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Engineering, Environmental
Miscanthus is a valuable renewable feedstock and has a significant potential for the manufacture of diverse products based on macromolecules such as cellulose, hemicelluloses and lignin.
- miscanthus
- renewable polymers
- biofuel
- bacterial cellulose
1. Introduction
The perpetually increasing atmospheric carbon dioxide and global warming are a serious threat to humankind. Hence, actions are required to mitigate the climate change consequences, and there is a need for the transition to a low-carbon economy in which biomass is the most common and available source of carbon [1].
A good many researchers consider the issue of global greenhouse gas emissions from the standpoint of trading and policy [2,3,4,5,6,7], which is undoubtedly important to combat the climate change. The other research studies are focused on a quantitative evaluation of the potential of various productions to minimize the consequences or reduce the CO2 emissions; for example, the estimations of biomass utilization for transport, power engineering, construction and iron-and-steel industry [8].
Miscanthus is a bio-pump [9] and has the potential of greenhouse gas emission reduction through soil carbon assimilation [10].
The studies [11,12,13] reported valuable results evaluating the life cycle of heat, electric power, ethanol and biogas productions from miscanthus, and demonstrated that the miscanthus cultivation and the manufacture of commodities from miscanthus are a good option for carbon footprint mitigation.
Miscanthus is a perennial rhizomatous grass with a high yield capacity and low nutrient requirements. Miscanthus has a life span up to 20 years, which is an advantage over annual plants. The merits of Miscanthus × giganteus may also include the anatomy of its stalks whose bast layer does not contain long fibers unlike some bast plants that require pruning of their bast fibers (for example, flax and hemp) [14]. Compared to other perennial crops, miscanthus yields a higher content of dry matter. Once planted, miscanthus requires no fertilizers or special care in the field but annual harvesting with standard farm machinery [15]. This crop with a high water use efficiency and ability to adapt to severe conditions along with its environmental functions such as soil remediation may have a vital part in the bioeconomic development of any nation [14,15]. Miscanthus is a frost-resistant crop and can grow on marginal, salinized and unused lands [16]. Considering the probability of further depletion of the world forest areas and the limitation on wood procurement due to the environment-conserving role of forests, miscanthus is being more frequently viewed as a potential feedstock to replace some of softwood and hardwood [14].
About 123,000 ha are utilized for the miscanthus biomass production across the world. The largest area is located in China, where approx. 100,000 ha are occupied by M. lutarioriparius in the wildlife at the Dongting Lake. The biomass yields constitute about 12 t/ha/year [17].
The machine learning study results [18] showed that globally there exist 3068.25 million ha marginal land resources eligible for M. × giganteus cultivation, which are basically located in Africa (902.05 million ha), Asia (620.32 million ha), South America (547.60 million ha) and North America (529.26 million ha). The countries with the largest land resources, Russia and Brazil, hold the first and second places based on the amount of marginal lands suitable for M. × giganteus, with areas of 373.35 and 332.37 million ha, respectively.
Miscanthus is a valuable renewable feedstock and has a significant potential for the manufacture of diverse biotechnology products based on macromolecules such as cellulose, hemicelluloses and lignin. The studies on the miscanthus chemical composition compared to the diverse vegetable world are constantly developing and show the advantages of miscanthus over many lignocellulosic resources, particularly by the content of cellulose, a polymer that is the most valuable for conversion.
2. Core Directions in Miscanthus Research
2.1. Miscanthus Selection
M. × giganteus is the most common worldwide among the Miscanthus species. The high yield capacity (10 t/ha/year) and life span (15–20 years) make miscanthus a promising bioenergy crop and an effective tool to combat the climate change. However, M. × giganteus is not free of shortcomings, i.e., it is sensitive to cold winter temperatures and drought, can only be reproduced through rhizome division, has a poor genetic diversity and is susceptible to soil pathogens. Hence, the other Miscanthus species and cultivars have become valuable sources of the genetic material for intraspecific and interspecific breeding. In breeding, a special focus is placed on achieving a higher yield capacity, quality and tolerance to antibiotic stressors. For instance, despite having a poorer aboveground biomass yield compared to M. × giganteus, M. sinensis is more tolerant to water stress and, hence, is more suitable for cultivation in a drier climate. M. lutarioriparius offers a high yield of biomass but is less resistant to cold and drought, and is therefore more suitable for regions that are less exposed to frequent water deficiency [25].
