Bioprocessing of Renewable Resources to Commodity Bioproducts

This book provides the vision of a successful biorefinery—the lignocelluloic biomass needs to be efficiently converted to its constituent monomers, comprising mainly of sugars such as glucose, xylose, mannose and arabinose. Accordingly, the first part of the book deals with aspects crucial for the pretreatment and hydrolysis of biomass to give sugars in high yield, as well as the general aspects of bioprocessing technologies which will enable the development of biorefineries through inputs of metabolic engineering, fermentation, downstream processing and formulation. The second part of the book gives the current status and future directions of the biological processes for production of ethanol (a biofuel as well as an important commodity raw material), solvents (butanol, isobutanol, butanediols, propanediols), organic acids (lactic acid, 3-hydroxy propionic acid, fumaric acid, succinic acid and adipic acid), and amino acid (glutamic acid). The commercial production of some of these commodity bioproducts in the near future will have a far reaching effect in realizing our goal of sustainable conversion of these renewable resources and realizing the concept of biorefinery.

Suitable for researchers, practitioners, graduate students and consultants in biochemical/ bioprocess engineering, industrial microbiology, bioprocess technology, metabolic engineering, environmental science and energy, the book offers:

• Exemplifies the application of metabolic engineering approaches for development of microbial cell factories
• Provides a unique perspective to the industry about the scientific problems and their possible solutions in making a bioprocess work for commercial production of commodity bioproducts
• Discusses the processing of renewable resources, such as plant biomass, for  mass production of commodity chemicals and liquid fuels to meet our ever- increasing demands
• Encourages sustainable green technologies for the utilization of renewable resources
• Offers timely solutions to help address the energy problem as non-renewable fossil oil will soon be unavailable

• Language: English
• Publisher: Wiley
• Release date: Apr 7, 2014
• ISBN: 9781118845332

Book preview

Part I

Enabling Processing Technologies

CHAPTER 1

Biorefineries—Concepts for Sustainability

Michael Sauer, Matthias Steiger, and Diethard Mattanovich

Department of Biotechnology, University of Natural Resources and Life Sciences, Vienna, Austria

Austrian Centre of Industrial Biotechnology, Vienna, Austria

Hans Marx

Department of Biotechnology, University of Natural Resources and Life Sciences, Vienna, Austria

1.1 Introduction

1.2 Three Levels for Biomass Use

1.3 The Sustainable Removal of Biomass from the Field is Crucial for a Successful Biorefinery

1.4 Making Order: Classification of Biorefineries

1.5 Quantities of Sustainably Available Biomass

1.6 Quantification of Sustainability

1.7 Starch- and Sugar-Based Biorefinery

1.7.1 Sugar Crop Raffination

1.7.2 Starch Crop Raffination

1.8 Oilseed Crops

1.9 Lignocellulosic Feedstock

1.9.1 Biochemical Biorefinery (Fractionation Biorefinery)

1.9.2 Syngas Biorefinery (Gasification Biorefinery)

1.10 Green Biorefinery

1.11 Microalgae

1.12 Future Prospects—Aiming for Higher Value from Biomass

References

Abstract

Our society is highly dependent on fossil non-renewable resources. Therefore the main driving force in establishing a new industrial system is sustainability. Biorefineries, which are based on renewable biomass, can contribute to such a system. However, many current endeavors focus on single technologies and feedstocks such as starch or vegetable oils that could compete with food or feed. Nevertheless, in future it will be necessary to consider carefully for which purpose land is used to balance the needs of mankind for food and energy. We need to create flexible, zero-waste biorefineries that can accept a variety of low-value local feedstocks. The challenges are the development of efficient processes for the collection, handling, and pretreatment of biomass and for the selective conversion of biomass feedstocks into value-added products.

1.1 Introduction

Sustainability is the capacity to endure through renewal, maintenance, or sustenance. This is in contrast to durability, which is the capacity to endure through unchanging resistance to change. For humans in eco (and social) systems, sustainability is based on long-term maintenance of responsibility. In other words, as the Brundtland Commission of the United Nations (1987) has coined it: sustainable development is development that meets the needs of the present without compromising the ability of future generations to meet their own needs.

For true sustainability this includes not only environmental, but also economic and social dimensions of resource use. All along the history access and availability of resources was a main driving force for the development of societies. But never in history was the demand for resources as high as it is now. Unfortunately, the development in the last 100 years was hardly sustainable because our energy demand is currently met mainly by fossil resources. While obviously based on natural carbon, the recycle time for the replenishment of fossil resources (estimated to be 280 million years; Liu et al., 2012a) is so high that they can be regarded as nonrenewable in context with human life. Use of this nonrenewable carbon sources is connected with two major problems: the first problem is the obvious limitation of these resources. Even avoiding any discussion about the time scale, the limitation of fossil resources is a fact due to the immense recycle time. This implies rising costs for energy and goods followed by increasing conflicts for access and distribution. The second problem inherently connected with prolonged use of fossil resources is the liberation of carbon dioxide—a greenhouse gas, which has been sequestered to the ground in ancient times. Its liberation is connected to various side effects, which shall not be discussed here in detail. However, we consider it commonly accepted that major pollution of the earth's atmosphere with greenhouse gasses is something unwanted, which we should strive to avoid for the sake of sustainability.

In this sense, CO2 and compounds, which are obtainable from atmospheric CO2, are the only renewable and thereby truly sustainable carbon sources. At present, this boils down to photosynthetic organisms as plants, algae, or cyanobacteria as basis for the production of carbon-containing goods, be it chemicals or fuels. In fact, the development of humanity started with the exclusive use of biomass as source for food, energy, and all goods. However, fossil resources have overtaken the role as a dominant energy and chemical source since the industrial revolution (Liu et al., 2012a).

