Lignocellulose biorefinery feedstock engineering

Lignocellulose biorefinery feedstock engineering 3.1

3

Overview

Lignocellulose is an aggregate of supramolecular structure—i.e. a combination of cellulose, hemicellulose, lignin, ash, and others—wherein cellulose is wrapped by the dense structure formed by hemicellulose and lignin, thus forming a complex spatial structure with a three-dimensional network (Zhang, 2008). The diversity of lignocellulosic feedstock composition determines that the engineering development processes differ for the agricultural and forestry products processing industry, the fermentation industry with food as raw materials, and traditional cellulose industries. Lignocellulose biorefinery engineering should be the process of “refining” with pluralistic, multifunctional conversion. Its purpose is not only to transfer the lignocellulose for biofuel or bio-based materials production, but also to make lignocellulose a general industrial raw material for the production of bio-based energy, materials, and chemicals. The dense spatial structure of lignocellulosic material makes it difficult to use it directly, hence feedstock pretreatment is necessary to achieve efficient conversion (Eggeman and Elander, 2005). However, from the perspective of full utilization of lignocellulose, the end-result of existing feedstock pretreatment technologies is still cellulose hydrolysis and fermentation, while high-value utilization of hemicellulose and lignin is rarely considered, and this seriously affects the healthy development of lignocellulosic feedstock utilization. Therefore, the term ‘pretreatment’ must first be given a new meaning and second, a new effective multicomponent utilization technology should be established to deal with the complexity of lignocellulosic feedstock; namely, the component separation technique. Component separation and directional conversion – further enhancing raw material pretreatment and achieving the conversion of cellulose, hemicelluloses, and lignin – is currently the most important highvalue lignocellulose utilization concept, and has made much significant research progress both at home and abroad (Jin and Chen, 2007; Kim and Lee, 2006; Sun and Chen, 2008). However, the idea of lignocellulose component separation and directional conversion is still hard to sell because of the problems of technical and economic efficiency in large-scale clean and efficient industrialization when lignocellulose is used as an industrial raw material. First of all, lignocellulose itself is a highenergy macromolecular structure, and all the existing component separation and directional conversion circuits need to first consume a certain amount of energy in order to break down the lignocellulose structure before conversion. This method does not take the functional needs of the product into account, and simply separates the components of the raw materials, which, for some products, increases its energy consumption and leads to a low atom economy of the raw material. Thus, if lignocellulosic resources are to become universal biological and chemical raw materials, the Lignocellulose Biorefinery Engineering. http://dx.doi.org/10.1016/B978-0-08-100135-6.00003-X Copyright © 2015 Elsevier Ltd. All rights reserved.

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Lignocellulose Biorefinery Engineering

Table 3.1 Comparison of three development stages of primary lignocellulosic material refining Development stage Pretreatment Components separation Selective structure split

Forms of technology

Goals of products

Single conversion technology of pretreatment Integration of various conversion technologies

Single product

Low-power and high-efficiency conversion technology according to the structure and function characteristics of the raw material

Full use of each component, and polygeneration Maximum value of the products and cleaner production

refining process and technology of high-value utilization that maintains the original structure of lignocellulose need to be developed (Chen et al., 2009). Faced with the fact that lignocellulose is a macromolecule of complex chemical composition and structure, Prof. Chen found that the biorefining process should be based on the requirements of the target end products and the structural characteristics of the materials; pretreatment of lignocellulosic feedstock, component separation and the selective structure deconstruction process should all be carried out according to product functional requirements. With energy consumption kept at a minimum, and optimum efficiency, maximum value, and clean conversion as the goals, we can achieve universal use of lignocellulose as the main raw material for a new generation of biological and chemical industries. Table 3.1 compares the technologies and product goals in the three development stages of primary lignocellulosic material refining. Hereinafter, the concepts of preprocessing, separation of components, and the selective structure deconstruction are described in detail (Chen, 2014).

3.2 3.2.1

Technologies of components fractionation Molecular level selective fractionation technology

Lignocellulosic feedstock is composed of cellulose, hemicellulose, and lignin. Cellulose connects with hemicellulose and lignin by hydrogen bonds, while hemicellulose and lignin connect with each other by covalent bonds. Therefore, the structure of the cell wall is tight and complex. Cellulose, lignin, and hemicellulose are composed of glucose, phenyl propane units, and pentose, respectively. It is obvious that the three main components are different from each other. Cellulose and hemicellulose are carbohydrates, while lignin is an aromatic polymer. The heterogeneous bioconversion of corn stalk at a component level involves two processes: hydrolysis and fermentation. In the hydrolysis process, hemicellulose and cellulose could be hydrolyzed by cellulase. However, lignin is regarded as recalcitrant

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to cellulase enzymatic hydrolysis (Himmel et al., 2007). On the one hand, lignin prevents cellulase contacting with the substrate. On the other hand, lignin absorbs cellulase nonproductively. In the fermentation process, glucose from cellulose could be used as the main carbon source. Pentose from hemicellulose could also be used by several kinds of microorganisms (Hahn-Ha¨gerdal et al., 2007). However, the hydrolysate of lignin, especially a small molecular one, has been proven to be an inhibitor for fermentation (Larsson et al., 2001). Therefore, if corn stalk is converted as a whole, the cellulose conversion rate would be low due to the lignin effect. It would be necessary to fractionate the stalk into different components and then convert them, respectively. For pulping, cellulose is extracted with different chemicals during the pretreatment process. It is worth noting that lignin is removed to improve the properties of the pulp. The chemical structures of cellulose, hemicellulose, and lignin change in different ways during pulping.

3.2.2

Cell-level selective fractionation technology

Cells in lignocellulosic material can be classified into two categories according to the thickness of the cell wall. The first category is the cell that only has a primary cell wall, such as parenchyma tissue cells, sieve cells, and companion cells. The second type is the cell that has both a primary cell wall and a secondary cell wall, such as sclerenchyma cells (including fiber and hardened cells), vessel cells, tracheids, and collenchyma cells. Although cellulose exists in both the primary cell wall and the secondary cell wall, the lignin content in the secondary cell wall is high. Lignin is regarded as the cause of the recalcitrance of cornstalk to hydrolysis (Himmel et al., 2007). For bioconversion of cornstalk, the properties of these two kinds of cells are therefore different. In addition, there are special cells with different cell wall structures (Evert, 2006). In some cells, the cell wall is often silicified. And the cell wall of dermal cells may undergo hornification. There are also secretory tissue cells including secretory cells, glandular hair, nectar, and secretory sacs. Secretory cells belong to parenchyma cells, and so their cell walls are rich in cellulose. Although there are few special cells in vascular plants, they can affect the bioconversion process. For example, hornification can result in recalcitrance to enzymatic hydrolysis (Himmel et al., 2007). Therefore, stalk is heterogeneous at the cell level because of the different components of the cell wall. Hence, it is necessary to study the different conversion properties of the different components. When corn stalk is pretreated with steam explosion integrated with Bauer screening, two fractions can result. The fraction bigger than a 28 mesh contains more than 89% fiber cells and the fraction smaller than a 200 mesh contains 64% parenchyma cells. Therefore, these two fractions are chosen to analyze the hydrolysis properties of fiber cells and parenchyma cells. The fraction bigger than 28 mesh is first crushed to smaller than a 200 mesh to remove the effects of particle size. When the two fractions are hydrolyzed for 48 h, the glucose concentration is 5.15 g/L for parenchyma cells,

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which is two times higher than that of fiber cells. In addition, the hydrolysis rate of parenchyma cells can reach 70%. If corn stalk is fractionated with steam explosion integrated with super grinding, a powder fraction and a residues fraction can be derived. The parenchyma cell content in the powder fraction is 26.6% higher than that in the residues fraction (area percentage). And the fiber cell content in the residue fraction is 26.4% higher than that in the powder fraction (Jin and Chen, 2006). The epidermal cell content is also different in the two fractions. Therefore, the two fractions derived from corn stalk pretreated by steam explosion integrated with super grinding can be used to analyze the hydrolysis properties of different cells. After hydrolysis for 24 h, reducing sugar content is 61.4% for the powder fraction, which is 3.8 times higher than that in the residues fraction (Jin and Chen, 2006). The cell content before and after 48 h of hydrolysis for the powder fraction has been analyzed. Results show that fiber cell content reduces by 22.8% after hydrolysis. Parenchyma cell percentage reduces from 54.2% to 7.3%, while epidermal cell percentage increases from 10.4% to 80.9%. These changes demonstrate that different cells have different hydrolysis properties. The enzyme hydrolysis properties could be arranged, from high to low, as parenchyma cell, fiber cell, and epidermal cell (Jin and Chen, 2006). In the process of pretreatment with steam explosion integrated with super grinding, moisture content could affect fractionation results. If the moisture content of the steam-exploded material is 40%, the fiber cell content in the residue fraction would be more than 60%. Therefore, the residue fraction could be used to analyze fiber cell conversion properties. The ethanol autocatalysis method is applied for pulping. The pulping process is carried out at 180  C for 2 h with 50% ethanol concentration and solid to liquid ratio of 0.8/10 (g/ml). This reveals that the crude pulp yield of the residue fraction could reach 61.4%. However, pulp yields of steam-exploded rice straw and untreated rice straw are 35.5% and 32.1%, respectively. So, it demonstrates that there is a positive correlation between fiber cell content and pulping properties. Therefore, an effective biorefinery method would be to fractionate different cells and then convert them.

3.2.3

Tissue-level selective fractionation technology

According to Sachs’s convenient classification (Sachs et al., 2011), the body of a vascular plant is composed of three tissue systems: the dermal, the vascular, and the fundamental (or ground). The dermal tissue system comprises the epidermis and the periderm. The vascular tissue system contains two kinds of conducting tissues: the phloem (food conduction) and the xylem (water conduction). The fundamental tissue system (or ground tissue system) includes parenchyma tissue, secretory tissue, collenchyma tissue, and sclerenchyma tissue. There is a cutin layer on the outside of the dermal tissue system (Evert, 2006) which is composed of cutin and wax. Cutin is mostly polymers composed of C16 and C18 monomers. The parenchyma cells under the dermal system are hardened, leading to a high lignin content. For the vascular tissue system, the vessel cells in the xylem are

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thickened so that the lignin content is high too. There are mainly parenchyma tissue cells in the ground tissue system. For parenchyma tissue cells, there is only primary cell wall, which is composed of cellulose. In terms of mass content, the vascular plant is mainly composed of the ground tissue system and the vascular tissue system. Therefore, vascular plants could be simply fractioned into a vascular tissue fraction, which is rich in lignin, and a ground tissue fraction, which is rich in cellulose. Corn stalk could be manually fractionated into a vascular tissue fraction and a parenchyma tissue fraction. The vascular tissue fraction includes fiber cells around vascular bundles because fiber cells are tightly connected with xylem and phloem. When both fractions are hydrolyzed with enzymes for 48 h, the glucose content from parenchyma tissue is 22.5% higher than that from vascular tissue. Corn stalk could be fractionated into vascular tissue and parenchyma tissue fractions using steam explosion integrated with mechanical carding. For rind and leaf, the enzyme hydrolysis rates of the parenchyma tissue fraction are 1.77 and 1.37 times higher than for the vascular tissue fraction, respectively. The vascular tissue fraction from corn stalk is tested by pulping with ethanol autocatalysis. The result reveals that pulp yields could reach 57.6% while pulp whiteness reaches 65.9% when the catalysis time is 2.0 h at 160  C (ethanol concentration 50% and solid to liquid ratio is 1:10). The whiteness could meet the requirement of printing paper (Chen and Liu, 2007b). Corn stalk has not been used in pulping because there are many nonfiber cells. Nonfiber cells are mainly parenchyma cells and dermal cells. Also, the fiber cell content in corn stalk is low. Moreover, the ratio of length to width for corn stalk fiber cells is smaller than that of other pulping materials. Therefore, if the whole corn stalk is applied for pulping, the pulp yield and the quality of paper could hardly meet requirements. If nonfiber cells could be removed and the fiber cell content was high, corn stalk would become another resource for pulping.

3.3 3.3.1

Technologies of selective structure deconstruction Introduction to selective structure deconstruction technologies

Lignocellulose is a carbon-rich organic resource, and can be used to develop recycled and reclaimed functional products (such as bio-based materials, liquid fuels, organic chemicals, etc.). Although the existing idea of lignocellulose component separation and orientation conversion can help to solve the problem of efficient use of lignocellulosic resources to a certain extent, it is still difficult to achieve economic and technical efficiency in large-scale, clean, and efficient industrial processes with lignocellulose as the raw material. Lignocellulose itself is a high-energy macromolecular structure, and the existing utilization routes of component separation and directional conversion need to consume a certain amount of energy prior to the destruction of the lignocellulose structure, this approach also does not take the functional requirements of the product into account. Therefore, the approach increases the energy

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consumption for some products, and results in a low atom economy of the raw material. In addition, the process is made more complicated by a higher risk of contamination. The root cause of this problem is the lack of full understanding of the structure–activity relationship between the complex structure and the functional use of lignocellulosic feedstock, resulting in the lack of both effective research ideas and an efficient conversion technology system. Therefore, the refining process and technology for maintaining the functional and high-value use of the original lignocellulose structure must be further improved and developed. Component separation proposed for raw material feedstock project development should be promoted to achieve selective degradation of components according to the product features, which should be based on the structural characteristics of the raw material and the requirements of target products. The purpose of this process is not only to obtain several products, but to achieve versatility of straw as the main raw material in biological and chemical industries with minimum energy consumption, optimum efficiency, maximum value, and clean conversion. Thus, if lignocellulosic resources are to become universal raw materials in the next generation of biological and chemical industries, an in-depth understanding of the following aspects must be achieved: first, the relationship between selective structure deconstruction of the raw material feedstock and functional utilization must be proven, which is the premise of refining lignocellulosic feedstock; second, the equations governing materials structural components and biological and chemical catalytic reactions must be revealed; and finally, new methods must be developed for refining and processing lignocellulosic raw materials with high efficiency and low cost to achieve process optimization and coupled integration (Chen and Qiu, 2009).

