Solubilization of microwave pretreated samples in subcritical water.
Abstract
Industrial production of a wide range of value‐added products heavily relies on fossil resources. Lignocellulosic biomass materials are receiving increased attention as a renewable, economical, and abundant alternative to fossil resources for the production of various value‐added products. Biomass feedstocks utilized for these productions include energy crops, agricultural biomass residues, forest biomass, and food‐based biomass wastes. Various conversion technologies are used for production value‐added products from biomass. Efficiencies of conversion technologies highly depend on the types of biomass used as raw materials that differ in contents and compositions of cellulose, hemicellulose, and lignin structures in biomass. In some conversion technologies, such as chemical, biochemical, and hydrothermal conversion techniques, biomass materials must be first broken down into smaller molecular weight components (e.g., oligosaccharides and monosaccharides) in order to be efficiently converted into target products. In this matter, pretreatment and hydrolysis play critical roles on the yield of the product(s). The chapter describes lignocellulosic materials that are used for production of top value‐added products and conversion technologies to produce products in high yields. Future developments in the conversion of lignocellulosic biomass into value‐added products are directly correlated to improvements of conversion technologies and selection the right types of biomass in the process.
Keywords
- biomass types
- value‐added products
- biofuels
- bioproducts
- conversion methods
1. Introduction
Being a nonedible portion of the plant, lignocellulosic biomass materials are attractively growing the attention as sustainable and renewable energy sources. Biomass materials can be used for producing a wide range of value‐added products, including biofuels (ethanol, hydrogen, etc.), bioproducts products (sugar and sugar alcohols, etc.), and industrially important chemicals (e.g., solvents) [1]. Conversion can be performed using a variety of methods, including chemical, biochemical, and thermochemical processes. Each method offers several advantages or disadvantages for high yielding of a certain product.
Biomass can be derived from forestry wastes such as residues of the trees and shrubs, energy crops like sorghum, miscanthus, kenaf, switchgrass, corn, sugarcane, and any agricultural residues such as corn stovers, wheat straw, etc. The diversity in the chemical composition of biomass (cellulose, hemicellulose, and lignin constituents) can affect the conversion technologies employed for production of high‐value products.
2. Top value‐added products from biomass
2.1. Biofuels
Biodiesel is another most widely used liquid biofuel; however, its production does not rely on lignocellulosic fraction of biomass. Biodiesel is produced from vegetable oil or animal fat with an alcohol and a catalyst through transesterification reaction.
2.2. Bioproducts and industrially important chemicals
Nonedible lignocellulosic biomass materials are attracting increasing attention as renewable, economical, and abundant resources to reduce dependency on petroleum resources and minimize energy and material feedstock costs. In addition to energy and fuels, biomass can be used to create valuable carbon‐based chemicals and materials, known as bioproducts. These products are sugars and sugar alcohols, glycerine, furfurals, cellulose fiber and derivatives, carbonaceous materials, resins, bioplastics, etc.
Addition to sugar alcohols, following C5 and C6 sugar‐derived
Biomass feedstocks are converted into plenty of
Lignocellulosic biomass‐based
3. Parameters affecting the product yield
3.1. Pretreatments
Despite their potential, the complex and rigid structures of biomass materials limit their use in many applications. Biomass materials must first be broken down into components with smaller molecular weights (e.g
In
Other common pretreatment methods used for biomass are
There are also some methods that use
On the other hand, microwave treatment has enhanced the surface disruption and the breaking of lignin structures in switchgrass and improved enzymatic saccharification 53% more compared to conventional heating [33]. Microwave pretreatment has positive effect on solubilization of switchgrass in subcritical water. Hydrolysis percentages and total organic carbons released into solution are higher in microwave‐treated biomass samples. When microwave pretreatment is applied at higher temperature, solubilization significantly increases (Table 1).