Currently, biomass-derived energy sources supply about 50 EJ (exajoules) of the world's energy. The global energy demand was 463 EJ in 2005 and is supposed to increase to 691 EJ in 2030 (Lal, 2010). About 10% of the global primary energy consumption per year is based on biomass; this corresponds to 75% of the energy derived from alternative renewable energy sources (Haberl, 2010). Only 2% of the biomass-derived energy sources are utilized in the transportation sector. The rest is consumed for household uses predominantly as firewood (Srirangan et al., 2012). Model calculations suggest that a significant fraction of the energy demand could be met by the use of biomass. The World Energy Council and World Energy Assessment project estimates that bioenergy could supply a maximum of 250–450 EJ/year (probably a quarter of the global energy demand) by the year 2050 (Ragauskas et al., 2006a,b).

Biomass production by photosynthesis obtains its energy from the sun and its carbon from atmospheric CO2. It is therefore a truly renewable source that can serve as food and feed, chemical and material, fuel and energy resource. A broad and valuable product mix can be created starting from biomass. The valorization possibilities in a biorefinery are at least as big as in fossil refineries. However, while the diversity of options allows a large range of configurations, this also implies different environmental and societal consequences or footprints. Decisions taken, regarding the valorization of biomass, should always take at least two critical points into account: the market impact (demands for products, possible displacements of products) and the ecology of the entire production chain (De Meester et al., 2011). Ideally, these decisions are well-thought-out and based on sound assessments to achieve an optimal sustainable development in the post fossil era.

1.2 Three Levels for Biomass Use

Biomass can be used on three different levels:

for food and feed,

for (bio-based) products, and

for (bio-) energy.

Food and feed inevitably rely on biomass. Materials and products are right now produced from fossil resources and biomass. If fossil resources are to be avoided or if they are depleted, biomass remains the only other basis for material production. For heat and power generation a variety of choices exist or are under development, such as solar power, wind energy, or geothermal energy. However, liquid fuels for transportation rely on fixed carbon, which again points to fossil resources or biomass.

It is obvious that limited land availability makes it unlikely that biomass will be able to cover all of these demands fully (Ponton, 2009, De Meester et al., 2011). Figure 1.1 outlines the trilemma. Maximal valorization of biomass is therefore a key issue in the future.

Figure 1.1 Food security, energy security, and climate change are centered around the limited availability of arable land. This constitutes a trilemma, which has to be addressed by our societies.

This means that no biomass fraction should be considered as waste. From an economic perspective this is a major opportunity for biorefineries. From a societal perspective this appears as the major challenge of the future. However, focusing on the question of food or fuel appears not helpful (Karp and Richter, 2011). The pertinent challenge is how the increasing demands for food and energy can be met in the future, particularly when water and land availability will be limited and considering that food production requires significant fuel inputs, which are constantly increasing with intensification of agriculture. Food security has once again risen to the top of government agendas. Nevertheless, energy security is arguably an equally important challenge impacting food security and climate change.

Exemplary, in the United Kingdom, agriculture accounts for only 2% of energy use. However, almost 20% of United Kingdom's total energy consumption is used throughout the whole food production chain (Barling et al., 2008). Consequently, rising fuel prices or fuel shortages have a significant impact on the cost of food production. Decisions over land use should therefore be considered within the context of the bigger framework of all the challenges that lie ahead (Karp and Richter, 2011).

Carbon efficiency and energy efficiency are key parameters that should be taken into account for such decisions. As mentioned before—carbon can be obtained from fossil resources and biomass, while a variety of energy sources are conceivable including wind, photovoltaic, photothermic, geothermic, or hydro power. The energy mixture of the future will be diverse and will also contain a substantial amount of energy from biomass, but energy should be seen in this context more as a byproduct of the biorefinery than the main driving force. (This is different for the time being.)

One reason for this consideration is the inherent energy content of biomass: essentially the energy stored in biomass is a chemically captured form of solar energy. Energy from biomass can therefore be directly compared with energy obtained from photovoltaic systems. Blankenship et al. (2011) recently reviewed the energy efficiency of both technologies. They showed that a photovoltaic system coupled with hydrogen generation might capture up to 10–11% of the total solar energy per used area. In contrast, the solar energy conversion efficiencies of conventional crop plants usually do not exceed 1% (Blankenship et al., 2011). Only for microalgae grown in bioreactors, yields up to 3% are reported. These low efficiencies require a cautious consideration of biomass as a mere energy product, and strengthen the importance of material products obtained from biorefineries as a primary goal.

1.3 The Sustainable Removal of Biomass from the Field is Crucial for a Successful Biorefinery

The key factor for sustainability of currently cultivated biomass is their resource footprint. This concerns not only the direct land use and transformation and amongst others the related use of fertilizers, pesticides, fuels, and water for farming, but also the mineral balance and quality of the soil (Cherubini, 2010a).

This means that the production chain for biomass-derived goods is more demanding than its fossil equivalent. In fact, a variety of studies suggest that the agricultural phase is often the main contributor to the environmental impact of the production chain of bio-based products (Zah et al., 2007). Sustainable production of biomass is therefore of utmost importance. However, since the demand for bio-feedstock is increasing, while the arable land remains limited, much emphasis is given to higher agricultural yields. However, innovations for yield increase are often not focused on the simultaneous acceleration of environmental protection (Cassmann and Liska, 2007). Often, a blind striving for higher yields tends to cause severe damage to the natural environment. One example is the use of field crop residues. These residues are often seen as a renewable resource, which is freely available. In reality such residues are often required to enhance soil quality and to prevent erosion and nutrient depletion. Using this part of biomass might thus actually turn out to be a very bad choice in the long term (Reijnders, 2006).