3.3.2 3.3.2.1

The history of the development of selective structure deconstruction technologies Overall conversion of functionality

Lignocellulose utilization has a long history, and the emergence of papermaking and charcoal manufacturing in ancient Egypt are successful examples of raw materials utilization. China is a typical agricultural country, which has long carried out strawbased recycling of lignocellulosic resources, including composting, returning to field, use as feed, fuel, and so on. However, due to technological limitations, lignocellulose conversion technology has been confined to simple and modest utilization for a long time. For example, burning stoves is the most primitive lignocellulose utilization method, and also the most common way in which lignocellulose is used, but its utilization rate is only 15–20%. With the development and improvement of conversion technology, the conversion rate is increasing. In the late twentieth century, helped by the energy crisis, research into lignocellulose utilization developed quickly, and new technologies of lignocellulose gasification and liquidation have been developed. The technologies of direct combustion, gasification, pyrolysis, and liquidation are all utilization processes that emphasize the entire functional conversion of lignocellulosic materials. However, lignocellulosic feedstock has a functional supramolecular body

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of complex structure; it is impossible to realize the maximum value of lignocellulosic material unless the entire functional conversion method is used for the primary refining of lignocellulose.

3.3.2.2 Complete conversion of functionality Lignocellulosic materials consist of cellulose, hemicellulose, and lignin. In addition to entire functional conversion such as direct combustion, and in order to realize more complete and full utilization, there has been some progress in the separate utilization of the three components. The separate conversion of the three components at the same time is a typical process of entire functional conversion. Nowadays, the papermaking industry mainly uses the cellulose in lignocellulose, and makes use of various methods to remove the other components in the materials (such as hemicellulose and lignin), as a result of which the other components cannot be utilized, thereby generating waste. Meanwhile, in the lignocellulose research of bioethanol, in order to obtain fermentative sugar, different pretreatment methods are taken to get high-purity cellulose and there is little consideration given to hemicellulose and lignin. Therefore, such pretreatment technologies are only halfway to being primary refining methods for lignocellulosic materials, and only realize the utilization of one or two main components, while other components are destroyed or wasted. In order to realize the entire utilization of lignocellulose in lignocellulosic materials, a set of effective comprehensive utilization technologies for multicomponent materials should be established. Component separation and oriented conversion is a refining technology for the comprehensive utilization of the three components; it is a further improvement of primary lignocellulosic material refining, i.e. not only a pretreatment method, but also a resource distribution process for the macromolecular components in lignocellulosic materials. It can realize the separate conversion and utilization of the three components, and is the main concept for high-value use of lignocellulosic resources.

3.3.2.3 Deconstruction and conversion of partial functionality The process of component separation and oriented conversion in the complete function conversion requires a certain level of energy to break the tight structure of lignocellulose prior to conversion. This method doesn’t consider the functional needs of the products, the breakdown of the material increases energy consumption for certain products, and the atom economy is low. Therefore, the idea of component separation and oriented conversion still can’t break through the inherent difficulties of economic and technical efficiency and other issues in large-scale, clean, and efficient industrialization with lignocellulose as the industrial raw material. Therefore, on the basis of summarizing the research results of the “973” Project numbered 2004CB719702 (China), Prof. Chen proposed that the pretreatment and components separation of lignocellulosic raw materials should be promoted to a selective structure deconstruction process according to the structural features and needs of target products, making lignocellulose a general industrial material for lignocellulose

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Lignocellulose Biorefinery Engineering

energy, materials, and chemicals in the future. The product engineering takes the product requirements as direction, and tries to develop a product satisfying the ultimate features of usability. Product engineering is currently an important research area. Lignocellulose product engineering is a process that can design, process, and produce lignocellulose products with abundant recycled lignocellulose resources as materials and taking the needs of people’s lives and production in the region as direction. The production process of lignocellulose product engineering refers to the lignocellulose refining process designed by one or several functional molecules in the lignocellulosic raw materials from the perspective of product function, which is the selective deconstruction process of lignocellulosic raw materials (Chen, 2014).

3.3.3

Current situation for selective structure deconstruction technologies

Biomass feedstock, such as lignocelluloses, are supramolecules consisting of cellulose, hemicellulose, and lignin, and the multicomponent structure determines that the products using lignocellulose as raw materials are diverse: e.g. lignocellulose energy, materials, and chemicals. Nowadays, some basic technologies using selective structure deconstruction theory have been developed for the lignocellulosic raw materials refining process, and preferable research results have been obtained (Chen and Qiu, 2009).

3.3.3.1

Partial structure deconstruction of hemp material to improve spinning function

Hemp is a type of lignocellulose in abundant supply; in order to make the hemp fiber spinnable, a degumming process must first be carried out. The traditional chemical degumming method causes several environmental pollution problems. What’s more, the low efficiency and high cost of microbial degumming can not meet the needs of industrial production. Considering the entire structure of hemp material, Prof. Chen’s research group came up with a selective structure deconstruction technology with steam explosion at its core according to the functional needs of the products. The steam explosion process can degrade 80% of hemicellulose and pectin to realize the goal of hemp spinnability; with further processing, high-count pure yarn and blended yarn with high spinning performance can be obtained. The steam explosion method can not only advance the spinnability rate but also improve physical mechanical properties, dyeing properties, and serviceability, and the whole industrialization process does not contribute to environmental pollution.

3.3.3.2

Functional modification of whole straw to prepare ecological board

The common method for making strawboard is to first conduct a simple heat treatment, followed by thermocuring molding, during which the appropriate adhesive must be added to obtain good products; however, the process leads to high costs and

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pollution. Considering straw structural features and product function, Prof. Chen’s research group utilized the active group with similar characteristics to polyphenol by lignin degradation during steam explosion, which leads to a cross-linking reaction with furfural produced from hemicellulose degradation, generating properties similar to phenolic resin. Thus, no extra adhesive is needed. The changes of straw aggregation structure during steam explosion can be fully utilized to hot-press the denaturized structure after treatment. Through controlling the moisture content during the process of hot-press forming, the hydrogen bonds of cellulose can be rearranged to produce ecological board with high mechanical strength. This method not only solves the pollution problems of the traditional method, but also maximizes the atom utilization of straw materials and results in a simple operation and a clean craft process.

3.3.3.3 Multifunction realized by partial structure deconstruction of straw Lignocellulosic feedstock such as straw consists of various components, each of which has different functional utilization methods to obtain various products. However, the current idea of component separation and conversion can not realize the efficient, low-cost, and clean conversion of straw materials. Through intensive studies, Prof. Chen’s research group found that the functional characteristics of different kinds of fibrocytes in different cellulose components were diverse. For example, the fiber structure of the fiber bundle tissue can compete with wood fiber, hence it can be used to replace a certain amount of wood for pulping. The pulping yield can reach 45% with macrofiber using ethanol, and the lignin content of the pulp is as low as 3.5%; while the parenchyma cell tissue has good fermentation properties, and the cellulase yield can reach 194.18 FPU/g dry substrate. Among other components, in addition to cellulose in straw, hemicellulose can be degraded during steam explosion, resulting in an excellent material for producing xylose and newtol; the separated lignin is high in purity and a widely used material; and the ash can further be used to prepare nanosilica. Therefore, full utilization of straw can be realized in a clean and pollution-free method. It is clear from the results above that selective structure deconstruction can not only realize maximum atom economy of materials, but also is the simplest technical method according to functional features of products, thus realizing low-cost, low energy-consumption, and clean production (Chen and Qiu, 2009).

3.3.4

Development trends of selective structure deconstruction technologies

China imports and consumes a large number of fossil resources, even though it already has abundant lignocellulosic resources. The biorefinery industry, international academia, and the industrial community in general should focus on and exploit theory and technology for the increased use and conversion of lignocellulose resources, and the development of a lignocellulose resource refining process for biological and chemical industries.

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In order to build up a new industrial mode with lignocellulose as a general raw material, the key refining problems must be resolved from the perspective of engineering that inhibit lignocellulose from becoming the new generation of biological and chemical general industrial materials. With basic science as the research method and the ecological recycling economic concept as a guide, it is necessary to develop lignocellulose refining technology to conserve the original structure and function and build a simple functional utilization process with the highest atom economy and the lowest energy consumption. In order to realize this target, the following three key scientific problems need to be solved: (1) Research into the relationship between material aggregation structures deconstruction and functional components utilization. Lignocellulosic feedstock is a functional resource with the characteristics of low carbon content, low energy density, and high inhomogeneity, and different ways of structural deconstruction produce different functional components. Therefore, to clarify the relationship between supramolecular structure deconstruction of raw materials aggregation state and functional utilization, and to improve the high-value utilization level of raw materials, has become one of the key scientific problems to make lignocellulose a common industrial raw material. (2) The relationship between the structural components of raw materials and the intrinsic characteristics of heterogeneous catalytic reactions. Lignocellulose biomass has a dense and impervious structure containing crystals, where direct catalytic conversion is difficult or affects the performance of the conversion products. Selective structural deconstruction can avoid the problems of high cost and environmental pollution in the existing process of “first component separation and then reconversion.” However, the relationship between material structure components and selective catalytic reaction must be recognized, and the reaction laws of catalytic conversion in components deconstruction should be represented; this is the key scientific issue in developing a refining system on the road to developing a new generation of biological and chemical industries based on lignocellulose material. (3) Optimization principles of an efficient and low-cost raw material refining process. Due to both the complex structure and high nonuniformity, the functional utilization of straw must be a reaction system and a flow path with multiple steps, bulk treatment, and utilization distinguished by pathway, in a cluster reaction system. In the lignocellulose biorefinery process, low energy consumption and high efficiency and value should be the goal. Although the selective structure deconstruction and product functional conversion is the most economic and efficient method, the cluster reaction system within the material refining system must be recognized. To analyze the refining system, focusing on a complex solid substrate and identifying the principle during each process is the third key problem to realize the high-efficiency refining of lignocellulosic feedstock.

The cellulose, hemicellulose, and lignin in lignocellulosic raw materials form a threedimensional network structure with cross-linking. There are essential differences in the reactions between chemical catalysis of single-composition materials, even pure cellulose, and solid complex lignocellulosic raw materials. The selective deconstruction and interaction among components and the control of target products in a lignocellulose complex structure are completely new areas of chemical research. Therefore, recognition of the relationship between the structural features of complex lignocellulose and biological conversion is the basic key to realizing the refining process. The high-value functions of lignocellulose belong to the reaction system of raw material cluster refining, which requires the most economical and effective steps

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and methods to achieve the conversion of raw materials. Therefore, understanding the cluster reaction mechanism of the lignocellulose stratified multistage directional conversion refining system for each process, resolving a complex three-dimensional resource utilization refining system of comprehensive solid-phase materials for lignocellulose, and matching the coupling principle of each process are the keys to achieve efficient refining of the raw material feedstock. With bioethanol as the product model, it is necessary to study: the refined lignocellulose material, the coupling effect, and the principle of optimality during the process of hydrolysis and biological degradation; the mass and heat transfer, the reaction principle, and rules of multifield coupling – including material, energy, phase change, and chemical reaction in the bioreactor; and to establish a continuous technology system with the integration of straw enzymolysis, fermentation, product online adsorption, and separation.

3.4 3.4.1

Steam explosion treatment technology Principles and equipment of steam explosion technology

Biorefining using lignocellulose as the main material needs pretreatment to break the complex compact structure and to realize structure deconstruction and functional components utilization, in order to strengthen the efficient and high-value conversion of the lignocellulose resource. Among the existing pretreatment methods, steam explosion technology is one with a long history and good performance. Steam explosion technology dates back to 1928, when steam explosion pulping technology was invented by W. H. Mason in the United States for fiberboard production. Since the 1970s, this technology had been widely used in the production of animal feed and the extraction of ethanol and other chemicals from wood fiber. In the late 1980s, this technology was further improved, when the Stake Technology Company in Canada developed continuous water vapor explosion technology and its associated equipment, and applied this technology to the pulping industry. Canadians E. A. Delong and G. S. Ritchie patented the use of saturation water vapor to explode wood chips with chemical pretreatment (DeLong and Ritchie, 1990). San Martin et al. (1995) and Moniruzzaman (1996) used pine tree and bagasse, respectively, as steam explosion materials, and concluded that steam explosion pretreatment was beneficial to enzymolysis. Research by Morjanoff and Gray (1987) showed that when the material underwent steam explosion, the hemicellulose hydrolysis rate could be improved with the addition of H2SO4 or by treatment with SO2 or H2SO4 before steam explosion, which released the enzymolysis inhibitors. Prof. Chen began to apply steam explosion technology to the straw treatment field. Because of the differences between straw and wood in terms of chemical composition and structure, Prof. Chen proposed a lowpressure and no-pollutant steam explosion technology without any chemical additions, exploiting a series of innovative methods such as clean pulp, clean hemp degumming, humic acid prepared by straw, activated xylooligosaccharide, and so on, and applied for several patents.

