predominantly triglycerides) obtained through the fermentation of lignocellulosic hydrolyzates with oleaginous microorganisms can be used for the production of free fatty acids, fatty alcohols (Metzger, 2006), and fatty acid methyl (or ethyl) esters, which can be further converted to several intermediates for the production of biodiesel, hydroprocessed esters and fatty acids, lubricants (Chowdhury, 2013), and surfactants. However, the commercialization of microbial lipids is still cost-prohibitive. More efficient and integrated processes are necessary for better techno-economics of this process (Gong et al., 2013).

Advanced biohydrocarbons are similar to conventional hydrocarbon fuels such as gasoline, diesel or jet fuels, but are produced from biomass feedstocks. Recently, Amyris engineered the isoprene metabolic pathways in some microorganisms for the production of terpenoids such as farnesene, a 15-carbon branched hydrocarbon that can be chemically hydrogenated to a drop-in diesel or jet fuel component (farnesane).

The major part of the current chemicals-based biorefineries use first-generation sugars but very little second-generation sugars. In Table 2.3, chemicals derived from lignocellulosic material are summarized together with selected process conditions.

2.2.5.1 Chemicals From Glycerol

The production of first-generation biodiesel will continue, particularly in Malaysia and Indonesia. Combining palm oil and biodiesel production would produce refined palm oil, biodiesel, and surplus electricity as primary products, as well as bleaching earth, palm fatty acid distillate (PFAD), and glycerol as byproducts. PFAD is a light brown solid at room temperature, which currently is used for soap and animal feed, but also could serve as feedstock for the oleochemical industry. PFAD also contains a significant amount of extractable vitamin E (Santosa, 2008).

Purified glycerol from biodiesel production is already a marketable product. However, vastly increased biodiesel production could lead to significant glycerol quantities that the market might not be able to absorb without causing a drop in glycerol price. If the most recent biodiesel targets are fulfilled in Indonesia, then the amount of glycerol will increase from 10,000 t in 2009 to 750,000 t in 2016. Vlysidis et al. (2011) conducted a techno-economic analysis of biodiesel refineries. Their results indicate the importance of glycerol as a key building block for the production of commodity chemicals and as feedstock for various fermentation processes (Fig. 2.4).

Another option for the utilization of glycerol is the catalytic dehydration of glycerol to acrolein, a versatile intermediate. Acrolein derivatives, such as acrylic acid, an ingredient for paints and coatings, offer high industrial value. Currently, acrolein is mostly used for the synthesis of methionine. Methionine is a sulfur-containing amino acid that is required as a supplement in animal feed. Methionine has a global annual demand of approximately 850,000 t (Willke, 2014) and an expected annual growth of 5% (Liu et al., 2012).

Food-grade L-methionine, mainly used in human nutrition and medicine, amounts to only 5% of the whole methionine market, but offers a substantially higher margin. Biobased methionine serves the animal feed market in organic farming, in which legislation prohibits or limits the use of fossil-based feed additives (Willke, 2014).

2.2.6 Thermochemical Conversion: Fast Pyrolysis

In addition to the biochemical conversion, thermal conversion for relatively dry, woody materials is a suitable option. Currently, fast pyrolysis is a promising thermal treatment option for converting lignocellulosic biomass into liquid energy carriers or as a drop-in compound in existing refineries.

In fast pyrolysis, biomass decomposes quickly at approximately 475°C in the absence of oxygen, generating primarily condensable vapors and aerosols, as well as smaller amounts of char and noncondensable gases. After cooling and condensation, a dark brown, homogeneous, mobile liquid with a heating value about half that of conventional fuel oil is formed. The essential features of the utilization of fast pyrolysis for producing liquids are:

The main product, bio-oil, is obtained at yields of up to 75 wt% on a dry feed basis, together with byproduct char and noncondensable gas, which can be used within the system to provide the process heat requirements. Flue gas and ash are the only waste products. However, liquid yield depends on biomass, temperature, hot vapor residence time, char separation, and biomass ash content and composition. The latter two have a catalytic effect on vapor cracking at high temperatures.

The basic fast pyrolysis process is depicted in Fig. 2.5. Fig. 2.5 also demonstrates the versatility of fast pyrolysis processes and the most relevant process option at each process stage. The product quality and composition can be influenced through:

2.2.6.2 Pyrolysis Liquid: Bio-Oil

Crude pyrolysis liquid, or bio-oil, is dark brown, viscous, and similar to biomass in elemental composition. Bio-oil is composed of a very complex mixture of oxygenated hydrocarbons with a relatively high proportion of water. It is typically unstable during storage.

Typical organic yields and their variation with temperature for a woody feedstock are shown in Fig. 2.6. Similar results are obtained for most biomass feedstocks, although the maximum yield can occur between 480°C and 520°C, depending on the feedstock. Grasses, for example, tend to give maximum bio-oil yields of around 55–60 wt% on a dry feed basis at the lower end of this temperature range.

Bio-oil is formed by rapidly quenching, and thus “freezing,” the intermediate products of flash degradation of hemicellulose, cellulose, and lignin. The bio-oil thus contains many reactive species, which contribute to its unique attributes. Bio-oil can be considered a microemulsion in which the continuous phase is an aqueous solution of holocellulose decomposition products, that stabilizes the discontinuous phase of pyrolytic lignin macromolecules through mechanisms such as hydrogen bonding. Aging or instability is believed to result from a breakdown in this microemulsion.

2.2.6.4 Applications of Bio-Oil

Bio-oil is primarily used in thermal and/or chemical applications. It can substitute for fuel oil in static applications such as boilers and turbines (Czernik and Bridgwater, 2004). Experiments have also been carried out with stationary diesel engines (Solantausta et al., 1994). Gasification of bio-oil is an ongoing activity at Karlsruhe Institute of Technology, Germany. In their bioliq process, bio-oil from straw pyrolysis is mixed with char to form a slurry which is subsequently gasified in a pressurized entrained flow gasifier. The resulting syngas is cleaned and used for the synthesis of dimethyl-ether, primarily used as transportation fuel (Dahmen et al., 2012).

Fig. 2.7 shows various use options of bio-oil. The only nonenergy application is the production of liquid smoke aroma and flavor enhancer. In the European Community, several companies have applied for product permission in the European market (Theobald et al., 2012). All other chemical applications are being studied at the laboratory or technical scale.