Biomassa | Hydrolysisb (%) | TOC (mg L−1) |
---|---|---|
Untreated | 49.01 ± 0.9 | 1132 ± 13 |
MW—120°C | 57.09 ± 0.6 | 1541 ± 14 |
MW—150°C | 62.91 ± 0.7 | 1679 ± 17 |
Microwave pretreatment was used for solubilization of lignocellulosic biomass in combination with acid and alkali treatments followed by enzymatic hydrolysis [33–35]. Chimentão et al. [36] investigated hydrolysis of dilute acid‐pretreated cellulose in a conventional oven and under microwave heating. Although the method was called “mild hydrothermal conditions,” the hydrolysis process was accelerated using acids (sulfuric and oxalic acids).
Radio frequency (RF) heating is another promising dielectric heating technology, which is used as an initial breakdown of the lignocellulosic matrix. Dielectric heating transforms electromagnetic energy into heat that is effective on breakdown of biomass structure. The electromagnetic field could generate nonthermal effects, which can also accelerate the destruction of the crystallinity structure [37]. RF heating prevents uncontrolled heating and overheating that protects the product from degradation. RF has large penetration depth (10–30 m) and higher energy efficiency than microwave [38–40]. Efficiency of pretreatment highly depends on temperature, frequency, and type of product/biomass (water content, chemical composition, etc.). Radio frequency‐assisted dielectric heating was usually combined with alkaline pretreatment for destruction of biomass materials [41, 42].
3.2. Hydrolysis of biomass materials
The major hydrolysis processes typically used for the solubilization of biomass require either use of toxic, corrosive, and hazardous chemicals (e.g., acid and alkali treatments) or longer retention times (e.g., enzymatic hydrolysis), which collectively make the process environmentally unsafe and/or expensive. Mineral acids are commonly used to dissolve hemicelluloses, whereas lignin is typically dissolved by alkaline or organosolv pretreatments [45, 49]. Recovery of the chemical catalyst is often crucial to the success of these processes [24]. On the other hand, generally harsh conditions (e.g., high temperatures and high acid concentrations) are needed to release glucose from biomass complex structures. Pyrolysis and other side reactions at higher temperatures become very important, and the amount of undesirable byproducts (tars) increases as the temperature is increased above 220°C [50].
Before treatment biomass exhibits rigid and highly ordered fibrils. The cell walls are visible creating a “brickwork‐like” appearance to surface (a). The compacted outer layer is partially removed at 200°C and a range of discrete droplet morphologies that contain lignin is appeared on the cell. The 250°C‐subcritical water treatment reduces and degrades lignocellulosic structure, leaving highly degraded solids. The maximum solubilization yield of wheat straw and kenaf biomass were found to be 70–75% in subcritical water medium (under 250°C and 27.58 MPa carbon dioxide pressure conditions) [11, 32]. However, the hydrolysates obtained in this process contained high molecular weight polysaccharides that were difficult to utilize for producing value‐added products such as gas biofuel hydrogen.