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3.4.1.1

Principles of steam explosion technology

Steam explosion technology, after decades of research, has been widely used in fields such as food processing, pharmaceuticals, bioenergy, materials, chemicals, and environmental protection; the last of which having application prospects and advantages that are becoming more and more obvious. In a steam explosion process, plant materials (with the cell structure having undergone a re-watering treatment vapor phase) are stewed at high temperature and pressure where high-pressure steam penetrates into the pores of the materials. Under the physical and chemical effects of high temperature and pressure, hemicellulose is partially hydrolyzed into soluble sugars, and lignin between the composite layers is softened and partially degraded, reducing fiber coupling strength and softening. When the pressure is suddenly released, media and materials work together to complete the physical energy releasing process. The gas media within the materials expand rapidly at the moment of ejection while the high-temperature liquid water within the material quickly explodes, completing the instantaneous adiabatic expansion, thus fragmenting the material into tiny fiber bundles. In the process of explosion, the expanding gas forms a shock wave, forming a shear deformation movement under the conditions of softening materials, in order to achieve the components separation of the raw material and structural changes. According to the research of Zhang and Chen (2012), through theoretical analysis and calculation, the steam explosion should be carried out at the cellular level in order to achieve the best explosion performance, indicating that each cell can be treated as a “micro steam explosion chamber” to establish the multilevel model at the phase of instantaneous pressure release at the cellular level (Figure 3.1). The breakage of physical and chemical structures during steam explosion mainly includes the following: (1) Similar acid hydrolysis and thermal degradation. During the process of steam explosion, the high temperature and pressure go into the cellulose materials and penetrate into the inner core of the cellulose. Because of the joint function of vapor and heat, cellulose and part of the active components undergo similar acid degradation and heat hydrolysis, and the low molecular material digests with the degree of polymerization of the cellulose decreased. (2) Similar mechanical breakage function. When the high pressure is released, the superheated vapor that has already penetrated into the cellulose releases rapidly and instantly from the closed space in the form of airflow. In this way, cellulose is mechanically broken to a State 1

P1,T1, r1,m0

State 2

Intermediate state v3

Pa,Ta

P3,T2, r3,m0+mw . q

Pa,T2

ms,v2 Pw,T2, r2,m0+mw,v1

d

Flash evaporation

mw ms Cell model

Figure 3.1 Simple model for steam explosion at the cellular level (Zhang and Chen, 2012).

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certain degree. Such breakage shows not only bond rupture, increased reducing groups, and internal hydrogen bond damage of cellulose in the cellulose macromolecule, but also the breakage of the amorphous region and part of the crystalline regions. (3) The breakage of hydrogen bonds. During steam explosion, vapor penetrates into the pores of the cellulose and forms hydrogen bonds with part of the hydroxyl on the cellulose chains. Meanwhile, high temperature, high pressure, and high moisture content conditions will increase the breakage of hydrogen bonds inside the cellulose, dissociating new hydroxyl and strengthening the hydrogen bonds in the cellulose. Meanwhile, cellulose cools down to room temperature, freezing the super molecule structure, and only a small proportion of the hydrogen bonds recombine. Then the solvent can easily enter, breaking the hydrogen bonds that are left and accelerating the total breakage of other crystalline regions until total dissolution. (4) The function of structural rearrangement. Under high temperature and pressure, the hydrogen bonds inside the cellulose molecule are broken to varying degrees, and the mobility of cellulose chains increases, which is beneficial to the ordered structure changes of the cellulose. Meanwhile, the breakage of the cellulose chain makes the rearrangement of the cellulose chain easier.

Generally speaking, during steam explosion, hemicellulose is partly autohydrolyzed to generate monosaccharide and oligomer, while lignin is partly degraded. Under the high temperature conditions, the acetyl hydrolyzed from the hemicellulose chains forms acetic acid that can strengthen the hydrolysis of glucosidic bonds from hemicellulose and ester bonds from lignin. Therefore, within limits, the stronger the processing strength, the stronger the degree of hemicellulose hydrolysis and lignin degradation, and the more obvious the component separation effect is. Drawn from previous studies, steam explosion is not only a single chemical reaction process, but also the result of its relation to physics and chemistry, as noted below. (1) The impact on the three components. In studies by Sun et al. (2005) and Li et al. (2007), the sugar, acid, and furfural degradation product categories were regarded as degradation products of hemicellulose, and the phenolic degradation was regarded as the product of lignin degradation. Also, the degradation rate of hemicellulose and lignin with steam explosion improved with the enhancement of explosion conditions (temperature and pressure). Under high temperature and pressure, as a plasticizer, water rearranges part of the cellulose crystalline regions by heating, and a portion of the amorphous region converts to a crystalline region, thus improving the crystallinity of cellulose (Chen and Li, 1999). (2) The material characteristics of porosity and permeability. Chen (He and Chen, 2013) and Kohler (Kohler and Nebel, 2006), through comparisons of SEM pictures, specific surface, and Mercury Injection Apparatus before and after steam explosion, drew the conclusion that compared with straw without steam explosion, the porosity and permeability was improved, the pore area increased, and the total pore volume of the straw decreased through steam explosion. Cellulose is the skeleton of cell walls, and hemicellulose and lignin are the filler, with lignin and hemicellulose apparently degraded after steam explosion. With the degradation of fillers, the skeleton is exposed, the porosity of the cell increases, and the original mesopores (100–10,000 nm) and micropores (5–100 nm) turn into a macropore (10,000–100,000 nm) structure. Because the area of macropores (100,000–400,000 nm) is small and the total area content of mesopores and micropores is over 90%, the area of total pores decreases. (3) The enzymolysis rate of the material. The removal of hemicellulose after steam explosion and the increase of specific surface area improves the accessibility of enzymes to cellulose

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Lignocellulose Biorefinery Engineering

(Piccolo et al., 2010). Hendriks and Zeeman (2009) reported that the enzymolysis rate was improved by six times after steam explosion. Adding acid and alkali (Li et al., 2005), removing lignin degradation inhibitor (Wang and Chen, 2011), or adopting a two-stage steam explosion technology can inhibit the degradation of hemicellulose, improving the enzymolysis rate. (4) The extraction rate of active ingredients. Yuan and Chen (2006) utilized vapor-coupled steam explosion to treat Ephedra to extract ephedrine, and the extraction rate was improved by 3.5 times compared to steam explosion alone, and the yield of raw materials was only 0.35%. Chen et al. studied the effect of steam explosion on the flavones extraction rate of Rhus fruit, and the extraction rate can reach 19.65 mg/g of dry material under steam explosion of 220  C for 5 min (Chen and Chen, 2011). Compared with untreated materials, the extraction rate was improved by eight times, and it can be stopped within 15 min, shortening the extraction time by 180 min. This is because the plant structure was broken after steam explosion and the protective cell wall was separated, reducing mass transfer resistance during the internal diffusion. The actively effective constituents could contact, dissolve, and spread fully with the solvent, reaching the high extraction rate in a short time.

For the choice of steam explosion conditions, the main factors are explosion pressure and maintaining time, so the main expression equation is still the equations proposed by Overend et al. in 1987 (Overend et al., 1987). However, the one-sided equations do not generalize all the factors impacting pretreatment performance. Therefore, Chum et al. (1990), Belkacemi et al. (1991), Abatzoglou et al. (1992), Montane´ et al. (1998), Hosseini and Shah (2009), and Zhang and Chen (2012) further enriched the steam explosion strength equation considering the properties of materials, vapor, and the tank. During the actual process of steam explosion, the results are correlated with material variety, application of the exploded material, and tank properties. For instance, the explosion conditions for wheat straw and wood are different. The requirements for pulping, chemical fiber, and biological conversion are different for plant materials. In order to obtain high-yield pulp, related measures should be taken to avoid composition degradation. From the total utilization of lignocellulose, the goal is to obtain the three components with the lowest costs. Considering the steam explosion conditions from the prospective of engineering, it is mainly the homogeneity of the gas stewing and gas transfer property, which is related to the initial water content of material, particle size, bulk density, and coefficient of charge. The physical properties of the explosion tank have a great effect on the process. For example, the ratio of the discharge port diameter of the steam explosion tank and tank diameter, decompression speed, pipe shape of discharge hole, and stabilization tank volume have an effect on explosion results.

3.4.1.2

Steam explosion equipment

Batch steam explosion equipment Batch steam explosion equipment (Figure 3.2) consists of a vapor generator, a steam explosion tank, and a stabilization tank. Under some conditions, there is a cooler after the stabilization tank. Based on these, our group designed steam explosion equipment at the lab level, which was used for the detection and analysis of data during explosion. There are pressure sensors, temperature sensors, and a sight glass window on the pressure-maintained tank; and a solid flow meter on the discharge hole, equipped with

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2

20 19

5 6

22

11 21 7 8 9

1

17

3

18

4 12 10

16 13 14

15

Figure 3.2 Batch steam explosion equipment. (1) Steam valve; (2) inlet ball valve; (3) discharge ball valve; (4) solids flow meters; (5, 8, 12) pressure sensor; (6, 9, 13) temperature sensor; (7) pH sensor; (10) pipe buffer; (11) sight glass; (14) buffer tank; (15) valve; (16) gas condensation system; (17) dynamic data acquisition and analysis system; (18) computer; (19) thermometer; (20) pressure gauge; (21) pressure maintaining tank; (22) image acquisition device.

a computer for dynamic data acquisition and analysis. The data provided by these devices are convenient for the calculation. The devices gather the necessary data during steam explosion and the kinetic model can be used to assess the steam explosion effects and other parameters; this is beneficial for the engineering enlargement design and development of application ranges of the steam explosion devices. Steam explosion equipment has been gradually magnified to the scale of industrial application, from 0.5 L to 1, 5, and 50 m3 (Figure 3.3). The steam explosion tank Figure 3.3 Amplifying 0.5, 5, and 50 m3 steam explosion vessels.

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Lignocellulose Biorefinery Engineering

designed by Prof. Chen includes the tank body, cover, feeding port, and the discharge port slip. The layout of the steam distribution pipes is optimized using heat distribution; a quick-opening door steam explosion vessel is established and pneumatic control valves are used instead of a manual ball valve (Chen and Liu, 2007a). In the course of processing, the tank can ensure the desired pressure, and vapor explosion temperature can be controlled within a certain range. As a result, the vapor explosion with pneumatic control can quickly open jars using a cylinder control lever, to ensure enough gas pressure to meet production requirements and improve production efficiency. For the processing of Chinese herbal medicine, agricultural waste, and other products, compared with ordinary vapor explosion, steam explosion can achieve production characteristics of high-pressure with a cryogenic tank and rapid valve opening. Based on the different optimum temperatures of the lignocellulosic feedstock and the different requirements of steam explosion, the steam explosion method of a mixed atmosphere instead of water vapor, and in situ methods and corresponding equipment are proposed for steam explosion and materials. Figure 3.4 shows a thin-walled steel tube (5) welded on the top of the steam explosion tank (13) with dual intake and a quick-opening door; a cover (14) is on the top of

4 14

1

3 11

2

5

6 13

7 12

10 15 9 8

Figure 3.4 Structure diagram of dual intake and quick-opening door steam explosion vessel. (1) Feed port; (2) feed valve; (3) push rod; (4) double-acting cylinder; (5) pipe; (6) lugs; (7, 15) nozzle; (8) discharging port; (9) residue discharging port; (10) ring; (11) wheel; (12) manhole; (13) steam explosion tank; (14) cover.

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the quick-opening door, and two lugs (6) for fixing the tank and a ring (10) are welded on the body of the tank (13). The shell of the gas explosion tank can withstand bursting pressure of 1–1.7 MPa, and the temperature of the gas inside the tank can be controlled in the range of 180–206  C. Vapor explosion material is added by hopper (1) into the vapor explosion tank (14), and the feed valve (2) opens or closes by pushing the rod (3) and a small wheel (11) and double-acting cylinder (4). The action of the push rod (3) to the cylinder (4) is push and pull, and the small wheel (11) on the end of the rod (4) can slide a valve (2), making the valve open or close, and there are two sets of valve seals. The saturated steam goes through pipe (5) and the nozzle (7) into the steam explosion tank, and the nozzle is full of high-pressure gas mixture, to ensure that the reaction system has the desired reaction temperature and pressure. The resulting vapor explosion materials flow out through the discharge port (8), while the discharge reaction residue and other waste materials discharge through the slag port (9). After a certain reaction time, the tank can be inspected through the manhole (12).

Continuous steam explosion equipment Stake Technology Company has conducted in-depth research and developed the technology related to equipment for vapor-phase steam explosion, and has applied for a number of patents. For example, the key equipment feeding system, antispray devices, and unloading devices all have many unique properties. Feeding system. Two spiral feeding systems (Co-Ax-Model) are used for steam explosion and are driven by piston hydraulic drive devices (Brown, 1980). There are four kinds of approved shape products, and the specific standards are listed in Table 3.2 (Chen and Liu, 2007a). Stake Technology Digester. Stake Technology has four kinds of technology digesters for industrial production, and the specific standards are shown in Table 3.3 (Chen and Liu, 2007a). The discharge system of water steam explosion pulping technology is also critical, and Stake Technology Company has successfully designed and developed a pulp discharge system to adapt to this technology. Figure 3.5 is a crosssectional view of the discharge system, and Figure 3.6 is a side view of the valve of the discharge system.