Summing up, it is the production and supply of biomass rather than the demand for fuel or materials which limits the use of biomass as a renewable resource. In this context it is important to note that chemical production requires far lower amounts of carbon than fuel production. For example, in the United States, the chemical products segment consumed just over 3% of the total US petroleum consumption in 2007 (FitzPatrick et al., 2010). This opens an economic opportunity for the development of bio-sourced chemical products since the value of the chemical industry is comparable to the fuel industry, but requires only a fraction of the biomass (FitzPatrick et al., 2010).

So while the current industrial systems are split into three sectors namely food, bioenergy, and the chemical industry, these three sectors should come together and strive to valorize the used feedstock to the fullest to obtain the lowest resource footprint per (combined) output product(s).

The biorefinery approach is the promising concept, ideally combining the production of food, materials, and energy from biomass. Following the International Energy Association's (IEA) definition, a biorefinery is the sustainable processing of biomass into a spectrum of marketable products (food, feed, materials, chemicals) and energy (fuels, power, heat) (De Jong et al., 2009).

The core technologies of a biorefinery are biochemical, microbial, and thermochemical processing. Energy and waste streams are internally recycled. Biochemical processes have the advantage of high selectivity at low processing temperatures. However, they normally require elaborate preprocessing stages and long processing times (FitzPatrick et al., 2010; Macrelli et al., 2012). Complementary, thermochemical routes include gasification, pyrolysis, and direct combustion to produce oils and gases. These are fast but nonspecific and generally require a high energy input. Biochemical/microbial and thermochemical processing complement each other. In an integrated system they can deliver significant advantages in terms of specificity of products, flexibility, and efficiency.

The biorefinery concept that has emerged is analogous to today's petroleum refineries. However, many current endeavors focus on single technologies and feedstock such as starch or vegetable oils that could compete with food or feed. We need to create flexible, zero-waste biorefineries that can accept a variety of low-value local feedstock. Biorefineries will then be able to compete with existing industries (Clark et al., 2012). Further down the value chain the development of green chemistry fills the gap between the sustainable resource and the product (Poliakoff and License, 2007).

1.4 Making Order: Classification of Biorefineries

A variety of classifications of biorefineries have been proposed: some of them consider feedstock, products, or processes. A very simple overview and classification of biorefineries subdivides them into three types: Phase I, Phase II, and Phase III (Clark et al., 2012).

Phase I biorefineries are integrated facilities limited to a single feedstock (e.g., corn or oils) which is converted into a single major product (e.g., ethanol or biodiesel).

Phase II biorefineries produce various end products from a single feedstock. They might be more flexible depending on product demand, prices, or others. An example is a biorefinery generating multiple products, ranging from sugar to ethanol, polymer precursors to animal feed, by utilizing sugar beet as the single feedstock. Exemplary, a biorefinery in Pomacle, France produces both ethanol and succinic acid in addition to beet sugar and glucose from a single facility with many processing streams (Hatti-Kaul et al., 2007; Le Henaff and Huc, 2008).

Finally, Phase III biorefineries are the most advanced, as they use a variety of biomass feedstock to yield a mix of products (Figure 1.2). Such biorefineries employ a combination of technologies, among them are chemical and/or biological transformations, extractions, and separations. Examples for Phase III biorefineries include whole-crop biorefineries encompassing an array of transformations of feedstock (e.g., corn, or rapeseed). The most promising type of Phase III biorefineries are based on lignocellulosic feedstock (e.g., wood, corn stover/cobs) to produce chemicals, fuels, energy, and other valuable outputs. Lignocellulosic biorefineries can be subclassified into various intermediate concepts based on the employed processes. This includes thermochemical biorefineries such as syngas platforms, or biochemical or microbial biorefineries such as sugar platforms. Sustainability is in the long run only obtainable with Phase III biorefineries. They are expected to expand the range and volume of bioproducts on the market as well as to improve the economics of biorefinery plants. At the same time the expectation is that they optimize the energy and environmental performance and enhance the cost competitiveness of bio-derived products. However, the development of such advanced integrated biorefineries is still ongoing.

Figure 1.2 Schematic overview about the processing strategy of different feedstock used in biorefineries and the various products obtained. Processes leading to an energy product are shown as dashed lines. Fermentation sludge is the microbial biomass produced during the bioconversion processes.

More precise and detailed classifications of biorefineries rely on four main features: platforms, products, feedstock, and processes (Clark et al., 2012).

Platforms are defined as key intermediates between raw material and final products. They are considered as particularly relevant as these can be used to link different biorefinery concepts. Typical platforms are sugars (C6 and/or C5), lignin, syngas, or pyrolysis oils.

In terms of products, biorefineries can be broadly grouped into energy-driven and product-driven biorefineries. The main goal of energy-driven biorefineries is the production of one or more energy carriers (fuels, power, and/or heat) from biomass. The economic profitability of the plant is subsequently maximized by an upgrade and valorization of process residues. On the other hand, product-driven biorefineries are dedicated to the generation of one or more bio-based products. The economic profitability is maximized by production of bioenergy from process residues.

Biorefineries can be further classified based on their feedstock. For example, the feedstock can be subdivided into the main classes according to their origin, such as agriculture, forestry, industries, households, and aquaculture (Cherubini, 2010b).

Clearly, the feedstock for biorefineries will change further with the ongoing technical developments. First-generation biorefineries are based on edible feedstock from the agricultural sector (Srirangan et al., 2012), such as sugar and starch. Second-generation biorefineries are based on non-edible feedstock, comprising raw material derived from lignocellulosic biomass and crop waste residues from various agricultural and forestry processes (Nigam and Singh, 2011; Srirangan et al., 2012). Third-generation biorefineries make direct use of photosynthetic bacteria and algae, which can be cultivated in bioreactors and are thus independent of crop land (Nigam and Singh, 2011).