3.3. Chemical composition of biomass materials
Chemical composition of biomass and structures of biopolymers (cellulose, hemicellulose, and lignin) are two important factors affecting the yield of the biofuels, bioproducts, and chemicals produced from biomass. Composition of lignin, cellulose, and hemicellulose in biomass materials significantly differ among biomass species. For instance, some biomass materials such as hardwoods contain more cellulose in their structures, whereas others such as straws and leaves have more of hemicelluloses. It is known that lignin content of herbaceous plants such as grasses is very low compared to softwoods, which are known to have highest amount of lignin in their structures [54]. On the other hand, polymerization degree and(or) structures of biopolymers can also considerably varies among biomass species. For instance, the chemical structure of lignin is based on syringyl (S), guaiacyl (G), and p‐hydroxyphenyl (H) units. Softwood lignins are mainly composed of residues derived from guaiacyl units (lignin type G), whereas hardwood lignins contain both syringyl and guaiacyl units with minor amount of p‐hydroxyphenyl (lignin type GS). Lignins from grasses are composed of the three basic precursors (lignin type HGS) [55, 56]. Hemicellulose fractions of softwoods mainly have D‐mannose‐derived structures such as galactoglucomannans, whereas hemicelluloses in hardwoods have D‐xylose‐derived structures such as arabinoglucuronoxylan. Xylan is a polypentose hemicellulose structure in biomass materials that displays a wide range of compositions, molecular sizes, and structures depending upon its source [57, 58]. This diversity among biomass materials can significantly affect the yield of value‐added products directly produced from biomass as raw materials. On the other hand, this diversity can also affect solubilization efficiency of the biomass materials and, therefore, contents and compositions of the biomass components in the hydrolysates. The differences in the hydrolysates will considerably change the yield of the target compounds and byproducts produced from these biomass hydrolysates. For instance, molecular weight distribution of carbohydrates in the hydrolysates can significantly affect the method employed for biofuel or useful chemicals production. The more degraded organics containing hydrolysates are preferable for the production of certain various value‐added products from biomass. For example, high‐yielding hydrogen gas production from biomass hydrolysates requires reduced molecular weight oxygenated compounds containing biomass hydrolysates as feeds in aqueous‐phase reforming gasification process [59].
3.4. Conversion methods
3.4.1. Chemical
Sugars released from biomass can be hydrogenated to C5‐6 polyols (
Furfural is a valuable compound for a variety of chemical applications and it serves as a precursor for the synthesis of many fine chemicals and biofuels. It is produced industrially by acid‐catalyzed hydrolysis and dehydration of pentoses (mainly xylose) in lignocellulosic feedstocks (sugarcane bagasse, corn cobs, sunflower stalk, etc.) at temperatures ranging from 153 to 240°C [61]. During the initial stage, the hemicellulose is hydrolyzed to xylans, which generate pentose carbohydrates to be further converted into furfural. Commercially, furfural is produced using sulfuric acid as a homogeneous catalyst. Significant quantities of steam are used in the process in order to strip the furfural and to avoid its further degradation.
Xylitol is currently produced through chemical reduction of xylose derived from birch wood chips and sugarcane bagasse hemicellulose hydrolysate. The chemical process adapted for xylitol production from xylan‐rich biomass demands high production costs in terms of temperature and pressure input, as well as the formation of byproducts that require expensive separation and purification steps [62].
3.4.2. Thermochemical
Thermochemical conversion processes are combustion, pyrolysis, gasification, and liquefaction.
Lignocellulosic biomass can also be converted into liquid and gaseous fuels by
Various value‐added products can be produced or synthesis from biomass or biomass‐derived compounds by thermochemical methods. For instance, sugars can be reduced into sugar alcohols by thermochemical reduction. In industrial reaction, sorbitol production by thermochemical reduction of glucose is mainly carried out discontinuously in stirred tank reactors at 100–180°C under 5–15 MPa of H2 pressure in the presence of a catalyst, usually raney‐type nickel, or ruthenium catalysts. Table 2 shows thermochemical sugar alcohol production from glucose, simple biomass model compound, at various processing temperatures under 2.0 MPa H2 pressure in the presence of carbon supported ruthenium catalyst. The reaction time was 60 min [71].
The results showed that 80°C hydrogenation temperature is not enough for complete reduction of glucose solution. Further increase of the process temperature causes complete reduction of all glucose; however, the contents and compositions of reduced products (sugar alcohols) significantly change as temperature increased. The main reduction product of glucose is sorbitol and its highest yield is at 100°C at the processing conditions described above.