Table 3.2 Technical characteristics of the feeding systems (Chen and Liu, 2007a)

Standards

Capacity for treating wood (ODT/h)

Capacity for treating wheat straw (ODT/h)

6¢¢(Ф150) 8¢¢(Ф200) 10¢¢(Ф250) 14¢¢(Ф300)

0.75 2.0 5.0 15.0

0.45 1.2 3.0 9.0

Transporting concentration (%)

Transporting pressure (kg/cm2)

35–90

45

54

Table 3.3

Lignocellulose Biorefinery Engineering

Production capacity of different digesters (Chen and Liu,

2007a)

Standards

Capacity for treating wood (ODT/h)

Capacity for treating wheat straw (ODT/h)

Transporting concentration (%)

Transporting temperature ( C)

12¢¢(Ф150) 24¢¢(Ф200) 36¢¢(Ф250) 96¢¢(Ф300)

0.75 2.0 5.0 15.0

0.45 1.2 3.0 9.0

45

233

Figure 3.5 Cross-sectional view of the discharge system (Brown, 1980). Figure 3.6 Side view of the valve of the discharge system (Brown, 1980).

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The explosion devices are divided into one-section types and two-section types. The highest vapor pressure during the explosion process is up to 6.0 MPa, and the maximum steam temperature can reach 275  C. A muffler is on the bottom of the reactor, and the noise caused by pressure release can be partially eliminated. The specific operation is as follows: a certain amount of wood chips is put into the reactor, under a high pressure for a time, and the ball valve installed at the bottom of the reactor is opened instantaneously. Drastic changes in pressure both inside and outside of the tank produced by the explosion crush the wood chips into fibers or filaments, and then the fibers and filaments fall into the cyclone receiver and divide into gas-, liquid-, and solid-phase materials. The gas-phase material is condensed for recovery, and the liquid- and solid-phase material is mashing material. The two-section explosion device type is modified from the one-section type for the purpose of suppressing the heat loss of a segment-type device, in order to reduce power consumption.

3.4.1.3 Steam explosion-related devices The main processing equipment required for straw and other nonwood fiber raw materials before and after steam explosion is shown in Figure 3.7. Cutting equipment for nonwood raw materials can be roughly divided into two categories: knife rolls and knife discs. The preferred cutting device for straw, wheat straw, and other materials has been the grass cutting machine of knife rolls, which consists of a knife roller, bottom knives, feed roller, tape used in delivering materials, and gearing. In a forced mechanical feed process, the raw material is sent into the cutting mechanism. The material entering is sent into a feeding port consisting of a feed roller with one roller on a vertically disposed axis and the other on a horizontal axis. The latter suppresses forage and constantly turns the forage to the second and third levels of the feed roller. Materials are then cut into certain lengths by the knife on the knife roller and the knife on the bottom of the feeding machine, then sent out by the material tape, and finally sent to the dust filtering system for packaging (Himmel et al., 2007). The straw baler is an environmentally friendly piece of equipment that compresses straw, rice straw, and other materials into blocks, which will help reduce transportation costs and are easy to use. Currently, there are many types of straw balers, mainly screw balers and hydraulic balers. In a screw baler, the nut sleeve is driven by a motor Straw raw materials

Straw cutting machine

Straw packing machine

Rehydration equipment

Transformation

Dehydration equipment

Straw powder machine

Straw crusher

Steam explosion equipment

Figure 3.7 Main processing equipment for steam explosion.

The carding equipment

Conveyor

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and rotates, while the screw is consistent with the sleeve and does not rotate, thus generating a force by moving up and down to press the materials. By comparison, in a hydraulic packer, the travel switch is pushed by the hydraulic pump driven by the motor and this compacts and packs the materials. After packing, the material is transported to be processed, overcoming the disadvantages of high transportation costs of bulk materials, inventory storage, exposure to fire, and possible air pollution. Straw bales, after entering the processing site, need to be bulk packaged, and artificial bulk packaging is a low-productivity, high labor-intensity activity, with often poor results. Therefore, bulk packaging machinery is necessary for larger-scale straw treatment. This machinery can improve work efficiency and lower labor costs with better loosening of the straw bag. The machine mainly consists of a feeding frame, feed section, bulk bag roll, and out-feed conveyor parts. The material bag is packaged on the conveyor to transport to the feed tank, and after two feed rollers the feed bag is initially evacuated and then delivered forward. After a high-speed rotation of the bulk bag roll, the straw bag is completely loose, and the spread material is passed down to the belt and sent out by the conveyor. Screw conveyors can be generally divided into one-part and two-part forms. The one-part screw conveyor with a large span has a wider range of applications in forage transfer, for example the spiral presoaking equipment for herbage material in the process of pulping, and spiral discharge equipment with large volume, and so on. The diameter of the spiral leaf is mainly 400–800 mm, with a span in the 6–13 mm range. The spiral conveyor is mainly used for tablet presoak, which mixes the forage pieces with chemicals after cutting the materials and before entering the digester and transport, in order to increase the material bulk density, harvest rate, and quantity of the chamber, while reducing dust in the process of delivery and reducing air pollution. It will convey the material by rotating spiral blade, and materials are moved along the screw axis, as with a nonrotating nut sleeve. The force that does not make the material rotate with the spiral leaf is the weight of the material itself and the friction resistance of the material to the shell. Compared with multistage screw conveyors, there is no suspending screw axis in a one-part conveyor and the materials are not plugged, owing to its better adaptability to a variety of materials and thus broader application prospects. Raw materials usually need pretreatment before steam explosion, including water pretreatment, acid leaching pretreatment, and alkali leaching (including liquid ammonia and dilute NaOH) pretreatment; the same pretreatment equipment can be used in each case. For larger materials, a cutting or crushing device is needed in the pretreatment. Pretreatment is done to improve the steam explosion effect on the material, and to enhance the separation of the components. The general process involves delivering the straw materials to the dip tank and wetting them by a nozzle according to the moisture content of the material. If the material contains more impurities such as dust, more water can be injected to clean the material. The acid- or alkali-leaching process is similar to water leaching, during which the material is sprayed with acid or alkali and sent to the steam explosion chamber directly after stirring for steam explosion. The acid and alkali concentration needed for a specific processing situation is determined by the processing effect and the purpose. It is worth pointing out that water catalytic hydrolysis without adding any acid or alkali is now a promising treatment method.

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If less presoak liquid is added during the material presoak treatment process, steam explosion can be carried out directly without dehydration. If the amount of leaching liquid used in the material presoak treatment process is excessive, as in total immersion stirring processing, the excess liquid needs to be removed in advance before it is sent into the steam explosion chamber. The equipment for removing preimpregnation liquid can be either simple or complex, according to the actual production situation. The main types are twin wire extrusion machines, roller pulp dehydration units, presstype dryers, and others. In the twin wire extrusion machine, the wet slurry material is dehydrated by gravity, then sent into the dual network for preliminary squeeze dewatering, and finally dehydrated through a mechanical pressure device, which can increase the slurry concentration to 30%. This machine has the advantages of large production capacity, compact structure, small floor space, and high work efficiency (Chen and Liu, 2007a). For a roller dehydration unit, the material is dehydrated by a squeezing roller before entering the roller, and this machine is suitable for batch production of small quantities. The press-type dryer, just as its name implies, squeezes out the moisture from the slurry through hydraulic extrusion. The device can lower the moisture content of presoak material to 45–48%, with low investment cost, simple structure, easy operation, small maintenance workload, and higher dehydration efficiency, which is suitable for larger factories. Steam explosion is helpful for separation of the fibrous tissue in materials, and through carding processing, the material can be used according to its fiber length or particle size classification, to improve the comprehensive utilization of raw materials. At present the equipment used for material carding processing after steam explosion mainly includes the Paul fiber screening instrument and airflow classified equipment. Of these, the Paul fiber screening instrument uses liquid flow to transfer the material across sieves with different meshes, and after a certain period of time, collects fiber in various screening slots separately. It is widely used in processing, quality control and research in wood fiber production and also in the production of other industrial materials such as glass fiber or natural synthetic fiber. An airflow grading device is a commonly used gradation method depending on the materials used, in which a grading machine, cyclone separator, dust collector, and induced draft fan form a hierarchical system. When particles and air are mixed in certain proportions and sent into the sizing chamber, according to a certain speed, the particle is affected by the three-dimensional force of the turbine blade’s centrifugal force, air resistance, and gravity. By changing parameters such as turbine speed and system air volume to adjust its classification, the separation of fine and coarse particle materials is realized. It is suitable for the detailed classification of dry micron-grade products, including spherical particles, flake, and irregularly shaped particles. The device can also classify particles of different densities. In addition, Prof. Chen proposed the dry classification method and its corresponding equipment for short- and long-fiber lignocellulose (Figure 3.8). The mechanical combing classification device consists of a high-speed rotating drum and a fixed arc concave board, where the materials move between the cylinder and concave plate, and are loosened by hitting, kneading, rolling, and comb brushing. Material is fed from one end of the roller, moving along the cylinder axis, and thrown from the other end of the cylinder, which explains why the device is called

58

Lignocellulose Biorefinery Engineering

Figure 3.8 Short- and long-fiber dry classification equipment.

an axial flow device. Because the separation device uses centrifugal force to separate short or long fibers, it can be made of a drum of axial flow and sieve plate or concave plate to realize the two functions of loosening and separating, thereby simplifying the structure. The main factors influencing the effect of carding points include the water content of the gas phase explosion material and drum rotational speed. Fiber bundle tissues and cells can be obtained after combing classification, and the long fiber obtained from the combination of steam explosion, mechanical combing, organization, and classification can be used for papermaking, textiles, and other industrial applications of high-quality fiber raw materials. Small fibers consisting of parenchyma cells can also be used for the production of fertilizer, feed protein, or enzyme fermentation to produce a series of biological chemical products such as ethanol and butanol, so as to realize the component fractionation, cleaning, and high-value use of lignocellulose fiber (Wang and Chen, 2013c).

3.4.2 3.4.2.1

Steam explosion technology types Low pressure and nonpolluting steam explosion technology

Natural resources from agricultural production include food, fruits and vegetables, nonstaple crops, agricultural waste straw, and so on. At present, the processing technology used for agricultural products is at a basic level with outdated equipment; for agricultural waste such as agricultural products, straw, and others, mechanical crushing or acid and alkali treatment is often used as pretreatment, and practical high-value utilization technologies are rare. This not only leads to high energy and material consumption, and wastewater pollution from the production process, but also results in the

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low utilization of resources and less variety from a single product form, from which it is difficult to form high-value products. Prof. Chen assessed the differences in chemical composition and structure for straw and wood; based on this assessment he improved the previous steam explosion technology and proposed a steam explosion technology with low pressure and no pollution. Chen also developed 3- and 5-m3 water vapor explosion tanks with quick-open doors, and applied for a number of national patents (Chen, 2013). The technology is under low pressure (usually <3 MPa), with saturated steam as the heat transfer media material, and after a period of time, it breaks down the material structure through quick evaporation of water by instantaneous explosion within these materials, in order to realize component separation. The novel steam explosion technology has the following features: (1) Compared with the traditional mechanical crushing and chemical reagent treatment, there is no need to add any chemical reagents during the process of explosion, which is beneficial for processing and can eliminate pollution. (2) The low steam pressure required lowers energy consumption, with generally 0.2–0.5 tons steam consumption per ton. Also, the reagent consumption cost and the human cost are significantly lower than those of other pretreatment technologies. More importantly, there is no pollution control cost for this technology. (3) The hemicellulose degraded in the process of steam explosion can produce high valueadded dichotomous growth factor and realize the goal of beneficial utilization. Therefore, it can fundamentally solve the pollution problem of the steam explosion process and greatly reduce the production cost. Currently, the nonpollutant steam explosion technology has broad application prospects in industry, such as comprehensive utilization of straw, tobacco processing, paper production, Chinese herbal medicine extraction, and clean hemp degumming.

This production process is a batch-type explosion process. The device consists of a steam generator, steam explosion tank, feeding valve, and receiver. The specific operation process used for components separation of wheat straw is shown in Figure 3.9. After dust removal, the wheat straw and other straws are sent into the steam workshop. Materials are first given a presoak treatment because differences in starting material moisture content will influence the time needed to reach the highest cooking temperature, which also affects steam explosion performance. Experiments proved that when the material moisture content was 34%, the steam explosion effect was better, with the highest hemicellulose recovery rate, and this was also conducive to the improvement of the performance of the pulping and paper pulp. Presoak processed materials are sent into a steam tank, and the ventilation is heated to the required pressure and temperature with high-pressure steam after the presoaked processed materials are sent into the steam tank, and after a certain pressure time, the outlet is quickly opened to release the pressure for explosion. In the actual process of steam explosion, adjustments can be made according to the properties and usages of the exploded materials and different steam explosion devices, in order to meet the required demands. The straw with 34% moisture content, after heat preservation for 4.5 min at 1.5 MPa pressure, undergoes steam explosion. Electron microscopy pictures of materials before and after steam explosion are shown in Figure 3.10. Extraction conditions

60

Lignocellulose Biorefinery Engineering

Wheat straw (water content: 34%)

Steam explosion 1.5 MPa, 4.5 min

Countercurrent extraction for times Washing Filtration

Temperature: 75 °C

Liquid

Purification

Fermentation

Decoloration: D412 resin

Solid/liquid =15/100 Sizing agent

Ethanol recycle

Ethanol extraction Temperature: 160 °C; Time: 1h Filtration

Ethanol: 70%

Liquid

Cryogenic distillation

Precipitate 0.3 mol/l HCl pH = 4.0

Solid/liquid = 1:50 Sizing agent

Lignin purification Andors Bjirkman Method

Cellulose

Figure 3.9 Components of the wheat straw separation process.