Processes employed in biorefinery concepts to convert biomass feedstock into marketable products include biochemical (e.g., anaerobic digestion, microbial fermentation, enzymatic conversion), chemical (e.g., hydrolysis, transesterification, hydrogenation, oxidation), and thermochemical (e.g., pyrolysis, gasification) processes.

1.5 Quantities of Sustainably Available Biomass

The actual availability of the resources is the basic question to answer, when taking decisions about which resources to use. However, the sustainable and usable amount of biomass, which is present or possibly present in the future, is difficult to assess. Here we would like to give an overview about some numbers, particularly the orders of magnitude.

Nature produces over 200 billion tons of biomass by photosynthesis each year (Tschan et al., 2012). Other models estimate the global net primary production (NPP) to yield between 77.5 billion tons (Dhillon and von Wuehlisch, 2013) to over 100 billion tons (Field et al., 1998) of fixed carbon per year. The contribution of the marine environment to the global NPP is estimated to be between 31.8% up to about 50%. The standing biomass, however, is very distinct; more than 99% of the carbon fixed in biomass is estimated to be on land and less than 1% is found in marine environments (Dhillon and von Wuehlisch, 2013). Woody biomass is regarded the most abundant organic source on earth—around 90% of the carbon in standing biomass (Field et al., 1998; Liu, 2012b; Dhillon and von Wuehlisch, 2013).

Generally speaking, about 75% of the total biomass produced belongs to the class of carbohydrates. However, only 3.5% of these compounds are actually used by mankind (Tschan et al. 2012). Clearly, the theoretical availability of biomass does not mean that it is economically feasible or environmentally viable to collect it for industrial use. Parikka (2004) estimated the sustainable worldwide biomass energy potential to be about 100 EJ/year. Only 40% of this biomass is currently used according to Parikka (2004).

Current global bio-based chemical and polymer production (excluding biofuels) is estimated to be around 50 million tonnes ( De Jong et al., 2012). The global petrochemical production of chemicals and polymers is estimated at around 330 million tonnes ( De Jong et al., 2012).

Figure 1.3, taken from Vennestrøm et al. (2011) compares the total US oil consumption with harvested non-food biomass on a weight basis. On a weight basis biomass has a lower energy and carbon density than crude oil. In fact, oil contains about twice the amount of carbon atoms and chemically stored energy as biomass. What becomes clear from this figure is that the orders of magnitude correspond to each other; however, with an increasing use of biomass in industry at some point biomass can become a scarce resource with increasing prizes. Feedstock which are very cheap at the time might become expensive when their industrial use is carried out on a scale that is comparable to that of current petrochemical processes. Large-scale use of biomass as feedstock will drastically alter the market. Thus, for long-term planning, the mature market must be considered instead of the current market, which is by no means a simple task. Anyhow, biomass has the potential to fully substitute petrol as carbon source for the chemical market, if the processes to produce and use them are sufficiently efficient.

Figure 1.3 Total US oil consumption compared to potential and currently harvested nonfood biomass divided into its main uses. The area of each circle is proportional to the consumed amount. From Vennestrøm et al. (2011).

1.6 Quantification of Sustainability

In the development of sustainable industries, researchers are challenged to find innovative technical solutions without losing sight of the economic, societal, and environmental impacts of their work (Jenkins and Alles, 2011). Scientific and quantifiable methods are needed to guide research and industry into the right direction. Sustainability assessments aimed at quantifying the economic, environmental, and societal impacts can help to move debates and decision finding to a factual level. Science-based methods such as life-cycle analysis (LCA), defined as a holistic approach to quantify environmental impacts throughout the value chain of a product (International Organization for Standardization, 2006), can be applied as a decision support tool. With LCA the feedstock selection for defined products and the decision of how to produce the feedstock and the product are set on a factual basis.

For the feedstock, there are logistical and sustainability concerns. Each potential biorefinery concept has specific coproduct and waste issues to consider. Transport is a general issue in this discussion. The biomass resource has to be transported to the refinery; subsequently the products have to be transported to the downstream industry and/or the consumer. Of interest is the approach of the company Nature-Works LLC that currently operates the largest biorefinery in the United States in Blair, Nebraska. The nameplate capacity of the polymer production plant is 140,000 tons of polymer per year. Corn is the basis for the production of the bioplastic polylactic acid (PLA) in a complex multistage process. Sixty percent of its corn feedstock is obtained from the local area (producers, located less than 40 kilometers from the plant). Several companies in an emergent network are now active on the Blair biorefinery campus (Wells and Zapata, 2012) reducing transportation from one industrial branch to the next one.

1.7 Starch- and Sugar-Based Biorefinery

Starch- and sugar-containing crops are quantitatively the most important products of today's agricultural system. Most of the existing biorefinery concepts are based on these plants and they are referred to as first-generation feedstock (vide supra), but they also constitute the backbone of human nutrition. General characteristics of this type of biorefinery are listed in Table 1.1.

Table 1.1 Characteristics of the Starch and Sugar Biorefinery

The polysaccharide starch is found in most plants as a storage compound; however, only five plants, namely maize, rice, wheat, potatoes, and cassava account for the majority of worldwide produced starch-containing plants. Roughly 2.7 billion tons of these crops are annually harvested. The class of sugar-containing crops contains only two plants namely sugar cane and sugar beet of which about 2 billion tons are annually harvested. The shares of each plant in the overall worldwide production are shown in Figure 1.4. From this graph, it can be depicted that the overall annual production of these crops is rising. However, the future growth rate will depend on three main parameters: land, fertilizer, and plant productivity. The production of nitrogen fertilizers requires a high energy input. This directly connects energy price with crop price.