Since the chemical composition of the feedstock is essential for the miscanthus conversion, Table 1 outlines exactly this aspect for some Miscanthus species from different geographical locations, as reported in the recent studies.
Table 1. Component content (%) of miscanthus.
| Miscanthus Species and Country of Habitat | Cellulose | Hemicelluloses | Lignin | Ash | Others | Ref. |
|---|---|---|---|---|---|---|
| M. × gigantus (France) | 41.08 | 24.52 | 27.00 | - | 7.4 | [26] |
| M. × giganteus (Canada) | 55.01 | 17.42 | 16.90 | - | 10.67 | [27] |
| M. × giganteus (UK) | 45.5 | 29.2 | 23.8 | - | - | [28] |
| M. × giganteus (different climate regions of Russia) | 43.2–55.5 | 17.9–22.9 | 17.1–25.1 | 0.90–2.95 | 0.3–1.2% | [29] |
| M. × giganteus (Poland) | 45.12 | 29.30 | 22.21 | - | - | [22] |
| M. sinensis (Poland) | 44.12 | 29.79 | 19.52 | - | - | |
| M. sacchariflorus (Poland) | 44.57 | 29.11 | 20.34 | - | - | |
| M. lutarioriparius (China) | 41.89 | 18.21 | 16.77 | - | - | [30] |
| M. sinensis (Russia) | 49.1 | 20.7 | 23.3 | 3.00 | 2.6 | [31] |
| M. sacchariflorus (Russia) | 53.3 | 21.3 | 28.1 | 5.66 | 2.4 | |
| M. sinensis (China) | 37.66 (av.), 48.52 (max.) |
22.94 (av.) | 17.35 (av.) | 2.47 (av.), 4.5 (max.), 1.43 (min) |
15.83 (av.) | [24] |
| M. floridulus (China) | 36.28 (av.) | 21.95 (av.) | 16.94 (av.) | 2.74 (av.) | 18.41 (av.) | |
| M. nudipes (China) | 36.07 (av.) | 22.39 (av.) | 17.21 (av.) | 2.51 (av.) | 21.10 (av.) | |
| M. sacchariflorus (China) | 39.25 (av.), | 26.35 (av.), 34.23 (max.) |
18.11 (av.), 23.75 (max.) |
2.51 (av.), | 11.62 (av), 5.38 (min.) |
|
| M. lutarioriparius (China) | 39.96 (av.) | 22.85 (av.) | 18.69 (av.) | 2.43 (av.) | 12.43 (av.) | |
| Hybrid (China) | 37.14 (av.) | 21.21 (av.), 15.71 (min.) |
16.37 (av.), 13.01 (min.) |
2.56 (av.) | 20.67 (av.), 34.88 (max.) |
Because of miscanthus having a rich genetic diversity, its lignocellulosic content varies widely; yet, many Miscanthus species are characterized by a high content of renewable polymers.
In recent years, the research initiatives have resulted in a range of miscanthus traits being identified, which can be optimized for various applications. For example, improved miscanthus varieties for bio-based applications were released that are less recalcitrant to destruction due to having less lignin and due to alterations in specific cell wall characteristics [32]. In contrast, transgenic miscanthus with enhanced lignin content was derived in order to improve the energy value [33].
2.2. Studies on Environmental Impact of Miscanthus
Wang et al. [34] summarized publications in this field in their review paper. An economic model for the estimation of greenhouse gas emissions in the miscanthus cultivation using the commercial practice adopted in the UK was reported recently as well [35].
2.3. Production of Various Products from Miscanthus
A great many works worldwide have been focused on the miscanthus processing. Some applications employ all fractions of the miscanthus biomass, for example, incineration for power generation [36,37,38] or pyrolysis for the production of bio-oil [39,40], biochar [41,42], hydrochar [43,44] and graphene oxide [45], for biopolyol synthesis [27] and for the production of composite materials [46,47,48], concrete [49], miscanthus-based mortar [50], fiber-reinforced screed [51] and bio-based PET [52].