Figure 3.10 Surface fiber (a) (b) and fiber bundle (c) (d) of the Ephedra herb before (a) (c) and after (b) (d) steam explosion (Chen and Liu, 2007a).

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of hemicellulose sugar degradation after steam explosion are as follows: the solid to liquid ratio is 15/100 (w/w), the temperature is 75  C, the materials are extracted countercurrent for three times, and the hemicellulose recovery rate is over 80%. Recycled hemicellulose monosaccharides and polysaccharides can be further purified for fermentation or as a product for sale. The remaining solid materials after extraction can react with 70% ethanol under 160  C for 1 h (solid to liquid ratio is 1:50), resulting in recycled alcohol-soluble lignin and high-purity pulp. Ethanol can be recycled, and lignin after further purification can be used in the synthesis of chemical products. Under these conditions, the lignin recovery rate can reach 75%. The obtained pulp can become higher-quality paper after subsequent processing such as bleaching, and it can also be directly used in sugar enzymolysis for liquid fuel ethanol fermentation, where the digestion rate can reach over 90%. Low-pressure and pollution-free steam explosion technology, which has been successfully amplified to an industrial scale (50 m3), has a good adaptability for straw with low raw material lignin and high acetyl content, and it has characteristics such as no chemical reagent presoak, low vapor pressure, low energy consumption, and no pollution. Thus, it fundamentally solves the problem of steam explosion pollution, not only by realizing comprehensive utilization of straw with no pollution, but also by being successfully applied in tobacco processing, the paper industry, Chinese herbal medicine extraction, and the clean degumming hemp industry, which helps to improve the environmental credentials of these industries.

3.4.2.2 Gas-phase explosion technology of mixed media When water is used as a medium for steam explosion, the two forms of water (namely gaseous and liquid water), are regarded as heat transfer media. The water mainly includes the moisture of materials, water derived from rehydration, and water condensed from steam. High temperature steam seeps into the organization and pores of materials to heat both the material and the liquid inside, and then forms a hightemperature and high-pressure water cooking environment to soften and degrade the components of materials. Rapid evaporation of water caused by the sudden release of pressure damages the compact structure of wood fiber, so as to achieve the aim of component separation and improve the utilization rate of the effective components. During this process, as well as being the transfer media, water is also the reaction media and causes many changes under high temperature and high pressure environments. However, such reactions result in the decomposition of target active ingredients in addition to the destruction of the wall-like barrier. Therefore, in dealing with materials with heat-sensitive composition, or material that is easily hydrolyzed in the presence of high-temperature and high-pressure water, the steam explosion medium needs to change correspondingly in order to achieve the desired treatment effect. The explosion medium is no longer purely the water in the steam explosion technology, but includes “gas explosion” collectively with steam explosion. Prof. Chen came up with the idea of putting air, carbon dioxide, ozone, liquid nitrogen, nitrogen, steam, supercritical CO2, and other media such as ethanol into the steam explosion medium, and

62

Lignocellulose Biorefinery Engineering

for the first time put forward the concept of “gas explosion” and expanded the scope of steam explosion operational conditions, broadening the application field of steam explosion. Mixed media gas phase explosion technology allows a gas medium of nonaqueous vapor, or bubbles into the nonaqueous gas medium first, and then allows steam to enter after reaching a certain pressure. After maintaining the pressure and temperature for a certain time, the pressure is reduced instantaneously. This gas phase explosion method has the following features: (1) under the original pressure condition, the operation temperature is lower, especially applicable for heat sensitive materials, such as Ephedra (Yuan and Chen, 2006), Eucommia, and other medicinal plants (Chen, 2010); (2) it avoids the presence of water on the target component, eliminating the hydrolysis in the process of steam explosion; (3) the use of low temperature and constant pressure breaks down lignocellulose components, reducing the high temperature and high pressure as well as the production of enzyme inhibitor, thus aiding enzymatic hydrolysis and fermentation (Zhang et al., 2013); and (4) some gas can be used as the pressure source for the explosion, also as a solvent gas for the extraction of target components or products in the process of steam explosion. Ephedra spp. are important in Chinese traditional medicine. Its chemical composition contains more than 10 kinds of alkaloids, mainly 1-ephedrine and d-pseudoephedrine. Its effects are numerous: soothes wheezing, antitussive, expectorant, diaphoresis, and diuretic. Ephedrine is mainly distributed inside the cortex fiber, and its extraction must overcome the mass transfer resistance of the epidermis, the fibrous layer, and especially the cell walls. The compact structure of cell walls is the main obstacle to the extraction of effective components, resulting in an ephedrine extraction rate that is not high by current industrial extraction methods. By using steam explosion treatments the plant cell walls can be broken, which is beneficial to the release of active ingredients. In the steam explosion process of Ephedra spp. for ephedrine extraction, the plant materials are stewed at high temperature (150–210  C). However, ephedrine is volatile and is easy to lose along with steam under high temperature conditions. To avoid volatilization and improve the extraction efficiency and extraction yield of ephedrine at the same time, our group carried out research on steam explosion pretreatment of air mixed with steam for ephedrine extraction from the Ephedra herb. This new steam explosion method reduces the steam temperature while keeping the same pressure during steam explosion. The steam explosion condition for the Ephedra herb is as follows: the pressure is 0.8 MPa with air inletting, then is quickly increased to 1.5 MPa, and then released for explosion after holding the pressure for 3 min. Under these conditions, the ephedrine extraction yield reached 0.345%; higher than that of direct extraction by 243%. By electron microscopy, organization changes of Ephedra before and after explosion steam were observed. The surface of the Ephedra herb without steam explosion was small and thin, while the holes on the surface of the Ephedra herb after steam explosion treatment were large and dense. Surface fiber and fiber bundles of the Ephedra herb after steam explosion were curly, folding, and becoming soft, and some fibers were fractured as shown in Figure 3.10. This was the root cause of steam explosion promoting the active ingredient extraction rate.

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3.4.2.3 Two-phase steam explosion technology For lignocellulose raw materials, especially crop straws, the organizational structure is extremely uneven, causing significant differences in the physical chemistry of different organizational structures occurring in the process of steam explosion. For example, corn stover contains leaves, pulp, and skins; the mechanical properties of leaves and pulp are weak, easily broken, while the skin mainly contains mechanical organization with strong mechanical performance that is not easily broken. In addition, leaves and pulp mainly contain miscellaneous cells with much more cell content, the chemical reaction is strong in the steam explosion process, and it is easier to generate inhibitors to subsequent sugar platform construction. Therefore, based on the mechanical property differences of different structures within the raw material itself and the differences in the stability of different organizational structures under the acid environment of high temperature and high pressure, Prof. Chen put forward a two-phase steam explosion and carding process (Chen, 2013). In the first stage of the process under the lower steam explosion strength, the structures with weaker mechanical properties are crushed first, while the crushing effects on the parts of the plant with strong organizational structure are not obvious. According to the different material forms after steam explosion, separation can be achieved through mechanical combing equipment. Because the first stage of steam explosion is a low-strength steam explosion, the production of inhibitor is low. At the same time, the first stage of the process achieves even rehydration of materials, as well as mechanical tearing effects, increasing the material porosity and fluffing out the organizational structure. The material used in the second explosion stage has no obvious effects from the first explosion process, and the second process is also under low-strength steam explosion. Because of the even rehydration of materials and the effects of mechanical tearing in the first stage of the process, the materials used in the second stage have an even water content and loose tissue, and the even explosion of materials can be realized under lower steam explosion strength. The two-phase steam explosion with carding process for corn stalk is expounded below as an example. (1) The feasibility analysis of two-phase steam explosion with a carding process. The nonuniformity of natural cellulose raw materials leads to differences in the ways of utilization and pretreatment. In the process of steam explosion, the extent of steam (as the solvent in the process of high-temperature cooking as well as the power source of the physical process of tearing) penetration into the cells inside raw materials and the resistance of the cell tearing strength can both affect steam explosion effects. Beyond the interference of external factors such as particle size and filling coefficient, the cell wall thickness of the materials and cell cavity size are the main factors influencing the steam infiltration, and the cell wall thickness is the main factor influencing the tearing strength of the cell. In view of the fact that the morphological structure of the thick cavity of fiber cell walls is small and the morphological structure of the thin cavity of the parenchyma cell walls is big, it can be shown that there are differences in the steam permeability of fiber cells and parenchyma cells.

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Moisture content (mass percentage)

As can be seen from Figure 3.11a, in the process of maintaining pressure, the rehydration properties of fiber cells and parenchyma cells are different: parenchyma cells have better rehydration properties than fiber cells, they absorb more and achieve earlier equilibrium. As can be seen from Figure 3.11b, the moisture contents of the two cells after the explosion of the steam are inconsistent, and the moisture content of parenchyma cells is higher than that of fiber cells. It can be seen from Figure 3.11 that, because the thick cavity of fiber cell walls is small and the thin cavity of parenchyma cells is big, the vapor permeability of fiber cells and parenchyma cells do not match, and steam, as a heat carrier in the process of steam explosion and a power source, is bound to affect the extent of the degradation and the degree of tearing of fiber cells and parenchyma cells. Therefore, without getting involved in organization and classification, if whole plant straw is regarded as the raw material, steam explosion pretreatment conditions can be varied to achieve a high hemicellulose degradation rate, a low entrainment rate, an increasing raw material specific surface area, and this may cause some excessive

0.7 0.6 0.5 0.4 0.3 0.2 Parenchyma cells Fiber cells

0.1 0.0

0

1

Moisture content (mass percentage)

(a) 0.7

2 3 Steam entering time (min)

4

Parenchyma cells after carding Fiber cells after carding

0.6 0.5 0.4 0.3 0.2 0.4

(b)

Figure 3.11 Comparison of rehydration conditions of fiber cells and parenchyma cells after the water blasting process. (a) The moisture content of fiber cells and hybrid cells under different steam admission times; (b) The moisture content of fiber cells and hybrid cells after water vapor blasting and carding (Zhang et al., 2012).

0.8

0.6

0.8

1.0

1.2

Holding pressure (MPa)

1.4

1.6

Feedstock engineering

65

parenchyma degradation, which increases the likelihood of inhibitor production. Therefore, appropriate steam explosion conditions can be chosen based on fiber cells and parenchyma cells, respectively, to achieve their best pretreatment effect. It can be seen from Figure 3.11 that steam explosion helps to achieve even rehydration of fiber cells and parenchyma cells. Zhang et al. found that steam explosion was beneficial in improving the degree of materials isolation, for example fiber cells and parenchyma cells (Zhang et al., 2012). Using this method, in the first period of steam explosion, milder conditions were taken to realize even rehydration, and based on the principle of centrifugal air weight classification to separate the parenchyma cells of high moisture content and the fiber cells of low moisture content, the fiber cells were isolated and taken to the second period of steam explosion in more moderate conditions. Therefore, reasonable design of this method is feasible. (2) The effect of two-phase steam explosion with a carding process on inhibitor content. It can be seen from Figure 3.12 that the inhibitor content of parenchyma cells after the first period of carding was higher than that of fiber cells, and the inhibitor content after the first period of steam explosion without combing is in between, which shows that under the same steam explosion conditions, the extent of degradation of parenchyma cells is greater than that of fiber cells. As a result, the lignocellulose nonuniformity of the organizational structure itself determines the diversity and selectivity of the pretreatment conditions, and the result also indicates the necessity of classification and selective pretreatment. In steam explosion, the materials obtained from the second stage had lower inhibitor contents had lower than the control group, with the lowest content at 1.1 MPa holding pressure (Figure 3.12). (The exception to this was the material held at

Conversion of inhibitor (%)

0.06

0.05

0.04

0.03

Fiber cells after ISFC Parenchyma cells after ISFC Control group (1.2 MPa/8 min) First-step steam explosion Second-step steam explosion Integrated process

0.02

0.01 0.4

0.6

1.0 1.2 0.8 Holding pressure (MPa)

1.4

1.6

Figure 3.12 Effect of the two-phase steam explosion carding process on inhibitor content (Zhang et al., 2012). ISFC, intermediate separation of fiber cells.

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Degree of seperation (fiber cells/parenchyma cells)

1.5 MPa which had higher inhibitor contents than the control group (control group: steam explosion conditions of 1.2 MPa and 8 min)). The reason for the lowest inhibitor content at 1.1 MPa may be that the fiber cells and parenchyma cells can achieve the best degree of separation (Figure 3.13) at 1.1 MPa, and after carding the amount of parenchyma cells in the materials at the second stage of steam explosion is the least with the lowest inhibitor content. In the holding pressure range from 0.5 to 0.9 MPa, the inhibitor content increases. This is firstly because the steam explosion strength is increasing, and the material degradation rate increases with more by-products; and secondly, because the degree of degradation of the raw materials is poor within the pressure range of 0.5–0.9 MPa, fiber cells and parenchyma cells do not achieve good separation. At pressures of 1.3–1.5 MPa, the degree of degradation of fiber cells and parenchyma cells is good, but because its strength is relatively violent, the inhibitor content increases. Combining with Figure 3.11, the best degree of separation is 1.6 when the inhibitor content of corn straw by pretreatment is the lowest under this condition. (3) The effect of the two-phase steam explosion with carding process on cellulose enzymolysis rate. It can be seen from Figure 3.14 that the enzymatic rate of parenchyma cells was higher than that of fiber cells after the first period of carding, and the digestion rate of the materials from the first-step steam explosion without combing was in between, which also suggests that under the same steam explosion conditions, the degree of degradation of parenchyma cells was greater than that of fiber cells. Compared with the control group, in the steam explosion materials obtained from the first step, the digestion rate was higher than the control group at 1.1 MPa holding pressure. In the steam explosion materials obtained from the second-step, apart from the 0.5 MPa holding pressure condition, the digestion rates of the other groups were higher than that of the control group, and with the increase of steam explosion strength, the digestion rate increased.