Figure 1.4 Worldwide production of the main sugar- and starch-containing crops. Data taken from the Food and Agriculture Organization of the United Nations (www.fao.org).

The starch and sugar processing industry is already highly developed and has the technology to readily deal with the conventional crops of today's agriculture. Its major limitation can be seen in the circumstance that only a part of the starch- or sugar-containing plant is implemented in the biorefinery approach. In the future, the whole crop including the stover has to be taken into account in order to develop a whole crop biorefinery concept (Kamm et al., 2006), keeping in mind that only a part can be used in a sustainable way.

The primary biorefinery products of the sugar and starch industry are glucose, fructose, gluconate, and bioethanol (Wagemann et al., 2012). At present, bioethanol is a fast-growing biorefinery energy product. Both in the United States and Brazil, bioethanol is produced in high quantities in order to substitute the dependence on fossil fuels in the transportation sector. In both cases, the biorefinery process can be structured in three steps: (1) obtainment of a solution of fermentable sugars; (2) bioconversion of sugars to ethanol; (3) ethanol separation and purification (Mussatto et al., 2010). Sugarcane is used as the main feedstock in Brazil whereas the majority of the bioethanol in the United States is produced from maize. More than 35% of harvested maize grain is used for bioethanol production in the United States (Perlack and Stokes, 2011) and by 2011, 52 million liters of ethanol were produced annually (Jerck et al., 2012). Two other important crops which may be used for biofuel production are cassava and sorghum. Cassava is grown as an annual crop in the tropical and subtropical countries and has the advantage that it is compatible with current corn ethanol technologies. Sorghum is a good alternative feedstock for dry regions, because of its lower water requirements compared to maize and sugarcane (Srirangan et al., 2012).

However, bioethanol is not the only biorefinery product which can be obtained from starch- and sugar-containing crops. Microorganisms can directly use sugars as a substrate and convert them to virtually any product. Today products like organic acids (e.g., citric acid, gluconic acid) and amino acids (e.g., glutamate, lysine) are already produced from sugars in high amounts and are not dependent on fossil resources as a substrate.

1.7.1 Sugar Crop Raffination

After harvesting, the main crop is further treated by a crushing and milling step that yields the sugar juice. As a by-product the insoluble lignocellulosic material of the plant is obtained. In case of sugar cane this residue is referred to as sugarcane bagasse. This feedstock is currently used (by burning) as an energy resource to drive the thermal requirements of the sugar plant. However, it is also considered a valuable feedstock for lignocellulosic biorefinery approaches (Dawson and Boopthay, 2008; Cherubini and Strømann 2011; Nigam and Singh, 2011; Macrelli et al., 2012). The sugar juice obtained can then either be directly used by the fermentation industry as a substrate or sugar is crystallized stepwise by water evaporation. The crystallized sugar can be used for various purposes including human nutrition and fermentation processes, if higher substrate purity is required.

1.7.2 Starch Crop Raffination

In a first milling step, the crop is broken up. In case of maize, a previous steeping step at 50°C enables a high yield and good starch quality. Starch is extracted with water and separated from the insoluble fibers, which are a potential feedstock for a lignocellulosic biorefinery. In successive steps the protein fraction is separated from the starch by either protein coagulation (heat or acid treatment) or centrifugation utilizing density differences. Starch can be readily hydrolyzed to fermentable glucose by means of amylases.

The processing of starch- and sugar-containing plants has a very long history and is directly connected to the main function of these crops as nutritional products. The highest potential for new biorefinery concepts based on sugar- and starch-containing crops can be expected in the various side products of this industry starting with plant residues, which are already separated from the crop on the field, and residues from milling and further processing steps. Those product streams often yield only a low value and are currently used as animal feed or for thermal processing (Nitayavardhana and Khanal, 2012). For example, current flour mills operate at 70–80% grain-to-flour yields. Various waste and by-product streams include bran, germ, and endosperm. These by-products contain a high proportion of starch (25–30%) that could be used for microbial bioconversion to produce valuable chemicals (Clark et al., 2012).

1.8 Oilseed Crops

Plants like soybean, sunflower, rapeseed, peanut, oil palm, and coconut contain a high fraction of lipids and are referred to as oilseed crops. Vegetable oils have a long tradition as edible oils and are of growing interest for the biofuel industry. Over the last 50 years the production of oilseed crops increased dramatically from around 100 million tons to over 800 million tons per year (Figure 1.5). Especially, the cultivation of oil palms and soybeans was significantly enlarged.

Figure 1.5 Development of the worldwide production of the major oil seed crops over the last 50 years normalized against the increase of the human population and compared to the development of cereals. Data taken from the Food and Agriculture Organization of the United Nations (www.fao.org).

General characteristics of this type of biorefinery are listed in Table 1.2

Table 1.2 Characteristics of the Oil Crop Biorefinery

Besides the application as nutritional product, those plants have an important application for production of biofuels also. The triglycerides can be modified by a transesterification reaction with short-chain alcohols to produce alkyl esters, mainly methyl and ethyl esters. The product obtained is referred to as biodiesel. In that case, a fundamental concept of biorefineries was neglected, which requires a suitable application for all byproducts because during the transesterification reaction glycerol is obtained as a byproduct in high amounts. The production of 10 tons of biodiesel generates 1 ton of glycerol. Its price decreases with increased biodiesel production. Therefore, new biorefinery concepts need to be developed to convert glycerol to an added-value product like 1,2- or 1,3-propanediol, acrolein, or lactic acid (Pflügl et al., 2012; Posada et al., 2012).

