Sugarcane straw as a potential second generation feedstock for biorefinery and white biotechnology applications
Introduction
In the last decades, many efforts have been made for the development of sustainable processes and replacement of global energy matrix toward renewable sources due to environmental problems, climate changes and greenhouse gas emissions [1,2]. The biorefinery concept is based on the use of the whole plant biomass, especially those that are not foodstuffs, in order to replace fossil resources and leads to the reduction of waste generation [3]. Its implementation is a potential approach for pollution prevention and a promising alternative for the development of the society addressing with circular economy proposition of processes without waste generation [4]. In this context the white biotechnology, science that aims the application of biological systems instead of classical chemical catalysts, e.g. enzymes, stands out as an important tool for the reduction of industries environmental impact and the development of biorefineries (Fig. 1) [5].
In a biorefinery, the integration of different technologies and facilities could be applied to produce more than 200 value-added products from the biomass main components: starch, cellulose, hemicellulose, lignin and oils [4]. Among the available biomass for biorefineries in Brazil, the most abundant came from sugarcane processing due to high bioethanol and sugar productions. The introduction of flex-fuel vehicles into the Brazilian automobile market in 2003 increased exponentially the demand for ethanol nationwide [6].
In 2019, Brazilian ethanol production reached 36 billion liters (both anhydrous and hydrated ethanol), which was 11% higher than the previous year's production. The total harvested area by the sugarcane sector was equal to 8.4 million hectares, which led to the processing of 654 million tons of sugarcane by Brazilian sugarcane mills. From this amount of sugarcane processed, 64.5% was used to produce ethanol and 35.5% was destined for sugar production. As a consequence of this production, Brazil became the largest sugarcane producer in the world [7].
Ethanol and sugar production from sugarcane generate a large amount of waste, mainly bagasse and straw, hence requiring appropriate disposal and recycling methods for these residues [8]. Sugarcane straw (SCS), also called trash [9,10], is composed by dry leaves (60%) and green tops (40%), and its recovered amount depends on the variety, sites, and crop circles [11]. SCS represents approximately one-third of the total primary energy of sugarcane as crop and it possesses quite similar characteristics to those of bagasse [8,12].
During the manual harvest of sugarcane, it is necessary to burn straw because it makes it easier to cut the sugarcane plant and improves productivity. However, the soot resulting from straw burning, settles on the ground in black shaped flakes composed of 70 harmful substances to the environment. These soot flakes release greenhouse gases that cause serious respiratory problems for the population and farm workers [12,13]. In addition, the burning of sugarcane fields before harvest has been prohibited by 2021 (law nº 11,241, from September 19, 2002, and according to decree nº 47,700 issued on March 11, 2003). Due to environmental, economic, and agronomic reasons, the sugarcane manual harvesting has gradually been replaced by mechanical harvesting [8], without the need for burn straw, and 15 Mg ha−1 of SCS on average are left on the field [14,15].
Keeping straw in the field is an option that can improve soil quality, contributing to soil organic matter increase, water storage, control of weed infestation and, soil erosion. In addition, it reduces the necessary amount of potassium and nitrogen fertilizers and increases carbon content in the soil. However, there are drawbacks of this practice, once there is a larger amount of straw in the field, the number of pests that can be harmful to crops in the future also increases. Moreover, high amounts of straw in the soil could offer a fire risk, and so, there will be more N2O emissions as a consequence [11,16].
According to Menandro et al. [11], the proportion of SCS per processed sugarcane is equal to 12%. The same work indicates that it is better to use dry leaves for bioproducts and electricity generation purposes, leaving the green parts in the field to use their nutrients. Taking this into account, and considering the production of sugarcane in 2019/2020 [7], around 78.48 million tons of straw were generated in Brazil in the last harvest, from which 60% (about 47 million tons) could be used for the production of bioproducts and/or electricity, and the other 40% (around 31 million tons) could be left in the field. On the other hand, Carvalho et al. [9] suggests that leaving 7 Mg ha−1 in the field may be sufficient to soil maintenance, which would still make a considerable amount of approximately 20 million tons of SCS available for biorefinery applications. Lately, SCS is gaining attention due to its potential as a feedstock for biofuels, value-added products and electrical power generation [17].
In general, biomass represents 9% of the power supplied by ANEEL (Brazilian Electricity Regulatory Agency) in the Brazilian electrical grid (167,341 MW). Furthermore, biomass is the third most important source of electricity generation in Brazil [18]. In respect of bioelectricity, the sugarcane sector has 11,356 MW of installed capacity, which is superior to the installed capacity of Belo Monte (Hydropower plant in Brazil). Moreover, the utilization of SCS for bioelectricity production could result in an amount of power from sugarcane mills at around 7% of supplied power in Brazil and 77% of power from biomass [18]. Considering economic and environmental aspects, the bioelectricity presents a lower cost and great potential to mitigate greenhouse gas emissions compared with fossil fuel–based electricity. Moreover, it reduces the country's dependence on fossil fuels [19]. However, currently, only a part of the generated sugarcane straw amount is available to be processed together with the bagasse for steam production and electricity generation [16] because SCS needs to be transported from the field to the mills.