6 Experiment groups Control group (1.2 MPa/8 min)

5

4

3

2

1 0.4

0.6

0.8

1.0

1.2

1.4

1.6

Holding pressure (MPa)

Figure 3.13 Degree of separation of fibers under different steam explosion conditions (Zhang et al., 2012).

Enzymatic hydrolyzation of materials (%)

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67

0.9

0.8

0.7

Control group (1.2 MPa/8 min) First-step steam explosion Parenchyma cells after ISFC Fiber cells after ISFC Second-step steam explosion Integrated process

0.6

0.5

0.4 0.4

0.6

0.8

1.0

1.2

1.4

1.6

Holding pressure (MPa)

Figure 3.14 Effect of different steam explosion methods on the digestion rate of materials (Zhang et al., 2012).

Based on Figures 3.12–3.14, the optimal process conditions of steam explosion with carding is 1.1 MPa\4 min-carding–1.2 MPa\4 min, when the fiber had moderate degradation and reduced to avoid excessive degradation of parenchyma tissue with the lowest inhibitor content by pretreatment (Zhang, 2012). (4) The effect of the two-phase steam explosion with carding process on 2,3butanediol yield. It can be seen from Figure 3.15 that the inhibitor content of the experimental group was the lowest while the content of 2,3-butanediol was the highest. Compared with one-phase steam explosion (1.2 MPa/8 min), the inhibitor content was reduced by 33%, and fermentation product 2,3-butanediol content increased by 209%. Compared with the 2,3-butanediol fermentation with only sugar as the carbon source, the 2,3-butanediol content of the experimental group was higher than that of the control group of sugar, while the sugar content of other groups was lower than the control group, which showed that inhibitor produced under the pretreatment condition of the experimental group not only had no inhibition to the production of 2,3-butanediol, but also promoted the conversion of 2,3-butanediol; while the inhibitors produced under the pretreatment condition in other groups inhibited the generation of 2,3-butanediol.

3.4.3

Technical evaluation of steam explosion technology

Prof. Chen established the steam explosion and component separation technology based on 10 years of research, which formed the lignocellulose refining technology platform. Based on this platform, a series of cost-effective lignocellulose refining processes were developed and bio-based energy, bio-based chemicals, and bio-based materials and many other products were produced.

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0.45 0.40

Conversion (%)

0.35 Inhibitor 2,3-BD 2,3-BD in fermentation control group (using glucose as the only carbon source)

0.30 0.25 0.20 0.15 0.10 0.05 1.1(4)–ISFC–1.2(4)

1.2(8)

1.1(4)–1.2(4)

1.5(4)

Conditions

Figure 3.15 Effect of the two-phase steam explosion carding method on 2,3-butanediol (Zhang et al., 2012). 1.1(4)-ISFC-1.2(4) indicates 1.1 MPa, 4 min-ISFC-1.2 MPa, 4 min, etc.

Lignocellulose refining has various advantages based on steam explosion as the core technology: (1) Steam explosion can be applied to a variety of plant lignocellulosic raw materials, and pretreatment conditions are easy to control. (2) Three components (i.e. hemicellulose, lignin, and cellulose) can be separated in three different separation processes to give water-soluble components, alkali-soluble components, and alkali-insoluble components, respectively. (3) Cellulose conversion rates in hydrolysis can reach the theoretical maximum. (4) After the steam explosion lignin can still be used for the production of other chemical products. (5) Sugars from hemicellulose can be completely used, and can be converted into liquid fuels. (6) Fermentation inhibitors generated in steam explosion pretreatment can be reduced by controlling the pretreatment conditions. (7) The temperature and pressure of the steam explosion process can be controlled separately. (8) In situ steam explosion of hemp material can be achieved, which avoids the entanglement and knotting of the material. (9) The costs of steam explosion pretreatment are relatively low compared with other pretreatments.

3.4.3.1

Comparison of different pretreatments

As shown in Table 3.4, based on the existing pretreatment techniques, the following conclusions can be drawn:

Feedstock engineering

Table 3.4

69

Comparison of different pretreatment technologies

Pretreatment methods Mechanical grinding

Pyrolysis

Liquid hot water High-energy radiation

Dilute acid

Advantages

Disadvantages

Decrease of particle size, increase of specific surface area, decrease of degree of crystallinity, increase of watersoluble material Using zinc chloride or sodium carbonate as a catalyst, pure cellulose can be obtained at low temperature

High energy consumption, high operating costs

Hemicelluloses recovery is above 80%; cellulose conversion can reach 90% No swelling of cellulosic materials; enhanced accessibility and reactivity; high sugars concentration; shortened time High hemicellulose yield, low catalytic cost

Dilute alkali

Cellulose swelling; reduced cleanliness; higher sugar yields after enzymatic hydrolysis

Ozone

Efficient removal of lignin; low toxic compounds concentration; low temperature and atmospheric pressure Low capital costs; low inhibitors production for obstruction of microbial growth, acidification hydrolysis, and fermentation Low energy consumption; environmentally friendly Shortened pretreatment time; high enzymatic hydrolysis conversion; low energy consumption; wide applications; no need for size reduction

Organic solvent

Biodegradation Steam explosion

Complex products; difficulty of separation; when the temperature reduces to low value, the decomposition rate slows down; the generation of volatile byproducts Low solid loading; high energy consumption; low production efficiency Radiation doses higher than 100 Mrad; high processing costs; difficult to industrialize; environmental pollution Hydrolysis is slow; the presence of acid recovery and other issues; severe equipment corrosion; by-products production High alkali consumption; difficulty of reagent recycling; neutralization, washing, and other issues; not suitable for mass production Requires a large amount of ozone; high production costs

Presence of corrosion and toxicity issues; causes environmental pollution Long processing time and long production cycle Washing unit reduced the sugar yields; inhibitors production

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Lignocellulose Biorefinery Engineering

(1) Traditional chemical treatment and mechanical pretreatment technologies consume more energy and result in environmental pollution, and so are not suitable for modern industrial production processes. (2) Although biological pretreatment is environmentally friendly and has low power consumption, its longer pretreatment time and production cycle limit its application in engineering. (3) Compared with other pretreatment methods, steam explosion pretreatment has various advantages such as low energy consumption and environmental friendliness, which is a favorable direction in the development of pretreatment technologies. However, its high temperature and long cooking time result in low pretreatment efficiency. Meanwhile, pretreatment conditions should be varied for the different feedstocks.

3.4.3.2

Pretreatment economy

The costs of steam explosion pretreatment are lower compared with other pretreatments. For example, the steam consumption of general materials is 0.2–0.5 tons steam per ton of feedstock, which indicates the low costs of energy and reagent consumption in steam explosion. Meanwhile, the labor costs are significantly lower than other pretreatment techniques. The capital costs of pretreatment are shown in Figure 3.16. Based on the structural characteristics of lignocellulosic feedstock and the requirements of the target product, lignocellulose utilization in modern industrial production processes can be achieved by introducing steam explosion pretreatment and selective

Pollution Maintenance Solvent Energy consumption Manpower

SE-4 SE-3 SE-2 SE-1 SE-acid-2 SE-acid-1 SE-ammonia Mechanical milling 0

200 300 100 Pretreatment cost (tty RMB/year)

400

500

Figure 3.16 Capital costs of different pretreatment technologies. The conditions of steam explosion (SE): 1/2/3, temperature 167  C, time 5 min, initial moisture content 0.7/0.5/0.3, charge coefficient 80 kg/m3; 4, temperature 167  C, time 5 min, initial moisture content 0.3, charge coefficient 87.6 kg/m3. The conditions of steam explosion with acid addition (SE-acid): 1/2, the ratio of SO2 to feedstock (dry weight) 1.6%, temperature 164  C, time 5 min, initial moisture content 0.5/0.3, charge coefficient 80 kg/m3. The conditions of steam explosion with ammonia addition (SE-ammonia): the ratio of SO2 to feedstock (dry weight) 1:10 (w/w), temperature 164  C, time 5 min, initial moisture content 0.5/0.3, charge coefficient 80 kg/m3. tty RMB/year stands for ten thousand yuan per year.

Feedstock engineering

71 Cellulose screening grinding

Fiber cells

Polyether glycol

Parenchymal cells

Enzymatic hydrolysis

Cellulose

Biomass

Steam explosion

Hemicellulose

Enzymatic hydrolysis

Washing liquid

Fermentation

Bioenergy

Fermentation

Chemical products

Solid residues

Pyrolysis

Bio-oil Lignin

Phenolic resin

Xylooligosaccharide

Figure 3.17 Schematic flow chart of steam explosion technology as the core technology in lignocellulose biorefining.

component deconstruction into the conversion process, which also results in minimal energy consumption, optimum conversion efficiency, and the maximum amount of clean production. Such an idea is the core concept for the lignocellulose biorefining process using steam explosion pretreatment technology, and aims to achieve the maximum economic benefits from lignocellulosic resources. A schematic diagram of raw material utilization after steam explosion is given in Figure 3.17.

3.5

Establishment of the sugar platform

The establishment of sugar platforms is the process whereby the constituents of lignocellulosic feedstock (including hemicellulose, cellulose, pectin, soluble sugars, and other ingredients) can be directly converted into sugar, which can then be used in an industrial process or as the basis for further conversion. Sucrose and starch are generally used directly as raw sugars for industrial production because they are easy to obtain and ferment, and are fit for industrial production. However, the current supply of sucrose and starch cannot meet the needs of modern industry. Cellulose, hemicellulose, and other lignocellulose components can be converted to sugar for industrial utilization due to their abundant reserves and renewability, and this will help to ensure food security.

3.5.1

Hemicellulose platform

3.5.1.1 Introduction Hemicellulose is a general term for a group of complex polysaccharides in plant cell walls; it is the second largest category of plant cell wall polysaccharides, and mainly exists in the primary walls and secondary walls of the plant cell walls. The

72

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structure of hemicellulose is complicated, and is primarily composed of several different types of pentoses (b-D-xylose, a-L-arabinose), hexoses (b-D-glucose, b-D-mannose, a-D-galactose, a-L-rhamnose, and a-L-fucose), and glycolytic acid (a-D-glucose fermentation acid, a-D-4-O-methyl-glucose acid, and a-D-galacturonic acid). Hemicellulose is formed by a glycosidic bond between the monosaccharide groups. Hemicellulose is closely attached to cellulose through an oxygen–hydrogen bond linkage and van der Waals forces, which is considered to be a bridge between the fiber fines. Hemicellulose and lignin form the hemicellulose–ferulic acid–ether bond–lignin bridge structure with ferulic acid. Some hemicellulose polysaccharide chains have two ferulic acid molecules, or even have a side chain connecting with coumaric acid molecules. In addition, there are also connections between hemicellulose and protein, most generally the hydroxyproline residue in the 3- or 4-position connecting to arabinose in hemicellulose. Compared with cellulose, hemicellulose has a naturally low molecular weight and is also multibranched. The average degree of polymerization (DP) of hemicellulose is 80–200. Hemicellulose has different structures according to its different biological origins.

3.5.1.2

Methods of establishing a hemicellulose platform

Alkali method The traditional separation of components is primarily achieved by industrial pulping. In the alkaline pulping process, more complete cellulose can be obtained, but hemicellulose and lignin are degraded into small molecules and discharged into the black liquor. The alkali method is the most common separation method for hemicellulose; the method not only swells the cellulose, but also breaks the connecting bonds (ester bonds, ether bonds) between hemicellulose, lignin, and hydroxy cinnamic acid, and hence dissolves hemicellulose from the cell wall.

Acid method Acid hydrolysis can be used for the preparation of monosaccharide. Hydrochloric acid, sulfuric acid, trifluoroacetic acid, formic acid, and nitric acid are commonly used in acid hydrolysis. Compared with other methods, acid hydrolysis results in a higher sugar yield and good reproducibility. However, acid hydrolysis will produce large amounts of degradation products from monosaccharides such as furfural generated from pentose and hydroxymethylfurfural produced from hexose (which further degrades into formic acid and levulinic acid). Therefore, the type of acid, pH, reaction temperature, and reaction time should be considered carefully in acid hydrolysis.

Autohydrolysis method The autohydrolysis method is also called the liquid hot water method, and is most commonly used in the production of oligosaccharides. Under conditions of high temperature and pressure, hydration oxygen ions in water and acetic acid generated from acetyl in hemicellulose cause the cleavage of glycosidic bonds, which are the backbone of hemicellulose, the degree of polymerization of hemicellulose reduces, and the desired products dissolve into the hydrolysis solution. Most of the cellulose and lignin

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73

are retained in the residue. Then the hydrolyzate is separated and purified, by-products and nonsugar components are removed, and high-purity oligosaccharides are obtained. The autohydrolysis method has many advantages including not requiring any chemicals other than water, noncorrosivity to equipment, higher enzymatic reaction rate, milder reaction conditions, and less degradation of the cellulose-rich residue. However, the hydrothermal treatment will produce degradation products from the monosaccharide, including formic acid, acetic acid, levulinic acid, furfural, and a few lignin degradation products.