The situation for oil crop plants is comparable with the starch and sugar processing industry. The technology to obtain primary products like vegetable oils and biofuels is already established and is commercially applied. However, secondary refinery streams like plant residues, seed cakes of pressing and filtration steps, or process byproducts like the above-mentioned glycerol need to be considered in future. Processes need to be developed to add additional value to those products. Furthermore, waste streams like cooking oil can be fed back into the biodiesel production pipeline (Wang et al., 2007).

1.9 Lignocellulosic Feedstock

The current use of lignocellulosic biomass is primarily the use of wood for combustion, construction, and furniture making; cellulose fibers are used for pulp and paper making or clothing (Möller et al., 2007). The global production of forest products has been estimated by the Food and Agriculture Organization of the United Nations (FAO) to be 3469 million m³ of round wood in 2011, where roughly 50% are used as wood fuel and 50% for industrial use. Furthermore, 406 million m³ are produced as sawn wood, 288 million m³ are wood-based panels, wood pulp accounts for 173 million tons, paper and paperboard production has been 403 million tons, and 211 million tons are produced from recovered paper. However, not only wood can serve as feedstock for lignocellulose biorefineries but residues from agriculture can also be taken into account. The lignocellulose feedstock report from the EPOBIO project (Möller et al., 2007) states that the most abundant agricultural residue in Europe is wheat straw, rice straw in Asia, and corn stover in North America.

As the systematic cultivation of crops for biorefinery purposes is gaining more importance the question arises as to which crops are the most desirable ones. To select the best possible feedstock several criteria have to be considered. From an economic point of view the crop has to have a very high biomass yield, low requirement for fertilizers and pesticides, the ability to grow on marginal lands, and the cell wall structure should allow an easy access for bioconversion methods. In addition the crop should also cover environmental criteria such as low impact on biodiversity, water and soil quality, low greenhouse gas emission, and high carbon sequestration. Based on these criteria Möller et al. (2007) selected four candidates, namely poplar, willow, Miscanthus, and wheat straw for the region of the European Union.

Compared to the availability of fossil resources, areas for biomass cultivation are globally more evenly distributed, thereby enhancing the security of supply. Biorefineries utilizing lignocellulosic feedstock may even help to a certain extent to combat the unemployment status of rural areas (Menon and Rao, 2012).

1.9.1 Biochemical Biorefinery (Fractionation Biorefinery)

The term lignocellulosic biomass describes the material that constitutes the plant cell wall. This includes primarily cellulose (30–50%), hemicellulose (15–35%), and lignin (10–30%). As a result of the organization and interaction between these polymeric structures, the plant cell wall is naturally recalcitrant to biological degradation (Himmel et al., 2007).

The lignocellulose biorefinery is one of the most desirable forms of a biorefinery. As mentioned earlier, a Phase III type biorefinery uses virtually any lignocellulosic feedstock like wood, corn stover/cobs, straw, bagasse, and other lignocellulose-rich waste streams from agriculture, forestry, and municipal areas. The accessibility of the desired fermentable sugars is severely hindered by the assembly of the lignocellulosic biomass itself; therefore, a pretreatment by milling and grinding followed by a treatment with high temperature and pressure is required to access the fibers composed of fermentable mono- and oligosaccharides. The addition of mild or harsh acids, bases, or organic substances can further enhance the pulping. Depending on the particle size and the composition of the biomass that is delivered to the biorefinery, a suitable flow chart of pretreatment steps has to be established. Pretreatment techniques can be categorized into physical (milling, irradiation, and extrusion), physicochemical (steam explosion, ammonia fiber explosion, ammonia recycle percolation, microwave chemical, and liquid hot-water pretreatment), chemical (acid, alkaline, green solvents), and biological processes. The pretreatment gives rise to a solid or fluid stream of the three biomass main components: cellulose, hemicellulose, and lignin. These streams are further subjected to enzymatic hydrolysis to gain sugars for microbial fermentation. Menon and Rao (2012) conclude that the choice of pretreatment should consider the overall compatibility of feedstock, enzymes, and organisms to be applied. A more detailed description of pretreatment processes and lignocellulose hydrolysis is given in the following chapters of this book. The enzymatic conversion of cellulose and hemicellulose to fermentable sugars opens the possibility for the microbial conversion to chemical building blocks for the synthesis of bio-based materials or the conversion to biofuels for transport or energy purposes. Saccharification of cellulose and hemicellulose to platform compounds D-glucose (from cellulose), D-xylose, L-arabinose, D-mannose, D-glucose, D-galactose, and D-glucuronic acid (from hemicellulose) involve a series of hydrolytic enzymes or enzyme complexes to which Chapters 4 and 5 in this book are dedicated.

Platform compounds are further converted to products by microbial fermentation processes. Saccharification and fermentation can be accomplished in a sequential process by separate hydrolysis and fermentation (SHF), or in a consolidated one-pot process known as simultaneous saccharification and fermentation (SSF) of single sugars or simultaneous saccharification and co-fermentation (SSCF) of all monosaccharides. Future developments might even combine the production of saccharolytic enzymes, the hydrolysis of cellulose, and hemicellulose to monomeric sugars and the fermentation of hexose and pentose sugars in a single process, the so-called consolidated biomass processing (CBP) (Menon and Rao 2012).

The most prominent product in the biorefineries nowadays is bioethanol with a global production volume over 20 billion gallons (or 75 billion liters) in 2011 (Alternative Fuels Data Center, 2013). Huge efforts in research and development are on the way to broaden the product portfolio of the biorefineries in the future. Screening for new production strains in nature, or microorganisms accessible for metabolic engineering for directed biosynthesis of bioproducts, or a combination of both approaches will achieve this aim. Part II of this book is dedicated to specific biocommodity products from biorefineries and future aspects for the production of those.

