Lignocellulosic biomass is a powerful approach to produce sustainable biofuels and the further achievement of the goal of biomass conversion into a second-generation clean energy that can cope with the depletion of fossil reserves and rising energy requirements. In the conversion process, a pretreatment is essential to overcome the recalcitrance of the lignocellulosic biomass; accelerate its disintegration into cellulose, hemicellulose, and lignin; and, in turn, obtain an optimal yield of fermentable sugars in the enzymatic hydrolysis. In addition to this, it should be industrially scalable and capable of enhancing fuel properties and feedstock processability. Here, steam explosion technology has stood out due to its results and advantages, such as wide applicability, high efficiency in the short term, or lack of contamination despite its conventionality.
1. Introduction
The necessity of finding alternative pathways for power supplementation in a more sustainable way has been promoted because of the current carbon emissions that contribute to climate change
[1]. Thus, the energy-from-waste (EfW) concept has been introduced as an alternative pathway where the use of refuse-derived flues, household waste and non-hazardous industrial by-products are considered as potential sources for energy production
[1]. This productive model fits with the sustainable model of circular economy in which the concept ‘end-of-life’ is replaced by reutilization, recycling and recovering. Hence, by-products or wastes from one industry become raw material for another. In addition, another productive model can be easily combined with bio-economy, which promotes the use renewables based on biomass by-products
[2]. Indeed, in the past years, the production of renewable fuels using lignocellulosic waste from agricultural activities has been considered as an alternative to traditional fuels
[3]. Agricultural residues are defined as unusable and unstable materials derived from agricultural production which are directly linked to the cultivation of crops, and these materials are characterized by their biodegradability and solid and lignocellulosic composition
[3,4][3][4]. In this way, the lignocellulosic non-edible biomass discarded by agriculture feedstock can be used as raw material to obtain biofuel, being considered second-generation (2G) biofuel
[1,5][1][5]. Lignocellulosic biomass is mainly composed by cellulose, hemicellulose, and lignin, which are low energy-density compounds, so a pretreatment step is necessary so plant-specific enzymes can release sugars for biofuel production
[5,6][5][6]. Moreover, 2G biofuels present several advantages compared to first generation biofuels, such as the cheaper non-edible matter used since it is the waste obtained as a result of an industrial activity; no fuel–food competition since non-edible matrices are used; and the reuse of by-products obtained after pretreatment of raw materials to obtain animal fodder to be used, for example, in livestock feed, thus reinforcing the circular economy. In this way, sugarcane bagasse (SCB), wheat, barley, rice and corn straw, and sorghum can be used for bioethanol production
[7]. In the specific case of sugarcane production, it has an associated production of residues that may reach up to 30% of the production
[8], which may represent more than 100 MT of residues. This huge volume of residues has several drawbacks. From an economic point of view, they require being effectively managed, which has an associated cost for the industry. From an environmental point of view, they represent a potential source of CO
2 since they are likely to be burned or, if not, accumulated in landfills, which threatens the quality of environmental (i.e., air and water pollution and noise, among others, and their potential consequences) and public health. Therefore, the reutilization of these by-products as part of a circular economy model is key to reduce management costs of residues to sugarcane industries and their negative environmental impact. In fact, their reutilization as raw material for bioethanol production has double benefits. It would avoid the release of this biomass to the environment, and it would promote the use of bioethanol produced using SCB, which is less carbon intensive compared with fossil fuel, so air pollution can be reduced
[9]. Therefore, these two approaches have the potential of reducing the footprint of sugarcane industries and fuel utilization.
2. Sugarcane Bagasse as a Potential Raw Material for Bioethanol Production
Sugarcane (
Saccharum oficinarum L.) is a tropical grass that belongs to the Gramineae family and the
Saccharum spp. genus
[22][10] and is characterized for being large and perennial. The sugarcane cultivation requirements are 6–12 months to grow with 60–100 cm
3 of water
[23][11]. Brazil, China and India are the main producers of SCB, with almost 500 MT generated every year from the sugarcane industry, which provides an important contribution to economic development
[9,22,24][9][10][12]. SCB yielded the highest crop straw production between 2012 and 2022 according to the Food and Agricultural Organization Corporate Statistical Database (FAOSTAT)
[25][13]. Sucrose is the main product of sugarcane, which accumulates in the internodes of the stalk
[9]. However, the percentage of waste generated during sugarcane production varies between 25 and 30%
[8], which may represent 125–150 MT of residues. The residues produced by the sugarcane industry are mainly two types, and they can be classified as straw, which is the harvest residue, and as bagasse, which is the fibrous fraction after the extraction of the sugarcane stem juice
[22][10]. These two by-products are characterized by their lignocellulosic composition, with cellulose, lignin and hemicellulose being the major components and having also extractants and ashes in their composition
[26][14]. SCB is composed of approximately 45–50% cellulose, 25–30% hemicellulose, 25% lignin and 2.4–9% ash
[9]. Nevertheless, this may vary depending on different factors such as chemical composition of the soil, climatic conditions and variety of the crop, among others
[9].
Table 1 shows variations in the composition of SCB, sugarcane fiber and sugarcane straw. The relative abundances of component units are usually calculated on the basis of the volume integration of the raw material and are expressed in percentage of dried weight
[27][15]. The chemical characterization of vegetal biomass is highly relevant for their consideration as potential sources of carbon to produce bioethanol. Indeed, for bioethanol production, it is important to evaluate the cellulose, hemicellulose and lignin composition of the raw material
[9]. Owing to its high yield of sugar and lignocellulosic biomass, SCB is regarded as an excellent alternative energy source to substitute fossil fuels
[28,29][16][17].
Table 1.
Composition of the main sugarcane chemical compounds expressed in percentage of dried weight.