3.5.1.3 Product development of the hemicellulose sugar platform Hexoses from hemicellulose can be used for ethanol production by fermentation and sorbitol preparation by reduction; these have important applications in the food, toothpaste, cosmetics, explosives manufacturing, and papermaking fields. Five-carbon sugars are used to produce multifeed yeast, bran acid, xylose, and xylitol. Xylitol can be produced from xylose by using a chemical method of oxygen catalysis under high-pressure.

3.5.2

Cellulose platform

Cellulose is the largest component of plant cell walls, and is composed of homogeneous b-1,4-glycosidic bonds. Commonly used conversion methods for cellulose include acid hydrolysis and enzymatic hydrolysis.

3.5.2.1 Methods of establishing a cellulose platform Acid hydrolysis Acid hydrolysis includes both concentrated acid hydrolysis and dilute acid hydrolysis. Concentrated acid hydrolysis was first reported in 1883. The principle of concentrated acid hydrolysis is that crystalline cellulose can be completely dissolved in 72% sulfuric acid or 42% hydrochloric acid or 77–83% phosphoric acid at a lower temperature, resulting in the homogeneous hydrolysis of cellulose. During this process, cellulose can be converted into several oligosaccharides containing glucose units, mainly cellotetraose (four-glucose polymers). With further dilution by water and heating for a certain time, cellotetraose is hydrolyzed to glucose. Concentrated sulfuric acid hydrolysis technology is more mature than other concentrated acid methods, and about 90% of sugars converted from cellulose and hemicellulose can be recycled. Two hundred grams of dry pulverized raw material is mixed with 500 g sulfuric acid, and then diluted to 20–33% sulfuric acid. The hydrolysis is conducted at 100  C for 30–120 min and the corresponding yield of sugar is 78–82%. The most obvious advantage of concentrated acid hydrolysis is its higher sugar yield. However, concentrated acid hydrolysis needs a long reaction time, and high concentrations of acid, which results in serious corrosion of the equipment. Meanwhile, high acid concentrations will lead to environment pollution which is difficult to recycle, further limiting the development of concentrated acid hydrolysis.

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Dilute acid hydrolysis is a process in which cellulose is hydrolyzed to glucose monosaccharide by using an inorganic acid such as sulfuric acid or hydrochloric acid as the catalyst, generally under a 10% (w/w) load. The hydrolysis reaction temperature is usually between 100 and 240  C, and the pressure is usually more than 1 MPa, which is above the saturated vapor pressure of the liquid. Dilute acid hydrolysis put forward by the Frenchman Mel Parsons in 1856, is a simple process. Dilute acid hydrolysis has the advantages of low acid concentration, low impact on the environment, simple waste processing, and low raw material costs. However, because the hydrolysis process requires an environment with high temperature and pressure, the development of the dilute acid hydrolysis process was initially very slow. With the development of chemicals and equipment, and the appearance of a variety of new materials that can tolerate violent temperature, pressure, and acid, dilute acid hydrolysis has developed quickly. Because the dilute acid hydrolysis process produces by-products such as formic acid, acetic acid, 5-hydroxymethylfurfural (5-HMF), and phenolic compounds, researchers should continue to study various physical, chemical, and biological methods to remove cellulose hydrolyzate and fermentation inhibitors to improve fermentation yield. Factors influencing dilute acid hydrolysis include the degree of grinding of the cellulosic feedstock, liquid-to-solid ratio, reaction temperature, reaction time, acid concentrations, and co-catalyst type and concentration. The higher the degree of grinding of the cellulosic feedstock the greater the contact area for the acid catalyst, resulting in better hydrolysis performance. When the reaction rate is fast, monosaccharide can be removed from the solid surface in a timely manner, which is favorable for the reaction. The liquid-to-solid ratio is the ratio of the volume of water to the quality of solid material. Generally, with an increase in the liquid-to-solid ratio, sugar yield from the cellulosic feedstock also increases. But the cost of hydrolysis increases, and the sugar concentration decreases, increasing the cost of the subsequent fermentation and distillation processes. The liquid-to-solid ratio of 5–20 mL/g is usually chosen for the conversion process. Temperature is an important factor affecting the rate of hydrolysis. Generally, when the temperature rises by 10  C, the hydrolysis rate will increase by 0.5–1 times. However, high temperature will accelerate the decomposition rate of the monosaccharide. Therefore, when high temperature hydrolysis is used, the reaction residence time should be reduced. Theoretically, if the other conditions remain unchanged, the concentration is double, and the acid hydrolysis time can be shortened by one-third to one-half. The increases of acid concentration lead to increased process costs, while the corrosion-resistance requirements of the equipment will increase, resulting in increased equipment costs, which is not suitable for industrial production. Generally, the acid concentration should not exceed 10%. Previous studies of acid hydrolysis focused on inorganic acids including hydrochloric acid and sulfuric acid. The efficiency of hydrochloric acid hydrolysis is superior to sulfuric acid, but the treatment of waste is more difficult than with sulfuric acid. Meanwhile, hydrochloric acid is more expensive than sulfuric acid and requires equipment with greater corrosion resistance.

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75

Enzymatic hydrolysis Enzymatic hydrolysis has been used since the 1950s. With the new interest in cellulose hydrolysis in biology, it has attracted increasing attention. Cellulose can be hydrolyzed to sugar using cellulase at 45–50  C and pH 4.8. Cellulase is not a single substance; it comprises three main components, i.e., endoglucanase, exoglucanase, and b-glucanase. Endoglucanase cuts the b-1,4 glycosidic bonds randomly, and the cellulose long strand is cracked, with the broken molecular chains having a reducing end and a nonreducing end; exoglucanase comprises two components, which cut glucose and cellobiose from the reducing end of the long chain, respectively; and the role of b-glucanase is to convert cellobiose and short-chain oligosaccharides into glucose. Enzyme hydrolysis can be carried out at normal temperature and pressure, reducing the energy consumption of the process. The enzyme has a high selectivity to produce a single product from the substrate, and a high yield of glucose (>95%) can be obtained. Because the cellulase has to enter the inner part of cellulose and then convert it into sugar, pretreatment should be done to remove hemicellulose and lignin or destroy the crystalline structure of cellulose. Typically, in order to facilitate enzymatic hydrolysis, the acid hydrolysis and enzymatic hydrolysis are combined with a pretreatment.

3.5.2.2 Products from cellulose sugar platforms Cellulose is composed of glucose, so the cellulose platform can meet the needs for conversion of glucose to industrial products, such as sorbitol, ethylene glycol, 1,2propanediol, and various fermented products.

3.6 3.6.1

Establishment of the lignin platform Introduction

Lignin is the cheapest raw material on earth that can provide aromatic compounds. Currently, in the industries using plant fiber as raw materials (such as papermaking, fiber textile, wood hydrolysis, and bio-fuels), lignin is discharged as waste; this is not only a waste of resources, but also an environmental pollutant. Therefore, with the increasingly short supply of petrochemical resources and worsening environmental pollution, recycling of natural lignin to replace oil and other nonrenewable resources can not only alleviate the situation of overreliance on petrochemicals, but also reduce environmental pollution. However, due to the complex structure and the wide range of molecular weights of lignin, it is unstable in its physical and chemical properties, which results in an inability to obtain products with costs comparable to those from petrochemicals. Therefore, lignin can only act as a partial replacement for fossil-based resources, taking advantage of its low price to make up for the deficiencies in product performance. It is not able to achieve the goal of high-value utilization due to the issues mentioned above. The first step toward changing the inefficient use of lignin is to overcome the unstable nature of lignin itself and to create relatively homogeneous

76

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and stable active products; this is also the most important step to achieve high-value utilization of lignin. Lignin, hemicellulose, and cellulose are the three main components of plant cell walls, and are widely present in vascular plants. The total amount of lignin on earth is exceeded only by cellulose. The chemical structure of lignin differs from cellulose and hemicellulose: it is a polymer formed by a three-dimensional network structure mainly taking aromatic benzene propane as monomer by connecting with the CdC bond or ether bond. Because of its differences from the other two kinds of polysaccharides, the development and utilization of lignin has often not been as good as the more highly valued cellulose and hemicellulose. However, because the lignin content in lignocellulosic feedstock is up to 30% and the energy content is as much as 40%, more attention should be paid to lignin in the lignocellulose biorefining process. We must have a comprehensive and detailed understanding of lignin before using it. In 1838, P. Payen in France isolated cellulose from wood, and he also found a compound that had a higher carbon content, which he called “real wood material” (the true woody material). After that, F. Schulze carefully isolated this compound, and it was named lignin, which was derived from the word for timber in Latin. However, due to the paucity of lignin applications, initial lignin chemical research made no great progress for decades. In the 1870s, with the industrialization of sulfite pulping, people became interested in chemical reactions in pulping, which led to an upsurge in research on lignin. Thereafter, due to the value of its research applications, lignin research developed rapidly. Although a variety of modern sophisticated analytical instruments have been gradually applied to lignin structure analysis, including twodimensional and three-dimensional nuclear magnetic resonance analysis, electron spin resonance absorption spectroscopy, and HPLC, there is no full understanding of its complicated structure. Based on current knowledge, the structure of lignin is generally composed of three benzene propane monomers, and a three-dimensional network polymer is built through CdC bonds and ether linkages bonds. The three monomers include guaiacyl propane (G), syringyl (S), and p-hydroxyphenyl (H) (Figure 3.18). Meanwhile, there are differences in the content of these three monomers in different plants. Lignin in softwood is mainly constructed by the guaiacyl structural unit; lignin in angiosperms is composed of guaiacyl structural and syringyl; and lignin in grassy plants is composed of guaiacyl, syringyl, and p-hydroxyphenyl. Covalently, bonds connecting the structural units include b-O-4 aryl ether; b-1, b-5, b-b, a-O-4 aryl ether; and diphenyl ether linkages. Due to the complex combination, there are significant differences in the structure of the lignin polymer molecules, leading to instability of subsequent products from lignin; this is an important reason for the difficulties in achieving highvalue utilization of lignin.

3.6.2

Methods of establishing a lignin platform

As mentioned above, the natural lignin in different plant cell walls has large differences, and the different separation methods will make the already complex polymer have even greater differences, and this determines its application in different fields.

Feedstock engineering

HO γ R

77 Lignin

Lignin

β O

α

OCH3 OH

OH γ

OCH3 OCH3 O

β

γ

α

Oα

β

O

α

Lignin O OCH3

Lignin

O

H3CO

O

O

OH

OCH3

(d)

(c) O

Lignin 6 5

OH

Lignin

2 OCH3 Lignin

H3CO

(e)

Lignin

OH

OCH3 Lignin

γ

OH

β

OH α

2

6

2

6

OCH3

HO

(b)

(a)

β γ

Lignin

Lignin

Lignin Lignin

(h)

(g)

(f)

Figure 3.18 Structures of lignin monomers and their linkages.

In fact, research on lignin is currently focused on two aspects. One is the analysis of lignin, including structural analysis in the separation process, reaction activity analysis of different lignin types, and analysis of the relationship between the structure of lignin and its physical and chemical properties. The second aspect is the application of lignin, which is the ultimate goal of theoretical lignin research. Here, the different lignin types are briefly introduced (shown in Table 3.5) based on structure, performance, and major applications.

Table 3.5 Separation reagents and applications of different lignin types

Analytical lignin

Industrial lignin

Reagent

Applications

Milling wood lignin Klason lignin

Water/dioxane

Lignin structure analysis

Sulfuric acid

Lignin quantitative analysis

Kraft lignin Alkali lignin

NaOH/NaS NaOH/ anthraquinone NaHSO3/Na2SO3

Burned for energy recovery Filler/binder/fertilizer

Lignosulfonate Organosolv lignin Hydrolysis lignin Steam-exploded lignin

Organic solvent NaOH/organic solvent NaOH/organic solvent

Binder/dispersant/retarder/ emulsifier Filler/phenol substitute Filler/sorbent/phenol substitute Filler/phenol substitute

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Analytical lignin

The reason for the relatively backward research situation of lignin is due to lack of clarity regarding lignin applications. Lignin’s complex structure is another main reason. Even so, researchers have proposed two commonly used separation methods to produce analytical lignin materials. Lignin obtained from these two methods is milling wood lignin and Klason lignin. The former is used for lignin structure (qualitative) analysis, and the latter is mainly used for the quantitative analysis of lignin.

Milling wood lignin One important reason why structural research of lignin has been impeded is the difficulty of lignin separation. Even today, it is not feasible to successfully isolate nondeteriorated lignin from plant tissues. This is because the lignin itself contains reactive groups that are prone to be degraded or self-condensed in the separation process. Almost pure natural cellulose is found on cottonseed cotton fibers, which advances the study of cellulose. If nearly pure lignin existed in plant organs, lignin research would be relatively easy to carry out. But because newly grown tissues in plants do not contain lignin, which only begins to appear after a certain period of time, such hopes of lignin acquisition are dashed. Therefore, in the study of the lignin structure in plant tissues, there are no analytic techniques that do not involve components separation. In 1957, Bjo¨rkman separated lignin from vibration milling wood flour with aqueous dioxane, and the lignin obtained was called milling wood lignin (Bjo¨rkman, 1956). Because the structural changes are relatively small in the separation process, milling wood lignin is currently suitable for lignin research.

Klason lignin Klason lignin is the insoluble residue portion after removing the ash by concentrated acid hydrolysis of the plant tissues, which is also an intuitive method for the determination of lignin content in plants. After the proposed determination methods of Klason lignin, a lot of research work was carried out to study the treatment conditions, and it was considered to be the most direct and the most reliable method for quantitative lignin analysis and was used as the standard method for lignin determination. The main steps are as follows: the lignocellulosic feedstock is extracted by benzene and ethanol, then 72% concentrated sulfuric acid is added and the reaction is conducted at 30  C for 4 h; the sulfuric acid is then diluted to 3% and reacted for 2 h with reflux, and the insoluble substrate is weighed as lignin (Sluiter et al., 2008).