The remaining lignin that is separated from the sugar stream during the pretreatment process can be used for the production of heat and power, which is done very frequently at the moment; nevertheless, it could be used in the future as a source of various aromatic compounds. The effective utilization of all the three components would play a significant role in the economic viability of an integrated biorefinery. General characteristics of this type of biorefinery are listed in Table 1.3.

Table 1.3 Characteristics of the Biochemical Biorefinery

Source: Adapted from Wagemann et al. (2012).

To evaluate the sustainability concerning economic, environmental, and social aspects the Department of Energy (DOE) is funding integrated biorefinery projects in pilot, demonstration, and commercial scale. The importance of biochemical conversion is underlined by the fact that four out of five commercial scale plants funded by the DOE are based on biochemical conversion (Department of Energy, 2012).

1.9.2 Syngas Biorefinery (Gasification Biorefinery)

Thermochemical conversion of biomass can be accomplished in three ways that differ in the amount of oxygen that is supplied to the process. By the supply of excess air the biomass is combusted to generate heat and power. When oxygen is depleted from the process, the biomass undergoes pyrolysis or liquefaction. In the liquefaction biomass is decomposed into small molecules which then polymerize to oily compounds. Liquids gained from pyrolysis or hydrothermal liquefaction can be further refined to gasoline, diesel fuel, jet fuel, or chemicals. Syngas is produced by gasification by means of partial combustion of biomass, where only a limited amount of oxygen is supplied to the process (Demirbas, 2010). Depending on the type of gasifier and biomass used for the gasification (at temperatures of 750–800°C) a mixture of carbon monoxide (CO), hydrogen (H2), methane (CH4), nitrogen (N2), carbon dioxide (CO2), and some higher hydrocarbons in varying amounts are generated. The main constituents of syngas are H2 (5–24%) and CO (14–67%) (Munasinghe and Khanal, 2010).

The H2 and CO from the syngas can be converted via the Fischer–Tropsch process to long-chain hydrocarbons catalyzed by cobalt or iron. Before the Fischer–Tropsch synthesis (FTS) the syngas has to be cleaned and conditioned. The FTS can be operated at low temperatures to produce heavy, waxy hydrocarbons or at higher temperatures to produce olefins. By further product upgrading the array of products from FTS ranges from diesel, gasoline, methane, ethane, and to light and heavy waxes (Demirbas, 2010).

The microbial conversion of syngas offers some interesting opportunities for the future production of biofuels and biochemicals as well. According to Munasinghe and Khanal (2010), the merits over biochemical biorefinery approaches are the eliminations of costly pretreatment steps and enzymes, as well as the usage of all fractions from biomass including the lignin part. Compared to the Fischer–Tropsch process microbial catalysts have a much higher specificity and the ratio of H2:CO is of minor importance.

Possible products from syngas fermentation can be ethanol, butanol, lactate, acetate, pyruvate, and butyrate. One of the most important factors in syngas fermentation is the microbial catalyst itself (anaerobic bacteria from the genera of Clostridium, Acetobacterium, Butyribacterium). The efficient conversion of syngas by the microbe can be negatively influenced by impurities like ethylene, ethane, sulfur, and nitrogen-containing gases as well as solid particles of tar, ash, and char. So these impurities have to be avoided by the appropriate choice of gasifier or the syngas has to be cleaned from these impurities before fermentation. The fermentation process is influenced by parameters such as pH (depending on the microorganism, optima are between 5.5 and 7.5), temperature (mesophilic organisms 37–40°C or thermophilic organisms 55–80°C), gas-to-liquid mass transfer in combination with reactor type (stirred tank, bubble column, membrane-based systems), and growth media (depending on the microorganism used). Despite the mass transfer limitations and the quality of the syngas Munasinghe and Khanal (2010) recommend, for future development of syngas fermentation, the genetic modification of existing syngas-fermenting microbes to high yield strains especially for solvent production, where the pathways to acid production have to be blocked. General characteristics of this type of biorefinery are listed in Table 1.4.

Table 1.4 Characteristics of the Syngas Biorefinery

Source: Adapted from Wagemann et al. (2012).

1.10 Green Biorefinery

In contrast to lignocellulose-feedstock biorefineries, where the composite of lignin, cellulose, and hemicellulose are very strong, the green biorefinery uses green biomass such as grasses, green crops like lucerne, clover, and green cereals. By wet fractionation, a fiber-rich press cake and a nutrient-rich green juice is obtained. The dried press cake can serve as fodder, as a raw material for hydrocarbons and chemicals or it can serve as a raw material for syngas production. By separation enzymes, dyes, flavorings, carbohydrates, and proteins can be recovered from the press juice. The press juice can also serve as a feedstock for fermentation where the fermentation broth is a source of lactic acid, amino acids, ethanol, and proteins (Kamm and Kamm, 2004).

1.11 Microalgae

Microalgae constitute a further source of industrially usable carbon fixed by photosynthesis. They offer a great potential for exploitation, such as biodiesel production, due to their oil content that can exceed 80% w/w (Amaro et al., 2011). Some possible advantages connected to microalgae as feedstock include that their cultivation is not linked to arable land (it is not linked to land at all, as off-shore cultivation is conceivable); they can grow in brackish or salty water and their efficiency in terms of energy use per hectare is potentially high. Following the classical concept of biorefineries, they are interesting because apart from their potential as oil for biodiesel producer, a variety of by-products are accumulated. These by-products include valuable omega-3-fatty acids, recombinant proteins, and algal meal containing high amounts of proteins (Subhadra and Grinson-George, 2011). The direct by-product of biodiesel production from oil is glycerol that can be used to grow more algae or which can be converted to higher-valued chemicals such as 1,3-propanediol.