Many authors have studied different methods to recover the straw from the field, considering technical and economic aspects. Okuno et al. [20] presented two possibilities. The first option was the straw baling and the second was the integral harvesting. Regards the latter, the straw is harvest and transported with the sugarcane stalks, whereas in the former, the straw stays on the field until it is dried, which usually takes two weeks after the harvesting. Despite the environmental benefits and the biomass role to reduce the global energy deficit, the feasibility of energy extraction from the straw still needs optimization [9].
New developments focus on adding value to SCS by using it in an economically, socially, and environmentally sustainable manner to produce high-value products in biorefineries [19,21]. There is a strong linkage between the potential of SCS as an industrial feedstock and its composition. Once it is mainly composed of cellulose and hemicelluloses, SCS is a great source of fermentable sugars. However, it has been less extensively studied than other biomass sources, such as sugarcane bagasse, due to a lack of long-term data [22]. For this reason, in this review we will focus on the main applications of SCS and bioproducts explored from this biomass in the biorefinery context and which are the main challenges that still need to be overcome for its application in a feasible industrial process.
Section snippets
Sugarcane straw composition
SCS composition must be understood in order to be utilized appropriately. Its biochemical composition varies according to the material collection, material collection site, weather conditions, plant development period and variety [23,24], i.e. there is a marked variation in chemical components as it can be seen in Table 1. Typically, SCS is composed of cellulose (31–45%), hemicelluloses (20–30%) and lignin (12–31%). Other constituents found in lesser amounts are extractives (4–16%) and ashes
Sugarcane straw pretreatment
The pretreatment of sugarcane straw pursues the breakdown of biomass hemicellulose-lignin-cellulose complex and the release of fermentable sugars from recalcitrant biomass fiber [40]. An ideal pretreatment must break lignin seal, solubilize hemicelluloses, and disrupt the crystalline structure of cellulose in order to improve enzymatic hydrolysis in the following process steps [36]. Various pretreatment technologies for biomass disruption have been proposed in literature for the production of
Cellulose enzymatic hydrolysis
To be used as a substrate in fermentation processes, hemicelluloses and cellulose must be converted into monomeric sugars, for that a pretreatment step is required. Generally, after the biomass pretreatment, there are two phases: a liquid one with hemicelluloses and a solid one containing cellulose (Fig. 2). In most cases, hemicelluloses are already solubilized as monomers (mainly xylose) due to its biochemical characteristics, but cellulose remains polymerized and requires a hydrolysis step [23
Second generation (2G) ethanol
The development of processes using lignocellulosic biomass as feedstock contributes to sustainability and protects the environment [54]. In this context, much attention has been given to the use of SCS for biofuel production in recent years. Using SCS for second generation bioethanol production in biorefineries could reduce costs and make conventional ethanol production processes more sustainable [29].
The majority of SCS studies for ethanol production have been conducted in Brazil. Some
Processes for obtaining cellulosic pulp from sugarcane straw and its applications
Cellulosic pulp generation can be carried out using different types of energy and processes [106], and non-wood fibers such as SCS as feedstock play a key role in supplying virgin cellulose and using regional agricultural waste products [10,107,108]. Pulping processes modify lignocellulosic materials, disrupt the structure of the plant's cell wall, remove, solubilize and/or fragment lignin by causing the least possible damage to carbohydrates and yielding the same amount. These aspects are
Polymeric composites reinforced with sugarcane straw fibers
In polymeric composites, fiber distribution is an important parameter for mechanical properties [123]. In general, mechanical properties of natural fiber-reinforced composites depend on some parameters as their fiber volume fraction, aspect ratio, fiber-matrix adhesion, interface stress transfer and fiber orientation [124]. Some studies on natural fiber-reinforced composites involve mechanical properties characterization as a function of fiber content, the effect of fiber treatments, and the
Thermochemical conversion of sugarcane straw
Lignocellulosic biomass can also be thermochemically converted into diverse value-added products such as biofuels, in addition to energy. Unlike biotechnological routes, thermochemical processes thoroughly use biomass; both carbohydrates and lignin are simultaneously converted [49]. The thermal and physicochemical properties of SCS are similar to those of bagasse [8] and, therefore, the former also has potential for generating heat and energy. In addition to biochemical composition (cellulose,
Environmental applications of SCS
SCS has been evaluated as an adsorbent for organic or inorganic cationic pollutants. However, a poor sorption capacity of biomasses due to lower specific surface area and reduced pore volume suggests improvements by physical or chemical modifications. The two alternatives, either with or without modification, have been tested to evaluate SCS as adsorbent. Farasati et al. [154] evaluated Phragmites australis and SCS with no modification to adsorb Cd2+ in the solution. Optimum pH values were 6
Conclusions
In the recent years, the necessity to establish new applications and valorization of SCS has emerged due to the substitution of the manual harvest of sugarcane, which led to a high generation of this substantially novel residue. In this paper, we presented SCS potential for applications into biorefinery and white biotechnology concepts. Besides SCS importance on soil enrichment, it is an attractive feedstock for application in many fields, such as bioenergy, biofuels, and composites due to its
Declaration of competing interest
The authors declare no conflict of interest.
Acknowledgements
The authors thank CAPES and CNPq. Thaís S. Milessi thanks São Paulo Research Foundation (FAPESP), grant #2016/10636-8. D. R. Mulinari thanks FAPERJ, grant #E26–260.026/2018.
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