3.6.2.2

Lignin for industrial applications

Industrial lignin consists of large quantities of lignin obtained from lignocellulosic feedstock by different treatments; applications include paper pulp, fiber textile, wood hydrolysis, and lignocellulose energy. Currently, in the field of lignocellulosic feedstock pulp, different lignin raw materials can be obtained according to different pulp methods, including Kraft lignin from Kraft pulping, lignosulfonate from sulfite pulping, alkali lignin from alkali pulp, and organic solvents lignin from organic solvent

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pulping. In recent years, due to the pressures regarding environmental protection and the shortage of petrochemical resources, bioconversion of lignocellulose for energy production has been a hot topic. Lignin from enzymatic hydrolysis residues of lignocellulose biomass has better prospects because it has more active groups due to the mild action of enzymatic hydrolysis. With the expanding application of steam explosion pretreatment in energy, textile materials, paper materials, and other fields, steam explosion lignin, whose structure is similar to the lignin generated from acid hydrolysis of wood, will be widely used in industry. Due to the effect of pretreatment on the removal of sugar, its sugar content is significantly lower than other industrial lignin materials.

Kraft lignin Kraft lignin is a kind of industrial lignin obtained from Kraft pulp, which accounts for about 85% of the total lignin production in the world. The Kraft pulp method is the main method for converting coniferous wood to pulp; the pulping yield is higher than other alkaline pulping methods. In the Kraft cooking process, about 90–95% of the lignin is dissolved into the cooking liquor containing sodium hydroxide and sodium sulfide. In the pulping process, the lignin macromolecules are fractured, the molecular weight is decreased, the lignin is dissolved in alkaline solution, making the solution turn dark brown. Currently, production of Kraft lignin is about 630,000 tons per year, it is mainly used in the form of combustion for heat recovery, resulting in low-value utilization. Although high-value utilization of Kraft lignin has been reported, there has been no good progress in industrialization.

Alkali lignin Alkali lignin is the lignin obtained from soda-anthraquinone pulping, during which grasses such as wheat straw, bagasse, and others are used as raw materials. The main difference between soda-anthraquinone pulping and Kraft pulping is that the lignin from the former does not contain sulfur. Owing to the characteristics of raw materials, both the viscosity of the black liquor of alkaline pulping and its silicon content is high, which adversely affects combustion and evaporation in the recovery system. However, higher purity lignin from the black liquor of alkaline pulping can be recovered by the acid precipitation method with repeated washing.

Lignosulfonate Lignosulfonate comes mainly from traditional sulfite pulping and other modified sulfite pulping processes. In the sulfite pulping process, sulfite ions are substituted for phenyl propane on the functional groups of side chains. The introduced sulfonate groups dissolve lignin, while most of the cellulose does not change and can be separated for paper and other fiber products. Meanwhile, there is a small amount of fiber and hemicellulose dissolved in the solution to form water-soluble monosaccharides and other substances, which are generally referred to as sulfite pulp waste. After classification by membrane filtration, high-purity lignosulfonate and small-molecule oligosaccharides or monosaccharides can be obtained by acid hydrolysis, and the former

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may be spray dried to obtain a solid product – lignosulfonate. Because lignosulfonate has prime sulfonic acid groups and is water soluble, it is widely used in industrial lignin utilization.

Organic solvent lignin Lignin extraction by organic solvents was adopted in the nineteenth century. Because the lignin in lignocellulose is removed using this method, it can be used in the pulp. However, the recovery rate of organic solvents is low, resulting in the high costs of this process. These results lead to this pulping technology not being widely used. Later, due to its low environmental pollution compared with other pulping processes and the high purity and activity of the lignin derived, organic solvent extraction has become a hot research area in recent years. Currently, there are two main organic solvent pulping methods. One is the ethanol method, during which the acetic acid generated from the cooking of raw materials is used as a catalyst, resulting in the depolymerization of lignin. The generated lignin is called ethanol lignin. The other method is the high-boiling point ethanol method, which is mainly based on the high boiling point of solvents and can be cooked at normal pressure, resulting in a high recycling yield. This lignin is called high-boiling solvent lignin.

Enzymatic hydrolysis lignin Enzymatic hydrolysis lignin is separated from lignocellulosic raw materials after enzymatic hydrolysis. The extraction methods used include aqueous alkaline extraction and organic solvent extraction. Because the reaction conditions of enzymatic hydrolysis are mild, enzymatic hydrolysis lignin has more active groups compared with pulp lignin, which can be used as green materials in polymer modification. Enzymatic hydrolysis lignin also helps to improve the economic efficiency of the enzymatic conversion process of lignocellulose.

Steam exploded lignin Steam exploded lignin is from lignocellulosic feedstock after steam explosion pretreatment. Some ether bonds are cracked, molecular weight is reduced, and the activity of phenolic hydroxyl groups is increased by weak acid hydrolysis in the high-temperature steam explosion process (Wang and Chen, 2013b). Accompanied by the hydrolysis of hemicellulose, the purity of lignin increases. At present, because steam explosion pretreatment of lignocellulosic feedstock is widely used in lignocellulose refining, steam exploded lignin as an additional product has also attracted a lot of attention (Wang and Chen, 2014).

3.6.3

Product development of the lignin platform

Lignin applications are important for lignocellulose refining. Due to the differences in the complex structure and the various extraction methods used for lignin, the chemical properties of the separated lignin are different, resulting in various

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utilization methods, low utilization efficiency, and difficulties of high-value utilization (Wang and Chen, 2013a).

3.6.3.1 Applications in agriculture Fertilizer modifier Lignin contains many active groups and can be slowly degraded by microorganisms in the soil, and converted to humus. Humus has a certain effect on the activity of soil urease, which improves soil quality and promotes fertilizer efficiency. With the change in agricultural production methods and the reduction of compost and manure application rates, farmland eventually becomes barren. Thus, the demand for organic and inorganic compound fertilizers is greatly increased. As a natural polymer compound, lignin will be widely applied in terms of improving agricultural fertilizers. Slow-release nitrogen fertilizer can be obtained by using ammoniated oxide-modified alkali lignin. Lignosulfonate can also be used as slow-release nitrogen fertilizer raw material; for example, calcium lignosulfonate has a certain inhibiting effect on urease activity, as well as nitrification and denitrification effects, and it can reduce the loss of ammonia volatilization (Clay et al., 1990). Chelate fertilizer or fertilizer can be prepared with lignin and other fertilizers as raw materials, such as lignin compound fertilizer and lignin chelate fertilizer.

Pesticide release agent Lignin has the natural property of inhibiting the release of agricultural chemicals. Based on the network structure of lignin, insecticides, herbicides, and biocides can be easily introduced into the lignin structure by physical or chemical methods. Pesticide formulation ingredients gradually spread from the base to the surface of the preparation, which plays the role of relieving the release of efficacy. Compared with ordinary particle pesticides, sustained-release pesticides made from strong polar pesticides had a long-lasting function during indoor rice control experiments. The releasing agent pesticides produced from lignin can provide a cost benefit of 200 Yuan per ton. It has many advantages including high efficacy, long-term efficacy, the saving of the number of chemicals used, and the reduction of pesticide loss (Yang, 2001).

Feed binding agent Lignin can be added to granular compound feed, which can play the role of binder and improve the granularity, reduce the powder in granular feedstuffs and the return rate of the powder, and reduce the production cost of the feed.

Liquid film Polyethylene plastic film has been widely used in agricultural production, but it cannot be degraded. It generates white pollution, which is a problem that needs to be addressed. Lignin is a soluble natural polymer. The film can be obtained by adding alkali, which also has certain strength. If a small amount of formaldehyde is added to the lignin solution as the cross-linking agent, the molecular weight and the strength of the film increase. In addition, a small amount of short fibers or other soluble

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polymer compound can be added to further increase the strength of the film. The advantages of this film include the fact that the film forms on the surface of the soil and that labor intensity is reduced due to the self-bursting effect. It will gradually degrade into humic acid fertilizer, and can improve the granular structure of soil. Before it degrades, the film covering on the soil surface prevents the evaporation of soil moisture and the growth of weeds. Because lignin has antimicrobial actions and the ability to absorb ultraviolet light, it can increase soil temperature and help the crops improve their resistance to disease. Pesticides and fertilizers can be added to this film to generate multifunctional composite film, and the cost is less than some synthetic mulches.

Soil ameliorant In recent decades, a large quantity of nitrogen has been used, and the content of phosphorus and potassium in the soil has become seriously unbalanced, combined with a lack of knowledge about the use of organic fertilizers and farm manures. These factors result in soil degradation, and hence some of the granular structure of the soil is severely damaged. Lignin can be slowly degraded into humic acid in the soil, so that the granular structure is regenerated or enhanced, which improves soil structure and ameliorates the effects of over-farming. For example, the ratio of lignin, NaOH, and hexamethylene tetramine of (100–120):(10–12):(20–30) (dry weight) can be used to prepare the soil conditioner, and it can be applied to many types of soils, such as forest tundra, sandy soils, or solid soils. Lignin’s molecular structure contains a hydroxyl group and a sulfonic acid group, which may bind the sand particles. Zaslavsky and Rozenberg reported on soil improver preparation methods that used lignosulfonate and graft copolymerization of vinyl monomers, and pointed out that the modified products can also be used to resist wind erosion (Zaslavsky and Rozenberg, 1981).

3.6.3.2

Applications in high polymer materials

Polymerization reactions with monomer/polymer materials Applications of lignin in the field of polymer materials include primarily two methods. One is reactions of lignin with monomers to generate polymer materials, and the other is mixing lignin with polymer materials directly. Because lignin has many reactive groups, which are capable of reacting to polymers with a higher activity – such as some monomeric formaldehydes, propylene oxide, and isocyanate – it can be used to prepare some modified wood polymer products. Research has been conducted to replace part of the phenol by lignin in the preparation of thermosetting phenolic resins. Lignin plays a similar role to the wood adhesive in plants, so the main application of the modified lignin phenolic resin is in the wood adhesives field. Its main purpose is to replace part of the relatively high-priced phenol in order to reduce the overall price of resin without reducing its adhesive properties. However, due to the complexity of the structure of lignin and the low reactivity of formaldehyde, the replacement rate of lignin is low. In addition, the modified lignin phenolic adhesives have a darker color, higher viscosity, and shorter storage time, and are subject to market price fluctuations of phenol; this results in poor market efficiency in the application of lignin as a phenol substitute in industrial adhesives. Recently, due to the excellent insulation and flame

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retardant performance of phenolic foam, increased use of lignin-modified phenolic foam materials has been reported. At the same time, the use of urea-formaldehyde resins and melamine resins modified by lignin in wood adhesives has been reported. The lignin can react with the free formaldehyde in adhesives, reducing the content of formaldehyde and increasing the market acceptance.

Lignin-based polymer blends Lignin is a natural macromolecule. Modified products can be directly formed by mixing lignin with a number of thermoplastic resins. Lignin, as a bio-based material, is renewable, which helps to reduce dependence on oil and other nonrenewable resources. Lignin has good resistance to oxidation and UV radiation, is flame retardant, nontoxic, and reduces microbial degradation. Polymeric materials prepared by mixing lignin with other polymers include polyethylene, polyvinyl chloride, polypropylene, polystyrene, and polyvinyl alcohol. The main problems of such blends are the compatibility issues of the two different polymer systems. The main solution to these problems is to improve the compatibility or increase the co-solvent through lignin modification. Lignin can be mixed with rubber, and replaces part of the carbon black. It can be used as a reinforcing agent, which is an important field of lignin applications. Preparation of lignin latex sediment rubber usually involves dissolving the lignin in alkali, and then adding the mixture to the rubber latex followed by heating with stirring. After the injection of acid, precipitated coprecipitated gel is prepared. Lignin is not easy to coalesce into wet particles in the latex dispersion. Under the action of the electrolyte, lignin can coprecipitate with the rubber. After removing the supernatant, the precipitate is dried, and then the lignin master batch is obtained. In order to improve the reinforcing effect of lignin on the rubber, an initiator can be added to induce polymerization between the lignin and rubber molecules, similar to the process of graft polymerization.

3.6.3.3 Applications in carbon materials Activated carbon The carbon content of lignin is up to 50%, so it is a suitable raw material to produce carbon materials. Preparation of activated carbon includes physical activation and chemical activation methods. During the physical activation process, material is carbonized, and then reacted with vapor and carbon dioxide at a certain temperature to develop the pores. The prepared activated carbon usually does not require a postoperation to remove impurities. However, the desired carbonization temperature is higher— greater than 800  C. During the chemical activation process, the activating agents such as phosphoric acid and zinc chloride are mixed at different temperatures and the activated carbon obtained needs cleaning by washing. The activation temperature is low, and the activated carbon pores are more developed.

Carbon fiber Carbon fiber has the properties of high strength, high modulus, high temperature resistance, corrosion resistance, fatigue and creep resistance, electrical conductivity, and thermal conductivity. It is mainly used for the preparation of composite materials.

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Currently, there are three main raw materials: polyacrylonitrile-based carbon fibers, pitch-based carbon fibers, and viscose-based carbon fibers. Because of the high carbon content of lignin, lignin can be used as the starting material for preparing carbon fibers. The preparation process is to first spin the lignin and the fibers form carbon fibers with a series of preoxidation, carbonization, and surface treatments.

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