The high oil productivity of microalgae cultures and the possible absence of competition for arable land and water resources justify the currently high investments into such projects. Nevertheless, care has to be taken with the evaluation of possible productivities. Various studies seem to claim unrealistically high numbers for the microalgal oil production potential. Decades of worldwide research have demonstrated that annual productivities beyond 100 tons of algal biomass per hectare appear not attainable at large scale, at least not with current strains and current technologies (Rodolfi et al., 2009). Thus, even under the best conditions a realistic oil yield will not exceed 40 tons per hectare per year. This compares to about 1000 liters of oil per hectare per year which can be typically produced by sunflower or rapeseed, and 6000 liters per hectare per year obtained with oil palms (Chisti, 2007).

Nevertheless, full commercialization of biodiesel from algae oil has not been realized yet. Till now the cost of algal biomass production of about US$5/kg is simply not compatible with the low costs required for biofuel production. One major problem connected to microalgae as oil resource is the large scale cultivation that has to be guaranteed the availability of light and CO2 at high cell density. The energy demand for mixing and pumping is very high. Light does not penetrate more than a few centimeters into a dense culture of algal cells, so scale-up depends on an increase of surface area and not volume as is the case for heterotrophic fermentations (Scott et al., 2010). The costs of scale-up are much debated—estimates of production and capital costs vary widely. This points to the current painful lack of data from real-life demonstrations. There is a pressing need to conduct pilot studies at realistic scale to assess productivities likely to be achieved in practice.

Further drawbacks hindering the large-scale use of microalgae as resource is the enormous water content of the harvested biomass and the intracellular localization of the desired oil. Drying and oil extraction are very costly, particularly when environmentally benign technologies are applied and sustainability is an aim of the endeavor (Singh and Olsen, 2011). Lardon et al. (2009) exemplarily calculated that 1 MJ of energy in biodiesel from Chlorella vulgaris required an energy input of 1.66 MJ for production. Use of the algal biomass for energy generation could turn the balance to the positive side; however, this shows the significance of technological advancement before industrial exploitation of microalgae. An important step regarding the downstream processing is that it should allow the product generation without drying the biomass. For example, Levine et al. (2010) have developed a biodiesel production process starting from wet algal biomass with 80% moisture.

It is out of question that algal biomass can be utilized for the production of various bioproducts. However, significant improvements in the efficiency, cost structure and ability to scale-up algal growth, and lipid extraction are required to establish a commercially viable microalgae-based biorefinery.

1.12 Future Prospects—Aiming for Higher Value from Biomass

The concepts for biomass valorization are manifold. Some existing examples for biochemical products from bio-derived resources are summarized in Table 1.5 (Vennestrøm et al., 2011). In summary, an upgrading of biomass to higher-value products is a reasonable approach to replace crude oil. For electrical (on-grid) energy production alternative sources are simply conceivable. It is liquid transportation fuels where most problems occur for the judgment if they can or should be replaced by biomass-derived products or not.

Table 1.5 Overview of Chemicals That Are Currently Produced, or Could Be Produced, From Biomass Together With Their Respective Market Type, Size of the Market, and Potential Biomass Feedstock

Source:: Vennestrøm et al. (2011).

aMarket size of an existing market is given as its current size including production from fossil resources; for emerging markets the expected market size is reported in parenthesis.

The problems inherently connected with the production of liquid transportation fuels from biomass are, the amount of available biomass and the relatively low value of fuels. Fuel production from biomass should be limited to applications for which substitution is not a feasible alternative. This is, for example, the case for aviation and maybe marine traffic. Many other forms of traffic can be more and more shifted to electric or other energy forms, for example, to batteries or fuel cells. Clearly, this requires an overall modification of the current transportation infrastructure and a general reconsideration of transportation, which will take time.

The currently available biomass appears to be sufficient to replace fossil resources for the production of chemicals (Vennestrøm et al., 2011). The challenges in this context are the development of efficient processes for the collection, handling, and pretreatment of biomass and for the selective conversion of biomass feedstock into the value-added products.

The extensive current research into second-generation biofuels will significantly benefit the future renewable chemical industry. While products such as ethanol as fuel do not appear as perfectly sustainable solutions in the long run, the technologies currently developed to produce them are valuable for biomass use aiming at other products. Furthermore, many of the compounds at present produced by the biofuels industry might serve as interesting platform chemicals for a green chemical industry in the future. For example, ethanol is a possible starting point for acetic acid, ethylene, or ethylene glycol production (Christensen et al., 2008; Vennestrøm, 2011). An already existing example for this is the Brazilian company Braskem, producing biopolyethylene from sugarcane-derived ethanol. The polyethylene produced at Braskem is widely used for automobiles, cosmetics, packaging, and toys. In 2010, the company claimed to be the world leader as it opened a US$320 million sugarcane ethanol processing plant, which has the capacity to produce 200,000 tonnes of bio polyethylene per year (Wells and Zapata, 2012).

The production of transportation fuels is thus a good way of establishing processes and infrastructure needed for large-scale industrial utilization of biomass aiming at higher value. However, the assumption that biomass is available in excess, forming the basis for the current production of transportation fuels, will likely not hold true in the future. Careful evaluation is therefore needed when allocating these resources.

A further thought in this context is that current platform chemicals have been developed because they were convenient to produce from fossil resources. While it appears tempting to simply replace such fossil-based molecules by producing them from biomass, the inherent functionality of bio-derived molecules should be utilized as much as possible in the long run for the sake of sustainability.

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