The other applications employ only certain parts of the cell wall for the transformation into products, for example, esterified lignin [53]. Acid hydrolysis of miscanthus has been studied for the synthesis of chemicals such as furfural, hydroxymethylfurfural [54], levulic acid [55] and other organic acids and ethylene glycol [56].
Cellulose, cellulose microfibers and paper [57,58,59,60], cellulose nanocrystals [61], oligosaccharides [62,63,64,65] and xylene [30] are derived from miscanthus. Pidlisnyuk et al. [66] comprehensively reviewed some products from miscanthus (agricultural products, insulation and composite materials, hemicelluloses, pulp and paper).
Many biotechnology products such as bioethanol, biogas, bacterial cellulose, enzymes, lactic acid, lipids, fumaric acid and polyhydroxyalkanoates are derived from miscanthus, as detailed in Section 3.
2.4. Miscanthus Pretreatment and Hydrolysis Processes
Furthermore, some studies are focused only on miscanthus pretreatment without end-product isolation [67]. The pretreatment of miscanthus biomass is highly requisite to obtain fermentable sugars and subsequent biotechnology products. Due to the heterogeneous structure, miscanthus has serious limitations with respect to the conversion and is recalcitrant to enzyme-assisted hydrolysis. The pretreatment step is chiefly meant to breakdown the structure composed of the three main renewable polymers, i.e., cellulose, hemicellulose and lignin, as well as minor non-structural constituents (extractives, ash).
Out of the three basic constituents, lignin is the most recalcitrant to degradation. Cellulose retains a significant crystallinity index and forms a rigid framework acting as a bearing structure of the cell wall. Hemicellulose, a heteropolymer of xylose, arabinose, galactose and other sugars, is not crystalline and therefore more amenable to hydrolysis than cellulose [68].
Similar to other lignocellulosic feedstocks, several pretreatment methods are applicable to miscanthus. Some methods are already reckoned to be conventional (ball milling, acid treatment, alkaline treatment, ammonia treatment, organosolv treatment, ionic liquid treatment, hot water treatment, steam explosion treatment), and new methods are under development (microwave, ultrasound, deep eutectic solvent, irradiation, high force-assisted pretreatment methods, biological pretreatment) [69,70]. That said, the conventional methods continue to be investigated for a deeper understanding of fractionation, optimization and process scale-up [71]. Furthermore, it is also proposed that a combination of two or more approaches for biomass pretreatment be used for maximum destruction of the biomass [72].
Figure 1 shows a schematic of the effect of pretreatment on biomasses [73].
Figure 1. Effect of pretreatment on biomasses (reproduced with permission from [73], MDPI, 2023).
The evaluation of different approaches demonstrates that successive efforts are still needed to develop an economical and eco-benign pretreatment strategy [68,72].
But, not all of the biotechnology products require that a biomass be pretreated; for instance, pretreatment is not mandatory for the biogas production and the use of lignocellulose as an inducer of enzyme production.
After pretreatment, cellulose and hemicelluloses can be hydrolyzed to monomeric sugars. Enzymatic hydrolysis of lignocellulosics is the most known and promising technique for biomass saccharification. Various hydrolytic enzymes produced by microorganisms are available in the market, as outlined in the tables below (columns “Enzymes for Hydrolysis”).
Enzymatic hydrolysis can liberate monomeric sugars in a very wide range, depending on the pretreatment method. For instance, Dai et al. [74] recently examined how pretreatment methods such as microwave, NaOH, CaO and microwave + NaOH/CaO influenced the sugar yield from miscanthus. The hexose yield showed a substantial range from 4.0 to 73.4% (% on a cellulose basis). The highest hexose yield was achieved by the 12% NaOH pretreatment and the lowest one by the 1% CaO + microwave pretreatment.
This entry is adapted from the peer-reviewed paper 10.3390/ijms241613001
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