Temperature (°C) | Sugar alcohols (% concentration) | ||||
---|---|---|---|---|---|
Glucose | Mannitol | Sorbitol | Xylitol | Other productst2 | |
80 | 8.8 | 0.9 | 90.3 | n.d. | n.d. |
100 | n.d.a | 2.2 | 97.3 | n.d. | n.d. |
120 | n.d. | 5.9 | 88.6 | n.d. | 5.5 |
140 | n.d. | 16.5 | 59.7 | 2.7 | 21.1 |
160 | n.d. | 21.8 | 41.0 | 3.6 | 33.6 |
180 | n.d. | 23.4 | 20.7 | 4.1 | 51.8 |
200 | n.d. | 19.7 | 12.1 | 2.1 | 66.1 |
Other types of value‐added products that can be produced by a thermochemical method are carbonaceous materials. Nonsolubilized residue containing mostly lignin and lignin destruction products can be utilized for production of carbon‐based materials such as activated carbon and mesoporous carbons, etc. [72].
3.4.3. Biochemical
Biochemical conversion can also be used for breaking down biomass into sugars that can then be converted into biofuels (gaseous or liquid fuels) and bioproducts through the use of microorganisms and enzymes. This process is usually used for treating high moisture containing organic wastes. Biochemical processes are relatively slow processes that require more time for conversion of biomass into useful compounds [73]. The most popular biochemical technologies are
4. Comparison of various types of biomass materials for high‐yielding value‐added products
4.1. Energy crops
Energy crops are specifically grown for its fuel value or to produce bioenergy. These plants usually require low cost and low maintenance to grow and they are utilized to make biofuels or directly exploited for their energy contents. Energy crops can be food crops (corn, sugarcane, sugar beet, sweet sorghum, etc.) or nonfood crops (poplar trees, switchgrass, miscanthus, kenaf, etc.). A major focus among them is nonfood energy crops.
4.2. Agricultural biomass residues
Agricultural residues from well‐established production chains are important sources of biomass that can provide a substantial amount of biomass for production of a wide range of value‐added products. Since these residues are a natural byproduct of the food crop, they can be used as promising low‐cost feedstocks without increasing the amount of land used for agriculture. Agricultural biomass includes
High
Biomass | Advantages and disadvantages |
---|---|
Switchgrass and miscanthus | Nonedible Require less fertilizer, water, and energy for production Do not need to be replanted each year Miscanthus produces more biomass than switchgrass |
Kenaf | Nonedible Grows fast in the tropics and subtropics Does not need special care for production produces large amount of biomass |
Corn | Edible Mostly used for ethanol production because of relatively low‐cost source of starch Large amount of cropland devoted to corn Excess corn production in the United States |
Sugarcane | Edible Grows in tropics and subtropics Needs little fertilizer Sugarcane juice is directly used in conversion techniques (no pretreatment needed) Waste product, bagasse, is an important feedstock for value‐added products |
Sweet sorghum | Needs little fertilizer Sugars in the juice can be directly utilized for value‐added products (no pretreatment needed) Required to squeeze the juice out immediately (not stable) Limited flexibility in harvesting and transportation costs |
Poplar trees | Nonedible Grow fast Produce large amount of biomass Can be harvested throughout the year Have many environmentally desirable applications such as reducing erosion |
Cereal straw | Nonedible Natural byproduct of the food crops rice, wheat, corn, etc. Produce large amounts of biomass |
Corn stover | Nonedible Natural byproduct of the food crop, corn Produce large amounts of biomass (more than cereals) |
Forest biomass | Nonedible Largest source of lignocellulosic biomass High costs of harvesting and transportation Widely used in combustion process but not in gasification, pyrolysis, and fermentation |
Food biomass wastes | Edible/nonedible Have low values Released in large amounts from food industries |
4.3. Forest biomass
Forests are the largest source of lignocellulosic biomass that can be substitute for fossil fuels in the production of energy and other value‐added products. Since it is a nonfood type of biomass, it is a promising feedstock for these conversions [93]. Forest biomass includes material left on logging sites (
4.4. Food biomass wastes
The food industry produces a large number of residues and byproducts that can be used as biomass energy sources.
Table 3 summarizes advantages and disadvantages of biomass materials for utilization as raw materials for production of value‐added products.
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